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.TH CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS 3 "1 Feb 2018" libcurl libcurl
.SH NAME
CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS \- head start for IPv6 for happy eyeballs
.SH SYNOPSIS
.nf
#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS,
                          long timeout);
.fi
.SH DESCRIPTION
Happy eyeballs is an algorithm that attempts to connect to both IPv4 and IPv6
addresses for dual-stack hosts, preferring IPv6 first for \fItimeout\fP
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 \fItimeout\fP 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.
.SH DEFAULT
CURL_HET_DEFAULT (currently defined as 200L)
.SH PROTOCOLS
All except FILE
.SH EXAMPLE
.nf
CURL *curl = curl_easy_init();
if(curl) {
  curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
  curl_easy_setopt(curl, CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS, 300L);

  curl_easy_perform(curl);

  /* always cleanup */
  curl_easy_cleanup(curl);
}
.fi
.SH AVAILABILITY
Added in 7.59.0
.SH RETURN VALUE
Returns CURLE_OK
.SH SEE ALSO
.BR CURLOPT_CONNECTTIMEOUT_MS "(3), "
.BR CURLOPT_TIMEOUT "(3), " CURLOPT_LOW_SPEED_LIMIT "(3), "