Question

My server creates a multicast socket which listens for UDP packets. It is receiving packets sent to the broadcast address of the network but I can’t get the ip address of the sender:

multisocket.getInetAddress().getHostAddress();

returns

"::" 

(I guess its because of null getInetAddress).

How can I get the IP address of the sender?

TIPS: I guess it has to do with the socket not being bound and basically the whole broadcasting because those packets arent sent exclusively to me but to the whole network, but shouldnt they hold the IP address of the sender? Enlighten me please.

Here is the code:

public void run() {
    try {
        Thread.sleep(5000);
        Log.i("SERVERUDP", "Connecting...");
        MulticastSocket multisocket = new MulticastSocket(SERVERPORT);
        multisocket.setBroadcast(true);
        Log.i("SERVERUDP","Server's IP is: " + multisocket.getLocalAddress().getHostAddress());
        getLocalIpAddress();
        while(true){
            byte[] b = new byte[65535];
            ByteArrayInputStream b_in = new ByteArrayInputStream(b);
            DatagramPacket dgram = new DatagramPacket(b, b.length);
            multisocket.receive(dgram); // blocks
            ObjectInputStream o_in = new ObjectInputStream(b_in);
            Object o = o_in.readObject();
            dgram.setLength(b.length);
            b_in.reset();
            if(o.getClass().getSimpleName().equalsIgnoreCase("Request")){
                Request request = (Request)o;
                String inetaddress = multisocket.getInetAddress().getHostAddress();
                Log.i("SERVERUDP-if", "Sending request to IP: " + inetaddress);
                new Thread(new ClientTCP(inetaddress, createRequestFromBroadcast(request))).start();
            }else if(o.getClass().getSimpleName().equalsIgnoreCase("String")){
                Log.e("SERVERUDP-elseif-string", "WTF received a string: " + (String)o);
            }else{
                Log.e("SERVERUDP-else", "Unrecognized object of type: " + o.getClass().getSimpleName());
            }
            o_in.close();
            //iteration done only once for testing!
            break;
        }
        multisocket.close();
    } catch (Exception e) {
        Log.e("SERVERUDP", "Error", e);
    }
}

Answer 1

Each packet that you receive might have a different source address. So I’m not sure why you’re trying to look at multisocket to figure out the source address.

I have to admit I haven’t tried this, but does dgram.getSocketAddress () give you what you want after the multisocket.receive call returns?