Socket Connector

Connect two TCP sockets and optionally display the traffic.

Features

TCP Sockets come in two flavours - a server and a client; you have to be one or the other. If you want to join two clients or two servers this package includes all the tools you need to connect servers to servers and clients to clients. Why would you need this type of service? To create a rendezvous service that two clients can connect to for example or to join two servers with a shared client. Also included is a client to server, this acts as a simple TCP proxy and as with all the services you can optionally set the verbose flag in order to see more info about what is happening, and the logTraffic flag in order to see the readable (ascii) characters that are being transmitted and received.

The relay is created via one of four factory methods on SocketConnector, depending on whether each side is a server (something connects to us) or a client (we connect out):

Method	Side A	Side B
serverToServer	binds/listens	binds/listens
serverToSocket	binds/listens	connects out
socketToServer	connects out	binds/listens
socketToSocket	connects out	connects out

Other capabilities:

• Authentication — pass a SocketAuthVerifier per side to gate a connection before any data is relayed. See example/socket_connector_with_authenticator.dart.
• Transformers — pass a DataTransformer (transformAtoB / transformBtoA) to rewrite the byte stream in flight (compression, framing, etc.).
• Stats — connector.stats reports sockets created, bytes relayed each way, and the remote addresses seen during the session.
• Lifecycle — connector.connectionStream emits each new Connection, and connector.done completes when the connector closes. See example/wait_for_connector_done_example.dart.

Getting started

dart pub add socket_connector

Usage

The following code will open two server sockets and connect them and display any traffic that goes between the sockets. You can test this using ncat to connect to the two listening ports in two sessions. You will see what is typed in one window appear in the other plus see the data via the Dart program.

The snippets below assume import 'dart:io'; (for InternetAddress) and import 'package:socket_connector/socket_connector.dart';.

  // Once running use ncat to check the sockets
  SocketConnector socketConnector = await SocketConnector.serverToServer(
    addressA: InternetAddress.anyIPv4,
    addressB: InternetAddress.anyIPv4,
    portA: 9000,
    portB: 8000,
    verbose: true,
    logTraffic: true,
  );
  print('Sender Port: ${socketConnector.sideAPort}'
      ' Receiver Port: ${socketConnector.sideBPort}');

ncat localhost 8000

ncat localhost 9000

TCP keep-alive

Every socket the connector accepts or creates has TCP keep-alive enabled by default (idle 60s, interval 10s, count 5). Override it by passing a SocketKeepAlive to any of the factory methods (serverToServer, socketToServer, socketToSocket, serverToSocket):

  SocketConnector socketConnector = await SocketConnector.serverToServer(
    addressA: InternetAddress.anyIPv4,
    addressB: InternetAddress.anyIPv4,
    portA: 9000,
    portB: 8000,
    keepAlive: SocketKeepAlive(
      idleSeconds: 30,
      intervalSeconds: 5,
      probeCount: 4,
    ),
  );

Use SocketKeepAlive.disabled to turn keep-alive off. On Windows only SO_KEEPALIVE is set (with the system-default timings) because the per-probe tuning requires an ioctl that dart:io does not expose.

Additional information

Excerpted from the SocketConnector class's documentation:

/// Typical usage is via the [serverToServer], [serverToSocket],
/// [socketToSocket] and [socketToServer] methods which are different flavours
/// of the same functionality - to relay information from one socket to another.
///
/// - Upon creation, a [Timer] will be created for [timeout] duration. The
///   timer callback, when it executes, calls [close] if [connections]
//    is empty
/// - When an established connection is closed, [close] will be called if
///   [connections] is empty
/// - New [Connection]s are added to [connections] when both
///   [pendingA] and [pendingB] have at least one entry
/// - When [verbose] is true, log messages will be logged to [logger]
/// - When [logTraffic] is true, socket traffic will be logged to [logger]