Comment by cyberax

> The only ways you'll get multiple addresses for a given host are to either hard-code them, or use DNS

mDNS exists. As does SIP and SMB.

I first discovered the ULA/GUA issue when my TimeMachine backup dropped mid-session when I was testing Internet connection failover. Apple later fixed it by allowing multiple SMB sessions in parallel and preferring IPv4 connections. I have a pure IPv6-only network, as I said.

The same issue happens with SIP, as phones send their own addresses during the media connection negotiation.

(I have another funny story about SIP: Asterisk used to send a Cartesian product of available codecs and IP addresses, so if you had IPv6 privacy addresses on, it eventually accumulated enough of them to overflow the MTU, causing connections to silently drop)