what should PTR recs look like for multihomed hosts

[Don Buchholz]

If your sites uses the Berkeley "r-commands" (rcp, rsh, rlogin, etc.),
you will probably want to do:

	foo     A       10.10.232.11
	                10.10.132.11
	foo-e0  A       10.10.232.11
	foo-e1  A       10.10.132.11

	10.10.232.11  PTR  foo.domain.
	10.10.132.11  PTR  foo.domain.


Otherwise your users need to put multiple hostnames in their .rhosts
files.  

NFS is also subject to this, too.  When you put lines in an 
"/etc/exports" file (or Solaris "dfstab"), you have the choice of

   /filesystem access,root=foo.domain

  vs.

   /filesystem access,root=foo-e0.domain:foo-e1.domain

Not a real problem with only 2 interfaces, but when faced with
a half-dozen or more it starts to get messy.

Similarly, for consistency, your hosts file(s) ought to look like

       10.10.132.11  foo.domain foo  foo-e1.domain foo-e1
       10.10.232.11  foo.domain foo  foo-e0.domain foo-e0

This way forward lookups on "foo.domain", "foo", "foo-e1.domain",
"foo-e1", "foo-e0.domain", and "foo-e0" all return pretty much the
same as if you'd asked the DNS system.  

And the reverse lookups for 10.10.132.11 and 10.10.232.11 always
return the fully qualified hostname regardless of which name service
was queried.

The only problem is when people want to ping, traceroute, netstat, snoop,
etc. and have the IP-address resolve back to a interface-specific name.
That's when I point them to a man-page and suggest they find the option
to suppress address->name resolution for their tool.

I standardized on this system about 5 years ago, and a whole bunch of
problems (inconsistencies) I had in my first 3 years of system
administration have disappeared.

- Don



On Wed, 19 Jan 2000, Barry Margolin wrote:

> In article <3886070B.3BD8999D at msdw.com>,
> Anthony Golia  <Anthony.Golia at msdw.com> wrote:
> >If my A recs look this for host foo:
> >
> >mydomain.com:
> >foo     A       10.10.232.11
> >                10.10.132.11
> >foo-e0  A       10.10.232.11
> >foo-e1  A       10.10.132.11
> >
> >What would most folks have the PTR recs look like?  
> 
> You should have PTR records pointing to foo-e0 and foo-e1.  Don't bother
> with PTR records pointing to foo.
> 
> >						     Does anyone know of
> >any probs. using multiple PTR recs for a host?
> 
> There shouldn't be any serious problems, but I recommend against it.  It
> will be unpredictable which name will show up when you do a reverse lookup.
> If you look up 10.10.232.11, sometimes you'll get foo (and you won't know
> which foo it's referring to) and other times you'll get foo-e0.
> 
> -- 
> Barry Margolin, barmar at bbnplanet.com
> GTE Internetworking, Powered by BBN, Burlington, MA
> *** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to newsgroups.
> Please DON'T copy followups to me -- I'll assume it wasn't posted to the group.
> 
>