Linux basics

Linux Operating
System

SANTOSH KHADSARE

Windows
Advantages:
• Easy to understand.
• GUI interface.

Disadvantages:
• Less Secure.
• More vulnerable.

Linux
Advantages:
• High security.
• More tools and automation.

Disadvantages:
• Hard to understand interface.
• Difficult to administrate.

Windows versus Linux
Server Type Windows Linux

Web Server IIS Apache

Database Server Spl Mysql,Postgresql

DNS Server Own Bind

FTP Server Own PROFTPD

Email Server MS exchange Sendmail, Qmail

Before Linux
• In 80’s, Microsoft’s DOS was the dominated OS
for PC
• Apple MAC was better, but expensive
• UNIX was much better, but much, much more
expensive. Only for minicomputer for commercial
applications
• People was looking for a UNIX based system,
which is cheaper and can run on PC
• Both DOS, MAC and UNIX were proprietary, i.e.,
the source code of their kernel is protected
• No modification is possible without paying high
license fees

GNU project

– Established in 1984 by Richard Stallman, who
believes that software should be free from
restrictions against copying or modification in
order to make better and efficient computer
programs
GNU is a recursive acronym for “GNU's Not Unix”
Aim at developing a complete Unix-like operating
system which is free for copying and modification
Companies make their money by maintaining and
distributing the software, e.g. optimally packaging the
software with different tools (Redhat, Slackware,
Mandrake, SuSE, etc)
Stallman built the first free GNU C Compiler in 1991.
But still, an OS was yet to be developed

Beginning of Linux

– A famous professor Andrew Tanenbaum
developed Minix, a simplified version of UNIX
that runs on PC
– Minix is for class teaching only. No intention for
commercial use
– In Sept 1991, Linus Torvalds, a second year
student of Computer Science at the University
of Helsinki, developed the preliminary kernel of
Linux, known as Linux version 0.0.1

What is good about using
Linux for forensics?

– Most tools are free of charge .
– Most tools are open source (Allows you to
know exactly what the tool is doing).
– Environment is very flexible.
– When tools are used properly, evidence should
stand up in court.

limitations of using Linux for
forensics?
– Most tools are free - you don’t get any tech
support and may not be able to call the author
in to court if necessary .
– Most tools are open source, which allows
those who oppose you in court to scour the
code for bugs and try to call the tool into
question.
– Environment is very flexible - it is often difficult
to decide what is the best way to do something
and there are no universally accepted
standard procedures.

limitations of using Linux for
forensics?
– Free tools are not as full featured as
commercial packages.
– Tools are significantly more complicated to
use than commercial packages and most
involve using the command line.
– In many operations, especially when imaging,
you can destroy your work or even your
original evidence with a typo.

Linux is Growing in Popularity
• Each new version becoming more user friendly.
– Disk installation no longer confusing.
– Installation interface more intuitive.
– Graphical environment becoming much more mature.
• More and more companies are embracing &
supporting Linux.
– IBM has teams of developers working on it.
– Apple’s OS now has a UNIX-like core.
– Novell is now in the Linux business.
• More and more devices are now running Linux
– Personal Devices: Cell Phones & PDA’s.
– Electronics: Video Recorders, MP3 Players.

Why Linux
• Linux is a powerful operating system.
– Many web sites use Linux as the operating system.
– Even Steve Ballmer of Microsoft said Linux has 60% of the server
market in 2008.
– Tolerant of a range of hardware platforms without special
configuration.
• Computer Forensics need to be able to consider server
forensics.
– Forensic issues can happen on server platforms too.
• Host-Based forensic tools often run on linux platforms.
– Free platform
– Flexible and reliable
– Easier to access low-level interfaces
– Good forensic qualities.
– Will consider Caine (a Linux live cd) for host-based forensics,
which runs The Forensic Toolkit and Autopsy.

Linux Shell
whoami
ls pwd
 Shell interprets the command and Bash, Tcsh, Zsh
request service from kernel
 Similar to DOS but DOS has only one
set of interface while Linux can select Kernel
different shell
– Bourne Again shell (Bash), TC shell
(Tcsh), Z shell (Zsh)
 Different shell has similar but different functionality
 Bash is the default for Linux
 Graphical user interface of Linux is in fact an application program work on
the shell

GRUB
• GNU GRUB (short for GNU GRand Unified
Bootloader) is a boot loader package from
the GNU Project.
• GRUB is the reference implementation of
the Multiboot Specification, which provides
a user the choice to boot one of multiple
operating systems installed on a computer
or select a specific kernel configuration
available on a particular operating system's
partitions.

KERNEL
• Typically, a kernel (or any comparable center of an operating
system) includes an interrupt handler that handles all
requests or completed I/O operations that compete for the
kernel's services, a scheduler that determines which
programs share the kernel's processing time in what order,
and a supervisor that actually gives use of the computer to
each process when it is scheduled.
• A kernel may also include a manager of the operating
system's address spaces in memory or storage, sharing
these among all components and other users of the kernel's
services. A kernel's services are requested by other parts of
the operating system or by application programs through a
specified set of program interfaces sometimes known as
system calls.

KERNEL VERSION
• UBUNTU 11.0 running
• What is kernel version?

KERNEL VERSION
• UBUNTU 11.0 –say company name
• TYPE umane –r to find the kernel .
• UBUNTU 11.0 has 2.6.38 kernel
version.
• We can also kind kernel when we
start a linux machine. (GRUB)

KDE and GNOME
• KDE and GNOME are two desktop
environments (collection of software that
provides certain functionality and a look and feel
for operating systems) that run on operating
systems that use X Window System (mostly
Unix, Linux, Solaris, FreeBSD, and Mac OS X).
•
KDE’s main programming language is C++. The
main reason for this is that the main functionality
of KDE is coded using QT, which is written in C+
+. It takes approximately 210MBs to install the
base system of KDE.

KDE and GNOME
• GNOME’s main programming language is C,
because the tool kit used to write GNOME is GTK+
and it is written in C. Approximately 180 MB is
required to install the base system of GNOME .

After the recent rebranding,“KDE” actually refers to
the whole collection of applications including the
desktop environment while GNOME refers to a
desktop environment alone.

Directory Tree
(root)

When you log on the the Linux OS
using your username you are
automatically located in your
home directory.

• /bin System binaries, including the command shell
• /boot Boot-up routines
• /dev Device files for all your peripherals
• /etc System configuration files
• /home User directories
• /lib Shared libraries and modules
• /lost+found Lost-cluster files, recovered from a disk-check
• /mnt Mounted file-systems
• /opt Optional software
•/proc Kernel-processes pseudo file-system
• /root Administrator’s home directory
• /sbin System administration binaries
•/usr User-oriented software
• /var Various other files: mail, spooling and logging

Directories
• /bin : This contains commands a user can run, like ‘ls’, but
which might be needed during boot.
• /dev : This contains devices, like the mouse.
• /home : This is where users store their files.
• /tmp : Temporary storage for users and the system
• /var : System files which can change.
• /etc : System config files which don’t change
• /lib : Where all the system libraries live
• /proc : Files which represent the running system (like
processes).
• /sbin : Commands which only an administrator would
want.
• /usr : Commands which are never needed during bootup.

The most important subdirectories
inside the root directory are:

• /bin : Important Linux commands available to the average user.
• /boot : The files necessary for the system to boot. Not all Linux
distributions use this one. Fedora does.
• /dev : All device drivers. Device drivers are the files that your
Linux system uses to talk to your hardware. For example,
there's a file in the /dev directory for your particular make and
model of monitor, and all of your Linux computer's
communications with the monitor go through that file.
• /etc : System configuration files.
• /home : Every user except root gets her own folder in here,
named for her login account. So, the user who logs in with linda
has the directory /home/linda, where all of her personal files are
kept.
• /lib : System libraries. Libraries are just bunches of
programming code that the programs on your system use to get
things done.

The most important subdirectories
inside the root directory are:

• /mnt : Mount points. When you temporarily load the contents of a
CD-ROM or USB drive, you typically use a special name under
/mnt. For example, many distributions (including Fedora) come, by
default, with the directory /mnt/cdrom, which is where your CD-ROM
drive's contents are made accessible.
• /root : The root user's home directory.
• /sbin : Essential commands that are only for the system
administrator.
• /tmp : Temporary files and storage space. Don't put anything in here
that you want to keep. Most Linux distributions (including Fedora)
are set up to delete any file that's been in this directory longer than
three days.
• /usr : Programs and data that can be shared across many systems
and don't need to be changed.
• /var : Data that changes constantly (log files that contain information
about what's happening on your system, data on its way to the
printer, and so on).

Important Subdirectories inside the
root directory are:

• /proc : virtual file sys(imp for forensics).
• /etc/passwd: contains all user specific info.
• /etc/shadow: stores password.

Home directory
• You can see what your home directory is
called by entering
• pwd (print current working directory)

Useful commands:
Command Description
ls List dir and files

dir See directory

Ifconfig See IP address config

Useradd Add new user

Reboot Reboot machine

Su Switch user

Wget To download any file for internet

Top See process activity

Ps Display processes

Useful commands:
Command Description
Vmstat Sys activity, hardware and sys info

Uptime Tell how long sys is running

Free Memory usages

Df –h See hard disk space

Cp Copy file or folder

Lostat Average CPU load, disk activity

Sar Collect and report sys activity

Ping Test contv

Netstat Network statistics

Useful commands:
Command Description
Iptraf Eal time network statistics

Tcpdump Detailed network traffic analysis

Cat Displays contents of the file

vim Opens text editor

Mount Mounts dir

Cd Change dir

Mkdir Make dir

Rmdir Remove dir

Pwdshow Show present working dir

The PROMPT
• Once you log into your machine, you are at
the prompt. Here you can perform your
commands.
• Everything on linux is either a file or a
directory.
• A file which is executed becomes a
process.
• Processes can be seen as files too.
• Devices, such as scanners and hard drives
are also files.

File System
●Windows uses letters of the alphabet to
represent different devices and different hard
disk partitions. Under Windows, you need to
know what volume (C:, D:,...) a file resides on
to select it, the file's physical location is part
of it's name.
●In Linux all directories are attached to the root
directory, which is identified by a forward-
slash, "/". - root.
● For example, below are some second-level
directories:

File System
●# - shell command.
●# fdisk -l /*list partitions*/
●/dev/sda1
 /dev – device
 /sda1 or /hda1
• sd – SATA /*SATA- tech to read/write data*/
• hd – IDE
• a

File System
 Sd ..a/b/c/d.....1/2/3.....
• a – primary master
• b – primary slave
• c – secondary master
• d – secondary slave
• 1/2/3... – first/second/third partition

File System
●# fdisk /dev/sda /*’m’ for help*/
●(if type ‘l’ it will list all available file sys with their
Id e.g. Windows -7 and Linux -83)
●(‘q’ to quit)
●/dev/sda1 - sys reserve
●/dev/sda2 - is C:
●(windows makes 2 partitions: 100 Mb(from 100 GB)
for sys reserve and remaining C: (100GB))

Redirection

• If you end a command with “>”, its output goes to
a file.
• If you end a command with “<“, its input comes
from a file.

Linux Help

●man
●info
●command –help
●Forums.

Man -k

• You can keyword search for commands
• For instance, what commands show a
calendar?

$ man -k calendar
cal (1) - displays a calendar
cal (1p) - print a calendar
difftime (3p) - compute the difference…

man command
– The “-k” option
● man –k print
– Manual pages are divided in 8 sections:
 User commands
 System calls
 Libc calls
 Devices
 File formats and protocols
 Games
 Conventions, macro packages and so
forth
 System administation
– To select correct section, add section
number:
● man 1 passwd, man 5 passwd

info command

●A program for reading documentation,
sometimes a replacement for manual pages
●Example : info ls

Linux Boot Sequence
• Start your computer that has Linux installed.

B IO S P o s t

L in u x B o o t L o a d e r

L i n u x K e r n e l I n it

I n it P ro g r a m

B a s h S h e ll S ta r t e d

RUN LEVELS
• They initiate sys call which interact with kernel.
• Run level 0 : shutdown.
• Run level 1 : single user interface.
• Run level 2 : multi user without NFS(new file sys) support.
• Run level 3 : multi user with NFS(new file sys) support.
• Run level 4 GUI.
• Run level 5 restart.

Boot sequence summary

•BIOS
•Master Boot Record (MBR)
•LILO or GRUB
•Kernel
•init
•Run Levels

The Linux Environment
What exactly is Linux?
• "Linux" is just an OS kernel
• the rest is additional Open Source Software
• together they are a "Linux Distribution"
• [Knoppix, Ubuntu, Redhat, Novell/SuSe]

Large choice of GUI and/or commandline
environments
• most popular are KDE and Gnome
• Unix-like, Mac-like, MSWindows-like, NeXT-like
• advanced shell environments
• web front-ends, GUI front-ends
• [KDE, Gnome, Windowmaker, bash, zsh, emacs, mc]

SECOND EXTENDED FILE SYSTEM(EXT2)

• Commonly used in Linux.
• Building blocks known as data blocks(4 Kb in
size) : similar to FAT32 file sys.
• Allocation of blocks similar to FAT32.Thus
physical sizes and logical sizes in EXT2 differ as
they do in FAT32.
• Main components:
 Directories.
 Inodes.
 Data blocks.


• Directories store the filenames in the file sys and
the inodes associated with the files.
• In EXT2, files are represented by inodes. An
inode contains attributes like file type, size,
access rights, timestamps address of data
blocks.
• An Inode is similar to a directory entry in FAT32.
Each inode has a unique no. identifying it.


• File sys made up of “block groups” a sequential
arrangement of a group of data blocks. The
entire file sys is thus managed as a series of
block groups.
• Block group meta data is provided by a ‘block
group descriptor’. It contains a copy of super
block(size of inode table and file sys), Block
bitmap(tracks allocation of each data block), part
of inode table and data blocks.

ext2 Characteristics
• Complicated internal structure to enhance
performance, but on-disk structure is
straightforward

From: Understanding the Linux Kernel (Bovet/Cesati)

ext2 Directory Structure

deleted


ext2/3: inode


ext3 Characteristics
• Binary compatible with ext2 on-disk
• Reason for existence: huge disks == huge
amounts of time to restore filesystem
consistency after improper shutdown
• (Must check inodes, etc.)
• Major improvement over ext2: log that stores
info about in-progress file operations
• On boot, can check log and quickly restore
filesystem consistency
• Journaling filesystems: DANGER!

File Deletion: Linux
• ext2 file deletion
– Adjust previous directory entry length to obscure deleted record
– No reorganization to make space in directories
– “first fit” for new directory entries, based on real name length
– Directory entry’s inode # is cleared
• ext3 file deletion
– Same as for ext2, but…
– inode is wiped on file deletion, so block numbers are lost
– Major anti-forensics issue!
– But directory entry’s inode # isn’t cleared…

Linux basics

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