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 <dan...@haxx.se> Sendt: fredag 22. august 2025 16:57 Til: Lars Sunde via curl-library <curl-library@lists.haxx.se> Kopi: Lars Sunde <laasu...@hotmail.com> 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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