A brief public service announcement to internet experimenters. If your machine thinks it has a global IPv6 address, it had better work – it better be able to send & receive traffic at that address.
Because there will be some daemon (I’m looking at you, bind9 in Fedora 27), that will prefer it exclusively, even over a fully functional IPv4 interface address. And it when it tries to collect anything like, say, DNSSEC keys, this will fail with a timeout. That’ll lead to unverificable DNS records, which lead to a SERVFAIL on many inquiries and/or very slow service. What fun!
So, if you dabbled in one of the Hurricane Electric IPv6 trials, and might have left a network-scripts/ifcfg-FOO file around, or an interface tagged with that address – keep it online, or clean it completely out.