iptables mangling rule

[David Eduardo Gomez Noguera <davidgn@servidor.unam.mx>]

Hello.
In a school, they want to set up a firewall that should filter pornsites
and the like.
Anyone can give me a hand there?

The solution I thought about was setting up a proxy, and using the proxy
to filter some sites. 
However, I couldt think how to use iptables to force every connection to
with destination port 80 to go to the proxy.

What rule could do the trick? I think it has to do with changing some
headers so that the proxy gets it.

Or is there a better solution to filtering sites?


Thank you