A "Null MX" No Service Resource Record for Domains That Accept No Mail

Versions: (draft-delany-nullmx)   00 01 02 03 04         Standards Track
          05 06 07 08 09 10 rfc7505                                     
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


   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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 21, 2014.

Copyright Notice

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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",
   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

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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

   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

   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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