Internet DRAFT - draft-ietf-appsawg-nullmx
draft-ietf-appsawg-nullmx
Network Working Group J. Levine
Internet-Draft Taughannock Networks
Intended status: Standards Track M. Delany
Expires: March 17, 2015 Apple Inc.
September 13, 2014
A "Null MX" No Service Resource Record for Domains that Accept No Mail
draft-ietf-appsawg-nullmx-10
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 no service MX RR, informally called null
MX, formalizes the existing mechanism by which a domain announces
that it accepts no mail, without having to provide a mail server,
which permits significant operational efficiencies.
Status of This Memo
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This Internet-Draft will expire on March 17, 2015.
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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to this document. Code Components extracted from this document must
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Table of Contents
1. Conventions Used in This Document . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. MX Resource Records Specifying Null MX . . . . . . . . . . . 3
4. Effects of Null MX . . . . . . . . . . . . . . . . . . . . . 3
4.1. SMTP Server Benefits . . . . . . . . . . . . . . . . . . 3
4.2. Sending Mail from Domains that Publish Null MX . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 6
A.1. Change to appsawg-nullmx-10 . . . . . . . . . . . . . . . 6
A.2. Change to appsawg-nullmx-09 . . . . . . . . . . . . . . . 6
A.3. Change to appsawg-nullmx-08 . . . . . . . . . . . . . . . 6
A.4. Change to appsawg-nullmx-07 . . . . . . . . . . . . . . . 6
A.5. Change to appsawg-nullmx-06 . . . . . . . . . . . . . . . 7
A.6. Change to appsawg-nullmx-05 . . . . . . . . . . . . . . . 7
A.7. Change to appsawg-nullmx-04 . . . . . . . . . . . . . . . 7
A.8. Change to appsawg-nullmx-03 . . . . . . . . . . . . . . . 7
A.9. Change to appsawg-nullmx-02 . . . . . . . . . . . . . . . 7
A.10. Change to appsawg-nullmx-1 . . . . . . . . . . . . . . . 7
A.11. Change to appsawg-nullmx-0 . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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].
The terms RFC5321.MailFrom and RFC5322.From are used as defined in
[RFC5598].
2. Introduction
This document defines the No Service MX, informally called null MX,
as a simple mechanism by which a domain can indicate that it does not
accept email.
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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
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. MX Resource Records Specifying Null MX
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 zero length label, written
in master files as ".", as the exchange domain, to denote that there
exists no mail exchanger for a domain. Since "." is not a valid host
name, a null MX record can not be confused with an ordinary MX
record. The use of "." as a pseudo-host name meaning no service
available is modeled on the SRV RR [RFC2782] where it has a similar
meaning.
A domain that advertises a null MX MUST NOT advertise any other MX
RR.
4. Effects of Null MX
The null MX record has a variety of efficiency and usability
benefits.
4.1. SMTP Server Benefits
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 Delivery Status Notifications (DSNs) and
other attempted responses to such mail to be disposed of efficiently.
The ability to detect domains that do not accept email offers
resource savings to an SMTP client. It will discover on the first
sending attempt that an address is not deliverable, avoiding queuing
and retries.
When a submission or SMTP relay server rejects an envelope recipient
due to a domain's null MX record, it SHOULD use a 556 reply
code[code521556] (Requested action not taken: domain does not accept
mail) and a 5.1.TBD enhanced status code (Permanent failure:
Recipient address has null MX).
A receiving SMTP server that chooses to reject email during the SMTP
conversation that presents an undeliverable RFC5321.MailFrom or
RFC5322.From domain can be more confident that for other messages a
subsequent attempt to send a DSN or other response will reach a
recipient SMTP server.
SMTP servers that reject mail because a RFC5321.MailFrom or
RFC5322.From domain has a null MX record SHOULD use a 550 reply code
(Requested action not taken: mailbox unavailable) and a 5.7.TBD
enhanced status code (Permanent failure: Sender address has null MX).
4.2. Sending Mail from Domains that Publish Null MX
Null MX is primarily intended for domains that do not send or receive
any mail, but have mail sent to them anyway due to mistakes or
malice. Many receiving systems reject mail that has an invalid
return address. Return addresses are needed to allow the sender to
handle message delivery errors. An invalid return address often
signals that the message is spam. Hence mail systems SHOULD NOT
publish a null MX record for domains that they use in
RFC5321.MailFrom or RFC5322.From addresses. If a server nonetheless
does so, it risks having its mail rejected.
Operators of domains that do not send mail can publish SPF -all
[RFC7208] policies to make an explicit declaration that the domains
send no mail.
Null MX is not intended to be a replacement for the null reverse path
described in RFC 5321 section 4.5.5 and does not change the meaning
or use of a null reverse path.
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5. Security Considerations
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
IANA is requested to add the following entries to the "Enumerated
Status Codes" sub-registry of the Simple Mail Transfer Protocol
(SMTP) Enhanced Status Codes Registry.
Code: X.1.TBD
Sample Text: Recipient address has null MX
Associated basic status code: 556
Description: This status code is returned when the associated
address is marked as invalid using a null MX.
Reference: [this document]
Submitter: [authors of this document]
Change controller: IESG
Code: X.7.TBD
Sample Text: Sender address has null MX
Associated basic status code: 550
Description: This status code is returned when the associated
sender address has a null MX, and the SMTP
receiver is configured to reject mail from such
sender (e.g. because it could not return a DSN).
Reference: [this document]
Submitter: [authors of this document]
Change controller: IESG
7. Acknowledgements
We thank Dave Crocker for his diligent and lengthy shepherding of
this document, and members of the appsawg working group for their
constructive suggestions.
8. References
8.1. 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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[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[code521556]
Klensin, J., "SMTP 521 and 556 Reply Codes", internet-
draft draft-klensin-smtp-521code, .
8.2. Informative References
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[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-10
Minor twiddle to clarify reference.
A.2. Change to appsawg-nullmx-09
Change 521 to 556, change reference.
A.3. Change to appsawg-nullmx-08
Fix name of IANA registry.
Yea, even yet more editorial cleanup.
A.4. Change to appsawg-nullmx-07
Add new enhanced status codes and ref for 521 return code.
Even yet more editorial cleanup.
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A.5. Change to appsawg-nullmx-06
Even more editorial cleanup.
Mention SRV
you SHOULD NOT put a null MX on domains that send mail
A.6. Change to appsawg-nullmx-05
Fix ID nits, add NULL IANA section. More editorial cleanup.
A.7. Change to appsawg-nullmx-04
Reorganize.
A.8. Change to appsawg-nullmx-03
Editorial nits per Murray.
A.9. 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.10. Change to appsawg-nullmx-1
Editorial improvements per D. Crocker's review.
A.11. 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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