Skip site navigation (1)Skip section navigation (2)
Date:      Sat, 15 Sep 2001 11:28:28 -0500
From:      Len Conrad <LConrad@Go2France.com>
To:        Freebsd-isp@freebsd.org
Subject:   Re: Mail Server - Round Robin Load Distribution
Message-ID:  <5.1.0.14.0.20010915110914.02ceeea8@mail.Go2France.com>
In-Reply-To: <3BA37C96.678DB083@buckhorn.net>
References:  <5.1.0.14.0.20010915091315.0a697b28@mail.Go2France.com>

next in thread | previous in thread | raw e-mail | index | archive | help

>Internet Explorer > V4.x, Outlook and Outlook Express > V4.x, Netscape >
>V4.x (On your Unix box, with netscape running, do a ps -ax... that dns
>helper is a caching resolver) Once any of these find a working name==ip,
>they will continue to use it until the pair fails.

hmm.  ok, application-level caching.

The corporate DNS admin who was desiging to roll out W2K and AD to 60K 
desktops made the point of W2K "resolver" doing caching.

>I was being overly simplistic. But using multiple RR's won't load
>balance, it causes [hopefully] load sharing

yes, if "balancing" implies load detection. Alternating RR physical 
sequence is dumb load sharing, load distribution.

>, assuming nothing between
>the client and the authoritative server caches the response from the
>authoritative server.

a caching BIND DNS will also respect its RRorder param.

>  More to the point of this thread, and using your
>example, all of aol's mail servers have separate names, and A records,
>but have the same MX priority. And on high traffic networks, DNS based
>load sharing won't work for a number of reasons, but primarily because
>of client caching,

it will and does work

>  and that this method of load distribution doesn't
>take server responsiveness into account.

yep, it´s dumb, but it´s a lot better than no load sharing.

>  For clarity on that last point,
>I'll use the example of 2 mail servers with MX records of equal
>preference. Each will handle every other request. But if every other
>request is a list

what´s a query for a "list" ?

>, one server is going to end up doing a lot more work
>than the other, possibly to the point of failure.

what?

>  While this tends to
>affect web servers more than mail servers, it's still the reason they
>build load balancers.

Note that DNS-based load balancers have extremely short TTL's, which will 
slow the average access time due to loss of caching.

>  There is also a problem with the authoritative
>name servers and timing. If I dig at aol.com 10 times in a row, I will
>get cyclic answers. But if I dig at aol.com once an hour for 10 hours
>(which is far more likely in the real world) I'm apt to get a much
>higher incidence of the same response.

why?

>Again, this is a much bigger problem on a high volume network.

why?

> > ok, your answer is right, for the wrong reasons. :))
>A different way of arriving at the same conclusion perhaps?

yes, my right way, and your wrong way.  :))

>Some place in the midst of this discussion, somebody ought to point out
>that no matter what you do, using CNAME's for mail servers is a bad
>idea.

CNAME´s are to be avoided.

>  Pick the MTA of your choice, go to their web site, and you are
>bound to find something about CNAME loops in the FAQ.

CNAME´s are to be avoided.

Much more common is an MX hostname being an ip address.

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


To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-isp" in the body of the message




Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?5.1.0.14.0.20010915110914.02ceeea8>