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Re: domain name definition in RFC2821

2007-03-21 18:18:39

--On Wednesday, March 21, 2007 21:42 +0100 Frank Ellermann <nobody(_at_)xyzzy(_dot_)claranet(_dot_)de> wrote:

Hi, Alex asked me to forward his reply to the SMTP list, see


Date: Tue, 20 Mar 2007 20:59:24 +0100
From: Alex van den Bogaerdt <alex(_at_)ergens(_dot_)op(_dot_)het(_dot_)net>
Content-Length: 1470

On Tue, Mar 20, 2007 at 05:41:55PM +0100, Frank Ellermann

> I do not believe that there is any remaining question
> about trailing periods on multiple-label FQDNs.  They
> are prohibited.

+1 wrt SMTP (wrt SPF policies it's a different story)


I'm not subscribed to that list, so forgive me for replying
directly to you.  Please forward it to the list if you think
it is appropriate.

If I may quote a part of RFC3696:

"The DNS specification also permits a trailing period to be
used to  denote the root, e.g., "a.b.c" and "a.b.c." are
equivalent, but the  latter is more explicit and is required
to be accepted by applications.  This convention is especially
important when a TLD name is being  referred to directly.

is required to be accepted<<<

Are the 2821bis folks really going to make the same mistakes?

A TLD is a domain like any other.  Poor judgement from large
companies and such doesn't make a TLD less valid.  Besides,
allowing a trailing dot may actually help them, to distinguish
between "" as part of domain
"" and "" as
the (existing) FQDN. (sorry for using an existing example!)

RFC 821 allowed one label (called "name").  It is 2821 that is

Together with the statement about trailing dot, <domain>
should be:

<domain> ::= <label> [ "." ] | <label> "." <domain>
domain = *(label ".") tld ["."]

But this is almost exactly what was in 2821bis-00, and people made me take it out. The question now is whether I didn't take out enough or took out a bit too much.

FWIW, I believe that, in retrospect, 821 should have permitted the trailing period. But it didn't, probably partially because of bad timing wrt other things. And 1123, which strengthened the trailing period rule for other cases, didn't apply it to email. The consensus when 2821bis-00 was posted was that permitting it now would be a disruptive incompatible change after too many years of established practice.