Network Working GroupInternet Engineering Task Force (IETF) J. LevineInternet-DraftRequest for Comments: 7505 Taughannock NetworksIntended status:Category: Standards Track M. DelanyExpires: March 17, 2015ISSN: 2070-1721 Apple Inc.September 13, 2014June 2015 A "Null MX" No Service Resource Record for DomainsthatThat Accept No Maildraft-ietf-appsawg-nullmx-10Abstract 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.UnfortunatelyUnfortunately, this means that theA/ AAAAA/AAAA record is taken to be mail server address even when that address does not accept mail. Theno serviceNo Service MX RR, informally callednull MX,"null MX", formalizes the existing mechanism by which a domain announces that it accepts no mail, without having to provide a mailserver, whichserver; this permits significant operational efficiencies. Status of This Memo ThisInternet-Draftissubmitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documentsan Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF).Note that other groups may also distribute working documents as Internet-Drafts. The listIt represents the consensus ofcurrent Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents validthe IETF community. It has received public review and has been approved fora maximumpublication 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 ofsix monthsthis document, any errata, and how to provide feedback on it may beupdated, replaced, or obsoleted by other documentsobtained atany time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on March 17, 2015.http://www.rfc-editor.org/info/rfc7505. Copyright Notice Copyright (c)20142015 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. Table of Contents 1.Conventions Used in This DocumentIntroduction . . . . . . . . . . . . . .2 2. 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 DomainsthatThat Publish Null MX . . . . . 4 5. Security Considerations . . . . . . . . . . . . . . . . . . .54 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 7.Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 8.References . . . . . . . . . . . . . . . . . . . . . . . . . 58.1.7.1. Normative References . . . . . . . . . . . . . . . . . . 58.2.7.2. Informative References . . . . . . . . . . . . . . . . . 6Appendix 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 . . . . . . . . . . . . . .Acknowledgements . . . . . . . . .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.. . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction This document defines the No Service MX, informally callednull MX,"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 indetail, butdetail; inessenceessence, the SMTP client first looks up a DNS MXRR andRR, and, if that is notfoundfound, it falls back to looking up a DNS A or AAAA RR.HenceHence, this overloadsan email service semantic ontoa DNS recordwith(that has a different primarymission.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 AAAArecord's addresses.records. If thereisare no SMTPlistenerlisteners at the A/AAAAaddress,addresses, message delivery will be attempted repeatedly for a long period, typically a week, before the sendingMTAMail Transfer Agent (MTA) gives up. This will delay notification to the sender in the case of misdirectedmail,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]. 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[RFC1035], section 3.3.9)Section 3.3.9 of [RFC1035]) with an RDATA section consisting of preference number0,0 and azero lengthzero-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 recordcan notcannot be confused with an ordinary MX record. The use of "." as apseudo-host namepseudo-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, toalice@www.example.com or alice@example.orgalice@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 replycode[code521556]code [RFC7504] (Requested action not taken: domain does not accept mail) and a5.1.TBD5.1.10 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 a5.7.TBD5.7.27 enhanced status code (Permanent failure: Sender address has null MX). 4.2. Sending Mail from DomainsthatThat 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.HenceHence, mail systems SHOULD NOT publish a null MX record for domains that they use in RFC5321.MailFrom or RFC5322.From addresses. If aserversystem nonetheless does so, it risks having its mail rejected. Operators of domains that do not send mail can publishSPF -all [RFC7208]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 nullreverse pathreverse-path described in Section 4.5.5 of RFC 5321section 4.5.5and does not change the meaning or use of a nullreverse path.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. 6. IANA Considerations IANAis requested to addhas added the following entries to the "Enumerated Status Codes"sub-registrysubregistry of theSimple"Simple Mail Transfer Protocol (SMTP) Enhanced Status CodesRegistry.Registry". Code:X.1.TBDX.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]This document Submitter:[authorsAuthors of thisdocument]document Change controller: IESG Code:X.7.TBDX.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.(e.g., because it could not return a DSN). Reference:[this document]This document Submitter:[authorsAuthors of thisdocument]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.References8.1.7.1. Normative References [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November1987.1987, <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, March1997.1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October2008. [code521556]2008, <http://www.rfc-editor.org/info/rfc5321>. [RFC7504] Klensin, J., "SMTP 521 and 556 Reply Codes",internet- draft draft-klensin-smtp-521code, . 8.2.RFC 7504, DOI 10.17487/RFC7504, June 2015, <http://www.rfc-editor.org/info/rfc7504>. 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, February2000.2000, <http://www.rfc-editor.org/info/rfc2782>. [RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, DOI 10.17487/RFC5598, July2009.2009, <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, April2014. Appendix A. Change Log *NOTE TO RFC EDITOR: This section may be removed upon publication2014, <http://www.rfc-editor.org/info/rfc7208>. Acknowledgements We thank Dave Crocker for his diligent and lengthy shepherding of thisdocument 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 codesdocument, andrefmembers of the APPSAWG working group for521 return code. Even yet more editorial cleanup. 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.their constructive suggestions. Authors' Addresses John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 United States Phone: +1 831 480 2300Email:EMail: standards@taugh.com URI: http://jl.ly Mark Delany Apple Inc. 1 Infinite Loop Cupertino, CA 95014Email:United States EMail: mx0dot@yahoo.com