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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

How to verify the ACE-CORBA service local windows machine

I'm able to create docker container for ACE-TAO service , and able to access it from parent windows machine using port-forwarding concept.
From browser i try to hit the localhost:forward-port and getting "ERR_EMPTY_RESPONSE" and TAO service is running in docker container.
If I want to verify in local, whether its connected properly or not.
How can I write Java code to verify?

The following java code connects to localhost:17500 and prints out a message saying whether or not it could create a tcp connection.
import java.io.*;
import java.net.*;
class TCPClient
{
public static void main(String argv[]) throws Exception
{
try {
Socket clientSocket = new Socket("localhost", 17500);
System.out.println("Could connect");
}
catch (ConnectException e) {
System.out.println("Cannot connect");
}
}
}

Java RMI client and remote server Connection Refused to localhost

I am not so experienced in Java RMI. I am trying to implement simple RMI communication, but faced with the problem.
Here my RMI client
public class MainClientApplication {
public static final String FILE_NAME_RMI_POLICY = "rmi.policy";
public static final String RMI_DOMAIN = "mydomain.net";
public static final String RMI_ENDPOINT = "/MD5Decrypter";
public static final String PROTOCOL = "rmi://";
public static void main(String[] args) {
System.out.println(System.getProperty("user.dir") + File.separator + FILE_NAME_RMI_POLICY);
System.setProperty("java.security.policy", System.getProperty("user.dir") + File.separator + FILE_NAME_RMI_POLICY);
String remoteHost = String.format("%s%s%s", PROTOCOL, RMI_DOMAIN, RMI_ENDPOINT);
System.setSecurityManager(new SecurityManager());
try {
ComputeEngine computeEngine = (ComputeEngine) Naming.lookup(remoteHost);
SwingUtilities.invokeLater(() -> new MainWindow("MD5 Decryption", computeEngine));
} catch (Exception ex) {
ex.printStackTrace();
}
}
}
Communication inerface
public interface ComputeEngine extends Remote {
Object executeTask(Task t) throws RemoteException;
}
My server
public class MainServer {
public static final int PORT = 1099;
public static final String RMI_ENDPOINT = "MD5Decrypter";
public static final String FILE_NAME_RMI_POLICY = "rmi.policy";
public static final String RMI_DOMAIN = "mydomain.net";
public static final String PROTOCOL = "rmi://";
public MainServer() {
try {
System.setProperty("java.security.policy", System.getProperty("user.dir") + File.separator + FILE_NAME_RMI_POLICY);
//System.setProperty("java.rmi.server.hostname", RMI_IP);
System.setProperty("java.rmi.server.hostname", RMI_DOMAIN);
System.setSecurityManager(new SecurityManager());
ComputeEngine computeEngine = new ComputeEngineExecutor();
// Naming.rebind(String.format("%s%s:%d/%s", PROTOCOL, RMI_DOMAIN, PORT, RMI_ENDPOINT), computeEngine);
Naming.rebind(RMI_ENDPOINT, computeEngine);
System.out.println("Successfully started RMI Server");
} catch (RemoteException e) {
e.printStackTrace();
} catch (MalformedURLException e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
MainServer mainServer = new MainServer();
}
}
And rmi.policy
grant {
permission java.net.SocketPermission "*:1024-65535", "connect, accept";
permission java.io.FilePermission "-", "read";
permission java.net.SocketPermission "*:80", "connect";
};
So I have started rmiregistry on my server rmiregistry 1099. And everything works well until calling remote object method.
Here are my steps
Lets consider that my domain is mydomain.net , I host it on my server at home with static IP.
Run rmiregistry on the server
Run server application on the server
Run client application on any PC
Click button to invoke remote method
So it seems that everything connects and binds successfully, because if force enter invalid domain, endpoint or refused if rmiregistry is not started - java.rmi.ConnectException is thrown.
But when I try to invoke any method on remote object, BTW which has been already retrieved successfully it throws a java.rmi.ConnectException
java.rmi.ConnectException: Connection refused to host: 127.0.1.1; nested exception is:
java.net.ConnectException: Connection refused
at sun.rmi.transport.tcp.TCPEndpoint.newSocket(TCPEndpoint.java:619)
at sun.rmi.transport.tcp.TCPChannel.createConnection(TCPChannel.java:216)
at sun.rmi.transport.tcp.TCPChannel.newConnection(TCPChannel.java:202)
at sun.rmi.server.UnicastRef.invoke(UnicastRef.java:130)
at java.rmi.server.RemoteObjectInvocationHandler.invokeRemoteMethod(RemoteObjectInvocationHandler.java:227)
at java.rmi.server.RemoteObjectInvocationHandler.invoke(RemoteObjectInvocationHandler.java:179)
at com.sun.proxy.$Proxy0.executeTask(Unknown Source)
at ui.MainWindow.lambda$initGui$0(MainWindow.java:49)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
As I can understand the problem is that it (server) forces me to connect to my localhost instead of remote.
P.S on localhost everything works perfectly (both client and server on the one PC)
Please help to solve this problem.
EDIT
I have forgotten to tell that I have tried to set property of java.rmi.server.hostname, in this case it takes a lot of time to start the server and ends with exception.

Leaving the 127.0.0.1 issue aside, which is solved by java.rmi.server.hostname, your problem now is connect timeouts. This will be caused by your firewall. You need to export your remote object on a specific port, and you need to open that TCP port in your firewall.

RMI connection failure detection on callback

I'm writing a distributed app by Java RMI. The RMI client registers event handler / callback to RMI server, and the server calls the client's callback function when required. Now the problem is, when network connection failure (for example, Ethernet cable plugged out...), the RMI server and client won't be notified, and the RMI server fails when attempts to call the client's registered callback function.The RMI server cannot notify the RMI client about this issue too. Even worse, when network connection recovers, the RMI client service will still lose contact with RMI server because nobody notify her to reconnect.
My current idea is to implement a ping() method in RMI client in separate thread.
This thread could wake up at regular intervals and check on the server.
if failed, then farce to reconnect.
Any other elegant solutions? Hope you guys can help !
the interface
import java.rmi.Remote;
import java.rmi.RemoteException;
public interface MyInterface extends Remote {
public int RegisterEventHandler(RemoteMyEventHandler eventHandler) throws RemoteException;
public void unRegisterEventHandler(int eventHandlerId) throws RemoteException;
}
the RMI Server impelementation
import java.rmi.RemoteException;
import com.me.MyInterface;
public class MyInterfaceImpl implements MyInterface {
{
public void init() {
try {
//... initialize RMI server....
//....
} catch (Exception ex) {
ex.printStackTrace();
}
}
#Override
public int RegisterEventHandler(RemoteMyEventHandler eventHandler)
throws RemoteException {
return MyEventHandlerImp.getInstance().addHandler(eventHandler);
}
#Override
public void unRegisterEventHandler(int eventHandlerId)
throws RemoteException {
MyEventHandlerImp.getInstance().removeHandler(eventHandlerId);
}
}
//handler.notifyEventSnap(events);
the RMI Client implementation
import java.rmi.NotBoundException;
import java.rmi.RemoteException;
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.util.Properties;
import org.apache.log4j.Logger;
import com.me.MyInterface;
public class MyService implements NotifyHandler{
{
private MyInterface client;
private MyEventHandler myEventHandler;
private void connectToServer() {
try {
//...
Registry registry = LocateRegistry.getRegistry(rmiHost, rmiPort);
client = (MyInterface) registry.lookup(MyCInterface.class.getName());
} catch (RemoteException er) {
} catch (NotBoundException en) {
} catch (Exception en) {
}
}
private void startService(){
//Attach my event handler
if(client != null)
{
myEventHandler = new MyEventHandler();
myEventHandlerId = client.RegisterEventHandler(myEventHandler);
}
}
}

when network connection failure (for example, Ethernet cable plugged out...), the RMI server and client won't be notified, and the RMI server fails when attempts to call the client's registered callback function.
Err, that is the notification to the server. The server just has to note this and try again later.
The RMI server cannot notify the RMI client about this issue too.
The client doesn't need to know.
Even worse, when network connection recovers, the RMI client service will still lose contact with RMI server because nobody notify her to reconnect.
The client doesn't have to 'reconnect'. There is no connect or reconnect step in RMI. As long as the client's JVM and remote objects remain up and exported respectively, the stubs at the server remain valid and can continue to be used by the server.
You're solving a non-problem.

You seem to be partially implementing a client/server session. This is a token that the server can track to ensure a client is valid. If there is an error while the server is communicating with the client the session should be ended and all references to the client removed.
Your server is already implementing a session with the integer used to unRegisterEventHandler. You should keep track of those integers somewhere like a Map. If the server cannot connect to a client it should simply unregister that client and make the session invalid by removing it from the map. The server should remove all references to the client and not attempt to communicate with the client until a new session is created.
If a client tries to communicate with the server it should get an InvalidException exception from the server. This way the client can attempt to make a new session by calling RegisterEventHandler in the catch block.
I worked on a project that dealt with this problem using a ping like you suggested at https://code.google.com/p/umuc-team-factor/
All client communication with the server was in a looped try catch block like
private void getSession() {
while(isRun()) {
try {
if(server == null) {
Logger.getLogger(JobClient.class.getName()).info("Server is null.");
setupServer();
}
UUID sid = server.getSession(this);
synchronized (this) {
id = sid;
}
Logger.getLogger(JobClient.class.getName()).info("Session id is " + id);
return;
} catch (RemoteException ex) {
Logger.getLogger(JobClient.class.getName()).info("Could not get session from server: " + ex + ". setting up server.");
setupServer();
}
}
}
This try to setup a session with the server until the program is stopped.
All server communication with the client should end the session for the client if there is a RemoteException thrown. c.status() is similar to a ping.
List<UUID> endSessions = new ArrayList<UUID>();
for (UUID id : copy.keySet()) {
ClientCallback c = copy.get(id).client;
try {
ClientStatus status = c.status();
Logger.getLogger(ProcessManager.class.getName()).info("got client status for " + id + ": " + status.getSessionID() + " -" + status.getJobStatus());
if (status.getSessionID() == null || !status.getSessionID().equals(id)) {
endSessions.add(id);
}
} catch (Exception ex) {
endSessions.add(id);
Logger.getLogger(ProcessManager.class.getName()).log(Level.SEVERE, null, ex);
}
}
for (UUID id : endSessions) {
try {
endSession(id);
} catch (SessionExpiredException ex) {
Logger.getLogger(ProcessManager.class.getName()).log(Level.SEVERE, null, ex);
}
}

Java - File Transfer FTP to Remote Server(Apache MINA)

I am implementing a Java program,
which has to connect to remote server
connected remote server should download a file from ftp
i am using Apache MINA lib's for this code
here is code, which connects to the remote server
public class filetrans
{
public static void main(String[] args) throws IOException, InterruptedException
{
SshClient client = null;
String login="user";
String password="password";
try
{
client = SshClient.setUpDefaultClient();
client.start();
ConnectFuture future = client.connect("myhost",myport);
future.await();
ClientSession session = (ClientSession) future.getSession();
boolean auth = session.authPassword(login, password).await().isSuccess();
if (auth)
{
System.out.println("Authenticated....");
ClientChannel channel = session.createChannel("shell");
channel.setIn(new NoCloseInputStream(System.in));
channel.setOut(new NoCloseOutputStream(System.out));
channel.setErr(new NoCloseOutputStream(System.err));
channel.open();
channel.waitFor(ClientChannel.CLOSED, 5000);
channel.close(true);
}
else
{
System.out.println("Authentication failed....");
}
}
catch (Throwable t)
{
System.out.println(t);
}
finally
{
client.stop();
}
}
}
I am successfully connecting to the Remote server. now i have to connect to the FTP server and download a file and to save in the Remote Server. I am Stuck here, any ideas how to implement further or any codes or any suggestion will be great. thanks