Made it possible to modify the server_conn.connection attribute,

using the serverconnect stub.

mitmproxy/models/connections.py

@@ -112,7 +112,7 @@ class ServerConnection(tcp.TCPClient, stateobject.StateObject):
     Attributes:
         address: Remote address. Can be both a domain or an IP address.
         ip_address: Resolved remote IP address.
-        source_address: Local IP address
+        source_address: Local IP address or client's source IP address.
         ssl_established: True if TLS is established, False otherwise
         cert: The certificate presented by the remote during the TLS handshake
         sni: Server Name Indication sent by the proxy during the TLS handshake

netlib/tcp.py

@@ -729,10 +729,15 @@ class TCPClient(_Connection):
 
     def connect(self):
         try:
-            connection = socket.socket(self.address.family, socket.SOCK_STREAM)
+            if not self.connection:
+                connection = socket.socket(self.address.family, socket.SOCK_STREAM)
+            else:
+                connection = self.connection
+
             if self.spoof_source_address:
                 try:
-                    connection.setsockopt(socket.SOL_IP, 19, 1)
+                    if not connection.getsockopt(socket.SOL_IP, 19):
+                        connection.setsockopt(socket.SOL_IP, 19, 1)
                 except socket.error as e:
                     raise exceptions.ProtocolException(
                         "Failed to spoof the source address: " + e.strerror)