In my specific example I am trying to use OpenVPN and need to port forward 1194 to my internal ubuntu machine on 192.168.1.12 from the Orange Livebox, but the port forwarding page wants an external IP address to enter on the port forwarding page whereas I want to allow all IPs to contact 1194 on that machine.

Any ideas?

I don't really want to put the machine in the DMZ as it runs all sorts of other stuff that I don't want public