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POWhat's the correct and working way to broadcast an UDP packet in Java?
 

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  1. POWhat's the correct and working way to broadcast an UDP packet in Java?
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    <p>I need to broadcast an UDP packet on every network interface. At first, I tried broadcasting to <code>255.255.255.255</code>, with no results, and I later discovered that this "has been deprecated for about 20 years". So I tried iterating on every network interface in order to get the broadcast address of the interface and then send an UDP packet to that address.</p> <p>Still, the following code:</p> <pre><code>public static Collection&lt;InetAddress&gt; getBroadcastAddresses() { try { Collection&lt;InetAddress&gt; result = new LinkedList&lt;InetAddress&gt;(); Enumeration&lt;NetworkInterface&gt; nets = NetworkInterface.getNetworkInterfaces(); for (NetworkInterface netint : Collections.list(nets)) for (InterfaceAddress address : netint.getInterfaceAddresses()) { InetAddress broadcastAddress = address.getBroadcast(); if (broadcastAddress != null) result.add(broadcastAddress); } return result; } catch (SocketException e) { throw new RuntimeException(e); } } public static void broadcast(int port, DatagramPacket packet, DatagramSocket socket, PrintWriter logger) throws IOException { packet.setPort(port); for (InetAddress address : getBroadcastAddresses()) { logger.println("Broadcasting to: "+address); packet.setAddress(address); socket.send(packet); } } </code></pre> <p>prints this stuff:</p> <pre><code>Broadcasting to: /0.255.255.255 Broadcasting to: /255.255.255.255 Broadcasting to: /255.255.255.255 Broadcasting to: /255.255.255.255 Broadcasting to: /255.255.255.255 </code></pre> <p>which is really annoying. Am I supposed to grab the IP address and netmask for every network interface and perform bitwise operations to "build" the correct broadcast address? <a href="https://stackoverflow.com/questions/683624/udp-broadcast-on-all-interfaces">This seems to me like Unix socket programming in C</a>... Is there a clean, Java way to neatly deliver a miserable UDP packet to all the buddies that crowd my network?</p> <p><strong>EDIT</strong>: searching the web, it turned out that this time my code is not broken. Instead, the JVM is. The data you get from <code>InterfaceAddress.getBroadcast()</code> is inconsistent, at least under Windows 7. See for example <a href="http://web.archiveorange.com/archive/v/9zwtbZAkSzOXBSu2TcLT" rel="nofollow noreferrer">this</a> and <a href="http://enigma2eureka.blogspot.com/2009/08/finding-your-ip-v4-broadcast-address.html" rel="nofollow noreferrer">this</a>: the solution seems to set a Java system property in order to make it prefer IPv4 over IPv6, but this doesn't work for me. Even with the suggested workaround, I get <strong>different results on every different run</strong>, and since the broadcast address I get is apparently random, I suspect I'm given data taken from undefined state memory locations. Serious, serious...</p> <p><strong>InterfaceAddress implementation is broken</strong>. Now I'm having a big problem on my side, because I don't know how to develop this network application. IP multicast is widely unsupported. I just want to broadcast some crap to the right UDP broadcast address without having the user write it on a text field.</p>
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