Network Working Group C. Newman
Request for Comments: 3848 Sun Microsystems
Category: Standards Track July 2004
ESMTP and LMTP Transmission Types Registration
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This registers seven new mail transmission types (ESMTPA, ESMTPS,
ESMTPSA, LMTP, LMTPA, LMTPS, LMTPSA) for use in the "with" clause of
a Received header in an Internet message.
1. IANA Considerations
As directed by SMTP [2], IANA maintains a registry [7] of "WITH
protocol types" for use in the "with" clause of the Received header
in an Internet message. This registry presently includes SMTP [6],
and ESMTP [2]. This specification updates the registry as follows:
o The new keyword "ESMTPA" indicates the use of ESMTP when the SMTP
AUTH [3] extension is also used and authentication is successfully
achieved.
o The new keyword "ESMTPS" indicates the use of ESMTP when STARTTLS
[1] is also successfully negotiated to provide a strong transport
encryption layer.
o The new keyword "ESMTPSA" indicates the use of ESMTP when both
STARTTLS and SMTP AUTH are successfully negotiated (the
combination of ESMTPS and ESMTPA).
o The new keyword "LMTP" indicates the use of LMTP [4].
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o The new keyword "LMTPA" indicates the use of LMTP when the SMTP
AUTH extension is also used and authentication is successfully
achieved.
o The new keyword "LMTPS" indicates the use of LMTP when STARTTLS is
also successfully negotiated to provide a strong transport
encryption layer.
o The new keyword "LMTPSA" indicates the use of LMTP when both
STARTTLS and SMTP AUTH are successfully negotiated (the
combination of LSMTPS and LSMTPA).
o The references for the ESMTP and SMTP entries in the registry
should be updated to the latest specification [2] since both RFC
821 and RFC 1869 [5] are obsoleted by RFC 2821.
2. Implementation Experience
The ESMTPA, ESMTPS and ESMTPSA keywords have been implemented in
deployed email server software for several years and no problems have
been reported with their use.
3. Security Considerations
Use of these additional keywords provides trace information to
indicate when various high-level security framing protocols are used
for hop-to-hop transport of email without exposing details of the
specifics of the security mechanism. This trace information provides
an informal way to track the deployment of these mechanisms on the
Internet and can assist after-the-fact diagnosis of email abuse.
These keywords are not normally protected in transport which means
they can be modified by an active attacker. They also do not
indicate the specifics of the mechanism used, and therefore do not
provide any real-world security assurance. They should not be used
for mail filtering or relaying decisions except in very controlled
environments. As they are both cryptic and hidden in trace headers
used primarily to diagnose email problems, it is not expected they
will mislead end users with a false sense of security. Information
with a higher degree of reliability can be obtained by correlating
the Received headers with the logs of the various Mail Transfer
Agents through which the message passed.
The trace information provided by these keywords and other parts of
the Received header provide a significant benefit when doing after-
the-fact diagnosis of email abuse or problems. Unfortunately, some
people in a misguided attempt to hide information about their
internal servers will strip Received headers of useful information
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and reduce their ability to correct security abuses after they
happen. The result of such misguided efforts is usually a reduction
of the overall security of the systems.
4. References
4.1. Normative References
[1] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[2] Klensin, J., Ed., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[3] Myers, J., "SMTP Service Extension for Authentication", RFC
2554, March 1999.
[4] Myers, J., "Local Mail Transfer Protocol", RFC 2033, October
1996.
4.2. Informative References
[5] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker,
"SMTP Service Extensions", STD 10, RFC 1869, November 1995.
[6] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
August 1982.
4.3. URIs
[7] <http://www.iana.org/assignments/mail-parameters>
Author's Address
Chris Newman
Sun Microsystems
1050 Lakes Drive
West Covina, CA 91790
US
EMail: chris.newman@sun.com
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Full Copyright Statement
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Acknowledgement
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