Basically what I want to do is kinda like a 50% proxy.

The source machine is going to be setup to use a web proxy for browsing. It will make a request to my linux box on port 3128. The linux box will forward that request out, but leave the source address as the original machine. So instead of coming back to the linux box, it will go straight to the original machine.

The source machine is a mobile phone and port 80 and 443 are blocked on this network. Have been using squid for a while, but it just doesn't work right. There is too much of a delay before I start getting the page on my phone, and sometimes half the page loads and then restarts.

Was actually going to start looking into squid to see if I can tweak it to make it better, then this idea popped into my head. Was looking into iptables, but not sure. Maybe someone can point me in the right direction?

Thanks.

No, that's impossible in a simple world. The proxy creates a different TCP stream completely, so you can't have a different address on it compared to the address of the proxy itself. You could possibly do some level of port redirection though if you can make the proxy server be the upstream router for the phone network. As this is a phone though, that seems very unlikely. Unless you mean wi-fi connections on a phone.