Slightly OT - MX RR Santity Check requested...
Kevin Darcy
kcd at daimlerchrysler.com
Thu Mar 29 02:39:22 UTC 2007
Barry Margolin wrote:
> In article <euf4o6$98o$1 at sf1.isc.org>,
> Mark Andrews <Mark_Andrews at isc.org> wrote:
>
>
>>> You mean "their" configuration is broken? The sending mail server is NOT
>>> ours. We're on the receiving end.
>>>
>> No. Your configuration is broken. The lowest preference
>> MXs MUST always be reachable. You cannot depend upon
>> fallback to higher preference MXs. The sending side is not
>> required to try them. It is required to try all the lowest
>> preference MXs.
>>
>
> RFC 2821 seems to contradict you:
>
> When the lookup succeeds, the mapping can result in a list of
> alternative delivery addresses rather than a single address, because
> of multiple MX records, multihoming, or both. To provide reliable
> mail transmission, the SMTP client MUST be able to try (and retry)
> each of the relevant addresses in this list in order, until a
> delivery attempt succeeds. However, there MAY also be a configurable
> limit on the number of alternate addresses that can be tried. In any
> case, the SMTP client SHOULD try at least two addresses.
>
The MUST of the "be able to try" is effectively overridden by the MAY of
the "configurable limit" (since there's nothing to prohibit a configured
limit of 0). Which leaves only the SHOULD, which doesn't create a
mandate. An implementation can try only a single address and still be
compliant (minimally) with this text. Blame the RFC authors.
- Kevin
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