MX question

[Barry Margolin]

In article <a9hf19$baj at pub3.rc.vix.com>,
Kevin Darcy  <kcd at daimlerchrysler.com> wrote:
>None of which explains why it is *necessary* to have multiple A records
>attached to
>each MX target, as opposed to simply having more MX targets. I understand
>the need
>for load-balancing and clustering. I just don't see why it is necessary for
>load-balancing and clustering to have such ugly repercussions in DNS.

Didn't I answer that in one of the first responses I sent?

Some systems have a limit on the number of equal-preference MX records that
they'll try.

Also, if you have *lots* of mail servers, you risk overflowing the 500-byte
limit of UDP DNS replies if they're all separate MX records.  If you
cluster them, the A records go into the Additional Records section, and an
overflow there doesn't require a TCP retry.

Most of these problems were first experienced by AOL about 5 years ago.  I
remember them experimenting with a number of different solutions, including
some that violated the DNS specs (they were desperate).

-- 
Barry Margolin, barmar at genuity.net
Genuity, Woburn, MA
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