Netty 4 and 5 - multicast not working for me

Question

I have been trying to get UDP multicast working with Netty 4.0.26 and 5.0.0.Alpha2, without any success. I have adapted some code from this post which in its edited form supposedly works, but does not for me. The attached code simply echoes "Send 1", "Send 2" etc. but the corresponding packets are never received.

In the expression localSocketAddr = new InetSocketAddress(localAddr, MCAST_PORT) I have tried port 0 as well, also without success. All kinds of other combinations of bind port and local address have been tried also.

The various socket options are copied from the other post I mentioned.

Can anyone tell me what I'm doing wrong? This is with Java 8 on WIndows 8.1.

Thanks very much,

Sean

import io.netty.bootstrap.Bootstrap;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelFactory;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelOption;
import io.netty.channel.SimpleChannelInboundHandler;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.DatagramChannel;
import io.netty.channel.socket.DatagramPacket;
import io.netty.channel.socket.InternetProtocolFamily;
import io.netty.channel.socket.nio.NioDatagramChannel;

import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.NetworkInterface;

public class MCast {

    private static final String LOCAL_ADDR = "192.168.0.18";
    private static final String MCAST_GROUP = "239.254.42.96";
    private static final int MCAST_PORT = 9796;

    public static void main(String[] args) throws Exception {
            Thread sender = new Thread(new Sender());
            Thread receiver = new Thread(new Receiver());

            receiver.start();
            sender.start();

            sender.join();
            receiver.join();
    }

    private static class MCastSupport {
            protected InetAddress localAddr;
            protected InetAddress remoteAddr;
            protected InetSocketAddress localSocketAddr;
            protected InetSocketAddress remoteSocketAddr;
            protected DatagramChannel chan;
            protected Bootstrap bootstrap;

            public MCastSupport() {
                    try {
                            localAddr = InetAddress.getByName(LOCAL_ADDR);
                            remoteAddr = InetAddress.getByName(MCAST_GROUP);

                            localSocketAddr = new InetSocketAddress(localAddr, MCAST_PORT);
                            remoteSocketAddr = new InetSocketAddress(remoteAddr, MCAST_PORT);

                            NetworkInterface nif = NetworkInterface.getByInetAddress(localAddr);

                            bootstrap = new Bootstrap()
                                    .group(new NioEventLoopGroup())
                                    .handler(new SimpleChannelInboundHandler<DatagramPacket>() {
                                            @Override
                                            protected void messageReceived(ChannelHandlerContext ctx, DatagramPacket msg) throws Exception {
                                                    System.out.println("Received: " + msg.content().getInt(0));
                                            }
                                    })
                                    .channelFactory(new ChannelFactory<NioDatagramChannel>() {
                                            @Override
                                            public NioDatagramChannel newChannel() {
                                                    return new NioDatagramChannel(InternetProtocolFamily.IPv4);
                                            }
                                    })
                                    .handler(new SimpleChannelInboundHandler<DatagramPacket>() {
                                            @Override
                                            protected void messageReceived(ChannelHandlerContext ctx, DatagramPacket msg) throws Exception {
                                                    System.out.println("Received: " + msg.content().getInt(0));
                                            }
                                    })
                            .option(ChannelOption.SO_BROADCAST, true)
                            .option(ChannelOption.SO_REUSEADDR, true)
                            .option(ChannelOption.IP_MULTICAST_LOOP_DISABLED, false)
                            .option(ChannelOption.SO_RCVBUF, 2048)
                            .option(ChannelOption.IP_MULTICAST_TTL, 255)
                            .option(ChannelOption.IP_MULTICAST_IF, nif);

                    chan = (DatagramChannel) bootstrap.bind(localSocketAddr).sync().channel();

                    chan.joinGroup(remoteSocketAddr, nif).sync();

                    } catch (Throwable t) {
                            System.err.println(t);
                            t.printStackTrace(System.err);
                    }
            }
    }

    private static class Sender extends MCastSupport implements Runnable {
            @Override
            public void run() {
                    try {
                            for (int seq = 1; seq <= 5; ++ seq) {
                                    ByteBuf buf = Unpooled.copyInt(seq);
                                    DatagramPacket dgram = new DatagramPacket(buf, remoteSocketAddr, localSocketAddr);
                                    chan.writeAndFlush(dgram);
                                    System.out.println("Send: " + seq);
                                    Thread.sleep(5000);
                            }

                    } catch (Throwable t) {
                            System.err.println(t);
                            t.printStackTrace(System.err);
                    }
            }
    }

    private static class Receiver extends MCastSupport implements Runnable {
            @Override
            public void run() {
                    try {
                            Thread.sleep(5 * 5000);
                    } catch (Throwable t) {
                            System.err.println(t);
                            t.printStackTrace(System.err);
                    }
            }
    }
}

Answer 1

The root of my issue was setting ChannelOption.IP_MULTICAST_LOOP_DISABLED to false. Leaving out that line (ie allowing IP_MULTICAST_LOOP_DISABLED to default to true) allows multicast to work as expected. Frankly, this doesn't make a lot of sense to me, but I have no more time to investigate.