DatagramSocket not picking up UDP broadcasts

Question

Hi All.
Opened a DatagramSocket on specific port. Verfied (w/ tcpdump) that other process is sending UDP packets to that port with broadcast address (128.0.3.255). {pseudocode}DatagramSocket.recieve(dgram) never picks up the packets.
If I change the sender to use specific IP (unicast), the recieve gets the packet and unblocks.
JRE1_5
Any ideas?
-K

Answer 1

Are you binding the socket to the wildcard address?
Is that really your broadcast address? The 128.0.0.0/16 address block is slightly unusual (RFC 3330). broadcast = ip_address | ~netmask; (or use 255.255.255.255).
Try these programs; do they work for you? If they do, what is different between these and your program:
import java.io.*;
import java.net.*;
public class BroadcastSend
    public static void main(String args[])
     throws Exception
     DatagramSocket socket = new DatagramSocket();
     socket.setBroadcast(true);
     socket.setReceiveBufferSize(64 * 1024);
     socket.setSendBufferSize(64 * 1024);
     int str_size = args.length == 0 ? 256 : Integer.parseInt(args[0]);
     StringBuffer str = new StringBuffer();
     while (str.length() < str_size)
         str.append("hello world");
     str.setLength(str_size);
     byte buf[] = str.toString().getBytes("ISO-8859-1");
     InetAddress address = InetAddress.getByName("255.255.255.255");
     DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 6666);
     socket.send(packet);
import java.io.*;
import java.net.*;
public class BroadcastReceive
    public static void main(String args[])
     throws Exception
     DatagramSocket socket = new DatagramSocket(6666);
     socket.setBroadcast(true);
     socket.setReceiveBufferSize(64 * 1024);
     socket.setSendBufferSize(64 * 1024);
     while (true) {
         byte buf[] = new byte[64 * 1024];
         DatagramPacket packet = new DatagramPacket(buf, buf.length);
         socket.receive(packet);
         System.out.println("got " + packet.getLength() +
                      " bytes from " + packet.getAddress());
}