On Mon, 19 Sep 2022, Dmitry Karpov via curl-library wrote:
Working with IPv6-enabled dual-stack libcurl, I noticed that for some Linux kernel configurations, it takes ~15-30ms more for dual-stack libcurl to establish IPv4 connections than for IPv4 single-stack libcurl. After looking into this issue, it turned out that the regression was caused by Curl_ipv6works() function enabled in IPv6-on builds.
The origin for that IPv6 check was that back in the 90s it happened that configure detected IPv6 support but then at runtime curl couldn't actually use it.
I dare to suggest that we don't see that kind of system setups and behavior anymore. I think maybe the time has come to make instead rather assume that it works when built IPv6 enabled. At least I think it is worth exploring that route first before we consider adding a new API function for it.
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