Em alguns segundos, googling dá tor iptables Lula
O comentário relevante é:
You can use the following configuration: user -> squid -> privoxy -> tor
The configuration of tor client and privoxy is standard. To redirect squid to privoxy you need the option cache_peer with never_direct parameter (see: http://www.visolve.com/squid/squid24s1/neighbour.php).