Re: [exim-dev] [Bug 1026] New: dnslists doesn't allow unders…

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Author: Phil Pennock
To: Sven Hartge
CC: exim-dev
Subject: Re: [exim-dev] [Bug 1026] New: dnslists doesn't allow underscores
On 2010-09-30 at 12:31 +0200, Sven Hartge wrote:
> They are just not allowed in hostnames. Have a look at

Actually, they are allowed in hostnames, but not if you're intending to
use them in an http: URL (and some other URL schemes), or to send mail
from that host.

RFC 1035 provides a guideline for reducing interoperability problems,
but that's not a mandate. The guideline is one it is wise to follow,
especially if you ever want to send mail from that host to a system not
under your control, or if you want to run a web-server on it, but that
does not mean "not allowed".

The only current constraints I know of are in RFC 2396, where http is a
Server-based Naming Authority and so underscores are disallowed in the
URI grammar, and in RFC 5321, where the EHLO/HELO command needs to give
a hostname but constrains it to the Domain grammar rule (*sigh*).