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

[Roy Marples <roy%marples.name@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.

When that was written we had rtadvd and in kernel RA as the norm.
So the statement holds true in that the NetBSD routing table only allows one default route (which IMHO is a good thing, keeps things simple).
If both interfaces have active carrier, and the router has the same preference then the first interface to get the RA wins.
In other words, it's not very deterministic which interface is going to work.

Fast forward to today, we have many interfaces as fairly normal and dhcpcd has replaced rtadvd and the kernel RA handling so that it's very determinstic.
As an added bonus, you can share the same IP address on different interfaces (on the same subnet) and it works!
dhcpcd has a clear set of rules (metric based) on which interface wins in a many interface scenario.

I would remove that statement as we want to autoconfigure all interfaces for IPv6 - there is literally no downside when dhcpcd runs in manager mode.
There *is* a downside for the same on IPv4 in that we need a BPF instance for DHCP so if there is no DHCP server answering then by default dhcpcd will keep one open per interface.
There are some exclusions like say tap(4) and bridge(4) where you need a manual config so generally this is fine.

Roy