"Your FreeBSD system comes with two built-in mechanisms for
inspecting IP packets: ipfw and ipfilter. Both have their own
peculiar syntax for creating rulesets to determine which packets to
allow and which packets to discard, so I'd like to demonstrate the
usage of both. Since you can only run one or the other, I'll start with
ipfw; once we've had a good look at it, I'll switch gears and move
on to ipfilter." From ONLamp.com.
April 27, 2001