MX question

Kevin Darcy kcd at daimlerchrysler.com
Tue Apr 16 19:39:20 UTC 2002


Barry Margolin wrote:

> 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.

Those systems are broken, IMO, if they can't deal with moderate quantities of
equal-preference MX records (6 in this case). Is Microsoft -- a mail server
software vendor, I'll note -- setting a good example by structuring their
MX records to accommodate brokenness?

> 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.

Again, we're only talking about 6 targets here. That's nowhere near overflowing
the UDP limit.

But, even in the hypothetical case where UDP overflow occurs, what's so bad about
a TCP retry? It seems to me that much if not most of the time a simple TCP retry
will actually use *less* resources overall than having to do "x" UDP queries for
all of the A records missing from the Additional Section.

(Yes, yes, I understand that some misconfigured firewalls may block TCP retries.
But now we're back into the "accommodating brokenness" syndrome...)

EDNS0 should greatly mitigate -- if not moot -- the whole "TCP retry" problem
anyway.


- Kevin





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