How to Stop bypassing Proxy server?

Some of the users at my location are accessing the Internet by
bypassing the in-house proxy server. They are just typing the upstream
proxy server IP address and port number in browser and accessing the
Internet. How can I prohibit such activities? My network is on
192.168.7.0 and IP address of upstream proxy server is 195.2.104.7
(take these IP addresses as an example). I can access 195.2.104.0
network from my location. I am using Squid Proxy server on Linux
RH8.Can anyone suggests use of IPTABLES to achieve this?

not sure with iptables but with ipchains it would look something like:

ipchains -I output -p tcp -s 192.168.7.0/24 -d 195.2.104.7/32 -j DENY


"ssp2000" <sspphp@indiatimes.com> wrote in message
news:62e8bd5a.0405302054.2ed83b44@posting.google.c om...
> Some of the users at my location are accessing the Internet by
> bypassing the in-house proxy server. They are just typing the upstream
> proxy server IP address and port number in browser and accessing the
> Internet. How can I prohibit such activities? My network is on
> 192.168.7.0 and IP address of upstream proxy server is 195.2.104.7
> (take these IP addresses as an example). I can access 195.2.104.0
> network from my location. I am using Squid Proxy server on Linux
> RH8.Can anyone suggests use of IPTABLES to achieve this?