FW: Prioritizing among MX records

Kevin Jackson kevin.jackson at tradermedia.co.uk
Tue Mar 9 09:05:30 UTC 2004


I replied personally instead of to the group - if this helps anyone...

Kev


>Jan,
>There is no correlation between the preference value and the time taken to
>retry delivery.  Your list could've been written as
>MX 1
>MX 5
>MX 78
>
>And it would've made no difference, the mailer would try the lowest
>preference value as you know.
>
>But, its not as clear cut as this - its not just a case of looking up
values
>and determining which is the smaller value -- the MX lookup algorithm does
>other operations like trying to guess which is the best candidates,
discards
>MX records and reorders the list accordingly if things can't be delivered
as
>stated.
>
>Mail sending is the old store and forward technique.  If mail can't get to
>primary.bogus.com and say can't get to backup.bogus.com it sends it to
>last-hope.bogus.com.  The mailer (configurable) will then retry after 5
>mins, 10 mins, etc up to 4 hours (say a default sendmail install) before
>sending a mail to the originator saying it can't be delivered) for 5 days
>before giving up (again, all configurable in your mailers settings).
>In that time, if it can't still get to primary.bogus.com and
>backup.bogus.com comes up - the MX lookup algorithm determines whether this
>gets sent on one step closer to primary.bogus.com -- to backup.bogus.com or
>not.  If backup.bogus.com is further away than last-hope.bogus.com then the
>MX algorithm discards backup.bogus.com and just waits for primary.bogus.com
>to come up.
>
>Hope this helps or sheds any light on the results you are seeing.  But the
>official line is that the number is irrelevant apart from determining your
>primary, secondary and tertiary mail exchangers.
>
>Kev
>
>
>
>-----Original Message-----
>From: Jan Koz=E1nek [mailto:jkozanek at totalservice.cz]=20
>Sent: 08 March 2004 08:55
>To: bind-users at isc.org
>Subject: Prioritizing among MX records
>
>Hello,
>
>I'd like to ask if anyone knows whether numbers in MX records are just=20
>positional values or real priority numbers.
>
>Let's say we have an example like this:
>
>bogus.com.    IN    MX    10    primary.bogus.com.
>bogus.com.    IN    MX    60    backup.bogus.com.
>bogus.com.    IN    MX    65    last-hope.bogus.com.
>


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