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Android VpnService to capture packets won't capture packets

Posted by: admin April 23, 2020 Leave a comment

Questions:

I been searching for my answer for a couple of hours now and I can’t figure it out. Please help.

What I want to do is to use the VpnService in Android to grab network packets like the application tPacketCapture

I started by using the ToyVpn sample code from google and modifying it so I don’t send the data to a server. However, I’m not sure if this is correct.

My configure method uses the wlan ip address for binder.addAddress() before calling establish(). I am using a nexus 7 and I used “adb shell netcfg | grep wlan0” to get the address:

wlan0 UP 192.168.0.6/24 0x00001043 10:bf:48:bf:5f:9d

And add it in my method:

    private void configure() throws Exception {
    // If the old interface has exactly the same parameters, use it!
    if (mInterface != null) {
        Log.i(TAG, "Using the previous interface");
        return;
    }

    // Configure a builder while parsing the parameters.
    Builder builder = new Builder();
    builder.setMtu(1500);
    builder.addAddress("192.168.0.6", 24);

    try {
        mInterface.close();
    } catch (Exception e) {
        // ignore
    }

    mInterface = builder.establish();
}

After calling this, I call the run method which I modified to pass a String instead of a InetSocketAddress and this is not important because I am not using it anywhere:

    private void run(String run) throws Exception {
    configure();

    FileInputStream in = new FileInputStream(mInterface.getFileDescriptor());

    // Allocate the buffer for a single packet.
    ByteBuffer packet = ByteBuffer.allocate(32767);

    // We use a timer to determine the status of the tunnel. It
    // works on both sides. A positive value means sending, and
    // any other means receiving. We start with receiving.
    int timer = 0;

    // We keep forwarding packets till something goes wrong.
    while (true) {
        // Assume that we did not make any progress in this iteration.
        boolean idle = true;

        // Read the outgoing packet from the input stream.
        int length = in.read(packet.array());
        if (length > 0) {

            Log.i(TAG,"************new packet");
            while (packet.hasRemaining()) {
                Log.i(TAG,""+packet.get());
                //System.out.print((char) packet.get());
            }

            // Write the outgoing packet to the tunnel.
            packet.limit(length);
            //  tunnel.write(packet);
            packet.clear();

            // There might be more outgoing packets.
            idle = false;

            // If we were receiving, switch to sending.
            if (timer < 1) {
                timer = 1;
            }
        }
    }
}

When I do adb logcat, nothing is happening. Am I going about this correctly? I feel like I am missing something.

Thank you!

EDIT:

From the logs I see the following lines:

I/ActivityManager(  460): START u0 {act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10000000 cmp=com.example.android.toyvpn/.ToyVpnClient} from pid 10247
I/ActivityManager(  460): Start proc com.example.android.toyvpn for activity com.example.android.toyvpn/.ToyVpnClient: pid=10287 uid=10122 gids={50122, 3003, 1028}
I/ActivityManager(  460): Displayed com.example.android.toyvpn/.ToyVpnClient: +1s144ms
I/Vpn     (  460): Switched from [Legacy VPN] to com.example.android.toyvpn
D/Vpn     (  460): setting state=IDLE, reason=prepare
I/ToyVpnService(10287): running vpnService
D/Vpn     (  460): setting state=CONNECTING, reason=establish
D/VpnJni  (  460): Address added on tun0: 192.168.0.6/24
I/Vpn     (  460): Established by com.example.android.toyvpn.ToyVpnService on tun0
W/ContextImpl(  460): Calling a method in the system process without a qualified user: android.app.ContextImpl.bindService:1406 com.android.server.connectivity.Vpn.establish:289 com.android.server.ConnectivityService.establishVpn:3263 android.net.IConnectivityManager$Stub.onTransact:504 android.os.Binder.execTransact:351 
D/Vpn     (  460): setting state=AUTHENTICATING, reason=establish

So it seems to be connecting.

Full source:

public class ToyVpnService extends VpnService implements Handler.Callback, Runnable {
    private static final String TAG = "ToyVpnService";

    private Handler mHandler;
    private Thread mThread;

    private ParcelFileDescriptor mInterface;

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        // The handler is only used to show messages.
        if (mHandler == null) {
            mHandler = new Handler(this);
        }

        // Stop the previous session by interrupting the thread.
        if (mThread != null) {
            mThread.interrupt();
        }

        // Start a new session by creating a new thread.
        mThread = new Thread(this, "ToyVpnThread");
        mThread.start();
        return START_STICKY;
    }

    @Override
    public void onDestroy() {
        if (mThread != null) {
            mThread.interrupt();
        }
    }

    @Override
    public boolean handleMessage(Message message) {
        if (message != null) {
            Toast.makeText(this, message.what, Toast.LENGTH_SHORT).show();
        }
        return true;
    }

    @Override
    public synchronized void run() {
        Log.i(TAG,"running vpnService");
        try {
            runVpnConnection();
        } catch (Exception e) {
            e.printStackTrace();
            //Log.e(TAG, "Got " + e.toString());
        } finally {
            try {
                mInterface.close();
            } catch (Exception e) {
                // ignore
            }
            mInterface = null;

            mHandler.sendEmptyMessage(R.string.disconnected);
            Log.i(TAG, "Exiting");
        }
    }

    private boolean runVpnConnection() throws Exception {

        configure();

        FileInputStream in = new FileInputStream(mInterface.getFileDescriptor());

        // Allocate the buffer for a single packet.
        ByteBuffer packet = ByteBuffer.allocate(32767);

        // We keep forwarding packets till something goes wrong.
        while (true) {
            // Assume that we did not make any progress in this iteration.
            boolean idle = true;

            // Read the outgoing packet from the input stream.
            int length = in.read(packet.array());
            if (length > 0) {

                Log.i(TAG,"************new packet");
                System.exit(-1);
                while (packet.hasRemaining()) {
                    Log.i(TAG,""+packet.get());
                    //System.out.print((char) packet.get());
                }
                packet.limit(length);
                //  tunnel.write(packet);
                packet.clear();

                // There might be more outgoing packets.
                idle = false;
            }
            Thread.sleep(50);
        }
    }

    public String getLocalIpAddress()
    {
        try {
            for (Enumeration<NetworkInterface> en = NetworkInterface.getNetworkInterfaces(); en.hasMoreElements();) {
                NetworkInterface intf = en.nextElement();
                for (Enumeration<InetAddress> enumIpAddr = intf.getInetAddresses(); enumIpAddr.hasMoreElements();) {
                    InetAddress inetAddress = enumIpAddr.nextElement();
                    Log.i(TAG,"****** INET ADDRESS ******");
                    Log.i(TAG,"address: "+inetAddress.getHostAddress());
                    Log.i(TAG,"hostname: "+inetAddress.getHostName());
                    Log.i(TAG,"address.toString(): "+inetAddress.getHostAddress().toString());
                    if (!inetAddress.isLoopbackAddress()) {
                        //IPAddresses.setText(inetAddress.getHostAddress().toString());
                        Log.i(TAG,"IS NOT LOOPBACK ADDRESS: "+inetAddress.getHostAddress().toString());
                        return inetAddress.getHostAddress().toString();
                    } else{
                        Log.i(TAG,"It is a loopback address");
                    }
                }
            }
        } catch (SocketException ex) {
            String LOG_TAG = null;
            Log.e(LOG_TAG, ex.toString());
        }

        return null;
    }

    private void configure() throws Exception {
        // If the old interface has exactly the same parameters, use it!
        if (mInterface != null) {
            Log.i(TAG, "Using the previous interface");
            return;
        }

        // Configure a builder while parsing the parameters.
        Builder builder = new Builder();
        builder.setMtu(1500);
        builder.addAddress("192.168.0.6", 24);
        try {
            mInterface.close();
        } catch (Exception e) {
            // ignore
        }

        mInterface = builder.establish();
    }
}
How to&Answers:

Ok, it was not easy at all but I figured out how to capture packets. Since I am not extremely familiar with networking (but this new job is requesting that I am) I had difficulty with setting everything correctly. Basically after setting the right route in the VpnService.builder I got to receiving packets correctly.

So:

builder.addAddress("192.168.0.6", 24); // was wrong, you need to put an internal IP (10.0.2.0 for example)

and

builder.addRoute("0.0.0.0", 0); // needs to be this.

you don’t need to set up a DnsServer through builder.addDnsServer() to make it work. Hope this helps anyone!

Answer:

My configure method uses the wlan ip address for binder.addAddress() before >calling establish(). I am using a nexus 7 and I used “adb shell netcfg | grep >wlan0” to get the address:

wlan0 UP 192.168.0.6/24 0x00001043 10:bf:48:bf:5f:9d

I have wrote a simple script in python to show you netcfg graphically from adb.
It is updating every second.

enter image description here
https://github.com/ilanben/graphical_netcfg

Enjoy 🙂