Introduction to iptables

  • Last updated on: 2015-12-08
  • Authored by: Rackspace Support

This article explains how to use IP tables for a simple firewall, and describes the process to open up holes in your firewall to necessary ports. An IP table is a firewall and networking tool available to all Linux distros and operates by analyzing packets at the kernel level as they are received.

For a comprehensive list of IP tables commands, parameters, and other options, see the MAN page and the IP tables how-to.

The Base Ruleset

  • List of the Current rules

    sudo iptables --list
    

Your output is going to look like the following if you haven’t made any changes:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Reading the Ruleset

In a default install you will see three predefined Chains that will take care of the three major activities: Incoming Traffic, Forwarded Traffic and Outgoing Traffic. The “policy” is probably the most important thing to take away from the above table. The policy is the default ruleset for that particular Chain, with a standard install all policies will be “Accept”.

Policies

The available policies and other options are extensive, if you would like to know more about them check out the manpage for IPtables. In the scope of this article I will only cover the following three policies which are the most common:

  • Accept - This is used to explicitly pass through as long as no target rules apply.

  • 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.

  • Drop - This policy will halt a connection to a host without any communication unless there is a target rule that applies.

Available Options

IPtables MAN page: http://linux.die.net/man/8/iptables

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

  • Commands - These options specify a specific action to perform.
  • Parameters - Parameters set rule specifications for the commands used.
  • Other Options - Other options can be specified, as necessary, for commands used.

Simple Firewall

Simple Rules:

  • Allow connections that are already connected to your server

      sudo iptables -A INPUT -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT
    
  • To allow connections on ServiceNet (required by other Rackspace Cloud products such as Cloud Backup and Cloud Monitoring)

       sudo iptables -A INPUT -i eth1 -m state --state ESTABLISHED,RELATED -j ACCEPT
    
  • Allow connections to SSH

Ok, in this case we can make a few different choices, the choices can be applied to other ports or situations to make customizations.

In this command, we will allow connections for all tcp connections attempts at SSH connections.

     sudo iptables -I INPUT 1 -p tcp  --dport 22 -j ACCEPT

In this command, we will allow connections only coming from a certain IP subnet using CIDR notation. In this example we are going to lockdown to any IP address lying in the range of 192.168.1.0 - 192.168.1.255

     sudo iptables -I INPUT 1 -p tcp --dport 22 -s 192.168.1.0/24 -j ACCEPT
  • Allowing connections to HTTP/HTTPS

The following iptables rules will allow connections from both port 80 (HTTP) and port 443 (HTTPS) from any connections.

     sudo iptables -I INPUT 1 -p tcp --dport 80 -j ACCEPT
     sudo iptables -I INPUT 1 -p tcp --dport 443 -j ACCEPT
  • Allowing connections to FTP

The following iptables rules will allow connections for FTP servers on port 21.

     sudo iptables -I INPUT 1 -p tcp --dport 21 -j ACCEPT
  • List of common Ports

Using what you’ve learned from the above use the following list of common ports from the link to create rules for any running server you have.

  • Changing the Default Policy

The only real policy change that we are going to make is going to effect incoming traffic, as a general rule we are going to Drop all connections, and only allow those we have deemed legit.

    sudo iptables -P INPUT DROP

Note: This rule should be run only after you have setup your access rules to allow you to ssh in.

Save your Ruleset

If your server reboots for any reason or you restart IPTables you will loose your changes. The rules that you input by hand are stored in volatile memory. Make sure that you save IPtables rules for any change you want to make permanent you will need one of the following commands:

  • for CentOS and Fedora

    /etc/init.d/iptables save
    
  • for Ubuntu

    iptables-save > /etc/iptables.rules
    
  • for all other Distros

    iptables-save > /etc/sysconfig/iptables
    

The above commands will create a file /etc/sysconfig/iptables that will be a flat file with human readable syntax that can be edited by hand if necessary. All edits to this file will be live whenever iptables is restarted.

Summary

Hopefully from this article you can create a simple firewall to protect your server from basic attacks. Keep in mind that IPTables is a very powerful tool that would take a book to fully go through all of its abilities. My next networking article will go through a few simple networking tricks like port forwarding and NATing. If you need help creating more rules, see Easy Firewall Generator for IPTables.

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