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Network Working Group P. Guenther, Ed.
Request for Comments: 5228 Sendmail, Inc.
Obsoletes: 3028 T. Showalter, Ed.
Category: Standards Track January 2008
Sieve: An Email Filtering Language
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 a language for filtering email messages at
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 (IMAP) servers, as the base language
has no variables, loops, or ability to shell out to external
programs.
Guenther & Showalter Standards Track [Page 1]
RFC 5228 Sieve: An Email Filtering Language January 2008
Table of Contents
1. Introduction ....................................................4
1.1. Conventions Used in This Document ..........................4
1.2. Example Mail Messages ......................................5
2. Design ..........................................................6
2.1. Form of the Language .......................................6
2.2. Whitespace .................................................7
2.3. Comments ...................................................7
2.4. Literal Data ...............................................7
2.4.1. Numbers .............................................7
2.4.2. Strings .............................................8
2.4.2.1. String Lists ...............................9
2.4.2.2. Headers ....................................9
2.4.2.3. Addresses .................................10
2.4.2.4. Encoding Characters Using
"encoded-character" .......................10
2.5. Tests .....................................................11
2.5.1. Test Lists .........................................12
2.6. Arguments .................................................12
2.6.1. Positional Arguments ...............................12
2.6.2. Tagged Arguments ...................................12
2.6.3. Optional Arguments .................................13
2.6.4. Types of Arguments .................................13
2.7. String Comparison .........................................13
2.7.1. Match Type .........................................14
2.7.2. Comparisons across Character Sets ..................15
2.7.3. Comparators ........................................15
2.7.4. Comparisons against Addresses ......................16
2.8. Blocks ....................................................17
2.9. Commands ..................................................17
2.10. Evaluation ...............................................18
2.10.1. Action Interaction ................................18
2.10.2. Implicit Keep .....................................18
2.10.3. Message Uniqueness in a Mailbox ...................19
2.10.4. Limits on Numbers of Actions ......................19
2.10.5. Extensions and Optional Features ..................19
2.10.6. Errors ............................................20
2.10.7. Limits on Execution ...............................20
3. Control Commands ...............................................21
3.1. Control if ................................................21
3.2. Control require ...........................................22
3.3. Control stop ..............................................22
4. Action Commands ................................................23
4.1. Action fileinto ...........................................23
4.2. Action redirect ...........................................23
4.3. Action keep ...............................................24
4.4. Action discard ............................................25
Guenther & Showalter Standards Track [Page 2]
RFC 5228 Sieve: An Email Filtering Language January 2008
5. Test Commands ..................................................26
5.1. Test address ..............................................26
5.2. Test allof ................................................27
5.3. Test anyof ................................................27
5.4. Test envelope .............................................27
5.5. Test exists ...............................................28
5.6. Test false ................................................28
5.7. Test header ...............................................29
5.8. Test not ..................................................29
5.9. Test size .................................................29
5.10. Test true ................................................30
6. Extensibility ..................................................30
6.1. Capability String .........................................31
6.2. IANA Considerations .......................................31
6.2.1. Template for Capability Registrations ..............32
6.2.2. Handling of Existing Capability Registrations ......32
6.2.3. Initial Capability Registrations ...................32
6.3. Capability Transport ......................................33
7. Transmission ...................................................33
8. Parsing ........................................................34
8.1. Lexical Tokens ............................................34
8.2. Grammar ...................................................36
8.3. Statement Elements ........................................36
9. Extended Example ...............................................37
10. Security Considerations .......................................38
11. Acknowledgments ...............................................39
12. Normative References ..........................................39
13. Informative References ........................................40
14. Changes from RFC 3028 .........................................41
Guenther & Showalter Standards Track [Page 3]
RFC 5228 Sieve: An Email Filtering Language January 2008
1. Introduction
This memo documents a language that can be used to create filters for
electronic mail. It is not tied to any particular operating system
or mail architecture. It requires the use of [IMAIL]-compliant
messages, but should otherwise generalize to many systems.
The language is powerful enough to be useful but limited in order to
allow for a safe server-side filtering system. The intention is to
make it impossible for users to do anything more complex (and
dangerous) than write simple mail filters, along with facilitating
the use of graphical user interfaces (GUIs) for filter creation and
manipulation. The base language was not designed to be Turing-
complete: it does not have a loop control structure or functions.
Scripts written in Sieve are executed during final delivery, when the
message is moved to the user-accessible mailbox. In systems where
the Mail Transfer Agent (MTA) does final delivery, such as
traditional Unix mail, it is reasonable to filter when the MTA
deposits mail into the user's mailbox.
There are a number of reasons to use a filtering system. Mail
traffic for most users has been increasing due to increased usage of
email, the emergence of unsolicited email as a form of advertising,
and increased usage of mailing lists.
Experience at Carnegie Mellon has shown that if a filtering system is
made available to users, many will make use of it in order to file
messages from specific users or mailing lists. However, many others
did not make use of the Andrew system's FLAMES filtering language
[FLAMES] due to difficulty in setting it up.
Because of the expectation that users will make use of filtering if
it is offered and easy to use, this language has been made simple
enough to allow many users to make use of it, but rich enough that it
can be used productively. However, it is expected that GUI-based
editors will be the preferred way of editing filters for a large
number of users.
1.1. Conventions Used in This Document
In the sections of this document that discuss the requirements of
various keywords and operators, the following conventions have been
adopted.
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].
Guenther & Showalter Standards Track [Page 4]
RFC 5228 Sieve: An Email Filtering Language January 2008
Each section on a command (test, action, or control) has a line
labeled "Usage:". This line describes the usage of the command,
including its name and its arguments. Required arguments are listed
inside angle brackets ("<" and ">"). Optional arguments are listed
inside square brackets ("[" and "]"). Each argument is followed by
its type, so "<key: string>" represents an argument called "key" that
is a string. Literal strings are represented with double-quoted
strings. Alternatives are separated with slashes, and parentheses
are used for grouping, similar to [ABNF].
In the "Usage:" line, there are three special pieces of syntax that
are frequently repeated, MATCH-TYPE, COMPARATOR, and ADDRESS-PART.
These are discussed in sections 2.7.1, 2.7.3, and 2.7.4,
respectively.
The formal grammar for these commands is defined in section 8 and is
the authoritative reference on how to construct commands, but the
formal grammar does not specify the order, semantics, number or types
of arguments to commands, or the legal command names. The intent is
to allow for extension without changing the grammar.
1.2. Example Mail Messages
The following mail messages will be used throughout this document in
examples.
Message A
-----------------------------------------------------------
Date: Tue, 1 Apr 1997 09:06:31 -0800 (PST)
From: coyote@desert.example.org
To: roadrunner@acme.example.com
Subject: I have a present for you
Look, I'm sorry about the whole anvil thing, and I really
didn't mean to try and drop it on you from the top of the
cliff. I want to try to make it up to you. I've got some
great birdseed over here at my place--top of the line
stuff--and if you come by, I'll have it all wrapped up
for you. I'm really sorry for all the problems I've caused
for you over the years, but I know we can work this out.
--
Wile E. Coyote "Super Genius" coyote@desert.example.org
-----------------------------------------------------------
Guenther & Showalter Standards Track [Page 5]
RFC 5228 Sieve: An Email Filtering Language January 2008
Message B
-----------------------------------------------------------
From: youcouldberich!@reply-by-postal-mail.invalid
Sender: b1ff@de.res.example.com
To: rube@landru.example.com
Date: Mon, 31 Mar 1997 18:26:10 -0800
Subject: $$ YOU, TOO, CAN BE A MILLIONAIRE! $$
YOU MAY HAVE ALREADY WON TEN MILLION DOLLARS, BUT I DOUBT
IT! SO JUST POST THIS TO SIX HUNDRED NEWSGROUPS! IT WILL
GUARANTEE THAT YOU GET AT LEAST FIVE RESPONSES WITH MONEY!
MONEY! MONEY! COLD HARD CASH! YOU WILL RECEIVE OVER
$20,000 IN LESS THAN TWO MONTHS! AND IT'S LEGAL!!!!!!!!!
!!!!!!!!!!!!!!!!!!111111111!!!!!!!11111111111!!1 JUST
SEND $5 IN SMALL, UNMARKED BILLS TO THE ADDRESSES BELOW!
-----------------------------------------------------------
2. Design
2.1. Form of the Language
The language consists of a set of commands. Each command consists of
a set of tokens delimited by whitespace. The command identifier is
the first token and it is followed by zero or more argument tokens.
Arguments may be literal data, tags, blocks of commands, or test
commands.
With the exceptions of strings and comments, the language is limited
to US-ASCII characters. Strings and comments may contain octets
outside the US-ASCII range. Specifically, they will normally be in
UTF-8, as specified in [UTF-8]. NUL (US-ASCII 0) is never permitted
in scripts, while CR and LF can only appear as the CRLF line ending.
Note: While this specification permits arbitrary octets to appear
in Sieve scripts inside strings and comments, this has made it
difficult to robustly handle Sieve scripts in programs that are
sensitive to the encodings used. The "encoded-character"
capability (section 2.4.2.4) provides an alternative means of
representing such octets in strings using just US-ASCII
characters. As such, the use of non-UTF-8 text in scripts should
be considered a deprecated feature that may be abandoned.
Tokens other than strings are considered case-insensitive.
Guenther & Showalter Standards Track [Page 6]
RFC 5228 Sieve: An Email Filtering Language January 2008
2.2. Whitespace
Whitespace is used to separate tokens. Whitespace is made up of
tabs, newlines (CRLF, never just CR or LF), and the space character.
The amount of whitespace used is not significant.
2.3. Comments
Two types of comments are offered. Comments are semantically
equivalent to whitespace and can be used anyplace that whitespace is
(with one exception in multi-line strings, as described in the
grammar).
Hash comments begin with a "#" character that is not contained within
a string and continue until the next CRLF.
Example: if size :over 100k { # this is a comment
discard;
}
Bracketed comments begin with the token "/*" and end with "*/"
outside of a string. Bracketed comments may span multiple lines.
Bracketed comments do not nest.
Example: if size :over 100K { /* this is a comment
this is still a comment */ discard /* this is a comment
*/ ;
}
2.4. Literal Data
Literal data means data that is not executed, merely evaluated "as
is", to be used as arguments to commands. Literal data is limited to
numbers, strings, and string lists.
2.4.1. Numbers
Numbers are given as ordinary decimal numbers. As a shorthand for
expressing larger values, such as message sizes, a suffix of "K",
"M", or "G" MAY be appended to indicate a multiple of a power of two.
To be comparable with the power-of-two-based versions of SI units
that computers frequently use, "K" specifies kibi-, or 1,024 (2^10)
times the value of the number; "M" specifies mebi-, or 1,048,576
(2^20) times the value of the number; and "G" specifies gibi-, or
1,073,741,824 (2^30) times the value of the number [BINARY-SI].
Guenther & Showalter Standards Track [Page 7]
RFC 5228 Sieve: An Email Filtering Language January 2008
Implementations MUST support integer values in the inclusive range
zero to 2,147,483,647 (2^31 - 1), but MAY support larger values.
Only non-negative integers are permitted by this specification.
2.4.2. Strings
Scripts involve large numbers of string values as they are used for
pattern matching, addresses, textual bodies, etc. Typically, short
quoted strings suffice for most uses, but a more convenient form is
provided for longer strings such as bodies of messages.
A quoted string starts and ends with a single double quote (the <">
character, US-ASCII 34). A backslash ("\", US-ASCII 92) inside of a
quoted string is followed by either another backslash or a double
quote. These two-character sequences represent a single backslash or
double quote within the value, respectively.
Scripts SHOULD NOT escape other characters with a backslash.
An undefined escape sequence (such as "\a" in a context where "a" has
no special meaning) is interpreted as if there were no backslash (in
this case, "\a" is just "a"), though that may be changed by
extensions.
Non-printing characters such as tabs, CRLF, and control characters
are permitted in quoted strings. Quoted strings MAY span multiple
lines. An unencoded NUL (US-ASCII 0) is not allowed in strings; see
section 2.4.2.4 for how it can be encoded.
As message header data is converted to [UTF-8] for comparison (see
section 2.7.2), most string values will use the UTF-8 encoding.
However, implementations MUST accept all strings that match the
grammar in section 8. The ability to use non-UTF-8 encoded strings
matches existing practice and has proven to be useful both in tests
for invalid data and in arguments containing raw MIME parts for
extension actions that generate outgoing messages.
For entering larger amounts of text, such as an email message, a
multi-line form is allowed. It starts with the keyword "text:",
followed by a CRLF, and ends with the sequence of a CRLF, a single
period, and another CRLF. The CRLF before the final period is
considered part of the value. In order to allow the message to
contain lines with a single dot, lines are dot-stuffed. That is,
when composing a message body, an extra '.' is added before each line
that begins with a '.'. When the server interprets the script, these
extra dots are removed. Note that a line that begins with a dot
followed by a non-dot character is not interpreted as dot-stuffed;
Guenther & Showalter Standards Track [Page 8]
RFC 5228 Sieve: An Email Filtering Language January 2008
that is, ".foo" is interpreted as ".foo". However, because this is
potentially ambiguous, scripts SHOULD be properly dot-stuffed so such
lines do not appear.
Note that a hashed comment or whitespace may occur in between the
"text:" and the CRLF, but not within the string itself. Bracketed
comments are not allowed here.
2.4.2.1. String Lists
When matching patterns, it is frequently convenient to match against
groups of strings instead of single strings. For this reason, a list
of strings is allowed in many tests, implying that if the test is
true using any one of the strings, then the test is true.
For instance, the test 'header :contains ["To", "Cc"]
["me@example.com", "me00@landru.example.com"]' is true if either a To
header or Cc header of the input message contains either of the email
addresses "me@example.com" or "me00@landru.example.com".
Conversely, in any case where a list of strings is appropriate, a
single string is allowed without being a member of a list: it is
equivalent to a list with a single member. This means that the test
'exists "To"' is equivalent to the test 'exists ["To"]'.
2.4.2.2. Headers
Headers are a subset of strings. In the Internet Message
Specification [IMAIL], each header line is allowed to have whitespace
nearly anywhere in the line, including after the field name and
before the subsequent colon. Extra spaces between the header name
and the ":" in a header field are ignored.
A header name never contains a colon. The "From" header refers to a
line beginning "From:" (or "From :", etc.). No header will match
the string "From:" due to the trailing colon.
Similarly, no header will match a syntactically invalid header name.
An implementation MUST NOT cause an error for syntactically invalid
header names in tests.
Header lines are unfolded as described in [IMAIL] section 2.2.3.
Interpretation of header data SHOULD be done according to [MIME3]
section 6.2 (see section 2.7.2 below for details).
Guenther & Showalter Standards Track [Page 9]
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2.4.2.3. Addresses
A number of commands call for email addresses, which are also a
subset of strings. When these addresses are used in outbound
contexts, addresses must be compliant with [IMAIL], but are further
constrained within this document. Using the symbols defined in
[IMAIL], section 3, the syntax of an address is:
sieve-address = addr-spec ; simple address
/ phrase "<" addr-spec ">" ; name & addr-spec
That is, routes and group syntax are not permitted. If multiple
addresses are required, use a string list. Named groups are not
permitted.
It is an error for a script to execute an action with a value for use
as an outbound address that doesn't match the "sieve-address" syntax.
2.4.2.4. Encoding Characters Using "encoded-character"
When the "encoded-character" extension is in effect, certain
character sequences in strings are replaced by their decoded value.
This happens after escape sequences are interpreted and dot-
unstuffing has been done. Implementations SHOULD support "encoded-
character".
Arbitrary octets can be embedded in strings by using the syntax
encoded-arb-octets. The sequence is replaced by the octets with the
hexadecimal values given by each hex-pair.
blank = WSP / CRLF
encoded-arb-octets = "${hex:" hex-pair-seq "}"
hex-pair-seq = *blank hex-pair *(1*blank hex-pair) *blank
hex-pair = 1*2HEXDIG
Where WSP and HEXDIG non-terminals are defined in Appendix B.1 of
[ABNF].
It may be inconvenient or undesirable to enter Unicode characters
verbatim, and for these cases the syntax encoded-unicode-char can be
used. The sequence is replaced by the UTF-8 encoding of the
specified Unicode characters, which are identified by the hexadecimal
value of unicode-hex.
encoded-unicode-char = "${unicode:" unicode-hex-seq "}"
unicode-hex-seq = *blank unicode-hex
*(1*blank unicode-hex) *blank
unicode-hex = 1*HEXDIG
Guenther & Showalter Standards Track [Page 10]
RFC 5228 Sieve: An Email Filtering Language January 2008
It is an error for a script to use a hexadecimal value that isn't in
either the range 0 to D7FF or the range E000 to 10FFFF. (The range
D800 to DFFF is excluded as those character numbers are only used as
part of the UTF-16 encoding form and are not applicable to the UTF-8
encoding that the syntax here represents.)
Note: Implementations MUST NOT raise an error for an out-of-range
Unicode value unless the sequence containing it is well-formed
according to the grammar.
The capability string for use with the require command is "encoded-
character".
In the following script, message B is discarded, since the specified
test string is equivalent to "$$".
Example: require "encoded-character";
if header :contains "Subject" "${hex:24 24}" {
discard;
}
The following examples demonstrate valid and invalid encodings and
how they are handled:
"${hex:40}" -> "$@"
"${hex: 40 }" -> "@"
"${HEX: 40}" -> "@"
"${hex:40" -> "${hex:40"
"${hex:400}" -> "${hex:400}"
"${hex:4${hex:30}}" -> "${hex:40}"
"${unicode:40}" -> "@"
"${ unicode:40}" -> "${ unicode:40}"
"${UNICODE:40}" -> "@"
"${UnICoDE:0000040}" -> "@"
"${Unicode:40}" -> "@"
"${Unicode:Cool}" -> "${Unicode:Cool}"
"${unicode:200000}" -> error
"${Unicode:DF01} -> error
2.5. Tests
Tests are given as arguments to commands in order to control their
actions. In this document, tests are given to if/elsif to decide
which block of code is run.
Guenther & Showalter Standards Track [Page 11]
RFC 5228 Sieve: An Email Filtering Language January 2008
2.5.1. Test Lists
Some tests ("allof" and "anyof", which implement logical "and" and
logical "or", respectively) may require more than a single test as an
argument. The test-list syntax element provides a way of grouping
tests as a comma-separated list in parentheses.
Example: if anyof (not exists ["From", "Date"],
header :contains "from" "fool@example.com") {
discard;
}
2.6. Arguments
In order to specify what to do, most commands take arguments. There
are three types of arguments: positional, tagged, and optional.
It is an error for a script, on a single command, to use conflicting
arguments or to use a tagged or optional argument more than once.
2.6.1. Positional Arguments
Positional arguments are given to a command that discerns their
meaning based on their order. When a command takes positional
arguments, all positional arguments must be supplied and must be in
the order prescribed.
2.6.2. Tagged Arguments
This document provides for tagged arguments in the style of
CommonLISP. These are also similar to flags given to commands in
most command-line systems.
A tagged argument is an argument for a command that begins with ":"
followed by a tag naming the argument, such as ":contains". This
argument means that zero or more of the next tokens have some
particular meaning depending on the argument. These next tokens may
be literal data, but they are never blocks.
Tagged arguments are similar to positional arguments, except that
instead of the meaning being derived from the command, it is derived
from the tag.
Tagged arguments must appear before positional arguments, but they
may appear in any order with other tagged arguments. For simplicity
of the specification, this is not expressed in the syntax definitions
Guenther & Showalter Standards Track [Page 12]
RFC 5228 Sieve: An Email Filtering Language January 2008
with commands, but they still may be reordered arbitrarily provided
they appear before positional arguments. Tagged arguments may be
mixed with optional arguments.
Tagged arguments SHOULD NOT take tagged arguments as arguments.
2.6.3. Optional Arguments
Optional arguments are exactly like tagged arguments except that they
may be left out, in which case a default value is implied. Because
optional arguments tend to result in shorter scripts, they have been
used far more than tagged arguments.
One particularly noteworthy case is the ":comparator" argument, which
allows the user to specify which comparator [COLLATION] will be used
to compare two strings, since different languages may impose
different orderings on UTF-8 [UTF-8] strings.
2.6.4. Types of Arguments
Abstractly, arguments may be literal data, tests, or blocks of
commands. In this way, an "if" control structure is merely a command
that happens to take a test and a block as arguments and may execute
the block of code.
However, this abstraction is ambiguous from a parsing standpoint.
The grammar in section 8.2 presents a parsable version of this:
Arguments are string lists (string-lists), numbers, and tags, which
may be followed by a test or a test list (test-list), which may be
followed by a block of commands. No more than one test or test list,
or more than one block of commands, may be used, and commands that
end with a block of commands do not end with semicolons.
2.7. String Comparison
When matching one string against another, there are a number of ways
of performing the match operation. These are accomplished with three
types of matches: an exact match, a substring match, and a wildcard
glob-style match. These are described below.
In order to provide for matches between character sets and case
insensitivity, Sieve uses the comparators defined in the Internet
Application Protocol Collation Registry [COLLATION].
Guenther & Showalter Standards Track [Page 13]
RFC 5228 Sieve: An Email Filtering Language January 2008
However, when a string represents the name of a header, the
comparator is never user-specified. Header comparisons are always
done with the "i;ascii-casemap" operator, i.e., case-insensitive
comparisons, because this is the way things are defined in the
message specification [IMAIL].
2.7.1. Match Type
Commands that perform string comparisons may have an optional match
type argument. The three match types in this specification are
":contains", ":is", and ":matches".
The ":contains" match type describes a substring match. If the value
argument contains the key argument as a substring, the match is true.
For instance, the string "frobnitzm" contains "frob" and "nit", but
not "fbm". The empty key ("") is contained in all values.
The ":is" match type describes an absolute match; if the contents of
the first string are absolutely the same as the contents of the
second string, they match. Only the string "frobnitzm" is the string
"frobnitzm". The empty key ("") only ":is" matches with the empty
value.
The ":matches" match type specifies a wildcard match using the
characters "*" and "?"; the entire value must be matched. "*"
matches zero or more characters in the value and "?" matches a single
character in the value, where the comparator that is used (see
section 2.7.3) defines what a character is. For example, the
comparators "i;octet" and "i;ascii-casemap" define a character to be
a single octet, so "?" will always match exactly one octet when one
of those comparators is in use. In contrast, a Unicode-based
comparator would define a character to be any UTF-8 octet sequence
encoding one Unicode character and thus "?" may match more than one
octet. "?" and "*" may be escaped as "\\?" and "\\*" in strings to
match against themselves. The first backslash escapes the second
backslash; together, they escape the "*". This is awkward, but it is
commonplace in several programming languages that use globs and
regular expressions.
In order to specify what type of match is supposed to happen,
commands that support matching take optional arguments ":matches",
":is", and ":contains". Commands default to using ":is" matching if
no match type argument is supplied. Note that these modifiers
interact with comparators; in particular, only comparators that
support the "substring match" operation are suitable for matching
with ":contains" or ":matches". It is an error to use a comparator
with ":contains" or ":matches" that is not compatible with it.
Guenther & Showalter Standards Track [Page 14]
RFC 5228 Sieve: An Email Filtering Language January 2008
It is an error to give more than one of these arguments to a given
command.
For convenience, the "MATCH-TYPE" syntax element is defined here as
follows:
Syntax: ":is" / ":contains" / ":matches"
2.7.2. Comparisons across Character Sets
Messages may involve a number of character sets. In order for
comparisons to work across character sets, implementations SHOULD
implement the following behavior:
Comparisons are performed on octets. Implementations convert text
from header fields in all charsets [MIME3] to Unicode, encoded as
UTF-8, as input to the comparator (see section 2.7.3).
Implementations MUST be capable of converting US-ASCII, ISO-8859-
1, the US-ASCII subset of ISO-8859-* character sets, and UTF-8.
Text that the implementation cannot convert to Unicode for any
reason MAY be treated as plain US-ASCII (including any [MIME3]
syntax) or processed according to local conventions. An encoded
NUL octet (character zero) SHOULD NOT cause early termination of
the header content being compared against.
If implementations fail to support the above behavior, they MUST
conform to the following:
No two strings can be considered equal if one contains octets
greater than 127.
2.7.3. Comparators
In order to allow for language-independent, case-independent matches,
the match type may be coupled with a comparator name. The Internet
Application Protocol Collation Registry [COLLATION] provides the
framework for describing and naming comparators.
All implementations MUST support the "i;octet" comparator (simply
compares octets) and the "i;ascii-casemap" comparator (which treats
uppercase and lowercase characters in the US-ASCII subset of UTF-8 as
the same). If left unspecified, the default is "i;ascii-casemap".
Some comparators may not be usable with substring matches; that is,
they may only work with ":is". It is an error to try to use a
comparator with ":matches" or ":contains" that is not compatible with
it.
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Sieve treats a comparator result of "undefined" the same as a result
of "no-match". That is, this base specification does not provide any
means to directly detect invalid comparator input.
A comparator is specified by the ":comparator" option with commands
that support matching. This option is followed by a string providing
the name of the comparator to be used. For convenience, the syntax
of a comparator is abbreviated to "COMPARATOR", and (repeated in
several tests) is as follows:
Syntax: ":comparator" <comparator-name: string>
So in this example,
Example: if header :contains :comparator "i;octet" "Subject"
"MAKE MONEY FAST" {
discard;
}
would discard any message with subjects like "You can MAKE MONEY
FAST", but not "You can Make Money Fast", since the comparator used
is case-sensitive.
Comparators other than "i;octet" and "i;ascii-casemap" must be
declared with require, as they are extensions. If a comparator
declared with require is not known, it is an error, and execution
fails. If the comparator is not declared with require, it is also an
error, even if the comparator is supported. (See section 2.10.5.)
Both ":matches" and ":contains" match types are compatible with the
"i;octet" and "i;ascii-casemap" comparators and may be used with
them.
It is an error to give more than one of these arguments to a given
command.
2.7.4. Comparisons against Addresses
Addresses are one of the most frequent things represented as strings.
These are structured, and being able to compare against the local-
part or the domain of an address is useful, so some tests that act
exclusively on addresses take an additional optional argument that
specifies what the test acts on.
These optional arguments are ":localpart", ":domain", and ":all",
which act on the local-part (left side), the domain-part (right
side), and the whole address.
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If an address is not syntactically valid, then it will not be matched
by tests specifying ":localpart" or ":domain".
The kind of comparison done, such as whether or not the test done is
case-insensitive, is specified as a comparator argument to the test.
If an optional address-part is omitted, the default is ":all".
It is an error to give more than one of these arguments to a given
command.
For convenience, the "ADDRESS-PART" syntax element is defined here as
follows:
Syntax: ":localpart" / ":domain" / ":all"
2.8. Blocks
Blocks are sets of commands enclosed within curly braces and supplied
as the final argument to a command. Such a command is a control
structure: when executed it has control over the number of times the
commands in the block are executed.
With the commands supplied in this memo, there are no loops. The
control structures supplied--if, elsif, and else--run a block either
once or not at all.
2.9. Commands
Sieve scripts are sequences of commands. Commands can take any of
the tokens above as arguments, and arguments may be either tagged or
positional arguments. Not all commands take all arguments.
There are three kinds of commands: test commands, action commands,
and control commands.
The simplest is an action command. An action command is an
identifier followed by zero or more arguments, terminated by a
semicolon. Action commands do not take tests or blocks as arguments.
The actions referenced in this document are:
- keep, to save the message in the default location
- fileinto, to save the message in a specific mailbox
- redirect, to forward the message to another address
- discard, to silently throw away the message
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A control command is a command that affects the parsing or the flow
of execution of the Sieve script in some way. A control structure is
a control command that ends with a block instead of a semicolon.
A test command is used as part of a control command. It is used to
specify whether or not the block of code given to the control command
is executed.
2.10. Evaluation
2.10.1. Action Interaction
Some actions cannot be used with other actions because the result
would be absurd. These restrictions are noted throughout this memo.
Extension actions MUST state how they interact with actions defined
in this specification.
2.10.2. Implicit Keep
Previous experience with filtering systems suggests that cases tend
to be missed in scripts. To prevent errors, Sieve has an "implicit
keep".
An implicit keep is a keep action (see section 4.3) performed in
absence of any action that cancels the implicit keep.
An implicit keep is performed if a message is not written to a
mailbox, redirected to a new address, or explicitly thrown out. That
is, if a fileinto, a keep, a redirect, or a discard is performed, an
implicit keep is not.
Some actions may be defined to not cancel the implicit keep. These
actions may not directly affect the delivery of a message, and are
used for their side effects. None of the actions specified in this
document meet that criteria, but extension actions may.
For instance, with any of the short messages offered above, the
following script produces no actions.
Example: if size :over 500K { discard; }
As a result, the implicit keep is taken.
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2.10.3. Message Uniqueness in a Mailbox
Implementations SHOULD NOT deliver a message to the same mailbox more
than once, even if a script explicitly asks for a message to be
written to a mailbox twice.
The test for equality of two messages is implementation-defined.
If a script asks for a message to be written to a mailbox twice, it
MUST NOT be treated as an error.
2.10.4. Limits on Numbers of Actions
Site policy MAY limit the number of actions taken and MAY impose
restrictions on which actions can be used together. In the event
that a script hits a policy limit on the number of actions taken for
a particular message, an error occurs.
Implementations MUST allow at least one keep or one fileinto. If
fileinto is not implemented, implementations MUST allow at least one
keep.
2.10.5. Extensions and Optional Features
Because of the differing capabilities of many mail systems, several
features of this specification are optional. Before any of these
extensions can be executed, they must be declared with the "require"
action.
If an extension is not enabled with "require", implementations MUST
treat it as if they did not support it at all. This protects scripts
from having their behavior altered by extensions that the script
author might not have even been aware of.
Implementations MUST NOT execute any Sieve script test or command
subsequent to "require" if one of the required extensions is
unavailable.
Note: The reason for this restriction is that prior experiences
with languages such as LISP and Tcl suggest that this is a
workable way of noting that a given script uses an extension.
Extensions that define actions MUST state how they interact with
actions discussed in the base specification.
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2.10.6. Errors
In any programming language, there are compile-time and run-time
errors.
Compile-time errors are ones in syntax that are detectable if a
syntax check is done.
Run-time errors are not detectable until the script is run. This
includes transient failures like disk full conditions, but also
includes issues like invalid combinations of actions.
When an error occurs in a Sieve script, all processing stops.
Implementations MAY choose to do a full parse, then evaluate the
script, then do all actions. Implementations might even go so far as
to ensure that execution is atomic (either all actions are executed
or none are executed).
Other implementations may choose to parse and run at the same time.
Such implementations are simpler, but have issues with partial
failure (some actions happen, others don't).
Implementations MUST perform syntactic, semantic, and run-time checks
on code that is actually executed. Implementations MAY perform those
checks or any part of them on code that is not reached during
execution.
When an error happens, implementations MUST notify the user that an
error occurred and which actions (if any) were taken, and do an
implicit keep.
2.10.7. Limits on Execution
Implementations may limit certain constructs. However, this
specification places a lower bound on some of these limits.
Implementations MUST support fifteen levels of nested blocks.
Implementations MUST support fifteen levels of nested test lists.
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3. Control Commands
Control structures are needed to allow for multiple and conditional
actions.
3.1. Control if
There are three pieces to if: "if", "elsif", and "else". Each is
actually a separate command in terms of the grammar. However, an
elsif or else MUST only follow an if or elsif. An error occurs if
these conditions are not met.
Usage: if <test1: test> <block1: block>
Usage: elsif <test2: test> <block2: block>
Usage: else <block3: block>
The semantics are similar to those of any of the many other
programming languages these control structures appear in. When the
interpreter sees an "if", it evaluates the test associated with it.
If the test is true, it executes the block associated with it.
If the test of the "if" is false, it evaluates the test of the first
"elsif" (if any). If the test of "elsif" is true, it runs the
elsif's block. An elsif may be followed by an elsif, in which case,
the interpreter repeats this process until it runs out of elsifs.
When the interpreter runs out of elsifs, there may be an "else" case.
If there is, and none of the if or elsif tests were true, the
interpreter runs the else's block.
This provides a way of performing exactly one of the blocks in the
chain.
In the following example, both messages A and B are dropped.
Example: require "fileinto";
if header :contains "from" "coyote" {
discard;
} elsif header :contains ["subject"] ["$$"] {
discard;
} else {
fileinto "INBOX";
}
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When the script below is run over message A, it redirects the message
to acm@example.com; message B, to postmaster@example.com; any other
message is redirected to field@example.com.
Example: if header :contains ["From"] ["coyote"] {
redirect "acm@example.com";
} elsif header :contains "Subject" "$$" {
redirect "postmaster@example.com";
} else {
redirect "field@example.com";
}
Note that this definition prohibits the "... else if ..." sequence
used by C. This is intentional, because this construct produces a
shift-reduce conflict.
3.2. Control require
Usage: require <capabilities: string-list>
The require action notes that a script makes use of a certain
extension. Such a declaration is required to use the extension, as
discussed in section 2.10.5. Multiple capabilities can be declared
with a single require.
The require command, if present, MUST be used before anything other
than a require can be used. An error occurs if a require appears
after a command other than require.
Example: require ["fileinto", "reject"];
Example: require "fileinto";
require "vacation";
3.3. Control stop
Usage: stop
The "stop" action ends all processing. If the implicit keep has not
been cancelled, then it is taken.
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4. Action Commands
This document supplies four actions that may be taken on a message:
keep, fileinto, redirect, and discard.
Implementations MUST support the "keep", "discard", and "redirect"
actions.
Implementations SHOULD support "fileinto".
Implementations MAY limit the number of certain actions taken (see
section 2.10.4).
4.1. Action fileinto
Usage: fileinto <mailbox: string>
The "fileinto" action delivers the message into the specified
mailbox. Implementations SHOULD support fileinto, but in some
environments this may be impossible. Implementations MAY place
restrictions on mailbox names; use of an invalid mailbox name MAY be
treated as an error or result in delivery to an implementation-
defined mailbox. If the specified mailbox doesn't exist, the
implementation MAY treat it as an error, create the mailbox, or
deliver the message to an implementation-defined mailbox. If the
implementation uses a different encoding scheme than UTF-8 for
mailbox names, it SHOULD reencode the mailbox name from UTF-8 to its
encoding scheme. For example, the Internet Message Access Protocol
[IMAP] uses modified UTF-7, such that a mailbox argument of "odds &
ends" would appear in IMAP as "odds &- ends".
The capability string for use with the require command is "fileinto".
In the following script, message A is filed into mailbox
"INBOX.harassment".
Example: require "fileinto";
if header :contains ["from"] "coyote" {
fileinto "INBOX.harassment";
}
4.2. Action redirect
Usage: redirect <address: string>
The "redirect" action is used to send the message to another user at
a supplied address, as a mail forwarding feature does. The
"redirect" action makes no changes to the message body or existing
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headers, but it may add new headers. In particular, existing
Received headers MUST be preserved and the count of Received headers
in the outgoing message MUST be larger than the same count on the
message as received by the implementation. (An implementation that
adds a Received header before processing the message does not need to
add another when redirecting.)
The message is sent back out with the address from the redirect
command as an envelope recipient. Implementations MAY combine
separate redirects for a given message into a single submission with
multiple envelope recipients. (This is not a Mail User Agent (MUA)-
style forward, which creates a new message with a different sender
and message ID, wrapping the old message in a new one.)
The envelope sender address on the outgoing message is chosen by the
sieve implementation. It MAY be copied from the message being
processed. However, if the message being processed has an empty
envelope sender address the outgoing message MUST also have an empty
envelope sender address. This last requirement is imposed to prevent
loops in the case where a message is redirected to an invalid address
when then returns a delivery status notification that also ends up
being redirected to the same invalid address.
A simple script can be used for redirecting all mail:
Example: redirect "bart@example.com";
Implementations MUST take measures to implement loop control,
possibly including adding headers to the message or counting Received
headers as specified in section 6.2 of [SMTP]. If an implementation
detects a loop, it causes an error.
Implementations MUST provide means of limiting the number of
redirects a Sieve script can perform. See section 10 for more
details.
Implementations MAY ignore a redirect action silently due to policy
reasons. For example, an implementation MAY choose not to redirect
to an address that is known to be undeliverable. Any ignored
redirect MUST NOT cancel the implicit keep.
4.3. Action keep
Usage: keep
The "keep" action is whatever action is taken in lieu of all other
actions, if no filtering happens at all; generally, this simply means
to file the message into the user's main mailbox. This command
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provides a way to execute this action without needing to know the
name of the user's main mailbox, providing a way to call it without
needing to understand the user's setup or the underlying mail system.
For instance, in an implementation where the IMAP server is running
scripts on behalf of the user at time of delivery, a keep command is
equivalent to a fileinto "INBOX".
Example: if size :under 1M { keep; } else { discard; }
Note that the above script is identical to the one below.
Example: if not size :under 1M { discard; }
4.4. Action discard
Usage: discard
Discard is used to silently throw away the message. It does so by
simply canceling the implicit keep. If discard is used with other
actions, the other actions still happen. Discard is compatible with
all other actions. (For instance, fileinto+discard is equivalent to
fileinto.)
Discard MUST be silent; that is, it MUST NOT return a non-delivery
notification of any kind ([DSN], [MDN], or otherwise).
In the following script, any mail from "idiot@example.com" is thrown
out.
Example: if header :contains ["from"] ["idiot@example.com"] {
discard;
}
While an important part of this language, "discard" has the potential
to create serious problems for users: Students who leave themselves
logged in to an unattended machine in a public computer lab may find
their script changed to just "discard". In order to protect users in
this situation (along with similar situations), implementations MAY
keep messages destroyed by a script for an indefinite period, and MAY
disallow scripts that throw out all mail.
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5. Test Commands
Tests are used in conditionals to decide which part(s) of the
conditional to execute.
Implementations MUST support these tests: "address", "allof",
"anyof", "exists", "false", "header", "not", "size", and "true".
Implementations SHOULD support the "envelope" test.
5.1. Test address
Usage: address [COMPARATOR] [ADDRESS-PART] [MATCH-TYPE]
<header-list: string-list> <key-list: string-list>
The "address" test matches Internet addresses in structured headers
that contain addresses. It returns true if any header contains any
key in the specified part of the address, as modified by the
comparator and the match keyword. Whether there are other addresses
present in the header doesn't affect this test; this test does not
provide any way to determine whether an address is the only address
in a header.
Like envelope and header, this test returns true if any combination
of the header-list and key-list arguments match and returns false
otherwise.
Internet email addresses [IMAIL] have the somewhat awkward
characteristic that the local-part to the left of the at-sign is
considered case sensitive, and the domain-part to the right of the
at-sign is case insensitive. The "address" command does not deal
with this itself, but provides the ADDRESS-PART argument for allowing
users to deal with it.
The address primitive never acts on the phrase part of an email
address or on comments within that address. It also never acts on
group names, although it does act on the addresses within the group
construct.
Implementations MUST restrict the address test to headers that
contain addresses, but MUST include at least From, To, Cc, Bcc,
Sender, Resent-From, and Resent-To, and it SHOULD include any other
header that utilizes an "address-list" structured header body.
Example: if address :is :all "from" "tim@example.com" {
discard;
}
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5.2. Test allof
Usage: allof <tests: test-list>
The "allof" test performs a logical AND on the tests supplied to it.
Example: allof (false, false) => false
allof (false, true) => false
allof (true, true) => true
The allof test takes as its argument a test-list.
5.3. Test anyof
Usage: anyof <tests: test-list>
The "anyof" test performs a logical OR on the tests supplied to it.
Example: anyof (false, false) => false
anyof (false, true) => true
anyof (true, true) => true
5.4. Test envelope
Usage: envelope [COMPARATOR] [ADDRESS-PART] [MATCH-TYPE]
<envelope-part: string-list> <key-list: string-list>
The "envelope" test is true if the specified part of the [SMTP] (or
equivalent) envelope matches the specified key. This specification
defines the interpretation of the (case insensitive) "from" and "to"
envelope-parts. Additional envelope-parts may be defined by other
extensions; implementations SHOULD consider unknown envelope parts an
error.
If one of the envelope-part strings is (case insensitive) "from",
then matching occurs against the FROM address used in the SMTP MAIL
command. The null reverse-path is matched against as the empty
string, regardless of the ADDRESS-PART argument specified.
If one of the envelope-part strings is (case insensitive) "to", then
matching occurs against the TO address used in the SMTP RCPT command
that resulted in this message getting delivered to this user. Note
that only the most recent TO is available, and only the one relevant
to this user.
The envelope-part is a string list and may contain more than one
parameter, in which case all of the strings specified in the key-list
are matched against all parts given in the envelope-part list.
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Like address and header, this test returns true if any combination of
the envelope-part list and key-list arguments match and returns false
otherwise.
All tests against envelopes MUST drop source routes.
If the SMTP transaction involved several RCPT commands, only the data
from the RCPT command that caused delivery to this user is available
in the "to" part of the envelope.
If a protocol other than SMTP is used for message transport,
implementations are expected to adapt this command appropriately.
The envelope command is optional. Implementations SHOULD support it,
but the necessary information may not be available in all cases. The
capability string for use with the require command is "envelope".
Example: require "envelope";
if envelope :all :is "from" "tim@example.com" {
discard;
}
5.5. Test exists
Usage: exists <header-names: string-list>
The "exists" test is true if the headers listed in the header-names
argument exist within the message. All of the headers must exist or
the test is false.
The following example throws out mail that doesn't have a From header
and a Date header.
Example: if not exists ["From","Date"] {
discard;
}
5.6. Test false
Usage: false
The "false" test always evaluates to false.
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5.7. Test header
Usage: header [COMPARATOR] [MATCH-TYPE]
<header-names: string-list> <key-list: string-list>
The "header" test evaluates to true if the value of any of the named
headers, ignoring leading and trailing whitespace, matches any key.
The type of match is specified by the optional match argument, which
defaults to ":is" if not specified, as specified in section 2.6.
Like address and envelope, this test returns true if any combination
of the header-names list and key-list arguments match and returns
false otherwise.
If a header listed in the header-names argument exists, it contains
the empty key (""). However, if the named header is not present, it
does not match any key, including the empty key. So if a message
contained the header
X-Caffeine: C8H10N4O2
these tests on that header evaluate as follows:
header :is ["X-Caffeine"] [""] => false
header :contains ["X-Caffeine"] [""] => true
Testing whether a given header is either absent or doesn't contain
any non-whitespace characters can be done using a negated "header"
test:
not header :matches "Cc" "?*"
5.8. Test not
Usage: not <test1: test>
The "not" test takes some other test as an argument, and yields the
opposite result. "not false" evaluates to "true" and "not true"
evaluates to "false".
5.9. Test size
Usage: size <":over" / ":under"> <limit: number>
The "size" test deals with the size of a message. It takes either a
tagged argument of ":over" or ":under", followed by a number
representing the size of the message.
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If the argument is ":over", and the size of the message is greater
than the number provided, the test is true; otherwise, it is false.
If the argument is ":under", and the size of the message is less than
the number provided, the test is true; otherwise, it is false.
Exactly one of ":over" or ":under" must be specified, and anything
else is an error.
The size of a message is defined to be the number of octets in the
[IMAIL] representation of the message.
Note that for a message that is exactly 4,000 octets, the message is
neither ":over" nor ":under" 4000 octets.
5.10. Test true
Usage: true
The "true" test always evaluates to true.
6. Extensibility
New control commands, actions, and tests can be added to the
language. Sites must make these features known to their users; this
document does not define a way to discover the list of extensions
supported by the server.
Any extensions to this language MUST define a capability string that
uniquely identifies that extension. Capability string are case-
sensitive; for example, "foo" and "FOO" are different capabilities.
If a new version of an extension changes the functionality of a
previously defined extension, it MUST use a different name.
Extensions may register a set of related capabilities by registering
just a unique prefix for them. The "comparator-" prefix is an
example of this. The prefix MUST end with a "-" and MUST NOT overlap
any existing registrations.
In a situation where there is a script submission protocol and an
extension advertisement mechanism aware of the details of this
language, scripts submitted can be checked against the mail server to
prevent use of an extension that the server does not support.
Extensions MUST state how they interact with constraints defined in
section 2.10, e.g., whether they cancel the implicit keep, and which
actions they are compatible and incompatible with. Extensions MUST
NOT change the behavior of the "require" control command or alter the
interpretation of the argument to the "require" control.
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Extensions that can submit new email messages or otherwise generate
new protocol requests MUST consider loop suppression, at least to
document any security considerations.
6.1. Capability String
Capability strings are typically short strings describing what
capabilities are supported by the server.
Capability strings beginning with "vnd." represent vendor-defined
extensions. Such extensions are not defined by Internet standards or
RFCs, but are still registered with IANA in order to prevent
conflicts. Extensions starting with "vnd." SHOULD be followed by the
name of the vendor and product, such as "vnd.acme.rocket-sled".
The following capability strings are defined by this document:
encoded-character The string "encoded-character" indicates that the
implementation supports the interpretation of
"${hex:...}" and "${unicode:...}" in strings.
envelope The string "envelope" indicates that the implementation
supports the "envelope" command.
fileinto The string "fileinto" indicates that the implementation
supports the "fileinto" command.
comparator- The string "comparator-elbonia" is provided if the
implementation supports the "elbonia" comparator.
Therefore, all implementations have at least the
"comparator-i;octet" and "comparator-i;ascii-casemap"
capabilities. However, these comparators may be used
without being declared with require.
6.2. IANA Considerations
In order to provide a standard set of extensions, a registry is
maintained by IANA. This registry contains both vendor-controlled
capability names (beginning with "vnd.") and IETF-controlled
capability names. Vendor-controlled capability names may be
registered on a first-come, first-served basis, by applying to IANA
with the form in the following section. Registration of capability
prefixes that do not begin with "vnd." REQUIRES a standards track or
IESG-approved experimental RFC.
Extensions designed for interoperable use SHOULD use IETF-controlled
capability names.
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6.2.1. Template for Capability Registrations
The following template is to be used for registering new Sieve
extensions with IANA.
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: [the string for use in the 'require' statement]
Description: [a brief description of what the extension adds
or changes]
RFC number: [for extensions published as RFCs]
Contact address: [email and/or physical address to contact for
additional information]
6.2.2. Handling of Existing Capability Registrations
In order to bring the existing capability registrations in line with
the new template, IANA has modified each as follows:
1. The "capability name" and "capability arguments" fields have been
eliminated
2. The "capability keyword" field have been renamed to "Capability
name"
3. An empty "Description" field has been added
4. The "Standards Track/IESG-approved experimental RFC number" field
has been renamed to "RFC number"
5. The "Person and email address to contact for further information"
field should be renamed to "Contact address"
6.2.3. Initial Capability Registrations
This RFC updates the following entries in the IANA registry for Sieve
extensions.
Capability name: encoded-character
Description: changes the interpretation of strings to allow
arbitrary octets and Unicode characters to be
represented using US-ASCII
RFC number: RFC 5228 (Sieve base spec)
Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>
Capability name: fileinto
Description: adds the 'fileinto' action for delivering to a
mailbox other than the default
RFC number: RFC 5228 (Sieve base spec)
Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>
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Capability name: envelope
Description: adds the 'envelope' test for testing the message
transport sender and recipient address
RFC number: RFC 5228 (Sieve base spec)
Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>
Capability name: comparator-* (anything starting with "comparator-")
Description: adds the indicated comparator for use with the
:comparator argument
RFC number: RFC 5228 (Sieve base spec) and [COLLATION]
Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>
6.3. Capability Transport
A method of advertising which capabilities an implementation supports
is difficult due to the wide range of possible implementations. Such
a mechanism, however, should have the property that the
implementation can advertise the complete set of extensions that it
supports.
7. Transmission
The [MIME] type for a Sieve script is "application/sieve".
The registration of this type for RFC 2048 requirements is updated as
follows:
Subject: Registration of MIME media type application/sieve
MIME media type name: application
MIME subtype name: sieve
Required parameters: none
Optional parameters: none
Encoding considerations: Most Sieve scripts will be textual,
written in UTF-8. When non-7bit characters are used,
quoted-printable is appropriate for transport systems
that require 7bit encoding.
Security considerations: Discussed in section 10 of this RFC.
Interoperability considerations: Discussed in section 2.10.5
of this RFC.
Published specification: this RFC.
Applications that use this media type: sieve-enabled mail
servers and clients
Additional information:
Magic number(s):
File extension(s): .siv .sieve
Macintosh File Type Code(s):
Guenther & Showalter Standards Track [Page 33]
RFC 5228 Sieve: An Email Filtering Language January 2008
Person & email address to contact for further information:
See the discussion list at ietf-mta-filters@imc.org.
Intended usage:
COMMON
Author/Change controller:
The SIEVE WG, delegated by the IESG.
8. Parsing
The Sieve grammar is separated into tokens and a separate grammar as
most programming languages are. Additional rules are supplied here
for common arguments to various language facilities.
8.1. Lexical Tokens
Sieve scripts are encoded in UTF-8. The following assumes a valid
UTF-8 encoding; special characters in Sieve scripts are all US-ASCII.
The following are tokens in Sieve:
- identifiers
- tags
- numbers
- quoted strings
- multi-line strings
- other separators
Identifiers, tags, and numbers are case-insensitive, while quoted
strings and multi-line strings are case-sensitive.
Blanks, horizontal tabs, CRLFs, and comments ("whitespace") are
ignored except as they separate tokens. Some whitespace is required
to separate otherwise adjacent tokens and in specific places in the
multi-line strings. CR and LF can only appear in CRLF pairs.
The other separators are single individual characters and are
mentioned explicitly in the grammar.
The lexical structure of sieve is defined in the following grammar
(as described in [ABNF]):
bracket-comment = "/*" *not-star 1*STAR
*(not-star-slash *not-star 1*STAR) "/"
; No */ allowed inside a comment.
; (No * is allowed unless it is the last
; character, or unless it is followed by a
; character that isn't a slash.)
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comment = bracket-comment / hash-comment
hash-comment = "#" *octet-not-crlf CRLF
identifier = (ALPHA / "_") *(ALPHA / DIGIT / "_")
multi-line = "text:" *(SP / HTAB) (hash-comment / CRLF)
*(multiline-literal / multiline-dotstart)
"." CRLF
multiline-literal = [ octet-not-period *octet-not-crlf ] CRLF
multiline-dotstart = "." 1*octet-not-crlf CRLF
; A line containing only "." ends the
; multi-line. Remove a leading '.' if
; followed by another '.'.
not-star = CRLF / %x01-09 / %x0B-0C / %x0E-29 / %x2B-FF
; either a CRLF pair, OR a single octet
; other than NUL, CR, LF, or star
not-star-slash = CRLF / %x01-09 / %x0B-0C / %x0E-29 / %x2B-2E /
%x30-FF
; either a CRLF pair, OR a single octet
; other than NUL, CR, LF, star, or slash
number = 1*DIGIT [ QUANTIFIER ]
octet-not-crlf = %x01-09 / %x0B-0C / %x0E-FF
; a single octet other than NUL, CR, or LF
octet-not-period = %x01-09 / %x0B-0C / %x0E-2D / %x2F-FF
; a single octet other than NUL,
; CR, LF, or period
octet-not-qspecial = %x01-09 / %x0B-0C / %x0E-21 / %x23-5B / %x5D-FF
; a single octet other than NUL,
; CR, LF, double-quote, or backslash
QUANTIFIER = "K" / "M" / "G"
quoted-other = "\" octet-not-qspecial
; represents just the octet-no-qspecial
; character. SHOULD NOT be used
quoted-safe = CRLF / octet-not-qspecial
; either a CRLF pair, OR a single octet other
; than NUL, CR, LF, double-quote, or backslash
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quoted-special = "\" (DQUOTE / "\")
; represents just a double-quote or backslash
quoted-string = DQUOTE quoted-text DQUOTE
quoted-text = *(quoted-safe / quoted-special / quoted-other)
STAR = "*"
tag = ":" identifier
white-space = 1*(SP / CRLF / HTAB) / comment
8.2. Grammar
The following is the grammar of Sieve after it has been lexically
interpreted. No whitespace or comments appear below. The start
symbol is "start".
argument = string-list / number / tag
arguments = *argument [ test / test-list ]
block = "{" commands "}"
command = identifier arguments (";" / block)
commands = *command
start = commands
string = quoted-string / multi-line
string-list = "[" string *("," string) "]" / string
; if there is only a single string, the brackets
; are optional
test = identifier arguments
test-list = "(" test *("," test) ")"
8.3. Statement Elements
These elements are collected from the "Syntax" sections elsewhere in
this document, and are provided here in [ABNF] syntax so that they
can be modified by extensions.
ADDRESS-PART = ":localpart" / ":domain" / ":all"
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RFC 5228 Sieve: An Email Filtering Language January 2008
COMPARATOR = ":comparator" string
MATCH-TYPE = ":is" / ":contains" / ":matches"
9. Extended Example
The following is an extended example of a Sieve script. Note that it
does not make use of the implicit keep.
#
# Example Sieve Filter
# Declare any optional features or extension used by the script
#
require ["fileinto"];
#
# Handle messages from known mailing lists
# Move messages from IETF filter discussion list to filter mailbox
#
if header :is "Sender" "owner-ietf-mta-filters@imc.org"
{
fileinto "filter"; # move to "filter" mailbox
}
#
# Keep all messages to or from people in my company
#
elsif address :DOMAIN :is ["From", "To"] "example.com"
{
keep; # keep in "In" mailbox
}
#
# Try and catch unsolicited email. If a message is not to me,
# or it contains a subject known to be spam, file it away.
#
elsif anyof (NOT address :all :contains
["To", "Cc", "Bcc"] "me@example.com",
header :matches "subject"
["*make*money*fast*", "*university*dipl*mas*"])
{
fileinto "spam"; # move to "spam" mailbox
}
else
{
# Move all other (non-company) mail to "personal"
# mailbox.
fileinto "personal";
}
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10. Security Considerations
Users must get their mail. It is imperative that whatever
implementations use to store the user-defined filtering scripts
protect them from unauthorized modification, to preserve the
integrity of the mail system. An attacker who can modify a script
can cause mail to be discarded, rejected, or forwarded to an
unauthorized recipient. In addition, it's possible that Sieve
scripts might expose private information, such as mailbox names, or
email addresses of favored (or disfavored) correspondents. Because
of that, scripts SHOULD also be protected from unauthorized
retrieval.
Several commands, such as "discard", "redirect", and "fileinto",
allow for actions to be taken that are potentially very dangerous.
Use of the "redirect" command to generate notifications may easily
overwhelm the target address, especially if it was not designed to
handle large messages.
Allowing a single script to redirect to multiple destinations can be
used as a means of amplifying the number of messages in an attack.
Moreover, if loop detection is not properly implemented, it may be
possible to set up exponentially growing message loops. Accordingly,
Sieve implementations:
(1) MUST implement facilities to detect and break message loops. See
section 6.2 of [SMTP] for additional information on basic loop
detection strategies.
(2) MUST provide the means for administrators to limit the ability of
users to abuse redirect. In particular, it MUST be possible to
limit the number of redirects a script can perform.
Additionally, if no use cases exist for using redirect to
multiple destinations, this limit SHOULD be set to 1. Additional
limits, such as the ability to restrict redirect to local users,
MAY also be implemented.
(3) MUST provide facilities to log use of redirect in order to
facilitate tracking down abuse.
(4) MAY use script analysis to determine whether or not a given
script can be executed safely. While the Sieve language is
sufficiently complex that full analysis of all possible scripts
is computationally infeasible, the majority of real-world scripts
are amenable to analysis. For example, an implementation might
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RFC 5228 Sieve: An Email Filtering Language January 2008
allow scripts that it has determined are safe to run unhindered,
block scripts that are potentially problematic, and subject
unclassifiable scripts to additional auditing and logging.
Allowing redirects at all may not be appropriate in situations where
email accounts are freely available and/or not trackable to a human
who can be held accountable for creating message bombs or other
abuse.
As with any filter on a message stream, if the Sieve implementation
and the mail agents 'behind' Sieve in the message stream differ in
their interpretation of the messages, it may be possible for an
attacker to subvert the filter. Of particular note are differences
in the interpretation of malformed messages (e.g., missing or extra
syntax characters) or those that exhibit corner cases (e.g., NUL
octets encoded via [MIME3]).
11. Acknowledgments
This document has been revised in part based on comments and
discussions that took place on and off the SIEVE mailing list.
Thanks to Sharon Chisholm, Cyrus Daboo, Ned Freed, Arnt Gulbrandsen,
Michael Haardt, Kjetil Torgrim Homme, Barry Leiba, Mark E. Mallett,
Alexey Melnikov, Eric Rescorla, Rob Siemborski, and Nigel Swinson for
reviews and suggestions.
12. Normative References
[ABNF] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", RFC 4234, October 2005.
[COLLATION] Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
Application Protocol Collation Registry", RFC 4790, March
2007.
[IMAIL] Resnick, P., Ed., "Internet Message Format", RFC 2822,
April 2001.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[MIME3] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII
Text", RFC 2047, November 1996.
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RFC 5228 Sieve: An Email Filtering Language January 2008
[SMTP] Klensin, J., Ed., "Simple Mail Transfer Protocol", RFC
2821, April 2001.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
13. Informative References
[BINARY-SI] "Standard IEC 60027-2: Letter symbols to be used in
electrical technology - Part 2: Telecommunications and
electronics", January 1999.
[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464, January
2003.
[FLAMES] Borenstein, N, and C. Thyberg, "Power, Ease of Use, and
Cooperative Work in a Practical Multimedia Message
System", Int. J. of Man-Machine Studies, April, 1991.
Reprinted in Computer-Supported Cooperative Work and
Groupware, Saul Greenberg, editor, Harcourt Brace
Jovanovich, 1991. Reprinted in Readings in Groupware and
Computer-Supported Cooperative Work, Ronald Baecker,
editor, Morgan Kaufmann, 1993.
[IMAP] Crispin, M., "Internet Message Access Protocol - version
4rev1", RFC 3501, March 2003.
[MDN] Hansen, T., Ed., and G. Vaudreuil, Ed., "Message
Disposition Notification", RFC 3798, May 2004.
[RFC3028] Showalter, T., "Sieve: A Mail Filtering Language", RFC
3028, January 2001.
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RFC 5228 Sieve: An Email Filtering Language January 2008
14. Changes from RFC 3028
This following list is a summary of the changes that have been made
in the Sieve language base specification from [RFC3028].
1. Removed ban on tests having side-effects
2. Removed reject extension (will be specified in a separate RFC)
3. Clarified description of comparators to match [COLLATION], the
new base specification for them
4. Require stripping of leading and trailing whitespace in "header"
test
5. Clarified or tightened handling of many minor items, including:
- invalid [MIME3] encoding
- invalid addresses in headers
- invalid header field names in tests
- 'undefined' comparator result
- unknown envelope parts
- null return-path in "envelope" test
6. Capability strings are case-sensitive
7. Clarified that fileinto should reencode non-ASCII mailbox
names to match the mailstore's conventions
8. Errors in the ABNF were corrected
9. The references were updated and split into normative and
informative
10. Added encoded-character capability and deprecated (but did not
remove) use of arbitrary binary octets in Sieve scripts.
11. Updated IANA registration template, and added IANA
considerations to permit capability prefix registrations.
12. Added .sieve as a valid extension for Sieve scripts.
Editors' Addresses
Philip Guenther
Sendmail, Inc.
6425 Christie St. Ste 400
Emeryville, CA 94608
EMail: guenther@sendmail.com
Tim Showalter
EMail: tjs@psaux.com
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Guenther & Showalter Standards Track [Page 42]