Code review comment for ~harald-jensas/cloud-init:bug/1806014

Thanks for sorting this out. One more element here is related to kernel defaults; at least on Ubuntu the kernel defaults the interfaces accept_ra value to 1; which allows RAs on interfaces that aren't forwarding.

The code below which defaults accept-ra to False is going to break that. I would like to suggest that only *if* accept-ra is set in the source config (for example in the openstack network_data.json where dhcp6 stateful is specified, then it makes sense to include accept-ra: true in the interface config.

In the renderers, if accept-ra is *not* defined, I'd like to avoid emitting any RA related config at all.

Now that cloud-init handles dhcp6-stateless/stateful; I think we only want to enable RA on stateful dhcp6, otherwise we should leave the kernel defaults.

Additionally, ifupdown (eni) can toggle these values as well:

accept_ra [0, 1, 2]
autoconf [0, 1]