Server not Receiving UDP Packet from Android

Question

I looked at some other articles that seemed similar, however I was unable to find any that were relevant and answered the question.

I am trying to send a UDP packet from an android application to an external server.

As of right now, the packet sends successfully (code is completed with no errors) yet the server is not receiving the packet.

1. I know that UDP is not guaranteed, so as a temporary fix, it sends the packet multiple times just in case.
2. It worked when the code was on a local machine, however once it moved to the app it stopped receiving.
3. I have tried several different port numbers resulting in the same effect
4. I have tried several different server programs, including ones I know work with a computer client that also don't work

5. I believe I have the necessary permissions (no permissions errors), however if I don't please let me know. All current permissions are as follows:

   • INTERNET
   • USE_CREDENTIALS
   • GET_ACCOUNTS
   • READ_PROFILE
   • READ_CONTACTS
   • INTERNET
   • ACCESS_NETWORK_STATE

Android Client:

@Override
    protected void onResume() {
        super.onResume();
        // TODO: following is done onResume only because it does not save its data yet

        // check for internet first
        ConnectivityManager connMgr = (ConnectivityManager)
                getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();

        if (networkInfo != null && networkInfo.isConnected()) {
            // download and display plant data
            new downloadPlantDataTask().execute("169.234.79.98");
        } else {
            // display error
            TextView viewErrors = (TextView) findViewById(R.id.view_errors);
            viewErrors.setText("No internet connection...");
        }
    }

public String[] downloadPlantData(String ip) throws IOException {
        Vector<String> plantsNShit = new Vector<String>();
        TextView viewErrors = (TextView) findViewById(R.id.view_errors); // TODO: temp(?)

        // get datagram socket
        DatagramSocket socket = new DatagramSocket();

        // setup info for request
        byte[] buf = new byte[256];
        buf[0] = 1;
        InetAddress address = InetAddress.getByName(ip);
        DatagramPacket packet;

        while (true) {
            // send request
            for (int i = 0; i < 5; i++) {
                viewErrors.setText("sending packet"); // TODO: temp
                packet = new DatagramPacket(buf, buf.length, address, 60000);
                try {
                    socket.send(packet);
                    Log.i("Packet message", "packet sent"); // TODO: temp
                } catch (Exception e) {
                    Log.e("Packet error", e.getMessage());
                }
            }

            // get response
            packet = new DatagramPacket(buf, buf.length);
            socket.receive(packet);
            viewErrors.setText("packet received"); // TODO: temp

            // store response
            String received = new String(packet.getData(), 0, packet.getLength());
            if (received == "DONE")
                break;
            plantsNShit.add(received); // TODO: temporarily storing in vector
        }

        viewErrors.setText("finished");
        socket.close();
        return plantsNShit.toArray(new String[plantsNShit.size()]);
    }

Server Code

import java.io.*;
import java.net.*;

public class DatagramServerThread extends Thread{

    protected DatagramSocket socket = null;
    protected BufferedReader in = null;
    protected boolean moreLines = true;

    public DatagramServerThread() throws IOException {
        this("DatagramServer");
    }

    public DatagramServerThread(String name) throws IOException {
        super(name);
        socket = new DatagramSocket(60000);

        try {
            in = new BufferedReader(new FileReader("pingas.txt"));
        } catch (FileNotFoundException e) {
            System.err.println("Couldn't open file. You dun messed up son.");
        }
    }

    public void run() {
        while (moreLines) {
            try {
                byte[] buf = new byte[256];
                // receive request
                System.out.println("waiting for packet");
                DatagramPacket packet = new DatagramPacket(buf, buf.length);
                socket.receive(packet);
                System.out.println("packet received");

                // figure out response
                String dString = null;
                if (in == null)
                    dString = "DONE";
                else
                    dString = getNextLine();
                buf = dString.getBytes();

                // send the response
                InetAddress address = packet.getAddress();
                int port = packet.getPort();
                packet = new DatagramPacket(buf, buf.length, address, port);
                socket.send(packet);
                System.out.println("packet sent");
            } catch (IOException e) {
                e.printStackTrace();
                moreLines = false;
            }
        }
        socket.close();
    }

    protected String getNextLine() {
        String returnValue = null;
        try {
            if ((returnValue = in.readLine()) == null) {
                in.close();
                moreLines = false;
                returnValue = "DONE";
            }
        } catch (IOException e) {
            returnValue = "IOException occurred in server.";
        }
        return returnValue;
    }
}

*Also, if I am showing too much or too little code, or violating any other rules and stuff please let me know so I can get better. Thank you.

Answer 1

Two items:

1. Assuming when your code did work, both the client and server were on the same machine, what's most likely happening is that your packets are being blocked by your machine's firewall, since your client (your phone) is now making requests from outside the machine.

   Not sure what OS you are using (Windows, OS X, Linux?), but make sure you go into the firewall settings, and open the corresponding port 5673.

2. In your client code, I don't see it specifically sending to the same port that your server is listening on. This is also important.