Issue 208

example/get_resource_multicast.dart

@@ -14,14 +14,22 @@
  */
 
 import 'dart:async';
+import 'dart:io';
 import 'package:coap/coap.dart';
 
 FutureOr<void> main() async {
-  final uri = Uri.parse('coap://${MulticastAddress.allNodesLinkLocalIPV6}');
+  final iface = Platform.environment['COAP_IFACE'];
+  final uri =
+      iface == null
+          ? Uri.parse('coap://${MulticastAddress.allCOAPNodesLinkLocalIPV6}')
+          : Uri.parse(
+            'coap://[${MulticastAddress.allCOAPNodesLinkLocalIPV6.address}%$iface]',
+          );
   final client = CoapClient(uri);
 
   try {
     final request = CoapRequest.get(Uri(path: '/.well-known/core'));
+    request.token = CoapConstants.emptyToken;
 
     await for (final response in client.sendMulticast(request)) {
       print(response.payloadString);

lib/src/coap_client.dart

@@ -661,12 +661,16 @@ class CoapClient {
     final String host,
     final InternetAddressType addressType,
   ) async {
-    final parsedAddress = InternetAddress.tryParse(host);
+    final decodedHost = Uri.decodeComponent(host);
+    final parsedAddress = InternetAddress.tryParse(decodedHost);
     if (parsedAddress != null) {
       return parsedAddress;
     }
 
-    final addresses = await InternetAddress.lookup(host, type: addressType);
+    final addresses = await InternetAddress.lookup(
+      decodedHost,
+      type: addressType,
+    );
     if (addresses.isNotEmpty) {
       return addresses.first;
     }

lib/src/network/coap_network_udp.dart

@@ -82,6 +82,44 @@ class CoapNetworkUDP implements CoapINetwork {
 
     // Use port 0 to generate a random source port
     _socket = await RawDatagramSocket.bind(bindAddress, 0);
+    _socket!.multicastLoopback = false;
+
+    if (address.isMulticast) {
+      final iface = await _findInterfaceFor(address);
+      if (iface != null) {
+        _socket!.setRawOption(
+          RawSocketOption.fromInt(
+            RawSocketOption.levelIPv6,
+            RawSocketOption.IPv6MulticastInterface,
+            iface.index,
+          ),
+        );
+        _socket!.joinMulticast(address, iface);
+      } else {
+        _socket!.joinMulticast(address);
+      }
+    }
+  }
+
+  Future<NetworkInterface?> _findInterfaceFor(
+    final InternetAddress multicastAddress,
+  ) async {
+    final zoneIndex = multicastAddress.address.split('%');
+    if (zoneIndex.length != 2) {
+      return null;
+    }
+
+    final ifaceName = zoneIndex.last;
+    final interfaces = await NetworkInterface.list(
+      type: InternetAddressType.any,
+      includeLinkLocal: true,
+    );
+
+    try {
+      return interfaces.firstWhere((final i) => i.name == ifaceName);
+    } on StateError {
+      return null;
+    }
   }
 
   void _receive() {