// Figure out the host and port we're going to talk to
String host = args[0];
int port = 13;
if (args.length > 1) port = Integer.parseInt(args[1]);
// Create a socket to use
DatagramSocket socket = new DatagramSocket();
// Specify a 1-second timeout so that receive() does not block forever.
socket.setSoTimeout(1000);
// This buffer will hold the response. On overflow, extra bytes are
// discarded: there is no possibility of a buffer overflow attack here.
byte[] buffer = new byte[512];
DatagramPacket packet = new DatagramPacket(buffer, buffer.length,
new InetSocketAddress(host,port));
// Try three times before giving up
for(int i = 0; i < 3; i++) {
try {
// Send an empty datagram to the specified host (and port)
packet.setLength(0); // make the packet empty
socket.send(packet); // send it out
// Wait for a response (or timeout after 1 second)
packet.setLength(buffer.length); // make room for the response
socket.receive(packet); // wait for the response
// Decode and print the response
System.out.print(new String(buffer, 0, packet.getLength(),
"US-ASCII"));
// We were successful so break out of the retry loop
break;
}
catch(SocketTimeoutException e) {
// If the receive call timed out, print error and retry
System.out.println("No response");
}
}
// We're done with the channel now
socket.close();
