Building a firewall with IPtables

Linux has supported a couple generations of packet filtering at the kernel level. The latest, netfilter, provides an extensible framework for building sophisticated firewalls with just about any Linux distro you can imagine. The iptables user-mode application gives an administrator the ability to create, destroy and modify filtering rules right from the command-line. InformIT has an excellent tutorial that explains how netfilter works and how you can use IPtables to take advantage of its sophisticated filtering architecture.