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On the UDF, one feature I like is immediately knowing the IP/port of an incoming client. I've been looking at implementing application level IP filtering. It also looks like TCPRecv will actually respond correctly if the connection is closed. As I recall the built in TCPRecv has been broken in this regard for years.



TCP Keepalive

I've run into problems where idle connections are dropped from stateful firewalls. Usually after an hour or so, or they drop off a list as being unused (if there's a maximum number of connections). The socket will still appear to be active on both sides, but if the server tries to send data it will fail and close the connection on that side. The client will still think it's connected until it tries to send data. As well if the server PC were to crash, the client would never know. Ideally there you should have an application level Keepalive, for example Telnet command 0xFFF1, sent from both sides periodically to determine if the link is down, and to keep the connection active in any intermediate routers and firewalls.


The TCP stack has an "SO_KEEPALIVE" option. An example of an application that can use this is the "Connection" configuration for PuTTY.  The time for no activity before firing a keepalive is set by the registry entry "KeepAliveTime". The default is 2 hours which is too long, 300,000ms (5 minutes)  is a good recommendation. I wouldn't want to venture below 1 minute. These keepalives should keep your connection active in any firewalls, and will alert your application if the connection is broken. This option is off by default and must be enabled on a per connection basis.


Here's a "_TCPKeepAlive()" function that's a good fit for this UDF. @j0kky's use of Winsock API let me bluff my way around DllCall.

; #FUNCTION# ====================================================================================================================
; Name...........: _TCPKeepAlive
; Description ...: Set SO_KEEPALIVE Socket Option to enable stack level keepalive
; Syntax.........: _TCPKeepAlive($iMainsocket)
; Parameters ....: $iMainsocket - The array as returned by _TCPAccept
;                                 or the connected socket identifier (SocketID) as returned by _TCPListen and _TCPConnect.
; Return values .: On success it returns:
;                  |0  - Success.
;                  On failure it returns -1 and sets @error to non zero:
;                  |-1 - internal error
;                  |-2 - missing DLL (Ws2_32.dll)
;                  |-3 - undefined error
;                  |Any Windows Socket Error Code retrieved by WSAGetLastError
; Author ........: TurionAltec
; Modified ......: 0.0.1
; Remarks .......: KeepAlive will send a keepalive packet without sending any data.
;                  Some Firewalls may drop idle connections which can cause problems if a connection is quiet for an hour+
;                  Can also be used to detect if the other end of a connection was dropped (computer abruptly powered off)
;                  Delay with no traffic before sending packet set globally from registry key:
;                  HKLM\System\CurrentControlSet\Services\Tcpip\Parameters\KeepAliveTime
;                  Default is 7200000 (1 hour) Recommended is 300000 (5 minutes)
;                  "On Windows Vista and later, the SO_KEEPALIVE socket option can only be set using the setsockopt function
;                  when the socket is in a well-known state not a transitional state."
; Links .........: SO_KEEPALIVE:            https://msdn.microsoft.com/en-us/library/windows/desktop/ee470551(v=vs.85).aspx
;                  setsockopt function:     https://msdn.microsoft.com/en-us/library/windows/desktop/ms740476(v=vs.85).aspx
;                  KeepAliveTime Reg Entry: https://technet.microsoft.com/en-us/library/dd349797(v=ws.10).aspx#BKMK_2
; ===============================================================================================================================
Func _TCPKeepAlive($iMainsocket)
    If IsArray($iMainsocket) And (UBound($iMainsocket, 0) = 1) And (UBound($iMainsocket) > 0) Then $iMainsocket = $iMainsocket[0]
    Local $SOL_SOCKET = 0xffff
    Local $SO_KEEPALIVE = 0x0008
    Local $hWs2 = DllOpen("Ws2_32.dll")
    If @error Then Return SetError(-2, 0, -1) ;missing DLL
    Local $bError = 0, $nCode = 0, $nExtended = 0

    If Not $bError Then
        $aRet = DllCall($hWs2, "int", "setsockopt", "uint", $iMainsocket, "int", $SOL_SOCKET, "int", $SO_KEEPALIVE, "int*", 1, "int", 4)
        ;MSDN calls for "char*" not "int*", but DLLCall returns @Error=1 for char
        If @error Then
            $bError = -1
        ElseIf $aRet[0] <> 0 Then ;SOCKET_ERROR
            $bError = 1
    If $bError < 0 Then
        $nCode = -1 ;internal error
        $nReturn = -1 ;failure

    ElseIf $bError > 0 Then
        If Not $nCode Then
            $aRet = DllCall($hWs2, "int", "WSAGetLastError")
            If @error Then
                $nCode = -1
                $nCode = $aRet[0]
            If $nCode = 0 Then $nCode = -3 ;undefined error
        If $nCode = -10 Then $nCode = 0
        $nReturn = -1

        $nReturn = $aRet
    Return SetError($nCode, 0, $nReturn)

EndFunc   ;==>_TCPKeepAlive


Windows Registry Editor Version 5.00


;Hex 493e0 = Decimal 300000
;300,000 ms = 5 minutes

I call _TCPKeepAlive() after _TCPAccept(), or _TCPConnect() return a good socket identifier. Note that MSDN says the sockets must not be in a transitional state…

I found the included example scripts a little wonky, but below is a Wireshark Trace. I had sent the KeepAliveTime to 10 seconds for demonstration on the server,



"A TCP keep-alive packet is simply an ACK with the sequence number set to one less than the current sequence number for the connection. A host receiving one of these ACKs will respond with an ACK for the current sequence number."

Now if only I can figure out how to set keepalive on sockets owned by another application!

Edited by TurionAltec

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could this be extended to utilize the setsockopt winsock function to add to multicast groups and enable sending multicast packets? I'd be very interested in that.

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