GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc7505

Internet Engineering Task Force (IETF) J. Levine Request for Comments: 7505 Taughannock Networks Category: Standards Track M. Delany ISSN: 2070-1721 Apple Inc.

                                                             June 2015

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

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; this permits significant operational efficiencies.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7505.

Copyright Notice

 Copyright (c) 2015 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
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 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.

Levine & Delany Standards Track [Page 1] RFC 7505 Null MX June 2015

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Conventions Used in This Document . . . . . . . . . . . . . .   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 . . . . . . . . . . . . . . . . . . .   4
 6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
   7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1. 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.
 SMTP clients have a prescribed sequence for identifying a server that
 accepts email for a domain.  Section 5 of [RFC5321] covers this in
 detail; 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 a DNS record (that has a different primary
 mission) with an email service semantic.
 If a domain has no MX records, senders will attempt to deliver mail
 to the hosts at the addresses in the domain's A or AAAA records.  If
 there are no SMTP listeners at the A/AAAA addresses, message delivery
 will be attempted repeatedly for a long period, typically a week,
 before the sending Mail Transfer Agent (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.

2. 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].

Levine & Delany Standards Track [Page 2] RFC 7505 Null MX June 2015

 The terms "RFC5321.MailFrom" and "RFC5322.From" are used as defined
 in [RFC5598].

3. MX Resource Records Specifying Null MX

 To indicate that a domain does not accept email, it advertises a
 single MX RR (see Section 3.3.9 of [RFC1035]) 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 cannot be confused with an ordinary MX record.
 The use of "." as a pseudo-hostname 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, 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.
 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
 [RFC7504] (Requested action not taken: domain does not accept mail)
 and a 5.1.10 enhanced status code (Permanent failure: Recipient
 address has null MX).

Levine & Delany Standards Track [Page 3] RFC 7505 Null MX June 2015

 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 5.7.27
 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 system nonetheless
 does so, it risks having its mail rejected.
 Operators of domains that do not send mail can publish Sender Policy
 Framework (SPF) "-all" policies [RFC7208] 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 Section 4.5.5 of RFC 5321 and does not change the
 meaning or use of a null reverse-path.

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.

Levine & Delany Standards Track [Page 4] RFC 7505 Null MX June 2015

6. IANA Considerations

 IANA has added the following entries to the "Enumerated Status Codes"
 subregistry of the "Simple Mail Transfer Protocol (SMTP) Enhanced
 Status Codes Registry".
 Code:              X.1.10
 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.7.27
 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. References

7.1. Normative References

 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
            November 1987, <http://www.rfc-editor.org/info/rfc1035>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
            DOI 10.17487/RFC5321, October 2008,
            <http://www.rfc-editor.org/info/rfc5321>.
 [RFC7504]  Klensin, J., "SMTP 521 and 556 Reply Codes", RFC 7504,
            DOI 10.17487/RFC7504, June 2015,
            <http://www.rfc-editor.org/info/rfc7504>.

Levine & Delany Standards Track [Page 5] RFC 7505 Null MX June 2015

7.2. Informative References

 [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
            specifying the location of services (DNS SRV)", RFC 2782,
            DOI 10.17487/RFC2782, February 2000,
            <http://www.rfc-editor.org/info/rfc2782>.
 [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
            DOI 10.17487/RFC5598, July 2009,
            <http://www.rfc-editor.org/info/rfc5598>.
 [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
            Authorizing Use of Domains in Email, Version 1", RFC 7208,
            DOI 10.17487/RFC7208, April 2014,
            <http://www.rfc-editor.org/info/rfc7208>.

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.

Authors' Addresses

 John Levine
 Taughannock Networks
 PO Box 727
 Trumansburg, NY  14886
 United States
 Phone: +1 831 480 2300
 Email: standards@taugh.com
 URI:   http://jl.ly
 Mark Delany
 Apple Inc.
 1 Infinite Loop
 Cupertino, CA  95014
 United States
 Email: mx0dot@yahoo.com

Levine & Delany Standards Track [Page 6]

/data/webs/external/dokuwiki/data/pages/rfc/rfc7505.txt · Last modified: 2015/06/30 23:24 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki