[ietf-smtp] MTS-STS validation when MX host points to a CNAME, violating RFC 2181 § 10.3
2021-03-31 16:46:40
Hello,
I'm having an affair with one of the vendors as a sending MTA, honoring
MTA-STS (RFC 8461). Their response:
* Our TDS validation shows MX lookup for n0.lt returns n0.lt instead of
mx.n0.lt. It is consistent with what we are seeing with production.
_remote server(451 4.4.8 MX hosts of 'n0.lt' failed MTA-STS
validation.)' 3/24/2021 3:36:19 PM - Server at n0.lt (0.0.0.0) returned
'450 4.4.317 Cannot connect to remote server [Message=451 4.4.8 MX
hosts of 'n0.lt' failed MTA-STS validation.]
[LastAttemptedServerName=n0.lt]_
* We can confirm that customer is not RFC compliant with MX pointing to
a CNAME and we don't think it is worth to change the logic to
accommodate that.
* Customer does have an easy fix on their side, just to modify their
STS Policy to include n0.lt as one of the supported MX record.
My objections:
I'm familiar with a general prohibition, pursuant to RFC 2181 § 10.3
[1], for MX records to point to CNAMEs. Despite that, I do not believe
that it should affect MX host validation in accordance with RFC 8
[2]461 §4.1 [2] when selecting a target MX host, for the reasons of:
*
RFC 2181 is an RFC on Clarifications to the DNS Specification, not
SMTP.
*
Selecting an MX target host is regulated in a different RFC, namely and
specifically in RFC 5321 §5.1 [3]:
If MX records are present, but none of them are usable, this situation
MUST be reported as an error.<...> When a domain name associated with
an MX RR is looked up and the associated data field obtained, the data
field of that response MUST contain a domain name. That domain name,
when queried, MUST return at least one address record (e.g., A or AAAA
RR) that gives the IP address of the SMTP server to which the message
should be directed. Any other response, specifically including a value
that will return a CNAME record when queried, lies outside the scope
of this Standard. The prohibition on labels in the data that resolve
to CNAMEs is discussed in more detail in RFC 2181, Section 10.3 [38]
*
Thus, the prohibition of CNAMEs is NOT an SMTP or MTA-STS issue. As per
the RFC 5321 §5.1, which is used to select an MX target host, pursuant
to RFC 8461 §4.1 (MX Host validation), vendors are NOT allowed to
choose a different host name (in my scenario, n0.lt instead of mx.n0.lt
which is found in MX record). The situation MUST be reported as an
error, if none of the found records are usuable. That MUST happened
even if the target domain has not deployed MTA-STS. And this doesn't
seem to be the case. When MTA-STS is not deployed, Microsoft, as a
sending MTA doesn't return any errors.
*
No major providers (including, but not limited to Gmail) nor publicly
available MTA-STS tests (including, but not limited to My Email
Communications Security Assessment (MECSA) by European Commission's
Joint Research Center) doesn't select n0.lt instead of mx.n0.lt which
is found in MX record nor does it show any errors, suggesting my
interpretation of different RFCs is most likely correct.
As advised by one of the authors of RFC 8461, I'm reaching out to the
IETF SMTP list for your opinions, namely if MTA-STS, in theory, should
fail to validate if an MX points to a CNAME.
Any insights would be much appreciated and thanked in advance. :)
--
Regards,
Kristijonas
Links:
------
[1] https://tools.ietf.org/html/rfc2181#section-10.3
[2] https://tools.ietf.org/html/rfc8461#section-4.1
[3] https://tools.ietf.org/html/rfc5321#section-5.1 _______________________________________________
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