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 probl

Answer 1

Sorry for uploading such a huge piece of code, but i guess this is very clearly explains how things work, and may be really useful. If you will have issues with this code, please let me know.

Note:

1. this is just a draft
2. (important) you MUST inform server with your local endpoint. if you will not do it you will not be able to communicate between two peers behind one NAT (for example, on one local machine) even if the server is out of NAT
3. you must close "puncher" client (at least, i did not manage to receive any packets until i did it). later you will be able to communicate with server using other UdpClient's
4. of cource it will not work with symmetric NAT
5. if you find that something in this code is "terrible practice", please tell me, i'm not network expert :)

---

Server.cs

using System;
using System.Collections.Generic;
using System.Net;
using System.Net.Sockets;
using HolePunching.Common;

namespace HolePunching.Server
{
    class Server
    {
        private static bool _isRunning;
        private static UdpClient _udpClient;
        private static readonly Dictionary<byte, PeerContext> Contexts = new Dictionary<byte, PeerContext>();

        private static readonly Dictionary<byte, byte> Mappings = new Dictionary<byte, byte>
        {
            {1, 2},
            {2, 1},
        };

        static void Main()
        {
            _udpClient = new UdpClient( Consts.UdpPort );
            ListenUdp();

            Console.ReadLine();
            _isRunning = false;
        }

        private static async void ListenUdp()
        {
            _isRunning = true;

            while ( _isRunning )
            {
                try
                {
                    var receivedResults = await _udpClient.ReceiveAsync();

                    if ( !_isRunning )
                    {
                        break;
                    }

                    ProcessUdpMessage( receivedResults.Buffer, receivedResults.RemoteEndPoint );
                }
                catch ( Exception ex )
                {
                    Console.WriteLine( $"Error: {ex.Message}" );
                }
            }
        }

        private static void ProcessUdpMessage( byte[] buffer, IPEndPoint remoteEndPoint )
        {
            if ( !UdpProtocol.UdpInfoMessage.TryParse( buffer, out UdpProtocol.UdpInfoMessage message ) )
            {
                Console.WriteLine( $" >>> Got shitty UDP [ {remoteEndPoint.Address} : {remoteEndPoint.Port} ]" );
                _udpClient.Send( new byte[] { 1 }, 1, remoteEndPoint );
                return;
            }

            Console.WriteLine( $" >>> Got UDP from {message.Id}. [ {remoteEndPoint.Address} : {remoteEndPoint.Port} ]" );

            if ( !Contexts.TryGetValue( message.Id, out PeerContext context ) )
            {
                context = new PeerContext
                {
                    PeerId = message.Id,
                    PublicUdpEndPoint = remoteEndPoint,
                    LocalUdpEndPoint = new IPEndPoint( message.LocalIp, message.LocalPort ),
                };

                Contexts.Add( context.PeerId, context );
            }

            byte partnerId = Mappings[context.PeerId];
            if ( !Contexts.TryGetValue( partnerId, out context ) )
            {
                _udpClient.Send( new byte[] { 1 }, 1, remoteEndPoint );
                return;
            }

            var response = UdpProtocol.PeerAddressMessage.GetMessage(
                partnerId,
                context.PublicUdpEndPoint.Address,
                context.PublicUdpEndPoint.Port,
                context.LocalUdpEndPoint.Address,
                context.LocalUdpEndPoint.Port );

            _udpClient.Send( response.Data, response.Data.Length, remoteEndPoint );

            Console.WriteLine( $" <<< Responsed to {message.Id}" );
        }
    }

    public class PeerContext
    {
        public byte PeerId { get; set; }
        public IPEndPoint PublicUdpEndPoint { get; set; }
        public IPEndPoint LocalUdpEndPoint { get; set; }
    }
}

Client.cs

using System;

namespace HolePunching.Client
{
    class Client
    {
        public const string ServerIp = "your.server.public.address";

        static void Main()
        {
            byte id = ReadIdFromConsole();

            // you need some smarter :)
            int localPort = id == 1 ? 61043 : 59912;
            var x = new Demo( ServerIp, id, localPort );
            x.Start();
        }

        private static byte ReadIdFromConsole()
        {
            Console.Write( "Peer id (1 or 2): " );

            var id = byte.Parse( Console.ReadLine() );

            Console.Title = $"Peer {id}";

            return id;
        }
    }
}

Demo.cs

using HolePunching.Common;
using System;
using System.Collections.Generic;
using System.Net;
using System.Net.Sockets;
using System.Threading;
using System.Threading.Tasks;

namespace HolePunching.Client
{
    public class Demo
    {
        private static bool _isRunning;

        private static UdpClient _udpPuncher;
        private static UdpClient _udpClient;
        private static UdpClient _extraUdpClient;
        private static bool _extraUdpClientConnected;

        private static byte _id;

        private static IPEndPoint _localEndPoint;
        private static IPEndPoint _serverUdpEndPoint;
        private static IPEndPoint _partnerPublicUdpEndPoint;
        private static IPEndPoint _partnerLocalUdpEndPoint;

        private static string GetLocalIp()
        {
            var host = Dns.GetHostEntry( Dns.GetHostName() );
            foreach ( var ip in host.AddressList )
            {
                if ( ip.AddressFamily == AddressFamily.InterNetwork )
                {
                    return ip.ToString();
                }
            }
            throw new Exception( "Failed to get local IP" );
        }

        public Demo( string serverIp, byte id, int localPort )
        {
            _serverUdpEndPoint = new IPEndPoint( IPAddress.Parse( serverIp ), Consts.UdpPort );
            _id = id;

            // we have to bind all our UdpClients to this endpoint
            _localEndPoint = new IPEndPoint( IPAddress.Parse( GetLocalIp() ), localPort );
        }

        public void Start(  )
        {
            _udpPuncher = new UdpClient(); // this guy is just for punching
            _udpClient = new UdpClient(); // this will keep hole alive, and also can send data
            _extraUdpClient = new UdpClient(); // i think, this guy is the best option for sending data (explained below)

            InitUdpClients( new[] { _udpPuncher, _udpClient, _extraUdpClient }, _localEndPoint );

            Task.Run( (Action) SendUdpMessages );
            Task.Run( (Action) ListenUdp );

            Console.ReadLine();
            _isRunning = false;
        }

        private void InitUdpClients(IEnumerable<UdpClient> clients, EndPoint localEndPoint)
        {
            // if you don't want to use explicit localPort, you should create here one more UdpClient (X) and send something to server (it will automatically bind X to free port). then bind all clients to this port and close X

            foreach ( var udpClient in clients )
            {
                udpClient.ExclusiveAddressUse = false;
                udpClient.Client.SetSocketOption( SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true );
                udpClient.Client.Bind( localEndPoint );
            }
        }

        private void SendUdpMessages()
        {
            _isRunning = true;

            var messageToServer = UdpProtocol.UdpInfoMessage.GetMessage( _id, _localEndPoint.Address, _localEndPoint.Port );
            var messageToPeer = UdpProtocol.P2PKeepAliveMessage.GetMessage();

            while ( _isRunning )
            {
                // while we dont have partner's address, we will send messages to server
                if ( _partnerPublicUdpEndPoint == null && _partnerLocalUdpEndPoint == null )
                {
                    _udpPuncher.Send( messageToServer.Data, messageToServer.Data.Length, _serverUdpEndPoint );
                    Console.WriteLine( $" >>> Sent UDP to server [ {_serverUdpEndPoint.Address} : {_serverUdpEndPoint.Port} ]" );
                }
                else
                {
                    // you can skip it. just demonstration, that you still can send messages to server
                    _udpClient.Send( messageToServer.Data, messageToServer.Data.Length, _serverUdpEndPoint );
                    Console.WriteLine( $" >>> Sent UDP to server [ {_serverUdpEndPoint.Address} : {_serverUdpEndPoint.Port} ]" );

                    // THIS is how we punching hole! very first this message should be dropped by partner's NAT, but it's ok.
                    // i suppose that this is good idea to send this "keep-alive" messages to peer even if you are connected already,
                    // because AFAIK "hole" for UDP lives ~2 minutes on NAT. so "we will let it die? NEVER!" (c)
                    _udpClient.Send( messageToPeer.Data, messageToPeer.Data.Length, _partnerPublicUdpEndPoint );
                    _udpClient.Send( messageToPeer.Data, messageToPeer.Data.Length, _partnerLocalUdpEndPoint );
                    Console.WriteLine( $" >>> Sent UDP to peer.public [ {_partnerPublicUdpEndPoint.Address} : {_partnerPublicUdpEndPoint.Port} ]" );
                    Console.WriteLine( $" >>> Sent UDP to peer.local [ {_partnerLocalUdpEndPoint.Address} : {_partnerLocalUdpEndPoint.Port} ]" );

                    // "connected" UdpClient sends data much faster, 
                    // so if you have something that your partner cant wait for (voice, for example), send it this way
                    if ( _extraUdpClientConnected )
                    {
                        _extraUdpClient.Send( messageToPeer.Data, messageToPeer.Data.Length );
                        Console.WriteLine( $" >>> Sent UDP to peer.received EP" );
                    }
                }

                Thread.Sleep( 3000 );
            }
        }

        private async void ListenUdp()
        {
            _isRunning = true;

            while ( _isRunning )
            {
                try
                {
                    // also important thing!
                    // when you did not punched hole yet, you must listen incoming packets using "puncher" (later we will close it).
                    // where you already have p2p connection (and "puncher" closed), use "non-puncher"
                    UdpClient udpClient = _partnerPublicUdpEndPoint == null ? _udpPuncher : _udpClient;

                    var receivedResults = await udpClient.ReceiveAsync();

                    if ( !_isRunning )
                    {
                        break;
                    }

                    ProcessUdpMessage( receivedResults.Buffer, receivedResults.RemoteEndPoint );
                }
                catch ( SocketException ex )
                {
                    // do something here...
                }
                catch ( Exception ex )
                {
                    Console.WriteLine( $"Error: {ex.Message}" );
                }
            }
        }

        private static void ProcessUdpMessage( byte[] buffer, IPEndPoint remoteEndPoint )
        {
            // if server sent partner's endpoinps, we will store it and (IMPORTANT) close "puncher"
            if ( UdpProtocol.PeerAddressMessage.TryParse( buffer, out UdpProtocol.PeerAddressMessage peerAddressMessage ) )
            {
                Console.WriteLine( " <<< Got response from server" );
                _partnerPublicUdpEndPoint = new IPEndPoint( peerAddressMessage.PublicIp, peerAddressMessage.PublicPort );
                _partnerLocalUdpEndPoint = new IPEndPoint( peerAddressMessage.LocalIp, peerAddressMessage.LocalPort );

                _udpPuncher.Close();
            }
            // since we got this message we know partner's endpoint for sure, 
            // and we can "connect" UdpClient to it, so it will work faster
            else if ( UdpProtocol.P2PKeepAliveMessage.TryParse( buffer ) )
            {
                Console.WriteLine( $"           IT WORKS!!! WOW!!!  [ {remoteEndPoint.Address} : {remoteEndPoint.Port} ]" );

                _extraUdpClientConnected = true;
                _extraUdpClient.Connect( remoteEndPoint );
            }
            else
            {
                Console.WriteLine( "???" );
            }
        }
    }
}

Protocol.cs

I'm not sure how good this approach, maybe something like protobuf can do it better

using System;
using System.Linq;
using System.Net;
using System.Text;

namespace HolePunching.Common
{
    public static class UdpProtocol
    {
        public static readonly int GuidLength = 16;
        public static readonly int PeerIdLength = 1;
        public static readonly int IpLength = 4;
        public static readonly int IntLength = 4;

        public static readonly byte[] Prefix = { 12, 23, 34, 45 };

        private static byte[] JoinBytes( params byte[][] bytes )
        {
            var result = new byte[bytes.Sum( x => x.Length )];
            int pos = 0;

            for ( int i = 0; i < bytes.Length; i++ )
            {
                for ( int j = 0; j < bytes[i].Length; j++, pos++ )
                {
                    result[pos] = bytes[i][j];
                }
            }

            return result;
        }

        #region Helper extensions

        private static bool StartsWith( this byte[] @this, byte[] value, int offset = 0 )
        {
            if ( @this == null || value == null || @this.Length < offset + value.Length )
            {
                return false;
            }

            for ( int i = 0; i < value.Length; i++ )
            {
                if ( @this[i + offset] < value[i] )
                {
                    return false;
                }
            }

            return true;
        }

        private static byte[] ToUnicodeBytes( this string @this )
        {
            return Encoding.Unicode.GetBytes( @this );
        }

        private static byte[] Take( this byte[] @this, int offset, int length )
        {
            return @this.Skip( offset ).Take( length ).ToArray();
        }

        public static bool IsSuitableUdpMessage( this byte[] @this )
        {
            return @this.StartsWith( Prefix );
        }

        public static int GetInt( this byte[] @this )
        {
            if ( @this.Length != 4 )
                throw new ArgumentException( "Byte array must be exactly 4 bytes to be convertible to uint." );

            return ( ( ( @this[0] << 8 ) + @this[1] << 8 ) + @this[2] << 8 ) + @this[3];
        }

        public static byte[] ToByteArray( this int value )
        {
            return new[]
            {
                (byte)(value >> 24),
                (byte)(value >> 16),
                (byte)(value >> 8),
                (byte)value
            };
        }

        #endregion

        #region Messages

        public abstract class UdpMessage
        {
            public byte[] Data { get; }

            protected UdpMessage( byte[] data )
            {
                Data = data;
            }
        }

        public class UdpInfoMessage : UdpMessage
        {
            private static readonly byte[] MessagePrefix = { 41, 57 };
            private static readonly int MessageLength = Prefix.Length + MessagePrefix.Length + PeerIdLength + IpLength + IntLength;

            public byte Id { get; }
            public IPAddress LocalIp { get; }
            public int LocalPort { get; }

            private UdpInfoMessage( byte[] data, byte id, IPAddress localIp, int localPort )
                : base( data )
            {
                Id = id;
                LocalIp = localIp;
                LocalPort = localPort;
            }

            public static UdpInfoMessage GetMessage( byte id, IPAddress localIp, int localPort )
            {
                var data = JoinBytes( Prefix, MessagePrefix, new[] { id }, localIp.GetAddressBytes(), localPort.ToByteArray() );

                return new UdpInfoMessage( data, id, localIp, localPort );
            }

            public static bool TryParse( byte[] data, out UdpInfoMessage message )
            {
                message = null;

                if ( !data.StartsWith( Prefix ) )
                    return false;
                if ( !data.StartsWith( MessagePrefix, Prefix.Length ) )
                    return false;
                if ( data.Length != MessageLength )
                    return false;

                int index = Prefix.Length + MessagePrefix.Length;
                byte id = data[index];

                index += PeerIdLength;
                byte[] localIpBytes = data.Take( index, IpLength );
                var localIp = new IPAddress( localIpBytes );

                index += IpLength;
                byte[] localPortBytes = data.Take( index, IntLength );
                int localPort = localPortBytes.GetInt();

                message = new UdpInfoMessage( data, id, localIp, localPort );

                return true;
            }
        }

        public class PeerAddressMessage : UdpMessage
        {
            private static readonly byte[] MessagePrefix = { 36, 49 };
            private static readonly int MessageLength = Prefix.Length + MessagePrefix.Length + PeerIdLength + ( IpLength + IntLength ) * 2;

            public byte Id { get; }
            public IPAddress PublicIp { get; }
            public int PublicPort { get; }
            public IPAddress LocalIp { get; }
            public int LocalPort { get; }

            private PeerAddressMessage( byte[] data, byte id, IPAddress publicIp, int publicPort, IPAddress localIp, int localPort )
                : base( data )
            {
                Id = id;
                PublicIp = publicIp;
                PublicPort = publicPort;
                LocalIp = localIp;
                LocalPort = localPort;
            }

            public static PeerAddressMessage GetMessage( byte id, IPAddress publicIp, int publicPort, IPAddress localIp, int localPort )
            {
                var data = JoinBytes( Prefix, MessagePrefix, new[] { id }, 
                    publicIp.GetAddressBytes(), publicPort.ToByteArray(),
                    localIp.GetAddressBytes(), localPort.ToByteArray() );

                return new PeerAddressMessage( data, id, publicIp, publicPort, localIp, localPort );
            }

            public static bool TryParse( byte[] data, out PeerAddressMessage message )
            {
                message = null;

                if ( !data.StartsWith( Prefix ) )
                    return false;
                if ( !data.StartsWith( MessagePrefix, Prefix.Length ) )
                    return false;
                if ( data.Length != MessageLength )
                    return false;

                int index = Prefix.Length + MessagePrefix.Length;
                byte id = data[index];

                index += PeerIdLength;
                byte[] publicIpBytes = data.Take( index, IpLength );
                var publicIp = new IPAddress( publicIpBytes );

                index += IpLength;
                byte[] publicPortBytes = data.Take( index, IntLength );
                int publicPort = publicPortBytes.GetInt();

                index += IntLength;
                byte[] localIpBytes = data.Take( index, IpLength );
                var localIp = new IPAddress( localIpBytes );

                index += IpLength;
                byte[] localPortBytes = data.Take( index, IntLength );
                int localPort = localPortBytes.GetInt();

                message = new PeerAddressMessage( data, id, publicIp, publicPort, localIp, localPort );

                return true;
            }
        }

        public class P2PKeepAliveMessage : UdpMessage
        {
            private static readonly byte[] MessagePrefix = { 11, 19 };
            private static P2PKeepAliveMessage _message;

            private P2PKeepAliveMessage( byte[] data )
                : base( data )
            {

            }

            public static bool TryParse( byte[] data )
            {
                if ( !data.StartsWith( Prefix ) )
                    return false;
                if ( !data.StartsWith( MessagePrefix, Prefix.Length ) )
                    return false;

                return true;
            }

            public static P2PKeepAliveMessage GetMessage()
            {
                if ( _message == null )
                {
                    var data = JoinBytes( Prefix, MessagePrefix );
                    _message = new P2PKeepAliveMessage( data );
                }

                return _message;
            }
        }

        #endregion
    }
}

Answer 2

Edit: After a lot more testing this doesn't seem to work at all for me unless I enable UPnP. So a lot of the things I wrote here you may find useful but many people don't have UPnP enabled (because it is a security risk) so it will not work for them.

Here is some code using PubNub as a relay server :). I don't recommend using this code without testing because it is not perfect (I'm not sure if it is even secure or the right way to do things? idk I'm not a networking expert) but it should give you an idea of what to do. It at least has worked for me so far in a hobby project. The things it is missing are:

• Testing if the client is on your LAN. I just send to both which works for your LAN and a device on another network but that is very inefficient.
• Testing when the client stops listening if, for example, they closed the program. Because this is UDP it is stateless so it doesn't matter if we are sending messages into the void but we probably shouldn't do that if noone is getting them
• I use Open.NAT to do port forwarding programatically but this might not work on some devices. Specifically, it uses UPnP which is a little insecure and requires UDP port 1900 to be port forwarded manually. Once they do this it is supported on most routers but many have not done this yet.

So first of all, you need a way to get your external and local IPs. Here is code for getting your local IP:

// From http://stackoverflow.com/questions/6803073/get-local-ip-address
public string GetLocalIp()
{
    var host = Dns.GetHostEntry(Dns.GetHostName());
    foreach (var ip in host.AddressList)
    {
        if (ip.AddressFamily == AddressFamily.InterNetwork)
        {
            return ip.ToString();
        }
    }
    throw new Exception("Failed to get local IP");
}

And here is some code for getting your external IP via trying a few websites that are designed to return your external IP

public string GetExternalIp()
{
    for (int i = 0; i < 2; i++)
    {
        string res = GetExternalIpWithTimeout(400);
        if (res != "")
        {
            return res;
        }
    }
    throw new Exception("Failed to get external IP");
}
private static string GetExternalIpWithTimeout(int timeoutMillis)
{
    string[] sites = new string[] {
      "http://ipinfo.io/ip",
      "http://icanhazip.com/",
      "http://ipof.in/txt",
      "http://ifconfig.me/ip",
      "http://ipecho.net/plain"
    };
    foreach (string site in sites)
    {
        try
        {
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(site);
            request.Timeout = timeoutMillis;
            using (var webResponse = (HttpWebResponse)request.GetResponse())
            {
                using (Stream responseStream = webResponse.GetResponseStream())
                {
                    using (StreamReader responseReader = new System.IO.StreamReader(responseStream, Encoding.UTF8))
                    {
                        return responseReader.ReadToEnd().Trim();
                    }
                }
            }
        }
        catch
        {
            continue;
        }
    }

    return "";

}

Now we need to find an open port and forward it to an external port. As mentioned above I used Open.NAT. First, you put together a list of ports that you think would be reasonable for your application to use after looking at registered UDP ports. Here are a few for example:

public static int[] ports = new int[]
{
  5283,
  5284,
  5285,
  5286,
  5287,
  5288,
  5289,
  5290,
  5291,
  5292,
  5293,
  5294,
  5295,
  5296,
  5297
};

Now we can loop through them and hopefully find one that is not in use to use port forwarding on:

public UdpClient GetUDPClientFromPorts(out Socket portHolder, out string localIp, out int localPort, out string externalIp, out int externalPort)
{
  localIp = GetLocalIp();
  externalIp = GetExternalIp();

  var discoverer = new Open.Nat.NatDiscoverer();
  var device = discoverer.DiscoverDeviceAsync().Result;

  IPAddress localAddr = IPAddress.Parse(localIp);
  int workingPort = -1;
  for (int i = 0; i < ports.Length; i++)
  {
      try
      {
          // You can alternatively test tcp with  nc -vz externalip 5293 in linux and
          // udp with  nc -vz -u externalip 5293 in linux
          Socket tempServer = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
          tempServer.Bind(new IPEndPoint(localAddr, ports[i]));
          tempServer.Close();
          workingPort = ports[i];
          break;
      }
      catch
      {
        // Binding failed, port is in use, try next one
      }
  }


  if (workingPort == -1)
  {
      throw new Exception("Failed to connect to a port");
  }


  int localPort = workingPort;

  // You could try a different external port if the below code doesn't work
  externalPort = workingPort;

  // Mapping ports
  device.CreatePortMapAsync(new Open.Nat.Mapping(Open.Nat.Protocol.Udp, localPort, externalPort));

  // Bind a socket to our port to "claim" it or cry if someone else is now using it
  try
  {
      portHolder = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
      portHolder.Bind(new IPEndPoint(localAddr, localPort));
  }
  catch
  {
      throw new Exception("Failed, someone is now using local port: " + localPort);
  }


  // Make a UDP Client that will use that port
  UdpClient udpClient = new UdpClient(localPort);
  return udpClient;
}

Now for the PubNub relay server code (P2PPeer will be defined later below). There is a lot here so I'm not really gonna explain it but hopefully the code is clear enough to help you understand what is going on

public delegate void NewPeerCallback(P2PPeer newPeer);
public event NewPeerCallback OnNewPeerConnection;

public Pubnub pubnub;
public string pubnubChannelName;
public string localIp;
public string externalIp;
public int localPort;
public int externalPort;
public UdpClient udpClient;
HashSet<string> uniqueIdsPubNubSeen;
object peerLock = new object();
Dictionary<string, P2PPeer> connectedPeers;
string myPeerDataString;

public void InitPubnub(string pubnubPublishKey, string pubnubSubscribeKey, string pubnubChannelName)
{
    uniqueIdsPubNubSeen = new HashSet<string>();
    connectedPeers = new Dictionary<string, P2PPeer>;
    pubnub = new Pubnub(pubnubPublishKey, pubnubSubscribeKey);
    myPeerDataString = localIp + " " + externalIp + " " + localPort + " " + externalPort + " " + pubnub.SessionUUID;
    this.pubnubChannelName = pubnubChannelName;
    pubnub.Subscribe<string>(
        pubnubChannelName,
        OnPubNubMessage,
        OnPubNubConnect,
        OnPubNubError);
    return pubnub;
}

//// Subscribe callbacks
void OnPubNubConnect(string res)
{
    pubnub.Publish<string>(pubnubChannelName, connectionDataString, OnPubNubTheyGotMessage, OnPubNubMessageFailed);
}

void OnPubNubError(PubnubClientError clientError)
{
    throw new Exception("PubNub error on subscribe: " + clientError.Message);
}

void OnPubNubMessage(string message)
{
    // The message will be the string ["localIp externalIp localPort externalPort","messageId","channelName"]
    string[] splitMessage = message.Trim().Substring(1, message.Length - 2).Split(new char[] { ',' });
    string peerDataString = splitMessage[0].Trim().Substring(1, splitMessage[0].Trim().Length - 2);

    // If you want these, I don't need them
    //string peerMessageId = splitMessage[1].Trim().Substring(1, splitMessage[1].Trim().Length - 2);
    //string channelName = splitMessage[2].Trim().Substring(1, splitMessage[2].Trim().Length - 2);


    string[] pieces = peerDataString.Split(new char[] { ' ', '\t' });
    string peerLocalIp = pieces[0].Trim();
    string peerExternalIp = pieces[1].Trim();
    string peerLocalPort = int.Parse(pieces[2].Trim());
    string peerExternalPort = int.Parse(pieces[3].Trim());
    string peerPubnubUniqueId = pieces[4].Trim();

    pubNubUniqueId = pieces[4].Trim();

    // If you are on the same device then you have to do this for it to work idk why
    if (peerLocalIp == localIp && peerExternalIp == externalIp)
    {
        peerLocalIp = "127.0.0.1";
    }


    // From me, ignore
    if (peerPubnubUniqueId == pubnub.SessionUUID)
    {
        return;
    }

    // We haven't set up our connection yet, what are we doing
    if (udpClient == null)
    {
        return;
    }


    // From someone else


    IPEndPoint peerEndPoint = new IPEndPoint(IPAddress.Parse(peerExternalIp), peerExternalPort);
    IPEndPoint peerEndPointLocal = new IPEndPoint(IPAddress.Parse(peerLocalIp), peerLocalPort);

    // First time we have heard from them
    if (!uniqueIdsPubNubSeen.Contains(peerPubnubUniqueId))
    {
        uniqueIdsPubNubSeen.Add(peerPubnubUniqueId);

        // Dummy messages to do UDP hole punching, these may or may not go through and that is fine
        udpClient.Send(new byte[10], 10, peerEndPoint);
        udpClient.Send(new byte[10], 10, peerEndPointLocal); // This is if they are on a LAN, we will try both
        pubnub.Publish<string>(pubnubChannelName, myPeerDataString, OnPubNubTheyGotMessage, OnPubNubMessageFailed);
    }
    // Second time we have heard from them, after then we don't care because we are connected
    else if (!connectedPeers.ContainsKey(peerPubnubUniqueId))
    {
        //bool isOnLan = IsOnLan(IPAddress.Parse(peerExternalIp)); TODO, this would be nice to test for
        bool isOnLan = false; // For now we will just do things for both
        P2PPeer peer = new P2PPeer(peerLocalIp, peerExternalIp, peerLocalPort, peerExternalPort, this, isOnLan);
        lock (peerLock)
        {
            connectedPeers.Add(peerPubnubUniqueId, peer);
        }

        // More dummy messages because why not
        udpClient.Send(new byte[10], 10, peerEndPoint);
        udpClient.Send(new byte[10], 10, peerEndPointLocal);


        pubnub.Publish<string>(pubnubChannelName, connectionDataString, OnPubNubTheyGotMessage, OnPubNubMessageFailed);
        if (OnNewPeerConnection != null)
        {
            OnNewPeerConnection(peer);
        }
    }
}

//// Publish callbacks
void OnPubNubTheyGotMessage(object result)
{

}

void OnPubNubMessageFailed(PubnubClientError clientError)
{
    throw new Exception("PubNub error on publish: " + clientError.Message);
}

And here is a P2PPeer

public class P2PPeer
{
    public string localIp;
    public string externalIp;
    public int localPort;
    public int externalPort;
    public bool isOnLan;

    P2PClient client;

    public delegate void ReceivedBytesFromPeerCallback(byte[] bytes);

    public event ReceivedBytesFromPeerCallback OnReceivedBytesFromPeer;


    public P2PPeer(string localIp, string externalIp, int localPort, int externalPort, P2PClient client, bool isOnLan)
    {
        this.localIp = localIp;
        this.externalIp = externalIp;
        this.localPort = localPort;
        this.externalPort = externalPort;
        this.client = client;
        this.isOnLan = isOnLan;



        if (isOnLan)
        {
            IPEndPoint endPointLocal = new IPEndPoint(IPAddress.Parse(localIp), localPort);
            Thread localListener = new Thread(() => ReceiveMessage(endPointLocal));
            localListener.IsBackground = true;
            localListener.Start();
        }

        else
        {
            IPEndPoint endPoint = new IPEndPoint(IPAddress.Parse(externalIp), externalPort);
            Thread externalListener = new Thread(() => ReceiveMessage(endPoint));
            externalListener.IsBackground = true;
            externalListener.Start();
        }
    }

    public void SendBytes(byte[] data)
    {
        if (client.udpClient == null)
        {
            throw new Exception("P2PClient doesn't have a udpSocket open anymore");
        }
        //if (isOnLan) // This would work but I'm not sure how to test if they are on LAN so I'll just use both for now
        {
            client.udpClient.Send(data, data.Length, new IPEndPoint(IPAddress.Parse(localIp), localPort));
        }
        //else
        {
            client.udpClient.Send(data, data.Length, new IPEndPoint(IPAddress.Parse(externalIp), externalPort));
        }
    }

    // Encoded in UTF8
    public void SendString(string str)
    {
        SendBytes(System.Text.Encoding.UTF8.GetBytes(str));
    }


    void ReceiveMessage(IPEndPoint endPoint)
    {
        while (client.udpClient != null)
        {
            byte[] message = client.udpClient.Receive(ref endPoint);
            if (OnReceivedBytesFromPeer != null)
            {
                OnReceivedBytesFromPeer(message);
            }
            //string receiveString = Encoding.UTF8.GetString(message);
            //Console.Log("got: " + receiveString);
        }
    }
}

Finally, here are all my usings:

using PubNubMessaging.Core; // Get from PubNub GitHub for C#, I used the Unity3D library
using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;

I'm open to comments and questions, feel free to give feedback if something here is bad practice or doesn't work. A few bugs were introduced in translation from my code that I'll fix here eventually but this should at least give you the idea of what to do.

Answer 3

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.

Answer 4

Update:

Whichever of the UdpClients binds first is the one that will be sent incoming packets by Windows. In your example try moving the code block that sets up the listening thread to the top.

Are you sure the problem is not just that the receive thread is only written to handle a single receive? Try replacing the receive thread with as below.

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 + ".");

    while (inEndPoint != null)
    {
        byte[] buffer = udpServer.Receive(ref inEndPoint);
        Console.WriteLine("Bytes received from " + inEndPoint + " " + Encoding.ASCII.GetString(buffer) + ".");
    }
});

Answer 5

Have you tried using the Async functions, here is a example of how you might get it to work it may need a bit of work to make it 100% functional:

    public void HolePunch(String ServerIp, Int32 Port)
    {
        IPEndPoint LocalPt = new IPEndPoint(Dns.GetHostEntry(Dns.GetHostName()).AddressList[0], Port);
        UdpClient Client = new UdpClient();
        Client.ExclusiveAddressUse = false;
        Client.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
        Client.Client.Bind(LocalPt);

        IPEndPoint RemotePt = new IPEndPoint(IPAddress.Parse(ServerIp), Port);

        // This Part Sends your local endpoint to the server so if the two peers are on the same nat they can bypass it, you can omit this if you wish to just use the remote endpoint.
        byte[] IPBuffer = System.Text.Encoding.UTF8.GetBytes(Dns.GetHostEntry(Dns.GetHostName()).AddressList[0].ToString());
        byte[] LengthBuffer = BitConverter.GetBytes(IPBuffer.Length);
        byte[] PortBuffer = BitConverter.GetBytes(Port);
        byte[] Buffer = new byte[IPBuffer.Length + LengthBuffer.Length + PortBuffer.Length];
        LengthBuffer.CopyTo(Buffer,0);
        IPBuffer.CopyTo(Buffer, LengthBuffer.Length);
        PortBuffer.CopyTo(Buffer, IPBuffer.Length + LengthBuffer.Length);
        Client.BeginSend(Buffer, Buffer.Length, RemotePt, new AsyncCallback(SendCallback), Client);

        // Wait to receve something
        BeginReceive(Client, Port);

        // you may want to use a auto or manual ResetEvent here and have the server send back a confirmation, the server should have now stored your local (you sent it) and remote endpoint.

        // you now need to work out who you need to connect to then ask the server for there remote and local end point then need to try to connect to the local first then the remote.
        // if the server knows who you need to connect to you could just have it send you the endpoints as the confirmation.

        // you may also need to keep this open with a keepalive packet untill it is time to connect to the peer or peers.

        // once you have the endpoints of the peer you can close this connection unless you need to keep asking the server for other endpoints

        Client.Close();
    }

    public void ConnectToPeer(String PeerIp, Int32 Port)
    {
        IPEndPoint LocalPt = new IPEndPoint(Dns.GetHostEntry(Dns.GetHostName()).AddressList[0], Port);
        UdpClient Client = new UdpClient();
        Client.ExclusiveAddressUse = false;
        Client.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
        Client.Client.Bind(LocalPt);
        IPEndPoint RemotePt = new IPEndPoint(IPAddress.Parse(PeerIp), Port);
        Client.Connect(RemotePt);
        //you may want to keep the peer client connections in a list.

        BeginReceive(Client, Port);
    }

    public void SendCallback(IAsyncResult ar)
    {
        UdpClient Client = (UdpClient)ar.AsyncState;
        Client.EndSend(ar);
    }

    public void BeginReceive(UdpClient Client, Int32 Port)
    {
        IPEndPoint ListenPt = new IPEndPoint(IPAddress.Any, Port);

        Object[] State = new Object[] { Client, ListenPt };

        Client.BeginReceive(new AsyncCallback(ReceiveCallback), State);
    }

    public void ReceiveCallback(IAsyncResult ar)
    {
        UdpClient Client = (UdpClient)((Object[])ar.AsyncState)[0];
        IPEndPoint ListenPt = (IPEndPoint)((Object[])ar.AsyncState)[0];

        Byte[] receiveBytes = Client.EndReceive(ar, ref ListenPt);
    }

I hope this helps.