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
The scenario describes seems to be an intended limitation in functionality designed to prevent end users from inappropriately opening holes in the firewall.
You may need to 1) Adjust you network config, for example by moving the server to the DMZ
2) Get a different ADSL modem or router
3) Get a firmware update.
external IP would be the IP address on the external interface on the router. Try: