How do I do TCP hole punching? - sockets

Question

Question is below. Here is my current test code which did not succeed.
static void Main(string[] args)
{
if (args.Count() != 3)
{
Console.WriteLine("Bad args");
}
var ep = new IPEndPoint(IPAddress.Parse(args[0]), int.Parse(args[1]));
var lp = new IPEndPoint(IPAddress.Any, int.Parse(args[2]));
var s = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
s.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
s.Bind(lp);
var c = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
c.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
c.Bind(lp);
Task.Run(() => { try { c.Connect(ep); } catch { } });
s.Listen(10);
var v = s.Accept();
v.Close();
}
How do I do TCP hole punching? I am testing using a remote server. I'm running wget local_public_ip:port/test. I have my router setup for port 80 so it doesn't need a hole punch. My code got a connection. Now I try on other ports and I can't exactly figure out how to punch the hole.
What I have done is (C# code)
var l = new TcpListener(8090);
l.Start();
try { var o = new TcpClient(); o.Connect("myserverip", 123); }
catch(Exception ex) {}
var e = l.AcceptSocket();
Console.WriteLine(e.RemoteEndPoint.AddressFamily);
I thought maybe I need to setup the local endpoint on the out tcp connection.
TcpClient(new System.Net.IPEndPoint(new System.Net.IPAddress(bytearray), port));
I made a mistake and got this exception
The requested address is not valid in its context
Fixing up the byte array to 192,168,1,5 it appears to make outgoing connects correctly. Now that I have a out connection to the remote IP using my listening port I thought wget would be able to connect to me. It wasn't the case
How do I do TCP hole punching?

Answer 1

I'd use the "sequential hole punching technique" detailed in http://www.bford.info/pub/net/p2pnat/index.html. It seems much simpler to do that simultaneous connections and socket reuse. It is not necessary for hole punching to do anything exactly simultaneously (that is a meaningless notion in distributed systems anyway).
I have implemented hole punching. My router seems not to like it. Wireshark shows the outbound hole punching SYN is correct but the remote party can't get through to me. I verifies all ports with TcpView.exe and disabled all firewalls. Must be a router issue. (It is a strange and invasive router.)
class HolePunchingTest
{
IPEndPoint localEndPoint;
IPEndPoint remoteEndPoint;
bool useParallelAlgorithm;
public static void Run()
{
var ipHostEntry = Dns.GetHostEntry("REMOTE_HOST");
new HolePunchingTest()
{
localEndPoint = new IPEndPoint(IPAddress.Parse("LOCAL_IP"), 1234),
remoteEndPoint = new IPEndPoint(ipHostEntry.AddressList.First().Address, 1235),
useParallelAlgorithm = true,
}.RunImpl();
}
void RunImpl()
{
if (useParallelAlgorithm)
{
Parallel.Invoke(() =>
{
while (true)
{
PunchHole();
}
},
() => RunServer());
}
else
{
PunchHole();
RunServer();
}
}
void PunchHole()
{
Console.WriteLine("Punching hole...");
using (var punchSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
{
EnableReuseAddress(punchSocket);
punchSocket.Bind(localEndPoint);
try
{
punchSocket.Connect(remoteEndPoint);
Debug.Assert(false);
}
catch (SocketException socketException)
{
Console.WriteLine("Punching hole: " + socketException.SocketErrorCode);
Debug.Assert(socketException.SocketErrorCode == SocketError.TimedOut || socketException.SocketErrorCode == SocketError.ConnectionRefused);
}
}
Console.WriteLine("Hole punch completed.");
}
void RunServer()
{
using (var listeningSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
{
EnableReuseAddress(listeningSocket);
listeningSocket.Bind(localEndPoint);
listeningSocket.Listen(0);
while (true)
{
var connectionSocket = listeningSocket.Accept();
Task.Run(() => ProcessConnection(connectionSocket));
}
}
}
void ProcessConnection(Socket connectionSocket)
{
Console.WriteLine("Socket accepted.");
using (connectionSocket)
{
connectionSocket.Shutdown(SocketShutdown.Both);
}
Console.WriteLine("Socket shut down.");
}
void EnableReuseAddress(Socket socket)
{
if (useParallelAlgorithm)
socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
}
}
You can try both values for useParallelAlgorithm. Both should work.
This code is for the server. It punches a hole into the local NAT. You can then connect from the remote side using any client that allows to pick the local port. I used curl.exe. Apparently, telnet on Windows does not support binding to a port. wget apparently neither.
Verify that the ports are correct on both sides using TcpView or Process Explorer. You can use Wireshark to verify packets. Set a filter like tcp.port = 1234.
When you "call out" to punch a hole you enable the tuple (your-ip, your-port, remote-ip, remote-port) to communicate. This means that all further communication must use those values. All sockets (inbound or outbound) must use these exact port numbers. In case you aren't aware: outgoing connections can control the local port as well. This is just uncommon.