Network Working Group J. Levine
Internet-Draft Taughannock Networks
Intended status: Standards Track M. Delany
Expires: November 21, 2014 Apple Inc.
May 20, 2014
A NULL MX Resource Record for Domains that Accept No Mail
draft-ietf-appsawg-nullmx-01
Abstract
Internet mail determines the address of a receiving server through
the DNS, first by looking for an MX record and then by looking for an
A/AAAA record as a fallback. Unfortunately this means that the A/
AAAA record is taken to be mail server address even when that address
does not accept mail. The NULL MX RR formalizes the existing
mechanism by which a domain announces that it accepts no mail, which
permits significant operational efficiencies.
Status of This Memo
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This Internet-Draft will expire on November 21, 2014.
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Table of Contents
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. SMTP server benefits . . . . . . . . . . . . . . . . . . . . 3
4. Parallel Considerations . . . . . . . . . . . . . . . . . . . 3
5. The NULL MX Resource Record . . . . . . . . . . . . . . . . . 4
6. Domains that do not send mail . . . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . 4
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Inforrmative References . . . . . . . . . . . . . . . . . 5
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 5
A.1. Change to appsawg-nullmx-1 . . . . . . . . . . . . . . . 5
A.2. Change to appsawg-nullmx-0 . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Introduction
This document formally defines the "NULL MX" as a simple mechanism by
which a domain can indicate that it will never accept email.
SMTP clients have a prescribed sequence for identifying aserver that
accepts email for a domain. Section 5 of [RFC5321] covers this in
detail, but in essence the SMTP client first looks up a DNS MX RR and
if that is not found it falls back to looking up a DNS A or AAAA RR.
Hence this overloads an email service semantic onto a DNS record with
a different primary mission.
If a domain has no MX records, senders will attempt to deliver mail
to the hosts at the domain's A or AAAA record's addresses. However
many domains do not accept email.
If there is no SMTP listener at the A/AAAA address, the message will
be attempted repeatedly for a long period, typically a week, before
the sending MTA gives up. This will delay notification to the sender
in the case of misdirected mail, and will consume resources at the
sender.
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A domain could set up an SMTP listener at that address that rejects
all connections (for instance with a 554 response as a connection-
opening response) or have an MX record pointing to such a listener,
to notify senders in a timely fashion. But resources (generating a
bounce) will still be consumed by the sender and it requires
additional services to be provided which provide little benefit to
the domain.
These resource usage problems are exacerbated when large volumes of
email are sent using forged email addresses from a domain which does
not accept email as its envelope sender, causing large numbers of
bounces to be generated and to consume large amounts of resources at
the sender of the bounces.
This document defines a NULL MX that will cause all mail delivery
attempts to a domain to fail immediately.
3. SMTP server benefits
The ability to detect domains that never accept email offers many
resource savings to an SMTP server. It can choose to reject email
during the SMTP conversation that presents an undeliverable
5321.MailFrom domain.
Also, if an SMTP server accepts a message, it can be more confident
that an attempt to send a Delivery Status Notification or other
response will reach a recipient SMTP server. This helps to reduce
non-delivery queues. Currently, a DSN for, e.g., www.example.net,
will sit in the queue for a full queue lifetime until the server's
attempts to deliver to www.example.net time out.
4. Parallel Considerations
Senders of abusive email often use return addresses with domain names
that do not accept mail. the perpetrators of such mail can adapt
such that the "vast class of email" that this mechanism helps
identify, simply move over to using 5321.MailFrom domains that have
valid MX RRs.
While this is true, the direct benefits to the SMTP server still
apply. When an SMTP server queues a non-delivery email, the target
domain will accept the email or give a definitive rejection so the
queue entry will be removed promptly, thus keeping the queues short.
There is also a fair amount of mail that is just misaddressed by
people who mistranscribed or misunderstood an e-mail address, for
example, alice@www.example.com or alice@examp1e.com rather than
alice@example.com. NULL MX allows a mail system to report the
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delivery failure when the user sends the message, rather than hours
or days later.
5. The NULL MX Resource Record
To indicate that a domain never accepts email, it advertises a single
MX RR with a RDATA section consisting of preference number 0, and a
dot, i.e., the DNS root, as the mail exchanger domain, to denote that
there exists no mail exchanger for a domain. (The DNS root is not a
valid host name, which avoids any possibility that a NULL MX record
could be confused with an ordinary MX record.)
The interpretation of a NULL MX RR only applies when the domain has a
single MX RR. If a domain advertises multiple MX RRs including a
NULL MX, the interpretation is as described in RFC5321.
6. Domains that do not send mail
The operator of an SMTP server might prefer to reject mail sent from
domains that publish NULL MX, since a response or non-delivery notice
will never be accepted, and legitimate mail rarely comes from domains
that do not accept replies.
SMTP servers that reject mail because a MAIL FROM domain has a NULL
MX record SHOULD use a 550 reply code.
A domain that does not accept mail, as declared by NULL MX, often
will also not send mail. Operators can publish SPF [RFC4408] -ALL
policies to make an explicit declaration that the domain is not valid
in the rfc5321.mailfrom command.
7. Security Considerations
SMTP mail is inherently insecure in that it is feasible for even
fairly casual users to negotiate directly with SMTP servers. This
specification is about eliminating one small section of SMTP
insecurity.
In the unlikely event that a domain legitimately sends email but
never wants to receive email, SMTP servers that reject mail from
domains that advertise a NULL MX risk losing email from those
domains. Note that the normal way to send mail for which a sender
wants no responses remains unchanged, by using an empty 5321.MailFrom
address.
Within the DNS, a NULL MX RR is an ordinary MX record and presents no
new security issues.
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8. References
8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
8.2. Inforrmative References
[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1", RFC
4408, April 2006.
Appendix A. Change Log
*NOTE TO RFC EDITOR: This section may be removed upon publication of
this document as an RFC.*
A.1. Change to appsawg-nullmx-1
Editorial improvements per D. Crocker's review.
A.2. Change to appsawg-nullmx-0
Fix typos.
Authors' Addresses
John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886
Phone: +1 831 480 2300
Email: standards@taugh.com
URI: http://jl.ly
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Mark Delany
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
Email: mx0dot@yahoo.com
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