Develop Reference

UDP packets sent by Android application over wifi sometimes not received by android devices

I have built a client-server application in Android(Compiling with java-6) to send and receive udp packets.
My client code is as follow:
DatagramSocket socket =new DatagramSocket();
socket.setBroadcast(true);
DatagramPacket packet = new DatagramPacket(dgram, dgramLength, host, port);
socket.send( packet );
socket.close();
My server code is as follows:
DatagramSocket socket =new DatagramSocket(this.port);
byte[] data=new byte[512];
DatagramPacket packet = new DatagramPacket(data, data.length) ;
// Wait for a response from the server
try
{
socket.receive(packet) ;
}
catch (Exception e)
{}
The issue that I am facing is that when I am working on a wired network then all the sent packets are being received by master properly but when I am running the same code over wifi then sometimes sent packets are being received and sometimes not. I want to know that whether it is the general behavior of a wifi network or not.
Any kind of help is appreciated.

How to read a socket command sent by VB.NET to Android?

I'm trying to send some commands to Android (client) from VB.NET (server) using sockets. I can connect the client to the server, but I don't know how to receive the commands sent by the server.
Here's a part of my Android code:
public void connect(View v){ //called on button click
SERVER_IP = ip.getText().toString(); //it gets the server's ip from an editText
SERVER_PORT = Integer.parseInt(port.getText().toString()); //and the port too
Toast.makeText(this, "Trying to connect to\n" + SERVER_IP + ":" + SERVER_PORT + "...", Toast.LENGTH_SHORT).show();
new Thread(new Runnable() {
public void run() {
InetAddress serverAddr;
try {
serverAddr = InetAddress.getByName(SERVER_IP);
socket = new Socket(serverAddr, SERVER_PORT); //It establishes the connection with the server
if(socket != null && socket.isConnected()){ //not sure if it is correct
BufferedReader input = new BufferedReader(new InputStreamReader(socket.getInputStream()));
//Here comes the problem, I don't know what to add...
}
} catch (Exception e) {
}
}
}).start();
}
And here's a part of my VB.NET send code:
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
send(TextBox1.text)
End Sub
Private Sub Send(ByVal command)
Dim temp() As Byte = UTF8.GetBytes(command) 'Is UTF8 right to use for that?
stream.Write(temp, 0, temp.Length)
stream.Flush()
End Sub
Question1: is it right to us UTF8 instead of for example ASCII encoding?
Question2: what would I change in the Android code if it wanted to use a timer that sends a command every second?
Thanks.

To read input from a BufferedReader you need to do something similiar to this:
BufferedReader input = new BufferedReader(new InputStreamReader(socket.getInputStream()));
String line;
while((line = input.readLine()) != null){
// do something with the input here
}
A nice tutorial on sockets is available from oracle in the docs: http://docs.oracle.com/javase/tutorial/networking/sockets/readingWriting.html
The default charset on Android is UTF-8 http://developer.android.com/reference/java/nio/charset/Charset.html, so no worries there but you can always send a byte stream from the server onto the client and decode it however you want.
To receive a byte stream you need to do this:
BufferedInputStream input = new BufferedInputStream(socket.getInputStream());
byte[] buffer = new byte[byteCount];
while(input.read(buffer, 0, byteCount) != -1 ){
// do something with the bytes
// for example decode it to string
String decoded = new String(buffer, Charset.forName("UTF-8"));
// keep in mind this string might not be a complete command it's just a decoded byteCount number of bytes
}
As you see it's much easier if you send strings instead of bytes.
If you want to receive input from the server periodically, one of the solutions would be to create a loop which opens a socket, receives input, process it, closes the socket, and then repeats, our you could just keep the loop running endlessly until some command like "STOP" is received.

Android receive DatagramPacket

I saw many pages, and try many advices, but still can't receive data via UDP on device (nexus 4). I work in my LAN, over wifi (3G is off).
I have a client, on PC (192.168.1.5) and consumer on Android device (192.168.1.3:54445)
Here is a client code:
String string = "hello udp";
DatagramSocket socket = null;
try {
address = InetAddress.getByName("192.168.1.3");
socket = new DatagramSocket();
DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 54445);
socket.send(packet);
Here is a consumer code:
DatagramSocket socket = null;
stop = false;
InetAddress address = null;
try {
socket = new DatagramSocket(54445);
while (!stop) {
byte[] buffer = new byte[minBufSize];
DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
socket.receive(packet);
Log.d(this.getClass().getSimpleName(), String.valueOf(packet.getLength()));
Debugger stop on socket.receive(packet). Consumer work in AsyncTask.
Thank you!

It's my fault, at first check you router settings.

android UDP connection, not receiving any data

Im a newbie in this so please escuse me if i ask dumb questions.
Im trying to make a UDP connection between Eclipse's PC Emulator & a android phone
(or between two android phone devices).
I have a router and the phone connects to the internet thru router's wifi network. The PC is on same network also (direct cable router-PC connection). Im trying to send some text data from Server thread to Client thread but nothing is received/sent. :(
The Server java class (RE-EDITED, Server receives msg. from Client):
public class server implements Runnable
{
// the Server's Port
public static final int SERVERPORT = 6000;
// running Server thread.
public void run()
{
Log.d("redwing","server thread started.");
DatagramSocket serverSocket = null;
try
{
// Open Server Port
serverSocket = new DatagramSocket(server.SERVERPORT);
byte[] receiveData = new byte[32];
byte[] sendData = new byte[32];
// loop until "server_finished" becomes False.
while(createserver.server_finished)
{
if(renderer.gGL!=null) // ignore me, just a null pointer checker
{
// waiting for the incoming client's message packet
DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
renderer.recv = new String(receivePacket.getData());
Log.d("server","packet received from client, ETA " + timing.getNow() + " " + renderer.recv); // timing getNow - just returns current system minute & second.
// server is replying to the client back with a message.
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
sendData = new String("server msg").getBytes();
DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port);
serverSocket.send(sendPacket);
renderer.sent = new String(sendData, 0, sendData.length);
Log.d("server","packet sent to client, ETA " + timing.getNow() + " " + renderer.sent); // timing getNow - just returns current system minute & second.
}
}
// close the socket
if(serverSocket!=null) serverSocket.close();
serverSocket = null;
}
catch (Exception e)
{
Log.e("server", "Error", e);
if(serverSocket!=null) serverSocket.close();
serverSocket = null;
}
finally
{
if(serverSocket!=null) serverSocket.close();
serverSocket = null;
}
Log.d("redwing","server thread terminated.");
}
}
and the Client java class (RE-EDITED, Client does not receive msg from Server) :
public class client implements Runnable
{
public static final int CLIENTPORT = 5000;
// running Client thread.
public void run()
{
Log.d("redwing","client thread started.");
DatagramSocket clientSocket = null;
try
{
// getting local address
clientSocket = new DatagramSocket(server.SERVERPORT);
InetAddress IPAddress = InetAddress.getByName("192.168.0.100");
// displaying IP & hostname.
Log.d("client", "IP: " + IPAddress.getHostAddress() + " Name: " + IPAddress.getHostName());
byte[] sendData = new byte[32];
byte[] receiveData = new byte[32];
// Loop until client_finished becomes False.
while(createclient.client_finished)
{
if(renderer.gGL!=null) // ignore me, just a null pointer checker
{
// sending a message to the server
sendData = timing.getNow().getBytes();
DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, client.CLIENTPORT);
clientSocket.send(sendPacket);
renderer.sent = new String(sendData,0,sendData.length);;
Log.d("client","packet sent to server, ETA " + timing.getNow());
// waiting for the server packet message.
DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
clientSocket.receive(receivePacket);
renderer.recv = new String(receivePacket.getData());
Log.d("client","packet received from server, ETA " + timing.getNow());
}
}
// close the socket
if(clientSocket!=null) clientSocket.close();
clientSocket = null;
}
catch (Exception e)
{
Log.e("client", "Error", e);
if(clientSocket!=null) clientSocket.close();
clientSocket = null;
}
finally
{
if(clientSocket!=null) clientSocket.close();
clientSocket = null;
}
Log.d("redwing","client thread terminated.");
}
}
the Permisions are enabled in the manifest:
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.INTERNET"/>
<user-permission android:name="android.permission.NETWORK" />
<uses-permission android:name="android.permission.CHANGE_WIFI_MULTICAST_STATE" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
I'm running the Server on the android PC Emulator, and the Client on my android Phone.
Both Client & Server threads run just fine but ... the problem is that i dont get any data exchanged between them. The client doesn't receive anyting and the server doesnt receive anything. The packets are sent but nothing received .
What im doing wrong ?
Please help me.
Thank you in advance.

After running your emulator, type it in command prompt - "telnet localhost ", then type "redir add udp:5000:6000". Connect client with port number 5000 and open udp server with port number 6000. Then you can receive client message from udp server.
Take a look for details
clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("<pc ip>"); // instead of "localhost"
public static final String SERVERIP = "127.0.0.1"; // 'Within' the emulator!
public static final int SERVERPORT = 6000;
/* Retrieve the ServerName */
InetAddress serverAddr = InetAddress.getByName(SERVERIP);
DatagramSocket socket = new DatagramSocket(SERVERPORT, serverAddr);

Android, problems with SocketAddress and sockets. Reverse lookup?

i have a problem with Android. I am trying to connect to a server with a proxy with no luck.
I have this code that works fine on normal Java. It only defines a proxy server and creates a socket that would connect to google with that proxy. It sends a simple GET request and then shows the response.
import java.net.InetSocketAddress;
import java.net.Proxy;
import java.net.Socket;
import java.net.SocketAddress;
public class Main {
public static void main(String[] args) {
try{
//create the proxy info
SocketAddress socketAddress = new InetSocketAddress("78.137.18.67" , 8364);
Proxy proxy = new Proxy(Proxy.Type.SOCKS, socketAddress);
// create the socket with the proxy
Socket socket = new Socket(proxy);
// connect to some address and send/receive data
socket.connect(new InetSocketAddress("www.google.com", 80));
socket.getOutputStream().write("GET /index.html HTTP/1.1\r\nHost: www.google.com\r\n\r\n".getBytes("UTF-8"));
byte result[] = new byte[1024];
socket.getInputStream().read(result);
socket.close();
System.out.println(new String(result));
}catch(Exception e){
e.printStackTrace();
}
}
}
The problem with android, with a code similar like that, is that the InetSocketAddress is doing something strange. It seems that it does a reverse lookup of the given ip, and then the socket created with the proxy tries to connect with the resolved host name, in this case is 78-137-18-67.dynamic-pool.mclaut.net.
This would not be a problem (except on performance) if the socket could resolve the hostname back to the ip address. The fact is that this hostname cannot be resolved to ip address with my internet connection (i don't know if others can do). So the reverse lookup is working fine but the normal lookups fails, so when the socket tries to connect through the proxy it raises the following exception:
08-25 19:26:46.332: ERROR/Microlog(3526): 40274 SocketConnection
[ERROR] Error establishing connection java.net.SocketException: SOCKS
connection failed: java.net.UnknownHostException:
78-137-18-67.dynamic-pool.mclaut.net
So the question is, why it is trying to connect with the hostname if i gave the ip address? Is there any way to avoid this lookup? I have tried with createUnresolved of InetSocketAddress but in this case the socket hangs on connection.
Is not a waste of time, internet connection, etc, to do a reverse DNS lookup to get the hostname (if any), and later when the socket needs to connect, resolve again the host to an ip address?
NOTE: this code is an example, the real app do not perform any http request in this way. It uses binary data packets.

To prevent a reverse lookup, you can create the InetAddress with getByAddress(byte[]).
Then pass the InetAddress instance into the InetSocketAddress constructor.
Alternatively, use the factory method InetSocketAddress.createUnresolved(String,int)

Yes it seems that the particular constructor of InetSocketAddress does a reverse DNS lookup: http://mailinglists.945824.n3.nabble.com/Android-and-reverse-DNS-lookup-issues-td3011461.html
Also, it seems that this does not happen anymore on Android 2.3.4.

In android you have to do everything with background process so that you do not write code for socket in onCreate method directly you have to do this in background so that your ui does not hangs
something like this
new Thread(new Runnable() {
#Override
public void run() {
try {
client = new Socket(ipaddress, port);
printwriter = new PrintWriter(client.getOutputStream(), true);
InputStream is = client.getInputStream();
printwriter.write(msg);
printwriter.flush();
byte[] buffer = new byte[2046];
int read;
while ((read = is.read(buffer)) != -1) {
final String output = new String(buffer, 0, read);
);
printwriter.close();
}
});
}
Log.e("message", "message send");
} catch (UnknownHostException e2) {
e2.printStackTrace();
} catch (IOException e1) {
e1.printStackTrace();
Log.d("Time out", "Time");
}