[Mimedefang] Revisit: Filtering on HELO

Kees Theunissen theuniss at rijnh.nl
Fri Mar 16 03:51:01 EST 2007

On Thu, 15 Mar 2007, Dirk the Daring wrote:

>   Which begs the question, what is the definition of "fully-qualified domain
> name"? It turns out the term is defined in RFC 1983, which states on Page 21:
> 	Fully Qualified Domain Name (FQDN)
> 		The FQDN is the full name of a system, rather than just
> 		its hostname.  For example, "venera" is a hostname and
> 		"venera.isi.edu" is an FQDN.

This only says that the FQDN includes all (sub)domains up to and including
the top level domain. It doen't say anything about the number of dots /
elements in the FQDN.

>   So, I stand by my original filter code, which would reject a "domain.tld"
> argument to EHLO (or HELO) as not being an FQDN. If the HELO is not an address
> literal, then it should be an FDQN.

In my perception any name specified up to and including the tld is a FQDN.
And any name, specified up to and including the tld, that has an A record
in DNS and a ptr record correctly pointing to that name, is a _valid_ FQDN.

This doesn't mean that a legitimate mail server is likely to use "domain.tld"
as the argument to EHLO, but that is an other question.



Kees Theunissen
F.O.M.-Institute for Plasma Physics Rijnhuizen, Nieuwegein, Netherlands
E-mail: theuniss at rijnh.nl,  Tel: (+31|0)306096724,  Fax: (+31|0)306031204

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