Server: Added an optional dead connection detection mechanism instead of simply disconnecting them after a specified duration of inactivity

SMBLibrary/SMBLibrary.csproj

@@ -256,6 +256,7 @@
     <Compile Include="Server\SMB2\ChangeNotifyHelper.cs" />
     <Compile Include="Server\SMB2\CloseHelper.cs" />
     <Compile Include="Server\SMB2\CreateHelper.cs" />
+    <Compile Include="Server\SMB2\EchoHelper.cs" />
     <Compile Include="Server\SMB2\IOCtlHelper.cs" />
     <Compile Include="Server\SMB2\LockHelper.cs" />
     <Compile Include="Server\SMB2\NegotiateHelper.cs" />

SMBLibrary/Server/ConnectionManager.cs

@@ -54,14 +54,30 @@ namespace SMBLibrary.Server
             }
         }
 
-        public void ReleaseInactiveConnections(TimeSpan inactivityDuration)
+        /// <summary>
+        /// Some broken NATs will reply to TCP KeepAlive even after the client initiating the connection has long gone,
+        /// This methods prevent such connections from hanging around indefinitely by sending an unsolicited ECHO response to make sure the connection is still alive.
+        /// </summary>
+        public void SendSMBKeepAlive(TimeSpan inactivityDuration)
         {
             List<ConnectionState> connections = new List<ConnectionState>(m_activeConnections);
             foreach (ConnectionState connection in connections)
             {
-                if (connection.LastReceiveDT.Add(inactivityDuration) < DateTime.UtcNow)
+                if (connection.LastReceiveDT.Add(inactivityDuration) < DateTime.UtcNow &&
+                    connection.LastSendDT.Add(inactivityDuration) < DateTime.UtcNow)
                 {
-                    ReleaseConnection(connection);
+                    if (connection is SMB1ConnectionState)
+                    {
+                        // [MS-CIFS] Clients SHOULD, at minimum, send an SMB_COM_ECHO to the server every few minutes.
+                        // This means that an unsolicited SMB_COM_ECHO reply is not likely to be sent on a connection that is alive.
+                        SMBLibrary.SMB1.SMB1Message echoReply = SMB1.EchoHelper.GetUnsolicitedEchoReply();
+                        SMBServer.EnqueueMessage(connection, echoReply);
+                    }
+                    else if (connection is SMB2ConnectionState)
+                    {
+                        SMBLibrary.SMB2.EchoResponse echoResponse = SMB2.EchoHelper.GetUnsolicitedEchoResponse();
+                        SMBServer.EnqueueResponse(connection, echoResponse);
+                    }
                 }
             }
         }

SMBLibrary/Server/SMB1/EchoHelper.cs

@@ -24,5 +24,28 @@ namespace SMBLibrary.Server.SMB1
             }
             return response;
         }
+
+        internal static SMB1Message GetUnsolicitedEchoReply()
+        {
+            // [MS-CIFS] 3.2.5.1 - If the PID and MID values of the received message are not found in the
+            // Client.Connection.PIDMIDList, the message MUST be discarded.
+            SMB1Header header = new SMB1Header();
+            header.Command = CommandName.SMB_COM_ECHO;
+            header.Status = NTStatus.STATUS_SUCCESS;
+            header.Flags = HeaderFlags.CaseInsensitive | HeaderFlags.CanonicalizedPaths | HeaderFlags.Reply;
+            // [MS-CIFS] SMB_FLAGS2_LONG_NAMES SHOULD be set to 1 when the negotiated dialect is NT LANMAN.
+            // [MS-CIFS] SMB_FLAGS2_UNICODE SHOULD be set to 1 when the negotiated dialect is NT LANMAN.
+            header.Flags2 = HeaderFlags2.LongNamesAllowed | HeaderFlags2.NTStatusCode | HeaderFlags2.Unicode;
+            header.UID = 0xFFFF;
+            header.TID = 0xFFFF;
+            header.PID = 0xFFFFFFFF;
+            header.MID = 0xFFFF;
+
+            EchoResponse response = new EchoResponse();
+            SMB1Message reply = new SMB1Message();
+            reply.Header = header;
+            reply.Commands.Add(response);
+            return reply;
+        }
     }
 }

SMBLibrary/Server/SMB2/EchoHelper.cs

@@ -0,0 +1,25 @@
+/* Copyright (C) 2017 Tal Aloni <tal.aloni.il@gmail.com>. All rights reserved.
+ * 
+ * You can redistribute this program and/or modify it under the terms of
+ * the GNU Lesser Public License as published by the Free Software Foundation,
+ * either version 3 of the License, or (at your option) any later version.
+ */
+using System;
+using System.Collections.Generic;
+using SMBLibrary.SMB2;
+using Utilities;
+
+namespace SMBLibrary.Server.SMB2
+{
+    internal class EchoHelper
+    {
+        internal static EchoResponse GetUnsolicitedEchoResponse()
+        {
+            // [MS-SMB2] 3.2.5.1.2 - If the MessageId is 0xFFFFFFFFFFFFFFFF, this is not a reply to a previous request, and the client MUST NOT attempt to locate the request, but instead process it as follows:
+            // If the command field in the SMB2 header is SMB2 OPLOCK_BREAK, it MUST be processed as specified in 3.2.5.19. Otherwise, the response MUST be discarded as invalid.
+            EchoResponse response = new EchoResponse();
+            response.Header.MessageID = 0xFFFFFFFFFFFFFFFF;
+            return response;
+        }
+    }
+}

SMBLibrary/Server/SMBServer.cs

@@ -32,7 +32,7 @@ namespace SMBLibrary.Server
         private Guid m_serverGuid;
 
         private ConnectionManager m_connectionManager;
-        private Thread m_monitorInactiveConnectionsThread;
+        private Thread m_sendSMBKeepAliveThread;
 
         private IPAddress m_serverAddress;
         private SMBTransportType m_transport;
@@ -64,9 +64,9 @@ namespace SMBLibrary.Server
         }
 
         /// <param name="connectionInactivityTimeout">
-        /// The duration after which a connection will be closed if no data has been received.
+        /// The duration after which an unsolicited ECHO response will be sent if no data has been sent or received.
         /// Some broken NATs will reply to TCP KeepAlive even after the client initiating the connection has long gone,
-        /// to prevent such connections from hanging around indefinitely, this parameter can be used to disconnect any connection that has not received data in a long while.
+        /// to prevent such connections from hanging around indefinitely, this parameter can be used.
         /// </param>
         /// <exception cref="System.Net.Sockets.SocketException"></exception>
         public void Start(IPAddress serverAddress, SMBTransportType transport, bool enableSMB1, bool enableSMB2, TimeSpan? connectionInactivityTimeout)
@@ -89,16 +89,16 @@ namespace SMBLibrary.Server
 
                 if (connectionInactivityTimeout.HasValue)
                 {
-                    m_monitorInactiveConnectionsThread = new Thread(delegate()
+                    m_sendSMBKeepAliveThread = new Thread(delegate()
                     {
                         while (m_listening)
                         {
                             Thread.Sleep(InactivityMonitoringInterval);
-                            m_connectionManager.ReleaseInactiveConnections(connectionInactivityTimeout.Value);
+                            m_connectionManager.SendSMBKeepAlive(connectionInactivityTimeout.Value);
                         }
                     });
-                    m_monitorInactiveConnectionsThread.IsBackground = true;
-                    m_monitorInactiveConnectionsThread.Start();
+                    m_sendSMBKeepAliveThread.IsBackground = true;
+                    m_sendSMBKeepAliveThread.Start();
                 }
             }
         }
@@ -107,9 +107,9 @@ namespace SMBLibrary.Server
         {
             Log(Severity.Information, "Stopping server");
             m_listening = false;
-            if (m_monitorInactiveConnectionsThread != null)
+            if (m_sendSMBKeepAliveThread != null)
             {
-                m_monitorInactiveConnectionsThread.Abort();
+                m_sendSMBKeepAliveThread.Abort();
             }
             SocketUtils.ReleaseSocket(m_listenerSocket);
             m_connectionManager.ReleaseAllConnections();