Multiple PTR records (was Setup QuickDNS)

Re: Multiple PTR records (was Re: Setup QuickDNS)

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Very interesting discussion. Let me try to answer all the questions raised,
in one message.

>shouldn't one use a CNAME instead of an A record for the second record? I
>thought it was not recommended to use multiple A records pointing to the
>same IP?

In this particular case, using a CNAME would have been an error. A name
that has subdomains can't have a CNAME. Thus, the following is illegal:
www.domain.com. A 192.168.0.1
domain.com. CNAME www

Having multiple A records for a given address is not an error. It does give
rise to PTR record "errors", but these can generally be ignored.

>It is kind of ironic, considering that both DNS and BIND and DNS Expert
>complain about using a CNAME for a mail server record, however what if
>you run DNS and Mail on the same machine? Are you supposed to have an A
>record for both,(hence pointing to the same IP) which is also a no, no?

If you don't want to run afoul of the CNAME restrictions, and you don't
want to run afoul of the PTR rules, there is a simple solution:
domain.com. A 192.168.0.1
domain.com. NS domain.com.
domain.com. MX 10 domain.com.

You don't have to name your mail server "mail", nor do you have to name
your DNS server "ns1" or any such thing.

However, using the name of your domain for all services means that you may
have to change more than an address if you ever split your services between
different machines.

>It would seem that having multiple PTR records would be a significant
>no-no...anyone trying to authenticate a machine name to an IP would see two
>different names...or would it matter? If it's just looking for the "name"
>and hoping that it's associated with a US IP, then it probably wouldn't be
>confused if another "name" had the same PTR record.

If you use multiple PTR records (officially an error), the DNS server will
serve them all out (in permuted order) for each request. The resolver will
generally only look at the first one, which is effectively randomly chosen.
The result is that you have *no* effective PTR records, because they don't
reliably come up.

>If you have two A records pointing to the same IP, let's say for mail and
>DNS, which one is more important to use in the PTR record? Does it matter if
>there's no PTR record for DNS?

The mail name is more important, because, as someone else pointed out, it
is necessary to get through anti-spam measures on other mail servers.

I hope this clears things up a bit.
____________________________________________________________________
Chris Buxton Men & Mice
cbuxton@menandmice.com http://www.menandmice.com

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