round robing / load balancind

[Barry Margolin]

In article <9ntc0m$eo7 at pub3.rc.vix.com>,
Sébastien WILLEMIJNS <sebastien at willemijns.com> wrote:
>i try to understand how DNS choose an MX server ie smtp.free.fr

DNS just returns all the MX records in response to the lookup that the
mailserver does.  It's the mailserver's job to choose the servers.

>Querying Mail routing information (mx) for free.fr
>;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 51213
>;; flags: qr rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 0, ADDITIONAL: 0
>;;	free.fr, type = MX, class = IN
>free.fr.		0S IN MX	50 mrelay2-2.free.fr.
>free.fr.		0S IN MX	90 ns1.proxad.net.
>free.fr.		0S IN MX	10 mx.free.fr.
>free.fr.		0S IN MX	20 mrelay1-1.free.fr.
>free.fr.		0S IN MX	30 mrelay1-2.free.fr.
>free.fr.		0S IN MX	40 mrelay2-1.free.fr.
>
>what is this number ? 50, 90, 10, 20...etc... 

The number is the preference level.  The mailer is supposed to try them in
increasing order.

>how to detect if a server FREE.FR in my example use round robing or load
>balancing ? must i connect to the free.fr name server to know this ?

If two MX records have the same preference level, the mailer is supposed to
try them in a random order, so you'll get load balancing that way.  Also,
if a hostname has multiple A records, they'll be returned in round-robin
order, so you'll get load balancing from that.

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