CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS(3) Library Functions Manual
NAME
CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS - timing of connect attempts
SYNOPSIS
#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS,
long timeout);
DESCRIPTION
Happy eyeballs is an algorithm that controls connecting to a host that
resolves to more than one IP address. A common setup is to expose an
IPv4 and IPv6 address (dual-stack). Other host offer a range of
addresses for one or both stacks.
IP Addresses
When curl is built with IPv6 support, it attempts to connect to
IPv6 first, when available. When that fails, another connect
attempt for the first IPv4 address (again, if available) is
started. Should that fail, the next IPv6 address is used, then
the next IPv4, etc. If there are only addresses for one stack,
those are tried one after the other.
When there is neither a positive nor negative response to an
attempt, another attempt is started after timeout has passed.
Then another, after timeout has passed again. As long as there
are addresses available.
When all addresses have been tried and failed, the transfer
fails. All attempts are aborted after
CURLOPT_CONNECTTIMEOUT_MS(3) has passed, counted from the first
attempt onward.
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.
As an example, for a host that resolves to 'a1_v4, a2_v4, a3_v6,
a4_v6' curl opens a socket to 'a3_v6' first. When that does not
report back, it opens another socket to 'a1_v4' after 200ms. The
first socket is left open and might still succeed. When 200ms
have gone by again, a socket for 'a4_v6' is opened. 200ms later,
'a2_v4' is tried.
At this point, there are 4 sockets open (unless the network has
reported anything back). That took 3 times the happy eyeballs
timeout, so 600ms in the default setting. When any of those four
report a success, that socket is used for the transfer and the
other three are closed.
There are situations where connect attempts fail, but the
failure is considered being inconclusive. The QUIC protocol may
encounter this. When a QUIC server restarts, it may send
replies indicating that it is not accepting new connections
right now, but maybe later.
Such "inclusive" connect attempt failures cause a restart of the
attempt, with the same address on a new socket, closing the
previous one. Repeatedly until CURLOPT_CONNECTTIMEOUT_MS(3)
strikes.
HTTPS When connection with the HTTPS protocol to a host that may talk
HTTP/3, HTTP/2 or HTTP/1.1, curl applies a similar happy
eyeballs strategy when attempting these versions.
When HTTPS only involves a TCP connection, the versions are
negotiated via ALPN, the TLS extension, in a single connect.
Since HTTP/3 runs on QUIC (which runs on UDP), it requires a
separate connect attempt.
The HTTP/3 attempt is started first and, after timeout expires,
the HTTP/2 (or 1.1) attempt is started in parallel.
DEFAULT
CURL_HET_DEFAULT (currently defined as 200L)
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
int main(void)
{
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);
}
}
AVAILABILITY
Added in curl 7.59.0
RETURN VALUE
curl_easy_setopt(3) returns a CURLcode indicating success or error.
CURLE_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3).
SEE ALSO
CURLOPT_CONNECTTIMEOUT_MS(3), CURLOPT_LOW_SPEED_LIMIT(3),
CURLOPT_TIMEOUT(3)
libcurl 2026-03-23
CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS(3)
curl 8.19.0 - Generated Fri Mar 27 13:49:27 CDT 2026