UNIX hosts and MX "best practice"

[Barry Margolin]

In article <38923337.923869FC at sas.upenn.edu>,
John H. Yates <yates at sas.upenn.edu> wrote:
>What is considered "best practice" for UNIX hosts these days? To
>assign a UNIX host its MX record as itself, or to leave it unspecified?

Any name that you expect to be used frequently after '@' in email addresses
should have an MX record.

>I've observed that it falls back to the A record if you don't bother
>specifying it, but it makes "nslookup" MX queries look "different" than
>I am used to, and I was just wondering what current "best practice" is.

Falling back to the A record is required, since the purpose of MX is to
specify an alternate place to send mail instead of sending it to the host
itself.  However, there are performance implications if you don't have an
MX record.  The sending machine always has to query for an MX record first.
If its local DNS server doesn't have the MX record in its cache, it can't
tell if this is because there is no MX record or because it just hasn't
looked it up recently enough to have it in cache, so it has to query an
authoritative server (negative caching improves this somewhat, since it
will remember recent failures, but many DNS servers don't implement ncache
or limit the negative cache timeout severely).  After this query fails, the
mailer will query for the A record, which will presumably be in its local
server's cache.

But if there's an MX record, the first query will succeed immediately with
the cached record.

-- 
Barry Margolin, barmar at bbnplanet.com
GTE Internetworking, Powered by BBN, Burlington, MA
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