c# - UDP hole punching implementation

Question

I am trying to accomplish UDP hole punching. I am basing my theory on this article and this WIKI page, but I am facing some issues with the C# coding of it. Here is my problem:

Using the code that was posted here I am now able to connect to a remote machine and listen on the same port for incoming connections (Bind 2 UDP clients to the same port).

For some reason the two bindings to the same port block each other from receiving any data. I have a UDP server that responds to my connection so if I connect to it first before binding any other client to the port I get its responses back.

If I bind another client to the port no data will be received on either clients.

Following are 2 code pieces that show my problem. The first connects to a remote server to create the rule on the NAT device and then a listener is started on a different thread to capture the incoming packets. The code then sends packets to the local IP so that the listener will get it. The second only sends packets to the local IP to make sure this works. I know this is not the actual hole punching as I am sending the packets to myself without living the NAT device at all. I am facing a problem at this point, and I don't imagine this will be any different if I use a computer out side the NAT device to connect.

[EDIT] 2/4/2012 I tried using another computer on my network and WireShark (packet sniffer) to test the listener. I see the packets incoming from the other computer but are not received by the listener UDP client (udpServer) or the sender UDP client (client).

[EDIT] 2/5/2010 I have now added a function call to close the first UDP client after the initial sending and receiving of packets only living the second UDP client to listen on the port. This works and I can receive packets from inside the network on that port. I will now try to send and receive packets from outside the network. I will post my findings as soon as I find something.

Using this code I get data on the listening client:

static void Main(string[] args)
{
    IPEndPoint localpt = new IPEndPoint(Dns.Resolve(Dns.GetHostName()).AddressList[0], 4545);

    ThreadPool.QueueUserWorkItem(delegate
    {
        UdpClient udpServer = new UdpClient();
        udpServer.ExclusiveAddressUse = false;
        udpServer.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
        udpServer.Client.Bind(localpt);

        IPEndPoint inEndPoint = new IPEndPoint(IPAddress.Any, 0);
        Console.WriteLine("Listening on " + localpt + ".");
        byte[] buffer = udpServer.Receive(ref inEndPoint); //this line will block forever
        Console.WriteLine("Receive from " + inEndPoint + " " + Encoding.ASCII.GetString(buffer) + ".");
    });

    Thread.Sleep(1000);

    UdpClient udpServer2 = new UdpClient(6000);

    // the following lines work and the data is received
    udpServer2.Connect(Dns.Resolve(Dns.GetHostName()).AddressList[0], 4545);
    udpServer2.Send(new byte[] { 0x41 }, 1);

    Console.Read();
}

If I use the following code, after the connection and data transfer between my client and server, the listening UDP client will not receive anything:

static void Main(string[] args)
{
    IPEndPoint localpt = new IPEndPoint(Dns.Resolve(Dns.GetHostName()).AddressList[0], 4545);

    //if the following lines up until serverConnect(); are removed all packets are received correctly
    client = new UdpClient();
    client.ExclusiveAddressUse = false;
    client.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
    client.Client.Bind(localpt);
    remoteServerConnect(); //connection to remote server is done here
                           //response is received correctly and printed to the console

    ThreadPool.QueueUserWorkItem(delegate
    {
        UdpClient udpServer = new UdpClient();
        udpServer.ExclusiveAddressUse = false;
        udpServer.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
        udpServer.Client.Bind(localpt);

        IPEndPoint inEndPoint = new IPEndPoint(IPAddress.Any, 0);
        Console.WriteLine("Listening on " + localpt + ".");
        byte[] buffer = udpServer.Receive(ref inEndPoint); //this line will block forever
        Console.WriteLine("Receive from " + inEndPoint + " " + Encoding.ASCII.GetString(buffer) + ".");
    });

    Thread.Sleep(1000);

    UdpClient udpServer2 = new UdpClient(6000);

    // I expected the following line to work and to receive this as well
    udpServer2.Connect(Dns.Resolve(Dns.GetHostName()).AddressList[0], 4545);
    udpServer2.Send(new byte[] { 0x41 }, 1);

    Console.Read();
}

Answer 1

If i understand correctly, you are trying to communicate peer-to-peer between 2 clients each behind a different NAT, using a mediation server for hole punching?

Few years ago i did the exact same thing in c#, i haven't found the code yet, but ill give you some pointers if you like:

First, I wouldn't use the Connect() function on the udpclient, since UDP is a connectionless protocol, all this function really does is hide the functionality of a UDP socket.

You should perfrom the following steps:

1. Open a UDP socket on a server with it's ports not blocked by a firewall, at a specific port (eg Bind this socket to a chosen port for example 23000)
2. Create a UDP socket on the first client, and send something to the server at 23000. Do not bind this socket. When a udp is used to send a packet, windows will automatically assign a free port to the socket
3. Do the same from the other client
4. The server has now received 2 packets from 2 clients at 2 different adresses with 2 different ports. Test if the server can send packets back on the same address and port. (If this doesn't work you did something wrong or your NAT isn't working. You know its working if you can play games without opening ports :D)
5. The server should now send the address and port of the other clients to each connected client.
6. A client should now be able to send packets using UDP to the adresses received from the server.

You should note that the port used on the nat is probably not the same port as on your client pc!! The server should distribute this external port to clients. You must use the external adresses and the external ports to send to!

Also note that your NAT might not support this kind of port forwarding. Some NAT's forward all incoming traffic on a assigned port to you client, which is what you want. But some nats do filtering on the incoming packets adresses so it might block the other clients packets. This is unlikely though when using a standard personal user router.