You can do transparent caching on Linux, Solaris, and BSD derivations.
The FAQ talks about this, see FAQ-5.html. It says:
The trick is to get the operating system to forward certain IP packets
to the application. This document currently contains only instruction
for configuring transparent caching on Linux and Solaris.
Here are the important settings in squid.conf:
httpd_accel virtual 80
However this way all hosts get indexed under their IP number instead of
under their real hostname, and that accounts for less efficiency.
The Debian version of squid-1.1.20 has been patched so that squid in
transparent proxy mode looks for the HTTP Host: header and uses that if
present. This patch is included in squid-1.1.21 and up. To turn on this
behaviour you also need:
Miquel van Smoorenburg <firstname.lastname@example.org> 18-Mar-1998