Re: IPv6 default route flapping

[Joerg Sonnenberger <joerg%bec.de@localhost>]

On Wed, Apr 21, 2021 at 12:54:36AM +0700, Robert Elz wrote:
> It seems as if what is happening, is that the router is sending RA's with
> the source-link addr option, which isn't being added to the neighbour
> cache.
> 
> Then NetBSD is doing a NS to discover the address it ignored from the RA,
> but instead of replying with a ND as would perhaps be expected, the router
> is simply sending another RA (containing the relevant addr info, which would
> answer the NS if processed).

I'm not entirely sure that the behavior of sending a RA as "answer" to a
NS is valid under RFC 4861, but I also don't understand why the existing
code (nd6_rtr_cache) doesn't cover this. That would be a good place to
debugging this.

> I'd suggest putting RA processing back into the kernel to avoid this kind
> of issue.

Except of course that it introduces back all the reasons for why it was
removed in first place and ignores that it shouldn't happen.

> 
> I have another reason for wanting that ... I run a reasonably current HEAD
> kernel (9.99.80 from mid Feb at the minute, though I can upgrade that as
> soon as there's a reason) but a fairly old userland (8.99.12 from late Jan, 
> 2018)
> 
> Upgrading the kernel without upgrading userland is supposed to be supported.

Are you really arguing that non-essential functionality should be kept
in the kernel just because it was once present? That doesn't strike me
as very useful at all...

Joerg