How to see what port was opened by router

Question

If I do the following:

UdpClient c = new UdpClient();
c.Connect(new System.Net.IPEndPoint(IPAddress.Parse("69.65.85.125"), 9900));
c.Send(new byte[] { 1,2,3,4,5 }, 5);

then I will be sending a packet to my router then my router will send that packet to the ip "69.65.85.125".

If I where to capture that packet on the computer that has the ip "69.65.85.125" I will be able to see the port that was oppened by the router (client.RemoteEndpoint). How will it be possible to see that information without capturing the packet at the other enpoint? Is there a way to query the router?

Answer 1

If your router supports it you can query it via UPnP. Here is a wrapper library for UPnP I found for .NET, I have never used it so I cant give you any advice if it is good or not.

Look at the ComponetsTest program for example code in the zip for the library. You will need to reference the UPnP documentation to find out what calls you will need to make to the service.

From the message board of the library of someone asking a how to find port mappings.

The WANPPPConnection and WANIPConnection services have actions called GetSpecificPortMappingEntry, simply call this iterating through the indexes from 0 until an error is returned, each call will return another UPnP port mapping, you can also get the static mappings with a different service.

Answer 2

In order to get the public IP, the remote device should respond by sending a UDP packet back to you that contains the IP address and port it saw. This is one of the most fundamental concepts behind a STUN server, commonly used in UDP hole-punching algorithms.

There are several free STUN servers available that do exactly this. Send one of them a "binding" request, and you will get back a response with your public IP address and port.

stun.l.google.com:19302
stun1.l.google.com:19302
stun2.l.google.com:19302
stun3.l.google.com:19302
stun4.l.google.com:19302
stun01.sipphone.com
stun.ekiga.net
stun.fwdnet.net
stun.ideasip.com
stun.iptel.org
stun.rixtelecom.se
stun.schlund.de
stunserver.org
stun.softjoys.com
stun.voiparound.com
stun.voipbuster.com
stun.voipstunt.com
stun.voxgratia.org
stun.xten.com

If you are truly interested in doing proper UDP hole-punching, check out ICE (Interactive Connectivity Establishment). It's a brilliant algorithm that uses STUN and another protocol called TURN to guarantee a successful connection between peers. (Apple uses it for Facetime video calls, among others.)

If you're interested, the company I work for has developed a product called IceLink that uses ICE/STUN/TURN to establish direct data streams between peers. SDKs are available for .NET, Mac, iOS, Android, Java, Windows Phone, Windows 8, Unity, Xamarin, and more, and it even includes full support for WebRTC audio/video streams.