There are lots and lots of things which can go wrong using a simple document like this, because there are plenty of special cases. The majority of possible problems adhere to the configuration of the internal and external network devices. I will try and respond to people with problems, figure out what went wrong and add links down here so that people with special case problems can track down help. Feel free to contact me at pramsey@refractions.net.
Some portions of ICQ work fine over masquerading. Other portions do not
work well at all. There is a
beta quality ICQ module under development, however, which addresses some
(but not all) of the deficiencies of running ICQ over masquerading. The README
file in the source code distribution describes how to compile the module. Once
you have it compiled and installed, invoke /sbin/modprobe ip_masq_icq.