Question

My Android app sends/retrieves data to/from the user s own PC using HTTP and it s working fine with a handful of beta testers. I now need to consider a situation where the PC is hibernating.

I ve never done this before but I ve googled to find info about the WOL magic packet and some simple source written in C (using CAsyncSocket at the client end). Doing this over a wi-fi connection on the user s home network is likely to be relatively straight-forward but ideally I want this to work over mobile internet (assuming the user can configure their home router to accept / forward the packet).

I m guessing I need to use some generic Java network code and I ve been looking at java.net.

At this point I can t decide whether I should be using java.net.Socket or java.net.DatagramSocket. So the question is, am I approaching this the right way and which of the two socket types should I be using (or would both suffice)? Many thanks.

Answer 1

Here is some C# code that I have used in the past. It should be relatively easy to convert into java and send using a DatagramPacket

namespace WakeOnLan
{
    class Program
    {
        static void Main(string[] args)
        {

            byte[] mac = new byte[] { mac address goes here i.e 0x00, and so on };
            WakeUp(mac);
        }

        private static void WakeUp(byte[] mac)
        {
            //
            // WOL packet is sent over UDP 255.255.255.0:40000.
            //
            Console.WriteLine("Waking Up.......");
            UdpClient client = new UdpClient();
            client.Connect(IPAddress.Broadcast, 40000);

            //
            // WOL packet contains a 6-bytes trailer and 16 times a 6-bytes sequence containing the MAC address.
            //
            byte[] packet = new byte[17 * 6];

            //
            // Trailer of 6 times 0xFF.
            //
            for (int i = 0; i < 6; i++)
                packet[i] = 0xFF;

            //
            // Body of magic packet contains 16 times the MAC address.
            //
            for (int i = 1; i <= 16; i++)
                for (int j = 0; j < 6; j++)
                    packet[i * 6 + j] = mac[j];

            //
            // Submit WOL packet.
            //
            client.Send(packet, packet.Length);
            Console.WriteLine("Machine Woke Up....");
        }
    }
}

Hope this helps

Answer 2

I can t take too much credit for it as its from this site

But this is a java version of wake on lan class:

public static final int PORT = 9;    

public static void main(String[] args) {
    
    if (args.length != 2) {
        System.out.println("Usage: java WakeOnLan <broadcast-ip> <mac-address>");
        System.out.println("Example: java WakeOnLan 192.168.0.255 00:0D:61:08:22:4A");
        System.out.println("Example: java WakeOnLan 192.168.0.255 00-0D-61-08-22-4A");
        System.exit(1);
    }
    
    String ipStr = args[0];
    String macStr = args[1];
    
    try {
        byte[] macBytes = getMacBytes(macStr);
        byte[] bytes = new byte[6 + 16 * macBytes.length];
        for (int i = 0; i < 6; i++) {
            bytes[i] = (byte) 0xff;
        }
        for (int i = 6; i < bytes.length; i += macBytes.length) {
            System.arraycopy(macBytes, 0, bytes, i, macBytes.length);
        }
        
        InetAddress address = InetAddress.getByName(ipStr);
        DatagramPacket packet = new DatagramPacket(bytes, bytes.length, address, PORT);
        try (DatagramSocket socket = new DatagramSocket()) {
            socket.send(packet);
        }
        
        System.out.println("Wake-on-LAN packet sent.");
    }
    catch (Exception e) {
        System.out.println("Failed to send Wake-on-LAN packet: " + e);
        System.exit(1);
    }
    
}

private static byte[] getMacBytes(String macStr) throws IllegalArgumentException {
    byte[] bytes = new byte[6];
    String[] hex = macStr.split("(\:|\-)");
    if (hex.length != 6) {
        throw new IllegalArgumentException("Invalid MAC address.");
    }
    try {
        for (int i = 0; i < 6; i++) {
            bytes[i] = (byte) Integer.parseInt(hex[i], 16);
        }
    }
    catch (NumberFormatException e) {
        throw new IllegalArgumentException("Invalid hex digit in MAC address.");
    }
    return bytes;
}

Of course you will need to modify this to work with android (very little work needed) but I found it works better than @Bear s answer.

Answer 3

java.net.DatagramSocket would probably work well enough, since WoL does not provide delivery confirmation. It s doubtful you ll be able to get it to work outside the local network, since WoL packets are broadcast across the network with the destination address as the MAC address of the target computer and most routers are setup to block broadcast packets from the WAN.

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