Network Working Group J. Levine
Internet-Draft Taughannock Networks
Intended status: Standards Track M. Delany
Expires: November 26, 2014 Apple Inc.
May 25, 2014
A NULL MX Resource Record for Domains that Accept No Mail
draft-ietf-appsawg-nullmx-02
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 26, 2014.
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Table of Contents
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. SMTP server benefits . . . . . . . . . . . . . . . . . . . . 3
4. Parallel Considerations . . . . . . . . . . . . . . . . . . . 3
5. The NULL MX Resource Record . . . . . . . . . . . . . . . . . 4
6. Domains that do not send mail . . . . . . . . . . . . . . . . 4
7. Security Considerations . . . . . . . . . . . . . . . . . . . 4
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Inforrmative References . . . . . . . . . . . . . . . . . 5
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 5
A.1. Change to appsawg-nullmx-02 . . . . . . . . . . . . . . . 5
A.2. Change to appsawg-nullmx-1 . . . . . . . . . . . . . . . 5
A.3. 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,
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.
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A domain could set up an SMTP listener at that address that rejects
all connections (for instance with a 554 response as a connection-
opening response) or have an MX record pointing to such a listener,
to notify senders in a timely fashion. But resources (generating a
bounce) will still be consumed by the sender and it requires
additional services to be provided which provide little benefit to
the domain.
These resource usage problems are exacerbated when large volumes of
email are sent using forged email addresses from a domain which does
not accept email as its envelope sender, causing large numbers of
bounces to be generated and to consume large amounts of resources at
the senders of the bounces.
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. SMTP server benefits
The ability to detect domains that do not accept email offers many
resource savings to an SMTP server. It can choose to reject email
during the SMTP conversation that presents an undeliverable
5321.MailFrom domain. A sending server will discover on the first
sending attempt that an address is not deliverable, avoiding queuing
and retries.
Also, if an SMTP server accepts a message, it can be more confident
that an attempt to send a Delivery Status Notification or other
response will reach a recipient SMTP server.
4. Parallel Considerations
Senders of abusive email often use return addresses with domain names
that do not accept mail. the perpetrators of such mail can adapt
such that the "vast class of email" that this mechanism helps
identify, simply move over to using 5321.MailFrom domains that have
valid MX RRs.
While this is true, the direct benefits to the SMTP server still
apply. When an SMTP server queues a non-delivery email, the target
domain will accept the email or give a definitive rejection so the
queue entry will be removed promptly, thus keeping the queues short.
There is also a fair amount of mail that is just misaddressed by
people who mistranscribed or misunderstood an e-mail address, for
example, alice@www.example.com or alice@example.org or
alice@examp1e.com rather than alice@example.com. NULL MX allows a
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mail system to report the delivery failure when the user sends the
message, rather than hours or days later.
5. The NULL MX Resource Record
To indicate that a domain does not accept email, it advertises a
single MX RR with a RDATA section consisting of preference number 0,
and a dot, i.e., the DNS root, as the mail exchanger 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.)
The interpretation of a NULL MX RR only applies when the domain has a
single MX RR. A domain SHOULD NOT advertise multiple MX RRs
including a NULL MX, but if it does, the interpretation is as
described in [RFC5321].
6. Domains that do not send mail
The operator of an SMTP server might prefer to reject mail sent from
domains that publish NULL MX, since a response or non-delivery notice
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.
7. Security Considerations
SMTP mail is inherently insecure in that it is feasible for even
fairly casual users to negotiate directly with SMTP servers. 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 5321.MailFrom address.
Within the DNS, a NULL MX RR is an ordinary MX record and presents no
new security issues.
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8. References
8.1. Normative References
[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.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
8.2. Inforrmative References
[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-02
Should not publish NULL MX with other MX.
Never say never.
Add 5.1.2 enhanced status code.
Minor editorial changes.
A.2. Change to appsawg-nullmx-1
Editorial improvements per D. Crocker's review.
A.3. Change to appsawg-nullmx-0
Fix typos.
Authors' Addresses
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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
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