Making a Port Forward work for an Internal PPTP VPN Service Running on a Windows Server

Solution

For a port forward to work for PPTP VPN services both TCP port 1723  and the GRE Protocol (Protocol 47) need to be forwarded to the internal server IP.

Both of these have been configured for the service entry called PPTP in the service listing, so just add that entry to the port forward to include both items in the port forward.