ifconfig prefer_source and IPv6 privacy extensions

[Jan Behrens]

On Sat, 14 Mar 2020 04:51:43 +0900 (JST)
Hiroki Sato <hrs at FreeBSD.org> wrote:

> Jan Behrens <jbe-mlist at magnetkern.de> wrote
>   in <20200313202833.cbbe8d1679ac0fd7a80788e1 at magnetkern.de>:
> 
> jb> Is it intended that "net.inet6.ip6.prefer_tempaddr" takes precedence
> jb> over "prefer_source"? If yes, why?
> 
>  Yes, and the reason is that RFC 6724 specifies that behavior.

I'm afraid my knowledge on IPv6 is not good enough yet to fully
understand the RFC. Would "net.inet6.ip6.prefer_tempaddr" correspond to
Rule 7 and "prefer_source" correspond to Rule 8 (that "MAY be
superseded if the implementation has other means of choosing among
source addresses")?

>  Why do you want to use fd<prefix>::2 over fd<prefix>:<random> as the
>  default source address?

I want to use fd<prefix>::2 as the preferred address in order to always
appear as fd<prefix>::2 in my local network even if I change my network
card. That address won't be used when I access the internet, even if it
is preferred (see below).

>  If prefer_source takes precedence, the tempaddr will nerver be used.

That depends on how "prefer_source" competes with other rules for
source address selection.

Even with current FreeBSD's behavior, if I disable privacy extensions,
then for reaching hosts in the internet,
2003:<prefix>:<modified EUI-64> is used, and only for connections to
my LAN, fd<prefix>::2 (the address with "prefer_source") is used, i.e.
the address with "prefer_source" set is NOT always used.

Moreover, preferring temporary addresses over addresses marked with
"prefer_source" doesn't seem to make much sense (even if the RFC would
demand it). This is because it doesn't seem to make much sense to mark
a temporary address as preferred. That assumed, the current behavior of
FreeBSD effectivly renders "prefer_source" useless if
net.inet6.ip6.prefer_tempaddr=1.

> 
> -- Hiroki

Regards,
Jan Behrens