ipv6.method=shared is very limited
The ipv6.method=shared setup has a lot of limitations that make it not very useful:
- There's no ability to set a prefix-delegation hint size on DHCPv6 requests - this is required for some ISPs to get anything other than a /64 for example.
- There's no ability to set the size prefix to try to assign to the shared (downstream) interface; only assigning a /64 means that no further delegation is possible.
- There's no ability to set a hint for assigning prefixes (like I might want prefix 0 from the PD block to go to a specific interface).
- I think NM is running a router-advertisement service? The docs don't say, but I can't see anything else running to do it. There's no configurability for it if so.
- There are no IPv6 events sent to the dispatcher for the shared (downstream) interface, not even for adding the local IP (so can't do things like update dynamic DNS entries).
- There's no dispatcher event for prefix-delegation, so no ability to start up a DHCPv6 server for further prefix delegation.
- It doesn't look like any of the IPv6 state is saved in /var/lib/NetworkManager, which seems likely to break things across a service restart.