Question

I’m having a problem with sockets.
I’ve written myself two simple applications, one server and one client.
The server simply waits for a UDP packet to arrive, printing out something in the console once that happens.
The client sends a UDP packet to a specific end point.

// server
Socket sock = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
sock.Bind(new IPEndPoint(IPAddress.Any, 1337));
IPEndPoint remote = new IPEndPoint(IPAddress.Any, 0);
byte[] buf = new byte[1024];
sock.ReceiveFrom(buf, ref remote);
Console.WriteLine("Received packet.");

// client
Socket sock = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
IPEndPoint remote = new IPEndPoint(IPAddress.Parse(Console.ReadLine()), UInt16.Parse(Console.ReadLine()));
byte[] buf = { 1, 2, 3, 4 }; // random data
sock.SendTo(buf, remote);

Now the weird thing is that when the client socket sends its packet to the public IP address of my router, the server socket only receives it if the client is not run on the same PC as the server. So if I start the server on my PC and then start the client on my PC, enter my public IP and port 1337, the server doesn’t receive anything.
However, if I send the client application to my friend and give him my IP address and port, it works perfectly fine.
It also works if I let the client connect to my local IP instead of my public one.

Am I the only one experiencing this behaviour?
Port 1337 is forwarded to the computer the server is ran on, btw.

Answer 1

Most routers and modems do not normally forward UDP traffic — see http://www.gotroot.com/blogpost4-Why-your-should-never-forward-UDP-out-of-your-firewall for an explanation. Also try attaching both the client and server to the same physical network (well, subnet) and to try sending datagrams directly to the server instead of forwarding them.