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Date:      Sat, 15 Sep 2001 09:12:04 -0500
From:      Len Conrad <LConrad@Go2France.com>
To:        Freebsd-questions@freebsd.org
Subject:   Re: Mail Server - Round Robin Load Distribution
Message-ID:  <5.1.0.14.0.20010915085600.03274008@mail.Go2France.com>
In-Reply-To: <3BA35C8B.8ACC74E6@buckhorn.net>
References:  <20010915080113.985.qmail@web20101.mail.yahoo.com> <20010915222844.A6415@plug.cx>

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>Multiple A records for a single MX, as well as CNAMES for the MX record
>won't do what you want, as clients and servers (DNS) tend to cache
>addresses.

BIND caches, of course. The only other non-recursive resolver that Iīve 
heard of is in W2K, MS anticipating that in their primary target market of 
AD + DHCP + DDNS intranets that they needed to reduce query loads.  What 
are the others?

>Since your DNS server will always pass along the first A
>record

false. eg, with BIND, the default RRset-order value is cyclic.

do

dig aol.com mx

10 times to confirm.

>, or the first CNAME, no balancing will occure.

false.  BINDīs RRset-order cyclic or random will cause effective load 
distribution among members of the RRset for resolvers that work strictly 
with the physical order of RRset records.

IN conrast, BIND's own recursive resolver uses RTT, round trip time, to 
determine, continuously and dynamically, which member of the RRset it will 
query.

>The priority flag for the MX record was designed for just this reason.

false.  the preference field is not for distributing load, but for 
providing backup MX's.

>domain.com.     IN      MX      10      mail1
>domain.com.     IN      MX      10      mail2
>mail1           IN      A               1.2.3.4
>mail2           IN      A               1.2.3.5

ok, your answer is right, for the wrong reasons. :))

Len


http://MenAndMice.com/DNS-training
http://BIND8NT.MEIway.com : ISC BIND 8.2.4 for NT4 & W2K
http://IMGate.MEIway.com  : Build free, hi-perf, anti-abuse mail gateways


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