Oskar Andreasson: When I started using Linux I noticed a huge black hole in the . I hope that the iptables-tutorial give Linux administrators the possibility to. Iptables Tutorial Oskar Andreasson [email protected] http://people. 10/06/ Oskar Andreasson . The above also implies that the rule-sets available with this tutorial are not written to deal with actual bugs inside Netfilter. The main goal of.

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Oskar Andreasson”, and with no Back-Cover Texts. She has supported me more than I ever can support her to any degree. I wish I could make you just as happy as you make me. Second andreawson all, I would like to dedicate this work to all of the incredibly hard working Linux developers and maintainers.

It is people like those who make this wonderful operating system possible. Why this document was written 1. How it was written 1. Terms used in this document 2.


Where to get iptables 2. Compiling the user-land applications 2. Installation on Red Hat 7. Traversing of tables and chains 3. The state machine 4.

The conntrack entries 4. Complex protocols and connection tracking 5. Saving and restoring large rule-sets 5. Drawbacks with restore 5. How a rule is built 6. Initial loading of extra modules 7. Displacement of rules to different chains 7. Setting up default policies 7. Setting up user specified chains in the filter table 7.

Detailed iptwbles of special commands A. Listing your active rule-set A. Updating and flushing your tables B.

Common problems and questions B. Problems loading modules B. Other resources and links E. Example scripts code-base I. Destination local host our own machine Source local host our own machine Limit match options MAC match options Mark match options Multiport match options Owner match options LOG target options MARK target options The new iptables is a good upgrade from the old ipchains in this regard. With ipchains, you could make a fairly secure network by dropping all incoming packages not destined for given ports.

They assign ports on the server, tell the client about it, and then let the client connect. There were some teething problems in the iptables code that I ran into in the beginning, and in some respects I found the code not quite ready for release in full production. Today, I’d recommend everyone who uses ipchains or even older ipfwadm etc. How to read This document was written purely so people can start to grasp the wonderful world of iptables.

New version of iptables and ipsysctl tutorials

It was never meant to contain information on specific security bugs in iptables or Netfilter. If you find peculiar bugs or behaviors in iptables or any of the subcomponents, you should contact the Netfilter mailing lists and tell them about the problem and they can tell you if this is a real bug or if it has already been fixed.

There are very rarely actual security related bugs found in iptables or Netfilter, however, one or two do slip by once in a while.

These are properly shown on the front page of the Netfilter main pageand that is where you should go to get information on such topics.


The above also implies that the rule-sets available with this tutorial are not written to deal with actual bugs inside Netfilter.

The main goal of them is to simply show how to set up rules in a nice simple fashion that deals with all problems we may run into. For example, this tutorial will not cover how we would close down the HTTP port for the simple reason that Apache happens to be vulnerable in version 1.

This document was simply written to give everyone a good and simple primer at how to get started with iptables, but at the same time it was created to be as complete as possible.

It does not contain any targets or matches that are in patch-o-matic for the simple reason that it would require too much effort to keep such a list updated. If you need information about the patch-o-matic updates, you should read the info that comes with it in patch-o-matic as well as the other documentations available on the Netfilter main page.

I have tried as much as possible to eradicate all prerequisites needed before fully grasping this document, but to some extent it is simply impossible to not need some previous knowledge.

Conventions used in this document The following conventions are used in this document when it comes to commands, files and other specific information. Code excerpts and command-outputs are printed like this, with all output in fixed width font and user-written commands in bold typeface: All system items such as hardware, and also kernel internals or abstract system items such as the loopback interface are all shown in an italic typeface.

Why this document was written Well, I found a big empty space in the HOWTO’s out there lacking in information about the iptables and Netfilter functions in the new Linux 2. Among other things, I’m going to try to answer questions that some might have about the new possibilities like state matching.

Most of this will be illustrated with an example rc. Also, there’s a small script that I wrote just in case you screw up as much as I did during the configuration available as rc. How it was written I’ve consulted Marc Boucher and others from the core Netfilter team. Many heartfelt thanks to them for their work and for their help on this tutorial, that I originally wrote for boingworld.

This document will guide you through the setup process step by step and hopefully help you to understand some more about the iptables package.

I will base most of the stuff here on the example rc. I have decided to just follow the basic chains and from there go down into each and one of the chains traversed in each due order.

iptables Tutorial 1

That way the tutorial is a little bit harder to follow, though this way is more logical. Whenever you find something that’s hard to understand, just come back to this tutorial.

Terms used in this document This document contains a few terms that may need more detailed explanations before you read them. This section will try to cover the most obvious ones and how I have chosen to use them within this document.


This is normally done by assigning different ports with a Internet routable IP address, and then tell the Linux router where to send the traffic. Stream – This term refers to a connection that sends and ipttables packets that are related to each other in some fashion.

Basically, I have used this term for any kind of connection that sends two or more packets in both directions. In other words, I use this tutirial very loosely. This refers to the techniques used to translate one source address to another in a packet. This is used to make it possible for several hosts to share a single Internet routable IP address, since there is currently a shortage of available IP addresses in IPv4 IPv6 will solve this.

Note that the used states internally, and externally, do not fully follow the RFC specification fully. The main reason is that Netfilter has to make several assumptions about the connections and packets. User space – With this term I mean everything and anything that takes place outside the kernel.

Kernel space – I;tables is more or less the opposite of User space. This implies the actions that take place within the kernel, and not outside of the kernel.

Userland – See User space. Preparations This chapter is aimed at getting you started and to help you understand the role Netfilter and iptables play in Linux today.

This chapter should hopefully get you set up and finished to go with your experimentation, and installation of your firewall. Given time and perseverance, you’ll then get it to perform exactly as you want it to. Where to get iptables The iptables user-space package can be downloaded from the http: Also a good place to start at when wondering what iptables and Netfilter is about. The iptables package also makes use of kernel space facilities which can iptxbles configured into the kernel during make configure.

The necessary steps will be discussed a iptxbles further down in this document. Kernel setup To run the pure basics of iptables you need to configure the following options into the kernel while doing make config or one of its related commands: Examples of such utilities are tcpdump or snort. If you do not want it, don’t include it. In other words, this is most definitely required for anything in this tutorial to work at all.

I assume you will want this, since you are reading this. And of course you need to add the proper drivers for your interfaces to work properly, i. The above will only add some of the pure basics in iptables. You won’t be able to do anything productive to be honest, it just adds the framework to the kernel.

If you want to use the more advanced options in Iptables, you need to set up the proper configuration options in your kernel.