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>
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>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
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