Multiple Domain Names/ One IP Address

[Phil Howard]

Martin McCormick writes:

> Barry Margolin writes:
> >Yes.  Just create different A records that all have the same IP address, or
> >use CNAME records if possible.  Pick one of the A records and use it in the
> >corresponding PTR record (multiple PTR records are legal, but I generally
> >don't recommend them).
> 
> 	This brings up a point which I am not clear about.  Exactly
> what happens when there are multiple PTR records?  We have several
> redundant A records at our site and I usually remove all but one of
> the corresponding PTR records.  Does the reverse lookup just get all
> of them the way it can for MX records that point to a score of mail
> servers?

They do all come through, within the constraints of size.  What the program
that queried for them does is another issue.  I've had problems with some
servers that did a reverse lookup, and used only one name.  Instead of doing
a forward lookup with that name (it would have given the correct IP) they
instead matched the name with something (often failing) like a list of
authorized hosts.  The Typhoon news server comes to mind as one of the
sources of this problem.

IMHO, the ideal is to take all PTR results and forward lookup each, and
among those that have any IP matching the connection, consider them valid
and proceed with whatever other authentication might be needed.  But there
may be reasons not to do this, too, such as that being a lot of work to do
which slows things down, not many PTRs can be delivered since the results
are potentially long strings, and you may incur the wrath of the standards
purists.

I do give different domain names for different functional roles.  If one
machine has many roles (happens more often than I'd like) then those names
obviously get an A record to that machine.  I do try to also make sure a
PTR list for that machine reflects all the names I have given to it.  But
it can be a problem with a big list, so I generally try to keep it limited
to what might be essential.

-- 
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