[clug] Single MX, multiple A records

Daniel Pittman daniel at rimspace.net
Fri Aug 1 06:23:53 GMT 2008

Michael James <michael at james.st> writes:
> On Fri, 1 Aug 2008 Michael James suggested:
>> > rather than have a single MX with multiple IP numbers
>> > I think a better DNS setup is to have multiple MXs, 
>> > then the specified behaviour for a mail sender
>> > is to work up the list till one accepts (or rejects) the mail.
> On Fri, 1 Aug 2008 01:48:28 pm Daniel Pittman wrote:
>> Your theory here does not reflect the option for listing multiple MX
>> hosts at the same priority level, in which case you face exactly the
>> same situation save in a protocol specific context.
> Multiple MXs at the same priority was the option I was thinking about.
> Wouldn't senders order the equal MXs at random within the list?

Oh.  I see that I misread your comments.  Yes, they should.


>> DNS Round Robin performance or redundancy works, but not as well as more
>> modern solutions that provide multiple systems behind a single IP, and
>> so have (sensibly) fallen out of vogue.
> DNS hosting is different from say providing an airline booking system.
> With such a simple largely read-only database we have the luxury of
> spreading servers across the world.  Gives us immunity from computer
> room fires, state-wide network blackouts, etc.  Short of having
> massive control of backbone links I can't imagine how you could do
> this with a single IP.

anycast, BGP arrangements for your AS, or provided by your upstream
service provider, tunnelling, or other solutions can take the place of
the DNS round-robin support.

By the time you need a service to be redundant against central location
failures you usually have solved enough costly and difficult problems
that those are not really going to substantially eat into your budget.


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