1 Long: happy-eyeballs-timeout-ms 2 Arg: <milliseconds> 3 Help: Time for IPv6 before trying IPv4 4 Added: 7.59.0 5 Category: connection 6 --- 7 Happy eyeballs is an algorithm that attempts to connect to both IPv4 and IPv6 8 addresses for dual-stack hosts, preferring IPv6 first for the number of 9 milliseconds. If the IPv6 address cannot be connected to within that time then 10 a connection attempt is made to the IPv4 address in parallel. The first 11 connection to be established is the one that is used. 12 13 The range of suggested useful values is limited. Happy Eyeballs RFC 6555 says 14 "It is RECOMMENDED that connection attempts be paced 150-250 ms apart to 15 balance human factors against network load." libcurl currently defaults to 16 200 ms. Firefox and Chrome currently default to 300 ms. 17 18 If this option is used several times, the last one will be used. 19