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

Sam Varshavchik wrote:

> Rodrigo Severo writes:
>
>> Let's see this example:
>>
>> 10 brsmtp02.br.abnamro.com
>> 10 brsmtp04.br.abnamro.com
>> 15 naxpf001.abnamro.com
>> 15 naxpf002.abnamro.com
>> 15 naxpf003.abnamro.com
>> 15 naxpf011.abnamro.com
>> 15 naxpf012.abnamro.com
>> 15 naxpf013.abnamro.com
>> 30 plum03ap.abnamro.com
>> 30 walnut001ap.abnamro.com
>>
>> In this case brsmtp02.br.abnamro.com has 9 out of 10 chances of being
>> contacted and brsmtp04.br.abnamro.com have the remaining 1 out of 10
>
>
> No.
>
> The MX records are always sorted according to their priority.

Ok.

>
> brsmtp02 will be contacted 50% of the time, with brsmtp04 taking the
> other 50%.

Being dependent on the way bind randomizes it's answers the distribution
won't have anything near a 50/50 distribution, I'm afraid.

Let me explain why. The way bind randomizes it's answers, only 10
different answers are possible:

Answer 1)
10 brsmtp02.br.abnamro.com
10 brsmtp04.br.abnamro.com
15 naxpf001.abnamro.com
15 naxpf002.abnamro.com
15 naxpf003.abnamro.com
15 naxpf011.abnamro.com
15 naxpf012.abnamro.com
15 naxpf013.abnamro.com
30 plum03ap.abnamro.com
30 walnut001ap.abnamro.com

Answer 2)
10 brsmtp04.br.abnamro.com
15 naxpf001.abnamro.com
15 naxpf002.abnamro.com
15 naxpf003.abnamro.com
15 naxpf011.abnamro.com
15 naxpf012.abnamro.com
15 naxpf013.abnamro.com
30 plum03ap.abnamro.com
30 walnut001ap.abnamro.com
10 brsmtp02.br.abnamro.com

Answer 3)
15 naxpf001.abnamro.com
15 naxpf002.abnamro.com
15 naxpf003.abnamro.com
15 naxpf011.abnamro.com
15 naxpf012.abnamro.com
15 naxpf013.abnamro.com
30 plum03ap.abnamro.com
30 walnut001ap.abnamro.com
10 brsmtp02.br.abnamro.com
10 brsmtp04.br.abnamro.com

Answer 4)
15 naxpf002.abnamro.com
15 naxpf003.abnamro.com
15 naxpf011.abnamro.com
15 naxpf012.abnamro.com
15 naxpf013.abnamro.com
30 plum03ap.abnamro.com
30 walnut001ap.abnamro.com
10 brsmtp02.br.abnamro.com
10 brsmtp04.br.abnamro.com
15 naxpf001.abnamro.com

and so on.

Please notice that only answer 2 will make Courier contact
brsmtp04.br.abnamro.com. All other 9 will make Courier contact
brsmtp02.br.abnamro.com. Obviously I'm referring to temporary delivery
failures situations, not to failed connections.

This is why I'm saying Courier won't get a 50/50 distribution as
expected but 1/9. I hope this is clearer now.

This is why I believe something else should be done to, in this
situation for example, get nearer a 50/50 distribution of the MX Courier
tries to contact.

>
> Which one will be tried for a given connection attempts depends on the
> response from the DNS server.

This is already understood.


Rodrigo



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