Internet DRAFT - draft-delany-nullmx
draft-delany-nullmx
Network Working Group J. Levine
Internet-Draft Taughannock Networks
Intended status: Standards Track M. Delany
Expires: July 7, 2014 Apple Inc.
January 3, 2014
A NULL MX Resource Record for Domains that Accept No Mail
draft-delany-nullmx-02
Abstract
When the 5321.MailFrom domain in an e-mail message has a DNS MX
Resource Record (RR), it is making an explicit statement that it is
willing to accept email. However, when the domain has just a DNS A
or AAAA RR, there mail clients cannot easily tell whether the domain
accepts mail, as many hosts on the Internet advertise an A or AAAA RR
regardless of whether they want to accept email.
The NULL MX RR formalizes the existing mechanism by which a domain
announces that it accepts no mail.
Status of This Memo
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This Internet-Draft will expire on July 7, 2014.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. SMTP server benefits . . . . . . . . . . . . . . . . . . . . 3
3. Parallel Considerations . . . . . . . . . . . . . . . . . . . 3
4. The NULL MX Resource Record . . . . . . . . . . . . . . . . . 4
5. Domains that do not send mail . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Inforrmative References . . . . . . . . . . . . . . . . . 5
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 5
A.1. Change from -01 to -02 . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
This document formally defines the "NULL MX" as a simple mechanism by
which a domain can indicate that it will never accept email.
SMTP clients have a prescribed sequence for resolving how to deliver
email to 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.
Many domains do not accept email, but do have A or AAAA records. If
they have no MX records, senders will attempt to deliver mail to
those A or AAAA records.
If there is no SMTP listener at that address, the message 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.
If the domain has an SMTP listener at that address that rejects all
connections (for instance with a 554 response as a connection-opening
response) or has MX records pointing to such a listener then the
sender will be notified 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.
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These resource usage problems are exacerbated when large volumes of
email are sent using forged email addresses in 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 sender of the bounces.
This document defines a NULL MX that will cause all mail delivery
attempts to a domain to fail immediately, without any reconfiguration
of existing MTAs.
2. SMTP server benefits
Being able to detect domains that never accept email offers many
resource savings to an SMTP server. In the first instance, it can
choose to reject email during the SMTP conversation that does not
present a deliverable 5321.MailFrom domain.
In the second instance, if an SMTP server accepts an email, it can be
confident that an attempt to send a non-delivery email will likely be
answered by another SMTP server. This greatly helps to reduce non-
delivery queues. This contrasts greatly with the current situation
where a non-delivery email for, e.g., www.example.net, will sit in
the queue for a full queue lifetime as SMTP connection attempts to
www.example.net simply time out.
3. Parallel Considerations
Clearly the perpetrators of abusive 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@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.
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4. The NULL MX Resource Record
To indicate that a domain never accepts 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. If a domain advertises multiple MX RRs including a
NULL MX, the interpretation is as described in RFC5321.
5. Domains that do not send mail
An SMTP server when presented with an "I never accept email" MX might
decline to accept such email as it knows that a response or non-
delivery notice will never be accepted, and that 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.
Although NULL MX may imply that a domain sends no mail, it does not
sat so explicitly. Operators may want to publish SPF [RFC4408] -ALL
policies to make an explicit statement.
6. Security Considerations
SMTP mail is inherently insecure in that it is feasible for even
fairly casual users to negotiate directly with SMTP servers. This
proposal is about eliminating one small section of SMTP insecurity.
In the unlikely event that a domain legitimately sends email but
never wants to receive email, SMTP servers that reject mail from
domains that advertise a NULL MX risk losing email from those
domains. Note that 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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7. References
7.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
7.2. Inforrmative References
[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1", RFC
4408, April 2006.
Appendix A. Change Log
*NOTE TO RFC EDITOR: This section may be removed upon publication of
this document as an RFC.*
A.1. Change from -01 to -02
Note that SPF -ALL is how you say you send no mail.
Authors' Addresses
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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