Our future IPv6 access control problems due to non-DHCP6 machines
Back almost two years ago, I wrote about how I suspected a lot of IPv6 hosts wouldn't have reverse DNS because they would be using stateless address autoconfiguration (SLAAC) where they essentially assign themselves one or more random IPv6 addresses when they show up on your network. For us, this presents a problem much larger than just DNS, because control over what hosts DHCP will give addresses to (and what addresses it will assign) are how we force machines to be registered on our laptop network and our wireless network before we give them network access.
The specific driver of IPv6 SLAAC is Android devices, which don't do DHCP6 at all; unfortunately this also includes ChromeOS, which means Chromebooks. But once you enable SLAAC on your network, any number of things may decide to grab themselves SLAAC addresses and then use them, even if they also do DHCP6 and so get whatever address you give them there (this is the iOS behavior I observed a couple of years ago; I don't know how Windows, macOS, and so on behave here). If the IPv6 address and routing they get via DHCP6 doesn't seem to work, I suspect that quite a lot of devices will be perfectly happy to route via their SLAAC address and route, and if that doesn't work, well, the Android and ChromeOS devices aren't getting on the Internet.
There are a number of approaches I can think of. One possible brute force answer is to simply not do SLAAC, only DHCP6 and (IPv4) DHCP. This would mean that SLAAC-only devices would only get IPv4 addresses, but that's not likely to be a practical problem for a long time to come. I think this is our most likely short term answer, because it's the easiest approach and we can always get more complicated later. The other brute force approach is some sort of MAC filtering on our firewalls, but we use OpenBSD and my understanding is that there are a number of issues around MAC filtering in OpenBSD PF.
The officially approved answer is probably to move to IEEE 802.1X on our networks that require this sort of access control. This is infeasible for multiple reasons, including that I believe it would require a wholesale replacement of our network switches on the affected networks. For extra bonus points we don't even run much of the infrastructure that provides our wireless network, which is one of the networks we need this access control on (this is not as crazy as it sounds, but that's another entry).
All of this is yet another reason why any migration to IPv6 will be neither fast nor easy for us, and thus why we still haven't done more than vaguely look in the direction of IPv6. Someday, maybe, when IPv6 appears to actually be important for something.
(And when we do start doing IPv6, it's highly likely to start out being only for a few servers with static IP addresses. Extending it to people's own 'client' devices is likely to be one of the last things we get around to.)