hi friends,
i have installed REDHAT centos on my server.now i have setup a squid-2.5-STABLE as a proy server on it.now we have a local area network in which their are 15 machines are present.
i have a linksys router through which internet connection is given to all machines.client use to access internet by putting gateway as the router ip(192.168.0.4) in their network properties.
the private ip given to this router is 192.168.0.4 and the ip address of my squid server is 192.168.0.5.
now it is expected that any request to the internet should forcefully go through proxy server without any proxy server setting in the internet properties of client machine.

will you please help me to solve this problem?

What you are looking for is Transparent proxying (http://tldp.org/HOWTO/TransparentProxy.html), however for this to work all the traffic has to go through the squid machine first.

You could look into making the linux box the gateway and it sends all traffic to the router. In my setup, my linux box also does DNS caching/DHCP serving so the router effectively becomes an ethernet modem.

At least this is how I've set mine up and it works well.

Hope this helps.

I have also done the same and it works well. Just ensure that your IP table rules are configured correctly.