I try to implement a UDP client server detection via broadcast. The idea is the following: I have a server, which is bound to port 12344 and a client which is bound to port 12345. Now, the client sends a broadcast package to 255.255.255.255 12344. The server should reply to this package with a other package to IPClient:12345.
The implementation uses Java nio.
The problem is, that on windows, the server gets the packages but cannot(?) send an answer (I don't see the answer in wireshark).
I have the following example code:
Client
public final class ASyncUDPClient {
public static void main(String[] args) throws IOException {
InetSocketAddress hostAddress = new InetSocketAddress("255.255.255.255", 12344);
System.out.println(hostAddress);
// Create a non-blocking socket channel
DatagramChannel channel = DatagramChannel.open();
channel.socket().setBroadcast(true);
channel.socket().bind(new InetSocketAddress(getAddress(), 12345));
channel.configureBlocking(false);
// Kick off connection establishment
channel.connect(hostAddress);
ByteBuffer buffer = getBuffer();
Selector selector = Selector.open();
channel.write(buffer);
System.out.println("data send");
channel.register(selector, SelectionKey.OP_READ);
while (true) {
final int select = selector.select();
System.out.println("select " + select);
Iterator selectedKeys = selector.selectedKeys().iterator();
while (selectedKeys.hasNext()) {
System.out.println("key selected");
SelectionKey key = (SelectionKey) selectedKeys.next();
selectedKeys.remove();
if (!key.isValid()) {
continue;
}
if (key.isReadable()) {
System.out.println("read");
} else if (key.isWritable()) {
System.out.println("write");
}
}
}
}
private static ByteBuffer getBuffer() throws CharacterCodingException {
return Charset.forName("UTF-8").newEncoder().encode(CharBuffer.wrap("1234"));
}
private static InetAddress getAddress() throws SocketException {
final Enumeration<NetworkInterface> networkInterfaces = NetworkInterface.getNetworkInterfaces();
NetworkInterface networkInterfaceToUse = null;
while (networkInterfaces.hasMoreElements()) {
final NetworkInterface networkInterface = networkInterfaces.nextElement();
if (networkInterface.getDisplayName().contains("Virtual")) continue;
if (networkInterface.isVirtual()) continue;
if (networkInterface.isLoopback()) continue;
if (!networkInterface.isUp()) continue;
networkInterfaceToUse = networkInterface;
System.out.println(networkInterfaceToUse);
}
return networkInterfaceToUse.getInterfaceAddresses().get(1).getAddress();
}
}
Server example
public class ASyncUDPSvr {
static int BUF_SZ = 1024;
static int port = 12344;
static public void main(String[] args) {
ASyncUDPSvr svr = new ASyncUDPSvr();
svr.process();
}
private static InetAddress getAddress() throws SocketException {
final Enumeration<NetworkInterface> networkInterfaces = NetworkInterface.getNetworkInterfaces();
NetworkInterface networkInterfaceToUse = null;
while (networkInterfaces.hasMoreElements()) {
final NetworkInterface networkInterface = networkInterfaces.nextElement();
if (networkInterface.getDisplayName().contains("Virtual")) continue;
if (networkInterface.isVirtual()) continue;
if (networkInterface.isLoopback()) continue;
if (!networkInterface.isUp()) continue;
networkInterfaceToUse = networkInterface;
System.out.println(networkInterfaceToUse);
}
return networkInterfaceToUse.getInterfaceAddresses().get(1).getAddress();
}
private void process() {
try {
Selector selector = Selector.open();
DatagramChannel channel = DatagramChannel.open();
InetSocketAddress isa = new InetSocketAddress(getAddress(), port);
channel.socket().bind(isa);
channel.configureBlocking(false);
SelectionKey clientKey = channel.register(selector, SelectionKey.OP_READ);
clientKey.attach(new Con());
while (true) {
try {
selector.select();
Iterator selectedKeys = selector.selectedKeys().iterator();
while (selectedKeys.hasNext()) {
try {
SelectionKey key = (SelectionKey) selectedKeys.next();
selectedKeys.remove();
if (!key.isValid()) {
continue;
}
if (key.isReadable()) {
read(key);
key.interestOps(SelectionKey.OP_WRITE);
} else if (key.isWritable()) {
write(key);
key.interestOps(SelectionKey.OP_READ);
}
} catch (IOException e) {
System.err.println("glitch, continuing... " + (e.getMessage() != null ? e.getMessage() : ""));
}
}
} catch (IOException e) {
System.err.println("glitch, continuing... " + (e.getMessage() != null ? e.getMessage() : ""));
}
}
} catch (IOException e) {
System.err.println("network error: " + (e.getMessage() != null ? e.getMessage() : ""));
}
}
private void read(SelectionKey key) throws IOException {
DatagramChannel chan = (DatagramChannel) key.channel();
Con con = (Con) key.attachment();
con.sa = chan.receive(con.req);
System.out.println("sender address: " + con.sa + "rcv: " + new String(con.req.array(), "UTF-8"));
con.resp = Charset.forName("UTF-8").newEncoder().encode(CharBuffer.wrap("send string"));
}
private void write(SelectionKey key) throws IOException {
DatagramChannel chan = (DatagramChannel) key.channel();
Con con = (Con) key.attachment();
System.out.println("sending data: " + new String(con.resp.array(), "UTF-8") + " to " + con.sa);
chan.send(con.resp, con.sa);
System.out.println("data send");
}
class Con {
ByteBuffer req;
ByteBuffer resp;
SocketAddress sa;
public Con() {
req = ByteBuffer.allocate(BUF_SZ);
}
}
}
