How can i solve "An existing connection was forcibly closed by a remote host." error?-CodePudding

I am trying to connect the server using ASP.Net Core 6. I published my Server project to FileZilla. However, i got this error message:

Socket connected.
Sent 19 bytes to server.
System.Net.Sockets.SocketException (10054): An existing connection was forcibly closed by a remote host. at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.ThrowException(SocketError error, CancellationToken cancellationToken) at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.System.Threading.Tasks.Sources.IValueTaskSource<System.Int32>.GetResult(Int16 token) at System.Threading.Tasks.ValueTask`1.ValueTaskSourceAsTask.<>c.<.cctor>b__4_0(Object state) --- End of stack trace from previous location --- at System.Threading.Tasks.TaskToApm.End[TResult](IAsyncResult asyncResult) at System.Net.Sockets.Socket.EndReceive(IAsyncResult asyncResult) at AsynchronousClient.ReceiveCallback(IAsyncResult ar) in C:\Users\rozer\source\repos\Client1\Client1\Program.cs:line 129

The error line:

int bytesRead = client.EndReceive(ar);

I understood that the connection has been made. But ı do not understand why it is forcibly closed by a remote host and I got this error. Is it about a firewall issue or something like that?

This is my Client project to check the connection:

using System.Net;
using System.Net.Sockets;
using System.Threading;
using System.Text;

// State object for receiving data from remote device.  
public class StateObject
{
    // Client socket.  
    public Socket workSocket = null;
    // Size of receive buffer.  
    public const int BufferSize = 256;
    // Receive buffer.  
    public byte[] buffer = new byte[BufferSize];
    // Received data string.  
    public StringBuilder sb = new StringBuilder();
}

public class AsynchronousClient
{
    // The port number for the remote device.  
    private const int port = 443;

    // ManualResetEvent instances signal completion.  
    private static ManualResetEvent connectDone =
        new ManualResetEvent(false);
    private static ManualResetEvent sendDone =
        new ManualResetEvent(false);
    private static ManualResetEvent receiveDone =
        new ManualResetEvent(false);

    // The response from the remote device.  
    private static String response = String.Empty;

    private static void StartClient()
    {
        // Connect to a remote device.  
        try
        {
            // Establish the remote endpoint for the socket.  
            // The name of the
            // remote device is "host.contoso.com".  
            IPHostEntry ipHostInfo = Dns.GetHostEntry("networks.net");
            IPAddress ipAddress = ipHostInfo.AddressList[0];
            IPEndPoint remoteEP = new IPEndPoint(ipAddress, port);

            // Create a TCP/IP socket.  
            Socket client = new Socket(ipAddress.AddressFamily,
                SocketType.Stream, ProtocolType.Tcp);

            // Connect to the remote endpoint.  
            client.BeginConnect(remoteEP,
                new AsyncCallback(ConnectCallback), client);
            connectDone.WaitOne();

            // Send test data to the remote device.  
            Send(client, "deneme fkfglb <EOF>");
            sendDone.WaitOne();

            // Receive the response from the remote device.  
            Receive(client);
            receiveDone.WaitOne();

            // Write the response to the console.  
            Console.WriteLine("Response received : {0}", response);

            // Release the socket.  
            client.Shutdown(SocketShutdown.Both);
            client.Close();

        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    private static void ConnectCallback(IAsyncResult ar)
    {
        try
        {
            // Retrieve the socket from the state object.  
            Socket client = (Socket)ar.AsyncState;

            // Complete the connection.  
            client.EndConnect(ar);

            Console.WriteLine("Socket connected to {0}",
                client.RemoteEndPoint.ToString());

            // Signal that the connection has been made.  
            connectDone.Set();
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    private static void Receive(Socket client)
    {
        try
        {
            // Create the state object.  
            StateObject state = new StateObject();
            state.workSocket = client;

            // Begin receiving the data from the remote device.  
            client.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0,
                new AsyncCallback(ReceiveCallback), state);
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    private static void ReceiveCallback(IAsyncResult ar)
    {
        try
        {
            // Retrieve the state object and the client socket
            // from the asynchronous state object.  
            StateObject state = (StateObject)ar.AsyncState;
            Socket client = state.workSocket;

            // Read data from the remote device.  
            int bytesRead = client.EndReceive(ar);

            if (bytesRead > 0)
            {
                // There might be more data, so store the data received so far.  
                state.sb.Append(Encoding.ASCII.GetString(state.buffer, 0, bytesRead));

                // Get the rest of the data.  
                client.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0,
                    new AsyncCallback(ReceiveCallback), state);
            }
            else
            {
                // All the data has arrived; put it in response.  
                if (state.sb.Length > 1)
                {
                    response = state.sb.ToString();
                }
                // Signal that all bytes have been received.  
                receiveDone.Set();
            }
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    private static void Send(Socket client, String data)
    {
        // Convert the string data to byte data using ASCII encoding.  
        byte[] byteData = Encoding.ASCII.GetBytes(data);

        // Begin sending the data to the remote device.  
        client.BeginSend(byteData, 0, byteData.Length, 0,
            new AsyncCallback(SendCallback), client);
    }

    private static void SendCallback(IAsyncResult ar)
    {
        try
        {
            // Retrieve the socket from the state object.  
            Socket client = (Socket)ar.AsyncState;

            // Complete sending the data to the remote device.  
            int bytesSent = client.EndSend(ar);
            Console.WriteLine("Sent {0} bytes to server.", bytesSent);

            // Signal that all bytes have been sent.  
            sendDone.Set();
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    public static int Main(String[] args)
    {
        StartClient();
        return 0;
    }
} ```

CodePudding user response：

There could be possible issues like wrong message format, firewall, unmatched library version between client and server, being not an admin user or unsupported connection format.

I would suggest to check EventViewer to see extended/better error.

Later, if you are unsure about connection, I would recommend to download an working sample from github or somewhere else and check if it works.

If works, it's probably not related with firewall but a potential library issue, if that sample do not work too, then it that case it could be firewall, anti-virus or not being an admin issue.

The other solution to find this error could be using WireShark to trace packages between server and client. If you can see any repeated RST package, that would be the sign that something is killing the connection.