--On Friday, July 18, 2014 06:46 -0700 Paul Vixie
<p...@redbarn.org> wrote:
>...
>> There are no addresses associated with the root, so the mail
>> server will immediately report a delivery error. RFC 5321
>> section 5.1 paragraph 2 final sentence.
>>
>> The SMTP server will not try to connect to the root name
>> servers, as your message suggested.
>
> true as stated.
>
> what's unstated here is that every SMTP sender who encounters
> such an MX without understanding its new meaning will do two
> or three lookups: ". MX", [". AAAA",] and ". A". if they are
> behind an RDNS that doesn't do negative caching (and there are
> many, though none of them are open source; the open source
> RDNS servers do negative caching right) then these two or
> three queries will flow through to the authority servers for
> "." which is to say the root name servers.
Paul,
Thanks. I obviously got the question wrong but, if it
accidentally called attention to an issue that deserves even
minimal consideration, I don't mind looking and feeling stupid.
I obviously knew about the multiple queries but, by thinking
about this from an SMTP context, was more concerned about the
connection and queuing part of the problem.
FWIW, many SMTP servers will deal with a negative answer from a
DNS query by queuing the message (for the historical reason that
a DNS change might not yet have caught up with the server
queried). While the port 25 issue that I incorrectly focused on
will not arise, the two or three queries you mention might,
depending on the behavior of the resolver on which that SMTP
server calls, be repeated at each queue retry interval.
Again, I'm not competent to evaluate whether that is an issue or
not.
thanks again,
john
p.s. Just one or two lookups against the root. The sequence
would be an MX lookup for the putative mail delivery domain
(which should not affect the root servers), then ". A" and,
optionally, ". AAAA", in either order. While some
implementations have historically done it, SMTP (at least as of
RFC 5321) forbids doing an MX lookup on the information obtained
from the DATA of a prior MX query.
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