On Mon, May 19, 2003 at 12:47:46PM -0500, David P. Discher wrote:
>
> I have to prove to someone that underscores are not valid characters in
> internet domain and hostnames. Is there an RFC I can quote ?
RFC2821, section 4.1.2: Command Argument Syntax:
(near the end)
To promote interoperability and consistent with long-standing
guidance about conservative use of the DNS in naming and applications
(e.g., see section 2.3.1 of the base DNS document, RFC1035 [22]),
characters outside the set of alphas, digits, and hyphen MUST NOT
appear in domain name labels for SMTP clients or servers. In
particular, the underscore character is not permitted. SMTP servers
that receive a command in which invalid character codes have been
employed, and for which there are no other reasons for rejection,
MUST reject that command with a 501 response.
This applies to EHLO, because EHLO is a command that uses the domain
name label of a server.
And indeed exim (4.14 at least) does just this:
# exim -bh 127.0.0.1
...
S: 220 mail.silverwraith.com ESMTP Exim 4.14 Mon, 19 May 2003 11:13:25 -0700
C: ehlo asd_asd
S: 501 Syntactically invalid EHLO argument(s)
S: LOG: rejected EHLO from localhost.silverwraith.com [127.0.0.1]:
syntactically invalid argument(s): asd_asd
