UNIX hosts and MX "best practice"

[Barry Margolin]

In article <7YLM$K2Xw-B at khms.westfalen.de>,
Kai Henningsen <kaih=7YLM$K2Xw-B at khms.westfalen.de> wrote:
>kcd at daimlerchrysler.com (Kevin Darcy)  wrote on 28.01.00 in
><38925106.B2C93E87 at daimlerchrysler.com>:
>
>> *some* purposes (canonicalizing host names), but if the ANY query fails to
>> turn up any MX records, it'll still do an explicit MX query when it comes
>
>Ugh. That sounds worse than querying for MX the first time around!

Maybe not.  Suppose the only reason you ever refer to something.com is when
sending mail there.  The first time the mailer performs an ANY query, there
won't be anything in the local server's cache, so it will forward the query
to the authoritative servers, which will return all the records.  If
they've followed "best practice", this will include the MX records, and the
Additional Records section will probably contain the A records for the MX
hosts.

However, if you've referred to a name in a domain in some other way
(perhaps you were looking up www.something.com, and in the process you
cached the NS records for something.com), you'll end up doing an additional
query: an ANY query that just returns the NS records, followed by the
explicit MX query.  Note, however, that the extra query is only between the
client machine and its local caching server; the purpose of the best
practice we're talking about is to reduce queries to the authoritative
servers.  The ANY query scheme has the same number of queries to the
authoritative server: first it queries for either ANY or MX, then it
possibly queries for A.

Before the web, this scheme probably worked pretty well.  In those days,
I'll bet 99% of references to remote names was in email, so you didn't end
up with lots of remote records cached that didn't include the MX records.

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