Keywords: spam, spoofing, phishing
Network Working Group E. Allman
Request for Comments: 4405 Sendmail, Inc.
Category: Experimental H. Katz
Microsoft Corp.
April 2006
SMTP Service Extension for
Indicating the Responsible Submitter of an E-Mail Message
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
IESG Note
The following documents (RFC 4405, RFC 4406, RFC 4407, and RFC 4408)
are published simultaneously as Experimental RFCs, although there is
no general technical consensus and efforts to reconcile the two
approaches have failed. As such, these documents have not received
full IETF review and are published "AS-IS" to document the different
approaches as they were considered in the MARID working group.
The IESG takes no position about which approach is to be preferred
and cautions the reader that there are serious open issues for each
approach and concerns about using them in tandem. The IESG believes
that documenting the different approaches does less harm than not
documenting them.
Note that the Sender ID experiment may use DNS records that may have
been created for the current SPF experiment or earlier versions in
this set of experiments. Depending on the content of the record,
this may mean that Sender-ID heuristics would be applied incorrectly
to a message. Depending on the actions associated by the recipient
with those heuristics, the message may not be delivered or may be
discarded on receipt.
Participants relying on Sender ID experiment DNS records are warned
that they may lose valid messages in this set of circumstances.
Participants publishing SPF experiment DNS records should consider
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the advice given in section 3.4 of RFC 4406 and may wish to publish
both v=spf1 and spf2.0 records to avoid the conflict.
Participants in the Sender-ID experiment need to be aware that the
way Resent-* header fields are used will result in failure to receive
legitimate email when interacting with standards-compliant systems
(specifically automatic forwarders which comply with the standards by
not adding Resent-* headers, and systems which comply with RFC 822
but have not yet implemented RFC 2822 Resent-* semantics). It would
be inappropriate to advance Sender-ID on the standards track without
resolving this interoperability problem.
The community is invited to observe the success or failure of the two
approaches during the two years following publication, in order that
a community consensus can be reached in the future.
Abstract
This memo defines an extension to the Simple Mail Transfer Protocol
(SMTP) service that allows an SMTP client to specify the responsible
submitter of an e-mail message. The responsible submitter is the
e-mail address of the entity most recently responsible for
introducing a message into the transport stream. This extension
helps receiving e-mail servers efficiently determine whether the SMTP
client is authorized to transmit mail on behalf of the responsible
submitter's domain.
Table of Contents
1. Introduction ....................................................3
1.1. Conventions Used in This Document ..........................4
2. The SUBMITTER Service Extension .................................4
3. The SUBMITTER Keyword of the EHLO Command .......................5
4. The SUBMITTER Parameter of the MAIL Command .....................5
4.1. Setting the SUBMITTER Parameter Value ......................5
4.2. Processing the SUBMITTER Parameter .........................5
4.3. Transmitting to a Non-SUBMITTER-Aware SMTP Server ..........6
5. Examples ........................................................6
5.1. Mail Submission ............................................7
5.2. Mail Forwarding ............................................7
5.3. Mobile User ................................................8
5.4. Guest E-Mail Service .......................................9
5.5. SUBMITTER Used on a Non-Delivery Report ...................11
6. Security Considerations ........................................11
7. Acknowledgements ...............................................12
8. IANA Considerations ............................................12
9. References .....................................................12
9.1. Normative References ......................................12
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1. Introduction
The practice of falsifying the identity of the sender of an e-mail
message, commonly called "spoofing", is a prevalent tactic used by
senders of unsolicited commercial e-mail, or "spam". This form of
abuse has highlighted the need to improve identification of the
"responsible submitter" of an e-mail message.
In this specification, the responsible submitter is the entity most
recently responsible for injecting a message into the e-mail
transport stream. The e-mail address of the responsible submitter
will be referred to as the Purported Responsible Address (PRA) of the
message. The Purported Responsible Domain (PRD) is the domain
portion of that address.
This specification codifies rules for encoding the purported
responsible address into the SMTP transport protocol. This will
permit receiving SMTP servers to efficiently validate whether or not
the SMTP client is authorized to transmit mail on behalf of the
responsible submitter's domain.
Broadly speaking, there are two possible approaches for determining
the purported responsible address: either from RFC 2821 [SMTP]
protocol data or from RFC 2822 [MSG-FORMAT] message headers. Each
approach has certain advantages and disadvantages.
Deriving the purported responsible domain from RFC 2821 data has the
advantage that validation can be performed before the SMTP client has
transmitted the message body. If spoofing is detected, then the SMTP
server has the opportunity, depending upon local policy, to reject
the message before it is ever transmitted. The disadvantage of this
approach is the risk of false positives, that is, incorrectly
concluding that the sender's e-mail address has been spoofed. There
are today legitimate reasons why the Internet domain names used in
RFC 2821 commands may be different from those of the sender of an e-
mail message.
Deriving the purported responsible domain from RFC 2822 headers has
the advantage that validation can usually be based on an identity
that is displayed to recipients by existing Mail User Agents (MUAs)
as the sender's identity. This aids in detection of a particularly
noxious form of spoofing known as "phishing" in which a malicious
sender attempts to fool a recipient into believing that a message
originates from an entity well known to the recipient. This approach
carries a lower risk of false positives since there are fewer
legitimate reasons for RFC 2822 headers to differ from the true
sender of the message. The disadvantage of this approach is that it
does require parsing and analysis of message headers. In practice,
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much if not all the message body is also transmitted since the SMTP
protocol described in RFC 2821 provides no mechanism to interrupt
message transmission after the DATA command has been issued.
It is desirable to unify these two approaches in a way that combines
the benefits of both while minimizing their respective disadvantages.
This specification describes just such a unified approach. It uses
the mechanism described in [SMTP] to describe an extension to the
SMTP protocol. Using this extension, an SMTP client can specify the
e-mail address of the entity most recently responsible for submitting
the message to the SMTP client in a new SUBMITTER parameter of the
SMTP MAIL command. SMTP servers can use this information to validate
that the SMTP client is authorized to transmit e-mail on behalf of
the Internet domain contained in the SUBMITTER parameter.
1.1. Conventions Used in This Document
In examples, "C:" and "S:" indicate lines sent by the client and
server, respectively.
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 RFC 2119 [KEYWORDS].
2. The SUBMITTER Service Extension
The following SMTP service extension is hereby defined:
(1) The name of this SMTP service extension is "Responsible
Submitter";
(2) The EHLO keyword value associated with this extension is
"SUBMITTER";
(3) The SUBMITTER keyword has no parameters;
(4) No additional SMTP verbs are defined by this extension;
(5) An optional parameter is added to the MAIL command using the
esmtp-keyword "SUBMITTER", and is used to specify the e-mail
address of the entity responsible for submitting the message for
delivery;
(6) This extension is appropriate for the submission protocol
[SUBMIT].
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3. The SUBMITTER Keyword of the EHLO Command
An SMTP server includes the SUBMITTER keyword in its EHLO response to
tell the SMTP client that the SUBMITTER service extension is
supported.
The SUBMITTER keyword has no parameters.
4. The SUBMITTER Parameter of the MAIL Command
The syntax of the SUBMITTER parameter is
"SUBMITTER=" Mailbox
where Mailbox is the Augmented Backus-Naur Form (ABNF) [ABNF]
production defined in Section 4.1.2 of [SMTP]. Characters such as
SP, "+", and "=" that may occur in Mailbox but are not permitted in
ESMTP parameter values MUST be encoded as "xtext" as described in
Section 4 of [DSN].
4.1. Setting the SUBMITTER Parameter Value
The purpose of the SUBMITTER parameter is to allow the SMTP client to
indicate to the server the purported responsible address of the
message directly in the RFC 2821 protocol.
Therefore, SMTP clients that support the Responsible Submitter
extension MUST include the SUBMITTER parameter on all messages. This
includes messages containing a null reverse-path in the MAIL command.
SMTP clients MUST set the SUBMITTER parameter value to the purported
responsible address of the message as defined in [PRA]. This also
applies to messages containing a null reverse-path.
In some circumstances, described in Section 7 of [SENDER-ID], SMTP
clients may need to add RFC 2822 headers to the message in order to
ensure that the correct SUBMITTER parameter value can be set.
4.2. Processing the SUBMITTER Parameter
Receivers of e-mail messages sent with the SUBMITTER parameter SHOULD
select the domain part of the SUBMITTER address value as the
purported responsible domain of the message, and SHOULD perform such
tests, including those defined in [SENDER-ID], as are deemed
necessary to determine whether the connecting SMTP client is
authorized to transmit e-mail messages on behalf of that domain.
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If these tests indicate that the connecting SMTP client is not
authorized to transmit e-mail messages on behalf of the SUBMITTER
domain, the receiving SMTP server SHOULD reject the message and when
rejecting MUST use "550 5.7.1 Submitter not allowed."
If the receiving SMTP server allows the connecting SMTP client to
transmit message data, then the server SHOULD determine the purported
responsible address of the message by examining the RFC 2822 message
headers as described in [PRA]. If this purported responsible address
does not match the address appearing in the SUBMITTER parameter, the
receiving SMTP server MUST reject the message and when rejecting MUST
use "550 5.7.1 Submitter does not match header."
If no purported responsible address is found according to the
procedure defined in [PRA], the SMTP server SHOULD reject the message
and when rejecting MUST use "554 5.7.7 Cannot verify submitter
address."
Verifying Mail Transfer Agents (MTAs) are strongly urged to validate
the SUBMITTER parameter against the RFC 2822 headers; otherwise, an
attacker can trivially defeat the algorithm.
Note that the presence of the SUBMITTER parameter on the MAIL command
MUST NOT change the effective reverse-path of a message. Any
delivery status notifications must be sent to the reverse-path, if
one exists, as per Section 3.7 of [SMTP] regardless of the presence
of a SUBMITTER parameter. If the reverse-path is null, delivery
status notifications MUST NOT be sent to the SUBMITTER address.
Likewise, the SUBMITTER parameter MUST NOT change the effective reply
address of a message. Replies MUST be sent to the From address or
the Reply-To address, if present, as described in Section 3.6.2 of
[MSG-FORMAT] regardless of the presence of a SUBMITTER parameter.
4.3. Transmitting to a Non-SUBMITTER-Aware SMTP Server
Notwithstanding the provisions of Section 4.1 above, when an MTA
transmits a message to another MTA that does not support the
SUBMITTER extension, the forwarding MTA MUST transmit the message
without the SUBMITTER parameter. This should involve no information
loss, since the SUBMITTER parameter is required to contain
information derived from the message headers.
5. Examples
This section provides examples of how the SUBMITTER parameter would
be used. The following dramatis personae appear in the examples:
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alice@example.com: the original sender of each e-mail message.
bob@company.com.example: the final recipient of each e-mail.
bob@almamater.edu.example: an e-mail address used by Bob that he has
configured to forward mail to his office account at
bob@company.com.example.
alice@mobile.net.example: an e-mail account provided to Alice by her
mobile e-mail network carrier.
5.1. Mail Submission
Under normal circumstances, Alice would configure her MUA to submit
her message to the mail system using the SUBMIT protocol [SUBMIT].
The MUA would transmit the message without the SUBMITTER parameter.
The SUBMIT server would validate that the MUA is allowed to submit a
message through some external scheme, perhaps SMTP Authentication
[SMTPAUTH]. Under most circumstances, this would look like a normal,
authenticated SMTP transaction. The SUBMIT server would extract her
name from the RFC 2822 headers for use in the SUBMITTER parameters of
subsequent transmissions of the message.
5.2. Mail Forwarding
When Alice sends a message to Bob at his almamater.edu.example
account, the SMTP session from her SUBMIT server might look something
like this:
S: 220 almamater.edu.example ESMTP server ready
C: EHLO example.com
S: 250-almamater.edu.example
S: 250-DSN
S: 250-AUTH
S: 250-SUBMITTER
S: 250 SIZE
C: MAIL FROM:<alice@example.com> SUBMITTER=alice@example.com
S: 250 <alice@example.com> sender ok
C: RCPT TO:<bob@almamater.edu.example>
S: 250 <bob@almamater.edu.example> recipient ok
C: DATA
S: 354 okay, send message
C: (message body goes here)
C: .
S: 250 message accepted
C: QUIT
S: 221 goodbye
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The almamater.edu.example MTA must now forward this message to
bob@company.com.example. Although the original sender of the message
is alice@example.com, Alice is not responsible for this most recent
retransmission of the message. That role is filled by
bob@almamater.edu.example, who established the forwarding of mail to
bob@company.com.example. Therefore, the almamater.edu.example MTA
determines a new purported responsible address for the message,
namely, bob@almamater.edu.example, and sets the SUBMITTER parameter
accordingly. The forwarding MTA also inserts a Resent-From header in
the message body to ensure the purported responsible address derived
from the RFC 2822 headers matches the SUBMITTER address.
S: 220 company.com.example ESMTP server ready
C: EHLO almamater.edu.example
S: 250-company.com.example
S: 250-DSN
S: 250-AUTH
S: 250-SUBMITTER
S: 250 SIZE
C: MAIL FROM:<alice@example.com>
SUBMITTER=bob@almamater.edu.example
S: 250 <alice@example.com> sender ok
C: RCPT TO:<bob@company.com.example>
S: 250 <bob@company.com.example> recipient ok
C: DATA
S: 354 okay, send message
C: Resent-From: bob@almamater.edu.example
C: Received By: ...
C: (message body goes here)
C: .
S: 250 message accepted
C: QUIT
S: 221 goodbye
5.3. Mobile User
Alice is at the airport and uses her mobile e-mail device to send a
message to Bob. The message travels through the carrier network
provided by mobile.net.example, but Alice uses her example.com
address on the From line of all her messages so that replies go to
her office mailbox.
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Here is an example of the SMTP session between the MTAs at
mobile.net.example and almamater.edu.example.
S: 220 almamater.edu.example ESMTP server ready
C: EHLO mobile.net.example
S: 250-almamater.edu.example
S: 250-DSN
S: 250-AUTH
S: 250-SUBMITTER
S: 250 SIZE
C: MAIL FROM:<alice@example.com>
SUBMITTER=alice@mobile.net.example
S: 250 <alice@example.com> sender ok
C: RCPT TO:<bob@almamater.edu.example>
S: 250 <bob@almamater.edu.example> recipient ok
C: DATA
S: 354 okay, send message
C: Sender: alice@mobile.net.example
C: Received By: ...
C: (message body goes here)
C: .
S: 250 message accepted
C: QUIT
S: 221 goodbye
Note that mobile.net.example uses the SUBMITTER parameter to
designate alice@mobile.net.example as the responsible submitter for
this message. Further, this MTA also inserts a Sender header to
ensure the purported responsible address derived from the RFC 2822
headers matches the SUBMITTER address.
Likewise, conventional ISPs may also choose to use the SUBMITTER
parameter to designate as the responsible submitter the user's
address on the ISP's network if that address is different from the
MAIL FROM address. This may be especially useful for ISPs that host
multiple domains or otherwise share MTAs among multiple domains.
When the message is subsequently forwarded by the
almamater.edu.example MTA, that MTA will replace the SUBMITTER
parameter with bob@almamater.edu.example as in Section 5.2 and add
its own Resent-From header.
5.4. Guest E-Mail Service
While on a business trip, Alice uses the broadband access facilities
provided by the Exemplar Hotel to connect to the Internet and send
e-mail. The hotel routes all outbound e-mail through its own SMTP
server, email.hotel.com.example.
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The SMTP session for Alice's message to Bob from the Exemplar Hotel
would look like this:
S: 220 almamater.edu.example ESMTP server ready
C: EHLO email.hotel.com.example
S: 250-almamater.edu.example
S: 250-DSN
S: 250-AUTH
S: 250-SUBMITTER
S: 250 SIZE
C: MAIL FROM:<alice@example.com>
SUBMITTER=guest.services@email.hotel.com.example
S: 250 <alice@example.com> sender ok
C: RCPT TO:<bob@almamater.edu.example>
S: 250 <bob@almamater.edu.example> recipient ok
C: DATA
S: 354 okay, send message
C: Resent-From: guest.services@email.hotel.com.example
C: Received By: ...
C: (message body goes here)
C: .
S: 250 message accepted
C: QUIT
S: 221 goodbye
Note that email.hotel.com.example uses the SUBMITTER parameter to
designate a generic account guest.services@email.hotel.com.example as
the responsible submitter address for this message. A generic
account is used since Alice herself does not have an account at that
domain. Furthermore, this client also inserts a Resent-From header
to ensure the purported responsible address derived from the RFC 2822
headers with the SUBMITTER address.
As before, when the message is subsequently forwarded by the
almamater.edu.example MTA, that MTA will replace the SUBMITTER
parameter with bob@almamater.edu.example as in Section 5.2 and add
its own Resent-From header.
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5.5. SUBMITTER Used on a Non-Delivery Report
Alice sends an incorrectly addressed e-mail message and receives a
non-delivery report from a SUBMITTER-compliant server.
S: 220 example.com ESMTP server ready
C: EHLO almamater.edu.example
S: 250-example.com
S: 250-DSN
S: 250-AUTH
S: 250-SUBMITTER
S: 250 SIZE
C: MAIL FROM:<> SUBMITTER=mailer-daemon@almamater.edu.example
S: 250 OK
C: RCPT TO:<alice@example.com>
S: 250 OK
C: DATA
S: 354 OK, send message
C: (message body goes here)
C: .
S: 250 message accepted
C: QUIT
S: 221 goodbye
6. Security Considerations
This extension provides an optimization to allow an SMTP client to
identify the responsible submitter of an e-mail message in the SMTP
protocol, and to enable SMTP servers to perform efficient validation
of that identity before the message contents are transmitted.
It is, however, quite possible for an attacker to forge the value of
the SUBMITTER parameter. Furthermore, it is possible for an attacker
to transmit an e-mail message whose SUBMITTER parameter does not
match the purported responsible address of the message as derived
from the RFC 2822 headers. Therefore, the presence of the SUBMITTER
parameter provides, by itself, no assurance of the authenticity of
the message or the responsible submitter. Rather, the SUBMITTER
parameter is intended to provide additional information to receiving
e-mail systems to enable them to efficiently determine the validity
of the responsible submitter, and specifically, whether the SMTP
client is authorized to transmit e-mail on behalf of the purported
responsible submitter's domain. Section 4.2 describes how receiving
e-mail systems should process the SUBMITTER parameter.
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7. Acknowledgements
The idea of an ESMTP extension to convey the identity of the
responsible sender of an e-mail message has many progenitors. Nick
Shelness suggested the idea in a private conversation with one of the
authors. Pete Resnick suggested a variant on the MARID mailing list.
The idea was also discussed on the Anti-Spam Research Group (ASRG)
mailing list.
The authors would also like to thank the participants of the MARID
working group and the following individuals for their comments and
suggestions, which greatly improved this document:
Robert Atkinson, Simon Attwell, Roy Badami, Greg Connor, Dave
Crocker, Matthew Elvey, Tony Finch, Ned Freed, Mark Lentczner, Jim
Lyon, Bruce McMillan, Sam Neely, Daryl Odnert, Margaret Olson,
Pete Resnick, Hector Santos, Nick Shelness, Rand Wacker, and Meng
Weng Wong.
8. IANA Considerations
The IANA has registered the SUBMITTER SMTP service extension.
9. References
9.1. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[DSN] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)", RFC
3461, January 2003.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[MSG-FORMAT] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
[PRA] Lyon, J., "Purported Responsible Address in E-Mail
Messages", RFC 4407, April 2006.
[SENDER-ID] Lyon, J. and M. Wong, "Sender ID: Authenticating E-
Mail", RFC 4406, April 2006.
[SUBMIT] Gellens, R. and J. Klensin, "Message Submission for
Mail", RFC 4409, April 2006.
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[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.
[SMTPAUTH] Myers, J., "SMTP Service Extension for Authentication",
RFC 2554, March 1999.
Authors' Addresses
Eric Allman
Sendmail, Inc.
6425 Christie Ave, Suite 400
Emeryville, CA 94608
USA
EMail: eric@sendmail.com
Harry Katz
Microsoft Corp.
1 Microsoft Way
Redmond, WA 98052
USA
EMail: hkatz@microsoft.com
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