Android 9 - Socket Connect TIMEOUT when Mobile data enabled

Stephen Orr :

I have a deployed app that is failing on Android 9. Part of its function is to configure a module over an Access Point network to allow that that module to connect to the users home network.

I have code that detects and connects to the correct WIFI network, but when I attempt to open a socket to the device, it fails - only on Android 9 and only if mobile data is enabled. If I manually disable mobile data on the device everything runs fine.

Socket open() {
    Socket sock = new Socket(Proxy.NO_PROXY);
    try {
        sock.bind(new InetSocketAddress(localIpAddress(), 50000));
    } catch (IOException e) {
        activity.logContent("Warning: Failed to bind socket : " + e.toString());
    }
    try {
        sock.connect(new InetSocketAddress("192.168.17.1", 5555), (int)5000);
    } catch (IOException e) {
        // This catch fires when Mobile Data is on.
        activity.logContent("Connected to " + activity.mWifiManager.getConnectionInfo().getSSID());
        activity.logContent("Couldn't open socket : " + e.toString());
    }
    return sock;
}

I have tried this with and without the Proxy.NO_PROXY and with and without the bind() call. If the bind call is missing the error implies that the socket is attempting to connect over the cell network. (Note: activity.logContent() is an on-screen log so it is easier to see what is happening when not connected to a debugger).

Any ideas what is going wrong?

Bronz :

After a few days of imprecations I believe I have come to the identification of the problem and therefore to the solution:

The problem occurs due to some changes in the version of android (I presume to be 9.0 even if other changes had occurred on API 21), in particular on the creation of the socket, if the system detects that there is a "better" network (access to internet, high signal, etc, etc) socket creation refers to that network and no longer to the wifi network you would like.

I looked for ways to force the creation of the socket on the wifi network (which is the network I want) and the only way I found is this:

Simply put instead of:

Socket sock = new Socket ();

Do:

ConnectivityManager connectivity = (ConnectivityManager) MyApp.getContext().getSystemService(Context.CONNECTIVITY_SERVICE);
                if (connectivity != null)
                {
                    for (Network network : connectivity.getAllNetworks())
                    {
                        NetworkInfo networkInfo = connectivity.getNetworkInfo(network);

                        if (networkInfo != null && networkInfo.getType() == ConnectivityManager.TYPE_WIFI)
                        {
                            if (networkInfo.isConnected())
                            {
                                Socket sock =  network.getSocketFactory().createSocket();
                            }
                        }
                    }
            }

Practically browse the networks present in the device and when you find your active wifi you do nothing but take advantage of this function to get the right socket for sure:

getSocketFactory().createSocket()

Now you have the working socket!

In my case it now works perfectly, if someone finds better solutions, it is welcome, but for now it is the only way I have found to make everything work as in the previous version of android.