What you're looking to create is called a proxy. An iframe is often used as a proxy to another site's content, but the problem you've run into is that browsers have to have access to the server at the iframe's declared URL. Since it's behind a firewall, as you've mentioned, a server side solution is the only way to go.
Check out Apache's mod_proxy or nginx's proxy capability to handle requests on the server side. They both require root privileges to modify http server configuration.