Instantly detect client disconnection from server socket

Question

How can I detect that a client has disconnected from my server   I have the following code in my AcceptCallBack method  static Socket handler   null  public static void AcceptCallback IAsyncResult ar        Accept incoming connection   Socket listener    Socket ar AsyncState    handler   listener EndAccept ar       I need to find a way to discover as soon as possible that the client has disconnected from the handler Socket   I ve tried    handler Available  handler Send new byte 1   0  SocketFlags None   handler Receive new byte 1   0  SocketFlags None     The above approaches work when you are connecting to a server and want to detect when the server disconnects but they do not work when you are the server and want to detect client disconnection   Any help will be appreciated

User · Answer

You can also check the  IsConnected property of the socket if you were to poll

User · Answer

I ve found quite useful  another workaround for that   If you use asynchronous methods for reading data from the network socket  I mean  use BeginReceive - EndReceive  methods   whenever a connection is terminated  one of these situations appear  Either a message is sent with no data  you can see it with Socket Available - even though BeginReceive is triggered  its value will be zero  or Socket Connected value becomes false in this call  don t try to use EndReceive then    I m posting the function I used  I think you can see what I meant from it better     private void OnRecieve IAsyncResult parameter         Socket sock    Socket parameter AsyncState      if  sock Connected    sock Available    0                   Connection is terminated  either by force or willingly         return             sock EndReceive parameter       sock BeginReceive                       new AsyncCallback OnRecieve   sock           To handle further commands sent by client               zones might change in your code

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That s just the way TCP works and you have to live with it    Yup  you re right  It s a fact of life I ve come to realize  You will see the same behavior exhibited even in professional applications utilizing this protocol  and even others   I ve even seen it occur in online games  you re buddy says  goodbye   and he appears to be online for another 1-2 minutes until the server  cleans house    You can use the suggested methods here  or implement a  heartbeat   as also suggested  I choose the former  But if I did choose the latter  I d simply have the server  ping  each client every so often with a single byte  and see if we have a timeout or no response  You could even use a background thread to achieve this with precise timing  Maybe even a combination could be implemented in some sort of options list  enum flags or something  if you re really worried about it  But it s no so big a deal to have a little delay in updating the server  as long as you DO update  It s the internet  and no one expects it to be magic

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Implementing heartbeat into your system might be a solution  This is only possible if both client and server are under your control  You can have a DateTime object keeping track of the time when the last bytes were received from the socket  And assume that the socket not responded over a certain interval are lost  This will only work if you have heartbeat custom keep alive implemented

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Expanding on comments by mbargiel and mycelo on the accepted answer  the following can be used with a non-blocking socket on the server end to inform whether the client has shut down  This approach does not suffer the race condition that affects the Poll method in the accepted answer     Determines whether the remote end has called Shutdown public bool HasRemoteEndShutDown       get               try                       int bytesRead   socket Receive new byte 1   SocketFlags Peek                if  bytesRead    0                  return true                    catch                          For a non-blocking socket  a SocketException with                 code 10035  WSAEWOULDBLOCK  indicates no data available                     return false           The approach is based on the fact that the Socket Receive method returns zero immediately after the remote end shuts down its socket and we ve read all of the data from it   From Socket Receive documentation   If the remote host shuts down the Socket connection with the Shutdown method  and all available data has been received  the Receive method will complete immediately and return zero bytes  If you are in non-blocking mode  and there is no data available in the protocol stack buffer  the Receive method will complete immediately and throw a SocketException   The second point explains the need for the try-catch  Use of the SocketFlags Peek flag leaves any received data untouched for a separate receive mechanism to read  The above will work with a blocking socket as well  but be aware that the code will block on the Receive call  until data is received or the receive timeout elapses  again resulting in a SocketException

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This worked for me  the key is you need a separate thread to analyze the socket state with polling  doing it in the same thread as the socket fails detection     open or receive a server socket - TODO your code here socket   new Socket           enable the keep alive so we can detect closure socket SetSocketOption SocketOptionLevel Socket  SocketOptionName KeepAlive  true      create a thread that checks every 5 seconds if the socket is still connected  TODO add your thread starting code void MonitorSocketsForClosureWorker         DateTime nextCheckTime   DateTime Now AddSeconds 5        while   exitSystem            if  nextCheckTime  lt  DateTime Now                try                   if  socket  null                        if socket Poll 5000  SelectMode SelectRead   amp  amp  socket Available    0                              socket not connected  close it if it s still running                         socket Close                            socket   null                            else                             socket still connected                                                         catch                  socket Close                  finally                   nextCheckTime   DateTime Now AddSeconds 5                                   Thread Sleep 1000

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The example code here http   msdn microsoft com en-us library system net sockets socket connected aspx shows how to determine whether the Socket is still connected without sending any data   If you called Socket BeginReceive   on the server program and then the client closed the connection  gracefully   your receive callback will be called and EndReceive   will return 0 bytes  These 0 bytes mean that the client  may  have disconnected  You can then use the technique shown in the MSDN example code to determine for sure whether the connection was closed

User · Answer

Can t you just use Select    Use select on a connected socket  If the select returns with your socket as Ready but the subsequent Receive returns 0 bytes that means the client disconnected the connection  AFAIK  that is the fastest way to determine if the client disconnected   I do not know C  so just ignore if my solution does not fit in C   C  does provide select though  or if I had misunderstood the context

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Since there are no events available to signal when the socket is disconnected  you will have to poll it at a frequency that is acceptable to you   Using this extension method  you can have a reliable method to detect if a socket is disconnected   static class SocketExtensions     public static bool IsConnected this Socket socket          try             return   socket Poll 1  SelectMode SelectRead   amp  amp  socket Available    0             catch  SocketException    return false

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i had same problem   try this     void client handler Socket client     set  KeepAlive  true       while  true                try                       if  client Connected                                           else                  client disconnected                 break                                  catch  Exception                        client Poll 4000  SelectMode SelectRead     try to get state

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This is in VB  but it seems to work well for me   It looks for a 0 byte return like the previous post   Private Sub RecData ByVal AR As IAsyncResult      Dim Socket As Socket   AR AsyncState      If Socket Connected   False And Socket Available   False Then         Debug Print  Detected Disconnected Socket -     Socket RemoteEndPoint ToString          Exit Sub     End If     Dim BytesRead As Int32   Socket EndReceive AR      If BytesRead   0 Then         Debug Print  Detected Disconnected Socket - Bytes Read   0 -     Socket RemoteEndPoint ToString          UpdateText  Client     Socket RemoteEndPoint ToString     has disconnected from Server            Socket Close           Exit Sub     End If     Dim msg As String   System Text ASCIIEncoding ASCII GetString ByteData      Erase ByteData     ReDim ByteData 1024      ClientSocket BeginReceive ByteData  0  ByteData Length  SocketFlags None  New AsyncCallback AddressOf RecData   ClientSocket      UpdateText msg  End Sub

User · Answer

This is simply not possible   There is no physical connection between you and the server  except in the extremely rare case where you are connecting between two compuers with a loopback cable      When the connection is closed gracefully  the other side is notified   But if the connection is disconnected some other way  say the users connection is dropped  then the server won t know until it times out  or tries to write to the connection and the ack times out    That s just the way TCP works and you have to live with it   Therefore   instantly  is unrealistic   The best you can do is within the timeout period  which depends on the platform the code is running on   EDIT  If you are only looking for graceful connections  then why not just send a  DISCONNECT  command to the server from your client

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Using the method SetSocketOption  you will be able to set KeepAlive that will let you know whenever a Socket gets disconnected  Socket  connectedSocket   this  sSocketEscucha EndAccept asyn                    connectedSocket SetSocketOption SocketOptionLevel Socket  SocketOptionName KeepAlive  1     http   msdn microsoft com en-us library 1011kecd v VS 90  aspx  Hope it helps  Ramiro Rinaldi

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Someone mentioned keepAlive capability of TCP Socket  Here it is nicely described  http   tldp org HOWTO TCP-Keepalive-HOWTO overview html I m using it this way  after the socket is connected  I m calling this function  which sets keepAlive on  The keepAliveTime parameter specifies the timeout  in milliseconds  with no activity until the first keep-alive packet is sent  The keepAliveInterval parameter specifies the interval  in milliseconds  between when successive keep-alive packets are sent if no acknowledgement is received      void SetKeepAlive bool on  uint keepAliveTime  uint keepAliveInterval                int size   Marshal SizeOf new uint              var inOptionValues   new byte size   3            BitConverter GetBytes  uint  on   1   0   CopyTo inOptionValues  0           BitConverter GetBytes  uint keepAliveTime  CopyTo inOptionValues  size           BitConverter GetBytes  uint keepAliveInterval  CopyTo inOptionValues  size   2            socket IOControl IOControlCode KeepAliveValues  inOptionValues  null          I m also using asynchronous reading  socket BeginReceive packet dataBuffer  0  128                      SocketFlags None  new AsyncCallback OnDataReceived   packet    And in callback  here is caught timeout SocketException  which raises when socket doesn t get ACK signal after keep-alive packet  public void OnDataReceived IAsyncResult asyn        try               SocketPacket theSockId    SocketPacket asyn AsyncState           int iRx   socket EndReceive asyn             catch  SocketException ex                SocketExceptionCaught ex            This way  I m able to safely detect disconnection between TCP client and server

[.net] Instantly detect client disconnection from server socket

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