Add new options.

Source/MQTTnet/Client/Options/MqttClientTcpOptions.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Net;
 using System.Net.Sockets;
 
 namespace MQTTnet.Client
@@ -10,6 +11,12 @@ namespace MQTTnet.Client
     {
         public AddressFamily AddressFamily { get; set; } = AddressFamily.Unspecified;
 
+        /// <summary>
+        /// Gets the local endpoint (network card) which is used by the client.
+        /// Set it to _null_ to let the OS select the network card.
+        /// </summary>
+        public EndPoint LocalEndpoint { get; set; }
+        
         public int BufferSize { get; set; } = 8192;
 
         public bool? DualMode { get; set; }

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -68,6 +68,11 @@ namespace MQTTnet.Implementations
                     socket = new CrossPlatformSocket(_tcpOptions.AddressFamily);
                 }
 
+                if (_tcpOptions.LocalEndpoint != null)
+                {
+                    socket.Bind(_tcpOptions.LocalEndpoint);
+                }
+                
                 socket.ReceiveBufferSize = _tcpOptions.BufferSize;
                 socket.SendBufferSize = _tcpOptions.BufferSize;
                 socket.SendTimeout = (int)_clientOptions.Timeout.TotalMilliseconds;