redirecting on a local machine

[saspan]

Hi there,

I have serious trouble configuring a LTSP-server, wanting it to 
work as a transparent proxy using squid in order to secure 
web-access for our children.

The problem is to redirect TCP-requests to the machine itself.

More detailed: I want the tcp-requests from users logged into 
the LTSP-machine being redirected to the machines' proxy. 
As mentioned above the proxy is running on the same system.

A simple

iptables -t -nat -A OUTPUT -m owner ! --owner-uid squid -p tcp 
--dport 80 -j REDIRECT --to-destination 3128

doesn't seem to work, cause a browser would not be expecting a 
reply from a transparent proxy when waiting for replies to its requests.

What am I doing wrong here? Do the packets have to masked 
locally before redirecting them to the proxy?

Any help is appreciated as I have been guessing on this problem 
for three days.

Regards,
Sascha