ipfw workaround for syn-loop attack, FreeBSD 2.2.5-STABLE

[robert () cyrus watson org (Robert Watson)]

Adding a rule for the interface denying packets from oneself appears to
defend against the new attack.

This rule worked:

03001 deny ip from 128.2.91.57 to 128.2.91.57 via ed0

Where 128.2.91.57 is the host's IP address on device ed0.

This presumably works on other versions of FreeBSD, and other systems with
ipfw/ipfirewall installed on them.  As always, if you are not familiar
with ipfw and don't know how it works, don't use this unless you are on
the console the first time!

Adding this to rc.firewall on FreeBSD is also a good idea.  Multi-homed
hosts require one entry per device, needless to say.

  Robert N Watson

Junior, Logic+Computation, Carnegie Mellon University  http://www.cmu.edu/
Network Administrator, SafePort Network Services  http://www.safeport.com/
robert () fledge watson org rwatson () safeport com http://www.watson.org/~robert/