One document matched: draft-ietf-sieve-refuse-reject-04.txt
Differences from draft-ietf-sieve-refuse-reject-03.txt
Internet Draft M. Elvey
Document: draft-ietf-sieve-refuse-reject-04 The Elvey Partnership,
LLC
Expires: April 2007 A. Melnikov
Isode Ltd
October 2006
The SIEVE mail filtering language - reject extension
draft-ietf-sieve-refuse-reject-04.txt
Status of this Memo
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Abstract
This memo updates the definition the SIEVE mail filtering language (RFC
<<3028bis>>) "reject" extension, originally defined in RFC 3028.
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 the 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 generally
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 a message is refused depends
on the capabilities of the 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 an action can not be taken or
fails, the interpreter should try the next item in the list:
1. 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.
2. Message delivery is refused by sending 5XX response code over
SMTP [SMTP] or LMTP [LMTP]. See section 3.1.1 for more details.
3. Message delivery is refused by sending a non delivery report
(DSN [DSN]). See section 3.1.2 for more details.
4. 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 the SMTP/LMTP protocol level
Sieve implementations that are able to reject messages at the
SMTP/LMTP level MUST do so and 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.
Note that SMTP [SMTP] doesn't allow for non-ASCII characters in the
SMTP response text. If non-ASCII characters appear in the "reason"
string, they can be sent if and only if the client and the server use
an SMTP extension that allows for transmission of non-ASCII reply
text. (One example of such an SMTP extension is described in
[UTF8-RESP].) In the absence of such an SMTP extension, the Sieve
engine MUST replace any reason string containing non-ASCII characters
with an implementation-defined ASCII-only string. Implementations
SHOULD notify the user that such replacement took place. Users that
don't like this behavior should consider using "reject :exacttext" as
described in Section 3.2, if available.
If the "reason" string consists of multiple CRLF separated lines,
then 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" {
reject 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 an 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 immediately sending of the 550 error
code by sending a 4XX error code on the first attempt to receive
the message.
3.1.2 Rejecting a message by sending a DSN
An implementation may receive 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.
Note that according to [DSN], Delivery Status Notifications MUST NOT
be generated if the MAIL FROM (return-path) is empty.
3.1.3 Rejecting a message by sending an MDN
When a Sieve engine is running inside an MUA, it has no ability to
reject a message at the SMTP/LMTP protocol level, as final delivery
(in the SMTP sense) is already complete. 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 MUST NOT implement "reject" by sending MDNs, they
SHOULD reject at the protocol level as described in section 3.1.1.
In the following script, a message is rejected and returned to the
sender.
Note that according to MDN MUST NOT be generated if the MAIL FROM
(Return-path) is empty.
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 :exacttext optional argument to reject action
SIEVE implementations that implement the :exacttext optional argument
to the "reject" action must advertise the "rejectexact" capability in
addition to the "reject" capability described above.
The :exacttext argument affects how reject processing described in
section 3.1.1 is performed. If this argument is present, and the
SMTP client and server don't both support an SMTP extension that
allows for transmission of non-ASCII reply text and there is
non-ASCII text in the reason string, then the reason string MUST NOT
be replaced with an implementation defined ASCII-only string as
defined in 3.1.1. Instead, the Sieve engine MUST try to generate
DSN, in order to preserve the exact text specified in the reason
string.
Example:
require ["reject", "rejectexact];
if size :over 100K {
reject :exacttext text:
Your message is to big. If you want to send me a big attachement,
put it on a public web site and send me an URL.
.
;
}
<<Pretend that the reason string above contains some non-ASCII text>>
NOTE: The :exacttext argument has no effect if the Sieve engine is
running in an MUA. The :exacttext argument also has no effect if the
Sieve engine is running in an MTA/MDA, but both client and server
support and negotiate use of an SMTP/LMTP extension for sending UTF-8
responses.
3.3 "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" MUST be incompatible with the "vacation" [VACATION]
action. It is NOT RECOMMENDED that implementations permit the use of
"reject" with actions that cause mail delivery, such as "keep",
"fileinto", "redirect".
Making "reject" compatible with actions that cause mail delivery
violates the RFC 2821 principle that a message is either delivered or
bounced back to the sender. So bouncing a message back (rejecting)
and delivering it will make the sender believe that the message was
not delivered.
However, there are existing laws requiring certain organizations to
archive all received messages, even the rejected ones. Also, it can
be quite useful to save copies of rejected messages for later
analysis.
Any action that would modify the message body will not have an effect
on the body of any message refused by "reject" using an SMTP response
code and MUST NOT have any effect on the content 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?>>
5.3 rejectexact extension registration
IANA is requested to add the following registration to the list of
Sieve extensions:
Capability name: rejectexact
Description: adds the ':exacttext' optional argument to the
reject action, which instructs the Sieve engine to
generate Delivery Status Notifications if rejection
reason string contains non-ASCII text.
RFC number: this RFC (Sieve base spec)
Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>
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.
[UTF8-RESP] A. Melnikov (Ed.), "SMTP Language Extension",
work in progress, draft-melnikov-smtp-lang-XX.txt
7. Acknowledgments
Thanks to Ned Freed, Cyrus Daboo, Arnt Gulbrandsen, Kristin Hubner,
Mark E. Mallett, Philip Guenther, Michael Haardt, Randy Gellens
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
1819 Polk-ietf St Ste 133
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.
06 (SIEVE WG 03) Explicitly list all actions incompatible with reject.
Added two paragraphs explaining why reject SHOULD (as opposed to
MUST/MAY) be incompatible with them. Clarified that if the reason
string contains non-ASCII and rejection over protocol is possible,
then the reason string MUST be replaced with an implementations
defined ASCII-only string. Added :exacttext optional argument that
preserves UTF-8 reason string by forcing generation of DSN.
07 (SIEVE WG 04) Removed special handling of empty return path.
Several editorial changes from Randy Gellens.
Clarified :exacttext applicability, removed redundancy. Reverted
SHOULD NOT send MDNs back to MUST NOT send MDNs of earlier drafts
(section 3.1.3).
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