Android - jmdns doesn't discover devices

Question

I\'m trying to implement a class to discover services on the network. I\'ve tried working with Android\'s NSD and it does discover the services fine, but it supports only AP

Answer 1

I am the author of ZeroConf Browser for Android and I use the open source Library JmDNS for all my resolving. It works great but there are a few tricks to getting it to work properly.

1. In your Android manifest.xml make sure you have these permissions at least.

2. Before starting the activity you must allow multi-cast packets by acquiring a multicast lock.

   @Override
   protected void onStart() {
       Log.i(TAG, "Starting ServiceActivity...");
       super.onStart();
       try {
           Log.i(TAG, "Starting Mutlicast Lock...");
           WifiManager wifi = (WifiManager) this.getSystemService(Context.WIFI_SERVICE);
           // get the device ip address
           final InetAddress deviceIpAddress = getDeviceIpAddress(wifi);
           multicastLock = wifi.createMulticastLock(getClass().getName());
           multicastLock.setReferenceCounted(true);
           multicastLock.acquire();
           Log.i(TAG, "Starting ZeroConf probe....");
           jmdns = JmDNS.create(deviceIpAddress, HOSTNAME);
           jmdns.addServiceTypeListener(this);
       } catch (IOException ex) {
           Log.e(TAG, ex.getMessage(), ex);
       }
       Log.i(TAG, "Started ZeroConf probe....");
   }
   
   private InetAddress getDeviceIpAddress(WifiManager wifi) {
      InetAddress result = null;
      try {
         // default to Android localhost
         result = InetAddress.getByName("10.0.0.2");
   
         // figure out our wifi address, otherwise bail
         WifiInfo wifiinfo = wifi.getConnectionInfo();
         int intaddr = wifiinfo.getIpAddress();
         byte[] byteaddr = new byte[] { (byte) (intaddr & 0xff), (byte) (intaddr >> 8 & 0xff),
             (byte) (intaddr >> 16 & 0xff), (byte) (intaddr >> 24 & 0xff) };
         result = InetAddress.getByAddress(byteaddr);
      } catch (UnknownHostException ex) {
         Log.w(TAG, String.format("getDeviceIpAddress Error: %s", ex.getMessage()));
      }
   
      return result;
   }
   

3. And don't forget on stopping the scan to unlock the multicast lock and shut down JmDNS.

   @Override
   protected void onStop() {
       Log.i(TAG, "Stopping ServiceActivity...");
       super.onStop();
   
       stopScan();
   }
   
   private static void stopScan() {
       try {
           if (jmdns != null) {
               Log.i(TAG, "Stopping ZeroConf probe....");
               jmdns.unregisterAllServices();
               jmdns.close();
               jmdns = null;
           }
           if (multicastLock != null) {
               Log.i(TAG, "Releasing Mutlicast Lock...");
               multicastLock.release();
               multicastLock = null;
           }
       } catch (Exception ex) {
           Log.e(TAG, ex.getMessage(), ex);
       }
   }
   

4. Most importanty don't use the default constructor. You must use the IP Address Constructor. I noticed in your code you are just doing JmDNS.create(). I think for some reason the only way it works on Android is to use the contructor below.

   jmdns = JmDNS.create(deviceIpAddress, HOSTNAME);