der Mouse wrote:
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.
Isn't the FQDN for a host the host name "dot" the domain name?
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.
The host gets its name after some buddy edits the zone file. Which
zone file? The domain's one. Yes, it is human, heuristic, and error
prone. (I never seriously meant to actually implement a zone cut
algorithm in MTA servers in order to derive domain names. However,
that was an early hypothesis for the SPF check algorithm, as an
alternative to requiring SPF records for each possible helo name.)
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?
If research.tjw.ibm.com had an MX, it would be a good candidate.
Otherwise... elementary my dear Watson. Is that worse than Bayesian
guesses?
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).
Yeah, John recently wrote something about .va sporting an MX (John
Levine, not john.vatican.va) while 2nd level co.uk has none. It is
much better if the domain is plainly told by the client rather than
badly guessed by the server. E.g. "VHLO domain.name".
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