Java, server client TCP communication ends with RST

Question

I'm trying to figure out if this is normal. Because without errors, a connection should be terminated by:

FIN ->
<- ACK
<- FIN
ACK ->

I get this at the end of a TCP connection (over SSL, but i also get it with non-encrypted):

         From     To
1494    server  client  TCP search-agent > 59185 [PSH, ACK] Seq=25974 Ack=49460 Win=63784 Len=50
1495    client  server  TCP 59185 > search-agent [ACK] Seq=49460 Ack=26024 Win=63565 Len=0
1496    client  server  TCP 59185 > search-agent [PSH, ACK] Seq=49460 Ack=26024 Win=63565 Len=23
1497    client  server  TCP 59185 > search-agent [FIN, ACK] Seq=49483 Ack=26024 Win=63565 Len=0
1498    server  client  TCP search-agent > 59185 [PSH, ACK] Seq=26024 Ack=49484 Win=63784 Len=23
1499    client  server  TCP 59185 > search-agent [RST, ACK] Seq=49484 Ack=26047 Win=0 Len=0

The client exits normally and reaches socket.close, shouldn't then the connection be shut down normally, without a reset?

I can't find anything about the TCP streams of java on google...

Here is my code:

Server:

package Security;

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

import javax.net.ServerSocketFactory;
import javax.net.ssl.*;
import java.util.*;

public class SSLDemoServer
{
    private static ServerSocket serverSocket;
    private static final int PORT = 1234;

    public static void main(String[] args) throws IOException
    {
        int received = 0;
        String returned; 
        ObjectInputStream input = null;
        PrintWriter output = null;
        Socket client;

        System.setProperty("javax.net.ssl.keyStore", "key.keystore");
        System.setProperty("javax.net.ssl.keyStorePassword", "vwpolo");
        System.setProperty("javax.net.ssl.trustStore", "key.keystore");
        System.setProperty("javax.net.ssl.trustStorePassword", "vwpolo");

        try
        {
            System.out.println("Trying to set up server ...");
            ServerSocketFactory factory = SSLServerSocketFactory.getDefault();
            serverSocket = factory.createServerSocket(PORT);

            System.out.println("Server started!\n");
        }
        catch (IOException ioEx)
        {
            System.out.println("Unable to set up port!");
            ioEx.printStackTrace();
            System.exit(1);
        }

        while(true)
        {
            client = serverSocket.accept();
            System.out.println("Client trying to connect...");

            try
            {
                System.out.println("Trying to create inputstream...");
                input = new ObjectInputStream(client.getInputStream());
                System.out.println("Trying to create outputstream...");
                output = new PrintWriter(client.getOutputStream(), true);
                System.out.println("Client successfully connected!");

                while( true )
                {
                    received = input.readInt();
                    returned = Integer.toHexString(received);
                    System.out.print(" " + received);
                    output.println(returned.toUpperCase());
                }
            }
            catch(SSLException sslEx)
            {
                System.out.println("Connection failed! (non-SSL connection?)\n");
                client.close();
                continue;
            }
            catch(EOFException eofEx)
            {
                System.out.println("\nEnd of client data.\n");
            }
            catch(IOException ioEx)
            {
                System.out.println("I/O problem! (correct inputstream?)");
            }

            try {
                input.close();
                output.close();
            }
            catch (Exception e) {
            }
            client.close();
            System.out.println("Client closed.\n");
        }
    }
}

Client:

package Security;

import java.io.*;
import java.net.*;
import javax.net.ssl.*;

import java.util.*;

public class SSLDemoClient
{
    private static InetAddress host;
    private static final int PORT = 1234;

    public static void main(String[] args)
    {
        System.setProperty("javax.net.ssl.keyStore", "key.keystore");
        System.setProperty("javax.net.ssl.keyStorePassword", "vwpolo");
        System.setProperty("javax.net.ssl.trustStore", "key.keystore");
        System.setProperty("javax.net.ssl.trustStorePassword", "vwpolo");

        System.out.println("\nCreating SSL socket ...");
        SSLSocket socket = null;
        try
        {
            host = InetAddress.getByName("192.168.56.101");
            SSLSocketFactory factory = (SSLSocketFactory)
            SSLSocketFactory.getDefault();
            socket = (SSLSocket) factory.createSocket(host, PORT);
            socket.startHandshake();
        }
        catch(UnknownHostException uhEx)
        {
            System.out.println("\nHost ID not found!\n");
            System.exit(1);
        }
        catch(SSLException sslEx)
        {
            System.out.println("\nHandshaking unsuccessful ...");
            System.exit(1);
        } 
        catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println("\nHandshaking succeeded ...\n");

        SSLClientThread client = new SSLClientThread(socket);
        SSLReceiverThread receiver = new SSLReceiverThread(socket);

        client.start();
        receiver.start();

        try
        {
            client.join();
            receiver.join();
            System.out.println("Trying to close...");
            socket.close();
        }
        catch(InterruptedException iEx)
        {
            iEx.printStackTrace();
        } 
        catch(IOException ioEx)
        {
            ioEx.printStackTrace();
        }

        System.out.println("\nClient finished.");
    }
}


class SSLClientThread extends Thread
{
    private SSLSocket socket;

    public SSLClientThread(SSLSocket s)
    {
        socket = s;
    }

    public void run() 
    {
        try
        {
            ObjectOutputStream output = new ObjectOutputStream(socket.getOutputStream());

            for( int i = 1; i < 1025; i++)
            {
                output.writeInt(i);
                sleep(10);
                output.flush();
            } 

            output.flush();
            sleep(1000);
            output.close();
        }
        catch(IOException ioEx)
        {
            System.out.println("Socket closed or unable to open socket.");
        }
        catch(InterruptedException iEx)
        {
            iEx.printStackTrace();
        }
    }
}


class SSLReceiverThread extends Thread
{
    private SSLSocket socket;

    public SSLReceiverThread(SSLSocket s)
    {
        socket = s;
    }

    public void run()
    {
        String response = null;
        BufferedReader input = null;

        try
        {
            input = new BufferedReader(
                    new InputStreamReader(socket.getInputStream()));

            try
            {
                response = input.readLine();
                while(!response.equals(null)) 
                {
                    System.out.print(response + " ");
                    response = input.readLine();
                } 
            }
            catch(Exception e)
            {
                System.out.println("\nEnd of server data.\n");
            }    

            input.close();

        }
        catch(IOException ioEx)
        {
            ioEx.printStackTrace();
        }
    }
}

Answer 1

Your packet log shows that the client is sending its last bit of data, and notifying the server of the connection closing.

The server then sends back it's last bit of data (the last converted integer, presumably). The client responds with a RST, which indicates that the client socket was closed for both reading and writing - the client is saying "sorry, I didn't catch that last bit of data, I'm not listening anymore".

I am not a Java guru, but it appears that calling the .close() method of the socket's OutputStream is resulting in the .close() method being called on the socket itself. What you actually want to happen here (after the client has written its last integer) is that the shutdownOutput() method is called on the socket.

Answer 2

As caf points out, your problem is that you are calling close when you should be calling shutdownOutput. This is because what you are trying to implement is what's called a half-close, in which each side of the connection closes it's output stream in order to indicate to the peer that there is no more data pending ( EOF ).

For an explanation of the difference between close and shutdown see this short SO answer "close vs. shutdown" and this longer article, "Socket shutdown versus socket close". Also see, Orderly Versus Abortive Connection Release in Java.

Answer 3

Thanks a lot, on a non-SSL connection, I now get a nice orderly connection shutdown without a reset.

BUT apparently this method is not supported by SSLSocket? When I try it in my SSL client, I get the following exception:

Exception in thread "Thread-0" java.lang.UnsupportedOperationException: The method shutdownOutput() is not supported in SSLSocket
    at com.sun.net.ssl.internal.ssl.BaseSSLSocketImpl.shutdownOutput(Unknown Source)
    at Security.SSLClientThread.run(SSLDemoClient.java:99)

My new code looks like this:

package Security;

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

import javax.net.SocketFactory;
import javax.net.ssl.*;

public class SSLDemoClient
{
    private static InetAddress host;
    private static Socket socket;
    private static final int PORT = 1234;

    public static void main(String[] args)
    {
        System.setProperty("javax.net.ssl.keyStore", "key.keystore");
        System.setProperty("javax.net.ssl.keyStorePassword", "vwpolo");
        System.setProperty("javax.net.ssl.trustStore", "key.keystore");
        System.setProperty("javax.net.ssl.trustStorePassword", "vwpolo");

        System.out.println("\nCreating SSL socket ...");

        try
        {
            host = InetAddress.getByName("192.168.56.101");
            SocketFactory socketFactory = SSLSocketFactory.getDefault();
            socket = socketFactory.createSocket(host, PORT);
        }
        catch(UnknownHostException uhEx)
        {
            System.out.println("\nHost ID not found!\n");
            System.exit(1);
        }
        catch(SSLException sslEx)
        {
            System.out.println("\nHandshaking unsuccessful ...");
            System.exit(1);
        } 
        catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println("\nHandshaking succeeded ...\n");

        SSLClientThread client = new SSLClientThread(socket);
        SSLReceiverThread receiver = new SSLReceiverThread(socket);

        client.start();
        receiver.start();

        try
        {
            client.join();
            receiver.join();
            System.out.println("Trying to close...");

            socket.close();
        }
        catch(InterruptedException iEx)
        {
            iEx.printStackTrace();
        } 
        catch(IOException ioEx)
        {
            ioEx.printStackTrace();
        }

        System.out.println("\nClient finished.");
    }
}


class SSLClientThread extends Thread
{
    private Socket socket;

    public SSLClientThread(Socket s)
    {
        socket = s;
    }

    public void run() 
    {
        try
        {
            ObjectOutputStream output = new ObjectOutputStream(socket.getOutputStream());

            for( int i = 1; i < 1025; i++)
            {
                output.writeInt(i);
                sleep(10);
                output.flush();
            } 

            output.flush();
            sleep(1000);
            socket.shutdownOutput();
        }
        catch(IOException ioEx)
        {
            System.out.println("Socket closed or unable to open socket.");
        }
        catch(InterruptedException iEx)
        {
            iEx.printStackTrace();
        }
    }
}


class SSLReceiverThread extends Thread
{
    private Socket socket;

    public SSLReceiverThread(Socket s)
    {
        socket = s;
    }

    public void run()
    {
        String response = null;
        BufferedReader input = null;

        try
        {
            input = new BufferedReader(
                    new InputStreamReader(socket.getInputStream()));

            try
            {
                response = input.readLine();
                while(!response.equals(null)) 
                {
                    System.out.print(response + " ");
                    response = input.readLine();
                } 
            }
            catch(Exception e)
            {
                System.out.println("\nEnd of server data.\n");
            }    

            socket.shutdownInput();

        }
        catch(IOException ioEx)
        {
            ioEx.printStackTrace();
        }
    }
}

Am I not using it correcty? Or is my SSLSocket not correct? Because with a non-SSL connection there is no problem.

Answer 4

The real problem here is that one end is writing after the other end has already closed its end of the connection. This is an application protocol error.