Internet Engineering Task Force (IETF) S. Bosch

Internet Engineering Task Force (IETF) S. Bosch
Request for Comments: 7352 September 2014
Category: Standards Track
ISSN: 2070-1721

Sieve Email Filtering: Detecting Duplicate Deliveries

Abstract

This document defines a new test command, "duplicate", for the Sieve
email filtering language. This test adds the ability to detect
duplications. The main application for this new test is handling
duplicate deliveries commonly caused by mailing list subscriptions or
redirected mail addresses. The detection is normally performed by
matching the message ID to an internal list of message IDs from
previously delivered messages. For more complex applications, the
"duplicate" test can also use the content of a specific header field
or other parts of the message.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7352.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Bosch Standards Track [Page 1]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

Table of Contents

1. Introduction ....................................................2
2. Conventions Used in This Document ...............................3
3. Test "duplicate" ................................................3
3.1. Arguments ":header" and ":uniqueid" ........................5
3.2. Argument ":handle" .........................................7
3.3. Arguments ":seconds" and ":last" ...........................8
3.4. Interaction with Other Sieve Extensions ....................9
4. Sieve Capability Strings ........................................9
5. Examples ........................................................9
5.1. Example 1 ..................................................9
5.2. Example 2 .................................................10
5.3. Example 3 .................................................11
5.4. Example 4 .................................................12
6. Security Considerations ........................................12
7. IANA Considerations ............................................13
8. Acknowledgements ...............................................14
9. References .....................................................14
9.1. Normative References ......................................14
9.2. Informative References ....................................15

1. Introduction

This document specifies an extension to the Sieve filtering language
defined by RFC 5228 [SIEVE]. It adds a test to track whether or not
a text string was seen before by the delivery agent in an earlier
execution of the Sieve script. This can be used to detect and handle
duplicate message deliveries.

Duplicate deliveries are a common side effect of being subscribed to
a mailing list. For example, if a member of the list decides to
reply to both the user and the mailing list itself, the user will
often get one copy of the message directly and another through the
mailing list. Also, if someone crossposts over several mailing lists
to which the user is subscribed, the user will likely receive a copy
from each of those lists. In another scenario, the user has several
redirected mail addresses all pointing to his main mail account. If
one of the user's contacts sends the message to more than one of
those addresses, the user will likely receive more than a single
copy. Using the "duplicate" extension, users have the means to
detect and handle such duplicates (e.g., by discarding them, marking
them as "seen", or putting them in a special folder).

Bosch Standards Track [Page 2]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

Duplicate messages are normally detected using the Message-ID header
field, which is required to be unique for each message. However, the
"duplicate" test is flexible enough to use different criteria for
defining what makes a message a duplicate (e.g., using the subject
line or parts of the message body). Other applications of this new
test command are also possible, as long as the tracked unique value
is a string.

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 [KEYWORDS].

Conventions for notations are as in Section 1.1 of [SIEVE], including
use of the "Usage:" label for the definition of action and tagged
arguments syntax.

3. Test "duplicate"

Usage: "duplicate" [":handle" <handle: string>]
[":header" <header-name: string> /
":uniqueid" <value: string>]
[":seconds" <timeout: number>] [":last"]

The "duplicate" test identifies the message by a "unique ID" and,
using that unique ID, keeps track of which messages were seen by a
"duplicate" test during an earlier Sieve execution. In its basic
form, the test gets the unique ID from the content of the message's
Message-ID header field. The "duplicate" test evaluates to "true"
if the message was seen before, and it evaluates to "false" if it
was not.

As a side effect, the "duplicate" test adds the unique ID to an
internal duplicate-tracking list once the Sieve execution finishes
successfully. The first time a particular unique ID is seen, the
message is not a duplicate, and the unique ID is added to the
tracking list. If a future Sieve execution sees a message whose
unique ID appears in the tracking list, that test will evaluate to
"true", and that message will be considered a duplicate.

Note that this side effect is performed only when the "duplicate"
test is actually evaluated. If the "duplicate" test is nested in a
control structure or if it is not the first item of an "allof" or
"anyof" test list, its evaluation depends on the result of preceding
tests, which may produce unexpected results.

Bosch Standards Track [Page 3]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

Implementations MUST only update the internal duplicate-tracking list
when the Sieve script execution finishes successfully. If failing
script executions add the unique ID to the duplicate-tracking list,
all "duplicate" tests in the Sieve script would erroneously yield
"true" for the next delivery attempt of the same message. This
can -- depending on the action taken for a duplicate -- easily lead
to discarding the message without further notice.

However, deferring the definitive modification of the tracking list
to the end of a successful Sieve script execution is not without
problems. It can cause a race condition when a duplicate message is
delivered in parallel before the tracking list is updated. This way,
a duplicate message could be missed by the "duplicate" test. More
complex implementations could use a locking mechanism to prevent this
problem. But, irrespective of what implementation is chosen,
situations in which the "duplicate" test erroneously yields "true"
MUST be prevented.

The "duplicate" test MUST only check for duplicates amongst unique ID
values encountered in previous executions of the Sieve script; it
MUST NOT consider ID values encountered earlier in the current Sieve
script execution as potential duplicates. This means that all
"duplicate" tests in a Sieve script execution, including those
located in scripts included using the "include" [INCLUDE] extension,
MUST always yield the same result if the arguments are identical.

The Message-ID header field is assumed to be globally unique as
required in Section 3.6.4 of RFC 5322 [IMAIL]. In practice, this
assumption may not always prove to be true. The "duplicate" test
does not deal with this situation, which means that false duplicates
may be detected in this case. However, the user can address such
situations by specifying an alternative means of message
identification using the ":header" or the ":uniqueid" argument, as
described in the next section.

Bosch Standards Track [Page 4]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

3.1. Arguments ":header" and ":uniqueid"

Duplicate tracking involves determining the unique ID for a
given message and checking whether that unique ID is in the
duplicate-tracking list. The unique ID for a message is
determined as follows:

o When neither the ":header" argument nor the ":uniqueid" argument
is used, the unique ID is the content of the message's Message-ID
header field.

o When the ":header" argument is used, the unique ID is the content
of the specified header field in the message. The header field
name is not part of the resulting unique ID; it consists only of
the field value.

o When the ":uniqueid" argument is used, the unique ID is the string
parameter that is specified with the argument.

The ":header" and ":uniqueid" arguments are mutually exclusive;
specifying both for a single "duplicate" test command MUST trigger an
error.

The syntax rules for the header name parameter of the ":header"
argument are specified in Section 2.4.2.2 of RFC 5228 [SIEVE]. Note
that implementations MUST NOT trigger an error for an invalid header
name. Instead, the "duplicate" test MUST yield "false"
unconditionally in this case. The parameter of the ":uniqueid"
argument can be any string.

If the tracked unique ID value is extracted directly from a message
header field (i.e., when the ":uniqueid" argument is not used), the
following operations MUST be performed before the actual duplicate
verification:

o Unfold the header line as described in Section 2.2.3 of RFC 5322
[IMAIL] (see also Section 2.4.2.2 of RFC 5228 [SIEVE]).

o If possible, convert the header value to Unicode, encoded as UTF-8
(see Section 2.7.2 of RFC 5228 [SIEVE]). If conversion is not
possible, the value is left unchanged.

o Trim leading and trailing whitespace from the header value (see
Section 2.2 of RFC 5228 [SIEVE]).

Bosch Standards Track [Page 5]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

Note that these rules also apply to the Message-ID header field used
by the basic "duplicate" test without a ":header" or ":uniqueid"
argument. When the ":uniqueid" argument is used, any normalization
needs to be done in the Sieve script itself as the unique ID is
created.

If the header field specified using the ":header" argument exists
multiple times in the message, extraction of the unique ID MUST use
only the first occurrence. This is true whether or not multiple
occurrences are allowed by Section 3.6 of RFC 5322 [IMAIL]. If the
specified header field is not present in the message, the "duplicate"
test MUST yield "false" unconditionally. In that case, the
duplicate-tracking list is left unmodified by this test, since no
unique ID value is available. The same rules apply with respect to
the Message-ID header field for the basic "duplicate" test without a
":header" or ":uniqueid" argument, since that header field could also
be missing or occur multiple times.

The string parameter of the ":uniqueid" argument can be composed from
arbitrary text extracted from the message using the "variables"
[VARIABLES] extension. To extract text from the message body, the
"foreverypart" and "extracttext" [SIEVE-MIME] extensions need to be
used as well. This provides the user with detailed control over how
the message's unique ID is created.

The unique ID MUST be matched case-sensitively with the contents of
the duplicate-tracking list, irrespective of how the unique ID was
determined. To achieve case-insensitive behavior when the
":uniqueid" argument is used, the "set" command added by the
"variables" [VARIABLES] extension can be used to normalize the unique
ID value to upper or lower case.

Bosch Standards Track [Page 6]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

3.2. Argument ":handle"

The "duplicate" test MUST track a unique ID value independent of its
source. This means that all values in the duplicate-tracking list
should be used for duplicate testing, regardless of whether they were
obtained from the Message-ID header field, from an arbitrary header
specified using the ":header" argument, or explicitly from the
":uniqueid" argument. The following three examples are equivalent
and match the same entry in the duplicate-tracking list:

require "duplicate";
if duplicate {
discard;
}

require "duplicate";
if duplicate :header "message-id" {
discard;
}

require ["duplicate", "variables"];
if header :matches "message-id" "*" {
if duplicate :uniqueid "${0}" {
discard;
}
}

The ":handle" argument can be used to override this default behavior.
The ":handle" argument separates a "duplicate" test from other
"duplicate" tests with a different or omitted ":handle" argument.
Using the ":handle" argument, unrelated "duplicate" tests can be
prevented from interfering with each other: a message is only
recognized as a duplicate when the tracked unique ID was seen before
in an earlier script execution by a "duplicate" test with the same
":handle" argument.

NOTE: The necessary mechanism to track duplicate messages is very
similar to the mechanism that is needed for tracking duplicate
responses for the "vacation" action [VACATION]. One way to
implement the necessary mechanism for the "duplicate" test is
therefore to store a hash of the tracked unique ID and, if
provided, the ":handle" argument.

Bosch Standards Track [Page 7]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

3.3. Arguments ":seconds" and ":last"

Implementations SHOULD let entries in the tracking list expire after
a short period of time. The user can explicitly control the length
of this expiration time by means of the ":seconds" argument, which
accepts an integer value specifying the timeout value in seconds. If
the ":seconds" argument is omitted, an appropriate default value MUST
be used. A default expiration time of around 7 days is usually
appropriate. Sites SHOULD impose a maximum limit on the expiration
time. If that limit is exceeded by the ":seconds" argument, the
maximum value MUST be silently substituted; exceeding the limit MUST
NOT produce an error. If the ":seconds" argument is zero, the
"duplicate" test MUST yield "false" unconditionally.

When the ":last" argument is omitted, the expiration time for entries
in the duplicate-tracking list MUST be measured relative to the
moment at which the entry was first created (i.e., at the end of the
successful script execution during which the "duplicate" test
returned "false" for a message with that particular unique ID value).
This means that subsequent duplicate messages have no influence on
the time at which the entry in the duplicate-tracking list finally
expires.

In contrast, when the ":last" argument is specified, the expiration
time MUST be measured relative to the last script execution during
which the "duplicate" test was used to check the entry's unique ID
value. This effectively means that the entry in the duplicate-
tracking list will not expire while duplicate messages with the
corresponding unique ID keep being delivered within intervals smaller
than the expiration time.

It is possible to write Sieve scripts where, during a single
execution, more than one "duplicate" test is evaluated with the same
unique ID value and ":handle" argument but different ":seconds" or
":last" arguments. The resulting behavior is left undefined by this
specification, so such constructs should be avoided. Implementations
MAY choose to use the ":seconds" and ":last" arguments from the
"duplicate" test that was evaluated last.

Bosch Standards Track [Page 8]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

3.4. Interaction with Other Sieve Extensions

The "duplicate" test does not support either the "index" [DATE-INDEX]
or "mime" [SIEVE-MIME] extensions directly, meaning that none of the
":index", ":mime", or associated arguments are added to the
"duplicate" test when these extensions are active. The ":uniqueid"
argument can be used in combination with the "variables" [VARIABLES]
extension to achieve the same result indirectly.

Normally, Sieve scripts are executed at final delivery. However,
with the "imapsieve" [IMAPSIEVE] extension, Sieve scripts are invoked
when the IMAP [IMAP] server performs operations on the message store
(e.g., when messages are uploaded, flagged, or moved to another
location). The "duplicate" test is devised for use at final
delivery, and the semantics in the "imapsieve" context are left
undefined. Therefore, implementations SHOULD NOT allow the
"duplicate" test to be used in the context of "imapsieve".

4. Sieve Capability Strings

A Sieve implementation that defines the "duplicate" test command will
advertise the capability string "duplicate".

5. Examples

5.1. Example 1

In this basic example, message duplicates are detected by tracking
the Message-ID header field. Duplicate deliveries are stored in a
special folder contained in the user's Trash folder. If the folder
does not exist, it is created automatically using the "mailbox"
[MAILBOX] extension. This way, the user has a chance to recover
messages when necessary. Messages that are not recognized as
duplicates are stored in the user's inbox as normal.

require ["duplicate", "fileinto", "mailbox"];

if duplicate {
fileinto :create "Trash/Duplicate";
}

Bosch Standards Track [Page 9]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

5.2. Example 2

This example shows a more complex use of the "duplicate" test. The
user gets network alerts from a set of remote automated monitoring
systems. Several notifications can be received about the same event
from different monitoring systems. The Message-ID header field of
these messages is different, because these are all distinct messages
from different senders. To avoid being notified more than a single
time about the same event, the user writes the following script:

require ["duplicate", "variables", "imap4flags",
"fileinto"];

if header :matches "subject" "ALERT: *" {
if duplicate :seconds 60 :uniqueid "${1}" {
setflag "\\seen";
}
fileinto "Alerts";
}

The subjects of the notification message are structured with a
predictable pattern that includes a description of the event. In the
script above, the "duplicate" test is used to detect duplicate alert
events. The message subject is matched against a pattern, and the
event description is extracted using the "variables" [VARIABLES]
extension. If a message with that event in the subject was received
before, but more than a minute ago, it is not detected as a duplicate
due to the specified ":seconds" argument. In the event of a
duplicate, the message is marked as "seen" using the "imap4flags"
[IMAP4FLAGS] extension. All alert messages are put into the "Alerts"
mailbox, irrespective of whether those messages are duplicates
or not.

Bosch Standards Track [Page 10]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

5.3. Example 3

This example shows how the "duplicate" test can be used to limit the
frequency of notifications sent using the "enotify" [NOTIFY]
extension. Consider the following scenario: a mail user receives
Extensible Messaging and Presence Protocol (XMPP) notifications
[NOTIFY-XMPP] about new mail through Sieve, but sometimes a single
contact sends many messages in a short period of time. Now the user
wants to prevent being notified of all of those messages. The user
wants to be notified about messages from each person at most once per
30 minutes and writes the following script:

require ["variables", "envelope", "enotify", "duplicate"];

if envelope :matches "from" "*" { set "sender" "${1}"; }
if header :matches "subject" "*" { set "subject" "${1}"; }

if not duplicate :seconds 1800 :uniqueid "${sender}"
{
notify :message "[SIEVE] ${sender}: ${subject}"
"xmpp:[email protected]";
}

The example shown above uses the message envelope sender rather than
the Message-ID header field as the unique ID for duplicate tracking.

The example can be extended to allow more messages from the same
sender in close succession as long as the discussed subject is
different. This can be achieved as follows:

require ["variables", "envelope", "enotify", "duplicate"];

if envelope :matches "from" "*" { set "sender" "${1}"; }
if header :matches "subject" "*" { set "subject" "${1}"; }

# account for 'Re:' prefix
if string :comparator "i;ascii-casemap"
:matches "${subject}" "Re:*"
{
set "subject" "${1}";
}
if not duplicate :seconds 1800
:uniqueid "${sender} ${subject}"
{
notify :message "[SIEVE] ${sender}: ${subject}"
"xmpp:[email protected]";
}

Bosch Standards Track [Page 11]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

This uses a combination of the message envelope sender and the
subject of the message as the unique ID for duplicate tracking.

5.4. Example 4

For this example, the mail user uses the "duplicate" test for two
separate applications: for discarding duplicate events from a
notification system and for marking certain follow-up messages in a
software support mailing as "seen" using the "imap4flags"
[IMAP4FLAGS] extension.

The two "duplicate" tests in the following example each use a
different header to identify messages. However, these "X-Event-ID"
and "X-Ticket-ID" headers can have similar values in this case (e.g.,
both based on a time stamp), meaning that one "duplicate" test can
erroneously detect duplicates based on ID values tracked by the
other. Therefore, the user wants to prevent the second "duplicate"
test from matching ID values tracked by the first "duplicate" test
and vice versa. This is achieved by specifying different ":handle"
arguments for these tests.

require ["duplicate", "imap4flags"];

if duplicate :header "X-Event-ID" :handle "notifier" {
discard;
}
if allof (
duplicate :header "X-Ticket-ID" :handle "support",
address "to" "[email protected]",
header :contains "subject" "fileserver")
{
setflag "\\seen";
}

6. Security Considerations

A flood of unique messages could cause the duplicate-tracking list to
grow indefinitely. Therefore, implementations SHOULD limit the
number of entries in the duplicate-tracking list. When limiting the
number of entries, implementations SHOULD discard the oldest ones
first.

Scripts using the "duplicate" test evaluation should be aware that
message IDs are not necessarily unique, either through the fault of
benign generators or attackers injecting a message with the
properties used by the duplicate Sieve filter at some point prior to

Bosch Standards Track [Page 12]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

the Sieve filter. Therefore, scripts are well advised to be
conservative with respect to actions taken when duplicate messages
are identified only by message ID.

The list of unique IDs used for duplicate tracking can include
privacy-sensitive information, such as message ID values, content of
subject lines, and content extracted from message bodies.
Implementations SHOULD protect that information by obscuring it
through hashing (see the note at the end of Section 3.2) and/or by
storing it with a level of access control equivalent to that of the
messages themselves.

These measures will not prevent an entity that has access to the
duplicate-tracking list from querying whether messages with certain
unique ID values were received. As this operation is the essence of
the "duplicate" test, this cannot be prevented and may violate the
expectations of the user. For example, a user who deletes a message
from the server may expect that no record of it remains on the
server, but that will not be true if its message ID is persisted on
the server in the duplicate-tracking list.

It's notable, however, that server logs will often store the
information present on the duplicate-tracking list anyway and
probably would expose plaintext message IDs for a much longer period
than this mechanism would. Users of email services that
intentionally delete such logs with the intent of limiting
traceability should be made aware that use of the duplicate-tracking
mechanism re-exposes this information for the duration of the expiry
interval. Therefore, a shorter default expiry interval may be
appropriate in those situations.

7. IANA Considerations

The following template specifies the IANA registration of the Sieve
extension specified in this document:

To: [email protected]
Subject: Registration of new Sieve extension

Capability name: duplicate
Description: Adds test 'duplicate' that can be used to test
whether a particular message is a duplicate,
i.e., whether a copy of it was seen before by
the delivery agent that is executing the Sieve
script.
RFC number: RFC 7352
Contact address: Sieve mailing list <[email protected]>

Bosch Standards Track [Page 13]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

This information has been added to the list of Sieve extensions given
on <http://www.iana.org/assignments/sieve-extensions>.

8. Acknowledgements

Thanks to Brian Carpenter, Cyrus Daboo, Arnt Gulbrandsen, Tony
Hansen, Kristin Hubner, Barry Leiba, Alexey Melnikov, Subramanian
Moonesamy, Tom Petch, Hector Santos, Robert Sparks, Aaron Stone, and
Stefan Winter for reviews and suggestions. Special thanks to Ned
Freed for his guidance and support.

9. References

9.1. Normative References

[DATE-INDEX]
Freed, N., "Sieve Email Filtering: Date and Index
Extensions", RFC 5260, July 2008.

[IMAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.

[IMAPSIEVE]
Leiba, B., "Support for Internet Message Access Protocol
(IMAP) Events in Sieve", RFC 6785, November 2012.

[INCLUDE] Daboo, C. and A. Stone, "Sieve Email Filtering: Include
Extension", RFC 6609, May 2012.

[KEYWORDS]
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[SIEVE] Guenther, P. and T. Showalter, "Sieve: An Email Filtering
Language", RFC 5228, January 2008.

[SIEVE-MIME]
Hansen, T. and C. Daboo, "Sieve Email Filtering: MIME Part
Tests, Iteration, Extraction, Replacement, and Enclosure",
RFC 5703, October 2009.

[VARIABLES]
Homme, K., "Sieve Email Filtering: Variables Extension",
RFC 5229, January 2008.

Bosch Standards Track [Page 14]

RFC 7352 Sieve: Detecting Duplicate Deliveries September 2014

9.2. Informative References

[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.

[IMAP4FLAGS]
Melnikov, A., "Sieve Email Filtering: Imap4flags
Extension", RFC 5232, January 2008.

[MAILBOX] Melnikov, A., "The Sieve Mail-Filtering Language --
Extensions for Checking Mailbox Status and Accessing
Mailbox Metadata", RFC 5490, March 2009.

[NOTIFY] Melnikov, A., Leiba, B., Segmuller, W., and T. Martin,
"Sieve Email Filtering: Extension for Notifications",
RFC 5435, January 2009.

[NOTIFY-XMPP]
Saint-Andre, P. and A. Melnikov, "Sieve Notification
Mechanism: Extensible Messaging and Presence Protocol
(XMPP)", RFC 5437, January 2009.

[VACATION] Showalter, T. and N. Freed, "Sieve Email Filtering:
Vacation Extension", RFC 5230, January 2008.

Author's Address

Stephan Bosch
Enschede
NL

EMail: [email protected]

Bosch Standards Track [Page 15]