[clug] Single MX, multiple A records

Daniel Pittman daniel at rimspace.net
Fri Aug 1 03:42:57 GMT 2008

Alex Satrapa <grail at goldweb.com.au> writes:
> On 01/08/2008, at 12:07 , Alex Osborne wrote:
>> From what I've seen, DNS servers will usually return multiple A
>> records in a random order each time they're queried.  

They do, to encourage round-robin access across clients, since many
resolvers return them in the same order they were received, and many
clients just try the first, or try them in order.

>> So likely even if the client host always just picks the first one, if
>> the first delivery attempt happens to hit the dud server, the second
>> or third attempt will hit the working server.
> That might work if DNS lookups aren't cached.

It works just fine, once you scale enough that it matters.  For a tiny
number of clients it isn't really varied, but the more clients you add
the closer to a genuinely round-robin result you get.


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