Android application Wi-Fi device - AP connectivity

Question

I am building an application which can transfer data between a mobile and a Wi-Fi device... The mobile has got the AP enabled (through code) and another device connects to t

Answer 1

You could read the /proc/net/arp file to read all the ARP entries. See the example in the blog post Android: Howto find the hardware MAC address of a remote host. In the ARP table, search for all the hosts that belong to your Wi-Fi network based on the IP address.

Here is example code, which counts the number of hosts connected to the AP. This code assumes that one ARP entry is for the phone connected to the network and the remaining ones are from hosts connected to the AP.

private int countNumMac()
{
    int macCount = 0;
    BufferedReader br = null;
    try {
        br = new BufferedReader(new FileReader("/proc/net/arp"));
        String line;
        while ((line = br.readLine()) != null) {
            String[] splitted = line.split(" +");
            if (splitted != null && splitted.length >= 4) {
                // Basic sanity check
                String mac = splitted[3];
                if (mac.matches("..:..:..:..:..:..")) {
                    macCount++;
                }
            }
        }
    }
    catch (Exception e) {
        e.printStackTrace();
    }
    finally {
        try {
            br.close();
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }

    if (macCount == 0)
        return 0;
    else
        return macCount-1; //One MAC address entry will be for the host.
}

Answer 2

You can use accesspoint:

WifiApControl apControl = WifiApControl.getInstance(context);

// These are cached and may no longer be connected, see
// WifiApControl.getReachableClients(int, ReachableClientListener)
List<WifiApControl.Client> clients = apControl.getClients()

Answer 3

You could ping the device if you know its host-name or its IP address.

    Runtime runtime = Runtime.getRuntime();
    Process proc = runtime.exec("ping -c 1   " + hostname);
    proc.waitFor();

You could do an IP address scan, trying every IP address on the network for a response using a ping like above or trying to connect using TCP or UDP.

If you know the MAC address, you could use the ARP table.

If you got some own software running on the devices, you could send out UDP packets on every device and listen for them on your Android device. See Sending and receiving UDP broadcast packets in Android on how to do this.