Question

I’m having some problems receiving a String from a multicast.
It’s the first time i’m using UDP multicast in Java.

I’m making a multiclient application over a LAN.
So i’m using Local Ip addresses.
I need the clients to find the server’s IP address so they can send their data, requests, etc.

I let the server sent out a multicast with his own IP as a string every 5 seconds.
The clients should be able to receive it.
The problem is that they don’t receive anything.
I’m testing with 2 devices so i don’t need to use localhost.

Here’s some of my code:
Server side:

public class MulticastIpSender extends Thread{

private String serverIp;
private int port;
private String multicastAddress;
private long WAITING_TIME = 5000;  // 5 seconden

private DatagramSocket socket;

public MulticastIpSender(String serverIp, int port, String multicastAddress) throws SocketException {
    super();
    this.serverIp = serverIp;
    this.port = port;
    this.multicastAddress = multicastAddress;
    socket = new DatagramSocket(port);
}

public void run() {
    while(true){
        try {
            byte[] buf = new byte[256];
            buf = serverIp.getBytes();
            InetAddress group = InetAddress.getByName(multicastAddress);
            DatagramPacket packet = new DatagramPacket(buf, buf.length, group, port);
            socket.send(packet);
            System.out.println("sent IP("+serverIp+") to group("+group+") on port "+port);
            sleep(WAITING_TIME);

        } catch (UnknownHostException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

}

}

I put this method in the main class:

private void sendIpAddressToListeners() {
    try {
        multicastIpSender = new MulticastIpSender(serverIp,PORT,"230.0.0.1");
        multicastIpSender.run();
    } catch (SocketException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

}

I get the serverIp like this:

    String hostName = InetAddress.getLocalHost().getHostName();
        InetAddress addrs[] = InetAddress.getAllByName(hostName);
if (!addr.isLoopbackAddress() && addr.isSiteLocalAddress()) {
        myIp = addr.getHostAddress();
        }
        }
        System.out.println ("\nIP = " + myIp);
        return myIp;

on this device it gives me 192.168.1.2

CLIENT SIDE:

multicastSocket = new MulticastSocket(PORT);
        InetAddress address = InetAddress.getByName(MULTICAST_ADDRESS);
        multicastSocket.joinGroup(address);

        DatagramPacket serverIpPacket;
        byte[] buf = new byte[256];
        serverIpPacket = new DatagramPacket(buf, buf.length);
        while(receivedIp ==null){
            multicastSocket.receive(serverIpPacket);
            receivedIp = new String(serverIpPacket.getData(), 0, serverIpPacket.getLength());
            System.out.println("received server ip: " + receivedIp);
        }

!! PORT = 4445 in both server and client

I hope somebody can help me with this or can explain a better way to do this.

Answer 1

Send a normal DatagramPacket on broadcast address, the packet will be received by all hosts in local network (with same network configuration, important part is mask)

Use calculator to check your broadcast address, this one works good: http://www.subnet-calculator.com/