"As I write this column, the world suffers infestation
by yet another Internet worm, and again the worms are intended to
attack Microsoft-based machines. Fortunately for my company, we
don't run any Microsoft-based machines, but the Nimda worm still
Nimda eats our bandwidth since other companies sharing our
subnet are running infectious Windows 98, NT, and 2000 machines.
These diseased, ill-maintained beasts from the land of insufficient
light are now trying to infest our stable, secure, long running,
easy-to-use, well-behaved Linux machines. (Can you tell I'm a bit
This has led me to start editing some of our firewall rules, and
in the process, consider that, as we have recently migrated to the
2.4 kernel, some people may be interested in how to create a Linux
2.4 kernel-based firewall quickly.
The following examples do not create the perfect firewall. What
I describe is designed to give a system administrator a little
piece of mind by fortifying a machine from obvious attacks."