Subject: Re: mutating IPv4 aliases on NetBSD 2 systems
To: der Mouse <mouse@Rodents.Montreal.QC.CA>
From: Thor Lancelot Simon <>
List: tech-net
Date: 02/24/2006 17:39:58
On Fri, Feb 24, 2006 at 04:25:15PM -0500, der Mouse wrote:
> > It is hard to imagine what sane meaning it could possibly have to
> > have two host addresses on the same physical segment with different
> > -- but overlapping -- netmasks.
> Your imagination is...peculiarly deficient, I would say.  It seems
> fairly straightforward to me.

Straightforward, perhaps.  However, it is certainly my opinion that
using multiple interface addresses with overlapping netmasks on the
same host, on the same segment, is not sane, for a number of reasons,
one of which is highlighted by the difficulty the original writer in
this thread experienced: if you don't bind a socket specifically to
either address, what source address do you get, and what destination
for broadcast packets?

I see little compelling reason why anyone would _want_ to run a
system this way, despite the fact that our networking code makes it,
at least, possible.

> > You can configure additional host addresses on lo0 with a /32 netmask
> > if you wish.
> And why not on any other interface?  What's magic about lo0 that /32
> aliases are acceptable on *it*, but not elsewhere?

You read into my message something that, quite simply, is not there.

Aliases with different, overlapping netmasks, on the same physical
segment, aren't okay anywhere.  If you're dumb enough to configure
one of those on lo0, however, it will hardly matter, since 127/8
is not reachable from outside the local host anyway. ;-)