Unreal Tournament 3 shipped with a campaign mode that was pretty meh if played alone, but much more fun if played with friends. Unfortunately, configuring one’s router to allow hosting of online co-op campaign games can be frustratingly complex. Epic has provided no official documentation on the subject, so UT3 owners have been left to figure out which ports to forward for themselves.
You could place your computer in your router’s DMZ, but that opens all ports on the computer to the Internet and presents a serious security risk.
Instead, here’s a list of UDP ports (not TCP) you’ll need to forward to get online co-op campaign play working:
I need to conduct further tests to determine whether all ports listed need to be forwarded to allow for online co-op campaign play, but I have personally tested these settings and they work for me when no other configurations I found during my long search worked.