![man pages
5.6. whereis
The location of a manpage can be revealed with whereis.
paul@laika:~$ whereis -m whois
whois: /usr/share/man/man1/whois.1.gz
This file is directly readable by man.
paul@laika:~$ man /usr/share/man/man1/whois.1.gz
5.7. man sections
By now you will have noticed the numbers between the round brackets. man man
will explain to you that these are section numbers. Executable programs and shell
commands reside in section one.
1 Executable programs or shell commands
2 System calls (functions provided by the kernel)
3 Library calls (functions within program libraries)
4 Special files (usually found in /dev)
5 File formats and conventions eg /etc/passwd
6 Games
7 Miscellaneous (including macro packages and conventions), e.g. man(7)
8 System administration commands (usually only for root)
9 Kernel routines [Non standard]
5.8. man $section $file
Therefor, when referring to the man page of the passwd command, you will see it
written as passwd(1); when referringto the passwd file, you will see it written as
passwd(5). The screenshot explains how to open the man page in the correct section.
[paul@RHEL52 ~]$ man passwd # opens the lowest section (1)
[paul@RHEL52 ~]$ man 5 passwd # opens a page from section 5
5.9. man man
If you want to know more about man, then Read The Fantastic Manual (RTFM).
Unfortunately, manual pages do not have the answer to everything...
paul@laika:~$ man woman
No manual entry for woman
21](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-27-320.jpg)
![working with files
7.7. mv
Use mv to rename a file or to move the file to another directory.
paul@laika:~/test$ touch file100
paul@laika:~/test$ ls
file100
paul@laika:~/test$ mv file100 ABC.txt
paul@laika:~/test$ ls
ABC.txt
paul@laika:~/test$
When you need to rename only one file then mv is the preferred command to use.
7.8. rename
The rename command can also be used but it has a more complex syntax to enable
renaming of many files at once. Below are two examples, the first switches all
occurrences of txt to png for all filenames ending in .txt. The second example switches
all occurrences of uppercase ABC in lowercase abc for all filenames ending in .png .
The following syntax will work on debian and ubuntu (prior to Ubuntu 7.10).
paul@laika:~/test$ ls
123.txt ABC.txt
paul@laika:~/test$ rename 's/txt/png/' *.txt
paul@laika:~/test$ ls
123.png ABC.png
paul@laika:~/test$ rename 's/ABC/abc/' *.png
paul@laika:~/test$ ls
123.png abc.png
paul@laika:~/test$
On Red Hat Enterprise Linux (and many other Linux distributions like Ubuntu 8.04),
the syntax of rename is a bit different. The first example below renames all *.conf
files replacing any occurrence of conf with bak. The second example renames all (*)
files replacing one with ONE.
[paul@RHEL4a test]$ ls
one.conf two.conf
[paul@RHEL4a test]$ rename conf bak *.conf
[paul@RHEL4a test]$ ls
one.bak two.bak
[paul@RHEL4a test]$ rename one ONE *
[paul@RHEL4a test]$ ls
ONE.bak two.bak
[paul@RHEL4a test]$
38](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-44-320.jpg)
![the Linux file tree
9.1. filesystem hierarchy standard
Many Linux distributions partially follow the Filesystem Hierarchy Standard. The
FHS may help make more Unix/Linux file system trees conform better in the future.
The FHS is available online at http://www.pathname.com/fhs/ where we read: "The
filesystem hierarchy standard has been designed to be used by Unix distribution
developers, package developers, and system implementors. However, it is primarily
intended to be a reference and is not a tutorial on how to manage a Unix filesystem
or directory hierarchy."
9.2. man hier
There are some differences in the filesystems between Linux distributions. For
help about your machine, enter man hier to find information about the file system
hierarchy. This manual will explain the directory structure on your computer.
9.3. the root directory /
All Linux systems have a directory structure that starts at the root directory. The
root directory is represented by a forward slash, like this: /. Everything that exists
on your Linux system can be found below this root directory. Let's take a brief look
at the contents of the root directory.
[paul@RHELv4u3 ~]$ ls /
bin dev home media mnt proc sbin srv tftpboot usr
boot etc lib misc opt root selinux sys tmp var
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![the Linux file tree
0: 13876877 IO-APIC-edge timer
1: 15 IO-APIC-edge i8042
8: 1 IO-APIC-edge rtc
9: 0 IO-APIC-level acpi
12: 67 IO-APIC-edge i8042
14: 128 IO-APIC-edge ide0
15: 124320 IO-APIC-edge ide1
169: 111993 IO-APIC-level ioc0
177: 2428 IO-APIC-level eth0
NMI: 0
LOC: 13878037
ERR: 0
MIS: 0
On a machine with two CPU's, the file looks like this.
paul@laika:~$ cat /proc/interrupts
CPU0 CPU1
0: 860013 0 IO-APIC-edge timer
1: 4533 0 IO-APIC-edge i8042
7: 0 0 IO-APIC-edge parport0
8: 6588227 0 IO-APIC-edge rtc
10: 2314 0 IO-APIC-fasteoi acpi
12: 133 0 IO-APIC-edge i8042
14: 0 0 IO-APIC-edge libata
15: 72269 0 IO-APIC-edge libata
18: 1 0 IO-APIC-fasteoi yenta
19: 115036 0 IO-APIC-fasteoi eth0
20: 126871 0 IO-APIC-fasteoi libata, ohci1394
21: 30204 0 IO-APIC-fasteoi ehci_hcd:usb1, uhci_hcd:usb2
22: 1334 0 IO-APIC-fasteoi saa7133[0], saa7133[0]
24: 234739 0 IO-APIC-fasteoi nvidia
NMI: 72 42
LOC: 860000 859994
ERR: 0
/proc/kcore
The physical memory is represented in /proc/kcore. Do not try to cat this file, instead
use a debugger. The size of /proc/kcore is the same as your physical memory, plus
four bytes.
paul@laika:~$ ls -lh /proc/kcore
-r-------- 1 root root 2.0G 2007-01-30 08:57 /proc/kcore
paul@laika:~$
/sys Linux 2.6 hot plugging
The /sys directory was created for the Linux 2.6 kernel. Since 2.6, Linux uses sysfs
to support usb and IEEE 1394 (FireWire) hot plug devices. See the manual pages
of udev(8) (the successor of devfs) and hotplug(8) for more info (or visit http://linux-
hotplug.sourceforge.net/ ).
Basically the /sys directory contains kernel information about hardware.
60](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-66-320.jpg)
![the Linux file tree
9.9. /var variable data
Files that are unpredictable in size, such as log, cache and spool files, should be
located in /var.
/var/log
The /var/log directory serves as a central point to contain all log files.
[paul@RHEL4b ~]$ ls /var/log
acpid cron.2 maillog.2 quagga secure.4
amanda cron.3 maillog.3 radius spooler
anaconda.log cron.4 maillog.4 rpmpkgs spooler.1
anaconda.syslog cups mailman rpmpkgs.1 spooler.2
anaconda.xlog dmesg messages rpmpkgs.2 spooler.3
audit exim messages.1 rpmpkgs.3 spooler.4
boot.log gdm messages.2 rpmpkgs.4 squid
boot.log.1 httpd messages.3 sa uucp
boot.log.2 iiim messages.4 samba vbox
boot.log.3 iptraf mysqld.log scrollkeeper.log vmware-tools-guestd
boot.log.4 lastlog news secure wtmp
canna mail pgsql secure.1 wtmp.1
cron maillog ppp secure.2 Xorg.0.log
cron.1 maillog.1 prelink.log secure.3 Xorg.0.log.old
/var/log/messages
A typical first file to check when troubleshooting is the /var/log/messages file. By
default this file will contain information on what just happened to the system.
[root@RHEL4b ~]# tail /var/log/messages
Jul 30 05:13:56 anacron: anacron startup succeeded
Jul 30 05:13:56 atd: atd startup succeeded
Jul 30 05:13:57 messagebus: messagebus startup succeeded
Jul 30 05:13:57 cups-config-daemon: cups-config-daemon startup succeeded
Jul 30 05:13:58 haldaemon: haldaemon startup succeeded
Jul 30 05:14:00 fstab-sync[3560]: removed all generated mount points
Jul 30 05:14:01 fstab-sync[3628]: added mount point /media/cdrom for...
Jul 30 05:14:01 fstab-sync[3646]: added mount point /media/floppy for...
Jul 30 05:16:46 sshd(pam_unix)[3662]: session opened for user paul by...
Jul 30 06:06:37 su(pam_unix)[3904]: session opened for user root by paul
/var/cache
The /var/cache directory can contain cache data for several applications.
paul@ubu1010:~$ ls /var/cache/
apt dictionaries-common gdm man software-center
binfmts flashplugin-installer hald pm-utils
cups fontconfig jockey pppconfig
debconf fonts ldconfig samba
63](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-69-320.jpg)
![commands and arguments
10.1. echo
This chapter frequently uses the echo command to demonstrate shell features. The
echo command is very simple: it echoes the input that it receives.
paul@laika:~$ echo Burtonville
Burtonville
paul@laika:~$ echo Smurfs are blue
Smurfs are blue
10.2. arguments
One of the primary features of a shell is to perform a command line scan. When
you enter a command at the shell's command prompt and press the enter key, then
the shell will start scanning that line, cutting it up in arguments. While scanning the
line, the shell may make many changes to the arguments you typed. This process
is called shell expansion. When the shell has finished scanning and modifying that
line, then it will be executed.
white space removal
Parts that are separated by one or more consecutive white spaces (or tabs) are
considered separate arguments, any white space is removed. The first argument is
the command to be executed, the other arguments are given to the command. The
shell effectively cuts your command into one or more arguments.
This explains why the following four different command lines are the same after shell
expansion.
[paul@RHELv4u3 ~]$ echo Hello World
Hello World
[paul@RHELv4u3 ~]$ echo Hello World
Hello World
[paul@RHELv4u3 ~]$ echo Hello World
Hello World
[paul@RHELv4u3 ~]$ echo Hello World
Hello World
The echo command will display each argument it receives from the shell. The echo
command will also add a new whitespace between the arguments it received.
single quotes
You can prevent the removal of white spaces by quoting the spaces. The contents of
the quoted string are considered as one argument. In the screenshot below the echo
receives only one argument.
[paul@RHEL4b ~]$ echo 'A line with single quotes'
A line with single quotes
71](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-77-320.jpg)
![commands and arguments
[paul@RHEL4b ~]$
double quotes
You can also prevent the removal of white spaces by double quoting the spaces. Same
as above, echo only receives one argument.
[paul@RHEL4b ~]$ echo "A line with double quotes"
A line with double quotes
[paul@RHEL4b ~]$
echo and quotes
Quoted lines can include special escaped characters recognized by the echo command
(when using echo -e). The screenshot below shows how to use n for a newline and
t for a tab (usually eight white spaces).
[paul@RHEL4b ~]$ echo -e "A line with na newline"
A line with
a newline
[paul@RHEL4b ~]$ echo -e 'A line with na newline'
A line with
a newline
[paul@RHEL4b ~]$ echo -e "A line with ta tab"
A line with a tab
[paul@RHEL4b ~]$ echo -e 'A line with ta tab'
A line with a tab
[paul@RHEL4b ~]$
The echo command can generate more than white spaces, tabs and newlines. Look
in the man page for a list of options.
10.3. commands
external or builtin commands ?
Not all commands are external to the shell, some are builtin. External commands
are programs that have their own binary and reside somewhere in the file system.
Many external commands are located in /bin or /sbin. Builtin commands are an
integral part of the shell program itself.
type
To find out whether a command given to the shell will be executed as an external
command or as a builtin command, use the type command.
paul@laika:~$ type cd
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![commands and arguments
cd is a shell builtin
paul@laika:~$ type cat
cat is /bin/cat
As you can see, the cd command is builtin and the cat command is external.
You can also use this command to show you whether the command is aliased or not.
paul@laika:~$ type ls
ls is aliased to `ls --color=auto'
running external commands
Some commands have both builtin and external versions. When one of these
commands is executed, the builtin version takes priority. To run the external version,
you must enter the full path to the command.
paul@laika:~$ type -a echo
echo is a shell builtin
echo is /bin/echo
paul@laika:~$ /bin/echo Running the external echo command...
Running the external echo command...
which
The which command will search for binaries in the $PATH environment variable
(variables will be explained later). In the screenshot below, it is determined that cd
is builtin, and ls, cp, rm, mv, mkdir, pwd, and which are external commands.
[root@RHEL4b ~]# which cp ls mv rm cd mkdir pwd which
/bin/cp
/bin/ls
/bin/mv
/bin/rm
/usr/bin/which: no cd in (/usr/kerberos/sbin:/usr/kerberos/bin:...
/bin/mkdir
/bin/pwd
/usr/bin/which
10.4. aliases
create an alias
The shell allows you to create aliases. Aliases are often used to create an easier to
remember name for an existing command or to easily supply parameters.
[paul@RHELv4u3 ~]$ cat count.txt
one
two
three
[paul@RHELv4u3 ~]$ alias dog=tac
[paul@RHELv4u3 ~]$ dog count.txt
73](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-79-320.jpg)
![commands and arguments
three
two
one
abbreviate commands
An alias can also be useful to abbreviate an existing command.
paul@laika:~$ alias ll='ls -lh --color=auto'
paul@laika:~$ alias c='clear'
paul@laika:~$
default options
Aliases can be used to supply commands with default options. The example below
shows how to set the -i option default when typing rm.
[paul@RHELv4u3 ~]$ rm -i winter.txt
rm: remove regular file `winter.txt'? no
[paul@RHELv4u3 ~]$ rm winter.txt
[paul@RHELv4u3 ~]$ ls winter.txt
ls: winter.txt: No such file or directory
[paul@RHELv4u3 ~]$ touch winter.txt
[paul@RHELv4u3 ~]$ alias rm='rm -i'
[paul@RHELv4u3 ~]$ rm winter.txt
rm: remove regular empty file `winter.txt'? no
[paul@RHELv4u3 ~]$
Some distributions enable default aliases to protect users from accidentally erasing
files ('rm -i', 'mv -i', 'cp -i')
viewing aliases
You can provide one or more aliases as arguments to the alias command to get their
definitions. Providing no arguments gives a complete list of current aliases.
paul@laika:~$ alias c ll
alias c='clear'
alias ll='ls -lh --color=auto'
unalias
You can undo an alias with the unalias command.
[paul@RHEL4b ~]$ which rm
/bin/rm
[paul@RHEL4b ~]$ alias rm='rm -i'
[paul@RHEL4b ~]$ which rm
alias rm='rm -i'
/bin/rm
[paul@RHEL4b ~]$ unalias rm
[paul@RHEL4b ~]$ which rm
/bin/rm
74](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-80-320.jpg)
![commands and arguments
[paul@RHEL4b ~]$
10.5. displaying shell expansion
You can display shell expansion with set -x, and stop displaying it with set +x. You
might want to use this further on in this course, or when in doubt about exactly what
the shell is doing with your command.
[paul@RHELv4u3 ~]$ set -x
++ echo -ne '033]0;paul@RHELv4u3:~007'
[paul@RHELv4u3 ~]$ echo $USER
+ echo paul
paul
++ echo -ne '033]0;paul@RHELv4u3:~007'
[paul@RHELv4u3 ~]$ echo $USER
+ echo '$USER'
$USER
++ echo -ne '033]0;paul@RHELv4u3:~007'
[paul@RHELv4u3 ~]$ set +x
+ set +x
[paul@RHELv4u3 ~]$ echo $USER
paul
75](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-81-320.jpg)
![control operators
11.1. ; semicolon
You can put two or more commands on the same line separated by a semicolon ; .
The shell will scan the line until it reaches the semicolon. All the arguments before
this semicolon will be considered a separate command from all the arguments after
the semicolon. Both series will be executed sequentially with the shell waiting for
each command to finish before starting the next one.
[paul@RHELv4u3 ~]$ echo Hello
Hello
[paul@RHELv4u3 ~]$ echo World
World
[paul@RHELv4u3 ~]$ echo Hello ; echo World
Hello
World
[paul@RHELv4u3 ~]$
11.2. & ampersand
When a line ends with an ampersand &, the shell will not wait for the command
to finish. You will get your shell prompt back, and the command is executed in
background. You will get a message when this command has finished executing in
background.
[paul@RHELv4u3 ~]$ sleep 20 &
[1] 7925
[paul@RHELv4u3 ~]$
...wait 20 seconds...
[paul@RHELv4u3 ~]$
[1]+ Done sleep 20
The technical explanation of what happens in this case is explained in the chapter
about processes.
11.3. $? dollar question mark
The exit code of the previous command is stored in the shell variable $?. Actually $?
is a shell parameter and not a variable, since you cannot assign a value to $?.
paul@debian5:~/test$ touch file1
paul@debian5:~/test$ echo $?
0
paul@debian5:~/test$ rm file1
paul@debian5:~/test$ echo $?
0
paul@debian5:~/test$ rm file1
rm: cannot remove `file1': No such file or directory
paul@debian5:~/test$ echo $?
1
paul@debian5:~/test$
81](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-87-320.jpg)
![control operators
11.4. && double ampersand
The shell will interprete && as a logical AND. When using && the second command
is executed only if the first one succeeds (returns a zero exit status).
paul@barry:~$ echo first && echo second
first
second
paul@barry:~$ zecho first && echo second
-bash: zecho: command not found
Another example of the same logical AND principle. This example starts with a
working cd followed by ls, then a non-working cd which is not followed by ls.
[paul@RHELv4u3 ~]$ cd gen && ls
file1 file3 File55 fileab FileAB fileabc
file2 File4 FileA Fileab fileab2
[paul@RHELv4u3 gen]$ cd gen && ls
-bash: cd: gen: No such file or directory
[paul@RHELv4u3 gen]$
11.5. || double vertical bar
The || represents a logical OR. The second command is executed only when the first
command fails (returns a non-zero exit status).
paul@barry:~$ echo first || echo second ; echo third
first
third
paul@barry:~$ zecho first || echo second ; echo third
-bash: zecho: command not found
second
third
paul@barry:~$
Another example of the same logical OR principle.
[paul@RHELv4u3 ~]$ cd gen || ls
[paul@RHELv4u3 gen]$ cd gen || ls
-bash: cd: gen: No such file or directory
file1 file3 File55 fileab FileAB fileabc
file2 File4 FileA Fileab fileab2
[paul@RHELv4u3 gen]$
11.6. combining && and ||
You can use this logical AND and logical OR to write an if-then-else structure on
the command line. This example uses echo to display whether the rm command was
successful.
paul@laika:~/test$ rm file1 && echo It worked! || echo It failed!
It worked!
paul@laika:~/test$ rm file1 && echo It worked! || echo It failed!
rm: cannot remove `file1': No such file or directory
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![control operators
It failed!
paul@laika:~/test$
11.7. # pound sign
Everything written after a pound sign (#) is ignored by the shell. This is useful to
write a shell comment, but has no influence on the command execution or shell
expansion.
paul@debian4:~$ mkdir test # we create a directory
paul@debian4:~$ cd test #### we enter the directory
paul@debian4:~/test$ ls # is it empty ?
paul@debian4:~/test$
11.8. escaping special characters
The backslash character enables the use of control characters, but without the shell
interpreting it, this is called escaping characters.
[paul@RHELv4u3 ~]$ echo hello ; world
hello ; world
[paul@RHELv4u3 ~]$ echo hello world
hello world
[paul@RHELv4u3 ~]$ echo escaping # & " '
escaping # & " '
[paul@RHELv4u3 ~]$ echo escaping ?*"'
escaping ?*"'
end of line backslash
Lines ending in a backslash are continued on the next line. The shell does not interpret
the newline character and will wait on shell expansion and execution of the command
line until a newline without backslash is encountered.
[paul@RHEL4b ~]$ echo This command line
> is split in three
> parts
This command line is split in three parts
[paul@RHEL4b ~]$
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![variables
12.1. about variables
$ dollar sign
Another important character interpreted by the shell is the dollar sign $. The shell
will look for an environment variable named like the string following the dollar
sign and replace it with the value of the variable (or with nothing if the variable does
not exist).
These are some examples using $HOSTNAME, $USER, $UID, $SHELL, and
$HOME.
[paul@RHELv4u3 ~]$ echo This is the $SHELL shell
This is the /bin/bash shell
[paul@RHELv4u3 ~]$ echo This is $SHELL on computer $HOSTNAME
This is /bin/bash on computer RHELv4u3.localdomain
[paul@RHELv4u3 ~]$ echo The userid of $USER is $UID
The userid of paul is 500
[paul@RHELv4u3 ~]$ echo My homedir is $HOME
My homedir is /home/paul
case sensitive
This example shows that shell variables are case sensitive!
[paul@RHELv4u3 ~]$ echo Hello $USER
Hello paul
[paul@RHELv4u3 ~]$ echo Hello $user
Hello
$PS1
The $PS1 variable determines your shell prompt. You can use backslash escaped
special characters like u for the username or w for the working directory. The bash
manual has a complete reference.
In this example we change the value of $PS1 a couple of times.
paul@deb503:~$ PS1=prompt
prompt
promptPS1='prompt '
prompt
prompt PS1='> '
>
> PS1='u@h$ '
paul@deb503$
paul@deb503$ PS1='u@h:W$'
paul@deb503:~$
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![variables
$PATH
The $PATH variable is determines where the shell is looking for commands to
execute (unless the command is builtin or aliased). This variable contains a list of
directories, separated by colons.
[[paul@RHEL4b ~]$ echo $PATH
/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:
The shell will not look in the current directory for commands to execute! (Looking
for executables in the current directory provided an easy way to hack PC-DOS
computers). If you want the shell to look in the current directory, then add a . at the
end of your $PATH.
[paul@RHEL4b ~]$ PATH=$PATH:.
[paul@RHEL4b ~]$ echo $PATH
/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:.
[paul@RHEL4b ~]$
Your path might be different when using su instead of su - because the latter will take
on the environment of the target user. The root user typically has /sbin directories
added to the $PATH variable.
[paul@RHEL3 ~]$ su
Password:
[root@RHEL3 paul]# echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin
[root@RHEL3 paul]# exit
[paul@RHEL3 ~]$ su -
Password:
[root@RHEL3 ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:
[root@RHEL3 ~]#
creating variables
This example creates the variable $MyVar and sets its value. It then uses echo to
verify the value.
[paul@RHELv4u3 gen]$ MyVar=555
[paul@RHELv4u3 gen]$ echo $MyVar
555
[paul@RHELv4u3 gen]$
12.2. quotes
Notice that double quotes still allow the parsing of variables, whereas single quotes
prevent this.
[paul@RHELv4u3 ~]$ MyVar=555
[paul@RHELv4u3 ~]$ echo $MyVar
555
[paul@RHELv4u3 ~]$ echo "$MyVar"
555
88](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-94-320.jpg)
![variables
[paul@RHELv4u3 ~]$ echo '$MyVar'
$MyVar
The bash shell will replace variables with their value in double quoted lines, but not
in single quoted lines.
paul@laika:~$ city=Burtonville
paul@laika:~$ echo "We are in $city today."
We are in Burtonville today.
paul@laika:~$ echo 'We are in $city today.'
We are in $city today.
12.3. set
You can use the set command to display a list of environment variables. On Ubuntu
and Debian systems, the set command will also list shell functions after the shell
variables. Use set | more to see the variables then.
12.4. unset
Use the unset command to remove a variable from your shell environment.
[paul@RHEL4b ~]$ MyVar=8472
[paul@RHEL4b ~]$ echo $MyVar
8472
[paul@RHEL4b ~]$ unset MyVar
[paul@RHEL4b ~]$ echo $MyVar
[paul@RHEL4b ~]$
12.5. env
The env command without options will display a list of exported variables. The
difference with set with options is that set lists all variables, inlcuding those not
exported to child shells.
But env can also be used to start a clean shell (a shell without any inherited
environment). The env -i command clears the environment for the subshell.
Notice in this screenshot that bash will set the $SHELL variable on startup.
[paul@RHEL4b ~]$ bash -c 'echo $SHELL $HOME $USER'
/bin/bash /home/paul paul
[paul@RHEL4b ~]$ env -i bash -c 'echo $SHELL $HOME $USER'
/bin/bash
[paul@RHEL4b ~]$
You can use the env command to set the $LANG, or any other, variable for just
one instance of bash with one command. The example below uses this to show the
influence of the $LANG variable on file globbing (see the chapter on file globbing).
[paul@RHEL4b test]$ env LANG=C bash -c 'ls File[a-z]'
89](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-95-320.jpg)
![variables
Filea Fileb
[paul@RHEL4b test]$ env LANG=en_US.UTF-8 bash -c 'ls File[a-z]'
Filea FileA Fileb FileB
[paul@RHEL4b test]$
12.6. export
You can export shell variables to other shells with the export command. This will
export the variable to child shells.
[paul@RHEL4b ~]$ var3=three
[paul@RHEL4b ~]$ var4=four
[paul@RHEL4b ~]$ export var4
[paul@RHEL4b ~]$ echo $var3 $var4
three four
[paul@RHEL4b ~]$ bash
[paul@RHEL4b ~]$ echo $var3 $var4
four
But it will not export to the parent shell (previous screenshot continued).
[paul@RHEL4b ~]$ export var5=five
[paul@RHEL4b ~]$ echo $var3 $var4 $var5
four five
[paul@RHEL4b ~]$ exit
exit
[paul@RHEL4b ~]$ echo $var3 $var4 $var5
three four
[paul@RHEL4b ~]$
12.7. delineate variables
Until now, we have seen that bash interpretes a variable starting from a dollar sign,
continuing until the first occurrence of a non-alphanumeric character that is not an
underscore. In some situations, this can be a problem. This issue can be resolved with
curly braces like in this example.
[paul@RHEL4b ~]$ prefix=Super
[paul@RHEL4b ~]$ echo Hello $prefixman and $prefixgirl
Hello and
[paul@RHEL4b ~]$ echo Hello ${prefix}man and ${prefix}girl
Hello Superman and Supergirl
[paul@RHEL4b ~]$
12.8. unbound variables
The example below tries to display the value of the $MyVar variable, but it fails
because the variable does not exist. By default the shell will display nothing when a
variable is unbound (does not exist).
[paul@RHELv4u3 gen]$ echo $MyVar
[paul@RHELv4u3 gen]$
90](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-96-320.jpg)
![variables
There is, however, the nounset shell option that you can use to generate an error
when a variable does not exist.
paul@laika:~$ set -u
paul@laika:~$ echo $Myvar
bash: Myvar: unbound variable
paul@laika:~$ set +u
paul@laika:~$ echo $Myvar
paul@laika:~$
In the bash shell set -u is identical to set -o nounset and likewise set +u is identical
to set +o nounset.
12.9. shell options
Both set and unset are builtin shell commands. They can be used to set options of
the bash shell itself. The next example will clarify this. By default, the shell will treat
unset variables as a variable having no value. By setting the -u option, the shell will
treat any reference to unset variables as an error. See the man page of bash for more
information.
[paul@RHEL4b ~]$ echo $var123
[paul@RHEL4b ~]$ set -u
[paul@RHEL4b ~]$ echo $var123
-bash: var123: unbound variable
[paul@RHEL4b ~]$ set +u
[paul@RHEL4b ~]$ echo $var123
[paul@RHEL4b ~]$
To list all the set options for your shell, use echo $-. The noclobber (or -C) option
will be explained later in this book (in the I/O redirection chapter).
[paul@RHEL4b ~]$ echo $-
himBH
[paul@RHEL4b ~]$ set -C ; set -u
[paul@RHEL4b ~]$ echo $-
himuBCH
[paul@RHEL4b ~]$ set +C ; set +u
[paul@RHEL4b ~]$ echo $-
himBH
[paul@RHEL4b ~]$
When typing set without options, you get a list of all variables without function when
the shell is on posix mode. You can set bash in posix mode typing set -o posix.
12.10. shell embedding
Shells can be embedded on the command line, or in other words, the command line
scan can spawn new processes containing a fork of the current shell. You can use
variables to prove that new shells are created. In the screenshot below, the variable
$var1 only exists in the (temporary) sub shell.
91](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-97-320.jpg)
![variables
[paul@RHELv4u3 gen]$ echo $var1
[paul@RHELv4u3 gen]$ echo $(var1=5;echo $var1)
5
[paul@RHELv4u3 gen]$ echo $var1
[paul@RHELv4u3 gen]$
You can embed a shell in an embedded shell, this is called nested embedding of
shells.
This screenshot shows an embedded shell inside an embedded shell.
paul@deb503:~$ A=shell
paul@deb503:~$ echo $C$B$A $(B=sub;echo $C$B$A; echo $(A=sub;echo $C$B$A))
shell subshell subsub
back ticks
Single embedding can be useful to avoid changing your current directory. The
screenshot below uses back ticks instead of dollar-bracket to embed.
[paul@RHELv4u3 ~]$ echo `cd /etc; ls -d * | grep pass`
passwd passwd- passwd.OLD
[paul@RHELv4u3 ~]$
You can only use the $() notation to nest embedded shells, backticks cannot do this.
back ticks or single quotes
Placing the embedding between backticks uses one character less than the dollar
and parenthesis combo. Be careful however, backticks are often confused with single
quotes. The technical difference between ' and ` is significant!
[paul@RHELv4u3 gen]$ echo `var1=5;echo $var1`
5
[paul@RHELv4u3 gen]$ echo 'var1=5;echo $var1'
var1=5;echo $var1
[paul@RHELv4u3 gen]$
92](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-98-320.jpg)
![variables
12.11. practice: shell variables
1. Use echo to display Hello followed by your username. (use a bash variable!)
2. Create a variable answer with a value of 42.
3. Copy the value of $LANG to $MyLANG.
4. List all current shell variables.
5. List all exported shell variables.
6. Do the env and set commands display your variable ?
6. Destroy your answer variable.
7. Find the list of shell options in the man page of bash. What is the difference
between set -u and set -o nounset?
8. Create two variables, and export one of them.
9. Display the exported variable in an interactive child shell.
10. Create a variable, give it the value 'Dumb', create another variable with value 'do'.
Use echo and the two variables to echo Dumbledore.
11. Activate nounset in your shell. Test that it shows an error message when using
non-existing variables.
12. Deactivate nounset.
13. Find the list of backslash escaped characters in the manual of bash. Add the time
to your PS1 prompt.
14. Execute cd /var and ls in an embedded shell.
15. Create the variable embvar in an embedded shell and echo it. Does the variable
exist in your current shell now ?
16. Explain what "set -x" does. Can this be useful ?
(optional)17. Given the following screenshot, add exactly four characters to that
command line so that the total output is FirstMiddleLast.
[paul@RHEL4b ~]$ echo First; echo Middle; echo Last
18. Display a long listing (ls -l) of the passwd command using the which command
inside back ticks.
93](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-99-320.jpg)
![variables
set -u
OR
set -o nounset
Both these lines have the same effect.
12. Deactivate nounset.
set +u
OR
set +o nounset
13. Find the list of backslash escaped characters in the manual of bash. Add the time
to your PS1 prompt.
PS1='t u@h W$ '
14. Execute cd /var and ls in an embedded shell.
echo $(cd /var ; ls)
The echo command is only needed to show the result of the ls command. Omitting
will result in the shell trying to execute the first file as a command.
15. Create the variable embvar in an embedded shell and echo it. Does the variable
exist in your current shell now ?
$(embvar=emb;echo $embvar) ; echo $embvar (the last echo fails).
$embvar does not exist in your current shell
16. Explain what "set -x" does. Can this be useful ?
It displays shell expansion for troubleshooting your command.
(optional)17. Given the following screenshot, add exactly four characters to that
command line so that the total output is FirstMiddleLast.
[paul@RHEL4b ~]$ echo First; echo Middle; echo Last
echo -n First; echo -n Middle; echo Last
18. Display a long listing (ls -l) of the passwd command using the which command
inside back ticks.
ls -l `which passwd`
95](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-101-320.jpg)
![Chapter 14. file globbing
Table of Contents
14.1. * asterisk .................................................................................................... 103
14.2. ? question mark .......................................................................................... 103
14.3. [] square brackets ....................................................................................... 103
14.4. a-z and 0-9 ranges ..................................................................................... 104
14.5. $LANG and square brackets ...................................................................... 104
14.6. preventing file globbing ............................................................................. 104
14.7. practice: shell globbing .............................................................................. 106
14.8. solution: shell globbing .............................................................................. 107
The shell is also responsible for file globbing (or dynamic filename generation). This
chapter will explain file globbing.
102](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-108-320.jpg)
![file globbing
14.1. * asterisk
The asterisk * is interpreted by the shell as a sign to generate filenames, matching
the asterisk to any combination of characters (even none). When no path is given,
the shell will use filenames in the current directory. See the man page of glob(7) for
more information. (This is part of LPI topic 1.103.3.)
[paul@RHELv4u3 gen]$ ls
file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc
[paul@RHELv4u3 gen]$ ls File*
File4 File55 FileA Fileab FileAB
[paul@RHELv4u3 gen]$ ls file*
file1 file2 file3 fileab fileabc
[paul@RHELv4u3 gen]$ ls *ile55
File55
[paul@RHELv4u3 gen]$ ls F*ile55
File55
[paul@RHELv4u3 gen]$ ls F*55
File55
[paul@RHELv4u3 gen]$
14.2. ? question mark
Similar to the asterisk, the question mark ? is interpreted by the shell as a sign to
generate filenames, matching the question mark with exactly one character.
[paul@RHELv4u3 gen]$ ls
file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc
[paul@RHELv4u3 gen]$ ls File?
File4 FileA
[paul@RHELv4u3 gen]$ ls Fil?4
File4
[paul@RHELv4u3 gen]$ ls Fil??
File4 FileA
[paul@RHELv4u3 gen]$ ls File??
File55 Fileab FileAB
[paul@RHELv4u3 gen]$
14.3. [] square brackets
The square bracket [ is interpreted by the shell as a sign to generate filenames,
matching any of the characters between [ and the first subsequent ]. The order in this
list between the brackets is not important. Each pair of brackets is replaced by exactly
one character.
[paul@RHELv4u3 gen]$ ls
file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc
[paul@RHELv4u3 gen]$ ls File[5A]
FileA
[paul@RHELv4u3 gen]$ ls File[A5]
FileA
[paul@RHELv4u3 gen]$ ls File[A5][5b]
File55
[paul@RHELv4u3 gen]$ ls File[a5][5b]
File55 Fileab
[paul@RHELv4u3 gen]$ ls File[a5][5b][abcdefghijklm]
ls: File[a5][5b][abcdefghijklm]: No such file or directory
103](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-109-320.jpg)
![file globbing
[paul@RHELv4u3 gen]$ ls file[a5][5b][abcdefghijklm]
fileabc
[paul@RHELv4u3 gen]$
You can also exclude characters from a list between square brackets with the
exclamation mark !. And you are allowed to make combinations of these wild cards.
[paul@RHELv4u3 gen]$ ls
file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc
[paul@RHELv4u3 gen]$ ls file[a5][!Z]
fileab
[paul@RHELv4u3 gen]$ ls file[!5]*
file1 file2 file3 fileab fileabc
[paul@RHELv4u3 gen]$ ls file[!5]?
fileab
[paul@RHELv4u3 gen]$
14.4. a-z and 0-9 ranges
The bash shell will also understand ranges of characters between brackets.
[paul@RHELv4u3 gen]$ ls
file1 file3 File55 fileab FileAB fileabc
file2 File4 FileA Fileab fileab2
[paul@RHELv4u3 gen]$ ls file[a-z]*
fileab fileab2 fileabc
[paul@RHELv4u3 gen]$ ls file[0-9]
file1 file2 file3
[paul@RHELv4u3 gen]$ ls file[a-z][a-z][0-9]*
fileab2
[paul@RHELv4u3 gen]$
14.5. $LANG and square brackets
But, don't forget the influence of the LANG variable. Some languages include
lowercase letters in an uppercase range (and vice versa).
paul@RHELv4u4:~/test$ ls [A-Z]ile?
file1 file2 file3 File4
paul@RHELv4u4:~/test$ ls [a-z]ile?
file1 file2 file3 File4
paul@RHELv4u4:~/test$ echo $LANG
en_US.UTF-8
paul@RHELv4u4:~/test$ LANG=C
paul@RHELv4u4:~/test$ echo $LANG
C
paul@RHELv4u4:~/test$ ls [a-z]ile?
file1 file2 file3
paul@RHELv4u4:~/test$ ls [A-Z]ile?
File4
paul@RHELv4u4:~/test$
14.6. preventing file globbing
The screenshot below should be no surprise. The echo * will echo a * when in an
empty directory. And it will echo the names of all files when the directory is not
empty.
104](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-110-320.jpg)
![file globbing
14.8. solution: shell globbing
1. Create a test directory and enter it.
mkdir testdir; cd testdir
2. Create files file1 file10 file11 file2 File2 File3 file33 fileAB filea fileA fileAAA
file( file 2 (the last one has 6 characters including a space)
touch file1 file10 file11 file2 File2 File3
touch file33 fileAB filea fileA fileAAA
touch "file("
touch "file 2"
3. List (with ls) all files starting with file
ls file*
4. List (with ls) all files starting with File
ls File*
5. List (with ls) all files starting with file and ending in a number.
ls file*[0-9]
6. List (with ls) all files starting with file and ending with a letter
ls file*[a-z]
7. List (with ls) all files starting with File and having a digit as fifth character.
ls File[0-9]*
8. List (with ls) all files starting with File and having a digit as fifth character and
nothing else.
ls File[0-9]
9. List (with ls) all files starting with a letter and ending in a number.
ls [a-z]*[0-9]
10. List (with ls) all files that have exactly five characters.
ls ?????
11. List (with ls) all files that start with f or F and end with 3 or A.
ls [fF]*[3A]
12. List (with ls) all files that start with f have i or R as second character and end
in a number.
ls f[iR]*[0-9]
13. List all files that do not start with the letter F.
ls [!F]*
107](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-113-320.jpg)
![redirection and pipes
15.1. stdin, stdout, and stderr
The shell (and almost every other Linux command) takes input from stdin (stream
0) and sends output to stdout (stream 1) and error messages to stderr (stream 2) .
The keyboard often server as stdin, stdout and stderr both go to the disply. The shell
allows you to redirect these streams.
15.2. output redirection
> stdout
stdout can be redirected with a greater than sign. While scanning the line, the shell
will see the > sign and will clear the file.
[paul@RHELv4u3 ~]$ echo It is cold today!
It is cold today!
[paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt
[paul@RHELv4u3 ~]$ cat winter.txt
It is cold today!
[paul@RHELv4u3 ~]$
Note that the > notation is in fact the abbreviation of 1> (stdout being referred to
as stream 1.
output file is erased
To repeat: While scanning the line, the shell will see the > sign and will clear the
file! This means that even when the command fails, the file will be cleared!
[paul@RHELv4u3 ~]$ cat winter.txt
It is cold today!
[paul@RHELv4u3 ~]$ zcho It is cold today! > winter.txt
-bash: zcho: command not found
[paul@RHELv4u3 ~]$ cat winter.txt
[paul@RHELv4u3 ~]$
noclobber
Erasing a file while using > can be prevented by setting the noclobber option.
[paul@RHELv4u3 ~]$ cat winter.txt
It is cold today!
[paul@RHELv4u3 ~]$ set -o noclobber
111](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-117-320.jpg)
![redirection and pipes
[paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt
-bash: winter.txt: cannot overwrite existing file
[paul@RHELv4u3 ~]$ set +o noclobber
[paul@RHELv4u3 ~]$
overruling noclobber
The noclobber can be overruled with >|.
[paul@RHELv4u3 ~]$ set -o noclobber
[paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt
-bash: winter.txt: cannot overwrite existing file
[paul@RHELv4u3 ~]$ echo It is very cold today! >| winter.txt
[paul@RHELv4u3 ~]$ cat winter.txt
It is very cold today!
[paul@RHELv4u3 ~]$
>> append
Use >> to append output to a file.
[paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt
[paul@RHELv4u3 ~]$ cat winter.txt
It is cold today!
[paul@RHELv4u3 ~]$ echo Where is the summer ? >> winter.txt
[paul@RHELv4u3 ~]$ cat winter.txt
It is cold today!
Where is the summer ?
[paul@RHELv4u3 ~]$
15.3. error redirection
2> stderr
Redirecting stderr is done with 2>. This can be very useful to prevent error messages
from cluttering your screen. The screenshot below shows redirection of stdout to a
file, and stderr to /dev/null. Writing 1> is the same as >.
[paul@RHELv4u3 ~]$ find / > allfiles.txt 2> /dev/null
[paul@RHELv4u3 ~]$
2>&1
To redirect both stdout and stderr to the same file, use 2>&1.
112](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-118-320.jpg)
![redirection and pipes
[paul@RHELv4u3 ~]$ find / > allfiles_and_errors.txt 2>&1
[paul@RHELv4u3 ~]$
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1
directs both standard output (file descriptor 1) and standard error (file descriptor 2)
to the file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error was made a
copy of the standard output before the standard output was redirected to dirlist.
15.4. input redirection
< stdin
Redirecting stdin is done with < (short for 0<).
[paul@RHEL4b ~]$ cat < text.txt
one
two
[paul@RHEL4b ~]$ tr 'onetw' 'ONEZZ' < text.txt
ONE
ZZO
[paul@RHEL4b ~]$
<< here document
The here document (sometimes called here-is-document) is a way to append input
until a certain sequence (usually EOF) is encountered. The EOF marker can be typed
literally or can be called with Ctrl-D.
[paul@RHEL4b ~]$ cat <<EOF > text.txt
> one
> two
> EOF
[paul@RHEL4b ~]$ cat text.txt
one
two
[paul@RHEL4b ~]$ cat <<brol > text.txt
> brel
> brol
[paul@RHEL4b ~]$ cat text.txt
brel
[paul@RHEL4b ~]$
113](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-119-320.jpg)
![filters
16.1. cat
When between two pipes, the cat command does nothing (except putting stdin on
stdout.
[paul@RHEL4b pipes]$ tac count.txt | cat | cat | cat | cat | cat
five
four
three
two
one
[paul@RHEL4b pipes]$
16.2. tee
Writing long pipes in Unix is fun, but sometimes you might want intermediate results.
This is were tee comes in handy. The tee filter puts stdin on stdout and also into a
file. So tee is almost the same as cat, except that it has two identical outputs.
[paul@RHEL4b pipes]$ tac count.txt | tee temp.txt | tac
one
two
three
four
five
[paul@RHEL4b pipes]$ cat temp.txt
five
four
three
two
one
[paul@RHEL4b pipes]$
16.3. grep
The grep filter is famous among Unix users. The most common use of grep is to
filter lines of text containing (or not containing) a certain string.
[paul@RHEL4b pipes]$ cat tennis.txt
Amelie Mauresmo, Fra
Kim Clijsters, BEL
Justine Henin, Bel
Serena Williams, usa
Venus Williams, USA
[paul@RHEL4b pipes]$ cat tennis.txt | grep Williams
Serena Williams, usa
Venus Williams, USA
You can write this without the cat.
[paul@RHEL4b pipes]$ grep Williams tennis.txt
Serena Williams, usa
Venus Williams, USA
One of the most useful options of grep is grep -i which filters in a case insensitive
way.
119](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-125-320.jpg)
![filters
[paul@RHEL4b pipes]$ grep Bel tennis.txt
Justine Henin, Bel
[paul@RHEL4b pipes]$ grep -i Bel tennis.txt
Kim Clijsters, BEL
Justine Henin, Bel
[paul@RHEL4b pipes]$
Another very useful option is grep -v which outputs lines not matching the string.
[paul@RHEL4b pipes]$ grep -v Fra tennis.txt
Kim Clijsters, BEL
Justine Henin, Bel
Serena Williams, usa
Venus Williams, USA
[paul@RHEL4b pipes]$
And of course, both options can be combined to filter all lines not containing a case
insensitive string.
[paul@RHEL4b pipes]$ grep -vi usa tennis.txt
Amelie Mauresmo, Fra
Kim Clijsters, BEL
Justine Henin, Bel
[paul@RHEL4b pipes]$
With grep -A1 one line after the result is also displayed.
paul@debian5:~/pipes$ grep -A1 Henin tennis.txt
Justine Henin, Bel
Serena Williams, usa
With grep -B1 one line before the result is also displayed.
paul@debian5:~/pipes$ grep -B1 Henin tennis.txt
Kim Clijsters, BEL
Justine Henin, Bel
With grep -C1 (context) one line before and one after are also displayed. All three
options (A,B, and C) can display any number of lines (using e.g. A2, B4 or C20).
paul@debian5:~/pipes$ grep -C1 Henin tennis.txt
Kim Clijsters, BEL
Justine Henin, Bel
Serena Williams, usa
16.4. cut
The cut filter can select columns from files, depending on a delimiter or a count of
bytes. The screenshot below uses cut to filter for the username and userid in the /etc/
passwd file. It uses the colon as a delimiter, and selects fields 1 and 3.
120](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-126-320.jpg)
![filters
[[paul@RHEL4b pipes]$ cut -d: -f1,3 /etc/passwd | tail -4
Figo:510
Pfaff:511
Harry:516
Hermione:517
[paul@RHEL4b pipes]$
When using a space as the delimiter for cut, you have to quote the space.
[paul@RHEL4b pipes]$ cut -d" " -f1 tennis.txt
Amelie
Kim
Justine
Serena
Venus
[paul@RHEL4b pipes]$
This example uses cut to display the second to the seventh character of /etc/passwd.
[paul@RHEL4b pipes]$ cut -c2-7 /etc/passwd | tail -4
igo:x:
faff:x
arry:x
ermion
[paul@RHEL4b pipes]$
16.5. tr
You can translate characters with tr. The screenshot shows the translation of all
occurrences of e to E.
[paul@RHEL4b pipes]$ cat tennis.txt | tr 'e' 'E'
AmEliE MaurEsmo, Fra
Kim ClijstErs, BEL
JustinE HEnin, BEl
SErEna Williams, usa
VEnus Williams, USA
Here we set all letters to uppercase by defining two ranges.
[paul@RHEL4b pipes]$ cat tennis.txt | tr 'a-z' 'A-Z'
AMELIE MAURESMO, FRA
KIM CLIJSTERS, BEL
JUSTINE HENIN, BEL
SERENA WILLIAMS, USA
VENUS WILLIAMS, USA
[paul@RHEL4b pipes]$
Here we translate all newlines to spaces.
[paul@RHEL4b pipes]$ cat count.txt
one
121](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-127-320.jpg)
![filters
two
three
four
five
[paul@RHEL4b pipes]$ cat count.txt | tr 'n' ' '
one two three four five [paul@RHEL4b pipes]$
The tr -s filter can also be used to squeeze multiple occurrences of a character to one.
[paul@RHEL4b pipes]$ cat spaces.txt
one two three
four five six
[paul@RHEL4b pipes]$ cat spaces.txt | tr -s ' '
one two three
four five six
[paul@RHEL4b pipes]$
You can also use tr to 'encrypt' texts with rot13.
[paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'nopqrstuvwxyzabcdefghijklm'
bar
gjb
guerr
sbhe
svir
[paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'n-za-m'
bar
gjb
guerr
sbhe
svir
[paul@RHEL4b pipes]$
This last example uses tr -d to delete characters.
paul@debian5:~/pipes$ cat tennis.txt | tr -d e
Amli Maursmo, Fra
Kim Clijstrs, BEL
Justin Hnin, Bl
Srna Williams, usa
Vnus Williams, USA
16.6. wc
Counting words, lines and characters is easy with wc.
[paul@RHEL4b pipes]$ wc tennis.txt
5 15 100 tennis.txt
[paul@RHEL4b pipes]$ wc -l tennis.txt
5 tennis.txt
[paul@RHEL4b pipes]$ wc -w tennis.txt
15 tennis.txt
[paul@RHEL4b pipes]$ wc -c tennis.txt
100 tennis.txt
122](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-128-320.jpg)
![filters
[paul@RHEL4b pipes]$
16.7. sort
The sort filter will default to an alphabetical sort.
paul@debian5:~/pipes$ cat music.txt
Queen
Brel
Led Zeppelin
Abba
paul@debian5:~/pipes$ sort music.txt
Abba
Brel
Led Zeppelin
Queen
But the sort filter has many options to tweak its usage. This example shows sorting
different columns (column 1 or column 2).
[paul@RHEL4b pipes]$ sort -k1 country.txt
Belgium, Brussels, 10
France, Paris, 60
Germany, Berlin, 100
Iran, Teheran, 70
Italy, Rome, 50
[paul@RHEL4b pipes]$ sort -k2 country.txt
Germany, Berlin, 100
Belgium, Brussels, 10
France, Paris, 60
Italy, Rome, 50
Iran, Teheran, 70
The screenshot below shows the difference between an alphabetical sort and a
numerical sort (both on the third column).
[paul@RHEL4b pipes]$ sort -k3 country.txt
Belgium, Brussels, 10
Germany, Berlin, 100
Italy, Rome, 50
France, Paris, 60
Iran, Teheran, 70
[paul@RHEL4b pipes]$ sort -n -k3 country.txt
Belgium, Brussels, 10
Italy, Rome, 50
France, Paris, 60
Iran, Teheran, 70
Germany, Berlin, 100
16.8. uniq
With uniq you can remove duplicates from a sorted list.
123](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-129-320.jpg)
![filters
Venus Williams, USA
Martina Hingis, SUI
Serena williams, USA
16.12. pipe examples
who | wc
How many users are logged on to this system ?
[paul@RHEL4b pipes]$ who
root tty1 Jul 25 10:50
paul pts/0 Jul 25 09:29 (laika)
Harry pts/1 Jul 25 12:26 (barry)
paul pts/2 Jul 25 12:26 (pasha)
[paul@RHEL4b pipes]$ who | wc -l
4
who | cut | sort
Display a sorted list of logged on users.
[paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort
Harry
paul
paul
root
Display a sorted list of logged on users, but every user only once.
[paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort | uniq
Harry
paul
root
grep | cut
Display a list of all bash user accounts on this computer. Users accounts are explained
in detail later.
paul@debian5:~$ grep bash /etc/passwd
root:x:0:0:root:/root:/bin/bash
paul:x:1000:1000:paul,,,:/home/paul:/bin/bash
serena:x:1001:1001::/home/serena:/bin/bash
paul@debian5:~$ grep bash /etc/passwd | cut -d: -f1
root
paul
serena
126](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-132-320.jpg)
![basic unix tools
17.1. find
The find command can be very useful at the start of a pipe to search for files. Here are
some examples. You might want to add 2>/dev/null to the command lines to avoid
cluttering your screen with error messages.
Find all files in /etc and put the list in etcfiles.txt
find /etc > etcfiles.txt
Find all files of the entire system and put the list in allfiles.txt
find / > allfiles.txt
Find files that end in .conf in the current directory (and all subdirs).
find . -name "*.conf"
Find files of type file (not directory, pipe or etc.) that end in .conf.
find . -type f -name "*.conf"
Find files of type directory that end in .bak .
find /data -type d -name "*.bak"
Find files that are newer than file42.txt
find . -newer file42.txt
Find can also execute another command on every file found. This example will look
for *.odf files and copy them to /backup/.
find /data -name "*.odf" -exec cp {} /backup/ ;
Find can also execute, after your confirmation, another command on every file found.
This example will remove *.odf files if you approve of it for every file found.
find /data -name "*.odf" -ok rm {} ;
17.2. locate
The locate tool is very different from find in that it uses an index to locate files. This
is a lot faster than traversing all the directories, but it also means that it is always
outdated. If the index does not exist yet, then you have to create it (as root on Red
Hat Enterprise Linux) with the updatedb command.
[paul@RHEL4b ~]$ locate Samba
warning: locate: could not open database: /var/lib/slocate/slocate.db:...
warning: You need to run the 'updatedb' command (as root) to create th...
Please have a look at /etc/updatedb.conf to enable the daily cron job.
[paul@RHEL4b ~]$ updatedb
fatal error: updatedb: You are not authorized to create a default sloc...
[paul@RHEL4b ~]$ su -
Password:
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![basic unix tools
[root@RHEL4b ~]# updatedb
[root@RHEL4b ~]#
Most Linux distributions will schedule the updatedb to run once every day.
17.3. date
The date command can display the date, time, timezone and more.
paul@rhel55 ~$ date
Sat Apr 17 12:44:30 CEST 2010
A date string can be customized to display the format of your choice. Check the man
page for more options.
paul@rhel55 ~$ date +'%A %d-%m-%Y'
Saturday 17-04-2010
Time on any Unix is calculated in number of seconds since 1969 (the first second
being the first second of the first of January 1970). Use date +%s to display Unix
time in seconds.
paul@rhel55 ~$ date +%s
1271501080
When will this seconds counter reach two thousand million ?
paul@rhel55 ~$ date -d '1970-01-01 + 2000000000 seconds'
Wed May 18 04:33:20 CEST 2033
17.4. cal
The cal command displays the current month, with the current day highlighted.
paul@rhel55 ~$ cal
April 2010
Su Mo Tu We Th Fr Sa
1 2 3
4 5 6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30
You can select any month in the past or the future.
paul@rhel55 ~$ cal 2 1970
February 1970
Su Mo Tu We Th Fr Sa
1 2 3 4 5 6 7
132](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-138-320.jpg)
![Introduction to vi
Table 18.11. searching
command action
/string forward search for string
?string backward search for string
n go to next occurrence of search string
/^string forward search string at beginning of line
/string$ forward search string at end of line
/br[aeio]l search for bral brel bril and brol
/<he> search for the word he (and not for here or the)
18.12. replace all ( :1,$ s/foo/bar/g )
To replace all occurrences of the string foo with bar, first switch to ex mode with : .
Then tell vi which lines to use, for example 1,$ will do the replace all from the first
to the last line. You can write 1,5 to only process the first five lines. The s/foo/bar/
g will replace all occurences of foo with bar.
Table 18.12. replace
command action
:4,8 s/foo/bar/g replace foo with bar on lines 4 to 8
:1,$ s/foo/bar/g replace foo with bar on all lines
18.13. reading files (:r :r !cmd)
When in command mode, :r foo will read the file named foo, :r !foo will execute the
command foo. The result will be put at the current location. Thus :r !ls will put a
listing of the current directory in your text file.
Table 18.13. read files and input
command action
:r fname (read) file fname and paste contents
:r !cmd execute cmd and paste its output
18.14. text buffers
There are 36 buffers in vi to store text. You can use them with the " character.
145](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-151-320.jpg)
![scripting introduction
19.1. scripts are everywhere
Shells like bash and Korn have support for programming constructs that can be saved
as scripts. These scripts in turn then become more shell commands. Many Linux
commands are scripts. User profile scripts are run when a user logs on and init
scripts are run when a daemon is stopped or started.
This means that system administrators also need basic knowledge of scripting to
understand how their servers and their applications are started, updated, upgraded,
patched, maintained, configured and removed, and also to understand how a user
environment is built.
19.2. hello world
Just like in every programming course, we start with a simple hello_world script.
The following script will output Hello World.
echo Hello World
After creating this simple script in vi or with echo, you'll have to chmod +x
hello_world to make it executable. And unless you add the scripts directory to your
path, you'll have to type the path to the script for the shell to be able to find it.
[paul@RHEL4a ~]$ echo echo Hello World > hello_world
[paul@RHEL4a ~]$ chmod +x hello_world
[paul@RHEL4a ~]$ ./hello_world
Hello World
[paul@RHEL4a ~]$
19.3. she-bang
Let's expand our example a little further by putting #!/bin/bash on the first line of
the script. The #! is called a she-bang (sometimes called sha-bang), where the she-
bang is the first two characters of the script.
#!/bin/bash
echo Hello World
You can never be sure which shell a user is running. A script that works flawlessly
in bash might not work in ksh, csh, or dash. To instruct a shell to run your script in
a certain shell, you can start your script with a she-bang followed by the shell it is
supposed to run in. This script will run in a bash shell.
#!/bin/bash
echo -n hello
echo A bash subshell `echo -n hello`
This script will run in a Korn shell (unless /bin/ksh is a hard link to /bin/bash). The
/etc/shells file contains a list of shells on your system.
152](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-158-320.jpg)
![scripting introduction
#!/bin/ksh
echo -n hello
echo a Korn subshell `echo -n hello`
19.4. comment
Let's expand our example a little further by adding comment lines.
#!/bin/bash
#
# Hello World Script
#
echo Hello World
19.5. variables
Here is a simple example of a variable inside a script.
#!/bin/bash
#
# simple variable in script
#
var1=4
echo var1 = $var1
Scripts can contain variables, but since scripts are run in their own shell, the variables
do not survive the end of the script.
[paul@RHEL4a ~]$ echo $var1
[paul@RHEL4a ~]$ ./vars
var1 = 4
[paul@RHEL4a ~]$ echo $var1
[paul@RHEL4a ~]$
19.6. sourcing a script
Luckily, you can force a script to run in the same shell; this is called sourcing a script.
[paul@RHEL4a ~]$ source ./vars
var1 = 4
[paul@RHEL4a ~]$ echo $var1
4
[paul@RHEL4a ~]$
The above is identical to the below.
[paul@RHEL4a ~]$ . ./vars
var1 = 4
[paul@RHEL4a ~]$ echo $var1
153](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-159-320.jpg)
![scripting introduction
4
[paul@RHEL4a ~]$
19.7. prevent setuid root spoofing
Some user may try to perform setuid based script root spoofing. This is a rare but
possible attack. To improve script security and to avoid interpreter spoofing, you need
to add -- after the #!/bin/bash, which disables further option processing so the shell
will not accept any options.
#!/bin/bash -
or
#!/bin/bash --
Any arguments after the -- are treated as filenames and arguments. An argument of
- is equivalent to --.
154](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-160-320.jpg)
![Chapter 20. scripting loops
Table of Contents
20.1. test [ ] ......................................................................................................... 158
20.2. if if, then then, or else ............................................................................... 158
20.3. for loop ....................................................................................................... 159
20.4. while loop .................................................................................................. 159
20.5. until loop .................................................................................................... 160
20.6. practice: scripting tests and loops .............................................................. 161
20.7. solution: scripting tests and loops .............................................................. 162
157](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-163-320.jpg)
![scripting loops
20.1. test [ ]
The test command can test whether something is true or false. Let's start by testing
whether 10 is greater than 55.
[paul@RHEL4b ~]$ test 10 -gt 55 ; echo $?
1
[paul@RHEL4b ~]$
The test command returns 1 if the test fails. And as you see in the next screenshot,
test returns 0 when a test succeeds.
[paul@RHEL4b ~]$ test 56 -gt 55 ; echo $?
0
[paul@RHEL4b ~]$
If you prefer true and false, then write the test like this.
[paul@RHEL4b ~]$ test 56 -gt 55 && echo true || echo false
true
[paul@RHEL4b ~]$ test 6 -gt 55 && echo true || echo false
false
The test command can also be written as square brackets, the screenshot below is
identical to the one above.
[paul@RHEL4b ~]$ [ 56 -gt 55 ] && echo true || echo false
true
[paul@RHEL4b ~]$ [ 6 -gt 55 ] && echo true || echo false
false
Below are some example tests. Take a look at man test to see more options for tests.
[ -d foo ] Does the directory foo exist ?
[ '/etc' = $PWD ] Is the string /etc equal to the variable $PWD ?
[ $1 != 'secret' ] Is the first parameter different from secret ?
[ 55 -lt $bar ] Is 55 less than the value of $bar ?
[ $foo -ge 1000 ] Is the value of $foo greater or equal to 1000 ?
[ "abc" < $bar ] Does abc sort before the value of $bar ?
[ -f foo ] Is foo a regular file ?
[ -r bar ] Is bar a readable file ?
[ foo -nt bar ] Is file foo newer than file bar ?
[ -o nounset ] Is the shell option nounset set ?
Tests can be combined with logical AND and OR.
paul@RHEL4b:~$ [ 66 -gt 55 -a 66 -lt 500 ] && echo true || echo false
true
paul@RHEL4b:~$ [ 66 -gt 55 -a 660 -lt 500 ] && echo true || echo false
false
paul@RHEL4b:~$ [ 66 -gt 55 -o 660 -lt 500 ] && echo true || echo false
true
20.2. if if, then then, or else
The if then else construction is about choice. If a certain condition is met, then
execute something, else execute something else. The example below tests whether a
file exists, and if the file exists then a proper message is echoed.
158](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-164-320.jpg)
![scripting loops
#!/bin/bash
if [ -f isit.txt ]
then echo isit.txt exists!
else echo isit.txt not found!
fi
If we name the above script 'choice', then it executes like this.
[paul@RHEL4a scripts]$ ./choice
isit.txt not found!
[paul@RHEL4a scripts]$ touch isit.txt
[paul@RHEL4a scripts]$ ./choice
isit.txt exists!
[paul@RHEL4a scripts]$
20.3. for loop
The example below shows the syntax of a classical for loop in bash.
for i in 1 2 4
do
echo $i
done
An example of a for loop combined with an embedded shell.
#!/bin/ksh
for counter in `seq 1 20`
do
echo counting from 1 to 20, now at $counter
sleep 1
done
This for loop use file globbing (from the shell expansion). Putting the instruction on
the command line has identical functionality.
kahlan@solexp11$ ls
count.ksh go.ksh
kahlan@solexp11$ for file in *.ksh ; do cp $file $file.backup ; done
kahlan@solexp11$ ls
count.ksh count.ksh.backup go.ksh go.ksh.backup
20.4. while loop
Below a simple example of a while loop.
let i=100;
while [ $i -ge 0 ] ;
do
echo Counting down, from 100 to 0, now at $i;
let i--;
done
Endless loops can be made with while true or while : , where the colon is the
equivalent of no operation in the Korn and bash shells.
#!/bin/ksh
159](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-165-320.jpg)
![scripting loops
# endless loop
while :
do
echo hello
sleep 1
done
20.5. until loop
Below a simple example of an until loop.
let i=100;
until [ $i -le 0 ] ;
do
echo Counting down, from 100 to 1, now at $i;
let i--;
done
160](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-166-320.jpg)
![scripting loops
20.7. solution: scripting tests and loops
1. Write a script that uses a for loop to count from 3 to 7.
#!/bin/bash
for i in 3 4 5 6 7
do
echo Counting from 3 to 7, now at $i
done
2. Write a script that uses a for loop to count from 1 to 17000.
#!/bin/bash
for i in `seq 1 17000`
do
echo Counting from 1 to 17000, now at $i
done
3. Write a script that uses a while loop to count from 3 to 7.
#!/bin/bash
i=3
while [ $i -le 7 ]
do
echo Counting from 3 to 7, now at $i
let i=i+1
done
4. Write a script that uses an until loop to count down from 8 to 4.
#!/bin/bash
i=8
until [ $i -lt 4 ]
do
echo Counting down from 8 to 4, now at $i
let i=i-1
done
5. Write a script that counts the number of files ending in .txt in the current directory.
#!/bin/bash
let i=0
for file in *.txt
do
let i++
done
echo "There are $i files ending in .txt"
6. Wrap an if statement around the script so it is also correct when there are zero files
ending in .txt.
#!/bin/bash
ls *.txt > /dev/null 2>&1
if [ $? -ne 0 ]
162](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-168-320.jpg)
![scripting parameters
21.1. script parameters
A bash shell script can have parameters. The numbering you see in the script below
continues if you have more parameters. You also have special parameters containing
the number of parameters, a string of all of them, and also the process id, and the last
return code. The man page of bash has a full list.
#!/bin/bash
echo The first argument is $1
echo The second argument is $2
echo The third argument is $3
echo $ $$ PID of the script
echo # $# count arguments
echo ? $? last return code
echo * $* all the arguments
Below is the output of the script above in action.
[paul@RHEL4a scripts]$ ./pars one two three
The first argument is one
The second argument is two
The third argument is three
$ 5610 PID of the script
# 3 count arguments
? 0 last return code
* one two three all the arguments
[paul@RHEL4a scripts]$ ./pars 1 2
The first argument is 1
The second argument is 2
The third argument is
$ 5612 PID of the script
# 2 count arguments
? 0 last return code
* 1 2 all the arguments
[paul@RHEL4a scripts]$
21.2. shift through parameters
The shift statement can parse all parameters one by one. This is a sample script.
kahlan@solexp11$ cat shift.ksh
#!/bin/ksh
if [ "$#" == "0" ]
then
echo You have to give at least one parameter.
exit 1
fi
while (( $# ))
do
echo You gave me $1
shift
done
165](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-171-320.jpg)
![scripting parameters
Below is some sample output of the script above.
kahlan@solexp11$ ./shift.ksh one
You gave me one
kahlan@solexp11$ ./shift.ksh one two three 1201 "33 42"
You gave me one
You gave me two
You gave me three
You gave me 1201
You gave me 33 42
kahlan@solexp11$ ./shift.ksh
You have to give at least one parameter.
21.3. runtime input
You can ask the user for input with the read command in a script.
#!/bin/bash
echo -n Enter a number:
read number
21.4. sourcing a config file
[paul@RHEL4a scripts]$ cat myApp.conf
# The config file of myApp
# Enter the path here
myAppPath=/var/myApp
# Enter the number of quines here
quines=5
[paul@RHEL4a scripts]$ cat myApp.bash
#!/bin/bash
#
# Welcome to the myApp application
#
. ./myApp.conf
echo There are $quines quines
[paul@RHEL4a scripts]$ ./myApp.bash
There are 5 quines
[paul@RHEL4a scripts]$
21.5. get script options with getopts
The getopts function allows you to parse options given to a command.
kahlan@solexp11$ cat options.ksh
#!/bin/ksh
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![scripting parameters
21.8. solution: parameters and options
1. Write a script that receives four parameters, and outputs them in reverse order.
echo $4 $3 $2 $1
2. Write a script that receives two parameters (two filenames) and outputs whether
those files exist.
#!/bin/bash
if [ -f $1 ]
then echo $1 exists!
else echo $1 not found!
fi
if [ -f $2 ]
then echo $2 exists!
else echo $2 not found!
fi
3. Write a script that asks for a filename. Verify existance of the file, then verify that
you own the file, and whether it is writable. If not, then make it writable.
4. Make a configuration file for the previous script. Put a logging switch in the config
file, logging means writing detailed output of everything the script does to a log file
in /tmp.
170](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-176-320.jpg)
![more scripting
22.1. eval
eval reads arguments as input to the shell (the resulting commands are executed).
This allows using the value of a variable as a variable.
paul@deb503:~/test42$ answer=42
paul@deb503:~/test42$ word=answer
paul@deb503:~/test42$ eval x=$$word ; echo $x
42
Both in bash and Korn the arguments can be quoted.
kahlan@solexp11$ answer=42
kahlan@solexp11$ word=answer
kahlan@solexp11$ eval "y=$$word" ; echo $y
42
22.2. (( ))
The (( )) allows for evaluation of numerical expressions.
paul@deb503:~/test42$ (( 42 > 33 )) && echo true || echo false
true
paul@deb503:~/test42$ (( 42 > 1201 )) && echo true || echo false
false
paul@deb503:~/test42$ var42=42
paul@deb503:~/test42$ (( 42 == var42 )) && echo true || echo false
true
paul@deb503:~/test42$ (( 42 == $var42 )) && echo true || echo false
true
paul@deb503:~/test42$ var42=33
paul@deb503:~/test42$ (( 42 == var42 )) && echo true || echo false
false
22.3. let
The let built-in shell function instructs the shell to perform an evaluation of arithmetic
expressions. It will return 0 unless the last arithmetic expression evaluates to 0.
[paul@RHEL4b ~]$ let x="3 + 4" ; echo $x
7
[paul@RHEL4b ~]$ let x="10 + 100/10" ; echo $x
20
[paul@RHEL4b ~]$ let x="10-2+100/10" ; echo $x
18
[paul@RHEL4b ~]$ let x="10*2+100/10" ; echo $x
30
The shell can also convert between different bases.
[paul@RHEL4b ~]$ let x="0xFF" ; echo $x
255
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![more scripting
[paul@RHEL4b ~]$ let x="0xC0" ; echo $x
192
[paul@RHEL4b ~]$ let x="0xA8" ; echo $x
168
[paul@RHEL4b ~]$ let x="8#70" ; echo $x
56
[paul@RHEL4b ~]$ let x="8#77" ; echo $x
63
[paul@RHEL4b ~]$ let x="16#c0" ; echo $x
192
There is a difference between assigning a variable directly, or using let to evaluate
the arithmetic expressions (even if it is just assigning a value).
kahlan@solexp11$ dec=15 ; oct=017 ; hex=0x0f
kahlan@solexp11$ echo $dec $oct $hex
15 017 0x0f
kahlan@solexp11$ let dec=15 ; let oct=017 ; let hex=0x0f
kahlan@solexp11$ echo $dec $oct $hex
15 15 15
22.4. case
You can sometimes simplify nested if statements with a case construct.
[paul@RHEL4b ~]$ ./help
What animal did you see ? lion
You better start running fast!
[paul@RHEL4b ~]$ ./help
What animal did you see ? dog
Don't worry, give it a cookie.
[paul@RHEL4b ~]$ cat help
#!/bin/bash
#
# Wild Animals Helpdesk Advice
#
echo -n "What animal did you see ? "
read animal
case $animal in
"lion" | "tiger")
echo "You better start running fast!"
;;
"cat")
echo "Let that mouse go..."
;;
"dog")
echo "Don't worry, give it a cookie."
;;
"chicken" | "goose" | "duck" )
echo "Eggs for breakfast!"
;;
"liger")
echo "Approach and say 'Ah you big fluffy kitty...'."
;;
"babelfish")
echo "Did it fall out your ear ?"
;;
*)
echo "You discovered an unknown animal, name it!"
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;;
esac
[paul@RHEL4b ~]$
22.5. shell functions
Shell functions can be used to group commands in a logical way.
kahlan@solexp11$ cat funcs.ksh
#!/bin/ksh
function greetings {
echo Hello World!
echo and hello to $USER to!
}
echo We will now call a function
greetings
echo The end
This is sample output from this script with a function.
kahlan@solexp11$ ./funcs.ksh
We will now call a function
Hello World!
and hello to kahlan to!
The end
A shell function can also receive parameters.
kahlan@solexp11$ cat addfunc.ksh
#!/bin/ksh
function plus {
let result="$1 + $2"
echo $1 + $2 = $result
}
plus 3 10
plus 20 13
plus 20 22
This script produces the following output.
kahlan@solexp11$ ./addfunc.ksh
3 + 10 = 13
20 + 13 = 33
20 + 22 = 42
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![more scripting
22.7. solution : more scripting
1. Write a script that asks for two numbers, and outputs the sum and product (as
shown here).
Enter a number: 5
Enter another number: 2
Sum: 5 + 2 = 7
Product: 5 x 2 = 10
#!/bin/bash
echo -n "Enter a number : "
read n1
echo -n "Enter another number : "
read n2
let sum="$n1+$n2"
let pro="$n1*$n2"
echo -e "Sumt: $n1 + $n2 = $sum"
echo -e "Productt: $n1 * $n2 = $pro"
2. Improve the previous script to test that the numbers are between 1 and 100, exit
with an error if necessary.
echo -n "Enter a number between 1 and 100 : "
read n1
if [ $n1 -lt 1 -o $n1 -gt 100 ]
then
echo Wrong number...
exit 1
fi
3. Improve the previous script to congratulate the user if the sum equals the product.
if [ $sum -eq $pro ]
then echo Congratulations $sum == $pro
fi
4. Write a script with a case insensitive case statement, using the shopt nocasematch
option. The nocasematch option is reset to the value it had before the scripts started.
#!/bin/bash
#
# Wild Animals Case Insensitive Helpdesk Advice
#
if shopt -q nocasematch; then
nocase=yes;
else
nocase=no;
shopt -s nocasematch;
fi
echo -n "What animal did you see ? "
read animal
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![more scripting
case $animal in
"lion" | "tiger")
echo "You better start running fast!"
;;
"cat")
echo "Let that mouse go..."
;;
"dog")
echo "Don't worry, give it a cookie."
;;
"chicken" | "goose" | "duck" )
echo "Eggs for breakfast!"
;;
"liger")
echo "Approach and say 'Ah you big fluffy kitty.'"
;;
"babelfish")
echo "Did it fall out your ear ?"
;;
*)
echo "You discovered an unknown animal, name it!"
;;
esac
if [ nocase = yes ] ; then
shopt -s nocasematch;
else
shopt -u nocasematch;
fi
5. If time permits (or if you are waiting for other students to finish this practice),
take a look at linux system scripts in /etc/init.d and /etc/rc.d and try to understand
them. Where does execution of a script start in /etc/init.d/samba ? There are also some
hidden scripts in ~, we will discuss them later.
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![users
23.1. identify yourself
whoami
The whoami command tells you your username.
[root@RHEL5 ~]# whoami
root
[root@RHEL5 ~]# su - paul
[paul@RHEL5 ~]$ whoami
paul
who
The who command will give you information about who is logged on the system.
[paul@RHEL5 ~]$ who
root tty1 2008-06-24 13:24
sandra pts/0 2008-06-24 14:05 (192.168.1.34)
paul pts/1 2008-06-24 16:23 (192.168.1.37)
who am i
With who am i the who command will display only the line pointing to your current
session.
[paul@RHEL5 ~]$ who am i
paul pts/1 2008-06-24 16:23 (192.168.1.34)
w
The w command shows you who is logged on and what they are doing.
$ w
05:13:36 up 3 min, 4 users, load average: 0.48, 0.72, 0.33
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
root tty1 - 05:11 2.00s 0.32s 0.27s find / -name shad
inge pts/0 192.168.1.33 05:12 0.00s 0.02s 0.02s -ksh
laura pts/1 192.168.1.34 05:12 46.00s 0.03s 0.03s -bash
paul pts/2 192.168.1.34 05:13 25.00s 0.07s 0.04s top
id
The id command will give you your user id, primary group id, and a list of the groups
that you belong to.
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23.2. users
user management
User management on any Unix can be done in three complimentary ways. You can
use the graphical tools provided by your distribution. These tools have a look and
feel that depends on the distribution. If you are a novice linux user on your home
system, then use the graphical tool that is provided by your distribution. This will
make sure that you do not run into problems.
Another option is to use command line tools like useradd, usermod, gpasswd, passwd
and others. Server administrators are likely to use these tools, since they are familiar
and very similar accross many different distributions. This chapter will focus on these
command line tools.
A third and rather extremist way is to edit the local configuration files directly using
vi (or vipw/vigr). Do not attempt this as a novice on production systems!
/etc/passwd
The local user database on Linux (and on most Unixes) is /etc/passwd.
[root@RHEL5 ~]# tail /etc/passwd
inge:x:518:524:art dealer:/home/inge:/bin/ksh
ann:x:519:525:flute player:/home/ann:/bin/bash
frederik:x:520:526:rubius poet:/home/frederik:/bin/bash
steven:x:521:527:roman emperor:/home/steven:/bin/bash
pascale:x:522:528:artist:/home/pascale:/bin/ksh
geert:x:524:530:kernel developer:/home/geert:/bin/bash
wim:x:525:531:master damuti:/home/wim:/bin/bash
sandra:x:526:532:radish stresser:/home/sandra:/bin/bash
annelies:x:527:533:sword fighter:/home/annelies:/bin/bash
laura:x:528:534:art dealer:/home/laura:/bin/ksh
As you can see, this file contains seven columns separated by a colon. The columns
contain the username, an x, the user id, the primary group id, a description, the name
of the home directory, and the login shell.
root
The root user also called the superuser is the most powerful account on your Linux
system. This user can do almost anything, including the creation of other users. The
root user always has userid 0 (regardless of the name of the account).
[root@RHEL5 ~]# head -1 /etc/passwd
root:x:0:0:root:/root:/bin/bash
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useradd
You can add users with the useradd command. The example below shows how to
add a user named yanina (last parameter) and at the same time forcing the creation
of the home directory (-m), setting the name of the home directory (-d), and setting
a description (-c).
[root@RHEL5 ~]# useradd -m -d /home/yanina -c "yanina wickmayer" yanina
[root@RHEL5 ~]# tail -1 /etc/passwd
yanina:x:529:529:yanina wickmayer:/home/yanina:/bin/bash
The user named yanina received userid 529 and primary group id 529.
/etc/default/useradd
Both Red Hat Enterprise Linux and Debian/Ubuntu have a file called /etc/default/
useradd that contains some default user options. Besides using cat to display this
file, you can also use useradd -D.
[root@RHEL4 ~]# useradd -D
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/bash
SKEL=/etc/skel
userdel
You can delete the user yanina with userdel. The -r option of userdel will also remove
the home directory.
[root@RHEL5 ~]# userdel -r yanina
usermod
You can modify the properties of a user with the usermod command. This example
uses usermod to change the description of the user harry.
[root@RHEL4 ~]# tail -1 /etc/passwd
harry:x:516:520:harry potter:/home/harry:/bin/bash
[root@RHEL4 ~]# usermod -c 'wizard' harry
[root@RHEL4 ~]# tail -1 /etc/passwd
harry:x:516:520:wizard:/home/harry:/bin/bash
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23.3. passwords
passwd
Passwords of users can be set with the passwd command. Users will have to provide
their old password before twice entering the new one.
[harry@RHEL4 ~]$ passwd
Changing password for user harry.
Changing password for harry
(current) UNIX password:
New UNIX password:
BAD PASSWORD: it's WAY too short
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully.
[harry@RHEL4 ~]$
As you can see, the passwd tool will do some basic verification to prevent users
from using too simple passwords. The root user does not have to follow these rules
(there will be a warning though). The root user also does not have to provide the old
password before entering the new password twice.
/etc/shadow
User passwords are encrypted and kept in /etc/shadow. The /etc/shadow file is read
only and can only be read by root. We will see in the file permissions section how it
is possible for users to change their password. For now, you will have to know that
users can change their password with the /usr/bin/passwd command.
[root@RHEL5 ~]# tail /etc/shadow
inge:$1$yWMSimOV$YsYvcVKqByFVYLKnU3ncd0:14054:0:99999:7:::
ann:!!:14054:0:99999:7:::
frederik:!!:14054:0:99999:7:::
steven:!!:14054:0:99999:7:::
pascale:!!:14054:0:99999:7:::
geert:!!:14054:0:99999:7:::
wim:!!:14054:0:99999:7:::
sandra:!!:14054:0:99999:7:::
annelies:!!:14054:0:99999:7:::
laura:$1$Tvby1Kpa$lL.WzgobujUS3LClIRmdv1:14054:0:99999:7:::
The /etc/shadow file contains nine colon separated columns. The nine fields contain
(from left to right) the user name, the encrypted password (note that only inge and
laura have an encrypted password), the day the password was last changed (day 1 is
January 1, 1970), number of days the password must be left unchanged, password
expiry day, warning number of days before password expiry, number of days after
expiry before disabling the account, and the day the account was disabled (again,
since 1970). The last field has no meaning yet.
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password encryption
encryption with passwd
Passwords are stored in an encrypted format. This encryption is done by the crypt
function. The easiest (and recommended) way to add a user with a password to the
system is to add the user with the useradd -m user command, and then set the user's
password with passwd.
[root@RHEL4 ~]# useradd -m xavier
[root@RHEL4 ~]# passwd xavier
Changing password for user xavier.
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully.
[root@RHEL4 ~]#
encryption with openssl
Another way to create users with a password is to use the -p option of useradd, but that
option requires an encrypted password. You can generate this encrypted password
with the openssl passwd command.
[root@RHEL4 ~]# openssl passwd stargate
ZZNX16QZVgUQg
[root@RHEL4 ~]# useradd -m -p ZZNX16QZVgUQg mohamed
encryption with crypt
A third option is to create your own C program using the crypt function, and compile
this into a command.
[paul@laika ~]$ cat MyCrypt.c
#include <stdio.h>
#define __USE_XOPEN
#include <unistd.h>
int main(int argc, char** argv)
{
if(argc==3)
{
printf("%sn", crypt(argv[1],argv[2]));
}
else
{
printf("Usage: MyCrypt $password $saltn" );
}
return 0;
}
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![users
This little program can be compiled with gcc like this.
[paul@laika ~]$ gcc MyCrypt.c -o MyCrypt -lcrypt
To use it, we need to give two parameters to MyCript. The first is the unencrypted
password, the second is the salt. The salt is used to perturb the encryption algorithm in
one of 4096 different ways. This variation prevents two users with the same password
from having the same entry in /etc/shadow.
paul@laika:~$ ./MyCrypt stargate 12
12L4FoTS3/k9U
paul@laika:~$ ./MyCrypt stargate 01
01Y.yPnlQ6R.Y
paul@laika:~$ ./MyCrypt stargate 33
330asFUbzgVeg
paul@laika:~$ ./MyCrypt stargate 42
42XFxoT4R75gk
Did you notice that the first two characters of the password are the salt ?
The standard output of the crypt function is using the DES algorithm which is old
and can be cracked in minutes. A better method is to use MD5 passwords which can
be recognized by a salt starting with $1$.
paul@laika:~$ ./MyCrypt stargate '$1$12'
$1$12$xUIQ4116Us.Q5Osc2Khbm1
paul@laika:~$ ./MyCrypt stargate '$1$01'
$1$01$yNs8brjp4b4TEw.v9/IlJ/
paul@laika:~$ ./MyCrypt stargate '$1$33'
$1$33$tLh/Ldy2wskdKAJR.Ph4M0
paul@laika:~$ ./MyCrypt stargate '$1$42'
$1$42$Hb3nvP0KwHSQ7fQmIlY7R.
The MD5 salt can be up to eight characters long. The salt is displayed in /etc/shadow
between the second and third $, so never use the password as the salt!
paul@laika:~$ ./MyCrypt stargate '$1$stargate'
$1$stargate$qqxoLqiSVNvGr5ybMxEVM1
password defaults
/etc/login.defs
The /etc/login.defs file contains some default settings for user passwords like
password aging and length settings. (You will also find the numerical limits of user
ids and group ids and whether or not a home directory should be created by default).
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[root@RHEL4 ~]# grep -i pass /etc/login.defs
# Password aging controls:
# PASS_MAX_DAYS Maximum number of days a password may be used.
# PASS_MIN_DAYS Minimum number of days allowed between password changes.
# PASS_MIN_LEN Minimum acceptable password length.
# PASS_WARN_AGE Number of days warning given before a password expires.
PASS_MAX_DAYS 99999
PASS_MIN_DAYS 0
PASS_MIN_LEN 5
PASS_WARN_AGE 7
chage
The chage command can be used to set an expiration date for a user account (-E),
set a mimimum (-m) and maximum (-M) password age, a password expiration date,
and set the number of warning days before the password expiration date. Much of
this functionality is also available from the passwd command. The -l option of chage
will list these settings for a user.
[root@RHEL4 ~]# chage -l harry
Minimum: 0
Maximum: 99999
Warning: 7
Inactive: -1
Last Change: Jul 23, 2007
Password Expires: Never
Password Inactive: Never
Account Expires: Never
[root@RHEL4 ~]#
disabling a password
Passwords in /etc/shadow cannot begin with an exclamation mark. When the second
field in /etc/passwd starts with an exclamation mark, then the password can not be
used.
Using this feature is often called locking, disabling, or suspending a user account.
Besides vi (or vipw) you can also accomplish this with usermod.
The first line in the next screenshot will disable the password of user harry, making
it impossible for harry to authenticate using this password.
[root@RHEL4 ~]# usermod -L harry
[root@RHEL4 ~]# tail -1 /etc/shadow
harry:!$1$143TO9IZ$RLm/FpQkpDrV4/Tkhku5e1:13717:0:99999:7:::
The root user (and users with sudo rights on su) still will be able to su to harry
(because the password is not needed here). Also note that harry will still be able to
login if he has set up passwordless ssh!
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![users
[root@RHEL4 ~]# su - harry
[harry@RHEL4 ~]$
You can unlock the account again with usermod -U.
Watch out for tiny differences in the command line options of passwd, usermod,
and useradd on different distributions! Verify the local files when using features like
"disabling, suspending, or locking" users and passwords!
editing local files
If you still want to manually edit the /etc/passwd or /etc/shadow, after knowing these
commands for password management, then use vipw instead of vi(m) directly. The
vipw tool will do proper locking of the file.
[root@RHEL5 ~]# vipw /etc/passwd
vipw: the password file is busy (/etc/ptmp present)
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![users
23.4. home directories
creating home directories
The easiest way to create a home directory is to supply the -m option with useradd
(it is likely set as a default option on Linux).
A less easy way is to create a home directory manually with mkdir which also
requires setting the owner and the permissions on the directory with chmod and
chown (both commands are discussed in detail in another chapter).
[root@RHEL5 ~]# mkdir /home/laura
[root@RHEL5 ~]# chown laura:laura /home/laura
[root@RHEL5 ~]# chmod 700 /home/laura
[root@RHEL5 ~]# ls -ld /home/laura/
drwx------ 2 laura laura 4096 Jun 24 15:17 /home/laura/
/etc/skel/
When using useradd with the -m option, the /etc/skel/ directory is copied to
the newly created home directory. The /etc/skel/ directory contains some (usually
hidden) files that contain profile settings and default values for applications. In this
way /etc/skel/ serves as a default home directory and as a default user profile.
[root@RHEL5 ~]# ls -la /etc/skel/
total 48
drwxr-xr-x 2 root root 4096 Apr 1 00:11 .
drwxr-xr-x 97 root root 12288 Jun 24 15:36 ..
-rw-r--r-- 1 root root 24 Jul 12 2006 .bash_logout
-rw-r--r-- 1 root root 176 Jul 12 2006 .bash_profile
-rw-r--r-- 1 root root 124 Jul 12 2006 .bashrc
deleting home directories
The -r option of userdel will make sure that the home directory is deleted together
with the user account.
[root@RHEL5 ~]# ls -ld /home/wim/
drwx------ 2 wim wim 4096 Jun 24 15:19 /home/wim/
[root@RHEL5 ~]# userdel -r wim
[root@RHEL5 ~]# ls -ld /home/wim/
ls: /home/wim/: No such file or directory
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23.5. user shell
login shell
The /etc/passwd file specifies the login shell for the user. In the screenshot below
you can see that user annelies will log in with the /bin/bash shell, and user laura with
the /bin/ksh shell.
[root@RHEL5 ~]# tail -2 /etc/passwd
annelies:x:527:533:sword fighter:/home/annelies:/bin/bash
laura:x:528:534:art dealer:/home/laura:/bin/ksh
You can use the usermod command to change the shell for a user.
[root@RHEL5 ~]# usermod -s /bin/bash laura
[root@RHEL5 ~]# tail -1 /etc/passwd
laura:x:528:534:art dealer:/home/laura:/bin/bash
chsh
Users can change their login shell with the chsh command. First, user harry obtains
a list of available shells (he could also have done a cat /etc/shells) and then changes
his login shell to the Korn shell (/bin/ksh). At the next login, harry will default into
ksh instead of bash.
[harry@RHEL4 ~]$ chsh -l
/bin/sh
/bin/bash
/sbin/nologin
/bin/ash
/bin/bsh
/bin/ksh
/usr/bin/ksh
/usr/bin/pdksh
/bin/tcsh
/bin/csh
/bin/zsh
[harry@RHEL4 ~]$ chsh -s /bin/ksh
Changing shell for harry.
Password:
Shell changed.
[harry@RHEL4 ~]$
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23.6. switch users with su
su to another user
The su command allows a user to run a shell as another user.
[paul@RHEL4b ~]$ su harry
Password:
[harry@RHEL4b paul]$
su to root
Yes you can alsu su to become root, when you know the root password.
[harry@RHEL4b paul]$ su root
Password:
[root@RHEL4b paul]#
su as root
Unless you are logged in as root, running a shell as another user requires that you
know the password of that user. The root user can become any user without knowing
the user's password.
[root@RHEL4b paul]# su serena
[serena@RHEL4b paul]$
su - $username
By default, the su command maintains the same shell environment. To become
another user and also get the target user's environment, issue the su - command
followed by the target username.
[paul@RHEL4b ~]$ su - harry
Password:
[harry@RHEL4b ~]$
su -
When no username is provided to su or su -, the command will assume root is the
target.
[harry@RHEL4b ~]$ su -
Password:
[root@RHEL4b ~]#
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![users
23.7. run a program as another user
about sudo
The sudo program allows a user to start a program with the credentials of another
user. Before this works, the system administrator has to set up the /etc/sudoers file.
This can be useful to delegate administrative tasks to another user (without giving
the root password).
The screenshot below shows the usage of sudo. User paul received the right to run
useradd with the credentials of root. This allows paul to create new users on the
system without becoming root and without knowing the root password.
paul@laika:~$ useradd -m inge
useradd: unable to lock password file
paul@laika:~$ sudo useradd -m inge
[sudo] password for paul:
paul@laika:~$
Image copied from xkcd.com.
setuid on sudo
The sudo binary has the setuid bit set, so any user can run it with the effective userid
of root.
192](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-198-320.jpg)
![users
23.10. shell environment
It is nice to have these preset and custom aliases and variables, but where do they all
come from ? The shell uses a number of startup files that are checked (and executed)
whenever the shell is invoked. What follows is an overview of startup scripts.
/etc/profile
Both the bash and the ksh shell will verify the existence of /etc/profile and execute
it if it exists.
When reading this script, you might notice (at least on Debian Lenny and on Red Hat
Enterprise Linux 5) that it builds the PATH environment variable. The script might
also change the PS1 variable, set the HOSTNAME and execute even more scripts
like /etc/inputrc
You can use this script to set aliases and variables for every user on the system.
~/.bash_profile
When this file exists in the users home directory, then bash will execute it. On Debian
Linux it does not exist by default.
RHEL5 uses a brief ~/.bash_profile where it checks for the existence of ~/.bashrc
and then executes it. It also adds $HOME/bin to the $PATH variable.
[serena@rhel53 ~]$ cat .bash_profile
# .bash_profile
# Get the aliases and functions
if [ -f ~/.bashrc ]; then
. ~/.bashrc
fi
# User specific environment and startup programs
PATH=$PATH:$HOME/bin
export PATH
~/.bash_login
When .bash_profile does not exist, then bash will check for ~/.bash_login and
execute it.
Neither Debian nor Red Hat have this file by default.
197](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-203-320.jpg)
![users
~/.profile
When neither ~/.bash_profile and ~/.bash_login exist, then bash will verify the
existence of ~/.profile and execute it. This file does not exist by default on Red Hat.
On Debian this script can execute ~/.bashrc and will add $HOME/bin to the $PATH
variable.
serena@deb503:~$ tail -12 .profile
# if running bash
if [ -n "$BASH_VERSION" ]; then
# include .bashrc if it exists
if [ -f "$HOME/.bashrc" ]; then
. "$HOME/.bashrc"
fi
fi
# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
PATH="$HOME/bin:$PATH"
fi
~/.bashrc
As seen in the previous points, the ~/.bashrc script might be executed by other scripts.
Let us take a look at what it does by default.
Red Hat uses a very simple ~/.bashrc, checking for /etc/bashrc and executing it. It
also leaves room for custom aliases and functions.
[serena@rhel53 ~]$ more .bashrc
# .bashrc
# Source global definitions
if [ -f /etc/bashrc ]; then
. /etc/bashrc
fi
# User specific aliases and functions
On Debian this script is quite a bit longer and configures $PS1, some history variables
and a number af active and inactive aliases.
serena@deb503:~$ ls -l .bashrc
-rw-r--r-- 1 serena serena 3116 2008-05-12 21:02 .bashrc
~/.bash_logout
When exiting bash, it can execute ~/.bash_logout. Debian and Red Hat both use this
opportunity to clear the screen.
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![users
serena@deb503:~$ cat .bash_logout
# ~/.bash_logout: executed by bash(1) when login shell exits.
# when leaving the console clear the screen to increase privacy
if [ "$SHLVL" = 1 ]; then
[ -x /usr/bin/clear_console ] && /usr/bin/clear_console -q
fi
[serena@rhel53 ~]$ cat .bash_logout
# ~/.bash_logout
/usr/bin/clear
Debian overview
Below is a table overview of when Debian is running any of these bash startup scripts.
Table 23.1. Debian User Environment
script su su - ssh gdm
~./bashrc no yes yes yes
~/.profile no yes yes yes
/etc/profile no yes yes yes
/etc/bash.bashrc yes no no yes
RHEL5 overview
Below is a table overview of when Red Hat Enterprise Linux 5 is running any of
these bash startup scripts.
Table 23.2. Red Hat User Environment
script su su - ssh gdm
~./bashrc yes yes yes yes
~/.bash_profile no yes yes yes
/etc/profile no yes yes yes
/etc/bashrc yes yes yes yes
199](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-205-320.jpg)
![groups
root@laika:~# tail -5 /etc/group
tennis:x:1006:inge,katrien
football:x:1007:
snooker:x:1008:sandra
formula1:x:1009:annelies
salsa:x:1010:katrien
root@laika:~#
Be careful when using usermod to add users to groups. By default, the usermod
command will remove the user from every group of which he is a member if the group
is not listed in the command! Using the -a (append) switch prevents this behaviour.
24.5. groupmod
You can change the group name with the groupmod command.
root@laika:~# groupmod -n darts snooker
root@laika:~# tail -5 /etc/group
tennis:x:1006:inge,katrien
football:x:1007:
formula1:x:1009:annelies
salsa:x:1010:katrien
darts:x:1008:sandra
24.6. groupdel
You can permanently remove a group with the groupdel command.
root@laika:~# groupdel tennis
root@laika:~#
24.7. groups
A user can type the groups command to see a list of groups where the user belongs to.
[harry@RHEL4b ~]$ groups
harry sports
[harry@RHEL4b ~]$
24.8. gpasswd
You can delegate control of group membership to another user with the gpasswd
command. In the example below we delegate permissions to add and remove group
members to serena for the sports group. Then we su to serena and add harry to the
sports group.
202](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-208-320.jpg)
![groups
[root@RHEL4b ~]# gpasswd -A serena sports
[root@RHEL4b ~]# su - serena
[serena@RHEL4b ~]$ id harry
uid=516(harry) gid=520(harry) groups=520(harry)
[serena@RHEL4b ~]$ gpasswd -a harry sports
Adding user harry to group sports
[serena@RHEL4b ~]$ id harry
uid=516(harry) gid=520(harry) groups=520(harry),522(sports)
[serena@RHEL4b ~]$ tail -1 /etc/group
sports:x:522:serena,venus,harry
[serena@RHEL4b ~]$
Group administrators do not have to be a member of the group. They can remove
themselves from a group, but this does not influence their ability to add or remove
members.
[serena@RHEL4b ~]$ gpasswd -d serena sports
Removing user serena from group sports
[serena@RHEL4b ~]$ exit
Information about group administrators is kept in the /etc/gshadow file.
[root@RHEL4b ~]# tail -1 /etc/gshadow
sports:!:serena:venus,harry
[root@RHEL4b ~]#
To remove all group administrators from a group, use the gpasswd command to set
an empty administrators list.
[root@RHEL4b ~]# gpasswd -A "" sports
24.9. vigr
Similar to vipw, the vigr command can be used to manually edit the /etc/group file,
since it will do proper locking of the file. Only experienced senior administrators
should use vi or vigr to manage groups.
203](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-209-320.jpg)
![standard file permissions
setting octal permissions
Most Unix administrators will use the old school octal system to talk about and set
permissions. Look at the triplet bitwise, equating r to 4, w to 2, and x to 1.
Table 25.4. Octal permissions
binary octal permission
000 0 ---
001 1 --x
010 2 -w-
011 3 -wx
100 4 r--
101 5 r-x
110 6 rw-
111 7 rwx
This makes 777 equal to rwxrwxrwx and by the same logic, 654 mean rw-r-xr-- . The
chmod command will accept these numbers.
paul@laika:~/perms$ chmod 777 permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxrwxrwx 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod 664 permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod 750 permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxr-x--- 1 paul paul 0 2007-02-07 22:34 permissions.txt
umask
When creating a file or directory, a set of default permissions are applied. These
default permissions are determined by the umask. The umask specifies permissions
that you do not want set on by default. You can display the umask with the umask
command.
[Harry@RHEL4b ~]$ umask
0002
[Harry@RHEL4b ~]$ touch test
[Harry@RHEL4b ~]$ ls -l test
-rw-rw-r-- 1 Harry Harry 0 Jul 24 06:03 test
[Harry@RHEL4b ~]$
As you can also see, the file is also not executable by default. This is a general security
feature among Unixes; newly created files are never executable by default. You have
212](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-218-320.jpg)
![advanced file permissions
26.5. solution: sticky, setuid and setgid bits
1a. Set up a directory, owned by the group sports.
groupadd sports
mkdir /home/sports
chown root:sports /home/sports
1b. Members of the sports group should be able to create files in this directory.
chmod 770 /home/sports
1c. All files created in this directory should be group-owned by the sports group.
chmod 2770 /home/sports
1d. Users should be able to delete only their own user-owned files.
chmod +t /home/sports
1e. Test that this works!
Log in with different users (group members and others and root), create files and
watch the permissions. Try changing and deleting files...
2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing
your password as a normal user. Reset the permissions back and try again.
root@deb503:~# ls -l /usr/bin/passwd
-rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd
root@deb503:~# chmod 755 /usr/bin/passwd
root@deb503:~# ls -l /usr/bin/passwd
-rwxr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd
A normal user cannot change password now.
root@deb503:~# chmod 4755 /usr/bin/passwd
root@deb503:~# ls -l /usr/bin/passwd
-rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd
3. If time permits (or if you are waiting for other students to finish this practice), read
about file attributes in the man page of chattr and lsattr. Try setting the i attribute on
a file and test that it works.
paul@laika:~$ sudo su -
[sudo] password for paul:
root@laika:~# mkdir attr
root@laika:~# cd attr/
root@laika:~/attr# touch file42
root@laika:~/attr# lsattr
------------------ ./file42
root@laika:~/attr# chattr +i file42
221](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-227-320.jpg)
![introduction to processes
29.2. basic process management
$$ and $PPID
Some shell environment variables contain information about processes. The $$
variable will hold your current process ID, and $PPID contains the parent PID.
Actually $$ is a shell parameter and not a variable, you cannot assign a value to it.
Below we use echo to display the values of $$ and $PPID.
[paul@RHEL4b ~]$ echo $$ $PPID
4224 4223
pidof
You can find all process id's by name using the pidof command.
root@rhel53 ~# pidof mingetty
2819 2798 2797 2796 2795 2794
parent and child
Processes have a parent-child relationship. Every process has a parent process.
When starting a new bash you can use echo to verify that the pid from before is the
ppid of the new shell. The child process from above is now the parent process.
[paul@RHEL4b ~]$ bash
[paul@RHEL4b ~]$ echo $$ $PPID
4812 4224
Typing exit will end the current process and brings us back to our original values
for $$ and $PPID.
[paul@RHEL4b ~]$ echo $$ $PPID
4812 4224
[paul@RHEL4b ~]$ exit
exit
[paul@RHEL4b ~]$ echo $$ $PPID
4224 4223
[paul@RHEL4b ~]$
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![introduction to processes
fork and exec
A process starts another process in two phases. First the process creates a fork of
itself, an identical copy. Then the forked process executes an exec to replace the
forked process with the target child process.
[paul@RHEL4b ~]$ echo $$
4224
[paul@RHEL4b ~]$ bash
[paul@RHEL4b ~]$ echo $$ $PPID
5310 4224
[paul@RHEL4b ~]$
exec
With the exec command, you can execute a process without forking a new process.
In the following screenshot a Korn shell (ksh) is started and is being replaced with a
bash shell using the exec command. The pid of the bash shell is the same as the pid
of the Korn shell. Exiting the child bash shell will get me back to the parent bash,
not to the Korn shell (which does not exist anymore).
[paul@RHEL4b ~]$ echo $$
4224 # PID of bash
[paul@RHEL4b ~]$ ksh
$ echo $$ $PPID
5343 4224 # PID of ksh and bash
$ exec bash
[paul@RHEL4b ~]$ echo $$ $PPID
5343 4224 # PID of bash and bash
[paul@RHEL4b ~]$ exit
exit
[paul@RHEL4b ~]$ echo $$
4224
ps
One of the most common tools on Linux to look at processes is ps. The following
screenshot shows the parent child relationship between three bash processes.
[paul@RHEL4b ~]$ echo $$ $PPID
4224 4223
[paul@RHEL4b ~]$ bash
[paul@RHEL4b ~]$ echo $$ $PPID
4866 4224
[paul@RHEL4b ~]$ bash
[paul@RHEL4b ~]$ echo $$ $PPID
4884 4866
[paul@RHEL4b ~]$ ps fx
PID TTY STAT TIME COMMAND
4223 ? S 0:01 sshd: paul@pts/0
238](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-244-320.jpg)
![introduction to processes
4224 pts/0 Ss 0:00 _ -bash
4866 pts/0 S 0:00 _ bash
4884 pts/0 S 0:00 _ bash
4902 pts/0 R+ 0:00 _ ps fx
[paul@RHEL4b ~]$ exit
exit
[paul@RHEL4b ~]$ ps fx
PID TTY STAT TIME COMMAND
4223 ? S 0:01 sshd: paul@pts/0
4224 pts/0 Ss 0:00 _ -bash
4866 pts/0 S 0:00 _ bash
4903 pts/0 R+ 0:00 _ ps fx
[paul@RHEL4b ~]$ exit
exit
[paul@RHEL4b ~]$ ps fx
PID TTY STAT TIME COMMAND
4223 ? S 0:01 sshd: paul@pts/0
4224 pts/0 Ss 0:00 _ -bash
4904 pts/0 R+ 0:00 _ ps fx
[paul@RHEL4b ~]$
On Linux, ps fax is often used. On Solaris ps -ef (which also works on Linux) is
common. Here is a partial output from ps fax.
[paul@RHEL4a ~]$ ps fax
PID TTY STAT TIME COMMAND
1 ? S 0:00 init [5]
...
3713 ? Ss 0:00 /usr/sbin/sshd
5042 ? Ss 0:00 _ sshd: paul [priv]
5044 ? S 0:00 _ sshd: paul@pts/1
5045 pts/1 Ss 0:00 _ -bash
5077 pts/1 R+ 0:00 _ ps fax
pgrep
Similar to the ps -C, you can also use pgrep to search for a process by its command
name.
[paul@RHEL5 ~]$ sleep 1000 &
[1] 32558
[paul@RHEL5 ~]$ pgrep sleep
32558
[paul@RHEL5 ~]$ ps -C sleep
PID TTY TIME CMD
32558 pts/3 00:00:00 sleep
You can also list the command name of the process with pgrep.
paul@laika:~$ pgrep -l sleep
9661 sleep
239](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-245-320.jpg)
![introduction to processes
29.3. signalling processes
kill
The kill command will kill (or stop) a process. The screenshot shows how to use a
standard kill to stop the process with pid 1942.
paul@ubuntu910:~$ kill 1942
paul@ubuntu910:~$
By using the kill we are sending a signal to the process.
list signals
Running processes can receive signals from each other or from the users. You can
have a list of signals by typing kill -l, that is a letter l, not the number 1.
[paul@RHEL4a ~]$ kill -l
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL
5) SIGTRAP 6) SIGABRT 7) SIGBUS 8) SIGFPE
9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2
13) SIGPIPE 14) SIGALRM 15) SIGTERM 17) SIGCHLD
18) SIGCONT 19) SIGSTOP 20) SIGTSTP 21) SIGTTIN
22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ
26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO
30) SIGPWR 31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1
36) SIGRTMIN+2 37) SIGRTMIN+3 38) SIGRTMIN+4 39) SIGRTMIN+5
40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8 43) SIGRTMIN+9
44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13
52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9
56) SIGRTMAX-8 57) SIGRTMAX-7 58) SIGRTMAX-6 59) SIGRTMAX-5
60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2 63) SIGRTMAX-1
64) SIGRTMAX
[paul@RHEL4a ~]$
kill -1 (SIGHUP)
It is common on Linux to use the first signal SIGHUP (or HUP or 1) to tell a process
that it should re-read its configuration file. Thus, the kill -1 1 command forces the
init process (init always runs with pid 1) to re-read its configuration file.
root@deb503:~# kill -1 1
root@deb503:~#
It is up to the developer of the process to decide whether the process can do this
running, or whether it needs to stop and start. It is up to the user to read the
documentation of the program.
241](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-247-320.jpg)
![introduction to processes
kill -15 (SIGTERM)
The SIGTERM signal is also called a standard kill. Whenever kill is executed
without specifying the signal, a kill -15 is assumed.
Both commands in the screenshot below are identical.
paul@ubuntu910:~$ kill 1942
paul@ubuntu910:~$ kill -15 1942
kill -9 (SIGKILL)
The SIGKILL is different from most other signals in that it is not being sent to the
process, but to the Linux kernel. A kill -9 is also called a sure kill. The kernel will
shoot down the process. As a developer you have no means to intercept a kill -9 signal.
root@rhel53 ~# kill -9 3342
killall
The killall command will also default to sending a signal 15 to the processes.
This command and its SysV counterpart killall5 can by used when shutting down
the system. This screenshot shows how Red Hat Enterprise Linux 5.3 uses killall5
when halting the system.
root@rhel53 ~# grep killall /etc/init.d/halt
action $"Sending all processes the TERM signal..." /sbin/killall5 -15
action $"Sending all processes the KILL signal..." /sbin/killall5 -9
pkill
You can use the pkill command to kill a process by its command name.
[paul@RHEL5 ~]$ sleep 1000 &
[1] 30203
[paul@RHEL5 ~]$ pkill sleep
[1]+ Terminated sleep 1000
[paul@RHEL5 ~]$
top
Inside top the k key allows you to select a signal and pid to kill. Below is a partial
screenshot of the line just below the summary in top after pressing k.
242](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-248-320.jpg)
![introduction to processes
PID to kill: 1932
Kill PID 1932 with signal [15]: 9
SIGSTOP and SIGCONT
A running process can be suspended when it receives a SIGSTOP signal. This is the
same as kill -19 on Linux, but might have a different number in other Unix systems.
A suspended process does not use any cpu cycles, but it stays in memory and can be
re-animated with a SIGCONT signal (kill -18 on Linux).
Both signals will be used in the section about background processes.
243](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-249-320.jpg)
![process priorities
30.1. priority and nice values
introduction
All processes have a priority and a nice value. Higher priority processes will get
more cpu time than lower priority processes. You can influence this with the nice
and renice commands.
pipes (mkfifo)
Processes can communicate with each other via pipes. These pipes can be created
with the mkfifo command.
The screenshots shows the creation of four distinct pipes (in a new directory).
paul@ubuntu910:~$ mkdir procs
paul@ubuntu910:~$ cd procs/
paul@ubuntu910:~/procs$ mkfifo pipe33a pipe33b pipe42a pipe42b
paul@ubuntu910:~/procs$ ls -l
total 0
prw-r--r-- 1 paul paul 0 2010-04-12 13:21 pipe33a
prw-r--r-- 1 paul paul 0 2010-04-12 13:21 pipe33b
prw-r--r-- 1 paul paul 0 2010-04-12 13:21 pipe42a
prw-r--r-- 1 paul paul 0 2010-04-12 13:21 pipe42b
paul@ubuntu910:~/procs$
some fun with cat
To demonstrate the use of the top and renice commands we will make the cat
command use the previously created pipes to generate a full load on the cpu.
The cat is copied with a distinct name to the current directory. (This enables us to
easily recognize the processes within top. You could do the same exercise without
copying the cat command, but using different users. Or you could just look at the pid
of each process.)
paul@ubuntu910:~/procs$ cp /bin/cat proj33
paul@ubuntu910:~/procs$ cp /bin/cat proj42
paul@ubuntu910:~/procs$ echo -n x | ./proj33 - pipe33a > pipe33b &
[1] 1670
paul@ubuntu910:~/procs$ ./proj33 <pipe33b >pipe33a &
[2] 1671
paul@ubuntu910:~/procs$ echo -n z | ./proj42 - pipe42a > pipe42b &
[3] 1673
paul@ubuntu910:~/procs$ ./proj42 <pipe42b >pipe42a &
[4] 1674
The commands you see above will create two proj33 processes that use cat to bounce
the x character between pipe33a and pipe33b. And ditto for the z character and
proj42.
248](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-254-320.jpg)
![process priorities
30.3. solution : process priorities
1. Create a new directory and create six pipes in that directory.
[paul@rhel53 ~]$ mkdir pipes ; cd pipes
[paul@rhel53 pipes]$ mkfifo p1 p2 p3 p4 p5 p6
[paul@rhel53 pipes]$ ls -l
total 0
prw-rw-r-- 1 paul paul 0 Apr 12 22:15 p1
prw-rw-r-- 1 paul paul 0 Apr 12 22:15 p2
prw-rw-r-- 1 paul paul 0 Apr 12 22:15 p3
prw-rw-r-- 1 paul paul 0 Apr 12 22:15 p4
prw-rw-r-- 1 paul paul 0 Apr 12 22:15 p5
prw-rw-r-- 1 paul paul 0 Apr 12 22:15 p6
2. Bounce a character between two pipes.
[paul@rhel53 pipes]$ echo -n x | cat - p1 > p2 &
[1] 4013
[paul@rhel53 pipes]$ cat <p2 >p1 &
[2] 4016
3. Use top and ps to display information (pid, ppid, priority, nice value, ...) about
these two cat processes.
top (probably the top two lines)
[paul@rhel53 pipes]$ ps -C cat
PID TTY TIME CMD
4013 pts/0 00:03:38 cat
4016 pts/0 00:01:07 cat
[paul@rhel53 pipes]$ ps fax | grep cat
4013 pts/0 R 4:00 | _ cat - p1
4016 pts/0 S 1:13 | _ cat
4044 pts/0 S+ 0:00 | _ grep cat
4. Bounce another character between two other pipes, but this time start the
commands nice. Verify that all cat processes are battling for the cpu. (Feel free to
fire up two more cats with the remaining pipes).
echo -n y | nice cat - p3 > p4 &
nice cat <p4 >p3 &
5. Use ps to verify that the two new cat processes have a nice value. Use the -o and
-C options of ps for this.
[paul@rhel53 pipes]$ ps -C cat -o pid,ppid,pri,ni,comm
PID PPID PRI NI COMMAND
4013 3947 14 0 cat
4016 3947 21 0 cat
4025 3947 13 10 cat
4026 3947 13 10 cat
6. Use renice te increase the nice value from 10 to 15. Notice the difference with
the usual commands.
[paul@rhel53 pipes]$ renice +15 4025
4025: old priority 10, new priority 15
[paul@rhel53 pipes]$ renice +15 4026
252](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-258-320.jpg)
![process priorities
4026: old priority 10, new priority 15
[paul@rhel53 pipes]$ ps -C cat -o pid,ppid,pri,ni,comm
PID PPID PRI NI COMMAND
4013 3947 14 0 cat
4016 3947 21 0 cat
4025 3947 9 15 cat
4026 3947 8 15 cat
253](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-259-320.jpg)
![background jobs
31.1. background processes
jobs
Stuff that runs in background of your current shell can be displayed with the jobs
command. By default you will not have any jobs running in background.
root@rhel53 ~# jobs
root@rhel53 ~#
This jobs command will be used several times in this section.
control-Z
Some processes can be suspended with the Ctrl-Z key combination. This sends
a SIGSTOP signal to the Linux kernel, effectively freezing the operation of the
process.
When doing this in vi(m), then vi(m) goes to the background. The background vi(m)
can be seen with the jobs command.
[paul@RHEL4a ~]$ vi procdemo.txt
[5]+ Stopped vim procdemo.txt
[paul@RHEL4a ~]$ jobs
[5]+ Stopped vim procdemo.txt
& ampersand
Processes that are started in background using the & character at the end of the
command line are also visible with the jobs command.
[paul@RHEL4a ~]$ find / > allfiles.txt 2> /dev/null &
[6] 5230
[paul@RHEL4a ~]$ jobs
[5]+ Stopped vim procdemo.txt
[6]- Running find / >allfiles.txt 2>/dev/null &
[paul@RHEL4a ~]$
jobs -p
An interesting option is jobs -p to see the process id of background processes.
[paul@RHEL4b ~]$ sleep 500 &
255](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-261-320.jpg)
![background jobs
[1] 4902
[paul@RHEL4b ~]$ sleep 400 &
[2] 4903
[paul@RHEL4b ~]$ jobs -p
4902
4903
[paul@RHEL4b ~]$ ps `jobs -p`
PID TTY STAT TIME COMMAND
4902 pts/0 S 0:00 sleep 500
4903 pts/0 S 0:00 sleep 400
[paul@RHEL4b ~]$
fg
Running the fg command will bring a background job to the foreground. The number
of the background job to bring forward is the parameter of fg.
[paul@RHEL5 ~]$ jobs
[1] Running sleep 1000 &
[2]- Running sleep 1000 &
[3]+ Running sleep 2000 &
[paul@RHEL5 ~]$ fg 3
sleep 2000
bg
Jobs that are suspended in background can be started in background with bg. The
bg will send a SIGCONT signal.
Below an example of the sleep command (suspended with Ctrl-Z) being reactivated
in background with bg.
[paul@RHEL5 ~]$ jobs
[paul@RHEL5 ~]$ sleep 5000 &
[1] 6702
[paul@RHEL5 ~]$ sleep 3000
[2]+ Stopped sleep 3000
[paul@RHEL5 ~]$ jobs
[1]- Running sleep 5000 &
[2]+ Stopped sleep 3000
[paul@RHEL5 ~]$ bg 2
[2]+ sleep 3000 &
[paul@RHEL5 ~]$ jobs
[1]- Running sleep 5000 &
[2]+ Running sleep 3000 &
[paul@RHEL5 ~]$
256](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-262-320.jpg)
![background jobs
31.2. practice : background processes
1. Use the jobs command to verify whether you have any processes running in
background.
2. Use vi to create a little text file. Suspend vi in background.
3. Verify with jobs that vi is suspended in background.
4. Start find / > allfiles.txt 2>/dev/null in foreground. Suspend it in background
before it finishes.
5. Start two long sleep processes in background.
6. Display all jobs in background.
7. Use the kill command to suspend the last sleep process.
8. Continue the find process in background (make sure it runs again).
9. Put one of the sleep commands back in foreground.
10. (if time permits, a general review question...) Explain in detail where the numbers
come from in the next screenshot. When are the variables replaced by their value ?
By which shell ?
[paul@RHEL4b ~]$ echo $$ $PPID
4224 4223
[paul@RHEL4b ~]$ bash -c "echo $$ $PPID"
4224 4223
[paul@RHEL4b ~]$ bash -c 'echo $$ $PPID'
5059 4224
[paul@RHEL4b ~]$ bash -c `echo $$ $PPID`
4223: 4224: command not found
257](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-263-320.jpg)
![background jobs
31.3. solution : background processes
1. Use the jobs command to verify whether you have any processes running in
background.
jobs (maybe the catfun is still running?)
2. Use vi to create a little text file. Suspend vi in background.
vi text.txt
(inside vi press ctrl-z)
3. Verify with jobs that vi is suspended in background.
[paul@rhel53 ~]$ jobs
[1]+ Stopped vim text.txt
4. Start find / > allfiles.txt 2>/dev/null in foreground. Suspend it in background
before it finishes.
[paul@rhel53 ~]$ find / > allfiles.txt 2>/dev/null
(press ctrl-z)
[2]+ Stopped find / > allfiles.txt 2> /dev/null
5. Start two long sleep processes in background.
sleep 4000 & ; sleep 5000 &
6. Display all jobs in background.
[paul@rhel53 ~]$ jobs
[1]- Stopped vim text.txt
[2]+ Stopped find / > allfiles.txt 2> /dev/null
[3] Running sleep 4000 &
[4] Running sleep 5000 &
7. Use the kill command to suspend the last sleep process.
[paul@rhel53 ~]$ kill -SIGSTOP 4519
[paul@rhel53 ~]$ jobs
[1] Stopped vim text.txt
[2]- Stopped find / > allfiles.txt 2> /dev/null
[3] Running sleep 4000 &
[4]+ Stopped sleep 5000
8. Continue the find process in background (make sure it runs again).
bg 2 (verify the job-id in your jobs list)
9. Put one of the sleep commands back in foreground.
fg 3 (again verify your job-id)
10. (if time permits, a general review question...) Explain in detail where the numbers
come from in the next screenshot. When are the variables replaced by their value ?
By which shell ?
258](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-264-320.jpg)
![background jobs
[paul@RHEL4b ~]$ echo $$ $PPID
4224 4223
[paul@RHEL4b ~]$ bash -c "echo $$ $PPID"
4224 4223
[paul@RHEL4b ~]$ bash -c 'echo $$ $PPID'
5059 4224
[paul@RHEL4b ~]$ bash -c `echo $$ $PPID`
4223: 4224: command not found
The current bash shell will replace the $$ and $PPID while scanning the line, and
before executing the echo command.
[paul@RHEL4b ~]$ echo $$ $PPID
4224 4223
The variables are now double quoted, but the current bash shell will replace $$ and
$PPID while scanning the line, and before executing the bach -c command.
[paul@RHEL4b ~]$ bash -c "echo $$ $PPID"
4224 4223
The variables are now single quoted. The current bash shell will not replace the $$
and the $PPID. The bash -c command will be executed before the variables replaced
with their value. This latter bash is the one replacing the $$ and $PPID with their
value.
[paul@RHEL4b ~]$ bash -c 'echo $$ $PPID'
5059 4224
With backticks the shell will still replace both variable before the embedded echo is
executed. The result of this echo is the two process id's. These are given as commands
to bash -c. But two numbers are not commands!
[paul@RHEL4b ~]$ bash -c `echo $$ $PPID`
4223: 4224: command not found
259](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-265-320.jpg)
![disk devices
32.1. terminology
platter, head, track, cylinder, sector
Data is commonly stored on magnetic or optical disk platters. The platters are rotated
(at high speeds). Data is read by heads, which are very close to the surface of the
platter, without touching it! The heads are mounted on an arm (sometimes called a
comb or a fork).
Data is written in concentric circles called tracks. Track zero is (usually) on the
outside. The time it takes to position the head over a certain track is called the seek
time. Often the platters are stacked on top of each other, hence the set of tracks
accessible at a certain position of the comb forms a cylinder. Tracks are divided into
512 byte sectors, with more unused space (gap) between the sectors on the outside
of the platter.
When you break down the advertised access time of a hard drive, you will notice
that most of that time is taken by movement of the heads (about 65%) and rotational
latency (about 30%).
block device
Random access hard disk devices have an abstraction layer called block device to
enable formatting in fixed-size (usually 512 bytes) blocks. Blocks can be accessed
independent of access to other blocks. A block device has the letter b to denote the
file type in the output of ls -l.
[root@RHEL4b ~]# ls -l /dev/sda*
brw-rw---- 1 root disk 8, 0 Aug 4 22:55 /dev/sda
brw-rw---- 1 root disk 8, 1 Aug 4 22:55 /dev/sda1
brw-rw---- 1 root disk 8, 2 Aug 4 22:55 /dev/sda2
[root@RHEL4b ~]#
Note that a character device is a constant stream of characters, being denoted by a
c in ls -l.
Note also that the ISO 9660 standard for cdrom uses a 2048 byte block size.
Old hard disks (and floppy disks) use cylinder-head-sector addressing to access a
sector on the disk. Most current disks use LBA (Logical Block Addressing).
ide or scsi
Actually, the title should be ata or scsi, since ide is an ata compatible device. Most
desktops use ata devices, most servers use scsi.
262](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-268-320.jpg)
![disk devices
32.3. discovering disk devices
/sbin/fdisk
You can start by using /sbin/fdisk to find out what kind of disks are seen by the
kernel. Below the result on Debian, with two ata-ide disks present.
root@barry:~# fdisk -l | grep Disk
Disk /dev/hda: 60.0 GB, 60022480896 bytes
Disk /dev/hdb: 81.9 GB, 81964302336 bytes
And here an example of sata disks on a laptop with Ubuntu. Remember that sata
disks are presented to you with the scsi /dev/sdx notation.
root@laika:~# fdisk -l | grep Disk
Disk /dev/sda: 100.0 GB, 100030242816 bytes
Disk /dev/sdb: 100.0 GB, 100030242816 bytes
Here is an overview of disks on a RHEL4u3 server with two real 72GB scsi disks.
This server is attached to a NAS with four NAS disks of half a terabyte. On the NAS
disks, four LVM (/dev/mdx) software RAID devices are configured.
[root@tsvtl1 ~]# fdisk -l | grep Disk
Disk /dev/sda: 73.4 GB, 73407488000 bytes
Disk /dev/sdb: 73.4 GB, 73407488000 bytes
Disk /dev/sdc: 499.0 GB, 499036192768 bytes
Disk /dev/sdd: 499.0 GB, 499036192768 bytes
Disk /dev/sde: 499.0 GB, 499036192768 bytes
Disk /dev/sdf: 499.0 GB, 499036192768 bytes
Disk /dev/md0: 271 MB, 271319040 bytes
Disk /dev/md2: 21.4 GB, 21476081664 bytes
Disk /dev/md3: 21.4 GB, 21467889664 bytes
Disk /dev/md1: 21.4 GB, 21476081664 bytes
You can also use fdisk to obtain information about one specific hard disk device.
[root@rhel4 ~]# fdisk -l /dev/sda
Disk /dev/sda: 12.8 GB, 12884901888 bytes
255 heads, 63 sectors/track, 1566 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sda1 * 1 13 104391 83 Linux
/dev/sda2 14 1566 12474472+ 8e Linux LVM
Later we will use fdisk to do dangerous stuff like creating and deleting partitions.
265](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-271-320.jpg)
![disk devices
/bin/dmesg
Kernel boot messages can be seen after boot with dmesg. Since hard disk devices
are detected by the kernel during boot, you can also use dmesg to find information
about disk devices.
root@barry:~# dmesg | grep "[hs]d[a-z]"
Kernel command line: root=/dev/hda1 ro
ide0: BM-DMA at 0xfc00-0xfc07, BIOS settings: hda:DMA, hdb:DMA
ide1: BM-DMA at 0xfc08-0xfc0f, BIOS settings: hdc:DMA, hdd:DMA
hda: ST360021A, ATA DISK drive
hdb: Maxtor 6Y080L0, ATA DISK drive
hdc: SONY DVD RW DRU-510A, ATAPI CD/DVD-ROM drive
hdd: SONY DVD RW DRU-810A, ATAPI CD/DVD-ROM drive
hda: max request size: 128KiB
hda: 117231408 sectors (60022 MB) w/2048KiB Cache, CHS=65535/16/63, UDMA
hda: hda1 hda2
hdb: max request size: 128KiB
hdb: 160086528 sectors (81964 MB) w/2048KiB Cache, CHS=65535/16/63, UDMA
hdb: hdb1 hdb2
hdc: ATAPI 32X DVD-ROM DVD-R CD-R/RW drive, 8192kB Cache, UDMA(33)
hdd: ATAPI 40X DVD-ROM DVD-R CD-R/RW drive, 2048kB Cache, UDMA(33)
...
Here's another example of dmesg (same computer as above, but with extra 200gb
disk now).
paul@barry:~$ dmesg | grep -i "ata disk"
[ 2.624149] hda: ST360021A, ATA DISK drive
[ 2.904150] hdb: Maxtor 6Y080L0, ATA DISK drive
[ 3.472148] hdd: WDC WD2000BB-98DWA0, ATA DISK drive
Third and last example of dmesg running on RHEL5.3.
root@rhel53 ~# dmesg | grep -i "scsi disk"
sd 0:0:2:0: Attached scsi disk sda
sd 0:0:3:0: Attached scsi disk sdb
sd 0:0:6:0: Attached scsi disk sdc
/sbin/lshw
The lshw tool will list hardware. With the right options lshw can show a lot of
information about disks (and partitions).
Below a truncated screenshot on Debian 5:
root@debian5:~# aptitude search lshw
p lshw - information about hardware configuration
p lshw-gtk - information about hardware configuration
root@debian5:~# aptitude install lshw
...
root@debian5:~# lshw -class volume
*-volume:0
266](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-272-320.jpg)
![disk devices
description: EXT3 volume
vendor: Linux
physical id: 1
bus info: ide@0.0,1
logical name: /dev/hda1
logical name: /
version: 1.0
serial: f327ca8a-8187-48c5-b760-956ec79d414b
size: 19GiB
capacity: 19GiB
capabilities: primary bootable journaled extended_attributes lar
ge_files huge_files recover ext3 ext2 initialized
configuration: created=2009-10-28 12:02:35 filesystem=ext3 ...
...
Below a screenshot of lshw running Ubuntu 10.10 on a macbook pro:
root@ubu1010:~# lshw -class volume
*-volume:0 UNCLAIMED
description: EFI GPT partition
physical id: 1
bus info: scsi@0:0.0.0,1
capacity: 2047KiB
capabilities: primary nofs
*-volume:1
description: EXT4 volume
vendor: Linux
physical id: 2
bus info: scsi@0:0.0.0,2
logical name: /dev/sda2
logical name: /
version: 1.0
serial: 101eb20f-3e25-4900-b988-4622c0ee4ff5
size: 142GiB
capacity: 142GiB
...
/sbin/lsscsi
The /sbin/lsscsi will give you a nice readable output of all scsi (and scsi emulated
devices). This first screenshot shows lsscsi on a SPARC system.
root@shaka:~# lsscsi
[0:0:0:0] disk Adaptec RAID5 V1.0 /dev/sda
[1:0:0:0] disk SEAGATE ST336605FSUN36G 0438 /dev/sdb
root@shaka:~#
Here is the same command, but run on a laptop with scsi emulated dvd writer and
scsi emulated usb.
paul@laika:~$ lsscsi
[0:0:0:0] disk ATA HTS721010G9SA00 MCZO /dev/sda
[1:0:0:0] disk ATA HTS721010G9SA00 MCZO /dev/sdb
[3:0:0:0] cd/dvd _NEC DVD_RW ND-7551A 1-02 /dev/scd0
[4:0:0:0] disk GENERIC USB Storage-CFC 019A /dev/sdc
[4:0:0:1] disk GENERIC USB Storage-SDC 019A /dev/sdd
[4:0:0:2] disk GENERIC USB Storage-SMC 019A /dev/sde
[4:0:0:3] disk GENERIC USB Storage-MSC 019A /dev/sdf
267](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-273-320.jpg)
![disk devices
32.7. solution: hard disk devices
1. Use dmesg to make a list of hard disk devices detected at boot-up.
Some possible answers...
dmesg | grep -i disk
Looking for ATA disks: dmesg | grep hd[abcd]
Looking for ATA disks: dmesg | grep -i "ata disk"
Looking for SCSI disks: dmesg | grep sd[a-f]
Looking for SCSI disks: dmesg | grep -i "scsi disk"
2. Use fdisk to find the total size of all hard disk devices on your system.
fdisk -l
3. Stop a virtual machine, add three virtual 1 gigabyte scsi hard disk devices and one
virtual 400 megabyte ide hard disk device. If possible, also add another virtual 400
megabyte ide disk.
This exercise happens in the settings of vmware or VirtualBox.
4. Use dmesg to verify that all the new disks are properly detected at boot-up.
See 1.
5. Verify that you can see the disk devices in /dev.
SCSI+SATA: ls -l /dev/sd*
ATA: ls -l /dev/hd*
6. Use fdisk (with grep and /dev/null) to display the total size of the new disks.
root@rhel53 ~# fdisk -l 2>/dev/null | grep [MGT]B
Disk /dev/hda: 21.4 GB, 21474836480 bytes
Disk /dev/hdb: 1073 MB, 1073741824 bytes
Disk /dev/sda: 2147 MB, 2147483648 bytes
Disk /dev/sdb: 2147 MB, 2147483648 bytes
Disk /dev/sdc: 2147 MB, 2147483648 bytes
7. Use badblocks to completely erase one of the smaller hard disks.
#Verify the device (/dev/sdc??) you want to erase before typing this.
#
root@rhel53 ~# badblocks -ws /dev/sdc
Testing with pattern 0xaa: done
Reading and comparing: done
Testing with pattern 0x55: done
Reading and comparing: done
Testing with pattern 0xff: done
Reading and comparing: done
Testing with pattern 0x00: done
Reading and comparing: done
8. Look at /proc/scsi/scsi.
root@rhel53 ~# cat /proc/scsi/scsi
272](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-278-320.jpg)
![disk devices
Attached devices:
Host: scsi0 Channel: 00 Id: 02 Lun: 00
Vendor: VBOX Model: HARDDISK Rev: 1.0
Type: Direct-Access ANSI SCSI revision: 05
Host: scsi0 Channel: 00 Id: 03 Lun: 00
Vendor: VBOX Model: HARDDISK Rev: 1.0
Type: Direct-Access ANSI SCSI revision: 05
Host: scsi0 Channel: 00 Id: 06 Lun: 00
Vendor: VBOX Model: HARDDISK Rev: 1.0
Type: Direct-Access ANSI SCSI revision: 05
9. If possible, install lsscsi, lshw and use them to list the disks.
Debian,Ubuntu: aptitude install lsscsi lshw
Fedora: yum install lsscsi lshw
root@rhel53 ~# lsscsi
[0:0:2:0] disk VBOX HARDDISK 1.0 /dev/sda
[0:0:3:0] disk VBOX HARDDISK 1.0 /dev/sdb
[0:0:6:0] disk VBOX HARDDISK 1.0 /dev/sdc
273](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-279-320.jpg)
![disk partitions
/dev/sdb1 1 14 112423+ 83 Linux
root@RHELv4u2:~#
33.4. about the partition table
master boot record
The partition table information (primary and extended partitions) is written in the
master boot record or mbr. You can use dd to copy the mbr to a file.
This example copies the master boot record from the first SCSI hard disk.
dd if=/dev/sda of=/SCSIdisk.mbr bs=512 count=1
The same tool can also be used to wipe out all information about partitions on a disk.
This example writes zeroes over the master boot record.
dd if=/dev/zero of=/dev/sda bs=512 count=1
Or to wipe out the whole partition or disk.
dd if=/dev/zero of=/dev/sda
partprobe
Don't forget that after restoring a master boot record with dd, that you need to force
the kernel to reread the partition table with partprobe. After running partprobe, the
partitions can be used again.
[root@RHEL5 ~]# partprobe
[root@RHEL5 ~]#
logical drives
The partition table does not contain information about logical drives. So the dd
backup of the mbr only works for primary and extended partitions. To backup the
partition table including the logical drives, you can use sfdisk.
This example shows how to backup all partition and logical drive information to a file.
sfdisk -d /dev/sda > parttable.sda.sfdisk
The following example copies the mbr and all logical drive info from /dev/sda to /
dev/sdb.
sfdisk -d /dev/sda | sfdisk /dev/sdb
279](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-285-320.jpg)
![file systems
34.3. putting a file system on a partition
We now have a fresh partition. The system binaries to make file systems can be found
with ls.
[root@RHEL4b ~]# ls -lS /sbin/mk*
-rwxr-xr-x 3 root root 34832 Apr 24 2006 /sbin/mke2fs
-rwxr-xr-x 3 root root 34832 Apr 24 2006 /sbin/mkfs.ext2
-rwxr-xr-x 3 root root 34832 Apr 24 2006 /sbin/mkfs.ext3
-rwxr-xr-x 3 root root 28484 Oct 13 2004 /sbin/mkdosfs
-rwxr-xr-x 3 root root 28484 Oct 13 2004 /sbin/mkfs.msdos
-rwxr-xr-x 3 root root 28484 Oct 13 2004 /sbin/mkfs.vfat
-rwxr-xr-x 1 root root 20313 Apr 10 2006 /sbin/mkinitrd
-rwxr-x--- 1 root root 15444 Oct 5 2004 /sbin/mkzonedb
-rwxr-xr-x 1 root root 15300 May 24 2006 /sbin/mkfs.cramfs
-rwxr-xr-x 1 root root 13036 May 24 2006 /sbin/mkswap
-rwxr-xr-x 1 root root 6912 May 24 2006 /sbin/mkfs
-rwxr-xr-x 1 root root 5905 Aug 3 2004 /sbin/mkbootdisk
[root@RHEL4b ~]#
It is time for you to read the manual pages of mkfs and mke2fs. In the example below,
you see the creation of an ext2 file system on /dev/sdb1. In real life, you might want
to use options like -m0 and -j.
root@RHELv4u2:~# mke2fs /dev/sdb1
mke2fs 1.35 (28-Feb-2004)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
28112 inodes, 112420 blocks
5621 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=67371008
14 block groups
8192 blocks per group, 8192 fragments per group
2008 inodes per group
Superblock backups stored on blocks:
8193, 24577, 40961, 57345, 73729
Writing inode tables: done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 37 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
34.4. tuning a file system
You can use tune2fs to list and set file system settings. The first screenshot lists the
reserved space for root (which is set at five percent).
[root@rhel4 ~]# tune2fs -l /dev/sda1 | grep -i "block count"
Block count: 104388
Reserved block count: 5219
[root@rhel4 ~]#
This example changes this value to ten percent. You can use tune2fs while the file
system is active, even if it is the root file system (as in this example).
286](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-292-320.jpg)
![file systems
[root@rhel4 ~]# tune2fs -m10 /dev/sda1
tune2fs 1.35 (28-Feb-2004)
Setting reserved blocks percentage to 10 (10430 blocks)
[root@rhel4 ~]# tune2fs -l /dev/sda1 | grep -i "block count"
Block count: 104388
Reserved block count: 10430
[root@rhel4 ~]#
34.5. checking a file system
The fsck command is a front end tool used to check a file system for errors.
[root@RHEL4b ~]# ls /sbin/*fsck*
/sbin/dosfsck /sbin/fsck /sbin/fsck.ext2 /sbin/fsck.msdos
/sbin/e2fsck /sbin/fsck.cramfs /sbin/fsck.ext3 /sbin/fsck.vfat
[root@RHEL4b ~]#
The last column in /etc/fstab is used to determine whether a file system should be
checked at boot-up.
[paul@RHEL4b ~]$ grep ext /etc/fstab
/dev/VolGroup00/LogVol00 / ext3 defaults 1 1
LABEL=/boot /boot ext3 defaults 1 2
[paul@RHEL4b ~]$
Manually checking a mounted file system results in a warning from fsck.
[root@RHEL4b ~]# fsck /boot
fsck 1.35 (28-Feb-2004)
e2fsck 1.35 (28-Feb-2004)
/dev/sda1 is mounted.
WARNING!!! Running e2fsck on a mounted filesystem may cause
SEVERE filesystem damage.
Do you really want to continue (y/n)? no
check aborted.
But after unmounting fsck and e2fsck can be used to check an ext2 file system.
[root@RHEL4b ~]# fsck /boot
fsck 1.35 (28-Feb-2004)
e2fsck 1.35 (28-Feb-2004)
/boot: clean, 44/26104 files, 17598/104388 blocks
[root@RHEL4b ~]# fsck -p /boot
fsck 1.35 (28-Feb-2004)
/boot: clean, 44/26104 files, 17598/104388 blocks
[root@RHEL4b ~]# e2fsck -p /dev/sda1
/boot: clean, 44/26104 files, 17598/104388 blocks
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![mounting
[root@RHEL52 ~]# cat /proc/filesystems | grep -v ^nodev
ext2
iso9660
ext3
35.2. displaying mounted file systems
To display all mounted file systems, issue the mount command. Or look at the files
/proc/mounts and /etc/mtab.
/bin/mount
The simplest and most common way to view all mounts is by issuing the mount
command without any arguments.
root@RHELv4u2:~# mount | grep /dev/sdb
/dev/sdb1 on /home/project55 type ext2 (rw)
/proc/mounts
The kernel provides the info in /proc/mounts in file form, but /proc/mounts does not
exist as a file on any hard disk. Looking at /proc/mounts is looking at information
that comes directly from the kernel.
root@RHELv4u2:~# cat /proc/mounts | grep /dev/sdb
/dev/sdb1 /home/project55 ext2 rw 0 0
/etc/mtab
The /etc/mtab file is not updated by the kernel, but is maintained by the mount
command. Do not edit /etc/mtab manually.
root@RHELv4u2:~# cat /etc/mtab | grep /dev/sdb
/dev/sdb1 /home/project55 ext2 rw 0 0
/bin/df
A more user friendly way to look at mounted file systems is df. The df (diskfree)
command has the added benefit of showing you the free space on each mounted disk.
Like a lot of Linux commands, df supports the -h switch to make the output more
human readable.
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![mounting
Note that you can still set the setuid bit on files.
root@rhel53 ~# mount -o nosuid /dev/hdb1 /home/project42
root@rhel53 ~# cp /bin/sleep /home/project42/
root@rhel53 ~# chmod 4555 /home/project42/sleep
root@rhel53 ~# ls -l /home/project42/sleep
-r-sr-xr-x 1 root root 19564 Jun 24 17:57 /home/project42/sleep
But users cannot exploit the setuid feature.
root@rhel53 ~# su - paul
[paul@rhel53 ~]$ /home/project42/sleep 500 &
[1] 2876
[paul@rhel53 ~]$ ps -f 2876
UID PID PPID C STIME TTY STAT TIME CMD
paul 2876 2853 0 17:58 pts/0 S 0:00 /home/project42/sleep 500
[paul@rhel53 ~]$
noacl
To prevent cluttering permissions with acl's, use the noacl option.
root@rhel53 ~# mount -o noacl /dev/hdb1 /home/project42
More mount options can be found in the manual page of mount.
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![introduction to uuid's
36.1. about unique objects
A uuid or universally unique identifier is used to uniquely identify objects. This
128bit standard allows anyone to create a unique uuid.
/sbin/vol_id
Below we use the vol_id utility to display the uuid of an ext3 file system.
root@laika:~# vol_id --uuid /dev/sda1
825d4b79-ec40-4390-8a71-9261df8d4c82
/lib/udev/vol_id
Red Hat Enterprise Linux 5 puts vol_id in /lib/udev/vol_id, which is not in the
$PATH. The syntax is also a bit different from Debian/Ubuntu.
root@rhel53 ~# /lib/udev/vol_id -u /dev/hda1
48a6a316-9ca9-4214-b5c6-e7b33a77e860
/sbin/tune2fs
We can also use tune2fs to find the uuid of a file system.
[root@RHEL5 ~]# tune2fs -l /dev/sda1 | grep UUID
Filesystem UUID: 11cfc8bc-07c0-4c3f-9f64-78422ef1dd5c
[root@RHEL5 ~]# /lib/udev/vol_id -u /dev/sda1
11cfc8bc-07c0-4c3f-9f64-78422ef1dd5c
36.2. uuid in /etc/fstab
You can use the uuid to make sure that a volume is universally uniquely identified
in /etc/fstab. The device name can change depending on the disk devices that are
present at boot time, but a uuid never changes.
First we use tune2fs to find the uuid.
[root@RHEL5 ~]# tune2fs -l /dev/sdc1 | grep UUID
Filesystem UUID: 7626d73a-2bb6-4937-90ca-e451025d64e8
Then we check that it is properly added to /etc/fstab, the uuid replaces the variable
devicename /dev/sdc1.
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![introduction to uuid's
[root@RHEL5 ~]# grep UUID /etc/fstab
UUID=7626d73a-2bb6-4937-90ca-e451025d64e8 /home/pro42 ext3 defaults 0 0
Now we can mount the volume using the mount point defined in /etc/fstab.
[root@RHEL5 ~]# mount /home/pro42
[root@RHEL5 ~]# df -h | grep 42
/dev/sdc1 397M 11M 366M 3% /home/pro42
The real test now, is to remove /dev/sdb from the system, reboot the machine and
see what happens. After the reboot, the disk previously known as /dev/sdc is now /
dev/sdb.
[root@RHEL5 ~]# tune2fs -l /dev/sdb1 | grep UUID
Filesystem UUID: 7626d73a-2bb6-4937-90ca-e451025d64e8
And thanks to the uuid in /etc/fstab, the mountpoint is mounted on the same disk
as before.
[root@RHEL5 ~]# df -h | grep sdb
/dev/sdb1 397M 11M 366M 3% /home/pro42
36.3. uuid in menu.lst
Recent incarnations of the Ubuntu distribution will use a uuid to identify the root file
system. This example shows how a root=/dev/sda1 is replaced with a uuid.
title Ubuntu 9.10, kernel 2.6.31-19-generic
uuid f001ba5d-9077-422a-9634-8d23d57e782a
kernel /boot/vmlinuz-2.6.31-19-generic
root=UUID=f001ba5d-9077-422a-9634-8d23d57e782a ro quiet splash
initrd /boot/initrd.img-2.6.31-19-generic
The screenshot above contains only four lines. The line starting with root= is the
continuation of the kernel line.
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![introduction to uuid's
36.5. solution: uuid and filesystems
1. Find the uuid of one of your ext3 partitions with tune2fs and vol_id.
root@rhel55:~# /lib/udev/vol_id -u /dev/hda1
60926898-2c78-49b4-a71d-c1d6310c87cc
root@ubu1004:~# tune2fs -l /dev/sda2 | grep UUID
Filesystem UUID: 3007b743-1dce-2d62-9a59-cf25f85191b7
2. Use this uuid in /etc/fstab and test that it works with a simple mount.
tail -1 /etc/fstab
UUID=60926898-2c78-49b4-a71d-c1d6310c87cc /home/pro42 ext3 defaults 0 0
3. (optional) Test it also by removing a disk (so the device name is changed). You
can edit settings in vmware/Virtualbox to remove a hard disk.
4. Display the root= directive in /boot/grub/menu.lst. (We see later in the course
how to maintain this file.)
paul@deb503:~$ grep ^[^#] /boot/grub/menu.lst | grep root=
kernel /boot/vmlinuz-2.6.26-2-686 root=/dev/hda1 ro selinux=1 quiet
kernel /boot/vmlinuz-2.6.26-2-686 root=/dev/hda1 ro selinux=1 single
5. (optional) Replace the /dev/xxx in /boot/grub/menu.lst with a uuid (use an extra
stanza for this). Test that it works.
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![introduction to raid
37.3. building a software raid5 array
do we have three disks?
First, you have to attach some disks to your computer. In this scenario, three brand
new disks of eight gigabyte each are added. Check with fdisk -l that they are
connected.
[root@rhel6c ~]# fdisk -l 2> /dev/null | grep MB
Disk /dev/sdb: 8589 MB, 8589934592 bytes
Disk /dev/sdc: 8589 MB, 8589934592 bytes
Disk /dev/sdd: 8589 MB, 8589934592 bytes
fd partition type
The next step is to create a partition of type fd on every disk. The fd type is to set the
partition as Linux RAID autodetect. See this (truncated) screenshot:
[root@rhel6c ~]# fdisk /dev/sdd
...
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-1044, default 1):
Using default value 1
Last cylinder, +cylinders or +size{K,M,G} (1-1044, default 1044):
Using default value 1044
Command (m for help): t
Selected partition 1
Hex code (type L to list codes): fd
Changed system type of partition 1 to fd (Linux raid autodetect)
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
verify all three partitions
Now all three disks are ready for raid 5, so we have to tell the system what to do
with these disks.
[root@rhel6c ~]# fdisk -l 2> /dev/null | grep raid
/dev/sdb1 1 1044 8385898+ fd Linux raid autodetect
/dev/sdc1 1 1044 8385898+ fd Linux raid autodetect
/dev/sdd1 1 1044 8385898+ fd Linux raid autodetect
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![introduction to raid
create the raid5
The next step used to be create the raid table in /etc/raidtab. Nowadays, you can just
issue the command mdadm with the correct parameters.
The command below is split on two lines to fit this print, but you should type it on
one line, without the backslash ().
[root@rhel6c ~]# mdadm --create /dev/md0 --chunk=64 --level=5 --raid-
devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
Below a partial screenshot how fdisk -l sees the raid 5.
[root@rhel6c ~]# fdisk -l /dev/md0
Disk /dev/md0: 17.2 GB, 17172135936 bytes
2 heads, 4 sectors/track, 4192416 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 65536 bytes / 131072 bytes
Disk identifier: 0x00000000
Disk /dev/md0 doesn't contain a valid partition table
We could use this software raid 5 array in the next topic: lvm.
/proc/mdstat
The status of the raid devices can be seen in /proc/mdstat. This example shows a
raid 5 in the process of rebuilding.
[root@rhel6c ~]# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sdd1[3] sdc1[1] sdb1[0]
16769664 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/2] [UU_]
[============>........] recovery = 62.8% (5266176/8384832) finish=0
.3min speed=139200K/sec
This example shows an active software raid 5.
[root@rhel6c ~]# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sdd1[3] sdc1[1] sdb1[0]
16769664 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/3] [UUU]
mdadm --detail
Use mdadm --detail to get information on a raid device.
[root@rhel6c ~]# mdadm --detail /dev/md0
/dev/md0:
Version : 1.2
Creation Time : Sun Jul 17 13:48:41 2011
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![introduction to raid
Raid Level : raid5
Array Size : 16769664 (15.99 GiB 17.17 GB)
Used Dev Size : 8384832 (8.00 GiB 8.59 GB)
Raid Devices : 3
Total Devices : 3
Persistence : Superblock is persistent
Update Time : Sun Jul 17 13:49:43 2011
State : clean
Active Devices : 3
Working Devices : 3
Failed Devices : 0
Spare Devices : 0
Layout : left-symmetric
Chunk Size : 64K
Name : rhel6c:0 (local to host rhel6c)
UUID : c10fd9c3:08f9a25f:be913027:999c8e1f
Events : 18
Number Major Minor RaidDevice State
0 8 17 0 active sync /dev/sdb1
1 8 33 1 active sync /dev/sdc1
3 8 49 2 active sync /dev/sdd1
removing a software raid
The software raid is visible in /proc/mdstat when active. To remove the raid
completely so you can use the disks for other purposes, you stop (de-activate) it with
mdadm.
[root@rhel6c ~]# mdadm --stop /dev/md0
mdadm: stopped /dev/md0
The disks can now be repartitioned.
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![introduction to raid
37.5. solution: raid
1. Add three virtual disks of 1GB each to a virtual machine.
2. Create a software raid 5 on the three disks. (It is not necessary to put a filesystem
on it)
3. Verify with fdisk and in /proc that the raid 5 exists.
4. (optional) Stop and remove the raid 5.
5. (optional) Create a raid 1 to mirror two disks.
[root@rhel6c ~]# mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 /dev/sdc1
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
[root@rhel6c ~]# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4] [raid1]
md0 : active raid1 sdc1[1] sdb1[0]
8384862 blocks super 1.2 [2/2] [UU]
[====>................] resync = 20.8% (1745152/8384862)
finish=0.5min speed=218144K/sec
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![logical volume management
38.4. example: extend a logical volume
A logical volume can be extended without unmounting the file system. Whether or
not a volume can be extended depends on the file system it uses. Volumes that are
mounted as vfat or ext2 cannot be extended, so in the example here we use the ext3
file system.
The fdisk command shows us newly added scsi-disks that will serve our lvm volume.
This volume will then be extended. First, take a look at these disks.
[root@RHEL5 ~]# fdisk -l | grep sd[bc]
Disk /dev/sdb doesn't contain a valid partition table
Disk /dev/sdc doesn't contain a valid partition table
Disk /dev/sdb: 1181 MB, 1181115904 bytes
Disk /dev/sdc: 429 MB, 429496320 bytes
You already know how to partition a disk, below the first disk is partitioned (in one
big primary partition), the second disk is left untouched.
[root@RHEL5 ~]# fdisk -l | grep sd[bc]
Disk /dev/sdc doesn't contain a valid partition table
Disk /dev/sdb: 1181 MB, 1181115904 bytes
/dev/sdb1 1 143 1148616 83 Linux
Disk /dev/sdc: 429 MB, 429496320 bytes
You also know how to prepare disks for lvm with pvcreate, and how to create a
volume group with vgcreate. This example adds both the partitioned disk and the
untouched disk to the volume group named vg2.
[root@RHEL5 ~]# pvcreate /dev/sdb1
Physical volume "/dev/sdb1" successfully created
[root@RHEL5 ~]# pvcreate /dev/sdc
Physical volume "/dev/sdc" successfully created
[root@RHEL5 ~]# vgcreate vg2 /dev/sdb1 /dev/sdc
Volume group "vg2" successfully created
You can use pvdisplay to verify that both the disk and the partition belong to the
volume group.
[root@RHEL5 ~]# pvdisplay | grep -B1 vg2
PV Name /dev/sdb1
VG Name vg2
--
PV Name /dev/sdc
VG Name vg2
And you are familiar both with the lvcreate command to create a small logical volume
and the mke2fs command to put ext2 on it.
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![logical volume management
[root@RHEL5 ~]# lvcreate --size 200m vg2
Logical volume "lvol0" created
[root@RHEL5 ~]# mke2fs -m20 -j /dev/vg2/lvol0
...
As you see, we end up with a mounted logical volume that according to df is almost
200 megabyte in size.
[root@RHEL5 ~]# mkdir /home/resizetest
[root@RHEL5 ~]# mount /dev/vg2/lvol0 /home/resizetest/
[root@RHEL5 ~]# df -h | grep resizetest
194M 5.6M 149M 4% /home/resizetest
Extending the volume is easy with lvextend.
[root@RHEL5 ~]# lvextend -L +100 /dev/vg2/lvol0
Extending logical volume lvol0 to 300.00 MB
Logical volume lvol0 successfully resized
But as you can see, there is a small problem: it appears that df is not able to display
the extended volume in its full size. This is because the filesystem is only set for the
size of the volume before the extension was added.
[root@RHEL5 ~]# df -h | grep resizetest
194M 5.6M 149M 4% /home/resizetest
With lvdisplay however we can see that the volume is indeed extended.
[root@RHEL5 ~]# lvdisplay /dev/vg2/lvol0 | grep Size
LV Size 300.00 MB
To finish the extension, you need resize2fs to span the filesystem over the full size
of the logical volume.
[root@RHEL5 ~]# resize2fs /dev/vg2/lvol0
resize2fs 1.39 (29-May-2006)
Filesystem at /dev/vg2/lvol0 is mounted on /home/resizetest; on-line re
sizing required
Performing an on-line resize of /dev/vg2/lvol0 to 307200 (1k) blocks.
The filesystem on /dev/vg2/lvol0 is now 307200 blocks long.
Congratulations, you just successfully expanded a logical volume.
[root@RHEL5 ~]# df -h | grep resizetest
291M 6.1M 225M 3% /home/resizetest
[root@RHEL5 ~]#
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![logical volume management
38.5. example: resize a physical Volume
This is a humble demonstration of how to resize a physical Volume with lvm (after
you resize it with fdisk). The demonstration starts with a 100MB partition named /
dev/sde1. We used fdisk to create it, and to verify the size.
[root@RHEL5 ~]# fdisk -l 2>/dev/null | grep sde1
/dev/sde1 1 100 102384 83 Linux
[root@RHEL5 ~]#
Now we can use pvcreate to create the Physical Volume, followed by pvs to verify
the creation.
[root@RHEL5 ~]# pvcreate /dev/sde1
Physical volume "/dev/sde1" successfully created
[root@RHEL5 ~]# pvs | grep sde1
/dev/sde1 lvm2 -- 99.98M 99.98M
[root@RHEL5 ~]#
The next step is ti use fdisk to enlarge the partition (actually deleting it and then
recreating /dev/sde1 with more cylinders).
[root@RHEL5 ~]# fdisk /dev/sde
Command (m for help): p
Disk /dev/sde: 858 MB, 858993152 bytes
64 heads, 32 sectors/track, 819 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Device Boot Start End Blocks Id System
/dev/sde1 1 100 102384 83 Linux
Command (m for help): d
Selected partition 1
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4):
Value out of range.
Partition number (1-4): 1
First cylinder (1-819, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-819, default 819): 200
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
[root@RHEL5 ~]#
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![logical volume management
When we now use fdisk and pvs to verify the size of the partition and the Physical
Volume, then there is a size difference. LVM is still using the old size.
[root@RHEL5 ~]# fdisk -l 2>/dev/null | grep sde1
/dev/sde1 1 200 204784 83 Linux
[root@RHEL5 ~]# pvs | grep sde1
/dev/sde1 lvm2 -- 99.98M 99.98M
[root@RHEL5 ~]#
Executing pvresize on the Physical Volume will make lvm aware of the size change
of the partition. The correct size can be displayed with pvs.
[root@RHEL5 ~]# pvresize /dev/sde1
Physical volume "/dev/sde1" changed
1 physical volume(s) resized / 0 physical volume(s) not resized
[root@RHEL5 ~]# pvs | grep sde1
/dev/sde1 lvm2 -- 199.98M 199.98M
[root@RHEL5 ~]#
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![logical volume management
38.6. example: mirror a logical volume
We start by creating three physical volumes for lvm. Then we verify the creation and
the size with pvs. Three physical disks because lvm uses two disks for the mirror and
a third disk for the mirror log!
[root@RHEL5 ~]# pvcreate /dev/sdb /dev/sdc /dev/sdd
Physical volume "/dev/sdb" successfully created
Physical volume "/dev/sdc" successfully created
Physical volume "/dev/sdd" successfully created
[root@RHEL5 ~]# pvs
PV VG Fmt Attr PSize PFree
/dev/sdb lvm2 -- 409.60M 409.60M
/dev/sdc lvm2 -- 409.60M 409.60M
/dev/sdd lvm2 -- 409.60M 409.60M
Then we create the Volume Group and verify again with pvs. Notice how the three
physical volumes now belong to vg33, and how the size is rounded down (in steps
of the extent size, here 4MB).
[root@RHEL5 ~]# vgcreate vg33 /dev/sdb /dev/sdc /dev/sdd
Volume group "vg33" successfully created
[root@RHEL5 ~]# pvs
PV VG Fmt Attr PSize PFree
/dev/sda2 VolGroup00 lvm2 a- 15.88G 0
/dev/sdb vg33 lvm2 a- 408.00M 408.00M
/dev/sdc vg33 lvm2 a- 408.00M 408.00M
/dev/sdd vg33 lvm2 a- 408.00M 408.00M
[root@RHEL5 ~]#
The last step is to create the Logical Volume with lvcreate. Notice the -m 1 switch to
create one mirror. Notice also the change in free space in all three Physical Volumes!
[root@RHEL5 ~]# lvcreate --size 300m -n lvmir -m 1 vg33
Logical volume "lvmir" created
[root@RHEL5 ~]# pvs
PV VG Fmt Attr PSize PFree
/dev/sda2 VolGroup00 lvm2 a- 15.88G 0
/dev/sdb vg33 lvm2 a- 408.00M 108.00M
/dev/sdc vg33 lvm2 a- 408.00M 108.00M
/dev/sdd vg33 lvm2 a- 408.00M 404.00M
You can see the copy status of the mirror with lvs. It currently shows a 100 percent
copy.
[root@RHEL5 ~]# lvs vg33/lvmir
LV VG Attr LSize Origin Snap% Move Log Copy%
lvmir vg33 mwi-ao 300.00M lvmir_mlog 100.00
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![logical volume management
38.7. example: snapshot a logical volume
A snapshot is a virtual copy of all the data at a point in time on a volume. A snapshot
Logical Volume will retain a copy of all changed files of the snapshotted Logical
Volume.
The example below creates a snapshot of the bigLV Logical Volume.
[root@RHEL5 ~]# lvcreate -L100M -s -n snapLV vg42/bigLV
Logical volume "snapLV" created
[root@RHEL5 ~]#
You can see with lvs that the snapshot snapLV is indeed a snapshot of bigLV.
Moments after taking the snapshot, there are few changes to bigLV (0.02 percent).
[root@RHEL5 ~]# lvs
LV VG Attr LSize Origin Snap% Move Log Copy%
bigLV vg42 owi-a- 200.00M
snapLV vg42 swi-a- 100.00M bigLV 0.02
[root@RHEL5 ~]#
But after using bigLV for a while, more changes are done. This means the snapshot
volume has to keep more original data (10.22 percent).
[root@RHEL5 ~]# lvs | grep vg42
bigLV vg42 owi-ao 200.00M
snapLV vg42 swi-a- 100.00M bigLV 10.22
[root@RHEL5 ~]#
You can now use regular backup tools (dump, tar, cpio, ...) to take a backup of the
snapshot Logical Volume. This backup will contain all data as it existed on bigLV
at the time the snapshot was taken. When the backup is done, you can remove the
snapshot.
[root@RHEL5 ~]# lvremove vg42/snapLV
Do you really want to remove active logical volume "snapLV"? [y/n]: y
Logical volume "snapLV" successfully removed
[root@RHEL5 ~]#
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![logical volume management
38.8. verifying existing physical volumes
lvmdiskscan
To get a list of block devices that can be used with LVM, use lvmdiskscan. The
example below uses grep to limit the result to SCSI devices.
[root@RHEL5 ~]# lvmdiskscan | grep sd
/dev/sda1 [ 101.94 MB]
/dev/sda2 [ 15.90 GB] LVM physical volume
/dev/sdb [ 409.60 MB]
/dev/sdc [ 409.60 MB]
/dev/sdd [ 409.60 MB] LVM physical volume
/dev/sde1 [ 95.98 MB]
/dev/sde5 [ 191.98 MB]
/dev/sdf [ 819.20 MB] LVM physical volume
/dev/sdg1 [ 818.98 MB]
[root@RHEL5 ~]#
pvs
The easiest way to verify whether devices are known to lvm is with the pvs command.
The screenshot below shows that only /dev/sda2 is currently known for use with
LVM. It shows that /dev/sda2 is part of Volgroup00 and is almost 16GB in size. It
also shows /dev/sdc and /dev/sdd as part of vg33. The device /dev/sdb is knwon to
lvm, but not linked to any Volume Group.
[root@RHEL5 ~]# pvs
PV VG Fmt Attr PSize PFree
/dev/sda2 VolGroup00 lvm2 a- 15.88G 0
/dev/sdb lvm2 -- 409.60M 409.60M
/dev/sdc vg33 lvm2 a- 408.00M 408.00M
/dev/sdd vg33 lvm2 a- 408.00M 408.00M
[root@RHEL5 ~]#
pvscan
The pvscan command will scan all disks for existing Physical Volumes. The
information is similar to pvs, plus you get a line with total sizes.
[root@RHEL5 ~]# pvscan
PV /dev/sdc VG vg33 lvm2 [408.00 MB / 408.00 MB free]
PV /dev/sdd VG vg33 lvm2 [408.00 MB / 408.00 MB free]
PV /dev/sda2 VG VolGroup00 lvm2 [15.88 GB / 0 free]
PV /dev/sdb lvm2 [409.60 MB]
Total: 4 [17.07 GB] / in use: 3 [16.67 GB] / in no VG: 1 [409.60 MB]
[root@RHEL5 ~]#
323](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-329-320.jpg)
![logical volume management
pvdisplay
Use pvdisplay to get more information about physical volumes. You can also use
pvdisplay without an argument to display information about all physical (lvm)
volumes.
[root@RHEL5 ~]# pvdisplay /dev/sda2
--- Physical volume ---
PV Name /dev/sda2
VG Name VolGroup00
PV Size 15.90 GB / not usable 20.79 MB
Allocatable yes (but full)
PE Size (KByte) 32768
Total PE 508
Free PE 0
Allocated PE 508
PV UUID TobYfp-Ggg0-Rf8r-xtLd-5XgN-RSPc-8vkTHD
[root@RHEL5 ~]#
324](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-330-320.jpg)
![logical volume management
38.9. verifying existing volume groups
vgs
Similar to pvs is the use of vgs to display a quick overview of all volume groups.
There is only one volume group in the screenshot below, it is named VolGroup00
and is almost 16GB in size.
[root@RHEL5 ~]# vgs
VG #PV #LV #SN Attr VSize VFree
VolGroup00 1 2 0 wz--n- 15.88G 0
[root@RHEL5 ~]#
vgscan
The vgscan command will scan all disks for existing Volume Groups. It will also
update the /etc/lvm/.cache file. This file contains a list of all current lvm devices.
[root@RHEL5 ~]# vgscan
Reading all physical volumes. This may take a while...
Found volume group "VolGroup00" using metadata type lvm2
[root@RHEL5 ~]#
LVM will run the vgscan automatically at boot-up, so if you add hot swap devices,
then you will need to run vgscan to update /etc/lvm/.cache with the new devices.
vgdisplay
The vgdisplay command will give you more detailed information about a volume
group (or about all volume groups if you omit the argument).
[root@RHEL5 ~]# vgdisplay VolGroup00
--- Volume group ---
VG Name VolGroup00
System ID
Format lvm2
Metadata Areas 1
Metadata Sequence No 3
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 2
Open LV 2
Max PV 0
Cur PV 1
Act PV 1
VG Size 15.88 GB
PE Size 32.00 MB
Total PE 508
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![logical volume management
Alloc PE / Size 508 / 15.88 GB
Free PE / Size 0 / 0
VG UUID qsXvJb-71qV-9l7U-ishX-FobM-qptE-VXmKIg
[root@RHEL5 ~]#
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![logical volume management
38.10. verifying existing logical volumes
lvs
Use lvs for a quick look at all existing logical volumes. Below you can see two logical
volumes named LogVol00 and LogVol01.
[root@RHEL5 ~]# lvs
LV VG Attr LSize Origin Snap% Move Log Copy%
LogVol00 VolGroup00 -wi-ao 14.88G
LogVol01 VolGroup00 -wi-ao 1.00G
[root@RHEL5 ~]#
lvscan
The lvscan command will scan all disks for existing Logical Volumes.
[root@RHEL5 ~]# lvscan
ACTIVE '/dev/VolGroup00/LogVol00' [14.88 GB] inherit
ACTIVE '/dev/VolGroup00/LogVol01' [1.00 GB] inherit
[root@RHEL5 ~]#
lvdisplay
More detailed information about logical volumes is available through the
lvdisplay(1) command.
[root@RHEL5 ~]# lvdisplay VolGroup00/LogVol01
--- Logical volume ---
LV Name /dev/VolGroup00/LogVol01
VG Name VolGroup00
LV UUID RnTGK6-xWsi-t530-ksJx-7cax-co5c-A1KlDp
LV Write Access read/write
LV Status available
# open 1
LV Size 1.00 GB
Current LE 32
Segments 1
Allocation inherit
Read ahead sectors 0
Block device 253:1
[root@RHEL5 ~]#
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![logical volume management
38.11. manage physical volumes
pvcreate
Use the pvcreate command to add devices to lvm. This example shows how to add
a disk (or hardware RAID device) to lvm.
[root@RHEL5 ~]# pvcreate /dev/sdb
Physical volume "/dev/sdb" successfully created
[root@RHEL5 ~]#
This example shows how to add a partition to lvm.
[root@RHEL5 ~]# pvcreate /dev/sdc1
Physical volume "/dev/sdc1" successfully created
[root@RHEL5 ~]#
You can also add multiple disks or partitions as target to pvcreate. This example adds
three disks to lvm.
[root@RHEL5 ~]# pvcreate /dev/sde /dev/sdf /dev/sdg
Physical volume "/dev/sde" successfully created
Physical volume "/dev/sdf" successfully created
Physical volume "/dev/sdg" successfully created
[root@RHEL5 ~]#
pvremove
Use the pvremove command to remove physical volumes from lvm. The devices
may not be in use.
[root@RHEL5 ~]# pvremove /dev/sde /dev/sdf /dev/sdg
Labels on physical volume "/dev/sde" successfully wiped
Labels on physical volume "/dev/sdf" successfully wiped
Labels on physical volume "/dev/sdg" successfully wiped
[root@RHEL5 ~]#
pvresize
When you used fdisk to resize a partition on a disk, then you must use pvresize to
make lvm recognize the new size of the physical volume that represents this partition.
[root@RHEL5 ~]# pvresize /dev/sde1
Physical volume "/dev/sde1" changed
1 physical volume(s) resized / 0 physical volume(s) not resized
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![logical volume management
pvchange
With pvchange you can prevent the allocation of a Physical Volume in a new Volume
Group or Logical Volume. This can be useful if you plan to remove a Physical
Volume.
[root@RHEL5 ~]# pvchange -xn /dev/sdd
Physical volume "/dev/sdd" changed
1 physical volume changed / 0 physical volumes not changed
[root@RHEL5 ~]#
To revert your previous decision, this example shows you how te re-enable the
Physical Volume to allow allocation.
[root@RHEL5 ~]# pvchange -xy /dev/sdd
Physical volume "/dev/sdd" changed
1 physical volume changed / 0 physical volumes not changed
[root@RHEL5 ~]#
pvmove
With pvmove you can move Logical Volumes from within a Volume Group to
another Physical Volume. This must be done before removing a Physical Volume.
[root@RHEL5 ~]# pvs | grep vg1
/dev/sdf vg1 lvm2 a- 816.00M 0
/dev/sdg vg1 lvm2 a- 816.00M 816.00M
[root@RHEL5 ~]# pvmove /dev/sdf
/dev/sdf: Moved: 70.1%
/dev/sdf: Moved: 100.0%
[root@RHEL5 ~]# pvs | grep vg1
/dev/sdf vg1 lvm2 a- 816.00M 816.00M
/dev/sdg vg1 lvm2 a- 816.00M 0
329](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-335-320.jpg)
![logical volume management
38.12. manage volume groups
vgcreate
Use the vgcreate command to create a volume group. You can immediately name all
the physical volumes that span the volume group.
[root@RHEL5 ~]# vgcreate vg42 /dev/sde /dev/sdf
Volume group "vg42" successfully created
[root@RHEL5 ~]#
vgextend
Use the vgextend command to extend an existing volume group with a physical
volume.
[root@RHEL5 ~]# vgextend vg42 /dev/sdg
Volume group "vg42" successfully extended
[root@RHEL5 ~]#
vgremove
Use the vgremove command to remove volume groups from lvm. The volume groups
may not be in use.
[root@RHEL5 ~]# vgremove vg42
Volume group "vg42" successfully removed
[root@RHEL5 ~]#
vgreduce
Use the vgreduce command to remove a Physical Volume from the Volume Group.
The following example adds Physical Volume /dev/sdg to the vg1 Volume Group
using vgextend. And then removes it again using vgreduce.
[root@RHEL5 ~]# pvs | grep sdg
/dev/sdg lvm2 -- 819.20M 819.20M
[root@RHEL5 ~]# vgextend vg1 /dev/sdg
Volume group "vg1" successfully extended
[root@RHEL5 ~]# pvs | grep sdg
/dev/sdg vg1 lvm2 a- 816.00M 816.00M
[root@RHEL5 ~]# vgreduce vg1 /dev/sdg
Removed "/dev/sdg" from volume group "vg1"
[root@RHEL5 ~]# pvs | grep sdg
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![logical volume management
/dev/sdg lvm2 -- 819.20M 819.20M
vgchange
Use the vgchange command to change parameters of a Volume Group.
This example shows how to prevent Physical Volumes from being added or removed
to the Volume Group vg1.
[root@RHEL5 ~]# vgchange -xn vg1
Volume group "vg1" successfully changed
[root@RHEL5 ~]# vgextend vg1 /dev/sdg
Volume group vg1 is not resizable.
You can also use vgchange to change most other properties of a Volume Group. This
example changes the maximum number of Logical Volumes and maximum number
of Physical Volumes that vg1 can serve.
[root@RHEL5 ~]# vgdisplay vg1 | grep -i max
MAX LV 0
Max PV 0
[root@RHEL5 ~]# vgchange -l16 vg1
Volume group "vg1" successfully changed
[root@RHEL5 ~]# vgchange -p8 vg1
Volume group "vg1" successfully changed
[root@RHEL5 ~]# vgdisplay vg1 | grep -i max
MAX LV 16
Max PV 8
vgmerge
Merging two Volume Groups into one is done with vgmerge. The following example
merges vg2 into vg1, keeping all the properties of vg1.
[root@RHEL5 ~]# vgmerge vg1 vg2
Volume group "vg2" successfully merged into "vg1"
[root@RHEL5 ~]#
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![logical volume management
38.13. manage logical volumes
lvcreate
Use the lvcreate command to create Logical Volumes in a Volume Group. This
example creates an 8GB Logical Volume in Volume Group vg42.
[root@RHEL5 ~]# lvcreate -L5G vg42
Logical volume "lvol0" created
[root@RHEL5 ~]#
As you can see, lvm automatically names the Logical Volume lvol0. The next
example creates a 200MB Logical Volume named MyLV in Volume Group vg42.
[root@RHEL5 ~]# lvcreate -L200M -nMyLV vg42
Logical volume "MyLV" created
[root@RHEL5 ~]#
The next example does the same thing, but with different syntax.
[root@RHEL5 ~]# lvcreate --size 200M -n MyLV vg42
Logical volume "MyLV" created
[root@RHEL5 ~]#
This example creates a Logical Volume that occupies 10 percent of the Volume
Group.
[root@RHEL5 ~]# lvcreate -l 10%VG -n MyLV2 vg42
Logical volume "MyLV2" created
[root@RHEL5 ~]#
This example creates a Logical Volume that occupies 30 percent of the remaining
free space in the Volume Group.
[root@RHEL5 ~]# lvcreate -l 30%FREE -n MyLV3 vg42
Logical volume "MyLV3" created
[root@RHEL5 ~]#
lvremove
Use the lvremove command to remove Logical Volumes from a Volume Group.
Removing a Logical Volume requires the name of the Volume Group.
[root@RHEL5 ~]# lvremove vg42/MyLV
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Do you really want to remove active logical volume "MyLV"? [y/n]: y
Logical volume "MyLV" successfully removed
[root@RHEL5 ~]#
Removing multiple Logical Volumes will request confirmation for each individual
volume.
[root@RHEL5 ~]# lvremove vg42/MyLV vg42/MyLV2 vg42/MyLV3
Do you really want to remove active logical volume "MyLV"? [y/n]: y
Logical volume "MyLV" successfully removed
Do you really want to remove active logical volume "MyLV2"? [y/n]: y
Logical volume "MyLV2" successfully removed
Do you really want to remove active logical volume "MyLV3"? [y/n]: y
Logical volume "MyLV3" successfully removed
[root@RHEL5 ~]#
lvextend
Extending the volume is easy with lvextend. This example extends a 200MB Logical
Volume with 100 MB.
[root@RHEL5 ~]# lvdisplay /dev/vg2/lvol0 | grep Size
LV Size 200.00 MB
[root@RHEL5 ~]# lvextend -L +100 /dev/vg2/lvol0
Extending logical volume lvol0 to 300.00 MB
Logical volume lvol0 successfully resized
[root@RHEL5 ~]# lvdisplay /dev/vg2/lvol0 | grep Size
LV Size 300.00 MB
The next example creates a 100MB Logical Volume, and then extends it to 500MB.
[root@RHEL5 ~]# lvcreate --size 100M -n extLV vg42
Logical volume "extLV" created
[root@RHEL5 ~]# lvextend -L 500M vg42/extLV
Extending logical volume extLV to 500.00 MB
Logical volume extLV successfully resized
[root@RHEL5 ~]#
This example doubles the size of a Logical Volume.
[root@RHEL5 ~]# lvextend -l+100%LV vg42/extLV
Extending logical volume extLV to 1000.00 MB
Logical volume extLV successfully resized
[root@RHEL5 ~]#
lvrename
Renaming a Logical Volume is done with lvrename. This example renames extLV
to bigLV in the vg42 Volume Group.
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![logical volume management
[root@RHEL5 ~]# lvrename vg42/extLV vg42/bigLV
Renamed "extLV" to "bigLV" in volume group "vg42"
[root@RHEL5 ~]#
334](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-340-320.jpg)
![bootloader
root@barry:~# ls -l /boot/grub/menu.lst
-rw-r--r-- 1 root root 5155 2009-03-31 18:20 /boot/grub/menu.lst
/boot/grub/grub.conf
Some distributions like Red Hat Enterprise Linux 5 use grub.conf and provide
a symbolic link to menu.lst. This is the same file, only the name changed from
grub.conf to menu.lst. Notice also in this screenshot that this file is a lot smaller
on Red Hat.
[root@RHEL52 grub]# ls -l grub.conf menu.lst
-rw------- 1 root root 1346 Jan 21 04:20 grub.conf
lrwxrwxrwx 1 root root 11 Oct 11 2008 menu.lst -> ./grub.conf
menu commands
The menu commands always have to be at the top of grub's configuration file.
default
The default command sets a default entry to start. The first entry has number 0.
default 0
fallback
In case the default does not boot, use the fallback entry instead.
fallback 1
timeout
The timeout will wait a number of seconds before booting the default entry.
timeout 5
hiddenmenu
The hiddenmenu will hide the grub menu unless the user presses Esc before the
timeout expires.
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![bootloader
39.5. solution : bootloader
1. Make a copy of the kernel, initrd and System.map files in /boot. Put the copies also
in /boot but replace 2.6.x with 3.0 (just imagine that Linux 3.0 is out.).
cd /boot
cp vmlinuz-2.6.18-8.e15 vmlinuz-3.0
cp initrd-2.6.18-8.e15.img initrd-3.0.img
cp System.map-2.6.18-8.e15 System.map-3.0
Do not forget the initrd file ends in .img .
2. Add a stanza in grub for the 3.0 files. Make sure the title is different.
[root@RHEL5 ~]# grep 3.0 /boot/grub/menu.lst
title Red Hat Enterprise Linux Server (3.0)
kernel /vmlinuz-3.0 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
initrd /initrd-3.0.img
3. Set the boot menu timeout to 30 seconds.
[root@RHEL5 ~]# grep time /boot/grub/menu.lst
timeout=30
4. Reboot and test the new stanza.
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![init and runlevels
40.1. about sysv init
Many Linux distributions use init scripts to start daemons in the same way that Unix
System V did. This chapter will explain in detail how that works.
Init starts daemons by using scripts, where each script starts one daemon, and where
each script waits for the previous script to finish. This serial process of starting
daemons is slow, and although slow booting is not a problem on servers where
uptime is measured in years, the recent uptake of Linux on the desktop results in user
complaints.
To improve Linux startup speed, Canonical has developed upstart, which was
first used in Ubuntu. Solaris also used init up to Solaris 9, for Solaris 10 Sun has
developed Service Management Facility. Both systems start daemons in parallel
and can replace the SysV init scripts. There is also an ongoing effort to create initng
(init next generation).
40.2. system init(ialization)
process id 1
The kernel receives system control from the bootloader. After a while the kernel starts
the init daemon. The init daemon (/sbin/init) is the first daemon that is started and
receives process id 1 (PID 1). Init never dies.
configuration in /etc/inittab
When /sbin/init is started, it will first read its configuration file /etc/inittab. In that
file, it will look for the value of initdefault (3 in the screenshot below).
[paul@rhel4 ~]$ grep ^id /etc/inittab
id:3:initdefault:
initdefault
The value found in initdefault indicates the default runlevel. Some Linux
distributions have a brief description of runlevels in /etc/inittab, like here on Red Hat
Enterprise Linux 4.
# Default runlevel. The runlevels used by RHS are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS (The same as 3, if you don't have network)
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![init and runlevels
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
Runlevel 0 means the system is shutting down. Runlevel 1 is used for
troubleshooting, only the root user can log on, and only at the console. Runlevel 3
is typical for servers, whereas runlevel 5 is typical for desktops (graphical logon).
Besides runlevels 0, 1 and 6, the use may vary depending on the distribution. Debian
and derived Linux systems have full network and GUI logon on runlevels 2 to 5. So
always verify the proper meaning of runlevels on your system.
sysinit script
/etc/rc.d/rc.sysinit
The next line in /etc/inittab in Red Hat and derivatives is the following.
si::sysinit:/etc/rc.d/rc.sysinit
This means that independent of the selected runlevel, init will run the /etc/rc.d/
rc.sysinit script. This script initializes hardware, sets some basic environment,
populates /etc/mtab while mounting file systems, starts swap and more.
[paul@rhel ~]$ egrep -e"^# Ini" -e"^# Sta" -e"^# Che" /etc/rc.d/rc.sysinit
# Check SELinux status
# Initialize hardware
# Start the graphical boot, if necessary; /usr may not be mounted yet...
# Initialiaze ACPI bits
# Check filesystems
# Start the graphical boot, if necessary and not done yet.
# Check to see if SELinux requires a relabel
# Initialize pseudo-random number generator
# Start up swapping.
# Initialize the serial ports.
That egrep command could also have been written with grep like this :
grep "^# (Ini|Sta|Che)".
/etc/init.d/rcS
Debian has the following line after initdefault.
si::sysinit:/etc/init.d/rcS
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![init and runlevels
The /etc/init.d/rcS script will always run on Debian (independent of the selected
runlevel). The script is actually running all scripts in the /etc/rcS.d/ directory in
alphabetical order.
root@barry:~# cat /etc/init.d/rcS
#! /bin/sh
#
# rcS
#
# Call all S??* scripts in /etc/rcS.d/ in numerical/alphabetical order
#
exec /etc/init.d/rc S
rc scripts
Init will continue to read /etc/inittab and meets this section on Debian Linux.
l0:0:wait:/etc/init.d/rc 0
l1:1:wait:/etc/init.d/rc 1
l2:2:wait:/etc/init.d/rc 2
l3:3:wait:/etc/init.d/rc 3
l4:4:wait:/etc/init.d/rc 4
l5:5:wait:/etc/init.d/rc 5
l6:6:wait:/etc/init.d/rc 6
On Red Hat Enterprise Linux it is identical except init.d is rc.d.
l0:0:wait:/etc/rc.d/rc 0
l1:1:wait:/etc/rc.d/rc 1
l2:2:wait:/etc/rc.d/rc 2
l3:3:wait:/etc/rc.d/rc 3
l4:4:wait:/etc/rc.d/rc 4
l5:5:wait:/etc/rc.d/rc 5
l6:6:wait:/etc/rc.d/rc 6
In both cases, this means that init will start the rc script with the runlevel as the
only parameter. Actually /etc/inittab has fields seperated by colons. The second field
determines the runlevel in which this line should be executed. So in both cases, only
one line of the seven will be executed, depending on the runlevel set by initdefault.
rc directories
When you take a look any of the /etc/rcX.d/ directories, then you will see a lot of
(links to) scripts who's name start with either uppercase K or uppercase S.
[root@RHEL52 rc3.d]# ls -l | tail -4
lrwxrwxrwx 1 root root 19 Oct 11 2008 S98haldaemon -> ../init.d/haldaemon
lrwxrwxrwx 1 root root 19 Oct 11 2008 S99firstboot -> ../init.d/firstboot
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![init and runlevels
lrwxrwxrwx 1 root root 11 Jan 21 04:16 S99local -> ../rc.local
lrwxrwxrwx 1 root root 16 Jan 21 04:17 S99smartd -> ../init.d/smartd
The /etc/rcX.d/ directories only contain links to scripts in /etc/init.d/. Links allow
for the script to have a different name. When entering a runlevel, all scripts that start
with uppercase K or uppercase S will be started in alphabetical order. Those that start
with K will be started first, with stop as the only parameter. The remaining scripts
with S will be started with start as the only parameter.
All this is done by the /etc/rc.d/rc script on Red Hat and by the /etc/init.d/rc script
on Debian.
mingetty
mingetty in /etc/inittab
Almost at the end of /etc/inittab there is a section to start and respawn several
mingetty daemons.
[root@RHEL4b ~]# grep getty /etc/inittab
# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2
3:2345:respawn:/sbin/mingetty tty3
4:2345:respawn:/sbin/mingetty tty4
5:2345:respawn:/sbin/mingetty tty5
6:2345:respawn:/sbin/mingetty tty6
mingetty and /bin/login
This /sbin/mingetty will display a message on a virtual console and allow you to
type a userid. Then it executes the /bin/login command with that userid. The /bin/
login program will verify whether that user exists in /etc/passwd and prompt for (and
verify) a password. If the password is correct, /bin/login passes control to the shell
listed in /etc/passwd.
respawning mingetty
The mingetty daemons are started by init and watched until they die (user exits the
shell and is logged out). When this happens, the init daemon will respawn a new
mingetty. So even if you kill a mingetty daemon, it will be restarted automatically.
This example shows that init respawns mingetty daemons. Look at the PID's of the
last two mingetty processes.
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![init and runlevels
[root@RHEL52 ~]# ps -C mingetty
PID TTY TIME CMD
2407 tty1 00:00:00 mingetty
2408 tty2 00:00:00 mingetty
2409 tty3 00:00:00 mingetty
2410 tty4 00:00:00 mingetty
2411 tty5 00:00:00 mingetty
2412 tty6 00:00:00 mingetty
When we kill the last two mingettys, then init will notice this and start them again
(with a different PID).
[root@RHEL52 ~]# kill 2411 2412
[root@RHEL52 ~]# ps -C mingetty
PID TTY TIME CMD
2407 tty1 00:00:00 mingetty
2408 tty2 00:00:00 mingetty
2409 tty3 00:00:00 mingetty
2410 tty4 00:00:00 mingetty
2821 tty5 00:00:00 mingetty
2824 tty6 00:00:00 mingetty
disabling a mingetty
You can disable a mingetty for a certain tty by removing the runlevel from the
second field in its line in /etc/inittab. Don't forget to tell init about the change of its
configuration file with kill -1 1.
The example below shows how to disable mingetty on tty3 to tty6 in runlevels 4 and 5.
[root@RHEL52 ~]# grep getty /etc/inittab
# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2
3:23:respawn:/sbin/mingetty tty3
4:23:respawn:/sbin/mingetty tty4
5:23:respawn:/sbin/mingetty tty5
6:23:respawn:/sbin/mingetty tty6
40.3. daemon or demon ?
A daemon is a process that runs in background, without a link to a GUI or terminal.
Daemons are usually started at system boot, and stay alive until the system shuts
down. In more recent technical writings, daemons are often refered to as services.
Unix daemons are not to be confused with demons. Evi Nemeth, co-author of the
UNIX System Administration Handbook has the following to say about daemons:
Many people equate the word "daemon" with the word "demon", implying some
kind of satanic connection between UNIX and the underworld. This is an egregious
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![init and runlevels
misunderstanding. "Daemon" is actually a much older form of "demon"; daemons
have no particular bias towards good or evil, but rather serve to help define a person's
character or personality. The ancient Greeks' concept of a "personal daemon" was
similar to the modern concept of a "guardian angel" ....
40.4. starting and stopping daemons
The K and S scripts are links to the real scripts in /etc/init.d/. These can also be used
when the system is running to start and stop daemons (or services). Most of them
accept the following parameters: start, stop, restart, status.
For example in this screenshot we restart the samba daemon.
root@laika:~# /etc/init.d/samba restart
* Stopping Samba daemons... [ OK ]
* Starting Samba daemons... [ OK ]
You can achieve the same result on RHEL/Fedora with the service command.
[root@RHEL4b ~]# service smb restart
Shutting down SMB services: [ OK ]
Shutting down NMB services: [ OK ]
Starting SMB services: [ OK ]
Starting NMB services: [ OK ]
You might also want to take a look at chkconfig, update-rc.d.
40.5. chkconfig
The purpose of chkconfig is to relieve system administrators of manually managing
all the links and scripts in /etc/init.d and /etc/rcX.d/.
chkconfig --list
Here we use chkconfig to list the status of a service in the different runlevels. You
can see that the crond daemon (or service) is only activated in runlevels 2 to 5.
[root@RHEL52 ~]# chkconfig --list crond
crond 0:off 1:off 2:on 3:on 4:on 5:on 6:off
When you compare the screenshot above with the one below, you can see that off
equals to a K link to the script, whereas on equals to an S link.
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![init and runlevels
[root@RHEL52 etc]# find ./rc?.d/ -name *crond -exec ls -l {} ;|cut -b40-
./rc0.d/K60crond -> ../init.d/crond
./rc1.d/K60crond -> ../init.d/crond
./rc2.d/S90crond -> ../init.d/crond
./rc3.d/S90crond -> ../init.d/crond
./rc4.d/S90crond -> ../init.d/crond
./rc5.d/S90crond -> ../init.d/crond
./rc6.d/K60crond -> ../init.d/crond
runlevel configuration
Here you see how to use chkconfig to disable (or enable) a service in a certain
runlevel.
This screenshot shows how to disable crond in runlevel 3.
[root@RHEL52 ~]# chkconfig --level 3 crond off
[root@RHEL52 ~]# chkconfig --list crond
crond 0:off 1:off 2:on 3:off 4:on 5:on 6:off
This screenshot shows how to enable crond in runlevels 3 and 4.
[root@RHEL52 ~]# chkconfig --level 34 crond on
[root@RHEL52 ~]# chkconfig --list crond
crond 0:off 1:off 2:on 3:on 4:on 5:on 6:off
chkconfig configuration
Every script in /etc/init.d/ can have (comment) lines to tell chkconfig what to do with
the service. The line with # chkconfig: contains the runlevels in which the service
should be started (2345), followed by the priority for start (90) and stop (60).
[root@RHEL52 ~]# head -9 /etc/init.d/crond | tail -5
# chkconfig: 2345 90 60
# description: cron is a standard UNIX program that runs user-specified
# programs at periodic scheduled times. vixie cron adds a
# number of features to the basic UNIX cron, including better
# security and more powerful configuration options.
enable and disable services
Services can be enabled or disabled in all runlevels with one command. Runlevels
0, 1 and 6 are always stopping services (or calling the scripts with stop) even when
their name starts with uppercase S.
[root@RHEL52 ~]# chkconfig crond off
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![init and runlevels
[root@RHEL52 ~]# chkconfig --list crond
crond 0:off 1:off 2:off 3:off 4:off 5:off 6:off
[root@RHEL52 ~]# chkconfig crond on
[root@RHEL52 ~]# chkconfig --list crond
crond 0:off 1:off 2:on 3:on 4:on 5:on 6:off
40.6. update-rc.d
about update-rc.d
The Debian equivalent of chkconfig is called update-rc.d. This tool is designed for
use in scripts, if you prefer a graphical tool then look at bum.
When there are existing links in /etc/rcX.d/ then update-rc.d does not do anything.
This is to avoid that post installation scripts using update-rc.d are overwriting
changes made by a system administrator.
root@barry:~# update-rc.d cron remove
update-rc.d: /etc/init.d/cron exists during rc.d purge (use -f to force)
As you can see in the next screenshot, nothing changed for the cron daemon.
root@barry:~# find /etc/rc?.d/ -name '*cron' -exec ls -l {} ;|cut -b44-
/etc/rc0.d/K11cron -> ../init.d/cron
/etc/rc1.d/K11cron -> ../init.d/cron
/etc/rc2.d/S89cron -> ../init.d/cron
/etc/rc3.d/S89cron -> ../init.d/cron
/etc/rc4.d/S89cron -> ../init.d/cron
/etc/rc5.d/S89cron -> ../init.d/cron
/etc/rc6.d/K11cron -> ../init.d/cron
removing a service
Here we remove cron from all runlevels. Remember that the proper way to disable
a service is to put K scripts oin all runlevels!
root@barry:~# update-rc.d -f cron remove
Removing any system startup links for /etc/init.d/cron ...
/etc/rc0.d/K11cron
/etc/rc1.d/K11cron
/etc/rc2.d/S89cron
/etc/rc3.d/S89cron
/etc/rc4.d/S89cron
/etc/rc5.d/S89cron
/etc/rc6.d/K11cron
root@barry:~# find /etc/rc?.d/ -name '*cron' -exec ls -l {} ;|cut -b44-
root@barry:~#
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![init and runlevels
40.8. runlevels
display the runlevel
You can see your current runlevel with the runlevel or who -r commands.
The runlevel command is typical Linux and will output the previous and the current
runlevel. If there was no previous runlevel, then it will mark it with the letter N.
[root@RHEL4b ~]# runlevel
N 3
The history of who -r dates back to Seventies Unix, it still works on Linux.
[root@RHEL4b ~]# who -r
run-level 3 Jul 28 09:15 last=S
changing the runlevel
You can switch to another runlevel with the telinit command. On Linux /sbin/telinit
is usually a (hard) link to /sbin/init.
This screenshot shows how to switch from runlevel 2 to runlevel 3 without reboot.
root@barry:~# runlevel
N 2
root@barry:~# init 3
root@barry:~# runlevel
2 3
/sbin/shutdown
The shutdown command is used to properly shut down a system.
Common switches used with shutdown are -a, -t, -h and -r.
The -a switch forces /sbin/shutdown to use /etc/shutdown.allow. The -t switch is
used to define the number of seconds between the sending of the TERM signal and
the KILL signal. The -h switch halts the system instead of changing to runlevel 1.
The -r switch tells /sbin/shutdown to reboot after shutting down.
This screenshot shows how to use shutdown with five seconds between TERM and
KILL signals.
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root@barry:~# shutdown -t5 -h now
The now is the time argument. This can be +m for the number of minutes to wait
before shutting down (with now as an alias for +0. The command will also accept
hh:mm instead of +m.
halt, reboot and poweroff
The binary /sbin/reboot is the same as /sbin/halt and /sbin/poweroff. Depending on
the name we use to call the command, it can behave differently.
When in runlevel 0 or 6 halt, reboot and poweroff will tell the kernel to halt, reboot
or poweroff the system.
When not in runlevel 0 or 6, typing reboot as root actually calls the shutdown
command with the -r switch and typing poweroff will switch off the power when
halting the system.
/var/log/wtmp
halt, reboot and poweroff all write to /var/log/wtmp. To look at /var/log/wtmp, we
need to use th last.
[root@RHEL52 ~]# last | grep reboot
reboot system boot 2.6.18-128.el5 Fri May 29 11:44 (192+05:01)
reboot system boot 2.6.18-128.el5 Wed May 27 12:10 (06:49)
reboot system boot 2.6.18-128.el5 Mon May 25 19:34 (1+15:59)
reboot system boot 2.6.18-128.el5 Mon Feb 9 13:20 (106+21:13)
Ctrl-Alt-Del
When rc is finished starting all those scripts, init will continue to read /etc/inittab.
The next line is about what to do when the user hits Ctrl-Alt-Delete on the keyboard.
Here is what Debian 4.0 does.
root@barry:~# grep -i ctrl /etc/inittab
# What to do when CTRL-ALT-DEL is pressed.
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
Which is very similar to the default Red Hat Enterprise Linux 5.2 action.
[root@RHEL52 ~]# grep -i ctrl /etc/inittab
# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now
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![init and runlevels
One noticable difference is that Debian forces shutdown to use /etc/shutdown.allow,
where Red Hat allows everyone to invoke shutdown pressing Ctrl-Alt-Delete.
UPS and loss of power
[root@RHEL52 ~]# grep ^p /etc/inittab
pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure; System Shutting Down"
pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Cancelled"
It will read commands on what to execute in case of powerfailure, powerok and
Ctrl-Alt-Delete. The init process never stops keeping an eye on power failures and
that triple key combo.
root@barry:~# grep ^p /etc/inittab
pf::powerwait:/etc/init.d/powerfail start
pn::powerfailnow:/etc/init.d/powerfail now
po::powerokwait:/etc/init.d/powerfail stop
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40.10. solution : init
1. Change /etc/inittab so that only two mingetty's are respawned. Kill the other
mingetty's and verify that they don't come back.
Killing the mingetty's will result in init respawning them. You can edit /etc/inittab
so it looks like the screenshot below. Don't forget to also run kill -1 1.
[root@RHEL5 ~]# grep tty /etc/inittab
# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2
3:2:respawn:/sbin/mingetty tty3
4:2:respawn:/sbin/mingetty tty4
5:2:respawn:/sbin/mingetty tty5
6:2:respawn:/sbin/mingetty tty6
[root@RHEL5 ~]#
2. Use the Red Hat Enterprise Linux virtual machine. Go to runlevel 5, display the
current and previous runlevel, then go back to runlevel 3.
init 5 (watch the console for the change taking place)
runlevel
init 3 (again you can follow this on the console)
3. Is the sysinit script on your computers setting or changing the PATH environment
variable ?
On Red Hat, grep for PATH in /etc/rc.sysinit, on Debian/Ubuntu check /etc/rc.local
and /etc/ini.t/rc.local. The answer is probably no, but on RHEL5 the rc.sysinit script
does set the HOSTNAME variable.
[root@RHEL5 etc]# grep HOSTNAME rc.sysinit
4. List all init.d scripts that are started in runlevel 2.
root@RHEL5 ~# chkconfig --list | grep '2:on'
5. Write a script that acts like a daemon script in /etc/init.d/. It should have a case
statement to act on start/stop/restart and status. Test the script!
The script could look something like this.
#!/bin/bash
#
# chkconfig: 345 99 01
# description: pold demo script
#
# /etc/init.d/pold
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![logging
42.1. login logging
To keep track of who is logging into the system, Linux can maintain the /var/log/
wtmp, /var/log/btmp, /var/run/utmp and /var/log/lastlog files.
/var/run/utmp (who)
Use the who command to see the /var/run/utmp file. This command is showing you
all the currently logged in users. Notice that the utmp file is in /var/run and not in /
var/log .
[root@rhel4 ~]# who
paul pts/1 Feb 14 18:21 (192.168.1.45)
sandra pts/2 Feb 14 18:11 (192.168.1.42)
inge pts/3 Feb 14 12:01 (192.168.1.33)
els pts/4 Feb 14 14:33 (192.168.1.19)
/var/log/wtmp (last)
The /var/log/wtmp file is updated by the login program. Use last to see the /var/
run/wtmp file.
[root@rhel4a ~]# last | head
paul pts/1 192.168.1.45 Wed Feb 14 18:39 still logged in
reboot system boot 2.6.9-42.0.8.ELs Wed Feb 14 18:21 (01:15)
nicolas pts/5 pc-dss.telematic Wed Feb 14 12:32 - 13:06 (00:33)
stefaan pts/3 pc-sde.telematic Wed Feb 14 12:28 - 12:40 (00:12)
nicolas pts/3 pc-nae.telematic Wed Feb 14 11:36 - 12:21 (00:45)
nicolas pts/3 pc-nae.telematic Wed Feb 14 11:34 - 11:36 (00:01)
dirk pts/5 pc-dss.telematic Wed Feb 14 10:03 - 12:31 (02:28)
nicolas pts/3 pc-nae.telematic Wed Feb 14 09:45 - 11:34 (01:48)
dimitri pts/5 rhel4 Wed Feb 14 07:57 - 08:38 (00:40)
stefaan pts/4 pc-sde.telematic Wed Feb 14 07:16 - down (05:50)
[root@rhel4a ~]#
The last command can also be used to get a list of last reboots.
[paul@rekkie ~]$ last reboot
reboot system boot 2.6.16-rekkie Mon Jul 30 05:13 (370+08:42)
wtmp begins Tue May 30 23:11:45 2006
[paul@rekkie ~]$
/var/log/lastlog (lastlog)
Use lastlog to see the /var/log/lastlog file.
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![logging
[root@rhel4a ~]# lastlog | tail
tim pts/5 10.170.1.122 Tue Feb 13 09:36:54 +0100 2007
rm pts/6 rhel4 Tue Feb 13 10:06:56 +0100 2007
henk **Never logged in**
stefaan pts/3 pc-sde.telematic Wed Feb 14 12:28:38 +0100 2007
dirk pts/5 pc-dss.telematic Wed Feb 14 10:03:11 +0100 2007
arsene **Never logged in**
nicolas pts/5 pc-dss.telematic Wed Feb 14 12:32:18 +0100 2007
dimitri pts/5 rhel4 Wed Feb 14 07:57:19 +0100 2007
bashuserrm pts/7 rhel4 Tue Feb 13 10:35:40 +0100 2007
kornuserrm pts/5 rhel4 Tue Feb 13 10:06:17 +0100 2007
[root@rhel4a ~]#
/var/log/btmp (lastb)
There is also the lastb command to display the /var/log/btmp file. This file is updated
by the login program when entering the wrong password, so it contains failed login
attempts. Many computers will not have this file, resulting in no logging of failed
login attempts.
[root@RHEL4b ~]# lastb
lastb: /var/log/btmp: No such file or directory
Perhaps this file was removed by the operator to prevent logging lastb
info.
[root@RHEL4b ~]#
The reason given for this is that users sometimes type their password by mistake
instead of their login, so this world readable file poses a security risk. You can
enable bad login logging by simply creating the file. Doing a chmod o-r /var/log/
btmp improves security.
[root@RHEL4b ~]# touch /var/log/btmp
[root@RHEL4b ~]# ll /var/log/btmp
-rw-r--r-- 1 root root 0 Jul 30 06:12 /var/log/btmp
[root@RHEL4b ~]# chmod o-r /var/log/btmp
[root@RHEL4b ~]# lastb
btmp begins Mon Jul 30 06:12:19 2007
[root@RHEL4b ~]#
Failed logins via ssh, rlogin or su are not registered in /var/log/btmp. Failed logins
via tty are.
[root@RHEL4b ~]# lastb
HalvarFl tty3 Mon Jul 30 07:10 - 07:10 (00:00)
Maria tty1 Mon Jul 30 07:09 - 07:09 (00:00)
Roberto tty1 Mon Jul 30 07:09 - 07:09 (00:00)
btmp begins Mon Jul 30 07:09:32 2007
[root@RHEL4b ~]#
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![logging
su and ssh logins
Depending on the distribution, you may also have the /var/log/secure file being filled
with messages from the auth and/or authpriv syslog facilities. This log will include
su and/or ssh failed login attempts. Some distributions put this in /var/log/auth.log,
verify the syslog configuration.
[root@RHEL4b ~]# cat /var/log/secure
Jul 30 07:09:03 sshd[4387]: Accepted publickey for paul from ::ffff:19
2.168.1.52 port 33188 ssh2
Jul 30 05:09:03 sshd[4388]: Accepted publickey for paul from ::ffff:19
2.168.1.52 port 33188 ssh2
Jul 30 07:22:27 sshd[4655]: Failed password for Hermione from ::ffff:1
92.168.1.52 port 38752 ssh2
Jul 30 05:22:27 sshd[4656]: Failed password for Hermione from ::ffff:1
92.168.1.52 port 38752 ssh2
Jul 30 07:22:30 sshd[4655]: Failed password for Hermione from ::ffff:1
92.168.1.52 port 38752 ssh2
Jul 30 05:22:30 sshd[4656]: Failed password for Hermione from ::ffff:1
92.168.1.52 port 38752 ssh2
Jul 30 07:22:33 sshd[4655]: Failed password for Hermione from ::ffff:1
92.168.1.52 port 38752 ssh2
Jul 30 05:22:33 sshd[4656]: Failed password for Hermione from ::ffff:1
92.168.1.52 port 38752 ssh2
Jul 30 08:27:33 sshd[5018]: Invalid user roberto from ::ffff:192.168.1
.52
Jul 30 06:27:33 sshd[5019]: input_userauth_request: invalid user rober
to
Jul 30 06:27:33 sshd[5019]: Failed none for invalid user roberto from
::ffff:192.168.1.52 port 41064 ssh2
Jul 30 06:27:33 sshd[5019]: Failed publickey for invalid user roberto
from ::ffff:192.168.1.52 port 41064 ssh2
Jul 30 08:27:36 sshd[5018]: Failed password for invalid user roberto f
rom ::ffff:192.168.1.52 port 41064 ssh2
Jul 30 06:27:36 sshd[5019]: Failed password for invalid user roberto f
rom ::ffff:192.168.1.52 port 41064 ssh2
[root@RHEL4b ~]#
You can enable this yourself, with a custom log file by adding the following line tot
syslog.conf.
auth.*,authpriv.* /var/log/customsec.log
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![logging
debug
info
notice
warning (warn)
err (error)
crit
alert
emerg (panic)
actions
The default action is to send a message to the username listed as action. When the
action is prefixed with a / then syslog will send the message to the file (which can be
a regular file, but also a printer or terminal). The @ sign prefix will send the message
on to another syslog server. Here is a list of all possible actions.
root,user1 list of users, separated by comma's
* message to all logged on users
/ file (can be a printer, a console, a tty, ...)
-/ file, but don't sync after every write
| named pipe
@ other syslog hostname
In addition, you can prefix actions with a - to omit syncing the file after every logging.
configuration
Below a sample configuration of custom local4 messages in /etc/syslog.conf.
local4.crit /var/log/critandabove
local4.=crit /var/log/onlycrit
local4.* /var/log/alllocal4
Don't forget to restart the server.
[root@rhel4a ~]# /etc/init.d/syslog restart
Shutting down kernel logger: [ OK ]
Shutting down system logger: [ OK ]
Starting system logger: [ OK ]
Starting kernel logger: [ OK ]
[root@rhel4a ~]#
378](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-384-320.jpg)
![logging
42.3. logger
The logger command can be used to generate syslog test messages. You can aslo use
it in scripts. An example of testing syslogd with the logger tool.
[root@rhel4a ~]# logger -p local4.debug "l4 debug"
[root@rhel4a ~]# logger -p local4.crit "l4 crit"
[root@rhel4a ~]# logger -p local4.emerg "l4 emerg"
[root@rhel4a ~]#
The results of the tests with logger.
[root@rhel4a ~]# cat /var/log/critandabove
Feb 14 19:55:19 rhel4a paul: l4 crit
Feb 14 19:55:28 rhel4a paul: l4 emerg
[root@rhel4a ~]# cat /var/log/onlycrit
Feb 14 19:55:19 rhel4a paul: l4 crit
[root@rhel4a ~]# cat /var/log/alllocal4
Feb 14 19:55:11 rhel4a paul: l4 debug
Feb 14 19:55:19 rhel4a paul: l4 crit
Feb 14 19:55:28 rhel4a paul: l4 emerg
[root@rhel4a ~]#
42.4. watching logs
You might want to use the tail -f command to look at the last lines of a log file. The
-f option will dynamically display lines that are appended to the log.
paul@ubu1010:~$ tail -f /var/log/udev
SEQNUM=1741
SOUND_INITIALIZED=1
ID_VENDOR_FROM_DATABASE=nVidia Corporation
ID_MODEL_FROM_DATABASE=MCP79 High Definition Audio
ID_BUS=pci
ID_VENDOR_ID=0x10de
ID_MODEL_ID=0x0ac0
ID_PATH=pci-0000:00:08.0
SOUND_FORM_FACTOR=internal
You can automatically repeat commands by preceding them with the watch
command. When executing the following:
[root@rhel6 ~]# watch who
Something similar to this, repeating the output of the who command every two
seconds, will appear on the screen.
Every 2.0s: who Sun Jul 17 15:31:03 2011
root tty1 2011-07-17 13:28
paul pts/0 2011-07-17 13:31 (192.168.1.30)
paul pts/1 2011-07-17 15:19 (192.168.1.30)
379](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-385-320.jpg)
![logging
7. Send the local5 messages to the syslog server of your neighbour. Test that it works.
On RHEL5, edit /etc/sysconfig/syslog to enable remote listening on the server.
On Debian/Ubuntu edit /etc/default/syslog or /etc/default/rsyslog.
on the client: logger -p local5.info "test local5 to neighbour"
8. Write a script that executes logger to local4 every 15 seconds (different message).
Use tail -f and watch on your local4 log files.
root@rhel53 scripts# cat logloop
#!/bin/bash
for i in `seq 1 10`
do
logger -p local4.info "local4.info test number $i"
sleep 15
done
root@rhel53 scripts# chmod +x logloop
root@rhel53 scripts# ./logloop &
[1] 8264
root@rhel53 scripts# tail -f /var/log/local4.all.log
Mar 28 13:13:36 rhel53 root: local4.info test number 1
Mar 28 13:13:51 rhel53 root: local4.info test number 2
...
383](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-389-320.jpg)
![memory management
43.3. monitoring memory with vmstat
You can find information about swap usage using vmstat.
Below a simple vmstat displaying information in megabytes.
paul@ubu1010:~$ vmstat -S m
procs ---------memory-------- ---swap-- -----io---- -system- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
0 0 87 225 46 2097 0 0 2 5 14 8 6 5 89 1
Below a sample vmstat when (in another terminal) root launches a find /. It generates
a lot of disk i/o (bi and bo are disk blocks in and out). There is no need for swapping
here.
paul@ubu1010:~$ vmstat 2 100
procs ----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
0 0 84984 1999436 53416 269536 0 0 2 5 2 10 6 5 89 1
0 0 84984 1999428 53416 269564 0 0 0 0 1713 2748 4 4 92 0
0 0 84984 1999552 53416 269564 0 0 0 0 1672 1838 4 6 90 0
0 0 84984 1999552 53424 269560 0 0 0 14 1587 2526 5 7 87 2
0 0 84984 1999180 53424 269580 0 0 0 100 1748 2193 4 6 91 0
1 0 84984 1997800 54508 269760 0 0 610 0 1836 3890 17 10 68 4
1 0 84984 1994620 55040 269748 0 0 250 168 1724 4365 19 17 56 9
0 1 84984 1978508 55292 269704 0 0 126 0 1957 2897 19 18 58 4
0 0 84984 1974608 58964 269784 0 0 1826 478 2605 4355 7 7 44 41
0 2 84984 1971260 62268 269728 0 0 1634 756 2257 3865 7 7 47 39
Below a sample vmstat when executing (on RHEL6) a simple memory leaking
program. Now you see a lot of memory being swapped (si is 'swapped in').
[paul@rhel6c ~]$ vmstat 2 100
procs ----------memory-------- ---swap-- ----io---- --system-- -----cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
0 3 245208 5280 232 1916 261 0 0 42 27 21 0 1 98 1 0
0 2 263372 4800 72 908 143840 128 0 1138 462 191 2 10 0 88 0
1 3 350672 4792 56 992 169280 256 0 1092 360 142 1 13 0 86 0
1 4 449584 4788 56 1024 95880 64 0 606 471 191 2 13 0 85 0
0 4 471968 4828 56 1140 44832 80 0 390 235 90 2 12 0 87 0
3 5 505960 4764 56 1136 68008 16 0 538 286 109 1 12 0 87 0
The code below was used to simulate a memory leak (and force swapping). This code
was found on wikipedia without author.
paul@mac:~$ cat memleak.c
#include <stdlib.h>
int main(void)
{
while (malloc(50));
return 0;
}
388](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-394-320.jpg)
![package management
44.2. rpm
about rpm
The Red Hat package manager can be used on the command line with rpm or in a
graphical way going to Applications--System Settings--Add/Remove Applications.
Type rpm --help to see some of the options.
Software distributed in the rpm format will be named foo-version.platform.rpm .
rpm -qa
To obtain a list of all installed software, use the rpm -qa command.
[root@RHEL52 ~]# rpm -qa | grep samba
system-config-samba-1.2.39-1.el5
samba-3.0.28-1.el5_2.1
samba-client-3.0.28-1.el5_2.1
samba-common-3.0.28-1.el5_2.1
rpm -q
To verify whether one package is installed, use rpm -q.
root@RHELv4u4:~# rpm -q gcc
gcc-3.4.6-3
root@RHELv4u4:~# rpm -q laika
package laika is not installed
rpm -q --redhatprovides
To check whether a package is provided by Red Hat, use the --redhatprovides option.
root@RHELv4u4:~# rpm -q --redhatprovides bash
bash-3.0-19.3
root@RHELv4u4:~# rpm -q --redhatprovides gcc
gcc-3.4.6-3
root@RHELv4u4:~# rpm -q --redhatprovides laika
no package provides laika
rpm -Uvh
To install or upgrade a package, use the -Uvh switches. The -U switch is the same as
-i for install, except that older versions of the software are removed. The -vh switches
are for nicer output.
393](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-399-320.jpg)
![package management
root@RHELv4u4:~# rpm -Uvh gcc-3.4.6-3
rpm -e
To remove a package, use the -e switch.
root@RHELv4u4:~# rpm -e gcc-3.4.6-3
rpm -e verifies dependencies, and thus will prevent you from accidentailly erasing
packages that are needed by other packages.
[root@RHEL52 ~]# rpm -e gcc-4.1.2-42.el5
error: Failed dependencies:
gcc = 4.1.2-42.el5 is needed by (installed) gcc-c++-4.1.2-42.el5.i386
gcc = 4.1.2-42.el5 is needed by (installed) gcc-gfortran-4.1.2-42.el5.i386
gcc is needed by (installed) systemtap-0.6.2-1.el5_2.2.i386
/var/lib/rpm
The rpm database is located at /var/lib/rpm. This database contains all meta
information about packages that are installed (via rpm). It keeps track of all files,
which enables complete removes of software.
rpm2cpio
We can use rpm2cpio to convert an rpm to a cpio archive.
[root@RHEL53 ~]# file kernel.src.rpm
kernel.src.rpm: RPM v3 src PowerPC kernel-2.6.18-92.1.13.el5
[root@RHEL53 ~]# rpm2cpio kernel.src.rpm > kernel.cpio
[root@RHEL53 ~]# file kernel.cpio
kernel.cpio: ASCII cpio archive (SVR4 with no CRC)
But why would you want to do this ?
Perhaps just to see of list of files in the rpm file.
[root@RHEL53 ~]# rpm2cpio kernel.src.rpm | cpio -t | head -5
COPYING.modules
Config.mk
Module.kabi_i686
Module.kabi_i686PAE
Module.kabi_i686xen
Or to extract one file from an rpm package.
[root@RHEL53 ~]# rpm2cpio kernel.src.rpm | cpio -iv Config.mk
Config.mk
246098 blocks
394](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-400-320.jpg)
![package management
44.3. yum
about yum
The Yellowdog Updater, Modified (yum) is an easier command to work with rpm
packages. It is installed by default on Fedora and Red Hat Enterprise Linux since
version 5.2.
yum list
Issue yum list available to see a list of available packages. The available parameter
is optional.
[root@rhel55 ~]# yum list | wc -l
2471
Issue yum list $package to get all versions (in different repositories) of one package.
[root@rhel55 ~]# yum list samba
Loaded plugins: rhnplugin, security
Installed Packages
samba.i386 3.0.33-3.28.el5 installed
Available Packages
samba.i386 3.0.33-3.29.el5_5 rhel-i386-server-5
yum search
To search for a package containing a certain string in the description or name use
yum search $string.
[root@rhel55 ~]# yum search gcc44
Loaded plugins: rhnplugin, security
========================== Matched: gcc44 ===========================
gcc44.i386 : Preview of GCC version 4.4
gcc44-c++.i386 : C++ support for GCC version 4.4
gcc44-gfortran.i386 : Fortran support for GCC 4.4 previe
yum provides
To search for a package containing a certain file (you might need for compiling
things) use yum provides $filename.
[root@rhel55 ~]# yum provides /usr/share/man/man1/gzip.1.gz
Loaded plugins: rhnplugin, security
Importing additional filelist information
gzip-1.3.5-9.el5.i386 : The GNU data compression program.
Repo : rhel-i386-server-5
Matched from:
Filename : /usr/share/man/man1/gzip.1.gz
...
396](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-402-320.jpg)
![package management
yum install
To install an application, use yum install $package. Naturally yum will install all
the necessary dependencies.
[root@rhel55 ~]# yum install sudo
Loaded plugins: rhnplugin, security
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package sudo.i386 0:1.7.2p1-7.el5_5 set to be updated
--> Finished Dependency Resolution
Dependencies Resolved
=======================================================================
Package Arch Version Repository Size
=======================================================================
Installing:
sudo i386 1.7.2p1-7.el5_5 rhel-i386-server-5 230 k
Transaction Summary
=======================================================================
Install 1 Package(s)
Upgrade 0 Package(s)
Total download size: 230 k
Is this ok [y/N]: y
Downloading Packages:
sudo-1.7.2p1-7.el5_5.i386.rpm | 230 kB 00:00
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
Installing : sudo 1/1
Installed:
sudo.i386 0:1.7.2p1-7.el5_5
Complete!
You can add more than one parameter here.
yum install $package1 $package2 $package3
yum update
To bring all applications up to date, by downloading and installing them, issue yum
update. All software that was installed via yum will be updated to the latest version
that is available in the repository.
yum update
If you only want to update one package, use yum update $package.
[root@rhel55 ~]# yum update sudo
Loaded plugins: rhnplugin, security
Skipping security plugin, no data
Setting up Update Process
397](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-403-320.jpg)
![package management
Resolving Dependencies
Skipping security plugin, no data
--> Running transaction check
---> Package sudo.i386 0:1.7.2p1-7.el5_5 set to be updated
--> Finished Dependency Resolution
Dependencies Resolved
=====================================================================
Package Arch Version Repository Size
=====================================================================
Updating:
sudo i386 1.7.2p1-7.el5_5 rhel-i386-server-5 230 k
Transaction Summary
=====================================================================
Install 0 Package(s)
Upgrade 1 Package(s)
Total download size: 230 k
Is this ok [y/N]: y
Downloading Packages:
sudo-1.7.2p1-7.el5_5.i386.rpm | 230 kB 00:00
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
Updating : sudo 1/2
Cleanup : sudo 2/2
Updated:
sudo.i386 0:1.7.2p1-7.el5_5
Complete!
yum software groups
Issue yum grouplist to see a list of all available software groups.
[root@rhel55 ~]# yum grouplist
Loaded plugins: rhnplugin, security
Setting up Group Process
Installed Groups:
Administration Tools
Authoring and Publishing
DNS Name Server
Development Libraries
Development Tools
Editors
GNOME Desktop Environment
GNOME Software Development
Graphical Internet
Graphics
Legacy Network Server
Legacy Software Development
Legacy Software Support
Mail Server
Network Servers
Office/Productivity
Printing Support
Server Configuration Tools
398](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-404-320.jpg)
![package management
System Tools
Text-based Internet
Web Server
Windows File Server
X Software Development
X Window System
Available Groups:
Engineering and Scientific
FTP Server
Games and Entertainment
Java Development
KDE (K Desktop Environment)
KDE Software Development
MySQL Database
News Server
OpenFabrics Enterprise Distribution
PostgreSQL Database
Sound and Video
Done
To install a set of applications, brought together via a group, use yum groupinstall
$groupname.
[root@rhel55 ~]# yum groupinstall 'Sound and video'
Loaded plugins: rhnplugin, security
Setting up Group Process
Package alsa-utils-1.0.17-1.el5.i386 already installed and latest version
Package sox-12.18.1-1.i386 already installed and latest version
Package 9:mkisofs-2.01-10.7.el5.i386 already installed and latest version
Package 9:cdrecord-2.01-10.7.el5.i386 already installed and latest version
Package cdrdao-1.2.1-2.i386 already installed and latest version
Resolving Dependencies
--> Running transaction check
---> Package cdda2wav.i386 9:2.01-10.7.el5 set to be updated
---> Package cdparanoia.i386 0:alpha9.8-27.2 set to be updated
---> Package sound-juicer.i386 0:2.16.0-3.el5 set to be updated
--> Processing Dependency: libmusicbrainz >= 2.1.0 for package: sound-juicer
--> Processing Dependency: libmusicbrainz.so.4 for package: sound-juicer
---> Package vorbis-tools.i386 1:1.1.1-3.el5 set to be updated
--> Processing Dependency: libao >= 0.8.4 for package: vorbis-tools
--> Processing Dependency: libao.so.2 for package: vorbis-tools
--> Running transaction check
---> Package libao.i386 0:0.8.6-7 set to be updated
---> Package libmusicbrainz.i386 0:2.1.1-4.1 set to be updated
--> Finished Dependency Resolution
...
Read the manual page of yum for more information about managing groups in yum.
/etc/yum.conf and repositories
The configuration of yum repositories is done in /etc/yum/yum.conf and /etc/yum/
repos.d/.
Configurating yum itself is done in /etc/yum.conf. This file will contain the location
of a log file and a cache directory for yum and can also contain a list of repositories.
Recently yum started accepting several repo files with each file containing a list of
repositories. These repo files are located in the /etc/yum.repos.d/ directory.
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![package management
One important flag for yum is enablerepo. Use this command if you want to use a
repository that is not enabled by default.
yum $command $foo --enablerepo=$repo
An example of the contents of the repo file: MyRepo.repo
[$repo]
name=My Repository
baseurl=http://path/to/MyRepo
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-MyRep
400](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-406-320.jpg)
![general networking
[Protocols in Frame: eth:ip:udp:bootp]
Sniffing for ntp (Network Time Protocol) packets gives us this line, which makes us
conclude to put ntp next to bootp in the protocol chart below.
[Protocols in Frame: eth:ip:udp:ntp]
Sniffing an arp broadcast makes us put arp next to ip. All these protocols are
explained later in this chapter.
[Protocols in Frame: eth:arp]
Below is a protocol chart based on wireshark's knowledge. It contains some very
common protocols that are discussed in this book. The chart does not contain all
protocols.
411](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-417-320.jpg)
![interface configuration
46.3. Red Hat/Fedora nic configuration
/etc/sysconfig/network
The /etc/sysconfig/network file is a global (across all network cards) configuration
file. It allows us to define whether we want networking (NETWORKING=yes|
no), what the hostname should be (HOSTNAME=) and which gateway to use
(GATEWAY=).
[root@rhel6 ~]# cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=rhel6
GATEWAY=192.168.1.1
There are a dozen more option settable in this file, details can be found in /usr/share/
doc/initscripts-*/sysconfig.txt.
/etc/sysconfig/network-scripts/ifcfg-
Each network card can be configured individually using the /etc/sysconfig/network-
scripts/ifcfg-* files. When you have only one network card, then this will probably
be /etc/sysconfig/network-scripts/ifcfg-eth0.
dhcp client
Below a screenshot of /etc/sysconfig/network-scripts/ifcfg-eth0 configured for
dhcp (BOOTPROTO="dhcp"). Note also the NM_CONTROLLED paramater to
disable control of this nic by Network Manager. This parameter is not explained (not
even mentioned) in /usr/share/doc/initscripts-*/sysconfig.txt, but many others are.
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
HWADDR="08:00:27:DD:0D:5C"
NM_CONTROLLED="no"
BOOTPROTO="dhcp"
ONBOOT="yes"
The BOOTPROTO variable can be set to either dhcp or bootp, anything else will
be considered static meaning there should be no protocol used at boot time to set the
interface values.
fixed ip
Below a screenshot of a fixed ip configuration in /etc/sysconfig/network-scripts/
ifcfg-eth0.
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
HWADDR="08:00:27:DD:0D:5C"
421](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-427-320.jpg)
![interface configuration
NM_CONTROLLED="no"
BOOTPROTO="none"
IPADDR="192.168.1.99"
NETMASK="255.255.255.0"
GATEWAY="192.168.1.1"
ONBOOT="yes"
The HWADDR can be used to make sure that each network card gets the correct
name when multiple network cards are present in the computer. It can not be used to
assign a mac address to a network card. For this, you need to specify the MACADDR
variable. Do not use HWADDR and MACADDR in the same ifcfg-ethx file.
The BROADCAST= and NETWORK= parameters from previous RHEL/Fedora
versions are obsoleted.
/sbin/ifup and /sbin/ifdown
The ifup and ifdown commands will set an interface up or down, using the
configuration discussed above. This is identical to their behaviour in Debian and
Ubuntu.
[root@rhel6 ~]# ifdown eth0 && ifup eth0
[root@rhel6 ~]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2452 errors:0 dropped:0 overruns:0 frame:0
TX packets:1881 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:257036 (251.0 KiB) TX bytes:184767 (180.4 KiB)
422](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-428-320.jpg)
![interface configuration
46.4. ifconfig
The use of /sbin/ifconfig without any arguments will present you with a list of all
active network interface cards, including wireless and the loopback interface. In the
screenshot below eth0 has no ip address.
root@ubu1010:~# ifconfig
eth0 Link encap:Ethernet HWaddr 00:26:bb:5d:2e:52
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
Interrupt:43 Base address:0xe000
eth1 Link encap:Ethernet HWaddr 00:26:bb:12:7a:5e
inet addr:192.168.1.30 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::226:bbff:fe12:7a5e/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:11141791 errors:202 dropped:0 overruns:0 frame:11580126
TX packets:6473056 errors:3860 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:3476531617 (3.4 GB) TX bytes:2114919475 (2.1 GB)
Interrupt:23
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:2879 errors:0 dropped:0 overruns:0 frame:0
TX packets:2879 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:486510 (486.5 KB) TX bytes:486510 (486.5 KB)
You can also use ifconfig to obtain information about just one network card.
[root@rhel6 ~]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2969 errors:0 dropped:0 overruns:0 frame:0
TX packets:1918 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:335942 (328.0 KiB) TX bytes:190157 (185.7 KiB)
When /sbin is not in the $PATH of a normal user you will have to type the full path,
as seen here on Debian.
paul@debian5:~$ /sbin/ifconfig eth3
eth3 Link encap:Ethernet HWaddr 08:00:27:ab:67:30
inet addr:192.168.1.29 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:feab:6730/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:27155 errors:0 dropped:0 overruns:0 frame:0
TX packets:30527 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:13095386 (12.4 MiB) TX bytes:25767221 (24.5 MiB)
423](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-429-320.jpg)
![interface configuration
up and down
You can also use ifconfig to bring an interface up or down. The difference with
ifup is that ifconfig eth0 up will re-activate the nic keeping its existing (current)
configuration, whereas ifup will read the correct file that contains a (possibly new)
configuration and use this config file to bring the interface up.
[root@rhel6 ~]# ifconfig eth0 down
[root@rhel6 ~]# ifconfig eth0 up
[root@rhel6 ~]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2995 errors:0 dropped:0 overruns:0 frame:0
TX packets:1927 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:339030 (331.0 KiB) TX bytes:191583 (187.0 KiB)
setting ip address
You can temporary set an ip address with ifconfig. This ip address is only valid until
the next ifup/ifdown cycle or until the next reboot.
[root@rhel6 ~]# ifconfig eth0 | grep 192
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
[root@rhel6 ~]# ifconfig eth0 192.168.33.42 netmask 255.255.0.0
[root@rhel6 ~]# ifconfig eth0 | grep 192
inet addr:192.168.33.42 Bcast:192.168.255.255 Mask:255.255.0.0
[root@rhel6 ~]# ifdown eth0 && ifup eth0
[root@rhel6 ~]# ifconfig eth0 | grep 192
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
setting mac address
You can also use ifconfig to set another mac address than the one hard coded in the
network card. This screenshot shows you how.
[root@rhel6 ~]# ifconfig eth0 | grep HWaddr
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
[root@rhel6 ~]# ifconfig eth0 hw ether 00:42:42:42:42:42
[root@rhel6 ~]# ifconfig eth0 | grep HWaddr
eth0 Link encap:Ethernet HWaddr 00:42:42:42:42:42
dhclient
Home and client Linux desktops often have /sbin/dhclient running. This is a daemon
that enables a network interface to lease an ip configuration from a dhcp server.
When your adapter is configured for dhcp or bootp, then /sbin/ifup will start the
dhclient daemon.
When a lease is renewed, dhclient will override your ifconfig set ip address!
424](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-430-320.jpg)
![interface configuration
46.5. hostname
Every host receives a hostname, often placed in a DNS name space forming the
fqdn or Fully Qualified Domain Name.
This screenshot shows the hostname command and the configuration of the hostname
on Red Hat/Fedora.
[root@rhel6 ~]# grep rhel /etc/sysconfig/network
HOSTNAME=rhel6
[root@rhel6 ~]# hostname
rhel6
Ubuntu/Debian uses the /etc/hostname file to configure the hostname.
paul@ubu1010:~$ cat /etc/hostname
ubu1010
paul@ubu1010:~$ hostname
ubu1010
On all Linux distributions you can change the hostname using the hostname
$newname command. This is not a permanent change.
[root@rhel6 ~]# hostname server42
[root@rhel6 ~]# hostname
server42
On any Linux you can use sysctl to display and set the hostname.
[root@rhel6 ~]# sysctl kernel.hostname
kernel.hostname = server42
[root@rhel6 ~]# sysctl kernel.hostname=rhel6
kernel.hostname = rhel6
[root@rhel6 ~]# sysctl kernel.hostname
kernel.hostname = rhel6
[root@rhel6 ~]# hostname
rhel6
425](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-431-320.jpg)
![interface configuration
46.6. arp
The ip to mac resolution is handled by the layer two broadcast protocol arp. The
arp table can be displayed with the arp tool. The screenshot below shows the list of
computers that this computer recently communicated with.
root@barry:~# arp -a
? (192.168.1.191) at 00:0C:29:3B:15:80 [ether] on eth1
agapi (192.168.1.73) at 00:03:BA:09:7F:D2 [ether] on eth1
anya (192.168.1.1) at 00:12:01:E2:87:FB [ether] on eth1
faith (192.168.1.41) at 00:0E:7F:41:0D:EB [ether] on eth1
kiss (192.168.1.49) at 00:D0:E0:91:79:95 [ether] on eth1
laika (192.168.1.40) at 00:90:F5:4E:AE:17 [ether] on eth1
pasha (192.168.1.71) at 00:03:BA:02:C3:82 [ether] on eth1
shaka (192.168.1.72) at 00:03:BA:09:7C:F9 [ether] on eth1
root@barry:~#
Anya is a Cisco Firewall, faith is a laser printer, kiss is a Kiss DP600, laika is a
laptop and Agapi, Shaka and Pasha are SPARC servers. The question mark is a Red
Hat Enterprise Linux server running on a virtual machine.
You can use arp -d to remove an entry from the arp table.
[root@rhel6 ~]# arp
Address HWtype HWaddress Flags Mask Iface
ubu1010 ether 00:26:bb:12:7a:5e C eth0
anya ether 00:02:cf:aa:68:f0 C eth0
[root@rhel6 ~]# arp -d anya
[root@rhel6 ~]# arp
Address HWtype HWaddress Flags Mask Iface
ubu1010 ether 00:26:bb:12:7a:5e C eth0
anya (incomplete) eth0
[root@rhel6 ~]# ping anya
PING anya (192.168.1.1) 56(84) bytes of data.
64 bytes from anya (192.168.1.1): icmp_seq=1 ttl=254 time=10.2 ms
...
[root@rhel6 ~]# arp
Address HWtype HWaddress Flags Mask Iface
ubu1010 ether 00:26:bb:12:7a:5e C eth0
anya ether 00:02:cf:aa:68:f0 C eth0
426](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-432-320.jpg)
![interface configuration
46.7. route
You can see the computer's local routing table with the /sbin/route command (and
also with netstat -r ).
root@RHEL4b ~]# netstat -r
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
192.168.1.0 * 255.255.255.0 U 0 0 0 eth0
[root@RHEL4b ~]# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.1.0 * 255.255.255.0 U 0 0 0 eth0
[root@RHEL4b ~]#
It appears this computer does not have a gateway configured, so we use route add
default gw to add a default gateway on the fly.
[root@RHEL4b ~]# route add default gw 192.168.1.1
[root@RHEL4b ~]# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.1.0 * 255.255.255.0 U 0 0 0 eth0
default 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
[root@RHEL4b ~]#
Unless you configure the gateway in one of the /etc/ file from the start of this chapter,
your computer will forget this gateway after a reboot.
46.8. ping
If you can ping to another host, then tcp/ip is configured.
[root@RHEL4b ~]# ping 192.168.1.5
PING 192.168.1.5 (192.168.1.5) 56(84) bytes of data.
64 bytes from 192.168.1.5: icmp_seq=0 ttl=64 time=1004 ms
64 bytes from 192.168.1.5: icmp_seq=1 ttl=64 time=1.19 ms
64 bytes from 192.168.1.5: icmp_seq=2 ttl=64 time=0.494 ms
64 bytes from 192.168.1.5: icmp_seq=3 ttl=64 time=0.419 ms
--- 192.168.1.5 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3009ms
rtt min/avg/max/mdev = 0.419/251.574/1004.186/434.520 ms, pipe 2
[root@RHEL4b ~]#
427](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-433-320.jpg)
![interface configuration
46.9. optional: ethtool
To display or change network card settings, use ethtool. The results depend on the
capabilities of your network card. The example shows a network that auto-negotiates
it's bandwidth.
root@laika:~# ethtool eth0
Settings for eth0:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Advertised auto-negotiation: Yes
Speed: 1000Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 0
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: pumbg
Wake-on: g
Current message level: 0x00000033 (51)
Link detected: yes
This example shows how to use ethtool to switch the bandwidth from 1000Mbit to
100Mbit and back. Note that some time passes before the nic is back to 1000Mbit.
root@laika:~# ethtool eth0 | grep Speed
Speed: 1000Mb/s
root@laika:~# ethtool -s eth0 speed 100
root@laika:~# ethtool eth0 | grep Speed
Speed: 100Mb/s
root@laika:~# ethtool -s eth0 speed 1000
root@laika:~# ethtool eth0 | grep Speed
Speed: 1000Mb/s
428](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-434-320.jpg)
![interface configuration
46.11. solution: interface configuration
1. Verify whether dhclient is running.
paul@debian5:~$ ps fax | grep dhclient
2. Display your current ip address(es).
paul@debian5:~$ /sbin/ifconfig | grep 'inet '
inet addr:192.168.1.31 Bcast:192.168.1.255 Mask:255.255.255.0
inet addr:127.0.0.1 Mask:255.0.0.0
3. Display the configuration file where this ip address is defined.
Ubuntu/Debian: cat /etc/network/interfaces
Redhat/Fedora: cat /etc/sysconfig/network-scripts/ifcfg-eth*
4. Follow the nic configuration in the book to change your ip address from dhcp
client to fixed. Keep the same ip address to avoid conflicts!
Ubuntu/Debian:
ifdown eth0
vi /etc/network/interfaces
ifup eth0
Redhat/Fedora:
ifdown eth0
vi /etc/sysconfig/network-scripts/ifcfg-eth0
ifup eth0
5. Did you also configure the correct gateway in the previous question ? If not, then
do this now.
6. Verify that you have a gateway.
paul@debian5:~$ /sbin/route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.1.0 * 255.255.255.0 U 0 0 0 eth0
default 192.168.1.1 0.0.0.0 UG 0 0 0 eth0
7. Verify that you can connect to the gateway, that it is alive.
paul@debian5:~$ ping -c3 192.168.1.1
PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data.
64 bytes from 192.168.1.1: icmp_seq=1 ttl=254 time=2.28 ms
64 bytes from 192.168.1.1: icmp_seq=2 ttl=254 time=2.94 ms
64 bytes from 192.168.1.1: icmp_seq=3 ttl=254 time=2.34 ms
--- 192.168.1.1 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2008ms
rtt min/avg/max/mdev = 2.283/2.524/2.941/0.296 ms
8. Change the last two digits of your mac address.
[root@rhel6 ~]# ifconfig eth0 hw ether 08:00:27:ab:67:XX
9. Which ports are used by http, pop3, ssh, telnet, nntp and ftp ?
root@rhel6 ~# grep ^'http ' /etc/services
430](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-436-320.jpg)
![interface configuration
http 80/tcp www www-http # WorldWideWeb HTTP
http 80/udp www www-http # HyperText Transfer Protocol
root@rhel6 ~# grep ^'smtp ' /etc/services
smtp 25/tcp mail
smtp 25/udp mail
root@rhel6 ~# grep ^'ssh ' /etc/services
ssh 22/tcp # The Secure Shell (SSH) Protocol
ssh 22/udp # The Secure Shell (SSH) Protocol
root@rhel6 ~# grep ^'telnet ' /etc/services
telnet 23/tcp
telnet 23/udp
root@rhel6 ~# grep ^'nntp ' /etc/services
nntp 119/tcp readnews untp # USENET News Transfer Protocol
nntp 119/udp readnews untp # USENET News Transfer Protocol
root@rhel6 ~# grep ^'ftp ' /etc/services
ftp 21/tcp
ftp 21/udp fsp fspd
Note that sctp was ommitted from the screenshot.
10. Explain why e-mail and websites are sent over tcp and not udp.
Because tcp is reliable and udp is not.
11. Display the hostname of your computer.
paul@debian5:~$ hostname
debian5
12. Which ip-addresses did your computer recently have contact with ?
root@rhel6 ~# arp -a
? (192.168.1.1) at 00:02:cf:aa:68:f0 [ether] on eth2
? (192.168.1.30) at 00:26:bb:12:7a:5e [ether] on eth2
? (192.168.1.31) at 08:00:27:8e:8a:a8 [ether] on eth2
431](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-437-320.jpg)
![binding and bonding
48.1. binding on Redhat/Fedora
binding extra ip addresses
To bind more than one ip address to the same interface, use ifcfg-eth0:0, where the
last zero can be anything else. Only two directives are required in the files.
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0:0
DEVICE="eth0:0"
IPADDR="192.168.1.133"
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0:1
DEVICE="eth0:0"
IPADDR="192.168.1.142"
enabling extra ip-addresses
To activate a virtual network interface, use ifup, to deactivate it, use ifdown.
[root@rhel6 ~]# ifup eth0:0
[root@rhel6 ~]# ifconfig | grep 'inet '
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet addr:192.168.1.133 Bcast:192.168.1.255 Mask:255.255.255.0
inet addr:127.0.0.1 Mask:255.0.0.0
[root@rhel6 ~]# ifup eth0:1
[root@rhel6 ~]# ifconfig | grep 'inet '
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet addr:192.168.1.133 Bcast:192.168.1.255 Mask:255.255.255.0
inet addr:192.168.1.142 Bcast:192.168.1.255 Mask:255.255.255.0
inet addr:127.0.0.1 Mask:255.0.0.0
verifying extra ip-addresses
Use ping from another computer to check the activation, or use ifconfig like in this
screenshot.
[root@rhel6 ~]# ifconfig
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1259 errors:0 dropped:0 overruns:0 frame:0
TX packets:545 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:115260 (112.5 KiB) TX bytes:84293 (82.3 KiB)
eth0:0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.133 Bcast:192.168.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
eth0:1 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.142 Bcast:192.168.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
439](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-445-320.jpg)
![binding and bonding
48.3. bonding on Redhat/Fedora
We start with ifconfig -a to get a list of all the network cards on our system.
[root@rhel6 network-scripts]# ifconfig -a | grep Ethernet
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
eth1 Link encap:Ethernet HWaddr 08:00:27:DA:C1:49
eth2 Link encap:Ethernet HWaddr 08:00:27:40:03:3B
In this demo we decide to bond eth1 and eth2.
We will name are bond bond0 and add this entry to modprobe so the kernel can load
the bonding module when we bring the interface up.
[root@rhel6 network-scripts]# cat /etc/modprobe.d/bonding.conf
alias bond0 bonding
Then we create /etc/sysconfig/network-scripts/ifcfg-bond0 to configure our bond0
interface.
[root@rhel6 network-scripts]# pwd
/etc/sysconfig/network-scripts
[root@rhel6 network-scripts]# cat ifcfg-bond0
DEVICE=bond0
IPADDR=192.168.1.199
NETMASK=255.255.255.0
ONBOOT=yes
BOOTPROTO=none
USERCTL=no
Next we create two files, one for each network card that we will use as slave in bond0.
[root@rhel6 network-scripts]# cat ifcfg-eth1
DEVICE=eth1
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SLAVE=yes
USERCTL=no
[root@rhel6 network-scripts]# cat ifcfg-eth2
DEVICE=eth2
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SLAVE=yes
USERCTL=no
Finally we bring the interface up with ifup bond0.
[root@rhel6 network-scripts]# ifup bond0
[root@rhel6 network-scripts]# ifconfig bond0
bond0 Link encap:Ethernet HWaddr 08:00:27:DA:C1:49
inet addr:192.168.1.199 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:feda:c149/64 Scope:Link
UP BROADCAST RUNNING MASTER MULTICAST MTU:1500 Metric:1
RX packets:251 errors:0 dropped:0 overruns:0 frame:0
TX packets:21 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:39852 (38.9 KiB) TX bytes:1070 (1.0 KiB)
The bond should also be visible in /proc/net/bonding.
441](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-447-320.jpg)
![binding and bonding
[root@rhel6 network-scripts]# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.5.0 (November 4, 2008)
Bonding Mode: load balancing (round-robin)
MII Status: up
MII Polling Interval (ms): 0
Up Delay (ms): 0
Down Delay (ms): 0
Slave Interface: eth1
MII Status: up
Link Failure Count: 0
Permanent HW addr: 08:00:27:da:c1:49
Slave Interface: eth2
MII Status: up
Link Failure Count: 0
Permanent HW addr: 08:00:27:40:03:3b
442](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-448-320.jpg)
![introduction to networking
49.1. introduction to iptables
iptables firewall
The Linux kernel has a built-in stateful firewall named iptables. To stop the iptables
firewall on Red Hat, use the service command.
root@RHELv4u4:~# service iptables stop
Flushing firewall rules: [ OK ]
Setting chains to policy ACCEPT: filter [ OK ]
Unloading iptables modules: [ OK ]
root@RHELv4u4:~#
The easy way to configure iptables, is to use a graphical tool like KDE's kmyfirewall
or Security Level Configuration Tool. You can find the latter in the graphical menu,
somewhere in System Tools - Security, or you can start it by typing system-config-
securitylevel in bash. These tools allow for some basic firewall configuration. You
can decide whether to enable or disable the firewall, and what typical standard ports
are allowed when the firewall is active. You can even add some custom ports. When
you are done, the configuration is written to /etc/sysconfig/iptables on Red Hat.
root@RHELv4u4:~# cat /etc/sysconfig/iptables
# Firewall configuration written by system-config-securitylevel
# Manual customization of this file is not recommended.
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:RH-Firewall-1-INPUT - [0:0]
-A INPUT -j RH-Firewall-1-INPUT
-A FORWARD -j RH-Firewall-1-INPUT
-A RH-Firewall-1-INPUT -i lo -j ACCEPT
-A RH-Firewall-1-INPUT -p icmp --icmp-type any -j ACCEPT
-A RH-Firewall-1-INPUT -p 50 -j ACCEPT
-A RH-Firewall-1-INPUT -p 51 -j ACCEPT
-A RH-Firewall-1-INPUT -p udp --dport 5353 -d 224.0.0.251 -j ACCEPT
-A RH-Firewall-1-INPUT -p udp -m udp --dport 631 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
-A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
-A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 21 -j ACCEPT
-A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 25 -j ACCEPT
-A RH-Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibited
COMMIT
root@RHELv4u4:~#
To start the service, issue the service iptables start command. You can configure
iptables to start at boot time with chkconfig.
root@RHELv4u4:~# service iptables start
Applying iptables firewall rules: [ OK ]
root@RHELv4u4:~# chkconfig iptables on
root@RHELv4u4:~#
448](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-454-320.jpg)
![introduction to networking
49.3. solution : iptables
1. Verify whether the firewall is running.
root@rhel55 ~# service iptables status | head
Table: filter
Chain INPUT (policy ACCEPT)
num target prot opt source destination
1 RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0
Chain FORWARD (policy ACCEPT)
num target prot opt source destination
1 RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0
Chain OUTPUT (policy ACCEPT)
2. Stop the running firewall.
root@rhel55 ~# service iptables stop
Flushing firewall rules: [ OK ]
Setting chains to policy ACCEPT: filter [ OK ]
Unloading iptables modules: [ OK ]
root@rhel55 ~# service iptables status
Firewall is stopped.
450](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-456-320.jpg)
![introduction to networking
49.4. xinetd and inetd
the superdaemon
Back when resources like RAM memory were limited, a super-server was devised
to listen to all sockets and start the appropriate daemon only when needed. Services
like swat, telnet and ftp are typically served by such a super-server. The xinetd
superdaemon is more recent than inetd. We will discuss the configuration both
daemons.
Recent Linux distributions like RHEL5 and Ubuntu10.04 do not activate inetd or
xinetd by default, unless an application requires it.
inetd or xinetd
First verify whether your computer is running inetd or xinetd. This Debian 4.0 Etch
is running inetd.
root@barry:~# ps fax | grep inet
3870 ? Ss 0:00 /usr/sbin/inetd
This Red Hat Enterprise Linux 4 update 4 is running xinetd.
[root@RHEL4b ~]# ps fax | grep inet
3003 ? Ss 0:00 xinetd -stayalive -pidfile /var/run/xinetd.pid
Both daemons have the same functionality (listening to many ports, starting other
daemons when they are needed), but they have different configuration files.
xinetd superdaemon
The xinetd daemon is often called a superdaemon because it listens to a lot of
incoming connections, and starts other daemons when they are needed. When a
connection request is received, xinetd will first check TCP wrappers (/etc/hosts.allow
and /etc/hosts.deny) and then give control of the connection to the other daemon. This
superdaemon is configured through /etc/xinetd.conf and the files in the directory /
etc/xinetd.d. Let's first take a look at /etc/xinetd.conf.
paul@RHELv4u2:~$ cat /etc/xinetd.conf
#
# Simple configuration file for xinetd
#
# Some defaults, and include /etc/xinetd.d/
defaults
{
451](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-457-320.jpg)
![introduction to networking
Configuration of ssh is done in the /etc/ssh directory and is pretty straightforward.
public and private keys
Imagine Alice and Bob, two people that like to communicate with each other.
Using public and private keys they can communicate with encryption and with
authentication.
When Alice wants to send an encrypted message to Bob, she uses the public key of
Bob. Bob shares his public key with Alice, but keeps his private key private! Since
Bob is the only one to have Bob's private key, Alice is sure that Bob is the only one
that can read the encrypted message.
When Bob wants to verify that the message came from Alice, Bob uses the public
key of Alice to verify that Alice signed the message with her private key. Since Alice
is the only one to have Alice's private key, Bob is sure the message came from Alice.
setting up passwordless ssh
To set up passwordless ssh authentication through public/private keys, use ssh-
keygen to generate a key pair without a passphrase, and then copy your public key
to the destination server. Let's do this step by step.
In the example that follows, we will set up ssh without password between Alice and
Bob. Alice has an account on a Red Hat Enterprise Linux server, Bob is using Ubuntu
on his laptop. Bob wants to give Alice access using ssh and the public and private
key system. This means that even if Bob changes his password on his laptop, Alice
will still have access.
ssh-keygen
The example below shows how Alice uses ssh-keygen to generate a key pair. Alice
does not enter a passphrase.
[alice@RHEL5 ~]$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/alice/.ssh/id_rsa):
Created directory '/home/alice/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/alice/.ssh/id_rsa.
Your public key has been saved in /home/alice/.ssh/id_rsa.pub.
The key fingerprint is:
9b:ac:ac:56:c2:98:e5:d9:18:c4:2a:51:72:bb:45:eb alice@RHEL5
[alice@RHEL5 ~]$
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~/.ssh
While ssh-keygen generates a public and a private key, it will also create a hidden
.ssh directory with proper permissions. If you create the .ssh directory manually, then
you need to chmod 700 it! Otherwise ssh will refuse to use the keys (world readable
private keys are not secure!).
As you can see, the .ssh directory is secure in Alice's home directory.
[alice@RHEL5 ~]$ ls -ld .ssh
drwx------ 2 alice alice 4096 May 1 07:38 .ssh
[alice@RHEL5 ~]$
Bob is using Ubuntu at home. He decides to manually create the .ssh directory, so
he needs to manually secure it.
bob@laika:~$ mkdir .ssh
bob@laika:~$ ls -ld .ssh
drwxr-xr-x 2 bob bob 4096 2008-05-14 16:53 .ssh
bob@laika:~$ chmod 700 .ssh/
bob@laika:~$
id_rsa and id_rsa.pub
The ssh-keygen command generate two keys in .ssh. The public key is named ~/.ssh/
id_rsa.pub. The private key is named ~/.ssh/id_rsa.
[alice@RHEL5 ~]$ ls -l .ssh/
total 16
-rw------- 1 alice alice 1671 May 1 07:38 id_rsa
-rw-r--r-- 1 alice alice 393 May 1 07:38 id_rsa.pub
[alice@RHEL5 ~]$
scp
To copy the public key from Alice's server tot Bob's laptop, Alice decides to use scp.
[alice@RHEL5 .ssh]$ scp id_rsa.pub bob@192.168.48.92:~/.ssh/authorized_keys
bob@192.168.48.92's password:
id_rsa.pub 100% 393 0.4KB/s 00:00
[alice@RHEL5 .ssh]$
Be careful when copying a second key! Do not overwrite the first key, instead append
the key to the same ~/.ssh/authorized_keys file!
authorized_keys
In your ~/.ssh directory, you can create a file called authorized_keys. This file can
contain one or more public keys from people you trust. Those trusted people can
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![introduction to networking
use their private keys to prove their identity and gain access to your account via ssh
(without password). The example shows Bob's authorized_keys file containing the
public key of Alice.
bob@laika:~$ cat .ssh/authorized_keys
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEApCQ9xzyLzJes1sR+hPyqW2vyzt1D4zTLqk
MDWBR4mMFuUZD/O583I3Lg/Q+JIq0RSksNzaL/BNLDou1jMpBe2Dmf/u22u4KmqlJBfDhe
yTmGSBzeNYCYRSMq78CT9l9a+y6x/shucwhaILsy8A2XfJ9VCggkVtu7XlWFDL2cum08/0
mRFwVrfc/uPsAn5XkkTscl4g21mQbnp9wJC40pGSJXXMuFOk8MgCb5ieSnpKFniAKM+tEo
/vjDGSi3F/bxu691jscrU0VUdIoOSo98HUfEf7jKBRikxGAC7I4HLa+/zX73OIvRFAb2hv
tUhn6RHrBtUJUjbSGiYeFTLDfcTQ== alice@RHEL5
bob@laika:~$
passwordless ssh
Alice can now use ssh to connect passwordless to Bob's laptop. In combination with
ssh's capability to execute commands on the remote host, this can be useful in pipes
across different machines.
[alice@RHEL5 ~]$ ssh bob@192.168.48.92 "ls -l .ssh"
total 4
-rw-r--r-- 1 bob bob 393 2008-05-14 17:03 authorized_keys
[alice@RHEL5 ~]$
X forwarding via SSH
The ssh protocol will remember the servers it connected to (and warn you in case
something suspicious happened), and will use strong 128-bit encryption. Another
popular feature of ssh is called X11 forwarding and is implemented with ssh -X.
Below an example of X11 forwarding: user paul logs in as user greet on her computer
to start the graphical application mozilla-thunderbird. Although the application will
run on the remote computer from greet, it will be displayed on the screen attached
locally to paul's computer.
paul@laika:~/PDF$ ssh -X greet@greet.dyndns.org -p 55555
Warning: Permanently added the RSA host key for IP address
'81.240.174.161' to the list of known hosts.
Password:
Linux raika 2.6.8-2-686 #1 Tue Aug 16 13:22:48 UTC 2005 i686 GNU/Linux
Last login: Thu Jan 18 12:35:56 2007
greet@raika:~$ ps fax | grep thun
greet@raika:~$ mozilla-thunderbird &
[1] 30336
troubleshooting ssh
Use ssh -v to get debug information about the ssh connection attempt.
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paul@laika:~$ ssh -v bert@192.168.1.192
OpenSSH_4.3p2 Debian-8ubuntu1, OpenSSL 0.9.8c 05 Sep 2006
debug1: Reading configuration data /home/paul/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to 192.168.1.192 [192.168.1.192] port 22.
debug1: Connection established.
debug1: identity file /home/paul/.ssh/identity type -1
debug1: identity file /home/paul/.ssh/id_rsa type 1
debug1: identity file /home/paul/.ssh/id_dsa type -1
debug1: Remote protocol version 1.99, remote software version OpenSSH_3
debug1: match: OpenSSH_3.9p1 pat OpenSSH_3.*
debug1: Enabling compatibility mode for protocol 2.0
...
49.7. practice: ssh
1. Create a user for your neighbour, then test ssh to your neighbour (by ip-address or
by hostname). (You might need to install the openssh-server with aptitude.)
2. Create a bookmark in Firefox, then close your firefox! Use ssh -X to run firefox
on your screen, but on your neighbour's computer. Do you see your neighbour's
bookmark ?
3. Verify in the ssh configuration files that only protocol version 2 is allowed.
4. Use ssh-keygen to create a key pair without passphrase. Setup passwordless ssh
between you and your neighbour. (or between the ubuntu and the Red Hat)
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49.8. network file system
protocol versions
The older nfs versions 2 and 3 are stateless (udp) by default, but they can use tcp.
Clients connect to the server using rpc (on Linux this is controlled by the portmap
daemon. Look at rpcinfo to verify that nfs and its related services are running.
root@RHELv4u2:~# /etc/init.d/portmap status
portmap (pid 1920) is running...
root@RHELv4u2:~# rpcinfo -p
program vers proto port
100000 2 tcp 111 portmapper
100000 2 udp 111 portmapper
100024 1 udp 32768 status
100024 1 tcp 32769 status
root@RHELv4u2:~# service nfs start
Starting NFS services: [ OK ]
Starting NFS quotas: [ OK ]
Starting NFS daemon: [ OK ]
Starting NFS mountd: [ OK ]
The same rpcinfo command when nfs is started.
root@RHELv4u2:~# rpcinfo -p
program vers proto port
100000 2 tcp 111 portmapper
100000 2 udp 111 portmapper
100024 1 udp 32768 status
100024 1 tcp 32769 status
100011 1 udp 985 rquotad
100011 2 udp 985 rquotad
100011 1 tcp 988 rquotad
100011 2 tcp 988 rquotad
100003 2 udp 2049 nfs
100003 3 udp 2049 nfs
100003 4 udp 2049 nfs
100003 2 tcp 2049 nfs
100003 3 tcp 2049 nfs
100003 4 tcp 2049 nfs
100021 1 udp 32770 nlockmgr
100021 3 udp 32770 nlockmgr
100021 4 udp 32770 nlockmgr
100021 1 tcp 32789 nlockmgr
100021 3 tcp 32789 nlockmgr
100021 4 tcp 32789 nlockmgr
100005 1 udp 1004 mountd
100005 1 tcp 1007 mountd
100005 2 udp 1004 mountd
100005 2 tcp 1007 mountd
100005 3 udp 1004 mountd
100005 3 tcp 1007 mountd
root@RHELv4u2:~#
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![Chapter 50. the Linux kernel
Table of Contents
50.1. about the Linux kernel ............................................................................... 463
50.2. Linux kernel source ................................................................................... 465
50.3. kernel boot files ......................................................................................... 469
50.4. Linux kernel modules ................................................................................ 470
50.5. compiling a kernel ..................................................................................... 474
50.6. compiling one module ............................................................................... 477
50.1. about the Linux kernel
kernel versions
In 1991 Linux Torvalds wrote (the first version of) the Linux kernel. He put it online,
and other people started contributing code. Over 4000 individuals contributed source
code to the latest kernel release (version 2.6.27 in November 2008).
Major Linux kernel versions used to come in even and odd numbers. Versions 2.0,
2.2, 2.4 and 2.6 are considered stable kernel versions. Whereas 2.1, 2.3 and 2.5 were
unstable (read development) versions. Since the release of 2.6.0 in January 2004, all
development has been done in the 2.6 tree. There is currently no v2.7.x and according
to Linus the even/stable vs odd/development scheme is abandoned forever.
uname -r
To see your current Linux kernel version, issue the uname -r command as shown
below.
This first example shows Linux major version 2.6 and minor version 24. The rest -22-
generic is specific to the distribution (Ubuntu in this case).
paul@laika:~$ uname -r
2.6.24-22-generic
The same command on Red Hat Enterprise Linux shows an older kernel (2.6.18) with
-92.1.17.el5 being specific to the distribution.
[paul@RHEL52 ~]$ uname -r
2.6.18-92.1.17.el5
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/proc/cmdline
The parameters that were passed to the kernel at boot time are in /proc/cmdline.
paul@RHELv4u4:~$ cat /proc/cmdline
ro root=/dev/VolGroup00/LogVol00 rhgb quiet
single user mode
When booting the kernel with the single parameter, it starts in single user mode.
Linux can start in a bash shell with the root user logged on (without password).
Some distributions prevent the use of this feature (at kernel compile time).
init=/bin/bash
Normally the kernel invokes init as the first daemon process. Adding init=/bin/bash
to the kernel parameters will instead invoke bash (again with root logged on without
providing a password).
/var/log/messages
The kernel reports during boot to syslog which writes a lot of kernel actions in /var/
log/messages. Looking at this file reveals when the kernel was started, including all
the devices that were detected at boot time.
[root@RHEL53 ~]# grep -A16 "syslogd 1.4.1:" /var/log/messages|cut -b24-
syslogd 1.4.1: restart.
kernel: klogd 1.4.1, log source = /proc/kmsg started.
kernel: Linux version 2.6.18-128.el5 (mockbuild@hs20-bc1-5.build.red...
kernel: BIOS-provided physical RAM map:
kernel: BIOS-e820: 0000000000000000 - 000000000009f800 (usable)
kernel: BIOS-e820: 000000000009f800 - 00000000000a0000 (reserved)
kernel: BIOS-e820: 00000000000ca000 - 00000000000cc000 (reserved)
kernel: BIOS-e820: 00000000000dc000 - 0000000000100000 (reserved)
kernel: BIOS-e820: 0000000000100000 - 000000001fef0000 (usable)
kernel: BIOS-e820: 000000001fef0000 - 000000001feff000 (ACPI data)
kernel: BIOS-e820: 000000001feff000 - 000000001ff00000 (ACPI NVS)
kernel: BIOS-e820: 000000001ff00000 - 0000000020000000 (usable)
kernel: BIOS-e820: 00000000fec00000 - 00000000fec10000 (reserved)
kernel: BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved)
kernel: BIOS-e820: 00000000fffe0000 - 0000000100000000 (reserved)
kernel: 0MB HIGHMEM available.
kernel: 512MB LOWMEM available.
This example shows how to use /var/log/messages to see kernel information about
/dev/sda.
[root@RHEL53 ~]# grep sda /var/log/messages | cut -b24-
kernel: SCSI device sda: 41943040 512-byte hdwr sectors (21475 MB)
kernel: sda: Write Protect is off
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kernel: sda: cache data unavailable
kernel: sda: assuming drive cache: write through
kernel: SCSI device sda: 41943040 512-byte hdwr sectors (21475 MB)
kernel: sda: Write Protect is off
kernel: sda: cache data unavailable
kernel: sda: assuming drive cache: write through
kernel: sda: sda1 sda2
kernel: sd 0:0:0:0: Attached scsi disk sda
kernel: EXT3 FS on sda1, internal journal
dmesg
The dmesg command prints out all the kernel bootup messages (from the last boot).
[root@RHEL53 ~]# dmesg | head
Linux version 2.6.18-128.el5 (mockbuild@hs20-bc1-5.build.redhat.com)
BIOS-provided physical RAM map:
BIOS-e820: 0000000000000000 - 000000000009f800 (usable)
BIOS-e820: 000000000009f800 - 00000000000a0000 (reserved)
BIOS-e820: 00000000000ca000 - 00000000000cc000 (reserved)
BIOS-e820: 00000000000dc000 - 0000000000100000 (reserved)
BIOS-e820: 0000000000100000 - 000000001fef0000 (usable)
BIOS-e820: 000000001fef0000 - 000000001feff000 (ACPI data)
BIOS-e820: 000000001feff000 - 000000001ff00000 (ACPI NVS)
BIOS-e820: 000000001ff00000 - 0000000020000000 (usable)
Thus to find information about /dev/sda, using dmesg will yield only kernel messages
from the last boot.
[root@RHEL53 ~]# dmesg | grep sda
SCSI device sda: 41943040 512-byte hdwr sectors (21475 MB)
sda: Write Protect is off
sda: Mode Sense: 5d 00 00 00
sda: cache data unavailable
sda: assuming drive cache: write through
SCSI device sda: 41943040 512-byte hdwr sectors (21475 MB)
sda: Write Protect is off
sda: Mode Sense: 5d 00 00 00
sda: cache data unavailable
sda: assuming drive cache: write through
sda: sda1 sda2
sd 0:0:0:0: Attached scsi disk sda
EXT3 FS on sda1, internal journal
50.2. Linux kernel source
ftp.kernel.org
The home of the Linux kernel source is ftp.kernel.org. It contains all official releases
of the Linux kernel source code from 1991. It provides free downloads over http, ftp
and rsync of all these releases, as well as changelogs and patches. More information
can be otained on the website www.kernel.org.
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drwxr-xr-x 23 root root 4096 2008-11-24 23:28 linux-source-2.6.24
paul@laika:~$ find /usr/src -name "e1000_main.c"
/usr/src/linux-source-2.6.24/drivers/net/e1000/e1000_main.c
Now take a look at /usr/src on Red Hat Enterprise Linux.
[paul@RHEL52 ~]$ ls -l /usr/src/
drwxr-xr-x 5 root root 4096 Dec 5 19:23 kernels
drwxr-xr-x 7 root root 4096 Oct 11 13:22 redhat
We will have to dig a little deeper to find the kernel source on Red Hat!
[paul@RHEL52 ~]$ cd /usr/src/redhat/BUILD/
[paul@RHEL52 BUILD]$ find . -name "e1000_main.c"
./kernel-2.6.18/linux-2.6.18.i686/drivers/net/e1000/e1000_main.c
downloading the kernel source
Debian
Installing the kernel source on Debian is really simple with aptitude install linux-
source. You can do a search for all linux-source packeges first, like in this screenshot.
root@barry:~# aptitude search linux-source
v linux-source -
v linux-source-2.6 -
id linux-source-2.6.15 - Linux kernel source for version 2.6.15
i linux-source-2.6.16 - Linux kernel source for version 2.6.16
p linux-source-2.6.18 - Linux kernel source for version 2.6.18
p linux-source-2.6.24 - Linux kernel source for version 2.6.24
And then use aptitude install to download and install the Debian Linux kernel source
code.
root@barry:~# aptitude install linux-source-2.6.24
When the aptitude is finished, you will see a new file named /usr/src/linux-source-
<version>.tar.bz2
root@barry:/usr/src# ls -lh
drwxr-xr-x 20 root root 4.0K 2006-04-04 22:12 linux-source-2.6.15
drwxr-xr-x 19 root root 4.0K 2006-07-15 17:32 linux-source-2.6.16
-rw-r--r-- 1 root root 45M 2008-12-02 10:56 linux-source-2.6.24.tar.bz2
Ubuntu
Ubuntu is based on Debian and also uses aptitude, so the task is very similar.
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root@laika:~# aptitude search linux-source
i linux-source - Linux kernel source with Ubuntu patches
v linux-source-2.6 -
i A linux-source-2.6.24 - Linux kernel source for version 2.6.24
root@laika:~# aptitude install linux-source
And when aptitude finishes, we end up with a /usr/src/linux-source-
<version>.tar.bz file.
oot@laika:~# ll /usr/src
total 45M
-rw-r--r-- 1 root root 45M 2008-11-24 23:30 linux-source-2.6.24.tar.bz2
Red Hat Enterprise Linux
The Red Hat kernel source is located on the fourth source cdrom. The file is called
kernel-2.6.9-42.EL.src.rpm (example for RHELv4u4). It is also available online
at ftp://ftp.redhat.com/pub/redhat/linux/enterprise/5Server/en/os/SRPMS/ (example
for RHEL5).
To download the kernel source on RHEL, use this long wget command (on one line,
without the trailing ).
wget ftp://ftp.redhat.com/pub/redhat/linux/enterprise/5Server/en/os/
SRPMS/kernel-`uname -r`.src.rpm
When the wget download is finished, you end up with a 60M .rpm file.
[root@RHEL52 src]# ll
total 60M
-rw-r--r-- 1 root root 60M Dec 5 20:54 kernel-2.6.18-92.1.17.el5.src.rpm
drwxr-xr-x 5 root root 4.0K Dec 5 19:23 kernels
drwxr-xr-x 7 root root 4.0K Oct 11 13:22 redhat
We will need to perform some more steps before this can be used as kernel source
code.
First, we issue the rpm -i kernel-2.6.9-42.EL.src.rpm command to install this Red
Hat package.
[root@RHEL52 src]# ll
total 60M
-rw-r--r-- 1 root root 60M Dec 5 20:54 kernel-2.6.18-92.1.17.el5.src.rpm
drwxr-xr-x 5 root root 4.0K Dec 5 19:23 kernels
drwxr-xr-x 7 root root 4.0K Oct 11 13:22 redhat
[root@RHEL52 src]# rpm -i kernel-2.6.18-92.1.17.el5.src.rpm
The we move to the SPECS directory and perform an rpmbuild.
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[root@RHEL52 ~]# cd /usr/src/redhat/SPECS
[root@RHEL52 SPECS]# rpmbuild -bp -vv --target=i686 kernel-2.6.spec
The rpmbuild command put the RHEL Linux kernel source code in /usr/src/redhat/
BUILD/kernel-<version>/.
[root@RHEL52 kernel-2.6.18]# pwd
/usr/src/redhat/BUILD/kernel-2.6.18
[root@RHEL52 kernel-2.6.18]# ll
total 20K
drwxr-xr-x 2 root root 4.0K Dec 6 2007 config
-rw-r--r-- 1 root root 3.1K Dec 5 20:58 Config.mk
drwxr-xr-x 20 root root 4.0K Dec 5 20:58 linux-2.6.18.i686
drwxr-xr-x 19 root root 4.0K Sep 20 2006 vanilla
drwxr-xr-x 8 root root 4.0K Dec 6 2007 xen
50.3. kernel boot files
vmlinuz
The vmlinuz file in /boot is the compressed kernel.
paul@barry:~$ ls -lh /boot | grep vmlinuz
-rw-r--r-- 1 root root 1.2M 2006-03-06 16:22 vmlinuz-2.6.15-1-486
-rw-r--r-- 1 root root 1.1M 2006-03-06 16:30 vmlinuz-2.6.15-1-686
-rw-r--r-- 1 root root 1.3M 2008-02-11 00:00 vmlinuz-2.6.18-6-686
paul@barry:~$
initrd
The kernel uses initrd (an initial RAM disk) at boot time. The initrd is mounted before
the kernel loads, and can contain additional drivers and modules. It is a compressed
cpio archive, so you can look at the contents in this way.
root@RHELv4u4:/boot# mkdir /mnt/initrd
root@RHELv4u4:/boot# cp initrd-2.6.9-42.0.3.EL.img TMPinitrd.gz
root@RHELv4u4:/boot# gunzip TMPinitrd.gz
root@RHELv4u4:/boot# file TMPinitrd
TMPinitrd: ASCII cpio archive (SVR4 with no CRC)
root@RHELv4u4:/boot# cd /mnt/initrd/
root@RHELv4u4:/mnt/initrd# cpio -i | /boot/TMPinitrd
4985 blocks
root@RHELv4u4:/mnt/initrd# ls -l
total 76
drwxr-xr-x 2 root root 4096 Feb 5 08:36 bin
drwxr-xr-x 2 root root 4096 Feb 5 08:36 dev
drwxr-xr-x 4 root root 4096 Feb 5 08:36 etc
-rwxr-xr-x 1 root root 1607 Feb 5 08:36 init
drwxr-xr-x 2 root root 4096 Feb 5 08:36 lib
drwxr-xr-x 2 root root 4096 Feb 5 08:36 loopfs
drwxr-xr-x 2 root root 4096 Feb 5 08:36 proc
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![the Linux kernel
/lib/modules
The modules are stored in the /lib/modules/<kernel-version> directory. There is a
separate directory for each kernel that was compiled for your system.
paul@laika:~$ ll /lib/modules/
total 12K
drwxr-xr-x 7 root root 4.0K 2008-11-10 14:32 2.6.24-16-generic
drwxr-xr-x 8 root root 4.0K 2008-12-06 15:39 2.6.24-21-generic
drwxr-xr-x 8 root root 4.0K 2008-12-05 12:58 2.6.24-22-generic
<module>.ko
The file containing the modules usually ends in .ko. This screenshot shows the
location of the isdn module files.
paul@laika:~$ find /lib/modules -name isdn.ko
/lib/modules/2.6.24-21-generic/kernel/drivers/isdn/i4l/isdn.ko
/lib/modules/2.6.24-22-generic/kernel/drivers/isdn/i4l/isdn.ko
/lib/modules/2.6.24-16-generic/kernel/drivers/isdn/i4l/isdn.ko
lsmod
To see a list of currently loaded modules, use lsmod. You see the name of each loaded
module, the size, the use count, and the names of other modules using this one.
[root@RHEL52 ~]# lsmod | head -5
Module Size Used by
autofs4 24517 2
hidp 23105 2
rfcomm 42457 0
l2cap 29505 10 hidp,rfcomm
/proc/modules
/proc/modules lists all modules loaded by the kernel. The output would be too long
to display here, so lets grep for the vm module.
We see that vmmon and vmnet are both loaded. You can display the same information
with lsmod. Actually lsmod only reads and reformats the output of /proc/modules.
paul@laika:~$ cat /proc/modules | grep vm
vmnet 36896 13 - Live 0xffffffff88b21000 (P)
vmmon 194540 0 - Live 0xffffffff88af0000 (P)
paul@laika:~$ lsmod | grep vm
vmnet 36896 13
vmmon 194540 0
paul@laika:~$
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![the Linux kernel
module dependencies
Some modules depend on others. In the following example, you can see that the nfsd
module is used by exportfs, lockd and sunrpc.
paul@laika:~$ cat /proc/modules | grep nfsd
nfsd 267432 17 - Live 0xffffffff88a40000
exportfs 7808 1 nfsd, Live 0xffffffff88a3d000
lockd 73520 3 nfs,nfsd, Live 0xffffffff88a2a000
sunrpc 185032 12 nfs,nfsd,lockd, Live 0xffffffff889fb000
paul@laika:~$ lsmod | grep nfsd
nfsd 267432 17
exportfs 7808 1 nfsd
lockd 73520 3 nfs,nfsd
sunrpc 185032 12 nfs,nfsd,lockd
paul@laika:~$
insmod
Kernel modules can be manually loaded with the insmod command. This is a very
simple (and obsolete) way of loading modules. The screenshot shows insmod loading
the fat module (for fat file system support).
root@barry:/lib/modules/2.6.17-2-686# pwd
/lib/modules/2.6.17-2-686
root@barry:/lib/modules/2.6.17-2-686# lsmod | grep fat
root@barry:/lib/modules/2.6.17-2-686# insmod kernel/fs/fat/fat.ko
root@barry:/lib/modules/2.6.17-2-686# lsmod | grep fat
fat 46588 0
insmod is not detecting dependencies, so it fails to load the isdn module (because the
isdn module depends on the slhc module).
[root@RHEL52 drivers]# pwd
/lib/modules/2.6.18-92.1.18.el5/kernel/drivers
[root@RHEL52 kernel]# insmod isdn/i4l/isdn.ko
insmod: error inserting 'isdn/i4l/isdn.ko': -1 Unknown symbol in module
modinfo
As you can see in the screenshot of modinfo below, the isdn module depends in the
slhc module.
[root@RHEL52 drivers]# modinfo isdn/i4l/isdn.ko | head -6
filename: isdn/i4l/isdn.ko
license: GPL
author: Fritz Elfert
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description: ISDN4Linux: link layer
srcversion: 99650346E708173496F6739
depends: slhc
modprobe
The big advantage of modprobe over insmod is that modprobe will load all
necessary modules, whereas insmod requires manual loading of dependencies.
Another advantage is that you don't need to point to the filename with full path.
This screenshot shows how modprobe loads the isdn module, automatically loading
slhc in background.
[root@RHEL52 kernel]# lsmod | grep isdn
[root@RHEL52 kernel]# modprobe isdn
[root@RHEL52 kernel]# lsmod | grep isdn
isdn 122433 0
slhc 10561 1 isdn
[root@RHEL52 kernel]#
/lib/modules/<kernel>/modules.dep
Module dependencies are stored in modules.dep.
[root@RHEL52 2.6.18-92.1.18.el5]# pwd
/lib/modules/2.6.18-92.1.18.el5
[root@RHEL52 2.6.18-92.1.18.el5]# head -3 modules.dep
/lib/modules/2.6.18-92.1.18.el5/kernel/drivers/net/tokenring/3c359.ko:
/lib/modules/2.6.18-92.1.18.el5/kernel/drivers/net/pcmcia/3c574_cs.ko:
/lib/modules/2.6.18-92.1.18.el5/kernel/drivers/net/pcmcia/3c589_cs.ko:
depmod
The modules.dep file can be updated (recreated) with the depmod command. In this
screenshot no modules were added, so depmod generates the same file.
root@barry:/lib/modules/2.6.17-2-686# ls -l modules.dep
-rw-r--r-- 1 root root 310676 2008-03-01 16:32 modules.dep
root@barry:/lib/modules/2.6.17-2-686# depmod
root@barry:/lib/modules/2.6.17-2-686# ls -l modules.dep
-rw-r--r-- 1 root root 310676 2008-12-07 13:54 modules.dep
rmmod
Similar to insmod, the rmmod command is rarely used anymore.
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[root@RHELv4u3 ~]# modprobe isdn
[root@RHELv4u3 ~]# rmmod slhc
ERROR: Module slhc is in use by isdn
[root@RHELv4u3 ~]# rmmod isdn
[root@RHELv4u3 ~]# rmmod slhc
[root@RHELv4u3 ~]# lsmod | grep isdn
[root@RHELv4u3 ~]#
modprobe -r
Contrary to rmmod, modprobe will automatically remove unneeded modules.
[root@RHELv4u3 ~]# modprobe isdn
[root@RHELv4u3 ~]# lsmod | grep isdn
isdn 133537 0
slhc 7233 1 isdn
[root@RHELv4u3 ~]# modprobe -r isdn
[root@RHELv4u3 ~]# lsmod | grep isdn
[root@RHELv4u3 ~]# lsmod | grep slhc
[root@RHELv4u3 ~]#
/etc/modprobe.conf
The /etc/modprobe.conf file and the /etc/modprobe.d directory can contain aliases
(used by humans) and options (for dependent modules) for modprobe.
[root@RHEL52 ~]# cat /etc/modprobe.conf
alias scsi_hostadapter mptbase
alias scsi_hostadapter1 mptspi
alias scsi_hostadapter2 ata_piix
alias eth0 pcnet32
alias eth2 pcnet32
alias eth1 pcnet32
50.5. compiling a kernel
extraversion
Enter into /usr/src/redhat/BUILD/kernel-2.6.9/linux-2.6.9/ and change the
extraversion in the Makefile.
[root@RHEL52 linux-2.6.18.i686]# pwd
/usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686
[root@RHEL52 linux-2.6.18.i686]# vi Makefile
[root@RHEL52 linux-2.6.18.i686]# head -4 Makefile
VERSION = 2
PATCHLEVEL = 6
SUBLEVEL = 18
EXTRAVERSION = -paul2008
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make mrproper
Now clean up the source from any previous installs with make mrproper. If this is
your first after downloading the source code, then this is not needed.
[root@RHEL52 linux-2.6.18.i686]# make mrproper
CLEAN scripts/basic
CLEAN scripts/kconfig
CLEAN include/config
CLEAN .config .config.old
.config
Now copy a working .config from /boot to our kernel directory. This file contains the
configuration that was used for your current working kernel. It determines whether
modules are included in compilation or not.
[root@RHEL52 linux-2.6.18.i686]# cp /boot/config-2.6.18-92.1.18.el5 .config
make menuconfig
Now run make menuconfig (or the graphical make xconfig). This tool allows you
to select whether to compile stuff as a module (m), as part of the kernel (*), or not
at all (smaller kernel size). If you remove too much, your kernel will not work. The
configuration will be stored in the hidden .config file.
[root@RHEL52 linux-2.6.18.i686]# make menuconfig
make clean
Issue a make clean to prepare the kernel for compile. make clean will remove most
generated files, but keeps your kernel configuration. Running a make mrproper at
this point would destroy the .config file that you built with make menuconfig.
[root@RHEL52 linux-2.6.18.i686]# make clean
make bzImage
And then run make bzImage, sit back and relax while the kernel compiles. You can
use time make bzImage to know how long it takes to compile, so next time you can
go for a short walk.
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[root@RHEL52 linux-2.6.18.i686]# time make bzImage
HOSTCC scripts/basic/fixdep
HOSTCC scripts/basic/docproc
HOSTCC scripts/kconfig/conf.o
HOSTCC scripts/kconfig/kxgettext.o
...
This command will end with telling you the location of the bzImage file (and with
time info if you also specified the time command.
Kernel: arch/i386/boot/bzImage is ready (#1)
real 13m59.573s
user 1m22.631s
sys 11m51.034s
[root@RHEL52 linux-2.6.18.i686]#
You can already copy this image to /boot with cp arch/i386/boot/bzImage /boot/
vmlinuz-<kernel-version>.
make modules
Now run make modules. It can take 20 to 50 minutes to compile all the modules.
[root@RHEL52 linux-2.6.18.i686]# time make modules
CHK include/linux/version.h
CHK include/linux/utsrelease.h
CC [M] arch/i386/kernel/msr.o
CC [M] arch/i386/kernel/cpuid.o
CC [M] arch/i386/kernel/microcode.o
make modules_install
To copy all the compiled modules to /lib/modules just run make modules_install
(takes about 20 seconds). Here's a screenshot from before the command.
[root@RHEL52 linux-2.6.18.i686]# ls -l /lib/modules/
total 20
drwxr-xr-x 6 root root 4096 Oct 15 13:09 2.6.18-92.1.13.el5
drwxr-xr-x 6 root root 4096 Nov 11 08:51 2.6.18-92.1.17.el5
drwxr-xr-x 6 root root 4096 Dec 6 07:11 2.6.18-92.1.18.el5
[root@RHEL52 linux-2.6.18.i686]# make modules_install
And here is the same directory after. Notice that make modules_install created a
new directory for the new kernel.
[root@RHEL52 linux-2.6.18.i686]# ls -l /lib/modules/
total 24
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drwxr-xr-x 6 root root 4096 Oct 15 13:09 2.6.18-92.1.13.el5
drwxr-xr-x 6 root root 4096 Nov 11 08:51 2.6.18-92.1.17.el5
drwxr-xr-x 6 root root 4096 Dec 6 07:11 2.6.18-92.1.18.el5
drwxr-xr-x 3 root root 4096 Dec 6 08:50 2.6.18-paul2008
/boot
We still need to copy the kernel, the System.map and our configuration file to /boot.
Strictly speaking the .config file is not obligatory, but it might help you in future
compilations of the kernel.
[root@RHEL52 ]# pwd
/usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686
[root@RHEL52 ]# cp System.map /boot/System.map-2.6.18-paul2008
[root@RHEL52 ]# cp .config /boot/config-2.6.18-paul2008
[root@RHEL52 ]# cp arch/i386/boot/bzImage /boot/vmlinuz-2.6.18-paul2008
mkinitrd
The kernel often uses an initrd file at bootup. We can use mkinitrd to generate this
file. Make sure you use the correct kernel name!
[root@RHEL52 ]# pwd
/usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686
[root@RHEL52 ]# mkinitrd /boot/initrd-2.6.18-paul2008 2.6.18-paul2008
bootloader
Compilation is now finished, don't forget to create an additional stanza in grub or lilo.
50.6. compiling one module
hello.c
A little C program that will be our module.
[root@rhel4a kernel_module]# cat hello.c
#include <linux/module.h>
#include <section>
int init_module(void)
{
printk(KERN_INFO "Start Hello World...n");
return 0;
}
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void cleanup_module(void)
{
printk(KERN_INFO "End Hello World... n");
}
Makefile
The make file for this module.
[root@rhel4a kernel_module]# cat Makefile
obj-m += hello.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
These are the only two files needed.
[root@rhel4a kernel_module]# ll
total 16
-rw-rw-r-- 1 paul paul 250 Feb 15 19:14 hello.c
-rw-rw-r-- 1 paul paul 153 Feb 15 19:15 Makefile
make
The running of the make command.
[root@rhel4a kernel_module]# make
make -C /lib/modules/2.6.9-paul-2/build M=~/kernel_module modules
make[1]: Entering dir... `/usr/src/redhat/BUILD/kernel-2.6.9/linux-2.6.9'
CC [M] /home/paul/kernel_module/hello.o
Building modules, stage 2.
MODPOST
CC /home/paul/kernel_module/hello.mod.o
LD [M] /home/paul/kernel_module/hello.ko
make[1]: Leaving dir... `/usr/src/redhat/BUILD/kernel-2.6.9/linux-2.6.9'
[root@rhel4a kernel_module]#
Now we have more files.
[root@rhel4a kernel_module]# ll
total 172
-rw-rw-r-- 1 paul paul 250 Feb 15 19:14 hello.c
-rw-r--r-- 1 root root 64475 Feb 15 19:15 hello.ko
-rw-r--r-- 1 root root 632 Feb 15 19:15 hello.mod.c
-rw-r--r-- 1 root root 37036 Feb 15 19:15 hello.mod.o
-rw-r--r-- 1 root root 28396 Feb 15 19:15 hello.o
-rw-rw-r-- 1 paul paul 153 Feb 15 19:15 Makefile
[root@rhel4a kernel_module]#
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hello.ko
Use modinfo to verify that it is really a module.
[root@rhel4a kernel_module]# modinfo hello.ko
filename: hello.ko
vermagic: 2.6.9-paul-2 SMP 686 REGPARM 4KSTACKS gcc-3.4
depends:
[root@rhel4a kernel_module]#
Good, so now we can load our hello module.
[root@rhel4a kernel_module]# lsmod | grep hello
[root@rhel4a kernel_module]# insmod ./hello.ko
[root@rhel4a kernel_module]# lsmod | grep hello
hello 5504 0
[root@rhel4a kernel_module]# tail -1 /var/log/messages
Feb 15 19:16:07 rhel4a kernel: Start Hello World...
[root@rhel4a kernel_module]# rmmod hello
[root@rhel4a kernel_module]#
Finally /var/log/messages has a little surprise.
[root@rhel4a kernel_module]# tail -2 /var/log/messages
Feb 15 19:16:07 rhel4a kernel: Start Hello World...
Feb 15 19:16:35 rhel4a kernel: End Hello World...
[root@rhel4a kernel_module]#
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53.44. verify ownership ...................................................................................... 522
53.45. common problems .................................................................................... 522
53.46. Practice : samba authentication ................................................................ 523
53.47. Solution: samba authentication ................................................................ 523
53.48. security based on user name .................................................................... 524
53.49. security based on ip-address .................................................................... 525
53.50. security through obscurity ........................................................................ 525
53.51. file system security .................................................................................. 526
53.52. Practice: securing shares .......................................................................... 527
53.53. Solution: securing shares ......................................................................... 528
53.54. changes in smb.conf ................................................................................. 529
53.55. joining an Active Directory domain ........................................................ 531
53.56. winbind ..................................................................................................... 531
53.57. wbinfo ...................................................................................................... 532
53.58. getent ........................................................................................................ 533
53.59. file ownership ........................................................................................... 533
53.60. Practice : samba domain member ............................................................ 534
53.61. About Domain Controllers ....................................................................... 534
53.62. About security modes .............................................................................. 535
53.63. About password backends ........................................................................ 535
53.64. [global] section in smb.conf .................................................................... 536
53.65. netlogon share .......................................................................................... 537
53.66. other [share] sections ............................................................................... 537
53.67. Users and Groups ..................................................................................... 538
53.68. tdbsam ...................................................................................................... 538
53.69. about computer accounts .......................................................................... 539
53.70. local or roaming profiles .......................................................................... 540
53.71. Groups in NTFS acls ............................................................................... 540
53.72. logon scripts ............................................................................................. 541
53.73. Practice ..................................................................................................... 541
53.74. Samba 4 alpha 6 ...................................................................................... 542
53.1. verify installed version
.rpm based distributions
To see the version of samba installed on RedHat, use rpm -q samba.
[root@RHEL52 ~]# rpm -q samba
samba-3.0.28-1.el5_2.1
The screenshots above and below this paragraph show that both RHEL4 and RHEL5
have Samba version 3.0 installed. The last number in the Samba version counts the
number of updates or patches. 3.0.28 in RHEL5 has a lot more updates than 3.0.10
in RHEL4.
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[paul@RHEL4b ~]$ rpm -q samba
samba-3.0.10-1.4E.9
.deb based distributions
Use dpkg -l on Debian or Ubuntu. Ubuntu 7.04 below uses Samba 3.0.24, but the
Samba server is not installed, only the common files are installed (for mounting smb
file systems).
paul@laika:~$ dpkg -l | grep samba
ii samba-common 3.0.24-2ubuntu1.2 Samba common files used by both...
Ubuntu 9.04 below uses Samba 3.3.2, and both the Samba server and the common
files are installed.
paul@laika:~$ dpkg -l | grep samba
ii samba 2:3.3.2-1ubuntu3 SMB/CIFS file, print, and login server
ii samba-common 2:3.3.2-1ubuntu3 common files used by both the Samba ser
53.2. installing samba
.rpm based distributions
Samba is installed by default on Red Hat Enterprise Linux. If Samba is not yet
installed, then you can use the graphical menu (Applications -- System Settings --
Add/Remove Applications) and select "Windows File Server" in the Server section.
The non-graphical way is to use rpm or yum.
When you downloaded the .rpm file, you can install Samba like this.
[paul@RHEL52 ~]$ rpm -i samba-3.0.28-1.el5_2.1.rpm
When you have a subscription to RHN (Red Hat Network), then yum is an easy tool
to use. This command also works on Fedora.
[paul@RHEL52 ~]$ yum install samba
.deb based distributions
Ubuntu and Debian users can use the aptitude program (or use a graphical tool like
Synaptic).
paul@laika:~$ aptitude install samba-server
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53.3. documentation
samba howto
Samba comes with excellent documentation in html and pdf format (and also as a free
download from Samba.org and it is for sale as a printed book). Red Hat Enterprise
Linux installs the html and pdf version in /usr/share/doc by default.
[paul@RHEL4b ~]$ locate Samba-HOWTO-Collection.pdf
/usr/share/doc/samba-3.0.10/Samba-HOWTO-Collection.pdf
Ubuntu packages the docs as a separate package from Samba.
root@laika:~# aptitude search samba | grep -i documentation
i samba-doc - Samba documentation
i samba-doc-pdf - Samba documentation (PDF format)
root@laika:~# find /usr/share/doc/samba-doc-pdf | grep -i howto
/usr/share/doc/samba-doc-pdf/Samba3-HOWTO.pdf.gz
samba by example
Besides the howto, there is also an excellent book called Samba By Example (again
available as printed edition in shops, and as a free pdf and html).
53.4. samba daemons
Samba 3 consists of three daemons, they are named nmbd, smbd and winbindd.
nmbd
The nmbd daemon takes care of all the names and naming. It registers and resolves
names, and handles browsing. According to the Samba documentation, it should be
the first daemon to start.
[root@RHEL52 ~]# ps -C nmbd
PID TTY TIME CMD
5681 ? 00:00:00 nmbd
smbd
The smbd daemon manages file transfers and authentication.
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[root@RHEL52 ~]# ps -C smbd
PID TTY TIME CMD
5678 ? 00:00:00 smbd
5683 ? 00:00:00 smbd
winbindd
The winbind daemon (winbindd) is only started to handle Microsoft Windows
domain membership.
Note that winbindd is started by the /etc/init.d/winbind script (two dd's for the
daemon and only one d for the script).
[root@RHEL52 ~]# /etc/init.d/winbind start
Starting Winbind services: [ OK ]
[root@RHEL52 ~]# ps -C winbindd
PID TTY TIME CMD
5752 ? 00:00:00 winbindd
5754 ? 00:00:00 winbindd
On Debian and Ubuntu, the winbindd daemon is installed via a seperate package
called winbind.
53.5. starting and stopping samba
You can start the daemons by invoking /etc/init.d/smb start (some systems use /etc/
init.d/samba) on any linux.
root@laika:~# /etc/init.d/samba stop
* Stopping Samba daemons [ OK ]
root@laika:~# /etc/init.d/samba start
* Starting Samba daemons [ OK ]
root@laika:~# /etc/init.d/samba restart
* Stopping Samba daemons [ OK ]
* Starting Samba daemons [ OK ]
root@laika:~# /etc/init.d/samba status
* SMBD is running [ OK ]
Red Hat derived systems are happy with service smb start.
[root@RHEL4b ~]# /etc/init.d/smb start
Starting SMB services: [ OK ]
Starting NMB services: [ OK ]
[root@RHEL4b ~]# service smb restart
Shutting down SMB services: [ OK ]
Shutting down NMB services: [ OK ]
Starting SMB services: [ OK ]
Starting NMB services: [ OK ]
[root@RHEL4b ~]#
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3. Verify networking by pinging the computer, edit the appropriate hosts files so you
can use names. Test the names by pinging them.
4. Make sure Samba is installed, write down the version of Samba.
5. Open the Official Samba-3 howto pdf file that is installed on your computer. How
many A4 pages is this file ? Then look at the same pdf on samba.org, it is updated
regularly.
6. Stop the Samba server.
53.8. /etc/samba/smb.conf
smbd -b
Samba configuration is done in the smb.conf file. The file can be edited manually,
or you can use a web based interface like webmin or swat to manage it. The file is
usually located in /etc/samba. You can find the exact location with smbd -b.
[root@RHEL4b ~]# smbd -b | grep CONFIGFILE
CONFIGFILE: /etc/samba/smb.conf
the default smb.conf
The default smb.conf file contains a lot of examples with explanations.
[paul@RHEL4b ~]$ ls -l /etc/samba/smb.conf
-rw-r--r-- 1 root root 10836 May 30 23:08 /etc/samba/smb.conf
Also on Ubuntu and Debian, smb.conf is packed with samples and explanations.
paul@laika:~$ ls -l /etc/samba/smb.conf
-rw-r--r-- 1 root root 10515 2007-05-24 00:21 /etc/samba/smb.conf
minimal smb.conf
Below is an example of a very minimalistic smb.conf. It allows samba to start, and to
be visible to other computers (Microsoft shows computers in Network Neighborhood
or My Network Places).
[paul@RHEL4b ~]$ cat /etc/samba/smb.conf
[global]
workgroup = WORKGROUP
[firstshare]
path = /srv/samba/public
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net view
Below is a screenshot of the net view command on Microsoft Windows Server 2003
sp2. It shows how a Red Hat Enterprise Linux 5.3 and a Ubuntu 9.04 Samba server,
both with a minimalistic smb.conf, are visible to Microsoft computers nearby.
C:Documents and SettingsAdministrator>net view
Server Name Remark
----------------------------------------------------------------------
LAIKA Samba 3.3.2
RHEL53 Samba 3.0.33-3.7.el5
W2003
The command completed successfully.
long lines in smb.conf
Some parameters in smb.conf can get a long list of values behind them. You can
continue a line (for clarity) on the next by ending the line with a backslash.
valid users = Serena, Venus, Lindsay
Kim, Justine, Sabine
Amelie, Marie, Suzanne
curious smb.conf
Curious but true: smb.conf accepts synonyms like create mode and create mask, and
(sometimes) minor spelling errors like browsable and browseable. And on occasion
you can even switch words, the guest only parameter is identical to only guest. And
writable = yes is the same as readonly = no.
man smb.conf
You can access a lot of documentation when typing man smb.conf.
[root@RHEL4b samba]# apropos samba
cupsaddsmb (8) - export printers to samba for windows clients
lmhosts (5) - The Samba NetBIOS hosts file
net (8) - Tool for administration of Samba and remote CIFS servers
pdbedit (8) - manage the SAM database (Database of Samba Users)
samba (7) - A Windows SMB/CIFS fileserver for UNIX
smb.conf [smb] (5) - The configuration file for the Samba suite
smbpasswd (5) - The Samba encrypted password file
smbstatus (1) - report on current Samba connections
swat (8) - Samba Web Administration Tool
tdbbackup (8) - tool for backing up and ... of samba .tdb files
[root@RHEL4b samba]#
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53.9. /usr/bin/testparm
syntax check smb.conf
To verify the syntax of the smb.conf file, you can use testparm.
[paul@RHEL4b ~]$ testparm
Load smb config files from /etc/samba/smb.conf
Processing section "[firstshare]"
Loaded services file OK.
Server role: ROLE_STANDALONE
Press enter to see a dump of your service definitions
testparm -v
An interesting option is testparm -v, which will output all the global options with
their default value.
[root@RHEL52 ~]# testparm -v | head
Load smb config files from /etc/samba/smb.conf
Processing section "[pub0]"
Processing section "[global$]"
Loaded services file OK.
Server role: ROLE_STANDALONE
Press enter to see a dump of your service definitions
[global]
dos charset = CP850
unix charset = UTF-8
display charset = LOCALE
workgroup = WORKGROUP
realm =
netbios name = TEACHER0
netbios aliases =
netbios scope =
server string = Samba 3.0.28-1.el5_2.1
...
There are about 350 default values for smb.conf parameters.
testparm -s
The samba daemons are constantly (once every 60 seconds) checking the smb.conf
file, so it is good practice to keep this file small. But it is also good practice to
document your samba configuration, and to explicitly set options that have the same
default values. The testparm -s option allows you to do both. It will output the
smallest possible samba configuration file, while retaining all your settings. The
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idea is to have your samba configuration in another file (like smb.conf.full) and
let testparm parse this for you. The screenshot below shows you how. First the
smb.conf.full file with the explicitly set option workgroup to WORKGROUP.
[root@RHEL4b samba]# cat smb.conf.full
[global]
workgroup = WORKGROUP
# This is a demo of a documented smb.conf
# These two lines are removed by testparm -s
server string = Public Test Server
[firstshare]
path = /srv/samba/public
Next, we execute testparm with the -s option, and redirect stdout to the real smb.conf
file.
[root@RHEL4b samba]# testparm -s smb.conf.full > smb.conf
Load smb config files from smb.conf.full
Processing section "[firstshare]"
Loaded services file OK.
And below is the end result. The two comment lines and the default option are no
longer there.
[root@RHEL4b samba]# cat smb.conf
# Global parameters
[global]
server string = Public Test Server
[firstshare]
path = /srv/samba/public
[root@RHEL4b samba]#
53.10. /usr/bin/smbclient
smbclient looking at Samba
With smbclient you can see browsing and share information from your smb server.
It will display all your shares, your workgroup, and the name of the Master Browser.
The -N switch is added to avoid having to enter an empty password. The -L switch
is followed by the name of the host to check.
[root@RHEL4b init.d]# smbclient -NL rhel4b
Anonymous login successful
Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.10-1.4E.9]
Sharename Type Comment
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--------- ---- -------
firstshare Disk
IPC$ IPC IPC Service (Public Test Server)
ADMIN$ IPC IPC Service (Public Test Server)
Anonymous login successful
Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.10-1.4E.9]
Server Comment
--------- -------
RHEL4B Public Test Server
WINXP
Workgroup Master
--------- -------
WORKGROUP WINXP
smbclient anonymous
The screenshot below uses smbclient to display information about a remote smb
server (in this case a Windows xp machine).
[root@RHEL4b init.d]# smbclient -NL winxp
Anonymous login successful
Domain=[WORKGROUP] OS=[Windows 5.1] Server=[Windows 2000 LAN Manager]
Sharename Type Comment
--------- ---- -------
Error returning browse list: NT_STATUS_ACCESS_DENIED
Anonymous login successful
Domain=[WORKGROUP] OS=[Windows 5.1] Server=[Windows 2000 LAN Manager]
Server Comment
--------- -------
RHEL4B Public Test Server
W2000
WINXP
Workgroup Master
--------- -------
WORKGROUP WINXP
smbclient with credentials
Windows versions after xp sp2 and 2003 sp1 do not accept guest access (the
NT_STATUS_ACCESS_DENIED error). This example shows how to provide
credentials with smbclient.
[paul@RHEL53 ~]$ smbclient -L w2003 -U administrator%stargate
Domain=[W2003] OS=[Windows Server 2003 3790 Service Pack 2] Server=...
Sharename Type Comment
--------- ---- -------
C$ Disk Default share
IPC$ IPC Remote IPC
ADMIN$ Disk Remote Admin
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...
53.11. /usr/bin/smbtree
Another useful tool to troubleshoot Samba or simply to browse the SMB network is
smbtree. In its simplest form, smbtree will do an anonymous browsing on the local
subnet. displaying all SMB computers and (if authorized) their shares.
Let's take a look at two screenshots of smbtree in action (with blank password).
The first one is taken immediately after booting four different computers (one MS
Windows 2000, one MS Windows xp, one MS Windows 2003 and one RHEL 4 with
Samba 3.0.10).
[paul@RHEL4b ~]$ smbtree
Password:
WORKGROUP
PEGASUS
WINXP
RHEL4B Pegasus Domain Member Server
Error connecting to 127.0.0.1 (Connection refused)
cli_full_connection: failed to connect to RHEL4B<20> (127.0.0.1)
HM2003
[paul@RHEL4b ~]$
The information displayed in the previous screenshot looks incomplete. The browsing
elections are still ongoing, the browse list is not yet distributed to all clients by the (to
be elected) browser master. The next screenshot was taken about one minute later.
And it shows even less.
[paul@RHEL4b ~]$ smbtree
Password:
WORKGROUP
W2000
[paul@RHEL4b ~]$
So we wait a while, and then run smbtree again, this time it looks a lot nicer.
[paul@RHEL4b ~]$ smbtree
Password:
WORKGROUP
W2000
PEGASUS
WINXP
RHEL4B Pegasus Domain Member Server
RHEL4BADMIN$ IPC Service (Pegasus Domain Member Server)
RHEL4BIPC$ IPC Service (Pegasus Domain Member Server)
RHEL4Bdomaindata Active Directory users only
HM2003
[paul@RHEL4b ~]$ smbtree --version
Version 3.0.10-1.4E.9
[paul@RHEL4b ~]$
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I added the version number of smbtree in the previous screenshot, to show you
the difference when using the latest version of smbtree (below a screenshot taken
from Ubuntu Feisty Fawn). The latest version shows a more complete overview of
machines and shares.
paul@laika:~$ smbtree --version
Version 3.0.24
paul@laika:~$ smbtree
Password:
WORKGROUP
W2000
W2000firstshare
W2000C$ Default share
W2000ADMIN$ Remote Admin
W2000IPC$ Remote IPC
PEGASUS
WINXP
cli_rpc_pipe_open: cli_nt_create failed on pipe srvsvc to machine WINXP.
Error was NT_STATUS_ACCESS_DENIED
RHEL4B Pegasus Domain Member Server
RHEL4BADMIN$ IPC Service (Pegasus Domain Member Server)
RHEL4BIPC$ IPC Service (Pegasus Domain Member Server)
RHEL4Bdomaindata Active Directory users only
HM2003
cli_rpc_pipe_open: cli_nt_create failed on pipe srvsvc to machine HM2003.
Error was NT_STATUS_ACCESS_DENIED
paul@laika:~$
The previous screenshot also provides useful errors on why we cannot see shared
info on computers winxp and w2003. Let us try the old smbtree version on our
RHEL server, but this time with Administrator credentials (which are the same on
all computers).
[paul@RHEL4b ~]$ smbtree -UAdministrator%Stargate1
WORKGROUP
W2000
PEGASUS
WINXP
WINXPC$ Default share
WINXPADMIN$ Remote Admin
WINXPshare55
WINXPIPC$ Remote IPC
RHEL4B Pegasus Domain Member Server
RHEL4BADMIN$ IPC Service (Pegasus Domain Member Server)
RHEL4BIPC$ IPC Service (Pegasus Domain Member Server)
RHEL4Bdomaindata Active Directory users only
HM2003
HM2003NETLOGON Logon server share
HM2003SYSVOL Logon server share
HM2003WSUSTemp A network share used by Local Publishing ...
HM2003ADMIN$ Remote Admin
HM2003tools
HM2003IPC$ Remote IPC
HM2003WsusContent A network share to be used by Local ...
HM2003C$ Default share
[paul@RHEL4b ~]$
As you can see, this gives a very nice overview of all SMB computers and their shares.
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53.12. server string
The comment seen by the net view and the smbclient commands is the default
value for the server string option. Simply adding this value to the global section in
smb.conf and restarting samba will change the option.
[root@RHEL53 samba]# testparm -s 2>/dev/null | grep server
server string = Red Hat Server in Paris
After a short while, the changed option is visible on the Microsoft computers.
C:Documents and SettingsAdministrator>net view
Server Name Remark
-------------------------------------------------------------------------------
LAIKA Ubuntu 9.04 server in Antwerp
RHEL53 Red Hat Server in Paris
W2003
53.13. Samba Web Administration Tool
(SWAT)
Samba comes with a web based tool to manage your samba configuration file. SWAT
is accessible with a web browser on port 901 of the host system. To enable the tool,
first find out whether your system is using the inetd or the xinetd superdaemon.
[root@RHEL4b samba]# ps fax | grep inet
15026 pts/0 S+ 0:00 _ grep inet
2771 ? Ss 0:00 xinetd -stayalive -pidfile /var/run/xinetd.pid
[root@RHEL4b samba]#
Then edit the inetd.conf or change the disable = yes line in /etc/xinetd.d/swat to
disable = no.
[root@RHEL4b samba]# cat /etc/xinetd.d/swat
# default: off
# description: SWAT is the Samba Web Admin Tool. Use swat
# to configure your Samba server. To use SWAT,
# connect to port 901 with your favorite web browser.
service swat
{
port = 901
socket_type = stream
wait = no
only_from = 127.0.0.1
user = root
server = /usr/sbin/swat
log_on_failure += USERID
disable = no
}
[root@RHEL4b samba]# /etc/init.d/xinetd restart
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Stopping xinetd: [ OK ]
Starting xinetd: [ OK ]
[root@RHEL4b samba]#
Change the only from value to enable swat from remote computers. This examples
shows how to provide swat access to all computers in a /24 subnet.
[root@RHEL53 xinetd.d]# grep only /etc/xinetd.d/swat
only_from = 192.168.1.0/24
Be careful when using SWAT, it erases all your manually edited comments in
smb.conf.
53.14. Practice: getting started with samba
1. Take a backup copy of the original smb.conf, name it smb.conf.orig
2. Enable SWAT and take a look at it.
3. Stop the Samba server.
4. Create a minimalistic smb.conf.minimal and test it with testparm.
5. Use tesparm -s to create /etc/samba/smb.conf from your smb.conf.minimal .
6. Start Samba with your minimal smb.conf.
7. Verify with smbclient that your Samba server works.
8. Verify that another (Microsoft) computer can see your Samba server.
9. Browse the network with net view, smbtree and with Windows Explorer.
10. Change the "Server String" parameter in smb.conf. How long does it take before
you see the change (net view, smbclient, My Network Places,...) ?
11. Will restarting Samba after a change to smb.conf speed up the change ?
12. Which computer is the master browser master in your workgroup ? What is the
master browser ?
13. If time permits (or if you are waiting for other students to finish this practice),
then install a sniffer (wireshark) and watch the browser elections.
53.15. Solution: getting started with samba
1. Take a backup copy of the original smb.conf, name it smb.conf.orig
cd /etc/samba ; cp smb.conf smb.conf.orig
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53.16. Setting up a directory to share
Let's start with setting up a very simple read only file server with Samba. Everyone
(even anonymous guests) will receive read access.
The first step is to create a directory and put some test files in it.
[root@RHEL52 ~]# mkdir -p /srv/samba/readonly
[root@RHEL52 ~]# cd /srv/samba/readonly/
[root@RHEL52 readonly]# echo "It is cold today." > winter.txt
[root@RHEL52 readonly]# echo "It is hot today." > summer.txt
[root@RHEL52 readonly]# ls -l
total 8
-rw-r--r-- 1 root root 17 Jan 21 05:49 summer.txt
-rw-r--r-- 1 root root 18 Jan 21 05:49 winter.txt
[root@RHEL52 readonly]#
53.17. configure the share
smb.conf [global] section
In this example the samba server is a member of WORKGROUP (the default
workgroup). We also set a descriptive server string, this string is visible to users
browsing the network with net view, windows explorer or smbclient.
[root@RHEL52 samba]# head -5 smb.conf
[global]
workgroup = WORKGROUP
server string = Public Anonymous File Server
netbios name = TEACHER0
security = share
You might have noticed the line with security = share. This line sets the default
security mode for our samba server. Setting the security mode to share will allow
clients (smbclient, any windows, another Samba server, ...) to provide a password for
each share. This is one way of using the SMB/CIFS protocol. The other way (called
user mode) will allow the client to provide a username/password combination, before
the server knows which share the client wants to access.
smb.conf [share] section
The share is called pubread and the path is set to our newly created directory.
Everyone is allowed access (guest ok = yes) and security is set to read only.
[pubread]
path = /srv/samba/readonly
comment = files to read
read only = yes
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guest ok = yes
53.18. restart the server
After testing with testparm, restart the samba server (so you don't have to wait).
[root@RHEL4b readonly]# service smb restart
Shutting down SMB services: [ OK ]
Shutting down NMB services: [ OK ]
Starting SMB services: [ OK ]
Starting NMB services: [ OK ]
53.19. verify the share
verify with smbclient
You can now verify the existence of the share with smbclient. Our pubread is listed
as the fourth share.
[root@RHEL52 samba]# smbclient -NL 127.0.0.1
Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.33-3.7.el5]
Sharename Type Comment
--------- ---- -------
IPC$ IPC IPC Service (Public Anonymous File Server)
global$ Disk
pub0 Disk
pubread Disk files to read
Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.33-3.7.el5]
Server Comment
--------- -------
TEACHER0 Samba 3.0.33-3.7.el5
W2003EE
Workgroup Master
--------- -------
WORKGROUP W2003EE
verify on windows
The final test is to go to a Microsoft windows computer and read a file on the
Samba server. First we use the net use command to mount the pubread share on the
driveletter k.
C:>net use K: teacher0pubread
The command completed successfully.
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The way this works, is by enabling netcat to listen on the windows computer to a
certain port, executing cmd.exe when a connection is received. Netcat is similar to
cat, in the way that cat does nothing, only netcat does nothing over the network.
To enable this connection, type the following on the windows computer (after
downloading netcat for windows).
nc -l -p 23 -t -e cmd.exe
And then connect to this machine with netcat from any Linux computer. You end up
with a cmd.exe prompt inside your Linux shell.
paul@laika:~$ nc 192.168.1.38 23
Microsoft Windows [Version 5.2.3790]
(C) Copyright 1985-2003 Microsoft Corp.
C:>net use k: /delete
net use k: /delete
k: was deleted successfully.
53.21. Practice: read only file server
1. Create a directory in a good location (FHS) to share files for everyone to read.
2. Make sure the directory is owned properly and is world accessible.
3. Put a textfile in this directory.
4. Share the directory with Samba.
5. Verify from your own and from another computer (smbclient, net use, ...) that the
share is accessible for reading.
6. Make a backup copy of your smb.conf, name it smb.conf.ReadOnlyFileServer.
53.22. Solution: read only file server
1. Create a directory in a good location (FHS) to share files for everyone to read.
choose one of these...
mkdir -p /srv/samba/readonly
mkdir -p /home/samba/readonly
/home/paul/readonly is wrong!!
/etc/samba/readonly is wrong!!
/readonly is wrong!!
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2. Make sure the directory is owned properly and is world accessible.
chown root:root /srv/samba/readonly
chmod 755 /srv/samba/readonly
3. Put a textfile in this directory.
echo Hello World > hello.txt
4. Share the directory with Samba.
You smb.conf.readonly could look like this:
[global]
workgroup = WORKGROUP
server string = Read Only File Server
netbios name = STUDENTx
security = share
[readonlyX]
path = /srv/samba/readonly
comment = read only file share
read only = yes
guest ok = yes
test with testparm before going in production!
5. Verify from your own and from another computer (smbclient, net use, ...) that the
share is accessible for reading.
On Linux: smbclient -NL 127.0.0.1
On Windows Explorer: browse to My Network Places
On Windows cmd.exe: net use L: //studentx/readonly
6. Make a backup copy of your smb.conf, name it smb.conf.ReadOnlyFileServer.
cp smb.conf smb.conf.ReadOnlyFileServer
53.23. set up a directory to share
In this second example, we will create a share where everyone can create files and
write to files. Again, we start by creating a directory
[root@RHEL52 samba]# mkdir -p /srv/samba/writable
[root@RHEL52 samba]# chmod 777 /srv/samba/writable/
53.24. share section in smb.conf
There are two parameters to make a share writable. We can use read only or writable.
This example shows how to use writable to give write access to a share.
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writable = yes
And this is an example of using the read only parameter to give write access to a
share.
read only = no
53.25. configure the share
Then we simply add a share to our file server by editing smb.conf. Below the check
with testparm. (We could have changed the description of the server...)
[root@RHEL52 samba]# testparm
Load smb config files from /etc/samba/smb.conf
Processing section "[pubwrite]"
Processing section "[pubread]"
Loaded services file OK.
Server role: ROLE_STANDALONE
Press enter to see a dump of your service definitions
[global]
netbios name = TEACHER0
server string = Public Anonymous File Server
security = SHARE
[pubwrite]
comment = files to write
path = /srv/samba/writable
read only = No
guest ok = Yes
[pubread]
comment = files to read
path = /srv/samba/readonly
guest ok = Yes
53.26. test connection with windows
We can now test the connection on a windows 2003 computer. We use the net use
for this.
C:>net use L: teacher0pubwrite
net use L: teacher0pubwrite
The command completed successfully.
53.27. test writing with windows
We mounted the pubwrite share on the L: drive in windows. Below we test that we
can write to this share.
L:>echo hoi > hoi.txt
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L:>dir
Volume in drive L is pubwrite
Volume Serial Number is 0C82-272A
Directory of L:
21/01/2009 06:11 <DIR> .
21/01/2009 06:11 <DIR> ..
21/01/2009 06:16 6 hoi.txt
1 File(s) 6 bytes
2 Dir(s) 13.496.238.080 bytes free
53.28. How is this possible ?
Linux (or any Unix) always needs a user account to gain access to a system. The
windows computer did not provide the samba server with a user account or a
password. Instead, the Linux owner of the files created through this writable share is
the Linux guest account (usually named nobody).
[root@RHEL52 samba]# ls -l /srv/samba/writable/
total 4
-rwxr--r-- 1 nobody nobody 6 Jan 21 06:16 hoi.txt
So this is not the cleanest solution. We will need to improve this.
53.29. Practice: writable file server
1. Create a directory and share it with Samba.
2. Make sure everyone can read and write files, test writing with smbclient and from
a Microsoft computer.
3. Verify the ownership of files created by (various) users.
53.30. Solution: writable file server
1. Create a directory and share it with Samba.
mkdir /srv/samba/writable
chmod 777 /srv/samba/writable
the share section in smb.conf can look like this:
[pubwrite]
path = /srv/samba/writable
comment = files to write
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read only = no
guest ok = yes
2. Make sure everyone can read and write files, test writing with smbclient and from
a Microsoft computer.
to test writing with smbclient:
echo one > count.txt
echo two >> count.txt
echo three >> count.txt
smbclient //localhost/pubwrite
Password:
smb: > put count.txt
3. Verify the ownership of files created by (various) users.
ls -l /srv/samba/writable
53.31. creating a samba user
We will create a user for our samba file server and make this user the owner of the
directory and all of its files. This anonymous user gets a clear description, but does
not get a login shell.
[root@RHEL52 samba]# useradd -s /bin/false sambanobody
[root@RHEL52 samba]# usermod -c "Anonymous Samba Access" sambanobody
[root@RHEL52 samba]# passwd sambanobody
Changing password for user sambanobody.
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully.
53.32. ownership of files
We can use this user as owner of files and directories, instead of using the root
account. This approach is clear and more secure.
[root@RHEL52 samba]# chown -R sambanobody:sambanobody /srv/samba/
[root@RHEL52 samba]# ls -al /srv/samba/writable/
total 12
drwxrwxrwx 2 sambanobody sambanobody 4096 Jan 21 06:11 .
drwxr-xr-x 6 sambanobody sambanobody 4096 Jan 21 06:11 ..
-rwxr--r-- 1 sambanobody sambanobody 6 Jan 21 06:16 hoi.txt
53.33. /usr/bin/smbpasswd
The sambanobody user account that we created in the previous examples is not yet
used by samba. It just owns the files and directories that we created for our shares.
The goal of this section is to force ownership of files created through the samba
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share to belong to our sambanobody user. Remember, our server is still accessible
to everyone, nobody needs to know this user account or password. We just want a
clean Linux server.
To accomplish this, we first have to tell Samba about this user. We can do this by
adding the account to smbpasswd.
[root@RHEL52 samba]# smbpasswd -a sambanobody
New SMB password:
Retype new SMB password:
Added user sambanobody.
53.34. /etc/samba/smbpasswd
To find out where Samba keeps this information (for now), use smbd -b. The
PRIVATE_DIR variable will show you where the smbpasswd database is located.
[root@RHEL52 samba]# smbd -b | grep PRIVATE
PRIVATE_DIR: /etc/samba
[root@RHEL52 samba]# ls -l smbpasswd
-rw------- 1 root root 110 Jan 21 06:19 smbpasswd
You can use a simple cat to see the contents of the smbpasswd database. The
sambanobody user does have a password (it is secret).
[root@RHEL52 samba]# cat smbpasswd
sambanobody:503:AE9 ... 9DB309C528E540978:[U ]:LCT-4976B05B:
53.35. forcing this user
Now that Samba knows about this user, we can adjust our writable share to force the
ownership of files created through it. For this we use the force user and force group
options. Now we can be sure that all files in the Samba writable share are owned by
the same sambanobody user.
Below is the renewed definition of our share in smb.conf.
[pubwrite]
path = /srv/samba/writable
comment = files to write
force user = sambanobody
force group = sambanobody
read only = no
guest ok = yes
When you reconnect to the share and write a file, then this sambanobody user will
own the newly created file (and nobody needs to know the password).
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53.36. Practice: first samba user account
1. Create a user account for use with samba.
2. Add this user to samba's user database.
3. Create a writable shared directory and use the "force user" and "force group"
directives to force ownership of files.
4. Test the working of force user with smbclient, net use and Windows Explorer.
53.37. Solution: first samba user account
1. Create a user account for use with samba.
useradd -s /bin/false smbguest
usermod -c 'samba guest'
passwd smbguest
2. Add this user to samba's user database.
smbpasswd -a smbguest
3. Create a writable shared directory and use the "force user" and "force group"
directives to force ownership of files.
[userwrite]
path = /srv/samba/userwrite
comment = everyone writes files owned by smbguest
read only = no
guest ok = yes
force user = smbguest
force group = smbguest
4. Test the working of force user with smbclient, net use and Windows Explorer.
ls -l /srv/samba/userwrite (and verify ownership)
53.38. creating the users on Linux
The goal of this example is to set up a file share accessible to a number of different
users. The users will need to authenticate with their password before access to this
share is granted. We will first create three randomly named users, each with their
own password. First we add these users to Linux.
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[root@RHEL52 ~]# useradd -c "Serena Williams" serena
[root@RHEL52 ~]# useradd -c "Justine Henin" justine
[root@RHEL52 ~]# useradd -c "Martina Hingis" martina
[root@RHEL52 ~]# passwd serena
Changing password for user serena.
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully.
[root@RHEL52 ~]# passwd justine
Changing password for user justine.
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully.
[root@RHEL52 ~]# passwd martina
Changing password for user martina.
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully.
53.39. creating the users on samba
Then we add them to the smbpasswd file, with the same password.
[root@RHEL52 ~]# smbpasswd -a serena
New SMB password:
Retype new SMB password:
Added user serena.
[root@RHEL52 ~]# smbpasswd -a justine
New SMB password:
Retype new SMB password:
Added user justine.
[root@RHEL52 ~]# smbpasswd -a martina
New SMB password:
Retype new SMB password:
Added user martina.
53.40. security = user
Remember that we set samba's security mode to share with the security = share
directive in the [global] section ? Since we now require users to always provide a
userid and password for access to our samba server, we will need to change this.
Setting security = user will require the client to provide samba with a valid userid
and password before giving access to a share.
Our [global] section now looks like this.
[global]
workgroup = WORKGROUP
netbios name = TEACHER0
server string = Samba File Server
security = user
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53.41. configuring the share
We add the following [share] section to our smb.conf (and we do not forget to create
the directory /srv/samba/authwrite).
[authwrite]
path = /srv/samba/authwrite
comment = authenticated users only
read only = no
guest ok = no
53.42. testing access with net use
After restarting samba, we test with different users from within Microsoft computers.
The screenshots use the net useFirst serena from Windows XP.
C:>net use m: teacher0authwrite stargate /user:serena
The command completed successfully.
C:>m:
M:>echo greetings from Serena > serena.txt
The next screenshot is martina on a Windows 2000 computer, she succeeds in writing
her files, but fails to overwrite the file from serena.
C:>net use k: teacher0authwrite stargate /user:martina
The command completed successfully.
C:>k:
K:>echo greetings from martina > Martina.txt
K:>echo test overwrite > serena.txt
Access is denied.
53.43. testing access with smbclient
You can also test connecting with authentication with smbclient. First we test with
a wrong password.
[root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina wrongpass
session setup failed: NT_STATUS_LOGON_FAILURE
Then we test with the correct password, and verify that we can access a file on the
share.
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[root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina stargate
Domain=[TEACHER0] OS=[Unix] Server=[Samba 3.0.33-3.7.el5]
smb: > more serena.txt
getting file serena.txt of size 14 as /tmp/smbmore.QQfmSN (6.8 kb/s)
one
two
three
smb: > q
53.44. verify ownership
We now have a simple standalone samba file server with authenticated access. And
the files in the shares belong to their proper owners.
[root@RHEL52 samba]# ls -l /srv/samba/authwrite/
total 8
-rwxr--r-- 1 martina martina 0 Jan 21 20:06 martina.txt
-rwxr--r-- 1 serena serena 14 Jan 21 20:06 serena.txt
-rwxr--r-- 1 serena serena 6 Jan 21 20:09 ser.txt
53.45. common problems
NT_STATUS_BAD_NETWORK_NAME
You can get NT_STATUS_BAD_NETWORK_NAME when you forget to create
the target directory.
[root@RHEL52 samba]# rm -rf /srv/samba/authwrite/
[root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina stargate
Domain=[TEACHER0] OS=[Unix] Server=[Samba 3.0.33-3.7.el5]
tree connect failed: NT_STATUS_BAD_NETWORK_NAME
NT_STATUS_LOGON_FAILURE
You can get NT_STATUS_LOGON_FAILURE when you type the wrong
password or when you type an unexisting username.
[root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina STARGATE
session setup failed: NT_STATUS_LOGON_FAILURE
usernames are (not) case sensitive
Remember that usernames om Linux are case sensitive.
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[root@RHEL52 samba]# su - MARTINA
su: user MARTINA does not exist
[root@RHEL52 samba]# su - martina
[martina@RHEL52 ~]$
But usernames on Microsoft computers are not case sensitive.
[root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina stargate
Domain=[TEACHER0] OS=[Unix] Server=[Samba 3.0.33-3.7.el5]
smb: > q
[root@RHEL52 samba]# smbclient //teacher0/authwrite -U MARTINA stargate
Domain=[TEACHER0] OS=[Unix] Server=[Samba 3.0.33-3.7.el5]
smb: > q
53.46. Practice : samba authentication
0. Make sure you have properly named backups of your smb.conf of the previous
practices.
1. Create three users (on the Linux and on the samba), remember their passwords!
2. Set up a shared directory that is only accessible to authenticated users.
3. Use smbclient and a windows computer to access your share, use more than one
user account (windows requires a logoff/logon for this).
4. Verify that files created by these users belong to them.
5. Try to change or delete a file from another user.
53.47. Solution: samba authentication
1. Create three users (on the Linux and on the samba), remember their passwords!
useradd -c 'SMB user1' userx
passwd userx
2. Set up a shared directory that is only accessible to authenticated users.
The shared section in smb.conf could look like this:
[authwrite]
path = /srv/samba/authwrite
comment = authenticated users only
read only = no
guest ok = no
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3. Use smbclient and a windows computer to access your share, use more than one
user account (windows requires a logoff/logon for this).
on Linux: smbclient //studentX/authwrite -U user1 password
on windows net use p: studentXauthwrite password /user:user2
4. Verify that files created by these users belong to them.
ls -l /srv/samba/authwrite
5. Try to change or delete a file from another user.
you should not be able to change or overwrite files from others.
53.48. security based on user name
valid users
To restrict users per share, you can use the valid users parameter. In the example
below, only the users listed as valid will be able to access the tennis share.
[tennis]
path = /srv/samba/tennis
comment = authenticated and valid users only
read only = No
guest ok = No
valid users = serena, kim, venus, justine
invalid users
If you are paranoia, you can also use invalid users to explicitely deny the listed users
access. When a user is in both lists, the user has no access!
[tennis]
path = /srv/samba/tennis
read only = No
guest ok = No
valid users = kim, serena, venus, justine
invalid users = venus
read list
On a writable share, you can set a list of read only users with the read list parameter.
[football]
path = /srv/samba/football
read only = No
guest ok = No
read list = martina, roberto
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write list
Even on a read only share, you can set a list of users that can write. Use the write
list parameter.
[football]
path = /srv/samba/golf
read only = Yes
guest ok = No
write list = eddy, jan
53.49. security based on ip-address
hosts allow
The hosts allow or allow hosts parameter is one of the key advantages of Samba. It
allows access control of shares on the ip-address level. To allow only specific hosts
to access a share, list the hosts, seperated by comma's.
allow hosts = 192.168.1.5, 192.168.1.40
Allowing entire subnets is done by ending the range with a dot.
allow hosts = 192.168.1.
Subnet masks can be added in the classical way.
allow hosts = 10.0.0.0/255.0.0.0
You can also allow an entire subnet with exceptions.
hosts allow = 10. except 10.0.0.12
hosts deny
The hosts deny or deny hosts parameter is the logical counterpart of the previous.
The syntax is the same as for hosts allow.
hosts deny = 192.168.1.55, 192.168.1.56
53.50. security through obscurity
hide unreadable
Setting hide unreadable to yes will prevent users from seeing files that cannot be
read by them.
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hide unreadable = yes
browsable
Setting the browseable = no directive will hide shares from My Network Places. But
it will not prevent someone from accessing the share (when the name of the share
is known).
Note that browsable and browseable are both correct syntax.
[pubread]
path = /srv/samba/readonly
comment = files to read
read only = yes
guest ok = yes
browseable = no
53.51. file system security
create mask
You can use create mask and directory mask to set the maximum allowed
permissions for newly created files and directories. The mask you set is an AND mask
(it takes permissions away).
[tennis]
path = /srv/samba/tennis
read only = No
guest ok = No
create mask = 640
directory mask = 750
force create mode
Similar to create mask, but different. Where the mask from above was a logical
AND, the mode you set here is a logical OR (so it adds permissions). You can use the
force create mode and force directory mode to set the minimal required permissions
for newly created files and directories.
[tennis]
path = /srv/samba/tennis
read only = No
guest ok = No
force create mode = 444
force directory mode = 550
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security mask
The security mask and directory security mask work in the same way as create
mask and directory mask, but apply only when a windows user is changing
permissions using the windows security dialog box.
force security mode
The force security mode and force directory security mode work in the same way
as force create mode and force directory mode, but apply only when a windows
user is changing permissions using the windows security dialog box.
inherit permissions
With inherit permissions = yes you can force newly created files and directories
to inherit permissions from their parent directory, overriding the create mask and
directory mask settings.
[authwrite]
path = /srv/samba/authwrite
comment = authenticated users only
read only = no
guest ok = no
create mask = 600
directory mask = 555
inherit permissions = yes
53.52. Practice: securing shares
1. Create a writable share called sales, and a readonly share called budget. Test that
it works.
2. Limit access to the sales share to ann, sandra and veronique.
3. Make sure that roberto cannot access the sales share.
4. Even though the sales share is writable, ann should only have read access.
5. Even though the budget share is read only, sandra should also have write access.
6. Limit one shared directory to the 192.168.1.0/24 subnet, and another share to the
two computers with ip-addresses 192.168.1.33 and 172.17.18.19.
7. Make sure the computer with ip 192.168.1.203 cannot access the budget share.
8. Make sure (on the budget share) that users can see only files and directories to
which they have access.
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accounts. The range is determined with the idmap uid and the idmap gid parameters.
The first Active Directory user to connect will receive Linux uid 20000.
idmap uid = 20000-22000
idmap gid = 20000-22000
winbind use default domain
The winbind use default domain parameter makes sure winbind also operates on
users without a domain component in their name.
winbind use default domain = yes
[global] section in smb.conf
Below is our new global section in smb.conf.
[global]
workgroup = STARGATE
security = Domain
server string = Stargate Domain Member Server
idmap uid = 20000-22000
idmap gid = 20000-22000
winbind use default domain = yes
realm in /etc/krb5.conf
To connect to a Windows 2003 sp2 (or later) you will need to adjust the kerberos
realm in /etc/krb5.conf and set both lookup statements to true.
[libdefaults]
default_realm = STARGATE.LOCAL
dns_lookup_realm = true
dns_lookup_kdc = true
[share] section in smb.conf
Nothing special is required for the share section in smb.conf. Remember that we do
not manually create users in smbpasswd or on the Linux (/etc/passwd). Only Active
Directory users are allowed access.
[domaindata]
path = /srv/samba/domaindata
comment = Active Directory users only
read only = No
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53.55. joining an Active Directory domain
While the Samba server is stopped, you can use net rpc join to join the Active
Directory domain.
[root@RHEL52 samba]# service smb stop
Shutting down SMB services: [ OK ]
Shutting down NMB services: [ OK ]
[root@RHEL52 samba]# net rpc join -U Administrator
Password:
Joined domain STARGATE.
We can verify in the aduc (Active Directory Users and Computers) that a computer
account is created for this samba server.
53.56. winbind
adding winbind to nsswitch.conf
The winbind daemon is talking with the Active Directory domain.
We need to update the /etc/nsswitch.conf file now, so user group and host names can
be resolved against the winbind daemon.
[root@RHEL52 samba]# vi /etc/nsswitch.conf
[root@RHEL52 samba]# grep winbind /etc/nsswitch.conf
passwd: files winbind
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group: files winbind
hosts: files dns winbind
starting samba and winbindd
Time to start Samba followed by winbindd.
[root@RHEL4b samba]# service smb start
Starting SMB services: [ OK ]
Starting NMB services: [ OK ]
[root@RHEL4b samba]# service winbind start
Starting winbindd services: [ OK ]
[root@RHEL4b samba]#
53.57. wbinfo
verify the trust
You can use wbinfo -t to verify the trust between your samba server and Active
Directory.
[root@RHEL52 ~]# wbinfo -t
checking the trust secret via RPC calls succeeded
list all users
We can obtain a list of all user with the wbinfo -u command. The domain is not
shown when the winbind use default domain parameter is set.
[root@RHEL52 ~]# wbinfo -u
TEACHER0serena
TEACHER0justine
TEACHER0martina
STARGATEadministrator
STARGATEguest
STARGATEsupport_388945a0
STARGATEpol
STARGATEkrbtgt
STARGATEarthur
STARGATEharry
list all groups
We can obtain a list of all domain groups with the wbinfo -g command. The domain
is not shown when the winbind use default domain parameter is set.
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[root@RHEL52 ~]# wbinfo -g
BUILTINadministrators
BUILTINusers
BATMANdomain computers
BATMANdomain controllers
BATMANschema admins
BATMANenterprise admins
BATMANdomain admins
BATMANdomain users
BATMANdomain guests
BATMANgroup policy creator owners
BATMANdnsupdateproxy
query a user
We can use wbinfo -a to verify authentication of a user against Active Directory.
Assuming a user account harry with password stargate is just created on the Active
Directory, we get the following screenshot.
[root@RHEL52 ~]# wbinfo -a harry%stargate
plaintext password authentication succeeded
challenge/response password authentication succeeded
53.58. getent
We can use getent to verify that winbindd is working and actually adding the Active
directory users to /etc/passwd.
[root@RHEL52 ~]# getent passwd harry
harry:*:20000:20008:harry potter:/home/BATMAN/harry:/bin/false
[root@RHEL52 ~]# getent passwd arthur
arthur:*:20001:20008:arthur dent:/home/BATMAN/arthur:/bin/false
[root@RHEL52 ~]# getent passwd bilbo
bilbo:*:20002:20008:bilbo baggins:/home/BATMAN/bilbo:/bin/false
If the user already exists locally, then the local user account is shown. This is because
winbind is configured in /etc/nsswitch.conf after files.
[root@RHEL52 ~]# getent passwd paul
paul:x:500:500:Paul Cobbaut:/home/paul:/bin/bash
All the Active Directory users can now easily connect to the Samba share. Files
created by them, belong to them.
53.59. file ownership
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[root@RHEL4b samba]# ll /srv/samba/domaindata/
total 0
-rwxr--r-- 1 justine 20000 0 Jun 22 19:54 create_by_justine_on_winxp.txt
-rwxr--r-- 1 venus 20000 0 Jun 22 19:55 create_by_venus.txt
-rwxr--r-- 1 maria 20000 0 Jun 22 19:57 Maria.txt
53.60. Practice : samba domain member
1. Verify that you have a working Active Directory (AD) domain.
2. Add the domain name and domain controller to /etc/hosts. Set the AD-DNS in /
etc/resolv.conf.
3. Setup Samba as a member server in the domain.
4. Verify the creation of a computer account in AD for your Samba server.
5. Verify the automatic creation of AD users in /etc/passwd with wbinfo and getent.
6. Connect to Samba shares with AD users, and verify ownership of their files.
53.61. About Domain Controllers
Windows NT4
Windows NT4 works with single master replication domain controllers. There is
exactly one PDC (Primary Domain Controller) in the domain, and zero or more BDC's
(Backup Domain Controllers). Samba 3 has all features found in Windows NT4 PDC
and BDC, and more. This includes file and print serving, domain control with single
logon, logon scripts, home directories and roaming profiles.
Windows 200x
With Windows 2000 came Active Directory. AD includes multimaster replication
and group policies. Samba 3 can only be a member server in Active Directory, it
cannot manage group policies. Samba 4 can do this (in beta).
Samba 3
Samba 3 can act as a domain controller in its own domain. In a Windows NT4
domain, with one Windows NT4 PDC and zero or more BDC's, Samba 3 can only
be a member server. The same is valid for Samba 3 in an Active Directory Domain.
In short, a Samba 3 domain controller can not share domain control with Windows
domain controllers.
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53.64. [global] section in smb.conf
Now is a good time to start adding comments in your smb.conf. First we will take a
look at the naming of our domain and server in the [global] section, and at the domain
controlling parameters.
security
The security must be set to user (which is the default). This mode will make samba
control the user accounts, so it will allow samba to act as a domain controller.
security = user
os level
A samba server is the most stable computer in the network, so it should win all
browser elections (os level above 32) to become the browser master
os level = 33
passdb backend
The passdb backend parameter will determine whether samba uses smbpasswd,
tdbsam or ldap.
passdb backend = tdbsam
preferred master
Setting the preferred master parameter to yes will make the nmbd daemon force an
election on startup.
preferred master = yes
domain logons
Setting the domain logons parameter will make this samba server a domain
controller.
domain logons = yes
domain master
Setting the domain master parameter can cause samba to claim the domain master
browser role for its workgroup. Don't use this parameter in a workgroup with an
active NT4 PDC.
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domain master = yes
[global] section
The screenshot below shows a sample [global] section for a samba domain controller.
[global]
# names
workgroup = SPORTS
netbios name = DCSPORTS
server string = Sports Domain Controller
# domain control parameters
security = user
os level = 33
preferred master = Yes
domain master = Yes
domain logons = Yes
53.65. netlogon share
Part of the microsoft definition for a domain controller is that it should have a
netlogon share. This is the relevant part of smb.conf to create this netlogon share
on Samba.
[netlogon]
comment = Network Logon Service
path = /srv/samba/netlogon
admin users = root
guest ok = Yes
browseable = No
53.66. other [share] sections
We create some sections for file shares, to test the samba server. Users can all access
the general sports file share, but only group members can access their own sports
share.
[sports]
comment = Information about all sports
path = /srv/samba/sports
valid users = @ntsports
read only = No
[tennis]
comment = Information about tennis
path = /srv/samba/tennis
valid users = @nttennis
read only = No
[football]
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comment = Information about football
path = /srv/samba/football
valid users = @ntfootball
read only = No
53.67. Users and Groups
To be able to use users and groups in the samba domain controller, we can first set
up some groups on the Linux computer.
[root@RHEL52 samba]# groupadd ntadmins
[root@RHEL52 samba]# groupadd ntsports
[root@RHEL52 samba]# groupadd ntfootball
[root@RHEL52 samba]# groupadd nttennis
This enables us to add group membership info to some new users for our samba
domain. Don't forget to give them a password.
[root@RHEL52 samba]# useradd -m -G ntadmins Administrator
[root@RHEL52 samba]# useradd -m -G ntsports,nttennis venus
[root@RHEL52 samba]# useradd -m -G ntsports,nttennis kim
[root@RHEL52 samba]# useradd -m -G ntsports,nttennis jelena
[root@RHEL52 samba]# useradd -m -G ntsports,ntfootball figo
[root@RHEL52 samba]# useradd -m -G ntsports,ntfootball ronaldo
[root@RHEL52 samba]# useradd -m -G ntsports,ntfootball pfaff
It is always safe to verify creation of users, groups and passwords in /etc/passwd, /
etc/shadow and /etc/group.
[root@RHEL52 samba]# tail -11 /etc/group
ntadmins:x:507:Administrator
ntsports:x:508:venus,kim,jelena,figo,ronaldo,pfaff
ntfootball:x:509:figo,ronaldo,pfaff
nttennis:x:510:venus,kim,jelena
Administrator:x:511:
venus:x:512:
kim:x:513:
jelena:x:514:
figo:x:515:
ronaldo:x:516:
pfaff:x:517:
53.68. tdbsam
Next we must make these users known to samba with the smbpasswd tool. When you
add the first user to tdbsam, the file /etc/samba/passdb.tdb will be created.
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[root@RHEL52 samba]# smbpasswd -a root
New SMB password:
Retype new SMB password:
tdbsam_open: Converting version 0 database to version 3.
Added user root.
Adding all the other users generates less output, because tdbsam is already created.
[root@RHEL4b samba]# smbpasswd -a root
New SMB password:
Retype new SMB password:
Added user root.
53.69. about computer accounts
Every NT computer (Windows NT, 2000, XP, Vista) can become a member of
a domain. Joining the domain (by right-clicking on My Computer) means that a
computer account will be created in the domain. This computer account also has a
password (but you cannot know it) to prevent other computers with the same name
from accidentally becoming member of the domain. The computer account created
by Samba is visible in the /etc/passwd file on Linux. Computer accounts appear as
a normal user account, but end their name with a dollar sign. Below a screenshot of
the windows 2003 computer account, created by Samba 3.
[root@RHEL52 samba]# tail -5 /etc/passwd
jelena:x:510:514::/home/jelena:/bin/bash
figo:x:511:515::/home/figo:/bin/bash
ronaldo:x:512:516::/home/ronaldo:/bin/bash
pfaff:x:513:517::/home/pfaff:/bin/bash
w2003ee$:x:514:518::/home/nobody:/bin/false
To be able to create the account, you will need to provide credentials of an account
with the permission to create accounts (by default only root can do this on Linux).
And we will have to tell Samba how to to this, by adding an add machine script to
the global section of smb.conf.
add machine script = /usr/sbin/useradd -s /bin/false -d /home/nobody %u
You can now join a Microsoft computer to the sports domain (with the root user).
After reboot of the Microsoft computer, you will be able to logon with Administrator
(password Stargate1), but you will get an error about your roaming profile. We will
fix this in the next section.
When joining the samba domain, you have to enter the credentials of a Linux account
that can create users (usually only root can do this). If the Microsoft computer
complains with The parameter is incorrect, then you possibly forgot to add the add
machine script.
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![samba
53.70. local or roaming profiles
For your information, if you want to force local profiles instead of roaming profiles,
then simply add the following two lines to the global section in smb.conf.
logon home =
logon path =
Microsoft computers store a lot of User Metadata and application data in a user
profile. Making this profile available on the network will enable users to keep their
Desktop and Application settings across computers. User profiles on the network
are called roaming profiles or roving profiles. The Samba domain controller can
manage these profiles. First we need to add the relevant section in smb.conf.
[Profiles]
comment = User Profiles
path = /srv/samba/profiles
readonly = No
profile acls = Yes
Besides the share section, we also need to set the location of the profiles share (this
can be another Samba server) in the global section.
logon path = %LProfiles%U
The %L variable is the name of this Samba server, the %U variable translates to the
username. After adding a user to smbpasswd and letting the user log on and off, the
profile of the user will look like this.
[root@RHEL4b samba]# ll /srv/samba/profiles/Venus/
total 568
drwxr-xr-x 4 Venus Venus 4096 Jul 5 10:03 Application Data
drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 Cookies
drwxr-xr-x 3 Venus Venus 4096 Jul 5 10:03 Desktop
drwxr-xr-x 3 Venus Venus 4096 Jul 5 10:03 Favorites
drwxr-xr-x 4 Venus Venus 4096 Jul 5 10:03 My Documents
drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 NetHood
-rwxr--r-- 1 Venus Venus 524288 Jul 5 2007 NTUSER.DAT
-rwxr--r-- 1 Venus Venus 1024 Jul 5 2007 NTUSER.DAT.LOG
-rw-r--r-- 1 Venus Venus 268 Jul 5 10:03 ntuser.ini
drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 PrintHood
drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 Recent
drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 SendTo
drwxr-xr-x 3 Venus Venus 4096 Jul 5 10:03 Start Menu
drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 Templates
53.71. Groups in NTFS acls
We have users on Unix, we have groups on Unix that contain those users.
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![samba
[root@RHEL4b samba]# grep nt /etc/group
...
ntadmins:x:506:Administrator
ntsports:x:507:Venus,Serena,Kim,Figo,Pfaff
nttennis:x:508:Venus,Serena,Kim
ntfootball:x:509:Figo,Pfaff
[root@RHEL4b samba]#
We already added Venus to the tdbsam with smbpasswd.
smbpasswd -a Venus
Does this mean that Venus can access the tennis and the sports shares ? Yes, all
access works fine on the Samba server. But the nttennis group is not available on
the windows machines. To make the groups available on windows (like in the ntfs
security tab of files and folders), we have to map unix groups to windows groups. To
do this, we use the net groupmap command.
[root@RHEL4b samba]# net groupmap add ntgroup="tennis" unixgroup=nttennis type=d
No rid or sid specified, choosing algorithmic mapping
Successully added group tennis to the mapping db
[root@RHEL4b samba]# net groupmap add ntgroup="football" unixgroup=ntfootball type=d
No rid or sid specified, choosing algorithmic mapping
Successully added group football to the mapping db
[root@RHEL4b samba]# net groupmap add ntgroup="sports" unixgroup=ntsports type=d
No rid or sid specified, choosing algorithmic mapping
Successully added group sports to the mapping db
[root@RHEL4b samba]#
Now you can use the Samba groups on all NTFS volumes on members of the domain.
53.72. logon scripts
Before testing a logon script, make sure it has the proper carriage returns that DOS
files have.
[root@RHEL4b netlogon]# cat start.bat
net use Z: DCSPORTS0SPORTS
[root@RHEL4b netlogon]# unix2dos start.bat
unix2dos: converting file start.bat to DOS format ...
[root@RHEL4b netlogon]#
Then copy the scripts to the netlogon share, and add the following parameter to
smb.conf.
logon script = start.bat
53.73. Practice
1. Setup Samba as a domain controller.
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![samba
make install
Then we can use the provision script to setup our realm. I used booi.schot as domain
name (instead of example.com).
./setup/provision --realm=BOOI.SCHOT --domain=BOOI --adminpass=stargate
--server-role='domain controller'
i added a simple share for testing
vi /usr/local/samba/etc/smb.conf
then i started samba
cd /usr/local/samba/sbin/
./samba
I tested with smbclient, it works
smbclient //localhost/test -Uadministrator%stargate
I checked that bind (and bind-chroot) were installed (yes), so copied the srv records
cp booi.schot.zone /var/named/chroot/etc/
then appended to named.conf
cat named.conf >> /var/named/chroot/etc/named.conf
I followed these steps in the howto4.txt
vi /etc/init.d/named [added two export lines right after start()]
chmod a+r /usr/local/samba/private/dns.keytab
cp krb5.conf /etc/
vi /var/named/chroot/etc/named.conf
--> remove a lot, but keep allow-update { any; };
restart bind (named!), then tested dns with dig, this works (stripped screenshot!)
[root@RHEL52 private]# dig _ldap._tcp.dc._msdcs.booi.schot SRV @localhost
; (1 server found)
;; global options: printcmd
;; Got answer:
;; -HEADER- opcode: QUERY, status: NXDOMAIN, id: 58186
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0
;; QUESTION SECTION:
;_ldap._tcp.dc._msdcs.booi.schot. IN SRV
;; AUTHORITY SECTION:
. 10800 IN SOA A.ROOT-SERVERS.NET....
;; Query time: 54 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Tue Jan 27 20:57:05 2009
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![samba
;; MSG SIZE rcvd: 124
[root@RHEL52 private]#
made sure /etc/resolv.conf points to himself
[root@RHEL52 private]# cat /etc/resolv.conf
search booi.schot
nameserver 127.0.0.1
start windows 2003 server, enter the samba4 as DNS!
ping the domain, if it doesn't work, then add your redhats hostname and your realm
to windows/system32/drivers/etc/hosts
join the windows computer to the domain
reboot the windows
log on with administrator stargate
start run dsa.msc to manage samba4
create an OU, a user and a GPO, test that it works
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![introduction to DNS
iterative or recursive query
A recursive query is a DNS query where the client that is submitting the query
expects a complete answer. An iterative query is a DNS query where the client
does not expect a complete answer. Iterative queries usually take place between name
servers. The root name servers do not respond to recursive queries.
54.4. old stuff....work in progress
Forward lookup zones are most common, they contain host or A records to translate
hostnames or Fully Qualified Domain Names (FQDN) to ip addresses. Reverse
lookup zones contain PTR records, they translate ip addresses to hostnames or
FQDN's.
The internet contains thirteen logical DNS servers for the top of the hierarchy.
This top is called the root, and is represented with a dot. Below the root are
the Top Level Domains (TLD's). There are common TLD's like .com, .net. .info.
aero. .museum, .gov, .mil, .edu and others. And there are country TLD's, like .be for
Belgium and .fr for France.
The internet root name servers will only answer iterative queries, most local DNS
servers will answer to recursive queries.
54.5. bind
One of the most common name servers on Linux is the Berkeley Internet Name
Domain (bind) server. Use rpm or dpkg to verify whether it is installed.
[root@RHEL4b etc]# rpm -qa | grep -i bind
ypbind-1.17.2-8
bind-chroot-9.2.4-16.EL4
bind-utils-9.2.4-16.EL4
bind-devel-9.2.4-16.EL4
bind-libs-9.2.4-16.EL4
bind-9.2.4-16.EL4
54.6. named
The software is called 'bind', the daemon runs as 'named' ! So look for the named
daemon, the named manual pages and /etc/named.conf to work with bind.
[root@RHEL4b etc]# apropos named | grep -i domain
named (8) - Internet domain name server
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![introduction to DNS
54.7. Caching only Name Server
A caching only name server is a DNS server that is not authoritative for any zone. It
forwards queries to other DNS servers and locally caches the results.
The default /etc/named.conf on RHEL is a caching only name server.
54.8. Our first zone
The way to set up zones in /etc/named.conf is to create a zone entry with a reference
to another file located in /var/named.
Here is an example of such an entry in /etc/named.conf
zone "classdemo.local" IN {
type master;
file "classdemo.local.zone";
allow-update { none; };
};
To create the zone file, the easy method is to copy an existing zone file (this is easier
than writing from scratch).
[root@RHEL4b named]# cd /var/named/
[root@RHEL4b named]# pwd
/var/named
[root@RHEL4b named]# cp localhost.zone classdemo.local.zone
[root@RHEL4b named]#
Here is an example of a zone file.
[root@RHEL4b named]# cat classdemo.local.zone
$TTL 86400
$ORIGIN classdemo.local.
@ IN SOA rhel4b.classdemo.local. admin.classdemo.local. (
2007083100 ; serial
3H ; refresh
900 ; retry
1W ; expiry
1D ) ; minimum
IN NS rhel4b.classdemo.local.
IN MX 10 mail.classdemo.local.
IN A 192.168.1.191
rhel4b IN A 192.168.1.191
mail IN A 192.168.1.191
www IN A 192.168.1.191
ftp IN A 192.168.1.191
server2 IN A 192.168.1.1
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![introduction to DNS
54.9. Starting the name server
When starting the name server, don't forget to look at the log file to verify that all
your zones are properly configured.
[root@RHEL4b etc]# service named restart
Stopping named: [ OK ]
Starting named: [ OK ]
[root@RHEL4b etc]# service named status
number of zones: 9
debug level: 0
xfers running: 0
xfers deferred: 0
soa queries in progress: 0
query logging is OFF
server is up and running
[root@RHEL4b etc]#
54.10. practice: DNS
1. Set up a working DNS server with your own zone. Test that it works.
2. Set up a master and a slave server.
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![Introduction to DNS
55.17. Practice: caching only DNS server
1a. installing DNS software on Debian/Ubuntu
root@ubu1010srv:~# dpkg -l | grep bind9
ii bind9-host 1:9.7.1.dfsg.P2-2ubuntu0.2 Version of 'host' bun
dled with BIND 9.X
ii libbind9-60 1:9.7.1.dfsg.P2-2ubuntu0.2 BIND9 Shared Library
used by BIND
root@ubu1010srv:~# aptitude install bind9
The following NEW packages will be installed:
bind9 bind9utils{a}
0 packages upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
Need to get 433kB of archives. After unpacking 1,352kB will be used.
Do you want to continue? [Y/n/?]
... output truncated ...
* Starting domain name service... bind9 [ OK ]
root@ubu1010srv:~# dpkg -l | grep bind9
ii bind9 1:9.7.1.dfsg.P2-2ubuntu0.2 Internet Domain Name Server
ii bind9-host 1:9.7.1.dfsg.P2-2ubuntu0.2 Version of 'host' bundled w
ith BIND 9.X
ii bind9utils 1:9.7.1.dfsg.P2-2ubuntu0.2 Utilities for BIND
ii libbind9-60 1:9.7.1.dfsg.P2-2ubuntu0.2 BIND9 Shared Library used b
y BIND
root@ubu1010srv:~#
1b. installing DNS software on RHEL/Fedora
[root@fedora14 ~]# rpm -qa | grep bind
samba-winbind-clients-3.5.8-74.fc14.i686
bind-utils-9.7.3-1.fc14.i686
PackageKit-device-rebind-0.6.12-2.fc14.i686
bind-libs-9.7.3-1.fc14.i686
[root@fedora14 ~]# yum install bind
Loaded plugins: langpacks, presto, refresh-packagekit
Adding en_US to language list
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package bind.i686 32:9.7.3-1.fc14 set to be installed
--> Finished Dependency Resolution
...output truncated
Running Transaction
Installing : 32:bind-9.7.3-1.fc14.i686 1/1
Installed:
bind.i686 32:9.7.3-1.fc14
Complete!
[root@fedora14 ~]# rpm -qa | grep bind
samba-winbind-clients-3.5.8-74.fc14.i686
bind-utils-9.7.3-1.fc14.i686
PackageKit-device-rebind-0.6.12-2.fc14.i686
bind-libs-9.7.3-1.fc14.i686
bind-9.7.3-1.fc14.i686
[root@fedora14 ~]#
2. Discover the default configuration files. Can you define the purpose of each file ?
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![Introduction to DNS
2a. On Fedora:
[root@fedora14 ~]# ls -ld /etc/named*
drwxr-x---. 2 root named 4096 Feb 18 16:07 /etc/named
-rw-r-----. 1 root named 1008 Jul 19 2010 /etc/named.conf
-rw-r--r--. 1 root named 2544 Feb 18 16:07 /etc/named.iscdlv.key
-rw-r-----. 1 root named 931 Jun 21 2007 /etc/named.rfc1912.zones
-rw-r--r--. 1 root named 487 Jul 19 2010 /etc/named.root.key
[root@fedora14 ~]# ls -l /var/named/
total 28
drwxrwx---. 2 named named 4096 Feb 18 16:07 data
drwxrwx---. 2 named named 4096 Feb 18 16:07 dynamic
-rw-r-----. 1 root named 1892 Feb 18 2008 named.ca
-rw-r-----. 1 root named 152 Dec 15 2009 named.empty
-rw-r-----. 1 root named 152 Jun 21 2007 named.localhost
-rw-r-----. 1 root named 168 Dec 15 2009 named.loopback
drwxrwx---. 2 named named 4096 Feb 18 16:07 slaves
2. On Ubuntu:
root@ubu1010srv:~# ls -l /etc/bind
total 52
-rw-r--r-- 1 root root 601 2011-02-23 16:22 bind.keys
-rw-r--r-- 1 root root 237 2011-02-23 16:22 db.0
-rw-r--r-- 1 root root 271 2011-02-23 16:22 db.127
-rw-r--r-- 1 root root 237 2011-02-23 16:22 db.255
-rw-r--r-- 1 root root 353 2011-02-23 16:22 db.empty
-rw-r--r-- 1 root root 270 2011-02-23 16:22 db.local
-rw-r--r-- 1 root root 2994 2011-02-23 16:22 db.root
-rw-r--r-- 1 root bind 463 2011-02-23 16:22 named.conf
-rw-r--r-- 1 root bind 490 2011-02-23 16:22 named.conf.default-zones
-rw-r--r-- 1 root bind 165 2011-02-23 16:22 named.conf.local
-rw-r--r-- 1 root bind 572 2011-02-23 16:22 named.conf.options
-rw-r----- 1 bind bind 77 2011-05-15 17:52 rndc.key
-rw-r--r-- 1 root root 1317 2011-02-23 16:22 zones.rfc1918
3. Setup caching only dns server. This is normally the default setup. A caching-only
name server will look up names for you and cache them. Most tutorials will tell you
to add a forwarder, so we first try without this!
root@ubu1010srv:/var/log# nslookup
> server 192.168.1.37
Default server: 192.168.1.37
Address: 192.168.1.37#53
>
> slashdot.org
Server: 192.168.1.37
Address: 192.168.1.37#53
Non-authoritative answer:
Name: slashdot.org
Address: 216.34.181.45
Hey this seems to work without a forwarder. Using a sniffer you can find out what
really happens (since the server is not using a cache, not using your dns-server (from /
etc/resolv.conf). So where is this information coming from, and what can you learn
from sniffing this dns traffic ?
4. Explain in detail what happens when you enable a caching only dns server without
forwarder. This wireshark screenshot can help, but you learn more by sniffing the
traffic yourself! I will choose two volunteers to explain this in front of the class.
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![Introduction to DNS
55.18. Practice: caching only with forwarder
5. Add a local dns-server as a forwarder (at my home this is 192.168.1.1, probably
different ip in a classroom!).
root@ubu1010srv:~# grep -A2 forwarder /etc/bind/named.conf.options| t
ail -3
forwarders {
192.168.1.1;
};
root@ubu1010srv:~# /etc/init.d/bind9 restart
* Stopping domain name service... bind9 [ OK ]
* Starting domain name service... bind9 [ OK ]
root@ubu1010srv:~#
6. Explain the purpose of adding the forwarder. What is our DNS server doing when it
receives a query ? Again the wireshark screenshot can help, you should see something
similar.
root@ubu1010srv:~# nslookup
> server
Default server: 192.168.1.4
Address: 192.168.1.4#53
> server 192.168.1.37
Default server: 192.168.1.37
Address: 192.168.1.37#53
>
> cobbaut.be
Server: 192.168.1.37
Address: 192.168.1.37#53
Non-authoritative answer:
Name: cobbaut.be
Address: 88.151.243.8
7. What happens when you query for the same domain name more than once ?
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![Introduction to DNS
55.19. Practice: primary authoritative server
1. Instead of only cachng the information from other servers, we will now make our
server authoritative for our own domain.
2. I choose the new TLD .paul and the domain cobbaut.paul and put the information
in /etc/bind/named.conf.local.
root@ubu1010srv:/etc/bind# grep -C1 cobbaut named.conf.local
zone "cobbaut.paul" {
type master;
file "/etc/bind/db.cobbaut.paul";
};
3. Also add a zone database file, similar to this one (add some A records for testing).
Set the Refresh and Retry values not too high so you can sniff this traffic (this
example makes the slave server contact the master every 300 seconds).
root@ubu1010srv:/etc/bind# cat db.cobbaut.paul
;
; BIND data file for domain cobbaut.paul
;
$TTL 604800
@ IN SOA ns.cobbaut.paul. root.cobbaut.paul. (
20110516 ; Serial
300 ; Refresh
200 ; Retry
2419200 ; Expire
604800 ) ; Negative Cache TTL
;
@ IN NS ns.cobbaut.paul.
ns IN A 192.168.1.37
ubu1010srv IN A 192.168.1.37
anya IN A 192.168.1.1
mac IN A 192.168.1.30
root@ubu1010srv:/etc/bind#
4. Restart the DNS server and check your zone in the error log.
root@ubu1010srv:/etc/bind# grep cobbaut /var/log/daemon.log
May 16 00:33:49 ubu1010srv named[25449]: zone cobbaut.paul/IN: loaded
serial 20110516
5. Use dig or nslookup (or even ping) to test your A records.
root@ubu1010srv:/etc/bind# ping mac.cobbaut.paul
PING mac.cobbaut.paul (192.168.1.30) 56(84) bytes of data.
64 bytes from 192.168.1.30: icmp_req=1 ttl=64 time=2.28 ms
64 bytes from 192.168.1.30: icmp_req=1 ttl=64 time=2.31 ms (DUP!)
^C
--- mac.cobbaut.paul ping statistics ---
1 packets transmitted, 1 received, +1 duplicates, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 2.282/2.296/2.310/0.014 ms
root@ubu1010srv:/etc/bind# dig anya.cobbaut.paul
; <<>> DiG 9.7.1-P2 <<>> anya.cobbaut.paul
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 38237
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![Introduction to DNS
55.21. Practice: a DNS slave server
1. A slave server transfers zone information over the network from a master server (a
slave can also be a master). A primary server maintains zone records in its local file
system. As an exercise, and to verify the work of all students, set up a slave server
of all the master servers in the classroom.
2. Before configuring the slave server, we have to allow transfers from our zone to
this server. Remember that this is not very secure since transfers are in clear text and
limited to an ip address. This example follows our demo from above. The ip of my
slave server is 192.168.1.31, yours is probably different.
root@ubu1010srv:/etc/bind# grep -A2 cobbaut named.conf.local
zone "cobbaut.paul" {
type master;
file "/etc/bind/db.cobbaut.paul";
allow-transfer { 192.168.1.31; };
};
root@ubu1010srv:/etc/bind#
3. My slave server is running Fedora 14. Bind configuration files are only a little
different. Below the addition of a slave zone to this server, note the ip address
(192.168.1.37) of my master dns server for the cobbaut.paul zone.
[root@fedora14 etc]# grep cobbaut -A2 named.conf
zone "cobbaut.paul" {
type slave;
file "/var/named/slaves/db.cobbaut.paul";
masters { 192.168.1.37; };
};
[root@fedora14 etc]#
4. You might need to add the ip-address of the server on Fedora to allow queries other
than from localhost.
[root@fedora14 etc]# grep 127 named.conf
listen-on port 53 { 127.0.0.1; 192.168.1.31; };
5. Restarting bind on the slave server should transfer the zone database file:
[root@fedora14 etc]# ls -l /var/named/slaves/
total 4
-rw-r--r--. 1 named named 387 May 16 03:23 db.cobbaut.paul
[root@fedora14 etc]#
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![Introduction to DNS
55.22. DNS History
Today, DNS or Domain Name System is a worldwide distributed hierarchical
database. It's primary function is to resolve names to ip addresses, and to point to
internet servers providing SMTP or LDAP services.
In the seventies, only a few hundred computers were connected to the internet. To
resolve names, computers had a flat file that contained a table to resolve hostnames
to ip-addresses. This local file was downloaded from hosts.txt on an ftp server in
Stanford.
In 1984 Paul Mockapetris created DNS, a distributed treelike hierarchical database.
Today ICANN controls DNS.
55.23. forward lookup zone example
The way to set up zones in /etc/named.conf is to create a zone entry with a reference
to another file located in /var/named.
Here is an example of such an entry in /etc/named.conf
zone "classdemo.local" IN {
type master;
file "classdemo.local.zone";
allow-update { none; };
};
To create the zone file, the easy method is to copy an existing zone file (this is easier
than writing from scratch).
[root@RHEL4b named]# cd /var/named/
[root@RHEL4b named]# pwd
/var/named
[root@RHEL4b named]# cp localhost.zone classdemo.local.zone
[root@RHEL4b named]#
Here is an example of a zone file.
[root@RHEL4b named]# cat classdemo.local.zone
$TTL 86400
$ORIGIN classdemo.local.
@ IN SOA rhel4b.classdemo.local. admin.classdemo.local. (
2007083100 ; serial
3H ; refresh
900 ; retry
1W ; expiry
1D ) ; minimum
IN NS rhel4b.classdemo.local.
IN MX 10 mail.classdemo.local.
IN A 192.168.1.191
rhel4b IN A 192.168.1.191
mail IN A 192.168.1.191
575](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-581-320.jpg)
![Introduction to DHCP
-rw-r--r-- 1 root root 3551 2011-04-10 21:23 /etc/dhcp3/dhcpd.conf
58.5. tekening DHCP
We hebben een klein netwerk met twee servers (DHCP-SRV1 en DHCP-SRV2) en
twee clients (SunWS1 en Mac42). In het midden staat een hub (of een switch) om
aan te tonen dat deze vier computers op hetzelfde netwerk, hetzelfde segment zitten.
Alle vier de computers hebben een kabel naar de hub (niet getekend hier).
1. De client SunWS1 start op en stuurt een DHCPDiscover op het netwerk. Alle
computers ontvangen deze broadcast.
2. Beide DHCP servers antwoorden met een DHCPOffer. DHCP-SRV1 is een
dedicated DHCP server en is sneller met zijn offer dan DHCP-SRV2 (die ook
fileserver is).
3. De client kiest het offer van DHCP-SRV1 en stuurt een DHCPRequest op het
netwerk.
4. DHCP-SRV1 antwoordt met een DHCPAck (een acknowledge).
Alle vier broadcasts (of vijf want er waren twee offers) zijn een layer 2 ethernet
broadcast naar ff:ff:ff:ff:ff:ff en een layer 3 ip broadcast naar 255.255.255.255. Alle
vier de computers hebben alle broadcasts ontvangen.
Dit verhaal staat ook mooi in rfc 2131.
58.6. dhcp server op Red Hat Enterprise
Linux
Eerste stap is om even te kijken in het /etc/dhcpd.conf bestand (om te zien of er al
iets gedefinieerd is). Er is nog geen bestaande config, het bestand wijst ons naar een
voorbeeld config genaamd dhcpd.conf.sample.
[root@localhost ~]# cat /etc/dhcpd.conf
#
# DHCP Server Configuration file.
# see /usr/share/doc/dhcp*/dhcpd.conf.sample
We passen het voorbeeldje een beetje aan en kopiëren de volgende configuratie naar
/etc/dhcpd.conf.
subnet 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.140 192.168.1.159
option routers 192.168.1.1;
option subnet-mask 255.255.255.0;
option domain-name "classdemo.local";
option domain-name-servers 192.168.1.1;
default-lease-time 21600;
600](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-606-320.jpg)
![Introduction to DHCP
58.14. Exercise DHCP and DDNS
1. Make sure you have a unique fixed ip address for your DNS and DHCP server
(easier on the same machine).
2. Install DHCP and browse the explanation in the default configuration file /etc/
dhcp/dhcpd.conf or /etc/dhcp3/dhcpd.conf.
3. Decide on a valid scope and activate it.
4. Test with a client that your DHCP server works.
5. Use wireshark to capture the four broadcasts when a client receives an ip (for the
first time).
6. Use wireshark to capture a DHCPNAK and a DHCPrelease.
7. Reserve a configuration for a particular client (using mac address).
8. Configure your DHCP/DNS server(s) with a proper hostname and
domainname (/etc/hosts, /etc/hostname, /etc/sysconfig/network on Fedora/RHEL, /
etc/resolv.conf ...). You may need to disable NetworkManager on *buntu-desktops.
9. Make sure your DNS server still works, and is master over (at least) one domain.
There are several ways to do steps 10-11-12. Google is your friend in exploring
DDNS with keys, with key-files or without keys.
10. Configure your DNS server to allow dynamic updates from your DHCP server.
11. Configure your DHCP server to send dynamic updates to your DNS server.
12. Test the working of Dynamic DNS.
58.15. Exercise DHCP in Packet Tracer
1. Setup a dhcp server and a client in packet tracer.
2. Test that it works, use simulation to sniff the four broadcasts.
3. Setup a relay agent in packet tracer.
58.16. Example config files
For dhcpd.conf on Fedora with dynamic updates for a DNS domain.
[root@fedora14 ~]# cat /etc/dhcp/dhcpd.conf
authoritative;
include "/etc/rndc.key";
603](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-609-320.jpg)
![Introduction to DHCP
log-facility local6;
server-identifier fedora14;
ddns-domainname "office.linux-training.be";
ddns-update-style interim;
ddns-updates on;
update-static-leases on;
option domain-name "office.linux-training.be";
option domain-name-servers 192.168.42.100;
option ip-forwarding off;
default-lease-time 1800;
max-lease-time 3600;
zone office.linux-training.be {
primary 192.168.42.100;
}
subnet 192.168.4.0 netmask 255.255.255.0 {
range 192.168.4.24 192.168.4.40;
}
Allowing any updates in the zone database (part of the named.conf configuration)
zone "office.linux-training.be" {
type master;
file "/var/named/db.office.linux-training.be";
allow-transfer { any; };
allow-update { any; };
};
Allowing secure key updates in the zone database (part of the named.conf
configuration)
zone "office.linux-training.be" {
type master;
file "/var/named/db.office.linux-training.be";
allow-transfer { any; };
allow-update { key mykey; };
};
Sample key file contents:
[root@fedora14 ~]# cat /etc/rndc.key
key "rndc-key" {
algorithm hmac-md5;
secret "4Ykd58uIeUr3Ve6ad1qTfQ==";
};
Generate your own keys with dnssec-keygen.
How to include a key in a config file:
include "/etc/bind/rndc.key";
Also make sure that bind can write to your db.zone file (using chmod/chown). For
Ubuntu this can be in /etc/bind, for Fedora in /var/named.
604](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-610-320.jpg)
![introduction to routers
travel from one network to another through its two network interfaces, it is said to
have enabled packet forwarding.
/proc/sys/net/ipv4/ip_forward
Whether a host is forwarding packets is defined in /proc/sys/net/ipv4/ip_forward.
The following screenshot shows how to enable packet forwarding on Linux.
[root@RHEL5 ~]# echo 1 > /proc/sys/net/ipv4/ip_forward
The next command shows how to disable packet forwarding.
[root@RHEL5 ~]# echo 0 > /proc/sys/net/ipv4/ip_forward
Use cat to check if packet forwarding is enabled.
[root@RHEL5 ~]# cat /proc/sys/net/ipv4/ip_forward
/etc/sysctl.conf
By default, most Linux computers are not configured for automatic packet
forwarding. To enable packet forwarding whenever the system starts, change the
net.ipv4.ip_forward variable in /etc/sysctl.conf to the value 1.
[root@RHEL5 ~]# grep ip_forward /etc/sysctl.conf
net.ipv4.ip_forward = 0
Practice: packet forwarding
1. Set up two dsl (Damn Small Linux) machines, one on vmnet1, the other on vmnet8.
Make sure they both get an ip-address in the correct subnet. These two machines will
be 'left' and 'right' from the 'router'.
2. Set up a RHEL server with two network cards, one on vmnet1, the other on vmnet8.
This computer will be the 'router'. Complete the table below with the relevant names,
ip-addresses and mac-addresses.
Table 59.1. Packet Forwarding Exercise
left: router: right:
MAC
IP
609](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-615-320.jpg)
![introduction to routers
3. How can you verify whether the RHEL will allow packet forwarding by default
or not ? Test that you can ping from the RHEL to the two dsl machines, and from
the two dsl machines to the RHEL. Use arp -a to make sure you are connected with
the correct MAC addresses.
4. Ping from one dsl to the other. Enable and/or disable packet forwarding on the
RHEL server and verify what happens to the ping between the two dsl machines. If
you do not succeed in pinging between the two dsl machines (on different subnets),
then use a sniffer like wireshark or tcpdump to discover the problem.
5. Use wireshark or tcpdump -xx to answer the following questions. Does the source
MAC change when a packet passes through the filter ? And the destination MAC ?
What about source and destination IP-addresses ?
Solution: packet forwarding
1. Set up two dsl (Damn Small Linux) machines, one on vmnet1, the other on vmnet8.
Make sure they both get an ip-address in the correct subnet. These two machines will
be 'left' and 'right' from the 'router'.
The configuration of the dsl machines can be similar to the following two
screenshots. Both machines must be in a different subnet (here 192.168.187.0/24 and
172.16.122.0/24)
root@ttyp1[root]# ifconfig eth0 | grep -A1 eth0
eth0 Link encap:Ethernet HWaddr 00:0C:29:08:F4:C1
inet addr:192.168.187.130 Bcast:192.168.187.255 Mask:255.255.255.0
root@ttyp1[root]# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.187.0 * 255.255.255.0 U 0 0 0 eth0
default 192.168.187.128 0.0.0.0 UG 0 0 0 eth0
root@ttyp1[root]#
root@ttyp1[root]# ifconfig eth0 | grep -A1 eth0
eth0 Link encap:Ethernet HWaddr 00:0C:29:6E:1A:AA
inet addr:172.16.122.129 Bcast:172.16.122.255 Mask:255.255.255.0
root@ttyp1[root]# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
172.16.122.0 * 255.255.255.0 U 0 0 0 eth0
default 172.16.122.128 0.0.0.0 UG 0 0 0 eth0
root@ttyp1[root]#
2. Set up a RHEL server with two network cards, one on vmnet1, the other on vmnet8.
This computer will be the 'router'.
The 'router' can be set up like this screenshot shows.
610](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-616-320.jpg)
![introduction to routers
[root@RHEL5 ~]# ifconfig | grep -A1 eth
eth1 Link encap:Ethernet HWaddr 00:0C:29:8C:90:49
inet addr:192.168.187.128 Bcast:192.168.187.255 Mask:255.255.255.0
--
eth2 Link encap:Ethernet HWaddr 00:0C:29:8C:90:53
inet addr:172.16.122.128 Bcast:172.16.122.255 Mask:255.255.255.0
[root@RHEL5 ~]#
Your setup may use different ip and mac addresses than the ones in the table below.
This table serves as a reference for the screenshots from this solution to the practice.
Table 59.2. Packet Forwarding Solution
left: dsl router: RHEL5 right: dsl
00:0c:29:08:f4:c1 00:0c:29:8c:90:49 00:0c:29:8c:90:53 00:0c:29:6e:1a:aa
192.168.187.130 192.168.187.128 172.16.122.128 172.16.122.129
3. How can you verify whether the RHEL will allow packet forwarding by default
or not ? Test that you can ping from the RHEL to the two dsl machines, and from
the two dsl machines to the RHEL. Use arp -a to make sure you are connected with
the correct MAC addresses.
This can be done with "grep ip_forward /etc/sysctl.conf" (1 is enabled, 0 is
disabled).
[root@RHEL5 ~]# grep ip_for /etc/sysctl.conf
net.ipv4.ip_forward = 0
4. Ping from one dsl to the other. Enable and/or disable packet forwarding on the
RHEL server and verify what happens to the ping between the two dsl machines. If
you do not succeed in pinging between the two dsl machines (on different subnets),
then use a sniffer like ethereal or tcpdump to discover the problem.
Did you forget to add a default gateway to the dsl machines ? Use route add default
gw 'ip-address'.
You should be able to ping when packet forwarding is enabled (and both default
gateways are properly configured). The ping will not work when packet forwarding
is disabled or when gateways are not configured correctly.
5. Use wireshark or tcpdump -xx to answer the following questions. Does the source
MAC change when a packet passes through the filter ? And the destination MAC ?
What about source and destination IP-addresses ?
Both MAC addresses are changed when passing the router. The screenshots below
show tcpdump -xx output on the router. The first one is taken on the eth1(vmnet1)
interface in the 192.168.187.0/24 network, the second one is from the other interface
(eth2 on vmnet8 in 172.16.122.0/24). The first six bytes are the destination MAC,
the next six are the source.
611](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-617-320.jpg)
![introduction to routers
[root@RHEL5 ~]# tcpdump -xx -i eth1
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
04:18:23.817854 IP 192.168.187.130 > 172.16.122.129: ICMP echo request...
0x0000: 000c 298c 9049 000c 2908 f4c1 0800 4500
0x0010: 0054 0000 4000 4001 97ec c0a8 bb82 ac10
0x0020: 7a81 0800 3b28 a717 0006 8059 d148 d614
0x0030: 0300 0809 0a0b 0c0d 0e0f 1011 1213 1415
0x0040: 1617 1819 1a1b 1c1d 1e1f 2021 2223 2425
0x0050: 2627 2829 2a2b 2c2d 2e2f 3031 3233 3435
04:18:23.817962 IP 172.16.122.129 > 192.168.187.130: ICMP echo reply...
0x0000: 000c 2908 f4c1 000c 298c 9049 0800 4500
0x0010: 0054 d364 0000 3f01 0588 ac10 7a81 c0a8
0x0020: bb82 0000 4328 a717 0006 8059 d148 d614
0x0030: 0300 0809 0a0b 0c0d 0e0f 1011 1213 1415
0x0040: 1617 1819 1a1b 1c1d 1e1f 2021 2223 2425
0x0050: 2627 2829 2a2b 2c2d 2e2f 3031 3233 3435
[root@RHEL5 ~]# tcpdump -xx -i eth2
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth2, link-type EN10MB (Ethernet), capture size 96 bytes
04:18:33.904697 IP 192.168.187.130 > 172.16.122.129: ICMP echo request...
0x0000: 000c 296e 1aaa 000c 298c 9053 0800 4500
0x0010: 0054 0000 4000 3f01 98ec c0a8 bb82 ac10
0x0020: 7a81 0800 2320 a717 0008 8a59 d148 e41a
0x0030: 0300 0809 0a0b 0c0d 0e0f 1011 1213 1415
0x0040: 1617 1819 1a1b 1c1d 1e1f 2021 2223 2425
0x0050: 2627 2829 2a2b 2c2d 2e2f 3031 3233 3435
04:18:33.944514 IP 172.16.122.129 > 192.168.187.130: ICMP echo reply...
0x0000: 000c 298c 9053 000c 296e 1aaa 0800 4500
0x0010: 0054 d366 0000 4001 0486 ac10 7a81 c0a8
0x0020: bb82 0000 2b20 a717 0008 8a59 d148 e41a
0x0030: 0300 0809 0a0b 0c0d 0e0f 1011 1213 1415
0x0040: 1617 1819 1a1b 1c1d 1e1f 2021 2223 2425
0x0050: 2627 2829 2a2b 2c2d 2e2f 3031 3233 3435
612](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-618-320.jpg)
![Chapter 60. Firewall: iptables
Table of Contents
60.1. about iptables ............................................................................................. 613
60.2. packet filtering ........................................................................................... 613
60.3. network address translation ........................................................................ 618
60.1. about iptables
Iptables is a user-space application that allows a user to configure the Linux kernel's
Netfilter. By default there are three tables in the kernel that contain sets of rules. The
filter table is used for packet filtering, the NAT table for address translation and the
mangle table for special-purpose processing of packets. Series of rules in each table
are called a chain.
The following screenshot shows how to stop and start iptables.
[root@RHEL5 ~]# /etc/init.d/iptables stop
[root@RHEL5 ~]# /etc/init.d/iptables start
[root@RHEL5 ~]#
60.2. packet filtering
about packet filtering
Packet filtering is a bit more than packet forwarding. Packet forwarding only uses
a routing table to make decisions, the kernel now also uses a list of rules. So with
packet filtering, the kernel will inspect each packet and decide based on iptables rules
to allow or drop a packet.
filter table
The filter table in iptables has three chains (sets of rules). The INPUT chain is used
for any packet coming into the system. The OUTPUT chain is for any packet leaving
the system. And the FORWARD chain is for packets that are forwarded (routed)
through the system.
The screenshot below shows how to list the filter table and all its rules.
[root@RHEL5 ~]# iptables -t filter -nL
Chain INPUT (policy ACCEPT)
target prot opt source destination
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![Firewall: iptables
Chain FORWARD (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
[root@RHEL5 ~]#
As you can see, all three chains in the filter table are set to ACCEPT everything.
ACCEPT is the default behaviour.
Changing default policy rules
To start, let's set the default policy for all three chains to drop everything. Note that
you might lose your connection when typing this over ssh ;-).
[root@RHEL5 ~]# iptables -P INPUT DROP
[root@RHEL5 ~]# iptables -P FORWARD DROP
[root@RHEL5 ~]# iptables -P OUTPUT DROP
Next, we allow the server to use its own loopback device (this allows the server to
access its services running on localhost). We first append a rule to the INPUT chain
to allow (ACCEPT) traffic from the lo (loopback) interface, then we do the same to
allow packets to leave the system through the loopback interface.
[root@RHEL5 ~]# iptables -A INPUT -i lo -j ACCEPT
[root@RHEL5 ~]# iptables -A OUTPUT -o lo -j ACCEPT
Looking at the filter table again (omitting -t filter because it is the default table).
[root@RHEL5 ~]# iptables -nL
Chain INPUT (policy DROP)
target prot opt source destination
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0
Chain FORWARD (policy DROP)
target prot opt source destination
Chain OUTPUT (policy DROP)
target prot opt source destination
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0
Allowing ssh over eth0
This example show how to add two rules to allow ssh access to your system from
outside.
[root@RHEL5 ~]# iptables -A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT
[root@RHEL5 ~]# iptables -A OUTPUT -o eth0 -p tcp --sport 22 -j ACCEPT
614](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-620-320.jpg)
![Firewall: iptables
The filter table will look something like this screenshot (note that -v is added for
more verbose output).
[root@RHEL5 ~]# iptables -nvL
Chain INPUT (policy DROP 7 packets, 609 bytes)
pkts bytes target prot opt in out source destination
0 0 ACCEPT all -- lo * 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:22
Chain FORWARD (policy DROP 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy DROP 3 packets, 228 bytes)
pkts bytes target prot opt in out source destination
0 0 ACCEPT all -- * lo 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT tcp -- * eth0 0.0.0.0/0 0.0.0.0/0 tcp spt:22
[root@RHEL5 ~]#
Allowing access from a subnet
This example shows how to allow access from any computer in the 10.1.1.0/24
network, but only through eth1. There is no port (application) limitation here.
[root@RHEL5 ~]# iptables -A INPUT -i eth1 -s 10.1.1.0/24 -p tcp -j ACCEPT
[root@RHEL5 ~]# iptables -A OUTPUT -o eth1 -d 10.1.1.0/24 -p tcp -j ACCEPT
Together with the previous examples, the policy is expanding.
[root@RHEL5 ~]# iptables -nvL
Chain INPUT (policy DROP 7 packets, 609 bytes)
pkts bytes target prot opt in out source destination
0 0 ACCEPT all -- lo * 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:22
0 0 ACCEPT tcp -- eth1 * 10.1.1.0/24 0.0.0.0/0
Chain FORWARD (policy DROP 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy DROP 3 packets, 228 bytes)
pkts bytes target prot opt in out source destination
0 0 ACCEPT all -- * lo 0.0.0.0/0 0.0.0.0/0
0 0 ACCEPT tcp -- * eth0 0.0.0.0/0 0.0.0.0/0 tcp spt:22
0 0 ACCEPT tcp -- * eth1 0.0.0.0/0 10.1.1.0/24
iptables save
Use iptables save to automatically implement these rules when the firewall is
(re)started.
[root@RHEL5 ~]# /etc/init.d/iptables save
Saving firewall rules to /etc/sysconfig/iptables: [ OK ]
[root@RHEL5 ~]#
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![Firewall: iptables
scripting example
You can write a simple script for these rules. Below is an example script that
implements the firewall rules that you saw before in this chapter.
#!/bin/bash
# first cleanup everything
iptables -t filter -F
iptables -t filter -X
iptables -t nat -F
iptables -t nat -X
# default drop
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT DROP
# allow loopback device
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
# allow ssh over eth0 from outside to system
iptables -A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT
iptables -A OUTPUT -o eth0 -p tcp --sport 22 -j ACCEPT
# allow any traffic from 10.1.1.0/24 to system
iptables -A INPUT -i eth1 -s 10.1.1.0/24 -p tcp -j ACCEPT
iptables -A OUTPUT -o eth1 -d 10.1.1.0/24 -p tcp -j ACCEPT
Allowing ICMP(ping)
When you enable iptables, you will get an 'Operation not permitted' message when
trying to ping other hosts.
[root@RHEL5 ~# ping 192.168.187.130
PING 192.168.187.130 (192.168.187.130) 56(84) bytes of data.
ping: sendmsg: Operation not permitted
ping: sendmsg: Operation not permitted
The screenshot below shows you how to setup iptables to allow a ping from or to
your machine.
[root@RHEL5 ~]# iptables -A INPUT -p icmp --icmp-type any -j ACCEPT
[root@RHEL5 ~]# iptables -A OUTPUT -p icmp --icmp-type any -j ACCEPT
The previous two lines do not allow other computers to route ping messages through
your router, because it only handles INPUT and OUTPUT. For routing of ping, you
will need to enable it on the FORWARD chain. The following command enables
routing of icmp messages between networks.
[root@RHEL5 ~]# iptables -A FORWARD -p icmp --icmp-type any -j ACCEPT
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![Firewall: iptables
# iptables -A INPUT -i eth2 -p tcp --dport 111 -j ACCEPT
# iptables -A OUTPUT -o eth2 -p tcp --sport 111 -j ACCEPT
60.3. network address translation
about NAT
A NAT device is a router that is also changing the source and/or target ip-address in
packets. It is typically used to connect multiple computers in a private address range
with the (public) internet. A NAT can hide private addresses from the internet.
NAT was developed to mitigate the use of real ip addresses, to allow private address
ranges to reach the internet and back, and to not disclose details about internal
networks to the outside.
The nat table in iptables adds two new chains. PREROUTING allows altering of
packets before they reach the INPUT chain. POSTROUTING allows altering packets
after they exit the OUTPUT chain.
Use iptables -t nat -nvL to look at the NAT table. The screenshot below shows an
empty NAT table.
[root@RHEL5 ~]# iptables -t nat -nL
Chain PREROUTING (policy ACCEPT)
target prot opt source destination
Chain POSTROUTING (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
[root@RHEL5 ~]#
SNAT (Source NAT)
The goal of source nat is to change the source address inside a packet before it leaves
the system (e.g. to the internet). The destination will return the packet to the NAT-
device. This means our NAT-device will need to keep a table in memory of all the
packets it changed, so it can deliver the packet to the original source (e.g. in the
private network).
Because SNAT is about packets leaving the system, it uses the POSTROUTING
chain.
Here is an example SNAT rule. The rule says that packets coming from 10.1.1.0/24
network and exiting via eth1 will get the source ip-address set to 11.12.13.14. (Note
that this is a one line command!)
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![Chapter 61. introduction to apache
Table of Contents
61.1. about apache .............................................................................................. 622
61.2. is apache installed ? ................................................................................... 622
61.3. is apache running ? .................................................................................... 623
61.4. apache configuration .................................................................................. 623
61.5. virtual hosts ................................................................................................ 624
61.6. aliases and redirects ................................................................................... 625
61.7. securing directories with htpasswd and .htaccess ...................................... 625
61.8. more on .htaccess ....................................................................................... 626
61.9. traffic .......................................................................................................... 626
61.10. practice: apache ........................................................................................ 626
61.1. about apache
According to NetCraft (http://news.netcraft.com/archives/web_server_survey.html)
about seventy percent of all web servers are running on Apache. Some people say
that the name is derived from a patchy web server, because of all the patches people
wrote for the NCSA httpd server.
61.2. is apache installed ?
To verify whether Apache is installed, use the proper tools (rpm, dpkg, ...) and grep
for apache or httpd.
This Red Hat Enterprise 4 Server has apache installed.
[paul@rhel4 ~]$ rpm -qa | grep -i httpd
httpd-2.0.52-25.ent
httpd-manual-2.0.52-25.ent
system-config-httpd-1.3.1-1
httpd-devel-2.0.52-25.ent
httpd-suexec-2.0.52-25.ent
This Ubuntu also has apache installed.
paul@laika:~$ dpkg -l | grep apache
ii apache2 2.2.3-3.2build1 Next generation, scalable, ...
ii apache2-mpm-prefork 2.2.3-3.2build1 Traditional model for Apach...
ii apache2-utils 2.2.3-3.2build1 utility programs for webser...
ii apache2.2-common 2.2.3-3.2build1 Next generation, scalable, ...
ii libapache2-mod-php5 5.2.1-0ubuntu1.2 server-side, HTML-embedded ...
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![introduction to apache
61.3. is apache running ?
This is how apache looks when it is installed on Red Hat Enterprise Linux 4, running
named as httpd.
[root@RHELv4u3 ~]# /etc/init.d/httpd status
httpd is stopped
[root@RHELv4u3 ~]# service httpd start
Starting httpd: [ OK ]
[root@RHELv4u3 ~]# ps -C httpd
PID TTY TIME CMD
4573 ? 00:00:00 httpd
4576 ? 00:00:00 httpd
4577 ? 00:00:00 httpd
4578 ? 00:00:00 httpd
4579 ? 00:00:00 httpd
4580 ? 00:00:00 httpd
4581 ? 00:00:00 httpd
4582 ? 00:00:00 httpd
4583 ? 00:00:00 httpd
[root@RHELv4u3 ~]#
And here is Apache running on Ubuntu, named as apache2.
root@laika:~# ps -C apache2
PID TTY TIME CMD
6170 ? 00:00:00 apache2
6248 ? 00:00:01 apache2
6249 ? 00:00:01 apache2
6250 ? 00:00:00 apache2
6251 ? 00:00:01 apache2
6252 ? 00:00:01 apache2
7520 ? 00:00:01 apache2
8943 ? 00:00:01 apache2
root@laika:~# /etc/init.d/apache2 status
* Usage: /etc/init.d/apache2 {start|stop|restart|reload|force-reload}
root@laika:~#
To verify that apache is running, open a web browser on the web server, and browse to
http://localhost. An Apache test page should be shown. The http://localhosts/manual
url will give you an extensive Apache manual. The second test is to connect to your
Apache from another computer.
61.4. apache configuration
Configuring Apache changed a bit the past couple of years. But it still takes place in
/etc/httpd or /etc/apache.
[root@RHELv4u3 ~]# cd /etc/httpd/
[root@RHELv4u3 httpd]# ll
total 32
lrwxrwxrwx 1 root root 25 Jan 24 09:28 build -> ../../usr/lib/httpd/build
drwxr-xr-x 7 root root 4096 Jan 24 08:48 conf
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![introduction to apache
drwxr-xr-x 2 root root 4096 Jan 24 09:29 conf.d
lrwxrwxrwx 1 root root 19 Jan 24 08:48 logs -> ../../var/log/httpd
lrwxrwxrwx 1 root root 27 Jan 24 08:48 modules -> ../../usr/lib/httpd/modules
lrwxrwxrwx 1 root root 13 Jan 24 08:48 run -> ../../var/run
[root@RHELv4u3 httpd]#
The main configuration file for the Apache server on RHEL is /etc/httpd/conf/
httpd.conf, on Ubuntu it is /etc/apache2/apache2.conf. The file explains itself, and
contains examples for how to set up virtual hosts or configure access.
61.5. virtual hosts
Virtual hosts can be defined by ip-address, by port or by name (host record). (The new
way of defining virtual hosts is through seperate config files in the conf.d directory.)
Below is a very simple virtual host definition.
[root@rhel4 conf]# tail /etc/httpd/conf/httpd.conf
#
# This is a small test website
#
<VirtualHost testsite.local:80>
ServerAdmin webmaster@testsite.local
DocumentRoot /var/www/html/testsite/
ServerName testsite.local
ErrorLog logs/testsite.local-error_log
CustomLog logs/testsite.local-access_log common
</VirtualHost>
[root@rhel4 conf]#
Should you put this little index.html file in the directory mentioned in the above
screenshot, then you can access this humble website.
[root@rhel4 conf]# cat /var/www/html/testsite/index.html
<html>
<head><title>Test Site</title></head>
<body>
<p>This is the test site.</p>
</body>
</html>
Below is a sample virtual host configuration. This virtual hosts overrules the default
Apache ErrorDocument directive.
<VirtualHost 83.217.76.245:80>
ServerName cobbaut.be
ServerAlias www.cobbaut.be
DocumentRoot /home/paul/public_html
ErrorLog /home/paul/logs/error_log
CustomLog /home/paul/logs/access_log common
ScriptAlias /cgi-bin/ /home/paul/cgi-bin/
<Directory /home/paul/public_html>
Options Indexes IncludesNOEXEC FollowSymLinks
allow from all
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![introduction to apache
</Directory>
ErrorDocument 404 http://www.cobbaut.be/cobbaut.php
</VirtualHost>
61.6. aliases and redirects
Apache supports aliases for directories, like this example shows.
Alias /paul/ "/home/paul/public_html/"
Similarly, content can be redirected to another website or web server.
Redirect permanent /foo http://www.foo.com/bar
61.7. securing directories with htpasswd
and .htaccess
You can secure files and directories in your website with a userid/password. First,
enter your website, and use the htpasswd command to create a .htpasswd file that
contains a userid and an (encrypted) password.
[root@rhel4 testsite]# htpasswd -c .htpasswd pol
New password:
Re-type new password:
Adding password for user pol
[root@rhel4 testsite]# cat .htpasswd
pol:x5vZlyw1V6KXE
[root@rhel4 testsite]#
You can add users to this file, just don't use the -c switch again.
[root@rhel4 testsite]# htpasswd .htpasswd kim
New password:
Re-type new password:
Adding password for user kim
[root@rhel4 testsite]# cat .htpasswd
pol:x5vZlyw1V6KXE
kim:6/RbvugwsgOI6
[root@rhel4 testsite]#
You have now defined two users. Next create a subsdirectory that you want to protect
with these two accounts. And put the following .htaccess file in that subdirectory.
[root@rhel4 kimonly]# pwd
/var/www/html/testsite/kimonly
[root@rhel4 kimonly]# cat .htaccess
AuthUserFile /var/www/html/testsite/.htpasswd
AuthGroupFile /dev/null
AuthName "test access title"
AuthType Basic
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![introduction to apache
<Limit GET POST>
require valid-user
</Limit>
[root@rhel4 kimonly]#
Finally, don't forget to verify that AllowOverride is set to All in the general Apache
configuration file.
# AllowOverride controls what directives may be placed in .htaccess files.
# It can be "All", "None", or any combination of the keywords:
# Options FileInfo AuthConfig Limit
#
AllowOverride All
From now on, when a user accesses a file in that subdirectory, that user will have to
provide a userid/password combo that is defined in your .htpasswd.
61.8. more on .htaccess
You can do much more with .htaccess. One example is to use .htaccess to prevent
people from certain domains to access your website. Like in this case, where a number
of referer spammers are blocked from the website.
paul@lounge:~/cobbaut.be$ cat .htaccess
# Options +FollowSymlinks
RewriteEngine On
RewriteCond %{HTTP_REFERER} ^http://(www.)?buy-adipex.fw.nu.*$ [OR]
RewriteCond %{HTTP_REFERER} ^http://(www.)?buy-levitra.asso.ws.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://(www.)?buy-tramadol.fw.nu.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://(www.)?buy-viagra.lookin.at.*$ [NC,OR]
...
RewriteCond %{HTTP_REFERER} ^http://(www.)?www.healthinsurancehelp.net.*$ [NC]
RewriteRule .* - [F,L]
paul@lounge:~/cobbaut.be$
61.9. traffic
Apache keeps a log of all visitors. The webalizer is often used to parse this log into
nice html statistics.
61.10. practice: apache
1. Verify that Apache is installed and running.
2. Browse to the Apache HTML manual from another computer.
3. Create a virtual hosts that listens to port 8247.
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![Chapter 62. introduction to squid
Table of Contents
62.1. about proxy servers .................................................................................... 628
62.2. squid proxy server ..................................................................................... 629
62.1. about proxy servers
usage
A proxy server is a server that caches the internet. Clients connect to the proxy server
with a request for an internet server. The proxy server will connect to the internet
server on behalf of the client. The proxy server will also cache the pages retrieved
from the internet server. A proxy server may provide pages from his cache to a client,
instead of connecting to the internet server to retrieve the (same) pages.
A proxy server has two main advantages. It improves web surfing speed when
returning cached data to clients, and it reduces the required bandwidth (cost) to the
internet.
Smaller organizations sometimes put the proxy server on the same physical computer
that serves as a NAT to the internet. In larger organizations, the proxy server is one
of many servers in the DMZ.
When web traffic passes via a proxy server, it is common practice to configure the
proxy with extra settings for access control. Access control in a proxy server can
mean user account access, but also website(url), ip-address or dns restrictions.
open proxy servers
You can find lists of open proxy servers on the internet that enable you to surf
anonymously. This works when the proxy server connects on your behalf to a website,
without logging your ip-address. But be careful, these (listed) open proxy servers
could be created in order to eavesdrop upon their users.
squid
This chapter is an introduction to the squid proxy server (http://www.squid-
cache.org). The version used is 2.5.
[root@RHEL4 ~]# rpm -qa | grep squid
squid-2.5.STABLE6-3.4E.12
[root@RHEL4 ~]#
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![introduction to squid
62.2. squid proxy server
/etc/squid/squid.conf
Squid's main configuration file is /etc/squid/squid.conf. The file explains every
parameter in great detail. It can be a good idea to start by creating a backup of this file.
[root@RHEL4 /etc/squid/]# cp squid.conf squid.conf.original
/var/spool/squid
The squid proxy server stores its cache by default in /var/spool/squid. This setting
is configurable in /etc/squid/squid.conf.
[root@RHEL4 ~]# grep "^# cache_dir" /etc/squid/squid.conf
# cache_dir ufs /var/spool/squid 100 16 256
It is possible that in a default setup where squid has never run, that the /var/spool/
squid directories do not exist.
[root@RHEL4 ~]# ls -al /var/spool/squid
ls: /var/spool/squid: No such file or directory
Running squid -z will create the necessary squid directories.
[root@RHEL4 ~]# squid -z
2008/09/22 14:07:47| Creating Swap Directories
[root@RHEL4 ~]# ls -al /var/spool/squid
total 80
drwxr-x--- 18 squid squid 4096 Sep 22 14:07 .
drwxr-xr-x 26 root root 4096 May 30 2007 ..
drwxr-xr-x 258 squid squid 4096 Sep 22 14:07 00
drwxr-xr-x 258 squid squid 4096 Sep 22 14:07 01
drwxr-xr-x 258 squid squid 4096 Sep 22 14:07 02
...
port 3128 or port 8080
By default the squid proxy server will bind to port 3128 to listen to incoming requests.
[root@RHEL4 ~]# grep "default port" /etc/squid/squid.conf
# The default port number is 3128.
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![introduction to squid
Many organizations use port 8080 instead.
[root@RHEL4 ~]# grep 8080 /etc/squid/squid.conf
http_port 8080
/var/log/squid
The standard log file location for squid is /var/log/squid.
[root@RHEL4 ~]# grep "/var/log" /etc/squid/squid.conf
# cache_access_log /var/log/squid/access.log
# cache_log /var/log/squid/cache.log
# cache_store_log /var/log/squid/store.log
access control
The default squid setup only allows localhost access. To enable access for a private
network range, look for the "INSERT YOUR OWN RULE(S) HERE..." sentence in
squid.conf and add two lines similar to the screenshot below.
# INSERT YOUR OWN RULE(S) HERE TO ALLOW ACCESS FROM YOUR CLIENTS
acl company_network src 192.168.1.0/24
http_access allow company_network
Restart the squid server, and now the local private network can use the proxy cache.
testing squid
First, make sure that the server running squid has access to the internet.
[root@RHEL4 ~]# wget -q http://linux-training.be/index.html
[root@RHEL4 ~]# ls -l index.html
-rw-r--r-- 1 root root 2269 Sep 18 13:18 index.html
[root@RHEL4 ~]#
Then configure a browser on a client to use the proxy server. OR you could set the
HTTP_PROXY (sometimes http_proxy) variable to point command line programs
to the proxy.
[root@fedora ~]# export HTTP_PROXY=http://192.168.1.39:8080
[root@ubuntu ~]# export http_proxy=http://192.168.1.39:8080
Testing a client machine can then be done with wget (wget -q is used to simplify the
screenshot).
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![introduction to squid
[root@RHEL5 ~]# > /etc/resolv.conf
[root@RHEL5 ~]# wget -q http://www.linux-training.be/index.html
[root@RHEL5 ~]# ls -l index.html
-rw-r--r-- 1 root root 2269 Sep 18 2008 index.html
[root@RHEL5 ~]#
name resolution
You need name resolution working on the squid server, but you don't need name
resolution on the clients.
[paul@RHEL5 ~]$ wget http://grep.be
--14:35:44-- http://grep.be
Resolving grep.be... failed: Temporary failure in name resolution.
[paul@RHEL5 ~]$ export http_proxy=http://192.168.1.39:8080
[paul@RHEL5 ~]$ wget http://grep.be
--14:35:49-- http://grep.be/
Connecting to 192.168.1.39:8080... connected.
Proxy request sent, awaiting response... 200 OK
Length: 5390 (5.3K) [text/html]
Saving to: `index.html.1'
100%[================================>] 5,390 --.-K/s in 0.1s
14:38:29 (54.8 KB/s) - `index.html' saved [5390/5390]
[paul@RHEL5 ~]$
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![Introduction to ipv6
63.8. 6to4
6to4 is defined in rfc's 2893 and 3056 as one possible way to transition between ipv4
and ipv6 by creating an ipv6 tunnel.
It encodes an ipv4 address in an ipv6 address that starts with 2002. For example
192.168.1.42/24 will be encoded as:
2002:c0a8:12a:18::1
You can use the command below to convert any ipv4 address to this range.
paul@ubu1010:~$ printf "2002:%02x%02x:%02x%02x:%04x::1n" `echo 192.168.1.42/24
|tr "./" " "`
2002:c0a8:012a:0018::1
63.9. ISP
Should you be so lucky to get an ipv6 address from an isp, then it will start with 2001:.
63.10. non routable addresses
Comparable to example.com for DNS, the following ipv6 address ranges are reserved
for examples, and not routable on the internet.
3fff:ffff::/32
2001:0db8::/32
63.11. ping6
Use ping6 to test connectivity between ipv6 hosts. You need to specify the interface
(there is no routing table for 'random' generated ipv6 link local addresses).
[root@fedora14 ~]# ping6 -I eth0 fe80::a00:27ff:fecd:7ffc
PING fe80::a00:27ff:fecd:7ffc(fe80::a00:27ff:fecd:7ffc) from fe80::a00:27ff:fe3c:4346 et
64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=1 ttl=64 time=0.586 ms
64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=2 ttl=64 time=3.95 ms
64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=3 ttl=64 time=1.53 ms
Below a multicast ping6 that recieves replies from three ip6 hosts on the same
network.
[root@fedora14 ~]# ping6 -I eth0 ff02::1
PING ff02::1(ff02::1) from fe80::a00:27ff:fe3c:4346 eth0: 56 data bytes
64 bytes from fe80::a00:27ff:fe3c:4346: icmp_seq=1 ttl=64 time=0.598 ms
64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=1 ttl=64 time=1.87 ms (DUP!)
64 bytes from fe80::8e7b:9dff:fed6:dff2: icmp_seq=1 ttl=64 time=535 ms (DUP!)
64 bytes from fe80::a00:27ff:fe3c:4346: icmp_seq=2 ttl=64 time=0.106 ms
64 bytes from fe80::8e7b:9dff:fed6:dff2: icmp_seq=2 ttl=64 time=1.79 ms (DUP!)
64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=2 ttl=64 time=2.48 ms (DUP!)
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![Chapter 64. mysql
Table of Contents
64.1. about mysql ................................................................................................ 643
64.2. installing mysql .......................................................................................... 643
64.3. mysql user management ............................................................................. 644
64.4. mysql databases ......................................................................................... 646
64.5. mysql tables ............................................................................................... 647
64.6. mysql records ............................................................................................. 649
64.7. mysql triggers ............................................................................................ 650
64.1. about mysql
MySQL is a database server that understands Structured Query Language (SQL).
MySQL was developed by the Swedish Company MySQL AB. The first release was
in 1995. In 2008 MySQL AB was bought by Sun Microsystems (which is now owned
by Oracle).
MySQL is very popular for websites in combination with PHP and Apache (the M
in LAMP servers), but mysql is also used in organizations with huge databases like
Facebook, Flickr, Google, Nokia, Wikipedia and Youtube.
64.2. installing mysql
On Debian/Ubuntu you can use aptitude install mysql-server to install the mysql
server and client. On this particular system the -R switch is used to avoid installation
of apparmor and exim4 (both are recommended but not required for mysql-server).
root@laika:~# aptitude -R install mysql-server
To verify the installed version, use dpkg -l on Debian/Ubuntu. This screenshot shows
version 5.0 installed.
paul@laika:~$ dpkg -l | grep mysql-server
ii mysql-server 5.0.51a-3ubuntu5.4 MySQL database server (meta...
ii mysql-server-5.0 5.0.51a-3ubuntu5.4 MySQL database server binaries
Issue rpm -q to get version information about MySQL on RHEL5.
[paul@RHEL52 ~]$ rpm -q mysql-server
mysql-server-5.0.45-7.el5
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![mysql
~/.my.cnf
You can save configuration in your home directory in the hidden file .my.cnf. In the
screenshot below we put the root password in .my.cnf.
paul@barry:~$ echo "[client]" > .my.cnf
paul@barry:~$ echo password=SecretPass >> .my.cnf
paul@barry:~$ cat .my.cnf
[client]
password=SecretPass
paul@barry:~$ mysql -u root
<cut MySQL Welcome Message>
mysql>
the mysql command line client
You can use the mysql command to take a look at the databases, and to execute SQL
queries on them. The screenshots below show you how. First, we log on to the mysql
server.
paul@laika:~$ mysql -u root -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or g.
Your MySQL connection id is 8
Server version: 5.0.51a-3ubuntu5.4 (Ubuntu)
Type 'help;' or 'h' for help. Type 'c' to clear the buffer.
mysql>
Next, we execute the command show databases. Every command must be terminated
by a delimiter. The default delimiter is ; (the semicolon).
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| demodb |
| mysql |
| netsec |
| test |
+--------------------+
5 rows in set (0.00 sec)
Type exit to get back to your Linux command prompt.
mysql> exit
Bye
paul@laika:~$
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![introduction to SELinux(draft)
65.2. selinux modes
selinux knows three modes: enforcing, permissive and disabled. The enforcing mode
will enforce policies, and may deny access based on selinux rules. The permissive
mode will not enforce policies, but can still log actions that would have been denied
in enforcing mode. The disabled mode disables selinux.
65.3. activating selinux
On RHEL you can use the GUI tool to activate selinux, on Debian there is the selinux-
activate command.Activation requires a reboot.
root@deb503:~# selinux-activate
Activating SE Linux
Searching for GRUB installation directory ... found: /boot/grub
Searching for default file ... found: /boot/grub/default
Testing for an existing GRUB menu.lst file ... found: /boot/grub/menu.lst
Searching for splash image ... none found, skipping ...
Found kernel: /boot/vmlinuz-2.6.26-2-686
Updating /boot/grub/menu.lst ... done
SE Linux is activated. You may need to reboot now.
65.4. getenforce
Use getenforce to verify whether selinux is enforced, disabled or permissive.
[root@rhel55 ~]# getenforce
Permissive
The /selinux/enforce file contains 1 when enforcing, and 0 when permissive mode
is ative.
root@fedora13 ~# cat /selinux/enforce
1root@fedora13 ~#
65.5. setenforce
You can use setenforce to switch between the Permissive or the Enforcing state
once selinux is activated..
[root@rhel55 ~]# setenforce Enforcing
[root@rhel55 ~]# getenforce
Enforcing
[root@rhel55 ~]# setenforce Permissive
654](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-660-320.jpg)
![introduction to SELinux(draft)
[root@rhel55 ~]# getenforce
Permissive
65.6. sestatus
You can see the current selinux status and policy with the sestatus command.
[root@rhel55 ~]# sestatus
SELinux status: enabled
SELinuxfs mount: /selinux
Current mode: permissive
Mode from config file: permissive
Policy version: 21
Policy from config file: targeted
65.7. logging
Verify that syslog is running and activated on boot to enable logging of deny
messages in /var/log/messages.
[root@rhel55 ~]# chkconfig --list syslog
syslog 0:off 1:off 2:on 3:on 4:on 5:on 6:off
Verify that auditd is running and activated on boot to enable logging of easier to read
messages in /var/log/audit/audit.log.
[root@rhel55 ~]# chkconfig --list auditd
auditd 0:off 1:off 2:on 3:on 4:on 5:on 6:off
If not activated, then run chkconfig --levels 2345 auditd on and service auditd start.
[root@rhel55 ~]# service auditd status
auditd (pid 1660) is running...
[root@rhel55 ~]# service syslog status
syslogd (pid 1688) is running...
klogd (pid 1691) is running...
The /var/log/messages log file will tell you that selinux is disabled.
root@deb503:~# grep -i selinux /var/log/messages
Jun 25 15:59:34 deb503 kernel: [ 0.084083] SELinux: Disabled at boot.
Or that it is enabled.
root@deb503:~# grep SELinux /var/log/messages | grep -i Init
655](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-661-320.jpg)
![introduction to SELinux(draft)
Jun 25 15:09:52 deb503 kernel: [ 0.084094] SELinux: Initializing.
65.8. DAC or MAC
Standard Unix permissions use Discretionary Access Control to set permissions on
files. This means that a user that owns a file, can make it world readable by typing
chmod 777 $file.
With selinux the kernel will enforce Mandatory Access Control which strictly
controls what processes or threads can do with files (superseding DAC). Processes
are confined by the kernel to the minimum access they require.
65.9. ls -Z
To see the DAC permissions on a file, use ls -l to display user and group owner and
permissions (here rw-r--r--).
root@deb503:~/selinux# touch test42.txt
root@deb503:~/selinux# ls -l
total 0
-rw-r--r-- 1 root root 0 2010-06-25 15:38 test42.txt
For MAC permissions there is new -Z option added to ls. The output shows
an selinux user named unconfined_u, a role named object_r, a type named
unconfined_home_t, and a level S0.
root@deb503:~/selinux# ls -Z
unconfined_u:object_r:unconfined_home_t:s0 test42.txt
65.10. /selinux
When selinux is active, there is a new virtual file system named /selinux. (You can
compare it to /proc and /dev.)
[root@RHEL5 ~]# ls /selinux/
access context mls
avc create null
booleans disable policyvers
checkreqprot enforce relabel
commit_pending_bools load user
compat_net member
Although some files in /selinux appear wih size 0, they often contain a boolean value.
Check /selinux/enforce to see if selinux is running in enforced mode.
656](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-662-320.jpg)
![introduction to SELinux(draft)
[root@RHEL5 ~]# ls -l /selinux/enforce
-rw-r--r-- 1 root root 0 Apr 29 08:21 /selinux/enforce
[root@RHEL5 ~]# echo `cat /selinux/enforce`
1
65.11. /etc/selinux/config
The main configuration file for selinux is /etc/selinux/config. When in permissive
mode, the file looks like this.
[root@rhel55 ~]# more /etc/selinux/config
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - SELinux is fully disabled.
SELINUX=permissive
# SELINUXTYPE= type of policy in use. Possible values are:
# targeted - Only targeted network daemons are protected.
# strict - Full SELinux protection.
SELINUXTYPE=targeted
65.12. identity
The SELinux Identity of a user is distinct from the user ID. An identity is part of
a security context, and (via domains) determines what you can do. The screenshot
shows user root having identity user_u.
[root@rhel55 ~]# id -Z
user_u:system_r:unconfined_t
65.13. type (or domain)
The selinux domain is the security context of a process. An selinux domain
determines what a process can do. The screenshot shows init running in domain init_t
and the mingetty's running in domain getty_t.
[root@RHEL5 ~]# ps fax -Z | grep init_t
system_u:system_r:init_t 1 ? Ss 0:01 init [3]
[root@RHEL5 ~]# ps fax -Z | grep getty_t
system_u:system_r:getty_t 2941 tty1 Ss+ 0:00 /sbin/mingetty tty1
system_u:system_r:getty_t 2942 tty2 Ss+ 0:00 /sbin/mingetty tty2
The selinux type is similar to an selinux domain, but refers to directories and files
instead of processes.
657](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-663-320.jpg)
![introduction to SELinux(draft)
65.14. role
The selinux role defines the domains that can be used. A role is denied to enter a
domain, unless the role is explicitely authorized to do so.
65.15. security context
The combination of identity, role and domain or type make up the selinux security
context. The id will show you your security context in the form identity:role:domain.
[paul@RHEL5 ~]$ id | cut -d' ' -f4
context=user_u:system_r:unconfined_t
The ls -Z command shows the security context for a file in the form identity:role:type.
[paul@RHEL5 ~]$ ls -Z test
-rw-rw-r-- paul paul user_u:object_r:user_home_t test
The security context for processes visible in /proc defines both the type (of the file
in /proc) and the domain (of the running process). Let's take a look at the init process
and /proc/1/ .
The init process runs in domain init_t.
[root@RHEL5 ~]# ps -ZC init
LABEL PID TTY TIME CMD
system_u:system_r:init_t 1 ? 00:00:01 init
The /proc/1/ directory, which identifies the init process, has type init_t.
[root@RHEL5 ~]# ls -Zd /proc/1/
dr-xr-xr-x root root system_u:system_r:init_t /proc/1/
It is not a coincidence that the domain of the init process and the type of /proc/1/
are both init_t.
Don't try to use chcon on /proc! It will not work.
65.16. transition
An selinux transition (aka an selinux labelling) determines the security context that
will be assigned. A transition of process domains is used when you execute a process.
A transition of file type happens when you create a file.
An example of file type transition.
658](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-664-320.jpg)
![introduction to SELinux(draft)
[paul@RHEL5 ~]$ touch test
[paul@RHEL5 ~]$ touch /tmp/test
[paul@RHEL5 ~]$ ls -Z test
-rw-rw-r-- paul paul user_u:object_r:user_home_t test
[paul@RHEL5 ~]$ ls -Z /tmp/test
-rw-rw-r-- paul paul user_u:object_r:tmp_t /tmp/test
[paul@RHEL5 ~]$
65.17. policy
Everything comes together in an selinux policy. Policies define user access to roles,
role access to domains and domain access to types.
65.18. extended attributes
Extended attributes are use by selinux to store security contexts. These attributes can
be viewed with ls when selinux is running.
[root@RHEL5 home]# ls --context
drwx------ paul paul system_u:object_r:user_home_dir_t paul
drwxr-xr-x root root user_u:object_r:user_home_dir_t project42
drwxr-xr-x root root user_u:object_r:user_home_dir_t project55
[root@RHEL5 home]# ls -Z
drwx------ paul paul system_u:object_r:user_home_dir_t paul
drwxr-xr-x root root user_u:object_r:user_home_dir_t project42
drwxr-xr-x root root user_u:object_r:user_home_dir_t project55
[root@RHEL5 home]#
When selinux is not running, then getfattr is the tool to use.
[root@RHEL5 etc]# getfattr -m . -d hosts
# file: hosts
security.selinux="system_u:object_r:etc_t:s0000"
65.19. process security context
A new option is added to ps to see the selinux security context of processes.
[root@RHEL5 etc]# ps -ZC mingetty
LABEL PID TTY TIME CMD
system_u:system_r:getty_t 2941 tty1 00:00:00 mingetty
system_u:system_r:getty_t 2942 tty2 00:00:00 mingetty
65.20. chcon
Use chcon to change the selinux security context.
659](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-665-320.jpg)
![introduction to SELinux(draft)
This example shows how to use chcon to change the type of a file.
[root@rhel55 ~]# ls -Z /var/www/html/test42.txt
-rw-r--r-- root root user_u:object_r:httpd_sys_content_t /var/www/html/test42.txt
[root@rhel55 ~]# chcon -t samba_share_t /var/www/html/test42.txt
[root@rhel55 ~]# ls -Z /var/www/html/test42.txt
-rw-r--r-- root root user_u:object_r:samba_share_t /var/www/html/test42.txt
65.21. a practical example
Tha apache webserver is by default targeted with selinux. The next screenshot
shows that any file created in /var/www/html will by default get the
http_sys_content_t type.
[root@rhel55 ~]# touch /var/www/html/test42.txt
[root@rhel55 ~]# ls -Z /var/www/html/test42.txt
-rw-r--r-- root root user_u:object_r:httpd_sys_content_t /var/www/html/test42.txt
Files created elsewhere do not get this type.
[root@rhel55 ~]# touch /root/test42.txt
[root@rhel55 ~]# ls -Z /root/test42.txt
-rw-r--r-- root root user_u:object_r:user_home_t /root/test42.txt
Make sure apache runs.
[root@rhel55 ~]# service httpd start
[ OK ]
Will this work ? Yes it does.
[root@rhel55 ~]# wget http://localhost/test42.txt
--2010-06-26 15:40:28-- http://localhost/test42.txt
Resolving localhost... 127.0.0.1
Connecting to localhost|127.0.0.1|:80... connected.
HTTP request sent, awaiting response... 200 OK
....
Why does this work ? Because apache runs in the httpd_t domain.
[root@rhel55 ~]# ps -ZC httpd
LABEL PID TTY TIME CMD
user_u:system_r:httpd_t 2979 ? 00:00:07 httpd
user_u:system_r:httpd_t 2981 ? 00:00:00 httpd
user_u:system_r:httpd_t 2982 ? 00:00:00 httpd
user_u:system_r:httpd_t 2983 ? 00:00:00 httpd
user_u:system_r:httpd_t 2984 ? 00:00:00 httpd
user_u:system_r:httpd_t 2985 ? 00:00:00 httpd
user_u:system_r:httpd_t 2986 ? 00:00:00 httpd
660](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-666-320.jpg)
![introduction to SELinux(draft)
user_u:system_r:httpd_t 2987 ? 00:00:00 httpd
user_u:system_r:httpd_t 2988 ? 00:00:00 httpd
So let's try to change the selinux type of this file.
[root@rhel55 ~]# chcon -t samba_share_t /var/www/html/test42.txt
[root@rhel55 ~]# ls -Z /var/www/html/test42.txt
-rw-r--r-- root root user_u:object_r:samba_share_t /var/www/html/test42.txt
There are two possibilities now: either it works, or it fails. It works when selinux is
in permissive mode, it fails when in enforcing mode.
[root@rhel55 ~]# wget http://localhost/test42.txt
--2010-06-26 15:41:33-- http://localhost/test42.txt
Resolving localhost... 127.0.0.1
Connecting to localhost|127.0.0.1|:80... connected.
HTTP request sent, awaiting response... 200 OK
...
The log file clearly shows that it would have failed in enforcing mode.
[root@rhel55 ~]# grep test42 /var/log/audit/audit.log
type=AVC msg=audit(1277559693.656:105): avc: denied { getattr } for
pid=2982 comm="httpd" path="/var/www/html/test42.txt" dev=dm-0 ino=1974
99 scontext=user_u:system_r:httpd_t:s0 tcontext=user_u:object_r:samba_s
hare_t:s0 tclass=file
type=AVC msg=audit(1277559693.658:106): avc: denied { read } for pid
=2982 comm="httpd" name="test42.txt" dev=dm-0 ino=197499 scontext=user_
u:system_r:httpd_t:s0 tcontext=user_u:object_r:samba_share_t:s0 tclass=
file
661](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-667-320.jpg)
![Appendix B. keyboard settings
B.1. about keyboard layout
Many people (like US-Americans) prefer the default US-qwerty keyboard layout. So
when you are not from the USA and want a local keyboard layout on your system,
then the best practice is to select this keyboard at installation time. Then the keyboard
layout will always be correct. Also, whenever you use ssh to remotely manage a linux
system, your local keyboard layout will be used, independent of the server keyboard
configuration. So you will not find much information on changing keyboard layout
on the fly on linux, because not many people need it. Below are some tips to help you.
B.2. X Keyboard Layout
This is the relevant portion in /etc/X11/xorg.conf, first for Belgian azerty, then for
US-qwerty.
[paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf
Option "XkbModel" "pc105"
Option "XkbLayout" "be"
[paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf
Option "XkbModel" "pc105"
Option "XkbLayout" "us"
When in Gnome or KDE or any other graphical environment, look in the graphical
menu in preferences, there will be a keyboard section to choose your layout. Use the
graphical menu instead of editing xorg.conf.
B.3. shell keyboard layout
When in bash, take a look in the /etc/sysconfig/keyboard file. Below a sample US-
qwerty configuration, followed by a Belgian azerty configuration.
[paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard
KEYBOARDTYPE="pc"
KEYTABLE="us"
[paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard
KEYBOARDTYPE="pc"
KEYTABLE="be-latin1"
The keymaps themselves can be found in /usr/share/keymaps or /lib/kbd/keymaps.
665](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-671-320.jpg)
![keyboard settings
[paul@RHEL5 ~]$ ls -l /lib/kbd/keymaps/
total 52
drwxr-xr-x 2 root root 4096 Apr 1 00:14 amiga
drwxr-xr-x 2 root root 4096 Apr 1 00:14 atari
drwxr-xr-x 8 root root 4096 Apr 1 00:14 i386
drwxr-xr-x 2 root root 4096 Apr 1 00:14 include
drwxr-xr-x 4 root root 4096 Apr 1 00:14 mac
lrwxrwxrwx 1 root root 3 Apr 1 00:14 ppc -> mac
drwxr-xr-x 2 root root 4096 Apr 1 00:14 sun
666](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-672-320.jpg)
![Appendix C. hardware
C.1. buses
about buses
Hardware components communicate with the Central Processing Unit or cpu over
a bus. The most common buses today are usb, pci, agp, pci-express and pcmcia aka
pc-card. These are all Plag and Play buses.
Older x86 computers often had isa buses, which can be confugred using jumpers or
dip switches.
/proc/bus
To list the buses recognized by the Linux kernel on your computer, look at the
contents of the /proc/bus/ directory (screenshot from Ubuntu 7.04 and RHEL4u4
below).
root@laika:~# ls /proc/bus/
input pccard pci usb
[root@RHEL4b ~]# ls /proc/bus/
input pci usb
Can you guess which of these two screenshots was taken on a laptop ?
/usr/sbin/lsusb
To list all the usb devices connected to your system, you could read the contents
of /proc/bus/usb/devices (if it exists) or you could use the more readable output of
lsusb, which is executed here on a SPARC system with Ubuntu.
root@shaka:~# lsusb
Bus 001 Device 002: ID 0430:0100 Sun Microsystems, Inc. 3-button Mouse
Bus 001 Device 003: ID 0430:0005 Sun Microsystems, Inc. Type 6 Keyboard
Bus 001 Device 001: ID 04b0:0136 Nikon Corp. Coolpix 7900 (storage)
root@shaka:~#
/var/lib/usbutils/usb.ids
The /var/lib/usbutils/usb.ids file contains a gzipped list of all known usb devices.
667](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-673-320.jpg)
![hardware
CPU0 CPU1
0: 1320048 555 IO-APIC-edge timer
1: 10224 7 IO-APIC-edge i8042
7: 0 0 IO-APIC-edge parport0
8: 2 1 IO-APIC-edge rtc
10: 3062 21 IO-APIC-fasteoi acpi
12: 131 2 IO-APIC-edge i8042
15: 47073 0 IO-APIC-edge ide1
18: 0 1 IO-APIC-fasteoi yenta
19: 31056 1 IO-APIC-fasteoi libata, ohci1394
20: 19042 1 IO-APIC-fasteoi eth0
21: 44052 1 IO-APIC-fasteoi uhci_hcd:usb1, uhci_hcd:usb2,...
22: 188352 1 IO-APIC-fasteoi ra0
23: 632444 1 IO-APIC-fasteoi nvidia
24: 1585 1 IO-APIC-fasteoi VIA82XX-MODEM, VIA8237
dmesg
You can also use dmesg to find irq's allocated at boot time.
paul@laika:~$ dmesg | grep "irq 1[45]"
[ 28.930069] ata3: PATA max UDMA/133 cmd 0x1f0 ctl 0x3f6 bmdma 0x2090 irq 14
[ 28.930071] ata4: PATA max UDMA/133 cmd 0x170 ctl 0x376 bmdma 0x2098 irq 15
C.3. io ports
about io ports
Communication in the other direction, from CPU to device, happens through IO
ports. The CPU writes data or control codes to the IO port of the device. But this is
not only a one way communication, the CPU can also use a device's IO port to read
status information about the device. Unlike interrupts, ports cannot be shared!
/proc/ioports
You can see a listing of your system's IO ports via /proc/ioports.
[root@RHEL4b ~]# cat /proc/ioports
0000-001f : dma1
0020-0021 : pic1
0040-0043 : timer0
0050-0053 : timer1
0060-006f : keyboard
0070-0077 : rtc
0080-008f : dma page reg
00a0-00a1 : pic2
00c0-00df : dma2
00f0-00ff : fpu
0170-0177 : ide1
02f8-02ff : serial
...
669](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-675-320.jpg)
![hardware
C.4. dma
about dma
A device that needs a lot of data, interrupts and ports can pose a heavy load on the
cpu. With dma or Direct Memory Access a device can gain (temporary) access to
a specific range of the ram memory.
/proc/dma
Looking at /proc/dma might not give you the information that you want, since it only
contains currently assigned dma channels for isa devices.
root@laika:~# cat /proc/dma
1: parport0
4: cascade
pci devices that are using dma are not listed in /proc/dma, in this case dmesg can be
useful. The screenshot below shows that during boot the parallel port received dma
channel 1, and the Infrared port received dma channel 3.
root@laika:~# dmesg | egrep -C 1 'dma 1|dma 3'
[ 20.576000] parport: PnPBIOS parport detected.
[ 20.580000] parport0: PC-style at 0x378 (0x778), irq 7, dma 1...
[ 20.764000] irda_init()
--
[ 21.204000] pnp: Device 00:0b activated.
[ 21.204000] nsc_ircc_pnp_probe() : From PnP, found firbase 0x2F8...
[ 21.204000] nsc-ircc, chip->init
670](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-676-320.jpg)
![Appendix F. introduction to vnc
F.1. About VNC
VNC can be configured in gnome or KDE using the Remote Desktop Preferences.
VNC can be used to run your desktop on another computer, and you can also use
it to see and take over the Desktop of another user. The last part can be useful for
help desks to show users how to do things. VNC has the added advantage of being
operating system independent, a lot of products (realvnc, tightvnc, xvnc, ...) use the
same protocol on Solaris, Linux, BSD and more.
F.2. VNC Server
Starting the vnc server for the first time.
[root@RHELv4u3 conf]# rpm -qa | grep -i vnc
vnc-server-4.0-8.1
vnc-4.0-8.1
[root@RHELv4u3 conf]# vncserver :2
You will require a password to access your desktops.
Password:
Verify:
xauth: creating new authority file /root/.Xauthority
New 'RHELv4u3.localdomain:2 (root)' desktop is RHELv4u3.localdomain:2
Creating default startup script /root/.vnc/xstartup
Starting applications specified in /root/.vnc/xstartup
Log file is /root/.vnc/RHELv4u3.localdomain:2.log
[root@RHELv4u3 conf]#
F.3. VNC Client
You can now use the vncviewer from another machine to connect to your vnc server.
It will default to a very simple graphical interface...
paul@laika:~$ vncviewer 192.168.1.49:2
VNC viewer version 3.3.7 - built Nov 20 2006 13:05:04
Copyright (C) 2002-2003 RealVNC Ltd.
Copyright (C) 1994-2000 AT&T Laboratories Cambridge.
See http://www.realvnc.com for information on VNC.
VNC server supports protocol version 3.8 (viewer 3.3)
Password:
VNC authentication succeeded
Desktop name "RHELv4u3.localdomain:2 (root)"
Connected to VNC server, using protocol version 3.3
...
675](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-681-320.jpg)
![introduction to vnc
If you don't like the simple twm window manager, you can comment out the last
two lines of ~/.vnc/xstartup and add a gnome-session & line to have vnc default to
gnome instead.
[root@RHELv4u3 ~]# cat .vnc/xstartup
#!/bin/sh
# Uncomment the following two lines for normal desktop:
# unset SESSION_MANAGER
# exec /etc/X11/xinit/xinitrc
[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
xsetroot -solid grey
vncconfig -iconic &
# xterm -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" &
# twm &
gnome-session &
[root@RHELv4u3 ~]#
Don't forget to restart your vnc server after changing this file.
[root@RHELv4u3 ~]# vncserver -kill :2
Killing Xvnc process ID 5785
[root@RHELv4u3 ~]# vncserver :2
New 'RHELv4u3.localdomain:2 (root)' desktop is RHELv4u3.localdomain:2
Starting applications specified in /root/.vnc/xstartup
Log file is /root/.vnc/RHELv4u3.localdomain:2.log
F.4. Practice VNC
1. Use VNC to connect from one machine to another.
676](https://image.slidesharecdn.com/linuxtraining-120930200745-phpapp01/85/Linux-training-682-320.jpg)
Linux training
- 2. Linux Fun Paul Cobbaut lt-2.0 Published Tue Sep 13 01:02:39 2011 Abstract This book is meant to be used in an instructor-led training. For self-study, the intent is to read this book next to a working Linux computer so you can immediately do every subject, practicing each command. This book is aimed at novice Linux system administrators (and might be interesting and useful for home users that want to know a bit more about their Linux system). However, this book is not meant as an introduction to Linux desktop applications like text editors, browsers, mail clients, multimedia or office applications. More information and free .pdf available at http://linux-training.be . Feel free to contact the authors: • Paul Cobbaut: paul.cobbaut@gmail.com, http://www.linkedin.com/in/cobbaut Contributors to the Linux Training project are: • Serge van Ginderachter: serge@ginsys.be, build scripts; infrastructure setup; minor stuff • Hendrik De Vloed: hendrik.devloed@ugent.be, buildheader.pl script We'd also like to thank our reviewers: • Wouter Verhelst: wouter@grep.be, http://grep.be • Geert Goossens: mail.goossens.geert@gmail.com, http://www.linkedin.com/in/geertgoossens • Elie De Brauwer: elie@de-brauwer.be, http://www.de-brauwer.be • Christophe Vandeplas: christophe@vandeplas.com, http://christophe.vandeplas.com • Bert Desmet: bert@devnox.be, http://bdesmet.be • Rich Yonts: richyonts@gmail.com, Copyright 2007-2011 Paul Cobbaut Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled 'GNU Free Documentation License'.
- 3. Table of Contents I. introduction to Linux ........................................................................................ 1 1. Unix history ................................................................................................ 2 2. licensing ...................................................................................................... 6 3. distributions ................................................................................................. 9 4. online help ................................................................................................ 15 II. first steps on the command line .................................................................... 18 5. man pages ................................................................................................. 19 6. working with directories ........................................................................... 23 7. working with files ..................................................................................... 33 8. working with file contents ........................................................................ 42 9. the Linux file tree ..................................................................................... 49 III. shell expansion .............................................................................................. 69 10. commands and arguments ....................................................................... 70 11. control operators ..................................................................................... 80 12. variables .................................................................................................. 86 13. shell history ............................................................................................. 96 14. file globbing .......................................................................................... 102 IV. pipes and commands .................................................................................. 109 15. redirection and pipes ............................................................................. 110 16. filters ..................................................................................................... 118 17. basic unix tools ..................................................................................... 130 V. vi ..................................................................................................................... 139 18. Introduction to vi .................................................................................. 140 VI. scripting ....................................................................................................... 150 19. scripting introduction ............................................................................ 151 20. scripting loops ....................................................................................... 157 21. scripting parameters .............................................................................. 164 22. more scripting ....................................................................................... 171 VII. local user management ............................................................................. 178 23. users ...................................................................................................... 179 24. groups .................................................................................................... 200 VIII. file security ............................................................................................... 206 25. standard file permissions ...................................................................... 207 26. advanced file permissions ..................................................................... 217 27. access control lists ................................................................................ 223 28. file links ................................................................................................ 227 IX. process management ................................................................................... 234 29. introduction to processes ...................................................................... 235 30. process priorities ................................................................................... 247 31. background jobs .................................................................................... 254 X. disk management .......................................................................................... 260 32. disk devices ........................................................................................... 261 33. disk partitions ........................................................................................ 274 34. file systems ........................................................................................... 282 35. mounting ............................................................................................... 290 36. introduction to uuid's ............................................................................ 299 iii
- 4. Linux Fun 37. introduction to raid ............................................................................... 304 38. logical volume management ................................................................. 312 XI. boot management ........................................................................................ 336 39. bootloader .............................................................................................. 337 40. init and runlevels .................................................................................. 349 XII. system management .................................................................................. 365 41. scheduling ............................................................................................. 366 42. logging .................................................................................................. 373 43. memory management ............................................................................ 384 44. package management ............................................................................ 391 XIII. network management .............................................................................. 407 45. general networking ................................................................................ 408 46. interface configuration .......................................................................... 417 47. network sniffing .................................................................................... 432 48. binding and bonding ............................................................................. 438 49. introduction to networking .................................................................... 447 XIV. kernel management ................................................................................. 462 50. the Linux kernel .................................................................................... 463 51. library management ............................................................................... 480 XV. backup management ................................................................................. 483 52. backup ................................................................................................... 484 XVI. samba ........................................................................................................ 493 53. samba .................................................................................................... 494 XVII. dns server ................................................................................................ 545 54. introduction to DNS .............................................................................. 546 55. Introduction to DNS ............................................................................. 553 56. naamresolutie en DNS .......................................................................... 577 57. advanced DNS ...................................................................................... 590 XVIII. dhcp server ............................................................................................ 597 58. Introduction to DHCP ........................................................................... 598 XIX. dhcp server ............................................................................................... 605 XX. iptables firewall ......................................................................................... 606 59. introduction to routers ........................................................................... 607 60. Firewall: iptables ................................................................................... 613 XXI. apache and squid ..................................................................................... 621 61. introduction to apache ........................................................................... 622 62. introduction to squid ............................................................................. 628 XXII. ipv6 .......................................................................................................... 632 63. Introduction to ipv6 .............................................................................. 633 XXIII. mysql database ...................................................................................... 642 64. mysql ..................................................................................................... 643 XXIV. selinux ..................................................................................................... 652 65. introduction to SELinux(draft) .............................................................. 653 XXV. Appendices .............................................................................................. 662 A. certifications ........................................................................................... 663 B. keyboard settings .................................................................................... 665 C. hardware ................................................................................................. 667 D. installing linux ....................................................................................... 671 iv
- 5. Linux Fun E. disk quotas .............................................................................................. 674 F. introduction to vnc .................................................................................. 675 G. cloning .................................................................................................... 677 H. License ................................................................................................... 679 Index .................................................................................................................... 686 v
- 6. List of Tables 1.1. Early Unix Timeline ........................................................................................ 3 1.2. Eighties Unix Timeline .................................................................................... 3 1.3. Current BSD Timeline ..................................................................................... 4 18.1. getting to command mode ......................................................................... 141 18.2. switch to insert mode ................................................................................. 141 18.3. replace and delete ...................................................................................... 142 18.4. undo and repeat .......................................................................................... 142 18.5. cut, copy and paste a line .......................................................................... 142 18.6. cut, copy and paste lines ............................................................................ 142 18.7. start and end of line ................................................................................... 143 18.8. join two lines ............................................................................................. 143 18.9. words .......................................................................................................... 144 18.10. save and exit vi ........................................................................................ 144 18.11. searching .................................................................................................. 145 18.12. replace ...................................................................................................... 145 18.13. read files and input .................................................................................. 145 18.14. text buffers ............................................................................................... 146 18.15. multiple files ............................................................................................ 146 18.16. abbreviations ............................................................................................ 146 23.1. Debian User Environment .......................................................................... 199 23.2. Red Hat User Environment ........................................................................ 199 25.1. Unix special files ....................................................................................... 209 25.2. standard Unix file permissions .................................................................. 209 25.3. Unix file permissions position ................................................................... 210 25.4. Octal permissions ....................................................................................... 212 32.1. ide device naming ...................................................................................... 264 32.2. scsi device naming ..................................................................................... 264 33.1. primary, extended and logical partitions .................................................... 275 33.2. Partition naming ......................................................................................... 275 38.1. disk partitioning example ........................................................................... 313 38.2. LVM Example ........................................................................................... 313 55.1. The first TLD's .......................................................................................... 557 55.2. new general purpose TLD's ....................................................................... 557 59.1. Packet Forwarding Exercise ...................................................................... 609 59.2. Packet Forwarding Solution ....................................................................... 611 vi
- 7. Part I. introduction to Linux
- 8. Chapter 1. Unix history Table of Contents 1.1. AT&T Bell Labs .............................................................................................. 3 1.2. The Unix Wars ................................................................................................ 3 1.3. University of California, Berkeley ................................................................... 4 1.4. GNU's not Unix ............................................................................................... 4 1.5. Linux ................................................................................................................ 4 This chapter briefly tells the history of Unix and where Linux fits in. 2
- 9. Unix history 1.1. AT&T Bell Labs In 1969 Dennis Ritchie and Ken Thompson wrote UNICS (Uniplexed Information and Computing System) at Bell Labs. Together with Douglas McIlroy they are seen as the creators of Unix. The name Unics is a play on the Multics Operating System for mainframe computers. Unics (later renamed to Unix) was written for mini-computers like the DEC PDP-series. In 1973 they decided to write Unix in C (instead of assembler), to make it portable to other computers. Unix was made available to universities, companies and the US government, including the full source code. This meant that every C programmer could make changes. By 1978 about 600 computers were running Unix. Table 1.1. Early Unix Timeline 1969-1977 1978-1980 1981 1982 4.1BSD BSD 4.1BSD SunOS 1.0 UNIX Time Sharing System Unix Unix System III 1.2. The Unix Wars The unity and openness that existed in the Unix world until 1977 was long gone by the end of the eighties. Different vendors of distinct versions of Unix tried to set the standard. Sun and AT&T joined the X/Open group to unify Unix. Other vendors joined the Open Software Foundation or OSF. These struggles were not good for Unix, allowing for new operating system families like OS/2, Novell Netware and Microsoft Windows NT to take big chunks of server market share in the early nineties. The table below shows the evolution of a united Unix into several Unixes in the eighties. Table 1.2. Eighties Unix Timeline 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 4.3BSD BSD Net/2 4.1BSD 4.3BSD NeXTSTEP SunOS1.0 SunOS3.2 Solaris SystemVr4 System V UnixWare System V AIX III + V HP-UX 3
- 10. Unix history 1.3. University of California, Berkeley Students of Berkeley were happy to join in the development of Bell Labs Unix, but were not so happy with the restrictive licensing. Unix was open source software, but it still required purchase of a license. So during the eighties, they rewrote all the Unix tools until they had a complete Unix-like operating system. By 1991, the BSD (Berkeley Software Distribution) branch of Unix was completely separate from the Bell Labs Unix. NetBSD, FreeBSD, and OpenBSD are three current Unix- like operating systems derived from the 1991 BSD Net/2 codebase. Sun Solaris, Microsoft Windows NT, and Apple Mac OS X all used source code from BSD. The table below shows operating systems still in use today that are in a way derived from the 1978-1981 BSD codebase. Table 1.3. Current BSD Timeline 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000-2008 FreeBSD BSD Net/2 NetBSD NetBSD OpenBSD NeXTSTEP Mac OS X Solaris 1.4. GNU's not Unix Largely because of discontent with the restrictive licensing on existing Unix implementations, Richard Stallman initiated the GNU Project in 1983. The GNU project aims to create free software. Development of the GNU operating system started, aiming to create a complete Unix-like branch, separate from the two other (BSD and Bell Labs). Today the GNU compiler gcc and most other GNU utilities (like bash) are among the most popular on many Unix-like systems. The official kernel of this project is GNU/Hurd, but you can hardly call that kernel a finished product. 1.5. Linux Where GNU/Hurd failed, the Linux kernel succeeded! In 1991 a Finnish student named Linus Torvalds started writing his own operating system for his intel 80386 computer. In January 1992, Linus decided to release Linux under the GNU GPL. Thanks to this, thousands of developers are now working on the Linux kernel. Linus Torvalds is in charge of the kernel developers. Contrary to popular belief, they are not all volunteers. Today big companies like Red Hat, Novell, IBM, Intel, SGI, Oracle, Montavista, Google, HP, NetApp, Cisco, Fujitsu, Broadcom, and others are actively paying developers to work on the Linux kernel. According to the Linux Foundation "over 3700 individual developers from 4
- 11. Unix history over 200 different companies have contributed to the kernel between 2005 and april 2008". 1057 developers from 186 different companies contributed code to make kernel version 2.6.23 into 2.6.24. 5
- 12. Chapter 2. licensing Table of Contents 2.1. proprietary ........................................................................................................ 7 2.2. bsd .................................................................................................................... 7 2.3. free software .................................................................................................... 7 2.4. GNU General Public License .......................................................................... 7 2.5. open source ...................................................................................................... 8 2.6. freeware ............................................................................................................ 8 2.7. more licenses... ................................................................................................. 8 This chapter briefly explains the different licenses used for distributing operating systems software. 6
- 13. licensing 2.1. proprietary IBM's aix, HP's hp-ux and Sun's Solaris 9 are delivered in binary form after purchase. You are not authorized to install or use these operating systems without paying a licensing fee. You are not authorized to distribute these copies to other people and you are not authorized to look at or change the closed source code of the operating system. This software is usually protected by copyright, patents and an extensive software license. 2.2. bsd bsd style licenses are close to the public domain. They essentially state that you can copy the software and do whatever you like with it. But you have to leave the copyright notice that refers to bsd. This license gives a lot of freedom but offers few protection to someone copying and selling your work. 2.3. free software Free software (not to be confused with freeware!) is defined as software with four freedoms. 0. The freedom to use the software for any purpose. 1. The freedom to study how the program works, and the freedom to make changes to it. (This requires access to the source code). 2. The freedom to distribute copies of the software to anyone else. 3. The freedom to distribute your modified copy (see 1). Detailed information about these freedoms can be found here http://www.gnu.org/ philosophy/free-sw.html. 2.4. GNU General Public License More and more software is being released under the GNU GPL (in 2006 Java was released under the GPL). The goal of the GPL is to guarantee that free software stays free. Everyone can work together on GPL software, knowing that the software will be freely available to everyone. The GPL can protect software, even in court. Free is to be understood as in freedom of speech, not to be confused with free as in not having to pay for your free beer. In other words, or even better, in other languages free software translates to vrije software (Dutch) or Logiciel Libre (French) whereas the free from free beer translates to gratis. 7
- 14. licensing Briefly explained, the GPL allows you to copy software, the GPL allows you to distribute (sell or give away) that software, and the GPL grants you the right to read and change the source code. But the person receiving or buying the software from you has the same rights. And should you decide to distribute GPL software of which you modified the source code, then you are obligated to put the same license on the modifications (and provide the source code of your modifications). 2.5. open source Because of the confusion between free software and freeware, the term open source has been coined instead of free software. In general, when talking open source software, we actually mean software with the four freedoms. 2.6. freeware Freeware is software that you can download and use without paying for it. This (usually) does not include source code (so the four freedoms do not apply) and does not give you any additional rights on copyrighted or patented parts of that software. For example the game Command and Conquer: Tiberian Sun was sold as proprietary in 1999 and is now (in 2011) available as freeware. 2.7. more licenses... There are many other licenses on operating system software (think cddl for Solaris 10). You should read and understand them before using any software. 8
- 15. Chapter 3. distributions Table of Contents 3.1. What is a distribution ? .................................................................................. 10 3.2. Linux Distributions ........................................................................................ 10 3.3. BSD Distributions .......................................................................................... 13 3.4. Major Vendors of Unix ................................................................................. 13 3.5. Solaris ............................................................................................................. 13 This chapter gives an overview of current Linux distributions and some Unix releases. 9
- 16. distributions 3.1. What is a distribution ? Unix comes in many flavors usually called distributions. A distribution (or in short distro) is a collection of software packages distributed on CD, online, or pre-installed on computers. All the software in a distribution is tested and integrated nicely into a common look and feel. Software is maintained (patched) by the distributor and is managed by an integrated package manager. Many distros have a central repository of approved software. Installing software from outside the distro can sometimes be cumbersome and may void your warranty on the system. 3.2. Linux Distributions There are hundreds of Linux distributions, just take a look at the distrowatch.com website. For many years, Red Hat, Suse, and Mandrake were considered the big three for end users. Red Hat is still the biggest commercial Linux vendor and since 2008 the most popular Linux distribution for home users is Ubuntu from Canonical. Linux distribution detection Depending on the distribution used, there are distinct files that contain the distribution version. The /etc/redhat-release file contains the Red Hat version on most of the Red Hat and Red Hat derived systems. Debian and Ubuntu systems contain /etc/debian-version. Note that Ubuntu was originally derived from Debian. paul@RHELv4u4:~$ cat /etc/redhat-release Red Hat Enterprise Linux AS release 4 (Nahant Update 4) serge@venusia:~$ cat /etc/debian_version lenny/sid The /etc/lsb-release file can be found on distributions that follow the Linux Standard Base. Other variations to these files are /etc/slackware-version, /etc/SuSE-release, /etc/gentoo-release and /etc/mandrake-release. serge@venusia:~$ cat /etc/lsb-release DISTRIB_ID=Ubuntu DISTRIB_RELEASE=8.04 DISTRIB_CODENAME=hardy DISTRIB_DESCRIPTION="Ubuntu 8.04.1" Red Hat Redhat has been a company since 1993. They distribute Red Hat Enterprise Linux (RHEL) to companies and manage the Fedora project. RHEL is probably the most popular Linux-based distro on servers. Fedora is a very popular and user friendly Linux-based distro, aimed towards home users. The company makes a profit of around one hundred million dollars a year selling support contracts. Red Hat contributes a lot to the Linux kernel and other free software projects. 10
- 17. distributions Red Hat Linux Red Hat Linux was distributed from 1994 until 2003. It was one of the oldest common Linux distributions. Red Hat Linux was the first distro to use the rpm package format. Many other distros are originally derived from Red Hat Linux. The company Red Hat, Inc. decided to split Red Hat Linux into Fedora and Red Hat Enterprise Linux. Fedora Fedora is sponsored by Red Hat which is aimed toward home users. There is no official support from Red Hat. Every six to eight months, there is a new version of Fedora. Fedora usually has more recent versions of kernel and applications than RHEL. Fedora 9 was released May 2008. Red Hat Enterprise Linux 4 Since 2005 Red Hat has distributed four different RHEL4 variants. RHEL AS is for mission-critical computer systems. RHEL ES is for small to mid-range servers. RHEL WS is for technical power user desktops and critical design. Red Hat Desktop is for multiple deployments of single user desktops. Red Hat does not give an explanation for the meaning of AS, ES and WS, but it might be Advanced Server, Entry-level Server, and Workstation. Red Hat Enterprise Linux 5 Red Hat Enterprise Linux version 5 has been available since March 2007. One of the notable new features is the inclusion of Xen. Xen is a free virtual machine application that allows NetBSD and Linux to serve as host for guest operating systems. Besides virtualization, RHEL 5 also has better SELinux support, clustering, network storage and smart card integration. CentOS and Unbreakable Linux Both CentOS and Oracle's Unbreakable Linux are directly derived from RHEL, but all references to Red Hat trademarks are removed. Companies are allowed to do this (GPL), and are hoping to make a profit selling support (without having the cost of developing and maintaining their own distribution). Red Hat is not really worried about this since they develop a lot on Linux and thus can offer much better support. The Oracle offering is still very recent, so let's wait and see how many organizations will buy a complete solution from Oracle. Ubuntu Ubuntu is a rather new distribution, based on Debian, and funded by South African developer and billionaire astronaut Mark Shuttleworth. Ubuntu is giving away free 11
- 18. distributions (as in beer and speech) CDs with Ubuntu, Linux for Human Beings. Many people consider Ubuntu to be the most user friendly Linux distribution. The company behind Ubuntu is Canonical which intends to make a profit of selling support soon. Ubuntu is probably the most popular Unix-like distribution on personal desktops. Image copied from xkcd.com. Novell Suse A couple of years ago, Novell bought the German company Suse. They are seen as the main competitor to Red Hat with their SLES (Suse Linux Enterprise Server) and SLED (Suse Linux Enterprise Desktop) versions of Suse Linux. Similar to Fedora, Novell hosts the OpenSUSE project as a testbed for upcoming SLED and SLES releases. Novell has signed a very controversial deal with Microsoft. Some high-profile open source developers have left the company because of this agreement and many people from the open source community are actively advocating to abandon Novell completely. Debian Debian is one of the most secure Linux distros. It is known to be stable and reliable, and runs on many architectures (including old ones like the Motorola 68k). Debian is maintained by many passionate volunteers that have a strong focus towards freedom. There is no company directly controlling and releasing Debian. You will not find patented technologies or non-free software in the standard Debian repositories. A lot of distributions (Ubuntu, Knoppix, ...) are derived from the Debian codebase. Debian has aptitude, which is considered the best package management system. Mandriva Mandriva is the unification of the Brazilian distro Conectiva with the French distro Mandrake. They are considered a user friendly distro with support from the French government. 12
- 19. distributions 3.3. BSD Distributions FreeBSD FreeBSD is a complete operating system. The kernel and all of the utilities are held in the same source code tree. FreeBSD runs on many architectures and is considered to be reliable and robust. Millions of websites are running on FreeBSD, including some large ones like yahoo.com, apache.org, sony.co.jp, netcraft, php.net, and freebsd.org. Apple's MacOSX contains the FreeBSD virtual file system, network stack, and more. NetBSD NetBSD development started around the same time (1993) as FreeBSD. NetBSD aims for maximum portability and thus runs on many architectures. NetBSD is often used in embedded devices. OpenBSD Co-founder Theo De Raadt from NetBSD founded the OpenBSD project in 1994. OpenBSD aims for maximum security. Over the past ten years, only two vulnerabilities have been found in the default install of OpenBSD. All source code is thoroughly checked. OpenBSD runs on sixteen different architectures and is commonly used for firewalls and IDS. The OpenBSD people also bring us OpenSSH. 3.4. Major Vendors of Unix We should at least mention IBM's AIX, Sun's Solaris, and Hewlett-Packard's HP- UX, which are all based on the original Unix from Bell Labs (Unix System V). Sun's SunOS, HP's Tru64 (originally from DEC), and Apple's MacOSX are more derived from the BSD branch. But most Unixes today may contain source code and implementations from both original Unix-branches. 3.5. Solaris Solaris 8 and Solaris 9 All Sun Solaris releases before Solaris 10 are proprietary binary only, just like IBM AIX and HP-UX. Solaris 10 Solaris 10 is the officially supported Sun distribution. It is a free (as in beer) download. Sun releases binary patches and updates. Sun would like a community 13
- 20. distributions built around the Solaris kernel, similar to the Linux community. Sun released the Solaris kernel under the CDDL, a license similar to the GPL, hoping this will happen. Nevada and Solaris Express Nevada is the codename for the next release of Solaris (Solaris 11). It is currently under development by Sun and is based on the OpenSolaris code. Solaris Express Community Edition is an official, free binary release including open source OpenSolaris and some closed source technologies, updated twice a month without any support from Sun. Solaris Express Developer Edition is the same, but with some support, thorough testing before release, and is released twice a year. OpenSolaris, Belenix and Nexenta OpenSolaris is an open source development project (yes, it is only source code). Future versions of the Solaris operating system are based on this source code. The Belenix LiveCD is based on OpenSolaris. Another famous OpenSolaris based distro is Nexenta. Nexenta (www.gnusolaris.org) looks like Ubuntu and feels like Debian. The goal of this GNU/Solaris project is to have the best Linux desktop (Ubuntu) including the aptitude package manager running on a Sun Solaris kernel. 14
- 21. Chapter 4. online help Table of Contents 4.1. Google ............................................................................................................ 16 4.2. Wikipedia ....................................................................................................... 16 4.3. The Linux Documentation Project ................................................................. 16 4.4. Red Hat .......................................................................................................... 17 4.5. Ubuntu ............................................................................................................ 17 4.6. linux-training.be ............................................................................................. 17 This chapters gives some tips on where to find help with Linux on the internet. 15
- 22. online help 4.1. Google Google is a powerful tool to find help about Unix, or anything else. Here are some tricks. Look for phrases instead of single words. Search only pages from the .be TLD (or substitute .be for any other Top Level Domain). You can also use "country:be" to search only pages from Belgium (based on ip rather than TLD). Search for pages inside one domain Search for pages not containing some words. 4.2. Wikipedia Wikipedia is a web-based, free-content encyclopedia. Its growth over the past two years has been astonishing. You have a good chance of finding a clear explanation by typing your search term behind http://en.wikipedia.org/wiki/ like this example shows. 4.3. The Linux Documentation Project You can find much documentation, faqs, howtos and man pages about Linux and many of the programs running on Linux on www.tldp.org . 16
- 23. online help 4.4. Red Hat Red Hat has a lot of info online at http://www.redhat.com/docs/manuals/ in both pdf and html format. These manuals are good, but unfortunately are not always up to date. 4.5. Ubuntu Help for every Ubuntu release is available at https://help.ubuntu.com. Ubuntu also has video of how to perform tasks on Ubuntu at http://screencasts.ubuntu.com. 4.6. linux-training.be This book is available for free in .pdf and .html. Download it at http://linux- training.be and learn more about Linux fundamentals, system administration, networking, storage, security and more. 17
- 24. Part II. first steps on the command line
- 25. Chapter 5. man pages Table of Contents 5.1. man $command .............................................................................................. 20 5.2. man $configfile .............................................................................................. 20 5.3. man $daemon ................................................................................................. 20 5.4. man -k (apropos) ............................................................................................ 20 5.5. whatis ............................................................................................................. 20 5.6. whereis ........................................................................................................... 21 5.7. man sections ................................................................................................... 21 5.8. man $section $file .......................................................................................... 21 5.9. man man ........................................................................................................ 21 5.10. mandb ........................................................................................................... 22 This chapter will explain the use of man pages (also called manual pages) on your Unix or Linux computer. You will learn the man command together with related commands like whereis, whatis and mandb. Most Unix files and commands have pretty good man pages to explain their use. Man pages also come in handy when you are using multiple flavors of Unix or several Linux distributions since options and parameters sometimes vary. 19
- 26. man pages 5.1. man $command Type man followed by a command (for which you want help) and start reading. Press q to quit the manpage. Some man pages contain examples (near the end). paul@laika:~$ man whois Reformatting whois(1), please wait... 5.2. man $configfile Most configuration files have their own manual. paul@laika:~$ man syslog.conf Reformatting syslog.conf(5), please wait... 5.3. man $daemon This is also true for most daemons (background programs) on your system.. paul@laika:~$ man syslogd Reformatting syslogd(8), please wait... 5.4. man -k (apropos) man -k (or apropos) shows a list of man pages containing a string. paul@laika:~$ man -k syslog lm-syslog-setup (8) - configure laptop mode to switch syslog.conf ... logger (1) - a shell command interface to the syslog(3) ... syslog-facility (8) - Setup and remove LOCALx facility for sysklogd syslog.conf (5) - syslogd(8) configuration file syslogd (8) - Linux system logging utilities. syslogd-listfiles (8) - list system logfiles 5.5. whatis To see just the description of a manual page, use whatis followed by a string. paul@u810:~$ whatis route route (8) - show / manipulate the IP routing table 20
- 27. man pages 5.6. whereis The location of a manpage can be revealed with whereis. paul@laika:~$ whereis -m whois whois: /usr/share/man/man1/whois.1.gz This file is directly readable by man. paul@laika:~$ man /usr/share/man/man1/whois.1.gz 5.7. man sections By now you will have noticed the numbers between the round brackets. man man will explain to you that these are section numbers. Executable programs and shell commands reside in section one. 1 Executable programs or shell commands 2 System calls (functions provided by the kernel) 3 Library calls (functions within program libraries) 4 Special files (usually found in /dev) 5 File formats and conventions eg /etc/passwd 6 Games 7 Miscellaneous (including macro packages and conventions), e.g. man(7) 8 System administration commands (usually only for root) 9 Kernel routines [Non standard] 5.8. man $section $file Therefor, when referring to the man page of the passwd command, you will see it written as passwd(1); when referringto the passwd file, you will see it written as passwd(5). The screenshot explains how to open the man page in the correct section. [paul@RHEL52 ~]$ man passwd # opens the lowest section (1) [paul@RHEL52 ~]$ man 5 passwd # opens a page from section 5 5.9. man man If you want to know more about man, then Read The Fantastic Manual (RTFM). Unfortunately, manual pages do not have the answer to everything... paul@laika:~$ man woman No manual entry for woman 21
- 28. man pages 5.10. mandb Should you be convinced that a man page exists, but you can't access it, then try running mandb. root@laika:~# mandb 0 man subdirectories contained newer manual pages. 0 manual pages were added. 0 stray cats were added. 0 old database entries were purged. 22
- 29. Chapter 6. working with directories Table of Contents 6.1. pwd ................................................................................................................. 24 6.2. cd .................................................................................................................... 24 6.3. absolute and relative paths ............................................................................. 25 6.4. path completion .............................................................................................. 26 6.5. ls ..................................................................................................................... 26 6.6. mkdir .............................................................................................................. 28 6.7. rmdir ............................................................................................................... 28 6.8. practice: working with directories ................................................................. 30 6.9. solution: working with directories ................................................................. 31 To explore the Linux file tree, you will need some basic tools. This chapter is small overview of the most common commands to work with directories : pwd, cd, ls, mkdir, rmdir. These commands are available on any Linux (or Unix) system. This chapter also discusses absolute and relative paths and path completion in the bash shell. 23
- 30. working with directories 6.1. pwd The you are here sign can be displayed with the pwd command (Print Working Directory). Go ahead, try it: Open a command line interface (like gnome-terminal, konsole, xterm, or a tty) and type pwd. The tool displays your current directory. paul@laika:~$ pwd /home/paul 6.2. cd You can change your current directory with the cd command (Change Directory). paul@laika$ cd /etc paul@laika$ pwd /etc paul@laika$ cd /bin paul@laika$ pwd /bin paul@laika$ cd /home/paul/ paul@laika$ pwd /home/paul cd ~ You can pull off a trick with cd. Just typing cd without a target directory, will put you in your home directory. Typing cd ~ has the same effect. paul@laika$ cd /etc paul@laika$ pwd /etc paul@laika$ cd paul@laika$ pwd /home/paul paul@laika$ cd ~ paul@laika$ pwd /home/paul cd .. To go to the parent directory (the one just above your current directory in the directory tree), type cd .. . paul@laika$ pwd /usr/share/games paul@laika$ cd .. paul@laika$ pwd 24
- 31. working with directories /usr/share paul@laika$ cd .. paul@laika$ cd .. paul@laika$ pwd / To stay in the current directory, type cd . ;-) We will see useful use of the . character representing the current directory later. cd - Another useful shortcut with cd is to just type cd - to go to the previous directory. paul@laika$ pwd /home/paul paul@laika$ cd /etc paul@laika$ pwd /etc paul@laika$ cd - /home/paul paul@laika$ cd - /etc 6.3. absolute and relative paths You should be aware of absolute and relative paths in the file tree. When you type a path starting with a slash (/), then the root of the file tree is assumed. If you don't start your path with a slash, then the current directory is the assumed starting point. The screenshot below first shows the current directory /home/paul. From within this directory, you have to type cd /home instead of cd home to go to the /home directory. paul@laika$ pwd /home/paul paul@laika$ cd home bash: cd: home: No such file or directory paul@laika$ cd /home paul@laika$ pwd /home When inside /home, you have to type cd paul instead of cd /paul to enter the subdirectory paul of the current directory /home. paul@laika$ pwd /home paul@laika$ cd /paul bash: cd: /paul: No such file or directory paul@laika$ cd paul paul@laika$ pwd /home/paul 25
- 32. working with directories In case your current directory is the root directory /, then both cd /home and cd home will get you in the /home directory. paul@laika$ cd / paul@laika$ pwd / paul@laika$ cd home paul@laika$ pwd /home paul@laika$ cd / paul@laika$ pwd / paul@laika$ cd /home paul@laika$ pwd /home This was the last screenshot with pwd statements. From now on, the current directory will often be displayed in the prompt. Later in this book we will explain how the shell variable $PS1 can be configured to show this. 6.4. path completion The tab key can help you in typing a path without errors. Typing cd /et followed by the tab key will expand the command line to cd /etc/. When typing cd /Et followed by the tab key, nothing will happen because you typed the wrong path (uppercase E). You will need fewer key strokes when using the tab key, and you will be sure your typed path is correct! 6.5. ls You can list the contents of a directory with ls. paul@pasha:~$ ls allfiles.txt dmesg.txt httpd.conf stuff summer.txt paul@pasha:~$ ls -a A frequently used option with ls is -a to show all files. Showing all files means including the hidden files. When a filename on a Unix file system starts with a dot, it is considered a hidden file and it doesn't show up in regular file listings. paul@pasha:~$ ls allfiles.txt dmesg.txt httpd.conf stuff summer.txt paul@pasha:~$ ls -a 26
- 33. working with directories . allfiles.txt .bash_profile dmesg.txt .lesshst stuff .. .bash_history .bashrc httpd.conf .ssh summer.txt paul@pasha:~$ ls -l Many times you will be using options with ls to display the contents of the directory in different formats or to display different parts of the directory. Typing just ls gives you a list of files in the directory. Typing ls -l (that is a letter L, not the number 1) gives you a long listing (more information on the contents). paul@pasha:~$ ls -l total 23992 -rw-r--r-- 1 paul paul 24506857 2006-03-30 22:53 allfiles.txt -rw-r--r-- 1 paul paul 14744 2006-09-27 11:45 dmesg.txt -rw-r--r-- 1 paul paul 8189 2006-03-31 14:01 httpd.conf drwxr-xr-x 2 paul paul 4096 2007-01-08 12:22 stuff -rw-r--r-- 1 paul paul 0 2006-03-30 22:45 summer.txt ls -lh Another frequently used ls option is -h. It shows the numbers (file sizes) in a more human readable format. Also shown below is some variation in the way you can give the options to ls. We will explain the details of the output later in this book. paul@pasha:~$ ls -l -h total 24M -rw-r--r-- 1 paul paul 24M 2006-03-30 22:53 allfiles.txt -rw-r--r-- 1 paul paul 15K 2006-09-27 11:45 dmesg.txt -rw-r--r-- 1 paul paul 8.0K 2006-03-31 14:01 httpd.conf drwxr-xr-x 2 paul paul 4.0K 2007-01-08 12:22 stuff -rw-r--r-- 1 paul paul 0 2006-03-30 22:45 summer.txt paul@pasha:~$ ls -lh total 24M -rw-r--r-- 1 paul paul 24M 2006-03-30 22:53 allfiles.txt -rw-r--r-- 1 paul paul 15K 2006-09-27 11:45 dmesg.txt -rw-r--r-- 1 paul paul 8.0K 2006-03-31 14:01 httpd.conf drwxr-xr-x 2 paul paul 4.0K 2007-01-08 12:22 stuff -rw-r--r-- 1 paul paul 0 2006-03-30 22:45 summer.txt paul@pasha:~$ ls -hl total 24M -rw-r--r-- 1 paul paul 24M 2006-03-30 22:53 allfiles.txt -rw-r--r-- 1 paul paul 15K 2006-09-27 11:45 dmesg.txt -rw-r--r-- 1 paul paul 8.0K 2006-03-31 14:01 httpd.conf drwxr-xr-x 2 paul paul 4.0K 2007-01-08 12:22 stuff -rw-r--r-- 1 paul paul 0 2006-03-30 22:45 summer.txt paul@pasha:~$ ls -h -l total 24M -rw-r--r-- 1 paul paul 24M 2006-03-30 22:53 allfiles.txt -rw-r--r-- 1 paul paul 15K 2006-09-27 11:45 dmesg.txt -rw-r--r-- 1 paul paul 8.0K 2006-03-31 14:01 httpd.conf drwxr-xr-x 2 paul paul 4.0K 2007-01-08 12:22 stuff -rw-r--r-- 1 paul paul 0 2006-03-30 22:45 summer.txt 27
- 34. working with directories 6.6. mkdir Walking around the Unix file tree is fun, but it is even more fun to create your own directories with mkdir. You have to give at least one parameter to mkdir, the name of the new directory to be created. Think before you type a leading / . paul@laika:~$ mkdir MyDir paul@laika:~$ cd MyDir paul@laika:~/MyDir$ ls -al total 8 drwxr-xr-x 2 paul paul 4096 2007-01-10 21:13 . drwxr-xr-x 39 paul paul 4096 2007-01-10 21:13 .. paul@laika:~/MyDir$ mkdir stuff paul@laika:~/MyDir$ mkdir otherstuff paul@laika:~/MyDir$ ls -l total 8 drwxr-xr-x 2 paul paul 4096 2007-01-10 21:14 otherstuff drwxr-xr-x 2 paul paul 4096 2007-01-10 21:14 stuff paul@laika:~/MyDir$ mkdir -p When given the option -p, then mkdir will create parent directories as needed. paul@laika:~$ mkdir -p MyDir2/MySubdir2/ThreeDeep paul@laika:~$ ls MyDir2 MySubdir2 paul@laika:~$ ls MyDir2/MySubdir2 ThreeDeep paul@laika:~$ ls MyDir2/MySubdir2/ThreeDeep/ 6.7. rmdir When a directory is empty, you can use rmdir to remove the directory. paul@laika:~/MyDir$ rmdir otherstuff paul@laika:~/MyDir$ ls stuff paul@laika:~/MyDir$ cd .. paul@laika:~$ rmdir MyDir rmdir: MyDir/: Directory not empty paul@laika:~$ rmdir MyDir/stuff paul@laika:~$ rmdir MyDir rmdir -p And similar to the mkdir -p option, you can also use rmdir to recursively remove directories. 28
- 35. working with directories paul@laika:~$ mkdir -p dir/subdir/subdir2 paul@laika:~$ rmdir -p dir/subdir/subdir2 paul@laika:~$ 29
- 36. working with directories 6.8. practice: working with directories 1. Display your current directory. 2. Change to the /etc directory. 3. Now change to your home directory using only three key presses. 4. Change to the /boot/grub directory using only eleven key presses. 5. Go to the parent directory of the current directory. 6. Go to the root directory. 7. List the contents of the root directory. 8. List a long listing of the root directory. 9. Stay where you are, and list the contents of /etc. 10. Stay where you are, and list the contents of /bin and /sbin. 11. Stay where you are, and list the contents of ~. 12. List all the files (including hidden files) in your home directory. 13. List the files in /boot in a human readable format. 14. Create a directory testdir in your home directory. 15. Change to the /etc directory, stay here and create a directory newdir in your home directory. 16. Create in one command the directories ~/dir1/dir2/dir3 (dir3 is a subdirectory from dir2, and dir2 is a subdirectory from dir1 ). 17. Remove the directory testdir. 18. If time permits (or if you are waiting for other students to finish this practice), use and understand pushd and popd. Use the man page of bash to find information about these commands. 30
- 37. working with directories 6.9. solution: working with directories 1. Display your current directory. pwd 2. Change to the /etc directory. cd /etc 3. Now change to your home directory using only three key presses. cd (and the enter key) 4. Change to the /boot/grub directory using only eleven key presses. cd /boot/grub (use the tab key) 5. Go to the parent directory of the current directory. cd .. (with space between cd and ..) 6. Go to the root directory. cd / 7. List the contents of the root directory. ls 8. List a long listing of the root directory. ls -l 9. Stay where you are, and list the contents of /etc. ls /etc 10. Stay where you are, and list the contents of /bin and /sbin. ls /bin /sbin 11. Stay where you are, and list the contents of ~. ls ~ 12. List all the files (including hidden files) in your home directory. ls -al ~ 13. List the files in /boot in a human readable format. ls -lh /boot 14. Create a directory testdir in your home directory. mkdir ~/testdir 15. Change to the /etc directory, stay here and create a directory newdir in your home directory. 31
- 38. working with directories cd /etc ; mkdir ~/newdir 16. Create in one command the directories ~/dir1/dir2/dir3 (dir3 is a subdirectory from dir2, and dir2 is a subdirectory from dir1 ). mkdir -p ~/dir1/dir2/dir3 17. Remove the directory testdir. rmdir testdir 18. If time permits (or if you are waiting for other students to finish this practice), use and understand pushd and popd. Use the man page of bash to find information about these commands. man bash The Bash shell has two built-in commands called pushd and popd. Both commands work with a common stack of previous directories. Pushd adds a directory to the stack and changes to a new current directory, popd removes a directory from the stack and sets the current directory. paul@laika:/etc$ cd /bin paul@laika:/bin$ pushd /lib /lib /bin paul@laika:/lib$ pushd /proc /proc /lib /bin paul@laika:/proc$ paul@laika:/proc$ popd /lib /bin paul@laika:/lib$ paul@laika:/lib$ paul@laika:/lib$ popd /bin paul@laika:/bin$ 32
- 39. Chapter 7. working with files Table of Contents 7.1. all files are case sensitive .............................................................................. 34 7.2. everything is a file ......................................................................................... 34 7.3. file .................................................................................................................. 34 7.4. touch ............................................................................................................... 35 7.5. rm ................................................................................................................... 35 7.6. cp .................................................................................................................... 36 7.7. mv .................................................................................................................. 38 7.8. rename ............................................................................................................ 38 7.9. practice: working with files ........................................................................... 39 7.10. solution: working with files ......................................................................... 40 In this chapter we learn how to recognise, create, remove, copy and move files using commands like file, touch, rm, cp, mv and rename. 33
- 40. working with files 7.1. all files are case sensitive Linux is case sensitive, this means that FILE1 is different from file1, and /etc/hosts is different from /etc/Hosts (the latter one does not exist on a typical Linux computer). This screenshot shows the difference between two files, one with uppercase W, the other with lowercase w. paul@laika:~/Linux$ ls winter.txt Winter.txt paul@laika:~/Linux$ cat winter.txt It is cold. paul@laika:~/Linux$ cat Winter.txt It is very cold! 7.2. everything is a file A directory is a special kind of file, but it is still a file. Even a terminal window (/ dev/pts/4) or a hard disk (/dev/sdb) is represented somewhere in the file system as a file. It will become clear throughout this course that everything on Linux is a file. 7.3. file The file utility determines the file type. Linux does not use extensions to determine the file type. Your editor does not care whether a file ends in .TXT or .DOC. As a system administrator, you should use the file command to determine the file type. Here are some examples on a typical Linux system. paul@laika:~$ file pic33.png pic33.png: PNG image data, 3840 x 1200, 8-bit/color RGBA, non-interlaced paul@laika:~$ file /etc/passwd /etc/passwd: ASCII text paul@laika:~$ file HelloWorld.c HelloWorld.c: ASCII C program text Here's another example of the file utility. It shows different type of binaries on different architectures. # Solaris 9 on Intel bash-2.05$ file /bin/date /bin/date: ELF 32-bit LSB executable 80386 Version 1, dynamically linked, stripped # Ubuntu Linux on AMD64 paul@laika:~$ file /bin/date /bin/date: ELF 64-bit LSB executable, AMD x86-64, version 1 (SYSV), for GNU/Linux 2.6.0, dynamically linked (uses shared libs), for GNU/Linux 2.6.0, stripped 34
- 41. working with files # Debian Sarge on SPARC paul@pasha:~$ file /bin/date /bin/date: ELF 32-bit MSB executable, SPARC, version 1 (SYSV), for GNU/ Linux 2.4.1, dynamically linked (uses shared libs), for GNU/Linux 2.4.1 , stripped # AIX on RS/6000 serena@AIX7 /home/serena$ file /bin/date /bin/date: executable (RISC System/6000) or object module The file command uses a magic file that contains patterns to recognize filetypes. The magic file is located in /usr/share/file/magic. Type man 5 magic for more information. 7.4. touch One easy way to create a file is with touch. (We will see many other ways for creating files later in this book.) paul@laika:~/test$ touch file1 paul@laika:~/test$ touch file2 paul@laika:~/test$ touch file555 paul@laika:~/test$ ls -l total 0 -rw-r--r-- 1 paul paul 0 2007-01-10 21:40 file1 -rw-r--r-- 1 paul paul 0 2007-01-10 21:40 file2 -rw-r--r-- 1 paul paul 0 2007-01-10 21:40 file555 touch -t Of course, touch can do more than just create files. Can you determine what by looking at the next screenshot? If not, check the manual for touch. paul@laika:~/test$ touch -t 200505050000 SinkoDeMayo paul@laika:~/test$ touch -t 130207111630 BigBattle paul@laika:~/test$ ls -l total 0 -rw-r--r-- 1 paul paul 0 1302-07-11 16:30 BigBattle -rw-r--r-- 1 paul paul 0 2005-05-05 00:00 SinkoDeMayo 7.5. rm When you no longer need a file, use rm to remove it. Unlike some graphical user interfaces, the command line in general does not have a waste bin or trashcan to recover files. When you use rm to remove a file, the file is gone. Therefore, be careful when removing files! paul@laika:~/test$ ls 35
- 42. working with files BigBattle SinkoDeMayo paul@laika:~/test$ rm BigBattle paul@laika:~/test$ ls SinkoDeMayo rm -i To prevent yourself from accidentally removing a file, you can type rm -i. paul@laika:~/Linux$ touch brel.txt paul@laika:~/Linux$ rm -i brel.txt rm: remove regular empty file `brel.txt'? y paul@laika:~/Linux$ rm -rf By default, rm will not remove non-empty directories. However rm accepts several options that will allow you to remove any directory. The rm -rf statement is famous because it will erase anything (providing that you have the permissions to do so). When you are logged on as root, be very careful with rm -rf (the f means force and the r means recursive) since being root implies that permissions don't apply to you, so you can literally erase your entire file system by accident. paul@laika:~$ ls test SinkoDeMayo paul@laika:~$ rm test rm: cannot remove `test': Is a directory paul@laika:~$ rm -rf test paul@laika:~$ ls test ls: test: No such file or directory 7.6. cp To copy a file, use cp with a source and a target argument. If the target is a directory, then the source files are copied to that target directory. paul@laika:~/test$ touch FileA paul@laika:~/test$ ls FileA paul@laika:~/test$ cp FileA FileB paul@laika:~/test$ ls FileA FileB paul@laika:~/test$ mkdir MyDir paul@laika:~/test$ ls FileA FileB MyDir paul@laika:~/test$ cp FileA MyDir/ paul@laika:~/test$ ls MyDir/ FileA 36
- 43. working with files cp -r To copy complete directories, use cp -r (the -r option forces recursive copying of all files in all subdirectories). paul@laika:~/test$ ls FileA FileB MyDir paul@laika:~/test$ ls MyDir/ FileA paul@laika:~/test$ cp -r MyDir MyDirB paul@laika:~/test$ ls FileA FileB MyDir MyDirB paul@laika:~/test$ ls MyDirB FileA cp multiple files to directory You can also use cp to copy multiple files into a directory. In this case, the last argument (aka the target) must be a directory. cp file1 file2 dir1/file3 dir1/file55 dir2 cp -i To prevent cp from overwriting existing files, use the -i (for interactive) option. paul@laika:~/test$ cp fire water paul@laika:~/test$ cp -i fire water cp: overwrite `water'? no paul@laika:~/test$ cp -p To preserve permissions and time stamps from source files, use cp -p. paul@laika:~/perms$ cp file* cp paul@laika:~/perms$ cp -p file* cpp paul@laika:~/perms$ ll * -rwx------ 1 paul paul 0 2008-08-25 13:26 file33 -rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42 cp: total 0 -rwx------ 1 paul paul 0 2008-08-25 13:34 file33 -rwxr-x--- 1 paul paul 0 2008-08-25 13:34 file42 cpp: total 0 -rwx------ 1 paul paul 0 2008-08-25 13:26 file33 -rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42 37
- 44. working with files 7.7. mv Use mv to rename a file or to move the file to another directory. paul@laika:~/test$ touch file100 paul@laika:~/test$ ls file100 paul@laika:~/test$ mv file100 ABC.txt paul@laika:~/test$ ls ABC.txt paul@laika:~/test$ When you need to rename only one file then mv is the preferred command to use. 7.8. rename The rename command can also be used but it has a more complex syntax to enable renaming of many files at once. Below are two examples, the first switches all occurrences of txt to png for all filenames ending in .txt. The second example switches all occurrences of uppercase ABC in lowercase abc for all filenames ending in .png . The following syntax will work on debian and ubuntu (prior to Ubuntu 7.10). paul@laika:~/test$ ls 123.txt ABC.txt paul@laika:~/test$ rename 's/txt/png/' *.txt paul@laika:~/test$ ls 123.png ABC.png paul@laika:~/test$ rename 's/ABC/abc/' *.png paul@laika:~/test$ ls 123.png abc.png paul@laika:~/test$ On Red Hat Enterprise Linux (and many other Linux distributions like Ubuntu 8.04), the syntax of rename is a bit different. The first example below renames all *.conf files replacing any occurrence of conf with bak. The second example renames all (*) files replacing one with ONE. [paul@RHEL4a test]$ ls one.conf two.conf [paul@RHEL4a test]$ rename conf bak *.conf [paul@RHEL4a test]$ ls one.bak two.bak [paul@RHEL4a test]$ rename one ONE * [paul@RHEL4a test]$ ls ONE.bak two.bak [paul@RHEL4a test]$ 38
- 45. working with files 7.9. practice: working with files 1. List the files in the /bin directory 2. Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd. 3a. Download wolf.jpg and book.pdf from http://www.linux-training.be (wget http:// linux-training.be/files/studentfiles/wolf.jpg and wget http://linux-training.be/files/ books/LinuxFun.pdf) 3b. Display the type of file of wolf.jpg and book.pdf 3c. Rename wolf.jpg to wolf.pdf (use mv). 3d. Display the type of file of wolf.pdf and book.pdf. 4. Create a directory ~/touched and enter it. 5. Create the files today.txt and yesterday.txt in touched. 6. Change the date on yesterday.txt to match yesterday's date. 7. Copy yesterday.txt to copy.yesterday.txt 8. Rename copy.yesterday.txt to kim 9. Create a directory called ~/testbackup and copy all files from ~/touched in it. 10. Use one command to remove the directory ~/testbackup and all files in it. 11. Create a directory ~/etcbackup and copy all *.conf files from /etc in it. Did you include all subdirectories of /etc ? 12. Use rename to rename all *.conf files to *.backup . (if you have more than one distro available, try it on all!) 39
- 46. working with files 7.10. solution: working with files 1. List the files in the /bin directory ls /bin 2. Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd. file /bin/cat /etc/passwd /usr/bin/passwd 3a. Download wolf.jpg and book.pdf from http://www.linux-training.be (wget http:// linux-training.be/files/studentfiles/wolf.jpg and wget http://linux-training.be/files/ books/LinuxFun.pdf) wget http://linux-training.be/files/studentfiles/wolf.jpg wget http://linux-training.be/files/studentfiles/wolf.png wget http://linux-training.be/files/books/LinuxFun.pdf 3b. Display the type of file of wolf.jpg and book.pdf file wolf.jpg book.pdf 3c. Rename wolf.jpg to wolf.pdf (use mv). mv wolf.jpg wolf.pdf 3d. Display the type of file of wolf.pdf and book.pdf. file wolf.pdf book.pdf 4. Create a directory ~/touched and enter it. mkdir ~/touched ; cd ~/touched 5. Create the files today.txt and yesterday.txt in touched. touch today.txt yesterday.txt 6. Change the date on yesterday.txt to match yesterday's date. touch -t 200810251405 yesterday.txt (substitute 20081025 with yesterday) 7. Copy yesterday.txt to copy.yesterday.txt cp yesterday.txt copy.yesterday.txt 8. Rename copy.yesterday.txt to kim mv copy.yesterday.txt kim 9. Create a directory called ~/testbackup and copy all files from ~/touched in it. mkdir ~/testbackup ; cp -r ~/touched ~/testbackup/ 10. Use one command to remove the directory ~/testbackup and all files in it. rm -rf ~/testbackup 11. Create a directory ~/etcbackup and copy all *.conf files from /etc in it. Did you include all subdirectories of /etc ? 40
- 47. working with files cp -r /etc/*.conf ~/etcbackup Only *.conf files that are directly in /etc/ are copied. 12. Use rename to rename all *.conf files to *.backup . (if you have more than one distro available, try it on all!) On RHEL: touch 1.conf 2.conf ; rename conf backup *.conf On Debian: touch 1.conf 2.conf ; rename 's/conf/backup/' *.conf 41
- 48. Chapter 8. working with file contents Table of Contents 8.1. head ................................................................................................................ 43 8.2. tail .................................................................................................................. 43 8.3. cat ................................................................................................................... 44 8.4. tac ................................................................................................................... 45 8.5. more and less ................................................................................................. 45 8.6. strings ............................................................................................................. 45 8.7. practice: file contents ..................................................................................... 47 8.8. solution: file contents ..................................................................................... 48 In this chapter we will look at the contents of text files with head, tail, cat, tac, more, less and strings. We will also get a glimpse of the possibilities of tools like cat on the command line. 42
- 49. working with file contents 8.1. head You can use head to display the first ten lines of a file. paul@laika:~$ head /etc/passwd root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh sync:x:4:65534:sync:/bin:/bin/sync games:x:5:60:games:/usr/games:/bin/sh man:x:6:12:man:/var/cache/man:/bin/sh lp:x:7:7:lp:/var/spool/lpd:/bin/sh mail:x:8:8:mail:/var/mail:/bin/sh news:x:9:9:news:/var/spool/news:/bin/sh paul@laika:~$ The head command can also display the first n lines of a file. paul@laika:~$ head -4 /etc/passwd root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh Head can also display the first n bytes. paul@laika:~$ head -c4 /etc/passwd rootpaul@laika:~$ 8.2. tail Similar to head, the tail command will display the last ten lines of a file. paul@laika:~$ tail /etc/services vboxd 20012/udp binkp 24554/tcp # binkp fidonet protocol asp 27374/tcp # Address Search Protocol asp 27374/udp csync2 30865/tcp # cluster synchronization tool dircproxy 57000/tcp # Detachable IRC Proxy tfido 60177/tcp # fidonet EMSI over telnet fido 60179/tcp # fidonet EMSI over TCP # Local services paul@laika:~$ You can give tail the number of lines you want to see. $ tail -3 count.txt six seven 43
- 50. working with file contents eight The tail command has other useful options, some of which we will use some of them during this course. 8.3. cat The cat command is one of the most universal tools. All it does is copy standard input to standard output. In combination with the shell this can be very powerful and diverse. Some examples will give a glimpse into the possibilities. The first example is simple, you can use cat to display a file on the screen. If the file is longer than the screen, it will scroll to the end. paul@laika:~$ cat /etc/resolv.conf nameserver 194.7.1.4 paul@laika:~$ concatenate cat is short for concatenate. One of the basic uses of cat is to concatenate files into a bigger (or complete) file. paul@laika:~$ echo one > part1 paul@laika:~$ echo two > part2 paul@laika:~$ echo three > part3 paul@laika:~$ cat part1 part2 part3 one two three paul@laika:~$ create files You can use cat to create files with one or more lines of text. Type the command as shown in the screenshot below. Then type one or more lines, finishing each line with the enter key. After the last line, type and hold the Control (Ctrl) key and press d. The Ctrl d key combination will send an EOF (End of File) to the running process ending the cat command. paul@laika:~/test$ cat > winter.txt It is very cold today! paul@laika:~/test$ cat winter.txt It is very cold today! paul@laika:~/test$ You can choose this end marker for cat with << as is shown in this screenshot. 44
- 51. working with file contents paul@laika:~/test$ cat > hot.txt <<stop > It is hot today! > Yes it is summer. > stop paul@laika:~/test$ cat hot.txt It is hot today! Yes it is summer. paul@laika:~/test$ copy files In the third example you will see that cat can be used to copy files. We will explain in detail what happens here in the bash shell chapter. paul@laika:~/test$ cat winter.txt It is very cold today! paul@laika:~/test$ cat winter.txt > cold.txt paul@laika:~/test$ cat cold.txt It is very cold today! paul@laika:~/test$ 8.4. tac Just one example will show you the purpose of tac (as the opposite of cat). paul@laika:~/test$ cat count one two three four paul@laika:~/test$ tac count four three two one paul@laika:~/test$ 8.5. more and less The more command is useful for displaying files that take up more than one screen. More will allow you to see the contents of the file page by page. Use the spacebar to see the next page, or q to quit. Some people prefer the less command to more. 8.6. strings With the strings command you can display readable ascii strings found in (binary) files. This example locates the ls binary then displays readable strings in the binary file (output is truncated). 45
- 52. working with file contents paul@laika:~$ which ls /bin/ls paul@laika:~$ strings /bin/ls /lib/ld-linux.so.2 librt.so.1 __gmon_start__ _Jv_RegisterClasses clock_gettime libacl.so.1 ... 46
- 53. working with file contents 8.7. practice: file contents 1. Display the first 12 lines of /etc/services. 2. Display the last line of /etc/passwd. 3. Use cat to create a file named count.txt that looks like this: One Two Three Four Five 4. Use cp to make a backup of this file to cnt.txt. 5. Use cat to make a backup of this file to catcnt.txt. 6. Display catcnt.txt, but with all lines in reverse order (the last line first). 7. Use more to display /var/log/messages. 8. Display the readable character strings from the /usr/bin/passwd command. 9. Use ls to find the biggest file in /etc. 47
- 54. working with file contents 8.8. solution: file contents 1. Display the first 12 lines of /etc/services. head -12 /etc/services 2. Display the last line of /etc/passwd. tail -1 /etc/passwd 3. Use cat to create a file named count.txt that looks like this: One Two Three Four Five cat > count.txt 4. Use cp to make a backup of this file to cnt.txt. cp count.txt cnt.txt 5. Use cat to make a backup of this file to catcnt.txt. cat count.txt > catcnt.txt 6. Display catcnt.txt, but with all lines in reverse order (the last line first). tac catcnt.txt 7. Use more to display /var/log/messages. more /var/log/messages 8. Display the readable character strings from the /usr/bin/passwd command. strings /usr/bin/passwd 9. Use ls to find the biggest file in /etc. ls -lrS /etc 48
- 55. Chapter 9. the Linux file tree Table of Contents 9.1. filesystem hierarchy standard ........................................................................ 50 9.2. man hier ......................................................................................................... 50 9.3. the root directory / ......................................................................................... 50 9.4. binary directories ........................................................................................... 51 9.5. configuration directories ................................................................................ 53 9.6. data directories ............................................................................................... 55 9.7. in memory directories .................................................................................... 57 9.8. /usr Unix System Resources .......................................................................... 61 9.9. /var variable data ............................................................................................ 63 9.10. practice: file system tree .............................................................................. 65 9.11. solution: file system tree .............................................................................. 67 This chapters takes a look at the most common directories in the Linux file tree. It also shows that on Unix everything is a file. 49
- 56. the Linux file tree 9.1. filesystem hierarchy standard Many Linux distributions partially follow the Filesystem Hierarchy Standard. The FHS may help make more Unix/Linux file system trees conform better in the future. The FHS is available online at http://www.pathname.com/fhs/ where we read: "The filesystem hierarchy standard has been designed to be used by Unix distribution developers, package developers, and system implementors. However, it is primarily intended to be a reference and is not a tutorial on how to manage a Unix filesystem or directory hierarchy." 9.2. man hier There are some differences in the filesystems between Linux distributions. For help about your machine, enter man hier to find information about the file system hierarchy. This manual will explain the directory structure on your computer. 9.3. the root directory / All Linux systems have a directory structure that starts at the root directory. The root directory is represented by a forward slash, like this: /. Everything that exists on your Linux system can be found below this root directory. Let's take a brief look at the contents of the root directory. [paul@RHELv4u3 ~]$ ls / bin dev home media mnt proc sbin srv tftpboot usr boot etc lib misc opt root selinux sys tmp var 50
- 57. the Linux file tree 9.4. binary directories Binaries are files that contain compiled source code (or machine code). Binaries can be executed on the computer. Sometimes binaries are called executables. /bin The /bin directory contains binaries for use by all users. According to the FHS the / bin directory should contain /bin/cat and /bin/date (among others). In the screenshot below you see common Unix/Linux commands like cat, cp, cpio, date, dd, echo, grep, and so on. Many of these will be covered in this book. paul@laika:~$ ls /bin archdetect egrep mt setupcon autopartition false mt-gnu sh bash fgconsole mv sh.distrib bunzip2 fgrep nano sleep bzcat fuser nc stralign bzcmp fusermount nc.traditional stty bzdiff get_mountoptions netcat su bzegrep grep netstat sync bzexe gunzip ntfs-3g sysfs bzfgrep gzexe ntfs-3g.probe tailf bzgrep gzip parted_devices tar bzip2 hostname parted_server tempfile bzip2recover hw-detect partman touch bzless ip partman-commit true bzmore kbd_mode perform_recipe ulockmgr cat kill pidof umount ... other /bin directories You can find a /bin subdirectory in many other directories. A user named Serena could put her own programs in /home/serena/bin. Some applications, often when installed directly from source will put themselves in /opt. A samba server installation can use /opt/samba/bin to store its binaries. /sbin /sbin contains binaries to configure the operating system. Many of the system binaries require root privilege to perform certain tasks. Below a screenshot containing system binaries to change the ip-address, partition a disk and create an ext4 file system. paul@ubu1010:~$ ls -l /sbin/ifconfig /sbin/fdisk /sbin/mkfs.ext4 -rwxr-xr-x 1 root root 97172 2011-02-02 09:56 /sbin/fdisk 51
- 58. the Linux file tree -rwxr-xr-x 1 root root 65708 2010-07-02 09:27 /sbin/ifconfig -rwxr-xr-x 5 root root 55140 2010-08-18 18:01 /sbin/mkfs.ext4 /lib Binaries found in /bin and /sbin often use shared libraries located in /lib. Below is a screenshot of the partial contents of /lib. paul@laika:~$ ls /lib/libc* /lib/libc-2.5.so /lib/libcfont.so.0.0.0 /lib/libcom_err.so.2.1 /lib/libcap.so.1 /lib/libcidn-2.5.so /lib/libconsole.so.0 /lib/libcap.so.1.10 /lib/libcidn.so.1 /lib/libconsole.so.0.0.0 /lib/libcfont.so.0 /lib/libcom_err.so.2 /lib/libcrypt-2.5.so /lib/modules Typically, the Linux kernel loads kernel modules from /lib/modules/$kernel- version/. This directory is discussed in detail in the Linux kernel chapter. /lib32 and /lib64 We currently are in a transition between 32-bit and 64-bit systems. Therefore, you may encounter directories named /lib32 and /lib64 which clarify the register size used during compilation time of the libraries. A 64-bit computer may have some 32-bit binaries and libraries for compatibility with legacy applications. This screenshot uses the file utility to demonstrate the difference. paul@laika:~$ file /lib32/libc-2.5.so /lib32/libc-2.5.so: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), for GNU/Linux 2.6.0, stripped paul@laika:~$ file /lib64/libcap.so.1.10 /lib64/libcap.so.1.10: ELF 64-bit LSB shared object, AMD x86-64, version 1 (SYSV), stripped The ELF (Executable and Linkable Format) is used in almost every Unix-like operating system since System V. /opt The purpose of /opt is to store optional software. In many cases this is software from outside the distribution repository. You may find an empty /opt directory on many systems. A large package can install all its files in /bin, /lib, /etc subdirectories within /opt/ $packagename/. If for example the package is called wp, then it installs in /opt/wp, putting binaries in /opt/wp/bin and manpages in /opt/wp/man. 52
- 59. the Linux file tree 9.5. configuration directories /boot The /boot directory contains all files needed to boot the computer. These files don't change very often. On Linux systems you typically find the /boot/grub directory here. /boot/grub contains /boot/grub/menu.lst (the grub configuration file is often linked to /boot/grub/grub.conf) which defines the boot menu that is displayed before the kernel starts. /etc All of the machine-specific configuration files should be located in /etc. Historically /etc stood for etcetera, today people often use the Editable Text Configuration backronym. Many times the name of a configuration files is the same as the application, daemon, or protocol with .conf added as the extension. paul@laika:~$ ls /etc/*.conf /etc/adduser.conf /etc/ld.so.conf /etc/scrollkeeper.conf /etc/brltty.conf /etc/lftp.conf /etc/sysctl.conf /etc/ccertificates.conf /etc/libao.conf /etc/syslog.conf /etc/cvs-cron.conf /etc/logrotate.conf /etc/ucf.conf /etc/ddclient.conf /etc/ltrace.conf /etc/uniconf.conf /etc/debconf.conf /etc/mke2fs.conf /etc/updatedb.conf /etc/deluser.conf /etc/netscsid.conf /etc/usplash.conf /etc/fdmount.conf /etc/nsswitch.conf /etc/uswsusp.conf /etc/hdparm.conf /etc/pam.conf /etc/vnc.conf /etc/host.conf /etc/pnm2ppa.conf /etc/wodim.conf /etc/inetd.conf /etc/povray.conf /etc/wvdial.conf /etc/kernel-img.conf /etc/resolv.conf paul@laika:~$ There is much more to be found in /etc. /etc/init.d/ A lot of Unix/Linux distributions have an /etc/init.d directory that contains scripts to start and stop daemons. This directory could dissapear as Linux migrates to systems that replace the old init way of starting all daemons. /etc/X11/ The graphical display (aka X Window System or just X) is driven by software from the X.org foundation. The configuration file for your graphical display is /etc/X11/ xorg.conf. 53
- 60. the Linux file tree /etc/skel/ The skeleton directory /etc/skel is copied to the home directory of a newly created user. It usually contains hidden files like a .bashrc script. /etc/sysconfig/ This directory, which is not mentioned in the FHS, contains a lot of Red Hat Enterprise Linux configuration files. We will discuss some of them in greater detail. The screenshot below is the /etc/sysconfig directory from RHELv4u4 with everything installed. paul@RHELv4u4:~$ ls /etc/sysconfig/ apmd firstboot irda network saslauthd apm-scripts grub irqbalance networking selinux authconfig hidd keyboard ntpd spamassassin autofs httpd kudzu openib.conf squid bluetooth hwconf lm_sensors pand syslog clock i18n mouse pcmcia sys-config-sec console init mouse.B pgsql sys-config-users crond installinfo named prelink sys-logviewer desktop ipmi netdump rawdevices tux diskdump iptables netdump_id_dsa rhn vncservers dund iptables-cfg netdump_id_dsa.p samba xinetd paul@RHELv4u4:~$ The file /etc/sysconfig/firstboot tells the Red Hat Setup Agent not to run at boot time. If you want to run the Red Hat Setup Agent at the next reboot, then simply remove this file, and run chkconfig --level 5 firstboot on. The Red Hat Setup Agent allows you to install the latest updates, create a user account, join the Red Hat Network and more. It will then create the /etc/sysconfig/firstboot file again. paul@RHELv4u4:~$ cat /etc/sysconfig/firstboot RUN_FIRSTBOOT=NO The /etc/sysconfig/harddisks file contains some parameters to tune the hard disks. The file explains itself. You can see hardware detected by kudzu in /etc/sysconfig/hwconf. Kudzu is software from Red Hat for automatic discovery and configuration of hardware. The keyboard type and keymap table are set in the /etc/sysconfig/keyboard file. For more console keyboard information, check the manual pages of keymaps(5), dumpkeys(1), loadkeys(1) and the directory /lib/kbd/keymaps/. root@RHELv4u4:/etc/sysconfig# cat keyboard KEYBOARDTYPE="pc" KEYTABLE="us" We will discuss networking files in this directory in the networking chapter. 54
- 61. the Linux file tree 9.6. data directories /home Users can store personal or project data under /home. It is common (but not mandatory by the fhs) practice to name the users home directory after the username in the format /home/$USERNAME. For example: paul@ubu606:~$ ls /home geert annik sandra paul tom Besides giving every user (or every project or group) a location to store personal files, the home directory of a user also serves as a location to store the user profile. A typical Unix user profile contains many hidden files (files whose filename starts with a dot). The hidden files of the Unix user profiles contain settings specific for that user. paul@ubu606:~$ ls -d /home/paul/.* /home/paul/. /home/paul/.bash_profile /home/paul/.ssh /home/paul/.. /home/paul/.bashrc /home/paul/.viminfo /home/paul/.bash_history /home/paul/.lesshst /root On many systems /root is the default location for personal data and profile of the root user. If it does not exist by default, then some administrators create it. /srv You may use /srv for data that is served by your system. The FHS suggests locating cvs, rsync, ftp and www data in this location. The FHS also approves administrative naming in /srv, like /srv/project55/ftp and /srv/sales/www. On Sun Solaris (or Oracle Solaris) /export is used for this purpose. /media The /media directory serves as a mount point for removable media devices such as CD-ROM's, digital cameras, and various usb-attached devices. Since /media is rather new in the Unix world, you could very well encounter systems running without this directory. Solaris 9 does not have it, Solaris 10 does. Most Linux distributions today mount all removable media in /media. paul@debian5:~$ ls /media/ cdrom cdrom0 usbdisk 55
- 62. the Linux file tree /mnt The /mnt directory should be empty and should only be used for temporary mount points (according to the FHS). Unix and Linux administrators used to create many directories here, like /mnt/ something/. You likely will encounter many systems with more than one directory created and/or mounted inside /mnt to be used for various local and remote filesystems. /tmp Applications and users should use /tmp to store temporary data when needed. Data stored in /tmp may use either disk space or RAM. Both of which are managed by the operating system. Never use /tmp to store data that is important or which you wish to archive. 56
- 63. the Linux file tree 9.7. in memory directories /dev Device files in /dev appear to be ordinary files, but are not actually located on the hard disk. The /dev directory is populated with files as the kernel is recognizing hardware. common physical devices Common hardware such as hard disk devices are represented by device files in /dev. Below a screenshot of SATA device files on a laptop and then IDE attached drives on a desktop. (The detailed meaning of these devices will be discussed later.) # # SATA or SCSI # paul@laika:~$ ls /dev/sd* /dev/sda /dev/sda1 /dev/sda2 /dev/sda3 /dev/sdb /dev/sdb1 /dev/sdb2 # # IDE or ATAPI # paul@barry:~$ ls /dev/hd* /dev/hda /dev/hda1 /dev/hda2 /dev/hdb /dev/hdb1 /dev/hdb2 /dev/hdc Besides representing physical hardware, some device files are special. These special devices can be very useful. /dev/tty and /dev/pts For example, /dev/tty1 represents a terminal or console attached to the system. (Don't break your head on the exact terminology of 'terminal' or 'console', what we mean here is a command line interface.) When typing commands in a terminal that is part of a graphical interface like Gnome or KDE, then your terminal will be represented as /dev/pts/1 (1 can be another number). /dev/null On Linux you will find other special devices such as /dev/null which can be considered a black hole; it has unlimited storage, but nothing can be retrieved from it. Technically speaking, anything written to /dev/null will be discarded. /dev/null can be useful to discard unwanted output from commands. /dev/null is not a good location to store your backups ;-). 57
- 64. the Linux file tree /proc conversation with the kernel /proc is another special directory, appearing to be ordinary files, but not taking up disk space. It is actually a view of the kernel, or better, what the kernel manages, and is a means to interact with it directly. /proc is a proc filesystem. paul@RHELv4u4:~$ mount -t proc none on /proc type proc (rw) When listing the /proc directory you will see many numbers (on any Unix) and some interesting files (on Linux) mul@laika:~$ ls /proc 1 2339 4724 5418 6587 7201 cmdline mounts 10175 2523 4729 5421 6596 7204 cpuinfo mtrr 10211 2783 4741 5658 6599 7206 crypto net 10239 2975 4873 5661 6638 7214 devices pagetypeinfo 141 29775 4874 5665 6652 7216 diskstats partitions 15045 29792 4878 5927 6719 7218 dma sched_debug 1519 2997 4879 6 6736 7223 driver scsi 1548 3 4881 6032 6737 7224 execdomains self 1551 30228 4882 6033 6755 7227 fb slabinfo 1554 3069 5 6145 6762 7260 filesystems stat 1557 31422 5073 6298 6774 7267 fs swaps 1606 3149 5147 6414 6816 7275 ide sys 180 31507 5203 6418 6991 7282 interrupts sysrq-trigger 181 3189 5206 6419 6993 7298 iomem sysvipc 182 3193 5228 6420 6996 7319 ioports timer_list 18898 3246 5272 6421 7157 7330 irq timer_stats 19799 3248 5291 6422 7163 7345 kallsyms tty 19803 3253 5294 6423 7164 7513 kcore uptime 19804 3372 5356 6424 7171 7525 key-users version 1987 4 5370 6425 7175 7529 kmsg version_signature 1989 42 5379 6426 7188 9964 loadavg vmcore 2 45 5380 6430 7189 acpi locks vmnet 20845 4542 5412 6450 7191 asound meminfo vmstat 221 46 5414 6551 7192 buddyinfo misc zoneinfo 2338 4704 5416 6568 7199 bus modules Let's investigate the file properties inside /proc. Looking at the date and time will display the current date and time showing the files are constantly updated (a view on the kernel). paul@RHELv4u4:~$ date Mon Jan 29 18:06:32 EST 2007 paul@RHELv4u4:~$ ls -al /proc/cpuinfo -r--r--r-- 1 root root 0 Jan 29 18:06 /proc/cpuinfo paul@RHELv4u4:~$ paul@RHELv4u4:~$ ...time passes... paul@RHELv4u4:~$ paul@RHELv4u4:~$ date Mon Jan 29 18:10:00 EST 2007 paul@RHELv4u4:~$ ls -al /proc/cpuinfo -r--r--r-- 1 root root 0 Jan 29 18:10 /proc/cpuinfo 58
- 65. the Linux file tree Most files in /proc are 0 bytes, yet they contain data--sometimes a lot of data. You can see this by executing cat on files like /proc/cpuinfo, which contains information about the CPU. paul@RHELv4u4:~$ file /proc/cpuinfo /proc/cpuinfo: empty paul@RHELv4u4:~$ cat /proc/cpuinfo processor : 0 vendor_id : AuthenticAMD cpu family : 15 model : 43 model name : AMD Athlon(tm) 64 X2 Dual Core Processor 4600+ stepping : 1 cpu MHz : 2398.628 cache size : 512 KB fdiv_bug : no hlt_bug : no f00f_bug : no coma_bug : no fpu : yes fpu_exception : yes cpuid level : 1 wp : yes flags : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge... bogomips : 4803.54 Just for fun, here is /proc/cpuinfo on a Sun Sunblade 1000... paul@pasha:~$ cat /proc/cpuinfo cpu : TI UltraSparc III (Cheetah) fpu : UltraSparc III integrated FPU promlib : Version 3 Revision 2 prom : 4.2.2 type : sun4u ncpus probed : 2 ncpus active : 2 Cpu0Bogo : 498.68 Cpu0ClkTck : 000000002cb41780 Cpu1Bogo : 498.68 Cpu1ClkTck : 000000002cb41780 MMU Type : Cheetah State: CPU0: online CPU1: online Most of the files in /proc are read only, some require root privileges, some files are writable, and many files in /proc/sys are writable. Let's discuss some of the files in / proc. /proc/interrupts On the x86 architecture, /proc/interrupts displays the interrupts. paul@RHELv4u4:~$ cat /proc/interrupts CPU0 59
- 66. the Linux file tree 0: 13876877 IO-APIC-edge timer 1: 15 IO-APIC-edge i8042 8: 1 IO-APIC-edge rtc 9: 0 IO-APIC-level acpi 12: 67 IO-APIC-edge i8042 14: 128 IO-APIC-edge ide0 15: 124320 IO-APIC-edge ide1 169: 111993 IO-APIC-level ioc0 177: 2428 IO-APIC-level eth0 NMI: 0 LOC: 13878037 ERR: 0 MIS: 0 On a machine with two CPU's, the file looks like this. paul@laika:~$ cat /proc/interrupts CPU0 CPU1 0: 860013 0 IO-APIC-edge timer 1: 4533 0 IO-APIC-edge i8042 7: 0 0 IO-APIC-edge parport0 8: 6588227 0 IO-APIC-edge rtc 10: 2314 0 IO-APIC-fasteoi acpi 12: 133 0 IO-APIC-edge i8042 14: 0 0 IO-APIC-edge libata 15: 72269 0 IO-APIC-edge libata 18: 1 0 IO-APIC-fasteoi yenta 19: 115036 0 IO-APIC-fasteoi eth0 20: 126871 0 IO-APIC-fasteoi libata, ohci1394 21: 30204 0 IO-APIC-fasteoi ehci_hcd:usb1, uhci_hcd:usb2 22: 1334 0 IO-APIC-fasteoi saa7133[0], saa7133[0] 24: 234739 0 IO-APIC-fasteoi nvidia NMI: 72 42 LOC: 860000 859994 ERR: 0 /proc/kcore The physical memory is represented in /proc/kcore. Do not try to cat this file, instead use a debugger. The size of /proc/kcore is the same as your physical memory, plus four bytes. paul@laika:~$ ls -lh /proc/kcore -r-------- 1 root root 2.0G 2007-01-30 08:57 /proc/kcore paul@laika:~$ /sys Linux 2.6 hot plugging The /sys directory was created for the Linux 2.6 kernel. Since 2.6, Linux uses sysfs to support usb and IEEE 1394 (FireWire) hot plug devices. See the manual pages of udev(8) (the successor of devfs) and hotplug(8) for more info (or visit http://linux- hotplug.sourceforge.net/ ). Basically the /sys directory contains kernel information about hardware. 60
- 67. the Linux file tree 9.8. /usr Unix System Resources Although /usr is pronounced like user, remember that it stands for Unix System Resources. The /usr hierarchy should contain sharable, read only data. Some people choose to mount /usr as read only. This can be done from its own partition or from a read only NFS share. /usr/bin The /usr/bin directory contains a lot of commands. paul@deb508:~$ ls /usr/bin | wc -l 1395 (On Solaris the /bin directory is a symbolic link to /usr/bin.) /usr/include The /usr/include directory contains general use include files for C. paul@ubu1010:~$ ls /usr/include/ aalib.h expat_config.h math.h search.h af_vfs.h expat_external.h mcheck.h semaphore.h aio.h expat.h memory.h setjmp.h AL fcntl.h menu.h sgtty.h aliases.h features.h mntent.h shadow.h ... /usr/lib The /usr/lib directory contains libraries that are not directly executed by users or scripts. paul@deb508:~$ ls /usr/lib | head -7 4Suite ao apt arj aspell avahi bonobo /usr/local The /usr/local directory can be used by an administrator to install software locally. paul@deb508:~$ ls /usr/local/ bin etc games include lib man sbin share src paul@deb508:~$ du -sh /usr/local/ 128K /usr/local/ 61
- 68. the Linux file tree /usr/share The /usr/share directory contains architecture independent data. As you can see, this is a fairly large directory. paul@deb508:~$ ls /usr/share/ | wc -l 263 paul@deb508:~$ du -sh /usr/share/ 1.3G /usr/share/ This directory typically contains /usr/share/man for manual pages. paul@deb508:~$ ls /usr/share/man cs fr hu it.UTF-8 man2 man6 pl.ISO8859-2 sv de fr.ISO8859-1 id ja man3 man7 pl.UTF-8 tr es fr.UTF-8 it ko man4 man8 pt_BR zh_CN fi gl it.ISO8859-1 man1 man5 pl ru zh_TW And it contains /usr/share/games for all static game data (so no high-scores or play logs). paul@ubu1010:~$ ls /usr/share/games/ openttd wesnoth /usr/src The /usr/src directory is the recommended location for kernel source files. paul@deb508:~$ ls -l /usr/src/ total 12 drwxr-xr-x 4 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-686 drwxr-xr-x 18 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-common drwxr-xr-x 3 root root 4096 2009-10-28 16:01 linux-kbuild-2.6.26 62
- 69. the Linux file tree 9.9. /var variable data Files that are unpredictable in size, such as log, cache and spool files, should be located in /var. /var/log The /var/log directory serves as a central point to contain all log files. [paul@RHEL4b ~]$ ls /var/log acpid cron.2 maillog.2 quagga secure.4 amanda cron.3 maillog.3 radius spooler anaconda.log cron.4 maillog.4 rpmpkgs spooler.1 anaconda.syslog cups mailman rpmpkgs.1 spooler.2 anaconda.xlog dmesg messages rpmpkgs.2 spooler.3 audit exim messages.1 rpmpkgs.3 spooler.4 boot.log gdm messages.2 rpmpkgs.4 squid boot.log.1 httpd messages.3 sa uucp boot.log.2 iiim messages.4 samba vbox boot.log.3 iptraf mysqld.log scrollkeeper.log vmware-tools-guestd boot.log.4 lastlog news secure wtmp canna mail pgsql secure.1 wtmp.1 cron maillog ppp secure.2 Xorg.0.log cron.1 maillog.1 prelink.log secure.3 Xorg.0.log.old /var/log/messages A typical first file to check when troubleshooting is the /var/log/messages file. By default this file will contain information on what just happened to the system. [root@RHEL4b ~]# tail /var/log/messages Jul 30 05:13:56 anacron: anacron startup succeeded Jul 30 05:13:56 atd: atd startup succeeded Jul 30 05:13:57 messagebus: messagebus startup succeeded Jul 30 05:13:57 cups-config-daemon: cups-config-daemon startup succeeded Jul 30 05:13:58 haldaemon: haldaemon startup succeeded Jul 30 05:14:00 fstab-sync[3560]: removed all generated mount points Jul 30 05:14:01 fstab-sync[3628]: added mount point /media/cdrom for... Jul 30 05:14:01 fstab-sync[3646]: added mount point /media/floppy for... Jul 30 05:16:46 sshd(pam_unix)[3662]: session opened for user paul by... Jul 30 06:06:37 su(pam_unix)[3904]: session opened for user root by paul /var/cache The /var/cache directory can contain cache data for several applications. paul@ubu1010:~$ ls /var/cache/ apt dictionaries-common gdm man software-center binfmts flashplugin-installer hald pm-utils cups fontconfig jockey pppconfig debconf fonts ldconfig samba 63
- 70. the Linux file tree /var/spool The /var/spool directory typically contains spool directories for mail and cron, but also serves as a parent directory for other spool files (for example print spool files). The Red Hat Update Agent for example uses files in /var/spool/up2date. This location is also used when files are downloaded from the Red Hat Network. /var/lib The /var/lib directory contains application state information. Red Hat Enterprise Linux for example keeps files pertaining to rpm in /var/lib/rpm/. /var/... /var also contains Process ID files in /var/run and temporary files that survive a reboot in /var/tmp and information about file locks in /var/lock. There will be more examples of /var usage further in this book. 64
- 71. the Linux file tree 9.10. practice: file system tree 1. Does the file /bin/cat exist ? What about /bin/dd and /bin/echo. What is the type of these files ? 2. What is the size of the Linux kernel file(s) (vmlinu*) in /boot ? 3. Create a directory ~/test. Then issue the following commands: cd ~/test dd if=/dev/zero of=zeroes.txt count=1 bs=100 od zeroes.txt dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file / dev/zero to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ? 4. Now issue the following command: dd if=/dev/random of=random.txt count=1 bs=100 ; od random.txt dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file / dev/random to ~/test/random.txt. Can you describe the functionality of /dev/random ? 5. Issue the following two commands, and look at the first character of each output line. ls -l /dev/sd* /dev/hd* ls -l /dev/tty* /dev/input/mou* The first ls will show block(b) devices, the second ls shows character(c) devices. Can you tell the difference between block and character devices ? 6. Use cat to display /etc/hosts and /etc/resolv.conf. What is your idea about the purpose of these files ? 7. Are there any files in /etc/skel/ ? Check also for hidden files. 8. Display /proc/cpuinfo. On what architecture is your Linux running ? 9. Display /proc/interrupts. What is the size of this file ? Where is this file stored ? 10. Can you enter the /root directory ? Are there (hidden) files ? 11. Are ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are these binaries in /sbin and not in /bin ? 12. Is /var/log a file or a directory ? What about /var/spool ? 13. Open two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (Ctrl- Alt-F1, Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from one terminal to the other. 65
- 72. the Linux file tree 14. Read the man page of random and explain the difference between /dev/random and /dev/urandom. 66
- 73. the Linux file tree 9.11. solution: file system tree 1. Does the file /bin/cat exist ? What about /bin/dd and /bin/echo. What is the type of these files ? ls /bin/cat ; file /bin/cat ls /bin/dd ; file /bin/dd ls /bin/echo ; file /bin/echo 2. What is the size of the Linux kernel file(s) (vmlinu*) in /boot ? ls -lh /boot/vm* 3. Create a directory ~/test. Then issue the following commands: cd ~/test dd if=/dev/zero of=zeroes.txt count=1 bs=100 od zeroes.txt dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file / dev/zero to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ? /dev/zero is a Linux special device. It can be considered a source of zeroes. You cannot send something to /dev/zero, but you can read zeroes from it. 4. Now issue the following command: dd if=/dev/random of=random.txt count=1 bs=100 ; od random.txt dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file / dev/random to ~/test/random.txt. Can you describe the functionality of /dev/random ? /dev/random acts as a random number generator on your Linux machine. 5. Issue the following two commands, and look at the first character of each output line. ls -l /dev/sd* /dev/hd* ls -l /dev/tty* /dev/input/mou* The first ls will show block(b) devices, the second ls shows character(c) devices. Can you tell the difference between block and character devices ? Block devices are always written to (or read from) in blocks. For hard disks, blocks of 512 bytes are common. Character devices act as a stream of characters (or bytes). Mouse and keyboard are typical character devices. 6. Use cat to display /etc/hosts and /etc/resolv.conf. What is your idea about the purpose of these files ? 67
- 74. the Linux file tree /etc/hosts contains hostnames with their ip address /etc/resolv.conf should contain the ip address of a DNS name server. 7. Are there any files in /etc/skel/ ? Check also for hidden files. Issue "ls -al /etc/skel/". Yes, there should be hidden files there. 8. Display /proc/cpuinfo. On what architecture is your Linux running ? The file should contain at least one line with Intel or other cpu. 9. Display /proc/interrupts. What is the size of this file ? Where is this file stored ? The size is zero, yet the file contains data. It is not stored anywhere because /proc is a virtual file system that allows you to talk with the kernel. (If you answered "stored in RAM-memory, that is also correct...). 10. Can you enter the /root directory ? Are there (hidden) files ? Try "cd /root". Yes there are (hidden) files there. 11. Are ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are these binaries in /sbin and not in /bin ? Because those files are only meant for system administrators. 12. Is /var/log a file or a directory ? What about /var/spool ? Both are directories. 13. Open two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (Ctrl- Alt-F1, Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from one terminal to the other. tty-terminal: echo Hello > /dev/tty1 pts-terminal: echo Hello > /dev/pts/1 14. Read the man page of random and explain the difference between /dev/random and /dev/urandom. man 4 random 68
- 75. Part III. shell expansion
- 76. Chapter 10. commands and arguments Table of Contents 10.1. echo .............................................................................................................. 71 10.2. arguments ..................................................................................................... 71 10.3. commands .................................................................................................... 72 10.4. aliases ........................................................................................................... 73 10.5. displaying shell expansion ........................................................................... 75 10.6. practice: commands and arguments ............................................................. 76 10.7. solution: commands and arguments ............................................................. 78 This chapter introduces you to shell expansion by taking a close look at commands and arguments. Knowing shell expansion is important because many commands on your Linux system are processed and most likely changed by the shell before they are executed. The command line interface or shell used on most Linux systems is called bash, which stands for Bourne again shell. The bash shell incorporates features from sh (the original Bourne shell), csh (the C shell), and ksh (the Korn shell). 70
- 77. commands and arguments 10.1. echo This chapter frequently uses the echo command to demonstrate shell features. The echo command is very simple: it echoes the input that it receives. paul@laika:~$ echo Burtonville Burtonville paul@laika:~$ echo Smurfs are blue Smurfs are blue 10.2. arguments One of the primary features of a shell is to perform a command line scan. When you enter a command at the shell's command prompt and press the enter key, then the shell will start scanning that line, cutting it up in arguments. While scanning the line, the shell may make many changes to the arguments you typed. This process is called shell expansion. When the shell has finished scanning and modifying that line, then it will be executed. white space removal Parts that are separated by one or more consecutive white spaces (or tabs) are considered separate arguments, any white space is removed. The first argument is the command to be executed, the other arguments are given to the command. The shell effectively cuts your command into one or more arguments. This explains why the following four different command lines are the same after shell expansion. [paul@RHELv4u3 ~]$ echo Hello World Hello World [paul@RHELv4u3 ~]$ echo Hello World Hello World [paul@RHELv4u3 ~]$ echo Hello World Hello World [paul@RHELv4u3 ~]$ echo Hello World Hello World The echo command will display each argument it receives from the shell. The echo command will also add a new whitespace between the arguments it received. single quotes You can prevent the removal of white spaces by quoting the spaces. The contents of the quoted string are considered as one argument. In the screenshot below the echo receives only one argument. [paul@RHEL4b ~]$ echo 'A line with single quotes' A line with single quotes 71
- 78. commands and arguments [paul@RHEL4b ~]$ double quotes You can also prevent the removal of white spaces by double quoting the spaces. Same as above, echo only receives one argument. [paul@RHEL4b ~]$ echo "A line with double quotes" A line with double quotes [paul@RHEL4b ~]$ echo and quotes Quoted lines can include special escaped characters recognized by the echo command (when using echo -e). The screenshot below shows how to use n for a newline and t for a tab (usually eight white spaces). [paul@RHEL4b ~]$ echo -e "A line with na newline" A line with a newline [paul@RHEL4b ~]$ echo -e 'A line with na newline' A line with a newline [paul@RHEL4b ~]$ echo -e "A line with ta tab" A line with a tab [paul@RHEL4b ~]$ echo -e 'A line with ta tab' A line with a tab [paul@RHEL4b ~]$ The echo command can generate more than white spaces, tabs and newlines. Look in the man page for a list of options. 10.3. commands external or builtin commands ? Not all commands are external to the shell, some are builtin. External commands are programs that have their own binary and reside somewhere in the file system. Many external commands are located in /bin or /sbin. Builtin commands are an integral part of the shell program itself. type To find out whether a command given to the shell will be executed as an external command or as a builtin command, use the type command. paul@laika:~$ type cd 72
- 79. commands and arguments cd is a shell builtin paul@laika:~$ type cat cat is /bin/cat As you can see, the cd command is builtin and the cat command is external. You can also use this command to show you whether the command is aliased or not. paul@laika:~$ type ls ls is aliased to `ls --color=auto' running external commands Some commands have both builtin and external versions. When one of these commands is executed, the builtin version takes priority. To run the external version, you must enter the full path to the command. paul@laika:~$ type -a echo echo is a shell builtin echo is /bin/echo paul@laika:~$ /bin/echo Running the external echo command... Running the external echo command... which The which command will search for binaries in the $PATH environment variable (variables will be explained later). In the screenshot below, it is determined that cd is builtin, and ls, cp, rm, mv, mkdir, pwd, and which are external commands. [root@RHEL4b ~]# which cp ls mv rm cd mkdir pwd which /bin/cp /bin/ls /bin/mv /bin/rm /usr/bin/which: no cd in (/usr/kerberos/sbin:/usr/kerberos/bin:... /bin/mkdir /bin/pwd /usr/bin/which 10.4. aliases create an alias The shell allows you to create aliases. Aliases are often used to create an easier to remember name for an existing command or to easily supply parameters. [paul@RHELv4u3 ~]$ cat count.txt one two three [paul@RHELv4u3 ~]$ alias dog=tac [paul@RHELv4u3 ~]$ dog count.txt 73
- 80. commands and arguments three two one abbreviate commands An alias can also be useful to abbreviate an existing command. paul@laika:~$ alias ll='ls -lh --color=auto' paul@laika:~$ alias c='clear' paul@laika:~$ default options Aliases can be used to supply commands with default options. The example below shows how to set the -i option default when typing rm. [paul@RHELv4u3 ~]$ rm -i winter.txt rm: remove regular file `winter.txt'? no [paul@RHELv4u3 ~]$ rm winter.txt [paul@RHELv4u3 ~]$ ls winter.txt ls: winter.txt: No such file or directory [paul@RHELv4u3 ~]$ touch winter.txt [paul@RHELv4u3 ~]$ alias rm='rm -i' [paul@RHELv4u3 ~]$ rm winter.txt rm: remove regular empty file `winter.txt'? no [paul@RHELv4u3 ~]$ Some distributions enable default aliases to protect users from accidentally erasing files ('rm -i', 'mv -i', 'cp -i') viewing aliases You can provide one or more aliases as arguments to the alias command to get their definitions. Providing no arguments gives a complete list of current aliases. paul@laika:~$ alias c ll alias c='clear' alias ll='ls -lh --color=auto' unalias You can undo an alias with the unalias command. [paul@RHEL4b ~]$ which rm /bin/rm [paul@RHEL4b ~]$ alias rm='rm -i' [paul@RHEL4b ~]$ which rm alias rm='rm -i' /bin/rm [paul@RHEL4b ~]$ unalias rm [paul@RHEL4b ~]$ which rm /bin/rm 74
- 81. commands and arguments [paul@RHEL4b ~]$ 10.5. displaying shell expansion You can display shell expansion with set -x, and stop displaying it with set +x. You might want to use this further on in this course, or when in doubt about exactly what the shell is doing with your command. [paul@RHELv4u3 ~]$ set -x ++ echo -ne '033]0;paul@RHELv4u3:~007' [paul@RHELv4u3 ~]$ echo $USER + echo paul paul ++ echo -ne '033]0;paul@RHELv4u3:~007' [paul@RHELv4u3 ~]$ echo $USER + echo '$USER' $USER ++ echo -ne '033]0;paul@RHELv4u3:~007' [paul@RHELv4u3 ~]$ set +x + set +x [paul@RHELv4u3 ~]$ echo $USER paul 75
- 82. commands and arguments 10.6. practice: commands and arguments 1. How many arguments are in this line (not counting the command itself). touch '/etc/cron/cron.allow' 'file 42.txt' "file 33.txt" 2. Is tac a shell builtin command ? 3. Is there an existing alias for rm ? 4. Read the man page of rm, make sure you understand the -i option of rm. Create and remove a file to test the -i option. 5. Execute: alias rm='rm -i' . Test your alias with a test file. Does this work as expected ? 6. List all current aliases. 7a. Create an alias called 'city' that echoes your hometown. 7b. Use your alias to test that it works. 8. Execute set -x to display shell expansion for every command. 9. Test the functionality of set -x by executing your city and rm aliases. 10 Execute set +x to stop displaying shell expansion. 11. Remove your city alias. 12. What is the location of the cat and the passwd commands ? 13. Explain the difference between the following commands: echo /bin/echo 14. Explain the difference between the following commands: echo Hello echo -n Hello 15. Display A B C with two spaces between B and C. (optional)16. Complete the following command (do not use spaces) to display exactly the following output: 4+4 =8 10+14 =24 18. Use echo to display the following exactly: ?? 76
- 83. commands and arguments 19. Use one echo command to display three words on three lines. 77
- 84. commands and arguments 10.7. solution: commands and arguments 1. How many arguments are in this line (not counting the command itself). touch '/etc/cron/cron.allow' 'file 42.txt' "file 33.txt" answer: three 2. Is tac a shell builtin command ? type tac 3. Is there an existing alias for rm ? alias rm 4. Read the man page of rm, make sure you understand the -i option of rm. Create and remove a file to test the -i option. man rm touch testfile rm -i testfile 5. Execute: alias rm='rm -i' . Test your alias with a test file. Does this work as expected ? touch testfile rm testfile (should ask for confirmation) 6. List all current aliases. alias 7a. Create an alias called 'city' that echoes your hometown. alias city='echo Antwerp' 7b. Use your alias to test that it works. city (it should display Antwerp) 8. Execute set -x to display shell expansion for every command. set -x 9. Test the functionality of set -x by executing your city and rm aliases. shell should display the resolved aliases and then execute the command: paul@deb503:~$ set -x paul@deb503:~$ city + echo antwerp antwerp 10 Execute set +x to stop displaying shell expansion. set +x 11. Remove your city alias. 78
- 85. commands and arguments unalias city 12. What is the location of the cat and the passwd commands ? which cat (probably /bin/cat) which passwd (probably /usr/bin/passwd) 13. Explain the difference between the following commands: echo /bin/echo The echo command will be interpreted by the shell as the built-in echo command. The /bin/echo command will make the shell execute the echo binary located in the /bin directory. 14. Explain the difference between the following commands: echo Hello echo -n Hello The -n option of the echo command will prevent echo from echoing a trailing newline. echo Hello will echo six characters in total, echo -n hello only echoes five characters. (The -n option might not work in the Korn shell.) 15. Display A B C with two spaces between B and C. echo "A B C" 16. Complete the following command (do not use spaces) to display exactly the following output: 4+4 =8 10+14 =24 The solution is to use tabs with t. echo -e "4+4t=8" ; echo -e "10+14t=24" 18. Use echo to display the following exactly: ?? echo '??' 19. Use one echo command to display three words on three lines. echo -e "one ntwo nthree" 79
- 86. Chapter 11. control operators Table of Contents 11.1. ; semicolon ................................................................................................... 81 11.2. & ampersand ................................................................................................ 81 11.3. $? dollar question mark ............................................................................... 81 11.4. && double ampersand ................................................................................. 82 11.5. || double vertical bar .................................................................................... 82 11.6. combining && and || ................................................................................... 82 11.7. # pound sign ................................................................................................ 83 11.8. escaping special characters ........................................................................ 83 11.9. practice: control operators ........................................................................... 84 11.10. solution: control operators ......................................................................... 85 In this chapter we put more than one command on the command line using control operators. We also briefly discuss related parameters ($?) and similar special characters(&). 80
- 87. control operators 11.1. ; semicolon You can put two or more commands on the same line separated by a semicolon ; . The shell will scan the line until it reaches the semicolon. All the arguments before this semicolon will be considered a separate command from all the arguments after the semicolon. Both series will be executed sequentially with the shell waiting for each command to finish before starting the next one. [paul@RHELv4u3 ~]$ echo Hello Hello [paul@RHELv4u3 ~]$ echo World World [paul@RHELv4u3 ~]$ echo Hello ; echo World Hello World [paul@RHELv4u3 ~]$ 11.2. & ampersand When a line ends with an ampersand &, the shell will not wait for the command to finish. You will get your shell prompt back, and the command is executed in background. You will get a message when this command has finished executing in background. [paul@RHELv4u3 ~]$ sleep 20 & [1] 7925 [paul@RHELv4u3 ~]$ ...wait 20 seconds... [paul@RHELv4u3 ~]$ [1]+ Done sleep 20 The technical explanation of what happens in this case is explained in the chapter about processes. 11.3. $? dollar question mark The exit code of the previous command is stored in the shell variable $?. Actually $? is a shell parameter and not a variable, since you cannot assign a value to $?. paul@debian5:~/test$ touch file1 paul@debian5:~/test$ echo $? 0 paul@debian5:~/test$ rm file1 paul@debian5:~/test$ echo $? 0 paul@debian5:~/test$ rm file1 rm: cannot remove `file1': No such file or directory paul@debian5:~/test$ echo $? 1 paul@debian5:~/test$ 81
- 88. control operators 11.4. && double ampersand The shell will interprete && as a logical AND. When using && the second command is executed only if the first one succeeds (returns a zero exit status). paul@barry:~$ echo first && echo second first second paul@barry:~$ zecho first && echo second -bash: zecho: command not found Another example of the same logical AND principle. This example starts with a working cd followed by ls, then a non-working cd which is not followed by ls. [paul@RHELv4u3 ~]$ cd gen && ls file1 file3 File55 fileab FileAB fileabc file2 File4 FileA Fileab fileab2 [paul@RHELv4u3 gen]$ cd gen && ls -bash: cd: gen: No such file or directory [paul@RHELv4u3 gen]$ 11.5. || double vertical bar The || represents a logical OR. The second command is executed only when the first command fails (returns a non-zero exit status). paul@barry:~$ echo first || echo second ; echo third first third paul@barry:~$ zecho first || echo second ; echo third -bash: zecho: command not found second third paul@barry:~$ Another example of the same logical OR principle. [paul@RHELv4u3 ~]$ cd gen || ls [paul@RHELv4u3 gen]$ cd gen || ls -bash: cd: gen: No such file or directory file1 file3 File55 fileab FileAB fileabc file2 File4 FileA Fileab fileab2 [paul@RHELv4u3 gen]$ 11.6. combining && and || You can use this logical AND and logical OR to write an if-then-else structure on the command line. This example uses echo to display whether the rm command was successful. paul@laika:~/test$ rm file1 && echo It worked! || echo It failed! It worked! paul@laika:~/test$ rm file1 && echo It worked! || echo It failed! rm: cannot remove `file1': No such file or directory 82
- 89. control operators It failed! paul@laika:~/test$ 11.7. # pound sign Everything written after a pound sign (#) is ignored by the shell. This is useful to write a shell comment, but has no influence on the command execution or shell expansion. paul@debian4:~$ mkdir test # we create a directory paul@debian4:~$ cd test #### we enter the directory paul@debian4:~/test$ ls # is it empty ? paul@debian4:~/test$ 11.8. escaping special characters The backslash character enables the use of control characters, but without the shell interpreting it, this is called escaping characters. [paul@RHELv4u3 ~]$ echo hello ; world hello ; world [paul@RHELv4u3 ~]$ echo hello world hello world [paul@RHELv4u3 ~]$ echo escaping # & " ' escaping # & " ' [paul@RHELv4u3 ~]$ echo escaping ?*"' escaping ?*"' end of line backslash Lines ending in a backslash are continued on the next line. The shell does not interpret the newline character and will wait on shell expansion and execution of the command line until a newline without backslash is encountered. [paul@RHEL4b ~]$ echo This command line > is split in three > parts This command line is split in three parts [paul@RHEL4b ~]$ 83
- 90. control operators 11.9. practice: control operators 0. Each question can be answered by one command line! 1. When you type passwd, which file is executed ? 2. What kind of file is that ? 3. Execute the pwd command twice. (remember 0.) 4. Execute ls after cd /etc, but only if cd /etc did not error. 5. Execute cd /etc after cd etc, but only if cd etc fails. 6. Echo it worked when touch test42 works, and echo it failed when the touch failed. All on one command line as a normal user (not root). Test this line in your home directory and in /bin/ . 7. Execute sleep 6, what is this command doing ? 8. Execute sleep 200 in background (do not wait for it to finish). 9. Write a command line that executes rm file55. Your command line should print 'success' if file55 is removed, and print 'failed' if there was a problem. (optional)10. Use echo to display "Hello World with strange' characters * [ } ~ ." (including all quotes) 84
- 91. control operators 11.10. solution: control operators 0. Each question can be answered by one command line! 1. When you type passwd, which file is executed ? which passwd 2. What kind of file is that ? file /usr/bin/passwd 3. Execute the pwd command twice. (remember 0.) pwd ; pwd 4. Execute ls after cd /etc, but only if cd /etc did not error. cd /etc && ls 5. Execute cd /etc after cd etc, but only if cd etc fails. cd etc || cd /etc 6. Echo it worked when touch test42 works, and echo it failed when the touch failed. All on one command line as a normal user (not root). Test this line in your home directory and in /bin/ . paul@deb503:~$ cd ; touch test42 && echo it worked || echo it failed it worked paul@deb503:~$ cd /bin; touch test42 && echo it worked || echo it failed touch: cannot touch `test42': Permission denied it failed 7. Execute sleep 6, what is this command doing ? pausing for six seconds 8. Execute sleep 200 in background (do not wait for it to finish). sleep 200 & 9. Write a command line that executes rm file55. Your command line should print 'success' if file55 is removed, and print 'failed' if there was a problem. rm file55 && echo success || echo failed (optional)10. Use echo to display "Hello World with strange' characters * [ } ~ ." (including all quotes) echo "Hello World with strange' characters * [ } ~ . " or echo ""Hello World with strange' characters * [ } ~ . "" 85
- 92. Chapter 12. variables Table of Contents 12.1. about variables ............................................................................................. 87 12.2. quotes ........................................................................................................... 88 12.3. set ................................................................................................................. 89 12.4. unset ............................................................................................................. 89 12.5. env ................................................................................................................ 89 12.6. export ........................................................................................................... 90 12.7. delineate variables ........................................................................................ 90 12.8. unbound variables ........................................................................................ 90 12.9. shell options ................................................................................................. 91 12.10. shell embedding ......................................................................................... 91 12.11. practice: shell variables .............................................................................. 93 12.12. solution: shell variables ............................................................................. 94 In this chapter we learn to manage environment variables in the shell. These variables are often read by applications. We also take a brief look at child shells, embedded shells and shell options. 86
- 93. variables 12.1. about variables $ dollar sign Another important character interpreted by the shell is the dollar sign $. The shell will look for an environment variable named like the string following the dollar sign and replace it with the value of the variable (or with nothing if the variable does not exist). These are some examples using $HOSTNAME, $USER, $UID, $SHELL, and $HOME. [paul@RHELv4u3 ~]$ echo This is the $SHELL shell This is the /bin/bash shell [paul@RHELv4u3 ~]$ echo This is $SHELL on computer $HOSTNAME This is /bin/bash on computer RHELv4u3.localdomain [paul@RHELv4u3 ~]$ echo The userid of $USER is $UID The userid of paul is 500 [paul@RHELv4u3 ~]$ echo My homedir is $HOME My homedir is /home/paul case sensitive This example shows that shell variables are case sensitive! [paul@RHELv4u3 ~]$ echo Hello $USER Hello paul [paul@RHELv4u3 ~]$ echo Hello $user Hello $PS1 The $PS1 variable determines your shell prompt. You can use backslash escaped special characters like u for the username or w for the working directory. The bash manual has a complete reference. In this example we change the value of $PS1 a couple of times. paul@deb503:~$ PS1=prompt prompt promptPS1='prompt ' prompt prompt PS1='> ' > > PS1='u@h$ ' paul@deb503$ paul@deb503$ PS1='u@h:W$' paul@deb503:~$ 87
- 94. variables $PATH The $PATH variable is determines where the shell is looking for commands to execute (unless the command is builtin or aliased). This variable contains a list of directories, separated by colons. [[paul@RHEL4b ~]$ echo $PATH /usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin: The shell will not look in the current directory for commands to execute! (Looking for executables in the current directory provided an easy way to hack PC-DOS computers). If you want the shell to look in the current directory, then add a . at the end of your $PATH. [paul@RHEL4b ~]$ PATH=$PATH:. [paul@RHEL4b ~]$ echo $PATH /usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:. [paul@RHEL4b ~]$ Your path might be different when using su instead of su - because the latter will take on the environment of the target user. The root user typically has /sbin directories added to the $PATH variable. [paul@RHEL3 ~]$ su Password: [root@RHEL3 paul]# echo $PATH /usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin [root@RHEL3 paul]# exit [paul@RHEL3 ~]$ su - Password: [root@RHEL3 ~]# echo $PATH /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin: [root@RHEL3 ~]# creating variables This example creates the variable $MyVar and sets its value. It then uses echo to verify the value. [paul@RHELv4u3 gen]$ MyVar=555 [paul@RHELv4u3 gen]$ echo $MyVar 555 [paul@RHELv4u3 gen]$ 12.2. quotes Notice that double quotes still allow the parsing of variables, whereas single quotes prevent this. [paul@RHELv4u3 ~]$ MyVar=555 [paul@RHELv4u3 ~]$ echo $MyVar 555 [paul@RHELv4u3 ~]$ echo "$MyVar" 555 88
- 95. variables [paul@RHELv4u3 ~]$ echo '$MyVar' $MyVar The bash shell will replace variables with their value in double quoted lines, but not in single quoted lines. paul@laika:~$ city=Burtonville paul@laika:~$ echo "We are in $city today." We are in Burtonville today. paul@laika:~$ echo 'We are in $city today.' We are in $city today. 12.3. set You can use the set command to display a list of environment variables. On Ubuntu and Debian systems, the set command will also list shell functions after the shell variables. Use set | more to see the variables then. 12.4. unset Use the unset command to remove a variable from your shell environment. [paul@RHEL4b ~]$ MyVar=8472 [paul@RHEL4b ~]$ echo $MyVar 8472 [paul@RHEL4b ~]$ unset MyVar [paul@RHEL4b ~]$ echo $MyVar [paul@RHEL4b ~]$ 12.5. env The env command without options will display a list of exported variables. The difference with set with options is that set lists all variables, inlcuding those not exported to child shells. But env can also be used to start a clean shell (a shell without any inherited environment). The env -i command clears the environment for the subshell. Notice in this screenshot that bash will set the $SHELL variable on startup. [paul@RHEL4b ~]$ bash -c 'echo $SHELL $HOME $USER' /bin/bash /home/paul paul [paul@RHEL4b ~]$ env -i bash -c 'echo $SHELL $HOME $USER' /bin/bash [paul@RHEL4b ~]$ You can use the env command to set the $LANG, or any other, variable for just one instance of bash with one command. The example below uses this to show the influence of the $LANG variable on file globbing (see the chapter on file globbing). [paul@RHEL4b test]$ env LANG=C bash -c 'ls File[a-z]' 89
- 96. variables Filea Fileb [paul@RHEL4b test]$ env LANG=en_US.UTF-8 bash -c 'ls File[a-z]' Filea FileA Fileb FileB [paul@RHEL4b test]$ 12.6. export You can export shell variables to other shells with the export command. This will export the variable to child shells. [paul@RHEL4b ~]$ var3=three [paul@RHEL4b ~]$ var4=four [paul@RHEL4b ~]$ export var4 [paul@RHEL4b ~]$ echo $var3 $var4 three four [paul@RHEL4b ~]$ bash [paul@RHEL4b ~]$ echo $var3 $var4 four But it will not export to the parent shell (previous screenshot continued). [paul@RHEL4b ~]$ export var5=five [paul@RHEL4b ~]$ echo $var3 $var4 $var5 four five [paul@RHEL4b ~]$ exit exit [paul@RHEL4b ~]$ echo $var3 $var4 $var5 three four [paul@RHEL4b ~]$ 12.7. delineate variables Until now, we have seen that bash interpretes a variable starting from a dollar sign, continuing until the first occurrence of a non-alphanumeric character that is not an underscore. In some situations, this can be a problem. This issue can be resolved with curly braces like in this example. [paul@RHEL4b ~]$ prefix=Super [paul@RHEL4b ~]$ echo Hello $prefixman and $prefixgirl Hello and [paul@RHEL4b ~]$ echo Hello ${prefix}man and ${prefix}girl Hello Superman and Supergirl [paul@RHEL4b ~]$ 12.8. unbound variables The example below tries to display the value of the $MyVar variable, but it fails because the variable does not exist. By default the shell will display nothing when a variable is unbound (does not exist). [paul@RHELv4u3 gen]$ echo $MyVar [paul@RHELv4u3 gen]$ 90
- 97. variables There is, however, the nounset shell option that you can use to generate an error when a variable does not exist. paul@laika:~$ set -u paul@laika:~$ echo $Myvar bash: Myvar: unbound variable paul@laika:~$ set +u paul@laika:~$ echo $Myvar paul@laika:~$ In the bash shell set -u is identical to set -o nounset and likewise set +u is identical to set +o nounset. 12.9. shell options Both set and unset are builtin shell commands. They can be used to set options of the bash shell itself. The next example will clarify this. By default, the shell will treat unset variables as a variable having no value. By setting the -u option, the shell will treat any reference to unset variables as an error. See the man page of bash for more information. [paul@RHEL4b ~]$ echo $var123 [paul@RHEL4b ~]$ set -u [paul@RHEL4b ~]$ echo $var123 -bash: var123: unbound variable [paul@RHEL4b ~]$ set +u [paul@RHEL4b ~]$ echo $var123 [paul@RHEL4b ~]$ To list all the set options for your shell, use echo $-. The noclobber (or -C) option will be explained later in this book (in the I/O redirection chapter). [paul@RHEL4b ~]$ echo $- himBH [paul@RHEL4b ~]$ set -C ; set -u [paul@RHEL4b ~]$ echo $- himuBCH [paul@RHEL4b ~]$ set +C ; set +u [paul@RHEL4b ~]$ echo $- himBH [paul@RHEL4b ~]$ When typing set without options, you get a list of all variables without function when the shell is on posix mode. You can set bash in posix mode typing set -o posix. 12.10. shell embedding Shells can be embedded on the command line, or in other words, the command line scan can spawn new processes containing a fork of the current shell. You can use variables to prove that new shells are created. In the screenshot below, the variable $var1 only exists in the (temporary) sub shell. 91
- 98. variables [paul@RHELv4u3 gen]$ echo $var1 [paul@RHELv4u3 gen]$ echo $(var1=5;echo $var1) 5 [paul@RHELv4u3 gen]$ echo $var1 [paul@RHELv4u3 gen]$ You can embed a shell in an embedded shell, this is called nested embedding of shells. This screenshot shows an embedded shell inside an embedded shell. paul@deb503:~$ A=shell paul@deb503:~$ echo $C$B$A $(B=sub;echo $C$B$A; echo $(A=sub;echo $C$B$A)) shell subshell subsub back ticks Single embedding can be useful to avoid changing your current directory. The screenshot below uses back ticks instead of dollar-bracket to embed. [paul@RHELv4u3 ~]$ echo `cd /etc; ls -d * | grep pass` passwd passwd- passwd.OLD [paul@RHELv4u3 ~]$ You can only use the $() notation to nest embedded shells, backticks cannot do this. back ticks or single quotes Placing the embedding between backticks uses one character less than the dollar and parenthesis combo. Be careful however, backticks are often confused with single quotes. The technical difference between ' and ` is significant! [paul@RHELv4u3 gen]$ echo `var1=5;echo $var1` 5 [paul@RHELv4u3 gen]$ echo 'var1=5;echo $var1' var1=5;echo $var1 [paul@RHELv4u3 gen]$ 92
- 99. variables 12.11. practice: shell variables 1. Use echo to display Hello followed by your username. (use a bash variable!) 2. Create a variable answer with a value of 42. 3. Copy the value of $LANG to $MyLANG. 4. List all current shell variables. 5. List all exported shell variables. 6. Do the env and set commands display your variable ? 6. Destroy your answer variable. 7. Find the list of shell options in the man page of bash. What is the difference between set -u and set -o nounset? 8. Create two variables, and export one of them. 9. Display the exported variable in an interactive child shell. 10. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use echo and the two variables to echo Dumbledore. 11. Activate nounset in your shell. Test that it shows an error message when using non-existing variables. 12. Deactivate nounset. 13. Find the list of backslash escaped characters in the manual of bash. Add the time to your PS1 prompt. 14. Execute cd /var and ls in an embedded shell. 15. Create the variable embvar in an embedded shell and echo it. Does the variable exist in your current shell now ? 16. Explain what "set -x" does. Can this be useful ? (optional)17. Given the following screenshot, add exactly four characters to that command line so that the total output is FirstMiddleLast. [paul@RHEL4b ~]$ echo First; echo Middle; echo Last 18. Display a long listing (ls -l) of the passwd command using the which command inside back ticks. 93
- 100. variables 12.12. solution: shell variables 1. Use echo to display Hello followed by your username. (use a bash variable!) echo Hello $USER 2. Create a variable answer with a value of 42. answer=42 3. Copy the value of $LANG to $MyLANG. MyLANG=$LANG 4. List all current shell variables. set set|more on Ubuntu/Debian 5. List all exported shell variables. env 6. Do the env and set commands display your variable ? env | more set | more 6. Destroy your answer variable. unset answer 7. Find the list of shell options in the man page of bash. What is the difference between set -u and set -o nounset? read the manual of bash (man bash), search for nounset -- both mean the same thing. 8. Create two variables, and export one of them. var1=1; export var2=2 9. Display the exported variable in an interactive child shell. bash echo $var2 10. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use echo and the two variables to echo Dumbledore. varx=Dumb; vary=do echo ${varx}le${vary}re solution by Yves from Dexia : echo $varx'le'$vary're' solution by Erwin from Telenet : echo "$varx"le"$vary"re 11. Activate nounset in your shell. Test that it shows an error message when using non-existing variables. 94
- 101. variables set -u OR set -o nounset Both these lines have the same effect. 12. Deactivate nounset. set +u OR set +o nounset 13. Find the list of backslash escaped characters in the manual of bash. Add the time to your PS1 prompt. PS1='t u@h W$ ' 14. Execute cd /var and ls in an embedded shell. echo $(cd /var ; ls) The echo command is only needed to show the result of the ls command. Omitting will result in the shell trying to execute the first file as a command. 15. Create the variable embvar in an embedded shell and echo it. Does the variable exist in your current shell now ? $(embvar=emb;echo $embvar) ; echo $embvar (the last echo fails). $embvar does not exist in your current shell 16. Explain what "set -x" does. Can this be useful ? It displays shell expansion for troubleshooting your command. (optional)17. Given the following screenshot, add exactly four characters to that command line so that the total output is FirstMiddleLast. [paul@RHEL4b ~]$ echo First; echo Middle; echo Last echo -n First; echo -n Middle; echo Last 18. Display a long listing (ls -l) of the passwd command using the which command inside back ticks. ls -l `which passwd` 95
- 102. Chapter 13. shell history Table of Contents 13.1. repeating the last command ......................................................................... 97 13.2. repeating other commands ........................................................................... 97 13.3. history ........................................................................................................... 97 13.4. !n ................................................................................................................... 97 13.5. Ctrl-r ............................................................................................................. 97 13.6. $HISTSIZE .................................................................................................. 98 13.7. $HISTFILE .................................................................................................. 98 13.8. $HISTFILESIZE .......................................................................................... 98 13.9. (optional)regular expressions ....................................................................... 98 13.10. (optional)repeating commands in ksh ........................................................ 99 13.11. practice: shell history ............................................................................... 100 13.12. solution: shell history ............................................................................... 101 The shell makes it easy for us to repeat commands, this chapter explains how. 96
- 103. shell history 13.1. repeating the last command To repeat the last command in bash, type !!. This is pronounced as bang bang. paul@debian5:~/test42$ echo this will be repeated > file42.txt paul@debian5:~/test42$ !! echo this will be repeated > file42.txt paul@debian5:~/test42$ 13.2. repeating other commands You can repeat other commands using one bang followed by one or more characters. The shell will repeat the last command that started with those characters. paul@debian5:~/test42$ touch file42 paul@debian5:~/test42$ cat file42 paul@debian5:~/test42$ !to touch file42 paul@debian5:~/test42$ 13.3. history To see older commands, use history to display the shell command history (or use history n to see the last n commands). paul@debian5:~/test$ history 10 38 mkdir test 39 cd test 40 touch file1 41 echo hello > file2 42 echo It is very cold today > winter.txt 43 ls 44 ls -l 45 cp winter.txt summer.txt 46 ls -l 47 history 10 13.4. !n When typing ! followed by the number preceding the command you want repeated, then the shell will echo the command and execute it. paul@debian5:~/test$ !43 ls file1 file2 summer.txt winter.txt 13.5. Ctrl-r Another option is to use Ctrl-r to search in the history. In the screenshot below i only typed Ctrl-r followed by four characters apti and it finds the last command containing these four consecutive characters. 97
- 104. shell history paul@debian5:~$ (reverse-i-search)`apti': sudo aptitude install screen 13.6. $HISTSIZE The $HISTSIZE variable determines the number of commands that will be remembered in your current environment. Most distributions default this variable to 500 or 1000. paul@debian5:~$ echo $HISTSIZE 500 You can change it to any value you like. paul@debian5:~$ HISTSIZE=15000 paul@debian5:~$ echo $HISTSIZE 15000 13.7. $HISTFILE The $HISTFILE variable points to the file that contains your history. The bash shell defaults this value to ~/.bash_history. paul@debian5:~$ echo $HISTFILE /home/paul/.bash_history A session history is saved to this file when you exit the session! Closing a gnome-terminal with the mouse, or typing reboot as root will NOT save your terminal's history. 13.8. $HISTFILESIZE The number of commands kept in your history file can be set using $HISTFILESIZE. paul@debian5:~$ echo $HISTFILESIZE 15000 13.9. (optional)regular expressions It is possible to use regular expressions when using the bang to repeat commands. The screenshot below switches 1 into 2. paul@deianb5:~/test$ cat file1 paul@debian5:~/test$ !c:s/1/2 cat file2 hello paul@debian5:~/test$ 98
- 105. shell history 13.10. (optional)repeating commands in ksh (just in case you end up on an AIX or Solaris computer that does not have bash installed) Repeating a command in the Korn shell is very similar. The Korn shell also has the history command, but uses the letter r to recall lines from history. This screenshot shows the history command. Note the different meaning of the parameter. $ history 17 17 clear 18 echo hoi 19 history 12 20 echo world 21 history 17 Repeating with r can be combined with the line numbers given by the history command, or with the first few letters of the command. $ r e echo world world $ cd /etc $ r cd /etc $ 99
- 106. shell history 13.11. practice: shell history 1. Issue the command echo The answer to the meaning of life, the universe and everything is 42. 2. Repeat the previous command using only two characters (there are two solutiones!) 3. Display the last 5 commands you typed. 4. Issue the long echo from question 1 again, using the line numbers you received from the command in question 3. 5. How many commands can be kept in memory for your current shell session ? 6. Where are these commands stored when exiting the shell ? 7. How many commands can be written to the history file when exiting your current shell session ? 8. Make sure your current bash shell remembers the next 5000 commands you type. 9. Open more than one console (press Ctrl-shift-t in gnome-terminal) with the same user account. When is command history written to the history file ? 100
- 107. shell history 13.12. solution: shell history 1. Issue the command echo The answer to the meaning of life, the universe and everything is 42. echo The answer to the meaning of life, the universe and everything is 42 2. Repeat the previous command using only two characters (there are two solutiones!) !! OR !e 3. Display the last 5 commands you typed. paul@ubu1010:~$ history 5 52 ls -l 53 ls 54 df -h | grep sda 55 echo The answer to the meaning of life, the universe and everything is 42 56 history 5 You will receive different line numbers. 4. Issue the long echo from question 1 again, using the line numbers you received from the command in question 3. paul@ubu1010:~$ !56 echo The answer to the meaning of life, the universe and everything is 42 The answer to the meaning of life, the universe and everything is 42 5. How many commands can be kept in memory for your current shell session ? echo $HISTSIZE 6. Where are these commands stored when exiting the shell ? echo $HISTFILE 7. How many commands can be written to the history file when exiting your current shell session ? echo $HISTFILESIZE 8. Make sure your current bash shell remembers the next 5000 commands you type. HISTSIZE=5000 9. Open more than one console (press Ctrl-shift-t in gnome-terminal) with the same user account. When is command history written to the history file ? when you type exit 101
- 108. Chapter 14. file globbing Table of Contents 14.1. * asterisk .................................................................................................... 103 14.2. ? question mark .......................................................................................... 103 14.3. [] square brackets ....................................................................................... 103 14.4. a-z and 0-9 ranges ..................................................................................... 104 14.5. $LANG and square brackets ...................................................................... 104 14.6. preventing file globbing ............................................................................. 104 14.7. practice: shell globbing .............................................................................. 106 14.8. solution: shell globbing .............................................................................. 107 The shell is also responsible for file globbing (or dynamic filename generation). This chapter will explain file globbing. 102
- 109. file globbing 14.1. * asterisk The asterisk * is interpreted by the shell as a sign to generate filenames, matching the asterisk to any combination of characters (even none). When no path is given, the shell will use filenames in the current directory. See the man page of glob(7) for more information. (This is part of LPI topic 1.103.3.) [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc [paul@RHELv4u3 gen]$ ls File* File4 File55 FileA Fileab FileAB [paul@RHELv4u3 gen]$ ls file* file1 file2 file3 fileab fileabc [paul@RHELv4u3 gen]$ ls *ile55 File55 [paul@RHELv4u3 gen]$ ls F*ile55 File55 [paul@RHELv4u3 gen]$ ls F*55 File55 [paul@RHELv4u3 gen]$ 14.2. ? question mark Similar to the asterisk, the question mark ? is interpreted by the shell as a sign to generate filenames, matching the question mark with exactly one character. [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc [paul@RHELv4u3 gen]$ ls File? File4 FileA [paul@RHELv4u3 gen]$ ls Fil?4 File4 [paul@RHELv4u3 gen]$ ls Fil?? File4 FileA [paul@RHELv4u3 gen]$ ls File?? File55 Fileab FileAB [paul@RHELv4u3 gen]$ 14.3. [] square brackets The square bracket [ is interpreted by the shell as a sign to generate filenames, matching any of the characters between [ and the first subsequent ]. The order in this list between the brackets is not important. Each pair of brackets is replaced by exactly one character. [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc [paul@RHELv4u3 gen]$ ls File[5A] FileA [paul@RHELv4u3 gen]$ ls File[A5] FileA [paul@RHELv4u3 gen]$ ls File[A5][5b] File55 [paul@RHELv4u3 gen]$ ls File[a5][5b] File55 Fileab [paul@RHELv4u3 gen]$ ls File[a5][5b][abcdefghijklm] ls: File[a5][5b][abcdefghijklm]: No such file or directory 103
- 110. file globbing [paul@RHELv4u3 gen]$ ls file[a5][5b][abcdefghijklm] fileabc [paul@RHELv4u3 gen]$ You can also exclude characters from a list between square brackets with the exclamation mark !. And you are allowed to make combinations of these wild cards. [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc [paul@RHELv4u3 gen]$ ls file[a5][!Z] fileab [paul@RHELv4u3 gen]$ ls file[!5]* file1 file2 file3 fileab fileabc [paul@RHELv4u3 gen]$ ls file[!5]? fileab [paul@RHELv4u3 gen]$ 14.4. a-z and 0-9 ranges The bash shell will also understand ranges of characters between brackets. [paul@RHELv4u3 gen]$ ls file1 file3 File55 fileab FileAB fileabc file2 File4 FileA Fileab fileab2 [paul@RHELv4u3 gen]$ ls file[a-z]* fileab fileab2 fileabc [paul@RHELv4u3 gen]$ ls file[0-9] file1 file2 file3 [paul@RHELv4u3 gen]$ ls file[a-z][a-z][0-9]* fileab2 [paul@RHELv4u3 gen]$ 14.5. $LANG and square brackets But, don't forget the influence of the LANG variable. Some languages include lowercase letters in an uppercase range (and vice versa). paul@RHELv4u4:~/test$ ls [A-Z]ile? file1 file2 file3 File4 paul@RHELv4u4:~/test$ ls [a-z]ile? file1 file2 file3 File4 paul@RHELv4u4:~/test$ echo $LANG en_US.UTF-8 paul@RHELv4u4:~/test$ LANG=C paul@RHELv4u4:~/test$ echo $LANG C paul@RHELv4u4:~/test$ ls [a-z]ile? file1 file2 file3 paul@RHELv4u4:~/test$ ls [A-Z]ile? File4 paul@RHELv4u4:~/test$ 14.6. preventing file globbing The screenshot below should be no surprise. The echo * will echo a * when in an empty directory. And it will echo the names of all files when the directory is not empty. 104
- 111. file globbing paul@ubu1010:~$ mkdir test42 paul@ubu1010:~$ cd test42 paul@ubu1010:~/test42$ echo * * paul@ubu1010:~/test42$ touch file42 file33 paul@ubu1010:~/test42$ echo * file33 file42 Globbing can be prevented using quotes or by escaping the special characters, as shown in this screenshot. paul@ubu1010:~/test42$ echo * file33 file42 paul@ubu1010:~/test42$ echo * * paul@ubu1010:~/test42$ echo '*' * paul@ubu1010:~/test42$ echo "*" * 105
- 112. file globbing 14.7. practice: shell globbing 1. Create a test directory and enter it. 2. Create files file1 file10 file11 file2 File2 File3 file33 fileAB filea fileA fileAAA file( file 2 (the last one has 6 characters including a space) 3. List (with ls) all files starting with file 4. List (with ls) all files starting with File 5. List (with ls) all files starting with file and ending in a number. 6. List (with ls) all files starting with file and ending with a letter 7. List (with ls) all files starting with File and having a digit as fifth character. 8. List (with ls) all files starting with File and having a digit as fifth character and nothing else. 9. List (with ls) all files starting with a letter and ending in a number. 10. List (with ls) all files that have exactly five characters. 11. List (with ls) all files that start with f or F and end with 3 or A. 12. List (with ls) all files that start with f have i or R as second character and end in a number. 13. List all files that do not start with the letter F. 14. Copy the value of $LANG to $MyLANG. 15. Show the influence of $LANG in listing A-Z or a-z ranges. 16. You receive information that one of your servers was cracked, the cracker probably replaced the ls command. You know that the echo command is safe to use. Can echo replace ls ? How can you list the files in the current directory with echo ? 17. Is there another command besides cd to change directories ? 106
- 113. file globbing 14.8. solution: shell globbing 1. Create a test directory and enter it. mkdir testdir; cd testdir 2. Create files file1 file10 file11 file2 File2 File3 file33 fileAB filea fileA fileAAA file( file 2 (the last one has 6 characters including a space) touch file1 file10 file11 file2 File2 File3 touch file33 fileAB filea fileA fileAAA touch "file(" touch "file 2" 3. List (with ls) all files starting with file ls file* 4. List (with ls) all files starting with File ls File* 5. List (with ls) all files starting with file and ending in a number. ls file*[0-9] 6. List (with ls) all files starting with file and ending with a letter ls file*[a-z] 7. List (with ls) all files starting with File and having a digit as fifth character. ls File[0-9]* 8. List (with ls) all files starting with File and having a digit as fifth character and nothing else. ls File[0-9] 9. List (with ls) all files starting with a letter and ending in a number. ls [a-z]*[0-9] 10. List (with ls) all files that have exactly five characters. ls ????? 11. List (with ls) all files that start with f or F and end with 3 or A. ls [fF]*[3A] 12. List (with ls) all files that start with f have i or R as second character and end in a number. ls f[iR]*[0-9] 13. List all files that do not start with the letter F. ls [!F]* 107
- 114. file globbing 14. Copy the value of $LANG to $MyLANG. MyLANG=$LANG 15. Show the influence of $LANG in listing A-Z or a-z ranges. see example in book 16. You receive information that one of your servers was cracked, the cracker probably replaced the ls command. You know that the echo command is safe to use. Can echo replace ls ? How can you list the files in the current directory with echo ? echo * 17. Is there another command besides cd to change directories ? pushd popd 108
- 115. Part IV. pipes and commands
- 116. Chapter 15. redirection and pipes Table of Contents 15.1. stdin, stdout, and stderr .............................................................................. 111 15.2. output redirection ....................................................................................... 111 15.3. error redirection ......................................................................................... 112 15.4. input redirection ......................................................................................... 113 15.5. confusing redirection .................................................................................. 114 15.6. quick file clear ........................................................................................... 114 15.7. swapping stdout and stderr ........................................................................ 114 15.8. pipes ........................................................................................................... 115 15.9. practice: redirection and pipes ................................................................... 116 15.10. solution: redirection and pipes ................................................................. 117 One of the powers of the Unix command line is the use of redirection and pipes. This chapter first explains redirection of input, output and error streams. It then introduces pipes that consist of several commands. 110
- 117. redirection and pipes 15.1. stdin, stdout, and stderr The shell (and almost every other Linux command) takes input from stdin (stream 0) and sends output to stdout (stream 1) and error messages to stderr (stream 2) . The keyboard often server as stdin, stdout and stderr both go to the disply. The shell allows you to redirect these streams. 15.2. output redirection > stdout stdout can be redirected with a greater than sign. While scanning the line, the shell will see the > sign and will clear the file. [paul@RHELv4u3 ~]$ echo It is cold today! It is cold today! [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ Note that the > notation is in fact the abbreviation of 1> (stdout being referred to as stream 1. output file is erased To repeat: While scanning the line, the shell will see the > sign and will clear the file! This means that even when the command fails, the file will be cleared! [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ zcho It is cold today! > winter.txt -bash: zcho: command not found [paul@RHELv4u3 ~]$ cat winter.txt [paul@RHELv4u3 ~]$ noclobber Erasing a file while using > can be prevented by setting the noclobber option. [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ set -o noclobber 111
- 118. redirection and pipes [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt -bash: winter.txt: cannot overwrite existing file [paul@RHELv4u3 ~]$ set +o noclobber [paul@RHELv4u3 ~]$ overruling noclobber The noclobber can be overruled with >|. [paul@RHELv4u3 ~]$ set -o noclobber [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt -bash: winter.txt: cannot overwrite existing file [paul@RHELv4u3 ~]$ echo It is very cold today! >| winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is very cold today! [paul@RHELv4u3 ~]$ >> append Use >> to append output to a file. [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ echo Where is the summer ? >> winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! Where is the summer ? [paul@RHELv4u3 ~]$ 15.3. error redirection 2> stderr Redirecting stderr is done with 2>. This can be very useful to prevent error messages from cluttering your screen. The screenshot below shows redirection of stdout to a file, and stderr to /dev/null. Writing 1> is the same as >. [paul@RHELv4u3 ~]$ find / > allfiles.txt 2> /dev/null [paul@RHELv4u3 ~]$ 2>&1 To redirect both stdout and stderr to the same file, use 2>&1. 112
- 119. redirection and pipes [paul@RHELv4u3 ~]$ find / > allfiles_and_errors.txt 2>&1 [paul@RHELv4u3 ~]$ Note that the order of redirections is significant. For example, the command ls > dirlist 2>&1 directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command ls 2>&1 > dirlist directs only the standard output to file dirlist, because the standard error was made a copy of the standard output before the standard output was redirected to dirlist. 15.4. input redirection < stdin Redirecting stdin is done with < (short for 0<). [paul@RHEL4b ~]$ cat < text.txt one two [paul@RHEL4b ~]$ tr 'onetw' 'ONEZZ' < text.txt ONE ZZO [paul@RHEL4b ~]$ << here document The here document (sometimes called here-is-document) is a way to append input until a certain sequence (usually EOF) is encountered. The EOF marker can be typed literally or can be called with Ctrl-D. [paul@RHEL4b ~]$ cat <<EOF > text.txt > one > two > EOF [paul@RHEL4b ~]$ cat text.txt one two [paul@RHEL4b ~]$ cat <<brol > text.txt > brel > brol [paul@RHEL4b ~]$ cat text.txt brel [paul@RHEL4b ~]$ 113
- 120. redirection and pipes 15.5. confusing redirection The shell will scan the whole line before applying redirection. The following command line is very readable and is correct. cat winter.txt > snow.txt 2> errors.txt But this one is also correct, but less readable. 2> errors.txt cat winter.txt > snow.txt Even this will be understood perfectly by the shell. < winter.txt > snow.txt 2> errors.txt cat 15.6. quick file clear So what is the quickest way to clear a file ? >foo And what is the quickest way to clear a file when the noclobber option is set ? >|bar 15.7. swapping stdout and stderr When filtering an output stream, e.g. through a regular pipe ( | ) you only can filter stdout. Say you want to filter out some unimportant error, out of the stderr stream. This cannot be done directly, and you need to 'swap' stdout and stderr. This can be done by using a 4th stream referred to with number 3: 3>&1 1>&2 2>&3 This Tower Of Hanoi like construction uses a temporary stream 3, to be able to swap stdout (1) and stderr (2). The following is an example of how to filter out all lines in the stderr stream, containing $uninterestingerror. $command 3>&1 1>&2 2>&3 | grep -v $error 3>&1 1>&2 2>&3 But in this example, it can be done in a much shorter way, by using a pipe on STDERR: /usr/bin/$somecommand |& grep -v $uninterestingerror 114
- 121. redirection and pipes 15.8. pipes One of the most powerful advantages of Linux is the use of pipes. A pipe takes stdout from the previous command and sends it as stdin to the next command. All commands in a pipe run simultaneously. | vertical bar Consider the following example. paul@debian5:~/test$ ls /etc > etcfiles.txt paul@debian5:~/test$ tail -4 etcfiles.txt X11 xdg xml xpdf paul@debian5:~/test$ This can be written in one command line using a pipe. paul@debian5:~/test$ ls /etc | tail -4 X11 xdg xml xpdf paul@debian5:~/test$ The pipe is represented by a vertical bar | between two commands. multiple pipes One command line can use multiple pipes. All commands in the pipe can run at the same time. paul@deb503:~/test$ ls /etc | tail -4 | tac xpdf xml xdg X11 115
- 122. redirection and pipes 15.9. practice: redirection and pipes 1. Use ls to output the contents of the /etc/ directory to a file called etc.txt. 2. Activate the noclobber shell option. 3. Verify that nocclobber is active by repeating your ls on /etc/. 4. When listing all shell options, which character represents the noclobber option ? 5. Deactivate the noclobber option. 6. Make sure you have two shells open on the same computer. Create an empty tailing.txt file. Then type tail -f tailing.txt. Use the second shell to append a line of text to that file. Verify that the first shell displays this line. 7. Create a file that contains the names of five people. Use cat and output redirection to create the file and use a here document to end the input. 116
- 123. redirection and pipes 15.10. solution: redirection and pipes 1. Use ls to output the contents of the /etc/ directory to a file called etc.txt. ls /etc > etc.txt 2. Activate the noclobber shell option. set -o noclobber 3. Verify that nocclobber is active by repeating your ls on /etc/. ls /etc > etc.txt (should not work) 4. When listing all shell options, which character represents the noclobber option ? echo $- (noclobber is visible as C) 5. Deactivate the noclobber option. set +o noclobber 6. Make sure you have two shells open on the same computer. Create an empty tailing.txt file. Then type tail -f tailing.txt. Use the second shell to append a line of text to that file. Verify that the first shell displays this line. paul@deb503:~$ > tailing.txt paul@deb503:~$ tail -f tailing.txt hello world in the other shell: paul@deb503:~$ echo hello >> tailing.txt paul@deb503:~$ echo world >> tailing.txt 7. Create a file that contains the names of five people. Use cat and output redirection to create the file and use a here document to end the input. paul@deb503:~$ cat > tennis.txt << ace > Justine Henin > Venus Williams > Serena Williams > Martina Hingis > Kim Clijsters > ace paul@deb503:~$ cat tennis.txt Justine Henin Venus Williams Serena Williams Martina Hingis Kim Clijsters paul@deb503:~$ 117
- 124. Chapter 16. filters Table of Contents 16.1. cat ............................................................................................................... 119 16.2. tee ............................................................................................................... 119 16.3. grep ............................................................................................................. 119 16.4. cut ............................................................................................................... 120 16.5. tr ................................................................................................................. 121 16.6. wc ............................................................................................................... 122 16.7. sort .............................................................................................................. 123 16.8. uniq ............................................................................................................. 123 16.9. comm .......................................................................................................... 124 16.10. od .............................................................................................................. 125 16.11. sed ............................................................................................................ 125 16.12. pipe examples ........................................................................................... 126 16.13. practice: filters ......................................................................................... 127 16.14. solution: filters ......................................................................................... 128 Commands that are created to be used with a pipe are often called filters. These filters are very small programs that do one specific thing very efficiently. They can be used as building blocks. This chapter will introduce you to the most common filters. The combination of simple commands and filters in a long pipe allows you to design elegant solutions. 118
- 125. filters 16.1. cat When between two pipes, the cat command does nothing (except putting stdin on stdout. [paul@RHEL4b pipes]$ tac count.txt | cat | cat | cat | cat | cat five four three two one [paul@RHEL4b pipes]$ 16.2. tee Writing long pipes in Unix is fun, but sometimes you might want intermediate results. This is were tee comes in handy. The tee filter puts stdin on stdout and also into a file. So tee is almost the same as cat, except that it has two identical outputs. [paul@RHEL4b pipes]$ tac count.txt | tee temp.txt | tac one two three four five [paul@RHEL4b pipes]$ cat temp.txt five four three two one [paul@RHEL4b pipes]$ 16.3. grep The grep filter is famous among Unix users. The most common use of grep is to filter lines of text containing (or not containing) a certain string. [paul@RHEL4b pipes]$ cat tennis.txt Amelie Mauresmo, Fra Kim Clijsters, BEL Justine Henin, Bel Serena Williams, usa Venus Williams, USA [paul@RHEL4b pipes]$ cat tennis.txt | grep Williams Serena Williams, usa Venus Williams, USA You can write this without the cat. [paul@RHEL4b pipes]$ grep Williams tennis.txt Serena Williams, usa Venus Williams, USA One of the most useful options of grep is grep -i which filters in a case insensitive way. 119
- 126. filters [paul@RHEL4b pipes]$ grep Bel tennis.txt Justine Henin, Bel [paul@RHEL4b pipes]$ grep -i Bel tennis.txt Kim Clijsters, BEL Justine Henin, Bel [paul@RHEL4b pipes]$ Another very useful option is grep -v which outputs lines not matching the string. [paul@RHEL4b pipes]$ grep -v Fra tennis.txt Kim Clijsters, BEL Justine Henin, Bel Serena Williams, usa Venus Williams, USA [paul@RHEL4b pipes]$ And of course, both options can be combined to filter all lines not containing a case insensitive string. [paul@RHEL4b pipes]$ grep -vi usa tennis.txt Amelie Mauresmo, Fra Kim Clijsters, BEL Justine Henin, Bel [paul@RHEL4b pipes]$ With grep -A1 one line after the result is also displayed. paul@debian5:~/pipes$ grep -A1 Henin tennis.txt Justine Henin, Bel Serena Williams, usa With grep -B1 one line before the result is also displayed. paul@debian5:~/pipes$ grep -B1 Henin tennis.txt Kim Clijsters, BEL Justine Henin, Bel With grep -C1 (context) one line before and one after are also displayed. All three options (A,B, and C) can display any number of lines (using e.g. A2, B4 or C20). paul@debian5:~/pipes$ grep -C1 Henin tennis.txt Kim Clijsters, BEL Justine Henin, Bel Serena Williams, usa 16.4. cut The cut filter can select columns from files, depending on a delimiter or a count of bytes. The screenshot below uses cut to filter for the username and userid in the /etc/ passwd file. It uses the colon as a delimiter, and selects fields 1 and 3. 120
- 127. filters [[paul@RHEL4b pipes]$ cut -d: -f1,3 /etc/passwd | tail -4 Figo:510 Pfaff:511 Harry:516 Hermione:517 [paul@RHEL4b pipes]$ When using a space as the delimiter for cut, you have to quote the space. [paul@RHEL4b pipes]$ cut -d" " -f1 tennis.txt Amelie Kim Justine Serena Venus [paul@RHEL4b pipes]$ This example uses cut to display the second to the seventh character of /etc/passwd. [paul@RHEL4b pipes]$ cut -c2-7 /etc/passwd | tail -4 igo:x: faff:x arry:x ermion [paul@RHEL4b pipes]$ 16.5. tr You can translate characters with tr. The screenshot shows the translation of all occurrences of e to E. [paul@RHEL4b pipes]$ cat tennis.txt | tr 'e' 'E' AmEliE MaurEsmo, Fra Kim ClijstErs, BEL JustinE HEnin, BEl SErEna Williams, usa VEnus Williams, USA Here we set all letters to uppercase by defining two ranges. [paul@RHEL4b pipes]$ cat tennis.txt | tr 'a-z' 'A-Z' AMELIE MAURESMO, FRA KIM CLIJSTERS, BEL JUSTINE HENIN, BEL SERENA WILLIAMS, USA VENUS WILLIAMS, USA [paul@RHEL4b pipes]$ Here we translate all newlines to spaces. [paul@RHEL4b pipes]$ cat count.txt one 121
- 128. filters two three four five [paul@RHEL4b pipes]$ cat count.txt | tr 'n' ' ' one two three four five [paul@RHEL4b pipes]$ The tr -s filter can also be used to squeeze multiple occurrences of a character to one. [paul@RHEL4b pipes]$ cat spaces.txt one two three four five six [paul@RHEL4b pipes]$ cat spaces.txt | tr -s ' ' one two three four five six [paul@RHEL4b pipes]$ You can also use tr to 'encrypt' texts with rot13. [paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'nopqrstuvwxyzabcdefghijklm' bar gjb guerr sbhe svir [paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'n-za-m' bar gjb guerr sbhe svir [paul@RHEL4b pipes]$ This last example uses tr -d to delete characters. paul@debian5:~/pipes$ cat tennis.txt | tr -d e Amli Maursmo, Fra Kim Clijstrs, BEL Justin Hnin, Bl Srna Williams, usa Vnus Williams, USA 16.6. wc Counting words, lines and characters is easy with wc. [paul@RHEL4b pipes]$ wc tennis.txt 5 15 100 tennis.txt [paul@RHEL4b pipes]$ wc -l tennis.txt 5 tennis.txt [paul@RHEL4b pipes]$ wc -w tennis.txt 15 tennis.txt [paul@RHEL4b pipes]$ wc -c tennis.txt 100 tennis.txt 122
- 129. filters [paul@RHEL4b pipes]$ 16.7. sort The sort filter will default to an alphabetical sort. paul@debian5:~/pipes$ cat music.txt Queen Brel Led Zeppelin Abba paul@debian5:~/pipes$ sort music.txt Abba Brel Led Zeppelin Queen But the sort filter has many options to tweak its usage. This example shows sorting different columns (column 1 or column 2). [paul@RHEL4b pipes]$ sort -k1 country.txt Belgium, Brussels, 10 France, Paris, 60 Germany, Berlin, 100 Iran, Teheran, 70 Italy, Rome, 50 [paul@RHEL4b pipes]$ sort -k2 country.txt Germany, Berlin, 100 Belgium, Brussels, 10 France, Paris, 60 Italy, Rome, 50 Iran, Teheran, 70 The screenshot below shows the difference between an alphabetical sort and a numerical sort (both on the third column). [paul@RHEL4b pipes]$ sort -k3 country.txt Belgium, Brussels, 10 Germany, Berlin, 100 Italy, Rome, 50 France, Paris, 60 Iran, Teheran, 70 [paul@RHEL4b pipes]$ sort -n -k3 country.txt Belgium, Brussels, 10 Italy, Rome, 50 France, Paris, 60 Iran, Teheran, 70 Germany, Berlin, 100 16.8. uniq With uniq you can remove duplicates from a sorted list. 123
- 130. filters paul@debian5:~/pipes$ cat music.txt Queen Brel Queen Abba paul@debian5:~/pipes$ sort music.txt Abba Brel Queen Queen paul@debian5:~/pipes$ sort music.txt |uniq Abba Brel Queen uniq can also count occurrences with the -c option. paul@debian5:~/pipes$ sort music.txt |uniq -c 1 Abba 1 Brel 2 Queen 16.9. comm Comparing streams (or files) can be done with the comm. By default comm will output three columns. In this example, Abba, Cure and Queen are in both lists, Bowie and Sweet are only in the first file, Turner is only in the second. paul@debian5:~/pipes$ cat > list1.txt Abba Bowie Cure Queen Sweet paul@debian5:~/pipes$ cat > list2.txt Abba Cure Queen Turner paul@debian5:~/pipes$ comm list1.txt list2.txt Abba Bowie Cure Queen Sweet Turner The output of comm can be easier to read when outputting only a single column. The digits point out which output columns should not be displayed. paul@debian5:~/pipes$ comm -12 list1.txt list2.txt Abba Cure Queen paul@debian5:~/pipes$ comm -13 list1.txt list2.txt Turner paul@debian5:~/pipes$ comm -23 list1.txt list2.txt 124
- 131. filters Bowie Sweet 16.10. od European humans like to work with ascii characters, but computers store files in bytes. The example below creates a simple file, and then uses od to show the contents of the file in hexadecimal bytes, in octal bytes and in ascii (or backslashed) characters. paul@laika:~/test$ cat > text.txt abcdefg 1234567 paul@laika:~/test$ od -t x1 text.txt 0000000 61 62 63 64 65 66 67 0a 31 32 33 34 35 36 37 0a 0000020 paul@laika:~/test$ od -b text.txt 0000000 141 142 143 144 145 146 147 012 061 062 063 064 065 066 067 012 0000020 paul@laika:~/test$ od -c text.txt 0000000 a b c d e f g n 1 2 3 4 5 6 7 n 0000020 paul@laika:~/test$ 16.11. sed The stream editor sed can perform editing functions in the stream, using regular expressions. paul@debian5:~/pipes$ echo level5 | sed 's/5/42/' level42 paul@debian5:~/pipes$ echo level5 | sed 's/level/jump/' jump5 Add g for global replacements (all occurrences of the string per line). paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/' jump5 level7 paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/g' jump5 jump7 With d you can remove lines from a stream containing a character. paul@debian5:~/test42$ cat tennis.txt Venus Williams, USA Martina Hingis, SUI Justine Henin, BE Serena williams, USA Kim Clijsters, BE Yanina Wickmayer, BE paul@debian5:~/test42$ cat tennis.txt | sed '/BE/d' 125
- 132. filters Venus Williams, USA Martina Hingis, SUI Serena williams, USA 16.12. pipe examples who | wc How many users are logged on to this system ? [paul@RHEL4b pipes]$ who root tty1 Jul 25 10:50 paul pts/0 Jul 25 09:29 (laika) Harry pts/1 Jul 25 12:26 (barry) paul pts/2 Jul 25 12:26 (pasha) [paul@RHEL4b pipes]$ who | wc -l 4 who | cut | sort Display a sorted list of logged on users. [paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort Harry paul paul root Display a sorted list of logged on users, but every user only once. [paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort | uniq Harry paul root grep | cut Display a list of all bash user accounts on this computer. Users accounts are explained in detail later. paul@debian5:~$ grep bash /etc/passwd root:x:0:0:root:/root:/bin/bash paul:x:1000:1000:paul,,,:/home/paul:/bin/bash serena:x:1001:1001::/home/serena:/bin/bash paul@debian5:~$ grep bash /etc/passwd | cut -d: -f1 root paul serena 126
- 133. filters 16.13. practice: filters 1. Put a sorted list of all bash users in bashusers.txt. 2. Put a sorted list of all logged on users in onlineusers.txt. 3. Make a list of all files in /etc that contain the string samba. 4. Make a sorted list of all files in /etc that contain the case insensitive string samba. 5. Look at the output of /sbin/ifconfig. Write a line that displays only ip address and the subnet mask. 6. Write a line that removes all non-letters from a stream. 7. Write a line that receives a textfile, and outputs all words on a separate line. 8. Write a spell checker on the command line. (There might be a dictionary in /usr/ share/dict/ .) 127
- 134. filters 16.14. solution: filters 1. Put a sorted list of all bash users in bashusers.txt. grep bash /etc/passwd | cut -d: -f1 | sort > bashusers.txt 2. Put a sorted list of all logged on users in onlineusers.txt. who | cut -d' ' -f1 | sort > onlineusers.txt 3. Make a list of all files in /etc that contain the string samba. ls /etc | grep samba 4. Make a sorted list of all files in /etc that contain the case insensitive string samba. ls /etc | grep -i samba | sort 5. Look at the output of /sbin/ifconfig. Write a line that displays only ip address and the subnet mask. /sbin/ifconfig | head -2 | grep 'inet ' | tr -s ' ' | cut -d' ' -f3,5 6. Write a line that removes all non-letters from a stream. paul@deb503:~$ cat text This is, yes really! , a text with ?&* too many str$ange# characters ;-) paul@deb503:~$ cat text | tr -d ',!$?.*&^%#@;()-' This is yes really a text with too many strange characters 7. Write a line that receives a textfile, and outputs all words on a separate line. paul@deb503:~$ cat text2 it is very cold today without the sun paul@deb503:~$ cat text2 | tr ' ' 'n' it is very cold today without the sun 8. Write a spell checker on the command line. (There might be a dictionary in /usr/ share/dict/ .) paul@rhel ~$ echo "The zun is shining today" > text paul@rhel ~$ cat > DICT is shining sun the today 128
- 135. filters paul@rhel ~$ cat text | tr 'A-Z ' 'a-zn' | sort | uniq | comm -23 - DICT zun You could also add the solution from question number 6 to remove non-letters, and tr -s ' ' to remove redundant spaces. 129
- 136. Chapter 17. basic unix tools Table of Contents 17.1. find ............................................................................................................. 131 17.2. locate .......................................................................................................... 131 17.3. date ............................................................................................................. 132 17.4. cal ............................................................................................................... 132 17.5. sleep ........................................................................................................... 133 17.6. time ............................................................................................................. 133 17.7. gzip - gunzip .............................................................................................. 133 17.8. zcat - zmore ............................................................................................... 134 17.9. bzip2 - bunzip2 .......................................................................................... 134 17.10. bzcat - bzmore ......................................................................................... 134 17.11. practice: basic Unix tools ........................................................................ 136 17.12. solution: basic Unix tools ........................................................................ 137 This chapter introduces commands to find or locate files and to compress files, together with other common tools that were not discussed before. While the tools discussed here are technically not considered filters, they can be used in pipes. 130
- 137. basic unix tools 17.1. find The find command can be very useful at the start of a pipe to search for files. Here are some examples. You might want to add 2>/dev/null to the command lines to avoid cluttering your screen with error messages. Find all files in /etc and put the list in etcfiles.txt find /etc > etcfiles.txt Find all files of the entire system and put the list in allfiles.txt find / > allfiles.txt Find files that end in .conf in the current directory (and all subdirs). find . -name "*.conf" Find files of type file (not directory, pipe or etc.) that end in .conf. find . -type f -name "*.conf" Find files of type directory that end in .bak . find /data -type d -name "*.bak" Find files that are newer than file42.txt find . -newer file42.txt Find can also execute another command on every file found. This example will look for *.odf files and copy them to /backup/. find /data -name "*.odf" -exec cp {} /backup/ ; Find can also execute, after your confirmation, another command on every file found. This example will remove *.odf files if you approve of it for every file found. find /data -name "*.odf" -ok rm {} ; 17.2. locate The locate tool is very different from find in that it uses an index to locate files. This is a lot faster than traversing all the directories, but it also means that it is always outdated. If the index does not exist yet, then you have to create it (as root on Red Hat Enterprise Linux) with the updatedb command. [paul@RHEL4b ~]$ locate Samba warning: locate: could not open database: /var/lib/slocate/slocate.db:... warning: You need to run the 'updatedb' command (as root) to create th... Please have a look at /etc/updatedb.conf to enable the daily cron job. [paul@RHEL4b ~]$ updatedb fatal error: updatedb: You are not authorized to create a default sloc... [paul@RHEL4b ~]$ su - Password: 131
- 138. basic unix tools [root@RHEL4b ~]# updatedb [root@RHEL4b ~]# Most Linux distributions will schedule the updatedb to run once every day. 17.3. date The date command can display the date, time, timezone and more. paul@rhel55 ~$ date Sat Apr 17 12:44:30 CEST 2010 A date string can be customized to display the format of your choice. Check the man page for more options. paul@rhel55 ~$ date +'%A %d-%m-%Y' Saturday 17-04-2010 Time on any Unix is calculated in number of seconds since 1969 (the first second being the first second of the first of January 1970). Use date +%s to display Unix time in seconds. paul@rhel55 ~$ date +%s 1271501080 When will this seconds counter reach two thousand million ? paul@rhel55 ~$ date -d '1970-01-01 + 2000000000 seconds' Wed May 18 04:33:20 CEST 2033 17.4. cal The cal command displays the current month, with the current day highlighted. paul@rhel55 ~$ cal April 2010 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 You can select any month in the past or the future. paul@rhel55 ~$ cal 2 1970 February 1970 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 132
- 139. basic unix tools 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 17.5. sleep The sleep command is sometimes used in scripts to wait a number of seconds. This example shows a five second sleep. paul@rhel55 ~$ sleep 5 paul@rhel55 ~$ 17.6. time The time command can display how long it takes to execute a command. The date command takes only a little time. paul@rhel55 ~$ time date Sat Apr 17 13:08:27 CEST 2010 real 0m0.014s user 0m0.008s sys 0m0.006s The sleep 5 command takes five real seconds to execute, but consumes little cpu time. paul@rhel55 ~$ time sleep 5 real 0m5.018s user 0m0.005s sys 0m0.011s This bzip2 command compresses a file and uses a lot of cpu time. paul@rhel55 ~$ time bzip2 text.txt real 0m2.368s user 0m0.847s sys 0m0.539s 17.7. gzip - gunzip Users never have enough disk space, so compression comes in handy. The gzip command can make files take up less space. 133
- 140. basic unix tools paul@rhel55 ~$ ls -lh text.txt -rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt paul@rhel55 ~$ gzip text.txt paul@rhel55 ~$ ls -lh text.txt.gz -rw-rw-r-- 1 paul paul 760K Apr 17 13:11 text.txt.gz You can get the original back with gunzip. paul@rhel55 ~$ gunzip text.txt.gz paul@rhel55 ~$ ls -lh text.txt -rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt 17.8. zcat - zmore Text files that are compressed with gzip can be viewed with zcat and zmore. paul@rhel55 ~$ head -4 text.txt / /opt /opt/VBoxGuestAdditions-3.1.6 /opt/VBoxGuestAdditions-3.1.6/routines.sh paul@rhel55 ~$ gzip text.txt paul@rhel55 ~$ zcat text.txt.gz | head -4 / /opt /opt/VBoxGuestAdditions-3.1.6 /opt/VBoxGuestAdditions-3.1.6/routines.sh 17.9. bzip2 - bunzip2 Files can also be compressed with bzip2 which takes a little more time than gzip, but compresses better. paul@rhel55 ~$ bzip2 text.txt paul@rhel55 ~$ ls -lh text.txt.bz2 -rw-rw-r-- 1 paul paul 569K Apr 17 13:11 text.txt.bz2 Files can be uncompressed again with bunzip2. paul@rhel55 ~$ bunzip2 text.txt.bz2 paul@rhel55 ~$ ls -lh text.txt -rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt 17.10. bzcat - bzmore And in the same way bzcat and bzmore can display files compressed with bzip2. 134
- 141. basic unix tools paul@rhel55 ~$ bzip2 text.txt paul@rhel55 ~$ bzcat text.txt.bz2 | head -4 / /opt /opt/VBoxGuestAdditions-3.1.6 /opt/VBoxGuestAdditions-3.1.6/routines.sh 135
- 142. basic unix tools 17.11. practice: basic Unix tools 1. Explain the difference between these two commands. This question is very important. If you don't know the answer, then look back at the shell chapter. find /data -name "*.txt" find /data -name *.txt 2. Explain the difference between these two statements. Will they both work when there are 200 .odf files in /data ? How about when there are 2 million .odf files ? find /data -name "*.odf" > data_odf.txt find /data/*.odf > data_odf.txt 3. Write a find command that finds all files created after January 30th 2010. 4. Write a find command that finds all *.odf files created in September 2009. 5. Count the number of *.conf files in /etc and all its subdirs. 6. Two commands that do the same thing: copy *.odf files to /backup/ . What would be a reason to replace the first command with the second ? Again, this is an important question. cp -r /data/*.odf /backup/ find /data -name "*.odf" -exec cp {} /backup/ ; 7. Create a file called loctest.txt. Can you find this file with locate ? Why not ? How do you make locate find this file ? 8. Use find and -exec to rename all .htm files to .html. 9. Issue the date command. Now display the date in YYYY/MM/DD format. 10. Issue the cal command. Display a calendar of 1582 and 1752. Notice anything special ? 136
- 143. basic unix tools 17.12. solution: basic Unix tools 1. Explain the difference between these two commands. This question is very important. If you don't know the answer, then look back at the shell chapter. find /data -name "*.txt" find /data -name *.txt When *.txt is quoted then the shell will not touch it. The find tool will look in the /data for all files ending in .txt. When *.txt is not quoted then the shell might expand this (when one or more files that ends in .txt exist in the current directory). The find might show a different result, or can result in a syntax error. 2. Explain the difference between these two statements. Will they both work when there are 200 .odf files in /data ? How about when there are 2 million .odf files ? find /data -name "*.odf" > data_odf.txt find /data/*.odf > data_odf.txt The first find will output all .odf filenames in /data and all subdirectories. The shell will redirect this to a file. The second find will output all files named .odf in /data and will also output all files that exist in directories named *.odf (in /data). With two million files the command line would be expanded beyond the maximum that the shell can accept. The last part of the command line would be lost. 3. Write a find command that finds all files created after January 30th 2010. touch -t 201001302359 marker_date find . -type f -newer marker_date There is another solution : find . -type f -newerat "20100130 23:59:59" 4. Write a find command that finds all *.odf files created in September 2009. touch -t 200908312359 marker_start touch -t 200910010000 marker_end find . -type f -name "*.odf" -newer marker_start ! -newer marker_end The exclamation mark ! -newer can be read as not newer. 5. Count the number of *.conf files in /etc and all its subdirs. find /etc -type f -name '*.conf' | wc -l 6. Two commands that do the same thing: copy *.odf files to /backup/ . What would be a reason to replace the first command with the second ? Again, this is an important question. cp -r /data/*.odf /backup/ 137
- 144. basic unix tools find /data -name "*.odf" -exec cp {} /backup/ ; The first might fail when there are too many files to fit on one command line. 7. Create a file called loctest.txt. Can you find this file with locate ? Why not ? How do you make locate find this file ? You cannot locate this with locate because it is not yet in the index. updatedb 8. Use find and -exec to rename all .htm files to .html. paul@rhel55 ~$ find . -name '*.htm' ./one.htm ./two.htm paul@rhel55 ~$ find . -name '*.htm' -exec mv {} {}l ; paul@rhel55 ~$ find . -name '*.htm*' ./one.html ./two.html 9. Issue the date command. Now display the date in YYYY/MM/DD format. date +%Y/%m/%d 10. Issue the cal command. Display a calendar of 1582 and 1752. Notice anything special ? cal 1582 The calendars are different depending on the country. Check http://linux-training.be/ files/studentfiles/dates.txt 138
- 145. Part V. vi
- 146. Chapter 18. Introduction to vi Table of Contents 18.1. command mode and insert mode ............................................................... 141 18.2. start typing (a A i I o O) ........................................................................... 141 18.3. replace and delete a character (r x X) ........................................................ 141 18.4. undo and repeat (u .) .................................................................................. 142 18.5. cut, copy and paste a line (dd yy p P) ....................................................... 142 18.6. cut, copy and paste lines (3dd 2yy) ........................................................... 142 18.7. start and end of a line (0 or ^ and $) ......................................................... 143 18.8. join two lines (J) and more ........................................................................ 143 18.9. words (w b) ................................................................................................ 143 18.10. save (or not) and exit (:w :q :q! ) ............................................................. 144 18.11. Searching (/ ?) .......................................................................................... 144 18.12. replace all ( :1,$ s/foo/bar/g ) ................................................................... 145 18.13. reading files (:r :r !cmd) ........................................................................... 145 18.14. text buffers ............................................................................................... 145 18.15. multiple files ............................................................................................ 146 18.16. abbreviations ............................................................................................ 146 18.17. key mappings ........................................................................................... 146 18.18. setting options .......................................................................................... 146 18.19. practice: vi(m) .......................................................................................... 148 18.20. solution: vi(m) .......................................................................................... 149 The vi editor is installed on almost every Unix. Linux will very often install vim (vi improved) which is similar. Every system administrator should know vi(m), because it is an easy tool to solve problems. The vi editor is not intuitive, but once you get to know it, vi becomes a very powerful application. Most Linux distributions will include the vimtutor which is a 45 minute lesson in vi(m). 140
- 147. Introduction to vi 18.1. command mode and insert mode The vi editor starts in command mode. In command mode, you can type commands. Some commands will bring you into insert mode. In insert mode, you can type text. The escape key will return you to command mode. Table 18.1. getting to command mode key action Esc set vi(m) in command mode. 18.2. start typing (a A i I o O) The difference between a A i I o and O is the location where you can start typing. a will append after the current character and A will append at the end of the line. i will insert before the current character and I will insert at the beginning of the line. o will put you in a new line after the current line and O will put you in a new line before the current line. Table 18.2. switch to insert mode command action a start typing after the current character A start typing at the end of the current line i start typing before the current character I start typing at the start of the current line o start typing on a new line after the current line O start typing on a new line before the current line 18.3. replace and delete a character (r x X) When in command mode (it doesn't hurt to hit the escape key more than once) you can use the x key to delete the current character. The big X key (or shift x) will delete the character left of the cursor. Also when in command mode, you can use the r key to replace one single character. The r key will bring you in insert mode for just one key press, and will return you immediately to command mode. 141
- 148. Introduction to vi Table 18.3. replace and delete command action x delete the character below the cursor X delete the character before the cursor r replace the character below the cursor p paste after the cursor (here the last deleted character) xp switch two characters 18.4. undo and repeat (u .) When in command mode, you can undo your mistakes with u. You can do your mistakes twice with . (in other words, the . will repeat your last command). Table 18.4. undo and repeat command action u undo the last action . repeat the last action 18.5. cut, copy and paste a line (dd yy p P) When in command mode, dd will cut the current line. yy will copy the current line. You can paste the last copied or cut line after (p) or before (P) the current line. Table 18.5. cut, copy and paste a line command action dd cut the current line yy (yank yank) copy the current line p paste after the current line P paste before the current line 18.6. cut, copy and paste lines (3dd 2yy) When in command mode, before typing dd or yy, you can type a number to repeat the command a number of times. Thus, 5dd will cut 5 lines and 4yy will copy (yank) 4 lines. That last one will be noted by vi in the bottom left corner as "4 line yanked". Table 18.6. cut, copy and paste lines command action 3dd cut three lines 4yy copy four lines 142
- 149. Introduction to vi 18.7. start and end of a line (0 or ^ and $) When in command mode, the 0 and the caret ^ will bring you to the start of the current line, whereas the $ will put the cursor at the end of the current line. You can add 0 and $ to the d command, d0 will delete every character between the current character and the start of the line. Likewise d$ will delete everything from the current character till the end of the line. Similarly y0 and y$ will yank till start and end of the current line. Table 18.7. start and end of line command action 0 jump to start of current line ^ jump to start of current line $ jump to end of current line d0 delete until start of line d$ delete until end of line 18.8. join two lines (J) and more When in command mode, pressing J will append the next line to the current line. With yyp you duplicate a line and with ddp you switch two lines. Table 18.8. join two lines command action J join two lines yyp duplicate a line ddp switch two lines 18.9. words (w b) When in command mode, w will jump to the next word and b will move to the previous word. w and b can also be combined with d and y to copy and cut words (dw db yw yb). 143
- 150. Introduction to vi Table 18.9. words command action w forward one word b back one word 3w forward three words dw delete one word yw yank (copy) one word 5yb yank five words back 7dw delete seven words 18.10. save (or not) and exit (:w :q :q! ) Pressing the colon : will allow you to give instructions to vi (technically speaking, typing the colon will open the ex editor). :w will write (save) the file, :q will quit an unchanged file without saving, and :q! will quit vi discarding any changes. :wq will save and quit and is the same as typing ZZ in command mode. Table 18.10. save and exit vi command action :w save (write) :w fname save as fname :q quit :wq save and quit ZZ save and quit :q! quit (discarding your changes) :w! save (and write to non-writable file!) The last one is a bit special. With :w! vi will try to chmod the file to get write permission (this works when you are the owner) and will chmod it back when the write succeeds. This should always work when you are root (and the file system is writable). 18.11. Searching (/ ?) When in command mode typing / will allow you to search in vi for strings (can be a regular expression). Typing /foo will do a forward search for the string foo and typing ?bar will do a backward search for bar. 144
- 151. Introduction to vi Table 18.11. searching command action /string forward search for string ?string backward search for string n go to next occurrence of search string /^string forward search string at beginning of line /string$ forward search string at end of line /br[aeio]l search for bral brel bril and brol /<he> search for the word he (and not for here or the) 18.12. replace all ( :1,$ s/foo/bar/g ) To replace all occurrences of the string foo with bar, first switch to ex mode with : . Then tell vi which lines to use, for example 1,$ will do the replace all from the first to the last line. You can write 1,5 to only process the first five lines. The s/foo/bar/ g will replace all occurences of foo with bar. Table 18.12. replace command action :4,8 s/foo/bar/g replace foo with bar on lines 4 to 8 :1,$ s/foo/bar/g replace foo with bar on all lines 18.13. reading files (:r :r !cmd) When in command mode, :r foo will read the file named foo, :r !foo will execute the command foo. The result will be put at the current location. Thus :r !ls will put a listing of the current directory in your text file. Table 18.13. read files and input command action :r fname (read) file fname and paste contents :r !cmd execute cmd and paste its output 18.14. text buffers There are 36 buffers in vi to store text. You can use them with the " character. 145
- 152. Introduction to vi Table 18.14. text buffers command action "add delete current line and put text in buffer a "g7yy copy seven lines into buffer g "ap paste from buffer a 18.15. multiple files You can edit multiple files with vi. Here are some tips. Table 18.15. multiple files command action vi file1 file2 file3 start editing three files :args lists files and marks active file :n start editing the next file :e toggle with last edited file :rew rewind file pointer to first file 18.16. abbreviations With :ab you can put abbreviations in vi. Use :una to undo the abbreviation. Table 18.16. abbreviations command action :ab str long string abbreviate str to be 'long string' :una str un-abbreviate str 18.17. key mappings Similarly to their abbreviations, you can use mappings with :map for command mode and :map! for insert mode. This example shows how to set the F6 function key to toggle between set number and set nonumber. The <bar> seperates the two commands, set number! toggles the state and set number? reports the current state. :map <F6> :set number!<bar>set number?<CR> 18.18. setting options Some options that you can set in vim. 146
- 153. Introduction to vi :set number ( also try :se nu ) :set nonumber :syntax on :syntax off :set all (list all options) :set tabstop=8 :set tx (CR/LF style endings) :set notx You can set these options (and much more) in ~/.vimrc for vim or in ~/.exrc for standard vi. paul@barry:~$ cat ~/.vimrc set number set tabstop=8 set textwidth=78 map <F6> :set number!<bar>set number?<CR> paul@barry:~$ 147
- 154. Introduction to vi 18.19. practice: vi(m) 1. Start the vimtutor and do some or all of the exercises. You might need to run aptitude install vim on xubuntu. 2. What 3 key combination in command mode will duplicate the current line. 3. What 3 key combination in command mode will switch two lines' place (line five becomes line six and line six becomes line five). 4. What 2 key combination in command mode will switch a character's place with the next one. 5. vi can understand macro's. A macro can be recorded with q followed by the name of the macro. So qa will record the macro named a. Pressing q again will end the recording. You can recall the macro with @ followed by the name of the macro. Try this example: i 1 'Escape Key' qa yyp 'Ctrl a' q 5@a (Ctrl a will increase the number with one). 6. Copy /etc/passwd to your ~/passwd. Open the last one in vi and press Ctrl v. Use the arrow keys to select a Visual Block, you can copy this with y or delete it with d. Try pasting it. 7. What does dwwP do when you are at the beginning of a word in a sentence ? 148
- 155. Introduction to vi 18.20. solution: vi(m) 1. Start the vimtutor and do some or all of the exercises. You might need to run aptitude install vim on xubuntu. vimtutor 2. What 3 key combination in command mode will duplicate the current line. yyp 3. What 3 key combination in command mode will switch two lines' place (line five becomes line six and line six becomes line five). ddp 4. What 2 key combination in command mode will switch a character's place with the next one. xp 5. vi can understand macro's. A macro can be recorded with q followed by the name of the macro. So qa will record the macro named a. Pressing q again will end the recording. You can recall the macro with @ followed by the name of the macro. Try this example: i 1 'Escape Key' qa yyp 'Ctrl a' q 5@a (Ctrl a will increase the number with one). 6. Copy /etc/passwd to your ~/passwd. Open the last one in vi and press Ctrl v. Use the arrow keys to select a Visual Block, you can copy this with y or delete it with d. Try pasting it. cp /etc/passwd ~ vi passwd (press Ctrl-V) 7. What does dwwP do when you are at the beginning of a word in a sentence ? dwwP can switch the current word with the next word. 149
- 156. Part VI. scripting
- 157. Chapter 19. scripting introduction Table of Contents 19.1. scripts are everywhere ............................................................................... 152 19.2. hello world ................................................................................................. 152 19.3. she-bang ..................................................................................................... 152 19.4. comment ..................................................................................................... 153 19.5. variables ..................................................................................................... 153 19.6. sourcing a script ......................................................................................... 153 19.7. prevent setuid root spoofing ...................................................................... 154 19.8. practice: introduction to scripting .............................................................. 155 19.9. solution: introduction to scripting .............................................................. 156 151
- 158. scripting introduction 19.1. scripts are everywhere Shells like bash and Korn have support for programming constructs that can be saved as scripts. These scripts in turn then become more shell commands. Many Linux commands are scripts. User profile scripts are run when a user logs on and init scripts are run when a daemon is stopped or started. This means that system administrators also need basic knowledge of scripting to understand how their servers and their applications are started, updated, upgraded, patched, maintained, configured and removed, and also to understand how a user environment is built. 19.2. hello world Just like in every programming course, we start with a simple hello_world script. The following script will output Hello World. echo Hello World After creating this simple script in vi or with echo, you'll have to chmod +x hello_world to make it executable. And unless you add the scripts directory to your path, you'll have to type the path to the script for the shell to be able to find it. [paul@RHEL4a ~]$ echo echo Hello World > hello_world [paul@RHEL4a ~]$ chmod +x hello_world [paul@RHEL4a ~]$ ./hello_world Hello World [paul@RHEL4a ~]$ 19.3. she-bang Let's expand our example a little further by putting #!/bin/bash on the first line of the script. The #! is called a she-bang (sometimes called sha-bang), where the she- bang is the first two characters of the script. #!/bin/bash echo Hello World You can never be sure which shell a user is running. A script that works flawlessly in bash might not work in ksh, csh, or dash. To instruct a shell to run your script in a certain shell, you can start your script with a she-bang followed by the shell it is supposed to run in. This script will run in a bash shell. #!/bin/bash echo -n hello echo A bash subshell `echo -n hello` This script will run in a Korn shell (unless /bin/ksh is a hard link to /bin/bash). The /etc/shells file contains a list of shells on your system. 152
- 159. scripting introduction #!/bin/ksh echo -n hello echo a Korn subshell `echo -n hello` 19.4. comment Let's expand our example a little further by adding comment lines. #!/bin/bash # # Hello World Script # echo Hello World 19.5. variables Here is a simple example of a variable inside a script. #!/bin/bash # # simple variable in script # var1=4 echo var1 = $var1 Scripts can contain variables, but since scripts are run in their own shell, the variables do not survive the end of the script. [paul@RHEL4a ~]$ echo $var1 [paul@RHEL4a ~]$ ./vars var1 = 4 [paul@RHEL4a ~]$ echo $var1 [paul@RHEL4a ~]$ 19.6. sourcing a script Luckily, you can force a script to run in the same shell; this is called sourcing a script. [paul@RHEL4a ~]$ source ./vars var1 = 4 [paul@RHEL4a ~]$ echo $var1 4 [paul@RHEL4a ~]$ The above is identical to the below. [paul@RHEL4a ~]$ . ./vars var1 = 4 [paul@RHEL4a ~]$ echo $var1 153
- 160. scripting introduction 4 [paul@RHEL4a ~]$ 19.7. prevent setuid root spoofing Some user may try to perform setuid based script root spoofing. This is a rare but possible attack. To improve script security and to avoid interpreter spoofing, you need to add -- after the #!/bin/bash, which disables further option processing so the shell will not accept any options. #!/bin/bash - or #!/bin/bash -- Any arguments after the -- are treated as filenames and arguments. An argument of - is equivalent to --. 154
- 161. scripting introduction 19.8. practice: introduction to scripting 0. Give each script a different name, keep them for later! 1. Write a script that outputs the name of a city. 2. Make sure the script runs in the bash shell. 3. Make sure the script runs in the Korn shell. 4. Create a script that defines two variables, and outputs their value. 5. The previous script does not influence your current shell (the variables do not exist outside of the script). Now run the script so that it influences your current shell. 6. Is there a shorter way to source the script ? 7. Comment your scripts so that you know what they are doing. 155
- 162. scripting introduction 19.9. solution: introduction to scripting 0. Give each script a different name, keep them for later! 1. Write a script that outputs the name of a city. $ echo 'echo Antwerp' > first.bash $ chmod +x first.bash $ ./first.bash Antwerp 2. Make sure the script runs in the bash shell. $ cat first.bash #!/bin/bash echo Antwerp 3. Make sure the script runs in the Korn shell. $ cat first.bash #!/bin/ksh echo Antwerp Note that while first.bash will technically work as a Korn shell script, the name ending in .bash is confusing. 4. Create a script that defines two variables, and outputs their value. $ cat second.bash #!/bin/bash var33=300 var42=400 echo $var33 $var42 5. The previous script does not influence your current shell (the variables do not exist outside of the script). Now run the script so that it influences your current shell. source second.bash 6. Is there a shorter way to source the script ? . ./second.bash 7. Comment your scripts so that you know what they are doing. $ cat second.bash #!/bin/bash # script to test variables and sourcing # define two variables var33=300 var42=400 # output the value of these variables echo $var33 $var42 156
- 163. Chapter 20. scripting loops Table of Contents 20.1. test [ ] ......................................................................................................... 158 20.2. if if, then then, or else ............................................................................... 158 20.3. for loop ....................................................................................................... 159 20.4. while loop .................................................................................................. 159 20.5. until loop .................................................................................................... 160 20.6. practice: scripting tests and loops .............................................................. 161 20.7. solution: scripting tests and loops .............................................................. 162 157
- 164. scripting loops 20.1. test [ ] The test command can test whether something is true or false. Let's start by testing whether 10 is greater than 55. [paul@RHEL4b ~]$ test 10 -gt 55 ; echo $? 1 [paul@RHEL4b ~]$ The test command returns 1 if the test fails. And as you see in the next screenshot, test returns 0 when a test succeeds. [paul@RHEL4b ~]$ test 56 -gt 55 ; echo $? 0 [paul@RHEL4b ~]$ If you prefer true and false, then write the test like this. [paul@RHEL4b ~]$ test 56 -gt 55 && echo true || echo false true [paul@RHEL4b ~]$ test 6 -gt 55 && echo true || echo false false The test command can also be written as square brackets, the screenshot below is identical to the one above. [paul@RHEL4b ~]$ [ 56 -gt 55 ] && echo true || echo false true [paul@RHEL4b ~]$ [ 6 -gt 55 ] && echo true || echo false false Below are some example tests. Take a look at man test to see more options for tests. [ -d foo ] Does the directory foo exist ? [ '/etc' = $PWD ] Is the string /etc equal to the variable $PWD ? [ $1 != 'secret' ] Is the first parameter different from secret ? [ 55 -lt $bar ] Is 55 less than the value of $bar ? [ $foo -ge 1000 ] Is the value of $foo greater or equal to 1000 ? [ "abc" < $bar ] Does abc sort before the value of $bar ? [ -f foo ] Is foo a regular file ? [ -r bar ] Is bar a readable file ? [ foo -nt bar ] Is file foo newer than file bar ? [ -o nounset ] Is the shell option nounset set ? Tests can be combined with logical AND and OR. paul@RHEL4b:~$ [ 66 -gt 55 -a 66 -lt 500 ] && echo true || echo false true paul@RHEL4b:~$ [ 66 -gt 55 -a 660 -lt 500 ] && echo true || echo false false paul@RHEL4b:~$ [ 66 -gt 55 -o 660 -lt 500 ] && echo true || echo false true 20.2. if if, then then, or else The if then else construction is about choice. If a certain condition is met, then execute something, else execute something else. The example below tests whether a file exists, and if the file exists then a proper message is echoed. 158
- 165. scripting loops #!/bin/bash if [ -f isit.txt ] then echo isit.txt exists! else echo isit.txt not found! fi If we name the above script 'choice', then it executes like this. [paul@RHEL4a scripts]$ ./choice isit.txt not found! [paul@RHEL4a scripts]$ touch isit.txt [paul@RHEL4a scripts]$ ./choice isit.txt exists! [paul@RHEL4a scripts]$ 20.3. for loop The example below shows the syntax of a classical for loop in bash. for i in 1 2 4 do echo $i done An example of a for loop combined with an embedded shell. #!/bin/ksh for counter in `seq 1 20` do echo counting from 1 to 20, now at $counter sleep 1 done This for loop use file globbing (from the shell expansion). Putting the instruction on the command line has identical functionality. kahlan@solexp11$ ls count.ksh go.ksh kahlan@solexp11$ for file in *.ksh ; do cp $file $file.backup ; done kahlan@solexp11$ ls count.ksh count.ksh.backup go.ksh go.ksh.backup 20.4. while loop Below a simple example of a while loop. let i=100; while [ $i -ge 0 ] ; do echo Counting down, from 100 to 0, now at $i; let i--; done Endless loops can be made with while true or while : , where the colon is the equivalent of no operation in the Korn and bash shells. #!/bin/ksh 159
- 166. scripting loops # endless loop while : do echo hello sleep 1 done 20.5. until loop Below a simple example of an until loop. let i=100; until [ $i -le 0 ] ; do echo Counting down, from 100 to 1, now at $i; let i--; done 160
- 167. scripting loops 20.6. practice: scripting tests and loops 1. Write a script that uses a for loop to count from 3 to 7. 2. Write a script that uses a for loop to count from 1 to 17000. 3. Write a script that uses a while loop to count from 3 to 7. 4. Write a script that uses an until loop to count down from 8 to 4. 5. Write a script that counts the number of files ending in .txt in the current directory. 6. Wrap an if statement around the script so it is also correct when there are zero files ending in .txt. 161
- 168. scripting loops 20.7. solution: scripting tests and loops 1. Write a script that uses a for loop to count from 3 to 7. #!/bin/bash for i in 3 4 5 6 7 do echo Counting from 3 to 7, now at $i done 2. Write a script that uses a for loop to count from 1 to 17000. #!/bin/bash for i in `seq 1 17000` do echo Counting from 1 to 17000, now at $i done 3. Write a script that uses a while loop to count from 3 to 7. #!/bin/bash i=3 while [ $i -le 7 ] do echo Counting from 3 to 7, now at $i let i=i+1 done 4. Write a script that uses an until loop to count down from 8 to 4. #!/bin/bash i=8 until [ $i -lt 4 ] do echo Counting down from 8 to 4, now at $i let i=i-1 done 5. Write a script that counts the number of files ending in .txt in the current directory. #!/bin/bash let i=0 for file in *.txt do let i++ done echo "There are $i files ending in .txt" 6. Wrap an if statement around the script so it is also correct when there are zero files ending in .txt. #!/bin/bash ls *.txt > /dev/null 2>&1 if [ $? -ne 0 ] 162
- 169. scripting loops then echo "There are 0 files ending in .txt" else let i=0 for file in *.txt do let i++ done echo "There are $i files ending in .txt" fi 163
- 170. Chapter 21. scripting parameters Table of Contents 21.1. script parameters ........................................................................................ 165 21.2. shift through parameters ............................................................................ 165 21.3. runtime input .............................................................................................. 166 21.4. sourcing a config file ................................................................................. 166 21.5. get script options with getopts ................................................................... 166 21.6. get shell options with shopt ....................................................................... 168 21.7. practice: parameters and options ................................................................ 169 21.8. solution: parameters and options ............................................................... 170 164
- 171. scripting parameters 21.1. script parameters A bash shell script can have parameters. The numbering you see in the script below continues if you have more parameters. You also have special parameters containing the number of parameters, a string of all of them, and also the process id, and the last return code. The man page of bash has a full list. #!/bin/bash echo The first argument is $1 echo The second argument is $2 echo The third argument is $3 echo $ $$ PID of the script echo # $# count arguments echo ? $? last return code echo * $* all the arguments Below is the output of the script above in action. [paul@RHEL4a scripts]$ ./pars one two three The first argument is one The second argument is two The third argument is three $ 5610 PID of the script # 3 count arguments ? 0 last return code * one two three all the arguments [paul@RHEL4a scripts]$ ./pars 1 2 The first argument is 1 The second argument is 2 The third argument is $ 5612 PID of the script # 2 count arguments ? 0 last return code * 1 2 all the arguments [paul@RHEL4a scripts]$ 21.2. shift through parameters The shift statement can parse all parameters one by one. This is a sample script. kahlan@solexp11$ cat shift.ksh #!/bin/ksh if [ "$#" == "0" ] then echo You have to give at least one parameter. exit 1 fi while (( $# )) do echo You gave me $1 shift done 165
- 172. scripting parameters Below is some sample output of the script above. kahlan@solexp11$ ./shift.ksh one You gave me one kahlan@solexp11$ ./shift.ksh one two three 1201 "33 42" You gave me one You gave me two You gave me three You gave me 1201 You gave me 33 42 kahlan@solexp11$ ./shift.ksh You have to give at least one parameter. 21.3. runtime input You can ask the user for input with the read command in a script. #!/bin/bash echo -n Enter a number: read number 21.4. sourcing a config file [paul@RHEL4a scripts]$ cat myApp.conf # The config file of myApp # Enter the path here myAppPath=/var/myApp # Enter the number of quines here quines=5 [paul@RHEL4a scripts]$ cat myApp.bash #!/bin/bash # # Welcome to the myApp application # . ./myApp.conf echo There are $quines quines [paul@RHEL4a scripts]$ ./myApp.bash There are 5 quines [paul@RHEL4a scripts]$ 21.5. get script options with getopts The getopts function allows you to parse options given to a command. kahlan@solexp11$ cat options.ksh #!/bin/ksh 166
- 173. scripting parameters while getopts ":afz" option; do case $option in a) echo received -a ;; f) echo received -f ;; z) echo received -z ;; *) echo "invalid option -$OPTARG" ;; esac done This is sample output from the script above. kahlan@solexp11$ ./options.ksh kahlan@solexp11$ ./options.ksh -af received -a received -f kahlan@solexp11$ ./options.ksh -zfg received -z received -f invalid option -g kahlan@solexp11$ ./options.ksh -a -b -z received -a invalid option -b received -z You can also check for options that need an argument, as this example shows. kahlan@solexp11$ cat argoptions.ksh #!/bin/ksh while getopts ":af:z" option; do case $option in a) echo received -a ;; f) echo received -f with $OPTARG ;; z) echo received -z ;; :) echo "option -$OPTARG needs an argument" ;; *) echo "invalid option -$OPTARG" ;; esac done This is sample output from the script above. kahlan@solexp11$ ./argoptions.ksh -a -f hello -z received -a 167
- 174. scripting parameters received -f with hello received -z kahlan@solexp11$ ./argoptions.ksh -zaf 42 received -z received -a received -f with 42 kahlan@solexp11$ ./argoptions.ksh -zf received -z option -f needs an argument 21.6. get shell options with shopt You can toggle the values of variables controlling optional shell behavior with the shopt built-in shell command. The example below first verifies whether the cdspell option is set; it is not. The next shopt command sets the value, and the third shopt command verifies that the option really is set. You can now use minor spelling mistakes in the cd command. The man page of bash has a complete list of options. paul@laika:~$ shopt -q cdspell ; echo $? 1 paul@laika:~$ shopt -s cdspell paul@laika:~$ shopt -q cdspell ; echo $? 0 paul@laika:~$ cd /Etc /etc 168
- 175. scripting parameters 21.7. practice: parameters and options 1. Write a script that receives four parameters, and outputs them in reverse order. 2. Write a script that receives two parameters (two filenames) and outputs whether those files exist. 3. Write a script that asks for a filename. Verify existance of the file, then verify that you own the file, and whether it is writable. If not, then make it writable. 4. Make a configuration file for the previous script. Put a logging switch in the config file, logging means writing detailed output of everything the script does to a log file in /tmp. 169
- 176. scripting parameters 21.8. solution: parameters and options 1. Write a script that receives four parameters, and outputs them in reverse order. echo $4 $3 $2 $1 2. Write a script that receives two parameters (two filenames) and outputs whether those files exist. #!/bin/bash if [ -f $1 ] then echo $1 exists! else echo $1 not found! fi if [ -f $2 ] then echo $2 exists! else echo $2 not found! fi 3. Write a script that asks for a filename. Verify existance of the file, then verify that you own the file, and whether it is writable. If not, then make it writable. 4. Make a configuration file for the previous script. Put a logging switch in the config file, logging means writing detailed output of everything the script does to a log file in /tmp. 170
- 177. Chapter 22. more scripting Table of Contents 22.1. eval ............................................................................................................. 172 22.2. (( )) ............................................................................................................. 172 22.3. let ................................................................................................................ 172 22.4. case ............................................................................................................. 173 22.5. shell functions ............................................................................................ 174 22.6. practice : more scripting ............................................................................ 175 22.7. solution : more scripting ............................................................................ 176 171
- 178. more scripting 22.1. eval eval reads arguments as input to the shell (the resulting commands are executed). This allows using the value of a variable as a variable. paul@deb503:~/test42$ answer=42 paul@deb503:~/test42$ word=answer paul@deb503:~/test42$ eval x=$$word ; echo $x 42 Both in bash and Korn the arguments can be quoted. kahlan@solexp11$ answer=42 kahlan@solexp11$ word=answer kahlan@solexp11$ eval "y=$$word" ; echo $y 42 22.2. (( )) The (( )) allows for evaluation of numerical expressions. paul@deb503:~/test42$ (( 42 > 33 )) && echo true || echo false true paul@deb503:~/test42$ (( 42 > 1201 )) && echo true || echo false false paul@deb503:~/test42$ var42=42 paul@deb503:~/test42$ (( 42 == var42 )) && echo true || echo false true paul@deb503:~/test42$ (( 42 == $var42 )) && echo true || echo false true paul@deb503:~/test42$ var42=33 paul@deb503:~/test42$ (( 42 == var42 )) && echo true || echo false false 22.3. let The let built-in shell function instructs the shell to perform an evaluation of arithmetic expressions. It will return 0 unless the last arithmetic expression evaluates to 0. [paul@RHEL4b ~]$ let x="3 + 4" ; echo $x 7 [paul@RHEL4b ~]$ let x="10 + 100/10" ; echo $x 20 [paul@RHEL4b ~]$ let x="10-2+100/10" ; echo $x 18 [paul@RHEL4b ~]$ let x="10*2+100/10" ; echo $x 30 The shell can also convert between different bases. [paul@RHEL4b ~]$ let x="0xFF" ; echo $x 255 172
- 179. more scripting [paul@RHEL4b ~]$ let x="0xC0" ; echo $x 192 [paul@RHEL4b ~]$ let x="0xA8" ; echo $x 168 [paul@RHEL4b ~]$ let x="8#70" ; echo $x 56 [paul@RHEL4b ~]$ let x="8#77" ; echo $x 63 [paul@RHEL4b ~]$ let x="16#c0" ; echo $x 192 There is a difference between assigning a variable directly, or using let to evaluate the arithmetic expressions (even if it is just assigning a value). kahlan@solexp11$ dec=15 ; oct=017 ; hex=0x0f kahlan@solexp11$ echo $dec $oct $hex 15 017 0x0f kahlan@solexp11$ let dec=15 ; let oct=017 ; let hex=0x0f kahlan@solexp11$ echo $dec $oct $hex 15 15 15 22.4. case You can sometimes simplify nested if statements with a case construct. [paul@RHEL4b ~]$ ./help What animal did you see ? lion You better start running fast! [paul@RHEL4b ~]$ ./help What animal did you see ? dog Don't worry, give it a cookie. [paul@RHEL4b ~]$ cat help #!/bin/bash # # Wild Animals Helpdesk Advice # echo -n "What animal did you see ? " read animal case $animal in "lion" | "tiger") echo "You better start running fast!" ;; "cat") echo "Let that mouse go..." ;; "dog") echo "Don't worry, give it a cookie." ;; "chicken" | "goose" | "duck" ) echo "Eggs for breakfast!" ;; "liger") echo "Approach and say 'Ah you big fluffy kitty...'." ;; "babelfish") echo "Did it fall out your ear ?" ;; *) echo "You discovered an unknown animal, name it!" 173
- 180. more scripting ;; esac [paul@RHEL4b ~]$ 22.5. shell functions Shell functions can be used to group commands in a logical way. kahlan@solexp11$ cat funcs.ksh #!/bin/ksh function greetings { echo Hello World! echo and hello to $USER to! } echo We will now call a function greetings echo The end This is sample output from this script with a function. kahlan@solexp11$ ./funcs.ksh We will now call a function Hello World! and hello to kahlan to! The end A shell function can also receive parameters. kahlan@solexp11$ cat addfunc.ksh #!/bin/ksh function plus { let result="$1 + $2" echo $1 + $2 = $result } plus 3 10 plus 20 13 plus 20 22 This script produces the following output. kahlan@solexp11$ ./addfunc.ksh 3 + 10 = 13 20 + 13 = 33 20 + 22 = 42 174
- 181. more scripting 22.6. practice : more scripting 1. Write a script that asks for two numbers, and outputs the sum and product (as shown here). Enter a number: 5 Enter another number: 2 Sum: 5 + 2 = 7 Product: 5 x 2 = 10 2. Improve the previous script to test that the numbers are between 1 and 100, exit with an error if necessary. 3. Improve the previous script to congratulate the user if the sum equals the product. 4. Write a script with a case insensitive case statement, using the shopt nocasematch option. The nocasematch option is reset to the value it had before the scripts started. 5. If time permits (or if you are waiting for other students to finish this practice), take a look at linux system scripts in /etc/init.d and /etc/rc.d and try to understand them. Where does execution of a script start in /etc/init.d/samba ? There are also some hidden scripts in ~, we will discuss them later. 175
- 182. more scripting 22.7. solution : more scripting 1. Write a script that asks for two numbers, and outputs the sum and product (as shown here). Enter a number: 5 Enter another number: 2 Sum: 5 + 2 = 7 Product: 5 x 2 = 10 #!/bin/bash echo -n "Enter a number : " read n1 echo -n "Enter another number : " read n2 let sum="$n1+$n2" let pro="$n1*$n2" echo -e "Sumt: $n1 + $n2 = $sum" echo -e "Productt: $n1 * $n2 = $pro" 2. Improve the previous script to test that the numbers are between 1 and 100, exit with an error if necessary. echo -n "Enter a number between 1 and 100 : " read n1 if [ $n1 -lt 1 -o $n1 -gt 100 ] then echo Wrong number... exit 1 fi 3. Improve the previous script to congratulate the user if the sum equals the product. if [ $sum -eq $pro ] then echo Congratulations $sum == $pro fi 4. Write a script with a case insensitive case statement, using the shopt nocasematch option. The nocasematch option is reset to the value it had before the scripts started. #!/bin/bash # # Wild Animals Case Insensitive Helpdesk Advice # if shopt -q nocasematch; then nocase=yes; else nocase=no; shopt -s nocasematch; fi echo -n "What animal did you see ? " read animal 176
- 183. more scripting case $animal in "lion" | "tiger") echo "You better start running fast!" ;; "cat") echo "Let that mouse go..." ;; "dog") echo "Don't worry, give it a cookie." ;; "chicken" | "goose" | "duck" ) echo "Eggs for breakfast!" ;; "liger") echo "Approach and say 'Ah you big fluffy kitty.'" ;; "babelfish") echo "Did it fall out your ear ?" ;; *) echo "You discovered an unknown animal, name it!" ;; esac if [ nocase = yes ] ; then shopt -s nocasematch; else shopt -u nocasematch; fi 5. If time permits (or if you are waiting for other students to finish this practice), take a look at linux system scripts in /etc/init.d and /etc/rc.d and try to understand them. Where does execution of a script start in /etc/init.d/samba ? There are also some hidden scripts in ~, we will discuss them later. 177
- 184. Part VII. local user management
- 185. Chapter 23. users Table of Contents 23.1. identify yourself ......................................................................................... 180 23.2. users ........................................................................................................... 182 23.3. passwords ................................................................................................... 184 23.4. home directories ......................................................................................... 189 23.5. user shell .................................................................................................... 190 23.6. switch users with su ................................................................................... 191 23.7. run a program as another user ................................................................... 192 23.8. practice: users ............................................................................................ 194 23.9. solution: users ............................................................................................ 195 23.10. shell environment ..................................................................................... 197 179
- 186. users 23.1. identify yourself whoami The whoami command tells you your username. [root@RHEL5 ~]# whoami root [root@RHEL5 ~]# su - paul [paul@RHEL5 ~]$ whoami paul who The who command will give you information about who is logged on the system. [paul@RHEL5 ~]$ who root tty1 2008-06-24 13:24 sandra pts/0 2008-06-24 14:05 (192.168.1.34) paul pts/1 2008-06-24 16:23 (192.168.1.37) who am i With who am i the who command will display only the line pointing to your current session. [paul@RHEL5 ~]$ who am i paul pts/1 2008-06-24 16:23 (192.168.1.34) w The w command shows you who is logged on and what they are doing. $ w 05:13:36 up 3 min, 4 users, load average: 0.48, 0.72, 0.33 USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT root tty1 - 05:11 2.00s 0.32s 0.27s find / -name shad inge pts/0 192.168.1.33 05:12 0.00s 0.02s 0.02s -ksh laura pts/1 192.168.1.34 05:12 46.00s 0.03s 0.03s -bash paul pts/2 192.168.1.34 05:13 25.00s 0.07s 0.04s top id The id command will give you your user id, primary group id, and a list of the groups that you belong to. 180
- 187. users root@laika:~# id uid=0(root) gid=0(root) groups=0(root) root@laika:~# su - brel brel@laika:~$ id uid=1001(brel) gid=1001(brel) groups=1001(brel),1008(chanson),11578(wolf) 181
- 188. users 23.2. users user management User management on any Unix can be done in three complimentary ways. You can use the graphical tools provided by your distribution. These tools have a look and feel that depends on the distribution. If you are a novice linux user on your home system, then use the graphical tool that is provided by your distribution. This will make sure that you do not run into problems. Another option is to use command line tools like useradd, usermod, gpasswd, passwd and others. Server administrators are likely to use these tools, since they are familiar and very similar accross many different distributions. This chapter will focus on these command line tools. A third and rather extremist way is to edit the local configuration files directly using vi (or vipw/vigr). Do not attempt this as a novice on production systems! /etc/passwd The local user database on Linux (and on most Unixes) is /etc/passwd. [root@RHEL5 ~]# tail /etc/passwd inge:x:518:524:art dealer:/home/inge:/bin/ksh ann:x:519:525:flute player:/home/ann:/bin/bash frederik:x:520:526:rubius poet:/home/frederik:/bin/bash steven:x:521:527:roman emperor:/home/steven:/bin/bash pascale:x:522:528:artist:/home/pascale:/bin/ksh geert:x:524:530:kernel developer:/home/geert:/bin/bash wim:x:525:531:master damuti:/home/wim:/bin/bash sandra:x:526:532:radish stresser:/home/sandra:/bin/bash annelies:x:527:533:sword fighter:/home/annelies:/bin/bash laura:x:528:534:art dealer:/home/laura:/bin/ksh As you can see, this file contains seven columns separated by a colon. The columns contain the username, an x, the user id, the primary group id, a description, the name of the home directory, and the login shell. root The root user also called the superuser is the most powerful account on your Linux system. This user can do almost anything, including the creation of other users. The root user always has userid 0 (regardless of the name of the account). [root@RHEL5 ~]# head -1 /etc/passwd root:x:0:0:root:/root:/bin/bash 182
- 189. users useradd You can add users with the useradd command. The example below shows how to add a user named yanina (last parameter) and at the same time forcing the creation of the home directory (-m), setting the name of the home directory (-d), and setting a description (-c). [root@RHEL5 ~]# useradd -m -d /home/yanina -c "yanina wickmayer" yanina [root@RHEL5 ~]# tail -1 /etc/passwd yanina:x:529:529:yanina wickmayer:/home/yanina:/bin/bash The user named yanina received userid 529 and primary group id 529. /etc/default/useradd Both Red Hat Enterprise Linux and Debian/Ubuntu have a file called /etc/default/ useradd that contains some default user options. Besides using cat to display this file, you can also use useradd -D. [root@RHEL4 ~]# useradd -D GROUP=100 HOME=/home INACTIVE=-1 EXPIRE= SHELL=/bin/bash SKEL=/etc/skel userdel You can delete the user yanina with userdel. The -r option of userdel will also remove the home directory. [root@RHEL5 ~]# userdel -r yanina usermod You can modify the properties of a user with the usermod command. This example uses usermod to change the description of the user harry. [root@RHEL4 ~]# tail -1 /etc/passwd harry:x:516:520:harry potter:/home/harry:/bin/bash [root@RHEL4 ~]# usermod -c 'wizard' harry [root@RHEL4 ~]# tail -1 /etc/passwd harry:x:516:520:wizard:/home/harry:/bin/bash 183
- 190. users 23.3. passwords passwd Passwords of users can be set with the passwd command. Users will have to provide their old password before twice entering the new one. [harry@RHEL4 ~]$ passwd Changing password for user harry. Changing password for harry (current) UNIX password: New UNIX password: BAD PASSWORD: it's WAY too short New UNIX password: Retype new UNIX password: passwd: all authentication tokens updated successfully. [harry@RHEL4 ~]$ As you can see, the passwd tool will do some basic verification to prevent users from using too simple passwords. The root user does not have to follow these rules (there will be a warning though). The root user also does not have to provide the old password before entering the new password twice. /etc/shadow User passwords are encrypted and kept in /etc/shadow. The /etc/shadow file is read only and can only be read by root. We will see in the file permissions section how it is possible for users to change their password. For now, you will have to know that users can change their password with the /usr/bin/passwd command. [root@RHEL5 ~]# tail /etc/shadow inge:$1$yWMSimOV$YsYvcVKqByFVYLKnU3ncd0:14054:0:99999:7::: ann:!!:14054:0:99999:7::: frederik:!!:14054:0:99999:7::: steven:!!:14054:0:99999:7::: pascale:!!:14054:0:99999:7::: geert:!!:14054:0:99999:7::: wim:!!:14054:0:99999:7::: sandra:!!:14054:0:99999:7::: annelies:!!:14054:0:99999:7::: laura:$1$Tvby1Kpa$lL.WzgobujUS3LClIRmdv1:14054:0:99999:7::: The /etc/shadow file contains nine colon separated columns. The nine fields contain (from left to right) the user name, the encrypted password (note that only inge and laura have an encrypted password), the day the password was last changed (day 1 is January 1, 1970), number of days the password must be left unchanged, password expiry day, warning number of days before password expiry, number of days after expiry before disabling the account, and the day the account was disabled (again, since 1970). The last field has no meaning yet. 184
- 191. users password encryption encryption with passwd Passwords are stored in an encrypted format. This encryption is done by the crypt function. The easiest (and recommended) way to add a user with a password to the system is to add the user with the useradd -m user command, and then set the user's password with passwd. [root@RHEL4 ~]# useradd -m xavier [root@RHEL4 ~]# passwd xavier Changing password for user xavier. New UNIX password: Retype new UNIX password: passwd: all authentication tokens updated successfully. [root@RHEL4 ~]# encryption with openssl Another way to create users with a password is to use the -p option of useradd, but that option requires an encrypted password. You can generate this encrypted password with the openssl passwd command. [root@RHEL4 ~]# openssl passwd stargate ZZNX16QZVgUQg [root@RHEL4 ~]# useradd -m -p ZZNX16QZVgUQg mohamed encryption with crypt A third option is to create your own C program using the crypt function, and compile this into a command. [paul@laika ~]$ cat MyCrypt.c #include <stdio.h> #define __USE_XOPEN #include <unistd.h> int main(int argc, char** argv) { if(argc==3) { printf("%sn", crypt(argv[1],argv[2])); } else { printf("Usage: MyCrypt $password $saltn" ); } return 0; } 185
- 192. users This little program can be compiled with gcc like this. [paul@laika ~]$ gcc MyCrypt.c -o MyCrypt -lcrypt To use it, we need to give two parameters to MyCript. The first is the unencrypted password, the second is the salt. The salt is used to perturb the encryption algorithm in one of 4096 different ways. This variation prevents two users with the same password from having the same entry in /etc/shadow. paul@laika:~$ ./MyCrypt stargate 12 12L4FoTS3/k9U paul@laika:~$ ./MyCrypt stargate 01 01Y.yPnlQ6R.Y paul@laika:~$ ./MyCrypt stargate 33 330asFUbzgVeg paul@laika:~$ ./MyCrypt stargate 42 42XFxoT4R75gk Did you notice that the first two characters of the password are the salt ? The standard output of the crypt function is using the DES algorithm which is old and can be cracked in minutes. A better method is to use MD5 passwords which can be recognized by a salt starting with $1$. paul@laika:~$ ./MyCrypt stargate '$1$12' $1$12$xUIQ4116Us.Q5Osc2Khbm1 paul@laika:~$ ./MyCrypt stargate '$1$01' $1$01$yNs8brjp4b4TEw.v9/IlJ/ paul@laika:~$ ./MyCrypt stargate '$1$33' $1$33$tLh/Ldy2wskdKAJR.Ph4M0 paul@laika:~$ ./MyCrypt stargate '$1$42' $1$42$Hb3nvP0KwHSQ7fQmIlY7R. The MD5 salt can be up to eight characters long. The salt is displayed in /etc/shadow between the second and third $, so never use the password as the salt! paul@laika:~$ ./MyCrypt stargate '$1$stargate' $1$stargate$qqxoLqiSVNvGr5ybMxEVM1 password defaults /etc/login.defs The /etc/login.defs file contains some default settings for user passwords like password aging and length settings. (You will also find the numerical limits of user ids and group ids and whether or not a home directory should be created by default). 186
- 193. users [root@RHEL4 ~]# grep -i pass /etc/login.defs # Password aging controls: # PASS_MAX_DAYS Maximum number of days a password may be used. # PASS_MIN_DAYS Minimum number of days allowed between password changes. # PASS_MIN_LEN Minimum acceptable password length. # PASS_WARN_AGE Number of days warning given before a password expires. PASS_MAX_DAYS 99999 PASS_MIN_DAYS 0 PASS_MIN_LEN 5 PASS_WARN_AGE 7 chage The chage command can be used to set an expiration date for a user account (-E), set a mimimum (-m) and maximum (-M) password age, a password expiration date, and set the number of warning days before the password expiration date. Much of this functionality is also available from the passwd command. The -l option of chage will list these settings for a user. [root@RHEL4 ~]# chage -l harry Minimum: 0 Maximum: 99999 Warning: 7 Inactive: -1 Last Change: Jul 23, 2007 Password Expires: Never Password Inactive: Never Account Expires: Never [root@RHEL4 ~]# disabling a password Passwords in /etc/shadow cannot begin with an exclamation mark. When the second field in /etc/passwd starts with an exclamation mark, then the password can not be used. Using this feature is often called locking, disabling, or suspending a user account. Besides vi (or vipw) you can also accomplish this with usermod. The first line in the next screenshot will disable the password of user harry, making it impossible for harry to authenticate using this password. [root@RHEL4 ~]# usermod -L harry [root@RHEL4 ~]# tail -1 /etc/shadow harry:!$1$143TO9IZ$RLm/FpQkpDrV4/Tkhku5e1:13717:0:99999:7::: The root user (and users with sudo rights on su) still will be able to su to harry (because the password is not needed here). Also note that harry will still be able to login if he has set up passwordless ssh! 187
- 194. users [root@RHEL4 ~]# su - harry [harry@RHEL4 ~]$ You can unlock the account again with usermod -U. Watch out for tiny differences in the command line options of passwd, usermod, and useradd on different distributions! Verify the local files when using features like "disabling, suspending, or locking" users and passwords! editing local files If you still want to manually edit the /etc/passwd or /etc/shadow, after knowing these commands for password management, then use vipw instead of vi(m) directly. The vipw tool will do proper locking of the file. [root@RHEL5 ~]# vipw /etc/passwd vipw: the password file is busy (/etc/ptmp present) 188
- 195. users 23.4. home directories creating home directories The easiest way to create a home directory is to supply the -m option with useradd (it is likely set as a default option on Linux). A less easy way is to create a home directory manually with mkdir which also requires setting the owner and the permissions on the directory with chmod and chown (both commands are discussed in detail in another chapter). [root@RHEL5 ~]# mkdir /home/laura [root@RHEL5 ~]# chown laura:laura /home/laura [root@RHEL5 ~]# chmod 700 /home/laura [root@RHEL5 ~]# ls -ld /home/laura/ drwx------ 2 laura laura 4096 Jun 24 15:17 /home/laura/ /etc/skel/ When using useradd with the -m option, the /etc/skel/ directory is copied to the newly created home directory. The /etc/skel/ directory contains some (usually hidden) files that contain profile settings and default values for applications. In this way /etc/skel/ serves as a default home directory and as a default user profile. [root@RHEL5 ~]# ls -la /etc/skel/ total 48 drwxr-xr-x 2 root root 4096 Apr 1 00:11 . drwxr-xr-x 97 root root 12288 Jun 24 15:36 .. -rw-r--r-- 1 root root 24 Jul 12 2006 .bash_logout -rw-r--r-- 1 root root 176 Jul 12 2006 .bash_profile -rw-r--r-- 1 root root 124 Jul 12 2006 .bashrc deleting home directories The -r option of userdel will make sure that the home directory is deleted together with the user account. [root@RHEL5 ~]# ls -ld /home/wim/ drwx------ 2 wim wim 4096 Jun 24 15:19 /home/wim/ [root@RHEL5 ~]# userdel -r wim [root@RHEL5 ~]# ls -ld /home/wim/ ls: /home/wim/: No such file or directory 189
- 196. users 23.5. user shell login shell The /etc/passwd file specifies the login shell for the user. In the screenshot below you can see that user annelies will log in with the /bin/bash shell, and user laura with the /bin/ksh shell. [root@RHEL5 ~]# tail -2 /etc/passwd annelies:x:527:533:sword fighter:/home/annelies:/bin/bash laura:x:528:534:art dealer:/home/laura:/bin/ksh You can use the usermod command to change the shell for a user. [root@RHEL5 ~]# usermod -s /bin/bash laura [root@RHEL5 ~]# tail -1 /etc/passwd laura:x:528:534:art dealer:/home/laura:/bin/bash chsh Users can change their login shell with the chsh command. First, user harry obtains a list of available shells (he could also have done a cat /etc/shells) and then changes his login shell to the Korn shell (/bin/ksh). At the next login, harry will default into ksh instead of bash. [harry@RHEL4 ~]$ chsh -l /bin/sh /bin/bash /sbin/nologin /bin/ash /bin/bsh /bin/ksh /usr/bin/ksh /usr/bin/pdksh /bin/tcsh /bin/csh /bin/zsh [harry@RHEL4 ~]$ chsh -s /bin/ksh Changing shell for harry. Password: Shell changed. [harry@RHEL4 ~]$ 190
- 197. users 23.6. switch users with su su to another user The su command allows a user to run a shell as another user. [paul@RHEL4b ~]$ su harry Password: [harry@RHEL4b paul]$ su to root Yes you can alsu su to become root, when you know the root password. [harry@RHEL4b paul]$ su root Password: [root@RHEL4b paul]# su as root Unless you are logged in as root, running a shell as another user requires that you know the password of that user. The root user can become any user without knowing the user's password. [root@RHEL4b paul]# su serena [serena@RHEL4b paul]$ su - $username By default, the su command maintains the same shell environment. To become another user and also get the target user's environment, issue the su - command followed by the target username. [paul@RHEL4b ~]$ su - harry Password: [harry@RHEL4b ~]$ su - When no username is provided to su or su -, the command will assume root is the target. [harry@RHEL4b ~]$ su - Password: [root@RHEL4b ~]# 191
- 198. users 23.7. run a program as another user about sudo The sudo program allows a user to start a program with the credentials of another user. Before this works, the system administrator has to set up the /etc/sudoers file. This can be useful to delegate administrative tasks to another user (without giving the root password). The screenshot below shows the usage of sudo. User paul received the right to run useradd with the credentials of root. This allows paul to create new users on the system without becoming root and without knowing the root password. paul@laika:~$ useradd -m inge useradd: unable to lock password file paul@laika:~$ sudo useradd -m inge [sudo] password for paul: paul@laika:~$ Image copied from xkcd.com. setuid on sudo The sudo binary has the setuid bit set, so any user can run it with the effective userid of root. 192
- 199. users paul@laika:~$ ls -l `which sudo` -rwsr-xr-x 2 root root 107872 2008-05-15 02:41 /usr/bin/sudo paul@laika:~$ visudo Check the man page of visudo before playing with the /etc/sudoers file. sudo su On some linux systems like Ubuntu and Kubuntu, the root user does not have a password set. This means that it is not possible to login as root (extra security). To perform tasks as root, the first user is given all sudo rights via the /etc/sudoers. In fact all users that are members of the admin group can use sudo to run all commands as root. root@laika:~# grep admin /etc/sudoers # Members of the admin group may gain root privileges %admin ALL=(ALL) ALL The end result of this is that the user can type sudo su - and become root without having to enter the root password. The sudo command does require you to enter your own password. Thus the password prompt in the screenshot below is for sudo, not for su. paul@laika:~$ sudo su - Password: root@laika:~# 193
- 200. users 23.8. practice: users 1. Create the users Serena Williams, Venus Williams and Justine Henin, all of them with password set to stargate, with username (lowercase!) as their first name, and their full name in the comment. Verify that the users and their home directory are properly created. 2. Create a user called kornuser, give him the Korn shell (/bin/ksh) as his default shell. Log on with this user (on a command line or in a tty). 3. Create a user named einstime without home directory, give him /bin/date as his default logon shell. What happens when you log on with this user ? Can you think of a useful real world example for changing a user's login shell to an application ? 4. Try the commands who, whoami, who am i, w, id, echo $USER $UID . 5a. Lock the venus user account with usermod. 5b. Use passwd -d to disable the serena password. Verify the serena line in /etc/ shadow before and after disabling. 5c. What is the difference between locking a user account and disabling a user account's password ? 6. As root change the password of einstime to stargate. 7. Now try changing the password of serena to serena as serena. 8. Make sure every new user needs to change his password every 10 days. 9. Set the warning number of days to four for the kornuser. 10a. Set the password of two separate users to stargate. Look at the encrypted stargate's in /etc/shadow and explain. 10b. Take a backup as root of /etc/shadow. Use vi to copy an encrypted stargate to another user. Can this other user now log on with stargate as a password ? 11. Put a file in the skeleton directory and check whether it is copied to user's home directory. When is the skeleton directory copied ? 12. Why use vipw instead of vi ? What could be the problem when using vi or vim ? 13. Use chsh to list all shells, and compare to cat /etc/shells. Change your login shell to the Korn shell, log out and back in. Now change back to bash. 14. Which useradd option allows you to name a home directory ? 15. How can you see whether the password of user harry is locked or unlocked ? Give a solution with grep and a solution with passwd. 194
- 201. users 23.9. solution: users 1. Create the users Serena Williams, Venus Williams and Justine Henin, all of them with password set to stargate, with username (lowercase) as their first name, and their full name in the comment. Verify that the users and their home directory are properly created. useradd -m -c "Serena Williams" serena ; passwd serena useradd -m -c "Venus Williams" venus ; passwd venus useradd -m -c "Justine Henin" justine ; passwd justine tail /etc/passwd ; tail /etc/shadow ; ls /home Keep user logon names in lowercase! 2. Create a user called kornuser, give him the Korn shell (/bin/ksh) as his default shell. Log on with this user (on a command line or in a tty). useradd -s /bin/ksh kornuser ; passwd kornuser 3. Create a user named einstime without home directory, give him /bin/date as his default logon shell. What happens when you log on with this user ? Can you think of a useful real world example for changing a user's login shell to an application ? useradd -s /bin/date einstime ; passwd einstime It can be useful when users need to access only one application on the server. Just logging on opens the application for them, and closing the application automatically logs them off. 4. Try the commands who, whoami, who am i, w, id, echo $USER $UID . who ; whoami ; who am i ; w ; id ; echo $USER $UID 5a. Lock the venus user account with usermod. usermod -L venus 5b. Use passwd -d to disable the serena password. Verify the serena line in /etc/ shadow before and after disabling. grep serena /etc/shadow; passwd -d serena ; grep serena /etc/shadow 5c. What is the difference between locking a user account and disabling a user account's password ? Locking will prevent the user from logging on to the system with his password (by putting a ! in front of the password in /etc/shadow). Disbling with passwd will erase the password from /etc/shadow. 6. As root change the password of einstime to stargate. Log on as root and type: passwd einstime 7. Now try changing the password of serena to serena as serena. log on as serena, then execute: passwd serena... it should fail! 195
- 202. users 8. Make sure every new user needs to change his password every 10 days. For an existing user: chage -M 10 serena For all new users: vi /etc/login.defs (and change PASS_MAX_DAYS to 10) 9. Set the warning number of days to four for the kornuser. chage -W 4 kornuser 10a. Set the password of two separate users to stargate. Look at the encrypted stargate's in /etc/shadow and explain. If you used passwd, then the salt will be different for the two encrypted passwords. 10b. Take a backup as root of /etc/shadow. Use vi to copy an encrypted stargate to another user. Can this other user now log on with stargate as a password ? Yes. 11. Put a file in the skeleton directory and check whether it is copied to user's home directory. When is the skeleton directory copied ? When you create a user account with a new home directory. 12. Why use vipw instead of vi ? What could be the problem when using vi or vim ? vipw will give a warning when someone else is already using that file. 13. Use chsh to list all shells, and compare to cat /etc/shells. Change your login shell to the Korn shell, log out and back in. Now change back to bash. On Red Hat Enterprise Linux: chsh -l On Debian/Ubuntu: cat /etc/shells 14. Which useradd option allows you to name a home directory ? -d 15. How can you see whether the password of user harry is locked or unlocked ? Give a solution with grep and a solution with passwd. grep harry /etc/shadow passwd -S harry 196
- 203. users 23.10. shell environment It is nice to have these preset and custom aliases and variables, but where do they all come from ? The shell uses a number of startup files that are checked (and executed) whenever the shell is invoked. What follows is an overview of startup scripts. /etc/profile Both the bash and the ksh shell will verify the existence of /etc/profile and execute it if it exists. When reading this script, you might notice (at least on Debian Lenny and on Red Hat Enterprise Linux 5) that it builds the PATH environment variable. The script might also change the PS1 variable, set the HOSTNAME and execute even more scripts like /etc/inputrc You can use this script to set aliases and variables for every user on the system. ~/.bash_profile When this file exists in the users home directory, then bash will execute it. On Debian Linux it does not exist by default. RHEL5 uses a brief ~/.bash_profile where it checks for the existence of ~/.bashrc and then executes it. It also adds $HOME/bin to the $PATH variable. [serena@rhel53 ~]$ cat .bash_profile # .bash_profile # Get the aliases and functions if [ -f ~/.bashrc ]; then . ~/.bashrc fi # User specific environment and startup programs PATH=$PATH:$HOME/bin export PATH ~/.bash_login When .bash_profile does not exist, then bash will check for ~/.bash_login and execute it. Neither Debian nor Red Hat have this file by default. 197
- 204. users ~/.profile When neither ~/.bash_profile and ~/.bash_login exist, then bash will verify the existence of ~/.profile and execute it. This file does not exist by default on Red Hat. On Debian this script can execute ~/.bashrc and will add $HOME/bin to the $PATH variable. serena@deb503:~$ tail -12 .profile # if running bash if [ -n "$BASH_VERSION" ]; then # include .bashrc if it exists if [ -f "$HOME/.bashrc" ]; then . "$HOME/.bashrc" fi fi # set PATH so it includes user's private bin if it exists if [ -d "$HOME/bin" ] ; then PATH="$HOME/bin:$PATH" fi ~/.bashrc As seen in the previous points, the ~/.bashrc script might be executed by other scripts. Let us take a look at what it does by default. Red Hat uses a very simple ~/.bashrc, checking for /etc/bashrc and executing it. It also leaves room for custom aliases and functions. [serena@rhel53 ~]$ more .bashrc # .bashrc # Source global definitions if [ -f /etc/bashrc ]; then . /etc/bashrc fi # User specific aliases and functions On Debian this script is quite a bit longer and configures $PS1, some history variables and a number af active and inactive aliases. serena@deb503:~$ ls -l .bashrc -rw-r--r-- 1 serena serena 3116 2008-05-12 21:02 .bashrc ~/.bash_logout When exiting bash, it can execute ~/.bash_logout. Debian and Red Hat both use this opportunity to clear the screen. 198
- 205. users serena@deb503:~$ cat .bash_logout # ~/.bash_logout: executed by bash(1) when login shell exits. # when leaving the console clear the screen to increase privacy if [ "$SHLVL" = 1 ]; then [ -x /usr/bin/clear_console ] && /usr/bin/clear_console -q fi [serena@rhel53 ~]$ cat .bash_logout # ~/.bash_logout /usr/bin/clear Debian overview Below is a table overview of when Debian is running any of these bash startup scripts. Table 23.1. Debian User Environment script su su - ssh gdm ~./bashrc no yes yes yes ~/.profile no yes yes yes /etc/profile no yes yes yes /etc/bash.bashrc yes no no yes RHEL5 overview Below is a table overview of when Red Hat Enterprise Linux 5 is running any of these bash startup scripts. Table 23.2. Red Hat User Environment script su su - ssh gdm ~./bashrc yes yes yes yes ~/.bash_profile no yes yes yes /etc/profile no yes yes yes /etc/bashrc yes yes yes yes 199
- 206. Chapter 24. groups Table of Contents 24.1. about groups ............................................................................................... 201 24.2. groupadd ..................................................................................................... 201 24.3. /etc/group .................................................................................................... 201 24.4. usermod ...................................................................................................... 201 24.5. groupmod ................................................................................................... 202 24.6. groupdel ...................................................................................................... 202 24.7. groups ......................................................................................................... 202 24.8. gpasswd ...................................................................................................... 202 24.9. vigr ............................................................................................................. 203 24.10. practice: groups ........................................................................................ 204 24.11. solution: groups ........................................................................................ 205 200
- 207. groups 24.1. about groups Users can be listed in groups. Groups allow you to set permissions on the group level instead of having to set permissions for every individual user. Every Unix or Linux distribution will have a graphical tool to manage groups. Novice users are advised to use this graphical tool. More experienced users can use command line tools to manage users, but be careful: Some distributions do not allow the mixed use of GUI and CLI tools to manage groups (YaST in Novell Suse). Senior administrators can edit the relevant files directly with vi or vigr. 24.2. groupadd Groups can be created with the groupadd command. The example below shows the creation of five (empty) groups. root@laika:~# groupadd tennis root@laika:~# groupadd football root@laika:~# groupadd snooker root@laika:~# groupadd formula1 root@laika:~# groupadd salsa 24.3. /etc/group Users can be a member of several groups. Group membership is defined by the /etc/ group file. root@laika:~# tail -5 /etc/group tennis:x:1006: football:x:1007: snooker:x:1008: formula1:x:1009: salsa:x:1010: root@laika:~# The first field is the group's name. The second field is the group's (encrypted) password (can be empty). The third field is the group identification or GID. The fourth field is the list of members, these groups have no members. 24.4. usermod Group membership can be modified with the useradd or usermod command. root@laika:~# usermod -a -G tennis inge root@laika:~# usermod -a -G tennis katrien root@laika:~# usermod -a -G salsa katrien root@laika:~# usermod -a -G snooker sandra root@laika:~# usermod -a -G formula1 annelies 201
- 208. groups root@laika:~# tail -5 /etc/group tennis:x:1006:inge,katrien football:x:1007: snooker:x:1008:sandra formula1:x:1009:annelies salsa:x:1010:katrien root@laika:~# Be careful when using usermod to add users to groups. By default, the usermod command will remove the user from every group of which he is a member if the group is not listed in the command! Using the -a (append) switch prevents this behaviour. 24.5. groupmod You can change the group name with the groupmod command. root@laika:~# groupmod -n darts snooker root@laika:~# tail -5 /etc/group tennis:x:1006:inge,katrien football:x:1007: formula1:x:1009:annelies salsa:x:1010:katrien darts:x:1008:sandra 24.6. groupdel You can permanently remove a group with the groupdel command. root@laika:~# groupdel tennis root@laika:~# 24.7. groups A user can type the groups command to see a list of groups where the user belongs to. [harry@RHEL4b ~]$ groups harry sports [harry@RHEL4b ~]$ 24.8. gpasswd You can delegate control of group membership to another user with the gpasswd command. In the example below we delegate permissions to add and remove group members to serena for the sports group. Then we su to serena and add harry to the sports group. 202
- 209. groups [root@RHEL4b ~]# gpasswd -A serena sports [root@RHEL4b ~]# su - serena [serena@RHEL4b ~]$ id harry uid=516(harry) gid=520(harry) groups=520(harry) [serena@RHEL4b ~]$ gpasswd -a harry sports Adding user harry to group sports [serena@RHEL4b ~]$ id harry uid=516(harry) gid=520(harry) groups=520(harry),522(sports) [serena@RHEL4b ~]$ tail -1 /etc/group sports:x:522:serena,venus,harry [serena@RHEL4b ~]$ Group administrators do not have to be a member of the group. They can remove themselves from a group, but this does not influence their ability to add or remove members. [serena@RHEL4b ~]$ gpasswd -d serena sports Removing user serena from group sports [serena@RHEL4b ~]$ exit Information about group administrators is kept in the /etc/gshadow file. [root@RHEL4b ~]# tail -1 /etc/gshadow sports:!:serena:venus,harry [root@RHEL4b ~]# To remove all group administrators from a group, use the gpasswd command to set an empty administrators list. [root@RHEL4b ~]# gpasswd -A "" sports 24.9. vigr Similar to vipw, the vigr command can be used to manually edit the /etc/group file, since it will do proper locking of the file. Only experienced senior administrators should use vi or vigr to manage groups. 203
- 210. groups 24.10. practice: groups 1. Create the groups tennis, football and sports. 2. In one command, make venus a member of tennis and sports. 3. Rename the football group to foot. 4. Use vi to add serena to the tennis group. 5. Use the id command to verify that serena is a member of tennis. 6. Make someone responsible for managing group membership of foot and sports. Test that it works. 204
- 211. groups 24.11. solution: groups 1. Create the groups tennis, football and sports. groupadd tennis ; groupadd football ; groupadd sports 2. In one command, make venus a member of tennis and sports. usermod -a -G tennis,sports venus 3. Rename the football group to foot. groupmod -n foot football 4. Use vi to add serena to the tennis group. vi /etc/group 5. Use the id command to verify that serena is a member of tennis. id (and after logoff logon serena should be member) 6. Make someone responsible for managing group membership of foot and sports. Test that it works. gpasswd -A (to make manager) gpasswd -a (to add member) 205
- 212. Part VIII. file security
- 213. Chapter 25. standard file permissions Table of Contents 25.1. file ownership ............................................................................................ 208 25.2. list of special files ...................................................................................... 209 25.3. permissions ................................................................................................. 209 25.4. practice: standard file permissions ............................................................. 214 25.5. solution: standard file permissions ............................................................. 215 207
- 214. standard file permissions 25.1. file ownership user owner and group owner The users and groups of a system can be locally managed in /etc/passwd and /etc/ group, or they can be in a NIS, LDAP, or Samba domain. These users and groups can own files. Actually, every file has a user owner and a group owner, as can be seen in the following screenshot. paul@RHELv4u4:~/test$ ls -l total 24 -rw-rw-r-- 1 paul paul 17 Feb 7 11:53 file1 -rw-rw-r-- 1 paul paul 106 Feb 5 17:04 file2 -rw-rw-r-- 1 paul proj 984 Feb 5 15:38 data.odt -rw-r--r-- 1 root root 0 Feb 7 16:07 stuff.txt paul@RHELv4u4:~/test$ User paul owns three files, two of those are also owned by the group paul; data.odt is owned by the group proj. The root user owns the file stuff.txt, as does the group root. chgrp You can change the group owner of a file using the chgrp command. root@laika:/home/paul# touch FileForPaul root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 root root 0 2008-08-06 14:11 FileForPaul root@laika:/home/paul# chgrp paul FileForPaul root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 root paul 0 2008-08-06 14:11 FileForPaul chown The user owner of a file can be changed with chown command. root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 root paul 0 2008-08-06 14:11 FileForPaul root@laika:/home/paul# chown paul FileForPaul root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul You can also use chown to change both the user owner and the group owner. root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul root@laika:/home/paul# chown root:project42 FileForPaul root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 root project42 0 2008-08-06 14:11 FileForPaul 208
- 215. standard file permissions 25.2. list of special files When you use ls -l, for each file you can see ten characters before the user and group owner. The first character tells us the type of file. Regular files get a -, directories get a d, symbolic links are shown with an l, pipes get a p, character devices a c, block devices a b, and sockets an s. Table 25.1. Unix special files first file type character - normal file d directory l symbolic link p named pipe b block device c character device s socket 25.3. permissions rwx The nine characters following the file type denote the permissions in three triplets. A permission can be r for read access, w for write access, and x for execute. You need the r permission to list (ls) the contents of a directory. You need the x permission to enter (cd) a directory. You need the w permission to create files in or remove files from a directory. Table 25.2. standard Unix file permissions permission on a file on a directory r (read) read file contents (cat) read directory contents (ls) w (write) change file contents (vi) create files in (touch) x (execute) execute the file enter the directory (cd) three sets of rwx We already know that the output of ls -l starts with ten characters for each file. This screenshot shows a regular file (because the first character is a - ). 209
- 216. standard file permissions paul@RHELv4u4:~/test$ ls -l proc42.bash -rwxr-xr-- 1 paul proj 984 Feb 6 12:01 proc42.bash Below is a table describing the function of all ten characters. Table 25.3. Unix file permissions position position characters function 1 - this is a regular file 2-4 rwx permissions for the user owner 5-7 r-x permissions for the group owner 8-10 r-- permissions for others When you are the user owner of a file, then the user owner permissions apply to you. The rest of the permissions have no influence on your access to the file. When you belong to the group that is the group owner of a file, then the group owner permissions apply to you. The rest of the permissions have no influence on your access to the file. When you are not the user owner of a file and you do not belong to the group owner, then the others permissions apply to you. The rest of the permissions have no influence on your access to the file. permission examples Some example combinations on files and directories are seen in this screenshot. The name of the file explains the permissions. paul@laika:~/perms$ ls -lh total 12K drwxr-xr-x 2 paul paul 4.0K 2007-02-07 22:26 AllEnter_UserCreateDelete -rwxrwxrwx 1 paul paul 0 2007-02-07 22:21 EveryoneFullControl.txt -r--r----- 1 paul paul 0 2007-02-07 22:21 OnlyOwnersRead.txt -rwxrwx--- 1 paul paul 0 2007-02-07 22:21 OwnersAll_RestNothing.txt dr-xr-x--- 2 paul paul 4.0K 2007-02-07 22:25 UserAndGroupEnter dr-x------ 2 paul paul 4.0K 2007-02-07 22:25 OnlyUserEnter paul@laika:~/perms$ To summarize, the first rwx triplet represents the permissions for the user owner. The second triplet corresponds to the group owner; it specifies permissions for all members of that group. The third triplet defines permissions for all other users that are not the user owner and are not a member of the group owner. setting permissions (chmod) Permissions can be changed with chmod. The first example gives the user owner execute permissions. 210
- 217. standard file permissions paul@laika:~/perms$ ls -l permissions.txt -rw-r--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod u+x permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxr--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt This example removes the group owners read permission. paul@laika:~/perms$ chmod g-r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx---r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt This example removes the others read permission. paul@laika:~/perms$ chmod o-r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx------ 1 paul paul 0 2007-02-07 22:34 permissions.txt This example gives all of them the write permission. paul@laika:~/perms$ chmod a+w permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx-w--w- 1 paul paul 0 2007-02-07 22:34 permissions.txt You don't even have to type the a. paul@laika:~/perms$ chmod +x permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx-wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt You can also set explicit permissions. paul@laika:~/perms$ chmod u=rw permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rw--wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt Feel free to make any kind of combination. paul@laika:~/perms$ chmod u=rw,g=rw,o=r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt Even fishy combinations are accepted by chmod. paul@laika:~/perms$ chmod u=rwx,ug+rw,o=r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxrw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt 211
- 218. standard file permissions setting octal permissions Most Unix administrators will use the old school octal system to talk about and set permissions. Look at the triplet bitwise, equating r to 4, w to 2, and x to 1. Table 25.4. Octal permissions binary octal permission 000 0 --- 001 1 --x 010 2 -w- 011 3 -wx 100 4 r-- 101 5 r-x 110 6 rw- 111 7 rwx This makes 777 equal to rwxrwxrwx and by the same logic, 654 mean rw-r-xr-- . The chmod command will accept these numbers. paul@laika:~/perms$ chmod 777 permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxrwxrwx 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod 664 permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod 750 permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxr-x--- 1 paul paul 0 2007-02-07 22:34 permissions.txt umask When creating a file or directory, a set of default permissions are applied. These default permissions are determined by the umask. The umask specifies permissions that you do not want set on by default. You can display the umask with the umask command. [Harry@RHEL4b ~]$ umask 0002 [Harry@RHEL4b ~]$ touch test [Harry@RHEL4b ~]$ ls -l test -rw-rw-r-- 1 Harry Harry 0 Jul 24 06:03 test [Harry@RHEL4b ~]$ As you can also see, the file is also not executable by default. This is a general security feature among Unixes; newly created files are never executable by default. You have 212
- 219. standard file permissions to explicitly do a chmod +x to make a file executable. This also means that the 1 bit in the umask has no meaning--a umask of 0022 is the same as 0033. 213
- 220. standard file permissions 25.4. practice: standard file permissions 1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there. 2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ? 3. As root, create a file in the users ~/permissions directory. 4. As normal user, look at who owns this file created by root. 5. Change the ownership of all files in ~/permissions to yourself. 6. Make sure you have all rights to these files, and others can only read. 7. With chmod, is 770 the same as rwxrwx--- ? 8. With chmod, is 664 the same as r-xr-xr-- ? 9. With chmod, is 400 the same as r-------- ? 10. With chmod, is 734 the same as rwxr-xr-- ? 11a. Display the umask in octal and in symbolic form. 11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works. 12. Create a file as root, give only read to others. Can a normal user read this file ? Test writing to this file with vi. 13a. Create a file as normal user, give only read to others. Can another normal user read this file ? Test writing to this file with vi. 13b. Can root read this file ? Can root write to this file with vi ? 14. Create a directory that belongs to a group, where every member of that group can read and write to files, and create files. Make sure that people can only delete their own files. 214
- 221. standard file permissions 25.5. solution: standard file permissions 1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there. mkdir ~/permissions ; touch ~/permissions/myfile.txt 2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ? cp /etc/hosts ~/permissions/ The copy is owned by you. 3. As root, create a file in the users ~/permissions directory. (become root)# touch /home/username/permissions/rootfile 4. As normal user, look at who owns this file created by root. ls -l ~/permissions The file created by root is owned by root. 5. Change the ownership of all files in ~/permissions to yourself. chown user ~/permissions/* You cannot become owner of the file that belongs to root. 6. Make sure you have all rights to these files, and others can only read. chmod 644 (on files) chmod 755 (on directories) 7. With chmod, is 770 the same as rwxrwx--- ? yes 8. With chmod, is 664 the same as r-xr-xr-- ? No 9. With chmod, is 400 the same as r-------- ? yes 10. With chmod, is 734 the same as rwxr-xr-- ? no 11a. Display the umask in octal and in symbolic form. umask ; umask -S 215
- 222. standard file permissions 11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works. umask -S u=rwx,go= 12. Create a file as root, give only read to others. Can a normal user read this file ? Test writing to this file with vi. (become root) # echo hello > /home/username/root.txt # chmod 744 /home/username/root.txt (become user) vi ~/root.txt 13a. Create a file as normal user, give only read to others. Can another normal user read this file ? Test writing to this file with vi. echo hello > file ; chmod 744 file Yes, others can read this file 13b. Can root read this file ? Can root write to this file with vi ? Yes, root can read and write to this file. Permissions do not apply to root. 14. Create a directory that belongs to a group, where every member of that group can read and write to files, and create files. Make sure that people can only delete their own files. mkdir /home/project42 ; groupadd project42 chgrp project42 /home/project42 ; chmod 775 /home/project42 You can not yet do the last part of this exercise... 216
- 223. Chapter 26. advanced file permissions Table of Contents 26.1. sticky bit on directory ................................................................................ 218 26.2. setgid bit on directory ................................................................................ 218 26.3. setgid and setuid on regular files ............................................................... 219 26.4. practice: sticky, setuid and setgid bits ....................................................... 220 26.5. solution: sticky, setuid and setgid bits ....................................................... 221 217
- 224. advanced file permissions 26.1. sticky bit on directory You can set the sticky bit on a directory to prevent users from removing files that they do not own as a user owner. The sticky bit is displayed at the same location as the x permission for others. The sticky bit is represented by a t (meaning x is also there) or a T (when there is no x for others). root@RHELv4u4:~# mkdir /project55 root@RHELv4u4:~# ls -ld /project55 drwxr-xr-x 2 root root 4096 Feb 7 17:38 /project55 root@RHELv4u4:~# chmod +t /project55/ root@RHELv4u4:~# ls -ld /project55 drwxr-xr-t 2 root root 4096 Feb 7 17:38 /project55 root@RHELv4u4:~# The sticky bit can also be set with octal permissions, it is binary 1 in the first of four triplets. root@RHELv4u4:~# chmod 1775 /project55/ root@RHELv4u4:~# ls -ld /project55 drwxrwxr-t 2 root root 4096 Feb 7 17:38 /project55 root@RHELv4u4:~# You will typically find the sticky bit on the /tmp directory. root@barry:~# ls -ld /tmp drwxrwxrwt 6 root root 4096 2009-06-04 19:02 /tmp 26.2. setgid bit on directory setgid can be used on directories to make sure that all files inside the directory are owned by the group owner of the directory. The setgid bit is displayed at the same location as the x permission for group owner. The setgid bit is represented by an s (meaning x is also there) or a S (when there is no x for the group owner). As this example shows, even though root does not belong to the group proj55, the files created by root in /project55 will belong to proj55 since the setgid is set. root@RHELv4u4:~# groupadd proj55 root@RHELv4u4:~# chown root:proj55 /project55/ root@RHELv4u4:~# chmod 2775 /project55/ root@RHELv4u4:~# touch /project55/fromroot.txt root@RHELv4u4:~# ls -ld /project55/ drwxrwsr-x 2 root proj55 4096 Feb 7 17:45 /project55/ root@RHELv4u4:~# ls -l /project55/ total 4 -rw-r--r-- 1 root proj55 0 Feb 7 17:45 fromroot.txt root@RHELv4u4:~# You can use the find command to find all setgid directories. 218
- 225. advanced file permissions paul@laika:~$ find / -type d -perm -2000 2> /dev/null /var/log/mysql /var/log/news /var/local ... 26.3. setgid and setuid on regular files These two permissions cause an executable file to be executed with the permissions of the file owner instead of the executing owner. This means that if any user executes a program that belongs to the root user, and the setuid bit is set on that program, then the program runs as root. This can be dangerous, but sometimes this is good for security. Take the example of passwords; they are stored in /etc/shadow which is only readable by root. (The root user never needs permissions anyway.) root@RHELv4u4:~# ls -l /etc/shadow -r-------- 1 root root 1260 Jan 21 07:49 /etc/shadow Changing your password requires an update of this file, so how can normal non-root users do this? Let's take a look at the permissions on the /usr/bin/passwd. root@RHELv4u4:~# ls -l /usr/bin/passwd -r-s--x--x 1 root root 21200 Jun 17 2005 /usr/bin/passwd When running the passwd program, you are executing it with root credentials. You can use the find command to find all setuid programs. paul@laika:~$ find /usr/bin -type f -perm -04000 /usr/bin/arping /usr/bin/kgrantpty /usr/bin/newgrp /usr/bin/chfn /usr/bin/sudo /usr/bin/fping6 /usr/bin/passwd /usr/bin/gpasswd ... In most cases, setting the setuid bit on executables is sufficient. Setting the setgid bit will result in these programs to run with the credentials of their group owner. 219
- 226. advanced file permissions 26.4. practice: sticky, setuid and setgid bits 1a. Set up a directory, owned by the group sports. 1b. Members of the sports group should be able to create files in this directory. 1c. All files created in this directory should be group-owned by the sports group. 1d. Users should be able to delete only their own user-owned files. 1e. Test that this works! 2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your password as a normal user. Reset the permissions back and try again. 3. If time permits (or if you are waiting for other students to finish this practice), read about file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and test that it works. 220
- 227. advanced file permissions 26.5. solution: sticky, setuid and setgid bits 1a. Set up a directory, owned by the group sports. groupadd sports mkdir /home/sports chown root:sports /home/sports 1b. Members of the sports group should be able to create files in this directory. chmod 770 /home/sports 1c. All files created in this directory should be group-owned by the sports group. chmod 2770 /home/sports 1d. Users should be able to delete only their own user-owned files. chmod +t /home/sports 1e. Test that this works! Log in with different users (group members and others and root), create files and watch the permissions. Try changing and deleting files... 2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your password as a normal user. Reset the permissions back and try again. root@deb503:~# ls -l /usr/bin/passwd -rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd root@deb503:~# chmod 755 /usr/bin/passwd root@deb503:~# ls -l /usr/bin/passwd -rwxr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd A normal user cannot change password now. root@deb503:~# chmod 4755 /usr/bin/passwd root@deb503:~# ls -l /usr/bin/passwd -rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd 3. If time permits (or if you are waiting for other students to finish this practice), read about file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and test that it works. paul@laika:~$ sudo su - [sudo] password for paul: root@laika:~# mkdir attr root@laika:~# cd attr/ root@laika:~/attr# touch file42 root@laika:~/attr# lsattr ------------------ ./file42 root@laika:~/attr# chattr +i file42 221
- 228. advanced file permissions root@laika:~/attr# lsattr ----i------------- ./file42 root@laika:~/attr# rm -rf file42 rm: cannot remove `file42': Operation not permitted root@laika:~/attr# chattr -i file42 root@laika:~/attr# rm -rf file42 root@laika:~/attr# 222
- 229. Chapter 27. access control lists Table of Contents 27.1. acl in /etc/fstab ........................................................................................... 224 27.2. getfacl ......................................................................................................... 224 27.3. setfacl ......................................................................................................... 224 27.4. remove an acl entry ................................................................................... 225 27.5. remove the complete acl ............................................................................ 225 27.6. the acl mask ............................................................................................... 225 27.7. eiciel ........................................................................................................... 226 Standard Unix permissions might not be enough for some organisations. This chapter introduces access control lists or acl's to further protect files and directories. 223
- 230. access control lists 27.1. acl in /etc/fstab File systems that support access control lists, or acls, have to be mounted with the acl option listed in /etc/fstab. In the example below, you can see that the root file system has acl support, whereas /home/data does not. root@laika:~# tail -4 /etc/fstab /dev/sda1 / ext3 acl,relatime 0 1 /dev/sdb2 /home/data auto noacl,defaults 0 0 pasha:/home/r /home/pasha nfs defaults 0 0 wolf:/srv/data /home/wolf nfs defaults 0 0 27.2. getfacl Reading acls can be done with /usr/bin/getfacl. This screenshot shows how to read the acl of file33 with getfacl. paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rw- group::r-- mask::rwx other::r-- 27.3. setfacl Writing or changing acls can be done with /usr/bin/setfacl. These screenshots show how to change the acl of file33 with setfacl. First we add user sandra with octal permission 7 to the acl. paul@laika:~/test$ setfacl -m u:sandra:7 file33 Then we add the group tennis with octal permission 6 to the acl of the same file. paul@laika:~/test$ setfacl -m g:tennis:6 file33 The result is visible with getfacl. paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul 224
- 231. access control lists user::rw- user:sandra:rwx group::r-- group:tennis:rw- mask::rwx other::r-- 27.4. remove an acl entry The -x option of the setfacl command will remove an acl entry from the targeted file. paul@laika:~/test$ setfacl -m u:sandra:7 file33 paul@laika:~/test$ getfacl file33 | grep sandra user:sandra:rwx paul@laika:~/test$ setfacl -x sandra file33 paul@laika:~/test$ getfacl file33 | grep sandra Note that omitting the u or g when defining the acl for an account will default it to a user account. 27.5. remove the complete acl The -b option of the setfacl command will remove the acl from the targeted file. paul@laika:~/test$ setfacl -b file33 paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rw- group::r-- other::r-- 27.6. the acl mask The acl mask defines the maximum effective permissions for any entry in the acl. This mask is calculated every time you execute the setfacl or chmod commands. You can prevent the calculation by using the --no-mask switch. paul@laika:~/test$ setfacl --no-mask -m u:sandra:7 file33 paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rw- user:sandra:rwx #effective:rw- group::r-- mask::rw- 225
- 232. access control lists other::r-- 27.7. eiciel Desktop users might want to use eiciel to manage acls with a graphical tool. You will need to install eiciel and nautilus-actions to have an extra tab in nautilus to manage acls. paul@laika:~$ sudo aptitude install eiciel nautilus-actions 226
- 233. Chapter 28. file links Table of Contents 28.1. inodes ......................................................................................................... 228 28.2. about directories ......................................................................................... 229 28.3. hard links ................................................................................................... 230 28.4. symbolic links ............................................................................................ 230 28.5. removing links ........................................................................................... 231 28.6. practice : links ............................................................................................ 232 28.7. solution : links ............................................................................................ 233 227
- 234. file links 28.1. inodes To understand links in a file system, you first have to understand what an inode is. inode contents An inode is a data structure that contains metadata about a file. When the file system stores a new file on the hard disk, it stores not only the contents (data) of the file, but also extra properties like the name of the file, the creation date, its permissions, the owner of the file, and more. All this information (except the name of the file and the contents of the file) is stored in the inode of the file. The ls -l command will display some of the inode contents, as seen in this screenshot. root@rhel53 ~# ls -ld /home/project42/ drwxr-xr-x 4 root pro42 4.0K Mar 27 14:29 /home/project42/ inode table The inode table contains all of the inodes and is created when you create the file system (with mkfs). You can use the df -i command to see how many inodes are used and free on mounted file systems. oot@rhel53 ~# df -i Filesystem Inodes IUsed IFree IUse% Mounted on /dev/mapper/VolGroup00-LogVol00 4947968 115326 4832642 3% / /dev/hda1 26104 45 26059 1% /boot tmpfs 64417 1 64416 1% /dev/shm /dev/sda1 262144 2207 259937 1% /home/project42 /dev/sdb1 74400 5519 68881 8% /home/project33 /dev/sdb5 0 0 0 - /home/sales /dev/sdb6 100744 11 100733 1% /home/research In the df -i screenshot above you can see the inode usage for several mounted file systems. You don't see numbers for /dev/sdb5 because it is a fat file system. inode number Each inode has a unique number (the inode number). You can see the inode numbers with the ls -li command. paul@RHELv4u4:~/test$ touch file1 paul@RHELv4u4:~/test$ touch file2 paul@RHELv4u4:~/test$ touch file3 paul@RHELv4u4:~/test$ ls -li total 12 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 228
- 235. file links 817267 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 paul@RHELv4u4:~/test$ These three files were created one after the other and got three different inodes (the first column). All the information you see with this ls command resides in the inode, except for the filename (which is contained in the directory). inode and file contents Let's put some data in one of the files. paul@RHELv4u4:~/test$ ls -li total 16 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 817270 -rw-rw-r-- 1 paul paul 92 Feb 5 15:42 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 paul@RHELv4u4:~/test$ cat file2 It is winter now and it is very cold. We do not like the cold, we prefer hot summer nights. paul@RHELv4u4:~/test$ The data that is displayed by the cat command is not in the inode, but somewhere else on the disk. The inode contains a pointer to that data. 28.2. about directories a directory is a table A directory is a special kind of file that contains a table which maps filenames to inodes. Listing our current directory with ls -ali will display the contents of the directory file. paul@RHELv4u4:~/test$ ls -ali total 32 817262 drwxrwxr-x 2 paul paul 4096 Feb 5 15:42 . 800768 drwx------ 16 paul paul 4096 Feb 5 15:42 .. 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 817270 -rw-rw-r-- 1 paul paul 92 Feb 5 15:42 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 paul@RHELv4u4:~/test$ . and .. You can see five names, and the mapping to their five inodes. The dot . is a mapping to itself, and the dotdot .. is a mapping to the parent directory. The three other names are mappings to different inodes. 229
- 236. file links 28.3. hard links creating hard links When we create a hard link to a file with ln, an extra entry is added in the directory. A new file name is mapped to an existing inode. paul@RHELv4u4:~/test$ ln file2 hardlink_to_file2 paul@RHELv4u4:~/test$ ls -li total 24 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 hardlink_to_file2 paul@RHELv4u4:~/test$ Both files have the same inode, so they will always have the same permissions and the same owner. Both files will have the same content. Actually, both files are equal now, meaning you can safely remove the original file, the hardlinked file will remain. The inode contains a counter, counting the number of hard links to itself. When the counter drops to zero, then the inode is emptied. finding hard links You can use the find command to look for files with a certain inode. The screenshot below shows how to search for all filenames that point to inode 817270. Remember that an inode number is unique to its partition. paul@RHELv4u4:~/test$ find / -inum 817270 2> /dev/null /home/paul/test/file2 /home/paul/test/hardlink_to_file2 28.4. symbolic links Symbolic links (sometimes called soft links) do not link to inodes, but create a name to name mapping. Symbolic links are created with ln -s. As you can see below, the symbolic link gets an inode of its own. paul@RHELv4u4:~/test$ ln -s file2 symlink_to_file2 paul@RHELv4u4:~/test$ ls -li total 32 817273 -rw-rw-r-- 1 paul paul 13 Feb 5 17:06 file1 817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 hardlink_to_file2 817267 lrwxrwxrwx 1 paul paul 5 Feb 5 16:55 symlink_to_file2 -> file2 paul@RHELv4u4:~/test$ 230
- 237. file links Permissions on a symbolic link have no meaning, since the permissions of the target apply. Hard links are limited to their own partition (because they point to an inode), symbolic links can link anywhere (other file systems, even networked). 28.5. removing links Links can be removed with rm. paul@laika:~$ touch data.txt paul@laika:~$ ln -s data.txt sl_data.txt paul@laika:~$ ln data.txt hl_data.txt paul@laika:~$ rm sl_data.txt paul@laika:~$ rm hl_data.txt 231
- 238. file links 28.6. practice : links 1. Create two files named winter.txt and summer.txt, put some text in them. 2. Create a hard link to winter.txt named hlwinter.txt. 3. Display the inode numbers of these three files, the hard links should have the same inode. 4. Use the find command to list the two hardlinked files 5. Everything about a file is in the inode, except two things : name them! 6. Create a symbolic link to summer.txt called slsummer.txt. 7. Find all files with inode number 2. What does this information tell you ? 8. Look at the directories /etc/init.d/ /etc/rc.d/ /etc/rc3.d/ ... do you see the links ? 9. Look in /lib with ls -l... 10. Use find to look in your home directory for regular files that do not(!) have one hard link. 232
- 239. file links 28.7. solution : links 1. Create two files named winter.txt and summer.txt, put some text in them. echo cold > winter.txt ; echo hot > summer.txt 2. Create a hard link to winter.txt named hlwinter.txt. ln winter.txt hlwinter.txt 3. Display the inode numbers of these three files, the hard links should have the same inode. ls -li winter.txt summer.txt hlwinter.txt 4. Use the find command to list the two hardlinked files find . -inum xyz 5. Everything about a file is in the inode, except two things : name them! The name of the file is in a directory, and the contents is somewhere on the disk. 6. Create a symbolic link to summer.txt called slsummer.txt. ln -s summer.txt slsummer.txt 7. Find all files with inode number 2. What does this information tell you ? It tells you there is more than one inode table (one for every formatted partition + virtual file systems) 8. Look at the directories /etc/init.d/ /etc/rc.d/ /etc/rc3.d/ ... do you see the links ? ls -l /etc/init.d ls -l /etc/rc.d ls -l /etc/rc3.d 9. Look in /lib with ls -l... ls -l /lib 10. Use find to look in your home directory for regular files that do not(!) have one hard link. find ~ ! -links 1 -type f 233
- 240. Part IX. process management
- 241. Chapter 29. introduction to processes Table of Contents 29.1. terminology ................................................................................................ 236 29.2. basic process management ......................................................................... 237 29.3. signalling processes .................................................................................... 241 29.4. practice : basic process management ......................................................... 244 29.5. solution : basic process management ......................................................... 245 235
- 242. introduction to processes 29.1. terminology process A process is compiled source code that is currently running on the system. PID All processes have a process id or PID. PPID Every process has a parent process (with a PPID). The child process is often started by the parent process. init The init process always has process ID 1. The init process is started by the kernel itself so technically it does not have a parent process. init serves as a foster parent for orphaned processes. kill When a process stops running, the process dies, when you want a process to die, you kill it. daemon Processes that start at system startup and keep running forever are called daemon processes or daemons. These daemons never die. zombie When a process is killed, but it still shows up on the system, then the process is referred to as zombie. You cannot kill zombies, because they are already dead. 236
- 243. introduction to processes 29.2. basic process management $$ and $PPID Some shell environment variables contain information about processes. The $$ variable will hold your current process ID, and $PPID contains the parent PID. Actually $$ is a shell parameter and not a variable, you cannot assign a value to it. Below we use echo to display the values of $$ and $PPID. [paul@RHEL4b ~]$ echo $$ $PPID 4224 4223 pidof You can find all process id's by name using the pidof command. root@rhel53 ~# pidof mingetty 2819 2798 2797 2796 2795 2794 parent and child Processes have a parent-child relationship. Every process has a parent process. When starting a new bash you can use echo to verify that the pid from before is the ppid of the new shell. The child process from above is now the parent process. [paul@RHEL4b ~]$ bash [paul@RHEL4b ~]$ echo $$ $PPID 4812 4224 Typing exit will end the current process and brings us back to our original values for $$ and $PPID. [paul@RHEL4b ~]$ echo $$ $PPID 4812 4224 [paul@RHEL4b ~]$ exit exit [paul@RHEL4b ~]$ echo $$ $PPID 4224 4223 [paul@RHEL4b ~]$ 237
- 244. introduction to processes fork and exec A process starts another process in two phases. First the process creates a fork of itself, an identical copy. Then the forked process executes an exec to replace the forked process with the target child process. [paul@RHEL4b ~]$ echo $$ 4224 [paul@RHEL4b ~]$ bash [paul@RHEL4b ~]$ echo $$ $PPID 5310 4224 [paul@RHEL4b ~]$ exec With the exec command, you can execute a process without forking a new process. In the following screenshot a Korn shell (ksh) is started and is being replaced with a bash shell using the exec command. The pid of the bash shell is the same as the pid of the Korn shell. Exiting the child bash shell will get me back to the parent bash, not to the Korn shell (which does not exist anymore). [paul@RHEL4b ~]$ echo $$ 4224 # PID of bash [paul@RHEL4b ~]$ ksh $ echo $$ $PPID 5343 4224 # PID of ksh and bash $ exec bash [paul@RHEL4b ~]$ echo $$ $PPID 5343 4224 # PID of bash and bash [paul@RHEL4b ~]$ exit exit [paul@RHEL4b ~]$ echo $$ 4224 ps One of the most common tools on Linux to look at processes is ps. The following screenshot shows the parent child relationship between three bash processes. [paul@RHEL4b ~]$ echo $$ $PPID 4224 4223 [paul@RHEL4b ~]$ bash [paul@RHEL4b ~]$ echo $$ $PPID 4866 4224 [paul@RHEL4b ~]$ bash [paul@RHEL4b ~]$ echo $$ $PPID 4884 4866 [paul@RHEL4b ~]$ ps fx PID TTY STAT TIME COMMAND 4223 ? S 0:01 sshd: paul@pts/0 238
- 245. introduction to processes 4224 pts/0 Ss 0:00 _ -bash 4866 pts/0 S 0:00 _ bash 4884 pts/0 S 0:00 _ bash 4902 pts/0 R+ 0:00 _ ps fx [paul@RHEL4b ~]$ exit exit [paul@RHEL4b ~]$ ps fx PID TTY STAT TIME COMMAND 4223 ? S 0:01 sshd: paul@pts/0 4224 pts/0 Ss 0:00 _ -bash 4866 pts/0 S 0:00 _ bash 4903 pts/0 R+ 0:00 _ ps fx [paul@RHEL4b ~]$ exit exit [paul@RHEL4b ~]$ ps fx PID TTY STAT TIME COMMAND 4223 ? S 0:01 sshd: paul@pts/0 4224 pts/0 Ss 0:00 _ -bash 4904 pts/0 R+ 0:00 _ ps fx [paul@RHEL4b ~]$ On Linux, ps fax is often used. On Solaris ps -ef (which also works on Linux) is common. Here is a partial output from ps fax. [paul@RHEL4a ~]$ ps fax PID TTY STAT TIME COMMAND 1 ? S 0:00 init [5] ... 3713 ? Ss 0:00 /usr/sbin/sshd 5042 ? Ss 0:00 _ sshd: paul [priv] 5044 ? S 0:00 _ sshd: paul@pts/1 5045 pts/1 Ss 0:00 _ -bash 5077 pts/1 R+ 0:00 _ ps fax pgrep Similar to the ps -C, you can also use pgrep to search for a process by its command name. [paul@RHEL5 ~]$ sleep 1000 & [1] 32558 [paul@RHEL5 ~]$ pgrep sleep 32558 [paul@RHEL5 ~]$ ps -C sleep PID TTY TIME CMD 32558 pts/3 00:00:00 sleep You can also list the command name of the process with pgrep. paul@laika:~$ pgrep -l sleep 9661 sleep 239
- 246. introduction to processes top Another popular tool on Linux is top. The top tool can order processes according to cpu usage or other properties. You can also kill processes from within top. Press h inside top for help. In case of trouble, top is often the first tool to fire up, since it also provides you memory and swap space information. 240
- 247. introduction to processes 29.3. signalling processes kill The kill command will kill (or stop) a process. The screenshot shows how to use a standard kill to stop the process with pid 1942. paul@ubuntu910:~$ kill 1942 paul@ubuntu910:~$ By using the kill we are sending a signal to the process. list signals Running processes can receive signals from each other or from the users. You can have a list of signals by typing kill -l, that is a letter l, not the number 1. [paul@RHEL4a ~]$ kill -l 1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP 6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP 21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ 26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR 31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3 38) SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8 43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13 48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7 58) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2 63) SIGRTMAX-1 64) SIGRTMAX [paul@RHEL4a ~]$ kill -1 (SIGHUP) It is common on Linux to use the first signal SIGHUP (or HUP or 1) to tell a process that it should re-read its configuration file. Thus, the kill -1 1 command forces the init process (init always runs with pid 1) to re-read its configuration file. root@deb503:~# kill -1 1 root@deb503:~# It is up to the developer of the process to decide whether the process can do this running, or whether it needs to stop and start. It is up to the user to read the documentation of the program. 241
- 248. introduction to processes kill -15 (SIGTERM) The SIGTERM signal is also called a standard kill. Whenever kill is executed without specifying the signal, a kill -15 is assumed. Both commands in the screenshot below are identical. paul@ubuntu910:~$ kill 1942 paul@ubuntu910:~$ kill -15 1942 kill -9 (SIGKILL) The SIGKILL is different from most other signals in that it is not being sent to the process, but to the Linux kernel. A kill -9 is also called a sure kill. The kernel will shoot down the process. As a developer you have no means to intercept a kill -9 signal. root@rhel53 ~# kill -9 3342 killall The killall command will also default to sending a signal 15 to the processes. This command and its SysV counterpart killall5 can by used when shutting down the system. This screenshot shows how Red Hat Enterprise Linux 5.3 uses killall5 when halting the system. root@rhel53 ~# grep killall /etc/init.d/halt action $"Sending all processes the TERM signal..." /sbin/killall5 -15 action $"Sending all processes the KILL signal..." /sbin/killall5 -9 pkill You can use the pkill command to kill a process by its command name. [paul@RHEL5 ~]$ sleep 1000 & [1] 30203 [paul@RHEL5 ~]$ pkill sleep [1]+ Terminated sleep 1000 [paul@RHEL5 ~]$ top Inside top the k key allows you to select a signal and pid to kill. Below is a partial screenshot of the line just below the summary in top after pressing k. 242
- 249. introduction to processes PID to kill: 1932 Kill PID 1932 with signal [15]: 9 SIGSTOP and SIGCONT A running process can be suspended when it receives a SIGSTOP signal. This is the same as kill -19 on Linux, but might have a different number in other Unix systems. A suspended process does not use any cpu cycles, but it stays in memory and can be re-animated with a SIGCONT signal (kill -18 on Linux). Both signals will be used in the section about background processes. 243
- 250. introduction to processes 29.4. practice : basic process management 1. Use ps to search for the init process by name. 2. What is the process id of the init process ? 3. Use the who am i command to determine your terminal name. 4. Using your terminal name from above, use ps to find all processes associated with your terminal. 5. What is the process id of your shell ? 6. What is the parent process id of your shell ? 7. Start two instances of the sleep 3342 in background. 8. Locate the process id of all sleep commands. 9. Display only those two sleep processes in top. Then quit top. 10. Use a standard kill to kill one of the sleep processes. 11. Use one command to kill all sleep processes. 244
- 251. introduction to processes 29.5. solution : basic process management 1. Use ps to search for the init process by name. root@rhel53 ~# ps -C init PID TTY TIME CMD 1 ? 00:00:04 init 2. What is the process id of the init process ? 1 3. Use the who am i command to determine your terminal name. root@rhel53 ~# who am i paul pts/0 2010-04-12 17:44 (192.168.1.38) 4. Using your terminal name from above, use ps to find all processes associated with your terminal. oot@rhel53 ~# ps fax | grep pts/0 2941 ? S 0:00 _ sshd: paul@pts/0 2942 pts/0 Ss 0:00 _ -bash 2972 pts/0 S 0:00 _ su - 2973 pts/0 S 0:00 _ -bash 3808 pts/0 R+ 0:00 _ ps fax 3809 pts/0 R+ 0:00 _ grep pts/0 or also root@rhel53 ~# ps -ef | grep pts/0 paul 2941 2939 0 17:44 ? 00:00:00 sshd: paul@pts/0 paul 2942 2941 0 17:44 pts/0 00:00:00 -bash root 2972 2942 0 17:45 pts/0 00:00:00 su - root 2973 2972 0 17:45 pts/0 00:00:00 -bash root 3816 2973 0 21:25 pts/0 00:00:00 ps -ef root 3817 2973 0 21:25 pts/0 00:00:00 grep pts/0 5. What is the process id of your shell ? 2973 in the screenshot above, probably different for you echo $$ should display same number as the one you found 6. What is the parent process id of your shell ? 2972 in the screenshot above, probably different for you in this example the PPID is from the su - command, but when inside gnome then for example gnome-terminal can be the parent process 7. Start two instances of the sleep 3342 in background. 245
- 252. introduction to processes sleep 3342 & sleep 3342 & 8. Locate the process id of all sleep commands. pidof sleep 9. Display only those two sleep processes in top. Then quit top. top -p pidx,pidy (replace pidx pidy with the actual numbers) 10. Use a standard kill to kill one of the sleep processes. kill pidx 11. Use one command to kill all sleep processes. pkill sleep 246
- 253. Chapter 30. process priorities Table of Contents 30.1. priority and nice values ............................................................................. 248 30.2. practice : process priorities ........................................................................ 251 30.3. solution : process priorities ........................................................................ 252 247
- 254. process priorities 30.1. priority and nice values introduction All processes have a priority and a nice value. Higher priority processes will get more cpu time than lower priority processes. You can influence this with the nice and renice commands. pipes (mkfifo) Processes can communicate with each other via pipes. These pipes can be created with the mkfifo command. The screenshots shows the creation of four distinct pipes (in a new directory). paul@ubuntu910:~$ mkdir procs paul@ubuntu910:~$ cd procs/ paul@ubuntu910:~/procs$ mkfifo pipe33a pipe33b pipe42a pipe42b paul@ubuntu910:~/procs$ ls -l total 0 prw-r--r-- 1 paul paul 0 2010-04-12 13:21 pipe33a prw-r--r-- 1 paul paul 0 2010-04-12 13:21 pipe33b prw-r--r-- 1 paul paul 0 2010-04-12 13:21 pipe42a prw-r--r-- 1 paul paul 0 2010-04-12 13:21 pipe42b paul@ubuntu910:~/procs$ some fun with cat To demonstrate the use of the top and renice commands we will make the cat command use the previously created pipes to generate a full load on the cpu. The cat is copied with a distinct name to the current directory. (This enables us to easily recognize the processes within top. You could do the same exercise without copying the cat command, but using different users. Or you could just look at the pid of each process.) paul@ubuntu910:~/procs$ cp /bin/cat proj33 paul@ubuntu910:~/procs$ cp /bin/cat proj42 paul@ubuntu910:~/procs$ echo -n x | ./proj33 - pipe33a > pipe33b & [1] 1670 paul@ubuntu910:~/procs$ ./proj33 <pipe33b >pipe33a & [2] 1671 paul@ubuntu910:~/procs$ echo -n z | ./proj42 - pipe42a > pipe42b & [3] 1673 paul@ubuntu910:~/procs$ ./proj42 <pipe42b >pipe42a & [4] 1674 The commands you see above will create two proj33 processes that use cat to bounce the x character between pipe33a and pipe33b. And ditto for the z character and proj42. 248
- 255. process priorities top Just running top without options or arguments will display all processes and an overview of innformation. The top of the top screen might look something like this. top - 13:59:29 up 48 min, 4 users, load average: 1.06, 0.25, 0.14 Tasks: 139 total, 3 running, 136 sleeping, 0 stopped, 0 zombie Cpu(s): 0.3%us, 99.7%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st Mem: 509352k total, 460040k used, 49312k free, 66752k buffers Swap: 746980k total, 0k used, 746980k free, 247324k cached Notice the cpu idle time (0.0%id) is zero. This is because our cat processes are consuming the whole cpu. Results can vary on systems with four or more cpu cores. top -p The top -p 1670,1671,1673,1674 screenshot below shows four processes, all of then using approximately 25 percent of the cpu. paul@ubuntu910:~$ top -p 1670,1671,1673,1674 PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1674 paul 20 0 2972 616 524 S 26.6 0.1 0:11.92 proj42 1670 paul 20 0 2972 616 524 R 25.0 0.1 0:23.16 proj33 1671 paul 20 0 2972 616 524 S 24.6 0.1 0:23.07 proj33 1673 paul 20 0 2972 620 524 R 23.0 0.1 0:11.48 proj42 All four processes have an equal priority (PR), and are battling for cpu time. On some systems the Linux kernel might attribute slightly varying priority values, but the result will still be four processes fighting for cpu time. renice Since the processes are already running, we need to use the renice command to change their nice value (NI). The screenshot shows how to use renice on both the proj33 processes. paul@ubuntu910:~$ renice +8 1670 1670: old priority 0, new priority 8 paul@ubuntu910:~$ renice +8 1671 1671: old priority 0, new priority 8 Normal users can attribute a nice value from zero to 20 to processes they own. Only the root user can use negative nice values. Be very careful with negative nice values, since they can make it impossible to use the keyboard or ssh to a system. 249
- 256. process priorities impact of nice values The impact of a nice value on running processes can vary. The screenshot below shows the result of our renice +8 command. Look at the %CPU values. PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1674 paul 20 0 2972 616 524 S 46.6 0.1 0:22.37 proj42 1673 paul 20 0 2972 620 524 R 42.6 0.1 0:21.65 proj42 1671 paul 28 8 2972 616 524 S 5.7 0.1 0:29.65 proj33 1670 paul 28 8 2972 616 524 R 4.7 0.1 0:29.82 proj33 Important to remember is to always make less important processes nice to more important processes. Using negative nice values can have a serere impact on a system's usability. nice The nice works identical to the renice but it is used when starting a command. The screenshot shows how to start a script with a nice value of five. paul@ubuntu910:~$ nice -5 ./backup.sh 250
- 257. process priorities 30.2. practice : process priorities 1. Create a new directory and create six pipes in that directory. 2. Bounce a character between two pipes. 3. Use top and ps to display information (pid, ppid, priority, nice value, ...) about these two cat processes. 4. Bounce another character between two other pipes, but this time start the commands nice. Verify that all cat processes are battling for the cpu. (Feel free to fire up two more cats with the remaining pipes). 5. Use ps to verify that the two new cat processes have a nice value. Use the -o and -C options of ps for this. 6. Use renice te increase the nice value from 10 to 15. Notice the difference with the usual commands. 251
- 258. process priorities 30.3. solution : process priorities 1. Create a new directory and create six pipes in that directory. [paul@rhel53 ~]$ mkdir pipes ; cd pipes [paul@rhel53 pipes]$ mkfifo p1 p2 p3 p4 p5 p6 [paul@rhel53 pipes]$ ls -l total 0 prw-rw-r-- 1 paul paul 0 Apr 12 22:15 p1 prw-rw-r-- 1 paul paul 0 Apr 12 22:15 p2 prw-rw-r-- 1 paul paul 0 Apr 12 22:15 p3 prw-rw-r-- 1 paul paul 0 Apr 12 22:15 p4 prw-rw-r-- 1 paul paul 0 Apr 12 22:15 p5 prw-rw-r-- 1 paul paul 0 Apr 12 22:15 p6 2. Bounce a character between two pipes. [paul@rhel53 pipes]$ echo -n x | cat - p1 > p2 & [1] 4013 [paul@rhel53 pipes]$ cat <p2 >p1 & [2] 4016 3. Use top and ps to display information (pid, ppid, priority, nice value, ...) about these two cat processes. top (probably the top two lines) [paul@rhel53 pipes]$ ps -C cat PID TTY TIME CMD 4013 pts/0 00:03:38 cat 4016 pts/0 00:01:07 cat [paul@rhel53 pipes]$ ps fax | grep cat 4013 pts/0 R 4:00 | _ cat - p1 4016 pts/0 S 1:13 | _ cat 4044 pts/0 S+ 0:00 | _ grep cat 4. Bounce another character between two other pipes, but this time start the commands nice. Verify that all cat processes are battling for the cpu. (Feel free to fire up two more cats with the remaining pipes). echo -n y | nice cat - p3 > p4 & nice cat <p4 >p3 & 5. Use ps to verify that the two new cat processes have a nice value. Use the -o and -C options of ps for this. [paul@rhel53 pipes]$ ps -C cat -o pid,ppid,pri,ni,comm PID PPID PRI NI COMMAND 4013 3947 14 0 cat 4016 3947 21 0 cat 4025 3947 13 10 cat 4026 3947 13 10 cat 6. Use renice te increase the nice value from 10 to 15. Notice the difference with the usual commands. [paul@rhel53 pipes]$ renice +15 4025 4025: old priority 10, new priority 15 [paul@rhel53 pipes]$ renice +15 4026 252
- 259. process priorities 4026: old priority 10, new priority 15 [paul@rhel53 pipes]$ ps -C cat -o pid,ppid,pri,ni,comm PID PPID PRI NI COMMAND 4013 3947 14 0 cat 4016 3947 21 0 cat 4025 3947 9 15 cat 4026 3947 8 15 cat 253
- 260. Chapter 31. background jobs Table of Contents 31.1. background processes ................................................................................ 255 31.2. practice : background processes ................................................................. 257 31.3. solution : background processes ................................................................ 258 254
- 261. background jobs 31.1. background processes jobs Stuff that runs in background of your current shell can be displayed with the jobs command. By default you will not have any jobs running in background. root@rhel53 ~# jobs root@rhel53 ~# This jobs command will be used several times in this section. control-Z Some processes can be suspended with the Ctrl-Z key combination. This sends a SIGSTOP signal to the Linux kernel, effectively freezing the operation of the process. When doing this in vi(m), then vi(m) goes to the background. The background vi(m) can be seen with the jobs command. [paul@RHEL4a ~]$ vi procdemo.txt [5]+ Stopped vim procdemo.txt [paul@RHEL4a ~]$ jobs [5]+ Stopped vim procdemo.txt & ampersand Processes that are started in background using the & character at the end of the command line are also visible with the jobs command. [paul@RHEL4a ~]$ find / > allfiles.txt 2> /dev/null & [6] 5230 [paul@RHEL4a ~]$ jobs [5]+ Stopped vim procdemo.txt [6]- Running find / >allfiles.txt 2>/dev/null & [paul@RHEL4a ~]$ jobs -p An interesting option is jobs -p to see the process id of background processes. [paul@RHEL4b ~]$ sleep 500 & 255
- 262. background jobs [1] 4902 [paul@RHEL4b ~]$ sleep 400 & [2] 4903 [paul@RHEL4b ~]$ jobs -p 4902 4903 [paul@RHEL4b ~]$ ps `jobs -p` PID TTY STAT TIME COMMAND 4902 pts/0 S 0:00 sleep 500 4903 pts/0 S 0:00 sleep 400 [paul@RHEL4b ~]$ fg Running the fg command will bring a background job to the foreground. The number of the background job to bring forward is the parameter of fg. [paul@RHEL5 ~]$ jobs [1] Running sleep 1000 & [2]- Running sleep 1000 & [3]+ Running sleep 2000 & [paul@RHEL5 ~]$ fg 3 sleep 2000 bg Jobs that are suspended in background can be started in background with bg. The bg will send a SIGCONT signal. Below an example of the sleep command (suspended with Ctrl-Z) being reactivated in background with bg. [paul@RHEL5 ~]$ jobs [paul@RHEL5 ~]$ sleep 5000 & [1] 6702 [paul@RHEL5 ~]$ sleep 3000 [2]+ Stopped sleep 3000 [paul@RHEL5 ~]$ jobs [1]- Running sleep 5000 & [2]+ Stopped sleep 3000 [paul@RHEL5 ~]$ bg 2 [2]+ sleep 3000 & [paul@RHEL5 ~]$ jobs [1]- Running sleep 5000 & [2]+ Running sleep 3000 & [paul@RHEL5 ~]$ 256
- 263. background jobs 31.2. practice : background processes 1. Use the jobs command to verify whether you have any processes running in background. 2. Use vi to create a little text file. Suspend vi in background. 3. Verify with jobs that vi is suspended in background. 4. Start find / > allfiles.txt 2>/dev/null in foreground. Suspend it in background before it finishes. 5. Start two long sleep processes in background. 6. Display all jobs in background. 7. Use the kill command to suspend the last sleep process. 8. Continue the find process in background (make sure it runs again). 9. Put one of the sleep commands back in foreground. 10. (if time permits, a general review question...) Explain in detail where the numbers come from in the next screenshot. When are the variables replaced by their value ? By which shell ? [paul@RHEL4b ~]$ echo $$ $PPID 4224 4223 [paul@RHEL4b ~]$ bash -c "echo $$ $PPID" 4224 4223 [paul@RHEL4b ~]$ bash -c 'echo $$ $PPID' 5059 4224 [paul@RHEL4b ~]$ bash -c `echo $$ $PPID` 4223: 4224: command not found 257
- 264. background jobs 31.3. solution : background processes 1. Use the jobs command to verify whether you have any processes running in background. jobs (maybe the catfun is still running?) 2. Use vi to create a little text file. Suspend vi in background. vi text.txt (inside vi press ctrl-z) 3. Verify with jobs that vi is suspended in background. [paul@rhel53 ~]$ jobs [1]+ Stopped vim text.txt 4. Start find / > allfiles.txt 2>/dev/null in foreground. Suspend it in background before it finishes. [paul@rhel53 ~]$ find / > allfiles.txt 2>/dev/null (press ctrl-z) [2]+ Stopped find / > allfiles.txt 2> /dev/null 5. Start two long sleep processes in background. sleep 4000 & ; sleep 5000 & 6. Display all jobs in background. [paul@rhel53 ~]$ jobs [1]- Stopped vim text.txt [2]+ Stopped find / > allfiles.txt 2> /dev/null [3] Running sleep 4000 & [4] Running sleep 5000 & 7. Use the kill command to suspend the last sleep process. [paul@rhel53 ~]$ kill -SIGSTOP 4519 [paul@rhel53 ~]$ jobs [1] Stopped vim text.txt [2]- Stopped find / > allfiles.txt 2> /dev/null [3] Running sleep 4000 & [4]+ Stopped sleep 5000 8. Continue the find process in background (make sure it runs again). bg 2 (verify the job-id in your jobs list) 9. Put one of the sleep commands back in foreground. fg 3 (again verify your job-id) 10. (if time permits, a general review question...) Explain in detail where the numbers come from in the next screenshot. When are the variables replaced by their value ? By which shell ? 258
- 265. background jobs [paul@RHEL4b ~]$ echo $$ $PPID 4224 4223 [paul@RHEL4b ~]$ bash -c "echo $$ $PPID" 4224 4223 [paul@RHEL4b ~]$ bash -c 'echo $$ $PPID' 5059 4224 [paul@RHEL4b ~]$ bash -c `echo $$ $PPID` 4223: 4224: command not found The current bash shell will replace the $$ and $PPID while scanning the line, and before executing the echo command. [paul@RHEL4b ~]$ echo $$ $PPID 4224 4223 The variables are now double quoted, but the current bash shell will replace $$ and $PPID while scanning the line, and before executing the bach -c command. [paul@RHEL4b ~]$ bash -c "echo $$ $PPID" 4224 4223 The variables are now single quoted. The current bash shell will not replace the $$ and the $PPID. The bash -c command will be executed before the variables replaced with their value. This latter bash is the one replacing the $$ and $PPID with their value. [paul@RHEL4b ~]$ bash -c 'echo $$ $PPID' 5059 4224 With backticks the shell will still replace both variable before the embedded echo is executed. The result of this echo is the two process id's. These are given as commands to bash -c. But two numbers are not commands! [paul@RHEL4b ~]$ bash -c `echo $$ $PPID` 4223: 4224: command not found 259
- 266. Part X. disk management
- 267. Chapter 32. disk devices Table of Contents 32.1. terminology ................................................................................................ 262 32.2. device naming ............................................................................................ 264 32.3. discovering disk devices ............................................................................ 265 32.4. erasing a hard disk ..................................................................................... 269 32.5. advanced hard disk settings ....................................................................... 270 32.6. practice: hard disk devices ......................................................................... 271 32.7. solution: hard disk devices ........................................................................ 272 This chapter teaches you how to locate and recognise hard disk devices. This prepares you for the next chapter, where we put partitions on these devices. 261
- 268. disk devices 32.1. terminology platter, head, track, cylinder, sector Data is commonly stored on magnetic or optical disk platters. The platters are rotated (at high speeds). Data is read by heads, which are very close to the surface of the platter, without touching it! The heads are mounted on an arm (sometimes called a comb or a fork). Data is written in concentric circles called tracks. Track zero is (usually) on the outside. The time it takes to position the head over a certain track is called the seek time. Often the platters are stacked on top of each other, hence the set of tracks accessible at a certain position of the comb forms a cylinder. Tracks are divided into 512 byte sectors, with more unused space (gap) between the sectors on the outside of the platter. When you break down the advertised access time of a hard drive, you will notice that most of that time is taken by movement of the heads (about 65%) and rotational latency (about 30%). block device Random access hard disk devices have an abstraction layer called block device to enable formatting in fixed-size (usually 512 bytes) blocks. Blocks can be accessed independent of access to other blocks. A block device has the letter b to denote the file type in the output of ls -l. [root@RHEL4b ~]# ls -l /dev/sda* brw-rw---- 1 root disk 8, 0 Aug 4 22:55 /dev/sda brw-rw---- 1 root disk 8, 1 Aug 4 22:55 /dev/sda1 brw-rw---- 1 root disk 8, 2 Aug 4 22:55 /dev/sda2 [root@RHEL4b ~]# Note that a character device is a constant stream of characters, being denoted by a c in ls -l. Note also that the ISO 9660 standard for cdrom uses a 2048 byte block size. Old hard disks (and floppy disks) use cylinder-head-sector addressing to access a sector on the disk. Most current disks use LBA (Logical Block Addressing). ide or scsi Actually, the title should be ata or scsi, since ide is an ata compatible device. Most desktops use ata devices, most servers use scsi. 262
- 269. disk devices ata An ata controller allows two devices per bus, one master and one slave. Unless your controller and devices support cable select, you have to set this manually with jumpers. With the introduction of sata (serial ata), the original ata was renamed to parallel ata. Optical drives often use atapi, which is an ATA interface using the SCSI communication protocol. scsi A scsi controller allows more than two devices. When using SCSI (small computer system interface), each device gets a unique scsi id. The scsi controller also needs a scsi id, do not use this id for a scsi-attached device. Older 8-bit SCSI is now called narrow, whereas 16-bit is wide. When the bus speeds was doubled to 10Mhz, this was known as fast SCSI. Doubling to 20Mhz made it ultra SCSI. Take a look at http://en.wikipedia.org/wiki/SCSI for more SCSI standards. 263
- 270. disk devices 32.2. device naming ata (ide) device naming All ata drives on your system will start with /dev/hd followed by a unit letter. The master hdd on the first ata controller is /dev/hda, the slave is /dev/hdb. For the second controller, the names of the devices are /dev/hdc and /dev/hdd. Table 32.1. ide device naming controller connection device name master /dev/hda ide0 slave /dev/hdb master /dev/hdc ide1 slave /dev/hdd It is possible to have only /dev/hda and /dev/hdd. The first one is a single ata hard disk, the second one is the cdrom (by default configured as slave). scsi device naming scsi drives follow a similar scheme, but all start with /dev/sd. When you run out of letters (after /dev/sdz), you can continue with /dev/sdaa and /dev/sdab and so on. (We will see later on that lvm volumes are commonly seen as /dev/md0, /dev/md1 etc.) Below a sample of how scsi devices on a linux can be named. Adding a scsi disk or raid controller with a lower scsi address will change the naming scheme (shifting the higher scsi addresses one letter further in the alphabet). Table 32.2. scsi device naming device scsi id device name disk 0 0 /dev/sda disk 1 1 /dev/sdb raid controller 0 5 /dev/sdc raid controller 1 6 /dev/sdd 264
- 271. disk devices 32.3. discovering disk devices /sbin/fdisk You can start by using /sbin/fdisk to find out what kind of disks are seen by the kernel. Below the result on Debian, with two ata-ide disks present. root@barry:~# fdisk -l | grep Disk Disk /dev/hda: 60.0 GB, 60022480896 bytes Disk /dev/hdb: 81.9 GB, 81964302336 bytes And here an example of sata disks on a laptop with Ubuntu. Remember that sata disks are presented to you with the scsi /dev/sdx notation. root@laika:~# fdisk -l | grep Disk Disk /dev/sda: 100.0 GB, 100030242816 bytes Disk /dev/sdb: 100.0 GB, 100030242816 bytes Here is an overview of disks on a RHEL4u3 server with two real 72GB scsi disks. This server is attached to a NAS with four NAS disks of half a terabyte. On the NAS disks, four LVM (/dev/mdx) software RAID devices are configured. [root@tsvtl1 ~]# fdisk -l | grep Disk Disk /dev/sda: 73.4 GB, 73407488000 bytes Disk /dev/sdb: 73.4 GB, 73407488000 bytes Disk /dev/sdc: 499.0 GB, 499036192768 bytes Disk /dev/sdd: 499.0 GB, 499036192768 bytes Disk /dev/sde: 499.0 GB, 499036192768 bytes Disk /dev/sdf: 499.0 GB, 499036192768 bytes Disk /dev/md0: 271 MB, 271319040 bytes Disk /dev/md2: 21.4 GB, 21476081664 bytes Disk /dev/md3: 21.4 GB, 21467889664 bytes Disk /dev/md1: 21.4 GB, 21476081664 bytes You can also use fdisk to obtain information about one specific hard disk device. [root@rhel4 ~]# fdisk -l /dev/sda Disk /dev/sda: 12.8 GB, 12884901888 bytes 255 heads, 63 sectors/track, 1566 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Device Boot Start End Blocks Id System /dev/sda1 * 1 13 104391 83 Linux /dev/sda2 14 1566 12474472+ 8e Linux LVM Later we will use fdisk to do dangerous stuff like creating and deleting partitions. 265
- 272. disk devices /bin/dmesg Kernel boot messages can be seen after boot with dmesg. Since hard disk devices are detected by the kernel during boot, you can also use dmesg to find information about disk devices. root@barry:~# dmesg | grep "[hs]d[a-z]" Kernel command line: root=/dev/hda1 ro ide0: BM-DMA at 0xfc00-0xfc07, BIOS settings: hda:DMA, hdb:DMA ide1: BM-DMA at 0xfc08-0xfc0f, BIOS settings: hdc:DMA, hdd:DMA hda: ST360021A, ATA DISK drive hdb: Maxtor 6Y080L0, ATA DISK drive hdc: SONY DVD RW DRU-510A, ATAPI CD/DVD-ROM drive hdd: SONY DVD RW DRU-810A, ATAPI CD/DVD-ROM drive hda: max request size: 128KiB hda: 117231408 sectors (60022 MB) w/2048KiB Cache, CHS=65535/16/63, UDMA hda: hda1 hda2 hdb: max request size: 128KiB hdb: 160086528 sectors (81964 MB) w/2048KiB Cache, CHS=65535/16/63, UDMA hdb: hdb1 hdb2 hdc: ATAPI 32X DVD-ROM DVD-R CD-R/RW drive, 8192kB Cache, UDMA(33) hdd: ATAPI 40X DVD-ROM DVD-R CD-R/RW drive, 2048kB Cache, UDMA(33) ... Here's another example of dmesg (same computer as above, but with extra 200gb disk now). paul@barry:~$ dmesg | grep -i "ata disk" [ 2.624149] hda: ST360021A, ATA DISK drive [ 2.904150] hdb: Maxtor 6Y080L0, ATA DISK drive [ 3.472148] hdd: WDC WD2000BB-98DWA0, ATA DISK drive Third and last example of dmesg running on RHEL5.3. root@rhel53 ~# dmesg | grep -i "scsi disk" sd 0:0:2:0: Attached scsi disk sda sd 0:0:3:0: Attached scsi disk sdb sd 0:0:6:0: Attached scsi disk sdc /sbin/lshw The lshw tool will list hardware. With the right options lshw can show a lot of information about disks (and partitions). Below a truncated screenshot on Debian 5: root@debian5:~# aptitude search lshw p lshw - information about hardware configuration p lshw-gtk - information about hardware configuration root@debian5:~# aptitude install lshw ... root@debian5:~# lshw -class volume *-volume:0 266
- 273. disk devices description: EXT3 volume vendor: Linux physical id: 1 bus info: ide@0.0,1 logical name: /dev/hda1 logical name: / version: 1.0 serial: f327ca8a-8187-48c5-b760-956ec79d414b size: 19GiB capacity: 19GiB capabilities: primary bootable journaled extended_attributes lar ge_files huge_files recover ext3 ext2 initialized configuration: created=2009-10-28 12:02:35 filesystem=ext3 ... ... Below a screenshot of lshw running Ubuntu 10.10 on a macbook pro: root@ubu1010:~# lshw -class volume *-volume:0 UNCLAIMED description: EFI GPT partition physical id: 1 bus info: scsi@0:0.0.0,1 capacity: 2047KiB capabilities: primary nofs *-volume:1 description: EXT4 volume vendor: Linux physical id: 2 bus info: scsi@0:0.0.0,2 logical name: /dev/sda2 logical name: / version: 1.0 serial: 101eb20f-3e25-4900-b988-4622c0ee4ff5 size: 142GiB capacity: 142GiB ... /sbin/lsscsi The /sbin/lsscsi will give you a nice readable output of all scsi (and scsi emulated devices). This first screenshot shows lsscsi on a SPARC system. root@shaka:~# lsscsi [0:0:0:0] disk Adaptec RAID5 V1.0 /dev/sda [1:0:0:0] disk SEAGATE ST336605FSUN36G 0438 /dev/sdb root@shaka:~# Here is the same command, but run on a laptop with scsi emulated dvd writer and scsi emulated usb. paul@laika:~$ lsscsi [0:0:0:0] disk ATA HTS721010G9SA00 MCZO /dev/sda [1:0:0:0] disk ATA HTS721010G9SA00 MCZO /dev/sdb [3:0:0:0] cd/dvd _NEC DVD_RW ND-7551A 1-02 /dev/scd0 [4:0:0:0] disk GENERIC USB Storage-CFC 019A /dev/sdc [4:0:0:1] disk GENERIC USB Storage-SDC 019A /dev/sdd [4:0:0:2] disk GENERIC USB Storage-SMC 019A /dev/sde [4:0:0:3] disk GENERIC USB Storage-MSC 019A /dev/sdf 267
- 274. disk devices /proc/scsi/scsi Another way to locate scsi devices is via the /proc/scsi/scsi file. root@shaka:~# cat /proc/scsi/scsi Attached devices: Host: scsi0 Channel: 00 Id: 00 Lun: 00 Vendor: Adaptec Model: RAID5 Rev: V1.0 Type: Direct-Access ANSI SCSI revision: 02 Host: scsi1 Channel: 00 Id: 00 Lun: 00 Vendor: SEAGATE Model: ST336605FSUN36G Rev: 0438 Type: Direct-Access ANSI SCSI revision: 03 root@shaka:~# /sbin/scsi_info and /sbin/scsiinfo There is also a scsi_info command, but this is not always installed by default. root@shaka:~# scsi_info /dev/sdb SCSI_ID="0,0,0" HOST="1" MODEL="SEAGATE ST336605FSUN36G" FW_REV="0438" root@shaka:~# Another simple tool is scsiinfo which is a part of scsitools (also not installed by default). root@debian5:~# scsiinfo -l /dev/sda /dev/sdb /dev/sdc 268
- 275. disk devices 32.4. erasing a hard disk Before selling your old hard disk on the internet, it might be a good idea to erase it. By simply repartitioning, by using the Microsoft Windows format utility, or even after an mkfs command, some people will still be able to read most of the data on the disk. Although technically the /sbin/badblocks tool is meant to look for bad blocks, you can use it to completely erase all data from a disk. Since this is really writing to every sector of the disk, it can take a long time! root@RHELv4u2:~# badblocks -ws /dev/sdb Testing with pattern 0xaa: done Reading and comparing: done Testing with pattern 0x55: done Reading and comparing: done Testing with pattern 0xff: done Reading and comparing: done Testing with pattern 0x00: done Reading and comparing: done 269
- 276. disk devices 32.5. advanced hard disk settings Tweaking of hard disk settings (dma, gap, ...) are not covered in this course. Several tools exists, hdparm and sdparm are two of them. /sbin/hdparm can be used to display or set information and parameters about an ATA (or SATA) hard disk device. The -i and -I options will give you even more information about the physical properties of the device. root@laika:~# hdparm /dev/sdb /dev/sdb: IO_support = 0 (default 16-bit) readonly = 0 (off) readahead = 256 (on) geometry = 12161/255/63, sectors = 195371568, start = 0 Below hdparm info about a 200GB IDE disk. root@barry:~# hdparm /dev/hdd /dev/hdd: multcount = 0 (off) IO_support = 0 (default) unmaskirq = 0 (off) using_dma = 1 (on) keepsettings = 0 (off) readonly = 0 (off) readahead = 256 (on) geometry = 24321/255/63, sectors = 390721968, start = 0 Here a screenshot of sdparm on Ubuntu 10.10. root@ubu1010:~# aptitude install sdparm ... root@ubu1010:~# sdparm /dev/sda | head -1 /dev/sda: ATA FUJITSU MJA2160B 0081 root@ubu1010:~# man sdparm Use hdparm and sdparm with care. 270
- 277. disk devices 32.6. practice: hard disk devices About this lab: To practice working with hard disks, you will need some hard disks. When there are no physical hard disk available, you can use virtual disks in vmware or VirtualBox. The teacher will help you in attaching a couple of ATA and/or SCSI disks to a virtual machine. The results of this lab can be used in the next three labs (partitions, file systems, mounting). It is adviced to attach at least one ide and three equally sized scsi disks to the virtual machine. 1. Use dmesg to make a list of hard disk devices detected at boot-up. 2. Use fdisk to find the total size of all hard disk devices on your system. 3. Stop a virtual machine, add three virtual 1 gigabyte scsi hard disk devices and one virtual 400 megabyte ide hard disk device. If possible, also add another virtual 400 megabyte ide disk. 4. Use dmesg to verify that all the new disks are properly detected at boot-up. 5. Verify that you can see the disk devices in /dev. 6. Use fdisk (with grep and /dev/null) to display the total size of the new disks. 7. Use badblocks to completely erase one of the smaller hard disks. 8. Look at /proc/scsi/scsi. 9. If possible, install lsscsi, lshw and use them to list the disks. 271
- 278. disk devices 32.7. solution: hard disk devices 1. Use dmesg to make a list of hard disk devices detected at boot-up. Some possible answers... dmesg | grep -i disk Looking for ATA disks: dmesg | grep hd[abcd] Looking for ATA disks: dmesg | grep -i "ata disk" Looking for SCSI disks: dmesg | grep sd[a-f] Looking for SCSI disks: dmesg | grep -i "scsi disk" 2. Use fdisk to find the total size of all hard disk devices on your system. fdisk -l 3. Stop a virtual machine, add three virtual 1 gigabyte scsi hard disk devices and one virtual 400 megabyte ide hard disk device. If possible, also add another virtual 400 megabyte ide disk. This exercise happens in the settings of vmware or VirtualBox. 4. Use dmesg to verify that all the new disks are properly detected at boot-up. See 1. 5. Verify that you can see the disk devices in /dev. SCSI+SATA: ls -l /dev/sd* ATA: ls -l /dev/hd* 6. Use fdisk (with grep and /dev/null) to display the total size of the new disks. root@rhel53 ~# fdisk -l 2>/dev/null | grep [MGT]B Disk /dev/hda: 21.4 GB, 21474836480 bytes Disk /dev/hdb: 1073 MB, 1073741824 bytes Disk /dev/sda: 2147 MB, 2147483648 bytes Disk /dev/sdb: 2147 MB, 2147483648 bytes Disk /dev/sdc: 2147 MB, 2147483648 bytes 7. Use badblocks to completely erase one of the smaller hard disks. #Verify the device (/dev/sdc??) you want to erase before typing this. # root@rhel53 ~# badblocks -ws /dev/sdc Testing with pattern 0xaa: done Reading and comparing: done Testing with pattern 0x55: done Reading and comparing: done Testing with pattern 0xff: done Reading and comparing: done Testing with pattern 0x00: done Reading and comparing: done 8. Look at /proc/scsi/scsi. root@rhel53 ~# cat /proc/scsi/scsi 272
- 279. disk devices Attached devices: Host: scsi0 Channel: 00 Id: 02 Lun: 00 Vendor: VBOX Model: HARDDISK Rev: 1.0 Type: Direct-Access ANSI SCSI revision: 05 Host: scsi0 Channel: 00 Id: 03 Lun: 00 Vendor: VBOX Model: HARDDISK Rev: 1.0 Type: Direct-Access ANSI SCSI revision: 05 Host: scsi0 Channel: 00 Id: 06 Lun: 00 Vendor: VBOX Model: HARDDISK Rev: 1.0 Type: Direct-Access ANSI SCSI revision: 05 9. If possible, install lsscsi, lshw and use them to list the disks. Debian,Ubuntu: aptitude install lsscsi lshw Fedora: yum install lsscsi lshw root@rhel53 ~# lsscsi [0:0:2:0] disk VBOX HARDDISK 1.0 /dev/sda [0:0:3:0] disk VBOX HARDDISK 1.0 /dev/sdb [0:0:6:0] disk VBOX HARDDISK 1.0 /dev/sdc 273
- 280. Chapter 33. disk partitions Table of Contents 33.1. about partitions ........................................................................................... 275 33.2. discovering partitions ................................................................................. 276 33.3. partitioning new disks ................................................................................ 277 33.4. about the partition table ............................................................................. 279 33.5. practice: partitions ...................................................................................... 280 33.6. solution: partitions ..................................................................................... 281 This chapter continues on the hard disk devices from the previous one. Here we will put partitions on those devices. This chapter prepares you for the next chapter, where we put file systems on our partitions. 274
- 281. disk partitions 33.1. about partitions primary, extended and logical Linux requires you to create one or more partitions. The next paragraphs will explain how to create and use partitions. A partition's geometry and size is usually defined by a starting and ending cylinder (sometimes by sector). Partitions can be of type primary (maximum four), extended (maximum one) or logical (contained within the extended partition). Each partition has a type field that contains a code. This determines the computers operating system or the partitions file system. Table 33.1. primary, extended and logical partitions Partition Type naming Primary (max 4) 1-4 Extended (max 1) 1-4 Logical 5- partition naming We saw before that hard disk devices are named /dev/hdx or /dev/sdx with x depending on the hardware configuration. Next is the partition number, starting the count at 1. Hence the four (possible) primary partitions are numbered 1 to 4. Logical partition counting always starts at 5. Thus /dev/hda2 is the second partition on the first ATA hard disk device, and /dev/hdb5 is the first logical partition on the second ATA hard disk device. Same for SCSI, /dev/sdb3 is the third partition on the second SCSI disk. Table 33.2. Partition naming partition device /dev/hda1 first primary partition on /dev/hda /dev/hda2 second primary or extended partition on /dev/hda /dev/sda5 first logical drive on /dev/sda /dev/sdb6 second logical on /dev/sdb 275
- 282. disk partitions 33.2. discovering partitions fdisk -l In the fdisk -l example below you can see that two partitions exist on /dev/sdb. The first partition spans 31 cylinders and contains a Linux swap partition. The second partition is much bigger. root@laika:~# fdisk -l /dev/sdb Disk /dev/sdb: 100.0 GB, 100030242816 bytes 255 heads, 63 sectors/track, 12161 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Device Boot Start End Blocks Id System /dev/sdb1 1 31 248976 82 Linux swap / Solaris /dev/sdb2 32 12161 97434225 83 Linux root@laika:~# /proc/partitions The /proc/partitions file contains a table with major and minor number of partitioned devices, their number of blocks and the device name in /dev. Verify with /proc/ devices to link the major number to the proper device. paul@RHELv4u4:~$ cat /proc/partitions major minor #blocks name 3 0 524288 hda 3 64 734003 hdb 8 0 8388608 sda 8 1 104391 sda1 8 2 8281507 sda2 8 16 1048576 sdb 8 32 1048576 sdc 8 48 1048576 sdd 253 0 7176192 dm-0 253 1 1048576 dm-1 The major number corresponds to the device type (or driver) and can be found in /proc/devices. In this case 3 corresponds to ide and 8 to sd. The major number determines the device driver to be used with this device. The minor number is a unique identification of an instance of this device type. The devices.txt file in the kernel tree contains a full list of major and minor numbers. other tools You might be interested in alternatives to fdisk like parted, cfdisk, sfdisk and gparted. This course mainly uses fdisk to partition hard disks. 276
- 283. disk partitions 33.3. partitioning new disks In the example below, we bought a new disk for our system. After the new hardware is properly attached, you can use fdisk and parted to create the necessary partition(s). This example uses fdisk, but there is nothing wrong with using parted. recognising the disk First, we check with fdisk -l whether Linux can see the new disk. Yes it does, the new disk is seen as /dev/sdb, but it does not have any partitions yet. root@RHELv4u2:~# fdisk -l Disk /dev/sda: 12.8 GB, 12884901888 bytes 255 heads, 63 sectors/track, 1566 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Device Boot Start End Blocks Id System /dev/sda1 * 1 13 104391 83 Linux /dev/sda2 14 1566 12474472+ 8e Linux LVM Disk /dev/sdb: 1073 MB, 1073741824 bytes 255 heads, 63 sectors/track, 130 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Disk /dev/sdb doesn't contain a valid partition table opening the disk with fdisk Then we create a partition with fdisk on /dev/sdb. First we start the fdisk tool with / dev/sdb as argument. Be very very careful not to partition the wrong disk!! root@RHELv4u2:~# fdisk /dev/sdb Device contains neither a valid DOS partition table, nor Sun, SGI... Building a new DOS disklabel. Changes will remain in memory only, until you decide to write them. After that, of course, the previous content won't be recoverable. Warning: invalid flag 0x0000 of partition table 4 will be corrected... empty partition table Inside the fdisk tool, we can issue the p command to see the current disks partition table. Command (m for help): p Disk /dev/sdb: 1073 MB, 1073741824 bytes 255 heads, 63 sectors/track, 130 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes 277
- 284. disk partitions Device Boot Start End Blocks Id System create a new partition No partitions exist yet, so we issue n to create a new partition. We choose p for primary, 1 for the partition number, 1 for the start cylinder and 14 for the end cylinder. Command (m for help): n Command action e extended p primary partition (1-4) p Partition number (1-4): 1 First cylinder (1-130, default 1): Using default value 1 Last cylinder or +size or +sizeM or +sizeK (1-130, default 130): 14 We can now issue p again to verify our changes, but they are not yet written to disk. This means we can still cancel this operation! But it looks good, so we use w to write the changes to disk, and then quit the fdisk tool. Command (m for help): p Disk /dev/sdb: 1073 MB, 1073741824 bytes 255 heads, 63 sectors/track, 130 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Device Boot Start End Blocks Id System /dev/sdb1 1 14 112423+ 83 Linux Command (m for help): w The partition table has been altered! Calling ioctl() to re-read partition table. Syncing disks. root@RHELv4u2:~# display the new partition Let's verify again with fdisk -l to make sure reality fits our dreams. Indeed, the screenshot below now shows a partition on /dev/sdb. root@RHELv4u2:~# fdisk -l Disk /dev/sda: 12.8 GB, 12884901888 bytes 255 heads, 63 sectors/track, 1566 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Device Boot Start End Blocks Id System /dev/sda1 * 1 13 104391 83 Linux /dev/sda2 14 1566 12474472+ 8e Linux LVM Disk /dev/sdb: 1073 MB, 1073741824 bytes 255 heads, 63 sectors/track, 130 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Device Boot Start End Blocks Id System 278
- 285. disk partitions /dev/sdb1 1 14 112423+ 83 Linux root@RHELv4u2:~# 33.4. about the partition table master boot record The partition table information (primary and extended partitions) is written in the master boot record or mbr. You can use dd to copy the mbr to a file. This example copies the master boot record from the first SCSI hard disk. dd if=/dev/sda of=/SCSIdisk.mbr bs=512 count=1 The same tool can also be used to wipe out all information about partitions on a disk. This example writes zeroes over the master boot record. dd if=/dev/zero of=/dev/sda bs=512 count=1 Or to wipe out the whole partition or disk. dd if=/dev/zero of=/dev/sda partprobe Don't forget that after restoring a master boot record with dd, that you need to force the kernel to reread the partition table with partprobe. After running partprobe, the partitions can be used again. [root@RHEL5 ~]# partprobe [root@RHEL5 ~]# logical drives The partition table does not contain information about logical drives. So the dd backup of the mbr only works for primary and extended partitions. To backup the partition table including the logical drives, you can use sfdisk. This example shows how to backup all partition and logical drive information to a file. sfdisk -d /dev/sda > parttable.sda.sfdisk The following example copies the mbr and all logical drive info from /dev/sda to / dev/sdb. sfdisk -d /dev/sda | sfdisk /dev/sdb 279
- 286. disk partitions 33.5. practice: partitions 1. Use fdisk -l to display existing partitions and sizes. 2. Use df -h to display existing partitions and sizes. 3. Compare the output of fdisk and df. 4. Create a 200MB primary partition on a small disk. 5. Create a 400MB primary partition and two 300MB logical drives on a big disk. 6. Use df -h and fdisk -l to verify your work. 7. Compare the output again of fdisk and df. Do both commands display the new partitions ? 8. Create a backup with dd of the mbr that contains your 200MB primary partition. 9. Take a backup of the partition table containing your 400MB primary and 300MB logical drives. Make sure the logical drives are in the backup. 10. (optional) Remove all your partitions with fdisk. Then restore your backups. 280
- 287. disk partitions 33.6. solution: partitions 1. Use fdisk -l to display existing partitions and sizes. as root: # fdisk -l 2. Use df -h to display existing partitions and sizes. df -h 3. Compare the output of fdisk and df. Some partitions will be listed in both outputs (maybe /dev/sda1 or /dev/hda1). 4. Create a 200MB primary partition on a small disk. Choose one of the disks you added (this example uses /dev/sdc). root@rhel53 ~# fdisk /dev/sdc ... Command (m for help): n Command action e extended p primary partition (1-4) p Partition number (1-4): 1 First cylinder (1-261, default 1): 1 Last cylinder or +size or +sizeM or +sizeK (1-261, default 261): +200m Command (m for help): w The partition table has been altered! Calling ioctl() to re-read partition table. Syncing disks. 5. Create a 400MB primary partition and two 300MB logical drives on a big disk. Choose one of the disks you added (this example uses /dev/sdb) fdisk /dev/sdb inside fdisk : n p 1 +400m enter --- n e 2 enter enter --- n l +300m (twice) 6. Use df -h and fdisk -l to verify your work. fdisk -l ; df -h 7. Compare the output again of fdisk and df. Do both commands display the new partitions ? The newly created partitions are visible with fdisk. But they are not displayed by df. 8. Create a backup with dd of the mbr that contains your 200MB primary partition. dd if=/dev/sdc of=bootsector.sdc.dd count=1 bs=512 9. Take a backup of the partition table containing your 400MB primary and 300MB logical drives. Make sure the logical drives are in the backup. sfdisk -d /dev/sdb > parttable.sdb.sfdisk 281
- 288. Chapter 34. file systems Table of Contents 34.1. about file systems ...................................................................................... 283 34.2. common file systems ................................................................................. 283 34.3. putting a file system on a partition ............................................................ 286 34.4. tuning a file system .................................................................................... 286 34.5. checking a file system ............................................................................... 287 34.6. practice: file systems .................................................................................. 288 34.7. solution: file systems ................................................................................. 289 When you are finished partitioning the hard disk, you can put a file system on each partition. This chapter builds on the partitions from the previous chapter, and prepares you for the next one where we will mount the filesystems. 282
- 289. file systems 34.1. about file systems A file system is a way of organizing files on your partition. Besides file-based storage, file systems usually include directories and access control, and contain meta information about files like access times, modification times and file ownership. The properties (length, character set, ...) of filenames are determined by the file system you choose. Directories are usually implemented as files, you will have to learn how this is implemented! Access control in file systems is tracked by user ownership (and group owner- and membership) in combination with one or more access control lists. The manual page about filesystems(5) is usually accessed by typing man fs. You can also look at /proc/filesystems for currently loaded file system drivers. root@rhel53 ~# cat /proc/filesystems | grep -v nodev ext2 iso9660 ext3 34.2. common file systems ext2 and ext3 Once the most common Linux file systems is the ext2 (the second extended) file system. A disadvantage is that file system checks on ext2 can take a long time. You will see that ext2 is being replaced by ext3 on most Linux machines. They are essentially the same, except for the journaling which is only present in ext3. Journaling means that changes are first written to a journal on the disk. The journal is flushed regularly, writing the changes in the file system. Journaling keeps the file system in a consistent state, so you don't need a file system check after an unclean shutdown or power failure. You can create these file systems with the /sbin/mkfs or /sbin/mke2fs commands. Use mke2fs -j to create an ext3 file system. You can convert an ext2 to ext3 with tune2fs -j. You can mount an ext3 file system as ext2, but then you lose the journaling. Do not forget to run mkinitrd if you are booting from this device. ext4 Since 2008 the newest incarnation of the ext file system is ext4 is available in the Linux kernel. ext4 support larger files (up to 16 terabyte) and larger file systems than ext3 (and many more features). 283
- 290. file systems vfat The vfat file system exists in a couple of forms : fat12 for floppy disks, fat16 on ms- dos, and fat32 for larger disks. The Linux vfat implementation supports all of these, but vfat lacks a lot of features like security and links. fat disks can be read by every operating system, and are used a lot for digital cameras, usb sticks and to exchange data between different OS'ses on a home user's computer. iso 9660 iso 9660 is the standard format for cdroms. Chances are you will encounter this file system also on your hard disk in the form of images of cdroms (often with the .iso extension). The iso 9660 standard limits filenames to the 8.3 format. The Unix world didn't like this, and thus added the rock ridge extensions, which allows for filenames up to 255 characters and Unix-style file-modes, ownership and symbolic links. Another extensions to iso 9660 is joliet, which adds 64 unicode characters to the filename. The el torito standard extends iso 9660 to be able to boot from CD- ROM's. udf Most optical media today (including cd's and dvd's) use udf, the Universal Disk Format. swap All things considered, swap is not a file system. But to use a partition as a swap partition it must be formatted and mounted as swap space. others... You might encounter reiserfs on older Linux systems. Maybe you will see Sun's zfs or the open source btrfs. This last one requires a chapter on itself. /proc/filesystems The /proc/filesystems file displays a list of supported file systems. When you mount a file system without explicitly defining one, then mount will first try to probe / etc/filesystems and then probe /proc/filesystems for all the filesystems without the nodev label. If /etc/filesystems ends with a line containing only an asterisk (*) then both files are probed. paul@RHELv4u4:~$ cat /proc/filesystems nodev sysfs 284
- 291. file systems nodev rootfs nodev bdev nodev proc nodev sockfs nodev binfmt_misc nodev usbfs nodev usbdevfs nodev futexfs nodev tmpfs nodev pipefs nodev eventpollfs nodev devpts ext2 nodev ramfs nodev hugetlbfs iso9660 nodev relayfs nodev mqueue nodev selinuxfs ext3 nodev rpc_pipefs nodev vmware-hgfs nodev autofs paul@RHELv4u4:~$ 285
- 292. file systems 34.3. putting a file system on a partition We now have a fresh partition. The system binaries to make file systems can be found with ls. [root@RHEL4b ~]# ls -lS /sbin/mk* -rwxr-xr-x 3 root root 34832 Apr 24 2006 /sbin/mke2fs -rwxr-xr-x 3 root root 34832 Apr 24 2006 /sbin/mkfs.ext2 -rwxr-xr-x 3 root root 34832 Apr 24 2006 /sbin/mkfs.ext3 -rwxr-xr-x 3 root root 28484 Oct 13 2004 /sbin/mkdosfs -rwxr-xr-x 3 root root 28484 Oct 13 2004 /sbin/mkfs.msdos -rwxr-xr-x 3 root root 28484 Oct 13 2004 /sbin/mkfs.vfat -rwxr-xr-x 1 root root 20313 Apr 10 2006 /sbin/mkinitrd -rwxr-x--- 1 root root 15444 Oct 5 2004 /sbin/mkzonedb -rwxr-xr-x 1 root root 15300 May 24 2006 /sbin/mkfs.cramfs -rwxr-xr-x 1 root root 13036 May 24 2006 /sbin/mkswap -rwxr-xr-x 1 root root 6912 May 24 2006 /sbin/mkfs -rwxr-xr-x 1 root root 5905 Aug 3 2004 /sbin/mkbootdisk [root@RHEL4b ~]# It is time for you to read the manual pages of mkfs and mke2fs. In the example below, you see the creation of an ext2 file system on /dev/sdb1. In real life, you might want to use options like -m0 and -j. root@RHELv4u2:~# mke2fs /dev/sdb1 mke2fs 1.35 (28-Feb-2004) Filesystem label= OS type: Linux Block size=1024 (log=0) Fragment size=1024 (log=0) 28112 inodes, 112420 blocks 5621 blocks (5.00%) reserved for the super user First data block=1 Maximum filesystem blocks=67371008 14 block groups 8192 blocks per group, 8192 fragments per group 2008 inodes per group Superblock backups stored on blocks: 8193, 24577, 40961, 57345, 73729 Writing inode tables: done Writing superblocks and filesystem accounting information: done This filesystem will be automatically checked every 37 mounts or 180 days, whichever comes first. Use tune2fs -c or -i to override. 34.4. tuning a file system You can use tune2fs to list and set file system settings. The first screenshot lists the reserved space for root (which is set at five percent). [root@rhel4 ~]# tune2fs -l /dev/sda1 | grep -i "block count" Block count: 104388 Reserved block count: 5219 [root@rhel4 ~]# This example changes this value to ten percent. You can use tune2fs while the file system is active, even if it is the root file system (as in this example). 286
- 293. file systems [root@rhel4 ~]# tune2fs -m10 /dev/sda1 tune2fs 1.35 (28-Feb-2004) Setting reserved blocks percentage to 10 (10430 blocks) [root@rhel4 ~]# tune2fs -l /dev/sda1 | grep -i "block count" Block count: 104388 Reserved block count: 10430 [root@rhel4 ~]# 34.5. checking a file system The fsck command is a front end tool used to check a file system for errors. [root@RHEL4b ~]# ls /sbin/*fsck* /sbin/dosfsck /sbin/fsck /sbin/fsck.ext2 /sbin/fsck.msdos /sbin/e2fsck /sbin/fsck.cramfs /sbin/fsck.ext3 /sbin/fsck.vfat [root@RHEL4b ~]# The last column in /etc/fstab is used to determine whether a file system should be checked at boot-up. [paul@RHEL4b ~]$ grep ext /etc/fstab /dev/VolGroup00/LogVol00 / ext3 defaults 1 1 LABEL=/boot /boot ext3 defaults 1 2 [paul@RHEL4b ~]$ Manually checking a mounted file system results in a warning from fsck. [root@RHEL4b ~]# fsck /boot fsck 1.35 (28-Feb-2004) e2fsck 1.35 (28-Feb-2004) /dev/sda1 is mounted. WARNING!!! Running e2fsck on a mounted filesystem may cause SEVERE filesystem damage. Do you really want to continue (y/n)? no check aborted. But after unmounting fsck and e2fsck can be used to check an ext2 file system. [root@RHEL4b ~]# fsck /boot fsck 1.35 (28-Feb-2004) e2fsck 1.35 (28-Feb-2004) /boot: clean, 44/26104 files, 17598/104388 blocks [root@RHEL4b ~]# fsck -p /boot fsck 1.35 (28-Feb-2004) /boot: clean, 44/26104 files, 17598/104388 blocks [root@RHEL4b ~]# e2fsck -p /dev/sda1 /boot: clean, 44/26104 files, 17598/104388 blocks 287
- 294. file systems 34.6. practice: file systems 1. List the filesystems that are known by your system. 2. Create an ext2 filesystem on the 200MB partition. 3. Create an ext3 filesystem on the 400MB partition and one of the 300MB logical drives. 4. Set the reserved space for root on the logical drive to 0 percent. 5. Verify your work with fdisk and df. 288
- 295. file systems 34.7. solution: file systems 1. List the filesystems that are known by your system. man fs cat /proc/filesystems cat /etc/filesystems (not on all Linux distributions) 2. Create an ext2 filesystem on the 200MB partition. mke2fs /dev/sdc1 (replace sdc1 with the correct partition) 3. Create an ext3 filesystem on the 400MB partition and one of the 300MB logical drives. mke2fs -j /dev/sdb1 (replace sdb1 with the correct partition) mke2fs -j /dev/sdb5 (replace sdb5 with the correct partition) 4. Set the reserved space for root on the logical drive to 0 percent. tune2fs -m 0 /dev/sdb5 5. Verify your work with fdisk and df. mkfs (mke2fs) makes no difference in the output of these commands The big change is in the next topic: mounting 289
- 296. Chapter 35. mounting Table of Contents 35.1. mounting local file systems ....................................................................... 291 35.2. displaying mounted file systems ................................................................ 292 35.3. permanent mounts ...................................................................................... 293 35.4. securing mounts ......................................................................................... 294 35.5. practice: mounting file systems ................................................................. 296 35.6. solution: mounting file systems ................................................................. 297 Once you've put a file system on a partition, you can mount it. Mounting a file system makes it available for use, usually as a directory. We say mounting a file system instead of mounting a partition because we will see later that we can also mount file systems that do not exists on partitions. 290
- 297. mounting 35.1. mounting local file systems On all Unix systems, every file and every directory is part of one big file tree. To access a file, you need to know the full path starting from the root directory. When adding a file system to your computer, you need to make it available somewhere in the file tree. The directory where you make a file system available is called a mount point. /bin/mkdir This example shows how to create a new mount point with mkdir. root@RHELv4u2:~# mkdir /home/project55 /bin/mount When the mount point is created, and a file system is present on the partition, then mount can mount the file system on the mount point directory. root@RHELv4u2:~# mount -t ext2 /dev/sdb1 /home/project55/ Once mounted, the new file system is accessible to users. /etc/filesystems Actually the explicit -t ext2 option to set the file system is not always necessary. The mount command is able to automatically detect a lot of file systems. When mounting a file system without specifying explicitly the file system, then mount will first probe /etc/filesystems. Mount will skip lines with the nodev directive. paul@RHELv4u4:~$ cat /etc/filesystems ext3 ext2 nodev proc nodev devpts iso9660 vfat hfs paul@RHELv4u4:~$ /proc/filesystems When /etc/filesystems does not exist, or ends with a single * on the last line, then mount will read /proc/filesystems. 291
- 298. mounting [root@RHEL52 ~]# cat /proc/filesystems | grep -v ^nodev ext2 iso9660 ext3 35.2. displaying mounted file systems To display all mounted file systems, issue the mount command. Or look at the files /proc/mounts and /etc/mtab. /bin/mount The simplest and most common way to view all mounts is by issuing the mount command without any arguments. root@RHELv4u2:~# mount | grep /dev/sdb /dev/sdb1 on /home/project55 type ext2 (rw) /proc/mounts The kernel provides the info in /proc/mounts in file form, but /proc/mounts does not exist as a file on any hard disk. Looking at /proc/mounts is looking at information that comes directly from the kernel. root@RHELv4u2:~# cat /proc/mounts | grep /dev/sdb /dev/sdb1 /home/project55 ext2 rw 0 0 /etc/mtab The /etc/mtab file is not updated by the kernel, but is maintained by the mount command. Do not edit /etc/mtab manually. root@RHELv4u2:~# cat /etc/mtab | grep /dev/sdb /dev/sdb1 /home/project55 ext2 rw 0 0 /bin/df A more user friendly way to look at mounted file systems is df. The df (diskfree) command has the added benefit of showing you the free space on each mounted disk. Like a lot of Linux commands, df supports the -h switch to make the output more human readable. 292
- 299. mounting root@RHELv4u2:~# df Filesystem 1K-blocks Used Available Use% Mounted on /dev/mapper/VolGroup00-LogVol00 11707972 6366996 4746240 58% / /dev/sda1 101086 9300 86567 10% /boot none 127988 0 127988 0% /dev/shm /dev/sdb1 108865 1550 101694 2% /home/project55 root@RHELv4u2:~# df -h Filesystem Size Used Avail Use% Mounted on /dev/mapper/VolGroup00-LogVol00 12G 6.1G 4.6G 58% / /dev/sda1 99M 9.1M 85M 10% /boot none 125M 0 125M 0% /dev/shm /dev/sdb1 107M 1.6M 100M 2% /home/project55 In the df -h example below you can see the size, free space, used gigabytes and percentage and mount point of a partition. root@laika:~# df -h | egrep -e "(sdb2|File)" Filesystem Size Used Avail Use% Mounted on /dev/sdb2 92G 83G 8.6G 91% /media/sdb2 root@laika:~# /bin/du The du command can summarize disk usage for files and directories. Preventing du to go into subdirectories with the -s option will give you a total for that directory. This option is often used together with -h, so du -sh on a mount point gives the total amount used in that partition. root@pasha:~# du -sh /home/reet 881G /home/reet 35.3. permanent mounts Until now, we performed all mounts manually. This works nice, until the next reboot. Luckily there is a way to tell your computer to automatically mount certain file systems during boot. /etc/fstab This is done using the file system table located in the /etc/fstab file. Below is a sample /etc/fstab file. root@RHELv4u2:~# cat /etc/fstab /dev/VolGroup00/LogVol00 / ext3 defaults 1 1 LABEL=/boot /boot ext3 defaults 1 2 293
- 300. mounting none /dev/pts devpts gid=5,mode=620 0 0 none /dev/shm tmpfs defaults 0 0 none /proc proc defaults 0 0 none /sys sysfs defaults 0 0 /dev/VolGroup00/LogVol01 swap swap defaults 0 0 By adding the following line, we can automate the mounting of a file system. /dev/sdb1 /home/project55 ext2 defaults 0 0 mount /mountpoint Adding an entry to /etc/fstab has the added advantage that you can simplify the mount command. The command in the screenshot below forces mount to look for the partition info in /etc/fstab. # mount /home/project55 35.4. securing mounts File systems can be secured with several mount options. Here are some examples. ro The ro option will mount a file system as read only, preventing anyone from writing. root@rhel53 ~# mount -t ext2 -o ro /dev/hdb1 /home/project42 root@rhel53 ~# touch /home/project42/testwrite touch: cannot touch `/home/project42/testwrite': Read-only file system noexec The noexec option will prevent the execution of binaries and scripts on the mounted file system. root@rhel53 ~# mount -t ext2 -o noexec /dev/hdb1 /home/project42 root@rhel53 ~# cp /bin/cat /home/project42 root@rhel53 ~# /home/project42/cat /etc/hosts -bash: /home/project42/cat: Permission denied root@rhel53 ~# echo echo hello > /home/project42/helloscript root@rhel53 ~# chmod +x /home/project42/helloscript root@rhel53 ~# /home/project42/helloscript -bash: /home/project42/helloscript: Permission denied nosuid The nosuid option will ignore setuid bit set binaries on the mounted file system. 294
- 301. mounting Note that you can still set the setuid bit on files. root@rhel53 ~# mount -o nosuid /dev/hdb1 /home/project42 root@rhel53 ~# cp /bin/sleep /home/project42/ root@rhel53 ~# chmod 4555 /home/project42/sleep root@rhel53 ~# ls -l /home/project42/sleep -r-sr-xr-x 1 root root 19564 Jun 24 17:57 /home/project42/sleep But users cannot exploit the setuid feature. root@rhel53 ~# su - paul [paul@rhel53 ~]$ /home/project42/sleep 500 & [1] 2876 [paul@rhel53 ~]$ ps -f 2876 UID PID PPID C STIME TTY STAT TIME CMD paul 2876 2853 0 17:58 pts/0 S 0:00 /home/project42/sleep 500 [paul@rhel53 ~]$ noacl To prevent cluttering permissions with acl's, use the noacl option. root@rhel53 ~# mount -o noacl /dev/hdb1 /home/project42 More mount options can be found in the manual page of mount. 295
- 302. mounting 35.5. practice: mounting file systems 1. Mount the small 200MB partition on /home/project22. 2. Mount the big 400MB primary partition on /mnt, the copy some files to it (everything in /etc). Then umount, and mount the file system as read only on /srv/ nfs/salesnumbers. Where are the files you copied ? 3. Verify your work with fdisk, df and mount. Also look in /etc/mtab and /proc/ mounts. 4. Make both mounts permanent, test that it works. 5. What happens when you mount a file system on a directory that contains some files ? 6. What happens when you mount two file systems on the same mount point ? 7. (optional) Describe the difference between these file searching commands: find, locate, updatedb, whereis, apropos and which. 8. (optional) Perform a file system check on the partition mounted at /srv/nfs/ salesnumbers. 296
- 303. mounting 35.6. solution: mounting file systems 1. Mount the small 200MB partition on /home/project22. mkdir /home/project22 mount /dev/sdc1 /home/project22 2. Mount the big 400MB primary partition on /mnt, the copy some files to it (everything in /etc). Then umount, and mount the file system as read only on /srv/ nfs/salesnumbers. Where are the files you copied ? mount /dev/sdb1 /mnt cp -r /etc /mnt ls -l /mnt umount /mnt ls -l /mnt mkdir -p /srv/nfs/salesnumbers mount /dev/sdb1 /srv/nfs/salesnumbers You see the files in /srv/nfs/salenumbers now... But physically they are on ext3 on partition /dev/sdb1 3. Verify your work with fdisk, df and mount. Also look in /etc/mtab and /proc/ mounts. fdisk -l df -h mount All three the above commands should show your mounted partitions. grep project22 /etc/mtab grep project22 /proc/mounts 4. Make both mounts permanent, test that it works. add the following lines to /etc/fstab /dev/sdc1 /home/project22 auto defaults 0 0 /dev/sdb1 /srv/nfs/salesnumbers auto defaults 0 0 5. What happens when you mount a file system on a directory that contains some files ? The files are hidden until umount. 6. What happens when you mount two file systems on the same mount point ? Only the last mounted fs is visible. 7. (optional) Describe the difference between these file searching commands: find, locate, updatedb, whereis, apropos and which. man is your friend 8. (optional) Perform a file system check on the partition mounted at /srv/nfs/ salesnumbers. 297
- 304. mounting better to unmount first before # fsck /dev/sdb1 298
- 305. Chapter 36. introduction to uuid's Table of Contents 36.1. about unique objects .................................................................................. 300 36.2. uuid in /etc/fstab ......................................................................................... 300 36.3. uuid in menu.lst ......................................................................................... 301 36.4. practice: uuid and filesystems .................................................................... 302 36.5. solution: uuid and filesystems ................................................................... 303 299
- 306. introduction to uuid's 36.1. about unique objects A uuid or universally unique identifier is used to uniquely identify objects. This 128bit standard allows anyone to create a unique uuid. /sbin/vol_id Below we use the vol_id utility to display the uuid of an ext3 file system. root@laika:~# vol_id --uuid /dev/sda1 825d4b79-ec40-4390-8a71-9261df8d4c82 /lib/udev/vol_id Red Hat Enterprise Linux 5 puts vol_id in /lib/udev/vol_id, which is not in the $PATH. The syntax is also a bit different from Debian/Ubuntu. root@rhel53 ~# /lib/udev/vol_id -u /dev/hda1 48a6a316-9ca9-4214-b5c6-e7b33a77e860 /sbin/tune2fs We can also use tune2fs to find the uuid of a file system. [root@RHEL5 ~]# tune2fs -l /dev/sda1 | grep UUID Filesystem UUID: 11cfc8bc-07c0-4c3f-9f64-78422ef1dd5c [root@RHEL5 ~]# /lib/udev/vol_id -u /dev/sda1 11cfc8bc-07c0-4c3f-9f64-78422ef1dd5c 36.2. uuid in /etc/fstab You can use the uuid to make sure that a volume is universally uniquely identified in /etc/fstab. The device name can change depending on the disk devices that are present at boot time, but a uuid never changes. First we use tune2fs to find the uuid. [root@RHEL5 ~]# tune2fs -l /dev/sdc1 | grep UUID Filesystem UUID: 7626d73a-2bb6-4937-90ca-e451025d64e8 Then we check that it is properly added to /etc/fstab, the uuid replaces the variable devicename /dev/sdc1. 300
- 307. introduction to uuid's [root@RHEL5 ~]# grep UUID /etc/fstab UUID=7626d73a-2bb6-4937-90ca-e451025d64e8 /home/pro42 ext3 defaults 0 0 Now we can mount the volume using the mount point defined in /etc/fstab. [root@RHEL5 ~]# mount /home/pro42 [root@RHEL5 ~]# df -h | grep 42 /dev/sdc1 397M 11M 366M 3% /home/pro42 The real test now, is to remove /dev/sdb from the system, reboot the machine and see what happens. After the reboot, the disk previously known as /dev/sdc is now / dev/sdb. [root@RHEL5 ~]# tune2fs -l /dev/sdb1 | grep UUID Filesystem UUID: 7626d73a-2bb6-4937-90ca-e451025d64e8 And thanks to the uuid in /etc/fstab, the mountpoint is mounted on the same disk as before. [root@RHEL5 ~]# df -h | grep sdb /dev/sdb1 397M 11M 366M 3% /home/pro42 36.3. uuid in menu.lst Recent incarnations of the Ubuntu distribution will use a uuid to identify the root file system. This example shows how a root=/dev/sda1 is replaced with a uuid. title Ubuntu 9.10, kernel 2.6.31-19-generic uuid f001ba5d-9077-422a-9634-8d23d57e782a kernel /boot/vmlinuz-2.6.31-19-generic root=UUID=f001ba5d-9077-422a-9634-8d23d57e782a ro quiet splash initrd /boot/initrd.img-2.6.31-19-generic The screenshot above contains only four lines. The line starting with root= is the continuation of the kernel line. 301
- 308. introduction to uuid's 36.4. practice: uuid and filesystems 1. Find the uuid of one of your ext3 partitions with tune2fs and vol_id. 2. Use this uuid in /etc/fstab and test that it works with a simple mount. 3. (optional) Test it also by removing a disk (so the device name is changed). You can edit settings in vmware/Virtualbox to remove a hard disk. 4. Display the root= directive in /boot/grub/menu.lst. (We see later in the course how to maintain this file.) 5. (optional) Replace the /dev/xxx in /boot/grub/menu.lst with a uuid (use an extra stanza for this). Test that it works. 302
- 309. introduction to uuid's 36.5. solution: uuid and filesystems 1. Find the uuid of one of your ext3 partitions with tune2fs and vol_id. root@rhel55:~# /lib/udev/vol_id -u /dev/hda1 60926898-2c78-49b4-a71d-c1d6310c87cc root@ubu1004:~# tune2fs -l /dev/sda2 | grep UUID Filesystem UUID: 3007b743-1dce-2d62-9a59-cf25f85191b7 2. Use this uuid in /etc/fstab and test that it works with a simple mount. tail -1 /etc/fstab UUID=60926898-2c78-49b4-a71d-c1d6310c87cc /home/pro42 ext3 defaults 0 0 3. (optional) Test it also by removing a disk (so the device name is changed). You can edit settings in vmware/Virtualbox to remove a hard disk. 4. Display the root= directive in /boot/grub/menu.lst. (We see later in the course how to maintain this file.) paul@deb503:~$ grep ^[^#] /boot/grub/menu.lst | grep root= kernel /boot/vmlinuz-2.6.26-2-686 root=/dev/hda1 ro selinux=1 quiet kernel /boot/vmlinuz-2.6.26-2-686 root=/dev/hda1 ro selinux=1 single 5. (optional) Replace the /dev/xxx in /boot/grub/menu.lst with a uuid (use an extra stanza for this). Test that it works. 303
- 310. Chapter 37. introduction to raid Table of Contents 37.1. hardware or software ................................................................................. 304 37.2. raid levels ................................................................................................... 305 37.3. building a software raid5 array .................................................................. 307 37.4. practice: raid .............................................................................................. 310 37.5. solution: raid .............................................................................................. 311 37.1. hardware or software Redundant Array of Independent (originally Inexpensive) Disks or RAID can be set up using hardware or software. Hardware RAID is more expensive, but offers better performance. Software RAID is cheaper and easier to manage, but it uses your CPU and your memory. Where ten years ago nobody was arguing about the best choice being hardware RAID, this has changed since technologies like mdadm, lvm and even zfs focus more on managability. The workload on the cpu for software RAID used to be high, but cpu's have gotten a lot faster. 304
- 311. introduction to raid 37.2. raid levels raid 0 raid 0 uses two or more disks, and is often called striping (or stripe set, or striped volume). Data is divided in chunks, those chunks are evenly spread across every disk in the array. The main advantage of raid 0 is that you can create larger drives. raid 0 is the only raid without redundancy. jbod jbod uses two or more disks, and is often called concatenating (spanning, spanned set, or spanned volume). Data is written to the first disk, until it is full. Then data is written to the second disk... The main advantage of jbod (Just a Bunch of Disks) is that you can create larger drives. JBOD offers no redundancy. raid 1 raid 1 uses exactly two disks, and is often called mirroring (or mirror set, or mirrored volume). All data written to the array is written on each disk. The main advantage of raid 1 is redundancy. The main disadvantage is that you lose at least half of your available disk space (in other words, you at least double the cost). raid 2, 3 and 4 ? raid 2 uses bit level striping, raid 3 byte level, and raid 4 is the same as raid 5, but with a dedicated parity disk. This is actually slower than raid 5, because every write would have to write parity to this one (bottleneck) disk. It is unlikely that you will ever see these raid levels in production. raid 5 raid 5 uses three or more disks, each divided into chunks. Every time chunks are written to the array, one of the disks will receive a parity chunk. Unlike raid 4, the parity chunk will alternate between all disks. The main advantage of this is that raid 5 will allow for full data recovery in case of one hard disk failure. raid 6 raid 6 is very similar to raid 5, but uses two parity chunks. raid 6 protects against two hard disk failures. Oracle Solaris zfs calls this raidz2 (and also had raidz3 with triple parity). 305
- 312. introduction to raid raid 0+1 raid 0+1 is a mirror(1) of stripes(0). This means you first create two raid 0 stripe sets, and then you set them up as a mirror set. For example, when you have six 100GB disks, then the stripe sets are each 300GB. Combined in a mirror, this makes 300GB total. raid 0+1 will survive one disk failure. It will only survive the second disk failure if this disk is in the same stripe set as the previous failed disk. raid 1+0 raid 1+0 is a stripe(0) of mirrors(1). For example, when you have six 100GB disks, then you first create three mirrors of 100GB each. You then stripe them together into a 300GB drive. In this example, as long as not all disks in the same mirror fail, it can survive up to three hard disk failures. raid 50 raid 5+0 is a stripe(0) of raid 5 arrays. Suppose you have nine disks of 100GB, then you can create three raid 5 arrays of 200GB each. You can then combine them into one large stripe set. many others There are many other nested raid combinations, like raid 30, 51, 60, 100, 150, ... 306
- 313. introduction to raid 37.3. building a software raid5 array do we have three disks? First, you have to attach some disks to your computer. In this scenario, three brand new disks of eight gigabyte each are added. Check with fdisk -l that they are connected. [root@rhel6c ~]# fdisk -l 2> /dev/null | grep MB Disk /dev/sdb: 8589 MB, 8589934592 bytes Disk /dev/sdc: 8589 MB, 8589934592 bytes Disk /dev/sdd: 8589 MB, 8589934592 bytes fd partition type The next step is to create a partition of type fd on every disk. The fd type is to set the partition as Linux RAID autodetect. See this (truncated) screenshot: [root@rhel6c ~]# fdisk /dev/sdd ... Command (m for help): n Command action e extended p primary partition (1-4) p Partition number (1-4): 1 First cylinder (1-1044, default 1): Using default value 1 Last cylinder, +cylinders or +size{K,M,G} (1-1044, default 1044): Using default value 1044 Command (m for help): t Selected partition 1 Hex code (type L to list codes): fd Changed system type of partition 1 to fd (Linux raid autodetect) Command (m for help): w The partition table has been altered! Calling ioctl() to re-read partition table. Syncing disks. verify all three partitions Now all three disks are ready for raid 5, so we have to tell the system what to do with these disks. [root@rhel6c ~]# fdisk -l 2> /dev/null | grep raid /dev/sdb1 1 1044 8385898+ fd Linux raid autodetect /dev/sdc1 1 1044 8385898+ fd Linux raid autodetect /dev/sdd1 1 1044 8385898+ fd Linux raid autodetect 307
- 314. introduction to raid create the raid5 The next step used to be create the raid table in /etc/raidtab. Nowadays, you can just issue the command mdadm with the correct parameters. The command below is split on two lines to fit this print, but you should type it on one line, without the backslash (). [root@rhel6c ~]# mdadm --create /dev/md0 --chunk=64 --level=5 --raid- devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1 mdadm: Defaulting to version 1.2 metadata mdadm: array /dev/md0 started. Below a partial screenshot how fdisk -l sees the raid 5. [root@rhel6c ~]# fdisk -l /dev/md0 Disk /dev/md0: 17.2 GB, 17172135936 bytes 2 heads, 4 sectors/track, 4192416 cylinders Units = cylinders of 8 * 512 = 4096 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 65536 bytes / 131072 bytes Disk identifier: 0x00000000 Disk /dev/md0 doesn't contain a valid partition table We could use this software raid 5 array in the next topic: lvm. /proc/mdstat The status of the raid devices can be seen in /proc/mdstat. This example shows a raid 5 in the process of rebuilding. [root@rhel6c ~]# cat /proc/mdstat Personalities : [raid6] [raid5] [raid4] md0 : active raid5 sdd1[3] sdc1[1] sdb1[0] 16769664 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/2] [UU_] [============>........] recovery = 62.8% (5266176/8384832) finish=0 .3min speed=139200K/sec This example shows an active software raid 5. [root@rhel6c ~]# cat /proc/mdstat Personalities : [raid6] [raid5] [raid4] md0 : active raid5 sdd1[3] sdc1[1] sdb1[0] 16769664 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/3] [UUU] mdadm --detail Use mdadm --detail to get information on a raid device. [root@rhel6c ~]# mdadm --detail /dev/md0 /dev/md0: Version : 1.2 Creation Time : Sun Jul 17 13:48:41 2011 308
- 315. introduction to raid Raid Level : raid5 Array Size : 16769664 (15.99 GiB 17.17 GB) Used Dev Size : 8384832 (8.00 GiB 8.59 GB) Raid Devices : 3 Total Devices : 3 Persistence : Superblock is persistent Update Time : Sun Jul 17 13:49:43 2011 State : clean Active Devices : 3 Working Devices : 3 Failed Devices : 0 Spare Devices : 0 Layout : left-symmetric Chunk Size : 64K Name : rhel6c:0 (local to host rhel6c) UUID : c10fd9c3:08f9a25f:be913027:999c8e1f Events : 18 Number Major Minor RaidDevice State 0 8 17 0 active sync /dev/sdb1 1 8 33 1 active sync /dev/sdc1 3 8 49 2 active sync /dev/sdd1 removing a software raid The software raid is visible in /proc/mdstat when active. To remove the raid completely so you can use the disks for other purposes, you stop (de-activate) it with mdadm. [root@rhel6c ~]# mdadm --stop /dev/md0 mdadm: stopped /dev/md0 The disks can now be repartitioned. 309
- 316. introduction to raid 37.4. practice: raid 1. Add three virtual disks of 1GB each to a virtual machine. 2. Create a software raid 5 on the three disks. (It is not necessary to put a filesystem on it) 3. Verify with fdisk and in /proc that the raid 5 exists. 4. (optional) Stop and remove the raid 5. 5. (optional) Create a raid 1 to mirror two disks. 310
- 317. introduction to raid 37.5. solution: raid 1. Add three virtual disks of 1GB each to a virtual machine. 2. Create a software raid 5 on the three disks. (It is not necessary to put a filesystem on it) 3. Verify with fdisk and in /proc that the raid 5 exists. 4. (optional) Stop and remove the raid 5. 5. (optional) Create a raid 1 to mirror two disks. [root@rhel6c ~]# mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 /dev/sdc1 mdadm: Defaulting to version 1.2 metadata mdadm: array /dev/md0 started. [root@rhel6c ~]# cat /proc/mdstat Personalities : [raid6] [raid5] [raid4] [raid1] md0 : active raid1 sdc1[1] sdb1[0] 8384862 blocks super 1.2 [2/2] [UU] [====>................] resync = 20.8% (1745152/8384862) finish=0.5min speed=218144K/sec 311
- 318. Chapter 38. logical volume management Table of Contents 38.1. introduction to lvm .................................................................................... 313 38.2. lvm terminology ......................................................................................... 314 38.3. example: using lvm .................................................................................... 315 38.4. example: extend a logical volume ............................................................. 317 38.5. example: resize a physical Volume ........................................................... 319 38.6. example: mirror a logical volume .............................................................. 321 38.7. example: snapshot a logical volume .......................................................... 322 38.8. verifying existing physical volumes .......................................................... 323 38.9. verifying existing volume groups .............................................................. 325 38.10. verifying existing logical volumes ........................................................... 327 38.11. manage physical volumes ........................................................................ 328 38.12. manage volume groups ............................................................................ 330 38.13. manage logical volumes ........................................................................... 332 38.14. practice : lvm ........................................................................................... 335 312
- 319. logical volume management 38.1. introduction to lvm problems with standard partitions There are some problems when working with hard disks and standard partitions. Consider a system with a small and a large hard disk device, partitioned like this. The first disk (/dev/sda) is partitioned in two, the second disk (/dev/sdb) has three partitions. Table 38.1. disk partitioning example /dev/sda /dev/sdb /dev/sda1 /dev/sda2 /dev/sdb1 /dev/sdb2 /dev/sdb3 unused /boot / /var /home /project42 ext2 ext3 ext2 reiserfs ext3 In the example above, consider the options when you want to enlarge the space available for /project42. What can you do ? The solution will always force you to unmount the filesystem, take a backup of the data, remove and recreate partitions, and then restore the data and remount the file system. solution with lvm Using lvm will create a virtual layer between the mounted file systems and the hardware devices. This virtual layer will allow for an administrator to enlarge a mounted file system in use. When lvm is properly used, then there is no need to unmount the file system to enlarge it. Table 38.2. LVM Example /dev/sda /dev/sdb Volume Group /boot / /var /home /project42 ext2 ext3 ext2 reiserfs ext3 about lvm Most lvm implementations support physical storage grouping, logical volume resizing and data migration. Physical storage grouping is a fancy name for grouping multiple physical devices (hard disks) into a logical mass storage device. To enlarge this physical group, hard disks or even single partitions can be added at a later time. The size of lvm volumes on this physical group is independent of the individual size of the components. The total size of the group is the limit. 313
- 320. logical volume management One of the nicest features of lvm is the logical volume resizing. You can increase the size of an lvm volume, sometimes even without any downtime. Additionally, you can migrate data away from a failing hard disk device. 38.2. lvm terminology physical volume (pv) A physical volume is a disk, a partition or a (hardware or software) RAID device. All these devices can become a member of a Volume Group. volume group (vg) A Volume Group is an abstraction layer between Physical Devices and Logical Volumes. logical volume (lv) A Logical Volume is created in a Volume Group. Logical Volumes that contain a file system can be mounted. The use of logical volumes is similar to the use of partitions (both are standard block devices) and is accomplished with the same standard commands (mkfs, mount, fsck, df, ...). 314
- 321. logical volume management 38.3. example: using lvm This example shows how you can use a device (in this case /dev/sdc, but it could have been /dev/sdb or any other disk or partition) with lvm, how to create a volume group (vg) and how to create and use a logical volume (vg/lvol0). First thing to do, is create physical volumes that can join the volume group with pvcreate. This command makes a disk or partition available for use in Volume Groups. The screenshot shows how to present the SCSI Disk device to LVM. root@RHEL4:~# pvcreate /dev/sdc Physical volume "/dev/sdc" successfully created Note for home users: lvm will work fine when using the complete disk, but another operating system on the same computer will not recognize lvm and will mark the disk as being empty! You can avoid this by creating a partition that spans the whole disk, then run pvcreate on the partition instead of the disk. Then vgcreate creates a volume group using one device. Note that more devices could be added to the volume group. root@RHEL4:~# vgcreate vg /dev/sdc Volume group "vg" successfully created The last step lvcreate creates a logical volume. root@RHEL4:~# lvcreate --size 500m vg Logical volume "lvol0" created The logical volume /dev/vg/lvol0 can now be formatted with ext2, and mounted for normal use. root@RHELv4u2:~# mke2fs -m0 -j /dev/vg/lvol0 mke2fs 1.35 (28-Feb-2004) Filesystem label= OS type: Linux Block size=1024 (log=0) Fragment size=1024 (log=0) 128016 inodes, 512000 blocks 0 blocks (0.00%) reserved for the super user First data block=1 Maximum filesystem blocks=67633152 63 block groups 8192 blocks per group, 8192 fragments per group 2032 inodes per group Superblock backups stored on blocks: 8193, 24577, 40961, 57345, 73729, 204801, 221185, 401409 Writing inode tables: done Creating journal (8192 blocks): done Writing superblocks and filesystem accounting information: done 315
- 322. logical volume management This filesystem will be automatically checked every 37 mounts or 180 days, whichever comes first. Use tune2fs -c or -i to override. root@RHELv4u2:~# mkdir /home/project10 root@RHELv4u2:~# mount /dev/vg/lvol0 /home/project10/ root@RHELv4u2:~# df -h | grep proj /dev/mapper/vg-lvol0 485M 11M 474M 3% /home/project10 A logical volume is very similar to a partition, it can be formatted with a file system, and can be mounted so users can access it. 316
- 323. logical volume management 38.4. example: extend a logical volume A logical volume can be extended without unmounting the file system. Whether or not a volume can be extended depends on the file system it uses. Volumes that are mounted as vfat or ext2 cannot be extended, so in the example here we use the ext3 file system. The fdisk command shows us newly added scsi-disks that will serve our lvm volume. This volume will then be extended. First, take a look at these disks. [root@RHEL5 ~]# fdisk -l | grep sd[bc] Disk /dev/sdb doesn't contain a valid partition table Disk /dev/sdc doesn't contain a valid partition table Disk /dev/sdb: 1181 MB, 1181115904 bytes Disk /dev/sdc: 429 MB, 429496320 bytes You already know how to partition a disk, below the first disk is partitioned (in one big primary partition), the second disk is left untouched. [root@RHEL5 ~]# fdisk -l | grep sd[bc] Disk /dev/sdc doesn't contain a valid partition table Disk /dev/sdb: 1181 MB, 1181115904 bytes /dev/sdb1 1 143 1148616 83 Linux Disk /dev/sdc: 429 MB, 429496320 bytes You also know how to prepare disks for lvm with pvcreate, and how to create a volume group with vgcreate. This example adds both the partitioned disk and the untouched disk to the volume group named vg2. [root@RHEL5 ~]# pvcreate /dev/sdb1 Physical volume "/dev/sdb1" successfully created [root@RHEL5 ~]# pvcreate /dev/sdc Physical volume "/dev/sdc" successfully created [root@RHEL5 ~]# vgcreate vg2 /dev/sdb1 /dev/sdc Volume group "vg2" successfully created You can use pvdisplay to verify that both the disk and the partition belong to the volume group. [root@RHEL5 ~]# pvdisplay | grep -B1 vg2 PV Name /dev/sdb1 VG Name vg2 -- PV Name /dev/sdc VG Name vg2 And you are familiar both with the lvcreate command to create a small logical volume and the mke2fs command to put ext2 on it. 317
- 324. logical volume management [root@RHEL5 ~]# lvcreate --size 200m vg2 Logical volume "lvol0" created [root@RHEL5 ~]# mke2fs -m20 -j /dev/vg2/lvol0 ... As you see, we end up with a mounted logical volume that according to df is almost 200 megabyte in size. [root@RHEL5 ~]# mkdir /home/resizetest [root@RHEL5 ~]# mount /dev/vg2/lvol0 /home/resizetest/ [root@RHEL5 ~]# df -h | grep resizetest 194M 5.6M 149M 4% /home/resizetest Extending the volume is easy with lvextend. [root@RHEL5 ~]# lvextend -L +100 /dev/vg2/lvol0 Extending logical volume lvol0 to 300.00 MB Logical volume lvol0 successfully resized But as you can see, there is a small problem: it appears that df is not able to display the extended volume in its full size. This is because the filesystem is only set for the size of the volume before the extension was added. [root@RHEL5 ~]# df -h | grep resizetest 194M 5.6M 149M 4% /home/resizetest With lvdisplay however we can see that the volume is indeed extended. [root@RHEL5 ~]# lvdisplay /dev/vg2/lvol0 | grep Size LV Size 300.00 MB To finish the extension, you need resize2fs to span the filesystem over the full size of the logical volume. [root@RHEL5 ~]# resize2fs /dev/vg2/lvol0 resize2fs 1.39 (29-May-2006) Filesystem at /dev/vg2/lvol0 is mounted on /home/resizetest; on-line re sizing required Performing an on-line resize of /dev/vg2/lvol0 to 307200 (1k) blocks. The filesystem on /dev/vg2/lvol0 is now 307200 blocks long. Congratulations, you just successfully expanded a logical volume. [root@RHEL5 ~]# df -h | grep resizetest 291M 6.1M 225M 3% /home/resizetest [root@RHEL5 ~]# 318
- 325. logical volume management 38.5. example: resize a physical Volume This is a humble demonstration of how to resize a physical Volume with lvm (after you resize it with fdisk). The demonstration starts with a 100MB partition named / dev/sde1. We used fdisk to create it, and to verify the size. [root@RHEL5 ~]# fdisk -l 2>/dev/null | grep sde1 /dev/sde1 1 100 102384 83 Linux [root@RHEL5 ~]# Now we can use pvcreate to create the Physical Volume, followed by pvs to verify the creation. [root@RHEL5 ~]# pvcreate /dev/sde1 Physical volume "/dev/sde1" successfully created [root@RHEL5 ~]# pvs | grep sde1 /dev/sde1 lvm2 -- 99.98M 99.98M [root@RHEL5 ~]# The next step is ti use fdisk to enlarge the partition (actually deleting it and then recreating /dev/sde1 with more cylinders). [root@RHEL5 ~]# fdisk /dev/sde Command (m for help): p Disk /dev/sde: 858 MB, 858993152 bytes 64 heads, 32 sectors/track, 819 cylinders Units = cylinders of 2048 * 512 = 1048576 bytes Device Boot Start End Blocks Id System /dev/sde1 1 100 102384 83 Linux Command (m for help): d Selected partition 1 Command (m for help): n Command action e extended p primary partition (1-4) p Partition number (1-4): Value out of range. Partition number (1-4): 1 First cylinder (1-819, default 1): Using default value 1 Last cylinder or +size or +sizeM or +sizeK (1-819, default 819): 200 Command (m for help): w The partition table has been altered! Calling ioctl() to re-read partition table. Syncing disks. [root@RHEL5 ~]# 319
- 326. logical volume management When we now use fdisk and pvs to verify the size of the partition and the Physical Volume, then there is a size difference. LVM is still using the old size. [root@RHEL5 ~]# fdisk -l 2>/dev/null | grep sde1 /dev/sde1 1 200 204784 83 Linux [root@RHEL5 ~]# pvs | grep sde1 /dev/sde1 lvm2 -- 99.98M 99.98M [root@RHEL5 ~]# Executing pvresize on the Physical Volume will make lvm aware of the size change of the partition. The correct size can be displayed with pvs. [root@RHEL5 ~]# pvresize /dev/sde1 Physical volume "/dev/sde1" changed 1 physical volume(s) resized / 0 physical volume(s) not resized [root@RHEL5 ~]# pvs | grep sde1 /dev/sde1 lvm2 -- 199.98M 199.98M [root@RHEL5 ~]# 320
- 327. logical volume management 38.6. example: mirror a logical volume We start by creating three physical volumes for lvm. Then we verify the creation and the size with pvs. Three physical disks because lvm uses two disks for the mirror and a third disk for the mirror log! [root@RHEL5 ~]# pvcreate /dev/sdb /dev/sdc /dev/sdd Physical volume "/dev/sdb" successfully created Physical volume "/dev/sdc" successfully created Physical volume "/dev/sdd" successfully created [root@RHEL5 ~]# pvs PV VG Fmt Attr PSize PFree /dev/sdb lvm2 -- 409.60M 409.60M /dev/sdc lvm2 -- 409.60M 409.60M /dev/sdd lvm2 -- 409.60M 409.60M Then we create the Volume Group and verify again with pvs. Notice how the three physical volumes now belong to vg33, and how the size is rounded down (in steps of the extent size, here 4MB). [root@RHEL5 ~]# vgcreate vg33 /dev/sdb /dev/sdc /dev/sdd Volume group "vg33" successfully created [root@RHEL5 ~]# pvs PV VG Fmt Attr PSize PFree /dev/sda2 VolGroup00 lvm2 a- 15.88G 0 /dev/sdb vg33 lvm2 a- 408.00M 408.00M /dev/sdc vg33 lvm2 a- 408.00M 408.00M /dev/sdd vg33 lvm2 a- 408.00M 408.00M [root@RHEL5 ~]# The last step is to create the Logical Volume with lvcreate. Notice the -m 1 switch to create one mirror. Notice also the change in free space in all three Physical Volumes! [root@RHEL5 ~]# lvcreate --size 300m -n lvmir -m 1 vg33 Logical volume "lvmir" created [root@RHEL5 ~]# pvs PV VG Fmt Attr PSize PFree /dev/sda2 VolGroup00 lvm2 a- 15.88G 0 /dev/sdb vg33 lvm2 a- 408.00M 108.00M /dev/sdc vg33 lvm2 a- 408.00M 108.00M /dev/sdd vg33 lvm2 a- 408.00M 404.00M You can see the copy status of the mirror with lvs. It currently shows a 100 percent copy. [root@RHEL5 ~]# lvs vg33/lvmir LV VG Attr LSize Origin Snap% Move Log Copy% lvmir vg33 mwi-ao 300.00M lvmir_mlog 100.00 321
- 328. logical volume management 38.7. example: snapshot a logical volume A snapshot is a virtual copy of all the data at a point in time on a volume. A snapshot Logical Volume will retain a copy of all changed files of the snapshotted Logical Volume. The example below creates a snapshot of the bigLV Logical Volume. [root@RHEL5 ~]# lvcreate -L100M -s -n snapLV vg42/bigLV Logical volume "snapLV" created [root@RHEL5 ~]# You can see with lvs that the snapshot snapLV is indeed a snapshot of bigLV. Moments after taking the snapshot, there are few changes to bigLV (0.02 percent). [root@RHEL5 ~]# lvs LV VG Attr LSize Origin Snap% Move Log Copy% bigLV vg42 owi-a- 200.00M snapLV vg42 swi-a- 100.00M bigLV 0.02 [root@RHEL5 ~]# But after using bigLV for a while, more changes are done. This means the snapshot volume has to keep more original data (10.22 percent). [root@RHEL5 ~]# lvs | grep vg42 bigLV vg42 owi-ao 200.00M snapLV vg42 swi-a- 100.00M bigLV 10.22 [root@RHEL5 ~]# You can now use regular backup tools (dump, tar, cpio, ...) to take a backup of the snapshot Logical Volume. This backup will contain all data as it existed on bigLV at the time the snapshot was taken. When the backup is done, you can remove the snapshot. [root@RHEL5 ~]# lvremove vg42/snapLV Do you really want to remove active logical volume "snapLV"? [y/n]: y Logical volume "snapLV" successfully removed [root@RHEL5 ~]# 322
- 329. logical volume management 38.8. verifying existing physical volumes lvmdiskscan To get a list of block devices that can be used with LVM, use lvmdiskscan. The example below uses grep to limit the result to SCSI devices. [root@RHEL5 ~]# lvmdiskscan | grep sd /dev/sda1 [ 101.94 MB] /dev/sda2 [ 15.90 GB] LVM physical volume /dev/sdb [ 409.60 MB] /dev/sdc [ 409.60 MB] /dev/sdd [ 409.60 MB] LVM physical volume /dev/sde1 [ 95.98 MB] /dev/sde5 [ 191.98 MB] /dev/sdf [ 819.20 MB] LVM physical volume /dev/sdg1 [ 818.98 MB] [root@RHEL5 ~]# pvs The easiest way to verify whether devices are known to lvm is with the pvs command. The screenshot below shows that only /dev/sda2 is currently known for use with LVM. It shows that /dev/sda2 is part of Volgroup00 and is almost 16GB in size. It also shows /dev/sdc and /dev/sdd as part of vg33. The device /dev/sdb is knwon to lvm, but not linked to any Volume Group. [root@RHEL5 ~]# pvs PV VG Fmt Attr PSize PFree /dev/sda2 VolGroup00 lvm2 a- 15.88G 0 /dev/sdb lvm2 -- 409.60M 409.60M /dev/sdc vg33 lvm2 a- 408.00M 408.00M /dev/sdd vg33 lvm2 a- 408.00M 408.00M [root@RHEL5 ~]# pvscan The pvscan command will scan all disks for existing Physical Volumes. The information is similar to pvs, plus you get a line with total sizes. [root@RHEL5 ~]# pvscan PV /dev/sdc VG vg33 lvm2 [408.00 MB / 408.00 MB free] PV /dev/sdd VG vg33 lvm2 [408.00 MB / 408.00 MB free] PV /dev/sda2 VG VolGroup00 lvm2 [15.88 GB / 0 free] PV /dev/sdb lvm2 [409.60 MB] Total: 4 [17.07 GB] / in use: 3 [16.67 GB] / in no VG: 1 [409.60 MB] [root@RHEL5 ~]# 323
- 330. logical volume management pvdisplay Use pvdisplay to get more information about physical volumes. You can also use pvdisplay without an argument to display information about all physical (lvm) volumes. [root@RHEL5 ~]# pvdisplay /dev/sda2 --- Physical volume --- PV Name /dev/sda2 VG Name VolGroup00 PV Size 15.90 GB / not usable 20.79 MB Allocatable yes (but full) PE Size (KByte) 32768 Total PE 508 Free PE 0 Allocated PE 508 PV UUID TobYfp-Ggg0-Rf8r-xtLd-5XgN-RSPc-8vkTHD [root@RHEL5 ~]# 324
- 331. logical volume management 38.9. verifying existing volume groups vgs Similar to pvs is the use of vgs to display a quick overview of all volume groups. There is only one volume group in the screenshot below, it is named VolGroup00 and is almost 16GB in size. [root@RHEL5 ~]# vgs VG #PV #LV #SN Attr VSize VFree VolGroup00 1 2 0 wz--n- 15.88G 0 [root@RHEL5 ~]# vgscan The vgscan command will scan all disks for existing Volume Groups. It will also update the /etc/lvm/.cache file. This file contains a list of all current lvm devices. [root@RHEL5 ~]# vgscan Reading all physical volumes. This may take a while... Found volume group "VolGroup00" using metadata type lvm2 [root@RHEL5 ~]# LVM will run the vgscan automatically at boot-up, so if you add hot swap devices, then you will need to run vgscan to update /etc/lvm/.cache with the new devices. vgdisplay The vgdisplay command will give you more detailed information about a volume group (or about all volume groups if you omit the argument). [root@RHEL5 ~]# vgdisplay VolGroup00 --- Volume group --- VG Name VolGroup00 System ID Format lvm2 Metadata Areas 1 Metadata Sequence No 3 VG Access read/write VG Status resizable MAX LV 0 Cur LV 2 Open LV 2 Max PV 0 Cur PV 1 Act PV 1 VG Size 15.88 GB PE Size 32.00 MB Total PE 508 325
- 332. logical volume management Alloc PE / Size 508 / 15.88 GB Free PE / Size 0 / 0 VG UUID qsXvJb-71qV-9l7U-ishX-FobM-qptE-VXmKIg [root@RHEL5 ~]# 326
- 333. logical volume management 38.10. verifying existing logical volumes lvs Use lvs for a quick look at all existing logical volumes. Below you can see two logical volumes named LogVol00 and LogVol01. [root@RHEL5 ~]# lvs LV VG Attr LSize Origin Snap% Move Log Copy% LogVol00 VolGroup00 -wi-ao 14.88G LogVol01 VolGroup00 -wi-ao 1.00G [root@RHEL5 ~]# lvscan The lvscan command will scan all disks for existing Logical Volumes. [root@RHEL5 ~]# lvscan ACTIVE '/dev/VolGroup00/LogVol00' [14.88 GB] inherit ACTIVE '/dev/VolGroup00/LogVol01' [1.00 GB] inherit [root@RHEL5 ~]# lvdisplay More detailed information about logical volumes is available through the lvdisplay(1) command. [root@RHEL5 ~]# lvdisplay VolGroup00/LogVol01 --- Logical volume --- LV Name /dev/VolGroup00/LogVol01 VG Name VolGroup00 LV UUID RnTGK6-xWsi-t530-ksJx-7cax-co5c-A1KlDp LV Write Access read/write LV Status available # open 1 LV Size 1.00 GB Current LE 32 Segments 1 Allocation inherit Read ahead sectors 0 Block device 253:1 [root@RHEL5 ~]# 327
- 334. logical volume management 38.11. manage physical volumes pvcreate Use the pvcreate command to add devices to lvm. This example shows how to add a disk (or hardware RAID device) to lvm. [root@RHEL5 ~]# pvcreate /dev/sdb Physical volume "/dev/sdb" successfully created [root@RHEL5 ~]# This example shows how to add a partition to lvm. [root@RHEL5 ~]# pvcreate /dev/sdc1 Physical volume "/dev/sdc1" successfully created [root@RHEL5 ~]# You can also add multiple disks or partitions as target to pvcreate. This example adds three disks to lvm. [root@RHEL5 ~]# pvcreate /dev/sde /dev/sdf /dev/sdg Physical volume "/dev/sde" successfully created Physical volume "/dev/sdf" successfully created Physical volume "/dev/sdg" successfully created [root@RHEL5 ~]# pvremove Use the pvremove command to remove physical volumes from lvm. The devices may not be in use. [root@RHEL5 ~]# pvremove /dev/sde /dev/sdf /dev/sdg Labels on physical volume "/dev/sde" successfully wiped Labels on physical volume "/dev/sdf" successfully wiped Labels on physical volume "/dev/sdg" successfully wiped [root@RHEL5 ~]# pvresize When you used fdisk to resize a partition on a disk, then you must use pvresize to make lvm recognize the new size of the physical volume that represents this partition. [root@RHEL5 ~]# pvresize /dev/sde1 Physical volume "/dev/sde1" changed 1 physical volume(s) resized / 0 physical volume(s) not resized 328
- 335. logical volume management pvchange With pvchange you can prevent the allocation of a Physical Volume in a new Volume Group or Logical Volume. This can be useful if you plan to remove a Physical Volume. [root@RHEL5 ~]# pvchange -xn /dev/sdd Physical volume "/dev/sdd" changed 1 physical volume changed / 0 physical volumes not changed [root@RHEL5 ~]# To revert your previous decision, this example shows you how te re-enable the Physical Volume to allow allocation. [root@RHEL5 ~]# pvchange -xy /dev/sdd Physical volume "/dev/sdd" changed 1 physical volume changed / 0 physical volumes not changed [root@RHEL5 ~]# pvmove With pvmove you can move Logical Volumes from within a Volume Group to another Physical Volume. This must be done before removing a Physical Volume. [root@RHEL5 ~]# pvs | grep vg1 /dev/sdf vg1 lvm2 a- 816.00M 0 /dev/sdg vg1 lvm2 a- 816.00M 816.00M [root@RHEL5 ~]# pvmove /dev/sdf /dev/sdf: Moved: 70.1% /dev/sdf: Moved: 100.0% [root@RHEL5 ~]# pvs | grep vg1 /dev/sdf vg1 lvm2 a- 816.00M 816.00M /dev/sdg vg1 lvm2 a- 816.00M 0 329
- 336. logical volume management 38.12. manage volume groups vgcreate Use the vgcreate command to create a volume group. You can immediately name all the physical volumes that span the volume group. [root@RHEL5 ~]# vgcreate vg42 /dev/sde /dev/sdf Volume group "vg42" successfully created [root@RHEL5 ~]# vgextend Use the vgextend command to extend an existing volume group with a physical volume. [root@RHEL5 ~]# vgextend vg42 /dev/sdg Volume group "vg42" successfully extended [root@RHEL5 ~]# vgremove Use the vgremove command to remove volume groups from lvm. The volume groups may not be in use. [root@RHEL5 ~]# vgremove vg42 Volume group "vg42" successfully removed [root@RHEL5 ~]# vgreduce Use the vgreduce command to remove a Physical Volume from the Volume Group. The following example adds Physical Volume /dev/sdg to the vg1 Volume Group using vgextend. And then removes it again using vgreduce. [root@RHEL5 ~]# pvs | grep sdg /dev/sdg lvm2 -- 819.20M 819.20M [root@RHEL5 ~]# vgextend vg1 /dev/sdg Volume group "vg1" successfully extended [root@RHEL5 ~]# pvs | grep sdg /dev/sdg vg1 lvm2 a- 816.00M 816.00M [root@RHEL5 ~]# vgreduce vg1 /dev/sdg Removed "/dev/sdg" from volume group "vg1" [root@RHEL5 ~]# pvs | grep sdg 330
- 337. logical volume management /dev/sdg lvm2 -- 819.20M 819.20M vgchange Use the vgchange command to change parameters of a Volume Group. This example shows how to prevent Physical Volumes from being added or removed to the Volume Group vg1. [root@RHEL5 ~]# vgchange -xn vg1 Volume group "vg1" successfully changed [root@RHEL5 ~]# vgextend vg1 /dev/sdg Volume group vg1 is not resizable. You can also use vgchange to change most other properties of a Volume Group. This example changes the maximum number of Logical Volumes and maximum number of Physical Volumes that vg1 can serve. [root@RHEL5 ~]# vgdisplay vg1 | grep -i max MAX LV 0 Max PV 0 [root@RHEL5 ~]# vgchange -l16 vg1 Volume group "vg1" successfully changed [root@RHEL5 ~]# vgchange -p8 vg1 Volume group "vg1" successfully changed [root@RHEL5 ~]# vgdisplay vg1 | grep -i max MAX LV 16 Max PV 8 vgmerge Merging two Volume Groups into one is done with vgmerge. The following example merges vg2 into vg1, keeping all the properties of vg1. [root@RHEL5 ~]# vgmerge vg1 vg2 Volume group "vg2" successfully merged into "vg1" [root@RHEL5 ~]# 331
- 338. logical volume management 38.13. manage logical volumes lvcreate Use the lvcreate command to create Logical Volumes in a Volume Group. This example creates an 8GB Logical Volume in Volume Group vg42. [root@RHEL5 ~]# lvcreate -L5G vg42 Logical volume "lvol0" created [root@RHEL5 ~]# As you can see, lvm automatically names the Logical Volume lvol0. The next example creates a 200MB Logical Volume named MyLV in Volume Group vg42. [root@RHEL5 ~]# lvcreate -L200M -nMyLV vg42 Logical volume "MyLV" created [root@RHEL5 ~]# The next example does the same thing, but with different syntax. [root@RHEL5 ~]# lvcreate --size 200M -n MyLV vg42 Logical volume "MyLV" created [root@RHEL5 ~]# This example creates a Logical Volume that occupies 10 percent of the Volume Group. [root@RHEL5 ~]# lvcreate -l 10%VG -n MyLV2 vg42 Logical volume "MyLV2" created [root@RHEL5 ~]# This example creates a Logical Volume that occupies 30 percent of the remaining free space in the Volume Group. [root@RHEL5 ~]# lvcreate -l 30%FREE -n MyLV3 vg42 Logical volume "MyLV3" created [root@RHEL5 ~]# lvremove Use the lvremove command to remove Logical Volumes from a Volume Group. Removing a Logical Volume requires the name of the Volume Group. [root@RHEL5 ~]# lvremove vg42/MyLV 332
- 339. logical volume management Do you really want to remove active logical volume "MyLV"? [y/n]: y Logical volume "MyLV" successfully removed [root@RHEL5 ~]# Removing multiple Logical Volumes will request confirmation for each individual volume. [root@RHEL5 ~]# lvremove vg42/MyLV vg42/MyLV2 vg42/MyLV3 Do you really want to remove active logical volume "MyLV"? [y/n]: y Logical volume "MyLV" successfully removed Do you really want to remove active logical volume "MyLV2"? [y/n]: y Logical volume "MyLV2" successfully removed Do you really want to remove active logical volume "MyLV3"? [y/n]: y Logical volume "MyLV3" successfully removed [root@RHEL5 ~]# lvextend Extending the volume is easy with lvextend. This example extends a 200MB Logical Volume with 100 MB. [root@RHEL5 ~]# lvdisplay /dev/vg2/lvol0 | grep Size LV Size 200.00 MB [root@RHEL5 ~]# lvextend -L +100 /dev/vg2/lvol0 Extending logical volume lvol0 to 300.00 MB Logical volume lvol0 successfully resized [root@RHEL5 ~]# lvdisplay /dev/vg2/lvol0 | grep Size LV Size 300.00 MB The next example creates a 100MB Logical Volume, and then extends it to 500MB. [root@RHEL5 ~]# lvcreate --size 100M -n extLV vg42 Logical volume "extLV" created [root@RHEL5 ~]# lvextend -L 500M vg42/extLV Extending logical volume extLV to 500.00 MB Logical volume extLV successfully resized [root@RHEL5 ~]# This example doubles the size of a Logical Volume. [root@RHEL5 ~]# lvextend -l+100%LV vg42/extLV Extending logical volume extLV to 1000.00 MB Logical volume extLV successfully resized [root@RHEL5 ~]# lvrename Renaming a Logical Volume is done with lvrename. This example renames extLV to bigLV in the vg42 Volume Group. 333
- 340. logical volume management [root@RHEL5 ~]# lvrename vg42/extLV vg42/bigLV Renamed "extLV" to "bigLV" in volume group "vg42" [root@RHEL5 ~]# 334
- 341. logical volume management 38.14. practice : lvm 1. Create a volume group that contains a complete disk and a partition on another disk. 2. Create two logical volumes (a small one and a bigger one) in this volumegroup. Format them wih ext3, mount them and copy some files to them. 3. Verify usage with fdisk, mount, pvs, vgs, lvs, pvdisplay, vgdisplay, lvdisplay and df. Does fdisk give you any information about lvm? 4. Enlarge the small logical volume by 50 percent, and verify your work! 5. Take a look at other commands that start with vg* , pv* or lv*. 6. Create a mirror and a striped Logical Volume. 7. Convert a linear logical volume to a mirror. 8. Convert a mirror logical volume to a linear. 9. Create a snapshot of a Logical Volume, take a backup of the snapshot. Then delete some files on the Logical Volume, then restore your backup. 10. Move your volume group to another disk (keep the Logical Volumes mounted). 11. If time permits, split a Volume Group with vgsplit, then merge it again with vgmerge. 335
- 342. Part XI. boot management
- 343. Chapter 39. bootloader Table of Contents 39.1. boot terminology ........................................................................................ 338 39.2. grub ............................................................................................................ 340 39.3. lilo .............................................................................................................. 345 39.4. practice : bootloader ................................................................................... 347 39.5. solution : bootloader ................................................................................... 348 337
- 344. bootloader 39.1. boot terminology The exact order of things that happen when starting a computer system, depends on the hardware architecture (Intel x86 is different from Sun Sparc etc), on the boot loader (grub is different from lilo) and on the operating system (Linux, Solaris, BSD etc). Most of this chapter is focused on booting Linux on Intel x86 with grub. post A computer starts booting the moment you turn on the power (no kidding). This first process is called post or power on self test. If all goes well then this leads to the bios. If all goes not so well, then you might hear nothing, or hear beeping, or see an error message on the screen, or maybe see smoke coming out of the computer (burning hardware smells bad!). bios All Intel x86 computers will have a basic input/output system or bios to detect, identify and initialize hardware. The bios then goes looking for a boot device. This can be a floppy, hard disk, cdrom, network card or usb drive. During the bios you can see a message on the screen telling you which key (often Del or F2) to press to enter the bios setup. 338
- 345. bootloader openboot Sun sparc systems start with openboot to test the hardware and to boot the operating system. Bill Callkins explains openboot in his Solaris System Administration books. The details of openboot are not the focus of this course. boot password The bios allows you to set a password. Do not forget this password, or you will have to open up the hardware to reset it. You can sometimes set a password to boot the system, and another password to protect the bios from being modified. boot device The bios will look for a boot device in the order configured in the bios setup. Usually an operating system on a production server boots of a hard disk. master boot record The master boot record or mbr is the first sector of a hard disk. The partitioning of a disk in primary partitions, and the active partition are defined in the mbr. The mbr is 512 bytes long and can be copied with dd. dd if=/dev/sda of=bootsect.mbr count=1 bs=512 339
- 346. bootloader bootloader The mbr is executed by the bios and contains either (a small) bootloader or code to load a bootloader. Looking at the mbr with od can reveal information about the bootloader. paul@laika:~$ sudo dd if=/dev/sda count=1 bs=16 skip=24 2>/dev/null|od -c 0000000 376 G R U B 0 G e o m 0 H a r d 0000020 There are a variety of bootloaders available, most common on Intel architecture is grub, which is replacing lilo in many places. When installing Linux on sparc architecture, you can choose silo, Itanium systems can use elilo, IBM S/390 and zSeries use z/IPL, Alpha uses milo and PowerPC architectures use yaboot (yet another boot loader). Bootable cd's and dvd's often use syslinux. kernel The goal of all this is to load an operating system, or rather the kernel of an operating system. A typical bootloader like grub will copy a kernel from hard disk to memory, and will then hand control of the computer to the kernel (execute the kernel). Once the Linux kernel is loaded, the bootloader turns control over to it. From that moment on, the kernel is in control of the system. After discussing bootloaders, we continue with the init system that starts all the daemons. 39.2. grub about grub The most common bootloader on linux systems today is grub. On almost all Intel based systems grub is replacing lilo (the Linux loader). Even Solaris switched to grub on x86 architecture. One of the big advantages of grub over lilo is the capability to change the configuration during boot (by pressing e to edit the boot command line). /boot/grub/menu.lst grub's configuration file is called menu.lst and is located in /boot/grub. The screenshot below show the location and size of menu.lst on Debian. 340
- 347. bootloader root@barry:~# ls -l /boot/grub/menu.lst -rw-r--r-- 1 root root 5155 2009-03-31 18:20 /boot/grub/menu.lst /boot/grub/grub.conf Some distributions like Red Hat Enterprise Linux 5 use grub.conf and provide a symbolic link to menu.lst. This is the same file, only the name changed from grub.conf to menu.lst. Notice also in this screenshot that this file is a lot smaller on Red Hat. [root@RHEL52 grub]# ls -l grub.conf menu.lst -rw------- 1 root root 1346 Jan 21 04:20 grub.conf lrwxrwxrwx 1 root root 11 Oct 11 2008 menu.lst -> ./grub.conf menu commands The menu commands always have to be at the top of grub's configuration file. default The default command sets a default entry to start. The first entry has number 0. default 0 fallback In case the default does not boot, use the fallback entry instead. fallback 1 timeout The timeout will wait a number of seconds before booting the default entry. timeout 5 hiddenmenu The hiddenmenu will hide the grub menu unless the user presses Esc before the timeout expires. 341
- 348. bootloader hiddenmenu title With title we can start a new entry or stanza. title Debian Lenny password You can add a password to prevent interactive selection of a boot environment while grub is running. password --md5 $1$Ec.id/$T2C2ahI/EG3WRRsmmu/HN/ Use the grub interactive shell to create the password hash. grub> md5crypt Password: ******** Encrypted: $1$Ec.id/$T2C2ahI/EG3WRRsmmu/HN/ stanza commands Every operating system or kernel that you want to boot with grub will have a stanza aka an entry of a couple of lines. Listed here are some of the common stanza commands. boot Technically the boot command is only mandatory when running the grub command line. This command does not have any parameters and can only be set as the last command of a stanza. boot kernel The kernel command points to the location of the kernel. To boot Linux this means booting a gzip compressed zImage or bzip2 compressed bzImage. This screenshot shows a typical kernel command used to load a Debian kernel. 342
- 349. bootloader kernel /boot/vmlinuz-2.6.17-2-686 root=/dev/hda1 ro And this is how Red Hat uses the kernel command. kernel /vmlinuz-2.6.18-128.el5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet initrd Many Linux installations will need an initial ramdisk at boot time. This can be set in grub with the initrd command. Here a screenshot of Debian 4.0 initrd /boot/initrd.img-2.6.17-2-686 And the same for Red Hat Enterprise Linux 5.3 initrd /initrd-2.6.18-128.el5.img root The root command accepts the root device as a parameter. The root command will point to the hard disk and partition to use, with hd0 as the first hard disk device and hd1 as the second hard disk device. The same numbering is used for partitions, so hd0,0 is the first partition on the first disk and hd0,1 is the second partition on that disk. root (hd0,0) savedefault The savedefault command can be used together with default saved as a menu command. This combination will set the currently booted stanza as the next default stanza to boot. default saved timeout 10 title Linux root (hd0,0) kernel /boot/vmlinuz savedefault 343
- 350. bootloader title DOS root (hd0,1) makeactive chainloader +1 savedefault chainloading With grub booting, there are two choices: loading an operating system or chainloading another bootloader. The chainloading feature of grub loads the bootsector of a partition (that contains an operating system). Some older operating systems require a primary partition that is set as active. Only one partition can be set active so grub can do this on the fly just before chainloading. This screenshot shows how to set the first primary partition active with grub. root (hd0,0) makeactive Chainloading refers to grub loading another operating system's bootloader. The chainloader switch receives one option: the number of sectors to read and boot. For DOS and OS/2 one sector is enough. Note that DOS requires the boot/root partition to be active! Here is a complete example to chainload an old operating system. title MS-DOS 6.22 root (hd0,1) makeactive chainloader +1 stanza examples This is a screenshot of a typical Debian 4.0 stanza. title Debian GNU/Linux, kernel 2.6.17-2-686 root (hd0,0) kernel /boot/vmlinuz-2.6.17-2-686 root=/dev/hda1 ro initrd /boot/initrd.img-2.6.17-2-686 Here a screenshot of a typical Red Hat Enterprise Linux stanza. title Red Hat Enterprise Linux Server (2.6.18-128.el5) root (hd0,0) kernel /vmlinuz-2.6.18-98.el5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet initrd /initrd-2.6.18-98.el5.img 344
- 351. bootloader editing grub at boot time At boot time, when the grub menu is displayed, you can type e to edit the current stanza. This enables you to add parameters to the kernel. One such parameter, useful when you lost the root password, is single. This will boot the kernel in single user mode (although some distributions will still require you to type the root password. kernel /boot/vmlinuz-2.6.17-2-686 root=/dev/hda1 ro single Another option to reset a root password is to use an init=/bin/bash parameter. kernel /boot/vmlinuz-2.6.17-2-686 root=/dev/hda1 ro init=/bin/bash installing grub Run the grub-install command to install grub. The command requires a destination for overwriting the boot sector or mbr. # grub-install /dev/hda 39.3. lilo Linux loader lilo used to be the most used Linux bootloader, but is steadily being replaced in x86 with grub. lilo.conf Here is an example of a typical lilo.conf file. The delay switch receives a number in tenths of a second. So the delay below is three seconds, not thirty! boot = /dev/hda delay = 30 image = /boot/vmlinuz root = /dev/hda1 label = Red Hat 5.2 image = /boot/vmlinuz root = /dev/hda2 label = S.U.S.E. 8.0 345
- 352. bootloader other = /dev/hda4 table = /dev/hda label = MS-DOS 6.22 The configration file shows three example stanzas. The first one boots Red Hat from the first partition on the first disk (hda1). The second stanza boots Suse 8.0 from the next partition. The last one loads MS-DOS. 346
- 353. bootloader 39.4. practice : bootloader 1. Make a copy of the kernel, initrd and System.map files in /boot. Put the copies also in /boot but replace 2.6.x with 3.0 (just imagine that Linux 3.0 is out.). 2. Add a stanza in grub for the 3.0 files. Make sure the title is different. 3. Set the boot menu timeout to 30 seconds. 4. Reboot and test the new stanza. 347
- 354. bootloader 39.5. solution : bootloader 1. Make a copy of the kernel, initrd and System.map files in /boot. Put the copies also in /boot but replace 2.6.x with 3.0 (just imagine that Linux 3.0 is out.). cd /boot cp vmlinuz-2.6.18-8.e15 vmlinuz-3.0 cp initrd-2.6.18-8.e15.img initrd-3.0.img cp System.map-2.6.18-8.e15 System.map-3.0 Do not forget the initrd file ends in .img . 2. Add a stanza in grub for the 3.0 files. Make sure the title is different. [root@RHEL5 ~]# grep 3.0 /boot/grub/menu.lst title Red Hat Enterprise Linux Server (3.0) kernel /vmlinuz-3.0 ro root=/dev/VolGroup00/LogVol00 rhgb quiet initrd /initrd-3.0.img 3. Set the boot menu timeout to 30 seconds. [root@RHEL5 ~]# grep time /boot/grub/menu.lst timeout=30 4. Reboot and test the new stanza. 348
- 355. Chapter 40. init and runlevels Table of Contents 40.1. about sysv init ............................................................................................ 350 40.2. system init(ialization) ................................................................................. 350 40.3. daemon or demon ? .................................................................................... 354 40.4. starting and stopping daemons ................................................................... 355 40.5. chkconfig .................................................................................................... 355 40.6. update-rc.d .................................................................................................. 357 40.7. bum ............................................................................................................. 358 40.8. runlevels ..................................................................................................... 359 40.9. practice: init ............................................................................................... 362 40.10. solution : init ............................................................................................ 363 349
- 356. init and runlevels 40.1. about sysv init Many Linux distributions use init scripts to start daemons in the same way that Unix System V did. This chapter will explain in detail how that works. Init starts daemons by using scripts, where each script starts one daemon, and where each script waits for the previous script to finish. This serial process of starting daemons is slow, and although slow booting is not a problem on servers where uptime is measured in years, the recent uptake of Linux on the desktop results in user complaints. To improve Linux startup speed, Canonical has developed upstart, which was first used in Ubuntu. Solaris also used init up to Solaris 9, for Solaris 10 Sun has developed Service Management Facility. Both systems start daemons in parallel and can replace the SysV init scripts. There is also an ongoing effort to create initng (init next generation). 40.2. system init(ialization) process id 1 The kernel receives system control from the bootloader. After a while the kernel starts the init daemon. The init daemon (/sbin/init) is the first daemon that is started and receives process id 1 (PID 1). Init never dies. configuration in /etc/inittab When /sbin/init is started, it will first read its configuration file /etc/inittab. In that file, it will look for the value of initdefault (3 in the screenshot below). [paul@rhel4 ~]$ grep ^id /etc/inittab id:3:initdefault: initdefault The value found in initdefault indicates the default runlevel. Some Linux distributions have a brief description of runlevels in /etc/inittab, like here on Red Hat Enterprise Linux 4. # Default runlevel. The runlevels used by RHS are: # 0 - halt (Do NOT set initdefault to this) # 1 - Single user mode # 2 - Multiuser, without NFS (The same as 3, if you don't have network) 350
- 357. init and runlevels # 3 - Full multiuser mode # 4 - unused # 5 - X11 # 6 - reboot (Do NOT set initdefault to this) Runlevel 0 means the system is shutting down. Runlevel 1 is used for troubleshooting, only the root user can log on, and only at the console. Runlevel 3 is typical for servers, whereas runlevel 5 is typical for desktops (graphical logon). Besides runlevels 0, 1 and 6, the use may vary depending on the distribution. Debian and derived Linux systems have full network and GUI logon on runlevels 2 to 5. So always verify the proper meaning of runlevels on your system. sysinit script /etc/rc.d/rc.sysinit The next line in /etc/inittab in Red Hat and derivatives is the following. si::sysinit:/etc/rc.d/rc.sysinit This means that independent of the selected runlevel, init will run the /etc/rc.d/ rc.sysinit script. This script initializes hardware, sets some basic environment, populates /etc/mtab while mounting file systems, starts swap and more. [paul@rhel ~]$ egrep -e"^# Ini" -e"^# Sta" -e"^# Che" /etc/rc.d/rc.sysinit # Check SELinux status # Initialize hardware # Start the graphical boot, if necessary; /usr may not be mounted yet... # Initialiaze ACPI bits # Check filesystems # Start the graphical boot, if necessary and not done yet. # Check to see if SELinux requires a relabel # Initialize pseudo-random number generator # Start up swapping. # Initialize the serial ports. That egrep command could also have been written with grep like this : grep "^# (Ini|Sta|Che)". /etc/init.d/rcS Debian has the following line after initdefault. si::sysinit:/etc/init.d/rcS 351
- 358. init and runlevels The /etc/init.d/rcS script will always run on Debian (independent of the selected runlevel). The script is actually running all scripts in the /etc/rcS.d/ directory in alphabetical order. root@barry:~# cat /etc/init.d/rcS #! /bin/sh # # rcS # # Call all S??* scripts in /etc/rcS.d/ in numerical/alphabetical order # exec /etc/init.d/rc S rc scripts Init will continue to read /etc/inittab and meets this section on Debian Linux. l0:0:wait:/etc/init.d/rc 0 l1:1:wait:/etc/init.d/rc 1 l2:2:wait:/etc/init.d/rc 2 l3:3:wait:/etc/init.d/rc 3 l4:4:wait:/etc/init.d/rc 4 l5:5:wait:/etc/init.d/rc 5 l6:6:wait:/etc/init.d/rc 6 On Red Hat Enterprise Linux it is identical except init.d is rc.d. l0:0:wait:/etc/rc.d/rc 0 l1:1:wait:/etc/rc.d/rc 1 l2:2:wait:/etc/rc.d/rc 2 l3:3:wait:/etc/rc.d/rc 3 l4:4:wait:/etc/rc.d/rc 4 l5:5:wait:/etc/rc.d/rc 5 l6:6:wait:/etc/rc.d/rc 6 In both cases, this means that init will start the rc script with the runlevel as the only parameter. Actually /etc/inittab has fields seperated by colons. The second field determines the runlevel in which this line should be executed. So in both cases, only one line of the seven will be executed, depending on the runlevel set by initdefault. rc directories When you take a look any of the /etc/rcX.d/ directories, then you will see a lot of (links to) scripts who's name start with either uppercase K or uppercase S. [root@RHEL52 rc3.d]# ls -l | tail -4 lrwxrwxrwx 1 root root 19 Oct 11 2008 S98haldaemon -> ../init.d/haldaemon lrwxrwxrwx 1 root root 19 Oct 11 2008 S99firstboot -> ../init.d/firstboot 352
- 359. init and runlevels lrwxrwxrwx 1 root root 11 Jan 21 04:16 S99local -> ../rc.local lrwxrwxrwx 1 root root 16 Jan 21 04:17 S99smartd -> ../init.d/smartd The /etc/rcX.d/ directories only contain links to scripts in /etc/init.d/. Links allow for the script to have a different name. When entering a runlevel, all scripts that start with uppercase K or uppercase S will be started in alphabetical order. Those that start with K will be started first, with stop as the only parameter. The remaining scripts with S will be started with start as the only parameter. All this is done by the /etc/rc.d/rc script on Red Hat and by the /etc/init.d/rc script on Debian. mingetty mingetty in /etc/inittab Almost at the end of /etc/inittab there is a section to start and respawn several mingetty daemons. [root@RHEL4b ~]# grep getty /etc/inittab # Run gettys in standard runlevels 1:2345:respawn:/sbin/mingetty tty1 2:2345:respawn:/sbin/mingetty tty2 3:2345:respawn:/sbin/mingetty tty3 4:2345:respawn:/sbin/mingetty tty4 5:2345:respawn:/sbin/mingetty tty5 6:2345:respawn:/sbin/mingetty tty6 mingetty and /bin/login This /sbin/mingetty will display a message on a virtual console and allow you to type a userid. Then it executes the /bin/login command with that userid. The /bin/ login program will verify whether that user exists in /etc/passwd and prompt for (and verify) a password. If the password is correct, /bin/login passes control to the shell listed in /etc/passwd. respawning mingetty The mingetty daemons are started by init and watched until they die (user exits the shell and is logged out). When this happens, the init daemon will respawn a new mingetty. So even if you kill a mingetty daemon, it will be restarted automatically. This example shows that init respawns mingetty daemons. Look at the PID's of the last two mingetty processes. 353
- 360. init and runlevels [root@RHEL52 ~]# ps -C mingetty PID TTY TIME CMD 2407 tty1 00:00:00 mingetty 2408 tty2 00:00:00 mingetty 2409 tty3 00:00:00 mingetty 2410 tty4 00:00:00 mingetty 2411 tty5 00:00:00 mingetty 2412 tty6 00:00:00 mingetty When we kill the last two mingettys, then init will notice this and start them again (with a different PID). [root@RHEL52 ~]# kill 2411 2412 [root@RHEL52 ~]# ps -C mingetty PID TTY TIME CMD 2407 tty1 00:00:00 mingetty 2408 tty2 00:00:00 mingetty 2409 tty3 00:00:00 mingetty 2410 tty4 00:00:00 mingetty 2821 tty5 00:00:00 mingetty 2824 tty6 00:00:00 mingetty disabling a mingetty You can disable a mingetty for a certain tty by removing the runlevel from the second field in its line in /etc/inittab. Don't forget to tell init about the change of its configuration file with kill -1 1. The example below shows how to disable mingetty on tty3 to tty6 in runlevels 4 and 5. [root@RHEL52 ~]# grep getty /etc/inittab # Run gettys in standard runlevels 1:2345:respawn:/sbin/mingetty tty1 2:2345:respawn:/sbin/mingetty tty2 3:23:respawn:/sbin/mingetty tty3 4:23:respawn:/sbin/mingetty tty4 5:23:respawn:/sbin/mingetty tty5 6:23:respawn:/sbin/mingetty tty6 40.3. daemon or demon ? A daemon is a process that runs in background, without a link to a GUI or terminal. Daemons are usually started at system boot, and stay alive until the system shuts down. In more recent technical writings, daemons are often refered to as services. Unix daemons are not to be confused with demons. Evi Nemeth, co-author of the UNIX System Administration Handbook has the following to say about daemons: Many people equate the word "daemon" with the word "demon", implying some kind of satanic connection between UNIX and the underworld. This is an egregious 354
- 361. init and runlevels misunderstanding. "Daemon" is actually a much older form of "demon"; daemons have no particular bias towards good or evil, but rather serve to help define a person's character or personality. The ancient Greeks' concept of a "personal daemon" was similar to the modern concept of a "guardian angel" .... 40.4. starting and stopping daemons The K and S scripts are links to the real scripts in /etc/init.d/. These can also be used when the system is running to start and stop daemons (or services). Most of them accept the following parameters: start, stop, restart, status. For example in this screenshot we restart the samba daemon. root@laika:~# /etc/init.d/samba restart * Stopping Samba daemons... [ OK ] * Starting Samba daemons... [ OK ] You can achieve the same result on RHEL/Fedora with the service command. [root@RHEL4b ~]# service smb restart Shutting down SMB services: [ OK ] Shutting down NMB services: [ OK ] Starting SMB services: [ OK ] Starting NMB services: [ OK ] You might also want to take a look at chkconfig, update-rc.d. 40.5. chkconfig The purpose of chkconfig is to relieve system administrators of manually managing all the links and scripts in /etc/init.d and /etc/rcX.d/. chkconfig --list Here we use chkconfig to list the status of a service in the different runlevels. You can see that the crond daemon (or service) is only activated in runlevels 2 to 5. [root@RHEL52 ~]# chkconfig --list crond crond 0:off 1:off 2:on 3:on 4:on 5:on 6:off When you compare the screenshot above with the one below, you can see that off equals to a K link to the script, whereas on equals to an S link. 355
- 362. init and runlevels [root@RHEL52 etc]# find ./rc?.d/ -name *crond -exec ls -l {} ;|cut -b40- ./rc0.d/K60crond -> ../init.d/crond ./rc1.d/K60crond -> ../init.d/crond ./rc2.d/S90crond -> ../init.d/crond ./rc3.d/S90crond -> ../init.d/crond ./rc4.d/S90crond -> ../init.d/crond ./rc5.d/S90crond -> ../init.d/crond ./rc6.d/K60crond -> ../init.d/crond runlevel configuration Here you see how to use chkconfig to disable (or enable) a service in a certain runlevel. This screenshot shows how to disable crond in runlevel 3. [root@RHEL52 ~]# chkconfig --level 3 crond off [root@RHEL52 ~]# chkconfig --list crond crond 0:off 1:off 2:on 3:off 4:on 5:on 6:off This screenshot shows how to enable crond in runlevels 3 and 4. [root@RHEL52 ~]# chkconfig --level 34 crond on [root@RHEL52 ~]# chkconfig --list crond crond 0:off 1:off 2:on 3:on 4:on 5:on 6:off chkconfig configuration Every script in /etc/init.d/ can have (comment) lines to tell chkconfig what to do with the service. The line with # chkconfig: contains the runlevels in which the service should be started (2345), followed by the priority for start (90) and stop (60). [root@RHEL52 ~]# head -9 /etc/init.d/crond | tail -5 # chkconfig: 2345 90 60 # description: cron is a standard UNIX program that runs user-specified # programs at periodic scheduled times. vixie cron adds a # number of features to the basic UNIX cron, including better # security and more powerful configuration options. enable and disable services Services can be enabled or disabled in all runlevels with one command. Runlevels 0, 1 and 6 are always stopping services (or calling the scripts with stop) even when their name starts with uppercase S. [root@RHEL52 ~]# chkconfig crond off 356
- 363. init and runlevels [root@RHEL52 ~]# chkconfig --list crond crond 0:off 1:off 2:off 3:off 4:off 5:off 6:off [root@RHEL52 ~]# chkconfig crond on [root@RHEL52 ~]# chkconfig --list crond crond 0:off 1:off 2:on 3:on 4:on 5:on 6:off 40.6. update-rc.d about update-rc.d The Debian equivalent of chkconfig is called update-rc.d. This tool is designed for use in scripts, if you prefer a graphical tool then look at bum. When there are existing links in /etc/rcX.d/ then update-rc.d does not do anything. This is to avoid that post installation scripts using update-rc.d are overwriting changes made by a system administrator. root@barry:~# update-rc.d cron remove update-rc.d: /etc/init.d/cron exists during rc.d purge (use -f to force) As you can see in the next screenshot, nothing changed for the cron daemon. root@barry:~# find /etc/rc?.d/ -name '*cron' -exec ls -l {} ;|cut -b44- /etc/rc0.d/K11cron -> ../init.d/cron /etc/rc1.d/K11cron -> ../init.d/cron /etc/rc2.d/S89cron -> ../init.d/cron /etc/rc3.d/S89cron -> ../init.d/cron /etc/rc4.d/S89cron -> ../init.d/cron /etc/rc5.d/S89cron -> ../init.d/cron /etc/rc6.d/K11cron -> ../init.d/cron removing a service Here we remove cron from all runlevels. Remember that the proper way to disable a service is to put K scripts oin all runlevels! root@barry:~# update-rc.d -f cron remove Removing any system startup links for /etc/init.d/cron ... /etc/rc0.d/K11cron /etc/rc1.d/K11cron /etc/rc2.d/S89cron /etc/rc3.d/S89cron /etc/rc4.d/S89cron /etc/rc5.d/S89cron /etc/rc6.d/K11cron root@barry:~# find /etc/rc?.d/ -name '*cron' -exec ls -l {} ;|cut -b44- root@barry:~# 357
- 364. init and runlevels enable a service This screenshot shows how to use update-rc.d to enable a service in runlevels 2, 3, 4 and 5 and disable the service in runlevels 0, 1 and 6. root@barry:~# update-rc.d cron defaults Adding system startup for /etc/init.d/cron ... /etc/rc0.d/K20cron -> ../init.d/cron /etc/rc1.d/K20cron -> ../init.d/cron /etc/rc6.d/K20cron -> ../init.d/cron /etc/rc2.d/S20cron -> ../init.d/cron /etc/rc3.d/S20cron -> ../init.d/cron /etc/rc4.d/S20cron -> ../init.d/cron /etc/rc5.d/S20cron -> ../init.d/cron customize a service And here is an example on how to set your custom configuration for the cron daemon. root@barry:~# update-rc.d -n cron start 11 2 3 4 5 . stop 89 0 1 6 . Adding system startup for /etc/init.d/cron ... /etc/rc0.d/K89cron -> ../init.d/cron /etc/rc1.d/K89cron -> ../init.d/cron /etc/rc6.d/K89cron -> ../init.d/cron /etc/rc2.d/S11cron -> ../init.d/cron /etc/rc3.d/S11cron -> ../init.d/cron /etc/rc4.d/S11cron -> ../init.d/cron /etc/rc5.d/S11cron -> ../init.d/cron 40.7. bum This screenshot shows bum in advanced mode. 358
- 365. init and runlevels 40.8. runlevels display the runlevel You can see your current runlevel with the runlevel or who -r commands. The runlevel command is typical Linux and will output the previous and the current runlevel. If there was no previous runlevel, then it will mark it with the letter N. [root@RHEL4b ~]# runlevel N 3 The history of who -r dates back to Seventies Unix, it still works on Linux. [root@RHEL4b ~]# who -r run-level 3 Jul 28 09:15 last=S changing the runlevel You can switch to another runlevel with the telinit command. On Linux /sbin/telinit is usually a (hard) link to /sbin/init. This screenshot shows how to switch from runlevel 2 to runlevel 3 without reboot. root@barry:~# runlevel N 2 root@barry:~# init 3 root@barry:~# runlevel 2 3 /sbin/shutdown The shutdown command is used to properly shut down a system. Common switches used with shutdown are -a, -t, -h and -r. The -a switch forces /sbin/shutdown to use /etc/shutdown.allow. The -t switch is used to define the number of seconds between the sending of the TERM signal and the KILL signal. The -h switch halts the system instead of changing to runlevel 1. The -r switch tells /sbin/shutdown to reboot after shutting down. This screenshot shows how to use shutdown with five seconds between TERM and KILL signals. 359
- 366. init and runlevels root@barry:~# shutdown -t5 -h now The now is the time argument. This can be +m for the number of minutes to wait before shutting down (with now as an alias for +0. The command will also accept hh:mm instead of +m. halt, reboot and poweroff The binary /sbin/reboot is the same as /sbin/halt and /sbin/poweroff. Depending on the name we use to call the command, it can behave differently. When in runlevel 0 or 6 halt, reboot and poweroff will tell the kernel to halt, reboot or poweroff the system. When not in runlevel 0 or 6, typing reboot as root actually calls the shutdown command with the -r switch and typing poweroff will switch off the power when halting the system. /var/log/wtmp halt, reboot and poweroff all write to /var/log/wtmp. To look at /var/log/wtmp, we need to use th last. [root@RHEL52 ~]# last | grep reboot reboot system boot 2.6.18-128.el5 Fri May 29 11:44 (192+05:01) reboot system boot 2.6.18-128.el5 Wed May 27 12:10 (06:49) reboot system boot 2.6.18-128.el5 Mon May 25 19:34 (1+15:59) reboot system boot 2.6.18-128.el5 Mon Feb 9 13:20 (106+21:13) Ctrl-Alt-Del When rc is finished starting all those scripts, init will continue to read /etc/inittab. The next line is about what to do when the user hits Ctrl-Alt-Delete on the keyboard. Here is what Debian 4.0 does. root@barry:~# grep -i ctrl /etc/inittab # What to do when CTRL-ALT-DEL is pressed. ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now Which is very similar to the default Red Hat Enterprise Linux 5.2 action. [root@RHEL52 ~]# grep -i ctrl /etc/inittab # Trap CTRL-ALT-DELETE ca::ctrlaltdel:/sbin/shutdown -t3 -r now 360
- 367. init and runlevels One noticable difference is that Debian forces shutdown to use /etc/shutdown.allow, where Red Hat allows everyone to invoke shutdown pressing Ctrl-Alt-Delete. UPS and loss of power [root@RHEL52 ~]# grep ^p /etc/inittab pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure; System Shutting Down" pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Cancelled" It will read commands on what to execute in case of powerfailure, powerok and Ctrl-Alt-Delete. The init process never stops keeping an eye on power failures and that triple key combo. root@barry:~# grep ^p /etc/inittab pf::powerwait:/etc/init.d/powerfail start pn::powerfailnow:/etc/init.d/powerfail now po::powerokwait:/etc/init.d/powerfail stop 361
- 368. init and runlevels 40.9. practice: init 1. Change /etc/inittab so that only two mingetty's are respawned. Kill the other mingetty's and verify that they don't come back. 2. Use the Red Hat Enterprise Linux virtual machine. Go to runlevel 5, display the current and previous runlevel, then go back to runlevel 3. 3. Is the sysinit script on your computers setting or changing the PATH environment variable ? 4. List all init.d scripts that are started in runlevel 2. 5. Write a script that acts like a daemon script in /etc/init.d/. It should have a case statement to act on start/stop/restart and status. Test the script! 6. Use chkconfig to setup your script to start in runlevels 3,4 and 5, and to stop in any other runlevel. 362
- 369. init and runlevels 40.10. solution : init 1. Change /etc/inittab so that only two mingetty's are respawned. Kill the other mingetty's and verify that they don't come back. Killing the mingetty's will result in init respawning them. You can edit /etc/inittab so it looks like the screenshot below. Don't forget to also run kill -1 1. [root@RHEL5 ~]# grep tty /etc/inittab # Run gettys in standard runlevels 1:2345:respawn:/sbin/mingetty tty1 2:2345:respawn:/sbin/mingetty tty2 3:2:respawn:/sbin/mingetty tty3 4:2:respawn:/sbin/mingetty tty4 5:2:respawn:/sbin/mingetty tty5 6:2:respawn:/sbin/mingetty tty6 [root@RHEL5 ~]# 2. Use the Red Hat Enterprise Linux virtual machine. Go to runlevel 5, display the current and previous runlevel, then go back to runlevel 3. init 5 (watch the console for the change taking place) runlevel init 3 (again you can follow this on the console) 3. Is the sysinit script on your computers setting or changing the PATH environment variable ? On Red Hat, grep for PATH in /etc/rc.sysinit, on Debian/Ubuntu check /etc/rc.local and /etc/ini.t/rc.local. The answer is probably no, but on RHEL5 the rc.sysinit script does set the HOSTNAME variable. [root@RHEL5 etc]# grep HOSTNAME rc.sysinit 4. List all init.d scripts that are started in runlevel 2. root@RHEL5 ~# chkconfig --list | grep '2:on' 5. Write a script that acts like a daemon script in /etc/init.d/. It should have a case statement to act on start/stop/restart and status. Test the script! The script could look something like this. #!/bin/bash # # chkconfig: 345 99 01 # description: pold demo script # # /etc/init.d/pold 363
- 370. init and runlevels # case "$1" in start) echo -n "Starting pold..." sleep 1; touch /var/lock/subsys/pold echo "done." echo pold started >> /var/log/messages ;; stop) echo -n "Stopping pold..." sleep 1; rm -rf /var/lock/subsys/pold echo "done." echo pold stopped >> /var/log/messages ;; *) echo "Usage: /etc/init.d/pold {start|stop}" exit 1 ;; esac exit 0 The touch /var/lock/subsys/pold is mandatory and must be the same filename as the script name, if you want the stop sequence (the K01pold link) to be run. 6. Use chkconfig to setup your script to start in runlevels 3,4 and 5, and to stop in any other runlevel. chkconfig --add pold The command above will only work when the # chkconfig: and # description: lines in the pold script are there. 364
- 371. Part XII. system management
- 372. Chapter 41. scheduling Table of Contents 41.1. one time jobs with at ................................................................................. 367 41.2. cron ............................................................................................................. 369 41.3. practice : scheduling ................................................................................... 371 41.4. solution : scheduling .................................................................................. 372 Linux administrators use the at to schedule one time jobs. Recurring jobs are better scheduled with cron. The next two sections will discuss both tools. 366
- 373. scheduling 41.1. one time jobs with at at Simple scheduling can be done with the at command. This screenshot shows the scheduling of the date command at 22:01 and the sleep command at 22:03. root@laika:~# at 22:01 at> date at> <EOT> job 1 at Wed Aug 1 22:01:00 2007 root@laika:~# at 22:03 at> sleep 10 at> <EOT> job 2 at Wed Aug 1 22:03:00 2007 root@laika:~# In real life you will hopefully be scheduling more useful commands ;-) atq It is easy to check when jobs are scheduled with the atq or at -l commands. root@laika:~# atq 1 Wed Aug 1 22:01:00 2007 a root 2 Wed Aug 1 22:03:00 2007 a root root@laika:~# at -l 1 Wed Aug 1 22:01:00 2007 a root 2 Wed Aug 1 22:03:00 2007 a root root@laika:~# The at command understands English words like tomorrow and teatime to schedule commands the next day and at four in the afternoon. root@laika:~# at 10:05 tomorrow at> sleep 100 at> <EOT> job 5 at Thu Aug 2 10:05:00 2007 root@laika:~# at teatime tomorrow at> tea at> <EOT> job 6 at Thu Aug 2 16:00:00 2007 root@laika:~# atq 6 Thu Aug 2 16:00:00 2007 a root 5 Thu Aug 2 10:05:00 2007 a root root@laika:~# atrm Jobs in the at queue can be removed with atrm. 367
- 374. scheduling root@laika:~# atq 6 Thu Aug 2 16:00:00 2007 a root 5 Thu Aug 2 10:05:00 2007 a root root@laika:~# atrm 5 root@laika:~# atq 6 Thu Aug 2 16:00:00 2007 a root root@laika:~# at.allow and at.deny You can also use the /etc/at.allow and /etc/at.deny files to manage who can schedule jobs with at. The /etc/at.allow file can contain a list of users that are allowed to schedule at jobs. When /etc/at.allow does not exist, then everyone can use at unless their username is listed in /etc/at.deny. If none of these files exist, then everyone can use at. 368
- 375. scheduling 41.2. cron crontab file The crontab(1) command can be used to maintain the crontab(5) file. Each user can have their own crontab file to schedule jobs at a specific time. This time can be specified with five fields in this order: minute, hour, day of the month, month and day of the week. If a field contains an asterisk (*), then this means all values of that field. The following example means : run script42 eight minutes after two, every day of the month, every month and every day of the week. 8 14 * * * script42 Run script8472 every month on the first of the month at 25 past midnight. 25 0 1 * * script8472 Run this script33 every two minutes on Sunday (both 0 and 7 refer to Sunday). */2 * * * 0 Instead of these five fields, you can also type one of these: @reboot, @yearly or @annually, @monthly, @weekly, @daily or @midnight, and @hourly. crontab command Users should not edit the crontab file directly, instead they should type crontab -e which will use the editor defined in the EDITOR or VISUAL environment variable. Users can display their cron table with crontab -l. cron.allow and cron.deny The cron daemon crond is reading the cron tables, taking into account the /etc/ cron.allow and /etc/cron.deny files. These files work in the same way as at.allow and at.deny. When the cron.allow file exists, then your username has to be in it, otherwise you cannot use cron. When the cron.allow file does not exists, then your username cannot be in the cron.deny file if you want to use cron. /etc/crontab The /etc/crontab file contains entries for when to run hourly/daily/weekly/monthly tasks. It will look similar to this output. 369
- 376. scheduling SHELL=/bin/sh PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin 20 3 * * * root run-parts --report /etc/cron.daily 40 3 * * 7 root run-parts --report /etc/cron.weekly 55 3 1 * * root run-parts --report /etc/cron.monthly /etc/cron.* The directories shown in the next screenshot contain the tasks that are run at the times scheduled in /etc/crontab. The /etc/cron.d directory is for special cases, to schedule jobs that require finer control than hourly/daily/weekly/monthly. paul@laika:~$ ls -ld /etc/cron.* drwxr-xr-x 2 root root 4096 2008-04-11 09:14 /etc/cron.d drwxr-xr-x 2 root root 4096 2008-04-19 15:04 /etc/cron.daily drwxr-xr-x 2 root root 4096 2008-04-11 09:14 /etc/cron.hourly drwxr-xr-x 2 root root 4096 2008-04-11 09:14 /etc/cron.monthly drwxr-xr-x 2 root root 4096 2008-04-11 09:14 /etc/cron.weekly 370
- 377. scheduling 41.3. practice : scheduling 1. Schedule two jobs with at, display the at queue and remove a job. 2. As normal user, use crontab -e to schedule a script to run every four minutes. 3. As root, display the crontab file of your normal user. 4. As the normal user again, remove your crontab file. 5. Take a look at the cron files and directories in /etc and understand them. What is the run-parts command doing ? 371
- 378. scheduling 41.4. solution : scheduling 1. Schedule two jobs with at, display the at queue and remove a job. root@rhel55 ~# at 9pm today at> echo go to bed >> /root/todo.txt at> <EOT> job 1 at 2010-11-14 21:00 root@rhel55 ~# at 17h31 today at> echo go to lunch >> /root/todo.txt at> <EOT> job 2 at 2010-11-14 17:31 root@rhel55 ~# atq 2 2010-11-14 17:31 a root 1 2010-11-14 21:00 a root root@rhel55 ~# atrm 1 root@rhel55 ~# atq 2 2010-11-14 17:31 a root root@rhel55 ~# date Sun Nov 14 17:31:01 CET 2010 root@rhel55 ~# cat /root/todo.txt go to lunch 2. As normal user, use crontab -e to schedule a script to run every four minutes. paul@rhel55 ~$ crontab -e no crontab for paul - using an empty one crontab: installing new crontab 3. As root, display the crontab file of your normal user. root@rhel55 ~# crontab -l -u paul */4 * * * * echo `date` >> /home/paul/crontest.txt 4. As the normal user again, remove your crontab file. paul@rhel55 ~$ crontab -r paul@rhel55 ~$ crontab -l no crontab for paul 5. Take a look at the cron files and directories in /etc and understand them. What is the run-parts command doing ? run-parts runs a script in a directory 372
- 379. Chapter 42. logging Table of Contents 42.1. login logging .............................................................................................. 374 42.2. syslogd ....................................................................................................... 377 42.3. logger ......................................................................................................... 379 42.4. watching logs ............................................................................................. 379 42.5. rotating logs ............................................................................................... 380 42.6. practice : logging ........................................................................................ 381 42.7. solution : logging ....................................................................................... 382 This chapter has three distinct subjects. First we look at login logging ; how can we find out who is logging in to the system, when and from where. And who is not logging in, who fails at su or ssh. Second we discuss how to configure the syslog daemon, and how to test it with logger. The last part is mostly about rotating logs and mentions the tail -f and watch commands for watching logs. 373
- 380. logging 42.1. login logging To keep track of who is logging into the system, Linux can maintain the /var/log/ wtmp, /var/log/btmp, /var/run/utmp and /var/log/lastlog files. /var/run/utmp (who) Use the who command to see the /var/run/utmp file. This command is showing you all the currently logged in users. Notice that the utmp file is in /var/run and not in / var/log . [root@rhel4 ~]# who paul pts/1 Feb 14 18:21 (192.168.1.45) sandra pts/2 Feb 14 18:11 (192.168.1.42) inge pts/3 Feb 14 12:01 (192.168.1.33) els pts/4 Feb 14 14:33 (192.168.1.19) /var/log/wtmp (last) The /var/log/wtmp file is updated by the login program. Use last to see the /var/ run/wtmp file. [root@rhel4a ~]# last | head paul pts/1 192.168.1.45 Wed Feb 14 18:39 still logged in reboot system boot 2.6.9-42.0.8.ELs Wed Feb 14 18:21 (01:15) nicolas pts/5 pc-dss.telematic Wed Feb 14 12:32 - 13:06 (00:33) stefaan pts/3 pc-sde.telematic Wed Feb 14 12:28 - 12:40 (00:12) nicolas pts/3 pc-nae.telematic Wed Feb 14 11:36 - 12:21 (00:45) nicolas pts/3 pc-nae.telematic Wed Feb 14 11:34 - 11:36 (00:01) dirk pts/5 pc-dss.telematic Wed Feb 14 10:03 - 12:31 (02:28) nicolas pts/3 pc-nae.telematic Wed Feb 14 09:45 - 11:34 (01:48) dimitri pts/5 rhel4 Wed Feb 14 07:57 - 08:38 (00:40) stefaan pts/4 pc-sde.telematic Wed Feb 14 07:16 - down (05:50) [root@rhel4a ~]# The last command can also be used to get a list of last reboots. [paul@rekkie ~]$ last reboot reboot system boot 2.6.16-rekkie Mon Jul 30 05:13 (370+08:42) wtmp begins Tue May 30 23:11:45 2006 [paul@rekkie ~]$ /var/log/lastlog (lastlog) Use lastlog to see the /var/log/lastlog file. 374
- 381. logging [root@rhel4a ~]# lastlog | tail tim pts/5 10.170.1.122 Tue Feb 13 09:36:54 +0100 2007 rm pts/6 rhel4 Tue Feb 13 10:06:56 +0100 2007 henk **Never logged in** stefaan pts/3 pc-sde.telematic Wed Feb 14 12:28:38 +0100 2007 dirk pts/5 pc-dss.telematic Wed Feb 14 10:03:11 +0100 2007 arsene **Never logged in** nicolas pts/5 pc-dss.telematic Wed Feb 14 12:32:18 +0100 2007 dimitri pts/5 rhel4 Wed Feb 14 07:57:19 +0100 2007 bashuserrm pts/7 rhel4 Tue Feb 13 10:35:40 +0100 2007 kornuserrm pts/5 rhel4 Tue Feb 13 10:06:17 +0100 2007 [root@rhel4a ~]# /var/log/btmp (lastb) There is also the lastb command to display the /var/log/btmp file. This file is updated by the login program when entering the wrong password, so it contains failed login attempts. Many computers will not have this file, resulting in no logging of failed login attempts. [root@RHEL4b ~]# lastb lastb: /var/log/btmp: No such file or directory Perhaps this file was removed by the operator to prevent logging lastb info. [root@RHEL4b ~]# The reason given for this is that users sometimes type their password by mistake instead of their login, so this world readable file poses a security risk. You can enable bad login logging by simply creating the file. Doing a chmod o-r /var/log/ btmp improves security. [root@RHEL4b ~]# touch /var/log/btmp [root@RHEL4b ~]# ll /var/log/btmp -rw-r--r-- 1 root root 0 Jul 30 06:12 /var/log/btmp [root@RHEL4b ~]# chmod o-r /var/log/btmp [root@RHEL4b ~]# lastb btmp begins Mon Jul 30 06:12:19 2007 [root@RHEL4b ~]# Failed logins via ssh, rlogin or su are not registered in /var/log/btmp. Failed logins via tty are. [root@RHEL4b ~]# lastb HalvarFl tty3 Mon Jul 30 07:10 - 07:10 (00:00) Maria tty1 Mon Jul 30 07:09 - 07:09 (00:00) Roberto tty1 Mon Jul 30 07:09 - 07:09 (00:00) btmp begins Mon Jul 30 07:09:32 2007 [root@RHEL4b ~]# 375
- 382. logging su and ssh logins Depending on the distribution, you may also have the /var/log/secure file being filled with messages from the auth and/or authpriv syslog facilities. This log will include su and/or ssh failed login attempts. Some distributions put this in /var/log/auth.log, verify the syslog configuration. [root@RHEL4b ~]# cat /var/log/secure Jul 30 07:09:03 sshd[4387]: Accepted publickey for paul from ::ffff:19 2.168.1.52 port 33188 ssh2 Jul 30 05:09:03 sshd[4388]: Accepted publickey for paul from ::ffff:19 2.168.1.52 port 33188 ssh2 Jul 30 07:22:27 sshd[4655]: Failed password for Hermione from ::ffff:1 92.168.1.52 port 38752 ssh2 Jul 30 05:22:27 sshd[4656]: Failed password for Hermione from ::ffff:1 92.168.1.52 port 38752 ssh2 Jul 30 07:22:30 sshd[4655]: Failed password for Hermione from ::ffff:1 92.168.1.52 port 38752 ssh2 Jul 30 05:22:30 sshd[4656]: Failed password for Hermione from ::ffff:1 92.168.1.52 port 38752 ssh2 Jul 30 07:22:33 sshd[4655]: Failed password for Hermione from ::ffff:1 92.168.1.52 port 38752 ssh2 Jul 30 05:22:33 sshd[4656]: Failed password for Hermione from ::ffff:1 92.168.1.52 port 38752 ssh2 Jul 30 08:27:33 sshd[5018]: Invalid user roberto from ::ffff:192.168.1 .52 Jul 30 06:27:33 sshd[5019]: input_userauth_request: invalid user rober to Jul 30 06:27:33 sshd[5019]: Failed none for invalid user roberto from ::ffff:192.168.1.52 port 41064 ssh2 Jul 30 06:27:33 sshd[5019]: Failed publickey for invalid user roberto from ::ffff:192.168.1.52 port 41064 ssh2 Jul 30 08:27:36 sshd[5018]: Failed password for invalid user roberto f rom ::ffff:192.168.1.52 port 41064 ssh2 Jul 30 06:27:36 sshd[5019]: Failed password for invalid user roberto f rom ::ffff:192.168.1.52 port 41064 ssh2 [root@RHEL4b ~]# You can enable this yourself, with a custom log file by adding the following line tot syslog.conf. auth.*,authpriv.* /var/log/customsec.log 376
- 383. logging 42.2. syslogd about syslog The standard method of logging on Linux is through the syslogd daemon. Syslog was developed by Eric Allman for sendmail, but quickly became a standard among many Unix applications and was much later written as rfc 3164. The syslog daemon can receive messages on udp port 514 from many applications (and appliances), and can append to log files, print, display messages on terminals and forward logs to other syslogd daemons on other machines. The syslogd daemon is configured in / etc/syslog.conf. Each line in the configuration file uses a facility to determine where the message is coming from. It also contains a level for the severity of the message, and an action to decide on what to do with the message. facilities The man syslog.conf will explain the different default facilities for certain daemons, such as mail, lpr, news and kern(el) messages. The local0 to local7 facility can be used for appliances (or any networked device that supports syslog). Here is a list of all facilities for syslog.conf version 1.3. The security keyword is deprecated. auth (security) authpriv cron daemon ftp kern lpr mail mark (internal use only) news syslog user uucp local0-7 levels The worst severity a message can have is emerg followed by alert and crit. Lowest priority should go to info and debug messages. Specifying a severity will also log all messages with a higher severity. You can prefix the severity with = to obtain only messages that match that severity. You can also specify .none to prevent a specific action from any message from a certain facility. Here is a list of all levels, in ascending order. The keywords warn, error and panic are deprecated. 377
- 384. logging debug info notice warning (warn) err (error) crit alert emerg (panic) actions The default action is to send a message to the username listed as action. When the action is prefixed with a / then syslog will send the message to the file (which can be a regular file, but also a printer or terminal). The @ sign prefix will send the message on to another syslog server. Here is a list of all possible actions. root,user1 list of users, separated by comma's * message to all logged on users / file (can be a printer, a console, a tty, ...) -/ file, but don't sync after every write | named pipe @ other syslog hostname In addition, you can prefix actions with a - to omit syncing the file after every logging. configuration Below a sample configuration of custom local4 messages in /etc/syslog.conf. local4.crit /var/log/critandabove local4.=crit /var/log/onlycrit local4.* /var/log/alllocal4 Don't forget to restart the server. [root@rhel4a ~]# /etc/init.d/syslog restart Shutting down kernel logger: [ OK ] Shutting down system logger: [ OK ] Starting system logger: [ OK ] Starting kernel logger: [ OK ] [root@rhel4a ~]# 378
- 385. logging 42.3. logger The logger command can be used to generate syslog test messages. You can aslo use it in scripts. An example of testing syslogd with the logger tool. [root@rhel4a ~]# logger -p local4.debug "l4 debug" [root@rhel4a ~]# logger -p local4.crit "l4 crit" [root@rhel4a ~]# logger -p local4.emerg "l4 emerg" [root@rhel4a ~]# The results of the tests with logger. [root@rhel4a ~]# cat /var/log/critandabove Feb 14 19:55:19 rhel4a paul: l4 crit Feb 14 19:55:28 rhel4a paul: l4 emerg [root@rhel4a ~]# cat /var/log/onlycrit Feb 14 19:55:19 rhel4a paul: l4 crit [root@rhel4a ~]# cat /var/log/alllocal4 Feb 14 19:55:11 rhel4a paul: l4 debug Feb 14 19:55:19 rhel4a paul: l4 crit Feb 14 19:55:28 rhel4a paul: l4 emerg [root@rhel4a ~]# 42.4. watching logs You might want to use the tail -f command to look at the last lines of a log file. The -f option will dynamically display lines that are appended to the log. paul@ubu1010:~$ tail -f /var/log/udev SEQNUM=1741 SOUND_INITIALIZED=1 ID_VENDOR_FROM_DATABASE=nVidia Corporation ID_MODEL_FROM_DATABASE=MCP79 High Definition Audio ID_BUS=pci ID_VENDOR_ID=0x10de ID_MODEL_ID=0x0ac0 ID_PATH=pci-0000:00:08.0 SOUND_FORM_FACTOR=internal You can automatically repeat commands by preceding them with the watch command. When executing the following: [root@rhel6 ~]# watch who Something similar to this, repeating the output of the who command every two seconds, will appear on the screen. Every 2.0s: who Sun Jul 17 15:31:03 2011 root tty1 2011-07-17 13:28 paul pts/0 2011-07-17 13:31 (192.168.1.30) paul pts/1 2011-07-17 15:19 (192.168.1.30) 379
- 386. logging 42.5. rotating logs A lot of log files are always growing in size. To keep this within bounds, you might want to use logrotate to rotate, compress, remove and mail log files. More info on the logrotate command in /etc/logrotate.conf.. Individual configurations can be found in the /etc/logrotate.d/ directory. In this screenshot the configuration file for the logfiles from aptitude to configure monthly rotates, keeping the last six and compressing old logs. paul@ubu1010:/var/log$ cat /etc/logrotate.d/aptitude /var/log/aptitude { rotate 6 monthly compress missingok notifempty } And this screenshot is the resolt of the above configuration, for the logfile from aptitude. paul@ubu1010:/var/log$ ls -l /var/log/aptitude* -rw-r--r-- 1 root root 18298 2011-07-17 13:32 /var/log/aptitude -rw-r--r-- 1 root root 8163 2011-07-01 01:43 /var/log/aptitude.1.gz -rw-r--r-- 1 root root 8163 2011-06-01 01:43 /var/log/aptitude.2.gz -rw-r--r-- 1 root root 8163 2011-05-01 01:43 /var/log/aptitude.3.gz 380
- 387. logging 42.6. practice : logging 1. Display the /var/run/utmp file with the proper command (not with cat or vi). 2. Display the /var/log/wtmp file. 3. Use the lastlog and lastb commands, understand the difference. 4. Examine syslog to find the location of the log file containing ssh failed logins. 5. Configure syslog to put local4.error and above messages in /var/log/l4e.log and local4.info only .info in /var/log/l4i.log. Test that it works with the logger tool! 6. Configure /var/log/Mysu.log, all the su to root messages should go in that log. Test that it works! 7. Send the local5 messages to the syslog server of your neighbour. Test that it works. 8. Write a script that executes logger to local4 every 15 seconds (different message). Use tail -f and watch on your local4 log files. 381
- 388. logging 42.7. solution : logging 1. Display the /var/run/utmp file. who 2. Display the /var/log/wtmp file. last 3. Use the lastlog and lastb commands, understand the difference. lastlog : when users last logged on lastb: failed (bad) login attempts 4. Examine syslog to find the location of the log file containing ssh failed logins. root@rhel53 ~# grep authpriv /etc/syslog.conf authpriv.* /var/log/secure Debian/Ubuntu: /var/log/auth.log Ubuntu 9.10 and Debian Lenny have switched to using rsyslog. root@ubuntu910:~# grep authpriv /etc/rsyslog.d/50-default.conf auth,authpriv.* /var/log/auth.log root@deb503:~# grep authpriv /etc/rsyslog.conf auth,authpriv.* /var/log/auth.log 5. Configure syslog to put local4.error and above messages in /var/log/l4e.log and local4.info only .info in /var/log/l4i.log. Test that it works with the logger tool! echo local4.error /var/log/l4e.log >> /etc/syslog.conf echo local4.=info /var/log/l4i.log >> /etc/syslog.conf /etc/init.d/syslog restart logger -p local4.error "l4 error test" logger -p local4.alert "l4 alert test" logger -p local4.info "l4 info test" cat /var/log/l4e.log cat /var/log/l4i.log 6. Configure /var/log/Mysu.log, all the su to root messages should go in that log. Test that it works! echo authpriv.* /var/log/Mysu.log >> /etc/syslog.conf This will log more than just the su usage. 382
- 389. logging 7. Send the local5 messages to the syslog server of your neighbour. Test that it works. On RHEL5, edit /etc/sysconfig/syslog to enable remote listening on the server. On Debian/Ubuntu edit /etc/default/syslog or /etc/default/rsyslog. on the client: logger -p local5.info "test local5 to neighbour" 8. Write a script that executes logger to local4 every 15 seconds (different message). Use tail -f and watch on your local4 log files. root@rhel53 scripts# cat logloop #!/bin/bash for i in `seq 1 10` do logger -p local4.info "local4.info test number $i" sleep 15 done root@rhel53 scripts# chmod +x logloop root@rhel53 scripts# ./logloop & [1] 8264 root@rhel53 scripts# tail -f /var/log/local4.all.log Mar 28 13:13:36 rhel53 root: local4.info test number 1 Mar 28 13:13:51 rhel53 root: local4.info test number 2 ... 383
- 390. Chapter 43. memory management Table of Contents 43.1. displaying memory and cache ................................................................... 385 43.2. managing swap space ................................................................................ 386 43.3. monitoring memory with vmstat ................................................................ 388 43.4. practice : memory ...................................................................................... 389 43.5. solution : memory ...................................................................................... 390 This chapter will tell you how to manage RAM memory and cache. We start with some simple tools to display information about memory: free -om, top and cat /proc/meminfo. We continue with managing swap space, using terms like swapping, paging and virtual memory. The last part is about using vmstat to monitor swap usage. 384
- 391. memory management 43.1. displaying memory and cache /proc/meminfo Displaying /proc/meminfo will tell you a lot about the memory on your Linux computer. paul@ubu1010:~$ cat /proc/meminfo MemTotal: 3830176 kB MemFree: 244060 kB Buffers: 41020 kB Cached: 2035292 kB SwapCached: 9892 kB ... The first line contains the total amount of physical RAM, the second line is the unused RAM. Buffers is RAM used for buffering files, cached is the amount of RAM used as cache and SwapCached is the amount of swap used as cache. The file gives us much more information outside of the scope of this course. free The free tool can display the information provided by /proc/meminfo in a more readable format. The example below displays brief memory information in megabytes. paul@ubu1010:~$ free -om total used free shared buffers cached Mem: 3740 3519 221 0 42 1994 Swap: 6234 82 6152 top The top tool is often used to look at processes consuming most of the cpu, but it also displays memory information on line four and five (which can be toggled by pressing m). Below a screenshot of top on the same ubu1010 from above. top - 10:44:34 up 16 days, 9:56, 6 users, load average: 0.13, 0.09, 0.12 Tasks: 166 total, 1 running, 165 sleeping, 0 stopped, 0 zombie Cpu(s): 5.1%us, 4.6%sy, 0.6%ni, 88.7%id, 0.8%wa, 0.0%hi, 0.3%si, 0.0%st Mem: 3830176k total, 3613720k used, 216456k free, 45452k buffers Swap: 6384636k total, 84988k used, 6299648k free, 2050948k cached 385
- 392. memory management 43.2. managing swap space about swap space When the operating system needs more memory than physically present in RAM, it can use swap space. Swap space is located on slower but cheaper memory. Notice that, although hard disks are commonly used for swap space, their access times are one hundred thousand times slower. The swap space can be a file, a partition, or a combination of files and partitions. You can see the swap space with the free command, or with cat /proc/swaps. paul@ubu1010:~$ free -o | grep -v Mem total used free shared buffers cached Swap: 6384636 84988 6299648 paul@ubu1010:~$ cat /proc/swaps Filename Type Size Used Priority /dev/sda3 partition 6384636 84988 -1 The amount of swap space that you need depends heavily on the services that the computer provides. creating a swap partition You can activate or deactivate swap space with the swapon and swapoff commands. New swap space can be created with the mkswap command. The screenshot below shows the creation and activation of a swap partition. root@RHELv4u4:~# fdisk -l 2> /dev/null | grep hda Disk /dev/hda: 536 MB, 536870912 bytes /dev/hda1 1 1040 524128+ 83 Linux root@RHELv4u4:~# mkswap /dev/hda1 Setting up swapspace version 1, size = 536702 kB root@RHELv4u4:~# swapon /dev/hda1 Now you can see that /proc/swaps displays all swap spaces separately, whereas the free -om command only makes a human readable summary. root@RHELv4u4:~# cat /proc/swaps Filename Type Size Used Priority /dev/mapper/VolGroup00-LogVol01 partition 1048568 0 -1 /dev/hda1 partition 524120 0 -2 root@RHELv4u4:~# free -om total used free shared buffers cached Mem: 249 245 4 0 125 54 Swap: 1535 0 1535 creating a swap file Here is one more example showing you how to create a swap file. On Solaris you can use mkfile instead of dd. 386
- 393. memory management root@RHELv4u4:~# dd if=/dev/zero of=/smallswapfile bs=1024 count=4096 4096+0 records in 4096+0 records out root@RHELv4u4:~# mkswap /smallswapfile Setting up swapspace version 1, size = 4190 kB root@RHELv4u4:~# swapon /smallswapfile root@RHELv4u4:~# cat /proc/swaps Filename Type Size Used Priority /dev/mapper/VolGroup00-LogVol01 partition 1048568 0 -1 /dev/hda1 partition 524120 0 -2 /smallswapfile file 4088 0 -3 swap space in /etc/fstab If you like these swaps to be permanent, then don't forget to add them to /etc/fstab. The lines in /etc/fstab will be similar to the following. /dev/hda1 swap swap defaults 0 0 /smallswapfile swap swap defaults 0 0 387
- 394. memory management 43.3. monitoring memory with vmstat You can find information about swap usage using vmstat. Below a simple vmstat displaying information in megabytes. paul@ubu1010:~$ vmstat -S m procs ---------memory-------- ---swap-- -----io---- -system- ----cpu---- r b swpd free buff cache si so bi bo in cs us sy id wa 0 0 87 225 46 2097 0 0 2 5 14 8 6 5 89 1 Below a sample vmstat when (in another terminal) root launches a find /. It generates a lot of disk i/o (bi and bo are disk blocks in and out). There is no need for swapping here. paul@ubu1010:~$ vmstat 2 100 procs ----------memory---------- ---swap-- -----io---- -system-- ----cpu---- r b swpd free buff cache si so bi bo in cs us sy id wa 0 0 84984 1999436 53416 269536 0 0 2 5 2 10 6 5 89 1 0 0 84984 1999428 53416 269564 0 0 0 0 1713 2748 4 4 92 0 0 0 84984 1999552 53416 269564 0 0 0 0 1672 1838 4 6 90 0 0 0 84984 1999552 53424 269560 0 0 0 14 1587 2526 5 7 87 2 0 0 84984 1999180 53424 269580 0 0 0 100 1748 2193 4 6 91 0 1 0 84984 1997800 54508 269760 0 0 610 0 1836 3890 17 10 68 4 1 0 84984 1994620 55040 269748 0 0 250 168 1724 4365 19 17 56 9 0 1 84984 1978508 55292 269704 0 0 126 0 1957 2897 19 18 58 4 0 0 84984 1974608 58964 269784 0 0 1826 478 2605 4355 7 7 44 41 0 2 84984 1971260 62268 269728 0 0 1634 756 2257 3865 7 7 47 39 Below a sample vmstat when executing (on RHEL6) a simple memory leaking program. Now you see a lot of memory being swapped (si is 'swapped in'). [paul@rhel6c ~]$ vmstat 2 100 procs ----------memory-------- ---swap-- ----io---- --system-- -----cpu----- r b swpd free buff cache si so bi bo in cs us sy id wa st 0 3 245208 5280 232 1916 261 0 0 42 27 21 0 1 98 1 0 0 2 263372 4800 72 908 143840 128 0 1138 462 191 2 10 0 88 0 1 3 350672 4792 56 992 169280 256 0 1092 360 142 1 13 0 86 0 1 4 449584 4788 56 1024 95880 64 0 606 471 191 2 13 0 85 0 0 4 471968 4828 56 1140 44832 80 0 390 235 90 2 12 0 87 0 3 5 505960 4764 56 1136 68008 16 0 538 286 109 1 12 0 87 0 The code below was used to simulate a memory leak (and force swapping). This code was found on wikipedia without author. paul@mac:~$ cat memleak.c #include <stdlib.h> int main(void) { while (malloc(50)); return 0; } 388
- 395. memory management 43.4. practice : memory 1. Use dmesg to find the total amount of memory in your computer. 2. Use free to display memory usage in kilobytes (then in megabytes). 3. On a virtual machine, create a swap partition (you might need an extra virtual disk for this). 4. Add a 20 megabyte swap file to the system. 5. Put all swap spaces in /etc/fstab and activate them. Test with a reboot that they are mounted. 6. Use free to verify usage of current swap. 7. (optional) Display the usage of swap with vmstat and free -s during a memory leak. 389
- 396. memory management 43.5. solution : memory 1. Use dmesg to find the total amount of memory in your computer. dmesg | grep Memory 2. Use free to display memory usage in kilobytes (then in megabytes). free ; free -m 3. On a virtual machine, create a swap partition (you might need an extra virtual disk for this). mkswap /dev/sdd1 ; swapon /dev/sdd1 4. Add a 20 megabyte swap file to the system. dd if=/dev/zero of=/swapfile20mb bs=1024 count=20000 mkswap /swapfile20mb swapon /swapfile20mb 5. Put all swap spaces in /etc/fstab and activate them. Test with a reboot that they are mounted. root@computer# tail -2 /etc/fstab /dev/sdd1 swap swap defaults 0 0 /swapfile20mb swap swap defaults 0 0 6. Use free to verify usage of current swap. free -om 7. (optional) Display the usage of swap with vmstat and free -s during a memory leak. 390
- 397. Chapter 44. package management Table of Contents 44.1. terminology ................................................................................................ 392 44.2. rpm ............................................................................................................. 393 44.3. yum ............................................................................................................. 396 44.4. deb .............................................................................................................. 401 44.5. aptitude ....................................................................................................... 402 44.6. alien ............................................................................................................ 402 44.7. downloading software outside the repository ............................................ 404 44.8. compiling software ..................................................................................... 404 44.9. practice: package management .................................................................. 405 44.10. solution: package management ................................................................ 406 Most Linux distributions have a package management system with online repositories of the most common software packages. This makes it very easy to install and remove software. We discuss the two most used package formats and tools in this chapter, rpm based and deb based. We also briefly discuss the option of downloading source code and compiling the software yourself. 391
- 398. package management 44.1. terminology repositories Most software for your Linux distribution is available in a central distributed repository. This means that applications in the repository are tested for your distribution and very easy to install with a GUI or command line installer. The GUI is available via the standard menu (look for Add/Remove Software or something similar). The command line is explained below in detail. rpm based Red Hat, Fedora, OpenSUSE, Mandriva, Red Flag and others use .rpm packages. The tools to manage software packages are called yum and rpm. deb based Debian, Ubuntu, Mint and all derivatives from Debian and Ubuntu use .deb packages. To manage software on these systems, you can use aptitude. aptitude is a front end for dpkg. dependency Some packages need other packages to function. Tools like aptitude and yum will install all dependencies you need. When using dpkg or the rpm command, or when building from source, you will need to install dependencies yourself. 392
- 399. package management 44.2. rpm about rpm The Red Hat package manager can be used on the command line with rpm or in a graphical way going to Applications--System Settings--Add/Remove Applications. Type rpm --help to see some of the options. Software distributed in the rpm format will be named foo-version.platform.rpm . rpm -qa To obtain a list of all installed software, use the rpm -qa command. [root@RHEL52 ~]# rpm -qa | grep samba system-config-samba-1.2.39-1.el5 samba-3.0.28-1.el5_2.1 samba-client-3.0.28-1.el5_2.1 samba-common-3.0.28-1.el5_2.1 rpm -q To verify whether one package is installed, use rpm -q. root@RHELv4u4:~# rpm -q gcc gcc-3.4.6-3 root@RHELv4u4:~# rpm -q laika package laika is not installed rpm -q --redhatprovides To check whether a package is provided by Red Hat, use the --redhatprovides option. root@RHELv4u4:~# rpm -q --redhatprovides bash bash-3.0-19.3 root@RHELv4u4:~# rpm -q --redhatprovides gcc gcc-3.4.6-3 root@RHELv4u4:~# rpm -q --redhatprovides laika no package provides laika rpm -Uvh To install or upgrade a package, use the -Uvh switches. The -U switch is the same as -i for install, except that older versions of the software are removed. The -vh switches are for nicer output. 393
- 400. package management root@RHELv4u4:~# rpm -Uvh gcc-3.4.6-3 rpm -e To remove a package, use the -e switch. root@RHELv4u4:~# rpm -e gcc-3.4.6-3 rpm -e verifies dependencies, and thus will prevent you from accidentailly erasing packages that are needed by other packages. [root@RHEL52 ~]# rpm -e gcc-4.1.2-42.el5 error: Failed dependencies: gcc = 4.1.2-42.el5 is needed by (installed) gcc-c++-4.1.2-42.el5.i386 gcc = 4.1.2-42.el5 is needed by (installed) gcc-gfortran-4.1.2-42.el5.i386 gcc is needed by (installed) systemtap-0.6.2-1.el5_2.2.i386 /var/lib/rpm The rpm database is located at /var/lib/rpm. This database contains all meta information about packages that are installed (via rpm). It keeps track of all files, which enables complete removes of software. rpm2cpio We can use rpm2cpio to convert an rpm to a cpio archive. [root@RHEL53 ~]# file kernel.src.rpm kernel.src.rpm: RPM v3 src PowerPC kernel-2.6.18-92.1.13.el5 [root@RHEL53 ~]# rpm2cpio kernel.src.rpm > kernel.cpio [root@RHEL53 ~]# file kernel.cpio kernel.cpio: ASCII cpio archive (SVR4 with no CRC) But why would you want to do this ? Perhaps just to see of list of files in the rpm file. [root@RHEL53 ~]# rpm2cpio kernel.src.rpm | cpio -t | head -5 COPYING.modules Config.mk Module.kabi_i686 Module.kabi_i686PAE Module.kabi_i686xen Or to extract one file from an rpm package. [root@RHEL53 ~]# rpm2cpio kernel.src.rpm | cpio -iv Config.mk Config.mk 246098 blocks 394
- 401. package management 395
- 402. package management 44.3. yum about yum The Yellowdog Updater, Modified (yum) is an easier command to work with rpm packages. It is installed by default on Fedora and Red Hat Enterprise Linux since version 5.2. yum list Issue yum list available to see a list of available packages. The available parameter is optional. [root@rhel55 ~]# yum list | wc -l 2471 Issue yum list $package to get all versions (in different repositories) of one package. [root@rhel55 ~]# yum list samba Loaded plugins: rhnplugin, security Installed Packages samba.i386 3.0.33-3.28.el5 installed Available Packages samba.i386 3.0.33-3.29.el5_5 rhel-i386-server-5 yum search To search for a package containing a certain string in the description or name use yum search $string. [root@rhel55 ~]# yum search gcc44 Loaded plugins: rhnplugin, security ========================== Matched: gcc44 =========================== gcc44.i386 : Preview of GCC version 4.4 gcc44-c++.i386 : C++ support for GCC version 4.4 gcc44-gfortran.i386 : Fortran support for GCC 4.4 previe yum provides To search for a package containing a certain file (you might need for compiling things) use yum provides $filename. [root@rhel55 ~]# yum provides /usr/share/man/man1/gzip.1.gz Loaded plugins: rhnplugin, security Importing additional filelist information gzip-1.3.5-9.el5.i386 : The GNU data compression program. Repo : rhel-i386-server-5 Matched from: Filename : /usr/share/man/man1/gzip.1.gz ... 396
- 403. package management yum install To install an application, use yum install $package. Naturally yum will install all the necessary dependencies. [root@rhel55 ~]# yum install sudo Loaded plugins: rhnplugin, security Setting up Install Process Resolving Dependencies --> Running transaction check ---> Package sudo.i386 0:1.7.2p1-7.el5_5 set to be updated --> Finished Dependency Resolution Dependencies Resolved ======================================================================= Package Arch Version Repository Size ======================================================================= Installing: sudo i386 1.7.2p1-7.el5_5 rhel-i386-server-5 230 k Transaction Summary ======================================================================= Install 1 Package(s) Upgrade 0 Package(s) Total download size: 230 k Is this ok [y/N]: y Downloading Packages: sudo-1.7.2p1-7.el5_5.i386.rpm | 230 kB 00:00 Running rpm_check_debug Running Transaction Test Finished Transaction Test Transaction Test Succeeded Running Transaction Installing : sudo 1/1 Installed: sudo.i386 0:1.7.2p1-7.el5_5 Complete! You can add more than one parameter here. yum install $package1 $package2 $package3 yum update To bring all applications up to date, by downloading and installing them, issue yum update. All software that was installed via yum will be updated to the latest version that is available in the repository. yum update If you only want to update one package, use yum update $package. [root@rhel55 ~]# yum update sudo Loaded plugins: rhnplugin, security Skipping security plugin, no data Setting up Update Process 397
- 404. package management Resolving Dependencies Skipping security plugin, no data --> Running transaction check ---> Package sudo.i386 0:1.7.2p1-7.el5_5 set to be updated --> Finished Dependency Resolution Dependencies Resolved ===================================================================== Package Arch Version Repository Size ===================================================================== Updating: sudo i386 1.7.2p1-7.el5_5 rhel-i386-server-5 230 k Transaction Summary ===================================================================== Install 0 Package(s) Upgrade 1 Package(s) Total download size: 230 k Is this ok [y/N]: y Downloading Packages: sudo-1.7.2p1-7.el5_5.i386.rpm | 230 kB 00:00 Running rpm_check_debug Running Transaction Test Finished Transaction Test Transaction Test Succeeded Running Transaction Updating : sudo 1/2 Cleanup : sudo 2/2 Updated: sudo.i386 0:1.7.2p1-7.el5_5 Complete! yum software groups Issue yum grouplist to see a list of all available software groups. [root@rhel55 ~]# yum grouplist Loaded plugins: rhnplugin, security Setting up Group Process Installed Groups: Administration Tools Authoring and Publishing DNS Name Server Development Libraries Development Tools Editors GNOME Desktop Environment GNOME Software Development Graphical Internet Graphics Legacy Network Server Legacy Software Development Legacy Software Support Mail Server Network Servers Office/Productivity Printing Support Server Configuration Tools 398
- 405. package management System Tools Text-based Internet Web Server Windows File Server X Software Development X Window System Available Groups: Engineering and Scientific FTP Server Games and Entertainment Java Development KDE (K Desktop Environment) KDE Software Development MySQL Database News Server OpenFabrics Enterprise Distribution PostgreSQL Database Sound and Video Done To install a set of applications, brought together via a group, use yum groupinstall $groupname. [root@rhel55 ~]# yum groupinstall 'Sound and video' Loaded plugins: rhnplugin, security Setting up Group Process Package alsa-utils-1.0.17-1.el5.i386 already installed and latest version Package sox-12.18.1-1.i386 already installed and latest version Package 9:mkisofs-2.01-10.7.el5.i386 already installed and latest version Package 9:cdrecord-2.01-10.7.el5.i386 already installed and latest version Package cdrdao-1.2.1-2.i386 already installed and latest version Resolving Dependencies --> Running transaction check ---> Package cdda2wav.i386 9:2.01-10.7.el5 set to be updated ---> Package cdparanoia.i386 0:alpha9.8-27.2 set to be updated ---> Package sound-juicer.i386 0:2.16.0-3.el5 set to be updated --> Processing Dependency: libmusicbrainz >= 2.1.0 for package: sound-juicer --> Processing Dependency: libmusicbrainz.so.4 for package: sound-juicer ---> Package vorbis-tools.i386 1:1.1.1-3.el5 set to be updated --> Processing Dependency: libao >= 0.8.4 for package: vorbis-tools --> Processing Dependency: libao.so.2 for package: vorbis-tools --> Running transaction check ---> Package libao.i386 0:0.8.6-7 set to be updated ---> Package libmusicbrainz.i386 0:2.1.1-4.1 set to be updated --> Finished Dependency Resolution ... Read the manual page of yum for more information about managing groups in yum. /etc/yum.conf and repositories The configuration of yum repositories is done in /etc/yum/yum.conf and /etc/yum/ repos.d/. Configurating yum itself is done in /etc/yum.conf. This file will contain the location of a log file and a cache directory for yum and can also contain a list of repositories. Recently yum started accepting several repo files with each file containing a list of repositories. These repo files are located in the /etc/yum.repos.d/ directory. 399
- 406. package management One important flag for yum is enablerepo. Use this command if you want to use a repository that is not enabled by default. yum $command $foo --enablerepo=$repo An example of the contents of the repo file: MyRepo.repo [$repo] name=My Repository baseurl=http://path/to/MyRepo gpgcheck=1 gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-MyRep 400
- 407. package management 44.4. deb about deb Most people use aptitude or apt-get to manage their Debian/Ubuntu family of Linux distributions. Both are a front end for dpkg and are themselves a back end for synaptic or other graphical tools. This is a screenshot of synaptic running on Debian 4.0. dpkg -l The low level tool to work with .deb packages is dpkg. Here you see how to obtain a list of all installed packages. The ii at the beginning means the package is installed. root@laika:~# dpkg -l | grep gcc-4.2 ii gcc-4.2 4.2.4-1ubuntu3 The GNU C compiler ii gcc-4.2-base 4.2.4-1ubuntu3 The GNU Compiler Collection (base package) dpkg You could use dpkg -i to install a package and dpkg -r to remove a package, but you'd have to manually keep track of dependencies. 401
- 408. package management 44.5. aptitude Most people use aptitude for package management on Debian and Ubuntu Systems. To synchronize with the repositories. aptitude update To patch and upgrade all software to the latest version on Debian. aptitude upgrade To patch and upgrade all software to the latest version on Ubuntu and Mint. aptitude safe-upgrade To install an application with all dependencies. aptitude install $package To search the repositories for applications that contain a certain string in their name or description. aptitude search $string To remove an application and all unused files. aptitude remove $package apt-get We could also use apt-get, but aptitude is better at handling dependencies than apt- get. Whenever you see apt-get in a howto, feel free to type aptitude. /etc/apt/sources.list The resource list for both apt-get and aptitude is located in /etc/apt/sources.list. This file contains a list of http or ftp sources where packages for the distribution can be downloaded. root@barry:~# cat /etc/apt/sources.list deb http://ftp.be.debian.org/debian/ etch main non-free contrib # deb http://ftp.be.debian.org/debian/ sarge main non-free contrib # deb http://ftp.be.debian.org/debian/ unstable main non-free contrib # deb-src http://ftp.be.debian.org/debian/ etch main non-free contrib deb http://security.debian.org/ etch/updates main # deb-src http://security.debian.org/ etch/updates main 44.6. alien alien is experimental software that converts between rpm and deb package formats (and others). 402
- 409. package management Below an example of how to use alien to convert an rpm package to a deb package. paul@barry:~$ ls -l netcat* -rw-r--r-- 1 paul paul 123912 2009-06-04 14:58 netcat-0.7.1-1.i386.rpm paul@barry:~$ alien --to-deb netcat-0.7.1-1.i386.rpm netcat_0.7.1-2_i386.deb generated paul@barry:~$ ls -l netcat* -rw-r--r-- 1 paul paul 123912 2009-06-04 14:58 netcat-0.7.1-1.i386.rpm -rw-r--r-- 1 root root 125236 2009-06-04 14:59 netcat_0.7.1-2_i386.deb In real life, use the netcat tool provided by your distribution, or use the .deb file from their website. 403
- 410. package management 44.7. downloading software outside the repository First and most important, whenever you download software, start by reading the README file! Normally the readme will explain what to do after download. You will probably receive a .tar.gz or a .tgz file. Read the documentation, then put the compressed file in a directory. You can use the following to find out where the package wants to install. tar tvzpf $downloadedFile.tgz You unpack them like with tar xzf, it will create a directory called applicationName-1.2.3 tar xzf $applicationName.tgz Replace the z with a j when the file ends in .tar.bz2. The tar, gzip and bzip2 commands are explained in detail in the Linux Fundamentals course. If you download a .deb file, then you'll have to use dpkg to install it, .rpm's can be installed with the rpm command. 44.8. compiling software First and most important, whenever you download source code for installation, start by reading the README file! Usually the steps are always the same three : running ./configure followed by make (which is the actual compiling) and then by make install to copy the files to their proper location. ./configure make make install 404
- 411. package management 44.9. practice: package management 1. Find the Graphical application on all computers to add and remove applications. 2. Verify on both systems whether gcc is installed. 3. Use aptitude or yum to search for and install the 'dict', 'samba' and 'wesnoth' applications. Did you find all them all ? 4. Search the internet for 'webmin' and install it. 5. If time permits, uninstall Samba from the ubuntu machine, download the latest version from samba.org and install it. 405
- 412. package management 44.10. solution: package management 1. Find the Graphical application on all computers to add and remove applications. 2. Verify on both systems whether gcc is installed. dpkg -l | grep gcc rpm -qa | grep gcc 3. Use aptitude or yum to search for and install the 'dict', 'samba' and 'wesnoth' applications. Did you find all them all ? aptitude search wesnoth (Debian, Ubuntu and family) yum search wesnoth (Red Hat and family) 4. Search the internet for 'webmin' and install it. Google should point you to webmin.com. There are several formats available there choose .rpm, .deb or .tgz . 5. If time permits, uninstall Samba from the ubuntu machine, download the latest version from samba.org and install it. 406
- 413. Part XIII. network management
- 414. Chapter 45. general networking Table of Contents 45.1. network layers ............................................................................................ 409 45.2. unicast, multicast, broadcast, anycast ........................................................ 412 45.3. lan-wan-man ............................................................................................... 414 45.4. internet - intranet - extranet ....................................................................... 415 45.5. tcp/ip ........................................................................................................... 416 While this chapter is not directly about Linux, it does contain general networking concepts that will help you in troubleshooting networks on Linux. 408
- 415. general networking 45.1. network layers seven OSI layers When talking about protocol layers, people usually mention the seven layers of the osi protocol (Application, Presentation, Session, Transport, Network, Data Link and Physical). We will discuss layers 2 and 3 in depth, and focus less on the other layers. The reason is that these layers are important for understanding networks. You will hear administrators use words like "this is a layer 2 device" or "this is a layer 3 broadcast", and you should be able to understand what they are talking about. four DoD layers The DoD (or tcp/ip) model has only four layers, roughly mapping its network access layer to OSI layers 1 and 2 (Physical and Datalink), its internet (IP) layer to the OSI network layer, its host-to-host (tcp, udp) layer to OSI layer 4 (transport) and its application layer to OSI layers 5, 6 and 7. Below an attempt to put OSI and DoD layers next to some protocols and devices. short introduction to the physical layer The physical layer, or layer 1, is all about voltage, electrical signals and mechanical connections. Some networks might still use coax cables, but most will have migrated to utp (cat 5 or better) with rj45 connectors. 409
- 416. general networking Devices like repeaters and hubs are part of this layer. You cannot use software to 'see' a repeater or hub on the network. The only thing these devices are doing is amplifying electrical signals on cables. Passive hubs are multiport amplifiers that amplify an incoming electrical signal on all other connections. Active hubs do this by reading and retransmitting bits, without interpreting any meaning in those bits. Network technologies like csma/cd and token ring are defined on this layer. This is all we have to say about layer 1 in this book. short introduction to the data link layer The data link layer, or layer 2 is about frames. A frame has a crc (cyclic redundancy check). In the case of ethernet (802.3), each network card is identifiable by a unique 48-bit mac address (media access control address). On this layer we find devices like bridges and switches. A bridge is more intelligent than a hub because a bridge can make decisions based on the mac address of computers. A switch also understands mac addresses. In this book we will discuss commands like arp and ifconfig to explore this layer. short introduction to the network layer Layer 3 is about ip packets. This layer gives every host a unique 32-bit ip address. But ip is not the only protocol on this layer, there is also icmp, igmp, ipv6 and more. A complete list can be found in the /etc/protocols file. On this layer we find devices like routers and layer 3 switches, devices that know (and have) an ip address. In tcp/ip this layer is commonly referred to as the internet layer. short introduction to the transport layer We will discuss the tcp and udp protocols in the context of layer 4. The DoD model calls this the host-to-host layer. layers 5, 6 and 7 The tcp/ip application layer includes layers 5, 6 and 7. Details on the difference between these layers are out of scope of this course. network layers in this book Stacking of layers in this book is based on the Protocols in Frame explanation in the wireshark sniffer. When sniffing a dhcp packet, we notice the following in the sniffer. 410
- 417. general networking [Protocols in Frame: eth:ip:udp:bootp] Sniffing for ntp (Network Time Protocol) packets gives us this line, which makes us conclude to put ntp next to bootp in the protocol chart below. [Protocols in Frame: eth:ip:udp:ntp] Sniffing an arp broadcast makes us put arp next to ip. All these protocols are explained later in this chapter. [Protocols in Frame: eth:arp] Below is a protocol chart based on wireshark's knowledge. It contains some very common protocols that are discussed in this book. The chart does not contain all protocols. 411
- 418. general networking 45.2. unicast, multicast, broadcast, anycast unicast A unicast communication originates from one computer and is destined for exactly one other computer (or host). It is common for computers to have many unicast communications. multicast A multicast is destined for a group (of computers). Some examples of multicast are Realplayer (.sdp files) and ripv2 (a routing protocol). 412
- 419. general networking broadcast A broadcast is meant for everyone. Typical example here is the BBC (British Broadcasting Corporation) broadcasting to everyone. In datacommunications a broadcast is most common confined to the lan. Careful, a layer 2 broadcast is very different from a layer 3 broadcast. A layer two broadcast is received by all network cards on the same segment (it does not pass any router), whereas a layer 3 broadcast is received by all hosts in the same ip subnet. anycast The root name servers of the internet use anycast. An anycast signal goes the the (geographically) nearest of a well defined group. With thanks to the nice anonymous wikipedia contributor to put these pictures in the public domain. 413
- 420. general networking 45.3. lan-wan-man The term lan is used for local area networks, as opposed to a wan for wide area networks. The difference between the two is determined by the distance between the computers, and not by the number of computers in a network. Some protocols like atm are designed for use in a wan, others like ethernet are designed for use in a lan. lan A lan (Local Area Network) is a local network. This can be one room, or one floor, or even one big building. We say lan as long as computers are close to each other. You can also define a lan when all computers are ethernet connected. A lan can contain multiple smaller lan's. The picture below shows three lan's that together make up one lan. wan A wan (Wide Area Network) is a network with a lot of distance between the computers (or hosts). These hosts are often connected by leased lines. A wan does not use ethernet, but protocols like fddi, frame relay, ATM or X.25 to connect computers (and networks). The picture below shows a branch office that is connected through Frame Relay with headquarters. 414
- 421. general networking The acronym wan is also used for large surface area networks like the internet. Cisco is known for their wan technology. They make routers that connect many lan networks using wan protocols. man A man (Metropolitan Area Network) is something inbetween a lan and a wan, often comprising several buildings on the same campus or in the same city. A man can use fddi or ethernet or other protocols for connectivity. pan-wpan Your home network is called a pan (Personal Area Network). A wireless pan is a wpan. 45.4. internet - intranet - extranet The internet is a global network. It connects many networks using the tcp/ip protocol stack. The origin of the internet is the arpanet. The arpanet was created in 1969, that year only four computers were connected in the network. In 1971 the first e-mail was sent over the arpanet. E-mail took 75 percent of all arpanet traffic in 1973. 1973 was also the year ftp was introduced, and saw the connection of the first European countries (Norway and UK). In 2009 the internet was available to 25 percent of the world population. In 2011 it is estimated that only a quarter of internet webpages are in English. An intranet is a private tcp/ip network. An intranet uses the same protocols as the internet, but is only accessible to people from within one organization. An extranet is similar to an intranet, but some trusted organizations (partners/ clients/suppliers/...) also get access. 415
- 422. general networking 45.5. tcp/ip history of tcp/ip In the Sixties development of the tcp/ip protocol stack was started by the US Department of Defense. In the Eighties a lot of commercial enterprises developed their own protocol stack: IBM created sna, Novell had ipx/spx, Microsoft completed netbeui and Apple worked with appletalk. All the efforts from the Eighties failed to survive the Nineties. By the end of the Nineties, almost all computers in the world were able to speak tcp/ip. In my humble opinion, the main reason for the survival of tcp/ip over all the other protocols is its openness. Everyone is free to develop and use the tcp/ip protocol suite. rfc (request for comment) The protocols that are used on the internet are defined in rfc's. An rfc or request for comment describes the inner working of all internet protocols. The IETF (Internet Engineering Task Force) is the sole publisher of these protocols since 1986. The official website for the rfc's is http://www.rfc-editor.org. This website contains all rfc's in plain text, for example rfc2132 (which defines dhcp and bootp) is accessible at http://www.rfc-editor.org/rfc/rfc2132.txt. many protocols For reliable connections, you use tcp, whereas udp is connectionless but faster. The icmp error messages are used by ping, multicast groups are managed by igmp. These protocols are visible in the protocol field of the ip header, and are listed in the /etc/protocols file. paul@debian5:~$ grep tcp /etc/protocols tcp 6 TCP # transmission control protocol many services Network cards are uniquely identified by their mac address, hosts by their ip address and applications by their port number. Common application level protocols like smtp, http, ssh, telnet and ftp have fixed port numbers. There is a list of port numbers in /etc/services. paul@ubu1010:~$ grep ssh /etc/services ssh 22/tcp # SSH Remote Login Protocol ssh 22/udp 416
- 423. Chapter 46. interface configuration Table of Contents 46.1. to gui or not to gui .................................................................................... 418 46.2. Debian/Ubuntu nic configuration ............................................................... 419 46.3. Red Hat/Fedora nic configuration .............................................................. 421 46.4. ifconfig ....................................................................................................... 423 46.5. hostname .................................................................................................... 425 46.6. arp ............................................................................................................... 426 46.7. route ........................................................................................................... 427 46.8. ping ............................................................................................................. 427 46.9. optional: ethtool ......................................................................................... 428 46.10. practice: interface configuration .............................................................. 429 46.11. solution: interface configuration .............................................................. 430 This chapter explains how to configure network interface cards to work with tcp/ip. 417
- 424. interface configuration 46.1. to gui or not to gui Recent Linux distributions often include a graphical application to configure the network. Some people complain that these applications mess networking configurations up when used simultaneously with command line configurations. Notably Network Manager (often replaced by wicd) and yast are known to not care about configuration changes via the command line. Since the goal of this course is server administration, we will assume our Linux servers are always administered through the command line. This chapter only focuses on using the command line for network interface configuration! Unfortunately there is no single combination of Linux commands and /etc files that works on all Linux distributions. We discuss networking on two (large but distinct) Linux distribution families. We start with Debian/Ubuntu, then continue with Fedora/RHEL. 418
- 425. interface configuration 46.2. Debian/Ubuntu nic configuration /etc/network/interfaces The /etc/network/interfaces file is a core network interface card configuration file on Ubuntu and Debian. dhcp client The screenshot below shows that our current Ubuntu 11.04 is configured for dhcp on eth0 (the first network interface card or nic). root@ubu1104srv:~# cat /etc/network/interfaces # This file describes the network interfaces available on your system # and how to activate them. For more information, see interfaces(5). # The loopback network interface auto lo iface lo inet loopback # The primary network interface auto eth0 iface eth0 inet dhcp Configuring network cards for dhcp is good practice for clients, but servers usually require a fixed ip address. fixed ip The screenshot below shows /etc/network/interfaces configured with a fixed ip address. root@ubu1104srv:~# cat /etc/network/interfaces # This file describes the network interfaces available on your system # and how to activate them. For more information, see interfaces(5). # The loopback network interface auto lo iface lo inet loopback # The primary network interface auto eth0 iface eth0 inet static address 192.168.33.100 network 192.168.33.0 netmask 255.255.255.0 gateway 192.168.33.1 The screenshot above also shows that you can provide more configuration than just the ip address. See interfaces(5) for help on setting a gateway, netmask or any of the other options. 419
- 426. interface configuration /sbin/ifdown It is adviced (but not mandatory) to down an interface before changing its configuration. This can be done with the ifdown command. The command will not give any output when downing an interface with a fixed ip address. However ifconfig will no longer show the interface. root@ubu1104srv:~# ifdown eth0 root@ubu1104srv:~# ifconfig lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:106 errors:0 dropped:0 overruns:0 frame:0 TX packets:106 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:11162 (11.1 KB) TX bytes:11162 (11.1 KB) An interface that is down cannot be used to connect to the network. /sbin/ifup Below a screenshot of ifup bringing the eth0 ethernet interface up using dhcp. (Note that this is a Ubuntu 10.10 screenshot, Ubuntu 11.04 omits ifup output by default.) root@ubu1010srv:/etc/network# ifup eth0 Internet Systems Consortium DHCP Client V3.1.3 Copyright 2004-2009 Internet Systems Consortium. All rights reserved. For info, please visit https://www.isc.org/software/dhcp/ Listening on LPF/eth0/08:00:27:cd:7f:fc Sending on LPF/eth0/08:00:27:cd:7f:fc Sending on Socket/fallback DHCPREQUEST of 192.168.1.34 on eth0 to 255.255.255.255 port 67 DHCPNAK from 192.168.33.100 DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 3 DHCPOFFER of 192.168.33.77 from 192.168.33.100 DHCPREQUEST of 192.168.33.77 on eth0 to 255.255.255.255 port 67 DHCPACK of 192.168.33.77 from 192.168.33.100 bound to 192.168.33.77 -- renewal in 95 seconds. ssh stop/waiting ssh start/running, process 1301 root@ubu1010srv:/etc/network# The details of dhcp are covered in a separate chapter in the Linux Servers course. 420
- 427. interface configuration 46.3. Red Hat/Fedora nic configuration /etc/sysconfig/network The /etc/sysconfig/network file is a global (across all network cards) configuration file. It allows us to define whether we want networking (NETWORKING=yes| no), what the hostname should be (HOSTNAME=) and which gateway to use (GATEWAY=). [root@rhel6 ~]# cat /etc/sysconfig/network NETWORKING=yes HOSTNAME=rhel6 GATEWAY=192.168.1.1 There are a dozen more option settable in this file, details can be found in /usr/share/ doc/initscripts-*/sysconfig.txt. /etc/sysconfig/network-scripts/ifcfg- Each network card can be configured individually using the /etc/sysconfig/network- scripts/ifcfg-* files. When you have only one network card, then this will probably be /etc/sysconfig/network-scripts/ifcfg-eth0. dhcp client Below a screenshot of /etc/sysconfig/network-scripts/ifcfg-eth0 configured for dhcp (BOOTPROTO="dhcp"). Note also the NM_CONTROLLED paramater to disable control of this nic by Network Manager. This parameter is not explained (not even mentioned) in /usr/share/doc/initscripts-*/sysconfig.txt, but many others are. [root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0 DEVICE="eth0" HWADDR="08:00:27:DD:0D:5C" NM_CONTROLLED="no" BOOTPROTO="dhcp" ONBOOT="yes" The BOOTPROTO variable can be set to either dhcp or bootp, anything else will be considered static meaning there should be no protocol used at boot time to set the interface values. fixed ip Below a screenshot of a fixed ip configuration in /etc/sysconfig/network-scripts/ ifcfg-eth0. [root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0 DEVICE="eth0" HWADDR="08:00:27:DD:0D:5C" 421
- 428. interface configuration NM_CONTROLLED="no" BOOTPROTO="none" IPADDR="192.168.1.99" NETMASK="255.255.255.0" GATEWAY="192.168.1.1" ONBOOT="yes" The HWADDR can be used to make sure that each network card gets the correct name when multiple network cards are present in the computer. It can not be used to assign a mac address to a network card. For this, you need to specify the MACADDR variable. Do not use HWADDR and MACADDR in the same ifcfg-ethx file. The BROADCAST= and NETWORK= parameters from previous RHEL/Fedora versions are obsoleted. /sbin/ifup and /sbin/ifdown The ifup and ifdown commands will set an interface up or down, using the configuration discussed above. This is identical to their behaviour in Debian and Ubuntu. [root@rhel6 ~]# ifdown eth0 && ifup eth0 [root@rhel6 ~]# ifconfig eth0 eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:2452 errors:0 dropped:0 overruns:0 frame:0 TX packets:1881 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:257036 (251.0 KiB) TX bytes:184767 (180.4 KiB) 422
- 429. interface configuration 46.4. ifconfig The use of /sbin/ifconfig without any arguments will present you with a list of all active network interface cards, including wireless and the loopback interface. In the screenshot below eth0 has no ip address. root@ubu1010:~# ifconfig eth0 Link encap:Ethernet HWaddr 00:26:bb:5d:2e:52 UP BROADCAST MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:0 (0.0 B) TX bytes:0 (0.0 B) Interrupt:43 Base address:0xe000 eth1 Link encap:Ethernet HWaddr 00:26:bb:12:7a:5e inet addr:192.168.1.30 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::226:bbff:fe12:7a5e/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:11141791 errors:202 dropped:0 overruns:0 frame:11580126 TX packets:6473056 errors:3860 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:3476531617 (3.4 GB) TX bytes:2114919475 (2.1 GB) Interrupt:23 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:2879 errors:0 dropped:0 overruns:0 frame:0 TX packets:2879 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:486510 (486.5 KB) TX bytes:486510 (486.5 KB) You can also use ifconfig to obtain information about just one network card. [root@rhel6 ~]# ifconfig eth0 eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:2969 errors:0 dropped:0 overruns:0 frame:0 TX packets:1918 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:335942 (328.0 KiB) TX bytes:190157 (185.7 KiB) When /sbin is not in the $PATH of a normal user you will have to type the full path, as seen here on Debian. paul@debian5:~$ /sbin/ifconfig eth3 eth3 Link encap:Ethernet HWaddr 08:00:27:ab:67:30 inet addr:192.168.1.29 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::a00:27ff:feab:6730/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:27155 errors:0 dropped:0 overruns:0 frame:0 TX packets:30527 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:13095386 (12.4 MiB) TX bytes:25767221 (24.5 MiB) 423
- 430. interface configuration up and down You can also use ifconfig to bring an interface up or down. The difference with ifup is that ifconfig eth0 up will re-activate the nic keeping its existing (current) configuration, whereas ifup will read the correct file that contains a (possibly new) configuration and use this config file to bring the interface up. [root@rhel6 ~]# ifconfig eth0 down [root@rhel6 ~]# ifconfig eth0 up [root@rhel6 ~]# ifconfig eth0 eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:2995 errors:0 dropped:0 overruns:0 frame:0 TX packets:1927 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:339030 (331.0 KiB) TX bytes:191583 (187.0 KiB) setting ip address You can temporary set an ip address with ifconfig. This ip address is only valid until the next ifup/ifdown cycle or until the next reboot. [root@rhel6 ~]# ifconfig eth0 | grep 192 inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0 [root@rhel6 ~]# ifconfig eth0 192.168.33.42 netmask 255.255.0.0 [root@rhel6 ~]# ifconfig eth0 | grep 192 inet addr:192.168.33.42 Bcast:192.168.255.255 Mask:255.255.0.0 [root@rhel6 ~]# ifdown eth0 && ifup eth0 [root@rhel6 ~]# ifconfig eth0 | grep 192 inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0 setting mac address You can also use ifconfig to set another mac address than the one hard coded in the network card. This screenshot shows you how. [root@rhel6 ~]# ifconfig eth0 | grep HWaddr eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C [root@rhel6 ~]# ifconfig eth0 hw ether 00:42:42:42:42:42 [root@rhel6 ~]# ifconfig eth0 | grep HWaddr eth0 Link encap:Ethernet HWaddr 00:42:42:42:42:42 dhclient Home and client Linux desktops often have /sbin/dhclient running. This is a daemon that enables a network interface to lease an ip configuration from a dhcp server. When your adapter is configured for dhcp or bootp, then /sbin/ifup will start the dhclient daemon. When a lease is renewed, dhclient will override your ifconfig set ip address! 424
- 431. interface configuration 46.5. hostname Every host receives a hostname, often placed in a DNS name space forming the fqdn or Fully Qualified Domain Name. This screenshot shows the hostname command and the configuration of the hostname on Red Hat/Fedora. [root@rhel6 ~]# grep rhel /etc/sysconfig/network HOSTNAME=rhel6 [root@rhel6 ~]# hostname rhel6 Ubuntu/Debian uses the /etc/hostname file to configure the hostname. paul@ubu1010:~$ cat /etc/hostname ubu1010 paul@ubu1010:~$ hostname ubu1010 On all Linux distributions you can change the hostname using the hostname $newname command. This is not a permanent change. [root@rhel6 ~]# hostname server42 [root@rhel6 ~]# hostname server42 On any Linux you can use sysctl to display and set the hostname. [root@rhel6 ~]# sysctl kernel.hostname kernel.hostname = server42 [root@rhel6 ~]# sysctl kernel.hostname=rhel6 kernel.hostname = rhel6 [root@rhel6 ~]# sysctl kernel.hostname kernel.hostname = rhel6 [root@rhel6 ~]# hostname rhel6 425
- 432. interface configuration 46.6. arp The ip to mac resolution is handled by the layer two broadcast protocol arp. The arp table can be displayed with the arp tool. The screenshot below shows the list of computers that this computer recently communicated with. root@barry:~# arp -a ? (192.168.1.191) at 00:0C:29:3B:15:80 [ether] on eth1 agapi (192.168.1.73) at 00:03:BA:09:7F:D2 [ether] on eth1 anya (192.168.1.1) at 00:12:01:E2:87:FB [ether] on eth1 faith (192.168.1.41) at 00:0E:7F:41:0D:EB [ether] on eth1 kiss (192.168.1.49) at 00:D0:E0:91:79:95 [ether] on eth1 laika (192.168.1.40) at 00:90:F5:4E:AE:17 [ether] on eth1 pasha (192.168.1.71) at 00:03:BA:02:C3:82 [ether] on eth1 shaka (192.168.1.72) at 00:03:BA:09:7C:F9 [ether] on eth1 root@barry:~# Anya is a Cisco Firewall, faith is a laser printer, kiss is a Kiss DP600, laika is a laptop and Agapi, Shaka and Pasha are SPARC servers. The question mark is a Red Hat Enterprise Linux server running on a virtual machine. You can use arp -d to remove an entry from the arp table. [root@rhel6 ~]# arp Address HWtype HWaddress Flags Mask Iface ubu1010 ether 00:26:bb:12:7a:5e C eth0 anya ether 00:02:cf:aa:68:f0 C eth0 [root@rhel6 ~]# arp -d anya [root@rhel6 ~]# arp Address HWtype HWaddress Flags Mask Iface ubu1010 ether 00:26:bb:12:7a:5e C eth0 anya (incomplete) eth0 [root@rhel6 ~]# ping anya PING anya (192.168.1.1) 56(84) bytes of data. 64 bytes from anya (192.168.1.1): icmp_seq=1 ttl=254 time=10.2 ms ... [root@rhel6 ~]# arp Address HWtype HWaddress Flags Mask Iface ubu1010 ether 00:26:bb:12:7a:5e C eth0 anya ether 00:02:cf:aa:68:f0 C eth0 426
- 433. interface configuration 46.7. route You can see the computer's local routing table with the /sbin/route command (and also with netstat -r ). root@RHEL4b ~]# netstat -r Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 192.168.1.0 * 255.255.255.0 U 0 0 0 eth0 [root@RHEL4b ~]# route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 192.168.1.0 * 255.255.255.0 U 0 0 0 eth0 [root@RHEL4b ~]# It appears this computer does not have a gateway configured, so we use route add default gw to add a default gateway on the fly. [root@RHEL4b ~]# route add default gw 192.168.1.1 [root@RHEL4b ~]# route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 192.168.1.0 * 255.255.255.0 U 0 0 0 eth0 default 192.168.1.1 0.0.0.0 UG 0 0 0 eth0 [root@RHEL4b ~]# Unless you configure the gateway in one of the /etc/ file from the start of this chapter, your computer will forget this gateway after a reboot. 46.8. ping If you can ping to another host, then tcp/ip is configured. [root@RHEL4b ~]# ping 192.168.1.5 PING 192.168.1.5 (192.168.1.5) 56(84) bytes of data. 64 bytes from 192.168.1.5: icmp_seq=0 ttl=64 time=1004 ms 64 bytes from 192.168.1.5: icmp_seq=1 ttl=64 time=1.19 ms 64 bytes from 192.168.1.5: icmp_seq=2 ttl=64 time=0.494 ms 64 bytes from 192.168.1.5: icmp_seq=3 ttl=64 time=0.419 ms --- 192.168.1.5 ping statistics --- 4 packets transmitted, 4 received, 0% packet loss, time 3009ms rtt min/avg/max/mdev = 0.419/251.574/1004.186/434.520 ms, pipe 2 [root@RHEL4b ~]# 427
- 434. interface configuration 46.9. optional: ethtool To display or change network card settings, use ethtool. The results depend on the capabilities of your network card. The example shows a network that auto-negotiates it's bandwidth. root@laika:~# ethtool eth0 Settings for eth0: Supported ports: [ TP ] Supported link modes: 10baseT/Half 10baseT/Full 100baseT/Half 100baseT/Full 1000baseT/Full Supports auto-negotiation: Yes Advertised link modes: 10baseT/Half 10baseT/Full 100baseT/Half 100baseT/Full 1000baseT/Full Advertised auto-negotiation: Yes Speed: 1000Mb/s Duplex: Full Port: Twisted Pair PHYAD: 0 Transceiver: internal Auto-negotiation: on Supports Wake-on: pumbg Wake-on: g Current message level: 0x00000033 (51) Link detected: yes This example shows how to use ethtool to switch the bandwidth from 1000Mbit to 100Mbit and back. Note that some time passes before the nic is back to 1000Mbit. root@laika:~# ethtool eth0 | grep Speed Speed: 1000Mb/s root@laika:~# ethtool -s eth0 speed 100 root@laika:~# ethtool eth0 | grep Speed Speed: 100Mb/s root@laika:~# ethtool -s eth0 speed 1000 root@laika:~# ethtool eth0 | grep Speed Speed: 1000Mb/s 428
- 435. interface configuration 46.10. practice: interface configuration 1. Verify whether dhclient is running. 2. Display your current ip address(es). 3. Display the configuration file where this ip address is defined. 4. Follow the nic configuration in the book to change your ip address from dhcp client to fixed. Keep the same ip address to avoid conflicts! 5. Did you also configure the correct gateway in the previous question ? If not, then do this now. 6. Verify that you have a gateway. 7. Verify that you can connect to the gateway, that it is alive. 8. Change the last two digits of your mac address. 9. Which ports are used by http, pop3, ssh, telnet, nntp and ftp ? Note that sctp was ommitted from the screenshot. 10. Explain why e-mail and websites are sent over tcp and not udp. 11. Display the hostname of your computer. 12. Which ip-addresses did your computer recently have contact with ? 429
- 436. interface configuration 46.11. solution: interface configuration 1. Verify whether dhclient is running. paul@debian5:~$ ps fax | grep dhclient 2. Display your current ip address(es). paul@debian5:~$ /sbin/ifconfig | grep 'inet ' inet addr:192.168.1.31 Bcast:192.168.1.255 Mask:255.255.255.0 inet addr:127.0.0.1 Mask:255.0.0.0 3. Display the configuration file where this ip address is defined. Ubuntu/Debian: cat /etc/network/interfaces Redhat/Fedora: cat /etc/sysconfig/network-scripts/ifcfg-eth* 4. Follow the nic configuration in the book to change your ip address from dhcp client to fixed. Keep the same ip address to avoid conflicts! Ubuntu/Debian: ifdown eth0 vi /etc/network/interfaces ifup eth0 Redhat/Fedora: ifdown eth0 vi /etc/sysconfig/network-scripts/ifcfg-eth0 ifup eth0 5. Did you also configure the correct gateway in the previous question ? If not, then do this now. 6. Verify that you have a gateway. paul@debian5:~$ /sbin/route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 192.168.1.0 * 255.255.255.0 U 0 0 0 eth0 default 192.168.1.1 0.0.0.0 UG 0 0 0 eth0 7. Verify that you can connect to the gateway, that it is alive. paul@debian5:~$ ping -c3 192.168.1.1 PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data. 64 bytes from 192.168.1.1: icmp_seq=1 ttl=254 time=2.28 ms 64 bytes from 192.168.1.1: icmp_seq=2 ttl=254 time=2.94 ms 64 bytes from 192.168.1.1: icmp_seq=3 ttl=254 time=2.34 ms --- 192.168.1.1 ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 2008ms rtt min/avg/max/mdev = 2.283/2.524/2.941/0.296 ms 8. Change the last two digits of your mac address. [root@rhel6 ~]# ifconfig eth0 hw ether 08:00:27:ab:67:XX 9. Which ports are used by http, pop3, ssh, telnet, nntp and ftp ? root@rhel6 ~# grep ^'http ' /etc/services 430
- 437. interface configuration http 80/tcp www www-http # WorldWideWeb HTTP http 80/udp www www-http # HyperText Transfer Protocol root@rhel6 ~# grep ^'smtp ' /etc/services smtp 25/tcp mail smtp 25/udp mail root@rhel6 ~# grep ^'ssh ' /etc/services ssh 22/tcp # The Secure Shell (SSH) Protocol ssh 22/udp # The Secure Shell (SSH) Protocol root@rhel6 ~# grep ^'telnet ' /etc/services telnet 23/tcp telnet 23/udp root@rhel6 ~# grep ^'nntp ' /etc/services nntp 119/tcp readnews untp # USENET News Transfer Protocol nntp 119/udp readnews untp # USENET News Transfer Protocol root@rhel6 ~# grep ^'ftp ' /etc/services ftp 21/tcp ftp 21/udp fsp fspd Note that sctp was ommitted from the screenshot. 10. Explain why e-mail and websites are sent over tcp and not udp. Because tcp is reliable and udp is not. 11. Display the hostname of your computer. paul@debian5:~$ hostname debian5 12. Which ip-addresses did your computer recently have contact with ? root@rhel6 ~# arp -a ? (192.168.1.1) at 00:02:cf:aa:68:f0 [ether] on eth2 ? (192.168.1.30) at 00:26:bb:12:7a:5e [ether] on eth2 ? (192.168.1.31) at 08:00:27:8e:8a:a8 [ether] on eth2 431
- 438. Chapter 47. network sniffing Table of Contents 47.1. wireshark .................................................................................................... 433 47.2. tcpdump ...................................................................................................... 435 47.3. practice: network sniffing .......................................................................... 436 47.4. solution: network sniffing .......................................................................... 437 A good network administrator should be able to use a sniffer like wireshark or tcpdump to troubleshoot network problems. A good student will often use a sniffer to learn about networking. This chapter introduces you to network sniffing. 432
- 439. network sniffing 47.1. wireshark installing wireshark This example shows how to install wireshark on .deb based distributions like Ubuntu and Debian. aptitude install wireshark On .rpm based distributions you can use yum to install wireshark. yum install wireshark selecting interface When you first fire up wireshark, you will need to select an interface to sniff. You will see a dialog box that looks similar to this. Choose the interface that you want to sniff. On some distributions only root is allowed to sniff the network. You might need to use sudo wireshark. start sniffing In this example here, we sniffed a ping between two computers. The top pane shows that wireshark recognizes the icmp protocol, and captured all the ping packets between the two computers. 433
- 440. network sniffing looking inside packets The middle can be expanded. When selecting a line in this panel, you can see the corresponding bytes in the frame in the bottom panel. use filters You might get lost in too many packets. A quick solution to see only the packets that are of interest to you is to apply filters. When you type arp and click apply, you will only see arp packets displayed. You can combine two protocols with a logical or between them. The example below shows how to filter only arp and bootp (or dhcp) packets. This example shows how to filter for dns traffic containing a certain ip address. 434
- 441. network sniffing 47.2. tcpdump Sniffing on the command line can be done with tcpdump. Here are some examples. Using the tcpdump host $ip command displays all traffic with one host (192.168.1.38 in this example). root@ubuntu910:~# tcpdump host 192.168.1.38 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes Capturing only ssh (tcp port 22) traffic can be done with tcpdump tcp port $port. This screenshot is cropped to 76 characters for readability in the pdf. root@deb503:~# tcpdump tcp port 22 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes 14:22:20.716313 IP deb503.local.37973 > rhel53.local.ssh: P 666050963:66605 14:22:20.719936 IP rhel53.local.ssh > deb503.local.37973: P 1:49(48) ack 48 14:22:20.720922 IP rhel53.local.ssh > deb503.local.37973: P 49:113(64) ack 14:22:20.721321 IP rhel53.local.ssh > deb503.local.37973: P 113:161(48) ack 14:22:20.721820 IP deb503.local.37973 > rhel53.local.ssh: . ack 161 win 200 14:22:20.722492 IP rhel53.local.ssh > deb503.local.37973: P 161:225(64) ack 14:22:20.760602 IP deb503.local.37973 > rhel53.local.ssh: . ack 225 win 200 14:22:23.108106 IP deb503.local.54424 > ubuntu910.local.ssh: P 467252637:46 14:22:23.116804 IP ubuntu910.local.ssh > deb503.local.54424: P 1:81(80) ack 14:22:23.116844 IP deb503.local.54424 > ubuntu910.local.ssh: . ack 81 win 2 ^C 10 packets captured 10 packets received by filter 0 packets dropped by kernel Same as above, but write the output to a file with the tcpdump -w $filename command. root@ubuntu910:~# tcpdump -w sshdump.tcpdump tcp port 22 tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes ^C 17 packets captured 17 packets received by filter 0 packets dropped by kernel With tcpdump -r $filename the file created above can be displayed. root@ubuntu910:~# tcpdump -r sshdump.tcpdump Many more examples can be found in the manual page of tcpdump. 435
- 442. network sniffing 47.3. practice: network sniffing 1. Install wireshark on your computer (not inside a virtual machine). 2. Start a ping between your computer and another computer. 3. Start sniffing the network. 4. Display only the ping echo's in the top pane using a filter. 5. Now ping to a name (like www.linux-training.be) and try to sniff the DNS query and response. Which DNS server was used ? Was it a tcp or udp query and response ? 436
- 443. network sniffing 47.4. solution: network sniffing 1. Install wireshark on your computer (not inside a virtual machine). Debian/Ubuntu: aptitude install wireshark Red Hat/Mandriva/Fedora: yum install wireshark 2. Start a ping between your computer and another computer. ping $ip_address 3. Start sniffing the network. (sudo) wireshark select an interface (probably eth0) 4. Display only the ping echo's in the top pane using a filter. type 'icmp' (without quotes) in the filter box, and then click 'apply' 5. Now ping to a name (like www.linux-training.be) and try to sniff the DNS query and response. Which DNS server was used ? Was it a tcp or udp query and response ? First start the sniffer. Enter 'dns' in the filter box and click apply. root@ubuntu910:~# ping www.linux-training.be PING www.linux-training.be (88.151.243.8) 56(84) bytes of data. 64 bytes from fosfor.openminds.be (88.151.243.8): icmp_seq=1 ttl=58 time=14.9 ms 64 bytes from fosfor.openminds.be (88.151.243.8): icmp_seq=2 ttl=58 time=16.0 ms ^C --- www.linux-training.be ping statistics --- 2 packets transmitted, 2 received, 0% packet loss, time 1002ms rtt min/avg/max/mdev = 14.984/15.539/16.095/0.569 ms The wireshark screen should look something like this. The details in wireshark will say the DNS query was inside a udp packet. 437
- 444. Chapter 48. binding and bonding Table of Contents 48.1. binding on Redhat/Fedora .......................................................................... 439 48.2. binding on Debian/Ubuntu ......................................................................... 440 48.3. bonding on Redhat/Fedora ......................................................................... 441 48.4. bonding on Debian/Ubuntu ........................................................................ 443 48.5. practice: binding and bonding ................................................................... 445 48.6. solution: binding and bonding ................................................................... 446 Sometimes a server needs more than one ip address on the same network card, we call this binding ip addresses. Linux can also activate multiple network cards behind the same ip address, this is called bonding. This chapter will teach you how to configure binding and bonding on the most common Linux distributions. 438
- 445. binding and bonding 48.1. binding on Redhat/Fedora binding extra ip addresses To bind more than one ip address to the same interface, use ifcfg-eth0:0, where the last zero can be anything else. Only two directives are required in the files. [root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0:0 DEVICE="eth0:0" IPADDR="192.168.1.133" [root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0:1 DEVICE="eth0:0" IPADDR="192.168.1.142" enabling extra ip-addresses To activate a virtual network interface, use ifup, to deactivate it, use ifdown. [root@rhel6 ~]# ifup eth0:0 [root@rhel6 ~]# ifconfig | grep 'inet ' inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0 inet addr:192.168.1.133 Bcast:192.168.1.255 Mask:255.255.255.0 inet addr:127.0.0.1 Mask:255.0.0.0 [root@rhel6 ~]# ifup eth0:1 [root@rhel6 ~]# ifconfig | grep 'inet ' inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0 inet addr:192.168.1.133 Bcast:192.168.1.255 Mask:255.255.255.0 inet addr:192.168.1.142 Bcast:192.168.1.255 Mask:255.255.255.0 inet addr:127.0.0.1 Mask:255.0.0.0 verifying extra ip-addresses Use ping from another computer to check the activation, or use ifconfig like in this screenshot. [root@rhel6 ~]# ifconfig eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:1259 errors:0 dropped:0 overruns:0 frame:0 TX packets:545 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:115260 (112.5 KiB) TX bytes:84293 (82.3 KiB) eth0:0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C inet addr:192.168.1.133 Bcast:192.168.1.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 eth0:1 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C inet addr:192.168.1.142 Bcast:192.168.1.255 Mask:255.255.255.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 439
- 446. binding and bonding 48.2. binding on Debian/Ubuntu binding extra ip addresses The configuration of multiple ip addresses on the same network card is done in /etc/ network/interfaces by adding eth0:x devices. Adding the netmask is mandatory. debian5:~# cat /etc/network/interfaces # This file describes the network interfaces available on your system # and how to activate them. For more information, see interfaces(5). # The loopback network interface auto lo iface lo inet loopback # The primary network interface iface eth0 inet static address 192.168.1.34 network 192.168.1.0 netmask 255.255.255.0 gateway 192.168.1.1 auto eth0 auto eth0:0 iface eth0:0 inet static address 192.168.1.233 netmask 255.255.255.0 auto eth0:1 iface eth0:1 inet static address 192.168.1.242 netmask 255.255.255.0 enabling extra ip-addresses Use ifup to enable the extra addresses. debian5:~# ifup eth0:0 debian5:~# ifup eth0:1 verifying extra ip-addresses Use ping from another computer to check the activation, or use ifconfig like in this screenshot. debian5:~# ifconfig | grep 'inet ' inet addr:192.168.1.34 Bcast:192.168.1.255 Mask:255.255.255.0 inet addr:192.168.1.233 Bcast:192.168.1.255 Mask:255.255.255.0 inet addr:192.168.1.242 Bcast:192.168.1.255 Mask:255.255.255.0 inet addr:127.0.0.1 Mask:255.0.0.0 440
- 447. binding and bonding 48.3. bonding on Redhat/Fedora We start with ifconfig -a to get a list of all the network cards on our system. [root@rhel6 network-scripts]# ifconfig -a | grep Ethernet eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C eth1 Link encap:Ethernet HWaddr 08:00:27:DA:C1:49 eth2 Link encap:Ethernet HWaddr 08:00:27:40:03:3B In this demo we decide to bond eth1 and eth2. We will name are bond bond0 and add this entry to modprobe so the kernel can load the bonding module when we bring the interface up. [root@rhel6 network-scripts]# cat /etc/modprobe.d/bonding.conf alias bond0 bonding Then we create /etc/sysconfig/network-scripts/ifcfg-bond0 to configure our bond0 interface. [root@rhel6 network-scripts]# pwd /etc/sysconfig/network-scripts [root@rhel6 network-scripts]# cat ifcfg-bond0 DEVICE=bond0 IPADDR=192.168.1.199 NETMASK=255.255.255.0 ONBOOT=yes BOOTPROTO=none USERCTL=no Next we create two files, one for each network card that we will use as slave in bond0. [root@rhel6 network-scripts]# cat ifcfg-eth1 DEVICE=eth1 BOOTPROTO=none ONBOOT=yes MASTER=bond0 SLAVE=yes USERCTL=no [root@rhel6 network-scripts]# cat ifcfg-eth2 DEVICE=eth2 BOOTPROTO=none ONBOOT=yes MASTER=bond0 SLAVE=yes USERCTL=no Finally we bring the interface up with ifup bond0. [root@rhel6 network-scripts]# ifup bond0 [root@rhel6 network-scripts]# ifconfig bond0 bond0 Link encap:Ethernet HWaddr 08:00:27:DA:C1:49 inet addr:192.168.1.199 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::a00:27ff:feda:c149/64 Scope:Link UP BROADCAST RUNNING MASTER MULTICAST MTU:1500 Metric:1 RX packets:251 errors:0 dropped:0 overruns:0 frame:0 TX packets:21 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:39852 (38.9 KiB) TX bytes:1070 (1.0 KiB) The bond should also be visible in /proc/net/bonding. 441
- 448. binding and bonding [root@rhel6 network-scripts]# cat /proc/net/bonding/bond0 Ethernet Channel Bonding Driver: v3.5.0 (November 4, 2008) Bonding Mode: load balancing (round-robin) MII Status: up MII Polling Interval (ms): 0 Up Delay (ms): 0 Down Delay (ms): 0 Slave Interface: eth1 MII Status: up Link Failure Count: 0 Permanent HW addr: 08:00:27:da:c1:49 Slave Interface: eth2 MII Status: up Link Failure Count: 0 Permanent HW addr: 08:00:27:40:03:3b 442
- 449. binding and bonding 48.4. bonding on Debian/Ubuntu We start with ifconfig -a to get a list of all the network cards on our system. debian5:~# ifconfig -a | grep Ethernet eth0 Link encap:Ethernet HWaddr 08:00:27:bb:18:a4 eth1 Link encap:Ethernet HWaddr 08:00:27:63:9a:95 eth2 Link encap:Ethernet HWaddr 08:00:27:27:a4:92 In this demo we decide to bond eth1 and eth2. We also need to install the ifenslave package. debian5:~# aptitude search ifenslave p ifenslave - Attach and detach slave interfaces to a bonding device p ifenslave-2.6 - Attach and detach slave interfaces to a bonding device debian5:~# aptitude install ifenslave Reading package lists... Done ... Next we update the /etc/network/interfaces file with information about the bond0 interface. debian5:~# tail -7 /etc/network/interfaces iface bond0 inet static address 192.168.1.42 netmask 255.255.255.0 gateway 192.168.1.1 slaves eth1 eth2 bond-mode active-backup bond_primary eth1 On older version of Debian/Ubuntu you needed to modprobe bonding, but this is no longer required. Use ifup to bring the interface up, then test that it works. debian5:~# ifup bond0 debian5:~# ifconfig bond0 bond0 Link encap:Ethernet HWaddr 08:00:27:63:9a:95 inet addr:192.168.1.42 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::a00:27ff:fe63:9a95/64 Scope:Link UP BROADCAST RUNNING MASTER MULTICAST MTU:1500 Metric:1 RX packets:212 errors:0 dropped:0 overruns:0 frame:0 TX packets:39 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:31978 (31.2 KiB) TX bytes:6709 (6.5 KiB) The bond should also be visible in /proc/net/bonding. debian5:~# cat /proc/net/bonding/bond0 Ethernet Channel Bonding Driver: v3.2.5 (March 21, 2008) Bonding Mode: fault-tolerance (active-backup) Primary Slave: eth1 Currently Active Slave: eth1 MII Status: up MII Polling Interval (ms): 0 Up Delay (ms): 0 Down Delay (ms): 0 Slave Interface: eth1 MII Status: up 443
- 450. binding and bonding Link Failure Count: 0 Permanent HW addr: 08:00:27:63:9a:95 Slave Interface: eth2 MII Status: up Link Failure Count: 0 Permanent HW addr: 08:00:27:27:a4:92 444
- 451. binding and bonding 48.5. practice: binding and bonding 1. Add an extra ip address to one of your network cards. Test that it works (have your neighbour ssh to it)! 2. Use ifdown to disable this extra ip address. 3. Make sure your neighbour also succeeded in binding an extra ip address before you continue. 4. Add an extra network card (or two) to your virtual machine and use the theory to bond two network cards. 445
- 452. binding and bonding 48.6. solution: binding and bonding 1. Add an extra ip address to one of your network cards. Test that it works (have your neighbour ssh to it)! Redhat/Fedora: add an /etc/sysconfig/network-scripts/ifcfg-ethX:X file as shown in the theory Debian/Ubuntu: expand the /etc/network/interfaces file as shown in the theory 2. Use ifdown to disable this extra ip address. ifdown eth0:0 3. Make sure your neighbour also succeeded in binding an extra ip address before you continue. ping $extra_ip_neighbour or ssh $extra_ip_neighbour 4. Add an extra network card (or two) to your virtual machine and use the theory to bond two network cards. Redhat/Fedora: add ifcfg-ethX and ifcfg-bondX files in /etc/sysconfig/network-scripts as shown in the theory and don't forget the modprobe.conf Debian/Ubuntu: expand the /etc/network/interfaces file as shown in the theory and don't forget to install the ifenslave package 446
- 453. Chapter 49. introduction to networking Table of Contents 49.1. introduction to iptables .............................................................................. 448 49.2. practice : iptables ....................................................................................... 449 49.3. solution : iptables ....................................................................................... 450 49.4. xinetd and inetd ......................................................................................... 451 49.5. practice : inetd and xinetd .......................................................................... 453 49.6. openssh ....................................................................................................... 454 49.7. practice: ssh ................................................................................................ 458 49.8. network file system .................................................................................... 459 49.9. practice : network file system .................................................................... 461 447
- 454. introduction to networking 49.1. introduction to iptables iptables firewall The Linux kernel has a built-in stateful firewall named iptables. To stop the iptables firewall on Red Hat, use the service command. root@RHELv4u4:~# service iptables stop Flushing firewall rules: [ OK ] Setting chains to policy ACCEPT: filter [ OK ] Unloading iptables modules: [ OK ] root@RHELv4u4:~# The easy way to configure iptables, is to use a graphical tool like KDE's kmyfirewall or Security Level Configuration Tool. You can find the latter in the graphical menu, somewhere in System Tools - Security, or you can start it by typing system-config- securitylevel in bash. These tools allow for some basic firewall configuration. You can decide whether to enable or disable the firewall, and what typical standard ports are allowed when the firewall is active. You can even add some custom ports. When you are done, the configuration is written to /etc/sysconfig/iptables on Red Hat. root@RHELv4u4:~# cat /etc/sysconfig/iptables # Firewall configuration written by system-config-securitylevel # Manual customization of this file is not recommended. *filter :INPUT ACCEPT [0:0] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [0:0] :RH-Firewall-1-INPUT - [0:0] -A INPUT -j RH-Firewall-1-INPUT -A FORWARD -j RH-Firewall-1-INPUT -A RH-Firewall-1-INPUT -i lo -j ACCEPT -A RH-Firewall-1-INPUT -p icmp --icmp-type any -j ACCEPT -A RH-Firewall-1-INPUT -p 50 -j ACCEPT -A RH-Firewall-1-INPUT -p 51 -j ACCEPT -A RH-Firewall-1-INPUT -p udp --dport 5353 -d 224.0.0.251 -j ACCEPT -A RH-Firewall-1-INPUT -p udp -m udp --dport 631 -j ACCEPT -A RH-Firewall-1-INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT -A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT -A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT -A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 21 -j ACCEPT -A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 25 -j ACCEPT -A RH-Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibited COMMIT root@RHELv4u4:~# To start the service, issue the service iptables start command. You can configure iptables to start at boot time with chkconfig. root@RHELv4u4:~# service iptables start Applying iptables firewall rules: [ OK ] root@RHELv4u4:~# chkconfig iptables on root@RHELv4u4:~# 448
- 455. introduction to networking One of the nice features of iptables is that it displays extensive status information when queried with the service iptables status command. root@RHELv4u4:~# service iptables status Table: filter Chain INPUT (policy ACCEPT) target prot opt source destination RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0 Chain FORWARD (policy ACCEPT) target prot opt source destination RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0 Chain OUTPUT (policy ACCEPT) target prot opt source destination Chain RH-Firewall-1-INPUT (2 references) target prot opt source destination ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 ACCEPT icmp -- 0.0.0.0/0 0.0.0.0/0 icmp type 255 ACCEPT esp -- 0.0.0.0/0 0.0.0.0/0 ACCEPT ah -- 0.0.0.0/0 0.0.0.0/0 ACCEPT udp -- 0.0.0.0/0 224.0.0.251 udp dpt:5353 ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 udp dpt:631 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:22 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:80 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:21 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:25 REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited root@RHELv4u4:~# Mastering firewall configuration requires a decent knowledge of tcp/ip. Good iptables tutorials can be found online here http://iptables-tutorial.frozentux.net/ iptables-tutorial.html and here http://tldp.org/HOWTO/IP-Masquerade-HOWTO/. 49.2. practice : iptables 1. Verify whether the firewall is running. 2. Stop the running firewall. 449
- 456. introduction to networking 49.3. solution : iptables 1. Verify whether the firewall is running. root@rhel55 ~# service iptables status | head Table: filter Chain INPUT (policy ACCEPT) num target prot opt source destination 1 RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0 Chain FORWARD (policy ACCEPT) num target prot opt source destination 1 RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0 Chain OUTPUT (policy ACCEPT) 2. Stop the running firewall. root@rhel55 ~# service iptables stop Flushing firewall rules: [ OK ] Setting chains to policy ACCEPT: filter [ OK ] Unloading iptables modules: [ OK ] root@rhel55 ~# service iptables status Firewall is stopped. 450
- 457. introduction to networking 49.4. xinetd and inetd the superdaemon Back when resources like RAM memory were limited, a super-server was devised to listen to all sockets and start the appropriate daemon only when needed. Services like swat, telnet and ftp are typically served by such a super-server. The xinetd superdaemon is more recent than inetd. We will discuss the configuration both daemons. Recent Linux distributions like RHEL5 and Ubuntu10.04 do not activate inetd or xinetd by default, unless an application requires it. inetd or xinetd First verify whether your computer is running inetd or xinetd. This Debian 4.0 Etch is running inetd. root@barry:~# ps fax | grep inet 3870 ? Ss 0:00 /usr/sbin/inetd This Red Hat Enterprise Linux 4 update 4 is running xinetd. [root@RHEL4b ~]# ps fax | grep inet 3003 ? Ss 0:00 xinetd -stayalive -pidfile /var/run/xinetd.pid Both daemons have the same functionality (listening to many ports, starting other daemons when they are needed), but they have different configuration files. xinetd superdaemon The xinetd daemon is often called a superdaemon because it listens to a lot of incoming connections, and starts other daemons when they are needed. When a connection request is received, xinetd will first check TCP wrappers (/etc/hosts.allow and /etc/hosts.deny) and then give control of the connection to the other daemon. This superdaemon is configured through /etc/xinetd.conf and the files in the directory / etc/xinetd.d. Let's first take a look at /etc/xinetd.conf. paul@RHELv4u2:~$ cat /etc/xinetd.conf # # Simple configuration file for xinetd # # Some defaults, and include /etc/xinetd.d/ defaults { 451
- 458. introduction to networking instances = 60 log_type = SYSLOG authpriv log_on_success = HOST PID log_on_failure = HOST cps = 25 30 } includedir /etc/xinetd.d paul@RHELv4u2:~$ According to the settings in this file, xinetd can handle 60 client requests at once. It uses the authpriv facility to log the host ip-address and pid of successful daemon spawns. When a service (aka protocol linked to daemon) gets more than 25 cps (connections per second), it holds subsequent requests for 30 seconds. The directory /etc/xinetd.d contains more specific configuration files. Let's also take a look at one of them. paul@RHELv4u2:~$ ls /etc/xinetd.d amanda chargen-udp echo klogin rexec talk amandaidx cups-lpd echo-udp krb5-telnet rlogin telnet amidxtape daytime eklogin kshell rsh tftp auth daytime-udp finger ktalk rsync time chargen dbskkd-cdb gssftp ntalk swat time-udp paul@RHELv4u2:~$ cat /etc/xinetd.d/swat # default: off # description: SWAT is the Samba Web Admin Tool. Use swat # to configure your Samba server. To use SWAT, # connect to port 901 with your favorite web browser. service swat { port = 901 socket_type = stream wait = no only_from = 127.0.0.1 user = root server = /usr/sbin/swat log_on_failure += USERID disable = yes } paul@RHELv4u2:~$ The services should be listed in the /etc/services file. Port determines the service port, and must be the same as the port specified in /etc/services. The socket_type should be set to stream for tcp services (and to dgram for udp). The log_on_failure += concats the userid to the log message formatted in /etc/xinetd.conf. The last setting disable can be set to yes or no. Setting this to no means the service is enabled! Check the xinetd and xinetd.conf manual pages for many more configuration options. inetd superdaemon This superdaemon has only one configuration file /etc/inetd.conf. Every protocol or daemon that it is listening for, gets one line in this file. 452
- 459. introduction to networking root@barry:~# grep ftp /etc/inetd.conf tftp dgram udp wait nobody /usr/sbin/tcpd /usr/sbin/in.tftpd /boot/tftp root@barry:~# You can disable a service in inetd.conf above by putting a # at the start of that line. Here an example of the disabled vmware web interface (listening on tcp port 902). paul@laika:~$ grep vmware /etc/inetd.conf #902 stream tcp nowait root /usr/sbin/vmware-authd vmware-authd 49.5. practice : inetd and xinetd 1. Verify on all systems whether they are using xinetd or inetd. 2. Look at the configuration files. 3. (If telnet is installable, then replace swat in these questions with telnet) Is swat installed ? If not, then install swat and look at the changes in the (x)inetd configuration. Is swat enabled or disabled ? 4. Disable swat, test it. Enable swat, test it. 453
- 460. introduction to networking 49.6. openssh secure shell Avoid using telnet, rlogin and rsh to remotely connect to your servers. These older protocols do not encrypt the login session, which means your user id and password can be sniffed by tools like wireshark or tcpdump. To securely connect to your servers, use ssh. An ssh connection always starts with a cryptographic handshake, followed by encryption of the transport layer using a symmetric cypher. Then authentication takes place (using user id/password or public/private keys) and communication can take place over the encrypted connection. In other words, the tunnel is encrypted before you start typing anything. The openssh package is maintained by the OpenBSD people and is distributed with a lot of operating systems (it may even be the most popular package in the world). Below sample use of ssh to connect from one server (RHELv4u2) to another one (RHELv4u4). paul@RHELv4u2:~$ ssh 192.168.1.220 The authenticity of host '192.168.1.220' can't be established. RSA key fingerprint is c4:3c:52:e6:d8:8b:ce:17:8b:c9:78:5a:f3:51:06:4f. Are you sure you want to continue connecting (yes/no)? yes Warning: Permanently added '192.168.1.220' (RSA) to the list of known... paul@192.168.1.220's password: Last login: Sun Jan 21 07:16:26 2007 from 192.168.1.40 paul@RHELv4u4:~$ The second time ssh remembers the connection. It added an entry to the ~/.ssh/ known_hosts file. paul@RHELv4u2:~$ ssh 192.168.1.220 paul@192.168.1.220's password: Last login: Sun Jan 21 08:49:19 2007 from 192.168.1.222 paul@RHELv4u4:~$ ssh protocol versions The ssh protocol has two versions (1 and 2). Avoid using version 1 anywhere, since it contains some known vulnerabilities. You can control the protocol version via /etc/ ssh/ssh_config for the client side and /etc/ssh/sshd_config for the openssh-server daemon. root@laika:/etc/ssh# grep Protocol ssh_config # Protocol 2,1 root@laika:/etc/ssh# grep Protocol sshd_config Protocol 2 root@laika:/etc/ssh# 454
- 461. introduction to networking Configuration of ssh is done in the /etc/ssh directory and is pretty straightforward. public and private keys Imagine Alice and Bob, two people that like to communicate with each other. Using public and private keys they can communicate with encryption and with authentication. When Alice wants to send an encrypted message to Bob, she uses the public key of Bob. Bob shares his public key with Alice, but keeps his private key private! Since Bob is the only one to have Bob's private key, Alice is sure that Bob is the only one that can read the encrypted message. When Bob wants to verify that the message came from Alice, Bob uses the public key of Alice to verify that Alice signed the message with her private key. Since Alice is the only one to have Alice's private key, Bob is sure the message came from Alice. setting up passwordless ssh To set up passwordless ssh authentication through public/private keys, use ssh- keygen to generate a key pair without a passphrase, and then copy your public key to the destination server. Let's do this step by step. In the example that follows, we will set up ssh without password between Alice and Bob. Alice has an account on a Red Hat Enterprise Linux server, Bob is using Ubuntu on his laptop. Bob wants to give Alice access using ssh and the public and private key system. This means that even if Bob changes his password on his laptop, Alice will still have access. ssh-keygen The example below shows how Alice uses ssh-keygen to generate a key pair. Alice does not enter a passphrase. [alice@RHEL5 ~]$ ssh-keygen -t rsa Generating public/private rsa key pair. Enter file in which to save the key (/home/alice/.ssh/id_rsa): Created directory '/home/alice/.ssh'. Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/alice/.ssh/id_rsa. Your public key has been saved in /home/alice/.ssh/id_rsa.pub. The key fingerprint is: 9b:ac:ac:56:c2:98:e5:d9:18:c4:2a:51:72:bb:45:eb alice@RHEL5 [alice@RHEL5 ~]$ 455
- 462. introduction to networking ~/.ssh While ssh-keygen generates a public and a private key, it will also create a hidden .ssh directory with proper permissions. If you create the .ssh directory manually, then you need to chmod 700 it! Otherwise ssh will refuse to use the keys (world readable private keys are not secure!). As you can see, the .ssh directory is secure in Alice's home directory. [alice@RHEL5 ~]$ ls -ld .ssh drwx------ 2 alice alice 4096 May 1 07:38 .ssh [alice@RHEL5 ~]$ Bob is using Ubuntu at home. He decides to manually create the .ssh directory, so he needs to manually secure it. bob@laika:~$ mkdir .ssh bob@laika:~$ ls -ld .ssh drwxr-xr-x 2 bob bob 4096 2008-05-14 16:53 .ssh bob@laika:~$ chmod 700 .ssh/ bob@laika:~$ id_rsa and id_rsa.pub The ssh-keygen command generate two keys in .ssh. The public key is named ~/.ssh/ id_rsa.pub. The private key is named ~/.ssh/id_rsa. [alice@RHEL5 ~]$ ls -l .ssh/ total 16 -rw------- 1 alice alice 1671 May 1 07:38 id_rsa -rw-r--r-- 1 alice alice 393 May 1 07:38 id_rsa.pub [alice@RHEL5 ~]$ scp To copy the public key from Alice's server tot Bob's laptop, Alice decides to use scp. [alice@RHEL5 .ssh]$ scp id_rsa.pub bob@192.168.48.92:~/.ssh/authorized_keys bob@192.168.48.92's password: id_rsa.pub 100% 393 0.4KB/s 00:00 [alice@RHEL5 .ssh]$ Be careful when copying a second key! Do not overwrite the first key, instead append the key to the same ~/.ssh/authorized_keys file! authorized_keys In your ~/.ssh directory, you can create a file called authorized_keys. This file can contain one or more public keys from people you trust. Those trusted people can 456
- 463. introduction to networking use their private keys to prove their identity and gain access to your account via ssh (without password). The example shows Bob's authorized_keys file containing the public key of Alice. bob@laika:~$ cat .ssh/authorized_keys ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEApCQ9xzyLzJes1sR+hPyqW2vyzt1D4zTLqk MDWBR4mMFuUZD/O583I3Lg/Q+JIq0RSksNzaL/BNLDou1jMpBe2Dmf/u22u4KmqlJBfDhe yTmGSBzeNYCYRSMq78CT9l9a+y6x/shucwhaILsy8A2XfJ9VCggkVtu7XlWFDL2cum08/0 mRFwVrfc/uPsAn5XkkTscl4g21mQbnp9wJC40pGSJXXMuFOk8MgCb5ieSnpKFniAKM+tEo /vjDGSi3F/bxu691jscrU0VUdIoOSo98HUfEf7jKBRikxGAC7I4HLa+/zX73OIvRFAb2hv tUhn6RHrBtUJUjbSGiYeFTLDfcTQ== alice@RHEL5 bob@laika:~$ passwordless ssh Alice can now use ssh to connect passwordless to Bob's laptop. In combination with ssh's capability to execute commands on the remote host, this can be useful in pipes across different machines. [alice@RHEL5 ~]$ ssh bob@192.168.48.92 "ls -l .ssh" total 4 -rw-r--r-- 1 bob bob 393 2008-05-14 17:03 authorized_keys [alice@RHEL5 ~]$ X forwarding via SSH The ssh protocol will remember the servers it connected to (and warn you in case something suspicious happened), and will use strong 128-bit encryption. Another popular feature of ssh is called X11 forwarding and is implemented with ssh -X. Below an example of X11 forwarding: user paul logs in as user greet on her computer to start the graphical application mozilla-thunderbird. Although the application will run on the remote computer from greet, it will be displayed on the screen attached locally to paul's computer. paul@laika:~/PDF$ ssh -X greet@greet.dyndns.org -p 55555 Warning: Permanently added the RSA host key for IP address '81.240.174.161' to the list of known hosts. Password: Linux raika 2.6.8-2-686 #1 Tue Aug 16 13:22:48 UTC 2005 i686 GNU/Linux Last login: Thu Jan 18 12:35:56 2007 greet@raika:~$ ps fax | grep thun greet@raika:~$ mozilla-thunderbird & [1] 30336 troubleshooting ssh Use ssh -v to get debug information about the ssh connection attempt. 457
- 464. introduction to networking paul@laika:~$ ssh -v bert@192.168.1.192 OpenSSH_4.3p2 Debian-8ubuntu1, OpenSSL 0.9.8c 05 Sep 2006 debug1: Reading configuration data /home/paul/.ssh/config debug1: Reading configuration data /etc/ssh/ssh_config debug1: Applying options for * debug1: Connecting to 192.168.1.192 [192.168.1.192] port 22. debug1: Connection established. debug1: identity file /home/paul/.ssh/identity type -1 debug1: identity file /home/paul/.ssh/id_rsa type 1 debug1: identity file /home/paul/.ssh/id_dsa type -1 debug1: Remote protocol version 1.99, remote software version OpenSSH_3 debug1: match: OpenSSH_3.9p1 pat OpenSSH_3.* debug1: Enabling compatibility mode for protocol 2.0 ... 49.7. practice: ssh 1. Create a user for your neighbour, then test ssh to your neighbour (by ip-address or by hostname). (You might need to install the openssh-server with aptitude.) 2. Create a bookmark in Firefox, then close your firefox! Use ssh -X to run firefox on your screen, but on your neighbour's computer. Do you see your neighbour's bookmark ? 3. Verify in the ssh configuration files that only protocol version 2 is allowed. 4. Use ssh-keygen to create a key pair without passphrase. Setup passwordless ssh between you and your neighbour. (or between the ubuntu and the Red Hat) 458
- 465. introduction to networking 49.8. network file system protocol versions The older nfs versions 2 and 3 are stateless (udp) by default, but they can use tcp. Clients connect to the server using rpc (on Linux this is controlled by the portmap daemon. Look at rpcinfo to verify that nfs and its related services are running. root@RHELv4u2:~# /etc/init.d/portmap status portmap (pid 1920) is running... root@RHELv4u2:~# rpcinfo -p program vers proto port 100000 2 tcp 111 portmapper 100000 2 udp 111 portmapper 100024 1 udp 32768 status 100024 1 tcp 32769 status root@RHELv4u2:~# service nfs start Starting NFS services: [ OK ] Starting NFS quotas: [ OK ] Starting NFS daemon: [ OK ] Starting NFS mountd: [ OK ] The same rpcinfo command when nfs is started. root@RHELv4u2:~# rpcinfo -p program vers proto port 100000 2 tcp 111 portmapper 100000 2 udp 111 portmapper 100024 1 udp 32768 status 100024 1 tcp 32769 status 100011 1 udp 985 rquotad 100011 2 udp 985 rquotad 100011 1 tcp 988 rquotad 100011 2 tcp 988 rquotad 100003 2 udp 2049 nfs 100003 3 udp 2049 nfs 100003 4 udp 2049 nfs 100003 2 tcp 2049 nfs 100003 3 tcp 2049 nfs 100003 4 tcp 2049 nfs 100021 1 udp 32770 nlockmgr 100021 3 udp 32770 nlockmgr 100021 4 udp 32770 nlockmgr 100021 1 tcp 32789 nlockmgr 100021 3 tcp 32789 nlockmgr 100021 4 tcp 32789 nlockmgr 100005 1 udp 1004 mountd 100005 1 tcp 1007 mountd 100005 2 udp 1004 mountd 100005 2 tcp 1007 mountd 100005 3 udp 1004 mountd 100005 3 tcp 1007 mountd root@RHELv4u2:~# 459
- 466. introduction to networking nfs version 4 requires tcp (port 2049) and supports Kerberos user authentication as an option. nfs authentication only takes place when mounting the share. nfs versions 2 and 3 authenticate only the host. server configuration nfs is configured in /etc/exports. Here is a sample /etc/exports to explain the syntax. You need some way (NIS domain or LDAP) to synchronize userid's across computers when using nfs a lot. The rootsquash option will change UID 0 to the UID of the nfsnobody user account. The sync option will write writes to disk before completing the client request. paul@laika:~$ cat /etc/exports # Everyone can read this share /mnt/data/iso *(ro) # Only the computers barry and pasha can readwrite this one /var/www pasha(rw) barry(rw) # same, but without root squashing for barry /var/ftp pasha(rw) barry(rw,no_root_squash) # everyone from the netsec.lan domain gets access /var/backup *.netsec.lan(rw) # ro for one network, rw for the other /var/upload 192.168.1.0/24(ro) 192.168.5.0/24(rw) You don't need to restart the nfs server to start exporting your newly created exports. You can use the exportfs -va command to do this. It will write the exported directories to /var/lib/nfs/etab, where they are immediately applied. client configuration We have seen the mount command and the /etc/fstab file before. root@RHELv4u2:~# mount -t nfs barry:/mnt/data/iso /home/project55/ root@RHELv4u2:~# cat /etc/fstab | grep nfs barry:/mnt/data/iso /home/iso nfs defaults 0 0 root@RHELv4u2:~# Here is another simple example. Suppose the project55 people tell you they only need a couple of CD-ROM images, and you already have them available on an nfs server. You could issue the following command to mount this storage on their /home/ project55 mount point. root@RHELv4u2:~# mount -t nfs 192.168.1.40:/mnt/data/iso /home/project55/ root@RHELv4u2:~# ls -lh /home/project55/ total 3.6G drwxr-xr-x 2 1000 1000 4.0K Jan 16 17:55 RHELv4u1 460
- 467. introduction to networking drwxr-xr-x 2 1000 1000 4.0K Jan 16 14:14 RHELv4u2 drwxr-xr-x 2 1000 1000 4.0K Jan 16 14:54 RHELv4u3 drwxr-xr-x 2 1000 1000 4.0K Jan 16 11:09 RHELv4u4 -rw-r--r-- 1 root root 1.6G Oct 13 15:22 sled10-vmwarews5-vm.zip root@RHELv4u2:~# 49.9. practice : network file system 1. Create two directories with some files. Use nfs to share one of them as read only, the other must be writable. Have your neighbour connect to them to test. 2. Investigate the user owner of the files created by your neighbour. 3. Protect a share by ip-address or hostname, so only your neighbour can connect. 461
- 468. Part XIV. kernel management
- 469. Chapter 50. the Linux kernel Table of Contents 50.1. about the Linux kernel ............................................................................... 463 50.2. Linux kernel source ................................................................................... 465 50.3. kernel boot files ......................................................................................... 469 50.4. Linux kernel modules ................................................................................ 470 50.5. compiling a kernel ..................................................................................... 474 50.6. compiling one module ............................................................................... 477 50.1. about the Linux kernel kernel versions In 1991 Linux Torvalds wrote (the first version of) the Linux kernel. He put it online, and other people started contributing code. Over 4000 individuals contributed source code to the latest kernel release (version 2.6.27 in November 2008). Major Linux kernel versions used to come in even and odd numbers. Versions 2.0, 2.2, 2.4 and 2.6 are considered stable kernel versions. Whereas 2.1, 2.3 and 2.5 were unstable (read development) versions. Since the release of 2.6.0 in January 2004, all development has been done in the 2.6 tree. There is currently no v2.7.x and according to Linus the even/stable vs odd/development scheme is abandoned forever. uname -r To see your current Linux kernel version, issue the uname -r command as shown below. This first example shows Linux major version 2.6 and minor version 24. The rest -22- generic is specific to the distribution (Ubuntu in this case). paul@laika:~$ uname -r 2.6.24-22-generic The same command on Red Hat Enterprise Linux shows an older kernel (2.6.18) with -92.1.17.el5 being specific to the distribution. [paul@RHEL52 ~]$ uname -r 2.6.18-92.1.17.el5 463
- 470. the Linux kernel /proc/cmdline The parameters that were passed to the kernel at boot time are in /proc/cmdline. paul@RHELv4u4:~$ cat /proc/cmdline ro root=/dev/VolGroup00/LogVol00 rhgb quiet single user mode When booting the kernel with the single parameter, it starts in single user mode. Linux can start in a bash shell with the root user logged on (without password). Some distributions prevent the use of this feature (at kernel compile time). init=/bin/bash Normally the kernel invokes init as the first daemon process. Adding init=/bin/bash to the kernel parameters will instead invoke bash (again with root logged on without providing a password). /var/log/messages The kernel reports during boot to syslog which writes a lot of kernel actions in /var/ log/messages. Looking at this file reveals when the kernel was started, including all the devices that were detected at boot time. [root@RHEL53 ~]# grep -A16 "syslogd 1.4.1:" /var/log/messages|cut -b24- syslogd 1.4.1: restart. kernel: klogd 1.4.1, log source = /proc/kmsg started. kernel: Linux version 2.6.18-128.el5 (mockbuild@hs20-bc1-5.build.red... kernel: BIOS-provided physical RAM map: kernel: BIOS-e820: 0000000000000000 - 000000000009f800 (usable) kernel: BIOS-e820: 000000000009f800 - 00000000000a0000 (reserved) kernel: BIOS-e820: 00000000000ca000 - 00000000000cc000 (reserved) kernel: BIOS-e820: 00000000000dc000 - 0000000000100000 (reserved) kernel: BIOS-e820: 0000000000100000 - 000000001fef0000 (usable) kernel: BIOS-e820: 000000001fef0000 - 000000001feff000 (ACPI data) kernel: BIOS-e820: 000000001feff000 - 000000001ff00000 (ACPI NVS) kernel: BIOS-e820: 000000001ff00000 - 0000000020000000 (usable) kernel: BIOS-e820: 00000000fec00000 - 00000000fec10000 (reserved) kernel: BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved) kernel: BIOS-e820: 00000000fffe0000 - 0000000100000000 (reserved) kernel: 0MB HIGHMEM available. kernel: 512MB LOWMEM available. This example shows how to use /var/log/messages to see kernel information about /dev/sda. [root@RHEL53 ~]# grep sda /var/log/messages | cut -b24- kernel: SCSI device sda: 41943040 512-byte hdwr sectors (21475 MB) kernel: sda: Write Protect is off 464
- 471. the Linux kernel kernel: sda: cache data unavailable kernel: sda: assuming drive cache: write through kernel: SCSI device sda: 41943040 512-byte hdwr sectors (21475 MB) kernel: sda: Write Protect is off kernel: sda: cache data unavailable kernel: sda: assuming drive cache: write through kernel: sda: sda1 sda2 kernel: sd 0:0:0:0: Attached scsi disk sda kernel: EXT3 FS on sda1, internal journal dmesg The dmesg command prints out all the kernel bootup messages (from the last boot). [root@RHEL53 ~]# dmesg | head Linux version 2.6.18-128.el5 (mockbuild@hs20-bc1-5.build.redhat.com) BIOS-provided physical RAM map: BIOS-e820: 0000000000000000 - 000000000009f800 (usable) BIOS-e820: 000000000009f800 - 00000000000a0000 (reserved) BIOS-e820: 00000000000ca000 - 00000000000cc000 (reserved) BIOS-e820: 00000000000dc000 - 0000000000100000 (reserved) BIOS-e820: 0000000000100000 - 000000001fef0000 (usable) BIOS-e820: 000000001fef0000 - 000000001feff000 (ACPI data) BIOS-e820: 000000001feff000 - 000000001ff00000 (ACPI NVS) BIOS-e820: 000000001ff00000 - 0000000020000000 (usable) Thus to find information about /dev/sda, using dmesg will yield only kernel messages from the last boot. [root@RHEL53 ~]# dmesg | grep sda SCSI device sda: 41943040 512-byte hdwr sectors (21475 MB) sda: Write Protect is off sda: Mode Sense: 5d 00 00 00 sda: cache data unavailable sda: assuming drive cache: write through SCSI device sda: 41943040 512-byte hdwr sectors (21475 MB) sda: Write Protect is off sda: Mode Sense: 5d 00 00 00 sda: cache data unavailable sda: assuming drive cache: write through sda: sda1 sda2 sd 0:0:0:0: Attached scsi disk sda EXT3 FS on sda1, internal journal 50.2. Linux kernel source ftp.kernel.org The home of the Linux kernel source is ftp.kernel.org. It contains all official releases of the Linux kernel source code from 1991. It provides free downloads over http, ftp and rsync of all these releases, as well as changelogs and patches. More information can be otained on the website www.kernel.org. 465
- 472. the Linux kernel Anyone can anonymously use an ftp client to access ftp.kernel.org paul@laika:~$ ftp ftp.kernel.org Connected to pub3.kernel.org. 220 Welcome to ftp.kernel.org. Name (ftp.kernel.org:paul): anonymous 331 Please specify the password. Password: 230- Welcome to the 230- 230- LINUX KERNEL ARCHIVES 230- ftp.kernel.org All the Linux kernel versions are located in the pub/linux/kernel/ directory. ftp> ls pub/linux/kernel/v* 200 PORT command successful. Consider using PASV. 150 Here comes the directory listing. drwxrwsr-x 2 536 536 4096 Mar 20 2003 v1.0 drwxrwsr-x 2 536 536 20480 Mar 20 2003 v1.1 drwxrwsr-x 2 536 536 8192 Mar 20 2003 v1.2 drwxrwsr-x 2 536 536 40960 Mar 20 2003 v1.3 drwxrwsr-x 3 536 536 16384 Feb 08 2004 v2.0 drwxrwsr-x 2 536 536 53248 Mar 20 2003 v2.1 drwxrwsr-x 3 536 536 12288 Mar 24 2004 v2.2 drwxrwsr-x 2 536 536 24576 Mar 20 2003 v2.3 drwxrwsr-x 5 536 536 28672 Dec 02 08:14 v2.4 drwxrwsr-x 4 536 536 32768 Jul 14 2003 v2.5 drwxrwsr-x 7 536 536 110592 Dec 05 22:36 v2.6 226 Directory send OK. ftp> /usr/src On your local computer, the kernel source is located in /usr/src. Note though that the structure inside /usr/src might be different depending on the distribution that you are using. First let's take a look at /usr/src on Debian. There appear to be two versions of the complete Linux source code there. Looking for a specific file (e1000_main.c) with find reveals it's exact location. paul@barry:~$ ls -l /usr/src/ drwxr-xr-x 20 root root 4096 2006-04-04 22:12 linux-source-2.6.15 drwxr-xr-x 19 root root 4096 2006-07-15 17:32 linux-source-2.6.16 paul@barry:~$ find /usr/src -name e1000_main.c /usr/src/linux-source-2.6.15/drivers/net/e1000/e1000_main.c /usr/src/linux-source-2.6.16/drivers/net/e1000/e1000_main.c This is very similar to /usr/src on Ubuntu, except there is only one kernel here (and it is newer). paul@laika:~$ ls -l /usr/src/ 466
- 473. the Linux kernel drwxr-xr-x 23 root root 4096 2008-11-24 23:28 linux-source-2.6.24 paul@laika:~$ find /usr/src -name "e1000_main.c" /usr/src/linux-source-2.6.24/drivers/net/e1000/e1000_main.c Now take a look at /usr/src on Red Hat Enterprise Linux. [paul@RHEL52 ~]$ ls -l /usr/src/ drwxr-xr-x 5 root root 4096 Dec 5 19:23 kernels drwxr-xr-x 7 root root 4096 Oct 11 13:22 redhat We will have to dig a little deeper to find the kernel source on Red Hat! [paul@RHEL52 ~]$ cd /usr/src/redhat/BUILD/ [paul@RHEL52 BUILD]$ find . -name "e1000_main.c" ./kernel-2.6.18/linux-2.6.18.i686/drivers/net/e1000/e1000_main.c downloading the kernel source Debian Installing the kernel source on Debian is really simple with aptitude install linux- source. You can do a search for all linux-source packeges first, like in this screenshot. root@barry:~# aptitude search linux-source v linux-source - v linux-source-2.6 - id linux-source-2.6.15 - Linux kernel source for version 2.6.15 i linux-source-2.6.16 - Linux kernel source for version 2.6.16 p linux-source-2.6.18 - Linux kernel source for version 2.6.18 p linux-source-2.6.24 - Linux kernel source for version 2.6.24 And then use aptitude install to download and install the Debian Linux kernel source code. root@barry:~# aptitude install linux-source-2.6.24 When the aptitude is finished, you will see a new file named /usr/src/linux-source- <version>.tar.bz2 root@barry:/usr/src# ls -lh drwxr-xr-x 20 root root 4.0K 2006-04-04 22:12 linux-source-2.6.15 drwxr-xr-x 19 root root 4.0K 2006-07-15 17:32 linux-source-2.6.16 -rw-r--r-- 1 root root 45M 2008-12-02 10:56 linux-source-2.6.24.tar.bz2 Ubuntu Ubuntu is based on Debian and also uses aptitude, so the task is very similar. 467
- 474. the Linux kernel root@laika:~# aptitude search linux-source i linux-source - Linux kernel source with Ubuntu patches v linux-source-2.6 - i A linux-source-2.6.24 - Linux kernel source for version 2.6.24 root@laika:~# aptitude install linux-source And when aptitude finishes, we end up with a /usr/src/linux-source- <version>.tar.bz file. oot@laika:~# ll /usr/src total 45M -rw-r--r-- 1 root root 45M 2008-11-24 23:30 linux-source-2.6.24.tar.bz2 Red Hat Enterprise Linux The Red Hat kernel source is located on the fourth source cdrom. The file is called kernel-2.6.9-42.EL.src.rpm (example for RHELv4u4). It is also available online at ftp://ftp.redhat.com/pub/redhat/linux/enterprise/5Server/en/os/SRPMS/ (example for RHEL5). To download the kernel source on RHEL, use this long wget command (on one line, without the trailing ). wget ftp://ftp.redhat.com/pub/redhat/linux/enterprise/5Server/en/os/ SRPMS/kernel-`uname -r`.src.rpm When the wget download is finished, you end up with a 60M .rpm file. [root@RHEL52 src]# ll total 60M -rw-r--r-- 1 root root 60M Dec 5 20:54 kernel-2.6.18-92.1.17.el5.src.rpm drwxr-xr-x 5 root root 4.0K Dec 5 19:23 kernels drwxr-xr-x 7 root root 4.0K Oct 11 13:22 redhat We will need to perform some more steps before this can be used as kernel source code. First, we issue the rpm -i kernel-2.6.9-42.EL.src.rpm command to install this Red Hat package. [root@RHEL52 src]# ll total 60M -rw-r--r-- 1 root root 60M Dec 5 20:54 kernel-2.6.18-92.1.17.el5.src.rpm drwxr-xr-x 5 root root 4.0K Dec 5 19:23 kernels drwxr-xr-x 7 root root 4.0K Oct 11 13:22 redhat [root@RHEL52 src]# rpm -i kernel-2.6.18-92.1.17.el5.src.rpm The we move to the SPECS directory and perform an rpmbuild. 468
- 475. the Linux kernel [root@RHEL52 ~]# cd /usr/src/redhat/SPECS [root@RHEL52 SPECS]# rpmbuild -bp -vv --target=i686 kernel-2.6.spec The rpmbuild command put the RHEL Linux kernel source code in /usr/src/redhat/ BUILD/kernel-<version>/. [root@RHEL52 kernel-2.6.18]# pwd /usr/src/redhat/BUILD/kernel-2.6.18 [root@RHEL52 kernel-2.6.18]# ll total 20K drwxr-xr-x 2 root root 4.0K Dec 6 2007 config -rw-r--r-- 1 root root 3.1K Dec 5 20:58 Config.mk drwxr-xr-x 20 root root 4.0K Dec 5 20:58 linux-2.6.18.i686 drwxr-xr-x 19 root root 4.0K Sep 20 2006 vanilla drwxr-xr-x 8 root root 4.0K Dec 6 2007 xen 50.3. kernel boot files vmlinuz The vmlinuz file in /boot is the compressed kernel. paul@barry:~$ ls -lh /boot | grep vmlinuz -rw-r--r-- 1 root root 1.2M 2006-03-06 16:22 vmlinuz-2.6.15-1-486 -rw-r--r-- 1 root root 1.1M 2006-03-06 16:30 vmlinuz-2.6.15-1-686 -rw-r--r-- 1 root root 1.3M 2008-02-11 00:00 vmlinuz-2.6.18-6-686 paul@barry:~$ initrd The kernel uses initrd (an initial RAM disk) at boot time. The initrd is mounted before the kernel loads, and can contain additional drivers and modules. It is a compressed cpio archive, so you can look at the contents in this way. root@RHELv4u4:/boot# mkdir /mnt/initrd root@RHELv4u4:/boot# cp initrd-2.6.9-42.0.3.EL.img TMPinitrd.gz root@RHELv4u4:/boot# gunzip TMPinitrd.gz root@RHELv4u4:/boot# file TMPinitrd TMPinitrd: ASCII cpio archive (SVR4 with no CRC) root@RHELv4u4:/boot# cd /mnt/initrd/ root@RHELv4u4:/mnt/initrd# cpio -i | /boot/TMPinitrd 4985 blocks root@RHELv4u4:/mnt/initrd# ls -l total 76 drwxr-xr-x 2 root root 4096 Feb 5 08:36 bin drwxr-xr-x 2 root root 4096 Feb 5 08:36 dev drwxr-xr-x 4 root root 4096 Feb 5 08:36 etc -rwxr-xr-x 1 root root 1607 Feb 5 08:36 init drwxr-xr-x 2 root root 4096 Feb 5 08:36 lib drwxr-xr-x 2 root root 4096 Feb 5 08:36 loopfs drwxr-xr-x 2 root root 4096 Feb 5 08:36 proc 469
- 476. the Linux kernel lrwxrwxrwx 1 root root 3 Feb 5 08:36 sbin -> bin drwxr-xr-x 2 root root 4096 Feb 5 08:36 sys drwxr-xr-x 2 root root 4096 Feb 5 08:36 sysroot root@RHELv4u4:/mnt/initrd# System.map The System.map contains the symbol table and changes with every kernel compile. The symbol table is also present in /proc/kallsyms (pre 2.6 kernels name this file / proc/ksyms). root@RHELv4u4:/boot# head System.map-`uname -r` 00000400 A __kernel_vsyscall 0000041a A SYSENTER_RETURN_OFFSET 00000420 A __kernel_sigreturn 00000440 A __kernel_rt_sigreturn c0100000 A _text c0100000 T startup_32 c01000c6 t checkCPUtype c0100147 t is486 c010014e t is386 c010019f t L6 root@RHELv4u4:/boot# head /proc/kallsyms c0100228 t _stext c0100228 t calibrate_delay_direct c0100228 t stext c0100337 t calibrate_delay c01004db t rest_init c0100580 t do_pre_smp_initcalls c0100585 t run_init_process c01005ac t init c0100789 t early_param_test c01007ad t early_setup_test root@RHELv4u4:/boot# .config The last file copied to the /boot directory is the kernel configuration used for compilation. This file is not necessary in the /boot directory, but it is common practice to put a copy there. It allows you to recompile a kernel, starting from the same configuration as an existing working one. 50.4. Linux kernel modules about kernel modules The Linux kernel is a monolithic kernel with loadable modules. These modules contain parts of the kernel used typically for device drivers, file systems and network protocols. Most of the time the necessary kernel modules are loaded automatically and dynamically without administrator interaction. 470
- 477. the Linux kernel /lib/modules The modules are stored in the /lib/modules/<kernel-version> directory. There is a separate directory for each kernel that was compiled for your system. paul@laika:~$ ll /lib/modules/ total 12K drwxr-xr-x 7 root root 4.0K 2008-11-10 14:32 2.6.24-16-generic drwxr-xr-x 8 root root 4.0K 2008-12-06 15:39 2.6.24-21-generic drwxr-xr-x 8 root root 4.0K 2008-12-05 12:58 2.6.24-22-generic <module>.ko The file containing the modules usually ends in .ko. This screenshot shows the location of the isdn module files. paul@laika:~$ find /lib/modules -name isdn.ko /lib/modules/2.6.24-21-generic/kernel/drivers/isdn/i4l/isdn.ko /lib/modules/2.6.24-22-generic/kernel/drivers/isdn/i4l/isdn.ko /lib/modules/2.6.24-16-generic/kernel/drivers/isdn/i4l/isdn.ko lsmod To see a list of currently loaded modules, use lsmod. You see the name of each loaded module, the size, the use count, and the names of other modules using this one. [root@RHEL52 ~]# lsmod | head -5 Module Size Used by autofs4 24517 2 hidp 23105 2 rfcomm 42457 0 l2cap 29505 10 hidp,rfcomm /proc/modules /proc/modules lists all modules loaded by the kernel. The output would be too long to display here, so lets grep for the vm module. We see that vmmon and vmnet are both loaded. You can display the same information with lsmod. Actually lsmod only reads and reformats the output of /proc/modules. paul@laika:~$ cat /proc/modules | grep vm vmnet 36896 13 - Live 0xffffffff88b21000 (P) vmmon 194540 0 - Live 0xffffffff88af0000 (P) paul@laika:~$ lsmod | grep vm vmnet 36896 13 vmmon 194540 0 paul@laika:~$ 471
- 478. the Linux kernel module dependencies Some modules depend on others. In the following example, you can see that the nfsd module is used by exportfs, lockd and sunrpc. paul@laika:~$ cat /proc/modules | grep nfsd nfsd 267432 17 - Live 0xffffffff88a40000 exportfs 7808 1 nfsd, Live 0xffffffff88a3d000 lockd 73520 3 nfs,nfsd, Live 0xffffffff88a2a000 sunrpc 185032 12 nfs,nfsd,lockd, Live 0xffffffff889fb000 paul@laika:~$ lsmod | grep nfsd nfsd 267432 17 exportfs 7808 1 nfsd lockd 73520 3 nfs,nfsd sunrpc 185032 12 nfs,nfsd,lockd paul@laika:~$ insmod Kernel modules can be manually loaded with the insmod command. This is a very simple (and obsolete) way of loading modules. The screenshot shows insmod loading the fat module (for fat file system support). root@barry:/lib/modules/2.6.17-2-686# pwd /lib/modules/2.6.17-2-686 root@barry:/lib/modules/2.6.17-2-686# lsmod | grep fat root@barry:/lib/modules/2.6.17-2-686# insmod kernel/fs/fat/fat.ko root@barry:/lib/modules/2.6.17-2-686# lsmod | grep fat fat 46588 0 insmod is not detecting dependencies, so it fails to load the isdn module (because the isdn module depends on the slhc module). [root@RHEL52 drivers]# pwd /lib/modules/2.6.18-92.1.18.el5/kernel/drivers [root@RHEL52 kernel]# insmod isdn/i4l/isdn.ko insmod: error inserting 'isdn/i4l/isdn.ko': -1 Unknown symbol in module modinfo As you can see in the screenshot of modinfo below, the isdn module depends in the slhc module. [root@RHEL52 drivers]# modinfo isdn/i4l/isdn.ko | head -6 filename: isdn/i4l/isdn.ko license: GPL author: Fritz Elfert 472
- 479. the Linux kernel description: ISDN4Linux: link layer srcversion: 99650346E708173496F6739 depends: slhc modprobe The big advantage of modprobe over insmod is that modprobe will load all necessary modules, whereas insmod requires manual loading of dependencies. Another advantage is that you don't need to point to the filename with full path. This screenshot shows how modprobe loads the isdn module, automatically loading slhc in background. [root@RHEL52 kernel]# lsmod | grep isdn [root@RHEL52 kernel]# modprobe isdn [root@RHEL52 kernel]# lsmod | grep isdn isdn 122433 0 slhc 10561 1 isdn [root@RHEL52 kernel]# /lib/modules/<kernel>/modules.dep Module dependencies are stored in modules.dep. [root@RHEL52 2.6.18-92.1.18.el5]# pwd /lib/modules/2.6.18-92.1.18.el5 [root@RHEL52 2.6.18-92.1.18.el5]# head -3 modules.dep /lib/modules/2.6.18-92.1.18.el5/kernel/drivers/net/tokenring/3c359.ko: /lib/modules/2.6.18-92.1.18.el5/kernel/drivers/net/pcmcia/3c574_cs.ko: /lib/modules/2.6.18-92.1.18.el5/kernel/drivers/net/pcmcia/3c589_cs.ko: depmod The modules.dep file can be updated (recreated) with the depmod command. In this screenshot no modules were added, so depmod generates the same file. root@barry:/lib/modules/2.6.17-2-686# ls -l modules.dep -rw-r--r-- 1 root root 310676 2008-03-01 16:32 modules.dep root@barry:/lib/modules/2.6.17-2-686# depmod root@barry:/lib/modules/2.6.17-2-686# ls -l modules.dep -rw-r--r-- 1 root root 310676 2008-12-07 13:54 modules.dep rmmod Similar to insmod, the rmmod command is rarely used anymore. 473
- 480. the Linux kernel [root@RHELv4u3 ~]# modprobe isdn [root@RHELv4u3 ~]# rmmod slhc ERROR: Module slhc is in use by isdn [root@RHELv4u3 ~]# rmmod isdn [root@RHELv4u3 ~]# rmmod slhc [root@RHELv4u3 ~]# lsmod | grep isdn [root@RHELv4u3 ~]# modprobe -r Contrary to rmmod, modprobe will automatically remove unneeded modules. [root@RHELv4u3 ~]# modprobe isdn [root@RHELv4u3 ~]# lsmod | grep isdn isdn 133537 0 slhc 7233 1 isdn [root@RHELv4u3 ~]# modprobe -r isdn [root@RHELv4u3 ~]# lsmod | grep isdn [root@RHELv4u3 ~]# lsmod | grep slhc [root@RHELv4u3 ~]# /etc/modprobe.conf The /etc/modprobe.conf file and the /etc/modprobe.d directory can contain aliases (used by humans) and options (for dependent modules) for modprobe. [root@RHEL52 ~]# cat /etc/modprobe.conf alias scsi_hostadapter mptbase alias scsi_hostadapter1 mptspi alias scsi_hostadapter2 ata_piix alias eth0 pcnet32 alias eth2 pcnet32 alias eth1 pcnet32 50.5. compiling a kernel extraversion Enter into /usr/src/redhat/BUILD/kernel-2.6.9/linux-2.6.9/ and change the extraversion in the Makefile. [root@RHEL52 linux-2.6.18.i686]# pwd /usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686 [root@RHEL52 linux-2.6.18.i686]# vi Makefile [root@RHEL52 linux-2.6.18.i686]# head -4 Makefile VERSION = 2 PATCHLEVEL = 6 SUBLEVEL = 18 EXTRAVERSION = -paul2008 474
- 481. the Linux kernel make mrproper Now clean up the source from any previous installs with make mrproper. If this is your first after downloading the source code, then this is not needed. [root@RHEL52 linux-2.6.18.i686]# make mrproper CLEAN scripts/basic CLEAN scripts/kconfig CLEAN include/config CLEAN .config .config.old .config Now copy a working .config from /boot to our kernel directory. This file contains the configuration that was used for your current working kernel. It determines whether modules are included in compilation or not. [root@RHEL52 linux-2.6.18.i686]# cp /boot/config-2.6.18-92.1.18.el5 .config make menuconfig Now run make menuconfig (or the graphical make xconfig). This tool allows you to select whether to compile stuff as a module (m), as part of the kernel (*), or not at all (smaller kernel size). If you remove too much, your kernel will not work. The configuration will be stored in the hidden .config file. [root@RHEL52 linux-2.6.18.i686]# make menuconfig make clean Issue a make clean to prepare the kernel for compile. make clean will remove most generated files, but keeps your kernel configuration. Running a make mrproper at this point would destroy the .config file that you built with make menuconfig. [root@RHEL52 linux-2.6.18.i686]# make clean make bzImage And then run make bzImage, sit back and relax while the kernel compiles. You can use time make bzImage to know how long it takes to compile, so next time you can go for a short walk. 475
- 482. the Linux kernel [root@RHEL52 linux-2.6.18.i686]# time make bzImage HOSTCC scripts/basic/fixdep HOSTCC scripts/basic/docproc HOSTCC scripts/kconfig/conf.o HOSTCC scripts/kconfig/kxgettext.o ... This command will end with telling you the location of the bzImage file (and with time info if you also specified the time command. Kernel: arch/i386/boot/bzImage is ready (#1) real 13m59.573s user 1m22.631s sys 11m51.034s [root@RHEL52 linux-2.6.18.i686]# You can already copy this image to /boot with cp arch/i386/boot/bzImage /boot/ vmlinuz-<kernel-version>. make modules Now run make modules. It can take 20 to 50 minutes to compile all the modules. [root@RHEL52 linux-2.6.18.i686]# time make modules CHK include/linux/version.h CHK include/linux/utsrelease.h CC [M] arch/i386/kernel/msr.o CC [M] arch/i386/kernel/cpuid.o CC [M] arch/i386/kernel/microcode.o make modules_install To copy all the compiled modules to /lib/modules just run make modules_install (takes about 20 seconds). Here's a screenshot from before the command. [root@RHEL52 linux-2.6.18.i686]# ls -l /lib/modules/ total 20 drwxr-xr-x 6 root root 4096 Oct 15 13:09 2.6.18-92.1.13.el5 drwxr-xr-x 6 root root 4096 Nov 11 08:51 2.6.18-92.1.17.el5 drwxr-xr-x 6 root root 4096 Dec 6 07:11 2.6.18-92.1.18.el5 [root@RHEL52 linux-2.6.18.i686]# make modules_install And here is the same directory after. Notice that make modules_install created a new directory for the new kernel. [root@RHEL52 linux-2.6.18.i686]# ls -l /lib/modules/ total 24 476
- 483. the Linux kernel drwxr-xr-x 6 root root 4096 Oct 15 13:09 2.6.18-92.1.13.el5 drwxr-xr-x 6 root root 4096 Nov 11 08:51 2.6.18-92.1.17.el5 drwxr-xr-x 6 root root 4096 Dec 6 07:11 2.6.18-92.1.18.el5 drwxr-xr-x 3 root root 4096 Dec 6 08:50 2.6.18-paul2008 /boot We still need to copy the kernel, the System.map and our configuration file to /boot. Strictly speaking the .config file is not obligatory, but it might help you in future compilations of the kernel. [root@RHEL52 ]# pwd /usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686 [root@RHEL52 ]# cp System.map /boot/System.map-2.6.18-paul2008 [root@RHEL52 ]# cp .config /boot/config-2.6.18-paul2008 [root@RHEL52 ]# cp arch/i386/boot/bzImage /boot/vmlinuz-2.6.18-paul2008 mkinitrd The kernel often uses an initrd file at bootup. We can use mkinitrd to generate this file. Make sure you use the correct kernel name! [root@RHEL52 ]# pwd /usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686 [root@RHEL52 ]# mkinitrd /boot/initrd-2.6.18-paul2008 2.6.18-paul2008 bootloader Compilation is now finished, don't forget to create an additional stanza in grub or lilo. 50.6. compiling one module hello.c A little C program that will be our module. [root@rhel4a kernel_module]# cat hello.c #include <linux/module.h> #include <section> int init_module(void) { printk(KERN_INFO "Start Hello World...n"); return 0; } 477
- 484. the Linux kernel void cleanup_module(void) { printk(KERN_INFO "End Hello World... n"); } Makefile The make file for this module. [root@rhel4a kernel_module]# cat Makefile obj-m += hello.o all: make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules clean: make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean These are the only two files needed. [root@rhel4a kernel_module]# ll total 16 -rw-rw-r-- 1 paul paul 250 Feb 15 19:14 hello.c -rw-rw-r-- 1 paul paul 153 Feb 15 19:15 Makefile make The running of the make command. [root@rhel4a kernel_module]# make make -C /lib/modules/2.6.9-paul-2/build M=~/kernel_module modules make[1]: Entering dir... `/usr/src/redhat/BUILD/kernel-2.6.9/linux-2.6.9' CC [M] /home/paul/kernel_module/hello.o Building modules, stage 2. MODPOST CC /home/paul/kernel_module/hello.mod.o LD [M] /home/paul/kernel_module/hello.ko make[1]: Leaving dir... `/usr/src/redhat/BUILD/kernel-2.6.9/linux-2.6.9' [root@rhel4a kernel_module]# Now we have more files. [root@rhel4a kernel_module]# ll total 172 -rw-rw-r-- 1 paul paul 250 Feb 15 19:14 hello.c -rw-r--r-- 1 root root 64475 Feb 15 19:15 hello.ko -rw-r--r-- 1 root root 632 Feb 15 19:15 hello.mod.c -rw-r--r-- 1 root root 37036 Feb 15 19:15 hello.mod.o -rw-r--r-- 1 root root 28396 Feb 15 19:15 hello.o -rw-rw-r-- 1 paul paul 153 Feb 15 19:15 Makefile [root@rhel4a kernel_module]# 478
- 485. the Linux kernel hello.ko Use modinfo to verify that it is really a module. [root@rhel4a kernel_module]# modinfo hello.ko filename: hello.ko vermagic: 2.6.9-paul-2 SMP 686 REGPARM 4KSTACKS gcc-3.4 depends: [root@rhel4a kernel_module]# Good, so now we can load our hello module. [root@rhel4a kernel_module]# lsmod | grep hello [root@rhel4a kernel_module]# insmod ./hello.ko [root@rhel4a kernel_module]# lsmod | grep hello hello 5504 0 [root@rhel4a kernel_module]# tail -1 /var/log/messages Feb 15 19:16:07 rhel4a kernel: Start Hello World... [root@rhel4a kernel_module]# rmmod hello [root@rhel4a kernel_module]# Finally /var/log/messages has a little surprise. [root@rhel4a kernel_module]# tail -2 /var/log/messages Feb 15 19:16:07 rhel4a kernel: Start Hello World... Feb 15 19:16:35 rhel4a kernel: End Hello World... [root@rhel4a kernel_module]# 479
- 486. Chapter 51. library management Table of Contents 51.1. introduction ................................................................................................ 480 51.2. /lib and /usr/lib ........................................................................................... 480 51.3. ldd ............................................................................................................... 480 51.4. ltrace ........................................................................................................... 481 51.5. dpkg -S and debsums ................................................................................. 481 51.6. rpm -qf and rpm -V ................................................................................... 482 51.1. introduction With libraries we are talking about dynamically linked libraries (aka shared objects). These are binaries that contain functions and are not started themselves as programs, but are called by other binaries. Several programs can use the same library. The name of the library file usually starts with lib, followed by the actual name of the library, then the chracters .so and finally a version number. 51.2. /lib and /usr/lib When you look at the /lib or the /usr/lib directory, you will see a lot of symbolic links. Most libraries have a detailed version number in their name, but receive a symbolic link from a filename which only contains the major version number. root@rhel53 ~# ls -l /lib/libext* lrwxrwxrwx 1 root root 16 Feb 18 16:36 /lib/libext2fs.so.2 -> libext2fs.so.2.4 -rwxr-xr-x 1 root root 113K Jun 30 2009 /lib/libext2fs.so.2.4 51.3. ldd Many programs have dependencies on the installation of certain libraries. You can display these dependencies with ldd. This example shows the dependencies of the su command. paul@RHEL5 ~$ ldd /bin/su linux-gate.so.1 => (0x003f7000) libpam.so.0 => /lib/libpam.so.0 (0x00d5c000) libpam_misc.so.0 => /lib/libpam_misc.so.0 (0x0073c000) libcrypt.so.1 => /lib/libcrypt.so.1 (0x00aa4000) libdl.so.2 => /lib/libdl.so.2 (0x00800000) libc.so.6 => /lib/libc.so.6 (0x00ec1000) 480
- 487. library management libaudit.so.0 => /lib/libaudit.so.0 (0x0049f000) /lib/ld-linux.so.2 (0x4769c000) 51.4. ltrace The ltrace program allows to see all the calls made to library functions by a program. The example below uses the -c option to get only a summary count (there can be many calls), and the -l option to only show calls in one library file. All this to see what calls are made when executing su - serena as root. root@deb503:~# ltrace -c -l /lib/libpam.so.0 su - serena serena@deb503:~$ exit logout % time seconds usecs/call calls function ------ ----------- ----------- --------- -------------------- 70.31 0.014117 14117 1 pam_start 12.36 0.002482 2482 1 pam_open_session 5.17 0.001039 1039 1 pam_acct_mgmt 4.36 0.000876 876 1 pam_end 3.36 0.000675 675 1 pam_close_session 3.22 0.000646 646 1 pam_authenticate 0.48 0.000096 48 2 pam_set_item 0.27 0.000054 54 1 pam_setcred 0.25 0.000050 50 1 pam_getenvlist 0.22 0.000044 44 1 pam_get_item ------ ----------- ----------- --------- -------------------- 100.00 0.020079 11 total 51.5. dpkg -S and debsums Find out on Debian/Ubuntu to which package a library belongs. paul@deb503:/lib$ dpkg -S libext2fs.so.2.4 e2fslibs: /lib/libext2fs.so.2.4 You can then verify the integrity of all files in this package using debsums. paul@deb503:~$ debsums e2fslibs /usr/share/doc/e2fslibs/changelog.Debian.gz OK /usr/share/doc/e2fslibs/copyright OK /lib/libe2p.so.2.3 OK /lib/libext2fs.so.2.4 OK Should a library be broken, then reinstall it with aptitude reinstall $package. root@deb503:~# aptitude reinstall e2fslibs Reading package lists... Done Building dependency tree Reading state information... Done Reading extended state information Initializing package states... Done 481
- 488. library management Reading task descriptions... Done The following packages will be REINSTALLED: e2fslibs ... 51.6. rpm -qf and rpm -V Find out on Red Hat/Fedora to which package a library belongs. paul@RHEL5 ~$ rpm -qf /lib/libext2fs.so.2.4 e2fsprogs-libs-1.39-8.el5 You can then use rpm -V to verify all files in this package. In the example below the output shows that the Size and the Time stamp of the file have changed since installation. root@rhel53 ~# rpm -V e2fsprogs-libs prelink: /lib/libext2fs.so.2.4: prelinked file size differs S.?....T /lib/libext2fs.so.2.4 You can then use yum reinstall $package to overwrite the existing library with an original version. root@rhel53 lib# yum reinstall e2fsprogs-libs Loaded plugins: rhnplugin, security Setting up Reinstall Process Resolving Dependencies --> Running transaction check ---> Package e2fsprogs-libs.i386 0:1.39-23.el5 set to be erased ---> Package e2fsprogs-libs.i386 0:1.39-23.el5 set to be updated --> Finished Dependency Resolution ... The package verification now reports no problems with the library. root@rhel53 lib# rpm -V e2fsprogs-libs root@rhel53 lib# 482
- 489. Part XV. backup management
- 490. Chapter 52. backup Table of Contents 52.1. About tape devices ..................................................................................... 484 52.2. Compression ............................................................................................... 485 52.3. tar ............................................................................................................... 486 52.4. Backup Types ............................................................................................ 488 52.5. dump and restore ....................................................................................... 488 52.6. cpio ............................................................................................................. 489 52.7. dd ................................................................................................................ 489 52.8. split ............................................................................................................. 491 52.9. practice: backup ......................................................................................... 491 52.1. About tape devices Don't forget that the name of a device strictly speaking has no meaning since the kernel will use the major and minor number to find the hardware! See the man page of mknod and the devices.txt file in the linux kernel source for more info. SCSI tapes On the official Linux device list (http://www.lanana.org/docs/device-list/) we find the names for SCSI tapes (major 9 char). SCSI tape devices are located underneath / dev/st and are numbered starting with 0 for the first tape device. /dev/st0 First tape device /dev/st1 Second tape device /dev/st2 Third tape device To prevent automatic rewinding of tapes, prefix them with the letter n. /dev/nst0 First no rewind tape device /dev/nst1 Second no rewind tape device /dev/nst2 Third no rewind tape device By default, SCSI tapes on linux will use the highest hardware compression that is supported by the tape device. To lower the compression level, append one of the letters l (low), m (medium) or a (auto) to the tape name. /dev/st0l First low compression tape device /dev/st0m First medium compression tape device /dev/nst2m Third no rewind medium compression tape device 484
- 491. backup IDE tapes On the official Linux device list (http://www.lanana.org/docs/device-list/) we find the names for IDE tapes (major 37 char). IDE tape devices are located underneath /dev/ht and are numbered starting with 0 for the first tape device. No rewind and compression is similar to SCSI tapes. /dev/ht0 First IDE tape device /dev/nht0 Second no rewind IDE tape device /dev/ht0m First medium compression IDE tape device mt To manage your tapes, use mt (Magnetic Tape). Some examples. To receive information about the status of the tape. mt -f /dev/st0 status To rewind a tape... mt -f /dev/st0 rewind To rewind and eject a tape... mt -f /dev/st0 eject To erase a tape... mt -f /dev/st0 erase 52.2. Compression It can be beneficial to compress files before backup. The two most popular tools for compression of regular files on linux are gzip/gunzip and bzip2/bunzip2. Below you can see gzip in action, notice that it adds the .gz extension to the file. paul@RHELv4u4:~/test$ ls -l allfiles.tx* -rw-rw-r-- 1 paul paul 8813553 Feb 27 05:38 allfiles.txt paul@RHELv4u4:~/test$ gzip allfiles.txt paul@RHELv4u4:~/test$ ls -l allfiles.tx* -rw-rw-r-- 1 paul paul 931863 Feb 27 05:38 allfiles.txt.gz paul@RHELv4u4:~/test$ gunzip allfiles.txt.gz paul@RHELv4u4:~/test$ ls -l allfiles.tx* -rw-rw-r-- 1 paul paul 8813553 Feb 27 05:38 allfiles.txt paul@RHELv4u4:~/test$ In general, gzip is much faster than bzip2, but the latter one compresses a lot better. Let us compare the two. paul@RHELv4u4:~/test$ cp allfiles.txt bllfiles.txt 485
- 492. backup paul@RHELv4u4:~/test$ time gzip allfiles.txt real 0m0.050s user 0m0.041s sys 0m0.009s paul@RHELv4u4:~/test$ time bzip2 bllfiles.txt real 0m5.968s user 0m5.794s sys 0m0.076s paul@RHELv4u4:~/test$ ls -l ?llfiles.tx* -rw-rw-r-- 1 paul paul 931863 Feb 27 05:38 allfiles.txt.gz -rw-rw-r-- 1 paul paul 708871 May 12 10:52 bllfiles.txt.bz2 paul@RHELv4u4:~/test$ 52.3. tar The tar utility gets its name from Tape ARchive. This tool will receive and send files to a destination (typically a tape or a regular file). The c option is used to create a tar archive (or tarfile), the f option to name/create the tarfile. The example below takes a backup of /etc into the file /backup/etc.tar . root@RHELv4u4:~# tar cf /backup/etc.tar /etc root@RHELv4u4:~# ls -l /backup/etc.tar -rw-r--r-- 1 root root 47800320 May 12 11:47 /backup/etc.tar root@RHELv4u4:~# Compression can be achieved without pipes since tar uses the z flag to compress with gzip, and the j flag to compress with bzip2. root@RHELv4u4:~# tar czf /backup/etc.tar.gz /etc root@RHELv4u4:~# tar cjf /backup/etc.tar.bz2 /etc root@RHELv4u4:~# ls -l /backup/etc.ta* -rw-r--r-- 1 root root 47800320 May 12 11:47 /backup/etc.tar -rw-r--r-- 1 root root 6077340 May 12 11:48 /backup/etc.tar.bz2 -rw-r--r-- 1 root root 8496607 May 12 11:47 /backup/etc.tar.gz root@RHELv4u4:~# The t option is used to list the contents of a tar file. Verbose mode is enabled with v (also useful when you want to see the files being archived during archiving). root@RHELv4u4:~# tar tvf /backup/etc.tar drwxr-xr-x root/root 0 2007-05-12 09:38:21 etc/ -rw-r--r-- root/root 2657 2004-09-27 10:15:03 etc/warnquota.conf -rw-r--r-- root/root 13136 2006-11-03 17:34:50 etc/mime.types drwxr-xr-x root/root 0 2004-11-03 13:35:50 etc/sound/ ... To list a specific file in a tar archive, use the t option, added with the filename (without leading /). 486
- 493. backup root@RHELv4u4:~# tar tvf /backup/etc.tar etc/resolv.conf -rw-r--r-- root/root 77 2007-05-12 08:31:32 etc/resolv.conf root@RHELv4u4:~# Use the x flag to restore a tar archive, or a single file from the archive. Remember that by default tar will restore the file in the current directory. root@RHELv4u4:~# tar xvf /backup/etc.tar etc/resolv.conf etc/resolv.conf root@RHELv4u4:~# ls -l /etc/resolv.conf -rw-r--r-- 2 root root 40 May 12 12:05 /etc/resolv.conf root@RHELv4u4:~# ls -l etc/resolv.conf -rw-r--r-- 1 root root 77 May 12 08:31 etc/resolv.conf root@RHELv4u4:~# You can preserve file permissions with the p flag. And you can exclude directories or file with --exclude. root ~# tar cpzf /backup/etc_with_perms.tgz /etc root ~# tar cpzf /backup/etc_no_sysconf.tgz /etc --exclude /etc/sysconfig root ~# ls -l /backup/etc_* -rw-r--r-- 1 root root 8434293 May 12 12:48 /backup/etc_no_sysconf.tgz -rw-r--r-- 1 root root 8496591 May 12 12:48 /backup/etc_with_perms.tgz root ~# You can also create a text file with names of files and directories to archive, and then supply this file to tar with the -T flag. root@RHELv4u4:~# find /etc -name *.conf > files_to_archive.txt root@RHELv4u4:~# find /home -name *.pdf >> files_to_archive.txt root@RHELv4u4:~# tar cpzf /backup/backup.tgz -T files_to_archive.txt The tar utility can receive filenames from the find command, with the help of xargs. find /etc -type f -name "*.conf" | xargs tar czf /backup/confs.tar.gz You can also use tar to copy a directory, this is more efficient than using cp -r. (cd /etc; tar -cf - . ) | (cd /backup/copy_of_etc/; tar -xpf - ) Another example of tar, this copies a directory securely over the network. (cd /etc;tar -cf - . )|(ssh user@srv 'cd /backup/cp_of_etc/; tar -xf - ') tar can be used together with gzip and copy a file to a remote server through ssh cat backup.tar | gzip | ssh bashuser@192.168.1.105 "cat - > backup.tgz" 487
- 494. backup Compress the tar backup when it is on the network, but leave it uncompressed at the destination. cat backup.tar | gzip | ssh user@192.168.1.105 "gunzip|cat - > backup.tar" Same as the previous, but let ssh handle the compression cat backup.tar | ssh -C bashuser@192.168.1.105 "cat - > backup.tar" 52.4. Backup Types Linux uses multilevel incremental backups using distinct levels. A full backup is a backup at level 0. A higher level x backup will include all changes since the last level x-1 backup. Suppose you take a full backup on Monday (level 0) and a level 1 backup on Tuesday, then the Tuesday backup will contain all changes since Monday. Taking a level 2 on Wednesday will contain all changes since Tuesday (the last level 2-1). A level 3 backup on Thursday will contain all changes since Wednesday (the last level 3-1). Another level 3 on Friday will also contain all changes since Wednesday. A level 2 backup on Saturday would take all changes since the last level 1 from Tuesday. 52.5. dump and restore While dump is similar to tar, it is also very different because it looks at the file system. Where tar receives a lists of files to backup, dump will find files to backup by itself by examining ext2. Files found by dump will be copied to a tape or regular file. In case the target is not big enough to hold the dump (end-of-media), it is broken into multiple volumes. Restoring files that were backed up with dump is done with the restore command. In the example below we take a full level 0 backup of two partitions to a SCSI tape. The no rewind is mandatory to put the volumes behind each other on the tape. dump 0f /dev/nst0 /boot dump 0f /dev/nst0 / Listing files in a dump archive is done with dump -t, and you can compare files with dump -C. You can omit files from a dump by changing the dump attribute with the chattr command. The d attribute on ext will tell dump to skip the file, even during a full backup. In the following example, /etc/hosts is excluded from dump archives. chattr +d /etc/hosts 488
- 495. backup To restore the complete file system with restore, use the -r option. This can be useful to change the size or block size of a file system. You should have a clean file system mounted and cd'd into it. Like this example shows. mke2fs /dev/hda3 mount /dev/hda3 /mnt/data cd /mnt/data restore rf /dev/nst0 To extract only one file or directory from a dump, use the -x option. restore -xf /dev/st0 /etc 52.6. cpio Different from tar and dump is cpio (Copy Input and Output). It can be used to receive filenames, but copies the actual files. This makes it an easy companion with find! Some examples below. find sends filenames to cpio, which puts the files in an archive. find /etc -depth -print | cpio -oaV -O archive.cpio The same, but compressed with gzip find /etc -depth -print | cpio -oaV | gzip -c > archive.cpio.gz Now pipe it through ssh (backup files to a compressed file on another machine) find /etc -depth -print|cpio -oaV|gzip -c|ssh server "cat - > etc.cpio.gz" find sends filenames to cpio | cpio sends files to ssh | ssh sends files to cpio 'cpio extracts files' find /etc -depth -print | cpio -oaV | ssh user@host 'cpio -imVd' the same but reversed: copy a dir from the remote host to the local machine ssh user@host "find path -depth -print | cpio -oaV" | cpio -imVd 52.7. dd About dd Some people use dd to create backups. This can be very powerful, but dd backups can only be restored to very similar partitions or devices. There are however a lot of useful things possible with dd. Some examples. 489
- 496. backup Create a CDROM image The easiest way to create a .ISO file from any CD. The if switch means Input File, of is the Output File. Any good tool can burn a copy of the CD with this .ISO file. dd if=/dev/cdrom of=/path/to/cdrom.ISO Create a floppy image A little outdated maybe, but just in case : make an image file from a 1.44MB floppy. Blocksize is defined by bs, and count contains the number of blocks to copy. dd if=/dev/floppy of=/path/to/floppy.img bs=1024 count=1440 Copy the master boot record Use dd to copy the MBR (Master Boot Record) of hard disk /dev/hda to a file. dd if=/dev/hda of=/MBR.img bs=512 count=1 Copy files This example shows how dd can copy files. Copy the file summer.txt to copy_of_summer.txt . dd if=~/summer.txt of=~/copy_of_summer.txt Image disks or partitions And who needs ghost when dd can create a (compressed) image of a partition. dd if=/dev/hdb2 of=/image_of_hdb2.IMG dd if=/dev/hdb2 | gzip > /image_of_hdb2.IMG.gz Create files of a certain size dd can be used to create a file of any size. The first example creates a one MEBIbyte file, the second a one MEGAbyte file. dd if=/dev/zero of=file1MB count=1024 bs=1024 dd if=/dev/zero of=file1MB count=1000 bs=1024 CDROM server example And there are of course endless combinations with ssh and bzip2. This example puts a bzip2 backup of a cdrom on a remote server. 490
- 497. backup dd if=/dev/cdrom |bzip2|ssh user@host "cat - > /backups/cd/cdrom.iso.bz2" 52.8. split The split command is useful to split files into smaller files. This can be useful to fit the file onto multiple instances of a medium too small to contain the complete file. In the example below, a file of size 5000 bytes is split into three smaller files, with maximum 2000 bytes each. paul@laika:~/test$ ls -l total 8 -rw-r--r-- 1 paul paul 5000 2007-09-09 20:46 bigfile1 paul@laika:~/test$ split -b 2000 bigfile1 splitfile. paul@laika:~/test$ ls -l total 20 -rw-r--r-- 1 paul paul 5000 2007-09-09 20:46 bigfile1 -rw-r--r-- 1 paul paul 2000 2007-09-09 20:47 splitfile.aa -rw-r--r-- 1 paul paul 2000 2007-09-09 20:47 splitfile.ab -rw-r--r-- 1 paul paul 1000 2007-09-09 20:47 splitfile.ac 52.9. practice: backup !! Careful with tar options and the position of the backup file, mistakes can destroy your system!! 1. Create a directory (or partition if you like) for backups. Link (or mount) it under / mnt/backup. 2a. Use tar to backup /etc in /mnt/backup/etc_date.tgz, the backup must be gzipped. (Replace date with the current date) 2b. Use tar to backup /bin to /mnt/backup/bin_date.tar.bz2, the backup must be bzip2'd. 2c. Choose a file in /etc and /bin and verify with tar that the file is indeed backed up. 2d. Extract those two files to your home directory. 3a. Create a backup directory for your neighbour, make it accessible under /mnt/ neighbourName 3b. Combine ssh and tar to put a backup of your /boot on your neighbours computer in /mnt/YourName 4a. Combine find and cpio to create a cpio archive of /etc. 4b. Choose a file in /etc and restore it from the cpio archive into your home directory. 491
- 498. backup 5. Use dd and ssh to put a backup of the master boot record on your neighbours computer. 6. (On the real computer) Create and mount an ISO image of the ubuntu cdrom. 7. Combine dd and gzip to create a 'ghost' image of one of your partitions on another partition. 8. Use dd to create a five megabyte file in ~/testsplit and name it biggest. Then split this file in smaller two megabyte parts. mkdir testsplit dd if=/dev/zero of=~/testsplit/biggest count=5000 bs=1024 split -b 2000000 biggest parts 492
- 499. Part XVI. samba
- 500. Chapter 53. samba Table of Contents 53.1. verify installed version ............................................................................... 495 53.2. installing samba ......................................................................................... 496 53.3. documentation ............................................................................................ 497 53.4. samba daemons .......................................................................................... 497 53.5. starting and stopping samba ....................................................................... 498 53.6. the SMB protocol ....................................................................................... 499 53.7. Practice: introduction to samba .................................................................. 499 53.8. /etc/samba/smb.conf ................................................................................... 500 53.9. /usr/bin/testparm ......................................................................................... 502 53.10. /usr/bin/smbclient ..................................................................................... 503 53.11. /usr/bin/smbtree ........................................................................................ 505 53.12. server string .............................................................................................. 507 53.13. Samba Web Administration Tool (SWAT) .............................................. 507 53.14. Practice: getting started with samba ........................................................ 508 53.15. Solution: getting started with samba ........................................................ 508 53.16. Setting up a directory to share ................................................................. 510 53.17. configure the share ................................................................................... 510 53.18. restart the server ....................................................................................... 511 53.19. verify the share ........................................................................................ 511 53.20. a note on netcat ........................................................................................ 512 53.21. Practice: read only file server .................................................................. 513 53.22. Solution: read only file server ................................................................. 513 53.23. set up a directory to share ....................................................................... 514 53.24. share section in smb.conf ......................................................................... 514 53.25. configure the share ................................................................................... 515 53.26. test connection with windows .................................................................. 515 53.27. test writing with windows ........................................................................ 515 53.28. How is this possible ? .............................................................................. 516 53.29. Practice: writable file server .................................................................... 516 53.30. Solution: writable file server ................................................................... 516 53.31. creating a samba user ............................................................................... 517 53.32. ownership of files .................................................................................... 517 53.33. /usr/bin/smbpasswd .................................................................................. 517 53.34. /etc/samba/smbpasswd .............................................................................. 518 53.35. forcing this user ....................................................................................... 518 53.36. Practice: first samba user account ............................................................ 519 53.37. Solution: first samba user account ........................................................... 519 53.38. creating the users on Linux ..................................................................... 519 53.39. creating the users on samba ..................................................................... 520 53.40. security = user .......................................................................................... 520 53.41. configuring the share ............................................................................... 521 53.42. testing access with net use ....................................................................... 521 53.43. testing access with smbclient ................................................................... 521 494
- 501. samba 53.44. verify ownership ...................................................................................... 522 53.45. common problems .................................................................................... 522 53.46. Practice : samba authentication ................................................................ 523 53.47. Solution: samba authentication ................................................................ 523 53.48. security based on user name .................................................................... 524 53.49. security based on ip-address .................................................................... 525 53.50. security through obscurity ........................................................................ 525 53.51. file system security .................................................................................. 526 53.52. Practice: securing shares .......................................................................... 527 53.53. Solution: securing shares ......................................................................... 528 53.54. changes in smb.conf ................................................................................. 529 53.55. joining an Active Directory domain ........................................................ 531 53.56. winbind ..................................................................................................... 531 53.57. wbinfo ...................................................................................................... 532 53.58. getent ........................................................................................................ 533 53.59. file ownership ........................................................................................... 533 53.60. Practice : samba domain member ............................................................ 534 53.61. About Domain Controllers ....................................................................... 534 53.62. About security modes .............................................................................. 535 53.63. About password backends ........................................................................ 535 53.64. [global] section in smb.conf .................................................................... 536 53.65. netlogon share .......................................................................................... 537 53.66. other [share] sections ............................................................................... 537 53.67. Users and Groups ..................................................................................... 538 53.68. tdbsam ...................................................................................................... 538 53.69. about computer accounts .......................................................................... 539 53.70. local or roaming profiles .......................................................................... 540 53.71. Groups in NTFS acls ............................................................................... 540 53.72. logon scripts ............................................................................................. 541 53.73. Practice ..................................................................................................... 541 53.74. Samba 4 alpha 6 ...................................................................................... 542 53.1. verify installed version .rpm based distributions To see the version of samba installed on RedHat, use rpm -q samba. [root@RHEL52 ~]# rpm -q samba samba-3.0.28-1.el5_2.1 The screenshots above and below this paragraph show that both RHEL4 and RHEL5 have Samba version 3.0 installed. The last number in the Samba version counts the number of updates or patches. 3.0.28 in RHEL5 has a lot more updates than 3.0.10 in RHEL4. 495
- 502. samba [paul@RHEL4b ~]$ rpm -q samba samba-3.0.10-1.4E.9 .deb based distributions Use dpkg -l on Debian or Ubuntu. Ubuntu 7.04 below uses Samba 3.0.24, but the Samba server is not installed, only the common files are installed (for mounting smb file systems). paul@laika:~$ dpkg -l | grep samba ii samba-common 3.0.24-2ubuntu1.2 Samba common files used by both... Ubuntu 9.04 below uses Samba 3.3.2, and both the Samba server and the common files are installed. paul@laika:~$ dpkg -l | grep samba ii samba 2:3.3.2-1ubuntu3 SMB/CIFS file, print, and login server ii samba-common 2:3.3.2-1ubuntu3 common files used by both the Samba ser 53.2. installing samba .rpm based distributions Samba is installed by default on Red Hat Enterprise Linux. If Samba is not yet installed, then you can use the graphical menu (Applications -- System Settings -- Add/Remove Applications) and select "Windows File Server" in the Server section. The non-graphical way is to use rpm or yum. When you downloaded the .rpm file, you can install Samba like this. [paul@RHEL52 ~]$ rpm -i samba-3.0.28-1.el5_2.1.rpm When you have a subscription to RHN (Red Hat Network), then yum is an easy tool to use. This command also works on Fedora. [paul@RHEL52 ~]$ yum install samba .deb based distributions Ubuntu and Debian users can use the aptitude program (or use a graphical tool like Synaptic). paul@laika:~$ aptitude install samba-server 496
- 503. samba 53.3. documentation samba howto Samba comes with excellent documentation in html and pdf format (and also as a free download from Samba.org and it is for sale as a printed book). Red Hat Enterprise Linux installs the html and pdf version in /usr/share/doc by default. [paul@RHEL4b ~]$ locate Samba-HOWTO-Collection.pdf /usr/share/doc/samba-3.0.10/Samba-HOWTO-Collection.pdf Ubuntu packages the docs as a separate package from Samba. root@laika:~# aptitude search samba | grep -i documentation i samba-doc - Samba documentation i samba-doc-pdf - Samba documentation (PDF format) root@laika:~# find /usr/share/doc/samba-doc-pdf | grep -i howto /usr/share/doc/samba-doc-pdf/Samba3-HOWTO.pdf.gz samba by example Besides the howto, there is also an excellent book called Samba By Example (again available as printed edition in shops, and as a free pdf and html). 53.4. samba daemons Samba 3 consists of three daemons, they are named nmbd, smbd and winbindd. nmbd The nmbd daemon takes care of all the names and naming. It registers and resolves names, and handles browsing. According to the Samba documentation, it should be the first daemon to start. [root@RHEL52 ~]# ps -C nmbd PID TTY TIME CMD 5681 ? 00:00:00 nmbd smbd The smbd daemon manages file transfers and authentication. 497
- 504. samba [root@RHEL52 ~]# ps -C smbd PID TTY TIME CMD 5678 ? 00:00:00 smbd 5683 ? 00:00:00 smbd winbindd The winbind daemon (winbindd) is only started to handle Microsoft Windows domain membership. Note that winbindd is started by the /etc/init.d/winbind script (two dd's for the daemon and only one d for the script). [root@RHEL52 ~]# /etc/init.d/winbind start Starting Winbind services: [ OK ] [root@RHEL52 ~]# ps -C winbindd PID TTY TIME CMD 5752 ? 00:00:00 winbindd 5754 ? 00:00:00 winbindd On Debian and Ubuntu, the winbindd daemon is installed via a seperate package called winbind. 53.5. starting and stopping samba You can start the daemons by invoking /etc/init.d/smb start (some systems use /etc/ init.d/samba) on any linux. root@laika:~# /etc/init.d/samba stop * Stopping Samba daemons [ OK ] root@laika:~# /etc/init.d/samba start * Starting Samba daemons [ OK ] root@laika:~# /etc/init.d/samba restart * Stopping Samba daemons [ OK ] * Starting Samba daemons [ OK ] root@laika:~# /etc/init.d/samba status * SMBD is running [ OK ] Red Hat derived systems are happy with service smb start. [root@RHEL4b ~]# /etc/init.d/smb start Starting SMB services: [ OK ] Starting NMB services: [ OK ] [root@RHEL4b ~]# service smb restart Shutting down SMB services: [ OK ] Shutting down NMB services: [ OK ] Starting SMB services: [ OK ] Starting NMB services: [ OK ] [root@RHEL4b ~]# 498
- 505. samba 53.6. the SMB protocol brief history Development of this protocol was started by IBM in the early eighties. By the end of the eighties, most develpment was done by Microsoft. SMB is an application level protocol designed to run on top of NetBIOS/NetBEUI, but can also be run on top of tcp/ip. In 1996 Microsoft was asked to document the protocol. They submitted CIFS (Common Internet File System) as an internet draft, but it never got final rfc status. In 2004 the European Union decided Microsoft should document the protocol to enable other developers to write compatible software. December 20th 2007 Microsoft came to an agreement. The Samba team now has access to SMB/CIFS, Windows for Workgroups and Active Directory documentation. broadcasting protocol SMB uses the NetBIOS service location protocol, which is a broadcasting protocol. This means that NetBIOS names have to be unique on the network (even when you have different IP-addresses). Having duplicate names on an SMB network can seriously harm communications. NetBIOS names NetBIOS names are similar to hostnames, but are always uppercase and only 15 characters in length. Microsoft Windows computers and Samba servers will broadcast this name on the network. network bandwidth Having many broadcasting SMB/CIFS computers on your network can cause bandwidth issues. A solution can be the use of a NetBIOS name server (NBNS) like WINS (Windows Internet Naming Service). 53.7. Practice: introduction to samba 0. !! Make sure you know your student number, anything *ANYTHING* you name must include your student number! 1. Verify that you can logon to a Linux/Unix computer. Write down the name and ip address of this computer. 2. Do the same for all the other (virtual) machines available to you. 499
- 506. samba 3. Verify networking by pinging the computer, edit the appropriate hosts files so you can use names. Test the names by pinging them. 4. Make sure Samba is installed, write down the version of Samba. 5. Open the Official Samba-3 howto pdf file that is installed on your computer. How many A4 pages is this file ? Then look at the same pdf on samba.org, it is updated regularly. 6. Stop the Samba server. 53.8. /etc/samba/smb.conf smbd -b Samba configuration is done in the smb.conf file. The file can be edited manually, or you can use a web based interface like webmin or swat to manage it. The file is usually located in /etc/samba. You can find the exact location with smbd -b. [root@RHEL4b ~]# smbd -b | grep CONFIGFILE CONFIGFILE: /etc/samba/smb.conf the default smb.conf The default smb.conf file contains a lot of examples with explanations. [paul@RHEL4b ~]$ ls -l /etc/samba/smb.conf -rw-r--r-- 1 root root 10836 May 30 23:08 /etc/samba/smb.conf Also on Ubuntu and Debian, smb.conf is packed with samples and explanations. paul@laika:~$ ls -l /etc/samba/smb.conf -rw-r--r-- 1 root root 10515 2007-05-24 00:21 /etc/samba/smb.conf minimal smb.conf Below is an example of a very minimalistic smb.conf. It allows samba to start, and to be visible to other computers (Microsoft shows computers in Network Neighborhood or My Network Places). [paul@RHEL4b ~]$ cat /etc/samba/smb.conf [global] workgroup = WORKGROUP [firstshare] path = /srv/samba/public 500
- 507. samba net view Below is a screenshot of the net view command on Microsoft Windows Server 2003 sp2. It shows how a Red Hat Enterprise Linux 5.3 and a Ubuntu 9.04 Samba server, both with a minimalistic smb.conf, are visible to Microsoft computers nearby. C:Documents and SettingsAdministrator>net view Server Name Remark ---------------------------------------------------------------------- LAIKA Samba 3.3.2 RHEL53 Samba 3.0.33-3.7.el5 W2003 The command completed successfully. long lines in smb.conf Some parameters in smb.conf can get a long list of values behind them. You can continue a line (for clarity) on the next by ending the line with a backslash. valid users = Serena, Venus, Lindsay Kim, Justine, Sabine Amelie, Marie, Suzanne curious smb.conf Curious but true: smb.conf accepts synonyms like create mode and create mask, and (sometimes) minor spelling errors like browsable and browseable. And on occasion you can even switch words, the guest only parameter is identical to only guest. And writable = yes is the same as readonly = no. man smb.conf You can access a lot of documentation when typing man smb.conf. [root@RHEL4b samba]# apropos samba cupsaddsmb (8) - export printers to samba for windows clients lmhosts (5) - The Samba NetBIOS hosts file net (8) - Tool for administration of Samba and remote CIFS servers pdbedit (8) - manage the SAM database (Database of Samba Users) samba (7) - A Windows SMB/CIFS fileserver for UNIX smb.conf [smb] (5) - The configuration file for the Samba suite smbpasswd (5) - The Samba encrypted password file smbstatus (1) - report on current Samba connections swat (8) - Samba Web Administration Tool tdbbackup (8) - tool for backing up and ... of samba .tdb files [root@RHEL4b samba]# 501
- 508. samba 53.9. /usr/bin/testparm syntax check smb.conf To verify the syntax of the smb.conf file, you can use testparm. [paul@RHEL4b ~]$ testparm Load smb config files from /etc/samba/smb.conf Processing section "[firstshare]" Loaded services file OK. Server role: ROLE_STANDALONE Press enter to see a dump of your service definitions testparm -v An interesting option is testparm -v, which will output all the global options with their default value. [root@RHEL52 ~]# testparm -v | head Load smb config files from /etc/samba/smb.conf Processing section "[pub0]" Processing section "[global$]" Loaded services file OK. Server role: ROLE_STANDALONE Press enter to see a dump of your service definitions [global] dos charset = CP850 unix charset = UTF-8 display charset = LOCALE workgroup = WORKGROUP realm = netbios name = TEACHER0 netbios aliases = netbios scope = server string = Samba 3.0.28-1.el5_2.1 ... There are about 350 default values for smb.conf parameters. testparm -s The samba daemons are constantly (once every 60 seconds) checking the smb.conf file, so it is good practice to keep this file small. But it is also good practice to document your samba configuration, and to explicitly set options that have the same default values. The testparm -s option allows you to do both. It will output the smallest possible samba configuration file, while retaining all your settings. The 502
- 509. samba idea is to have your samba configuration in another file (like smb.conf.full) and let testparm parse this for you. The screenshot below shows you how. First the smb.conf.full file with the explicitly set option workgroup to WORKGROUP. [root@RHEL4b samba]# cat smb.conf.full [global] workgroup = WORKGROUP # This is a demo of a documented smb.conf # These two lines are removed by testparm -s server string = Public Test Server [firstshare] path = /srv/samba/public Next, we execute testparm with the -s option, and redirect stdout to the real smb.conf file. [root@RHEL4b samba]# testparm -s smb.conf.full > smb.conf Load smb config files from smb.conf.full Processing section "[firstshare]" Loaded services file OK. And below is the end result. The two comment lines and the default option are no longer there. [root@RHEL4b samba]# cat smb.conf # Global parameters [global] server string = Public Test Server [firstshare] path = /srv/samba/public [root@RHEL4b samba]# 53.10. /usr/bin/smbclient smbclient looking at Samba With smbclient you can see browsing and share information from your smb server. It will display all your shares, your workgroup, and the name of the Master Browser. The -N switch is added to avoid having to enter an empty password. The -L switch is followed by the name of the host to check. [root@RHEL4b init.d]# smbclient -NL rhel4b Anonymous login successful Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.10-1.4E.9] Sharename Type Comment 503
- 510. samba --------- ---- ------- firstshare Disk IPC$ IPC IPC Service (Public Test Server) ADMIN$ IPC IPC Service (Public Test Server) Anonymous login successful Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.10-1.4E.9] Server Comment --------- ------- RHEL4B Public Test Server WINXP Workgroup Master --------- ------- WORKGROUP WINXP smbclient anonymous The screenshot below uses smbclient to display information about a remote smb server (in this case a Windows xp machine). [root@RHEL4b init.d]# smbclient -NL winxp Anonymous login successful Domain=[WORKGROUP] OS=[Windows 5.1] Server=[Windows 2000 LAN Manager] Sharename Type Comment --------- ---- ------- Error returning browse list: NT_STATUS_ACCESS_DENIED Anonymous login successful Domain=[WORKGROUP] OS=[Windows 5.1] Server=[Windows 2000 LAN Manager] Server Comment --------- ------- RHEL4B Public Test Server W2000 WINXP Workgroup Master --------- ------- WORKGROUP WINXP smbclient with credentials Windows versions after xp sp2 and 2003 sp1 do not accept guest access (the NT_STATUS_ACCESS_DENIED error). This example shows how to provide credentials with smbclient. [paul@RHEL53 ~]$ smbclient -L w2003 -U administrator%stargate Domain=[W2003] OS=[Windows Server 2003 3790 Service Pack 2] Server=... Sharename Type Comment --------- ---- ------- C$ Disk Default share IPC$ IPC Remote IPC ADMIN$ Disk Remote Admin 504
- 511. samba ... 53.11. /usr/bin/smbtree Another useful tool to troubleshoot Samba or simply to browse the SMB network is smbtree. In its simplest form, smbtree will do an anonymous browsing on the local subnet. displaying all SMB computers and (if authorized) their shares. Let's take a look at two screenshots of smbtree in action (with blank password). The first one is taken immediately after booting four different computers (one MS Windows 2000, one MS Windows xp, one MS Windows 2003 and one RHEL 4 with Samba 3.0.10). [paul@RHEL4b ~]$ smbtree Password: WORKGROUP PEGASUS WINXP RHEL4B Pegasus Domain Member Server Error connecting to 127.0.0.1 (Connection refused) cli_full_connection: failed to connect to RHEL4B<20> (127.0.0.1) HM2003 [paul@RHEL4b ~]$ The information displayed in the previous screenshot looks incomplete. The browsing elections are still ongoing, the browse list is not yet distributed to all clients by the (to be elected) browser master. The next screenshot was taken about one minute later. And it shows even less. [paul@RHEL4b ~]$ smbtree Password: WORKGROUP W2000 [paul@RHEL4b ~]$ So we wait a while, and then run smbtree again, this time it looks a lot nicer. [paul@RHEL4b ~]$ smbtree Password: WORKGROUP W2000 PEGASUS WINXP RHEL4B Pegasus Domain Member Server RHEL4BADMIN$ IPC Service (Pegasus Domain Member Server) RHEL4BIPC$ IPC Service (Pegasus Domain Member Server) RHEL4Bdomaindata Active Directory users only HM2003 [paul@RHEL4b ~]$ smbtree --version Version 3.0.10-1.4E.9 [paul@RHEL4b ~]$ 505
- 512. samba I added the version number of smbtree in the previous screenshot, to show you the difference when using the latest version of smbtree (below a screenshot taken from Ubuntu Feisty Fawn). The latest version shows a more complete overview of machines and shares. paul@laika:~$ smbtree --version Version 3.0.24 paul@laika:~$ smbtree Password: WORKGROUP W2000 W2000firstshare W2000C$ Default share W2000ADMIN$ Remote Admin W2000IPC$ Remote IPC PEGASUS WINXP cli_rpc_pipe_open: cli_nt_create failed on pipe srvsvc to machine WINXP. Error was NT_STATUS_ACCESS_DENIED RHEL4B Pegasus Domain Member Server RHEL4BADMIN$ IPC Service (Pegasus Domain Member Server) RHEL4BIPC$ IPC Service (Pegasus Domain Member Server) RHEL4Bdomaindata Active Directory users only HM2003 cli_rpc_pipe_open: cli_nt_create failed on pipe srvsvc to machine HM2003. Error was NT_STATUS_ACCESS_DENIED paul@laika:~$ The previous screenshot also provides useful errors on why we cannot see shared info on computers winxp and w2003. Let us try the old smbtree version on our RHEL server, but this time with Administrator credentials (which are the same on all computers). [paul@RHEL4b ~]$ smbtree -UAdministrator%Stargate1 WORKGROUP W2000 PEGASUS WINXP WINXPC$ Default share WINXPADMIN$ Remote Admin WINXPshare55 WINXPIPC$ Remote IPC RHEL4B Pegasus Domain Member Server RHEL4BADMIN$ IPC Service (Pegasus Domain Member Server) RHEL4BIPC$ IPC Service (Pegasus Domain Member Server) RHEL4Bdomaindata Active Directory users only HM2003 HM2003NETLOGON Logon server share HM2003SYSVOL Logon server share HM2003WSUSTemp A network share used by Local Publishing ... HM2003ADMIN$ Remote Admin HM2003tools HM2003IPC$ Remote IPC HM2003WsusContent A network share to be used by Local ... HM2003C$ Default share [paul@RHEL4b ~]$ As you can see, this gives a very nice overview of all SMB computers and their shares. 506
- 513. samba 53.12. server string The comment seen by the net view and the smbclient commands is the default value for the server string option. Simply adding this value to the global section in smb.conf and restarting samba will change the option. [root@RHEL53 samba]# testparm -s 2>/dev/null | grep server server string = Red Hat Server in Paris After a short while, the changed option is visible on the Microsoft computers. C:Documents and SettingsAdministrator>net view Server Name Remark ------------------------------------------------------------------------------- LAIKA Ubuntu 9.04 server in Antwerp RHEL53 Red Hat Server in Paris W2003 53.13. Samba Web Administration Tool (SWAT) Samba comes with a web based tool to manage your samba configuration file. SWAT is accessible with a web browser on port 901 of the host system. To enable the tool, first find out whether your system is using the inetd or the xinetd superdaemon. [root@RHEL4b samba]# ps fax | grep inet 15026 pts/0 S+ 0:00 _ grep inet 2771 ? Ss 0:00 xinetd -stayalive -pidfile /var/run/xinetd.pid [root@RHEL4b samba]# Then edit the inetd.conf or change the disable = yes line in /etc/xinetd.d/swat to disable = no. [root@RHEL4b samba]# cat /etc/xinetd.d/swat # default: off # description: SWAT is the Samba Web Admin Tool. Use swat # to configure your Samba server. To use SWAT, # connect to port 901 with your favorite web browser. service swat { port = 901 socket_type = stream wait = no only_from = 127.0.0.1 user = root server = /usr/sbin/swat log_on_failure += USERID disable = no } [root@RHEL4b samba]# /etc/init.d/xinetd restart 507
- 514. samba Stopping xinetd: [ OK ] Starting xinetd: [ OK ] [root@RHEL4b samba]# Change the only from value to enable swat from remote computers. This examples shows how to provide swat access to all computers in a /24 subnet. [root@RHEL53 xinetd.d]# grep only /etc/xinetd.d/swat only_from = 192.168.1.0/24 Be careful when using SWAT, it erases all your manually edited comments in smb.conf. 53.14. Practice: getting started with samba 1. Take a backup copy of the original smb.conf, name it smb.conf.orig 2. Enable SWAT and take a look at it. 3. Stop the Samba server. 4. Create a minimalistic smb.conf.minimal and test it with testparm. 5. Use tesparm -s to create /etc/samba/smb.conf from your smb.conf.minimal . 6. Start Samba with your minimal smb.conf. 7. Verify with smbclient that your Samba server works. 8. Verify that another (Microsoft) computer can see your Samba server. 9. Browse the network with net view, smbtree and with Windows Explorer. 10. Change the "Server String" parameter in smb.conf. How long does it take before you see the change (net view, smbclient, My Network Places,...) ? 11. Will restarting Samba after a change to smb.conf speed up the change ? 12. Which computer is the master browser master in your workgroup ? What is the master browser ? 13. If time permits (or if you are waiting for other students to finish this practice), then install a sniffer (wireshark) and watch the browser elections. 53.15. Solution: getting started with samba 1. Take a backup copy of the original smb.conf, name it smb.conf.orig cd /etc/samba ; cp smb.conf smb.conf.orig 508
- 515. samba 2. Enable SWAT and take a look at it. vi /etc/inetd.d/samba (and set disable to no) 3. Stop the Samba server. /etc/init.d/smb stop (Red Hat) /etc/init.d/samba stop (Debian) 4. Create a minimalistic smb.conf.minimal and test it with testparm. cd /etc/samba ; mkdir my_smb_confs ; cd my_smb_confs vi smb.conf.minimal testparm smb.conf.minimal 5. Use tesparm -s to create /etc/samba/smb.conf from your smb.conf.minimal . testparm -s smb.conf.minimal > ../smb.conf 6. Start Samba with your minimal smb.conf. /etc/init.d/smb restart (Red Hat) /etc/init.d/samba restart (Debian) 7. Verify with smbclient that your Samba server works. smbclient -NL 127.0.0.1 8. Verify that another computer can see your Samba server. smbclient -NL 'ip-address' (on a Linux) 9. Browse the network with net view, smbtree and with Windows Explorer. on Linux: smbtree on Windows: net view (and WindowsKey + e) 10. Change the "Server String" parameter in smb.conf. How long does it take before you see the change (net view, smbclient, My Network Places,...) ? vi /etc/samba/smb.conf (should take only seconds when restarting samba) 11. Will restarting Samba after a change to smb.conf speed up the change ? yes 12. Which computer is the master browser master in your workgroup ? What is the master browser ? The computer that won the elections. This machine will make the list of computers in the network 13. If time permits (or if you are waiting for other students to finish this practice), then install a sniffer (wireshark) and watch the browser elections. On ubuntu: sudo aptitude install wireshark then: sudo wireshark, select interface 509
- 516. samba 53.16. Setting up a directory to share Let's start with setting up a very simple read only file server with Samba. Everyone (even anonymous guests) will receive read access. The first step is to create a directory and put some test files in it. [root@RHEL52 ~]# mkdir -p /srv/samba/readonly [root@RHEL52 ~]# cd /srv/samba/readonly/ [root@RHEL52 readonly]# echo "It is cold today." > winter.txt [root@RHEL52 readonly]# echo "It is hot today." > summer.txt [root@RHEL52 readonly]# ls -l total 8 -rw-r--r-- 1 root root 17 Jan 21 05:49 summer.txt -rw-r--r-- 1 root root 18 Jan 21 05:49 winter.txt [root@RHEL52 readonly]# 53.17. configure the share smb.conf [global] section In this example the samba server is a member of WORKGROUP (the default workgroup). We also set a descriptive server string, this string is visible to users browsing the network with net view, windows explorer or smbclient. [root@RHEL52 samba]# head -5 smb.conf [global] workgroup = WORKGROUP server string = Public Anonymous File Server netbios name = TEACHER0 security = share You might have noticed the line with security = share. This line sets the default security mode for our samba server. Setting the security mode to share will allow clients (smbclient, any windows, another Samba server, ...) to provide a password for each share. This is one way of using the SMB/CIFS protocol. The other way (called user mode) will allow the client to provide a username/password combination, before the server knows which share the client wants to access. smb.conf [share] section The share is called pubread and the path is set to our newly created directory. Everyone is allowed access (guest ok = yes) and security is set to read only. [pubread] path = /srv/samba/readonly comment = files to read read only = yes 510
- 517. samba guest ok = yes 53.18. restart the server After testing with testparm, restart the samba server (so you don't have to wait). [root@RHEL4b readonly]# service smb restart Shutting down SMB services: [ OK ] Shutting down NMB services: [ OK ] Starting SMB services: [ OK ] Starting NMB services: [ OK ] 53.19. verify the share verify with smbclient You can now verify the existence of the share with smbclient. Our pubread is listed as the fourth share. [root@RHEL52 samba]# smbclient -NL 127.0.0.1 Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.33-3.7.el5] Sharename Type Comment --------- ---- ------- IPC$ IPC IPC Service (Public Anonymous File Server) global$ Disk pub0 Disk pubread Disk files to read Domain=[WORKGROUP] OS=[Unix] Server=[Samba 3.0.33-3.7.el5] Server Comment --------- ------- TEACHER0 Samba 3.0.33-3.7.el5 W2003EE Workgroup Master --------- ------- WORKGROUP W2003EE verify on windows The final test is to go to a Microsoft windows computer and read a file on the Samba server. First we use the net use command to mount the pubread share on the driveletter k. C:>net use K: teacher0pubread The command completed successfully. 511
- 518. samba Then we test looking at the contents of the share, and reading the files. C:>dir k: Volume in drive K is pubread Volume Serial Number is 0C82-11F2 Directory of K: 21/01/2009 05:49 <DIR> . 21/01/2009 05:49 <DIR> .. 21/01/2009 05:49 17 summer.txt 21/01/2009 05:49 18 winter.txt 2 File(s) 35 bytes 2 Dir(s) 13.496.242.176 bytes free Just to be on the safe side, let us try writing. K:>echo very cold > winter.txt Access is denied. K:> Or you can use windows explorer... 53.20. a note on netcat The Windows command line screenshot is made in a Linux console, using netcat as a pipe to a Windows command shell. 512
- 519. samba The way this works, is by enabling netcat to listen on the windows computer to a certain port, executing cmd.exe when a connection is received. Netcat is similar to cat, in the way that cat does nothing, only netcat does nothing over the network. To enable this connection, type the following on the windows computer (after downloading netcat for windows). nc -l -p 23 -t -e cmd.exe And then connect to this machine with netcat from any Linux computer. You end up with a cmd.exe prompt inside your Linux shell. paul@laika:~$ nc 192.168.1.38 23 Microsoft Windows [Version 5.2.3790] (C) Copyright 1985-2003 Microsoft Corp. C:>net use k: /delete net use k: /delete k: was deleted successfully. 53.21. Practice: read only file server 1. Create a directory in a good location (FHS) to share files for everyone to read. 2. Make sure the directory is owned properly and is world accessible. 3. Put a textfile in this directory. 4. Share the directory with Samba. 5. Verify from your own and from another computer (smbclient, net use, ...) that the share is accessible for reading. 6. Make a backup copy of your smb.conf, name it smb.conf.ReadOnlyFileServer. 53.22. Solution: read only file server 1. Create a directory in a good location (FHS) to share files for everyone to read. choose one of these... mkdir -p /srv/samba/readonly mkdir -p /home/samba/readonly /home/paul/readonly is wrong!! /etc/samba/readonly is wrong!! /readonly is wrong!! 513
- 520. samba 2. Make sure the directory is owned properly and is world accessible. chown root:root /srv/samba/readonly chmod 755 /srv/samba/readonly 3. Put a textfile in this directory. echo Hello World > hello.txt 4. Share the directory with Samba. You smb.conf.readonly could look like this: [global] workgroup = WORKGROUP server string = Read Only File Server netbios name = STUDENTx security = share [readonlyX] path = /srv/samba/readonly comment = read only file share read only = yes guest ok = yes test with testparm before going in production! 5. Verify from your own and from another computer (smbclient, net use, ...) that the share is accessible for reading. On Linux: smbclient -NL 127.0.0.1 On Windows Explorer: browse to My Network Places On Windows cmd.exe: net use L: //studentx/readonly 6. Make a backup copy of your smb.conf, name it smb.conf.ReadOnlyFileServer. cp smb.conf smb.conf.ReadOnlyFileServer 53.23. set up a directory to share In this second example, we will create a share where everyone can create files and write to files. Again, we start by creating a directory [root@RHEL52 samba]# mkdir -p /srv/samba/writable [root@RHEL52 samba]# chmod 777 /srv/samba/writable/ 53.24. share section in smb.conf There are two parameters to make a share writable. We can use read only or writable. This example shows how to use writable to give write access to a share. 514
- 521. samba writable = yes And this is an example of using the read only parameter to give write access to a share. read only = no 53.25. configure the share Then we simply add a share to our file server by editing smb.conf. Below the check with testparm. (We could have changed the description of the server...) [root@RHEL52 samba]# testparm Load smb config files from /etc/samba/smb.conf Processing section "[pubwrite]" Processing section "[pubread]" Loaded services file OK. Server role: ROLE_STANDALONE Press enter to see a dump of your service definitions [global] netbios name = TEACHER0 server string = Public Anonymous File Server security = SHARE [pubwrite] comment = files to write path = /srv/samba/writable read only = No guest ok = Yes [pubread] comment = files to read path = /srv/samba/readonly guest ok = Yes 53.26. test connection with windows We can now test the connection on a windows 2003 computer. We use the net use for this. C:>net use L: teacher0pubwrite net use L: teacher0pubwrite The command completed successfully. 53.27. test writing with windows We mounted the pubwrite share on the L: drive in windows. Below we test that we can write to this share. L:>echo hoi > hoi.txt 515
- 522. samba L:>dir Volume in drive L is pubwrite Volume Serial Number is 0C82-272A Directory of L: 21/01/2009 06:11 <DIR> . 21/01/2009 06:11 <DIR> .. 21/01/2009 06:16 6 hoi.txt 1 File(s) 6 bytes 2 Dir(s) 13.496.238.080 bytes free 53.28. How is this possible ? Linux (or any Unix) always needs a user account to gain access to a system. The windows computer did not provide the samba server with a user account or a password. Instead, the Linux owner of the files created through this writable share is the Linux guest account (usually named nobody). [root@RHEL52 samba]# ls -l /srv/samba/writable/ total 4 -rwxr--r-- 1 nobody nobody 6 Jan 21 06:16 hoi.txt So this is not the cleanest solution. We will need to improve this. 53.29. Practice: writable file server 1. Create a directory and share it with Samba. 2. Make sure everyone can read and write files, test writing with smbclient and from a Microsoft computer. 3. Verify the ownership of files created by (various) users. 53.30. Solution: writable file server 1. Create a directory and share it with Samba. mkdir /srv/samba/writable chmod 777 /srv/samba/writable the share section in smb.conf can look like this: [pubwrite] path = /srv/samba/writable comment = files to write 516
- 523. samba read only = no guest ok = yes 2. Make sure everyone can read and write files, test writing with smbclient and from a Microsoft computer. to test writing with smbclient: echo one > count.txt echo two >> count.txt echo three >> count.txt smbclient //localhost/pubwrite Password: smb: > put count.txt 3. Verify the ownership of files created by (various) users. ls -l /srv/samba/writable 53.31. creating a samba user We will create a user for our samba file server and make this user the owner of the directory and all of its files. This anonymous user gets a clear description, but does not get a login shell. [root@RHEL52 samba]# useradd -s /bin/false sambanobody [root@RHEL52 samba]# usermod -c "Anonymous Samba Access" sambanobody [root@RHEL52 samba]# passwd sambanobody Changing password for user sambanobody. New UNIX password: Retype new UNIX password: passwd: all authentication tokens updated successfully. 53.32. ownership of files We can use this user as owner of files and directories, instead of using the root account. This approach is clear and more secure. [root@RHEL52 samba]# chown -R sambanobody:sambanobody /srv/samba/ [root@RHEL52 samba]# ls -al /srv/samba/writable/ total 12 drwxrwxrwx 2 sambanobody sambanobody 4096 Jan 21 06:11 . drwxr-xr-x 6 sambanobody sambanobody 4096 Jan 21 06:11 .. -rwxr--r-- 1 sambanobody sambanobody 6 Jan 21 06:16 hoi.txt 53.33. /usr/bin/smbpasswd The sambanobody user account that we created in the previous examples is not yet used by samba. It just owns the files and directories that we created for our shares. The goal of this section is to force ownership of files created through the samba 517
- 524. samba share to belong to our sambanobody user. Remember, our server is still accessible to everyone, nobody needs to know this user account or password. We just want a clean Linux server. To accomplish this, we first have to tell Samba about this user. We can do this by adding the account to smbpasswd. [root@RHEL52 samba]# smbpasswd -a sambanobody New SMB password: Retype new SMB password: Added user sambanobody. 53.34. /etc/samba/smbpasswd To find out where Samba keeps this information (for now), use smbd -b. The PRIVATE_DIR variable will show you where the smbpasswd database is located. [root@RHEL52 samba]# smbd -b | grep PRIVATE PRIVATE_DIR: /etc/samba [root@RHEL52 samba]# ls -l smbpasswd -rw------- 1 root root 110 Jan 21 06:19 smbpasswd You can use a simple cat to see the contents of the smbpasswd database. The sambanobody user does have a password (it is secret). [root@RHEL52 samba]# cat smbpasswd sambanobody:503:AE9 ... 9DB309C528E540978:[U ]:LCT-4976B05B: 53.35. forcing this user Now that Samba knows about this user, we can adjust our writable share to force the ownership of files created through it. For this we use the force user and force group options. Now we can be sure that all files in the Samba writable share are owned by the same sambanobody user. Below is the renewed definition of our share in smb.conf. [pubwrite] path = /srv/samba/writable comment = files to write force user = sambanobody force group = sambanobody read only = no guest ok = yes When you reconnect to the share and write a file, then this sambanobody user will own the newly created file (and nobody needs to know the password). 518
- 525. samba 53.36. Practice: first samba user account 1. Create a user account for use with samba. 2. Add this user to samba's user database. 3. Create a writable shared directory and use the "force user" and "force group" directives to force ownership of files. 4. Test the working of force user with smbclient, net use and Windows Explorer. 53.37. Solution: first samba user account 1. Create a user account for use with samba. useradd -s /bin/false smbguest usermod -c 'samba guest' passwd smbguest 2. Add this user to samba's user database. smbpasswd -a smbguest 3. Create a writable shared directory and use the "force user" and "force group" directives to force ownership of files. [userwrite] path = /srv/samba/userwrite comment = everyone writes files owned by smbguest read only = no guest ok = yes force user = smbguest force group = smbguest 4. Test the working of force user with smbclient, net use and Windows Explorer. ls -l /srv/samba/userwrite (and verify ownership) 53.38. creating the users on Linux The goal of this example is to set up a file share accessible to a number of different users. The users will need to authenticate with their password before access to this share is granted. We will first create three randomly named users, each with their own password. First we add these users to Linux. 519
- 526. samba [root@RHEL52 ~]# useradd -c "Serena Williams" serena [root@RHEL52 ~]# useradd -c "Justine Henin" justine [root@RHEL52 ~]# useradd -c "Martina Hingis" martina [root@RHEL52 ~]# passwd serena Changing password for user serena. New UNIX password: Retype new UNIX password: passwd: all authentication tokens updated successfully. [root@RHEL52 ~]# passwd justine Changing password for user justine. New UNIX password: Retype new UNIX password: passwd: all authentication tokens updated successfully. [root@RHEL52 ~]# passwd martina Changing password for user martina. New UNIX password: Retype new UNIX password: passwd: all authentication tokens updated successfully. 53.39. creating the users on samba Then we add them to the smbpasswd file, with the same password. [root@RHEL52 ~]# smbpasswd -a serena New SMB password: Retype new SMB password: Added user serena. [root@RHEL52 ~]# smbpasswd -a justine New SMB password: Retype new SMB password: Added user justine. [root@RHEL52 ~]# smbpasswd -a martina New SMB password: Retype new SMB password: Added user martina. 53.40. security = user Remember that we set samba's security mode to share with the security = share directive in the [global] section ? Since we now require users to always provide a userid and password for access to our samba server, we will need to change this. Setting security = user will require the client to provide samba with a valid userid and password before giving access to a share. Our [global] section now looks like this. [global] workgroup = WORKGROUP netbios name = TEACHER0 server string = Samba File Server security = user 520
- 527. samba 53.41. configuring the share We add the following [share] section to our smb.conf (and we do not forget to create the directory /srv/samba/authwrite). [authwrite] path = /srv/samba/authwrite comment = authenticated users only read only = no guest ok = no 53.42. testing access with net use After restarting samba, we test with different users from within Microsoft computers. The screenshots use the net useFirst serena from Windows XP. C:>net use m: teacher0authwrite stargate /user:serena The command completed successfully. C:>m: M:>echo greetings from Serena > serena.txt The next screenshot is martina on a Windows 2000 computer, she succeeds in writing her files, but fails to overwrite the file from serena. C:>net use k: teacher0authwrite stargate /user:martina The command completed successfully. C:>k: K:>echo greetings from martina > Martina.txt K:>echo test overwrite > serena.txt Access is denied. 53.43. testing access with smbclient You can also test connecting with authentication with smbclient. First we test with a wrong password. [root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina wrongpass session setup failed: NT_STATUS_LOGON_FAILURE Then we test with the correct password, and verify that we can access a file on the share. 521
- 528. samba [root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina stargate Domain=[TEACHER0] OS=[Unix] Server=[Samba 3.0.33-3.7.el5] smb: > more serena.txt getting file serena.txt of size 14 as /tmp/smbmore.QQfmSN (6.8 kb/s) one two three smb: > q 53.44. verify ownership We now have a simple standalone samba file server with authenticated access. And the files in the shares belong to their proper owners. [root@RHEL52 samba]# ls -l /srv/samba/authwrite/ total 8 -rwxr--r-- 1 martina martina 0 Jan 21 20:06 martina.txt -rwxr--r-- 1 serena serena 14 Jan 21 20:06 serena.txt -rwxr--r-- 1 serena serena 6 Jan 21 20:09 ser.txt 53.45. common problems NT_STATUS_BAD_NETWORK_NAME You can get NT_STATUS_BAD_NETWORK_NAME when you forget to create the target directory. [root@RHEL52 samba]# rm -rf /srv/samba/authwrite/ [root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina stargate Domain=[TEACHER0] OS=[Unix] Server=[Samba 3.0.33-3.7.el5] tree connect failed: NT_STATUS_BAD_NETWORK_NAME NT_STATUS_LOGON_FAILURE You can get NT_STATUS_LOGON_FAILURE when you type the wrong password or when you type an unexisting username. [root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina STARGATE session setup failed: NT_STATUS_LOGON_FAILURE usernames are (not) case sensitive Remember that usernames om Linux are case sensitive. 522
- 529. samba [root@RHEL52 samba]# su - MARTINA su: user MARTINA does not exist [root@RHEL52 samba]# su - martina [martina@RHEL52 ~]$ But usernames on Microsoft computers are not case sensitive. [root@RHEL52 samba]# smbclient //teacher0/authwrite -U martina stargate Domain=[TEACHER0] OS=[Unix] Server=[Samba 3.0.33-3.7.el5] smb: > q [root@RHEL52 samba]# smbclient //teacher0/authwrite -U MARTINA stargate Domain=[TEACHER0] OS=[Unix] Server=[Samba 3.0.33-3.7.el5] smb: > q 53.46. Practice : samba authentication 0. Make sure you have properly named backups of your smb.conf of the previous practices. 1. Create three users (on the Linux and on the samba), remember their passwords! 2. Set up a shared directory that is only accessible to authenticated users. 3. Use smbclient and a windows computer to access your share, use more than one user account (windows requires a logoff/logon for this). 4. Verify that files created by these users belong to them. 5. Try to change or delete a file from another user. 53.47. Solution: samba authentication 1. Create three users (on the Linux and on the samba), remember their passwords! useradd -c 'SMB user1' userx passwd userx 2. Set up a shared directory that is only accessible to authenticated users. The shared section in smb.conf could look like this: [authwrite] path = /srv/samba/authwrite comment = authenticated users only read only = no guest ok = no 523
- 530. samba 3. Use smbclient and a windows computer to access your share, use more than one user account (windows requires a logoff/logon for this). on Linux: smbclient //studentX/authwrite -U user1 password on windows net use p: studentXauthwrite password /user:user2 4. Verify that files created by these users belong to them. ls -l /srv/samba/authwrite 5. Try to change or delete a file from another user. you should not be able to change or overwrite files from others. 53.48. security based on user name valid users To restrict users per share, you can use the valid users parameter. In the example below, only the users listed as valid will be able to access the tennis share. [tennis] path = /srv/samba/tennis comment = authenticated and valid users only read only = No guest ok = No valid users = serena, kim, venus, justine invalid users If you are paranoia, you can also use invalid users to explicitely deny the listed users access. When a user is in both lists, the user has no access! [tennis] path = /srv/samba/tennis read only = No guest ok = No valid users = kim, serena, venus, justine invalid users = venus read list On a writable share, you can set a list of read only users with the read list parameter. [football] path = /srv/samba/football read only = No guest ok = No read list = martina, roberto 524
- 531. samba write list Even on a read only share, you can set a list of users that can write. Use the write list parameter. [football] path = /srv/samba/golf read only = Yes guest ok = No write list = eddy, jan 53.49. security based on ip-address hosts allow The hosts allow or allow hosts parameter is one of the key advantages of Samba. It allows access control of shares on the ip-address level. To allow only specific hosts to access a share, list the hosts, seperated by comma's. allow hosts = 192.168.1.5, 192.168.1.40 Allowing entire subnets is done by ending the range with a dot. allow hosts = 192.168.1. Subnet masks can be added in the classical way. allow hosts = 10.0.0.0/255.0.0.0 You can also allow an entire subnet with exceptions. hosts allow = 10. except 10.0.0.12 hosts deny The hosts deny or deny hosts parameter is the logical counterpart of the previous. The syntax is the same as for hosts allow. hosts deny = 192.168.1.55, 192.168.1.56 53.50. security through obscurity hide unreadable Setting hide unreadable to yes will prevent users from seeing files that cannot be read by them. 525
- 532. samba hide unreadable = yes browsable Setting the browseable = no directive will hide shares from My Network Places. But it will not prevent someone from accessing the share (when the name of the share is known). Note that browsable and browseable are both correct syntax. [pubread] path = /srv/samba/readonly comment = files to read read only = yes guest ok = yes browseable = no 53.51. file system security create mask You can use create mask and directory mask to set the maximum allowed permissions for newly created files and directories. The mask you set is an AND mask (it takes permissions away). [tennis] path = /srv/samba/tennis read only = No guest ok = No create mask = 640 directory mask = 750 force create mode Similar to create mask, but different. Where the mask from above was a logical AND, the mode you set here is a logical OR (so it adds permissions). You can use the force create mode and force directory mode to set the minimal required permissions for newly created files and directories. [tennis] path = /srv/samba/tennis read only = No guest ok = No force create mode = 444 force directory mode = 550 526
- 533. samba security mask The security mask and directory security mask work in the same way as create mask and directory mask, but apply only when a windows user is changing permissions using the windows security dialog box. force security mode The force security mode and force directory security mode work in the same way as force create mode and force directory mode, but apply only when a windows user is changing permissions using the windows security dialog box. inherit permissions With inherit permissions = yes you can force newly created files and directories to inherit permissions from their parent directory, overriding the create mask and directory mask settings. [authwrite] path = /srv/samba/authwrite comment = authenticated users only read only = no guest ok = no create mask = 600 directory mask = 555 inherit permissions = yes 53.52. Practice: securing shares 1. Create a writable share called sales, and a readonly share called budget. Test that it works. 2. Limit access to the sales share to ann, sandra and veronique. 3. Make sure that roberto cannot access the sales share. 4. Even though the sales share is writable, ann should only have read access. 5. Even though the budget share is read only, sandra should also have write access. 6. Limit one shared directory to the 192.168.1.0/24 subnet, and another share to the two computers with ip-addresses 192.168.1.33 and 172.17.18.19. 7. Make sure the computer with ip 192.168.1.203 cannot access the budget share. 8. Make sure (on the budget share) that users can see only files and directories to which they have access. 527
- 534. samba 9. Make sure the sales share is not visible when browsing the network. 10. All files created in the sales share should have 640 permissions or less. 11. All directories created in the budget share should have 750 permissions or more. 12. Permissions for files on the sales share should never be set more than 664. 13. Permissions for files on the budget share should never be set less than 500. 14. If time permits (or if you are waiting for other students to finish this practice), then combine the "read only" and "writable" statements to check which one has priority. 15. If time permits then combine "read list", "write list", "hosts allow" and "hosts deny". Which of these has priority ? 53.53. Solution: securing shares 1. Create a writable share called sales, and a readonly share called budget. Test that it works. see previous solutions on how to do this... 2. Limit access to the sales share to ann, sandra and veronique. valid users = ann, sandra, veronique 3. Make sure that roberto cannot access the sales share. invalid users = roberto 4. Even though the sales share is writable, ann should only have read access. read list = ann 5. Even though the budget share is read only, sandra should also have write access. write list = sandra 6. Limit one shared directory to the 192.168.1.0/24 subnet, and another share to the two computers with ip-addresses 192.168.1.33 and 172.17.18.19. hosts allow = 192.168.1. hosts allow = 192.168.1.33, 172.17.18.19 7. Make sure the computer with ip 192.168.1.203 cannot access the budget share. hosts deny = 192.168.1.203 8. Make sure (on the budget share) that users can see only files and directories to which they have access. hide unreadable = yes 528
- 535. samba 9. Make sure the sales share is not visible when browsing the network. browsable = no 10. All files created in the sales share should have 640 permissions or less. create mask = 640 11. All directories created in the budget share should have 750 permissions or more. force directory mode = 750 12. Permissions for files on the sales share should never be set more than 664. security mask = 750 13. Permissions for files on the budget share should never be set less than 500. force security directory mask = 500 14. If time permits (or if you are waiting for other students to finish this practice), then combine the "read only" and "writable" statements to check which one has priority. 15. If time permits then combine "read list", "write list", "hosts allow" and "hosts deny". Which of these has priority ? 53.54. changes in smb.conf workgroup The workgroup option in the global section should match the netbios name of the Active Directory domain. workgroup = STARGATE security mode Authentication will not be handled by samba now, but by the Active Directory domain controllers, so we set the security option to domain. security = Domain Linux uid's Linux requires a user account for every user accessing its file system, we need to provide Samba with a range of uid's and gid's that it can use to create these user 529
- 536. samba accounts. The range is determined with the idmap uid and the idmap gid parameters. The first Active Directory user to connect will receive Linux uid 20000. idmap uid = 20000-22000 idmap gid = 20000-22000 winbind use default domain The winbind use default domain parameter makes sure winbind also operates on users without a domain component in their name. winbind use default domain = yes [global] section in smb.conf Below is our new global section in smb.conf. [global] workgroup = STARGATE security = Domain server string = Stargate Domain Member Server idmap uid = 20000-22000 idmap gid = 20000-22000 winbind use default domain = yes realm in /etc/krb5.conf To connect to a Windows 2003 sp2 (or later) you will need to adjust the kerberos realm in /etc/krb5.conf and set both lookup statements to true. [libdefaults] default_realm = STARGATE.LOCAL dns_lookup_realm = true dns_lookup_kdc = true [share] section in smb.conf Nothing special is required for the share section in smb.conf. Remember that we do not manually create users in smbpasswd or on the Linux (/etc/passwd). Only Active Directory users are allowed access. [domaindata] path = /srv/samba/domaindata comment = Active Directory users only read only = No 530
- 537. samba 53.55. joining an Active Directory domain While the Samba server is stopped, you can use net rpc join to join the Active Directory domain. [root@RHEL52 samba]# service smb stop Shutting down SMB services: [ OK ] Shutting down NMB services: [ OK ] [root@RHEL52 samba]# net rpc join -U Administrator Password: Joined domain STARGATE. We can verify in the aduc (Active Directory Users and Computers) that a computer account is created for this samba server. 53.56. winbind adding winbind to nsswitch.conf The winbind daemon is talking with the Active Directory domain. We need to update the /etc/nsswitch.conf file now, so user group and host names can be resolved against the winbind daemon. [root@RHEL52 samba]# vi /etc/nsswitch.conf [root@RHEL52 samba]# grep winbind /etc/nsswitch.conf passwd: files winbind 531
- 538. samba group: files winbind hosts: files dns winbind starting samba and winbindd Time to start Samba followed by winbindd. [root@RHEL4b samba]# service smb start Starting SMB services: [ OK ] Starting NMB services: [ OK ] [root@RHEL4b samba]# service winbind start Starting winbindd services: [ OK ] [root@RHEL4b samba]# 53.57. wbinfo verify the trust You can use wbinfo -t to verify the trust between your samba server and Active Directory. [root@RHEL52 ~]# wbinfo -t checking the trust secret via RPC calls succeeded list all users We can obtain a list of all user with the wbinfo -u command. The domain is not shown when the winbind use default domain parameter is set. [root@RHEL52 ~]# wbinfo -u TEACHER0serena TEACHER0justine TEACHER0martina STARGATEadministrator STARGATEguest STARGATEsupport_388945a0 STARGATEpol STARGATEkrbtgt STARGATEarthur STARGATEharry list all groups We can obtain a list of all domain groups with the wbinfo -g command. The domain is not shown when the winbind use default domain parameter is set. 532
- 539. samba [root@RHEL52 ~]# wbinfo -g BUILTINadministrators BUILTINusers BATMANdomain computers BATMANdomain controllers BATMANschema admins BATMANenterprise admins BATMANdomain admins BATMANdomain users BATMANdomain guests BATMANgroup policy creator owners BATMANdnsupdateproxy query a user We can use wbinfo -a to verify authentication of a user against Active Directory. Assuming a user account harry with password stargate is just created on the Active Directory, we get the following screenshot. [root@RHEL52 ~]# wbinfo -a harry%stargate plaintext password authentication succeeded challenge/response password authentication succeeded 53.58. getent We can use getent to verify that winbindd is working and actually adding the Active directory users to /etc/passwd. [root@RHEL52 ~]# getent passwd harry harry:*:20000:20008:harry potter:/home/BATMAN/harry:/bin/false [root@RHEL52 ~]# getent passwd arthur arthur:*:20001:20008:arthur dent:/home/BATMAN/arthur:/bin/false [root@RHEL52 ~]# getent passwd bilbo bilbo:*:20002:20008:bilbo baggins:/home/BATMAN/bilbo:/bin/false If the user already exists locally, then the local user account is shown. This is because winbind is configured in /etc/nsswitch.conf after files. [root@RHEL52 ~]# getent passwd paul paul:x:500:500:Paul Cobbaut:/home/paul:/bin/bash All the Active Directory users can now easily connect to the Samba share. Files created by them, belong to them. 53.59. file ownership 533
- 540. samba [root@RHEL4b samba]# ll /srv/samba/domaindata/ total 0 -rwxr--r-- 1 justine 20000 0 Jun 22 19:54 create_by_justine_on_winxp.txt -rwxr--r-- 1 venus 20000 0 Jun 22 19:55 create_by_venus.txt -rwxr--r-- 1 maria 20000 0 Jun 22 19:57 Maria.txt 53.60. Practice : samba domain member 1. Verify that you have a working Active Directory (AD) domain. 2. Add the domain name and domain controller to /etc/hosts. Set the AD-DNS in / etc/resolv.conf. 3. Setup Samba as a member server in the domain. 4. Verify the creation of a computer account in AD for your Samba server. 5. Verify the automatic creation of AD users in /etc/passwd with wbinfo and getent. 6. Connect to Samba shares with AD users, and verify ownership of their files. 53.61. About Domain Controllers Windows NT4 Windows NT4 works with single master replication domain controllers. There is exactly one PDC (Primary Domain Controller) in the domain, and zero or more BDC's (Backup Domain Controllers). Samba 3 has all features found in Windows NT4 PDC and BDC, and more. This includes file and print serving, domain control with single logon, logon scripts, home directories and roaming profiles. Windows 200x With Windows 2000 came Active Directory. AD includes multimaster replication and group policies. Samba 3 can only be a member server in Active Directory, it cannot manage group policies. Samba 4 can do this (in beta). Samba 3 Samba 3 can act as a domain controller in its own domain. In a Windows NT4 domain, with one Windows NT4 PDC and zero or more BDC's, Samba 3 can only be a member server. The same is valid for Samba 3 in an Active Directory Domain. In short, a Samba 3 domain controller can not share domain control with Windows domain controllers. 534
- 541. samba Samba 4 Samba 4 can be a domain controller in an Active Directory domain, including managing group policies. As of this writing, Samba 4 is not released for production! 53.62. About security modes security = share The 'Windows for Workgroups' way of working, a client requests connection to a share and provides a password for that connection. Aanyone who knows a password for a share can access that share. This security model was common in Windows 3.11, Windows 95, Windows 98 and Windows ME. security = user The client will send a userid + password before the server knows which share the client wants to access. This mode should be used whenever the samba server is in control of the user database. Both for standalone and samba domain controllers. security = domain This mode will allow samba to verify user credentials using NTLM in Windows NT4 and in all Active Directory domains. This is similar to Windows NT4 BDC's joining a native Windows 2000/3 Active Directory domain. security = ads This mode will make samba use Kerberos to connect to the Active Directory domain. security = server This mode is obsolete, it can be used to forward authentication to another server. 53.63. About password backends The previous chapters all used the smbpasswd user database. For domain control we opt for the tdbsam password backend. Another option would be to use LDAP. Larger domains will benefit from using LDAP instead of the not so scalable tdbsam. When you need more than one Domain Controller, then the Samba team advises to not use tdbsam. 535
- 542. samba 53.64. [global] section in smb.conf Now is a good time to start adding comments in your smb.conf. First we will take a look at the naming of our domain and server in the [global] section, and at the domain controlling parameters. security The security must be set to user (which is the default). This mode will make samba control the user accounts, so it will allow samba to act as a domain controller. security = user os level A samba server is the most stable computer in the network, so it should win all browser elections (os level above 32) to become the browser master os level = 33 passdb backend The passdb backend parameter will determine whether samba uses smbpasswd, tdbsam or ldap. passdb backend = tdbsam preferred master Setting the preferred master parameter to yes will make the nmbd daemon force an election on startup. preferred master = yes domain logons Setting the domain logons parameter will make this samba server a domain controller. domain logons = yes domain master Setting the domain master parameter can cause samba to claim the domain master browser role for its workgroup. Don't use this parameter in a workgroup with an active NT4 PDC. 536
- 543. samba domain master = yes [global] section The screenshot below shows a sample [global] section for a samba domain controller. [global] # names workgroup = SPORTS netbios name = DCSPORTS server string = Sports Domain Controller # domain control parameters security = user os level = 33 preferred master = Yes domain master = Yes domain logons = Yes 53.65. netlogon share Part of the microsoft definition for a domain controller is that it should have a netlogon share. This is the relevant part of smb.conf to create this netlogon share on Samba. [netlogon] comment = Network Logon Service path = /srv/samba/netlogon admin users = root guest ok = Yes browseable = No 53.66. other [share] sections We create some sections for file shares, to test the samba server. Users can all access the general sports file share, but only group members can access their own sports share. [sports] comment = Information about all sports path = /srv/samba/sports valid users = @ntsports read only = No [tennis] comment = Information about tennis path = /srv/samba/tennis valid users = @nttennis read only = No [football] 537
- 544. samba comment = Information about football path = /srv/samba/football valid users = @ntfootball read only = No 53.67. Users and Groups To be able to use users and groups in the samba domain controller, we can first set up some groups on the Linux computer. [root@RHEL52 samba]# groupadd ntadmins [root@RHEL52 samba]# groupadd ntsports [root@RHEL52 samba]# groupadd ntfootball [root@RHEL52 samba]# groupadd nttennis This enables us to add group membership info to some new users for our samba domain. Don't forget to give them a password. [root@RHEL52 samba]# useradd -m -G ntadmins Administrator [root@RHEL52 samba]# useradd -m -G ntsports,nttennis venus [root@RHEL52 samba]# useradd -m -G ntsports,nttennis kim [root@RHEL52 samba]# useradd -m -G ntsports,nttennis jelena [root@RHEL52 samba]# useradd -m -G ntsports,ntfootball figo [root@RHEL52 samba]# useradd -m -G ntsports,ntfootball ronaldo [root@RHEL52 samba]# useradd -m -G ntsports,ntfootball pfaff It is always safe to verify creation of users, groups and passwords in /etc/passwd, / etc/shadow and /etc/group. [root@RHEL52 samba]# tail -11 /etc/group ntadmins:x:507:Administrator ntsports:x:508:venus,kim,jelena,figo,ronaldo,pfaff ntfootball:x:509:figo,ronaldo,pfaff nttennis:x:510:venus,kim,jelena Administrator:x:511: venus:x:512: kim:x:513: jelena:x:514: figo:x:515: ronaldo:x:516: pfaff:x:517: 53.68. tdbsam Next we must make these users known to samba with the smbpasswd tool. When you add the first user to tdbsam, the file /etc/samba/passdb.tdb will be created. 538
- 545. samba [root@RHEL52 samba]# smbpasswd -a root New SMB password: Retype new SMB password: tdbsam_open: Converting version 0 database to version 3. Added user root. Adding all the other users generates less output, because tdbsam is already created. [root@RHEL4b samba]# smbpasswd -a root New SMB password: Retype new SMB password: Added user root. 53.69. about computer accounts Every NT computer (Windows NT, 2000, XP, Vista) can become a member of a domain. Joining the domain (by right-clicking on My Computer) means that a computer account will be created in the domain. This computer account also has a password (but you cannot know it) to prevent other computers with the same name from accidentally becoming member of the domain. The computer account created by Samba is visible in the /etc/passwd file on Linux. Computer accounts appear as a normal user account, but end their name with a dollar sign. Below a screenshot of the windows 2003 computer account, created by Samba 3. [root@RHEL52 samba]# tail -5 /etc/passwd jelena:x:510:514::/home/jelena:/bin/bash figo:x:511:515::/home/figo:/bin/bash ronaldo:x:512:516::/home/ronaldo:/bin/bash pfaff:x:513:517::/home/pfaff:/bin/bash w2003ee$:x:514:518::/home/nobody:/bin/false To be able to create the account, you will need to provide credentials of an account with the permission to create accounts (by default only root can do this on Linux). And we will have to tell Samba how to to this, by adding an add machine script to the global section of smb.conf. add machine script = /usr/sbin/useradd -s /bin/false -d /home/nobody %u You can now join a Microsoft computer to the sports domain (with the root user). After reboot of the Microsoft computer, you will be able to logon with Administrator (password Stargate1), but you will get an error about your roaming profile. We will fix this in the next section. When joining the samba domain, you have to enter the credentials of a Linux account that can create users (usually only root can do this). If the Microsoft computer complains with The parameter is incorrect, then you possibly forgot to add the add machine script. 539
- 546. samba 53.70. local or roaming profiles For your information, if you want to force local profiles instead of roaming profiles, then simply add the following two lines to the global section in smb.conf. logon home = logon path = Microsoft computers store a lot of User Metadata and application data in a user profile. Making this profile available on the network will enable users to keep their Desktop and Application settings across computers. User profiles on the network are called roaming profiles or roving profiles. The Samba domain controller can manage these profiles. First we need to add the relevant section in smb.conf. [Profiles] comment = User Profiles path = /srv/samba/profiles readonly = No profile acls = Yes Besides the share section, we also need to set the location of the profiles share (this can be another Samba server) in the global section. logon path = %LProfiles%U The %L variable is the name of this Samba server, the %U variable translates to the username. After adding a user to smbpasswd and letting the user log on and off, the profile of the user will look like this. [root@RHEL4b samba]# ll /srv/samba/profiles/Venus/ total 568 drwxr-xr-x 4 Venus Venus 4096 Jul 5 10:03 Application Data drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 Cookies drwxr-xr-x 3 Venus Venus 4096 Jul 5 10:03 Desktop drwxr-xr-x 3 Venus Venus 4096 Jul 5 10:03 Favorites drwxr-xr-x 4 Venus Venus 4096 Jul 5 10:03 My Documents drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 NetHood -rwxr--r-- 1 Venus Venus 524288 Jul 5 2007 NTUSER.DAT -rwxr--r-- 1 Venus Venus 1024 Jul 5 2007 NTUSER.DAT.LOG -rw-r--r-- 1 Venus Venus 268 Jul 5 10:03 ntuser.ini drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 PrintHood drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 Recent drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 SendTo drwxr-xr-x 3 Venus Venus 4096 Jul 5 10:03 Start Menu drwxr-xr-x 2 Venus Venus 4096 Jul 5 10:03 Templates 53.71. Groups in NTFS acls We have users on Unix, we have groups on Unix that contain those users. 540
- 547. samba [root@RHEL4b samba]# grep nt /etc/group ... ntadmins:x:506:Administrator ntsports:x:507:Venus,Serena,Kim,Figo,Pfaff nttennis:x:508:Venus,Serena,Kim ntfootball:x:509:Figo,Pfaff [root@RHEL4b samba]# We already added Venus to the tdbsam with smbpasswd. smbpasswd -a Venus Does this mean that Venus can access the tennis and the sports shares ? Yes, all access works fine on the Samba server. But the nttennis group is not available on the windows machines. To make the groups available on windows (like in the ntfs security tab of files and folders), we have to map unix groups to windows groups. To do this, we use the net groupmap command. [root@RHEL4b samba]# net groupmap add ntgroup="tennis" unixgroup=nttennis type=d No rid or sid specified, choosing algorithmic mapping Successully added group tennis to the mapping db [root@RHEL4b samba]# net groupmap add ntgroup="football" unixgroup=ntfootball type=d No rid or sid specified, choosing algorithmic mapping Successully added group football to the mapping db [root@RHEL4b samba]# net groupmap add ntgroup="sports" unixgroup=ntsports type=d No rid or sid specified, choosing algorithmic mapping Successully added group sports to the mapping db [root@RHEL4b samba]# Now you can use the Samba groups on all NTFS volumes on members of the domain. 53.72. logon scripts Before testing a logon script, make sure it has the proper carriage returns that DOS files have. [root@RHEL4b netlogon]# cat start.bat net use Z: DCSPORTS0SPORTS [root@RHEL4b netlogon]# unix2dos start.bat unix2dos: converting file start.bat to DOS format ... [root@RHEL4b netlogon]# Then copy the scripts to the netlogon share, and add the following parameter to smb.conf. logon script = start.bat 53.73. Practice 1. Setup Samba as a domain controller. 541
- 548. samba 2. Create the shares salesdata, salespresentations and meetings. Salesdata must be accessible to all sales people and to all managers. SalesPresentations is only for all sales people. Meetings is only accessible to all managers. Use groups to accomplish this. 3. Join a Microsoft computer to your domain. Verify the creation of a computer account in /etc/passwd. 4. Setup and verify the proper working of roaming profiles. 5. Find information about home directories for users, set them up and verify that users receive their home directory mapped under the H:-drive in MS Windows Explorer. 6. Use a couple of samba domain groups with members to set acls on ntfs. Verify that it works! 7. Knowing that the %m variable contains the computername, create a seperate log file for every computer(account). 8. Knowing that %s contains the client operating system, include a smb.%s.conf file that contains a share. (The share will only be visible to clients with that OS). 9. If time permits (or if you are waiting for other students to finish this practice), then combine "valid users" and "invalid users" with groups and usernames with "hosts allow" and "hosts deny" and make a table of which get priority over which. 53.74. Samba 4 alpha 6 A quick view on Samba 4 alpha 6 (January 2009). You can also follow this guide http://wiki.samba.org/index.php/Samba4/HOWTO Remove old Samba from Red Hat yum remove samba set a fix ip address (Red Hat has an easy GUI) download and untar samba.org, click 'download info', choose mirror, dl samba4 latest alpha once untarred, enter the directory and read the howto4.txt cd samba-4.0.0alpha6/ more howto4.txt first we have to configure, compile and install samba4 cd source4/ ./configure make 542
- 549. samba make install Then we can use the provision script to setup our realm. I used booi.schot as domain name (instead of example.com). ./setup/provision --realm=BOOI.SCHOT --domain=BOOI --adminpass=stargate --server-role='domain controller' i added a simple share for testing vi /usr/local/samba/etc/smb.conf then i started samba cd /usr/local/samba/sbin/ ./samba I tested with smbclient, it works smbclient //localhost/test -Uadministrator%stargate I checked that bind (and bind-chroot) were installed (yes), so copied the srv records cp booi.schot.zone /var/named/chroot/etc/ then appended to named.conf cat named.conf >> /var/named/chroot/etc/named.conf I followed these steps in the howto4.txt vi /etc/init.d/named [added two export lines right after start()] chmod a+r /usr/local/samba/private/dns.keytab cp krb5.conf /etc/ vi /var/named/chroot/etc/named.conf --> remove a lot, but keep allow-update { any; }; restart bind (named!), then tested dns with dig, this works (stripped screenshot!) [root@RHEL52 private]# dig _ldap._tcp.dc._msdcs.booi.schot SRV @localhost ; (1 server found) ;; global options: printcmd ;; Got answer: ;; -HEADER- opcode: QUERY, status: NXDOMAIN, id: 58186 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 ;; QUESTION SECTION: ;_ldap._tcp.dc._msdcs.booi.schot. IN SRV ;; AUTHORITY SECTION: . 10800 IN SOA A.ROOT-SERVERS.NET.... ;; Query time: 54 msec ;; SERVER: 127.0.0.1#53(127.0.0.1) ;; WHEN: Tue Jan 27 20:57:05 2009 543
- 550. samba ;; MSG SIZE rcvd: 124 [root@RHEL52 private]# made sure /etc/resolv.conf points to himself [root@RHEL52 private]# cat /etc/resolv.conf search booi.schot nameserver 127.0.0.1 start windows 2003 server, enter the samba4 as DNS! ping the domain, if it doesn't work, then add your redhats hostname and your realm to windows/system32/drivers/etc/hosts join the windows computer to the domain reboot the windows log on with administrator stargate start run dsa.msc to manage samba4 create an OU, a user and a GPO, test that it works 544
- 551. Part XVII. dns server
- 552. Chapter 54. introduction to DNS Table of Contents 54.1. DNS History .............................................................................................. 546 54.2. DNS Structure ............................................................................................ 546 54.3. How DNS works ........................................................................................ 547 54.4. old stuff....work in progress ....................................................................... 550 54.5. bind ............................................................................................................. 550 54.6. named ......................................................................................................... 550 54.7. Caching only Name Server ........................................................................ 551 54.8. Our first zone ............................................................................................. 551 54.9. Starting the name server ............................................................................ 552 54.10. practice: DNS ........................................................................................... 552 54.1. DNS History Today, DNS or Domain Name System is a worldwide distributed hierarchical database. It's primary function is to resolve names to ip addresses, and to point to internet servers providing SMTP or LDAP services. In the seventies, only a few hundred computers were connected to the internet. To resolve names, computers had a flat file that contained a table to resolve hostnames to ip-addresses. This local file was downloaded from hosts.txt on an ftp server in Stanford. In 1984 Paul Mockapetris created DNS, a distributed treelike hierarchical database. ICANN............... 54.2. DNS Structure root DNS is a tree structure. The top of the tree is called the root. There are thirteen root servers on the internet, they are named A to M. Journalist often refer to these servers as the master servers of the internet, because if these servers go down, then nobody can (use names to) connect to websites. The root servers are not thirteen physical machines, in fact ... (expand later with mirror info...) top level domains (TLD) Below the root level are the top level domains or TLD's. Originally there were only seven 546
- 553. introduction to DNS defined: .com(mercial), .edu(cational), .gov(ernment), .int(ernational), .mil(ilitary), .net(work) and .org for non-commercial organizations. The .arpa domain was also used, it will be explained later. Country TLD's were defined for individual countries, like .be for Belgium and .fr for France. In the 21st century new TLD's were defined like .museum .info .biz and .aero. domains One level below the top level domains are the domains. Examples of domain names are google.com or linux-training.be. Domains can have subdomains (also called child domains). fully qualified domain name The fully qualified domain name or FQDN is the combination of the hostname of a machine appended with its domain name. If for example a system is called wolf and it is in the domain stargate.local, then the FQDN of this system is wolf.stargate.local. 54.3. How DNS works zone A zone is a portion of the DNS tree. A DNS server that is controlling a zone, is said to be the authoritative DNS server for that zone. A zone is a collection of resource records. There are several types of resource records, for example A, PTR, NS, MX, SOA and CNAME. A record The A record, which is also called a host record contains the ipv4-address of a computer. When a DNS client queries a DNS server for an A record, then the DNS server will resolve the hostname in the query to an ip-address. An AAAA record is similar but contains an ipv6 address instead of ipv4. PTR record A PTR record is the reverse of an A record. It contains the name of a computer and can be used to resolve an ip-address to a hostname. 547
- 554. introduction to DNS NS record A NS record or nameserver record is a record that points to a DNS name server (in this zone). You can list all your name servers for your DNS zone in distinct NS records. SOA record The SOA record of a zone contains meta information about the zone itself. The contents of the SOA record is explained in detail in the section about zone transfers. There is exactly one SOA record for each zone. CNAME record A CNAME record maps a hostname to a hostname, creating effectively an alias for an existing hostname. The name of the mail server is often aliased to mail or smtp, and the name of a web server to www. MX record The MX points to an SMTP server. When you send an email to another domain, then your mail server will need the MX record of the target domain's mail server. master and slave There are several reasons to create more than one name server in a zone. One server might not be able to answer to all queries, or you might want some fault tolerance to mitigate the impact of hardware failure. When adding a secondary DNS server to a zone, then you will configure this server as a slave server to the primary server. The primary server then becomes the master server of the slave server. Very often the primary DNS server is the master server of all slaves. Sometimes a slave server is master server for a second line slave server. zone transfers The slave server receives a copy of the zone database using a zone transfer. Zone transfers are requested by the slave servers at regular intervals. Those intervals are defined in the SOA record. The SOA record contains a refresh value. If this is set to 30 minutes, then the slave server will request a copy of the zone file every 30 minutes. There is also a retry value. The retry value is used when the master server did not reply to the last zone transfer request. The value for expiry time says how long the slave server will answer to queries, without receiving a zone update. 548
- 555. introduction to DNS Zone transfers only occur when the zone database was updated (meaning when one or more resource records were added, removed or changed on the master server). The slave server will compare the serial number of its own copy of the SOA record with the serial number of its master's SOA record. When both serial numbers are the same, then no update is needed (because no records were added, removed or deleted). When the slave has a lower serial number than its master, then a zone transfer is requested. full or incremental zone transfers When a zone tranfer occurs, this can be either a full zone transfer or an incremental zone transfer. The decision depends on the size of the transfer that is needed to completely update the zone on the slave server. An incremental zone transfer is prefered when the total size of changes is smaller than the size of the zone database. Full zone transfers use the axfr protocol, incremental zone transfer use the ixfr protocol. DNS cache DNS is a caching protocol. When a client queries its local DNS server, and the local DNS server is not authoritative for the query, then this server will go looking for an authoritative name server in the DNS tree. The local name server will first query a root server, then a TLD server and then a domain server. When the local name server resolves the query, then it will relay this information to the client that submitted the query, and it will also keep a copy of these queries in its cache. So when a(nother) client submits the same query to this name server, then it will retrieve this information form its cache. For example, a client queries for the A record on www.linux-training.be to its local server. This is the first query ever received by this local server. The local server checks that it is not authoritative for the linux-training.be domain, nor for the .be TLD, and it is also not a root server. So the local server will use the root hints to send an iterative query to a root server. The root server will reply with a reference to the server that is authoritative for the .be domain (root DNS servers do not resolve fqdn's, and root servers do not respond to recursive queries). The local server will then sent an iterative query to the authoritative server for the .be TLD. This server will respond with a reference to the name server that is authoritative for the linux-training.be domain. The local server will then sent the query for www.linux-training.be to the authoritative server (or one of its slave servers) for the linux-training.be domain. When the local server receives the ip-address for www.linux-training.be, then it will provide this information to the client that submitted this query. Besides caching the A record for www.linux-training.be, the local server will also cache the NS and A record for the linux-training.be name server and the .be name server. caching only server A DNS server that is set up without its own zone, but that is connected to other name servers and caches the queries is called a caching only name server. 549
- 556. introduction to DNS iterative or recursive query A recursive query is a DNS query where the client that is submitting the query expects a complete answer. An iterative query is a DNS query where the client does not expect a complete answer. Iterative queries usually take place between name servers. The root name servers do not respond to recursive queries. 54.4. old stuff....work in progress Forward lookup zones are most common, they contain host or A records to translate hostnames or Fully Qualified Domain Names (FQDN) to ip addresses. Reverse lookup zones contain PTR records, they translate ip addresses to hostnames or FQDN's. The internet contains thirteen logical DNS servers for the top of the hierarchy. This top is called the root, and is represented with a dot. Below the root are the Top Level Domains (TLD's). There are common TLD's like .com, .net. .info. aero. .museum, .gov, .mil, .edu and others. And there are country TLD's, like .be for Belgium and .fr for France. The internet root name servers will only answer iterative queries, most local DNS servers will answer to recursive queries. 54.5. bind One of the most common name servers on Linux is the Berkeley Internet Name Domain (bind) server. Use rpm or dpkg to verify whether it is installed. [root@RHEL4b etc]# rpm -qa | grep -i bind ypbind-1.17.2-8 bind-chroot-9.2.4-16.EL4 bind-utils-9.2.4-16.EL4 bind-devel-9.2.4-16.EL4 bind-libs-9.2.4-16.EL4 bind-9.2.4-16.EL4 54.6. named The software is called 'bind', the daemon runs as 'named' ! So look for the named daemon, the named manual pages and /etc/named.conf to work with bind. [root@RHEL4b etc]# apropos named | grep -i domain named (8) - Internet domain name server 550
- 557. introduction to DNS 54.7. Caching only Name Server A caching only name server is a DNS server that is not authoritative for any zone. It forwards queries to other DNS servers and locally caches the results. The default /etc/named.conf on RHEL is a caching only name server. 54.8. Our first zone The way to set up zones in /etc/named.conf is to create a zone entry with a reference to another file located in /var/named. Here is an example of such an entry in /etc/named.conf zone "classdemo.local" IN { type master; file "classdemo.local.zone"; allow-update { none; }; }; To create the zone file, the easy method is to copy an existing zone file (this is easier than writing from scratch). [root@RHEL4b named]# cd /var/named/ [root@RHEL4b named]# pwd /var/named [root@RHEL4b named]# cp localhost.zone classdemo.local.zone [root@RHEL4b named]# Here is an example of a zone file. [root@RHEL4b named]# cat classdemo.local.zone $TTL 86400 $ORIGIN classdemo.local. @ IN SOA rhel4b.classdemo.local. admin.classdemo.local. ( 2007083100 ; serial 3H ; refresh 900 ; retry 1W ; expiry 1D ) ; minimum IN NS rhel4b.classdemo.local. IN MX 10 mail.classdemo.local. IN A 192.168.1.191 rhel4b IN A 192.168.1.191 mail IN A 192.168.1.191 www IN A 192.168.1.191 ftp IN A 192.168.1.191 server2 IN A 192.168.1.1 551
- 558. introduction to DNS 54.9. Starting the name server When starting the name server, don't forget to look at the log file to verify that all your zones are properly configured. [root@RHEL4b etc]# service named restart Stopping named: [ OK ] Starting named: [ OK ] [root@RHEL4b etc]# service named status number of zones: 9 debug level: 0 xfers running: 0 xfers deferred: 0 soa queries in progress: 0 query logging is OFF server is up and running [root@RHEL4b etc]# 54.10. practice: DNS 1. Set up a working DNS server with your own zone. Test that it works. 2. Set up a master and a slave server. 552
- 559. Chapter 55. Introduction to DNS Table of Contents 55.1. What is DNS ? ........................................................................................... 554 55.2. forward and reverse lookup queries ........................................................... 554 55.3. /etc/resolv.conf ........................................................................................... 555 55.4. DNS namespace ......................................................................................... 556 55.5. root servers ................................................................................................. 556 55.6. top level domains (TLD) ........................................................................... 557 55.7. DNS domains ............................................................................................. 558 55.8. fully qualified domain name ...................................................................... 558 55.9. hostname and domainname commands ...................................................... 558 55.10. zones and records ..................................................................................... 559 55.11. caching only server .................................................................................. 561 55.12. primary and secondary ............................................................................. 563 55.13. master and slave ....................................................................................... 563 55.14. zone transfers ........................................................................................... 564 55.15. full or incremental zone transfers ............................................................ 564 55.16. DNS cache ............................................................................................... 564 55.17. Practice: caching only DNS server .......................................................... 566 55.18. Practice: caching only with forwarder ..................................................... 569 55.19. Practice: primary authoritative server ...................................................... 571 55.20. Practice: reverse DNS .............................................................................. 573 55.21. Practice: a DNS slave server ................................................................... 574 55.22. DNS History ............................................................................................ 575 55.23. forward lookup zone example .................................................................. 575 553
- 560. Introduction to DNS 55.1. What is DNS ? DNS is a service on a tcp/ip network that enables clients to translate names into ip- addresses. DNS is much more than that, but let's keep it simple for now. When you use a browser to go to a website, then you type the name of that website in the url bar. But for your computer to actually communicate with the web server hosting said website, your computer needs the ip-address of that web server. That is where DNS comes in. In wireshark you can use the dns filter to see this traffic. 55.2. forward and reverse lookup queries The question a client asks a dns server is called a query. When a client queries for an ip-address, this is called a forward lookup query (as seen in the picture above). The reverse, a query for the name of a host, is called a reverse lookup query. Below a picture of a reverse lookup query. Here is a screenshot of a reverse lookup query in nslookup. paul@ubu1010:~$ nslookup 554
- 561. Introduction to DNS > set type=PTR > 178.63.30.100 Server: 212.71.8.10 Address: 212.71.8.10#53 Non-authoritative answer: 100.30.63.178.in-addr.arpa name = antares.ginsys.net. This is what a reverse lookup looks like when sniffing with wireshark. 55.3. /etc/resolv.conf A client computer needs to know the ip-address of the dns server to be able to send queries to it. This is either provided by a dhcp server or manually entered. Linux clients keep this information in /etc/resolv.conf. paul@ubu1010:~$ cat /etc/resolv.conf nameserver 212.71.8.10 555
- 562. Introduction to DNS 55.4. DNS namespace The dns namespace is hierarchical tree structure, with the root servers (aka dot- servers) at the top. The root servers are usually represented by a dot. Below the root-servers are the Top Level Domains or TLD's. There are more TLD's than shown in the picture. Currently about 200 countries have a TLD. And there are several general TLD's like .com, .edu, .org, .gov, .net, .mil, .int and more recently also .aero, .info, .museum, ... 55.5. root servers There are thirteen root servers on the internet, they are named A to M. Journalists often refer to these servers as the master servers of the internet, because if these servers go down, then nobody can (use names to) connect to websites. The root servers are not thirteen physical machines, they are many more. For example the F root server (http://f.root-servers.org) consists of 46 physical machines that all behave as one (using anycast). More information at http://root-servers.org and http:// en.wikipedia.org/wiki/Root_nameserver. 556
- 563. Introduction to DNS 55.6. top level domains (TLD) Below the root level are the top level domains or TLD's. Originally there were only seven defined: Table 55.1. The first TLD's year TLD purpose 1985 .arpa Reverse lookup via in-addr.arpa 1985 .com Commercial Organizations 1985 .edu US Educational Institutions 1985 .gov US Government Institutions 1985 .mil US Military 1985 .net Internet Service Providers, Internet Infrastructure 1985 .org Non profit Organizations 1988 .int International Treaties like nato.int Country TLD's were defined for individual countries, like .uk in 1985 for Great Britain (yes really), .be for Belgium in 1988 and .fr for France in 1986. See RFC 1591 for more info. In 1998 seven new general purpose TLD's where chosen, they became active in the 21st century. Table 55.2. new general purpose TLD's year TLD purpose 2002 .aero aviation related 2001 .biz businesses 2001 .coop for co-operatives 2001 .info informative internet resources 2001 .museum for museums 2001 .name for all kinds of names, pseudonyms and labels... 2004 .pro for professionals Many people were surprised by the choices, claiming not much use for them and wanting a separate .xxx domain (introduced in 2011) for adult content, and .kidz a save haven for children. In the meantime more useless TLD's were create like .travel (for travel agents) and .tel (for internet communications) and .jobs (for jobs sites). 557
- 564. Introduction to DNS 55.7. DNS domains One level below the top level domains are the domains. Domains can have subdomains (also called child domains). This picture shows dns domains like google.com, chess.com, linux-training.be (there are millions more). DNS domains are registered at the TLD servers, the TLD servers are registered at the dot servers. 55.8. fully qualified domain name The fully qualified domain name or FQDN is the combination of the hostname of a machine appended with its domain name. If for example a system is called wolf and it is in the domain linux-training.be, then the FQDN of this system is wolf.linux-training.be. 55.9. hostname and domainname commands On Linux systems you can use the hostname and domainname commands to verify this information. paul@shared-010:~$ hostname shared-010 paul@shared-010:~$ hostname --fqdn shared-010.openminds.be 558
- 565. Introduction to DNS 55.10. zones and records A zone (aka a zone of authority) is a portion of the DNS tree. The picture below represents zones as blue ovals. Some zones will contain only one dns domain while others might have a whole tree of domains. In a zone are records, also called resource records. We will list some of those records here, and look at them in more detail later. A record The A record, which is also called a host record contains the ipv4-address of a computer. When a DNS client queries a DNS server for an A record, then the DNS server will resolve the hostname in the query to an ip-address. An AAAA record is similar but contains an ipv6 address instead of ipv4. PTR record A PTR record is the reverse of an A record. It contains the name of a computer and can be used to resolve an ip-address to a hostname. NS record A NS record or nameserver record is a record that points to a DNS name server (in this zone). You can list all your name servers for your DNS zone in distinct NS records. 559
- 566. Introduction to DNS glue A record An A record that maps the name of an NS record to an ip address is said to be a glue record. SOA record The SOA record of a zone contains meta information about the zone itself. The contents of the SOA record is explained in detail in the section about zone transfers. There is exactly one SOA record for each zone. CNAME record A CNAME record maps a hostname to a hostname, creating effectively an alias for an existing hostname. The name of the mail server is often aliased to mail or smtp, and the name of a web server to www. MX record The MX record points to an smtp server. When you send an email to another domain, then your mail server will need the MX record of the target domain's mail server. 560
- 567. Introduction to DNS 55.11. caching only server A DNS server that is set up without its own zone, but that is connected to other name servers and caches the queries is called a caching only name server. Caching only name servers do not have a zone database with resource records. Instead they connect to other name servers and cache that information. There are two kinds of caching only name servers. Those with a forwarder, and those without. caching only server with forwarder A caching only server with a forwarder is a DNS server that will get all its information from the forwarder. The forwarder can be the DNS server of an ISP. This picture shows a DNS server on the company LAN that has set the DNS server from their ISP as a forwarder. If the ip address of the ISP DNS server is 212.71.8.10, then the following lines would occur in the named.conf file of the company DNS server: forwarders { 212.71.8.10; }; 561
- 568. Introduction to DNS caching only server without forwarder A caching only server without forwarder will have to get information elsewhere. When it receives a query from a client, then it will consult one of the root servers. The root server will refer it to a TLD server, which will refer it to another dns server. That last server might know the answer to the query, or may refer to yet another dns server. In the end, our hard working dns server will find an answer and report this back to the client. In the picture below, the clients asks for the ip address of linux-training.be. Our caching only server will contact the root server, and be refered to the .be server. It will then contact the .be server and be refered to one of the name servers of Openminds. One of these name servers (in this cas ns1.openminds.be) will answer the query with the ip-address of linux-training.be. When our caching only server reports this to the client, then the client can connect to this website. iterative or recursive query A recursive query is a DNS query where the client that is submitting the query expects a complete answer (Like the fat red arrow above going from the Macbook to the DNS server). An iterative query is a DNS query where the client does not expect a complete answer (the three black arrows originating from the DNS server in the picture above). Iterative queries usually take place between name servers. The root name servers do not respond to recursive queries. 562
- 569. Introduction to DNS 55.12. primary and secondary A DNS server that is controlling a zone, is said to be the authoritative DNS server for that zone. Remember that a zone is a collection of resource records. When you set up the first authoritative dns server for a zone, the this is called the primary dns server. This server will have a readable and writable copy of the zone database. For reasons of fault tolerance, performance or load balancing you may decide to set up another dns server with authority over that zone. This is called a secondary dns server. 55.13. master and slave When adding a secondary DNS server to a zone, then you will configure this server as a slave server to the primary server. The primary server then becomes the master server of the slave server. Often the primary DNS server is the master server of all slaves. Sometimes a slave server is master server for a second line slave server. In the picture below ns1 is the primary dns server and ns2, ns3 and ns4 are secondaries. The master for slaves ns2 and ns3 is ns1, but the master for ns4 is ns3. 563
- 570. Introduction to DNS 55.14. zone transfers The slave server receives a copy of the zone database using a zone transfer. Zone transfers are requested by the slave servers at regular intervals. Those intervals are defined in the SOA record. The SOA record contains a refresh value. If this is set to 30 minutes, then the slave server will request a copy of the zone file every 30 minutes. There is also a retry value. The retry value is used when the master server did not reply to the last zone transfer request. The value for expiry time says how long the slave server will answer to queries, without receiving a zone update. Zone transfers only occur when the zone database was updated (meaning when one or more resource records were added, removed or changed on the master server). The slave server will compare the serial number of its own copy of the SOA record with the serial number of its master's SOA record. When both serial numbers are the same, then no update is needed (because no records were added, removed or deleted). When the slave has a lower serial number than its master, then a zone transfer is requested. Below a zone transfer captured in wireshark. 55.15. full or incremental zone transfers When a zone tranfer occurs, this can be either a full zone transfer or an incremental zone transfer. The decision depends on the size of the transfer that is needed to completely update the zone on the slave server. An incremental zone transfer is prefered when the total size of changes is smaller than the size of the zone database. Full zone transfers use the axfr protocol, incremental zone transfer use the ixfr protocol. 55.16. DNS cache DNS is a caching protocol. When a client queries its local DNS server, and the local DNS server is not authoritative for the query, then this server will go looking for an authoritative name server in the DNS tree. The local name server will first query a 564
- 571. Introduction to DNS root server, then a TLD server and then a domain server. When the local name server resolves the query, then it will relay this information to the client that submitted the query, and it will also keep a copy of these queries in its cache. So when a(nother) client submits the same query to this name server, then it will retrieve this information form its cache. For example, a client queries for the A record on www.linux-training.be to its local server. This is the first query ever received by this local server. The local server checks that it is not authoritative for the linux-training.be domain, nor for the .be TLD, and it is also not a root server. So the local server will use the root hints to send an iterative query to a root server. The root server will reply with a reference to the server that is authoritative for the .be domain (root DNS servers do not resolve fqdn's, and root servers do not respond to recursive queries). The local server will then sent an iterative query to the authoritative server for the .be TLD. This server will respond with a reference to the name server that is authoritative for the linux-training.be domain. The local server will then sent the query for www.linux-training.be to the authoritative server (or one of its slave servers) for the linux-training.be domain. When the local server receives the ip-address for www.linux-training.be, then it will provide this information to the client that submitted this query. Besides caching the A record for www.linux-training.be, the local server will also cache the NS and A record for the linux-training.be name server and the .be name server. 565
- 572. Introduction to DNS 55.17. Practice: caching only DNS server 1a. installing DNS software on Debian/Ubuntu root@ubu1010srv:~# dpkg -l | grep bind9 ii bind9-host 1:9.7.1.dfsg.P2-2ubuntu0.2 Version of 'host' bun dled with BIND 9.X ii libbind9-60 1:9.7.1.dfsg.P2-2ubuntu0.2 BIND9 Shared Library used by BIND root@ubu1010srv:~# aptitude install bind9 The following NEW packages will be installed: bind9 bind9utils{a} 0 packages upgraded, 2 newly installed, 0 to remove and 0 not upgraded. Need to get 433kB of archives. After unpacking 1,352kB will be used. Do you want to continue? [Y/n/?] ... output truncated ... * Starting domain name service... bind9 [ OK ] root@ubu1010srv:~# dpkg -l | grep bind9 ii bind9 1:9.7.1.dfsg.P2-2ubuntu0.2 Internet Domain Name Server ii bind9-host 1:9.7.1.dfsg.P2-2ubuntu0.2 Version of 'host' bundled w ith BIND 9.X ii bind9utils 1:9.7.1.dfsg.P2-2ubuntu0.2 Utilities for BIND ii libbind9-60 1:9.7.1.dfsg.P2-2ubuntu0.2 BIND9 Shared Library used b y BIND root@ubu1010srv:~# 1b. installing DNS software on RHEL/Fedora [root@fedora14 ~]# rpm -qa | grep bind samba-winbind-clients-3.5.8-74.fc14.i686 bind-utils-9.7.3-1.fc14.i686 PackageKit-device-rebind-0.6.12-2.fc14.i686 bind-libs-9.7.3-1.fc14.i686 [root@fedora14 ~]# yum install bind Loaded plugins: langpacks, presto, refresh-packagekit Adding en_US to language list Setting up Install Process Resolving Dependencies --> Running transaction check ---> Package bind.i686 32:9.7.3-1.fc14 set to be installed --> Finished Dependency Resolution ...output truncated Running Transaction Installing : 32:bind-9.7.3-1.fc14.i686 1/1 Installed: bind.i686 32:9.7.3-1.fc14 Complete! [root@fedora14 ~]# rpm -qa | grep bind samba-winbind-clients-3.5.8-74.fc14.i686 bind-utils-9.7.3-1.fc14.i686 PackageKit-device-rebind-0.6.12-2.fc14.i686 bind-libs-9.7.3-1.fc14.i686 bind-9.7.3-1.fc14.i686 [root@fedora14 ~]# 2. Discover the default configuration files. Can you define the purpose of each file ? 566
- 573. Introduction to DNS 2a. On Fedora: [root@fedora14 ~]# ls -ld /etc/named* drwxr-x---. 2 root named 4096 Feb 18 16:07 /etc/named -rw-r-----. 1 root named 1008 Jul 19 2010 /etc/named.conf -rw-r--r--. 1 root named 2544 Feb 18 16:07 /etc/named.iscdlv.key -rw-r-----. 1 root named 931 Jun 21 2007 /etc/named.rfc1912.zones -rw-r--r--. 1 root named 487 Jul 19 2010 /etc/named.root.key [root@fedora14 ~]# ls -l /var/named/ total 28 drwxrwx---. 2 named named 4096 Feb 18 16:07 data drwxrwx---. 2 named named 4096 Feb 18 16:07 dynamic -rw-r-----. 1 root named 1892 Feb 18 2008 named.ca -rw-r-----. 1 root named 152 Dec 15 2009 named.empty -rw-r-----. 1 root named 152 Jun 21 2007 named.localhost -rw-r-----. 1 root named 168 Dec 15 2009 named.loopback drwxrwx---. 2 named named 4096 Feb 18 16:07 slaves 2. On Ubuntu: root@ubu1010srv:~# ls -l /etc/bind total 52 -rw-r--r-- 1 root root 601 2011-02-23 16:22 bind.keys -rw-r--r-- 1 root root 237 2011-02-23 16:22 db.0 -rw-r--r-- 1 root root 271 2011-02-23 16:22 db.127 -rw-r--r-- 1 root root 237 2011-02-23 16:22 db.255 -rw-r--r-- 1 root root 353 2011-02-23 16:22 db.empty -rw-r--r-- 1 root root 270 2011-02-23 16:22 db.local -rw-r--r-- 1 root root 2994 2011-02-23 16:22 db.root -rw-r--r-- 1 root bind 463 2011-02-23 16:22 named.conf -rw-r--r-- 1 root bind 490 2011-02-23 16:22 named.conf.default-zones -rw-r--r-- 1 root bind 165 2011-02-23 16:22 named.conf.local -rw-r--r-- 1 root bind 572 2011-02-23 16:22 named.conf.options -rw-r----- 1 bind bind 77 2011-05-15 17:52 rndc.key -rw-r--r-- 1 root root 1317 2011-02-23 16:22 zones.rfc1918 3. Setup caching only dns server. This is normally the default setup. A caching-only name server will look up names for you and cache them. Most tutorials will tell you to add a forwarder, so we first try without this! root@ubu1010srv:/var/log# nslookup > server 192.168.1.37 Default server: 192.168.1.37 Address: 192.168.1.37#53 > > slashdot.org Server: 192.168.1.37 Address: 192.168.1.37#53 Non-authoritative answer: Name: slashdot.org Address: 216.34.181.45 Hey this seems to work without a forwarder. Using a sniffer you can find out what really happens (since the server is not using a cache, not using your dns-server (from / etc/resolv.conf). So where is this information coming from, and what can you learn from sniffing this dns traffic ? 4. Explain in detail what happens when you enable a caching only dns server without forwarder. This wireshark screenshot can help, but you learn more by sniffing the traffic yourself! I will choose two volunteers to explain this in front of the class. 567
- 574. Introduction to DNS 568
- 575. Introduction to DNS 55.18. Practice: caching only with forwarder 5. Add a local dns-server as a forwarder (at my home this is 192.168.1.1, probably different ip in a classroom!). root@ubu1010srv:~# grep -A2 forwarder /etc/bind/named.conf.options| t ail -3 forwarders { 192.168.1.1; }; root@ubu1010srv:~# /etc/init.d/bind9 restart * Stopping domain name service... bind9 [ OK ] * Starting domain name service... bind9 [ OK ] root@ubu1010srv:~# 6. Explain the purpose of adding the forwarder. What is our DNS server doing when it receives a query ? Again the wireshark screenshot can help, you should see something similar. root@ubu1010srv:~# nslookup > server Default server: 192.168.1.4 Address: 192.168.1.4#53 > server 192.168.1.37 Default server: 192.168.1.37 Address: 192.168.1.37#53 > > cobbaut.be Server: 192.168.1.37 Address: 192.168.1.37#53 Non-authoritative answer: Name: cobbaut.be Address: 88.151.243.8 7. What happens when you query for the same domain name more than once ? 569
- 576. Introduction to DNS 8. Why does it say "non-authoritative answer" ? When is a dns server authoritative ? 9. You can also use dig instead of nslookup. dig @192.168.1.37 linux-training.be 10. How can we avoid having to set the server in dig or nslookup ? root@ubu1010srv:~# cat /etc/resolv.conf nameserver 127.0.0.1 11. When you use dig for the first time for a domain, where is the answer coming from ? And the second time ? How can you tell ? 570
- 577. Introduction to DNS 55.19. Practice: primary authoritative server 1. Instead of only cachng the information from other servers, we will now make our server authoritative for our own domain. 2. I choose the new TLD .paul and the domain cobbaut.paul and put the information in /etc/bind/named.conf.local. root@ubu1010srv:/etc/bind# grep -C1 cobbaut named.conf.local zone "cobbaut.paul" { type master; file "/etc/bind/db.cobbaut.paul"; }; 3. Also add a zone database file, similar to this one (add some A records for testing). Set the Refresh and Retry values not too high so you can sniff this traffic (this example makes the slave server contact the master every 300 seconds). root@ubu1010srv:/etc/bind# cat db.cobbaut.paul ; ; BIND data file for domain cobbaut.paul ; $TTL 604800 @ IN SOA ns.cobbaut.paul. root.cobbaut.paul. ( 20110516 ; Serial 300 ; Refresh 200 ; Retry 2419200 ; Expire 604800 ) ; Negative Cache TTL ; @ IN NS ns.cobbaut.paul. ns IN A 192.168.1.37 ubu1010srv IN A 192.168.1.37 anya IN A 192.168.1.1 mac IN A 192.168.1.30 root@ubu1010srv:/etc/bind# 4. Restart the DNS server and check your zone in the error log. root@ubu1010srv:/etc/bind# grep cobbaut /var/log/daemon.log May 16 00:33:49 ubu1010srv named[25449]: zone cobbaut.paul/IN: loaded serial 20110516 5. Use dig or nslookup (or even ping) to test your A records. root@ubu1010srv:/etc/bind# ping mac.cobbaut.paul PING mac.cobbaut.paul (192.168.1.30) 56(84) bytes of data. 64 bytes from 192.168.1.30: icmp_req=1 ttl=64 time=2.28 ms 64 bytes from 192.168.1.30: icmp_req=1 ttl=64 time=2.31 ms (DUP!) ^C --- mac.cobbaut.paul ping statistics --- 1 packets transmitted, 1 received, +1 duplicates, 0% packet loss, time 0ms rtt min/avg/max/mdev = 2.282/2.296/2.310/0.014 ms root@ubu1010srv:/etc/bind# dig anya.cobbaut.paul ; <<>> DiG 9.7.1-P2 <<>> anya.cobbaut.paul ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 38237 571
- 578. Introduction to DNS ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 1 ;; QUESTION SECTION: ;anya.cobbaut.paul. IN A ;; ANSWER SECTION: anya.cobbaut.paul. 604800 IN A 192.168.1.1 ;; AUTHORITY SECTION: cobbaut.paul. 604800 IN NS ns.cobbaut.paul. ;; ADDITIONAL SECTION: ns.cobbaut.paul. 604800 IN A 192.168.1.37 ;; Query time: 1 msec ;; SERVER: 127.0.0.1#53(127.0.0.1) ;; WHEN: Mon May 16 00:38:22 2011 ;; MSG SIZE rcvd: 84 root@ubu1010srv:/etc/bind# 6. Our primary server appears to be up and running. Note the information here: server os : Ubuntu 10.10 ip : 192.168.1.37 domain name: cobbaut.paul server name: ns.cobbaut.paul 572
- 579. Introduction to DNS 55.20. Practice: reverse DNS 1. We can add ip to name resolution to our dns-server using a reverse dns zone. 2. Start by adding a .arpa zone to /etc/bind/named.conf.local like this (we set notify to no to avoid sending of notify messages to other name servers): root@ubu1010srv:/etc/bind# grep -A4 arpa named.conf.local zone "1.168.192.in-addr.arpa" { type master; notify no; file "/etc/bind/db.192"; }; 3. Also create a zone database file for this reverse lookup zone. root@ubu1010srv:/etc/bind# cat db.192 ; ; BIND reverse data file for 192.168.1.0/24 network ; $TTL 604800 @ IN SOA ns.cobbaut.paul root.cobbaut.paul. ( 20110516 ; Serial 604800 ; Refresh 86400 ; Retry 2419200 ; Expire 604800 ) ; Negative Cache TTL ; @ IN NS ns. 37 IN PTR ns.cobbaut.paul. 1 IN PTR anya.cobbaut.paul. 30 IN PTR mac.cobbaut.paul. root@ubu1010srv:/etc/bind# 4. Test with nslookup or dig: root@ubu1010srv:/etc/bind# dig 1.168.192.in-addr.arpa AXFR 573
- 580. Introduction to DNS 55.21. Practice: a DNS slave server 1. A slave server transfers zone information over the network from a master server (a slave can also be a master). A primary server maintains zone records in its local file system. As an exercise, and to verify the work of all students, set up a slave server of all the master servers in the classroom. 2. Before configuring the slave server, we have to allow transfers from our zone to this server. Remember that this is not very secure since transfers are in clear text and limited to an ip address. This example follows our demo from above. The ip of my slave server is 192.168.1.31, yours is probably different. root@ubu1010srv:/etc/bind# grep -A2 cobbaut named.conf.local zone "cobbaut.paul" { type master; file "/etc/bind/db.cobbaut.paul"; allow-transfer { 192.168.1.31; }; }; root@ubu1010srv:/etc/bind# 3. My slave server is running Fedora 14. Bind configuration files are only a little different. Below the addition of a slave zone to this server, note the ip address (192.168.1.37) of my master dns server for the cobbaut.paul zone. [root@fedora14 etc]# grep cobbaut -A2 named.conf zone "cobbaut.paul" { type slave; file "/var/named/slaves/db.cobbaut.paul"; masters { 192.168.1.37; }; }; [root@fedora14 etc]# 4. You might need to add the ip-address of the server on Fedora to allow queries other than from localhost. [root@fedora14 etc]# grep 127 named.conf listen-on port 53 { 127.0.0.1; 192.168.1.31; }; 5. Restarting bind on the slave server should transfer the zone database file: [root@fedora14 etc]# ls -l /var/named/slaves/ total 4 -rw-r--r--. 1 named named 387 May 16 03:23 db.cobbaut.paul [root@fedora14 etc]# 574
- 581. Introduction to DNS 55.22. DNS History Today, DNS or Domain Name System is a worldwide distributed hierarchical database. It's primary function is to resolve names to ip addresses, and to point to internet servers providing SMTP or LDAP services. In the seventies, only a few hundred computers were connected to the internet. To resolve names, computers had a flat file that contained a table to resolve hostnames to ip-addresses. This local file was downloaded from hosts.txt on an ftp server in Stanford. In 1984 Paul Mockapetris created DNS, a distributed treelike hierarchical database. Today ICANN controls DNS. 55.23. forward lookup zone example The way to set up zones in /etc/named.conf is to create a zone entry with a reference to another file located in /var/named. Here is an example of such an entry in /etc/named.conf zone "classdemo.local" IN { type master; file "classdemo.local.zone"; allow-update { none; }; }; To create the zone file, the easy method is to copy an existing zone file (this is easier than writing from scratch). [root@RHEL4b named]# cd /var/named/ [root@RHEL4b named]# pwd /var/named [root@RHEL4b named]# cp localhost.zone classdemo.local.zone [root@RHEL4b named]# Here is an example of a zone file. [root@RHEL4b named]# cat classdemo.local.zone $TTL 86400 $ORIGIN classdemo.local. @ IN SOA rhel4b.classdemo.local. admin.classdemo.local. ( 2007083100 ; serial 3H ; refresh 900 ; retry 1W ; expiry 1D ) ; minimum IN NS rhel4b.classdemo.local. IN MX 10 mail.classdemo.local. IN A 192.168.1.191 rhel4b IN A 192.168.1.191 mail IN A 192.168.1.191 575
- 582. Introduction to DNS www IN A 192.168.1.191 ftp IN A 192.168.1.191 server2 IN A 192.168.1.1 576
- 583. Chapter 56. naamresolutie en DNS Table of Contents 56.1. inleiding ...................................................................................................... 577 56.2. logische DNS structuur .............................................................................. 580 56.3. DNS caching .............................................................................................. 581 56.4. praktijkvoorbeeld ....................................................................................... 582 56.5. DNS records ............................................................................................... 582 56.6. DNS reverse lookup ................................................................................... 582 56.7. zone ............................................................................................................ 582 56.8. DNS server software .................................................................................. 583 56.9. name resolving ........................................................................................... 583 56.10. DNS IDN ................................................................................................. 583 56.11. Domeinnamen registreren ........................................................................ 583 56.12. DNS round robin ..................................................................................... 583 56.13. tekening DNS ........................................................................................... 583 56.14. tekening DNS en routerke ....................................................................... 585 56.15. screenshots DNS in actie ......................................................................... 585 56.16. oefeningen DNS ....................................................................................... 588 56.1. inleiding Wat is naamresolutie ? Om twee computers op een ip netwerk (zoals het internet) met elkaar te laten communiceren, hebben ze elkaars ip-adres nodig. Maar als mens is het eenvoudiger om namen zoals linux-training.be te onthouden i.p.v. 88.151.243.8 of 178.63.30.100. Naamresolutie is het zoeken van een ip-adres dat hoort bij deze naam van een computer. Een computer kan beschikken over een tekstbestand dat de relatie tussen namen en ip-adressen bevat, of kan verbonden zijn met een nameserver, bijvoorbeeld een dns server. geschiedenis Voordat dns bestond, gebruikten computers op arpanet een HOSTS.TXT bestand dat op een centrale server stond in Stanford. Dit bestand werd op regelmatige tijden gekopieerd naar de lokale machine. Dit systeem was op termijn onhoudbaar, zeker toen er begin jaren 80 duizenden computernamen in het bestand stonden en de groei exponentieel werd. In 1983 ontwikkelde de Griek Paul Mockapetris het Domain Name System (rfc 882/883, later rfc 1034/1035) als alternatief voor dit centrale hosts bestand. 577
- 584. naamresolutie en DNS dns is een gedistribueerde hiërarchische naamgevende database voor diensten en computers op tcp/ip netwerken. Een van de voor de hand liggende functies is het vertalen van host- en domeinnamen naar ip-adressen (net zoals het /etc/hosts bestand dat al deed). dns is al enkele keren uitgebreid (tik maar eens dns als zoekterm op www.rfc- editor.org), onder andere om het veilig te maken en om dns compatibel te maken met ipv6 (AAAA records). /etc/hosts We hebben vorige les reeds kennis gemaakt met het bestand /etc/hosts (alle Unix, incluis Linux en MacOSX) of %systemroot%/system32/drivers/etc/hosts (Microsoft). We hebben gezien hoe dit een tabel is die namen vertaalt naar ip- adressen. De inhoud van het bestand ziet er bijvoorbeeld zo uit: paul@laika:~$ cat /etc/hosts 127.0.0.1 localhost 127.0.1.1 laika 192.168.1.1 illyria 192.168.1.2 pasha 192.168.1.33 barry 192.168.1.34 laika 192.168.1.35 faith 192.168.1.36 wolf 192.168.1.38 rekkie 192.168.1.44 kiss 192.168.1.29 hero Dankzij dit bestand zal een ping commando naar een van deze namen resulteren in een ping naar het ip-adres dat voor deze naam staat. paul@laika:~$ ping illyria PING illyria (192.168.1.1) 56(84) bytes of data. 64 bytes from illyria (192.168.1.1): icmp_seq=1 ttl=254 time=0.596 ms 64 bytes from illyria (192.168.1.1): icmp_seq=2 ttl=254 time=0.588 ms Meer info over het hosts bestand vind je hier: http://en.wikipedia.org/wiki/Hosts_file volgorde Met twee technische oplossingen voor hetzelfde probleem (hosts tekstbestand en dns) moeten er duidelijke afspraken zijn over de prioriteit die beide systemen krijgen. Met andere woorden, als we een ping doen naar computer42, wordt dan eerst dns geraadpleegd ? Of eerst /etc/hosts ? Om dit te configureren moeten we een onderscheid maken tussen Microsoft, Mac OS X en de rest (Unix/Solaris/Linux/BSD...) 578
- 585. naamresolutie en DNS Microsoft Op Microsoft systemen is er geen optie om de volgorde in te stellen. Het lokale hosts bestand heeft volgens Microsoft altijd voorrang op dns. (Zie http:// support.microsoft.com/kb/172218 en gelijkaardige artikels voor W200X etc). De hostnames in deze lijst werken niet op Microsoft computers als je ze plaatst in het hosts bestand: www.msdn.com msdn.com www.msn.com msn.com go.microsoft.com msdn.microsoft.com office.microsoft.com microsoftupdate.microsoft.com wustats.microsoft.com support.microsoft.com www.microsoft.com microsoft.com update.microsoft.com download.microsoft.com microsoftupdate.com windowsupdate.com windowsupdate.microsoft.com Meer info hierover vind je hier: http://archive.cert.uni-stuttgart.de/bugtraq/2006/04/msg00291.html We hebben ook commando's als ipconfig /all, ipconfig /displaydns en ipconfig / flushdns gezien in de vorige lessen. ipconfig /all toont je heel wat informatie over je ip-conguratie, incluis het ip-adres van je dns-server. ipconfig /displaydns geeft een lijst van namen die recent vertaald zijn aar ip- adressen, en hoelang ze nog in de lokale cache blijven zitten. ipconfig /flushdns maakt de lokale dns cache leeg. Mac OS X Mac OS X gebruikt hiervoor twee .plist bestanden in de directory /Library/ Preferences/DirectoryService/ . Unix Linux Solaris BSD Zowat alle andere besturingssystemen gebruiken een name service switch bestand genaamd /etc/nsswitch.conf . In dit bestand staat voor heel wat name services de volgorde waarin verschillende diensten gebruikt worden. 579
- 586. naamresolutie en DNS Zo zal bijvoorbeeld de volgende lijn bepalen dat eerst het lokale hosts bestand moet geraadpleegd worden, en dan pas dns. paul@laika:~$ grep hosts /etc/nsswitch.conf hosts: files dns Als je dit omdraait naar dns files dan krijgt dns onmiddellijk voorrang op /etc/hosts. Sommige applicaties zullen dit bestand ook aanmaken op Mac OS X. 56.2. logische DNS structuur DNS root servers De namespace van dns is een boomstructuur met bovenaan een puntje. Dit puntje wordt de root van de dns namespace genoemd. Deze nameservers zijn DNS root servers, en worden in de (IT)volksmond ook gewoon root servers of dot servers genoemd. Er zijn logisch gezien 13 root name servers op het internet genaamd A-root tot M-root. De inhoud ervan wordt bepaald door ICANN, het beheer is in handen van verschillende organisaties (kijk maar op http://root-servers.org) en ze werken op eentje na allemaal met de bind software. Elke root server bestaat uit meerdere fysieke machines. Zo is de K-root server verspreid over alle continenten en aanwezig in 18 steden. De J-root server heeft 62 nodes. http://root-servers.org/ Journalisten noemen dit wel eens de root servers of master servers van het internet. In 2002 en in 2007 was er een poging om alle root servers plat te leggen met een ddos (distributed denial of service) aanval. http://en.wikipedia.org/wiki/Distributed_denial_of_service_attacks_on_root_nameservers top level domains Onder de dot servers vinden we de top level domains of TLD's. Oorspronkelijk waren dit er zeven generieke: .COM .NET .ORG .MIL .GOV .EDU 580
- 587. naamresolutie en DNS .INT (nato.int) Er zijn ook enkele honderden TLD's voor individuele landen, zoals .BE .NL .FR .US. Tussen haakjes, domeinnamen zijn hoofdletterongevoelig dus je kan zowel .be als .BE schrijven. Sinds enkele jaren zijn er enkele nieuwe generieke TLD's bijgekomen zoals .aero, .biz, .coop, .info, .museum, .name en .pro , maar deze worden behalve .info nauwelijks gebruikt. domeinnamen of domeinen Onder de TLD's in de boomstructuur vinden we dan de domeinnamen zoals google.com of linux-training.be. Indien we op een dns server een domeinnaam instellen, noemen we dit een zone. fqdn De combinatie van een hostname met een domeinnaam noemt men de fqdn ofte Fully Qualified Domain Name. De computer genaamd rhel5 in het classroom.local domein heeft dus rhel5.classroom.local als fqdn. Belgische domein namen Oorspronkelijk stond de Belgische .be server op de eerste verdieping van het computerwetenschappengebouw van de KULeuven in de Celestijnenlaan. De beheerder was professor Verbaeten en de enige manier om een .be te registreren was door een brief met BTW-nr te schrijven naar dit departement. Op 1 januari 2000 werd dotbe overgenomen door de vzw dns.be. Op 11 december 2000 was er een serieuze liberalisering van de regels voor aanvraag van een Belgische domeinnaam, vanaf dan konden voor het eerst ook particulieren hun eigen naam registreren. Voor meer info: http://dns.be/nl/home.php?n=22 56.3. DNS caching dns is een caching protocol. Je kan zelfs caching only servers installeren. Dat zijn dns servers die zelf geen autoriteit hebben over een domeinnaam. 581
- 588. naamresolutie en DNS Het adsl routerke dat bij jullie thuis staat, is waarschijnlijk een caching only dns server. 56.4. praktijkvoorbeeld De rol van DNS (en andere servers) bij het bezoeken van een website of het sturen van een e-mail tekenen we even op het bord. 56.5. DNS records In de dns database zitten verschillende resource records. Hieronder een beknopt overzicht van enkele van deze records. A : vertaalt een naam naar een ip-adres - ook wel host record genoemd - of ook forward lookup record AAAA : idem voor ipv6 NS : Nameserver record, wijst naar een nameserver MX : Mail Exchange record, wijst naar een mailserver SOA : bevat TTL, serienummer en andere info CNAME : een alias van een naam naar een andere naam SRV : wijst naar een dienst (of service) op het netwerk 56.6. DNS reverse lookup Al de records uit de vorige lijst vind je terug in een forward lookup zone, een zone die namen vertaalt naar ip-adressen. Een reverse lookup zone is het omgekeerde, en vertaalt dus ip-adressen naar namen. Deze zone bevat naast SOA en NS records vooral PTR records. PTR : vertaalt ip-adres naar naam - ook wel reverse lookup record genoemd 56.7. zone Een zone is een domeinnaam die op een dns server ingesteld is als primair (aanpasbaar) of secundair (read only copy). Een dns server kan verantwoordelijk zijn voor de inhoud (de resource records) van een bepaalde zone (aka domeinnaam), deze dns server heeft dan autoriteit over deze zone. Een zone is ofwel een forward lookup zone zoals linux-training.be of classroom.local, ofwel een reverse lookup zone. Zones kunnen primary of secondary zijn. Primary zones zitten op DNS servers en kunnen daar aangepast worden (records in de zone kunnen aanepast worden). Secondary zones krijgen kopies van een primary zone (en kunnen bijgevolg niet rechtstreeks aangepast worden). 582
- 589. naamresolutie en DNS 56.8. DNS server software Heel wat name servers op internet maken gebruik van de bind software. In het intranet van bedrijven wordt hiernaast ook regelmatig Microsoft DNS Server gebruikt ter ondersteuning van Microsoft servers. Dankzij de rfc's kunnen beide producten samenwerken. 56.9. name resolving Je computer vindt een DNS server dankzij zijn ip-adres dat genoteerd staat in /etc/ resolv.conf (of in de registry voor Microsoft clients). Meestal voorziet een DHCP server de client van het ip-adres van een DNS server. DNS queries van een client naar een server zijn recursief, clients verwachten een volledig antwoord. Queries tussen DNS servers onderling kunnen iteratief zijn. Iteratieve queries verwachten een referentie naar een andere server i.p.v. een volledig antwoord. De root servers antwoorden niet op recursieve queries. Forward lookup queries kennen een naam en vragen een ip-adres, reverse lookup queries kennen het ip-adres en willen de naam weten. Een zone zoals linux- training.be is een forward lookup zone. 56.10. DNS IDN unicode i.p.v. ascii, spoofing en squatting van homoniemen of homografen nog niet mogelijk in .be ! 56.11. Domeinnamen registreren Bij dns.be als je registrar ben, anders via je isp of via je registrar. 56.12. DNS round robin DNS kan de workload van een website verdelen over meerdere fysieke servers. 56.13. tekening DNS Hieronder een DNS tekening zoals ik ze gemaakt heb in de klas. 0. Ik surf thuis op mijn laptop naar http://www.linux-training.be, we volgen het DNS verhaal om tot aan de webserver te geraken waar deze website staat. De 583
- 590. naamresolutie en DNS laptop staat helemaal links op de tekening, de webserver staat helemaal rechts (ip 88.151.243.8). De webserver heeft meerdere namen (meerdere A records wijzen naar deze webserver). Voor deze test heb ik even rechtstreeks het ip-adres van de EDPNET DNS server (212.71.8.10) in de laptop gezet. Alternatief had ik ook het ip-adres van mijn routerke (alias dns/dhcp/adslmodem/nat) kunnen ingeven, een situatie die je kan bekijken in de volgende tekening. Hieronder dezelfde tekening zoals ik ze vorig jaar heb gemaakt met Dia (nu gebruik ik Inkscape). 1. De laptop vraagt aan de DNS server van mijn ISP EDPNET de A record van www.linux-training.be. We gaan er even van uit dat ik de eerste ben vandaag die naar deze website surft. DNS is een caching protocol, dus als vijf minuten geleden een andere klant van EDPNET naar deze website is geweest, dan antwoord de DNS server gewoon uit zijn cache. 2. De DNS server van EDPNET vraagt in een iteratieve query aan een root server (hier de H root server in de USA) waar zich www.linux-training.be bevindt. 3. De root server antwoordt met een verwijzing naar de DNS server die verantwoordelijk is voor de hele .be zone. Als je snift dan zie je dat er een hele lijst van .be servers wordt gegeven en kan je zien dat deze lijst zowel namen als IP- addressen bevat. 4. De DNS server van EDPNET vraagt, wederom iteratief, waar www.linux- training.be is aan een van de .be DNS servers (de eerste uit de lijst van de vorige stap was a.ns.dns.be). 5. De .be DNS server antwoordt met een reeks namen en bijhorende ip-adressen van openminds (het hosting bedrijf waar de www.linux-training.be) webserver staat. 6. De DNS server van EDPNET heeft nu de DNS server gevonden die verantwoordelijk (authoritative) is voor linux-training.be en vraagt aan deze de A record voor www.linux-training.be. 7. De DNS server ns1.openminds.be antwoordt met de gezochte A record 88.151.243.8. 8. De DNS server van EDPNET geeft dit antwoord aan de laptop. De laptop kan nu rechtstreeks de webserver bereiken (maar dat is een ander verhaal). In dit verhaal heeft de laptop een recursieve query gedaan, hij wil een volledig antwoord. De root servers en de .be servers antwoorden niet op recursieve queries, enkel op iteratieve! In dit verhaal werden enkel udp packetjes gebruikt, er is dus nergens een tcp sessie opgezet. 584
- 591. naamresolutie en DNS 56.14. tekening DNS en routerke Deze tekening is nagenoeg de zelfde als de vorige, alleen hebben we nu ons thuis routerke/adsl-modem/nat/dhcp-ke ertussen gezet. Op de macbook staat nu dus 192.168.1.1 als DNS server. Je weet (hopelijk) dat het routerke twee ip-adressen heeft, eentje aan de binnenkant (de kant van het thuisnetwerk 192.168.1.1) en eentje aan de buitenkant (de internet- kant). 0. De laptop doet een recursieve DNS query naar het routerke. 1. Het routerke doet een recursieve DNS query naar de DNS server van de ISP. 2. tot 7. De DNS server van de ISP EDPNET doet al het werk. 8. Het routerke krijgt een definitief antwoord, en houdt dit antwoord bij in zijn cache. 9. De laptop krijgt een definitief antwoord van het routerke. Als even later een andere (thuis)computer dezelfde vraag stelt aan het routerke, dan kan dit antwoorden vanuit zijn cache. 56.15. screenshots DNS in actie demo DNS Hieronder enkele screenshots van het opzetten en testen van een DNS server op Red Hat Enterprise Linux 5.3 en Microsoft Windows Server 2003. Om de twee eens naast elkaar te zetten heb ik geopteerd om de DNS setup op RHEL5 ook grafisch te doen, ook al zullen de meeste Unix sysadmins dit eerder doen door een bestaande zone database file (die in de laatste screenshot getoond wordt) te copieren en aan te passen. Beide OS-sen zijn geinstalleerd in een Virtualbox virtuele machine. software installeren We zoeken in beide OS-sen de GUI om software te installeren. Microsoft noemt dit Add or Remove Programs. Gemakshalve zijn we hier aangelogd als Administrator. 585
- 592. naamresolutie en DNS Red Hat noemt dit Add/Remove Software. Ook hier zijn we gemakshalve aangelogd als root. Alternatief kan je op Linux gebruik maken van de command line yum of aptitude. root@RHEL5:~# yum install bind root@debian:~# aptitude install bind9 installatie wizard Microsoft Windows beschouwt Domain Name System als een Windows Component die je gratis krijgt bij aankoop van een Windows Server. Red Hat gebruikt de vrije applicatie genaamd BIND. BIND is sinds de ontwikkeling van DNS in 1983 de meest gebruikte DNS server. GUI tool voor DNS beheer Microsoft heeft samen met de DNS server software ook een snapin voor DNS server beheer geinstalleerd en toegevoegd aan het start menu. Red Hat heeft ook een GUI DNS tool, maar deze moet je wel apart installeren. Unix mensen zijn al jaren gewoon om rechtstreeks met de text bestanden te werken. DNS voor zichzelf Hierzie de tcp/ip properties zodat Windows DNS server kan zijn voor zichzelf. En we doen hetzelfde op Red Hat. Alternatief kunnen we dit op zowat elke Unix ook zo doen: echo nameserver 192.168.1.200 > /etc/resolv.conf nieuwe zone maken Microsoft voorziet een wizard om een nieuwe zone aan te maken. In tegenstelling tot Windows 2000 Server wordt standaard geen root-zone meer aangemaakt. 586
- 593. naamresolutie en DNS BIND levert standaard een aantal zones mee. Zoals eerder gezegd, zullen Unix mensen meestal een bestaande zone-bestand copieren en aanpassen. zone database file Microsoft geeft hier mooi de naam van de zone database file. Hoewel de naam vrij te kiezen is, is het formaat van dit tekstbestand wel vastgelegd in een rfc. Red Hat vraagt eveneens de naam van de aan te maken zone, en gebruikt deze naam om de zone database file aan te maken. nslookup Hier gebruiken we nslookup om de DNS server te testen. We vragen de A record van w2003.classdemo.local en www.classdemo.local (die laatste bestaat niet). Microsoft start vanzelf de DNS server, Red Hat wacht totdat we dit zelf doen. U ziet wederom nslookup met twee queries voor twee A records. A record Na aanmaken van de A record, wordt die ook geresolved door nslookup. Na aanmaken van de A record, wordt die ook geresolved door nslookup. MX record Een MX record wijst naar de smtp server van een domeinnaam. Deze mailserver kan zelf in een andere domein zitten. In de nslookup prompt kan je set type=MX typen om MX records te vinden. reverse lookup Zonder reverse lookup zone kan je geen ip-to-name vertaling doen. Met een PTR record in een reverse lookup zone kan dat wel. 587
- 594. naamresolutie en DNS Op Linux wordt behalve nslookup ook meer en meer gebruikt gemaakt van dig. zone database file formaat Het formaat van de zone database file is vastgelegd in rfc 1034 (en opvolgers). Knippen en plakken in deze files kan zelfs tussen Unix en Microsoft systemen. 56.16. oefeningen DNS 1. Noteer hieronder welke DNS server je nu gebruikt. Je kan dit in Windows vinden met ipconfig /all of met de Control Panel GUI, in Linux met cat /etc/resolv.conf. IP adres DNS server : 2. Toon de lokale dns-cache op Windows met ipconfig /displaydns, ledig deze cache met ipconfig /flushdns (Standaard Linux isntallaties hebben geen lokale dns-cache). 3. Forceer een lookup van een A-record (je hebt toch genoteerd wat een A-record is in de vorige les?). Welke packetjes zie je op het netwerk alvorens er een antwoord komt op deze query ? 4. Welke poort gebruikt de client om de DNS server te bereiken ? 5. Gebruikt DNS tcp of udp ? 6. Wat is een AAAA record ? 7. Waarom zie je soms eerst een AAAA query voor een A query ? 8. Start nslookup interactief. Wat kan je met deze tool doen ? 9. Toon een A record via nslookup. 10. Verander van DNS-server (enkel in nslookup) naar de RHEL5 server (ip-adres staat op bord). 11. Wat is het ip-adres van www.classdemo.local volgens deze server ? 588
- 595. naamresolutie en DNS 12. Hoe kan je deze DNS server als default instellen in Windows XP ? 13. Kan je nog op internet als je deze DNS-server instelt in Windows XP ? 14. Is dns een internet standaard ? Zo ja, waar is die neergeschreven ? 589
- 596. Chapter 57. advanced DNS Table of Contents 57.1. DNS round robin ....................................................................................... 591 57.2. DNS delegation .......................................................................................... 592 57.3. DNS load balancing ................................................................................... 593 57.4. DNS notify ................................................................................................. 593 57.5. testing IXFR and AXFR ............................................................................ 593 57.6. DDNS integration with DHCP .................................................................. 593 57.7. reverse is forward in-addr.arpa .................................................................. 594 57.8. ipv6 ............................................................................................................. 594 57.9. split-horizon dns ......................................................................................... 594 57.10. DNS security : file corruption .................................................................. 594 57.11. DNS security : zone transfers .................................................................. 594 57.12. DNS security : zone transfers, ip spoofing .............................................. 595 57.13. DNS security : queries ............................................................................. 595 57.14. DNS security : chrooted bind .................................................................. 595 57.15. DNS security : DNSSEC ......................................................................... 595 57.16. DNS security : root .................................................................................. 596 590
- 597. advanced DNS 57.1. DNS round robin When you create multiple A records for the same name, then bind will do a round robin of the order in which the records are returned. This allows the use of DNS as a load balancer between hosts, since clients will usually take the first ip-address offered. This is what it looks like in the zone configuration file. faith IN A 192.168.1.20 faith IN A 192.168.1.22 Below a screenshot of nslookup querying a load balanced A record. Notice the order of ip-addresses returned. > server 192.168.1.35 Default server: 192.168.1.35 Address: 192.168.1.35#53 > faith.cobbaut.paul Server: 192.168.1.35 Address: 192.168.1.35#53 Name: faith.cobbaut.paul Address: 192.168.1.20 Name: faith.cobbaut.paul Address: 192.168.1.22 > faith.cobbaut.paul Server: 192.168.1.35 Address: 192.168.1.35#53 Name: faith.cobbaut.paul Address: 192.168.1.22 Name: faith.cobbaut.paul Address: 192.168.1.20 > faith.cobbaut.paul Server: 192.168.1.35 Address: 192.168.1.35#53 Name: faith.cobbaut.paul Address: 192.168.1.20 Name: faith.cobbaut.paul Address: 192.168.1.22 591
- 598. advanced DNS 57.2. DNS delegation You can delegate a child domain to another DNS server. The child domain then becomes a new zone, with authority at the new dns server. This is a screenshot of the zone database file with delegation. root@ubu1010srv:/etc/bind# cat db.linux-training.be $TTL 3d ; default ttl set to three days $ORIGIN linux-training.be. @ IN SOA ns1.linux-training.be. paul.linux-training.be. ( 20110524 300 300 10000 20000 ) IN NS ns1.linux-training.be. IN NS ns2.linux-training.be. IN NS ns3.linux-training.be. IN MX 10 smtp.openminds.be. ns1 IN A 192.168.1.35 ns2 IN A 192.168.1.36 ns3 IN A 192.168.1.37 www IN A 192.168.1.35 mac IN A 192.168.1.30 $ORIGIN office.linux-training.be. @ IN NS ns4.office.linux-training.be. ; or replace those two lines with: ; office.linux-training.com IN NS ns4.office.linux-training.be IN NS ns1.linux-training.be. ; in case this is a slave ns4 IN A 192.168.1.33 ; the glue record ; ns4.office.linux-training.be A 192.168.1.33 ; also ok! 592
- 599. advanced DNS 57.3. DNS load balancing Not as above. When you have more than one DNS server authoritative for a zone, you can spread queries amongst all server. One way to do this is by creating NS records for all servers that participate in the load balancing of external queries. You could also configure different name servers on internal clients. 57.4. DNS notify The original design of DNS in rfc 1034 and rfc 1035 implemented a refresh time in the SOA record to configure a time loop for slaves to query their master server. This can result in a lot of useless pull requests, or in a significant lag between updates. For this reason dns notify (rfc 1996) was designed. The server will now notify slaves whenever there is an update. By default this feature is activated in bind. Notify can be disabled as in this screenshot. zone "1.168.192.in-addr.arpa" { type master; notify no; file "/etc/bind/db.192"; }; 57.5. testing IXFR and AXFR Full zone transfers (AXFR) are initiated when you restart the bind server, or when you manually update the zone database file directly. With nsupdate you can update a zone database and initiate an incremental zone transfer. You need DDNS allowed for nsupdate to work. root@ubu1010srv:/etc/bind# nsupdate > server 127.0.0.1 > update add mac14.linux-training.be 86400 A 192.168.1.23 > send update failed: REFUSED 57.6. DDNS integration with DHCP Some organizations like to have all their client computers in DNS. This can be cumbersome to maintain. Luckily rfc 2136 describes integration of DHCP servers with a DNS server. Whenever DHCP acknowledges a client ip configuration, it can notify DNS with this clients ip-address and name. This is called dynamic updates or DDNS. 593
- 600. advanced DNS 57.7. reverse is forward in-addr.arpa Reverse lookup is actually iomplemented as a forward lookup in the in-addr.arpa domain. This domain has 256 child domains (from 0.in-addr.arpa to 255.in- addr.arpa), with each child domain having again 256 child domains. And this twice more to a structure of over four billion (2 to the power 32) domains. 57.8. ipv6 With rfc 3596 came ipv6 extensions for DNS. There is the AAAA record for ipv6 hosts on the network, and there is the ip6.int domain for reverse lookup (having 16 child domains from 0.ip6.int to f.ip6.int, each of those having again 16 child domains...and this 16 times. 57.9. split-horizon dns You can use the view clause in bind to give different results to different clients. view "antwerp" { match-clients { 172.16.42/24; }; // the network in Antwerp zone "cobbaut.paul" { type master; file "/etc/bind/db.cobbaut.paul.antwerp"; // www=172.16.42.9 }; }; view "brussels" { match-clients { 172.16.33/24; }; // the Brussels network zone "cobbaut.paul" { type master; file "/etc/bind/db.cobbaut.paul.brussels"; // www=172.16.33.4 }; }; 57.10. DNS security : file corruption To mitigate file corruption on the zone files and the bind configuration files protect them with Unix permissions and take regular backups. 57.11. DNS security : zone transfers Limit zone transfers to certain ip addresses instead of to any. Nevermind that ip- addresses can be spoofed, still use this. 594
- 601. advanced DNS 57.12. DNS security : zone transfers, ip spoofing You could setup DNSSEC (which is not the easiest to maintain) and with rfc 2845(tsig?) and with rfc 2930(tkey, but this is open to brute force), or you could disable all zone transfers and use a script with ssh to copy them manually. 57.13. DNS security : queries Allow recursion only from the local network, and iterative queries from outside only when necessary. This can be configured on master and slave servers. view "internal" { match-clients { 192.168.42/24; }; recursion yes; ... }; view "external" { match-clients { any; }; recursion no; ... }; Or allow only queries from the local network. options { allow-query { 192.168.42.0/24; localhost; }; }; zone "cobbaut.paul" { allow-query { any; }; }; Or only allow recursive queries from internal clients. options { allow-recursion { 192.168.42.0/24; localhost; }; }; 57.14. DNS security : chrooted bind Most Linux distributions allow an easy setup of bind in a chrooted environment. 57.15. DNS security : DNSSEC DNSSEC uses public/private keys to secure communications, this is described in rfc's 4033, 4034 and 4035. 595
- 602. advanced DNS 57.16. DNS security : root Do not run bind as root. Do not run any application daemon as root. 596
- 603. Part XVIII. dhcp server
- 604. Chapter 58. Introduction to DHCP Table of Contents 58.1. Introduction to dhcp ................................................................................... 599 58.2. vier broadcasts ........................................................................................... 599 58.3. dhcp options ............................................................................................... 599 58.4. installing dhcp ............................................................................................ 599 58.5. tekening DHCP .......................................................................................... 600 58.6. dhcp server op Red Hat Enterprise Linux ................................................. 600 58.7. dhcp server op windows ............................................................................ 601 58.8. dhcp client .................................................................................................. 601 58.9. client reservations ...................................................................................... 601 58.10. 80/20 rule ................................................................................................. 602 58.11. relay agent ................................................................................................ 602 58.12. rogue dhcp servers ................................................................................... 602 58.13. DHCP and DDNS .................................................................................... 602 58.14. Exercise DHCP and DDNS ..................................................................... 603 58.15. Exercise DHCP in Packet Tracer ............................................................. 603 58.16. Example config files ................................................................................ 603 598
- 605. Introduction to DHCP 58.1. Introduction to dhcp DHCP is a standard tcp/ip protocol that distributes ip configurations to clients. DHCP is defined in rfc 2131 (before this time DHCP it was defined as an update to bootp in rfc 1531/1541. The alternative to DHCP is manually entering the ip configuration on each client computer. 58.2. vier broadcasts dhcp werkt met broadcasts. Als een dhcp client opstart, dan stuurt die een DHCPdiscover. Alle dhcp servers antwoorden met een DHCPoffer. De client kiest een van de offers (volgens de rfc het eerste offer) en stuurt een DHCPrequest. De server antwoordt dan (meestal) met een DHCPack(knowledge). U ziet een sniff hiervan hieronder. Pas na deze vier broadcasts mag de client de ip-configuratie die hij gekregen heeft gebruiken. Dit tot aan het einde van de lease periode. 58.3. dhcp options Options can be set on the global, scope, client-reservation level. option subnet-mask 255.255.255.0; option domain-name "linux-training.be"; option domain-name-servers "ns1.openminds.be"; option routers 192.168.42.1; 58.4. installing dhcp On Debian/Ubuntu debian5:~# aptitude install dhcp3-server Reading package lists... Done Building dependency tree Reading state information... Done Reading extended state information Initializing package states... Done Reading task descriptions... Done The following NEW packages will be installed: dhcp3-server You get a configuration file with many examples. debian5:~# ls -l /etc/dhcp3/dhcpd.conf 599
- 606. Introduction to DHCP -rw-r--r-- 1 root root 3551 2011-04-10 21:23 /etc/dhcp3/dhcpd.conf 58.5. tekening DHCP We hebben een klein netwerk met twee servers (DHCP-SRV1 en DHCP-SRV2) en twee clients (SunWS1 en Mac42). In het midden staat een hub (of een switch) om aan te tonen dat deze vier computers op hetzelfde netwerk, hetzelfde segment zitten. Alle vier de computers hebben een kabel naar de hub (niet getekend hier). 1. De client SunWS1 start op en stuurt een DHCPDiscover op het netwerk. Alle computers ontvangen deze broadcast. 2. Beide DHCP servers antwoorden met een DHCPOffer. DHCP-SRV1 is een dedicated DHCP server en is sneller met zijn offer dan DHCP-SRV2 (die ook fileserver is). 3. De client kiest het offer van DHCP-SRV1 en stuurt een DHCPRequest op het netwerk. 4. DHCP-SRV1 antwoordt met een DHCPAck (een acknowledge). Alle vier broadcasts (of vijf want er waren twee offers) zijn een layer 2 ethernet broadcast naar ff:ff:ff:ff:ff:ff en een layer 3 ip broadcast naar 255.255.255.255. Alle vier de computers hebben alle broadcasts ontvangen. Dit verhaal staat ook mooi in rfc 2131. 58.6. dhcp server op Red Hat Enterprise Linux Eerste stap is om even te kijken in het /etc/dhcpd.conf bestand (om te zien of er al iets gedefinieerd is). Er is nog geen bestaande config, het bestand wijst ons naar een voorbeeld config genaamd dhcpd.conf.sample. [root@localhost ~]# cat /etc/dhcpd.conf # # DHCP Server Configuration file. # see /usr/share/doc/dhcp*/dhcpd.conf.sample We passen het voorbeeldje een beetje aan en kopiëren de volgende configuratie naar /etc/dhcpd.conf. subnet 192.168.1.0 netmask 255.255.255.0 { range 192.168.1.140 192.168.1.159 option routers 192.168.1.1; option subnet-mask 255.255.255.0; option domain-name "classdemo.local"; option domain-name-servers 192.168.1.1; default-lease-time 21600; 600
- 607. Introduction to DHCP } 58.7. dhcp server op windows De installatie verloopt geheel gelijk aan die van Microsoft Windows 2003 DNS server, we beperken ons tot een enkel screenshot hier. Zoals de meeste Windows settings wordt je ook hier een wizard kado gedaan die je begeleidt door de setup van een nieuwe scope (of range). Elke setting krijgt een aparte pagina in de wizard. Hieronder de vraag naar de range van ip-adressen. Als een client een ip-adres (of een ip-configuratie met subnet+router+dns+...) krijgt, dan is dit altijd tijdelijk. Als de helft van deze lease time bereikt is, dan zal de client terug een DHCPrequest broadcasten. Meestal komt er dan een DHCPack van de server, met een nieuwe (meestal dezelfde) lease time. Eens de wizard doorlopen, en de scope geactiveerd, kan je alle settings en alle leases bekijken via de DHCP tool (eigenlijk een dhcp snapin voor de mmc). 58.8. dhcp client Hieronder een voorbeeldje van een Windows 2003 computer die client is van een RHEL5 dhcp server. 58.9. client reservations You can reserve an ip configuration for a client using the mac address. host pc42 { hardware ethernet 11:22:33:44:55:66; fixed-address 192.168.42.42; } You can add individual options to this reservation. host pc42 { hardware ethernet 11:22:33:44:55:66; fixed-address 192.168.42.42; option domain-name "linux-training.be"; option routers 192.168.42.1; } 601
- 608. Introduction to DHCP 58.10. 80/20 rule DHCP servers should not be a single point of failure. Let us discuss redundant dhcp server setups. 58.11. relay agent To avoid having to place a dhcp server on every segment, we can use dhcp relay agents. 58.12. rogue dhcp servers Rogue dhcp servers are a problem without a solution. For example accidental connection of a (believed to be simple) hub/switch to a network with an internal dhcp server. 58.13. DHCP and DDNS DHCP can dynamically update DNS when it configures a client computer. DDNS can be used with or without secure keys. When set up properly records can be added automaticall to the zone file: root@fedora14~# tail -2 /var/named/db.office.linux-training.be ubu1010srv A 192.168.42.151 TXT "00dfbb15e144a273c3cf2d6ae933885782" 602
- 609. Introduction to DHCP 58.14. Exercise DHCP and DDNS 1. Make sure you have a unique fixed ip address for your DNS and DHCP server (easier on the same machine). 2. Install DHCP and browse the explanation in the default configuration file /etc/ dhcp/dhcpd.conf or /etc/dhcp3/dhcpd.conf. 3. Decide on a valid scope and activate it. 4. Test with a client that your DHCP server works. 5. Use wireshark to capture the four broadcasts when a client receives an ip (for the first time). 6. Use wireshark to capture a DHCPNAK and a DHCPrelease. 7. Reserve a configuration for a particular client (using mac address). 8. Configure your DHCP/DNS server(s) with a proper hostname and domainname (/etc/hosts, /etc/hostname, /etc/sysconfig/network on Fedora/RHEL, / etc/resolv.conf ...). You may need to disable NetworkManager on *buntu-desktops. 9. Make sure your DNS server still works, and is master over (at least) one domain. There are several ways to do steps 10-11-12. Google is your friend in exploring DDNS with keys, with key-files or without keys. 10. Configure your DNS server to allow dynamic updates from your DHCP server. 11. Configure your DHCP server to send dynamic updates to your DNS server. 12. Test the working of Dynamic DNS. 58.15. Exercise DHCP in Packet Tracer 1. Setup a dhcp server and a client in packet tracer. 2. Test that it works, use simulation to sniff the four broadcasts. 3. Setup a relay agent in packet tracer. 58.16. Example config files For dhcpd.conf on Fedora with dynamic updates for a DNS domain. [root@fedora14 ~]# cat /etc/dhcp/dhcpd.conf authoritative; include "/etc/rndc.key"; 603
- 610. Introduction to DHCP log-facility local6; server-identifier fedora14; ddns-domainname "office.linux-training.be"; ddns-update-style interim; ddns-updates on; update-static-leases on; option domain-name "office.linux-training.be"; option domain-name-servers 192.168.42.100; option ip-forwarding off; default-lease-time 1800; max-lease-time 3600; zone office.linux-training.be { primary 192.168.42.100; } subnet 192.168.4.0 netmask 255.255.255.0 { range 192.168.4.24 192.168.4.40; } Allowing any updates in the zone database (part of the named.conf configuration) zone "office.linux-training.be" { type master; file "/var/named/db.office.linux-training.be"; allow-transfer { any; }; allow-update { any; }; }; Allowing secure key updates in the zone database (part of the named.conf configuration) zone "office.linux-training.be" { type master; file "/var/named/db.office.linux-training.be"; allow-transfer { any; }; allow-update { key mykey; }; }; Sample key file contents: [root@fedora14 ~]# cat /etc/rndc.key key "rndc-key" { algorithm hmac-md5; secret "4Ykd58uIeUr3Ve6ad1qTfQ=="; }; Generate your own keys with dnssec-keygen. How to include a key in a config file: include "/etc/bind/rndc.key"; Also make sure that bind can write to your db.zone file (using chmod/chown). For Ubuntu this can be in /etc/bind, for Fedora in /var/named. 604
- 611. Part XIX. dhcp server
- 612. Part XX. iptables firewall
- 613. Chapter 59. introduction to routers Table of Contents 59.1. terminology ................................................................................................ 607 59.2. packet forwarding ...................................................................................... 608 59.1. terminology router or firewall A router is a device that connects two networks. A firewall is a device that besides acting as a router, also contains (and implements) rules to determine whether packets are allowed to travel from one network to another. A firewall can be configured to block access based on networks, hosts, protocols and ports. Firewalls can also change the contents of packets while forwarding them. packet forwarding Packet forwarding means allowing packets to go from one network to another. When a multihomed host is connected to two different networks, and it allows packets to travel from one network to another through its two network interfaces, it is said to have enabled packet forwarding. packet filtering Packet filtering is very similar to packet forwarding, but every packet is individually tested against rules that decide on allowing or dropping the packet. The rules are stored by iptables. stateful A stateful firewall is an advancement over stateless firewalls that inspect every individual packet. A stateful firewall will keep a table of active connections, and is knowledgeable enough to recognise when new connections are part of an active session. Linux iptables is a stateful firewall. NAT (network address translation) A NAT device is a router that is also changing the source and/or target ip-address in packets. It is typically used to connect multiple computers in a private address 607
- 614. introduction to routers range (rfc 1918) with the (public) internet. A NAT can hide private addresses from the internet. It is important to understand that people and vendors do not always use the right term when referring to a certain type of NAT. Be sure you talk about the same thing. We can distuinguish several types of NAT. PAT (port address translation) NAT often includes PAT. A PAT device is a router that is also changing the source and/or target tcp/udp port in packets. PAT is Cisco terminology and is used by SNAT, DNAT, masquerading and port forwarding in Linux. RFC 3022 calls it NAPT and defines the NAT/PAT combo as "traditional NAT". A device sold to you as a NAT- device will probably do NAT and PAT. SNAT (source network address translation) A SNAT device is changing the source ip-address when a packet passes our NAT. SNAT configuration with iptables includes a fixed target source address. masquerading Masquerading is a form of SNAT that will hide the (private) source ip-addresses of your private network using a public ip-address. Masquerading is common on dynamic internet interfaces (broadband modem/routers). Masquerade configuration with iptables uses a dynamic target source address. DNAT (destination network address translation) A DNAT device is changing the destination ip-address when a packet passes our NAT. port forwarding When static DNAT is set up in a way that allows outside connections to enter our private network, then we call it port forwarding. 59.2. packet forwarding about packet forwarding Packet forwarding means allowing packets to go from one network to another. When a multihomed host is connected to two different networks, and it allows packets to 608
- 615. introduction to routers travel from one network to another through its two network interfaces, it is said to have enabled packet forwarding. /proc/sys/net/ipv4/ip_forward Whether a host is forwarding packets is defined in /proc/sys/net/ipv4/ip_forward. The following screenshot shows how to enable packet forwarding on Linux. [root@RHEL5 ~]# echo 1 > /proc/sys/net/ipv4/ip_forward The next command shows how to disable packet forwarding. [root@RHEL5 ~]# echo 0 > /proc/sys/net/ipv4/ip_forward Use cat to check if packet forwarding is enabled. [root@RHEL5 ~]# cat /proc/sys/net/ipv4/ip_forward /etc/sysctl.conf By default, most Linux computers are not configured for automatic packet forwarding. To enable packet forwarding whenever the system starts, change the net.ipv4.ip_forward variable in /etc/sysctl.conf to the value 1. [root@RHEL5 ~]# grep ip_forward /etc/sysctl.conf net.ipv4.ip_forward = 0 Practice: packet forwarding 1. Set up two dsl (Damn Small Linux) machines, one on vmnet1, the other on vmnet8. Make sure they both get an ip-address in the correct subnet. These two machines will be 'left' and 'right' from the 'router'. 2. Set up a RHEL server with two network cards, one on vmnet1, the other on vmnet8. This computer will be the 'router'. Complete the table below with the relevant names, ip-addresses and mac-addresses. Table 59.1. Packet Forwarding Exercise left: router: right: MAC IP 609
- 616. introduction to routers 3. How can you verify whether the RHEL will allow packet forwarding by default or not ? Test that you can ping from the RHEL to the two dsl machines, and from the two dsl machines to the RHEL. Use arp -a to make sure you are connected with the correct MAC addresses. 4. Ping from one dsl to the other. Enable and/or disable packet forwarding on the RHEL server and verify what happens to the ping between the two dsl machines. If you do not succeed in pinging between the two dsl machines (on different subnets), then use a sniffer like wireshark or tcpdump to discover the problem. 5. Use wireshark or tcpdump -xx to answer the following questions. Does the source MAC change when a packet passes through the filter ? And the destination MAC ? What about source and destination IP-addresses ? Solution: packet forwarding 1. Set up two dsl (Damn Small Linux) machines, one on vmnet1, the other on vmnet8. Make sure they both get an ip-address in the correct subnet. These two machines will be 'left' and 'right' from the 'router'. The configuration of the dsl machines can be similar to the following two screenshots. Both machines must be in a different subnet (here 192.168.187.0/24 and 172.16.122.0/24) root@ttyp1[root]# ifconfig eth0 | grep -A1 eth0 eth0 Link encap:Ethernet HWaddr 00:0C:29:08:F4:C1 inet addr:192.168.187.130 Bcast:192.168.187.255 Mask:255.255.255.0 root@ttyp1[root]# route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 192.168.187.0 * 255.255.255.0 U 0 0 0 eth0 default 192.168.187.128 0.0.0.0 UG 0 0 0 eth0 root@ttyp1[root]# root@ttyp1[root]# ifconfig eth0 | grep -A1 eth0 eth0 Link encap:Ethernet HWaddr 00:0C:29:6E:1A:AA inet addr:172.16.122.129 Bcast:172.16.122.255 Mask:255.255.255.0 root@ttyp1[root]# route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 172.16.122.0 * 255.255.255.0 U 0 0 0 eth0 default 172.16.122.128 0.0.0.0 UG 0 0 0 eth0 root@ttyp1[root]# 2. Set up a RHEL server with two network cards, one on vmnet1, the other on vmnet8. This computer will be the 'router'. The 'router' can be set up like this screenshot shows. 610
- 617. introduction to routers [root@RHEL5 ~]# ifconfig | grep -A1 eth eth1 Link encap:Ethernet HWaddr 00:0C:29:8C:90:49 inet addr:192.168.187.128 Bcast:192.168.187.255 Mask:255.255.255.0 -- eth2 Link encap:Ethernet HWaddr 00:0C:29:8C:90:53 inet addr:172.16.122.128 Bcast:172.16.122.255 Mask:255.255.255.0 [root@RHEL5 ~]# Your setup may use different ip and mac addresses than the ones in the table below. This table serves as a reference for the screenshots from this solution to the practice. Table 59.2. Packet Forwarding Solution left: dsl router: RHEL5 right: dsl 00:0c:29:08:f4:c1 00:0c:29:8c:90:49 00:0c:29:8c:90:53 00:0c:29:6e:1a:aa 192.168.187.130 192.168.187.128 172.16.122.128 172.16.122.129 3. How can you verify whether the RHEL will allow packet forwarding by default or not ? Test that you can ping from the RHEL to the two dsl machines, and from the two dsl machines to the RHEL. Use arp -a to make sure you are connected with the correct MAC addresses. This can be done with "grep ip_forward /etc/sysctl.conf" (1 is enabled, 0 is disabled). [root@RHEL5 ~]# grep ip_for /etc/sysctl.conf net.ipv4.ip_forward = 0 4. Ping from one dsl to the other. Enable and/or disable packet forwarding on the RHEL server and verify what happens to the ping between the two dsl machines. If you do not succeed in pinging between the two dsl machines (on different subnets), then use a sniffer like ethereal or tcpdump to discover the problem. Did you forget to add a default gateway to the dsl machines ? Use route add default gw 'ip-address'. You should be able to ping when packet forwarding is enabled (and both default gateways are properly configured). The ping will not work when packet forwarding is disabled or when gateways are not configured correctly. 5. Use wireshark or tcpdump -xx to answer the following questions. Does the source MAC change when a packet passes through the filter ? And the destination MAC ? What about source and destination IP-addresses ? Both MAC addresses are changed when passing the router. The screenshots below show tcpdump -xx output on the router. The first one is taken on the eth1(vmnet1) interface in the 192.168.187.0/24 network, the second one is from the other interface (eth2 on vmnet8 in 172.16.122.0/24). The first six bytes are the destination MAC, the next six are the source. 611
- 618. introduction to routers [root@RHEL5 ~]# tcpdump -xx -i eth1 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes 04:18:23.817854 IP 192.168.187.130 > 172.16.122.129: ICMP echo request... 0x0000: 000c 298c 9049 000c 2908 f4c1 0800 4500 0x0010: 0054 0000 4000 4001 97ec c0a8 bb82 ac10 0x0020: 7a81 0800 3b28 a717 0006 8059 d148 d614 0x0030: 0300 0809 0a0b 0c0d 0e0f 1011 1213 1415 0x0040: 1617 1819 1a1b 1c1d 1e1f 2021 2223 2425 0x0050: 2627 2829 2a2b 2c2d 2e2f 3031 3233 3435 04:18:23.817962 IP 172.16.122.129 > 192.168.187.130: ICMP echo reply... 0x0000: 000c 2908 f4c1 000c 298c 9049 0800 4500 0x0010: 0054 d364 0000 3f01 0588 ac10 7a81 c0a8 0x0020: bb82 0000 4328 a717 0006 8059 d148 d614 0x0030: 0300 0809 0a0b 0c0d 0e0f 1011 1213 1415 0x0040: 1617 1819 1a1b 1c1d 1e1f 2021 2223 2425 0x0050: 2627 2829 2a2b 2c2d 2e2f 3031 3233 3435 [root@RHEL5 ~]# tcpdump -xx -i eth2 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth2, link-type EN10MB (Ethernet), capture size 96 bytes 04:18:33.904697 IP 192.168.187.130 > 172.16.122.129: ICMP echo request... 0x0000: 000c 296e 1aaa 000c 298c 9053 0800 4500 0x0010: 0054 0000 4000 3f01 98ec c0a8 bb82 ac10 0x0020: 7a81 0800 2320 a717 0008 8a59 d148 e41a 0x0030: 0300 0809 0a0b 0c0d 0e0f 1011 1213 1415 0x0040: 1617 1819 1a1b 1c1d 1e1f 2021 2223 2425 0x0050: 2627 2829 2a2b 2c2d 2e2f 3031 3233 3435 04:18:33.944514 IP 172.16.122.129 > 192.168.187.130: ICMP echo reply... 0x0000: 000c 298c 9053 000c 296e 1aaa 0800 4500 0x0010: 0054 d366 0000 4001 0486 ac10 7a81 c0a8 0x0020: bb82 0000 2b20 a717 0008 8a59 d148 e41a 0x0030: 0300 0809 0a0b 0c0d 0e0f 1011 1213 1415 0x0040: 1617 1819 1a1b 1c1d 1e1f 2021 2223 2425 0x0050: 2627 2829 2a2b 2c2d 2e2f 3031 3233 3435 612
- 619. Chapter 60. Firewall: iptables Table of Contents 60.1. about iptables ............................................................................................. 613 60.2. packet filtering ........................................................................................... 613 60.3. network address translation ........................................................................ 618 60.1. about iptables Iptables is a user-space application that allows a user to configure the Linux kernel's Netfilter. By default there are three tables in the kernel that contain sets of rules. The filter table is used for packet filtering, the NAT table for address translation and the mangle table for special-purpose processing of packets. Series of rules in each table are called a chain. The following screenshot shows how to stop and start iptables. [root@RHEL5 ~]# /etc/init.d/iptables stop [root@RHEL5 ~]# /etc/init.d/iptables start [root@RHEL5 ~]# 60.2. packet filtering about packet filtering Packet filtering is a bit more than packet forwarding. Packet forwarding only uses a routing table to make decisions, the kernel now also uses a list of rules. So with packet filtering, the kernel will inspect each packet and decide based on iptables rules to allow or drop a packet. filter table The filter table in iptables has three chains (sets of rules). The INPUT chain is used for any packet coming into the system. The OUTPUT chain is for any packet leaving the system. And the FORWARD chain is for packets that are forwarded (routed) through the system. The screenshot below shows how to list the filter table and all its rules. [root@RHEL5 ~]# iptables -t filter -nL Chain INPUT (policy ACCEPT) target prot opt source destination 613
- 620. Firewall: iptables Chain FORWARD (policy ACCEPT) target prot opt source destination Chain OUTPUT (policy ACCEPT) target prot opt source destination [root@RHEL5 ~]# As you can see, all three chains in the filter table are set to ACCEPT everything. ACCEPT is the default behaviour. Changing default policy rules To start, let's set the default policy for all three chains to drop everything. Note that you might lose your connection when typing this over ssh ;-). [root@RHEL5 ~]# iptables -P INPUT DROP [root@RHEL5 ~]# iptables -P FORWARD DROP [root@RHEL5 ~]# iptables -P OUTPUT DROP Next, we allow the server to use its own loopback device (this allows the server to access its services running on localhost). We first append a rule to the INPUT chain to allow (ACCEPT) traffic from the lo (loopback) interface, then we do the same to allow packets to leave the system through the loopback interface. [root@RHEL5 ~]# iptables -A INPUT -i lo -j ACCEPT [root@RHEL5 ~]# iptables -A OUTPUT -o lo -j ACCEPT Looking at the filter table again (omitting -t filter because it is the default table). [root@RHEL5 ~]# iptables -nL Chain INPUT (policy DROP) target prot opt source destination ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 Chain FORWARD (policy DROP) target prot opt source destination Chain OUTPUT (policy DROP) target prot opt source destination ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 Allowing ssh over eth0 This example show how to add two rules to allow ssh access to your system from outside. [root@RHEL5 ~]# iptables -A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT [root@RHEL5 ~]# iptables -A OUTPUT -o eth0 -p tcp --sport 22 -j ACCEPT 614
- 621. Firewall: iptables The filter table will look something like this screenshot (note that -v is added for more verbose output). [root@RHEL5 ~]# iptables -nvL Chain INPUT (policy DROP 7 packets, 609 bytes) pkts bytes target prot opt in out source destination 0 0 ACCEPT all -- lo * 0.0.0.0/0 0.0.0.0/0 0 0 ACCEPT tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:22 Chain FORWARD (policy DROP 0 packets, 0 bytes) pkts bytes target prot opt in out source destination Chain OUTPUT (policy DROP 3 packets, 228 bytes) pkts bytes target prot opt in out source destination 0 0 ACCEPT all -- * lo 0.0.0.0/0 0.0.0.0/0 0 0 ACCEPT tcp -- * eth0 0.0.0.0/0 0.0.0.0/0 tcp spt:22 [root@RHEL5 ~]# Allowing access from a subnet This example shows how to allow access from any computer in the 10.1.1.0/24 network, but only through eth1. There is no port (application) limitation here. [root@RHEL5 ~]# iptables -A INPUT -i eth1 -s 10.1.1.0/24 -p tcp -j ACCEPT [root@RHEL5 ~]# iptables -A OUTPUT -o eth1 -d 10.1.1.0/24 -p tcp -j ACCEPT Together with the previous examples, the policy is expanding. [root@RHEL5 ~]# iptables -nvL Chain INPUT (policy DROP 7 packets, 609 bytes) pkts bytes target prot opt in out source destination 0 0 ACCEPT all -- lo * 0.0.0.0/0 0.0.0.0/0 0 0 ACCEPT tcp -- eth0 * 0.0.0.0/0 0.0.0.0/0 tcp dpt:22 0 0 ACCEPT tcp -- eth1 * 10.1.1.0/24 0.0.0.0/0 Chain FORWARD (policy DROP 0 packets, 0 bytes) pkts bytes target prot opt in out source destination Chain OUTPUT (policy DROP 3 packets, 228 bytes) pkts bytes target prot opt in out source destination 0 0 ACCEPT all -- * lo 0.0.0.0/0 0.0.0.0/0 0 0 ACCEPT tcp -- * eth0 0.0.0.0/0 0.0.0.0/0 tcp spt:22 0 0 ACCEPT tcp -- * eth1 0.0.0.0/0 10.1.1.0/24 iptables save Use iptables save to automatically implement these rules when the firewall is (re)started. [root@RHEL5 ~]# /etc/init.d/iptables save Saving firewall rules to /etc/sysconfig/iptables: [ OK ] [root@RHEL5 ~]# 615
- 622. Firewall: iptables scripting example You can write a simple script for these rules. Below is an example script that implements the firewall rules that you saw before in this chapter. #!/bin/bash # first cleanup everything iptables -t filter -F iptables -t filter -X iptables -t nat -F iptables -t nat -X # default drop iptables -P INPUT DROP iptables -P FORWARD DROP iptables -P OUTPUT DROP # allow loopback device iptables -A INPUT -i lo -j ACCEPT iptables -A OUTPUT -o lo -j ACCEPT # allow ssh over eth0 from outside to system iptables -A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT iptables -A OUTPUT -o eth0 -p tcp --sport 22 -j ACCEPT # allow any traffic from 10.1.1.0/24 to system iptables -A INPUT -i eth1 -s 10.1.1.0/24 -p tcp -j ACCEPT iptables -A OUTPUT -o eth1 -d 10.1.1.0/24 -p tcp -j ACCEPT Allowing ICMP(ping) When you enable iptables, you will get an 'Operation not permitted' message when trying to ping other hosts. [root@RHEL5 ~# ping 192.168.187.130 PING 192.168.187.130 (192.168.187.130) 56(84) bytes of data. ping: sendmsg: Operation not permitted ping: sendmsg: Operation not permitted The screenshot below shows you how to setup iptables to allow a ping from or to your machine. [root@RHEL5 ~]# iptables -A INPUT -p icmp --icmp-type any -j ACCEPT [root@RHEL5 ~]# iptables -A OUTPUT -p icmp --icmp-type any -j ACCEPT The previous two lines do not allow other computers to route ping messages through your router, because it only handles INPUT and OUTPUT. For routing of ping, you will need to enable it on the FORWARD chain. The following command enables routing of icmp messages between networks. [root@RHEL5 ~]# iptables -A FORWARD -p icmp --icmp-type any -j ACCEPT 616
- 623. Firewall: iptables Practice: packet filtering 1. Make sure you can ssh to your router-system when iptables is active. 2. Make sure you can ping to your router-system when iptables is active. 3. Define one of your networks as 'internal' and the other as 'external'. Configure the router to allow visits to a website (http) to go from the internal network to the external network (but not in the other direction). 4. Make sure the internal network can ssh to the external, but not the other way around. Solution: packet filtering A possible solution, where dsl is the internal and dsr is the external network. #!/bin/bash # first cleanup everything iptables -t filter -F iptables -t filter -X iptables -t nat -F iptables -t nat -X # default drop iptables -P INPUT DROP iptables -P FORWARD DROP iptables -P OUTPUT DROP # allow loopback device iptables -A INPUT -i lo -j ACCEPT iptables -A OUTPUT -o lo -j ACCEPT # question 1: allow ssh over eth0 iptables -A INPUT -i eth0 -p tcp --dport 22 -j ACCEPT iptables -A OUTPUT -o eth0 -p tcp --sport 22 -j ACCEPT # question 2: Allow icmp(ping) anywhere iptables -A INPUT -p icmp --icmp-type any -j ACCEPT iptables -A FORWARD -p icmp --icmp-type any -j ACCEPT iptables -A OUTPUT -p icmp --icmp-type any -j ACCEPT # question 3: allow http from internal(dsl) to external(dsr) iptables -A FORWARD -i eth1 -o eth2 -p tcp --dport 80 -j ACCEPT iptables -A FORWARD -i eth2 -o eth1 -p tcp --sport 80 -j ACCEPT # question 4: allow ssh from internal(dsl) to external(dsr) iptables -A FORWARD -i eth1 -o eth2 -p tcp --dport 22 -j ACCEPT iptables -A FORWARD -i eth2 -o eth1 -p tcp --sport 22 -j ACCEPT # allow http from external(dsr) to internal(dsl) # iptables -A FORWARD -i eth2 -o eth1 -p tcp --dport 80 -j ACCEPT # iptables -A FORWARD -i eth1 -o eth2 -p tcp --sport 80 -j ACCEPT # allow rpcinfo over eth0 from outside to system 617
- 624. Firewall: iptables # iptables -A INPUT -i eth2 -p tcp --dport 111 -j ACCEPT # iptables -A OUTPUT -o eth2 -p tcp --sport 111 -j ACCEPT 60.3. network address translation about NAT A NAT device is a router that is also changing the source and/or target ip-address in packets. It is typically used to connect multiple computers in a private address range with the (public) internet. A NAT can hide private addresses from the internet. NAT was developed to mitigate the use of real ip addresses, to allow private address ranges to reach the internet and back, and to not disclose details about internal networks to the outside. The nat table in iptables adds two new chains. PREROUTING allows altering of packets before they reach the INPUT chain. POSTROUTING allows altering packets after they exit the OUTPUT chain. Use iptables -t nat -nvL to look at the NAT table. The screenshot below shows an empty NAT table. [root@RHEL5 ~]# iptables -t nat -nL Chain PREROUTING (policy ACCEPT) target prot opt source destination Chain POSTROUTING (policy ACCEPT) target prot opt source destination Chain OUTPUT (policy ACCEPT) target prot opt source destination [root@RHEL5 ~]# SNAT (Source NAT) The goal of source nat is to change the source address inside a packet before it leaves the system (e.g. to the internet). The destination will return the packet to the NAT- device. This means our NAT-device will need to keep a table in memory of all the packets it changed, so it can deliver the packet to the original source (e.g. in the private network). Because SNAT is about packets leaving the system, it uses the POSTROUTING chain. Here is an example SNAT rule. The rule says that packets coming from 10.1.1.0/24 network and exiting via eth1 will get the source ip-address set to 11.12.13.14. (Note that this is a one line command!) 618
- 625. Firewall: iptables iptables -t nat -A POSTROUTING -o eth1 -s 10.1.1.0/24 -j SNAT --to-source 11.12.13.14 Of course there must exist a proper iptables filter setup to allow the packet to traverse from one network to the other. SNAT example setup This example script uses a typical nat setup. The internal (eth0) network has access via SNAT to external (eth1) webservers (port 80). #!/bin/bash # # iptables script for simple classic nat websurfing # eth0 is internal network, eth1 is internet # echo 0 > /proc/sys/net/ipv4/ip_forward iptables -P INPUT ACCEPT iptables -P OUTPUT ACCEPT iptables -P FORWARD DROP iptables -A FORWARD -i eth0 -o eth1 -s 10.1.1.0/24 -p tcp --dport 80 -j ACCEPT iptables -A FORWARD -i eth1 -o eth0 -d 10.1.1.0/24 -p tcp --sport 80 -j ACCEPT iptables -t nat -A POSTROUTING -o eth1 -s 10.1.1.0/24 -j SNAT --to-source 11.12.13.14 echo 1 > /proc/sys/net/ipv4/ip_forward IP masquerading IP masquerading is very similar to SNAT, but is meant for dynamic interfaces. Typical example are broadband 'router/modems' connected to the internet and receiving a different ip-address from the isp, each time they are cold-booted. The only change needed to convert the SNAT script to a masquerading is one line. iptables -t nat -A POSTROUTING -o eth1 -s 10.1.1.0/24 -j MASQUERADE DNAT (Destination NAT) DNAT is typically used to allow packets from the internet to be redirected to an internal server (in your DMZ) and in a private address range that is inaccessible directly form the internet. This example script allows internet users to reach your internal (192.168.1.99) server via ssh (port 22). #!/bin/bash 619
- 626. Firewall: iptables # # iptables script for DNAT # eth0 is internal network, eth1 is internet # echo 0 > /proc/sys/net/ipv4/ip_forward iptables -P INPUT ACCEPT iptables -P OUTPUT ACCEPT iptables -P FORWARD DROP iptables -A FORWARD -i eth0 -o eth1 -s 10.1.1.0/24 -j ACCEPT iptables -A FORWARD -i eth1 -o eth0 -p tcp --dport 22 -j ACCEPT iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 22 -j DNAT --to-destination 10.1.1.99 echo 1 > /proc/sys/net/ipv4/ip_forward 620
- 627. Part XXI. apache and squid
- 628. Chapter 61. introduction to apache Table of Contents 61.1. about apache .............................................................................................. 622 61.2. is apache installed ? ................................................................................... 622 61.3. is apache running ? .................................................................................... 623 61.4. apache configuration .................................................................................. 623 61.5. virtual hosts ................................................................................................ 624 61.6. aliases and redirects ................................................................................... 625 61.7. securing directories with htpasswd and .htaccess ...................................... 625 61.8. more on .htaccess ....................................................................................... 626 61.9. traffic .......................................................................................................... 626 61.10. practice: apache ........................................................................................ 626 61.1. about apache According to NetCraft (http://news.netcraft.com/archives/web_server_survey.html) about seventy percent of all web servers are running on Apache. Some people say that the name is derived from a patchy web server, because of all the patches people wrote for the NCSA httpd server. 61.2. is apache installed ? To verify whether Apache is installed, use the proper tools (rpm, dpkg, ...) and grep for apache or httpd. This Red Hat Enterprise 4 Server has apache installed. [paul@rhel4 ~]$ rpm -qa | grep -i httpd httpd-2.0.52-25.ent httpd-manual-2.0.52-25.ent system-config-httpd-1.3.1-1 httpd-devel-2.0.52-25.ent httpd-suexec-2.0.52-25.ent This Ubuntu also has apache installed. paul@laika:~$ dpkg -l | grep apache ii apache2 2.2.3-3.2build1 Next generation, scalable, ... ii apache2-mpm-prefork 2.2.3-3.2build1 Traditional model for Apach... ii apache2-utils 2.2.3-3.2build1 utility programs for webser... ii apache2.2-common 2.2.3-3.2build1 Next generation, scalable, ... ii libapache2-mod-php5 5.2.1-0ubuntu1.2 server-side, HTML-embedded ... 622
- 629. introduction to apache 61.3. is apache running ? This is how apache looks when it is installed on Red Hat Enterprise Linux 4, running named as httpd. [root@RHELv4u3 ~]# /etc/init.d/httpd status httpd is stopped [root@RHELv4u3 ~]# service httpd start Starting httpd: [ OK ] [root@RHELv4u3 ~]# ps -C httpd PID TTY TIME CMD 4573 ? 00:00:00 httpd 4576 ? 00:00:00 httpd 4577 ? 00:00:00 httpd 4578 ? 00:00:00 httpd 4579 ? 00:00:00 httpd 4580 ? 00:00:00 httpd 4581 ? 00:00:00 httpd 4582 ? 00:00:00 httpd 4583 ? 00:00:00 httpd [root@RHELv4u3 ~]# And here is Apache running on Ubuntu, named as apache2. root@laika:~# ps -C apache2 PID TTY TIME CMD 6170 ? 00:00:00 apache2 6248 ? 00:00:01 apache2 6249 ? 00:00:01 apache2 6250 ? 00:00:00 apache2 6251 ? 00:00:01 apache2 6252 ? 00:00:01 apache2 7520 ? 00:00:01 apache2 8943 ? 00:00:01 apache2 root@laika:~# /etc/init.d/apache2 status * Usage: /etc/init.d/apache2 {start|stop|restart|reload|force-reload} root@laika:~# To verify that apache is running, open a web browser on the web server, and browse to http://localhost. An Apache test page should be shown. The http://localhosts/manual url will give you an extensive Apache manual. The second test is to connect to your Apache from another computer. 61.4. apache configuration Configuring Apache changed a bit the past couple of years. But it still takes place in /etc/httpd or /etc/apache. [root@RHELv4u3 ~]# cd /etc/httpd/ [root@RHELv4u3 httpd]# ll total 32 lrwxrwxrwx 1 root root 25 Jan 24 09:28 build -> ../../usr/lib/httpd/build drwxr-xr-x 7 root root 4096 Jan 24 08:48 conf 623
- 630. introduction to apache drwxr-xr-x 2 root root 4096 Jan 24 09:29 conf.d lrwxrwxrwx 1 root root 19 Jan 24 08:48 logs -> ../../var/log/httpd lrwxrwxrwx 1 root root 27 Jan 24 08:48 modules -> ../../usr/lib/httpd/modules lrwxrwxrwx 1 root root 13 Jan 24 08:48 run -> ../../var/run [root@RHELv4u3 httpd]# The main configuration file for the Apache server on RHEL is /etc/httpd/conf/ httpd.conf, on Ubuntu it is /etc/apache2/apache2.conf. The file explains itself, and contains examples for how to set up virtual hosts or configure access. 61.5. virtual hosts Virtual hosts can be defined by ip-address, by port or by name (host record). (The new way of defining virtual hosts is through seperate config files in the conf.d directory.) Below is a very simple virtual host definition. [root@rhel4 conf]# tail /etc/httpd/conf/httpd.conf # # This is a small test website # <VirtualHost testsite.local:80> ServerAdmin webmaster@testsite.local DocumentRoot /var/www/html/testsite/ ServerName testsite.local ErrorLog logs/testsite.local-error_log CustomLog logs/testsite.local-access_log common </VirtualHost> [root@rhel4 conf]# Should you put this little index.html file in the directory mentioned in the above screenshot, then you can access this humble website. [root@rhel4 conf]# cat /var/www/html/testsite/index.html <html> <head><title>Test Site</title></head> <body> <p>This is the test site.</p> </body> </html> Below is a sample virtual host configuration. This virtual hosts overrules the default Apache ErrorDocument directive. <VirtualHost 83.217.76.245:80> ServerName cobbaut.be ServerAlias www.cobbaut.be DocumentRoot /home/paul/public_html ErrorLog /home/paul/logs/error_log CustomLog /home/paul/logs/access_log common ScriptAlias /cgi-bin/ /home/paul/cgi-bin/ <Directory /home/paul/public_html> Options Indexes IncludesNOEXEC FollowSymLinks allow from all 624
- 631. introduction to apache </Directory> ErrorDocument 404 http://www.cobbaut.be/cobbaut.php </VirtualHost> 61.6. aliases and redirects Apache supports aliases for directories, like this example shows. Alias /paul/ "/home/paul/public_html/" Similarly, content can be redirected to another website or web server. Redirect permanent /foo http://www.foo.com/bar 61.7. securing directories with htpasswd and .htaccess You can secure files and directories in your website with a userid/password. First, enter your website, and use the htpasswd command to create a .htpasswd file that contains a userid and an (encrypted) password. [root@rhel4 testsite]# htpasswd -c .htpasswd pol New password: Re-type new password: Adding password for user pol [root@rhel4 testsite]# cat .htpasswd pol:x5vZlyw1V6KXE [root@rhel4 testsite]# You can add users to this file, just don't use the -c switch again. [root@rhel4 testsite]# htpasswd .htpasswd kim New password: Re-type new password: Adding password for user kim [root@rhel4 testsite]# cat .htpasswd pol:x5vZlyw1V6KXE kim:6/RbvugwsgOI6 [root@rhel4 testsite]# You have now defined two users. Next create a subsdirectory that you want to protect with these two accounts. And put the following .htaccess file in that subdirectory. [root@rhel4 kimonly]# pwd /var/www/html/testsite/kimonly [root@rhel4 kimonly]# cat .htaccess AuthUserFile /var/www/html/testsite/.htpasswd AuthGroupFile /dev/null AuthName "test access title" AuthType Basic 625
- 632. introduction to apache <Limit GET POST> require valid-user </Limit> [root@rhel4 kimonly]# Finally, don't forget to verify that AllowOverride is set to All in the general Apache configuration file. # AllowOverride controls what directives may be placed in .htaccess files. # It can be "All", "None", or any combination of the keywords: # Options FileInfo AuthConfig Limit # AllowOverride All From now on, when a user accesses a file in that subdirectory, that user will have to provide a userid/password combo that is defined in your .htpasswd. 61.8. more on .htaccess You can do much more with .htaccess. One example is to use .htaccess to prevent people from certain domains to access your website. Like in this case, where a number of referer spammers are blocked from the website. paul@lounge:~/cobbaut.be$ cat .htaccess # Options +FollowSymlinks RewriteEngine On RewriteCond %{HTTP_REFERER} ^http://(www.)?buy-adipex.fw.nu.*$ [OR] RewriteCond %{HTTP_REFERER} ^http://(www.)?buy-levitra.asso.ws.*$ [NC,OR] RewriteCond %{HTTP_REFERER} ^http://(www.)?buy-tramadol.fw.nu.*$ [NC,OR] RewriteCond %{HTTP_REFERER} ^http://(www.)?buy-viagra.lookin.at.*$ [NC,OR] ... RewriteCond %{HTTP_REFERER} ^http://(www.)?www.healthinsurancehelp.net.*$ [NC] RewriteRule .* - [F,L] paul@lounge:~/cobbaut.be$ 61.9. traffic Apache keeps a log of all visitors. The webalizer is often used to parse this log into nice html statistics. 61.10. practice: apache 1. Verify that Apache is installed and running. 2. Browse to the Apache HTML manual from another computer. 3. Create a virtual hosts that listens to port 8247. 626
- 633. introduction to apache 4. Create a virtual hosts that listens on another ip-address. 5. Test from another computer that all virtual hosts work. 6. Protect a subdirectory of a website with .htpasswd and .htaccess. 627
- 634. Chapter 62. introduction to squid Table of Contents 62.1. about proxy servers .................................................................................... 628 62.2. squid proxy server ..................................................................................... 629 62.1. about proxy servers usage A proxy server is a server that caches the internet. Clients connect to the proxy server with a request for an internet server. The proxy server will connect to the internet server on behalf of the client. The proxy server will also cache the pages retrieved from the internet server. A proxy server may provide pages from his cache to a client, instead of connecting to the internet server to retrieve the (same) pages. A proxy server has two main advantages. It improves web surfing speed when returning cached data to clients, and it reduces the required bandwidth (cost) to the internet. Smaller organizations sometimes put the proxy server on the same physical computer that serves as a NAT to the internet. In larger organizations, the proxy server is one of many servers in the DMZ. When web traffic passes via a proxy server, it is common practice to configure the proxy with extra settings for access control. Access control in a proxy server can mean user account access, but also website(url), ip-address or dns restrictions. open proxy servers You can find lists of open proxy servers on the internet that enable you to surf anonymously. This works when the proxy server connects on your behalf to a website, without logging your ip-address. But be careful, these (listed) open proxy servers could be created in order to eavesdrop upon their users. squid This chapter is an introduction to the squid proxy server (http://www.squid- cache.org). The version used is 2.5. [root@RHEL4 ~]# rpm -qa | grep squid squid-2.5.STABLE6-3.4E.12 [root@RHEL4 ~]# 628
- 635. introduction to squid 62.2. squid proxy server /etc/squid/squid.conf Squid's main configuration file is /etc/squid/squid.conf. The file explains every parameter in great detail. It can be a good idea to start by creating a backup of this file. [root@RHEL4 /etc/squid/]# cp squid.conf squid.conf.original /var/spool/squid The squid proxy server stores its cache by default in /var/spool/squid. This setting is configurable in /etc/squid/squid.conf. [root@RHEL4 ~]# grep "^# cache_dir" /etc/squid/squid.conf # cache_dir ufs /var/spool/squid 100 16 256 It is possible that in a default setup where squid has never run, that the /var/spool/ squid directories do not exist. [root@RHEL4 ~]# ls -al /var/spool/squid ls: /var/spool/squid: No such file or directory Running squid -z will create the necessary squid directories. [root@RHEL4 ~]# squid -z 2008/09/22 14:07:47| Creating Swap Directories [root@RHEL4 ~]# ls -al /var/spool/squid total 80 drwxr-x--- 18 squid squid 4096 Sep 22 14:07 . drwxr-xr-x 26 root root 4096 May 30 2007 .. drwxr-xr-x 258 squid squid 4096 Sep 22 14:07 00 drwxr-xr-x 258 squid squid 4096 Sep 22 14:07 01 drwxr-xr-x 258 squid squid 4096 Sep 22 14:07 02 ... port 3128 or port 8080 By default the squid proxy server will bind to port 3128 to listen to incoming requests. [root@RHEL4 ~]# grep "default port" /etc/squid/squid.conf # The default port number is 3128. 629
- 636. introduction to squid Many organizations use port 8080 instead. [root@RHEL4 ~]# grep 8080 /etc/squid/squid.conf http_port 8080 /var/log/squid The standard log file location for squid is /var/log/squid. [root@RHEL4 ~]# grep "/var/log" /etc/squid/squid.conf # cache_access_log /var/log/squid/access.log # cache_log /var/log/squid/cache.log # cache_store_log /var/log/squid/store.log access control The default squid setup only allows localhost access. To enable access for a private network range, look for the "INSERT YOUR OWN RULE(S) HERE..." sentence in squid.conf and add two lines similar to the screenshot below. # INSERT YOUR OWN RULE(S) HERE TO ALLOW ACCESS FROM YOUR CLIENTS acl company_network src 192.168.1.0/24 http_access allow company_network Restart the squid server, and now the local private network can use the proxy cache. testing squid First, make sure that the server running squid has access to the internet. [root@RHEL4 ~]# wget -q http://linux-training.be/index.html [root@RHEL4 ~]# ls -l index.html -rw-r--r-- 1 root root 2269 Sep 18 13:18 index.html [root@RHEL4 ~]# Then configure a browser on a client to use the proxy server. OR you could set the HTTP_PROXY (sometimes http_proxy) variable to point command line programs to the proxy. [root@fedora ~]# export HTTP_PROXY=http://192.168.1.39:8080 [root@ubuntu ~]# export http_proxy=http://192.168.1.39:8080 Testing a client machine can then be done with wget (wget -q is used to simplify the screenshot). 630
- 637. introduction to squid [root@RHEL5 ~]# > /etc/resolv.conf [root@RHEL5 ~]# wget -q http://www.linux-training.be/index.html [root@RHEL5 ~]# ls -l index.html -rw-r--r-- 1 root root 2269 Sep 18 2008 index.html [root@RHEL5 ~]# name resolution You need name resolution working on the squid server, but you don't need name resolution on the clients. [paul@RHEL5 ~]$ wget http://grep.be --14:35:44-- http://grep.be Resolving grep.be... failed: Temporary failure in name resolution. [paul@RHEL5 ~]$ export http_proxy=http://192.168.1.39:8080 [paul@RHEL5 ~]$ wget http://grep.be --14:35:49-- http://grep.be/ Connecting to 192.168.1.39:8080... connected. Proxy request sent, awaiting response... 200 OK Length: 5390 (5.3K) [text/html] Saving to: `index.html.1' 100%[================================>] 5,390 --.-K/s in 0.1s 14:38:29 (54.8 KB/s) - `index.html' saved [5390/5390] [paul@RHEL5 ~]$ 631
- 638. Part XXII. ipv6
- 639. Chapter 63. Introduction to ipv6 Table of Contents 63.1. about ipv6 .................................................................................................. 634 63.2. network id and host id ............................................................................... 634 63.3. host part generation .................................................................................... 634 63.4. ipv4 mapped ipv6 address ......................................................................... 635 63.5. link local addresses .................................................................................... 635 63.6. unique local addresses ............................................................................... 635 63.7. globally unique unicast addresses .............................................................. 635 63.8. 6to4 ............................................................................................................. 636 63.9. ISP .............................................................................................................. 636 63.10. non routable addresses ............................................................................. 636 63.11. ping6 ......................................................................................................... 636 63.12. Belgium and ipv6 ..................................................................................... 637 63.13. other websites ........................................................................................... 637 63.14. 6to4 gateways ........................................................................................... 639 63.15. ping6 and dns ........................................................................................... 639 63.16. ipv6 and tcp/http ...................................................................................... 639 63.17. ipv6 PTR record ....................................................................................... 639 63.18. 6to4 setup on Linux ................................................................................. 639 633
- 640. Introduction to ipv6 63.1. about ipv6 The ipv6 protocol is designed to replace ipv4. Where ip version 4 supports a maximum of four billion unique addresses, ip version 6 expands this to four billion times four billion times four billion times four billion unique addresses. This is more than 100.000.000.000.000.000.000 ipv6 addresses per square cm on our planet. That should be enough, even if every cell phone, every coffee machine and every pair of socks gets an address. Technically speaking ipv6 uses 128-bit addresses (instead of the 32-bit from ipv4). 128-bit addresses are huge numbers. In decimal it would amount up to 39 digits, in hexadecimal it looks like this: fe80:0000:0000:0000:0a00:27ff:fe8e:8aa8 Luckily ipv6 allows us to omit leading zeroes. Our address from above then becomes: fe80:0:0:0:a00:27ff:fe8e:8aa8 When a 16-bit block is zero, it can be written as ::. Consecutive 16-bit blocks that are zero can also be written as ::. So our address can from above can be shortened to: fe80::a00:27ff:fe8e:8aa8 This :: can only occur once! The following is not a valid ipv6 address: fe80::20:2e4f::39ac The ipv6 localhost address is 0000:0000:0000:0000:0000:0000:0000:0001, which can be abbreviated to ::1. paul@debian5:~/github/lt/images$ /sbin/ifconfig lo | grep inet6 inet6 addr: ::1/128 Scope:Host 63.2. network id and host id One of the few similarities between ipv4 and ipv6 is that addresses have a host part and a network part determined by a subnet mask. Using the cidr notation this looks like this: fe80::a00:27ff:fe8e:8aa8/64 The above address has 64 bits for the host id, theoretically allowing for 4 billion times four billion hosts. The localhost address looks like this with cidr: ::1/128 63.3. host part generation The host part of an automatically generated (stateless) ipv6 address contains part of the hosts mac address: 634
- 641. Introduction to ipv6 paul@debian5:~$ /sbin/ifconfig | head -3 eth3 Link encap:Ethernet HWaddr 08:00:27:ab:67:30 inet addr:192.168.1.29 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::a00:27ff:feab:6730/64 Scope:Link Some people are concerned about privacy here... 63.4. ipv4 mapped ipv6 address Some applications use ipv4 addresses embedded in an ipv6 address. (Yes there will be an era of migration with both ipv4 and ipv6 in use.) The ipv6 address then looks like this: ::ffff:192.168.1.42/96 Indeed a mix of decimal and hexadecimal characters... 63.5. link local addresses ipv6 addresses starting with fe8. can only be used on the local segment (replace the dot with an hexadecimal digit). This is the reason you see Scope:Link behind the address in this screenshot. This address serves only the local link. paul@deb503:~$ /sbin/ifconfig | grep inet6 inet6 addr: fe80::a00:27ff:fe8e:8aa8/64 Scope:Link inet6 addr: ::1/128 Scope:Host These link local addresses all begin with fe8.. Every ipv6 enabled nic will get an address in this range. 63.6. unique local addresses The now obsolete system of site local addresses similar to ipv4 private ranges is replaced with a system of globally unique local ipv6 addresses. This to prevent duplicates when joining of networks within site local ranges. All unique local addresses strat with fd... 63.7. globally unique unicast addresses Since ipv6 was designed to have multiple ip addresses per interface, the global ipv6 address can be used next to the link local address. These globally unique addresses all begin with 2... or 3... as the first 16-bits. 635
- 642. Introduction to ipv6 63.8. 6to4 6to4 is defined in rfc's 2893 and 3056 as one possible way to transition between ipv4 and ipv6 by creating an ipv6 tunnel. It encodes an ipv4 address in an ipv6 address that starts with 2002. For example 192.168.1.42/24 will be encoded as: 2002:c0a8:12a:18::1 You can use the command below to convert any ipv4 address to this range. paul@ubu1010:~$ printf "2002:%02x%02x:%02x%02x:%04x::1n" `echo 192.168.1.42/24 |tr "./" " "` 2002:c0a8:012a:0018::1 63.9. ISP Should you be so lucky to get an ipv6 address from an isp, then it will start with 2001:. 63.10. non routable addresses Comparable to example.com for DNS, the following ipv6 address ranges are reserved for examples, and not routable on the internet. 3fff:ffff::/32 2001:0db8::/32 63.11. ping6 Use ping6 to test connectivity between ipv6 hosts. You need to specify the interface (there is no routing table for 'random' generated ipv6 link local addresses). [root@fedora14 ~]# ping6 -I eth0 fe80::a00:27ff:fecd:7ffc PING fe80::a00:27ff:fecd:7ffc(fe80::a00:27ff:fecd:7ffc) from fe80::a00:27ff:fe3c:4346 et 64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=1 ttl=64 time=0.586 ms 64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=2 ttl=64 time=3.95 ms 64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=3 ttl=64 time=1.53 ms Below a multicast ping6 that recieves replies from three ip6 hosts on the same network. [root@fedora14 ~]# ping6 -I eth0 ff02::1 PING ff02::1(ff02::1) from fe80::a00:27ff:fe3c:4346 eth0: 56 data bytes 64 bytes from fe80::a00:27ff:fe3c:4346: icmp_seq=1 ttl=64 time=0.598 ms 64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=1 ttl=64 time=1.87 ms (DUP!) 64 bytes from fe80::8e7b:9dff:fed6:dff2: icmp_seq=1 ttl=64 time=535 ms (DUP!) 64 bytes from fe80::a00:27ff:fe3c:4346: icmp_seq=2 ttl=64 time=0.106 ms 64 bytes from fe80::8e7b:9dff:fed6:dff2: icmp_seq=2 ttl=64 time=1.79 ms (DUP!) 64 bytes from fe80::a00:27ff:fecd:7ffc: icmp_seq=2 ttl=64 time=2.48 ms (DUP!) 636
- 643. Introduction to ipv6 63.12. Belgium and ipv6 A lot of information on ipv6 in Belgium can be found at www.ipv6council.be. Sites like ipv6.belgium.be, www.bipt.be and www.bricozone.be are enabled for ipv6. Some Universities also: fundp.ac.be (Namur) and ulg.ac.be (Liege). 63.13. other websites Other useful websites for testing ipv6 are: test-ipv6.com ipv6-test.com Going to the ipv6-test.com website will test whether you have a valid accessible ipv6 address. Going to the test-ipv6.com website will also test whether you have a valid accessible ipv6 address. 637
- 644. Introduction to ipv6 638
- 645. Introduction to ipv6 63.14. 6to4 gateways To access ipv4 only websites when on ipv6 you can use sixxs.net (more specifically http://www.sixxs.net/tools/gateway/) as a gatway. For example use http://www.slashdot.org.sixxs.org/ instead of http://slashdot.org 63.15. ping6 and dns Below a screenshot of a ping6 from behind a 6to4 connection. 63.16. ipv6 and tcp/http Below a screenshot of a tcp handshake and http connection over ipv6. 63.17. ipv6 PTR record As seen in the DNS chapter, ipv6 PTR records are in the ip6.net domain, and have 32 generations of child domains. 63.18. 6to4 setup on Linux Below a transcript of a 6to4 setup on Linux. 639
- 646. Introduction to ipv6 Thanks to http://www.anyweb.co.nz/tutorial/v6Linux6to4 and http:// mirrors.bieringer.de/Linux+IPv6-HOWTO/ and tldp.org! root@mac:~# ifconfig eth0 Link encap:Ethernet HWaddr 00:26:bb:5d:2e:52 inet addr:81.165.101.125 Bcast:255.255.255.255 Mask:255.255.248.0 inet6 addr: fe80::226:bbff:fe5d:2e52/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:5926044 errors:0 dropped:0 overruns:0 frame:0 TX packets:2985892 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:4274849823 (4.2 GB) TX bytes:237002019 (237.0 MB) Interrupt:43 Base address:0x8000 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:598 errors:0 dropped:0 overruns:0 frame:0 TX packets:598 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:61737 (61.7 KB) TX bytes:61737 (61.7 KB) root@mac:~# sysctl -w net.ipv6.conf.default.forwarding=1 net.ipv6.conf.default.forwarding = 1 root@mac:~# ip tunnel add tun6to4 mode sit remote any local 81.165.101.125 root@mac:~# ip link set dev tun6to4 mtu 1472 up root@mac:~# ip link show dev tun6to4 10: tun6to4: <NOARP,UP,LOWER_UP> mtu 1472 qdisc noqueue state UNKNOWN link/sit 81.165.101.125 brd 0.0.0.0 root@mac:~# ip -6 addr add dev tun6to4 2002:51a5:657d:0::1/64 root@mac:~# ip -6 addr add dev eth0 2002:51a5:657d:1::1/64 root@mac:~# ip -6 addr add dev eth0 fdcb:43c1:9c18:1::1/64 root@mac:~# ifconfig eth0 Link encap:Ethernet HWaddr 00:26:bb:5d:2e:52 inet addr:81.165.101.125 Bcast:255.255.255.255 Mask:255.255.248.0 inet6 addr: fe80::226:bbff:fe5d:2e52/64 Scope:Link inet6 addr: fdcb:43c1:9c18:1::1/64 Scope:Global inet6 addr: 2002:51a5:657d:1::1/64 Scope:Global UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:5927436 errors:0 dropped:0 overruns:0 frame:0 TX packets:2986025 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:4274948430 (4.2 GB) TX bytes:237014619 (237.0 MB) Interrupt:43 Base address:0x8000 lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:598 errors:0 dropped:0 overruns:0 frame:0 TX packets:598 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:61737 (61.7 KB) TX bytes:61737 (61.7 KB) tun6to4 Link encap:IPv6-in-IPv4 inet6 addr: ::81.165.101.125/128 Scope:Compat inet6 addr: 2002:51a5:657d::1/64 Scope:Global UP RUNNING NOARP MTU:1472 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:0 (0.0 B) TX bytes:0 (0.0 B) 640
- 647. Introduction to ipv6 root@mac:~# ip -6 route add 2002::/16 dev tun6to4 root@mac:~# ip -6 route add ::/0 via ::192.88.99.1 dev tun6to4 metric 1 root@mac:~# ip -6 route show ::/96 via :: dev tun6to4 metric 256 mtu 1472 advmss 1412 hoplimit 0 2002:51a5:657d::/64 dev tun6to4 proto kernel metric 256 mtu 1472 advmss 1412 hoplimit 2002:51a5:657d:1::/64 dev eth0 proto kernel metric 256 mtu 1500 advmss 1440 hoplimit 2002::/16 dev tun6to4 metric 1024 mtu 1472 advmss 1412 hoplimit 0 fdcb:43c1:9c18:1::/64 dev eth0 proto kernel metric 256 mtu 1500 advmss 1440 hoplimit fe80::/64 dev eth0 proto kernel metric 256 mtu 1500 advmss 1440 hoplimit 0 fe80::/64 dev tun6to4 proto kernel metric 256 mtu 1472 advmss 1412 hoplimit 0 default via ::192.88.99.1 dev tun6to4 metric 1 mtu 1472 advmss 1412 hoplimit 0 root@mac:~# ping6 ipv6-test.com PING ipv6-test.com(ipv6-test.com) 56 data bytes 64 bytes from ipv6-test.com: icmp_seq=1 ttl=57 time=42.4 ms 64 bytes from ipv6-test.com: icmp_seq=2 ttl=57 time=43.0 ms 64 bytes from ipv6-test.com: icmp_seq=3 ttl=57 time=43.5 ms 64 bytes from ipv6-test.com: icmp_seq=4 ttl=57 time=43.9 ms 64 bytes from ipv6-test.com: icmp_seq=5 ttl=57 time=45.6 ms ^C --- ipv6-test.com ping statistics --- 5 packets transmitted, 5 received, 0% packet loss, time 4006ms rtt min/avg/max/mdev = 42.485/43.717/45.632/1.091 ms 641
- 648. Part XXIII. mysql database
- 649. Chapter 64. mysql Table of Contents 64.1. about mysql ................................................................................................ 643 64.2. installing mysql .......................................................................................... 643 64.3. mysql user management ............................................................................. 644 64.4. mysql databases ......................................................................................... 646 64.5. mysql tables ............................................................................................... 647 64.6. mysql records ............................................................................................. 649 64.7. mysql triggers ............................................................................................ 650 64.1. about mysql MySQL is a database server that understands Structured Query Language (SQL). MySQL was developed by the Swedish Company MySQL AB. The first release was in 1995. In 2008 MySQL AB was bought by Sun Microsystems (which is now owned by Oracle). MySQL is very popular for websites in combination with PHP and Apache (the M in LAMP servers), but mysql is also used in organizations with huge databases like Facebook, Flickr, Google, Nokia, Wikipedia and Youtube. 64.2. installing mysql On Debian/Ubuntu you can use aptitude install mysql-server to install the mysql server and client. On this particular system the -R switch is used to avoid installation of apparmor and exim4 (both are recommended but not required for mysql-server). root@laika:~# aptitude -R install mysql-server To verify the installed version, use dpkg -l on Debian/Ubuntu. This screenshot shows version 5.0 installed. paul@laika:~$ dpkg -l | grep mysql-server ii mysql-server 5.0.51a-3ubuntu5.4 MySQL database server (meta... ii mysql-server-5.0 5.0.51a-3ubuntu5.4 MySQL database server binaries Issue rpm -q to get version information about MySQL on RHEL5. [paul@RHEL52 ~]$ rpm -q mysql-server mysql-server-5.0.45-7.el5 643
- 650. mysql You will need at least version 5.0 to work with triggers. 64.3. mysql user management Linux users The installation of mysql creates a user account in /etc/passwd and a group account in /etc/group. paul@barry:~$ grep mysql /etc/passwd mysql:x:103:105:MySQL Server,,,:/var/lib/mysql:/bin/false paul@barry:~$ grep mysql /etc/group mysql:x:105: The mysql daemon mysqld will run with the credentials of this user and group. mysql root user With the mysqladmin command you can create a root user to administer your mysql server. It is advised to use a different password than the one for your Unix root account. After doing this, any user can issue mysql root commands using the -u root argument on the commandline. root@laika:~# mysqladmin -u root password SecretPass mysql client application You can now use mysql from the commandline by just typing mysql -u root -p and you 'll be asked for the password (of the mysql root account). In the screenshot below, the welcome message is cut. # Linux barry 2.6.17-2-686 #1 SMP Wed Sep 13 16:34:10 UTC 2006 i686 GNU/Linux paul@barry:~$ mysql -u root -p Enter password: <cut MySQL Welcome Message> mysql> exit Bye You could also put the password in clear text on the command line, but that would not be very secure. Anyone with access to your bash history would be able to read your mysql root password. paul@barry:~$ mysql -u root -pSecretPass <cut MySQL Welcome Message> mysql> exit Bye 644
- 651. mysql ~/.my.cnf You can save configuration in your home directory in the hidden file .my.cnf. In the screenshot below we put the root password in .my.cnf. paul@barry:~$ echo "[client]" > .my.cnf paul@barry:~$ echo password=SecretPass >> .my.cnf paul@barry:~$ cat .my.cnf [client] password=SecretPass paul@barry:~$ mysql -u root <cut MySQL Welcome Message> mysql> the mysql command line client You can use the mysql command to take a look at the databases, and to execute SQL queries on them. The screenshots below show you how. First, we log on to the mysql server. paul@laika:~$ mysql -u root -p Enter password: Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 8 Server version: 5.0.51a-3ubuntu5.4 (Ubuntu) Type 'help;' or 'h' for help. Type 'c' to clear the buffer. mysql> Next, we execute the command show databases. Every command must be terminated by a delimiter. The default delimiter is ; (the semicolon). mysql> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | demodb | | mysql | | netsec | | test | +--------------------+ 5 rows in set (0.00 sec) Type exit to get back to your Linux command prompt. mysql> exit Bye paul@laika:~$ 645
- 652. mysql 64.4. mysql databases listing all databases You can use the mysql command to take a look at the databases, and to execute SQL queries on them. The screenshots below show you how. First, we log on to our MySQL server and execute the command show databases to see which databases exist on our mysql server. paul@barry:~$ mysql -u root ... mysql> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | mysql | | wikidb | +--------------------+ 3 rows in set (0.01 sec) using a database Next we tell mysql to use one particular database with the use $database command. This screenshot shows how to make wikidb the current database (in use). mysql> use wikidb; Reading table information for completion of table and column names You can turn off this feature to get a quicker startup with -A Database changed creating databases You can create a new database with the create database command. mysql> create database demodb; Query OK, 1 row affected (0.01 sec) mysql> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | demodb | | mysql | +--------------------+ 3 rows in set (0.00 sec) 646
- 653. mysql access to databases To give everyone access to a mysql database, use the grant command. mysql> grant all on demodb.* to paul@localhost IDENTIFIED BY "stargate"; Query OK, 0 rows affected (0.00 sec) deleting databases When a database is no longer needed, you can permanently remove it with the drop database command. mysql> drop database demodb; Query OK, 1 row affected (0.09 sec) backup and restore databases You can take a backup of a database, or move it to another computer using the mysql and mysqldump commands. In the screenshot below, we take a backup of the wikidb database on the computer named laika. paul@laika:~$ mysqldump -u root wikidb > wikidb_backup_20070101.sql Then we ssh to another computer named barry and copy (with scp) the backup to barry. paul@laika:~$ ssh barry paul@barry:~$ scp laika:~/wikidb_backup_20070101.sql . wikidb_backup_20070101.sql 100% 1629KB 542.9KB/s 00:03 Then we restore the backup of wikidb on barry. paul@barry:~$ mysql -u root wikidb < wikidb_backup_20070101.sql 64.5. mysql tables listing tables You can see a list of tables in the current database with the show tables command. Our demodb is still empty. 647
- 654. mysql mysql> use demodb; Database changed mysql> show tables; Empty set (0.00 sec) Whereas the wikidb database in this truncated screenshot has 31 tables defined. mysql> use wikidb; Database changed mysql> show tables; +------------------+ | Tables_in_wikidb | +------------------+ | archive | | categorylinks | | externallinks | | filearchive | | hitcounter | | image | ... 31 rows in set (0.00 sec) creating a table The create table command will create a new table. This screenshot shows the creation of a country table. We use the country code as a primary key (all country codes are uniquely defined). Most country codes are two or three letters, so a char of three uses less space than a varchar of three. The country name and the name of the capital are both defined as varchar. The population can be seen as an integer. mysql> create table country ( -> countrycode char(3) NOT NULL, -> name varchar(50) NOT NULL, -> population int, -> capitalcity varchar(30), -> primary key (countrycode) -> ); Query OK, 0 rows affected (0.07 sec) You are allowed to type the create table command on one long line, but administrators often use multiple lines to improve readability. describing a table To see a description of the structure of a table, issue the describe $tablename command as shown below. mysql> describe country; +-------------+-------------+------+-----+---------+-------+ 648
- 655. mysql | Field | Type | Null | Key | Default | Extra | +-------------+-------------+------+-----+---------+-------+ | countrycode | char(3) | NO | PRI | NULL | | | name | varchar(50) | NO | | NULL | | | population | int(11) | YES | | NULL | | | capitalcity | varchar(30) | YES | | NULL | | +-------------+-------------+------+-----+---------+-------+ 4 rows in set (0.03 sec) removing a table To remove a table from a database, issue the drop table $tablename command as shown below. mysql> drop table country; Query OK, 0 rows affected (0.00 sec) 64.6. mysql records creating records Use insert to enter data into the table. The screenshot shows an insert statement that inserts values depending on the position of the data in the statement. mysql> insert into country values ('BE','Belgium','11000000','Brussels'); Query OK, 1 row affected (0.03 sec) Some administrators prefer to use uppercase for sql keywords. The mysql client accepts both. mysql> INSERT INTO country VALUES ('FR','France','65000000','Paris'); Query OK, 1 row affected (0.00 sec) viewing all records And finally an example of a simple select query to look at the contents of a table. mysql> select * from country; +-------------+---------+------------+-------------+ | countrycode | name | population | capitalcity | +-------------+---------+------------+-------------+ | BE | Belgium | 11000000 | Brussels | | FR | France | 65000000 | Paris | +-------------+---------+------------+-------------+ 2 rows in set (0.00 sec) 649
- 656. mysql updating records Consider the following insert statement. The capital of Spain is not Barcelona, it is Madrid. mysql> INSERT INTO country VALUES ('ES','Spain','46000000','Barcelona'); Query OK, 1 row affected (0.01 sec) Using an update statement, the record can be updated. mysql> update country set capitalcity='Madrid' where countrycode='ES'; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 viewing selected records Using a where clause in a select statement, you can specify which record(s) you want to see. mysql> select * from country where name='Spain'; +-------------+-------+------------+-------------+ | countrycode | name | population | capitalcity | +-------------+-------+------------+-------------+ | ES | Spain | 46000000 | Madrid | +-------------+-------+------------+-------------+ 1 row in set (0.00 sec) deleting records You can use the delete to permanently remove a record from a table. mysql> delete from country where countrycode='ES'; Query OK, 1 row affected (0.00 sec) mysql> select * from country where name='Spain'; Empty set (0.00 sec) 64.7. mysql triggers using a before trigger Consider the following create table command. The last field (amount) is the multiplication of the two fields named unitprice and unitcount. 650
- 657. mysql mysql> create table invoices ( -> id char(8) NOT NULL, -> customerid char(3) NOT NULL, -> unitprice int, -> unitcount smallint, -> amount int ); Query OK, 0 rows affected (0.00 sec) We can let mysql do the calculation for that by using a before trigger. The screenshot below shows the creation of a trigger that calculates the amount by multiplying two fields that are about to be inserted. mysql> create trigger total_amount before INSERT on invoices -> for each row set new.amount = new.unitprice * new.unitcount ; Query OK, 0 rows affected (0.02 sec) Here we verify that the trigger works by inserting a new record, without providing the total amount. mysql> insert into invoices values ('20090526','ABC','199','10',''); Query OK, 1 row affected (0.02 sec) Looking at the record proves that the trigger works. mysql> select * from invoices; +----------+------------+-----------+-----------+--------+ | id | customerid | unitprice | unitcount | amount | +----------+------------+-----------+-----------+--------+ | 20090526 | ABC | 199 | 10 | 1990 | +----------+------------+-----------+-----------+--------+ 1 row in set (0.00 sec) removing a trigger When a trigger is no longer needed, you can delete it with the drop trigger command. mysql> drop trigger total_amount; Query OK, 0 rows affected (0.00 sec) 651
- 658. Part XXIV. selinux
- 659. Chapter 65. introduction to SELinux(draft) Table of Contents 65.1. about selinux .............................................................................................. 653 65.2. selinux modes ............................................................................................ 654 65.3. activating selinux ....................................................................................... 654 65.4. getenforce ................................................................................................... 654 65.5. setenforce ................................................................................................... 654 65.6. sestatus ....................................................................................................... 655 65.7. logging ....................................................................................................... 655 65.8. DAC or MAC ............................................................................................ 656 65.9. ls -Z ............................................................................................................ 656 65.10. /selinux ..................................................................................................... 656 65.11. /etc/selinux/config .................................................................................... 657 65.12. identity ...................................................................................................... 657 65.13. type (or domain) ...................................................................................... 657 65.14. role ........................................................................................................... 658 65.15. security context ........................................................................................ 658 65.16. transition ................................................................................................... 658 65.17. policy ........................................................................................................ 659 65.18. extended attributes ................................................................................... 659 65.19. process security context ........................................................................... 659 65.20. chcon ........................................................................................................ 659 65.21. a practical example .................................................................................. 660 65.1. about selinux Security Enhanced Linux or SELinux is a set of modifications developed by the United States National Security Agency (NSA) to provide a variety of security policies for Linux. SELinux was released as open source at the end of 2000. Since kernel version 2.6 it is an integrated part of Linux. SELinux offers security! SELinux can control what kind of access users have to files and processes. Even when a file received chmod 777, SELinux can still prevent users from accessing it (unix file permissions are checked first!). SELinux does this by placing users in roles that represent a security context. Administrators have very strict control on access permissions granted to roles. SELinux is present in the latest versions of Red Hat Enterprise Linux, Debian, Fedora, Ubuntu, Yellow Dog Linux and Hardened Gentoo. There is currently (2008) limited support in Suse and Slackware. 653
- 660. introduction to SELinux(draft) 65.2. selinux modes selinux knows three modes: enforcing, permissive and disabled. The enforcing mode will enforce policies, and may deny access based on selinux rules. The permissive mode will not enforce policies, but can still log actions that would have been denied in enforcing mode. The disabled mode disables selinux. 65.3. activating selinux On RHEL you can use the GUI tool to activate selinux, on Debian there is the selinux- activate command.Activation requires a reboot. root@deb503:~# selinux-activate Activating SE Linux Searching for GRUB installation directory ... found: /boot/grub Searching for default file ... found: /boot/grub/default Testing for an existing GRUB menu.lst file ... found: /boot/grub/menu.lst Searching for splash image ... none found, skipping ... Found kernel: /boot/vmlinuz-2.6.26-2-686 Updating /boot/grub/menu.lst ... done SE Linux is activated. You may need to reboot now. 65.4. getenforce Use getenforce to verify whether selinux is enforced, disabled or permissive. [root@rhel55 ~]# getenforce Permissive The /selinux/enforce file contains 1 when enforcing, and 0 when permissive mode is ative. root@fedora13 ~# cat /selinux/enforce 1root@fedora13 ~# 65.5. setenforce You can use setenforce to switch between the Permissive or the Enforcing state once selinux is activated.. [root@rhel55 ~]# setenforce Enforcing [root@rhel55 ~]# getenforce Enforcing [root@rhel55 ~]# setenforce Permissive 654
- 661. introduction to SELinux(draft) [root@rhel55 ~]# getenforce Permissive 65.6. sestatus You can see the current selinux status and policy with the sestatus command. [root@rhel55 ~]# sestatus SELinux status: enabled SELinuxfs mount: /selinux Current mode: permissive Mode from config file: permissive Policy version: 21 Policy from config file: targeted 65.7. logging Verify that syslog is running and activated on boot to enable logging of deny messages in /var/log/messages. [root@rhel55 ~]# chkconfig --list syslog syslog 0:off 1:off 2:on 3:on 4:on 5:on 6:off Verify that auditd is running and activated on boot to enable logging of easier to read messages in /var/log/audit/audit.log. [root@rhel55 ~]# chkconfig --list auditd auditd 0:off 1:off 2:on 3:on 4:on 5:on 6:off If not activated, then run chkconfig --levels 2345 auditd on and service auditd start. [root@rhel55 ~]# service auditd status auditd (pid 1660) is running... [root@rhel55 ~]# service syslog status syslogd (pid 1688) is running... klogd (pid 1691) is running... The /var/log/messages log file will tell you that selinux is disabled. root@deb503:~# grep -i selinux /var/log/messages Jun 25 15:59:34 deb503 kernel: [ 0.084083] SELinux: Disabled at boot. Or that it is enabled. root@deb503:~# grep SELinux /var/log/messages | grep -i Init 655
- 662. introduction to SELinux(draft) Jun 25 15:09:52 deb503 kernel: [ 0.084094] SELinux: Initializing. 65.8. DAC or MAC Standard Unix permissions use Discretionary Access Control to set permissions on files. This means that a user that owns a file, can make it world readable by typing chmod 777 $file. With selinux the kernel will enforce Mandatory Access Control which strictly controls what processes or threads can do with files (superseding DAC). Processes are confined by the kernel to the minimum access they require. 65.9. ls -Z To see the DAC permissions on a file, use ls -l to display user and group owner and permissions (here rw-r--r--). root@deb503:~/selinux# touch test42.txt root@deb503:~/selinux# ls -l total 0 -rw-r--r-- 1 root root 0 2010-06-25 15:38 test42.txt For MAC permissions there is new -Z option added to ls. The output shows an selinux user named unconfined_u, a role named object_r, a type named unconfined_home_t, and a level S0. root@deb503:~/selinux# ls -Z unconfined_u:object_r:unconfined_home_t:s0 test42.txt 65.10. /selinux When selinux is active, there is a new virtual file system named /selinux. (You can compare it to /proc and /dev.) [root@RHEL5 ~]# ls /selinux/ access context mls avc create null booleans disable policyvers checkreqprot enforce relabel commit_pending_bools load user compat_net member Although some files in /selinux appear wih size 0, they often contain a boolean value. Check /selinux/enforce to see if selinux is running in enforced mode. 656
- 663. introduction to SELinux(draft) [root@RHEL5 ~]# ls -l /selinux/enforce -rw-r--r-- 1 root root 0 Apr 29 08:21 /selinux/enforce [root@RHEL5 ~]# echo `cat /selinux/enforce` 1 65.11. /etc/selinux/config The main configuration file for selinux is /etc/selinux/config. When in permissive mode, the file looks like this. [root@rhel55 ~]# more /etc/selinux/config # This file controls the state of SELinux on the system. # SELINUX= can take one of these three values: # enforcing - SELinux security policy is enforced. # permissive - SELinux prints warnings instead of enforcing. # disabled - SELinux is fully disabled. SELINUX=permissive # SELINUXTYPE= type of policy in use. Possible values are: # targeted - Only targeted network daemons are protected. # strict - Full SELinux protection. SELINUXTYPE=targeted 65.12. identity The SELinux Identity of a user is distinct from the user ID. An identity is part of a security context, and (via domains) determines what you can do. The screenshot shows user root having identity user_u. [root@rhel55 ~]# id -Z user_u:system_r:unconfined_t 65.13. type (or domain) The selinux domain is the security context of a process. An selinux domain determines what a process can do. The screenshot shows init running in domain init_t and the mingetty's running in domain getty_t. [root@RHEL5 ~]# ps fax -Z | grep init_t system_u:system_r:init_t 1 ? Ss 0:01 init [3] [root@RHEL5 ~]# ps fax -Z | grep getty_t system_u:system_r:getty_t 2941 tty1 Ss+ 0:00 /sbin/mingetty tty1 system_u:system_r:getty_t 2942 tty2 Ss+ 0:00 /sbin/mingetty tty2 The selinux type is similar to an selinux domain, but refers to directories and files instead of processes. 657
- 664. introduction to SELinux(draft) 65.14. role The selinux role defines the domains that can be used. A role is denied to enter a domain, unless the role is explicitely authorized to do so. 65.15. security context The combination of identity, role and domain or type make up the selinux security context. The id will show you your security context in the form identity:role:domain. [paul@RHEL5 ~]$ id | cut -d' ' -f4 context=user_u:system_r:unconfined_t The ls -Z command shows the security context for a file in the form identity:role:type. [paul@RHEL5 ~]$ ls -Z test -rw-rw-r-- paul paul user_u:object_r:user_home_t test The security context for processes visible in /proc defines both the type (of the file in /proc) and the domain (of the running process). Let's take a look at the init process and /proc/1/ . The init process runs in domain init_t. [root@RHEL5 ~]# ps -ZC init LABEL PID TTY TIME CMD system_u:system_r:init_t 1 ? 00:00:01 init The /proc/1/ directory, which identifies the init process, has type init_t. [root@RHEL5 ~]# ls -Zd /proc/1/ dr-xr-xr-x root root system_u:system_r:init_t /proc/1/ It is not a coincidence that the domain of the init process and the type of /proc/1/ are both init_t. Don't try to use chcon on /proc! It will not work. 65.16. transition An selinux transition (aka an selinux labelling) determines the security context that will be assigned. A transition of process domains is used when you execute a process. A transition of file type happens when you create a file. An example of file type transition. 658
- 665. introduction to SELinux(draft) [paul@RHEL5 ~]$ touch test [paul@RHEL5 ~]$ touch /tmp/test [paul@RHEL5 ~]$ ls -Z test -rw-rw-r-- paul paul user_u:object_r:user_home_t test [paul@RHEL5 ~]$ ls -Z /tmp/test -rw-rw-r-- paul paul user_u:object_r:tmp_t /tmp/test [paul@RHEL5 ~]$ 65.17. policy Everything comes together in an selinux policy. Policies define user access to roles, role access to domains and domain access to types. 65.18. extended attributes Extended attributes are use by selinux to store security contexts. These attributes can be viewed with ls when selinux is running. [root@RHEL5 home]# ls --context drwx------ paul paul system_u:object_r:user_home_dir_t paul drwxr-xr-x root root user_u:object_r:user_home_dir_t project42 drwxr-xr-x root root user_u:object_r:user_home_dir_t project55 [root@RHEL5 home]# ls -Z drwx------ paul paul system_u:object_r:user_home_dir_t paul drwxr-xr-x root root user_u:object_r:user_home_dir_t project42 drwxr-xr-x root root user_u:object_r:user_home_dir_t project55 [root@RHEL5 home]# When selinux is not running, then getfattr is the tool to use. [root@RHEL5 etc]# getfattr -m . -d hosts # file: hosts security.selinux="system_u:object_r:etc_t:s0000" 65.19. process security context A new option is added to ps to see the selinux security context of processes. [root@RHEL5 etc]# ps -ZC mingetty LABEL PID TTY TIME CMD system_u:system_r:getty_t 2941 tty1 00:00:00 mingetty system_u:system_r:getty_t 2942 tty2 00:00:00 mingetty 65.20. chcon Use chcon to change the selinux security context. 659
- 666. introduction to SELinux(draft) This example shows how to use chcon to change the type of a file. [root@rhel55 ~]# ls -Z /var/www/html/test42.txt -rw-r--r-- root root user_u:object_r:httpd_sys_content_t /var/www/html/test42.txt [root@rhel55 ~]# chcon -t samba_share_t /var/www/html/test42.txt [root@rhel55 ~]# ls -Z /var/www/html/test42.txt -rw-r--r-- root root user_u:object_r:samba_share_t /var/www/html/test42.txt 65.21. a practical example Tha apache webserver is by default targeted with selinux. The next screenshot shows that any file created in /var/www/html will by default get the http_sys_content_t type. [root@rhel55 ~]# touch /var/www/html/test42.txt [root@rhel55 ~]# ls -Z /var/www/html/test42.txt -rw-r--r-- root root user_u:object_r:httpd_sys_content_t /var/www/html/test42.txt Files created elsewhere do not get this type. [root@rhel55 ~]# touch /root/test42.txt [root@rhel55 ~]# ls -Z /root/test42.txt -rw-r--r-- root root user_u:object_r:user_home_t /root/test42.txt Make sure apache runs. [root@rhel55 ~]# service httpd start [ OK ] Will this work ? Yes it does. [root@rhel55 ~]# wget http://localhost/test42.txt --2010-06-26 15:40:28-- http://localhost/test42.txt Resolving localhost... 127.0.0.1 Connecting to localhost|127.0.0.1|:80... connected. HTTP request sent, awaiting response... 200 OK .... Why does this work ? Because apache runs in the httpd_t domain. [root@rhel55 ~]# ps -ZC httpd LABEL PID TTY TIME CMD user_u:system_r:httpd_t 2979 ? 00:00:07 httpd user_u:system_r:httpd_t 2981 ? 00:00:00 httpd user_u:system_r:httpd_t 2982 ? 00:00:00 httpd user_u:system_r:httpd_t 2983 ? 00:00:00 httpd user_u:system_r:httpd_t 2984 ? 00:00:00 httpd user_u:system_r:httpd_t 2985 ? 00:00:00 httpd user_u:system_r:httpd_t 2986 ? 00:00:00 httpd 660
- 667. introduction to SELinux(draft) user_u:system_r:httpd_t 2987 ? 00:00:00 httpd user_u:system_r:httpd_t 2988 ? 00:00:00 httpd So let's try to change the selinux type of this file. [root@rhel55 ~]# chcon -t samba_share_t /var/www/html/test42.txt [root@rhel55 ~]# ls -Z /var/www/html/test42.txt -rw-r--r-- root root user_u:object_r:samba_share_t /var/www/html/test42.txt There are two possibilities now: either it works, or it fails. It works when selinux is in permissive mode, it fails when in enforcing mode. [root@rhel55 ~]# wget http://localhost/test42.txt --2010-06-26 15:41:33-- http://localhost/test42.txt Resolving localhost... 127.0.0.1 Connecting to localhost|127.0.0.1|:80... connected. HTTP request sent, awaiting response... 200 OK ... The log file clearly shows that it would have failed in enforcing mode. [root@rhel55 ~]# grep test42 /var/log/audit/audit.log type=AVC msg=audit(1277559693.656:105): avc: denied { getattr } for pid=2982 comm="httpd" path="/var/www/html/test42.txt" dev=dm-0 ino=1974 99 scontext=user_u:system_r:httpd_t:s0 tcontext=user_u:object_r:samba_s hare_t:s0 tclass=file type=AVC msg=audit(1277559693.658:106): avc: denied { read } for pid =2982 comm="httpd" name="test42.txt" dev=dm-0 ino=197499 scontext=user_ u:system_r:httpd_t:s0 tcontext=user_u:object_r:samba_share_t:s0 tclass= file 661
- 668. Part XXV. Appendices
- 669. Appendix A. certifications A.1. Certification LPI: Linux Professional Institute LPIC Level 1 This is the junior level certification. You need to pass exams 101 and 102 to achieve LPIC 1 certification. To pass level one, you will need Linux command line, user management, backup and restore, installation, networking, and basic system administration skills. LPIC Level 2 This is the advanced level certification. You need to be LPIC 1 certified and pass exams 201 and 202 to achieve LPIC 2 certification. To pass level two, you will need to be able to administer medium sized Linux networks, including Samba, mail, news, proxy, firewall, web, and ftp servers. LPIC Level 3 This is the senior level certification. It contains one core exam (301) which tests advanced skills mainly about ldap. To achieve this level you also need LPIC Level 2 and pass a specialty exam (302 or 303). Exam 302 mainly focuses on Samba, and 303 on advanced security. More info on http://www.lpi.org. Ubuntu When you are LPIC Level 1 certified, you can take a LPI Ubuntu exam (199) and become Ubuntu certified. Red Hat Certified Engineer The big difference with most other certifications is that there are no multiple choice questions for RHCE. Red Hat Certified Engineers have to take a live exam consisting of two parts. First, they have to troubleshoot and maintain an existing but broken setup (scoring at least 80 percent), and second they have to install and configure a machine (scoring at least 70 percent). 663
- 670. certifications MySQL There are two tracks for MySQL certification; Certified MySQL 5.0 Developer (CMDEV) and Certified MySQL 5.0 DBA (CMDBA). The CMDEV is focused towards database application developers, and the CMDBA towards database administrators. Both tracks require two exams each. The MySQL cluster DBA certification requires CMDBA certification and passing the CMCDBA exam. Novell CLP/CLE To become a Novell Certified Linux Professional, you have to take a live practicum. This is a VNC session to a set of real SLES servers. You have to perform several tasks and are free to choose your method (commandline or YaST or ...). No multiple choice involved. Sun Solaris Sun uses the classical formula of multiple choice exams for certification. Passing two exams for an operating system gets you the Solaris Certified Administrator for Solaris X title. Other certifications There are many other lesser known certifications like EC council's Certified Ethical Hacker, CompTIA's Linux+, and Sair's Linux GNU. 664
- 671. Appendix B. keyboard settings B.1. about keyboard layout Many people (like US-Americans) prefer the default US-qwerty keyboard layout. So when you are not from the USA and want a local keyboard layout on your system, then the best practice is to select this keyboard at installation time. Then the keyboard layout will always be correct. Also, whenever you use ssh to remotely manage a linux system, your local keyboard layout will be used, independent of the server keyboard configuration. So you will not find much information on changing keyboard layout on the fly on linux, because not many people need it. Below are some tips to help you. B.2. X Keyboard Layout This is the relevant portion in /etc/X11/xorg.conf, first for Belgian azerty, then for US-qwerty. [paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf Option "XkbModel" "pc105" Option "XkbLayout" "be" [paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf Option "XkbModel" "pc105" Option "XkbLayout" "us" When in Gnome or KDE or any other graphical environment, look in the graphical menu in preferences, there will be a keyboard section to choose your layout. Use the graphical menu instead of editing xorg.conf. B.3. shell keyboard layout When in bash, take a look in the /etc/sysconfig/keyboard file. Below a sample US- qwerty configuration, followed by a Belgian azerty configuration. [paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard KEYBOARDTYPE="pc" KEYTABLE="us" [paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard KEYBOARDTYPE="pc" KEYTABLE="be-latin1" The keymaps themselves can be found in /usr/share/keymaps or /lib/kbd/keymaps. 665
- 672. keyboard settings [paul@RHEL5 ~]$ ls -l /lib/kbd/keymaps/ total 52 drwxr-xr-x 2 root root 4096 Apr 1 00:14 amiga drwxr-xr-x 2 root root 4096 Apr 1 00:14 atari drwxr-xr-x 8 root root 4096 Apr 1 00:14 i386 drwxr-xr-x 2 root root 4096 Apr 1 00:14 include drwxr-xr-x 4 root root 4096 Apr 1 00:14 mac lrwxrwxrwx 1 root root 3 Apr 1 00:14 ppc -> mac drwxr-xr-x 2 root root 4096 Apr 1 00:14 sun 666
- 673. Appendix C. hardware C.1. buses about buses Hardware components communicate with the Central Processing Unit or cpu over a bus. The most common buses today are usb, pci, agp, pci-express and pcmcia aka pc-card. These are all Plag and Play buses. Older x86 computers often had isa buses, which can be confugred using jumpers or dip switches. /proc/bus To list the buses recognized by the Linux kernel on your computer, look at the contents of the /proc/bus/ directory (screenshot from Ubuntu 7.04 and RHEL4u4 below). root@laika:~# ls /proc/bus/ input pccard pci usb [root@RHEL4b ~]# ls /proc/bus/ input pci usb Can you guess which of these two screenshots was taken on a laptop ? /usr/sbin/lsusb To list all the usb devices connected to your system, you could read the contents of /proc/bus/usb/devices (if it exists) or you could use the more readable output of lsusb, which is executed here on a SPARC system with Ubuntu. root@shaka:~# lsusb Bus 001 Device 002: ID 0430:0100 Sun Microsystems, Inc. 3-button Mouse Bus 001 Device 003: ID 0430:0005 Sun Microsystems, Inc. Type 6 Keyboard Bus 001 Device 001: ID 04b0:0136 Nikon Corp. Coolpix 7900 (storage) root@shaka:~# /var/lib/usbutils/usb.ids The /var/lib/usbutils/usb.ids file contains a gzipped list of all known usb devices. 667
- 674. hardware paul@barry:~$ zmore /var/lib/usbutils/usb.ids | head ------> /var/lib/usbutils/usb.ids <------ # # List of USB ID's # # Maintained by Vojtech Pavlik <vojtech@suse.cz> # If you have any new entries, send them to the maintainer. # The latest version can be obtained from # http://www.linux-usb.org/usb.ids # # $Id: usb.ids,v 1.225 2006/07/13 04:18:02 dbrownell Exp $ /usr/sbin/lspci To get a list of all pci devices connected, you could take a look at /proc/bus/pci or run lspci (partial output below). paul@laika:~$ lspci ... 00:06.0 FireWire (IEEE 1394): Texas Instruments TSB43AB22/A IEEE-139... 00:08.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-816... 00:09.0 Multimedia controller: Philips Semiconductors SAA7133/SAA713... 00:0a.0 Network controller: RaLink RT2500 802.11g Cardbus/mini-PCI 00:0f.0 RAID bus controller: VIA Technologies, Inc. VIA VT6420 SATA ... 00:0f.1 IDE interface: VIA Technologies, Inc. VT82C586A/B/VT82C686/A... 00:10.0 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.... 00:10.1 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.... ... C.2. interrupts about interrupts An interrupt request or IRQ is a request from a device to the CPU. A device raises an interrupt when it requires the attention of the CPU (could be because the device has data ready to be read by the CPU). Since the introduction of pci, irq's can be shared among devices. Interrupt 0 is always reserved for the timer, interrupt 1 for the keyboard. IRQ 2 is used as a channel for IRQ's 8 to 15, and thus is the same as IRQ 9. /proc/interrupts You can see a listing of interrupts on your system in /proc/interrupts. paul@laika:~$ cat /proc/interrupts 668
- 675. hardware CPU0 CPU1 0: 1320048 555 IO-APIC-edge timer 1: 10224 7 IO-APIC-edge i8042 7: 0 0 IO-APIC-edge parport0 8: 2 1 IO-APIC-edge rtc 10: 3062 21 IO-APIC-fasteoi acpi 12: 131 2 IO-APIC-edge i8042 15: 47073 0 IO-APIC-edge ide1 18: 0 1 IO-APIC-fasteoi yenta 19: 31056 1 IO-APIC-fasteoi libata, ohci1394 20: 19042 1 IO-APIC-fasteoi eth0 21: 44052 1 IO-APIC-fasteoi uhci_hcd:usb1, uhci_hcd:usb2,... 22: 188352 1 IO-APIC-fasteoi ra0 23: 632444 1 IO-APIC-fasteoi nvidia 24: 1585 1 IO-APIC-fasteoi VIA82XX-MODEM, VIA8237 dmesg You can also use dmesg to find irq's allocated at boot time. paul@laika:~$ dmesg | grep "irq 1[45]" [ 28.930069] ata3: PATA max UDMA/133 cmd 0x1f0 ctl 0x3f6 bmdma 0x2090 irq 14 [ 28.930071] ata4: PATA max UDMA/133 cmd 0x170 ctl 0x376 bmdma 0x2098 irq 15 C.3. io ports about io ports Communication in the other direction, from CPU to device, happens through IO ports. The CPU writes data or control codes to the IO port of the device. But this is not only a one way communication, the CPU can also use a device's IO port to read status information about the device. Unlike interrupts, ports cannot be shared! /proc/ioports You can see a listing of your system's IO ports via /proc/ioports. [root@RHEL4b ~]# cat /proc/ioports 0000-001f : dma1 0020-0021 : pic1 0040-0043 : timer0 0050-0053 : timer1 0060-006f : keyboard 0070-0077 : rtc 0080-008f : dma page reg 00a0-00a1 : pic2 00c0-00df : dma2 00f0-00ff : fpu 0170-0177 : ide1 02f8-02ff : serial ... 669
- 676. hardware C.4. dma about dma A device that needs a lot of data, interrupts and ports can pose a heavy load on the cpu. With dma or Direct Memory Access a device can gain (temporary) access to a specific range of the ram memory. /proc/dma Looking at /proc/dma might not give you the information that you want, since it only contains currently assigned dma channels for isa devices. root@laika:~# cat /proc/dma 1: parport0 4: cascade pci devices that are using dma are not listed in /proc/dma, in this case dmesg can be useful. The screenshot below shows that during boot the parallel port received dma channel 1, and the Infrared port received dma channel 3. root@laika:~# dmesg | egrep -C 1 'dma 1|dma 3' [ 20.576000] parport: PnPBIOS parport detected. [ 20.580000] parport0: PC-style at 0x378 (0x778), irq 7, dma 1... [ 20.764000] irda_init() -- [ 21.204000] pnp: Device 00:0b activated. [ 21.204000] nsc_ircc_pnp_probe() : From PnP, found firbase 0x2F8... [ 21.204000] nsc-ircc, chip->init 670
- 677. Appendix D. installing linux D.1. about The past couple of years the installation of linux has become a lot easier then before, at least for end users installing a distro like Ubuntu, Fedora, Debian or Mandrake on their home computer. Servers usually come pre-installed, and if not pre-installed, then setup of a linux server today is very easy. Linux can be installed in many different ways. End users most commonly use cdrom's or dvd's for installation, most of the time with a working internet connection te receive updates. Administrators might prefer network installations using protocols like tftp, bootp, rarp and/or nfs or response file solutions like Red Hat Kickstart or Solaris Jumpstart. D.2. installation by cdrom Installation of linux from cdrom is easy! Most distributions ask very few questions during install (keyboard type, language, username) and detect all the hardware themselves. There is usually no need to retrieve third-party drivers from the internet. The GUI installation gives options like Desktop (for end users), Workstation (for developers), Server or minimal (usually without graphical interface). D.3. installation with rarp and tftp Installing over the network involves powering on the machine, have it find a rarpd server to get an ip-address, then let it find an tftps server to get an installation image copied to the machine. This image can then boot. The procedure below demonstrates how to setup three Sun SPARC servers with Ubuntu Linux, using a Debian Linux machine to host the tftp, bootp and nfs daemons. First we need to configure the mac to ip resolution in the /etc/ethers configuration file. Each server will receive a unique ip-address during installation. root@laika:~# cat /etc/ethers 00:03:ba:02:c3:82 192.168.1.71 00:03:ba:09:7c:f9 192.168.1.72 00:03:ba:09:7f:d2 192.168.1.73 We need to install the rarpd and tftpd daemons on the (Debian) machine that will be hosting the install image. root@laika:~# aptitude install rarpd root@laika:~# aptitude install tftpd 671
- 678. installing linux The tftp services must be activated in inetd or xinetd. root@laika:~# cat /etc/inetd.conf | tail -1 tftp dgram udp wait nobody /usr/sbin/tcpd /usr/sbin/in.tftpd /srv/tftp And finally the linux install image must be present in the tftp served directory. The filename of the image must be the hex ip-address, this is accomplished with symbolic links. root@laika:~# ll /srv/tftp/ total 7.5M lrwxrwxrwx 1 root root 13 2007-03-02 21:49 C0A80147 -> ubuntu610.img lrwxrwxrwx 1 root root 13 2007-03-03 14:13 C0A80148 -> ubuntu610.img lrwxrwxrwx 1 root root 13 2007-03-02 21:49 C0A80149 -> ubuntu610.img -rw-r--r-- 1 paul paul 7.5M 2007-03-02 21:42 ubuntu610.img Time to enter boot net now in the openboot prompt. Twenty minutes later the three servers where humming with linux. D.4. about Red Hat kickstart Automating Linux installations with response files can be done with Red Hat kickstart. One way to set it up is by using the graphical tool /usr/sbin/system-config- kickstart. If you prefer to set it up manually, read on. You can modify the sample kickstart file RH-DOCS/sample.ks (can be found on the documentation dvd). Put this file so anaconda can read it. Anaconda is the Red Hat installer written in python. The name is chose because anacondas are lizard-eating pythons. Lizard is the name of the Caldera Linux installation program. Another option is to start with the /root/anaconda-ks.cfg file. This is a sample kickstart file that contains all the settings from your current installation. Do not change the order of the sections inside your kickstart file! The Red Hat System Administration Guide contains about 25 pages describing all the options, most of them are easy ti understand if you already performed a couple of installations. D.5. using kickstart To use kickstart, name your kickstart file ks.cfg and put it in the root directory of your installation cdrom (or on a usb stick or a floppy). For network based installations, name the file $ip-address-kickstart and place the following in dhcpd.conf. filename "/export/kickstart" next-server remote.installation.server 672
- 679. installing linux Leaving out the next-server line will result in the client looking for the file on the dhcp server itself. Booting from cdrom with kickstart requires the following command at the boot: prompt. linux ks=cdrom:/ks.cfg When the kickstart file is on the network, use nfs or http like in these examples. linux ks=nfs:servername:/path/to/ks.cfg linux ks=http://servername/path/to/ks.cfg 673
- 680. Appendix E. disk quotas E.1. About Disk Quotas To limit the disk space used by user, you can set up disk quotas. This requires adding usrquota and/or grpquota to one or more of the file systems in /etc/fstab. root@RHELv4u4:~# cat /etc/fstab | grep usrquota /dev/VolGroup00/LogVol02 /home ext3 usrquota,grpquota 0 0 Next you need to remount the file system. root@RHELv4u4:~# mount -o remount /home The next step is to build the quota.user and/or quota.group files. These files (called the quota files) contain the table of the disk usage on that file system. Use the quotacheck command to accomplish this. root@RHELv4u4:~# quotacheck -cug /home root@RHELv4u4:~# quotacheck -avug The -c is for create, u for user quota, g for group, a for checking all quota enabled file systems in /etc/fstab and v for verbose information. The next step is to edit individual user quotas with edquota or set a general quota on the file system with edquota -t. The tool will enable you to put hard (this is the real limit) and soft (allows a grace period) limits on blocks and inodes. The quota command will verify that quota for a user is set. You can have a nice overview with repquota. The final step (before your users start complaining about lack of disk space) is to enable quotas with quotaon(1). root@RHELv4u4:~# quotaon -vaug Issue the quotaoff command to stop all complaints. root@RHELv4u4:~# quotaoff -vaug E.2. Practice Disk quotas 1. Implement disk quotas on one of your new partitions. Limit one of your users to 10 megabyte. 2. Test that they work by copying many files to the quota'd partition. 674
- 681. Appendix F. introduction to vnc F.1. About VNC VNC can be configured in gnome or KDE using the Remote Desktop Preferences. VNC can be used to run your desktop on another computer, and you can also use it to see and take over the Desktop of another user. The last part can be useful for help desks to show users how to do things. VNC has the added advantage of being operating system independent, a lot of products (realvnc, tightvnc, xvnc, ...) use the same protocol on Solaris, Linux, BSD and more. F.2. VNC Server Starting the vnc server for the first time. [root@RHELv4u3 conf]# rpm -qa | grep -i vnc vnc-server-4.0-8.1 vnc-4.0-8.1 [root@RHELv4u3 conf]# vncserver :2 You will require a password to access your desktops. Password: Verify: xauth: creating new authority file /root/.Xauthority New 'RHELv4u3.localdomain:2 (root)' desktop is RHELv4u3.localdomain:2 Creating default startup script /root/.vnc/xstartup Starting applications specified in /root/.vnc/xstartup Log file is /root/.vnc/RHELv4u3.localdomain:2.log [root@RHELv4u3 conf]# F.3. VNC Client You can now use the vncviewer from another machine to connect to your vnc server. It will default to a very simple graphical interface... paul@laika:~$ vncviewer 192.168.1.49:2 VNC viewer version 3.3.7 - built Nov 20 2006 13:05:04 Copyright (C) 2002-2003 RealVNC Ltd. Copyright (C) 1994-2000 AT&T Laboratories Cambridge. See http://www.realvnc.com for information on VNC. VNC server supports protocol version 3.8 (viewer 3.3) Password: VNC authentication succeeded Desktop name "RHELv4u3.localdomain:2 (root)" Connected to VNC server, using protocol version 3.3 ... 675
- 682. introduction to vnc If you don't like the simple twm window manager, you can comment out the last two lines of ~/.vnc/xstartup and add a gnome-session & line to have vnc default to gnome instead. [root@RHELv4u3 ~]# cat .vnc/xstartup #!/bin/sh # Uncomment the following two lines for normal desktop: # unset SESSION_MANAGER # exec /etc/X11/xinit/xinitrc [ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup [ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources xsetroot -solid grey vncconfig -iconic & # xterm -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" & # twm & gnome-session & [root@RHELv4u3 ~]# Don't forget to restart your vnc server after changing this file. [root@RHELv4u3 ~]# vncserver -kill :2 Killing Xvnc process ID 5785 [root@RHELv4u3 ~]# vncserver :2 New 'RHELv4u3.localdomain:2 (root)' desktop is RHELv4u3.localdomain:2 Starting applications specified in /root/.vnc/xstartup Log file is /root/.vnc/RHELv4u3.localdomain:2.log F.4. Practice VNC 1. Use VNC to connect from one machine to another. 676
- 683. Appendix G. cloning G.1. About cloning You can have distinct goals for cloning a server. For instance a clone can be a cold iron backup system used for manual disaster recovery of a service. Or a clone can be created to serve in a test environment. Or you might want to make an almost identical server. Let's take a look at some offline and online ways to create a clone of a Linux server. G.2. About offline cloning The term offline cloning is used when you power off the running Linux server to create the clone. This method is easy since we don't have to consider open files and we don't have to skip virtual file systems like /dev or /sys . The offline cloning method c