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

der Mouse <mouse@Rodents-Montreal.ORG> Fri, 19 June 2009 12:33 UTC

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Subject: Re: [Asrg] What are the IPs that sends mail for a domain?
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>> The FQDN for a host is the host's FQDN.  As we've all noted, there's
>> lots of heuristics to guess domain names, none of which work.
> What about the other way around: given a domain and an IP address,
> can we say whether the IP address "is a member of" the domain?

We can.  We can also say the IP address and the domain live on the same
shelf in the supermarket, too; I'm not convinced either is a more
meaningful or useful statement than the other.

> Vhlo mentions the following three ways to determine that, without
> apparently resorting to heuristics.  I'm wondering how sound it is to
> rely on those, or similar, techniques.

If any of them results in a definition of "member of" that turns out to
be useful for whatever purpose you have in mind, sure.  I'm not sure
any of them does, but I'm also unclear on why you'd want this sort of
association between addresses and domains, so that doesn't mean much.

> * rDNS returns a name whose right part matches the domain name,
> * an MX record for the domain mentions a host with the given IP,
> * the IP address passes the SPF check for that domain.

One that's based on something designed for mail flowing to the domain;
one ditto for mail flowing from the domain; one that's based on
something not designed for mail at all.  Offhand, I'd guess that which
one is most appropriate depends on whether you're concerned with mail
flowing to the domain, mail flowing from the domain, or something other
than mail.

The things you list might be "without apparently resorting to
heuristics", and in a sense that's true, in that each one is
well-defined and has a mechanically testable definition. But that
doesn't keep them from being heuristics, depending on what you're using
them for.  Using MX records for anything other than determining what
host to connect to to deliver mail is, at best, a heuristic.  As is
each of the others, when used for anything other than its respective
purpose.

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