Re: [courier-users] MX randomizing: trying to understand the

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Rodrigo Severo writes:

> Sam Varshavchik wrote:
>
>> The DNS server already returns DNS records in random order.
>
> I believe that by "random order" you mean a different random order for
> each query. If this is the case I would like to ask to which DNS servers
> do you refer specifically?

Anyone heard of "bind"?

$ dig aol.com mx

; <<>> DiG 9.2.3 <<>> aol.com mx
;; global options: printcmd
;; Got answer:
;; =E2=86=92>HEADER<=E2=86=90 opcode: QUERY, status: NOERROR, id: 50613
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 4, ADDITIONAL: 18

;; QUESTION SECTION:
;aol.com. IN MX

;; ANSWER SECTION:
aol.com. 3600 IN MX 15 mailin-04.mx.aol.com.
aol.com. 3600 IN MX 15 mailin-01.mx.aol.com.
aol.com. 3600 IN MX 15 mailin-02.mx.aol.com.
aol.com. 3600 IN MX 15 mailin-03.mx.aol.com.

[ . . . ]

$ dig aol.com mx

; <<>> DiG 9.2.3 <<>> aol.com mx
;; global options: printcmd
;; Got answer:
;; =E2=86=92>HEADER<=E2=86=90 opcode: QUERY, status: NOERROR, id: 35832
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 4, ADDITIONAL: 18

;; QUESTION SECTION:
;aol.com. IN MX

;; ANSWER SECTION:
aol.com. 3559 IN MX 15 mailin-01.mx.aol.com.
aol.com. 3559 IN MX 15 mailin-02.mx.aol.com.
aol.com. 3559 IN MX 15 mailin-03.mx.aol.com.
aol.com. 3559 IN MX 15 mailin-04.mx.aol.com.

Oh, and the A records for mailin-01, mailin-02, et. al. are randomized too.

> Courier tries to contact MXs in exactly the order returned by the DNS
> server. Courier expects that DNS servers's answers came in a different,
> random, order for each query.

Correct.

> Could you please advise on how can I get a more random MX choosing by
> Courier?

Stop using broken DNS caches. By definition, the only function of a cache
is to speed up lookups. An application that uses a cache, be it a DNS
cache, a web cache, or any other kind of cache whatsoever, should NOT be
aware that it's using a cache, and not the real thing. It should get the
same reply that it would get from the real thing. Except that it would get
the reply faster.

Therefore, it is the caching software's complete and full responsibility to
behave and act in every way like the real thing, except that it's faster.

A real DNS server returns records in random order. That's a fundamental
function of DNS: load balancing. It makes no sense for every application
that queries DNS to go through the rigamarole of randomizing the results it
gets back. What sense is there in every application doing it, instead of
doing it just once, in the DNS server?

Real DNS servers return record in random order. Therefore, a properly
implemented cache will also return records in random order. If it doesn't,
it's broken and it needs to be fixed.



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