Listening for an Ethernet Cable Unplugging Event for a TCP Server Application

Question

I have a C# TCP server application. I detect disconnections of TCP clients when they disconnect from server but how can I detect a cable unplug event? When I unplug the ethernet cable I can't detect the disconnection.

score 1 · Accepted Answer · answered Apr 24 '12 at 14:14

You might want to apply "pinging" functionality, that will fail if there is TCP connection lose. Use this code to add extension method to Socket:

using System.Net.Sockets;

namespace Server.Sockets {
    public static class SocketExtensions {
        public static bool IsConnected(this Socket socket) {
            try {
                return !(socket.Poll(1, SelectMode.SelectRead) && socket.Available == 0);
            } catch(SocketException) {
                return false;
            }
        }
    }
}

Method will return false if there is no connection available. It should work to check if there is or no connection even if you had no SocketExceptions on Reveice / Send methods. Bear in mind that if you had exception that had error message that is related to connection lose, then you don't need check for connection anymore.
This method is meant to be used when socket is looks like connected but might be not like in your case.

Usage:

if (!socket.IsConnected()) {
    /* socket is disconnected */
}

No, it didnt't work. I can detect my cable unplug event but i can't detect tcp client's unplug event. — sanchop22, Apr 24 '12 at 14:21

score 0 · Answer 2 · answered Apr 24 '12 at 13:37

0

Try the NetworkAvailabilityChanged event.

answered Apr 24 '12 at 13:37

Eli Arbel

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It worked for cable unplug of server. How can i detect cable unplug of a client? – sanchop22 Apr 24 '12 at 13:46
1

@petre you cannot. there is no way for the server to know what's physically going on at the client. the only way is to ping it and see if it responds. or you could try sending a message from the client when the CableUnplugged event occurs to inform the server about it ;-) – Stefan Paul Noack Apr 24 '12 at 14:20

score 0 · Answer 3 · edited May 23 '17 at 12:15

I found this method here. It checks the different states of the connection and signals a disconnect. But does not detect an unplugged cable. After a further search and trial and error this is how I solved it finally.

As the Socket parameter I use on the server side the client socket from the accepted connection and on the client side the client that connected to the server.

public bool IsConnected(Socket socket)    
{
    try
    {
        // this checks whether the cable is still connected 
        // and the partner pc is reachable
        Ping p = new Ping();

        if (p.Send(this.PartnerName).Status != IPStatus.Success)
        {
            // you could also raise an event here to inform the user
            Debug.WriteLine("Cable disconnected!");
            return false;
        }

        // if the program on the other side went down at this point
        // the client or server will know after the failed ping 
        if (!socket.Connected)
        {
            return false;
        }

        // this part would check whether the socket is readable it reliably
        // detected if the client or server on the other connection site went offline
        // I used this part before I tried the Ping, now it becomes obsolete
        // return !(socket.Poll(1, SelectMode.SelectRead) && socket.Available == 0);

    }
    catch (SocketException) { return false; }
}

score 0 · Answer 4 · answered May 08 '19 at 13:44

This problem can also be resolved by setting the KeepAlive socket option like this:

   socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true);
   socket.SetKeepAliveValues(new SocketExtensions.KeepAliveValues
   {
       Enabled = true,
       KeepAliveTimeMilliseconds = 9000,
       KeepAliveIntervalMilliseconds = 1000
   });

These options can be tweaked to set how often checks are done to ensure the connection is valid. The sending of the Tcp KeepAlive will trigger the socket itself to detect the disconnect of the network cable.

Listening for an Ethernet Cable Unplugging Event for a TCP Server Application

4 Answers4

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