socketrace.cs

using System;
using System.Net;
using System.Net.Sockets;
using System.Threading;

public static class Program {
    public static void Main () {
        while (true) {
            Step();
            // Even though we're closing our listener and sockets, without this we run out of files in Mono on Linux
            // GC.Collect();
        }
    }

    const int FirstPort = 4010;
    const int LastPort  = 4040;

    static int Port = FirstPort;

    public static void Step () {        
        // listen with a new listener (IPv4 is the default)
        TcpListener inListener = new TcpListener (Port);
        inListener.Start();

        // connect to it from a new socket
        IPHostEntry hostent = Dns.GetHostByAddress (IPAddress.Loopback);
        Socket outSock = null;

        foreach (IPAddress address in hostent.AddressList) {
            if (address.AddressFamily != AddressFamily.InterNetwork)
                continue;

            /// Only keep IPv4 addresses, our Server is in IPv4 only mode.
            outSock = new Socket (
                address.AddressFamily, SocketType.Stream,
                ProtocolType.IP
            );
                
            IPEndPoint remote = new IPEndPoint (address, Port);
            outSock.Connect (remote);
            break;
        }

        if (!inListener.Pending()) {
            Thread.Sleep(50);

            if (!inListener.Pending())
                Console.Write("!");
            else
                Console.Write(",");
        } else {
            Console.Write(".");
        }

        Socket inSock = inListener.AcceptSocket ();
        // tidy up after ourselves
        inSock.Close ();
        outSock.Close ();

        // This quietly leaks a socket handle
        inListener.Stop ();

        Port += 1;
        if (Port > LastPort)
            Port = FirstPort;
    }
}