1---
2c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
3SPDX-License-Identifier: curl
4Long: happy-eyeballs-timeout-ms
5Arg: <ms>
6Help: Time for IPv6 before IPv4
7Added: 7.59.0
8Category: connection
9Multi: single
10See-also:
11  - max-time
12  - connect-timeout
13Example:
14  - --happy-eyeballs-timeout-ms 500 $URL
15---
16
17# `--happy-eyeballs-timeout-ms`
18
19Happy Eyeballs is an algorithm that attempts to connect to both IPv4 and IPv6
20addresses for dual-stack hosts, giving IPv6 a head-start of the specified
21number of milliseconds. If the IPv6 address cannot be connected to within that
22time, then a connection attempt is made to the IPv4 address in parallel. The
23first connection to be established is the one that is used.
24
25The range of suggested useful values is limited. Happy Eyeballs RFC 6555 says
26"It is RECOMMENDED that connection attempts be paced 150-250 ms apart to
27balance human factors against network load." libcurl currently defaults to
28200 ms. Firefox and Chrome currently default to 300 ms.
29