How to capture packets from a TCP connection using a proxy?

Question

I’m writing a TCP proxy to capture packets sent from my computer to a remote server and vice versa. I connected a fake test server on localhost and Proxy works well, intercepting and directing the incoming packets by a client, using telnet, to the test server (on my pc). The problem is that when I start the Proxy and connect to the real remote server, the Proxy does not capture the packets from that connection. How can I fix this?

proxy.Dart:

class Proxy {
  Proxy(String host, int port) {
    ServerSocket.bind(localhost, port).then((server) {
      print('Listening on $localhost:$port');
      server.listen((socket) async {
        print('Client connected to proxy');
        final clientConn = await Socket.connect(host /* localhost*/, port);

        final client2proxy = ClientToProxy(socket, clientConn);
        final proxy2server = ProxyToServer(clientConn, socket);
      });
    });
  }
}

client_to_proxy.Dart:

class ClientToProxy {
  Socket socket;
  Socket remote;
  String address;
  int port;

  ClientToProxy(Socket sock, Socket rem) {
    socket = sock;
    remote = rem;
    address = socket.remoteAddress.address;
    port = socket.remotePort;
    msgPattern = '[client] ($address:$port) ->';

    socket.listen(onDataHandler,
        onError: onErrorHandler, onDone: onDoneHandler);
  }

  void onDataHandler(List<int> data) {
    if (debug) print('$msgPattern Data: ${String.fromCharCodes(data)}');
    pipeSocket(remote, data);
  }

  void pipeSocket(Socket sock, dynamic data) => sock.add(data);
}

proxy_to_server.Dart:

class ProxyToServer {
  Socket socket;
  Socket remote;
  String address;
  int port;

  ProxyToServer(Socket sock, Socket rem) {
    socket = sock;
    remote = rem;
    address = socket.remoteAddress.address;
    port = socket.remotePort;
    msgPattern = '[server] ($address:$port) ->';

    socket.listen(onDataHandler,
        onError: onErrorHandler, onDone: onDoneHandler);
  }

  void onDataHandler(List<int> data) {
    if (debug) print('$msgPattern Data: ${String.fromCharCodes(data)}');
    pipeSocket(remote, data);
  }

  void pipeSocket(Socket sock, dynamic data) => sock.add(data);
}

How should I intercept these packages?

Answer 1

You are only binding on the localhost interface, you have to bind on all interfaces. To do this you use the ip address 0.0.0.0.

class Proxy {
  Proxy(String host, int port) {
    // ServerSocket.bind(localhost, port).then((server) { // <-----------------
    ServerSocket.bind("0.0.0.0", port).then((server) {
      print('Listening on $localhost:$port');
      server.listen((socket) async {
        print('Client connected to proxy');
        final clientConn = await Socket.connect(host /* localhost*/, port);

        final client2proxy = ClientToProxy(socket, clientConn);
        final proxy2server = ProxyToServer(clientConn, socket);
      });
    });
  }
}

obs. not tested, but if there is an error it will probably be in the Dart syntax.