Multicast packet not received by MulticastSocket on any localhost client

Question

Intro

There are a number of tutorials on creating a multicast publisher and receiver.

I used the one found here with a few modifications.

A few others:

• here
• here
• here

note that these tutorials are all quite similar.

Details:

The server runs on port 7777 and sends datagrams to 224.0.0.0 (I tested a couple of other ip's in the multicast range: 224.0.0.0 to 239.255.255.255 , but these didn't work)

Client then joins the multicast group 224.0.0.0 and waits for a packet response (run as a thread)

extra: I send a message like: 123.23.13.12[host-name]:1234 as the datagram data.

Problem:

Multicast packets from server (on localhost) not reaching client (on localhost).

Clients include a java console application (code found below) and Android application on Android Emulator. Both clients do not receive multicast packets.

I know that the multicast packets are being sent as this is shown in Wireshark

Below you will find a basic example of that which I have.

TL;DR : Server sends multicast packets (confirmed via Wireshark) but client doesn't receive them.

Suggestions are very welcome!

UPDATE

Based on Just another Java programmer 's comment, I check my firewall. Lo and behold, my firewall was dropping on the input and forward chains. I set this to accept all incoming (temporarily)

Based on Ron Maupin 's comments.

• I have changed the message sent to exclude the hostname, thus the message sent is 123.12.13.23:1234
• I have changed the multicast send address to 239.254.0.0 which is within the specified range (see Ron's comment)
• the multicast port is set to 7777
• the outgoing interface is set with s.setInterface(InetAddress.getLocalHost()) in the broadcastServer() try catch block

With these changes applied, the client(s) still do not receive any packets.

Code:

Server Side (Console App):

String multicastAddress = "239.254.0.0", multicastPort = 7777;

private void broadcastServer() {
    String message = null;
    MulticastSocket s = null;
    InetAddress local = null, group = null;
    InetAddress[] allByName;
    try {
        local = InetAddress.getLocalHost();
        s = new MulticastSocket(multicastPort);
        s.setReuseAddress(true);
        s.setInterface(local)
        s.joinGroup(InetAddress.getByName(multicastAddress));
        group = InetAddress.getByName(multicastAddress);
    } catch (SocketException e) {
        e.printStackTrace();
    } catch (UnknownHostException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

    MulticastSocket socket = s;

    // getNetworkIP() gets lan network ip
    // serverport = 1025
    message = local.getHostAddress() + ":" + String.valueOf(serverPort);
    byte[] buf = message.getBytes();
    DatagramPacket packet = new DatagramPacket(buf, buf.length, group, multicastPort);

    thdBroadcaster = new Thread(() -> {
        while (bRunServer) {
            try {
                Thread.sleep(1000);
                printf("[Broadcast] Broadcasting...");
                socket.send(packet);
                printf("OK\n");
                printf("");
            } catch (IOException e) {
                e.printStackTrace();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        socket.close();
    });
    thdBroadcaster.start();
}

Client Side (Console app):

String multicastAddress = "239.254.0.0", multicastPort = 7777;

private void startServerListenerThread() {
    Thread thdServerListener = new Thread(new Runnable() {
        @Override
        public void run() {
            MulticastSocket socket = null;
            InetAddress group = null;
            try {
                socket = new MulticastSocket(multicastPort);
                socket.setReuseAddress(true);
                group = InetAddress.getByName(multicastAddress);
                socket.joinGroup(group);

                handleServerBroadcasts(socket);
                socket.leaveGroup(group);
                socket.close();

            } catch (UnknownHostException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        private void handleServerBroadcasts(final MulticastSocket socket) {
            while (true){
                try {
                    byte[] buf = new byte[256];
                    DatagramPacket packet = new DatagramPacket(buf, buf.length);
                    socket.receive(packet);
                    String received = new String(packet.getData());

                    String address = received.substring(0, received.indexOf(":"));
                    String port = received.substring(received.indexOf(":") + 1);

                    System.out.println();
                } catch (IOException e) {
                    e.printStackTrace();
                } catch (Exception x){
                    x.printStackTrace();
                }
            }
        }
    });
    thdServerListener.start();
}

Answer 1

The solution ended up being a trivial change.

Resolving the issue was as simple as changing:

s = new MulticastSocket(multicastPort);

to

s = new MulticastSocket();

on the server side ONLY

Note: The firewall is a requirement, check if multicast packets are allowed through. The interface I used is localhost, not a requirement though. However this can be set by getting the specified interface you need.

UPDATE

EJP's comment:

I changed the port used to 8888 . This port was used to send server datagrams and for the client to connect on with their MulticastSocket

Answer 2

You are calling setReuseAddress() after binding the socket. It has no effect. You need to create an unbound MulticastSocket , call setReuseAddress() , and then bind it.