Question

I’m developing an app that uses wifi to broadcast UDP messages between all the mobiles that are in the same network that have my app.

I managed to send/receive packets from many cellphones having an external AP, that’s my router.

But given the case that there’s no AP, I want users to be able to use their phone’s Wifi Hotspot feature so they can still use my aplication. So one of the cellphones would be the wifi hotspot, and all the others would connect to that.

I require users to connect to a wifi by they own. Either to an external AP or to the said hotspot. Then when my application starts, it checks whether the phone is connected to a wifi network, calling the WifiManager.isWifiEnabled() and NetworkInfo.isConnected().

The problem is that if I call these functions in the cellphone that’s using hotspot, the function isConnected() will return false. And I can’t get the broadcast address using WifiManager.getDhcpInfo().
The other phones that are connected to the hotspot they do work perfectly. But the hotspot cellphone can’t send any broadcast, since WifiManager is disabled.

So, my question would be “Is there any way to check if a cellphone is currently being a wifi hotspot? And if so, is there any way I can get its broadcast address?”

Answer 1

First you can check what is your IP address :

public InetAddress getIpAddress() {
  InetAddress inetAddress = null;
  InetAddress myAddr = null;

  try {
    for (Enumeration<NetworkInterface> networkInterface = NetworkInterface
      .getNetworkInterfaces(); networkInterface.hasMoreElements();) {

      NetworkInterface singleInterface = networkInterface.nextElement();

      for (Enumeration<InetAddress> IpAddresses = singleInterface.getInetAddresses(); IpAddresses
        .hasMoreElements();) {
        inetAddress = IpAddresses.nextElement();

        if (!inetAddress.isLoopbackAddress() && (singleInterface.getDisplayName()
            .contains("wlan0") ||
            singleInterface.getDisplayName().contains("eth0") ||
            singleInterface.getDisplayName().contains("ap0"))) {

          myAddr = inetAddress;
        }
      }
    }

  } catch (SocketException ex) {
    Log.e(TAG, ex.toString());
  }
  return myAddr;
}

and I used this IP to get Broadcast this way:

public InetAddress getBroadcast(InetAddress inetAddr) {

    NetworkInterface temp;
    InetAddress iAddr = null;
    try {
        temp = NetworkInterface.getByInetAddress(inetAddr);
        List<InterfaceAddress> addresses = temp.getInterfaceAddresses();

        for (InterfaceAddress inetAddress: addresses)

            iAddr = inetAddress.getBroadcast();
        Log.d(TAG, "iAddr=" + iAddr);
        return iAddr;

    } catch (SocketException e) {

        e.printStackTrace();
        Log.d(TAG, "getBroadcast" + e.getMessage());
    }
    return null;
}

It can be done of course in one method, but in my implementation separating it into two method was useful.

To identify if Wifi Tether is on you can use this code:

WifiManager wifi = (WifiManager) getSystemService(Context.WIFI_SERVICE);
Method[] wmMethods = wifi.getClass().getDeclaredMethods();
for (Method method: wmMethods) {
    if (method.getName().equals("isWifiApEnabled")) {

        try {
            if ((Boolean) method.invoke(wifi)) {
                isInetConnOn = true;
                iNetMode = 2;

            } else {
                Log.d(TAG, "WifiTether off");
            }
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }

    }
}

If client devices needs to know if server device is mobile Hotspot, specific IP address can be used. As far as I know, all Tethering devices have the same address 192.168.43.1 It is the same on Android 2.3 and on 4.+, Checked on many phones and tablets. Of course it is not best solution but it is fast. In my application client devices checks (sends packet to this address) and my server devices response in predefined way like "yesIamInTheterModeIamYourServer".