[clug] Single MX, multiple A records

[Daniel Pittman]

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.

Regards,
        Daniel