Re: Can I autoconfigure both interfaces as slaac (accept_ra 1) on a non-router device?

[Taylor R Campbell <campbell+netbsd-tech-net%mumble.net@localhost>]

> Date: Wed, 21 Feb 2024 04:46:22 +0000
> From: Marinela Selseth <Marinela.Selseth%entrust.com@localhost>
> 
> Need clarification to the following statement
> 
>    Also, due to restrictions in the IPv6 specification, it is not recommended to autoconfigure a host with multiple external interfaces (like 2 ethernet interfaces).
> 
> from page
> https://www.netbsd.org/docs/network/ipv6/

If this statement is true, it ought to have a citation or explanation
to substantiate it.

Unfortunately, the person who wrote it back in 2000 was itojun@, so we
can't ask the author for clarification:

http://cvsweb.netbsd.org/bsdweb.cgi/htdocs/Documentation/network/ipv6/Attic/index.list?only_with_tag=MAIN#rev1.26

(itojun@ passed in 2007.)

So we'll have to figure it out and fix the documentation...

> Here is my question.
> 
> Not router host, set accept_ra 1, two interfaces, eth0, eth1
> The routing table ends up with two default link-local routes.
> 
> default via fe80::b60c:25ff:fee0:8011 dev eth1 proto ra metric 1024 expires 1726sec hoplimit 64 pref medium
> default via fe80::b60c:25ff:fee0:8011 dev eth0 proto ra metric 1024 expires 1787sec hoplimit 64 pref medium
> 
> Is this going to work, I am concerned about having two default routes, and if the statement above still applies.

This appears to be the output of `ip -6 route' on Linux, which
suggests you're asking about Linux, so I suspect the NetBSD networking
mailing list may not be a good place to get the rest of the answers
you're looking for.

That said, I don't think this routing table in itself is a problem:
you just have two alternate default routes which both work, and the
kernel will pick one arbitrarily for each packet going out (and
probably stick to the same choice for all packets in each network
flow).

In this case it happens that they route to the same gateway via the
same link-local address, but if the interfaces were on different
subnets with different gateways, you'd have the same situation of an
arbitrary choice of route for each packet.