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Re: [ietf-smtp] MX pointing to CNAME

2016-08-24 13:27:12

--On Wednesday, August 24, 2016 12:44 -0400 Hector Santos
<hsantos(_at_)isdg(_dot_)net> wrote:

On a related note, lately I've been seeing more MX records
pointing to "Localhost"  and "No.Longer.available" exchange
host names.   At least with LocalHost, it resolves to
something but you have to watch for the internal SMTP loop
backs.  I don't see RFC5321 guidance to best handle the
LocalHost MX exchanges.  Loop Detection (via Received lines)
is the suggested way, but I noticed a major DKIM signing loop
as well so I added code to skip the MX "LocalHost" exchange on
the outbound side and force a bounce.

I prefer the NXDOMAIN approach if people are going to be
creating "fake" "Stub" MX exchange records rather than getting
rid of them.

As you might remember, there was a proposal (or several) that
5321 contain explicit instructions for a DNS way to say "no SMTP
service here".  While I remember the "foo MX 0 ." approach that
Arnt suggests as being among the least obnoxious of the
proposals, I also remember another food fight with no
conclusions and, IIR, some people arguing that is was not the
role of the SMTP standard to specific how to deny support for
SMTP without a connection being open.

If anyone considers it important enough, I'd recommend a
separate draft to specify the DNS entry to indicate "that
service not supported for this domain".  It might reasonably
specify that for assorted SRV records or the like as well as MX.
Should such a draft get through the system and show a reasonable
level of deployment, I'd think it would be reasonable to include
a reference to it in 5321bis should we ever get around to that.


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