Tutorial: Design a stateful Linux firewall

This tutorial shows you how to use netfilter to set up a powerful Linux stateful firewall. All you need is an existing Linux system that's currently using a Linux 2.4.x or 2.6.x kernel. A laptop, workstation, router or server with at a Linux 2.4.x or 2.6.x kernel will do. You should be reasonably familiar with standard network terminology like IP addresses, source and destination port numbers, TCP, UDP and ICMP, etc. By the end of the tutorial, you'll understand how Linux stateful firewalls are put together and you'll have several example configurations to use in your own projects.