Can Windows Socket #10060 error (WSAETIMEDOUT - An attempt to connect timed out without establishing a connection) be a programming error at all?

I have Delphi application that uses Indy HTTP components (that uses Windows socket). And from time to time I am receiving #10060 socket error (WSAETIMEDOUT - An attempt to connect timed out without establishing a connection) upon execution of Indy procedure:

CheckForSocketError(IdWinsock2.Connect(ASocket, @LAddr, SizeOf(LAddr)));
...
connect : TconnectProc;
...
TconnectProc = function ( const s: TSocket; const name: PSockAddr; const namelen: Integer): Integer; stdcall;

Actually all this is just wrap around Windows connect function https://learn.microsoft.com/en-us/windows/win32/api/winsock2/nf-winsock2-connect that gives error and message WSAETIMEDOUT. So - my question is - can this be programming error at all? Even if I have server running on the other computer and even that server has trouble serving request, even in such cases the low level connect should execute normally, if the server can not serve the GET/POST request, then, of course, the errors should be but those errors should spring in only during execution of other Socket functions not in the connect function, isn't so?

I am trying to solve my problem https://serverfault.com/questions/973648/is-it-possible-that-unencrypted-traffic-can-cause-windows-socket-10057-10060?noredirect=1#comment1266907_973648 and now I am seeking whats happening in my code.

My server side code is very simple - it is just TIdHttpServer component with implemented (I provide event name only here):

MyForm.IdHTTPServerCommandGet(AContext: TIdContext;
  ARequestInfo: TIdHTTPRequestInfo; AResponseInfo: TIdHTTPResponseInfo);

So - what can be worng with my implemention, what can lead to the appearance of WSAETIMEDOUT for connect? Yes, my procedure can be long sometimes, but it for years it returned the answer sucessfully and there were no communication errors. And I guess, that connect function can even not depend (does not use/raise) OnCommandGet event, so, I have no control how the server side handles socket connect function from the client?

It may be possible that this is connected with TCP (not HTTP) keepalive, maybe some Windows updates have reduced client-side settings of Windows TCP keepalive for the clients and now this manifests as such error.

Solution

Indy TCP clients, like TIdHTTP, have a public ConnectTimeout property, which is set to 0 (infinite) by default. If no timeout is specified, a hard-coded 2 minute timeout is used if the client's TIdTCPClient.Connect() method is called in the main UI thread and TIdAntiFreeze is active, otherwise no timeout is used.

If a timeout is used, Indy calls Winsock's connect() function in a worker thread and waits for that thread to terminate. If Indy's timeout elapses, the socket is closed to abort the connect(), and then EIdConnectTimeout is raised to the caller. If connect() exits before Indy's timeout elapses, an exception is raised to the caller only if connect() failed.

If no timeout is used, Indy calls Winsock's connect() directly, waits for it to exit on its own accord, and then raises an exception only if failed.

So, the ONLY way you can get a WSATIMEDOUT error from Indy when it is calling Winsock's connect() function is if Winsock itself timed out internally before Indy's own timeout elapses. That does not necessarily indicate a problem in your code. It just means that the Host you are trying to connect to is simply not reachable at that moment in time. If the server were reachable, but could not accept your connection, you would get a different error, such as WSAECONNREFUSED.

If your server is behind a firewall or router, make sure it is not blocking connections from reaching your server. Try running a packet sniffer on the server machine, such as WireShark, and make sure the 3-way TCP handshake from TIdHTTP is reaching the server machine correctly.