IP Filter Based Firewalls HOWTO
IP Filter is a neat little firewall package. It does just about everything other free firewalls (ipfwadm, ipchains, ipfw) do, but it's also portable and does neat stuff the others don't. This document is intended to make some cohesive sense of the sparse documentation presently available for ipfilter.
The link for this article located at Brendan Conoboy is no longer available.