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 API levels 16 and up, and I can't seem to retrieve the txtRecord field within the service info (it returns null for some reason). Turns out it's a known problem...

So now I'm trying to work with jmDNS, which doesn't seem to find services at all. here's my class (I'm working with the AndroidAnnotations framework) MDnsHelper:

@EBean
public class MDnsHelper implements ServiceListener {

public static final String SERVICE_TYPE = "_http._tcp.local";

Activity activity;
private JmDNS jmdns;
private MulticastLock multicastLock;
WifiManager wm;
InetAddress bindingAddress;
boolean isDiscovering;

public void init(Activity activity) {
    this.activity = activity;
    isDiscovering = false;
    wm = (WifiManager) activity.getSystemService(Context.WIFI_SERVICE);
    multicastLock = wm.createMulticastLock(activity.getPackageName());
    multicastLock.setReferenceCounted(false);
}

@Background
public void startDiscovery() {
    if (isDiscovering)
        return;
    System.out.println("starting...");
    multicastLock.acquire();
    try {
        System.out.println("creating jmdns");
        jmdns = JmDNS.create();
        System.out.println("jmdns created");
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        if (jmdns != null) {
            jmdns.addServiceListener(SERVICE_TYPE, MDnsHelper.this);
            isDiscovering = true;
            System.out.println("discovering services of type: " + SERVICE_TYPE);
        }
    }
}

@Background
public void stopDiscovery() {
    if (!isDiscovering || jmdns == null)
        return;
    System.out.println("stopping...");
    multicastLock.release();
    jmdns.removeServiceListener(SERVICE_TYPE, MDnsHelper.this);
    System.out.println("listener for " + SERVICE_TYPE + " removed");
    try {
        jmdns.close();
        isDiscovering = false;
        System.out.println("jmdns closed");
    } catch (IOException e) {
        e.printStackTrace();
    }
}

@Override
public void serviceAdded(ServiceEvent service) {
    System.out.println("found: " + service.getInfo().toString());
}

@Override
public void serviceRemoved(ServiceEvent service) {
    System.out.println("lost: " + service.getInfo().toString());
}

@Override
public void serviceResolved(ServiceEvent service) {
    System.out.println("resolved: " + service.getInfo().toString());
}
}

And in my app I call:

init(getActivity());

And then startDiscovery(); to start scanning and stopDiscovery(); to stop scanning.

And of course, I gave the app the required permissions in the manifest... What am I missing here? If you need me to provide additional code/info - just ask. thanks!!

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.

   <uses-permission android:name="android.permission.INTERNET" />
   <uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
   <uses-permission android:name="android.permission.CHANGE_WIFI_MULTICAST_STATE" />
   

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);