[clug] Single MX, multiple A records

Craig Small csmall at enc.com.au
Fri Aug 1 07:41:13 GMT 2008

On Fri, Aug 01, 2008 at 11:30:49AM +1000, Michael James wrote:
> What is the standard behaviour
>  when a DNS name returns multiple A records?
Any and all, depending.

Strangely enough, having multiple entries can *reduce* availability for
certain clients. Some only try one and if they cannot get to it have
this stupidly long retry time.  Optusnet did (or used to) have this, so
if they couldn't get the mail out on the first go it got retried from
another set of servers.

The best way to do it is with dedicated GSLB (Global Server Load
Balancing) equipment which fudges the MX or A records. They can also do
clever things like give you the closest server etc.

Anything static will mean if one of your servers goes down then half of
the clients are going to try that server every time.  Some are sensible
and if they cannot get there they try another entry.  Others try one and
give up. So it is either a little delay, or a lot of delay or (for I'm
sure there is something that would do this) delay until the server comes

It's actually a little tricky to code because at resolve time you, as
the client, don't know what server is alive. Completely do-able of
course, but it's not just a matter of taking the head of the list and
connecting away.

 - Craig
Craig Small      GnuPG:1C1B D893 1418 2AF4 45EE  95CB C76C E5AC 12CA DFA5
http://www.enc.com.au/                             csmall at : enc.com.au
http://www.debian.org/          Debian GNU/Linux, software should be Free 

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