Network Working Group C. Hutzler
Internet-Draft America Online
Expires: March 24, 2006 D. Crocker
Brandenburg InternetWorking
P. Resnick
QUALCOMM Incorporated
R. Sanders
Earthlink, Inc.
E. Allman
Sendmail, Inc.
24 October 2005
Email Submission: Access and Accountability
draft-hutzler-spamops-05
Status of this Memo
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Abstract
Email has become a popular distribution service for a variety of
socially unacceptable, mass-effect purposes. The most obvious
ones include spam and worms. This note recommends conventions for
the operation of email submission and transport services between
independent operators, such as enterprises and Internet Service
Providers. Its goal is to improve lines of accountability for
controlling abusive uses of the Internet mail service.
Consequently the document offers recommendations for constructive
operational policies between independent operators of email
transmission services.
With the recent advent of email authentication technologies aimed
at providing assurances and traceability between internetworked
networks, the authors recognized that the initial submission of a
message became the weakest link. Consequently, the document offers
recommendations for constructive operational policies for the
first step of email sending, the submission (or posting) of email
into the transmission network. Relaying and delivery entail
policies that occur subsequent to submission and are outside the
scope of this document.
The document seeks BCP status. Comments and discussion of this
document should be addressed to the ietf-smtp@imc.org mailing
list.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Submission, Relaying, Delivery . . . . . . . . . . . . . . . 6
4. External Submission . . . . . . . . . . . . . . . . . . . . 8
5. Message Submission Authentication Technologies . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1 References -- Normative . . . . . . . . . . . . . . . . 11
7.2 References -- Informative . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 12
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . 14
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1. Introduction
The very characteristics that make email such a convenient
communications medium -- its near ubiquity, rapid delivery and low
cost -- have made it a fertile ground for the distribution of
unwanted or malicious content. Spam, fraud and worms have become
a serious problem, threatening the viability of email and costing
end users and providers millions of dollars in damages and lost
productivity. In recent years, independent operators including
enterprises and ISPs have turned to a number of different
technologies and procedures, in an attempt to combat these
problems, with varying effect and with vastly different impacts on
users and on the Internet mail infrastructure.
Email will often travel between multiple independent providers of
email transmission services, en route to its final destination.
They will generally have no prior arrangement with one another and
may employ different rules on the transmission. It is therefore
difficult both to debug problems that occur in mail transmission
and to assign accountability if undesired or malicious mail is
injected into the Internet mail infrastructure.
A wide variety of email authentication technologies has been
developed, and more are under development. They provide some
accountability and traceability between disparate networks. This
document aims to build on these technologies by exploring best
practices for authenticating and authorizing the first step of an
emails delivery from MUA to MSA, otherwise known as submission.
Without strong practices on email submission, the authentication
technologies provide limited benefit.
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This document specifies operational policies to be used for the
first step of email sending, the submission (or posting from an
MUA to an MSA as defined below) of email into the transmission
service. These policies will permit continued, smooth operation of
Internet email, with controls added to improve accountability.
Relaying and delivering employ policies that occur after
submission and are outside the scope of this document. The
policies listed here are appropriate for operators of all sizes
and may be implemented by operators independently, without regard
for whether the other side of an email exchange has implemented
them.
It is important to note that the adoption of these policies alone
will not solve the problems of spam and other undesirable email.
However they provide a useful step in clarifying lines of
accountability and interoperability between operators. This helps
raise the bar against abusers, and provides a foundation for
additional tools to preserve the utility of the Internet email
infrastructure.
This document does not delve into other anti-spam operational
issues such as standards for rejection of email. The authors note
that this would be a very valuable effort to undertake and suggest
that additional work under another BCP document should be embarked
upon.
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2. Terminology
The Internet email architecture distinguishes four message-handling
components:
o Mail User Agents (MUAs)
o Mail Submission Agents (MSAs)
o Mail Transfer Agents (MTAs)
o Mail Delivery Agents (MDAs)
At the origination end, an MUA works on behalf of end users to
create a message and perform initial "submission" into the
transmission infrastructure, via an MSA. An MSA accepts the
message submission, performs any necessary preprocessing on the
message and relays the message to an MTA for transmission. MTAs
relay messages to other MTAs, in a sequence reaching a
destination MDA that, in turn, delivers the email to the
recipient's inbox. The inbox is part of the recipient-side MUA
that works on behalf of the end-user to process received mail.
These architectural components are often compressed, such as
having the same software do MSA, MTA and MDA functions. However
the requirements for each of these components of the architecture
are becoming more extensive, so that their software and even
physical platform separation is increasingly common
Note: 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].
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3. Submission, Relaying, Delivery
The MSA, MTA and MDA functions used to be considered as the same
set of functions. This has been reflected in the history of
Internet mail by having MSA, MTA and MDA transfers all be
performed with SMTP [RFC2821] [RFC0821], over TCP Port 25.
Internet mail permits email to be exchanged with no prior
arrangement. Hence Port 25 exchanges occur without sender
authentication. That is, the confirmed identity of the originator
of the message is not necessarily known by the relaying MTAs or
the MDA.
It is important to distinguish MUA-to-MSA email submission, versus
MTA relaying, versus the final MTA-to-MDA transmission, prior to
MDA- to-MUA delivery. Submission typically does entail a pre-
established relationship between the user of the client and
operator of the server; equally, the MDA can determine that it
will be affecting final delivery and has an existing relationship
with the recipient. That is, MSAs and MDAs can take advantage of
having prior relationships with users, in order to constrain their
transfer activities.
Specifically, an MSA can choose to reject all postings from MUAs
for which it has no existing relationship. Similarly, an MDA can
choose to reject all mail to recipients for which that MDA has no
arrangement to perform delivery. Indeed, both of these policies
are already in common practice.
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BEST PRACTICES FOR SUBMISSION OPERATION:
Submission Port Availability:
MSAs MUST support the SUBMISSION port 587 [RFC2476] for MUA
access from outside the MSAs local environment. It is also
suggested that operators standardize on the SUBMISSION port
for both external AND LOCAL users for simplicity.
Submission Port Use:
MUAs SHOULD use the SUBMISSION port for message submission.
Submission Authentication:
MSAs MUST perform authentication on the identity asserted
during all mail transactions on the SUBMISSION port, even
for a message having a RCPT TO address that would not cause
the message to be relayed outside of the local
administrative environment.
Submission Authorization:
Operators of MSAs MUST perform authorization of the
authenticated identity, for the operations performed during
mail submission and based on an existing relationship with
the submitting entity. This requirement applies to all mail
submission mechanisms (MUA to MSA).
Submission Accountability after Submission:
Once a message has been submitted, the message SHOULD be
later traceable by the MSA operator to the authenticated
identity of the user who sent the message for a reasonable
period of time. Such tracing MAY be based on transactional
identifiers stored in the headers (received lines, etc) or
other fields in the message. The specific length of time,
after message submission, that traceability is supported is
not specified here. However issues regarding transit often
occur as much as one week after submission.
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4. External Submission
An MUA, desiring special services, may need to submit mail across
the Internet, rather than to a local MSA, in order to obtain
particular services. Examples include active privacy protection
against third-party content monitoring and timely processing.
Further the privacy requirement might reasonably include
protection against monitoring by the operator of the MUAs access
network. This requirement creates a challenge for the provider
operating the IP network through which the MUA gains access. It
makes that provider an involuntary recruit to the task of solving
mass-effect email problems: When the MUA participates in a
problem that affects large numbers of Internet users, the provider
is expected to effect remedies and is often expected to prevent
such occurrences.
A proactive technique used by some providers is to block all
outbound Port 25 SMTP traffic or to automatically redirect this
traffic through a local SMTP proxy, except for hosts that are
explicitly authorized. This can be problematic for some users,
notably legitimate mobile users attempting use their "home" MSA,
even though those users might already employ legitimate, Port 25-
based authentication.
This document offers no recommendation concerning the blocking of
SMTP Port 25 and similar practices for controlling abuse of the
standard anonymous mail transfer port. Rather, it pursues the
mutually constructive benefit of using the official SUBMISSION
Port 587 [RFC2476].
Note: However the authors wish to note that many established
practices for controlling abuse of outbound port25 traffic exist
including the proxy of smtp traffic to local hosts for screening
combined with various forms of rate limits. The authors suggest
that this topic should be addressed in a separate BCP that would
benefit the operational communities.
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BEST PRACTICES FOR SUPPORT OF EXTERNAL SUBMISSIONS:
Open Submission Port:
Access Providers MUST NOT block users from accessing the
external Internet using the SUBMISSION port 587 [RFC2476].
Traffic Identification -- External Posting Versus Relaying:
For email being received from outside their local
operational environment, email service providers MUST
distinguish between mail that will be delivered inside that
environment, versus mail that is to be relayed back out to
the internet. This allows the MTA to restrict this
operation, preventing the problem embodied by open
relays. Note that there are situations where this may not
apply such as secondary MXs and related implementations
internal to an operators network and within their control.
Delivery Authorization:
MDAs MUST NOT accept mail to recipients for which that MDA
has no arrangement to perform delivery.
Figure 1 depicts a local user (MUA.l) submitting a message to an
MSA (MSA). It also shows a remote user (MUA.r), such as might be
in a coffee shop offering "hotspot" wireless access, submitting a
message to their "home" MSA via an Authenticated Port 587
transaction.
HOME NETWORK DESTINATION
port -------- port
+-------+ +-----+ 25 +-----+ / \ 25 +-----+ +-----+
| MUA.l |--->| MSA |--->| MTA |--->| |--->| MTA |--->| MDA |
+-------+ +--^--+ +-----+ | INTERNET | +-----+ +-----+
port | | |
25/587 +-----------<------|----+ |
\ | /
--+---
| Port 587
+---+----+
| MUA.r |
+--------+
HOTSPOT
Figure 1: Example of Port 587 Usage Via Internet
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5. Message Submission Authentication/Authorization Technologies
There are many competent technologies and standards for
authenticating message submissions. Two mechanisms that have been
standardized include SMTP AUTH [RFC2554] and TLS [RFC3207].
Depending upon the environment, different mechanisms can be more
or less effective and convenient. Organizations SHOULD choose the
most secure approach that is practical.
This document does not provide recommendations on specific
security implementations. It simply provides a warning that
transmitting user credentials in clear text over insecure networks
SHOULD be avoided in all scenarios as this could allow attackers
to listen for this traffic and steal account data. In these cases,
it is strongly suggested that an appropriate security technology
MUST be used.
6. Security Considerations
Email transfer between independent administrations can be the
source of large volumes of unwanted email and email containing
malicious content designed to attack the recipient's system. This
document addresses the requirements and procedures to permit such
exchanges while reducing the likelihood that malicious mail will
be transmitted.
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7. References
7.1 References -- Normative
[RFC0821] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC 821, August 1982.
[RFC2476] Gellens, R. and J. Klensin, "Message Submission",
RFC 2476, December 1998.
[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
7.2 References -- Informative
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2554] Myers, J., "SMTP Service Extension for Authentication",
RFC 2554, March 1999.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
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Authors' Addresses
Carl Hutzler
America Online
12100 Sunrise Valley Drive
Reston, VA 20191
Phone: +1 703 265 5521
Email: cdhutzler@aol.com
Dave Crocker
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA 94086
USA
Phone: +1.408.246.8253
Email: dcrocker@bbiw.net
Peter W. Resnick
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego, CA 92121-1714
USA
Phone: +1 858 651 4478
Email: presnick@qualcomm.com
URI: http://www.qualcomm.com/~presnick/
Robert Sanders
Earthlink, Inc.
1375 Peachtree Street
Atlanta, GA 30309
USA
Phone: +1 404 748 7021
Email: sandersr@corp.earthlink.net
URI: http://home.mindspring.com/~rsanders/
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Eric Allman
Sendmail, Inc.
Emeryville, CA 94608
USA
Phone: +1 510 594 5501
Email: eric@sendmail.com
Appendix A. Acknowledgments
These recommendations were first formulated during informal
discussions among members of Anti-Spam Technical Alliance (ASTA) and
some participants from the Internet Research Task Force's Anti-Spam
Research Group (ASRG).
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