Movatterモバイル変換

Internet Engineering Task Force (IETF)                         J. LevineRequest for Comments: 7505                          Taughannock NetworksCategory: Standards Track                                      M. DelanyISSN: 2070-1721                                               Apple Inc.                                                               June 2015A "Null MX" No Service Resource Record for Domains That Accept No MailAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 2015Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions Used in This Document . . . . . . . . . . . . . .23.  MX Resource Records Specifying Null MX  . . . . . . . . . . .34.  Effects of Null MX  . . . . . . . . . . . . . . . . . . . . .34.1.  SMTP Server Benefits  . . . . . . . . . . . . . . . . . .34.2.  Sending Mail from Domains That Publish Null MX  . . . . .45.  Security Considerations . . . . . . . . . . . . . . . . . . .46.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .57.  References  . . . . . . . . . . . . . . . . . . . . . . . . .57.1.  Normative References  . . . . . . . . . . . . . . . . . .57.2.  Informative References  . . . . . . . . . . . . . . . . .6   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .6   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .61.  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 (seeSection 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 undeliverableRFC5321.MailFrom orRFC5322.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 aRFC5321.MailFrom orRFC5322.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 inRFC5321.MailFrom orRFC5322.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 inSection 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 20156.  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: IESG7.  References7.1.  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 20157.2.  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.comLevine & Delany              Standards Track                    [Page 6]