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POJava DatagramSocket(UDP Socket) only receives when previously have sent a packet
 

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  1. POJava DatagramSocket(UDP Socket) only receives when previously have sent a packet
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    <p>I have the following problem:</p> <p>I have an extremely simple Java Datagram client and server located on a remote machine. And the client will not receive anything from the server unless it sent a packet to the server earlier(it doesn't matter what information did the packet hold)</p> <p>Here is what my client looks like:</p> <pre><code> public static void TheClient() throws Exception { ds = new DatagramSocket(clientPort); while(true) { DatagramPacket p = new DatagramPacket(buffer, buffer.length); ds.receive(p); System.out.println(new String(p.getData(), 0, p.getLength())); } } </code></pre> <p>Basically all it does is listen on port clientPort and then prints whatever it receives. However it does not work. </p> <p>Now slightly modifying it solves the problem:</p> <pre><code>public static void TheClient() throws Exception { ds = new DatagramSocket(clientPort); //Sending an empty packet byte tempBuffer1[] = new byte[10]; InetAddress address = InetAddress.getByName(SERVER_IP); DatagramPacket packet1 = new DatagramPacket( tempBuffer1, tempBuffer1.length, address, serverPort); ds.send(packet1); while(true) { DatagramPacket p = new DatagramPacket(buffer, buffer.length); ds.receive(p); System.out.println(new String(p.getData(), 0, p.getLength())); } } </code></pre> <p>Does anyone know what might be causing this problem? While the workaround did solve the problem it doesn't make any sense to me why it wouldn't work originally so the solution might not actually work on all the computers.</p> <p>Also, it should be noted that the code works fine when both the client and the server are on the same machine. </p>
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    1. USSomych
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    1. COHave you determined that there isn't a firewall on the problematic client node? Some firewalls will open up UDP return traffic automatically, but only if initiated by the client first.
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      1. USChuck Adams
    2. COI just added a rule to my firewall to allow all the connections to that UDP port and that hasn't solved the problem. Besides, other application using UDP such as Skype worked fine on my machine without the need to tweak any firewall settings. I would prefer to do as much as possible on the programming side.
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      1. USSomych
    3. COAre you certain the server is sending you the packets?
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      1. USBevynQ
 

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