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