This might be a little off topic for this list but I am trying this
hack on a NetBSD box and I suspect that someone here has tried this.

I have an IP address, say 12.345.67.89 ( not the real one ).  An ISP
serves this as foo.bar.com, another place serves it as bar.foo.org.
In principle, an ARP for bar.foo.org should go to foo.org for
resolution and get handed the IP.  Similarly, a request for
foo.bar.com should get resolved at bar.com as the same IP.  So both
domains have an A record for 12.345.67.89.  I thought about CNAMEing
it but Sendmail wouldn't buy it.  I'm a little concerned about what's
going to happen with rarp or gethostbynumber().  Is this a cool thing
to do, or am I completely hacking DNS?

Feel free to respond in private, OTOH, other people may be
interested.

Thanks,
pjr