Network Working Group J. Levine Internet-Draft Taughannock Networks Intended status: Standards Track M. Delany Expires: December 20, 2014 Apple Inc. June 18, 2014 A NULL MX Resource Record for Domains that Accept No Mail draft-ietf-appsawg-nullmx-04 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any 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 December 20, 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 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 Levine & Delany Expires December 20, 2014 [Page 1] Internet-Draft NULL MX June 2014 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 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. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 6.1. Normative References . . . . . . . . . . . . . . . . . . 4 6.2. Inforrmative References . . . . . . . . . . . . . . . . . 5 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 5 A.1. Change to appsawg-nullmx-04 . . . . . . . . . . . . . . . 5 A.2. Change to appsawg-nullmx-03 . . . . . . . . . . . . . . . 5 A.3. Change to appsawg-nullmx-02 . . . . . . . . . . . . . . . 5 A.4. Change to appsawg-nullmx-1 . . . . . . . . . . . . . . . 5 A.5. Change to appsawg-nullmx-0 . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 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. However many domains do not accept email. If there is no SMTP listener at the A/AAAA address, message delivery will be attempted repeatedly for a long period, typically a week, Levine & Delany Expires December 20, 2014 [Page 2] Internet-Draft NULL MX June 2014 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. 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, which avoids any possibility that a NULL MX record could 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 thise 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. Levine & Delany Expires December 20, 2014 [Page 3] Internet-Draft NULL MX June 2014 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 [RFC7208] -all policies to make an explicit declaration that the domain is not valid in the RFC5321.MailFrom command. 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. References 6.1. 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. [RFC3463] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463, January 2003. Levine & Delany Expires December 20, 2014 [Page 4] Internet-Draft NULL MX June 2014 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. 6.2. Inforrmative 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-04 Reorganize. A.2. Change to appsawg-nullmx-03 Editorial nits per Murray. A.3. 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.4. Change to appsawg-nullmx-1 Editorial improvements per D. Crocker's review. A.5. Change to appsawg-nullmx-0 Fix typos. Authors' Addresses Levine & Delany Expires December 20, 2014 [Page 5] Internet-Draft NULL MX June 2014 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 Levine & Delany Expires December 20, 2014 [Page 6]