Re: [Asrg] What are the IPs that sends mail for a domain?

der Mouse <mouse@Rodents-Montreal.ORG> Wed, 17 June 2009 15:17 UTC

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Subject: Re: [Asrg] What are the IPs that sends mail for a domain?
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>> [...] I think what he is actually saying is that if you have a
>> machine that says "EHLO some.name" then there should be both a MX
>> record for some.name and a SRV record for _client._smtp.some.name
>> (i.e. a CSV/CSA record).
> However, the standard requires that it says "EHLO host-at.some.name".

Not quite.  It requires that the HELO/EHLO argument be a valid name for
the SMTP client host.  The presence or absence of any DNS zone cuts in
the vicinity is completely irrelevant.

> It is a seemingly simple task to drop the leftmost label(s) so as to
> obtain the mail domain, but doing that properly requires a zone cut
> algorithm that most servers miss.

...and which is wrong anyway.  The division of DNS names into "hosts"
and "domains" is purely a human one.  Dropping the first label from a
DNS name in an attempt to get "the domain" for it is, at best, a rough
heuristic.  Looking up the DNS tree for zone cuts also is nothing more
than a heuristic.

It's not even clear to me that there *is* a "_the_ domain".  What's
"the domain" for (to invent an example) mail.research.tjw.ibm.com?
There plausibly could be as many zone cuts as there are dots, there,
and I could argue for picking any of them as "the domain" for email
responsibility purposes (well, possibly excepting the TLD, but even
that is just a heuristic, likely to break soon).

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