CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS − head start for ipv6 for happy eyeballs
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS, long timeout);
Happy eyeballs is an algorithm that attempts to connect to both IPv4 and IPv6 addresses for dual-stack hosts, preferring IPv6 first for timeout milliseconds. If the IPv6 address cannot be connected to within that time then a connection attempt is made to the IPv4 address in parallel. The first connection to be established is the one that is used.
The range of suggested useful values for timeout is limited. Happy Eyeballs RFC 6555 says "It is RECOMMENDED that connection attempts be paced 150-250 ms apart to balance human factors against network load." libcurl currently defaults to 200 ms. Firefox and Chrome currently default to 300 ms.
CURL_HET_DEFAULT (currently defined as 200L)
CURL *curl =
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
curl_easy_setopt(curl, CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS, 300L);
Added in 7.59.0