Fixed socket dispose getting blocked issue with work around. More info on the issue: https://github.com/dotnet/runtime/issues/37873

DnsServerCore/Dhcp/DhcpServer.cs

@@ -695,7 +695,7 @@ namespace DnsServerCore.Dhcp
         {
             if (_udpListeners.TryRemove(dhcpEP.Address, out Socket socket))
             {
-                socket.Dispose();
+                socket.CloseWorkAround();
                 return true;
             }
 

DnsServerCore/Dns/DnsServer.cs

@@ -2292,20 +2292,67 @@ namespace DnsServerCore.Dns
                 _cacheMaintenanceTimer = null;
             }
 
-            foreach (Socket udpListener in _udpListeners)
-                udpListener.Dispose();
+            if (Environment.OSVersion.Platform == PlatformID.Win32NT)
+            {
+                foreach (Socket udpListener in _udpListeners)
+                    udpListener.Dispose();
 
-            foreach (Socket tcpListener in _tcpListeners)
-                tcpListener.Dispose();
+                foreach (Socket tcpListener in _tcpListeners)
+                    tcpListener.Dispose();
 
-            foreach (Socket httpListener in _httpListeners)
-                httpListener.Dispose();
+                foreach (Socket httpListener in _httpListeners)
+                    httpListener.Dispose();
 
-            foreach (Socket tlsListener in _tlsListeners)
-                tlsListener.Dispose();
+                foreach (Socket tlsListener in _tlsListeners)
+                    tlsListener.Dispose();
 
-            foreach (Socket httpsListener in _httpsListeners)
-                httpsListener.Dispose();
+                foreach (Socket httpsListener in _httpsListeners)
+                    httpsListener.Dispose();
+            }
+            else
+            {
+                //issue: https://github.com/dotnet/runtime/issues/37873
+
+                foreach (Socket udpListener in _udpListeners)
+                {
+                    ThreadPool.QueueUserWorkItem(delegate (object state)
+                    {
+                        udpListener.Dispose();
+                    });
+                }
+
+                foreach (Socket tcpListener in _tcpListeners)
+                {
+                    ThreadPool.QueueUserWorkItem(delegate (object state)
+                    {
+                        tcpListener.Dispose();
+                    });
+                }
+
+                foreach (Socket httpListener in _httpListeners)
+                {
+                    ThreadPool.QueueUserWorkItem(delegate (object state)
+                    {
+                        httpListener.Dispose();
+                    });
+                }
+
+                foreach (Socket tlsListener in _tlsListeners)
+                {
+                    ThreadPool.QueueUserWorkItem(delegate (object state)
+                    {
+                        tlsListener.Dispose();
+                    });
+                }
+
+                foreach (Socket httpsListener in _httpsListeners)
+                {
+                    ThreadPool.QueueUserWorkItem(delegate (object state)
+                    {
+                        httpsListener.Dispose();
+                    });
+                }
+            }
 
             _listenerThreads.Clear();
             _udpListeners.Clear();

DnsServerCore/SocketExtension.cs

@@ -0,0 +1,67 @@
+/*
+Technitium DNS Server
+Copyright (C) 2020  Shreyas Zare (shreyas@technitium.com)
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+*/
+
+using System;
+using System.Net;
+using System.Net.Sockets;
+using System.Threading;
+
+namespace DnsServerCore
+{
+    static class SocketExtension
+    {
+        public static void CloseWorkAround(this Socket socket)
+        {
+            //issue: https://github.com/dotnet/runtime/issues/37873
+
+            if (Environment.OSVersion.Platform == PlatformID.Win32NT)
+            {
+                socket.Dispose();
+            }
+            else
+            {
+                IPEndPoint localEP = socket.LocalEndPoint as IPEndPoint;
+                EventWaitHandle waitHandle = new AutoResetEvent(false);
+
+                ThreadPool.QueueUserWorkItem(delegate (object state)
+                {
+                    waitHandle.Set();
+                    socket.Dispose();
+                    waitHandle.Set();
+                });
+
+                waitHandle.WaitOne();
+                Thread.Sleep(1000); //wait to ensure the above thread has called socket.Dispose()
+
+                //send empty UDP packet to release thread blocking on Socket.ReceiveMessageFrom() call
+                using (Socket s = new Socket(localEP.AddressFamily, SocketType.Dgram, ProtocolType.Udp))
+                {
+                    if (localEP.Address.Equals(IPAddress.Any))
+                        localEP = new IPEndPoint(IPAddress.Loopback, localEP.Port);
+                    else if (localEP.Address.Equals(IPAddress.IPv6Any))
+                        localEP = new IPEndPoint(IPAddress.IPv6Loopback, localEP.Port);
+
+                    s.SendTo(new byte[] { }, localEP);
+                }
+
+                waitHandle.WaitOne();
+            }
+        }
+    }
+}