Tag mbufs with subnetwork source address? (PR 10495)

[Ignatios Souvatzis <is%netbsd.org@localhost>]

Hi,

I've sketched a proposal at the end of PR 10495 to (optionally)
add the subnetwork (link layer) source address as tagged data to
mbufs (on reception). This would be done in common code in arcsubr.c,
ethersubr.c, fddisubr.c, tokensubr.c.

On errors, the ARP code (and I think the IPv6 NDP, Appletalk ARP, 
ES-IS in NETISO) would be able to log this in addition to the
inner layer information, to allow for less easy spoofing of 
erroneous packets (at least, ARP doesn't need it as all the 
necessary information is on the inside).

There was no replay by the originator, and I wonder if such a
feature is useful in certain environments for debugging, or if
using bpf-code (e.g. tcpdump, arpwatch & friends) is good enough?

(That is, should I "just close" that PR, or should I implement?)

Regards,
        -is