Is there some particular reason you need to have 7777 forwarded to 12078, and can't just access router:12078 to forward to server:12078? My router (Linksys) can only do router:x -> server:x , not router:x -> server:y. I've personally never seen a router (in the "home user" category, anyway) that has that kind of flexibility.

If all else fails, you could have router:7777 forward to an Apache web server on server:7777, and use Apache's mod_proxy to "reverse proxy" to the other server on server:12078.