Iptablesrocks

An iptables guide & tutorial - iptablesrocks.org

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Welcome to Iptablesrocks.org

Welcome to iptablesrocks.org! Iptablesrocks was created to provide a publicly available, easy to understand guide and tutorial for the installation and
configuration of iptables. Iptables is a linux package for managing IPv4 (and optionally IPv6) packet filtering and NAT (Network Address Translation).
Iptables can be configured to function as a firewall, a NAT mechanism, a packet filter or all 3 at once. The diversity of capabilities possible with iptables
makes it a secure, stable and economical alterative to hardware based firewalls and routers or costly 3rd party firewall software. Iptables can also
serve as a rendundant firewall in conjunction with a hardware firewall, providing you with an extra layer of security. Iptablesrocks.org covers the
installation of iptables in a Redhat environment, the syntax and structure associated with iptables and a collection a pre-configured iptables
configurations for a variety of applications. This site is currently under development. Enjoy!

The iptablesrocks.org iptables firewall setup guide Quick links to need specific iptables configurations
An overview of the iptablesrocks firewall general web server firewall

Step 1: System requirements & Pre-configuration linux gateway w/DSL connection

Step 2: Deploying a safety net. linux gateway w/Cable connection

Step 3: The iptables firewall ruleset sever to server trusted connections

Step 4: Testing functionality

Step 5: Installing Iptables Log Analyzer

Step 6: Maintaining & modifying your firewall and starting it on boot

Step 7: Feedback

A word of advise: Before you make any changes to your iptables configuration, you should read about pre-configuration precautions!

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Iptablesrocks.org - Syntax & Structure

Syntax & Structure

The iptables manual page is the best resource for learning all of the possible syntax options for iptables:

iptables man page

An iptables syntax quick reference

Taking a look at what you have

iptables -L [chain] - Lists your current iptables configuration

Making modifications

iptables -A [chain] - Append a rule to a desired chain in the current configuration

iptables -D [chain] - Delete a rule from a desired chain in the current configuration

iptables -R [chain] - Replace an existing rule from a desired chain in the current configuration

iptables - I [chain] - (that's a capital I as in Insert) Insert a new rule into a desired chain of the current configuration

iptables -N [chain] - Create a new chain

iptables -X [chain] - Delete a chain

Getting rid of all rules and starting over

iptable -X - Delete all chains

iptables -F - Flush the table of all contents

Importing & Exporting

iptables-save > filename - exports the current iptables configuration to a flat file.

iptables-restore < filename - imports an iptables configuration from a flat file and overwrites the current configuration.

More syntax & structure information coming soon!


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Iptablesrocks.org - Syntax & Structure


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Iptablesrocks.org - Examples

Iptables Ruleset Examples
This page contains already configured examples of several types of iptables configurations for a variety of different purposes. Feel free to use them as
is or make any needed adjustments to suit your particular needs.

A word of caution: Before you make any changes to your iptables configuration, you should read about pre-configuration precautions!

Example 1 - iptables setup for a full service web server

Summary: A hearty iptables configuration geared for web servers running web services (http and https), mail services, mysql, ftp services
and dns services.

When running a typical web server, you need a firewall that is secure and offers protection, but you also need one that allows all needed traffic to be
able to enter and leave the server so that your server's day to day operations can carry on unimpeded. This particular iptables configuration does the
following:

1. Uses the "mangle" portion of the table to effectively block and/or confuse most port scans. These rules deal address FIN,SYN,RST,PSH,ACK,URG
FIN,PSH and URG methods.

2. Establishes a locked down and secure foundation by initially blocking all incoming, outgoing and forwarded packets COMPLETELY.

3. With all incoming packets dropped by default, it then grants incoming access to a select number of ports. These ports are ports that a typical web
server might allow.

Open inbound ports include: 20 & 21 (ftp), 22 (ssh), 25 (smtp), 53 (dns), 80 (http), 110 (pop), 143 (imap), 443 (https), 993 (imaps)

4. With all outbound packets dropped by default, it then grants outbound access to a select number of ports. These ports are posts that a typical web
server might allow.

Open outbound ports include: 20 & 21 (ftp), 22 (ssh), 25 (smtp), 43 (whois), 53 (dns), 80 (http), 110 (pop), 143 (imap), 443 (https), 993 (imaps)

5. Both inbound and outbound connections are required to be in a "related and established" state. The "related and established" state requirement will
require you to add the "ip_conntrack_ftp" module into your server's configuration. This is typically done with the following command:

insmod ip_conntrack_ftp

6. ICMP packet types 3, 8 and 11 are dropped by default for all inbound traffic. This makes your server unpingable and reduces the chance of ping
ICMP based attacks. Pings are allowed for all outbound traffic.

7. Unauthorized inbound and outbound traffic is logged.

Example 2 - iptables setup for linux gateway on DSL connection

Example 3- iptables setup for linux gateway on a cable internet connection



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Iptablesrocks.org - Examples

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iptablesrocks.org - FAQ

Frequently Asked Questions
q How can I view my current iptables ruleset?
q How do I import an iptables script into my server's iptables ruleset?
q How do I export my server's iptables rulset to flat iptables script?
q How can I open up a port on my firewall?
q How can I close a port on my firewall?
q How can I completely block a host from reaching my server?
q How can I block a host from a specific port on my server, while allowing access to all other ports?
q How can I make mty firewall start up automatically when my server is booted?
q I've installed the iptablesrocks.org firewall and it's working, but now I cannot telnet to my server! What's wrong?
q I've already installed the Qmailrocks.org qmail install on my server. How should my iptables firewall config look?

Q: How can I view my current iptables ruleset?
A: The command "iptables -L" will display your server's current iptables ruleset.

Q: How do I import an iptables script into my server's iptables ruleset?
A: The command "iptables-restore < /path/to/firewall_script" will import a flat file iptables script into the server's ruleset.

Q: How do I export my server's iptables rulset to flat iptables script?
A: The command "iptables-save > /path/to/firewall_script" will export the current iptables ruleset to a flat file called "firewall_script".

Q: I've installed the iptablesrocks.org firewall and it's working, but now I cannot telnet to my server! What's wrong?
A: The iptablesrocks.org firewall does not allow telnet connections on port 23 by default. Whty? Simple. Telnet is dumb and totally insecure. Don't use
it. If you've been using telnet in the past to connect to your server, it's time to grow up and start using SSH instead. However, if you insist on using
telnet, you can simply add a rule to your firewall script to allow connections on port 23 (telnet).

Q: I've already installed the Qmailrocks.org qmail install on my server. How should my iptables firewall config look?
A: If you've installed qmail according to qmailrocks.org, the default iptablesrocks firewall configuration should work just fine with one exception. You
will want to add a rule to open port 783 both on the INPUT and OUTPUT chains so that Spamassassin can work.



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Iptablesrocks.org - Contact

Contact Iptablesrocks.org

If you would like to contact me, please fill out the form below and then click "Send It!".

Your name:
Your e-mail:
Comments/Questions:

Send It!



http://www.iptablesrocks.org/contact/ [2/13/2004 8:04:45 PM]

Iptablesrocks.org - Links

Links
A collection of useful links.

The Netfilter/Iptables Project - http://www.netfilter.org/

Linuxguruz Iptables Tutorial - http://www.linuxguruz.com/iptables/howto/iptables-HOWTO.html

Qmailrocks.org - My qmail installation guide

Djbdnsrocks.org - My djbdns installation guide



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iptablesrocks.org - Maintaining, modifying and more...

The iptablesrocks.org iptables firewall setup guide
Maintaining & modifying your firewall and starting it on boot:

Maintenance

Maintaining your iptables firewall is pretty easy. The only thing you will want to keep an eye on is the logs at /var/log/firewall. The logs can let you know
when your server is being probed or when an attack is being attempted, this allowing you to take appropriate action. Once you start viewing your logs,
you are going to notice that almost every hour of every day someone is going to be trying to gain access to your server, find an exploit on your server
or attack your server. This is normal. Attackers simply scan entire networks for vulnerable servers, so don't take scans and attempted intrusions
personally. Every publicly available server out there gets probed and scanned all the time, so don't think you are special and that you need to call the
FBI or something rash like that. The fact is that anytime your see unsuccessful attempts to break into your server or attack it, that's a good thing
because it means that the attackers were not able to succeed. It means your server is secure and your firewall is working. In other words, failures that
are logged are a sign of a secure server.

You may also want to make sure that you have a log rotation schedule set up for the firewall logs. The logs can get quite large, so you will want to
keep them in check by rotating them out on a regular basis.

Modifications

OK, so you've got your firewall up and running. But what if you need to make a change to the rules? What if you need to open up another port? What if
you want to close a certain unused port? What if you want to block someone from your server? Get the answers right here.

What if I want to open another port?

While it is possible to append rules to your live iptables configuration, I find that the best way to make changes is to change the firewall script itself and
then re-import the script back into the iptables configuration. In terms of this site's setup, this means that you will want to edit the /root/primary_firewall
file. Within this config file you can add rules or remove rules and then import the new firewall into the iptables ruleset.

So, let's take an example

Example: I want to allow inbound "tcp" traffic to port 139

To do this you would add the following line to the "INPUT" section of the /root/primary_firewall script:

-A INPUT -p tcp -m tcp --dport 139 -j ACCEPT

Let's go over what this line does...

-A - this tells iptables to "append" the new rule to the current iptables ruleset.

INPUT - The new rule will be appended to the "INPUT" portion of the ruleset, which controls inbound server traffic.

-p - Indicates what protocol the rule applies to. Popular protocols are "tcp", "udp", "icmp" and several others.

-m - Indicates a matching protocal value. Usually, this is set to the same value as the -p flag.

--dport - Specifies the destination port to which the traffic will be directed. In this case, it's port 139.

-j - Instructs the firewall to "jump" to specified state. In this case, request to TCP port 139 "jump" to "ACCEPT" and are threfore accepted and allowed
to pass through the firewall.

ACCEPT - As denoted above, this is the state that the rule "jumps" to. In the example above, any inbound traffic to TCP port 139 will "jump" to an
"ACCEPT" state, and thus will be able to pass through the firewall.

So the breakdown above should tell you that the rule we are adding is going to allow all inbound traffic to TCP port 139. Once you add that line to the

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"INPUT" section of the firewall script, you can then import the new firewall with the following command:

iptables-restore < /root/primary_firewall

Wow, now wasn't that easy?

What if I want to close a port?

The answer to this is similar to the previous question. However, you need to remember that your firewall, as it stands now, automatically blocks ALL
inbound, outbound and fowarded traffic by default. The only way that a port gets opened is if there is a rule telling the firewall to open that port.

Let's take an example...

Example: I was to close TCP port 143 inbound.

In the firewall script that comes with this guide, you will notice the following entry that open up inbound traffic to TCP port 143 (For IMAP). That rule
looks like this:


Well, if you wanted to close port 143, all you would have to do is edit the firewall script at /root/primary_firewall and remove that line. Once the line is
gone and you've saved the changes, you would activate the change by re-importing the firewall script back into the iptables ruleset like so

:iptables-restore < /root/primary_firewall

Now wasn't that easy?

Now, let's take another example so that I can demonstrate something that you DON'T need to worry about.

Example: I want to block all inbound traffic to TCP port 3389.

Again, let's take the firewall that comes with this guide. All of the sudden you get it in your head that you need to close TCP port 3389. What to do?
Well, if you take a look at the firewall script you will notice that TCP port 3389 is not mentioned anywhere in the script and, to be more specific, it is not
mentioned in the "INPUT" portion of the script. Well, since our firewall is configured to block ALL traffic by default and only open ports on request, 3389
is ALREADY CLOSED. In other words, you don't need to worry about that port because it has been automatically closed by the firewall. The only way
it would be open is if you created a specific rule that rquested that the port be open. Get it?

What if I want to completely block someone from my server?

The procedure for this is just the same as the ones above. Basically, all you're going to do is edit the firewall script, add a rule to block whoever, save
the changes and then re-import the firewall script back into the server's ruleset. Easy as hell.

So let's take an example:

Example: I want to block anyone from the host 1.2.3.4 from accessing my server.

Open the /root/primary_firewall script and add the following line to the INPUT section of the script:

-A INPUT -s 1.2.3.4 -j DROP

Now let's break that down to see what this rule is doing..

-A - this tells iptables to "append" the new rule to the current iptables ruleset.

INPUT - The new rule will be appended to the "INPUT" portion of the ruleset, which controls inbound server traffic.

-s - Specifies the source address of the request. In this case we are specifying 1.2.3.4 as the source.

-j - Instructs the firewall to "jump" to specified state. In this case, request coming from source 1.2.3.4 "jump" to a DENY state, thus blocking anyone
from that addresss from accessing your server at all.



DROP - As denoted above, this is the state that the rule "jumps" to. In the example above, any traffic to coming from 1.2.3.4 will "jump" to an "DROP"
state, and thus will be blocked.

What if I only want to block someone from accessing a certain port, while allowing them to access all others?

This would just be a more specific version of the above rule. In this case, you would specify the destination port and the protocal type with the "-p", "-
m" and "--dport" flags. If you wanted to block anyone from 1.2.3.4 from accessing port 25 on your server, it would look like this:

-A INPUT -s 1.2.3.4 -p tcp -m tcp --dport 25 -j DROP

Easy enough, right?

The previous questions are only a sampling of the many, many modifications and customizations that can be made to your firewall. Check out the
iptables man page (man iptables) for the whole gamut.

As I said before, always make sure you enable your iptables "safetynet" before you start playing with your firewall setup. You don't want to accdientally
lock yourself out of your server!

Starting your fiewall on boot

The final part of this setup guide will cover the integration of your firewall into the server's boot process. If you would like your firewall to start on boot
and work correctly, you will want to do the following:

cp /root/primary_firewall /etc/sysconfig/iptables

This will cause your iptables config script to be loaded at boot time.The last step is to make sure that the "ip_conntrack_ftp" kernel module
loads on boot as well to ensure correct FTP functionality.

vi /etc/rc.local

add the following line:

/sbin/insmod ip_conntrack_ftp

Save and exit the file.

That's it! You're all done. Enjoy your new firewall! Please take a moment to go to the next page and provide me with some feedback and/or
commentary.

Proceed to the next step




iptablesrocks.org - Feedback

Feedback:

Now that you've finished the iptablesrocks.org iptables setup guide, please provide me with some feedback.

What do you think of the setup guide? It stinks! Not very good It was ok Very helpful Excellent

Your Name: (required)

Your E-mail: (required)
Any comments/suggestions?

May we post your comments on this website? Yes No
(your name & e-mail address will not be posted)

Submit Feedback

A note on privacy: Some people worry about providing their e-mail address on the form. You don't need to worry. The only
reason I require your e-mail address is so that I can personally reply to any questions or comments you may have. I won't put
your e-mail address on any list or anything like that. I'm hate spam just as much as you do. :)



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iptablesrocks.org - safety precautions

iptables pre-configuration safety precautions
Making changes to your server's firewall/routing configuration can be very tricky. One simple mistake can result in your being completely locked out of
your server. If the server you are working on is remotely hosted, this can cause prolonged downtime and a tech support nightmare. In short, it can be a
major pain in the ass.

For this reason, anytime you are making changes to your server's firewall or routing config via iptables, you should first take a few minutes to
implement some "disaster recovery" measures that can be automatically triggered should you accidentally lock yourself out of your server.

Whenver I'm making any major changes to my firewall, I like to implement a "disaster recovery" measure by means of a crontab entry. I do this by
creating a simple crontab entry that runs every 15 minutes or so. This crontab entry runs a command which simply resets my server's iptables
configuration to a completely open setting. That is, it sets my iptables configuration so that the server will accept all incoming, outgoing and forward
requests. In this way, should I screw up at some point and lock myself out of my server, I will only need to wait a maximum of 10 minutes before the
bad firewall rules will be dropped and I'll be able to shell into my server again. If you are a newbie at iptables, or even a seasoned pro, this little safety
measure can be a lifesaver. Everyone makes mistakes every now and then, and this measure will ensure that a mistake made in your firewall
configuration doesn't turn into a nightmare!

So here's how you do it....

Log into your server as the root user.

First, create a iptables import script that, when imported, will reset the iptables configuration to a default "accept all" state:

vi /root/iptables.open

*filter
:INPUT ACCEPT [164:15203]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [147:63028]
COMMIT
*mangle
:PREROUTING ACCEPT [164:15203]
:POSTROUTING ACCEPT [147:63028]
COMMIT
*nat
COMMIT

Now save and exit.

Now let's set up a crontab entry that will import the reset configuration every 15 minutes...

crontab -e

*/15 * * * * /sbin/iptables-restore < /root/iptables.open

Save and exit from the crontab editor

Ok. All done.

Now let's review what this is going to do to your server...

Once you enable the crontab, your server is going to reset it's iptables configuration every 15 minutes. This means that every 15 minutes you will lose

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iptablesrocks.org - safety precautions

any previous settings and you server will be in a "accept all" state where it accepts any incoming, outgoing and forward packet requests. You certainly
do not want to leave this crontab running if you are not actively working on your iptables configuration. The sole of this crontab is to provide a safety
net should you accidentally screw up and lock yourself out of your server while you are implementing or testing new iptables configurations. With this
crontab running, should you lock yourself out you will only have to wait a maximum of 15 minutes before the iptables configuration resets and you are
able to log into your server again.

You should always disable this crontab entry once you are finished modifying your iptables configuration.



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# import this saved configuration into your iptables configuration with the following command:
# iptables-restore < web_server.config

*nat
COMMIT

*mangle
:INPUT ACCEPT [444:43563] :FORWARD ACCEPT [0:0]
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG FIN,PSH,URG -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN,RST,PSH,ACK,URG NONE -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags SYN,RST SYN,RST -j DROP
-A PREROUTING -p tcp -m tcp --tcp-flags FIN,SYN FIN,SYN -j DROP
COMMIT

*filter
:INPUT DROP [1:242]
:FORWARD DROP [0:0]
:OUTPUT DROP [0:0]
:icmp_packets - [0:0]
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p udp -m udp --dport 53 -j ACCEPT
-A INPUT -s 127.0.0.1 -j ACCEPT
-A INPUT -p icmp -j icmp_packets
-A INPUT -j LOG --log-prefix "IPTABLES-IN Default Drop: " --log-level 7

-A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp -m tcp --dport 20 -j ACCEPT

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-A OUTPUT -p udp -m udp --dport 53 -j ACCEPT
-A OUTPUT -d 127.0.0.1 -j ACCEPT
-A OUTPUT -p icmp -j icmp_packets
-A OUTPUT -j LOG --log-prefix "IPTABLES-OUT Default Drop: " --log-level 7

-A icmp_packets -p icmp -m icmp --icmp-type 0 -j ACCEPT
-A icmp_packets -s 127.0.0.1 -p icmp -m icmp --icmp-type 8 -j ACCEPT
-A icmp_packets -p icmp -m icmp --icmp-type 8 -j DROP
COMMIT

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Manpage of IPTABLES

Content-type: text/html

IPTABLES
Section: (8)
Updated: Mar 09, 2002
Index Return to Main Contents

NAME
iptables - administration tool for IPv4 packet filtering and NAT

SYNOPSIS
iptables [-t table] -[ADC] chain rule-specification [options]
iptables [-t table] -I chain [rulenum] rule-specification [options]
iptables [-t table] -R chain rulenum rule-specification [options]
iptables [-t table] -D chain rulenum [options]
iptables [-t table] -[LFZ] [chain] [options]
iptables [-t table] -N chain
iptables [-t table] -X [chain]
iptables [-t table] -P chain target [options]
iptables [-t table] -E old-chain-name new-chain-name

DESCRIPTION
Iptables is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel.
Several different tables may be defined. Each table contains a number of built-in chains and may also
contain user-defined chains.

Each chain is a list of rules which can match a set of packets. Each rule specifies what to do with a packet
that matches. This is called a `target', which may be a jump to a user-defined chain in the same table.

TARGETS
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Manpage of IPTABLES

A firewall rule specifies criteria for a packet, and a target. If the packet does not match, the next rule in
the chain is the examined; if it does match, then the next rule is specified by the value of the target, which
can be the name of a user-defined chain or one of the special values ACCEPT, DROP, QUEUE, or
RETURN.

ACCEPT means to let the packet through. DROP means to drop the packet on the floor. QUEUE means
to pass the packet to userspace (if supported by the kernel). RETURN means stop traversing this chain
and resume at the next rule in the previous (calling) chain. If the end of a built-in chain is reached or a
rule in a built-in chain with target RETURN is matched, the target specified by the chain policy
determines the fate of the packet.

TABLES
There are currently three independent tables (which tables are present at any time depends on the kernel
configuration options and which modules are present).

-t, --table table
This option specifies the packet matching table which the command should operate on. If the
kernel is configured with automatic module loading, an attempt will be made to load the
appropriate module for that table if it is not already there.

The tables are as follows:
filter
This is the default table (if no -t option is passed). It contains the built-in chains INPUT (for
packets coming into the box itself), FORWARD (for packets being routed through the box), and
OUTPUT (for locally-generated packets).
nat
This table is consulted when a packet that creates a new connection is encountered. It consists of
three built-ins: PREROUTING (for altering packets as soon as they come in), OUTPUT (for
altering locally-generated packets before routing), and POSTROUTING (for altering packets as
they are about to go out).
mangle
This table is used for specialized packet alteration. Until kernel 2.4.17 it had two built-in chains:
PREROUTING (for altering incoming packets before routing) and OUTPUT (for altering locally-
generated packets before routing). Since kernel 2.4.18, three other built-in chains are also
supported: INPUT (for packets coming into the box itself), FORWARD (for altering packets
being routed through the box), and POSTROUTING (for altering packets as they are about to go
out).


Manpage of IPTABLES

OPTIONS
The options that are recognized by iptables can be divided into several different groups.

COMMANDS

These options specify the specific action to perform. Only one of them can be specified on the command
line unless otherwise specified below. For all the long versions of the command and option names, you
need to use only enough letters to ensure that iptables can differentiate it from all other options.

-A, --append chain rule-specification
Append one or more rules to the end of the selected chain. When the source and/or destination
names resolve to more than one address, a rule will be added for each possible address
combination.
-D, --delete chain rule-specification
-D, --delete chain rulenum
Delete one or more rules from the selected chain. There are two versions of this command: the rule
can be specified as a number in the chain (starting at 1 for the first rule) or a rule to match.
-I, --insert chain [rulenum] rule-specification
Insert one or more rules in the selected chain as the given rule number. So, if the rule number is 1,
the rule or rules are inserted at the head of the chain. This is also the default if no rule number is
specified.
-R, --replace chain rulenum rule-specification
Replace a rule in the selected chain. If the source and/or destination names resolve to multiple
addresses, the command will fail. Rules are numbered starting at 1.
-L, --list [chain]
List all rules in the selected chain. If no chain is selected, all chains are listed. As every other
iptables command, it applies to the specified table (filter is the default), so NAT rules get listed by

iptables -t nat -n -L
Please note that it is often used with the -n option, in order to avoid long reverse DNS lookups. It
is legal to specify the -Z (zero) option as well, in which case the chain(s) will be atomically listed
and zeroed. The exact output is affected by the other arguments given. The exact rules are
suppressed until you use

iptables -L -v
-F, --flush [chain]
Flush the selected chain (all the chains in the table if none is given). This is equivalent to deleting
all the rules one by one.
-Z, --zero [chain]
Zero the packet and byte counters in all chains. It is legal to specify the -L, --list (list) option as
well, to see the counters immediately before they are cleared. (See above.)


Manpage of IPTABLES

-N, --new-chain chain
Create a new user-defined chain by the given name. There must be no target of that name already.
-X, --delete-chain [chain]
Delete the optional user-defined chain specified. There must be no references to the chain. If there
are, you must delete or replace the referring rules before the chain can be deleted. If no argument
is given, it will attempt to delete every non-builtin chain in the table.
-P, --policy chain target
Set the policy for the chain to the given target. See the section TARGETS for the legal targets.
Only built-in (non-user-defined) chains can have policies, and neither built-in nor user-defined
chains can be policy targets.
-E, --rename-chain old-chain new-chain
Rename the user specified chain to the user supplied name. This is cosmetic, and has no effect on
the structure of the table.
-h
Help. Give a (currently very brief) description of the command syntax.

PARAMETERS

The following parameters make up a rule specification (as used in the add, delete, insert, replace and
append commands).

-p, --protocol [!] protocol
The protocol of the rule or of the packet to check. The specified protocol can be one of tcp, udp,
icmp, or all, or it can be a numeric value, representing one of these protocols or a different one. A
protocol name from /etc/protocols is also allowed. A "!" argument before the protocol inverts the
test. The number zero is equivalent to all. Protocol all will match with all protocols and is taken as
default when this option is omitted.
-s, --source [!] address[/mask]
Source specification. Address can be either a network name, a hostname (please note that
specifying any name to be resolved with a remote query such as DNS is a really bad idea), a
network IP address (with /mask), or a plain IP address. The mask can be either a network mask or
a plain number, specifying the number of 1's at the left side of the network mask. Thus, a mask of
24 is equivalent to 255.255.255.0. A "!" argument before the address specification inverts the
sense of the address. The flag --src is an alias for this option.
-d, --destination [!] address[/mask]
Destination specification. See the description of the -s (source) flag for a detailed description of
the syntax. The flag --dst is an alias for this option.
-j, --jump target
This specifies the target of the rule; i.e., what to do if the packet matches it. The target can be a
user-defined chain (other than the one this rule is in), one of the special builtin targets which
decide the fate of the packet immediately, or an extension (see EXTENSIONS below). If this


Manpage of IPTABLES

option is omitted in a rule, then matching the rule will have no effect on the packet's fate, but the
counters on the rule will be incremented.
-i, --in-interface [!] name
Name of an interface via which a packet is going to be received (only for packets entering the
INPUT, FORWARD and PREROUTING chains). When the "!" argument is used before the
interface name, the sense is inverted. If the interface name ends in a "+", then any interface which
begins with this name will match. If this option is omitted, any interface name will match.
-o, --out-interface [!] name
Name of an interface via which a packet is going to be sent (for packets entering the FORWARD,
OUTPUT and POSTROUTING chains). When the "!" argument is used before the interface
name, the sense is inverted. If the interface name ends in a "+", then any interface which begins
with this name will match. If this option is omitted, any interface name will match.
[!] -f, --fragment
This means that the rule only refers to second and further fragments of fragmented packets. Since
there is no way to tell the source or destination ports of such a packet (or ICMP type), such a
packet will not match any rules which specify them. When the "!" argument precedes the "-f" flag,
the rule will only match head fragments, or unfragmented packets.
-c, --set-counters PKTS BYTES
This enables the administrator to initialize the packet and byte counters of a rule (during INSERT,
APPEND, REPLACE operations).

OTHER OPTIONS

The following additional options can be specified:

-v, --verbose
Verbose output. This option makes the list command show the interface name, the rule options (if
any), and the TOS masks. The packet and byte counters are also listed, with the suffix 'K', 'M' or
'G' for 1000, 1,000,000 and 1,000,000,000 multipliers respectively (but see the -x flag to change
this). For appending, insertion, deletion and replacement, this causes detailed information on the
rule or rules to be printed.
-n, --numeric
Numeric output. IP addresses and port numbers will be printed in numeric format. By default, the
program will try to display them as host names, network names, or services (whenever applicable).
-x, --exact
Expand numbers. Display the exact value of the packet and byte counters, instead of only the
rounded number in K's (multiples of 1000) M's (multiples of 1000K) or G's (multiples of 1000M).
This option is only relevant for the -L command.
--line-numbers
When listing rules, add line numbers to the beginning of each rule, corresponding to that rule's


Manpage of IPTABLES

position in the chain.
--modprobe=command
When adding or inserting rules into a chain, use command to load any necessary modules (targets,
match extensions, etc).

MATCH EXTENSIONS
iptables can use extended packet matching modules. These are loaded in two ways: implicitly, when -p or
--protocol is specified, or with the -m or --match options, followed by the matching module name; after
these, various extra command line options become available, depending on the specific module. You can
specify multiple extended match modules in one line, and you can use the -h or --help options after the
module has been specified to receive help specific to that module.

The following are included in the base package, and most of these can be preceded by a ! to invert the
sense of the match.

tcp

These extensions are loaded if `--protocol tcp' is specified. It provides the following options:

--source-port [!] port[:port]
Source port or port range specification. This can either be a service name or a port number. An
inclusive range can also be specified, using the format port:port. If the first port is omitted, "0" is
assumed; if the last is omitted, "65535" is assumed. If the second port greater then the first they
will be swapped. The flag --sport is a convenient alias for this option.
--destination-port [!] port[:port]
Destination port or port range specification. The flag --dport is a convenient alias for this option.
--tcp-flags [!] mask comp
Match when the TCP flags are as specified. The first argument is the flags which we should
examine, written as a comma-separated list, and the second argument is a comma-separated list of
flags which must be set. Flags are: SYN ACK FIN RST URG PSH ALL NONE. Hence the
command

iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN
will only match packets with the SYN flag set, and the ACK, FIN and RST flags unset.
[!] --syn
Only match TCP packets with the SYN bit set and the ACK and FIN bits cleared. Such packets are
used to request TCP connection initiation; for example, blocking such packets coming in an
interface will prevent incoming TCP connections, but outgoing TCP connections will be
unaffected. It is equivalent to --tcp-flags SYN,RST,ACK SYN. If the "!" flag precedes the "--


Manpage of IPTABLES

syn", the sense of the option is inverted.
--tcp-option [!] number
Match if TCP option set.
--mss value[:value]
Match TCP SYN or SYN/ACK packets with the specified MSS value (or range), which control the
maximum packet size for that connection.

udp

These extensions are loaded if `--protocol udp' is specified. It provides the following options:

--source-port [!] port[:port]
Source port or port range specification. See the description of the --source-port option of the TCP
extension for details.
--destination-port [!] port[:port]
Destination port or port range specification. See the description of the --destination-port option of
the TCP extension for details.

icmp

This extension is loaded if `--protocol icmp' is specified. It provides the following option:

--icmp-type [!] typename
This allows specification of the ICMP type, which can be a numeric ICMP type, or one of the
ICMP type names shown by the command

iptables -p icmp -h

mac

--mac-source [!] address
Match source MAC address. It must be of the form XX:XX:XX:XX:XX:XX. Note that this only
makes sense for packets coming from an Ethernet device and entering the PREROUTING,
FORWARD or INPUT chains.


Manpage of IPTABLES

limit

This module matches at a limited rate using a token bucket filter. A rule using this extension will match
until this limit is reached (unless the `!' flag is used). It can be used in combination with the LOG target
to give limited logging, for example.

--limit rate
Maximum average matching rate: specified as a number, with an optional `/second', `/minute',
`/hour', or `/day' suffix; the default is 3/hour.
--limit-burst number
Maximum initial number of packets to match: this number gets recharged by one every time the
limit specified above is not reached, up to this number; the default is 5.

multiport

This module matches a set of source or destination ports. Up to 15 ports can be specified. It can only be
used in conjunction with -p tcp or -p udp.

--source-ports port[,port[,port...]]
Match if the source port is one of the given ports. The flag --sports is a convenient alias for this
option.
--destination-ports port[,port[,port...]]
Match if the destination port is one of the given ports. The flag --dports is a convenient alias for
this option.
--ports port[,port[,port...]]
Match if the both the source and destination ports are equal to each other and to one of the given
ports.

mark

This module matches the netfilter mark field associated with a packet (which can be set using the MARK
target below).

--mark value[/mask]
Matches packets with the given unsigned mark value (if a mask is specified, this is logically


Manpage of IPTABLES

ANDed with the mask before the comparison).

owner

This module attempts to match various characteristics of the packet creator, for locally-generated packets.
It is only valid in the OUTPUT chain, and even this some packets (such as ICMP ping responses) may
have no owner, and hence never match.

--uid-owner userid
Matches if the packet was created by a process with the given effective user id.
--gid-owner groupid
Matches if the packet was created by a process with the given effective group id.
--pid-owner processid
Matches if the packet was created by a process with the given process id.
--sid-owner sessionid
Matches if the packet was created by a process in the given session group.
--cmd-owner name
Matches if the packet was created by a process with the given command name. (this option is
present only if iptables was compiled under a kernel supporting this feature)

state

This module, when combined with connection tracking, allows access to the connection tracking state for
this packet.

--state state
Where state is a comma separated list of the connection states to match. Possible states are
INVALID meaning that the packet is associated with no known connection, ESTABLISHED
meaning that the packet is associated with a connection which has seen packets in both directions,
NEW meaning that the packet has started a new connection, or otherwise associated with a
connection which has not seen packets in both directions, and RELATED meaning that the packet
is starting a new connection, but is associated with an existing connection, such as an FTP data
transfer, or an ICMP error.

conntrack


Manpage of IPTABLES

This module, when combined with connection tracking, allows access to more connection tracking
information than the "state" match. (this module is present only if iptables was compiled under a kernel
supporting this feature)

--ctstate state
Where state is a comma separated list of the connection states to match. Possible states are
INVALID meaning that the packet is associated with no known connection, ESTABLISHED
meaning that the packet is associated with a connection which has seen packets in both directions,
NEW meaning that the packet has started a new connection, or otherwise associated with a
connection which has not seen packets in both directions, and RELATED meaning that the packet
is starting a new connection, but is associated with an existing connection, such as an FTP data
transfer, or an ICMP error. SNAT A virtual state, matching if the original source address differs
from the reply destination. DNAT A virtual state, matching if the original destination differs from
the reply source.
--ctproto proto
Protocol to match (by number or name)
--ctorigsrc [!] address[/mask]
Match against original source address
--ctorigdst [!] address[/mask]
Match against original destination address
--ctreplsrc [!] address[/mask]
Match against reply source address
--ctrepldst [!] address[/mask]
Match against reply destination address
--ctstatus [NONE|EXPECTED|SEEN_REPLY|ASSURED][,...]
Match against internal conntrack states
--ctexpire time[:time]
Match remaining lifetime in seconds against given value or range of values (inclusive)

dscp

This module matches the 6 bit DSCP field within the TOS field in the IP header. DSCP has superseded
TOS within the IETF.

--dscp value
Match against a numeric (decimal or hex) value [0-32].
--dscp-class DiffServ Class
Match the DiffServ class. This value may be any of the BE, EF, AFxx or CSx classes. It will then
be converted into it's according numeric value.


Manpage of IPTABLES

pkttype

This module matches the link-layer packet type.

--pkt-type [unicast|broadcast|multicast]

tos

This module matches the 8 bits of Type of Service field in the IP header (ie. including the precedence
bits).

--tos tos
The argument is either a standard name, (use

iptables -m tos -h
to see the list), or a numeric value to match.

ah

This module matches the SPIs in AH header of IPSec packets.

--ahspi [!] spi[:spi]

esp

This module matches the SPIs in ESP header of IPSec packets.

--espspi [!] spi[:spi]

length


Manpage of IPTABLES

This module matches the length of a packet against a specific value or range of values.

--length length[:length]

ttl

This module matches the time to live field in the IP header.

--ttl ttl
Matches the given TTL value.

unclean

This module takes no options, but attempts to match packets which seem malformed or unusual. This is
regarded as experimental.

TARGET EXTENSIONS
iptables can use extended target modules: the following are included in the standard distribution.

LOG

Turn on kernel logging of matching packets. When this option is set for a rule, the Linux kernel will print
some information on all matching packets (like most IP header fields) via the kernel log (where it can be
read with dmesg or syslogd(8)). This is a "non-terminating target", i.e. rule traversal continues at the next
rule. So if you want to LOG the packets you refuse, use two separate rules with the same matching
criteria, first using target LOG then DROP (or REJECT).

--log-level level
Level of logging (numeric or see syslog.conf(5)).
--log-prefix prefix
Prefix log messages with the specified prefix; up to 29 letters long, and useful for distinguishing
messages in the logs.
--log-tcp-sequence
Log TCP sequence numbers. This is a security risk if the log is readable by users.


Manpage of IPTABLES

--log-tcp-options
Log options from the TCP packet header.
--log-ip-options
Log options from the IP packet header.

MARK

This is used to set the netfilter mark value associated with the packet. It is only valid in the mangle table.
It can for example be used in conjunction with iproute2.

--set-mark mark

REJECT

This is used to send back an error packet in response to the matched packet: otherwise it is equivalent to
DROP so it is a terminating TARGET, ending rule traversal. This target is only valid in the INPUT,
FORWARD and OUTPUT chains, and user-defined chains which are only called from those chains. The
following option controls the nature of the error packet returned:

--reject-with type
The type given can be icmp-net-unreachable, icmp-host-unreachable, icmp-port-unreachable,
icmp-proto-unreachable, icmp-net-prohibited or icmp-host-prohibited, which return the
appropriate ICMP error message (port-unreachable is the default). The option tcp-reset can be
used on rules which only match the TCP protocol: this causes a TCP RST packet to be sent back.
This is mainly useful for blocking ident (113/tcp) probes which frequently occur when sending
mail to broken mail hosts (which won't accept your mail otherwise).

TOS

This is used to set the 8-bit Type of Service field in the IP header. It is only valid in the mangle table.

--set-tos tos
You can use a numeric TOS values, or use

iptables -j TOS -h


Manpage of IPTABLES

to see the list of valid TOS names.

MIRROR

This is an experimental demonstration target which inverts the source and destination fields in the IP
header and retransmits the packet. It is only valid in the INPUT, FORWARD and PREROUTING
chains, and user-defined chains which are only called from those chains. Note that the outgoing packets
are NOT seen by any packet filtering chains, connection tracking or NAT, to avoid loops and other
problems.

SNAT

This target is only valid in the nat table, in the POSTROUTING chain. It specifies that the source
address of the packet should be modified (and all future packets in this connection will also be mangled),
and rules should cease being examined. It takes one type of option:

--to-source ipaddr[-ipaddr][:port-port]
which can specify a single new source IP address, an inclusive range of IP addresses, and
optionally, a port range (which is only valid if the rule also specifies -p tcp or -p udp). If no port
range is specified, then source ports below 512 will be mapped to other ports below 512: those
between 512 and 1023 inclusive will be mapped to ports below 1024, and other ports will be
mapped to 1024 or above. Where possible, no port alteration will occur.
You can add several --to-source options. If you specify more
than one source address, either via an address range or multiple --to-source options, a simple round-
robin (one after another in cycle) takes place between these adresses.

DNAT

This target is only valid in the nat table, in the PREROUTING and OUTPUT chains, and user-defined
chains which are only called from those chains. It specifies that the destination address of the packet
should be modified (and all future packets in this connection will also be mangled), and rules should
cease being examined. It takes one type of option:

--to-destination ipaddr[-ipaddr][:port-port]
which can specify a single new destination IP address, an inclusive range of IP addresses, and
optionally, a port range (which is only valid if the rule also specifies -p tcp or -p udp). If no port
range is specified, then the destination port will never be modified.


Manpage of IPTABLES

You can add several --to-destination options. If you specify more
than one destination address, either via an address range or multiple --to-destination options, a
simple round-robin (one after another in cycle) load balancing takes place between these adresses.

MASQUERADE

This target is only valid in the nat table, in the POSTROUTING chain. It should only be used with
dynamically assigned IP (dialup) connections: if you have a static IP address, you should use the SNAT
target. Masquerading is equivalent to specifying a mapping to the IP address of the interface the packet is
going out, but also has the effect that connections are forgotten when the interface goes down. This is the
correct behavior when the next dialup is unlikely to have the same interface address (and hence any
established connections are lost anyway). It takes one option:

--to-ports port[-port]
This specifies a range of source ports to use, overriding the default SNAT source port-selection
heuristics (see above). This is only valid if the rule also specifies -p tcp or -p udp.

REDIRECT

This target is only valid in the nat table, in the PREROUTING and OUTPUT chains, and user-defined
chains which are only called from those chains. It alters the destination IP address to send the packet to
the machine itself (locally-generated packets are mapped to the 127.0.0.1 address). It takes one option:

--to-ports port[-port]
This specifies a destination port or range of ports to use: without this, the destination port is never
altered. This is only valid if the rule also specifies -p tcp or -p udp.

ULOG

This target provides userspace logging of matching packets. When this target is set for a rule, the Linux
kernel will multicast this packet through a netlink socket. One or more userspace processes may then
subscribe to various multicast groups and receive the packets. Like LOG, this is a "non-terminating
target", i.e. rule traversal continues at the next rule.

--ulog-nlgroup nlgroup


Manpage of IPTABLES

This specifies the netlink group (1-32) to which the packet is sent. Default value is 1.
--ulog-prefix prefix
Prefix log messages with the specified prefix; up to 32 characters long, and useful for
distinguishing messages in the logs.
--ulog-cprange size
Number of bytes to be copied to userspace. A value of 0 always copies the entire packet,
regardless of its size. Default is 0.
--ulog-qthreshold size
Number of packet to queue inside kernel. Setting this value to, e.g. 10 accumulates ten packets
inside the kernel and transmits them as one netlink multipart message to userspace. Default is 1
(for backwards compatibility).

TCPMSS

This target allows to alter the MSS value of TCP SYN packets, to control the maximum size for that
connection (usually limiting it to your outgoing interface's MTU minus 40). Of course, it can only be used
in conjunction with -p tcp.
This target is used to overcome criminally braindead ISPs or servers which block ICMP Fragmentation
Needed packets. The symptoms of this problem are that everything works fine from your Linux
firewall/router, but machines behind it can never exchange large packets:

1) Web browsers connect, then hang with no data received.

2) Small mail works fine, but large emails hang.

3) ssh works fine, but scp hangs after initial handshaking.
Workaround: activate this option and add a rule to your firewall configuration like:

iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN

-j TCPMSS --clamp-mss-to-pmtu

--set-mss value
Explicitly set MSS option to specified value.
--clamp-mss-to-pmtu
Automatically clamp MSS value to (path_MTU - 40).
These options are mutually exclusive.

DSCP


Manpage of IPTABLES

This target allows to alter the value of the DSCP bits within the TOS header of the IPv4 packet. As this
manipulates a packet, it can only be used in the mangle table.

--set-dscp value
Set the DSCP field to a numerical value (can be decimal or hex)
--set-dscp-class class
Set the DSCP field to a DiffServ class.

ECN

This target allows to selectively work around known ECN blackholes. It can only be used in the mangle
table.

--ecn-tcp-remove
Remove all ECN bits from the TCP header. Of course, it can only be used in conjunction with -p
tcp.

DIAGNOSTICS
Various error messages are printed to standard error. The exit code is 0 for correct functioning. Errors
which appear to be caused by invalid or abused command line parameters cause an exit code of 2, and
other errors cause an exit code of 1.

BUGS
Bugs? What's this? ;-) Well... the counters are not reliable on sparc64.

COMPATIBILITY WITH IPCHAINS
This iptables is very similar to ipchains by Rusty Russell. The main difference is that the chains INPUT
and OUTPUT are only traversed for packets coming into the local host and originating from the local
host respectively. Hence every packet only passes through one of the three chains; previously a forwarded
packet would pass through all three.


Manpage of IPTABLES

The other main difference is that -i refers to the input interface; -o refers to the output interface, and both
are available for packets entering the FORWARD chain.

iptables is a pure packet filter when using the default `filter' table, with optional extension modules. This
should simplify much of the previous confusion over the combination of IP masquerading and packet
filtering seen previously. So the following options are handled differently:

-j MASQ

-M -S

-M -L
There are several other changes in iptables.

SEE ALSO
iptables-save(8), iptables-restore(8), ip6tables(8), ip6tables-save(8), ip6tables-restore(8). The packet-
filtering-HOWTO details iptables usage for packet filtering, the NAT-HOWTO details NAT, the netfilter-
extensions-HOWTO details the extensions that are not in the standard distribution, and the netfilter-
hacking-HOWTO details the netfilter internals.
See http://www.netfilter.org/.

AUTHORS
Rusty Russell wrote iptables, in early consultation with Michael Neuling.

Marc Boucher made Rusty abandon ipnatctl by lobbying for a generic packet selection framework in
iptables, then wrote the mangle table, the owner match, the mark stuff, and ran around doing cool stuff
everywhere.

James Morris wrote the TOS target, and tos match.

Jozsef Kadlecsik wrote the REJECT target.

Harald Welte wrote the ULOG target, TTL, DSCP, ECN matches and targets.

The Netfilter Core Team is: Marc Boucher, Jozsef Kadlecsik, James Morris, Harald Welte and Rusty
Russell.

Man page written by Herve Eychenne <rv@wallfire.org>.


Manpage of IPTABLES

Index
NAME
SYNOPSIS
DESCRIPTION
TARGETS
TABLES
OPTIONS
COMMANDS
PARAMETERS
OTHER OPTIONS
MATCH EXTENSIONS
tcp
udp
icmp
mac
limit
multiport
mark
owner
state
conntrack
dscp
pkttype
tos
ah
esp
length
ttl
unclean
TARGET EXTENSIONS
LOG
MARK
REJECT
TOS


Manpage of IPTABLES

MIRROR
SNAT
DNAT
MASQUERADE
REDIRECT
ULOG
TCPMSS
DSCP
ECN
DIAGNOSTICS
BUGS
COMPATIBILITY WITH IPCHAINS
SEE ALSO
AUTHORS

This document was created by man2html, using the manual pages.
Time: 05:21:18 GMT, January 07, 2004


http://www.iptablesrocks.org/guide/

An overview:

The iptablesrocks.org iptables firewall is geared for a typical webserver running the following services:

SSH
HTTP and HTTPS
FTP
DNS
SMTP
POP3
IMAP and IMAPS
MySQL

When running a typical web server, you need a firewall that is secure and offers protection, but you also need one that allows all needed traffic to be
able to enter and leave the server so that your server's day to day operations can carry on unimpeded. This particular iptables configuration does the
following:

1. Uses the "mangle" portion of the table to effectively block and/or confuse most port scans. These rules deal with FIN,SYN,RST,PSH,ACK,URG
FIN,PSH and URG packets.

2. Establishes a secure foundation by initially blocking all incoming, outgoing and forwarded packets COMPLETELY.

3. With all incoming packets dropped by default, it then grants incoming access to a select number of ports. These ports are ports that a typical web
server might allow.

Open inbound ports include: 20 & 21 (ftp), 22 (ssh), 25 (smtp), 53 (dns), 80 (http), 110 (pop), 143 (imap), 443 (https), 993 (imaps)

4. With all outbound packets dropped by default, it then grants outbound access to a select number of ports. These ports are posts that a typical web
server might allow.

Open outbound ports include: 20 & 21 (ftp), 22 (ssh), 25 (smtp), 43 (whois), 53 (dns), 80 (http), 110 (pop), 143 (imap), 443 (https), 993 (imaps)

5. All requests to prohibited ports are logged to a custom log file. Additional logging is also possible.

6. An installation of "Iptables Log Analyzer", which you provides a web based tool to monitor the firewall logs.

Start the installation here



http://www.iptablesrocks.org/guide/ [2/13/2004 8:04:52 PM]

iptablesrocks.org - Preparation

System Requirements/Installation:

Iptables is geared for Linux type operating systems. This setup guide is geared for Redhat Linux and Redhat Enterprise Linux. However you can
probably apply this to any other Linux distro that Iptables will run on. In order to run this setup, you will obviously need to have iptables installed on
your server.

On a Redhat server, you can usually find out if you have the iptables RPM package installed by running the following commnand:

rpm -q iptables

If iptables is installed, you should get an output like this:

iptables-1.2.8-12

If iptables is not installed on your server, you can either download the RPM from Redhat's site or, if your server is registered with Redhat Network's
update service, you can install iptables directly from Redhat with the following command:

up2date -i iptables

Pre-configuration:

As part of the preparation for our new iptables firewall, we're going to adjust the kernel logging configuration so that the firewall will log its activites to a
custom logfile. Here's how you do it...

vi /etc/syslog.conf

Add the following to the syslog.conf file:

#IPTables logging
# kernel messages.
kern.debug;kern.info /var/log/firewall


Now let's activate the changes...

/etc/rc.d/init.d/syslog restart

Your kernel should now log the activity of the soon to be firewall to /var/log/firewall. Once we start up the iptables firewall, the /var/log/firewall file will be
the location of the logging output of the firewall. This output will, in turn, be analyzed by the "Iptables Log Analyzer" tool and put in to a web accesible
and user friendly format. Woohoo!

The last thing you will want to do is to add a module to your kernel. The "ip_conntrack_ftp" module will need to be added so that FTP sessions can
function correctly. If you don't add this module, the firewall will prevent proper FTP sessions. Fortunately, adding the module is easy. Jus run the
following command:

insmod ip_conntrack_ftp

Viola! The module is added. You may want to add that line to your server's /etc/rc.local file to ensure that the module loads on boot. We'll cover that
later in the guide.

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iptablesrocks.org - Preparation




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iptablesrocks.org - Deploying a Safetynet

Deploying a Safetynet:

This next step is of critical importance. Whenever you start toying around with the firewall settings of any server, you will want to deploy a means for
you to regain access to the server should you accidentally lock yourself out. This may sound silly at first, but it can be very easy to lock yourself out of
your server when making firewall adjustments, no matter how skilled you are. It only takes a tiny syntax error or a missed flag somewhere in the
firewall script to lock your server down to a point where you cannot access it. While locking yourself out of a server that you have physical access to is
not that big of a deal, it can be a nightmare if you lock yourself out of a server that you don't have physical access to. This is certainly the case if your
server is being hosted somewhere. If you don't believe me, try skipping this step and you may end up learning your lesson the hard way.

Fortunately, safeguarding against an accidental server lock down is not difficult. The procedure only takes a couple of minutes and can be a lifesaver.
So here's how you do it:

Our safety net is simply going to be a firewall reset script that runs of our server's crontab. We will set the script to run at regular intervals. In the way,
should you lock yourself out of your server, all you'll have to do is sit back and wait for the script to execute and for the firewall to reset.

So let's create the reset script...

vi /root/firewall_reset

# Iptables firewall reset script
*filter
COMMIT

*mangle
COMMIT

*nat
COMMIT

Save and exit the file

Now let's test the reset script. The following command will utilize the "iptables-import" tool, which is used to import an iptables config script directly into
the iptables ruleset. When we import the above iptables configuration into the ruleset, it will completely reset your firewall and put your server is a
totally "open" state where all packets are allowed to freely move in and out of your server. If you've locked yourself out of your server, this will drop any
mistakes and let you back in.

iptables-restore < /root/firewall_reset

If you don't get an error, the script worked. The last thing you will want to do is to actually look at the iptables status just to make sure that your server
is completely open as it should be. The following command will display your current iptables status:

iptables -L

An open firewall should look similar to this:

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iptablesrocks.org - Deploying a Safetynet

Chain INPUT (policy ACCEPT)
target prot opt source destination

Chain FORWARD (policy ACCEPT)

Chain OUTPUT (policy ACCEPT)

As you can see, the INPUT, OUTPUT and FORWARD chains of your firewall are now set to a default "ACCEPT" state. This means that your server
will accept any packets, be they OUTBOUDN, INBOUND or FORWARDED. In other words, you have no firewall at this point. So now that we know the
script works, let's put it into the server's crontab. We'll create a crontab entry that resets the firewall every 15 minutes.

crontab -e

0,15,30,45 * * * * /sbin/iptables-restore < /root/firewall_reset

That's it. Now save and exit out of the crontab editor.

That's it. Until the crontab is disabled, your server's iptables ruleset will reset every 15 minutes. This means that if you lock yourself out later on in this
setup guide, you'll only have to wait a maximum of 15 minutes before the firewall resets and the erroneous rules are dropped.

Note: Obviously, once you finally get your firewall safely configured you will want to disable this crontab entry. ;)

So now let's get down to business and deploy the firewall script!




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iptablesrocks.org - Deploying the firewall

Deploying the firewall:

Well, it's finally time to create the main firewall ruleset script. We will simply create an importable iptables ruleset script and then import it into the
iptables ruleset on your server. As soon as we import it, it instantly becomes active. So let's get started.

Create the firewall script...

vi /root/primary_firewall

#The NAT portion of the ruleset. Used for Network Address Transalation.
#Usually not needed on a typical web server, but it's there if you need it.
*nat
COMMIT

#The Mangle portion of the ruleset. Here is where unwanted packet types get dropped.
#This helps in making port scans against your server a bit more time consuming and difficult, but not impossible.
*mangle
COMMIT

#The FILTER section of the ruleset is where we initially drop all packets and then selectively open certain ports.
#We will also enable logging of all dropped requests.
*filter
:INPUT DROP [1:242]
:FORWARD DROP [0:0]
:OUTPUT DROP [0:0]
:LOG_DROP - [0:0]
:LOG_ACCEPT - [0:0]
:icmp_packets - [0:0]

#First, we cover the INPUT rules, or the rules for incoming requests.
#Note how at the end we log any incoming packets that are not accepted.
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

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-A INPUT -p tcp -m tcp --dport 22 -j LOG_ACCEPT
-A INPUT -p tcp -m tcp --dport 25 -j LOG_ACCEPT
#uncomment the next line if you are running Spamassassin on your server
#-A INPUT -p tcp -m tcp --dport 783 -j ACCEPT
-A INPUT -s 127.0.0.1 -j ACCEPT
-A INPUT -p icmp -j icmp_packets
-A INPUT -j LOG_DROP

#Next, we cover the OUTPUT rules, or the rules for all outgoing traffic.
#Note how at the end we log any outbound packets that are not accepted.
-A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
#uncomment the next line if you are running Spamassassin on your server
#-A OUTPUT -p tcp -m tcp --dport 783 -j ACCEPT
-A OUTPUT -d 127.0.0.1 -j ACCEPT
-A OUTPUT -p icmp -j icmp_packets
-A OUTPUT -j LOG_DROP

#Here we have 2 sets of logging rules. One for dropped packets to log all dropped requests and one for accepted packets, should we wish
to log any accepted requesets.
-A LOG_DROP -j LOG --log-prefix "[IPTABLES DROP] : " --log-tcp-options --log-ip-options
-A LOG_DROP -j DROP

-A LOG_ACCEPT -j LOG --log-prefix "[IPTABLES ACCEPT] : " --log-tcp-options --log-ip-options
-A LOG_ACCEPT -j ACCEPT

#And finally, a rule to deal with ICMP requests. We drop all ping requests except from our own server.
# Make sure you replace 1.2.3.4 with the IP address of your server.
-A icmp_packets -s 1.2.3.4 -p icmp -m icmp --icmp-type 8 -j ACCEPT
-A icmp_packets -p icmp -m icmp --icmp-type 8 -j DROP
COMMIT


Now import the firewall into your server's iptables ruleset...

iptables-restore < /root/primary_firewall

If you don't get any errors, your firewall should now be active. So let's take a look at the iptables status and see what it looks like.

iptables -L



You should see an output like this:

Chain INPUT (policy DROP)
ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED
ACCEPT tcp -- anywhere anywhere tcp dpt:ftp-data
ACCEPT tcp -- anywhere anywhere tcp dpt:ftp
LOG_ACCEPT tcp -- anywhere anywhere tcp dpt:ssh
LOG_ACCEPT tcp -- anywhere anywhere tcp dpt:smtp
ACCEPT tcp -- anywhere anywhere tcp dpt:nicname
ACCEPT udp -- anywhere anywhere udp dpt:domain
ACCEPT tcp -- anywhere anywhere tcp dpt:http
ACCEPT tcp -- anywhere anywhere tcp dpt:pop3
ACCEPT tcp -- anywhere anywhere tcp dpt:imap
ACCEPT tcp -- anywhere anywhere tcp dpt:https
ACCEPT tcp -- anywhere anywhere tcp dpt:imaps
ACCEPT tcp -- anywhere anywhere tcp dpt:mysql
ACCEPT all -- localhost.localdomain anywhere
icmp_packets icmp -- anywhere anywhere
LOG_DROP all -- anywhere anywhere

Chain FORWARD (policy DROP)

Chain OUTPUT (policy DROP)
ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED
ACCEPT tcp -- anywhere anywhere tcp dpt:ftp-data
ACCEPT tcp -- anywhere anywhere tcp dpt:ftp
ACCEPT tcp -- anywhere anywhere tcp dpt:ssh
ACCEPT tcp -- anywhere anywhere tcp dpt:telnet
ACCEPT tcp -- anywhere anywhere tcp dpt:smtp
ACCEPT tcp -- anywhere anywhere tcp dpt:nicname
ACCEPT udp -- anywhere anywhere udp dpt:domain
ACCEPT tcp -- anywhere anywhere tcp dpt:http
ACCEPT tcp -- anywhere anywhere tcp dpt:pop3
ACCEPT tcp -- anywhere anywhere tcp dpt:imap
ACCEPT tcp -- anywhere anywhere tcp dpt:https
ACCEPT tcp -- anywhere anywhere tcp dpt:imaps
ACCEPT tcp -- anywhere anywhere tcp dpt:mysql
ACCEPT all -- anywhere localhost.localdomain
icmp_packets icmp -- anywhere anywhere
LOG_DROP all -- anywhere anywhere

Chain LOG_ACCEPT (2 references)
LOG all -- anywhere anywhere LOG level warning tcp-options ip-options prefix `[IPTABLES ACCEPT] : '
ACCEPT all -- anywhere anywhere

Chain LOG_DROP (2 references)
LOG all -- anywhere anywhere LOG level warning tcp-options ip-options prefix `[IPTABLES DROP] : '
DROP all -- anywhere anywhere

Chain icmp_packets (2 references)
ACCEPT icmp -- anywhere anywhere icmp echo-reply
ACCEPT icmp -- yourhostname anywhere icmp echo-request
DROP icmp -- anywhere anywhere icmp echo-request
ACCEPT icmp -- anywhere anywhere icmp destination-unreachable
ACCEPT icmp -- anywhere anywhere icmp time-exceeded

Congratulations. Your server now has an active firewall. Technically speaking, you can stop here and you should have a decent firewall set up for your
web server. In the next few steps we will test the functionality of the firewall and we will also install a log analyzer that will provide us with a web based
log monitor. Additionally, I'll show you how to ensure that your new firewall starts up on boot.



Important Note: Now that your firewall is up and running, you will want to disable the "safetynet" crontab so that your firewall will remain up
and running.





iptablesrocks.org - Testing your new firewall

Testing your new firewall:

With you firewall now up and running, it's time to run a few simple tests on it to make sure it's doing it's job.

From a remote server, try the following:

1. SSH to your server. A successful connection will be logged in the firewall logs at /var/log/firewall. This will ensure that the ACCEPT logging functions
are working correctly.

2. Run the "nmap" utility against your server (nmap 1.2.3.4, where 1.2.3.4 is your server's IP). The default namp attempt will probably fail. This is good.
You can add other flags to the nmap probe to further test the server. While your firewall won't be albo to block ALL portscans, it should block many and
make the others a bit more time consuming and troublesome. A portion nmap probe's activities should be logged in the /var/log/firewall logfile. The log
entries will confirm that the server's DROP logging functions are working correctly.

3. Try to ping your server. This should fail, since all ping requests are being dropped by the firewall.

4. FTP to your server and test for full FTP functionality. If you can't get directly listings, you will need to add the "ip_conntrack_ftp" module with the
command "insmod ip_conntrack_ftp".

OK, assuming all your tests went smoothly your firewall should be in good shape. In the next step, we'll install the "Iptables Log Analyzer" package
which will allow you to view your log entries in a nice web based environment.




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iptablesrocks.org - Iptables Log Analyzer

Installing Iptables Log Analyzer:

Iptables Log Analyzer is a package that analyzes the log output from your iptables firewall, stores the info in a database and then produces a nice user
friendly web interface from where you can monitor your firewall log output at any time. The interface even lets you sort throught the logs and group
logs by category. The official site for Iptables Log Analyzer can be found at http://www.gege.org/iptables/.

So let's install it...

The first thing you will need to do is to download the Iptables Log Analyzer package. You can download the latest copy of the package right here:
http://www.iptablesrocks.org/downloads/iptables_logger_v0.4.tar.gz

Now unpack it...

tar zxvf iptables_logger_v0.4.tar.gz

And now let's start the installation...

cd iptables

Log into mysql and do the database work...

mysql -u root -p

create a database called "iptables"...

mysql> create database iptables;

Now create an admin user for the database...

mysql> grant all privileges on iptables.* to iptables_admin@localhost identified by 'xxxxx';

And a database user for the php interface...

mysql> grant all privileges on iptables.* to iptables_user@localhost identified by 'xxxxx';

All done. Let's exit out.

mysql> quit

Now import the "iptables" database information into the database...

cat db.sql | mysql -u iptables_admin -p iptables

OK, the database should be ready now. Next we'll Install the web interface.

cp -R web /path/to/webdocs/directory/firewall (Example: cp -R web /var/www/html/firewall)

Now you will need to configure the web interface. This is done via the "config.php" file within the interface files.

vi /path/to/webdocs/directory/firewall/config.php (Example: vi /var/www/html/firewall/config.php)

Make sure your configuration data is as follows:

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# Host of the MySQL database
$db_host="localhost";

# User of the MySQL database
$db_user="iptables_user";

# Password of the MySQL database
#Make sure you enter your "iptables_user" password in place of the red x's below
$db_password="xxxxxx";

# Name of the database
$db_name="iptables";

# URL Path to your installation
$url_base="/firewall/";

#debug mode
$debug=1;

#The default number of record displayed
$default_number=20;

#The default chain displayed
$default_chain="ALL";

#The default date for packets (10000 means any)
$default_date=10000;

#The default ignored ports
$default_ignored_ports= array();


Now we will configure and install the database feeder script.

vi scripts/feed_db.pl

Make sure the following configuration section is set properly. Make sure you enter the "iptables_user" mysql password where the x's are.

my $dsn = 'DBI:mysql:iptables:localhost';
my $db_user_name = 'iptables_admin';
my $db_password = 'xxxxx';
my $log_file = '/var/log/syslog';
my $pid_file = "/var/run/iptablelog.pid";


Now we will copy the database feeder script to a location from which it can be easily called:

cp feed_db.pl /usr/local/bin/

Next, copy the init.d script called "iptablelog" to your server's init.d directory..

cp scripts/iptablelog /etc/rc.d/init.d/

Note: I've already pre-configured the "iptableslog" init.d script, but you may want to customize it further depending on your needs.

So, let's start it up!

/etc/init.d/iptablelog start



Alright! That should be it. Iptables Log Analyzer should now be running! So, let's take a look at the web interface.

Open a browser and go to: http://www.yourdomain.com/firewall

You should get the Iptables Log Analyzer screen. It's pretty self explanatory, so just play around with it for a while and you'll figure it out. If you don't
see any logs yet, you can trigger a test by logging out of your server and then logging in again via SSH. The SSH connection will be recorded in the
iptables logs and this will appear on the Iptables Log Analyzer screen. If you need more help with Iptables Log Analyzer, check out the homepage at
http://www.gege.org/iptables/.

In the next step, we'll cover firewall maintenence as well as step to modify your firewall when needed. We'll also go over how to ensure that your
firewall starts up on boot.





Iptablesrocks

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