---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS
Section: 3
Source: libcurl
Protocol:
  - All
See-also:
  - CURLOPT_CONNECTTIMEOUT_MS (3)
  - CURLOPT_LOW_SPEED_LIMIT (3)
  - CURLOPT_TIMEOUT (3)
Added-in: 7.59.0
---

# NAME

CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS - timing of connect attempts

# SYNOPSIS

~~~c
#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 is a limit on the number of sockets opened for connect attempts. When
that limit is reached and more addresses are available, the oldest
attempt is discarded. This limit is currently 6. With the default
happy eyeballs timeout of 200ms, this closes attempts after 1.2 seconds
*as long as there are more addresses to try*.

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%

# EXAMPLE

~~~c
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%

# 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).
