Add a mDNS package

pkg/mdns/lib/mdns.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:collection';
+import 'dart:io';
+import 'dart:typed_data';
+
+import 'package:mdns/src/constants.dart';
+import 'package:mdns/src/lookup_resolver.dart';
+import 'package:mdns/src/packet.dart';
+
+/// Client for DNS lookup using the mDNS protocol.
+///
+/// This client only support "One-Shot Multicast DNS Queries" as described in
+/// section 5.1 of https://tools.ietf.org/html/rfc6762
+class MDNSClient {

[karlklose, 2015/10/28 10:14:30]

I think this should be 'MDnsClient'.

[Søren Gjesse, 2015/10/28 11:36:57]

Yes, you are probably right.

+  bool _starting = false;
+  bool _started = false;
+  RawDatagramSocket _incoming;
+  final List<RawDatagramSocket> _sockets = [];

[karlklose, 2015/10/28 10:14:30]

Consider adding a type to the list literal to catch type errors at runtime.

[Søren Gjesse, 2015/10/28 11:36:57]

Done.

+  final LookupResolver _resolver = new LookupResolver();
+  final Duration _defaultTimeout = new Duration(seconds: 5);

[karlklose, 2015/10/28 10:14:30]

Does it make sense to remove this field (it cannot be altered anyway) and make
'const Duration(seconds: 5)' the default value for timeout in  lookup?

[Søren Gjesse, 2015/10/28 11:36:57]

Absolutely. Done.

+
+  MDNSClient();

[karlklose, 2015/10/28 10:14:30]

Not needed.

[Søren Gjesse, 2015/10/28 11:36:57]

Removed.

+
+  /// Start the mDNS client.
+  Future start() async {
+    if (_started && _starting) {
+      throw new StateError('mDNS client already started');
+    }
+    _starting = true;
+
+    // Listen on all addresses.
+    _incoming = await RawDatagramSocket.bind(
+        InternetAddress.ANY_IP_V4, mDNSPort, reuseAddress: true);
+
+    // Find all network interfaces with an IPv4 address.
+    var interfaces =
+        await NetworkInterface.list(type: InternetAddressType.IP_V4);
+    var socket;

[karlklose, 2015/10/28 10:14:30]

Move declaration to line 45?

[Søren Gjesse, 2015/10/28 11:36:57]

Done.

+    for (int i = 0; i < interfaces.length; i++) {

[karlklose, 2015/10/28 10:14:30]

How about:

  for (NetworkInterface interface in interfaces) {
    ...
  }
?

[Søren Gjesse, 2015/10/28 11:36:57]

Sure, done.

+      // Create a socket for sending on each adapter.
+      socket = await RawDatagramSocket.bind(
+          interfaces[i].addresses[0], mDNSPort, reuseAddress: true);
+      _sockets.add(socket);
+
+      // Join multicast on this interface.
+      _incoming.joinMulticast(mDNSAddress, interfaces[i]);
+    }
+    _incoming.listen(_handleIncoming);
+
+    _starting = false;
+    _started = true;
+  }
+
+  /// Stop the mDNS client.
+  void stop() {
+    if (!_started) return;

[karlklose, 2015/10/28 10:14:30]

Should this method throw if '_starting == true'?

[Søren Gjesse, 2015/10/28 11:36:57]

Done.

+
+    _sockets.forEach((socket) => socket.close());
+    _incoming.close();
+
+    _started = false;
+  }
+
+  /// Lookup [hostname] using mDNS.
+  ///
+  /// The `hostname` must have the form `single-dns-label.local`,
+  /// e.g. `printer.local`.
+  ///
+  /// If no answer has been received within the specified [timeout]
+  /// this method will complete with the value `null`.
+  Future<InternetAddress> lookup(String hostname, {Duration timeout}) {
+    if (!_started) {
+      throw new StateError('mDNS client is not started');
+    }
+
+    // Add the pending request before sending the query.
+    var future = _resolver.addPendingRequest(
+        hostname, timeout != null ? timeout : _defaultTimeout);
+
+    // Send the request on all interfaces.
+    List<int> packet = encodeMDNSQuery(hostname);
+    for (int i = 0; i < _sockets.length; i++) {
+      _sockets[i].send(packet, mDNSAddress, mDNSPort);
+    }
+
+    return future;
+  }
+
+  // Process incoming datagrams.
+  _handleIncoming(event) {
+    if (event == RawSocketEvent.READ) {
+      var data = _incoming.receive();
+      var response = decodeMDNSResponse(data.data);
+      if (response != null) {
+        _resolver.handleResponse(response);
+      }
+    }
+  }
+}
+
+// Simple standalone test.
+main() async {
+  var client = new MDNSClient();
+  await client.start();
+  var address = await client.lookup('raspberrypi.local');
+  client.stop();
+  print(address);
+}