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Date:	Thu, 13 Mar 2003 19:20:03 PST8
From:	Peter Skye <pskye@peterskye.com >
Reply-To:	scoug-help@scoug.com
To:	scoug-help@scoug.com
Subject:	SCOUG-Help: The debacle of email (hi Steven)

Content Type: text/plain

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Steven Levine wrote:
>
> I was wondering what was causing the cycling through
> the servers. Either it's something Earthlink does
> for load balancing or Sundial cycling through the
> available MX records. I suspect the latter.

Neither. It's at the DNS. When Sundial asks for
the earthlink.net server it might get any of the
following seven Earthlink servers/frontends:

[G:\]nslookup
Default Server: vnsc-pri.sys.gtei.net
Address: 4.2.2.1
> set type=mx
> earthlink.net
Server: vnsc-pri.sys.gtei.net
Address: 4.2.2.1
Non-authoritative answer:
earthlink.net preference = 5, mail exchanger = mx02.earthlink.net
earthlink.net preference = 5, mail exchanger = mx03.earthlink.net
earthlink.net preference = 5, mail exchanger = mx04.earthlink.net
earthlink.net preference = 5, mail exchanger = mx05.earthlink.net
earthlink.net preference = 5, mail exchanger = mx06.earthlink.net
earthlink.net preference = 5, mail exchanger = mx00.earthlink.net
earthlink.net preference = 5, mail exchanger = mx01.earthlink.net
Authoritative answers can be found from:
earthlink.net nameserver = ns3.earthlink.net
earthlink.net nameserver = ns1.earthlink.net
earthlink.net nameserver = ns2.earthlink.net
mx00.earthlink.net internet address = 207.217.120.28
mx01.earthlink.net internet address = 207.217.120.29
mx02.earthlink.net internet address = 207.217.120.79
mx03.earthlink.net internet address = 207.217.120.78
mx04.earthlink.net internet address = 207.217.120.249
mx05.earthlink.net internet address = 207.217.120.31
mx06.earthlink.net internet address = 207.217.120.23
> exit
[G:\]

The Domain Name Server returns a round-robin value for the
requested MX (lurkers: Mail eXchanger) record. There are
other ways to set up the DNS but round-robin is quite common.

I'm not a DNS expert but I believe the "preference" value
(5 for all servers in this case) is the weight given to
each server. Thus, you can have some servers get more
hits than others.

Of course, there's some queueing theory that comes into
play here because there are a *lot* of different DNS
servers in play around this globe and it's possible to
have, say, 100,000 different DNS all decide to return
the value of mx03.earthlink.net to 100,000 different
requesters all at the same time. Poor, poor mx03.

- Peter

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