Linux Firewall
site: http://personal.atl.bellsouth.net/sdf/h/b/hburgiss/linux/ipchains.html
Ipchains is the utility that handles 'packet filtering' for Linux 2.2.x kernels. Ipchains can handle port blocking, logging, ~NAT, port forwarding, etc, and is quite flexible. This is my ipchains script -- more or less. Originally a canned script from nerdherd.net with some modifications here and there.

Linux IPCHAINS-HOWTO
site: http://www.tldp.org/HOWTO/IPCHAINS-HOWTO.html
This document aims to describe how to obtain, install and configure the enhanced IP firewalling chains software for Linux, and some ideas on how you might use them.