Pin Me

Linux iptables an Introduction

written by: Berry van der Linden•edited by: Lamar Stonecypher•updated: 9/9/2010

When you start out with Linux it can be quit daunting. You have to learn a new interface. Find out how to speak Linux. Figure out if you're hardware is going to work with Linux. And figure out how the security features of Linux work.

  • slide 1 of 5

    Introduction

    Windows has a standard firewall that according to some isn't very restricted. Some even say it's no good at all. In Linux you will not hear this at all. What you will hear instead is how bad a distribution is at setting up iptables restrictions correctly.

    If you are an Ubuntu user, then you are already using iptables. You never noticed it but in Ubuntu iptables is setup to let everything go out but let only a few connections in.

    For a Linux beginner it can be quite a task to figure out how to work with iptables. Here at Bright Hub we understand this. That is why we have written this introduction to iptables.

    After reading this article you will be able to add rules to iptables, open ports you want to have people connect to (think a webserver or a VPN server), and be able to administer your iptables.

    Now because I know not everybody is a command line guru, I will also point you towards a great GUI option.

  • slide 2 of 5

    How does iptables work?

    You can see iptables as a rule set. In iptables we talk about chains of rules. These chains of rules together build up your firewall. The chains are named:

    • PREROUTING - Rules in this chain will be applied before the packet gets routed.
    • INPUT - This packet will be locally delivered.
    • FORWARD - This chain applies after a packet is routed wen not delivered locally.
    • OUTPUT - Every connection going from this machine to other computers in the network or the Internet.
    • POSTROUTING - These rules apply after the packet is routed.

    This is what a rule might look like:

    iptables -A FORWARD -d 192.168.0.120 -p tcp --dport 80 -j ACCEPT

    In this case this rule belongs to the FORWARD chain. This rule forwards port 80 on this machine to IP address 192.168.0.120. We can use this rule to create a rule that will drop the connection to port 80.

    iptables -A INPUT -p tcp --dport 80 -j DROP

    This rule belongs to the chain INPUT. It will DROP all connections to port 80. If you want to see if this worked just run:

    sudo iptables -L

    This will show you a list per chain of current rules in use. Keep in mind that restriction is your best protection. Only open incoming ports if you have to. These rules are not preserved at boot. If you would reboot your computer now, all of the rules would be gone.

  • slide 3 of 5

    How to Preserve Rules On Boot

    The following will work on Ubuntu and possibly other Debian derivations. To accomplish this we need to save the rules sets first. Run:

    sudo iptables-save > /etc/iptables.rules

    To apply these rules every time Ubuntu boots we need to add some code to /etc/network/interfaces. This is a configuration file, so open it with your favorite text editor.

    sudo nano /etc/network/interfaces

    Add the following line to the interface you use for outward connections:

    pre-up iptables-restore < /etc/iptables.rules

    This will apply the rules in iptables.rules every time before the interface becomes active.

  • slide 4 of 5

    But What if you are not a Console Guru?

    There are several GUI options you could use. But I like to recommend an option that I have found easiest to use: Firestarter is a GTK (Gnome) GUI, but it also works great on Qt (KDE). Firestarter takes care of all of the iptables stuff described above; it saves the rules and applies them at boot of the operating system. It is easy to set new rules for outgoing and incoming connections. The default screen of Firestarter shows the status of the firewall, a blue icon means the firewall is active and a red one means the firewall is inactive. “Active connections" shows current connections to your computer. There are three tabs to Firestarter: Status, Events, and Policy. The Status tab you are now on. The Events tab shows all events currently going on. The Policy tab is where you create your rules. The tab has three sections, allowing you to allow connections from hosts, Allow services, and Forward services. Remember to hit the Apply Policy button to make sure the policy is in use. See screenshots below.

  • slide 5 of 5

    Images of Firestarter

    Firestarter Default WindowFirestarter Events windowPolicy Window in FirestarterInternet Connection Sharing Setup WizardFireStarter Tray Icon