I'll describe the problem simply. I have set up OpenVPN server (machine C) on Ubuntu inside of my LAN. I want to route all of the OpenVPN clients' internet traffic through this server. This seems to be working well.
However, when an outside machine B is connected to the OpenVPN server I can not RDP into B from another machine A. This makes sense; all of the traffic I send from A -> B goes from B -> C. Since machine C does not have remote desktop set up on it the connection fails.
I have drafted a schematic of what's going on here (I don't have enough reputation to include the image inline).
How can I configure machine C to reject all packets on port 3389 such that the RDP connection can be made successfully to B? I want traffic to flow like this.