Detecting TCP Client Disconnect

Question

Let s say I m running a simple server and have accept  ed a connection from a client   What is the best way to tell when the client has disconnected  Normally  a client is supposed to send a close command  but what if it disconnects manually or loses network connection altogether  How can the server detect or handle this

User · Accepted Answer

select  with the read mask set  will return with the handle signalled  but when you use ioctl  to check the number of bytes pending to be read  it will be zero  This is a sign that the socket has been disconnected   This is a great discussion on the various methods of checking that the client has disconnected  Stephen Cleary  Detection of Half-Open  Dropped  Connections     for Windows use ioctlsocket

User · Answer

TCP has  open  and a  close  procedures in the protocol   Once  opened   a connection is held until  closed   But there are lots of things that can stop the data flow abnormally   That being said  the techniques to determine if it is possible to use a link are highly dependent on the layers of software between the protocol and the application program   The ones mentioned above focus on a programmer attempting to use a socket in a non-invasive way  read or write 0 bytes  are perhaps the most common   Some layers in libraries will supply the  polling  for a programmer   For example Win32 asych  delayed  calls can Start a read that will return with no errors and 0 bytes to signal a socket that cannot be read any more  presumably a TCP FIN procedure    Other environments might use  events  as defined in their wrapping layers   There is no single answer to this question   The mechanism to detect when a socket cannot be used and should be closed depends on the wrappers supplied in the libraries   It is also worthy to note that sockets themselves can be reused by layers below an application library so it is wise to figure out how your environment deals with the Berkley Sockets interface

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tcp disconnect py Echo network data test program in python  This easily translates to C  amp  Java   A server program might want to confirm that a tcp client is still connected  before it sends a data  That is  detect if its connected without reading from socket  This will demonstrate how to detect a TCP client disconnect without reading data   The method to do this  1  select on socket as poll  no wait  2  if no recv data waiting  then client still connected 3  if recv data waiting  the read one char using PEEK flag  4  if PEEK data len 0  then client has disconnected  otherwise its connected  Note  the peek flag will read data without removing it from tcp queue   To see it in action  0  run this program on one computer 1  from another computer   connect via telnet port 12345  2  type a line of data 3  wait to see it echo   4  type another line  5  disconnect quickly  6  watch the program will detect the  disconnect and exit   John Masinter  17-Dec-2008      import socket import time import select  HOST              all local interfaces PORT   12345      port to listen    listen for new TCP connections s   socket socket socket AF INET  socket SOCK STREAM  s setsockopt socket SOL SOCKET  socket SO REUSEADDR  1  s bind  HOST  PORT   s listen 1    accept new conneciton conn  addr   s accept   print  Connected by   addr   loop reading echoing  until client disconnects try      conn send  Send me data  and I will echo it back after a short delay  n       while 1          data   conn recv 1024                             recv all data queued         if not data  break                                client disconnected         time sleep 3                                      simulate time consuming work           below will detect if client disconnects during sleep         r  w  e   select select  conn           0         more data waiting          print  select  r  s w  s e  s     r w e           debug output to command line         if r                                              yes  data avail to read              t   conn recv 1024  socket MSG PEEK           read without remove from queue             print  peek  len  d  data  s     len t  t     debug output             if len t   0                                  length of data peeked 0                  print  Client disconnected                client disconnected                 break                                     quit program         conn send  -- gt   data                              echo only if still connected finally      conn close

User · Answer

In python you can do a try-except statement like this   try    conn send   you can send anything to check connection    except BrokenPipeError    print  Client has Disconnected     This works because when the client server closes the program  python returns broken pip error to the server or client depending on who it was that disconnected

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It s really easy to do  reliable and not messy            Try             Clients Client Send BufferByte          Catch verror As Exception             BufferString   verror ToString         End Try         If BufferString  lt  gt     Then             EventLog Text  amp    User disconnected      vbNewLine             Clients Close           End If

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I toyed with a few solutions but this one seems to work best for detecting host and or client disconnection in Windows  It is for non-blocking sockets  and derived from IBM s example   char buf  int length recv socket   amp buf  0  0   int nError WSAGetLastError    if nError  WSAEWOULDBLOCK amp  amp nError  0       return 0       if  nError  0       if  length  0  return 0

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If you re using overlapped  i e  asynchronous  I O with completion routines or completion ports  you will be notified immediately  assuming you have an outstanding read  when the client side closes the connection

User · Answer

To expand on this a bit more   If you are running a server you either need to use TCP KEEPALIVE to monitor the client connections  or do something similar yourself  or have knowledge about the data protocol that you are running over the connection   Basically  if the connection gets killed  i e  not properly closed  then the server won t notice until it tries to write something to the client  which is what the keepalive achieves for you  Alternatively  if you know the protocol better  you could just disconnect on an inactivity timeout anyway

User · Answer

It s really easy to do  reliable and not messy            Try             Clients Client Send BufferByte          Catch verror As Exception             BufferString   verror ToString         End Try         If BufferString  lt  gt     Then             EventLog Text  amp    User disconnected      vbNewLine             Clients Close           End If

User · Answer

In TCP there is only one way to detect an orderly disconnect  and that is by getting zero as a return value from read   recv   recvXXX   when reading   There is also only one reliable way to detect a broken connection  by writing to it  After enough writes to a broken connection  TCP will have done enough retries and timeouts to know that it s broken and will eventually cause write   send   sendXXX   to return -1 with an errno WSAGetLastError   value of ECONNRESET  or in some cases  connection timed out   Note that the latter is different from  connect timeout   which can occur in the connect phase   You should also set a reasonable read timeout  and drop connections that fail it   The answer here about ioctl   and FIONREAD is compete nonsense  All that does is tell you how many bytes are presently in the socket receive buffer  available to be read without blocking  If a client doesn t send you anything for five minutes that doesn t constitute a disconnect  but it does cause FIONREAD to be zero  Not the same thing  not even close

User · Answer

If you re using overlapped  i e  asynchronous  I O with completion routines or completion ports  you will be notified immediately  assuming you have an outstanding read  when the client side closes the connection

User · Answer

TCP has  open  and a  close  procedures in the protocol   Once  opened   a connection is held until  closed   But there are lots of things that can stop the data flow abnormally   That being said  the techniques to determine if it is possible to use a link are highly dependent on the layers of software between the protocol and the application program   The ones mentioned above focus on a programmer attempting to use a socket in a non-invasive way  read or write 0 bytes  are perhaps the most common   Some layers in libraries will supply the  polling  for a programmer   For example Win32 asych  delayed  calls can Start a read that will return with no errors and 0 bytes to signal a socket that cannot be read any more  presumably a TCP FIN procedure    Other environments might use  events  as defined in their wrapping layers   There is no single answer to this question   The mechanism to detect when a socket cannot be used and should be closed depends on the wrappers supplied in the libraries   It is also worthy to note that sockets themselves can be reused by layers below an application library so it is wise to figure out how your environment deals with the Berkley Sockets interface

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The return value of receive will be -1 if connection is lost else it will be size of buffer   void ReceiveStream void  threadid        while true                while ch  0                        char buffer 1024               int newData              newData   recv thisSocket  buffer  sizeof buffer   0               if newData gt  0                                std  cout  lt  lt  buffer  lt  lt  std  endl                            else                               std  cout  lt  lt   Client disconnected   lt  lt  std  endl                  if  thisSocket                                         ifdef WIN32                         closesocket thisSocket                           WSACleanup                         endif                      ifdef LINUX                         close thisSocket                        endif                                   break                                  ch   1          StartSocket

User · Answer

I toyed with a few solutions but this one seems to work best for detecting host and or client disconnection in Windows  It is for non-blocking sockets  and derived from IBM s example   char buf  int length recv socket   amp buf  0  0   int nError WSAGetLastError    if nError  WSAEWOULDBLOCK amp  amp nError  0       return 0       if  nError  0       if  length  0  return 0

User · Answer

Try looking for EPOLLHUP or EPOLLERR   How do I check client connection is still alive  Reading and looking for 0 will work in some cases  but not all

User · Answer

If you re using overlapped  i e  asynchronous  I O with completion routines or completion ports  you will be notified immediately  assuming you have an outstanding read  when the client side closes the connection

User · Answer

In TCP there is only one way to detect an orderly disconnect  and that is by getting zero as a return value from read   recv   recvXXX   when reading   There is also only one reliable way to detect a broken connection  by writing to it  After enough writes to a broken connection  TCP will have done enough retries and timeouts to know that it s broken and will eventually cause write   send   sendXXX   to return -1 with an errno WSAGetLastError   value of ECONNRESET  or in some cases  connection timed out   Note that the latter is different from  connect timeout   which can occur in the connect phase   You should also set a reasonable read timeout  and drop connections that fail it   The answer here about ioctl   and FIONREAD is compete nonsense  All that does is tell you how many bytes are presently in the socket receive buffer  available to be read without blocking  If a client doesn t send you anything for five minutes that doesn t constitute a disconnect  but it does cause FIONREAD to be zero  Not the same thing  not even close

User · Answer

To expand on this a bit more   If you are running a server you either need to use TCP KEEPALIVE to monitor the client connections  or do something similar yourself  or have knowledge about the data protocol that you are running over the connection   Basically  if the connection gets killed  i e  not properly closed  then the server won t notice until it tries to write something to the client  which is what the keepalive achieves for you  Alternatively  if you know the protocol better  you could just disconnect on an inactivity timeout anyway

User · Answer

tcp disconnect py Echo network data test program in python  This easily translates to C  amp  Java   A server program might want to confirm that a tcp client is still connected  before it sends a data  That is  detect if its connected without reading from socket  This will demonstrate how to detect a TCP client disconnect without reading data   The method to do this  1  select on socket as poll  no wait  2  if no recv data waiting  then client still connected 3  if recv data waiting  the read one char using PEEK flag  4  if PEEK data len 0  then client has disconnected  otherwise its connected  Note  the peek flag will read data without removing it from tcp queue   To see it in action  0  run this program on one computer 1  from another computer   connect via telnet port 12345  2  type a line of data 3  wait to see it echo   4  type another line  5  disconnect quickly  6  watch the program will detect the  disconnect and exit   John Masinter  17-Dec-2008      import socket import time import select  HOST              all local interfaces PORT   12345      port to listen    listen for new TCP connections s   socket socket socket AF INET  socket SOCK STREAM  s setsockopt socket SOL SOCKET  socket SO REUSEADDR  1  s bind  HOST  PORT   s listen 1    accept new conneciton conn  addr   s accept   print  Connected by   addr   loop reading echoing  until client disconnects try      conn send  Send me data  and I will echo it back after a short delay  n       while 1          data   conn recv 1024                             recv all data queued         if not data  break                                client disconnected         time sleep 3                                      simulate time consuming work           below will detect if client disconnects during sleep         r  w  e   select select  conn           0         more data waiting          print  select  r  s w  s e  s     r w e           debug output to command line         if r                                              yes  data avail to read              t   conn recv 1024  socket MSG PEEK           read without remove from queue             print  peek  len  d  data  s     len t  t     debug output             if len t   0                                  length of data peeked 0                  print  Client disconnected                client disconnected                 break                                     quit program         conn send  -- gt   data                              echo only if still connected finally      conn close

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The return value of receive will be -1 if connection is lost else it will be size of buffer   void ReceiveStream void  threadid        while true                while ch  0                        char buffer 1024               int newData              newData   recv thisSocket  buffer  sizeof buffer   0               if newData gt  0                                std  cout  lt  lt  buffer  lt  lt  std  endl                            else                               std  cout  lt  lt   Client disconnected   lt  lt  std  endl                  if  thisSocket                                         ifdef WIN32                         closesocket thisSocket                           WSACleanup                         endif                      ifdef LINUX                         close thisSocket                        endif                                   break                                  ch   1          StartSocket

User · Answer

Try looking for EPOLLHUP or EPOLLERR   How do I check client connection is still alive  Reading and looking for 0 will work in some cases  but not all

User · Answer

If you re using overlapped  i e  asynchronous  I O with completion routines or completion ports  you will be notified immediately  assuming you have an outstanding read  when the client side closes the connection

User · Answer

In python you can do a try-except statement like this   try    conn send   you can send anything to check connection    except BrokenPipeError    print  Client has Disconnected     This works because when the client server closes the program  python returns broken pip error to the server or client depending on who it was that disconnected

User · Answer

To expand on this a bit more   If you are running a server you either need to use TCP KEEPALIVE to monitor the client connections  or do something similar yourself  or have knowledge about the data protocol that you are running over the connection   Basically  if the connection gets killed  i e  not properly closed  then the server won t notice until it tries to write something to the client  which is what the keepalive achieves for you  Alternatively  if you know the protocol better  you could just disconnect on an inactivity timeout anyway

User · Answer

To expand on this a bit more   If you are running a server you either need to use TCP KEEPALIVE to monitor the client connections  or do something similar yourself  or have knowledge about the data protocol that you are running over the connection   Basically  if the connection gets killed  i e  not properly closed  then the server won t notice until it tries to write something to the client  which is what the keepalive achieves for you  Alternatively  if you know the protocol better  you could just disconnect on an inactivity timeout anyway

[c++] Detecting TCP Client Disconnect

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