Android: get route info programmatically

Question

I am sending UDP packets to a remote, and I would like to know the IP of the local interface that will be used for that. In a shell I would run:

ip route get 192.168.1.54

and it would answer something like:

192.168.1.12 dev wlan0  table local_network  src 192.168.1.1

Where I'm interested in getting the src: 192.168.1.1.

I can do it by calling exec like this:

Runtime.getRuntime().exec("ip route get 192.168.1.54")

but that does not seem very elegant and I don't know which devices support that (it works on some of them, and others return an error code "1" though the command works in adb shell).

Is there a way to do that with the Android SDK? I looked at the ConnectivityManager but could not find a way...

Answer 1

It looks like you want the address of the network's gateway for your WiFi network.

This function is based on doc and some examples.

public InetAddress getGateway(Context context) {
    ConnectivityManager connectivityManager =
            (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
    if (Build.VERSION.SDK_INT < Build.VERSION_CODES.M) {
        // getActiveNetwork() requires M or later.
        return null;
    }
    Network activeNetwork = connectivityManager.getActiveNetwork();
    if (activeNetwork == null) {
        return null;
    }
    List<RouteInfo> routes = connectivityManager.getLinkProperties(activeNetwork).getRoutes();
    for (RouteInfo route : routes) {
        if (route.isDefaultRoute() && !(route.getGateway() instanceof Inet6Address)) {
           return route.getGateway();
        }
    }
    return null;
}

Answer 2

Using Android's DatagramSocket should directly provide you with source address:

val socket = DatagramSocket()
val targetAddress = InetAddress.getByName(destHostOrIp)
socket.connect(targetAddress, port)
val local = socket.getLocalAddress()