Comment 6 for bug 1732002

After some discussion, it appears that networkd is kinda of making a boolean (on|off) for IPV6 RA, when it's really a tri-state (on|off|kernel).

Upstream networkd indicates there is a tri-state; like so:

Enable or disable IPv6 Router Advertisement (RA) reception support for the interface. Takes a boolean parameter. If true, RAs are accepted; if false, RAs are ignored, independently of the local forwarding state. When not set, the kernel default is used, and RAs are accepted only when local forwarding is disabled for that interface. When RAs are accepted, they may trigger the start of the DHCPv6 client if the relevant flags are set in the RA data, or if no routers are found on the link.

While in netplan, we've only a boolean, which could be fine, except netplan defaults to AcceptRA=True which means we have no way of allowing the kernel configuration to work.

Netplan needs to know if the yaml includes an accept-ra key, and if so, uses the value set (off or on); but if the yaml does not specify an accept-ra key, it should *NOT* render a default value.

This allows hosts to defer the the kernel settings. This key was introduced as away to resolve a bug where "unconfigured" interfaces got an IPV6 address due to kernel setting and an IPV6 router present.

https://bugs.launchpad.net/maas/+bug/1655440