6️⃣ Backend: use dual-stack socket when possible

Listener on [::]:1080 now accepts traffic from both IPv4 and IPv6.

Shadowsocks.Net/TCPListener.cs

@@ -68,6 +68,7 @@ namespace Shadowsocks.Net
                 // Create a TCP/IP socket.
                 _tcpSocket = new Socket(_localEndPoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
                 _tcpSocket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
+                _tcpSocket.SetSocketOption(SocketOptionLevel.IPv6, SocketOptionName.IPv6Only, false);
 
                 // Bind the socket to the local endpoint and listen for incoming connections.
                 _tcpSocket.Bind(_localEndPoint);

Shadowsocks.Net/UDPListener.cs

@@ -63,6 +63,7 @@ namespace Shadowsocks.Net
             // Create a TCP/IP socket.
             _udpSocket = new Socket(_localEndPoint.AddressFamily, SocketType.Dgram, ProtocolType.Udp);
             _udpSocket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
+            _udpSocket.SetSocketOption(SocketOptionLevel.IPv6, SocketOptionName.IPv6Only, false);
 
             // Bind the socket to the local endpoint and listen for incoming connections.
             _udpSocket.Bind(_localEndPoint);