I'd say this is actually elegant, compared to NPTv6 which is a kludge and will break things (and isn't well-supported anyway).
For the most part it works today, if I stick to using ULA’s only in my zone file, and configure hosts to prefer the DHCPv6-provided ULA for connections in the ULA subnet, it’s fine. But suddenly if you connect to somehost.local instead of somehost.fqdn, the machine picks a GUA source address and you’re back to being unpredictable.
So although I say I want to use NPTv6 and be ULA-only, I don’t actually do that today, so I’m not super familiar with the downsides to the approach. But it does sound a lot cleaner to me in theory.
> if you're only using it for outbound connections then it changing isn't going to break anything.
A prefix change absolutely does break things in a lot of setups though. It happens something like:
- Your router reboots unexpectedly (no time to rescind the RA)
- Router comes up and gets a new prefix, starts advertising it
- Clients are brain dead and continue using the old prefix when making outbound connections.
I’ve had this happen and both Apple devices and Linux devices (I had no Windows machines) kept using the old prefix until I went around and rebooted them. So connecting to any IPv6 WAN address would fail, and only IPv4 was saving me from my internet being down until I went and manually rebooted everything.
There have since been RFC’s that come up with recommendations for routers to keep a stateful log of old prefixes, so that they can rescind them (advertise a zero TTL) when a new prefix arrives… but afaict none of them actually do this.