DNS CNAME round robin alternatives

[Barry Margolin]

In article <OF7A49DE05.8DF747B6-ON852567C5.0060E17B at factset.com>,
 <kmoriarty at factset.com> wrote:
>I thought multiple A records was a no-no?  If this is the 'right' ay to do
>this now, great, but I don't want to cause problems, but rather find a good

Multiple A records are perfectly fine.

>long term solution to this proble.  I see that it works, but other sites
>that do this cause problems for some DNS servers.  I seem to recall that
>when AOL started doing this, it caused problems for various DNS servers (or
>clients).  I don't remember the details though...

AOL's problems were pretty unique to their situation.  They had so many
mail servers that a single response with all the A records would overflow
the maximum size of a UDP DNS response.  Also, some mailers would try every
address, so they would waste lots of time trying to connect to AOL when
there was a general outage that affected them all.

Now AOL publishes 9 equal-preference MX records, and each one of them has 5
A records.  When you look up the MX records, the Additional Records section
of the response has room for the A records of the first 3 MX hosts.  Many
mailers will just try one address of each MX record, or all the addresses
of one MX record at each preference level, so you end up with reasonable
round-robin behavior without breaking anything.

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