How to go around a modem's port forwarding incapabilities

Question

This custom cable modem my ISP gave to me does not support forwarding an external port to a different internal port. (For example, I can't forward an incoming connection on port 80 to port 3389 on my main box and and an incoming connection on port 21 to port 3389 on my old box. I need to do this to avoid port blocking.)

I know I could go around this by changing the port of RDP on my machines but I can't because I already have an HTTP server which must run on 80 for internal testing purposes.

I could not find any software updates, I could not get any support from my ISP and I could not find any better cable modems. (Cable modems are very uncommmon compared to ADSL modems.)

So is there any way I can go around this limitation?

Answer 1

Use one computer as a port-translating relay. Configure the router to forward all required ports (unchanged) to that computer.

Use something like socat to bounce the connection over to a different port on the other computer as needed.

example

 socat TCP4-LISTEN:80 TCP4:otherpc:3389

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Update: Grawity points out below that using Windows you can do something like

c:\>netsh
netsh>interface portproxy
netsh interface portproxy>add v4tov4 listenport=80 connectaddress=192.168.0.100 connectport=3389 protocol=tcp

See Microsoft Technet