Comment by conradludgate

We enabled ipv6 for our APIs at work. Nothing broke immediately, but we've had a steady stream of unreachable host errors related to ipv6 since then.

Turns out there're many incorrect implementations of Happy Eyeballs that cancel the ipv4 connection attempts after the timeout, and then switch to trying the AAAA records and subsequently throwing a "Cannot reach host" error. For context, in Happy Eyeballs you're supposed to continue trying both network families in parallel.

This only impacts our customers who live far away from the region they're accessing, however, and there's usually a workaround - in Node you can force the network family to be v4 for instance