Re: Proxy ARP

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

On Fri, Feb 26, 2016 at 4:36 AM, Robert Elz <kre%munnari.oz.au@localhost> wrote:
> When a host is answering ARP queries for an IP address that is not its
> own, there are two ways (and reasons) for how it does that.
>
> It can send its own MAC address (kind of claiming ownership of the
> IP address, though all it will usually do with the packet is relay it
> elsewhere.)
>
> Or it can send someone else's MAC address - so that the host that sent
> the query gets told the MAC address of the owner of the IP address (one
> hopes.)  This was used in the ancient times Christos mentioned when some
> hosts existed that did not understand ARP (IP over ethernet existed before
> ARP was invented.)
>
> One of those was known as proxy ARP, the other wasn't - but I'm not sure
> the nomenclature has remained consistent over time.  Any rational need for
> the second form vasnished long long ago, I can't imagine any host still
> running which understands both IP and ethernet, but not ARP.
>
> I suspect from a (very very) brief look at the code, the "proxy" option
> to arp(8) is meaning the 2nd of the two.

I agree proxy ARP can be used that case and we can do so with NetBSD,
but in that case we also have to set up a machine to receive frames
that have the destination of someone's MAC address by say putting
an interface into promiscuous mode or a feature of MAC address
filtering that is implemented in recent NICs for virtual functions.

Anyway I'm not sure NetBSD's proxy ARP is intended for the case and
there are users for it.

Thanks,
  ozaki-r