Sv: ipv6 connect timeout

Thanks for the link to the blog which explain part of the problem. However, when running libCurl 8.5 in a VM on Windows 10 we sometimes see that "Trying [::1]:8010..." takes upto 10 seconds to complete which is quite a long time. We have setup curl with CURLOPT_TIMEOUT to 600 seconds, CURLOPT_CONNECTTIMEOUT to 10 seconds and CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS to 200. Windows 10 has a "Max SYN Retransmissions" parameter which is set to 4 in our environment but that does not really explain the delay. You can see this parameter via "netsh interface tcp show global" command.

How is the CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS option implemented in curl? Does it rely on the OS functionality to respond within a timeframe or does it use an internal timer? Has there been any changes\bug-fixes to happy eyeballs algorithm between version 8.5 and 8.15?

Regards, Lars
________________________________
Fra: Daniel Stenberg <[email protected]>
Sendt: fredag 22. august 2025 16:57
Til: Lars Sunde via curl-library <[email protected]>
Kopi: Lars Sunde <[email protected]>
Emne: Re: ipv6 connect timeout

On Fri, 22 Aug 2025, Lars Sunde via curl-library wrote:

> We are running C++ software that uses libCurl 8.5 on Windows 10 and 11 that
> perform HTTP operations.
>
> During testing I notice a difference in some rare situations when running
> the software in Window 11 and in a virtual machine. The virtual machine is
> Oracle Virtual Box with Windows 11 host and Windows 10 guest OS).
>
> We enabled CURLOPT_VERBOSE to debug the issue when the software attempts to
> perform a HTTP post operation.

I suspect you are seeing Windows working as designed. I believe this is more
or less the same problem I blogged aobut last year:

  https://daniel.haxx.se/blog/2024/08/14/slow-tcp-connect-on-windows/

--

  / daniel.haxx.se || https://rock-solid.curl.dev

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