report who accepted the connection in ListenAddressBindsOnlyThatAddress

src/log4net.Tests/Appender/TelnetAppenderTest.cs

@@ -23,6 +23,7 @@
 using System.Diagnostics;
 using System.IO;
 using System.Net;
+using System.Net.NetworkInformation;
 using System.Net.Sockets;
 using System.Text;
 using System.Threading;
@@ -139,6 +140,8 @@ void WaitFor(string what, string expected)
 
   private const string WelcomeMessage = "TelnetAppender";
 
+  private const string TooManyConnectionsMessage = "Sorry - Too many connections.";
+
   [Test]
   public void TelnetTest()
   {
@@ -398,7 +401,7 @@ private static LoggingEvent CreateEvent(string message)
 
   /// <summary>
   /// Binding to the loopback address has to keep the port unreachable from other machines, which
-  /// is what an operator asking for it wants.
+  /// is what an operator asking for it wants. A peer other than the appender counts as unreachable.
   /// </summary>
   [Test]
   [NonParallelizable]
@@ -431,7 +434,9 @@ public void ListenAddressBindsOnlyThatAddress()
       }
 
       using Socket external = new(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
-      Assert.That(() => external.Connect(new IPEndPoint(routable!, port)), Throws.TypeOf<SocketException>());
+      string greeting = ReadGreeting(external, new IPEndPoint(routable!, port));
+      Assert.That(greeting, Does.Not.StartWith(WelcomeMessage).And.Not.StartWith(TooManyConnectionsMessage),
+        () => DescribeUnexpectedConnection(external, port, greeting));
     }
     finally
     {
@@ -534,6 +539,60 @@ public override void Flush()
     }
   }
 
+  /// <summary>
+  /// Connects and returns the first line the peer sends, empty if it refuses, closes or stays silent.
+  /// </summary>
+  private static string ReadGreeting(Socket socket, IPEndPoint endPoint)
+  {
+    StringBuilder text = new();
+    try
+    {
+      socket.Connect(endPoint);
+      // Generous, since a slow accept must not pass as silence. Only a silent proxy pays for it.
+      socket.ReceiveTimeout = (int)_receiveTimeout.TotalMilliseconds;
+      byte[] buffer = new byte[256];
+      int read;
+      do
+      {
+        read = socket.Receive(buffer);
+        text.Append(Encoding.ASCII.GetString(buffer, 0, read));
+      }
+      while (read > 0 && Array.IndexOf(buffer, (byte)'\n', 0, read) < 0);
+    }
+    catch (SocketException)
+    {
+      // Refused, or silent until the timeout: either way the appender did not answer.
+    }
+    return text.ToString();
+  }
+
+  /// <summary>
+  /// Collects what a reviewer needs to tell who accepted a connection that should have been refused.
+  /// </summary>
+  private static string DescribeUnexpectedConnection(Socket connected, int port, string greeting)
+  {
+    StringBuilder text = new();
+    text.AppendLine($"peer sent: {greeting.Trim()}");
+    // A failing lookup must not hide the assertion this message belongs to.
+    try
+    {
+      text.AppendLine($"connected from {connected.LocalEndPoint} to {connected.RemoteEndPoint}");
+      text.AppendLine($"host name {Dns.GetHostName()} resolves to [{string.Join(", ", (object[])Dns.GetHostAddresses(Dns.GetHostName()))}]");
+      foreach (NetworkInterface nic in NetworkInterface.GetAllNetworkInterfaces())
+      {
+        string[] addresses = Array.ConvertAll([.. nic.GetIPProperties().UnicastAddresses], a => a.Address.ToString());
+        text.AppendLine($"interface {nic.Name} ({nic.NetworkInterfaceType}, {nic.OperationalStatus}): {string.Join(", ", addresses)}");
+      }
+      IPEndPoint[] listeners = Array.FindAll(IPGlobalProperties.GetIPGlobalProperties().GetActiveTcpListeners(), l => l.Port == port);
+      text.AppendLine($"listeners on port {port}: [{string.Join(", ", (object[])listeners)}]");
+    }
+    catch (Exception e) when (!e.IsFatal())
+    {
+      text.AppendLine($"diagnostics failed: {e.Message}");
+    }
+    return text.ToString();
+  }
+
   /// <summary>
   /// Asks the OS for a currently unused TCP port - a fixed port would collide with
   /// other tests or processes on the build machine.