Network Working Group J. Levine
Internet-Draft Taughannock Networks
Intended status: Standards Track M. Delany
Expires: December 29, 2014 Apple Inc.
June 27, 2014
A NULL MX Resource Record for Domains that Accept No Mail
draft-ietf-appsawg-nullmx-05
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on December 29, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. The NULL MX Resource Record . . . . . . . . . . . . . . . . . 3
4. Effects of NULL MX . . . . . . . . . . . . . . . . . . . . . 3
4.1. SMTP Server Benefits . . . . . . . . . . . . . . . . . . 3
4.2. Dealing with undeliverable and abusive mail . . . . . . . 3
4.3. Domains that Do Not Send Mail . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
7.1. Normative References . . . . . . . . . . . . . . . . . . 4
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 5
A.1. Change to appsawg-nullmx-05 . . . . . . . . . . . . . . . 5
A.2. Change to appsawg-nullmx-04 . . . . . . . . . . . . . . . 5
A.3. Change to appsawg-nullmx-03 . . . . . . . . . . . . . . . 5
A.4. Change to appsawg-nullmx-02 . . . . . . . . . . . . . . . 5
A.5. Change to appsawg-nullmx-1 . . . . . . . . . . . . . . . 5
A.6. Change to appsawg-nullmx-0 . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
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 does not accept email.
SMTP clients have a prescribed sequence for identifying a server 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. If there
is no SMTP listener at the A/AAAA address, message delivery will be
attempted repeatedly for a long period, typically a week, before the
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sending MTA gives up. This will delay notification to the sender in
the case of misdirected mail, and will consume resources at the
sender.
This document defines a NULL MX that will cause all mail delivery
attempts to a domain to fail immediately, without requiring domains
to create SMTP listeners dedicated to preventing delivery attempts.
3. The NULL MX Resource Record
To indicate that a domain does not accept email, it advertises a
single MX RR (see [RFC1035], section 3.3.9) with an RDATA section
consisting of preference number 0, and a dot, i.e., the DNS root, as
the exchange domain, to denote that there exists no mail exchanger
for a domain. The DNS root is not a valid host name, so a NULL MX
record can not be confused with an ordinary MX record.
A domain MUST NOT advertise multiple MX RRs including a NULL MX.
4. Effects of NULL MX
The NULL MX record has a variety of efficiency and usability
benefits.
4.1. SMTP Server Benefits
The ability to detect domains that do not accept email offers many
resource savings to an SMTP client. It will discover on the first
sending attempt that an address is not deliverable, avoiding queuing
and retries.
A receiving SMTP server that chooses to reject email during the SMTP
conversation that presents an undeliverable RFC5321.MailFrom or
RFC5322.From domain (see [RFC5598] for the definitions of these
terms) can be more confident that an attempt to send a Delivery
Status Notification or other response will reach a recipient SMTP
server.
4.2. Dealing with undeliverable and abusive mail
Mail often has an incorrect address due to user error, where the
address was mistranscribed or misunderstood, for example, to
alice@www.example.com or alice@example.org or alice@examp1e.com
rather than alice@example.com. NULL MX allows a mail system to
report the delivery failure when the user sends the message, rather
than hours or days later.
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Senders of abusive mail often use forged undeliverable return
addresses. NULL MX allows DSNs and other attempted responses to such
mail to be disposed of efficiently.
4.3. Domains that Do Not Send Mail
The operator of an SMTP server might prefer to reject mail with a
RFC5321.MailFrom or RFC5322.From domain that publishes NULL MX, since
a non-delivery notice or response will not 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 and a 5.1.2 enhanced status
code [RFC3463].
A domain that does not accept mail, as declared by NULL MX, often
will also not send mail. Operators can publish SPF -all[RFC7208]
policies to make an explicit declaration that domains send no mail.
5. Security Considerations
SMTP mail is inherently insecure since it does not validate any of
the e-mail addresses in the message or envelope. This specification
is about eliminating one small section of SMTP insecurity.
In the unlikely event that a domain legitimately sends email but does
not want to receive email, SMTP servers that reject mail from domains
that advertise a NULL MX risk losing email from those domains. The
normal way to send mail for which a sender wants no responses remains
unchanged, by using an empty RFC5321.MailFrom address.
Within the DNS, a NULL MX RR is an ordinary MX record and presents no
new security issues. If desired, it can be secured in the same
manner as any other DNS record using DNSSEC.
6. IANA Considerations
This document makes no requests of IANA.
7. References
7.1. Normative References
[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.
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[RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
3463, January 2003.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
7.2. Informative References
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July
2009.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
April 2014.
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-05
Fix ID nits, add NULL IANA section. More editorial cleanup.
A.2. Change to appsawg-nullmx-04
Reorganize.
A.3. Change to appsawg-nullmx-03
Editorial nits per Murray.
A.4. Change to appsawg-nullmx-02
Should not publish NULL MX with other MX.
Never say never.
Add 5.1.2 enhanced status code.
Minor editorial changes.
A.5. Change to appsawg-nullmx-1
Editorial improvements per D. Crocker's review.
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A.6. 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
Mark Delany
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
Email: mx0dot@yahoo.com
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