Unable to Discover BACnet Devices After Binding Android App to Specific Network Interface (Ethernet/Wi-Fi)

Question

` I’m developing an Android app that intended to use Ethernet to discover BACnet devices on the local network, while Wi-Fi is used for general internet connectivity. The device has both Ethernet and Wi-Fi interfaces connected simultaneously.

Issue: When I bind my app’s process to a specific network interface (Ethernet or Wi-Fi), the network binding appears to succeed, as indicated by the logs. However, I am unable to discover BACnet devices over Ethernet after binding, when both Ethernet and Wi-Fi are active. This issue occurs on Android 12 (API level 31). The same code works on older Android versions (like Android 7) where I can successfully discover devices using either interface.

Details: App Behavior:

When only one interface (Ethernet or Wi-Fi) is active, the app successfully discovers devices on that network. When both interfaces (Ethernet and Wi-Fi) are active, I bind to the required interface using bindProcessToNetwork(). The logs show that the process is successfully bound, but the device discovery fails over Ethernet (BACnet devices are discovered but I am unable to fetch objects).

Observed Behavior:

The app logs show that the process is bound to Ethernet (TRANSPORT_ETHERNET), but no BACnet devices are discovered. When I disable Wi-Fi, the devices on Ethernet are successfully discovered. The same code works perfectly on Android 7, where the app can discover devices using either interface, even when both are active.

Environment: Android Version: 12 (API level 31) App Requirement: Wi-Fi is needed for general internet access. Ethernet is used for BACnet device discovery on the local network.

• Implemented network binding logic using bindProcessToNetwork().
• Used DatagramSocket for BACnet device discovery after binding to Ethernet.
• Ensured permissions are set for network access (ACCESS_NETWORK_STATE, CHANGE_NETWORK_STATE).
• Verified that device discovery works when only one interface is active (Ethernet or Wi-Fi).
• Tested the same code on Android 7, where it works as expected.

Network Binding Code

 private static void bindToBestNetwork() {
        Network[] networks = connectivityManager.getAllNetworks();
        Network ethernetNetwork = null;
        Network wifiNetwork = null;

        // Check available networks and prioritize Ethernet
        for (Network network : networks) {
            NetworkCapabilities capabilities = connectivityManager.getNetworkCapabilities(network);
            if (capabilities != null) {
                if (capabilities.hasTransport(NetworkCapabilities.TRANSPORT_ETHERNET)) {
                    ethernetNetwork = network;
                } else if (capabilities.hasTransport(NetworkCapabilities.TRANSPORT_WIFI)) {
                    wifiNetwork = network;
                }
            }
        }

     
            // Prioritize Ethernet, fall back to Wi-Fi if Ethernet is not available
            if (ethernetNetwork != null) {
                WifiManager wifiManager = (WifiManager) context.getSystemService(Context.WIFI_SERVICE);
                wifiManager.setWifiEnabled(false);
                ConnectivityManager.setProcessDefaultNetwork(ethernetNetwork);
                boolean success = connectivityManager.bindProcessToNetwork(ethernetNetwork);
                wifiManager.disconnect();
                WifiManager.MulticastLock multicastLock = wifiManager.createMulticastLock("bacnetLock");
                multicastLock.setReferenceCounted(true);
                multicastLock.acquire();

                try {
                    Log.d(TAG,"Binding socket to Ethernet network");
                    DatagramSocket socket = new DatagramSocket();
                    ethernetNetwork.bindSocket(socket);
                } catch (Exception e) {
                    Log.d(TAG,"Exception while binding socket to Ethernet network");
                    e.printStackTrace();
                }
                if (success) {
                     Log.d(TAG, "Ethernet network successfully bound");
                } else {
                     Log.d(TAG, "Ethernet network binding failed");
                }
            } else if (wifiNetwork != null) {
                boolean success = connectivityManager.bindProcessToNetwork(wifiNetwork);
                if (success) {
                    Log.d(TAG, "Wi-Fi network successfully bound");
                } else {
                    Log.d(TAG, "Wi-Fi network binding failed");
                }
            } else {
                connectivityManager.bindProcessToNetwork(null);
            }
        
    }

    // Register a callback for Ethernet network
    private static void registerEthernetListener() {
        Log.d(TAG, "Registering Ethernet listener");
        NetworkRequest ethernetRequest = new NetworkRequest.Builder()
                .addTransportType(NetworkCapabilities.TRANSPORT_ETHERNET)
                .removeCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET)
                .build();

        ethernetCallback = new ConnectivityManager.NetworkCallback() {
            @Override
            public void onAvailable(Network network) {
                Log.d(TAG, "Ethernet network Available");
                bindToBestNetwork();
            }

            @Override
            public void onLost(Network network) {
                Log.d(TAG, "Ethernet network lost");

                bindToBestNetwork();
            }
        };

        connectivityManager.requestNetwork(ethernetRequest,ethernetCallback);
    }

    // Register a callback for Wi-Fi network
    private static void registerWifiListener() {
        Log.d(TAG, "Registering Wi-Fi listener");
        NetworkRequest wifiRequest = new NetworkRequest.Builder()
                .removeCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET)
                .build();

        wifiCallback = new ConnectivityManager.NetworkCallback() {
            @Override
            public void onAvailable(Network network) {
                Log.d(TAG, "Wi-Fi network Available");
                bindToBestNetwork();
            }

            @Override
            public void onLost(Network network) {
                Log.d(TAG, "Wi-Fi network lost");
                bindToBestNetwork();
            }
        };

        connectivityManager.requestNetwork(wifiRequest, wifiCallback);
    }

Answer 1

Sounds like the BACnet stack that you're using might also (now / for Android 12) need to be told which NIC (Network Interface Card) to use.

Try using (/can you use?) Wireshark to check/confirm what's happening.