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Connection refused error when using sockets between C# (server) and Java (Client) applications

I am trying to create a socket connection between a .Net server application and Java Client Application.
I am getting an error from the java client application:
Connection refused: connect
Notes:
Communicating with a .Net Client Application, works fine.
I have disables the windows firewall
Undoubtedly, I am running the server application in the background and then I am running the client application
Following are my server code (C#):
public class Server
{
public Server()
{
CreateListener();
}
public void CreateListener()
{
// Create an instance of the TcpListener class.
TcpListener tcpListener = null;
IPAddress ipAddress = Dns.GetHostEntry("localhost").AddressList[0];
string output;
try
{
// Set the listener on the local IP address
// and specify the port.
tcpListener = new TcpListener(ipAddress, 13);
tcpListener.Start();
output = "Waiting for a connection...";
}
catch (Exception e)
{
output = "Error: " + e.ToString();
MessageBox.Show(output);
}
}
}
and client application code (Java):
public class smtpClient {
public void Send() {
Socket smtpSocket = null;
DataOutputStream os = null;
DataInputStream is = null;
try {
smtpSocket = new Socket("localhost", 13); // FAILURE
os = new DataOutputStream(smtpSocket.getOutputStream());
is = new DataInputStream(smtpSocket.getInputStream());
} catch (UnknownHostException e) {
System.err.println("Don't know about host: hostname");
} catch (IOException e) {
System.err.println(e.getMessage());
}
}
It fails at the following line in the Java Client Application:
smtpSocket = new Socket("localhost", 13);

I can't tell what is the issue you are facing, but you need to start with a solid foundation to discover these issues.
As a rule of thumb, you should always write one piece (typically the server) first and verify connectivity (say using telnet) and then write the other piece (typically client) and verify its connectivity.
I always keep a Standard Client and Server handy to test whether its my code or its the environment/configuration.
Below is a sample code that works fine to test connectivity.
using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;
class ClientServer {
static void Main() {
new Thread(() => { StartServer("localhost", 5013); }).Start();
Thread.Sleep(100);
Console.WriteLine("\nPress enter to start the client...");
Console.ReadLine();
StartClient("localhost", 5013);
}
public static void StartServer(string serverInterface, int port) {
try {
IPHostEntry hostInfo = Dns.GetHostEntry(serverInterface);
string hostName = hostInfo.HostName;
IPAddress ipAddress = hostInfo.AddressList[0];
var server = new TcpListener(ipAddress, port);
server.Start();
Console.WriteLine($"Waiting for a connection at {server.LocalEndpoint}");
Console.WriteLine("Press ctrl+c to exit server...");
while (true) {
TcpClient client = server.AcceptTcpClient();
Console.WriteLine($"Server says - Client connected: {client.Client.RemoteEndPoint}");
ThreadPool.QueueUserWorkItem((state) => {
using (var _client = (TcpClient)state)
using (NetworkStream stream = _client.GetStream()) {
string msg = stream.ReadAsciiData();
if (msg == "Hello!") {
stream.WriteAsciiData($"Time:{DateTime.Now: yyyy/MM/dd HH:mm zzz}. Server name is {hostName}");
}
}
}, client);
}
} catch (Exception e) {
Console.WriteLine(e);
}
}
public static void StartClient(string serverInterface, int port) {
Console.WriteLine("Client started...");
try {
using (var client = new TcpClient(serverInterface, port))
using (NetworkStream stream = client.GetStream()) {
Console.WriteLine("Client says - Hello!");
stream.Write(Encoding.ASCII.GetBytes("Hello!"));
string msg = stream.ReadAsciiData();
Console.WriteLine($"Client says - Message from server: Server#{client.Client.RemoteEndPoint}: {msg}");
}
} catch (Exception e) {
Console.WriteLine(e);
}
Console.WriteLine("Client exited");
}
}
static class Utils {
public static void WriteAsciiData(this NetworkStream stream, string data) {
stream.Write(Encoding.ASCII.GetBytes(data));
}
public static string ReadAsciiData(this NetworkStream stream) {
var buffer = new byte[1024];
int read = stream.Read(buffer, 0, buffer.Length);
return Encoding.ASCII.GetString(buffer, 0, read);
}
public static void Write(this NetworkStream stream, byte[] data) {
stream.Write(data, 0, data.Length);
}
}
Now to your specific problem,
The choice of port 13, is not ideal for testing. Usually all ports below 1024 are considered privileged. i.e. a firewall or antivirus might block your attempt to listen on that port
Remember that IPV6 addresses plays a role. Your machine might have that enabled or disabled based on your configuration. You want to make sure that if your server is listening on a IPv6 interface, then your client also connects on the same
Which brings us to another related point: Irrespective of you are using IPv6 interface or not, the client needs to connect to the same interface the server is listening on. This might seem obvious, but is often missed. A typical machine
has at-least 2 interfaces: One for localhost (127...* called loopback interface) and another non local (typically 10...* or 192...*, but not restricted to it). It can so happen (especially when you pick the first available interface to bind your server without knowing which one it is) that server might be listening on non loopback interface like say 192.168.1.10 interface and the client might be connecting to 127.0.0.1, and you can see why the client will get "connection refused" errors
The sample code above works and you can test your code with it. You can us telnet for a client or just my sample code. You can play around changing the serverInterface values to some surprising discoveries which are accentuated by
ipAddress = hostInfo.AddressList[0] line
Hope this helps you with your debugging

Java-Client connecting to C# Server: Connection Timeout

I am trying to connect two Android devices to a c#-server and I always get a connection timeout.
The java-code is simple:
Socket socket = new Socket(mAddress, PORT);
If I start a java-server on the pc then the connection is successful. So its not a network/firewall problem.
But my c#-server just won't accept any connections, code:
private TcpListener serverSocket;
private TcpClient clientSocket1;
private TcpClient clientSocket2;
private void Form1_Load(object sender, EventArgs e)
{
serverSocket = new TcpListener(IPAddress.Any, port);
clientSocket1 = default(TcpClient);
clientSocket2 = default(TcpClient);
serverSocket.Start();
clientListenerThread = new Thread(wait4Clients);
clientListenerThread.Start();
}
private void wait4Clients()
{
logToConsole("Clientlistener started");
clientSocket1 = serverSocket.AcceptTcpClient();
logToConsole("Client No 1 started!");
clientSocket2 = serverSocket.AcceptTcpClient();
logToConsole("Client No 2 started!");
}
I also tried System.Net.Sockets.Socket instead of System.Net.Sockets.TcpClient, didnt worked either.
Thanks a lot
EDIT: Seems the code is perfectly fine. If I run the exe and not debug mode via Visual Studio, everything works. So the debug mode somehow prevents the server socket from working correctly. Any ideas why this happens?

// TCP Server C#, look at [msdn][1]
try
{
// Set the TcpListener on port 8080 of Any IP Address.
TcpListener server = new TcpListener(IPAddress.Any, 8080);
// Start listening for client requests.
server.Start();
// Buffer for reading data
Byte[] bytes = new Byte[1024];
String data = null;
// Enter the listening loop.
while(true) {
Console.Write("Waiting for a connection... ");
// Perform a blocking call to accept requests.
// You could also user server.AcceptSocket() here.
TcpClient client = server.AcceptTcpClient();
Console.WriteLine("Connected!");
// Get a stream object for reading and writing
data = null;
NetworkStream stream = client.GetStream();
int i;
// Loop to receive all the data sent by the client.
while((i = stream.Read(bytes, 0, bytes.Length))!=0)
{
// Translate data bytes to a ASCII string.
data = System.Text.Encoding.ASCII.GetString(bytes, 0, i);
Console.WriteLine("Received: {0}", data);
// Process the data sent by the client.
data = data.ToUpper();
byte[] msg = System.Text.Encoding.ASCII.GetBytes(data);
// Send back a response.
stream.Write(msg, 0, msg.Length);
Console.WriteLine("Sent: {0}", data);
}
}

Why AsyncTask to avoid "NetworkOnMainThreadException" is not working?

Trying to implement a simple UDP Client/Server datagram between an AndroidStudio-Java application as a client, to a Visual Studio C# Server. I am completely sure of the server side being working.
Here is a UDP Client and on a ButtonClick a UDP message should be sent to the localhost on port 15000 "for now".
my StackTrace popped the Android.os.NetworkOnMainThreadException Error. I found here that I can use an easy solution which is to import StrictMode and set a new policy to permitAll(). However still my application couldn't work and literally nothing happens on ButtonClick "No Exception to trace + No received message" and here is my code:
ButtonOne.setOnClickListener(
new Button.OnClickListener() {
public void onClick(View v) {
TextView TextOne = (TextView) findViewById(R.id.TestText);
TextOne.setText("Hi");
String host = "127.0.0.1"; // localhost
int port = 15000;
String message = "Test";
DatagramSocket dsocket = null;
if (android.os.Build.VERSION.SDK_INT > 9)
{
StrictMode.ThreadPolicy policy =
new StrictMode.ThreadPolicy.Builder().permitAll().build();
StrictMode.setThreadPolicy(policy);
}
try {
// Get the Internet address of the specified host
InetAddress address = InetAddress.getByName(host);
// wrap a packet
DatagramPacket packet = new DatagramPacket(
message.getBytes(),
message.length(),
address, port);
// Create a datagram socket, send the packet through it, close it.
dsocket = new DatagramSocket();
dsocket.send(packet);
dsocket.close();
} catch (Exception e) {
e.printStackTrace();
}
}
}
);
Then I found here here that it's strongly not recommended to use StrictMode and that I need to use AsyncTask. However on Android Documentation it says "AsyncTask must be subclassed to be used. The subclass will override at least one method (doInBackground(Params...)), and most often will override a second one (onPostExecute(Result).)" which I don't get because each time I add Async within MainActivity class I get errors and it's frustrating..
Is't okay to use StrictMode for this simple task? If yes, why it's not working? If No, can anyone tell me please how to import AsyncTask into this piece of code? and should I use Params, Progress, Result functions??

Since your packet is send and forget and you do not monitor its progress or do something at its end, you do not need async task. You need to start network activity in a new thread. The code is below, may have minor compilation issues since I do not have access to one right now.
ButtonOne.setOnClickListener(
new Button.OnClickListener() {
public void onClick(View v) {
TextView TextOne = (TextView) findViewById(R.id.TestText);
TextOne.setText("Hi");
String message = "Test";
Thread networkThread = new Thread() {
String host = "127.0.0.1"; // localhost
int port = 15000;
DatagramSocket dsocket = null;
public void run() {
try {
// Get the Internet address of the specified host
InetAddress address = InetAddress.getByName(host);
// wrap a packet
DatagramPacket packet = new DatagramPacket(
message.getBytes(),
message.length(),
address, port);
// Create a datagram socket, send the packet through it, close it.
dsocket = new DatagramSocket();
dsocket.send(packet);
dsocket.close();
} catch (Exception e) {
e.printStackTrace();
}//catch
}//run
};// Networkthread
networkThread.start();//networkThread.start()
}//onClick
}//onClickListener
);//setOnClickListener

Socket android client PC server

I have problem to connect my android client to my PC Server
here there are the codes
-->PC SERVER:
public class Server {
public static void main(String[] args) throws IOException, SQLException, ClassNotFoundException
{
ServerSocket server = new ServerSocket(4444);
System.out.println("Waiting for clients to connect...");
while (true)
{
Socket s = server.accept();
InetAddress clientAddress = s.getInetAddress();
System.out.println("Incoming connection from: " + clientAddress.getHostName() + "[" + clientAddress.getHostAddress() + "]");
s.close();
}
}
}
--->ANDROID CLIENT:
public class Main extends Activity {
Button b;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
b = (Button)findViewById(R.id.connect);
b.setOnClickListener(new View.OnClickListener(){
#Override
public void onClick(View arg0) {
try {
Socket client = new Socket("10.0.2.2", 4444); //connect to server
client.close(); //closing the connection
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
});
}
When on my android device i click the button in my server program(PC) doesn't view device connect...why? i've tried to insert
<uses-permission android:name="android.permission.INTERNET"></uses-permission>
into my AndroidManifest but when i click on button program crash
Android device and PC are connected to same network(router DSL).
Help me please

10.0.2.2 is an address that is used by your app running in an emulator to connect with a server running on your pc. The emulator is on the same pc. If you use a real devive you have to use the (W)LAN address of your pc which is in the same WLAN as your device. Often something like 192.168.0.12. Find out with ipconfig.

The address you are using is 10.0.2.2, which is non routable as private.
You should use an address that your device can resolve. Either a DNS name published in a DNS accessible from your phone (so a public one) or a routable IP. Your server should be reachable from the public network, which is unlikely to be the case of your PC unless you have specifically took care of that - I am not sure of what you mean by this:
Android device and PC are connected to same network(router DSL)
Please have a look at the exception that is thrown on phone side and edit your post with the resulting stack trace. It will help to dig into this further.

Your socket declaration should be as follow :
Socket client = new Socket(10.0.2.2, 4444); //Quotes removed

Android: NAT Traversal?

It appears to me that newer Android devices run behind a NAT, where the local address is an internal carrier or LAN address and the public address is the router or carrier assigned external address.
Nevertheless, newer phones don't return the same address using the NetworkInterface as when accessing an IP detection service.
Therefore, connecting via direct P2P SocketChannels inherently fails.
Are there any common workarounds to this problem designed for the Android platform? Can anyone clarify what is causing this NAT-like security issue?
Any links to Java NAT traversal tutorials or examples (NOT essays or theses) would also be appreciated as being helpful (as I'm not quite sure how to implement it in Java).
I will of course also accept any other solutions anyone has to offer!

Almost every phone or PC you will ever touch won't have a static public IP address, and therefore will require NAT traversal. It's not because of the device; the carrier or ISP put routers between your device and the public internet. Depending on your application, usually there are NAT-traversal libraries you can use, such as ice4j or STUNT.

I do that in my own project and have found this issue is not that complicated.
Here's a very simple UDP echo server in node.js
var dgram = require('dgram');
var socket =
dgram.createSocket('udp4');
socket
.on('listening', function()
{
var address = socket.address();
console.log('socket listening ' +
address.address + ':' + address.port);
})
.on('error', function(err)
{
console.log('socket error:\n' + err.stack);
socket.close();
})
.on('message', function(message, rinfo)
{
console.log('message: ' + message + ' from ' +
rinfo.address + ':' + rinfo.port);
var msg = new Buffer(rinfo.address + ':' + rinfo.port);
socket
.send(msg, 0, msg.length,
rinfo.port, rinfo.address,
function(err, bytes)
{
//socket.close();
});
})
.bind(15000);
An android client simply send a msg to this node server
System.out.println("UDP hole punching=======================");
class IOth extends Thread {
#Override
public void run() {
String sendMsg = "UDP hole punching";
byte[] buf = sendMsg.getBytes();
DatagramPacket packet;
System.out.println(HPremoteHost); // node server IP
System.out.println(HPremotePort); // 15000
try {
packet = new DatagramPacket(buf, buf.length, InetAddress.getByName(HPremoteHost), HPremotePort);
ds.send(packet);
} catch (Exception e) {
System.out.println("error================");
System.out.println(e);
}
}
}
IOth io00 = new IOth();
io00.start();
Android Client UDP listener to obtain general msg and your own Global ip&port via UDPholepunching
class IOLoop extends Thread {
#Override
public void run() {
try {
String msg = "Native.UDPserver.open";
SocketAddress sockAddress;
String address;
byte[] buf = new byte[1024];
DatagramPacket packet = new DatagramPacket(buf, buf.length);
while (true) {
try {
ds.receive(packet);
sockAddress = packet.getSocketAddress();
address = sockAddress.toString();
msg = new String(buf, 0, packet.getLength());
System.out.println(msg + " received !!! by " + address);
// this case is UDP HolePunching reaction
if (address.equals(HPaddress1)) {
System.out.println(msg + "hole punched");
// So you can obtain own Global ip& port here.
// exchange this information
// `remoteHost` `remotePort` to another client
// with some method (signaling server)
}
} catch (IOException e) {
}
}
} catch (Exception e) {
}
}
}
IOLoop io00 = new IOLoop();
io00.start();
Android Client UDP sender using other client's IP remoteHost remotePort
class IOth extends Thread {
#Override
public void run() {
String sendMsg = "This is a test message";
byte[] buf = sendMsg.getBytes();
DatagramPacket packet;
try {
packet = new DatagramPacket(buf, buf.length, InetAddress.getByName(remoteHost), remotePort);
ds.send(packet);
} catch (Exception e) {
}
}
}
IOth io00 = new IOth();
io00.start();

Look at http://sourceforge.net/projects/jnat-pmplib/
It is an implementation of NAT-PMP in java.

I've managed to establish sockets just by forwarding the sockets you're using during the connection in your router. It worked for me.
UPDATE
Find out your IP address through cmd.exe if your using Windows (ipconfig) or through a terminal session if your on Linux (ifconfig). Then connect to it through the browser and there should be a security section. Go to port forwarding and open up the ports your using when establishing you're ServerSocket and Socket. Use TCP as the protocol.
Please note that this only applies if you're trying to connect from outside your wlan.