Newbie Question: Is anyone actually using the Null MX (RFC 7505)?

Fri Feb 26 18:46:44 UTC 2021

Well, I bet my legacy system will bounce it for example...

Postfix 3.0: RFC 7505 ("Null MX" No Service Resource Record), Earlier 
Postfix versions will bounce mail because of a "Malformed DNS server reply". 

I cant speak about Sendmail, qmail, Exim.. when they started supporting it.

So, In my opinion changing already working standards in a way
that they arent full compat with old systems is imo bad aproach.

---------- Original message ----------

From: Suresh Ramasubramanian <ops.lists at gmail.com>
To: "borg at uu3.net" <borg at uu3.net>, "nanog at nanog.org" <nanog at nanog.org>
Subject: Re: Newbie Question: Is anyone actually using the Null MX (RFC 7505)?
Date: Fri, 26 Feb 2021 17:43:17 +0000

OK. In your experience, which legacy system is going to misinterpret this record?

The current RFC is from 2014-15 but the original idea from Mark Delany (then at Yahoo now at Apple) has been kicking around from 2006 or so. I remember contributing some text to the original draft RFC but can?t find any trace of it online right now.

It worked just fine even back then, I assure you. So if there is any legacy MTA that still doesn?t accept it, it probably relies on UUCP domain maps or similar.

--srs

From: NANOG <nanog-bounces+ops.lists=gmail.com at nanog.org> on behalf of borg at uu3.net <borg at uu3.net>
Date: Friday, 26 February 2021 at 10:51 PM
To: nanog at nanog.org <nanog at nanog.org>
Subject: Re: Newbie Question: Is anyone actually using the Null MX (RFC 7505)?
Thats cute, but remember that there are gazylion of legacy systems
on Internet as well. They might have no clue what do do with it..
Also remember that MTA is supposed to accept email to [ip] too.

On my opinion, its best to just have no MX record at all.
While MTA can fallback and try to do delivery by IN A record, I think
its not that big problem. You need to specify for what domains you
accept email anyway. And spammers will not care at all...

---------- Original message ----------

From: Pirawat WATANAPONGSE via NANOG <nanog at nanog.org>
To: nanog at nanog.org
Subject: Newbie Question: Is anyone actually using the Null MX (RFC 7505)?
Date: Fri, 26 Feb 2021 17:19:41 +0700

Dear all,

I put the ˙˙Null MX˙˙ Record (RFC 7505) into one of my domains yesterday,
then those online mail diagnostic tools out there start getting me worried:

It looks like most of those tools do not recognize the Null MX as a special
case; they just complain that they cannot find the mail server at ˙˙.˙˙
[Sarcasm: as if the root servers are going to provide mail service to a
mere mortal like me!]

Among a few shining exceptions (in a good way) is the good ol˙˙
https://bgp.he.net/ which does not show that domain as having any MX record.
[maybe it is also wrong, in the other direction?]

I fear that the MTAs are going to behave that same way, treating my Null MX
as a ˙˙misconfigured mail server name˙˙ and that my record will mean
unnecessary extra queries to the root servers. [well, minus cache hit]

So, here comes the questions:
1. Is there anyone actively using this Null MX? If so, may I please see
that actual record line (in BIND zone file format) just to satisfy myself
that I wrote mine correctly?
2. Which one makes more sense from the practical point-of-view: having a
Null MX Record for the no-mail domain, or having no MX record at all?

Thanks in advance for all advices,

--

Pirawat.