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

Alessandro Vesely <> Wed, 17 June 2009 16:50 UTC

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Subject: Re: [Asrg] What are the IPs that sends mail for a domain?
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der Mouse wrote:
>> However, the standard requires that it says "EHLO".
> 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)

If had an MX, it would be a good candidate. 
Otherwise... elementary my dear Watson. Is that worse than Bayesian 

> 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 while 2nd level 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".