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Differences from draft-ietf-sieve-refuse-reject-01.txt
Internet Draft M. Elvey
Document: draft-ietf-sieve-refuse-reject-02 The Elvey Partnership,
LLC
Expires: December 2006 A. Melnikov
Isode Ltd
June 2006
The SIEVE mail filtering language - reject extension
draft-ietf-sieve-refuse-reject
Status of this Memo
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Abstract
This memo defines the SIEVE mail filtering language (RFC
<<3028bis>>) "reject" extension.
A Joe-job is a spam run forged to appear as though it came from an
innocent party, who is then generally flooded by the bounces,
Message Disposition Notifications (MDNs) and messages with
complaints. The original Sieve "reject" action defined in RFC 3028
required use of MDNs for rejecting messages, thus contributing to
the flood of Joe-job spam to victims of Joe-jobs. This document
updates definition of "reject" to require rejecting messages during
the SMTP transaction (instead of accepting them and then sending
MDNs back to the alleged sender) wherever possible, thereby
reducing the problem.
Table of Contents
1. Introduction 2
2. Conventions Used in this Document 3
3. SIEVE "reject" extension 3
3.1 Action reject 3
3.2 "reject" compatibility with other actions 7
4. Security Considerations 7
5. IANA Considerations 7
5.1 reject extension registration 7
5.2 refuse extension registration 8
6. References 8
6.1 Normative References 8
6.2 Informative References 8
7. Acknowledgments 9
8. Author's Addresses 9
9. Intellectual Property Rights Statement 9
10. Full Copyright Statement 10
11. Changes from RFC 3028 11
12. Change Log 11
1. Introduction
The SIEVE mail filtering language [SIEVE] "reject" action defined
in RFC 3028 only allowed users to refuse delivery of a message by
sending an [MDN].
This document updates definition of the "reject" action to permit
users to handle unwanted email in a way that is sometimes
preferable to the existing 'discard' and the original 'reject'
capabilities. When a spam-detection system suspects a message is
spam, but isn't certain, discarding the email is considered too
risky for some users, for example, those who receive sales leads by
email. They are willing to use the reject command. Users are
willing to reject but not discard because the sender of an email
incorrectly marked as spam will receive a notification that the
email was refused, and will likely try again to contact the
intended recipient, perhaps via another method of communication.
Unfortunately, this usage is problematic, because in the usual
case, the email is indeed spam, and the alleged sender to whom an
MDN caused by the reject will be sent will often be an innocent Joe-
job victim. The updated "reject" is less likely to result in email
to an innocent victim, because it requires that an implemention
refuse to accept an email for delivery instead of accepting it and
then sending an MDN wherever possible. Much spam is sent through
open proxies, so SMTP level refusal reduces Joe-job bounces (AKA
backscatter) resulting from usage of MDNs. The updated "reject"
will also reduce Joe-jobs caused by virus self-propagation via
emails with false sender information. SMTP level refusal helps to
prevent the blacklisting of sources of backscatter and conserve
bandwidth, by reducing the number of MDNs sent. Further discussion
highlighting the risks of generating MDNs and the benefits of
protocol level refusal can be found in [Joe-DoS].
2. 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 RFC 2119.
Conventions for notations are as in [SIEVE] section 1.1.
This document does not attempt to define what exactly constitutes a
spam or virus containing email or how it should be identified.
3. SIEVE "reject" extension
SIEVE implementations that implement the "reject" action must use
the "reject" capability string.
3.1 Action reject
Usage: reject <reason: string>
The "reject" action cancels the implicit keep and refuses delivery
of a message. The reason string is a UTF-8 [UTF-8] string
specifying the reason for refusal. How message is refused depends
on capabilities of mail component (MUA, MDA or MTA) executing the
Sieve script. The Sieve interpreter must do one of the following
actions, as detailed by the following priority table (items listed
earlier take precedence). Note that if action can not be taken or
fails, the interpreter should try the next item in the list:
1. If message return-path (MAIL FROM) is empty the message SHOULD be
accepted and discarded.
2. If a "reject" implementation performs a return-path verification
and it clearly indicates that the message has a forged return-path,
the implementation need not refuse mail delivery, but rather MAY
accept and discard it.
3. Message delivery is refused by sending 5XX response code over
SMTP [SMTP] or LMTP [LMTP]. See section 3.1.1 for more details.
4. Message delivery is refused by sending a non delivery report
(DSN [DSN]). See section 3.1.2 for more details.
5. Message delivery is refused by sending a message disposition
notification report (MDN). See section 3.1.3 for more details.
3.1.1 Rejecting messages at SMTP/LMTP protocol level
Sieve implementations that are able to reject messages at SMTP/LMTP
level SHOULD use the 550 response code. Note that if a message is
arriving over SMTP and has multiple recipients, some of which have
accepted the message, or the Sieve implementation is part of an
MUA, section 3.1.2 and section 3.1.3 define how to reject such a
message.
<<Open issue: do we want to allow for non-ascii text below and do
we need a way to control "reject with DSN containing non-ascii
text" vs. "replace non-ascii characters with ?">>
Note that SMTP [SMTP] doesn't allow for non-ASCII characters in
SMTP response text. If non-ASCII characters appear in the "reason"
string, they may be sent if and only if the client and the server
use an SMTP extension that allows for transmission of non-ASCII
reply text. Otherwise, the implementation should either consider it
an error, or accept the message and generate DSN as described in
section 3.1.2.
If the "reason" string is multiline, than the reason text MUST be
returned as a multiline SMTP/LMTP response, per [SMTP], section
4.2.1. Any line MUST NOT exceed the SMTP limit on the maximal line
length. To make the reason string conform to any such limits the
server MAY insert CRLFs and turn the response into a multiline
response.
In the following script (which assumes support for the spamtest
[SPAMTEST] and fileinto extensions), messages that test highly
positive for spam are refused.
Example:
require ["reject", "spamtest",
"comparator-i;ascii-numeric", "fileinto"];
if spamtest :value "ge" :comparator "i;ascii-numeric" "6" {
refuse text:
AntiSpam engine thinks your message is spam.
It is therefore being refused.
Please call 1-900-PAY-US if you want to reach us.
.
;
} elsif spamtest :value "ge" :comparator "i;ascii-numeric" "4" {
fileinto "Suspect";
}
The following excerpt from an SMTP session shows it in action.
...
C: DATA
S: 354 Send message, ending in CRLF.CRLF.
...
C: .
S: 550-AntiSpam engine thinks your message is spam.
S: 550-It is therefore being refused.
S: 550 Please call 1-900-PAY-US if you want to reach us.
If the SMTP/LMTP server supports RFC 2034 [ENHANCED-CODES] it MUST
prepend an appropriate Enhanced Error Code to the "reason" text.
Enhanced Error code 5.7.1 or a more generic 5.7.0 are RECOMMENDED.
With Enhanced Error Code the response to DATA command in the SMTP
example below will look like:
S: 550-5.7.1 AntiSpam engine thinks your message is spam.
S: 550-5.7.1 It is therefore being refused.
S: 550 5.7.1 Please call 1-900-PAY-US if you want to reach us.
if the server selected "5.7.1" as appropriate.
If a Sieve implementation that supports "reject" doesn't wish to
immediately disclose the reason for rejection (for example that it
detected spam), it may delay immediate sending the 550 error code
by sending a 4XX error code on the first attempt to receive the
message.
3.1.2 Rejecting message by sending DSN
If the implementation receives a message via SMTP that has more
than one RCPT TO that has been accepted by the server, and at least
one but not all of them are refusing delivery (whether the refusal
is caused by execution of a Sieve "reject" or for another reason).
In this case, the server MUST accept the message and generate DSNs
for all recipients that are refusing it. Note that this exception
does not apply to LMTP, as LMTP is able to reject messages on a per-
recipient basis.
3.1.3 Rejecting message by sending MDN
When Sieve engine is running inside MUA it has no ability to reject
the message before it was delivered, as the message is already
delivered. In this case the client should send a Message
Disposition Notification [MDN] back to the sender. It resends the
message to the sender as specified in the Return-Path header field,
wrapping it in a "reject" form, noting that it was rejected by the
recipient.
MTAs and MDAs SHOULD NOT implement "reject" by sending MDNs, they
SHOULD reject at protocol level as described in section 3.1.1.
In the following script, a message is rejected and returned to the
sender.
Example:
require ["reject"];
if header :contains "from" "coyote@desert.example.org"
{
reject text:
I am not taking mail from you, and I don't
want your birdseed, either!"
.
;
}
A reject message MUST take the form of a failure MDN as specified
by [MDN]. The human-readable portion of the message, the first
component of the MDN, contains the human readable message
describing the error, and it SHOULD contain additional text
alerting the original sender that mail was refused by a filter.
This part of the MDN might appear as follows:
------------------------------------------------------------
The message was refused by the recipient's mail filtering program.
The reason
given was as follows:
I am not taking mail from you, and I don't want your birdseed,
either!
------------------------------------------------------------
The MDN action-value field as defined in the MDN specification MUST
be "deleted" and MUST have the MDN-sent-automatically and automatic-
action modes set.
3.2 "reject" compatibility with other actions
A "reject" action cancels the implicit keep.
Implementations MUST prohibit the execution of more than one reject
in a SIEVE script. "Reject" is also incompatible with the
"vacation" [VACATION] extensions. Implementations SHOULD prohibit
reject when used with other actions, in particular "reject" SHOULD
be incompatible with keep, fileinto, redirect and discard.
Any action that would modify the message body will not have effect
on the body of any message refused by "reject" using the 550 SMTP
response code and MUST NOT have any effect on context of generated
DSN/MDNs.
4. Security Considerations
The Introduction section talks about why rejecting messages before
delivery is better then accepting and bouncing them.
Security issues associated with mail auto-responders are fully
discussed in the security consideration section of [RFC3834]. This
document is believed not to introduce any additional security
considerations in this general area.
The "reject" extension does not raise any other security
considerations that are not already present in the base [SIEVE]
protocol, and these issues are discussed in [SIEVE].
5. IANA Considerations
The following section provides the IANA registrations for the Sieve
extensions specified in this document:
5.1 reject extension registration
IANA is requested to update the registration for the SIEVE "reject"
extension to point to this document.
IANA is also requested to update Tim Showalter's email address to
be
tjs@psaux.com
5.2 refuse extension registration
IANA is requested to remove registration of the refuse extension.
<<Should this be taken care of by talking directly to IANA?>>
6. References
6.1 Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[SIEVE] Showalter, T. and P. Guenther, "Sieve: An Email Filtering
Language", Work-in-progress, draft-ietf-sieve-3028bis-XX.txt
[SMTP] Klensin, J. (Editor), "Simple Mail Transfer Protocol", AT&T
Laboratories, RFC 2821, April 2001.
[LMTP] Myers, J., "Local Mail Transfer Protocol", Carnegie-Mellon
University, RFC 2033, October 1996.
[DSN] Moore , K., Vaudreuil, G., "An Extensible Message Format for
Delivery Status Notifications", University of Tennessee, Lucent
Technologies, RFC 3464, January 2003.
[MDN] Fajman, R., "An Extensible Message Format for Message
Disposition Notifications", National Institutes of Health, RFC
2298, March 1998.
[ENHANCED-CODES] Freed, N., "SMTP Service Extension for Returning
Enhanced Error Codes", Innosoft, RFC 2034, October 1996.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC 3629, November 2003.
[VACATION] Showalter, T. and N. Freed, "Sieve Email Filtering:
Vacation Extension", work in progress, draft-ietf-sieve-vacation-XX.txt.
6.2 Informative References
[Joe-DoS] Stefan Frei, Ivo Silvestri, Gunter Ollmann, "Mail Non
Delivery Message DDoS Attacks", 5 April 2004",
<http://www.techzoom.net/paper-mailbomb.asp>.
[SPAMTEST] Daboo, C., "SIEVE Email Filtering: Spamtest and
Virustest Extensions", work in progress, draft-ietf-sieve-
spamtestbis-XX.txt
<<Note to the RFC editor: this reference can be safely replaced
with RFC 3685.>>
[RFC3834] Moore, K., "Recommendations for Automatic Responses to
Electronic Mail", RFC 3834, August 2004.
7. Acknowledgments
Thanks to Ned Freed, Cyrus Daboo, Arnt Gulbrandsen, Kristin Hubner,
Mark E. Mallett, Philip Guenther and Michael Haardt for comments
and corrections.
The authors gratefully acknowledge the extensive work of Tim
Showalter as the author of the RFC 3028, which originally defined
the "reject" action.
8. Author's Addresses
Matthew Elvey
The Elvey Partnership, LLC
3042 Sacramento-ietf St Ste 04
San Francisco, CA
U.S.A.
Email: sieve3@matthew.elvey.com
Alexey Melnikov
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex, TW12 2BX
UK
Email: Alexey.Melnikov@isode.com
9. Intellectual Property Rights Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
10. Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
11. Changes from RFC 3028
Clarified that the "reject" action cancels the implicit keep.
Extended list of allowable actions on reject to include protocol
level message rejection and generation of DSNs.
12. Change Log
<<NOTE to the RFC editor: please delete this section before
publication.>>
00 First formal draft.
01 Explicit RFC 2034 support, disallow "refuse" in MUAs, typos
corrected, clarifications, etc.
02 Many insubstantial editorial changes (mostly rewording text for
readability). Added text regarding non-ASCII characters in the refuse
"reason" string. Added an exception allowing return-path forgery to
justify discarding a message.
03 (Renamed to be SIEVE WG 00) - Updated boilerplate, added reject
action from the base spec, acknowledged Tim as the author of "reject".
04 (SIEVE WG 01) Based on WGLC feedback, the refuse and the reject
actions were merged into a single action called reject. Text
reorganized as the result. Typos and examples corrected. Updated IANA
registration and Security Considerations sections.
05 (SIEVE WG 02) Copied some security considerations from Vacation
draft. Clarified that the "reason" string is in UTF-8. Clarified
interaction with "editheader" extension. Added text about sending of
4XX instead of 550. Corrected typos in several examples.
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