Network Working Group N. Freed

Network Working Group N. Freed
Request for Comments: 5260 Sun Microsystems
Category: Standards Track July 2008

Sieve Email Filtering: Date and Index Extensions

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.

Abstract

This document describes the "date" and "index" extensions to the
Sieve email filtering language. The "date" extension gives Sieve the
ability to test date and time values in various ways. The "index"
extension provides a means to limit header and address tests to
specific instances of header fields when header fields are repeated.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Capability Identifiers . . . . . . . . . . . . . . . . . . . . 3
4. Date Test . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Zone and Originalzone Arguments . . . . . . . . . . . . . 4
4.2. Date-part Argument . . . . . . . . . . . . . . . . . . . . 4
4.3. Comparator Interactions with Date-part Arguments . . . . . 5
4.4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Currentdate Test . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Index Extension . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Julian Date Conversions . . . . . . . . . . . . . . . 11
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 12

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1. Introduction

Sieve [RFC5228] is a language for filtering email messages at or
around the time of final delivery. It is designed to be
implementable on either a mail client or mail server. It is meant to
be extensible, simple, and independent of access protocol, mail
architecture, and operating system. It is suitable for running on a
mail server where users may not be allowed to execute arbitrary
programs, such as on black box Internet Message Access Protocol
[RFC3501] servers, as it does not have user-controlled loops or the
ability to run external programs.

The "date" extension provides a new date test to extract and match
date/time information from structured header fields. The date test
is similar in concept to the address test specified in [RFC5228],
which performs similar operations on addresses in header fields.

The "date" extension also provides a currentdate test that operates
on the date and time when the Sieve script is executed.

Some header fields containing date/time information, e.g., Received:,
naturally occur more than once in a single header. In such cases it
is useful to be able to restrict the date test to some subset of the
fields that are present. For example, it may be useful to apply a
date test to the last (earliest) Received: field. Additionally, it
may also be useful to apply similar restrictions to either the header
or address tests specified in [RFC5228].

For this reason, this specification also defines an "index"
extension. This extension adds two additional tagged arguments
:index and :last to the header, address, and date tests. If present,
these arguments specify which occurrence of the named header field is
to be tested.

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 [RFC2119].

The terms used to describe the various components of the Sieve
language are taken from Section 1.1 of [RFC5228]. Section 2 of the
same document describes basic Sieve language syntax and semantics.
The date-time syntactic element defined using ABNF notation [RFC5234]
in [RFC3339] is also used here.

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3. Capability Identifiers

The capability strings associated with the two extensions defined in
this document are "date" and "index".

4. Date Test

Usage: date [<":zone" <time-zone: string>> / ":originalzone"]
[COMPARATOR] [MATCH-TYPE] <header-name: string>
<date-part: string> <key-list: string-list>

The date test matches date/time information derived from headers
containing [RFC2822] date-time values. The date/time information is
extracted from the header, shifted to the specified time zone, and
the value of the given date-part is determined. The test returns
true if the resulting string matches any of the strings specified in
the key-list, as controlled by the comparator and match keywords.
The date test returns false unconditionally if the specified header
field does not exist, the field exists but does not contain a
syntactically valid date-time specification, the date-time isn't
valid according to the rules of the calendar system (e.g., January
32nd, February 29 in a non-leap year), or the resulting string fails
to match any key-list value.

The type of match defaults to ":is" and the default comparator is
"i;ascii-casemap".

Unlike the header and address tests, the date test can only be
applied to a single header field at a time. If multiple header
fields with the same name are present, only the first field that is
found is used. (Note, however, that this behavior can be modified
with the "index" extension defined below.) These restrictions
simplify the test and keep the meaning clear.

The "relational" extension [RFC5231] adds a match type called
":count". The count of a date test is 1 if the specified field
exists and contains a valid date; 0, otherwise.

Implementations MUST support extraction of RFC 2822 date-time
information that either makes up the entire header field (e.g., as it
does in a standard Date: header field) or appears at the end of a
header field following a semicolon (e.g., as it does in a standard
Received: header field). Implementations MAY support extraction of
date and time information in RFC2822 or other formats that appears in
other positions in header field content. In the case of a field
containing more than one date or time value, the last one that
appears SHOULD be used.

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4.1. Zone and Originalzone Arguments

The :originalzone argument specifies that the time zone offset
originally in the extracted date-time value should be retained. The
:zone argument specifies a specific time zone offset that the date-
time value is to be shifted to prior to testing. It is an error to
specify both :zone and :originalzone.

The value of time-zone MUST be an offset relative to UTC with the
following syntax:

time-zone = ( "+" / "-" ) 4DIGIT

The "+" or "-" indicates whether the time-of-day is ahead of (i.e.,
east of) or behind (i.e., west of) UTC. The first two digits
indicate the number of hours difference from Universal Time, and the
last two digits indicate the number of minutes difference from
Universal Time. Note that this agrees with the RFC 2822 format for
time zone offsets, not the ISO 8601 format.

If both the :zone and :originalzone arguments are omitted, the local
time zone MUST be used.

4.2. Date-part Argument

The date-part argument specifies a particular part of the resulting
date/time value to match against the key-list. Possible case-
insensitive values are:

"year" => the year, "0000" .. "9999".
"month" => the month, "01" .. "12".
"day" => the day, "01" .. "31".
"date" => the date in "yyyy-mm-dd" format.
"julian" => the Modified Julian Day, that is, the date
expressed as an integer number of days since
00:00 UTC on November 17, 1858 (using the Gregorian
calendar). This corresponds to the regular
Julian Day minus 2400000.5. Sample routines to
convert to and from modified Julian dates are
given in Appendix A.
"hour" => the hour, "00" .. "23".
"minute" => the minute, "00" .. "59".
"second" => the second, "00" .. "60".
"time" => the time in "hh:mm:ss" format.
"iso8601" => the date and time in restricted ISO 8601 format.
"std11" => the date and time in a format appropriate
for use in a Date: header field [RFC2822].

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"zone" => the time zone in use. If the user specified a
time zone with ":zone", "zone" will
contain that value. If :originalzone is specified
this value will be the original zone specified
in the date-time value. If neither argument is
specified the value will be the server's default
time zone in offset format "+hhmm" or "-hhmm". An
offset of 0 (Zulu) always has a positive sign.
"weekday" => the day of the week expressed as an integer between
"0" and "6". "0" is Sunday, "1" is Monday, etc.

The restricted ISO 8601 format is specified by the date-time ABNF
production given in [RFC3339], Section 5.6, with the added
restrictions that the letters "T" and "Z" MUST be in upper case, and
a time zone offset of zero MUST be represented by "Z" and not
"+00:00".

4.3. Comparator Interactions with Date-part Arguments

Not all comparators are suitable with all date-part arguments. In
general, the date-parts can be compared and tested for equality with
either "i;ascii-casemap" (the default) or "i;octet", but there are
two exceptions:

julian This is an integer, and may or may not have leading zeros.
As such, "i;ascii-numeric" is almost certainly the best
comparator to use with it.

std11 This is provided as a means to obtain date/time values in a
format appropriate for inclusion in email header fields. The
wide range of possible syntaxes for a std11 date/time --
which implementations of this extension are free to use when
composing a std11 string -- makes this format a poor choice
for comparisons. Nevertheless, if a comparison must be
performed, this is case-insensitive, and therefore "i;ascii-
casemap" needs to be used.

"year", "month", "day", "hour", "minute", "second" and "weekday" all
use fixed-width string representations of integers, and can therefore
be compared with "i;octet", "i;ascii-casemap", and "i;ascii-numeric"
with equivalent results.

"date" and "time" also use fixed-width string representations of
integers, and can therefore be compared with "i;octet" and "i;ascii-
casemap"; however, "i;ascii-numeric" can't be used with it, as
"i;ascii-numeric" doesn't allow for non-digit characters.

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4.4. Examples

The Date: field can be checked to test when the sender claims to have
created the message and act accordingly:

require ["date", "relational", "fileinto"];
if allof(header :is "from" "[email protected]",
date :value "ge" :originalzone "date" "hour" "09",
date :value "lt" :originalzone "date" "hour" "17")
{ fileinto "urgent"; }

Testing the initial Received: field can provide an indication of when
a message was actually received by the local system:

require ["date", "relational", "fileinto"];
if anyof(date :is "received" "weekday" "0",
date :is "received" "weekday" "6")
{ fileinto "weekend"; }

5. Currentdate Test

Usage: currentdate [":zone" <time-zone: string>]
[COMPARATOR] [MATCH-TYPE]
<date-part: string>
<key-list: string-list>

The currentdate test is similar to the date test, except that it
operates on the current date/time rather than a value extracted from
the message header. In particular, the ":zone" and date-part
arguments are the same as those in the date test.

All currentdate tests in a single Sieve script MUST refer to the same
point in time during execution of the script.

The :count value of a currentdate test is always 1.

5.1. Examples

The simplest use of currentdate is to have an action that only
operates at certain times. For example, a user might want to have
messages redirected to their pager after business hours and on
weekends:

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require ["date", "relational"];
if anyof(currentdate :is "weekday" "0",
currentdate :is "weekday" "6",
currentdate :value "lt" "hour" "09",
currentdate :value "ge" "hour" "17")
{ redirect "[email protected]"; }

Currentdate can be used to set up vacation [RFC5230] responses in
advance and to stop response generation automatically:

require ["date", "relational", "vacation"];
if allof(currentdate :value "ge" "date" "2007-06-30",
currentdate :value "le" "date" "2007-07-07")
{ vacation :days 7 "I'm away during the first week in July."; }

Currentdate may also be used in conjunction with the variables
extension to pass time-dependent arguments to other tests and
actions. The following Sieve places messages in a folder named
according to the current month and year:

require ["date", "variables", "fileinto"];
if currentdate :matches "month" "*" { set "month" "${1}"; }
if currentdate :matches "year" "*" { set "year" "${1}"; }
fileinto "${month}-${year}";

Finally, currentdate can be used in conjunction with the editheader
extension to insert a header-field containing date/time information:

require ["variables", "date", "editheader"];
if currentdate :matches "std11" "*"
{addheader "Processing-date" "${0}";}

6. Index Extension

The "index" extension, if specified, adds optional :index and :last
arguments to the header, address, and date tests as follows:

Syntax: date [":index" <fieldno: number> [":last"]]
[<":zone" <time-zone: string>> / ":originalzone"]
[COMPARATOR] [MATCH-TYPE] <header-name: string>
<date-part: string> <key-list: string-list>

Syntax: header [":index" <fieldno: number> [":last"]]
[COMPARATOR] [MATCH-TYPE]
<header-names: string-list> <key-list: string-list>

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Syntax: address [":index" <fieldno: number> [":last"]]
[ADDRESS-PART] [COMPARATOR] [MATCH-TYPE]
<header-list: string-list> <key-list: string-list>

If :index <fieldno> is specified, the attempts to match a value are
limited to the header field fieldno (beginning at 1, the first named
header field). If :last is also specified, the count is backwards; 1
denotes the last named header field, 2 the second to last, and so on.
Specifying :last without :index is an error.

:index only counts separate header fields, not multiple occurrences
within a single field. In particular, :index cannot be used to test
a specific address in an address list contained within a single
header field.

Both header and address allow the specification of more than one
header field name. If more than one header field name is specified,
all the named header fields are counted in the order specified by the
header-list.

6.1. Example

Mail delivery may involve multiple hops, resulting in the Received:
field containing information about when a message first entered the
local administrative domain being the second or subsequent field in
the message. As long as the field offset is consistent, it can be
tested:

# Implement the Internet-Draft cutoff date check assuming the
# second Received: field specifies when the message first
# entered the local email infrastructure.
require ["date", "relational", "index"];
if date :value "gt" :index 2 :zone "-0500" "received"
"iso8601" "2007-02-26T09:00:00-05:00",
{ redirect "[email protected]"; }

7. Security Considerations

The facilities defined here, like the facilities in the base Sieve
specification, operate on message header information that can easily
be forged. Note, however, that some fields are inherently more
reliable than others. For example, the Date: field is typically
inserted by the message sender and can be altered at any point. By
contrast, the uppermost Received: field is typically inserted by the
local mail system and is therefore difficult for the sender or an
intermediary to falsify.

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RFC 5260 Sieve Date and Index Extensions July 2008

Use of the currentdate test makes script behavior inherently less
predictable and harder to analyze. This may have consequences for
systems that use script analysis to try and spot problematic scripts.

All of the security considerations given in the base Sieve
specification also apply to these extensions.

8. IANA Considerations

The following templates specify the IANA registrations of the two
Sieve extensions specified in this document:

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

Capability name: date
Description: The "date" extension gives Sieve the ability
to test date and time values.
RFC number: RFC 5260
Contact address: Sieve discussion list <[email protected]>

Capability name: index
Description: The "index" extension provides a means to
limit header and address tests to specific
instances when more than one field of a
given type is present.
RFC number: RFC 5260
Contact address: Sieve discussion list <[email protected]>

9. References

9.1. Normative References

[CALGO199] Tantzen, R., "Algorithm 199: Conversions Between Calendar
Date and Julian Day Number", Collected Algorithms from
CACM 199.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.

[RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
Internet: Timestamps", RFC 3339, July 2002.

[RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering
Language", RFC 5228, January 2008.

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RFC 5260 Sieve Date and Index Extensions July 2008

[RFC5231] Segmuller, W. and B. Leiba, "Sieve Email Filtering:
Relational Extension", RFC 5231, January 2008.

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

9.2. Informative References

[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.

[RFC5230] Showalter, T. and N. Freed, "Sieve Email Filtering:
Vacation Extension", RFC 5230, January 2008.

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RFC 5260 Sieve Date and Index Extensions July 2008

Appendix A. Julian Date Conversions

The following C routines show how to translate day/month/year
information to and from modified Julian dates. These routines are
straightforward translations of the Algol routines specified in CACM
Algorithm 199 [CALGO199].

Given the day, month, and year, jday returns the modified Julian
date.

int jday(int year, int month, int day)
{
int j, c, ya;

if (month > 2)
month -= 3;
else
{
month += 9;
year--;
}
c = year / 100;
ya = year - c * 100;
return (c * 146097 / 4 + ya * 1461 / 4 + (month * 153 + 2) / 5 +
day + 1721119);
}

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Given j, the modified Julian date, jdate returns the day, month, and
year.

void jdate(int j, int *year, int *month, int *day)
{
int y, m, d;

j -= 1721119;
y = (j * 4 - 1) / 146097;
j = j * 4 - y * 146097 - 1;
d = j / 4;
j = (d * 4 + 3) / 1461;
d = d * 4 - j * 1461 + 3;
d = (d + 4) / 4;
m = (d * 5 - 3) / 153;
d = d * 5 - m * 153 - 3;
*day = (d + 5) / 5;
*year = y * 100 + j;
if (m < 10)
*month = m + 3;
else
{
*month = m - 9;
*year += 1;
}
}

Appendix B. Acknowledgements

Dave Cridland contributed the text describing the proper comparators
to use with different date-parts. Cyrus Daboo, Frank Ellerman,
Alexey Melnikov, Chris Newman, Dilyan Palauzov, and Aaron Stone
provided helpful suggestions and corrections.

Author's Address

Ned Freed
Sun Microsystems
800 Royal Oaks
Monrovia, CA 91016-6347
USA

Phone: +1 909 457 4293
EMail: [email protected]

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