DNS recommendations - the paper

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Quoth Randy Bush:

> not discussing MX RRs.  mailbox names.  the reason i mention piet's

I know, I just changed the subject. :) (Piet who, by the way?  It being a 
common Dutch name & all.)

>> (Remember that sendmail falls back to A records if it can't find any MX
>>  records for a host.)
> i think of it the other way, but with the same result.  mail will be sent
> to the (address in rdata of the) A RR unless some one put in an MX RR
> because the (interface denoted in the) A RR can not accept mail.

The same result, but you should add MX records if a host will at some
point receive mail (at its hostname).  The Bat Book advises you to
always add MX records; sendmail looks up the MX record anyway, first by
asking for "any" information, if there isn't any MX info it'll ask
specifically for it, causing more DNS traffic.

Say:

	example.com.	IN	A	1.2.3.4

Sending mail to nobody at example.com will cause one additional DNS lookup,
whereas if you added

	example.com.	IN	MX	10 example.com.

sendmail (or any mailer daemon following the RFCs for that matter) will
find the MX record immediately and get the A record at the same time, thus
knowing exactly where to send the mail in one try.

Unfortunately I don't have my RFCs greppable currently, else I could quote
the one that says that you should add an MX pointing elsewhere for a host
that is incapable of receiving mail, so that mail to postmaster@<host> has
a better chance of ending up somewhere where it'll be read, in case of
trouble with <host>.

I guess that practice was from before the explosion of dialup hosts, though.

Take care,


	-- Niels.