== Why have an MX record to yourself?

In a [[recent entry http://etbe.blogspot.com/2006/10/mx-vs-record.html]],
Russell Coker brings up an issue:

> One issue that has been the topic of some pointless discussion is
> whether a mail server should have an A record or an MX record.

It's harmless not to have an MX record that points to yourself,
but having one can save people a DNS query in many situations.

Answers to DNS queries have three sections: answer records, authority
records, and additional records. Authority records are the NS records of
the authoritative nameservers (and SOA records for negative answers);
additional records are A records for any NS or MX records in the rest of
the answer.

So if you have a self-pointing MX, anyone who queries your authoritative
nameservers will get your MX record and your A record in one query. If
you don't have an MX record, they will have to make two queries; one to
find out that you don't have an MX record, and the second to get your A
record.

(Similar clever tricks can be pulled through NS records. For example,
if you make your web server one of your nameservers, people who go to
your website will probably save a DNS lookup. But there are [[downsides
HowNotToDoDNSVI]] to such tricks.)

There are two flies in the ointment:

+ nameservers only return additional records *that they know at the time*;
  if a caching nameserver has discarded your A record but not your MX
  record, that's it.
+ some caching nameservers, including at least [[djbdns's dnscache
  http://cr.yp.to/djbdns/dnscache.html]], deliberately don't include
  authority records or additional records in their replies in order to
  make their replies smaller.