Change how PASV process local IP

Now use local ip for local connection

DearFTP/Connection/Commands/PassiveCommand.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Linq;
+using System.Net;
 
 namespace DearFTP.Connection.Commands
 {
@@ -16,11 +17,43 @@ namespace DearFTP.Connection.Commands
 
             int port = session.DataConnection.Port;
             var portBytes = BitConverter.GetBytes((ushort)port).Reverse().Select(x => x.ToString());
-            string remote = string.Join(',', session.Configuration.Server.Host.Split('.').Concat(portBytes));
+
+            string remote;
+
+            if (IsInternal(session.RemoteIP))
+            {
+                remote = string.Join(',', session.LocalIP.ToString().Split('.').Concat(portBytes));
+            }
+            else
+            {
+                remote = string.Join(',', session.Configuration.Server.Host.Split('.').Concat(portBytes));
+            }
 
             stream.Send(ResponseCode.PassiveMode, $"Entering Passive Mode ({remote})");
 
             session.DataConnection.AcceptClient();
         }
+
+        // Source: https://stackoverflow.com/a/39120248
+        /// <summary>
+        /// An extension method to determine if an IP address is internal, as specified in RFC1918
+        /// </summary>
+        /// <param name="toTest">The IP address that will be tested</param>
+        /// <returns>Returns true if the IP is internal, false if it is external</returns>
+        public bool IsInternal(IPAddress toTest)
+        {
+            byte[] bytes = toTest.GetAddressBytes();
+            switch (bytes[0])
+            {
+                case 10:
+                    return true;
+                case 172:
+                    return bytes[1] < 32 && bytes[1] >= 16;
+                case 192:
+                    return bytes[1] == 168;
+                default:
+                    return false;
+            }
+        }
     }
 }

DearFTP/Connection/Session.cs

@@ -23,10 +23,11 @@ namespace DearFTP.Connection
         public bool IsTlsProtected { get; private set; }
         public string CurrentWorkingDirectory => NavigablePath.CurrentDirectory;
 
-        public string IP => ((IPEndPoint)_client.Client.LocalEndPoint).Address.ToString();
+        public IPAddress LocalIP => ((IPEndPoint)_client.Client.LocalEndPoint).Address;
+        public IPAddress RemoteIP => ((IPEndPoint)_client.Client.RemoteEndPoint).Address;
 
-        private TcpClient _client;
+        private readonly TcpClient _client;
-        private NetworkStream _networkStream;
+        private readonly NetworkStream _networkStream;
         private SslStream _sslStream;
         private bool _isRunning = true;