Network Working Group T. Showalter
Internet Draft: Sieve Carnegie Mellon
Document: draft-showalter-sieve-04.txt August 1998
Expire in six months (31 January 1999)
Sieve -- a Mail Filtering Language
Status of this memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
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ftp.isi.edu (US West Coast).
The protocol discussed in this document is experimental and subject
to change. Persons planning on either implementing or using this
protocol are STRONGLY URGED to get in touch with the author before
embarking on such a project.
Copyright
Copyright (C) The Internet Society 1998. All Rights Reserved.
Abstract
This document describes a mail filtering language for filtering
messages at time of final delivery. It is designed to be independent
of protocol, and 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 IMAP servers, as
it has no variables, loops, or ability to shell out to external
programs.
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Table of Contents
Status of this memo
Copyright
Abstract
0. Meta-information on this draft
0.1. Discussion
0.2. Known Problems
0.2.1. Probable Extensions
0.2.2. Known Bugs
0.3. Open Issues
0.3. Noted Changes
1. Introduction
1.1. Conventions used in this document
1.2. Example mail messages
2. Design
2.1. Form of the language
2.2. Whitespace
2.3. Comments
2.4. Literal data
2.4.1. Numbers
2.4.2. Strings
2.4.2.1. String lists
2.4.2.2. Headers
2.4.2.3. Addresses
2.5. Tests
2.5.1. String Comparison
2.6.1. Match Keyword
2.6.2. Comparators
2.7. Tagged Arguments
2.8. Blocks
2.9. Commands
2.9.1. Positional Arguments
2.9.2. Optional Arguments
2.9.3. Blocks as Arguments
2.11. Evaluation
2.11.1. Implicit keep
3. Conditionals and Control Structures
4. Actions
4.1. Action reject
4.2. Action fileinto
4.3. Action forward
4.4. Action keep
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4.6. Action stop
4.7. Action discard
5. Tests
5.1. Test allof
5.2. Test anyof
5.3. Test exists
5.4. Test false
5.5. Test header
5.6. Test not
5.7. Test size
6. Errors in Processing a Script
7. Extensibility
7.1. Capability String
7.2. Registry
7.3. Capability Transport
8. Transmission
9. Acknowledgments
10. Formal Grammar
11. Security Considerations
12. Author's Address
Appendices
Appendix A. References
Appendix B. Full Copyright Statement
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0. Meta-information on this draft
This information is intended to facilitate discussion. It will be
removed when this document leaves the Internet-Draft stage.
0.1. Discussion
This draft is being discussed on the MTA Filters mailing list at
<ietf-mta-filters@imc.org>. Subscription requests can be sent to
<ietf-mta-filters-request@imc.org> (send an email message with the
word "subscribe" in the body). More information on the mailing list
along with a WWW archive of back messages is available at
<http://www.imc.org/ietf-mta-filters>.
0.2. Known Problems
0.2.1. Probable Extensions
The following suggestions have been made, and will probably be
addressed by extensions.
An extension for regular expressions will be written. While regular
expressions are of questionable utility for most users, the
programmers writing implementations desperately want regular
expressions.
Envelope-matching commands are not readily supported by all mail
systems, and putting them in the draft will result in a system that
cannot be implemented by a mail architecture that does not adequately
store envelopes.
"Detailed" addressing or "sub-addressing" (i.e., the "foo" in an
address "tjs+foo@andrew.cmu.edu") is not handled, and will be moved
to an extension for those systems that offer it.
A vacation command has been requested for an extension; a preliminary
draft exists and will be submitted to the internet-drafts repository.
Vacation functionality is isn't in the draft because having vacation
assumes you can store the addresses of people who have already
received vacation notifications, which isn't always the case.
A suggestion was made to set IMAP flags on delivery (e.g., \Flagged,
\Deleted, \Answered, \Seen).
An "include" command is not included, but has been suggested for an
extension.
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0.2.2. Known Bugs
The formal grammar probably still has some holes in it.
The reject command needs to be rechecked against the DSN
specification.
The error-handling clauses of this specification may not be
completely sensible, and may conflict.
The discussion of the limits of actions is not there. Only one
forward should be allowed per message. Keep and reject are mutually
exclusive.
0.3. Open Issues
In the event that there is an error while processing a script, what
happens? The draft implies you file into INBOX, but what if you've
already taken actions before you do this? (The parts of the draft
that require syntax checking stuff are all SHOULDs.)
I tried to fill in some of the blanks in previous versions; among
them, the description of what a rejected input message looks like,
but it's still nearly incomplete.
I moved the substring matching stuff out of the header command and
into a section of its own as it is reusable by extensions.
Suggestions on this section would be appreciated.
I tried to fill in the blanks in the section on extensibility and
borrowed some stuff from the ACAP spec (specifically, the comparator
registry), but it's probably not complete or good enough.
0.3. Noted Changes
This draft was unfortunately rushed and probably contains numerous
errors. This is purely the fault of the editor. For this reason,
readers are asked to subscribe to the mailing list noted above for
discussion, as well as some inevitable corrections.
The grammar has changed substantially to allow easier modification by
extensions at the expense of some additional definition in the rest
of the draft.
Support and reply have been removed, and the format of stringlists
has changed.
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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 otherwise should generalize to other systems that meet
these criteria.
The language is powerful enough to be useful, but limited in power 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 GUI-based editors. The language is not Turing-complete,
and provides no way to write a loop or a function. Variables are not
provided.
Implementations of the language are expected to take place at time of
final delivery, when the message is moved to the user-accessible
mailbox. In systems where the MTA does final delivery, such as and
traditional UNIX mail, is reasonable to sort when the MTA deposits
mail into the user's mailbox. If the MTA does not do final delivery,
or lacks the power to sort into separate mailboxes, as is the case
under POP3, the MUA must do filtering into local-disk folders.
There are a number of reasons to use a filtering system. Mail
traffic for most users has been increasing due both to increased
usage of e-mail, 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 [FLAMES] filtering
language 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 examples, line breaks have been inserted for readability.
In the sections of this document that discuss the requirements of
various keywords and operators, the following conventions have been
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adopted.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "CAN", and
"MAY" in this document are to be interpreted as defined in
[KEYWORDS].
Each section on a test, action, or control structure has a line
labeled "Syntax:". This line describes the syntax 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 "]"). However, the formal grammar
for these commands in section 10 and is the authoritative reference
on how to construct these commands.
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.org
To: roadrunner@birdseed.org
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@znic.net
-----------------------------------------------------------
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Message B
-----------------------------------------------------------
From: youcouldberich!@reply-by-postal-mail
Sender: b1ff@de.res.frobnitzm.edu
To: rube@landru.melon.net
Date: Mon, 31 Mar 1997 18:26:10 -0800 (PST)
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
This language is made up as a set of commands. Commands can take a
number of arguments; arguments can be either literal data, tests, or
blocks of commands.
2.2. Whitespace
Whitespace is used to separate commands. 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
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;
}
XXX this example is broken
2.4. Literal data
Literal data means data that is not executed and is supplied as
arguments, such as numbers and strings.
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2.4.1. Numbers
Numbers are given as ordinary decimal numbers. However, those
numbers that have a tendency to be fairly large, such as message
sizes, may have a "K", "M", or "G" appended to indicate a multiple of
a base-two number. To be comparable with the power-of-two-based
versions of SI units that computers frequently use, K specifies kilo,
or 1,024 (2^10) times the value of the number; M specifies mega, or
1,048,576 (2^20) times the value of the number; and G specifies giga,
or 1,073,741,824 (2^30) times the value of the number.
Implementations MUST provide 31 bits of magnitude in numbers, but may
provide more.
Negative, fractional, and decimal numbers are not permitted by this
specification.
2.4.2. Strings
Scripts involve large numbers of strings, as they are used for
pattern matching, addresses, and 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). A backslash ("\") inside of a quoted string is followed
by either another backslash or a double quote. This two-character
sequence represents a single backslash or double-quote within the
string, respectively.
Other escape sequences may be permitted depending on context. 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").
Non-printing characters such as tabs, CR and LF, and control
characters are permitted in strings. NUL (ASCII 0) is not allowed in
strings.
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. In order to allow the message to begin
lines with a single-dot, lines are dot-stuffed. That is, when
composing a message body, an extra `.' is added before each line
which begins with a `.'. When the server interprets the script,
these extra dots are removed.
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Note that a comment may occur in between the "text:" and the CRLF,
but not within the string itself.
2.4.2.1. String lists
When matching patterns, strings frequently come in groups. 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.
Implementations are encouraged to use short-circuit evaluation in
these cases.
For instance, the test `header ["To", "Cc"] contains
["me@frobnitzm.edu", "me00@landru.melon.edu"]' is true if either the
To header or Cc header of the input message contains either of the
e-mail addresses "me@frobnitzm.edu" or "me00@landru.melon.edu".
Conversely, in any case where a list of strings would be appropriate,
a single string is allowed without being a member of a list; it is
equivalent to a list with a single member. So 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 by the interpreter.
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.
2.4.2.3. Addresses
A number of commands call for email addresses, which are also a
subset of strings. These addresses must be compliant with [IMAIL].
Implementations MUST ensure the addresses are syntactically valid,
and need not ensure that they are actually deliverable.
2.5. Tests
Tests are given as arguments to commands in order to control how the
run. Generally, a test is used to decide if a block of code should
be evaluated.
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2.5.1. String Comparison
When matching one string against another, there are a number of ways
of performing the match. These are accomplished with three matches
-- an exact match, a substring match, and a wildcard glob-style
match. In order to provide for matches between character sets and
case insensitivity, Sieve borrows ACAP's comparator registry.
2.6.1. Match Keyword
There are two allowed match keywords describing the allowed match in
this draft; they are ":is" and ":contains". Match keywords are
supplied to those commands which allow them to specify whether the
match is to be a complete match or not.
These are used as tagged arguments to tests that perform string
comparison. Exactly one of them is necessary for a command.
The ":contains" version 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 null key ("") is contained in all values.
The ":is" version 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 null key only "is" the null value.
In order to specify what type of match is supposed to happen,
commands that support matching take optional tagged arguments ":is"
and ":contains". Commands default to using ":is" matching. Note
that these modifiers may interact with comparators; in particular,
some comparators are not suitable for matching with ":contains". It
is an error to use a comparator with ":contains" that is not
compatible with it.
2.6.2. Comparators
In order to allow for character set-independent matches, the match
keyword may be coupled with a comparator name. Comparators are
described for [ACAP]; a registry is defined for ACAP, and this
specification uses that registry.
ACAP defines multiple comparator types. Only equality types are used
in this specification.
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All implementations MUST support the "i;octet" comparator, which
simply compares one octet with the next. If left unspecified, the
default is octet.
Some comparators may not be usable with substring matches; that is,
they may only work with ":is". It is an error to try and use a
comparator with "matches" or "contains" that is not compatible with
it.
A comparator is specified with commands that support matching by the
":comparator" option. This option is followed by a string providing
the name of the comparator to be used. So in this example,
Example: if header "Subject" :contains :comparator "i;ascii-casemap"
"make money fast" {
discard;
would discard any message with subjects such as "Make Money Fast" and
"MaKe MONEY fAST".
OPEN: Are there any other comparators that SHOULD or MUST be
supported?
2.7. Tagged Arguments
This document provides for tagged arguments in the style of
CommonLISP.
A tagged argument is an an argument for a command that begins with
":", and consists of 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 numbers or strings, but are never blocks.
To keep the language simple, tagged arguments should not take tagged
arguments as arguments.
One case where this is useful is the ":comparator" argument, which
allows the user to specify which ACAP comparator will be used to
compare two strings, since different languages may impose different
orderings on UTF-8 [UTF-8] characters.
Tagged arguments may appear in any order, and may be interspersed
with positional arguments.
OPEN: Perhaps tagged arguments should always be before positional
arguments.
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2.8. Blocks
Blocks are sets of commands enclosed within curly braces. Blocks are
supplied to commands so that the commands can implement control
structures.
So a control structure is just a command that happens to take a test
and a block as its arguments; depending on the result of the control
structure, it runs the code in the block zero or more times. (Note
that by the commands supplied in the specification, there are no
loops, so the control structures supplied--if, elsif, and else--run a
block either once or not at all.)
2.9. Commands
Sieve scripts are made up of commands. Commands can take any of the
tokens above as arguments, and arguments may be either tagged or
positional arguments.
A command begins with a name, which is a simple token. It ends with
either a semicolon or a block. (Commands ending with blocks are used
to implement control structures.) Commands never take both a
semicolon and a block, nor do they ever take more than one block as
an argument.
2.9.1. Positional Arguments
Positional arguments are familiar from any programming language. A
command takes zero or more untagged positional arguments in order to
specify its behavior. Positional arguments are given their value
based on their order in the command.
2.9.2. Optional Arguments
Optional arguments are tagged arguments that may be omitted; when
omitted, they are given default values.
2.9.3. Blocks as Arguments
Commands may take blocks as arguments. A block is always the last
argument to a command, and when it exists, it replaces the semicolon
that would otherwise end the command.
2.11. Evaluation
Precedence is not important in any of the commands in this base
specification. However, as an extension might make order of
operation important, all arguments to rules MUST be evaluated in
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left-to-right order. Those operations that can implement short-
circuit evaluation (such as "allof" and "anyof") MUST do so.
Sieve imposes specific limits on actions; for instance, a rejected
message may not also be filed into a mailbox. These restrictions are
noted on a per-command basis.
OPEN: Or rather, they should be.
2.11.1. Implicit keep
If evaluation of a script fails to result in one "fileinto", "keep",
or "reject", a "keep" action is implicitly taken. So the message is
filed into the user's primary mailbox; that is,
For instance, with any of the short messages offered above, the
following script produces no actions.
Example: if size over 500K discard;
3. Conditionals and Control Structures
In order for a script to do more than one set of actions, control
structures are needed. In Sieve, a control structure is a command
that takes a block as an argument.
In this document, only the "if" control structure is provided. There
are three pieces to if: "if", "elsif", and "else".
Syntax: if <test> <command>
Syntax: elsif <test> <command>
Syntax: else <block>
The semantics are similar to any other programming language this
appears 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
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"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 case.
This provides a way of performing exactly one of the blocks in the
chain.
In the following example, both Message A and B are dropped.
Example: if header "from" contains "coyote" {
discard;
} elsif header ["subject"] :contains ["$$$"] {
discard;
} else fileinto "INBOX";
In the script below, when run over message A, forwards the message to
acm@frobnitzm.edu; message B, to postmaster@frobnitzm.edu; any other
message is forwarded to field@frobnitzm.edu.
Example: if header ["From"] contains ["coyote"] {
forward "acm@frobnitzm.edu";
} else if header "Subject" contains "$$$" {
forward "postmaster@frobnitzm.edu";
} else
forward "field@frobnitzm.edu";
4. Actions
This document supplies six actions that may be taken on a message:
keep, fileinto, forward, reject, discard, and stop.
4.1. Action reject
Syntax: reject <reason-string>
The "reject" action resends the message to the sender, wrapping it in
a "reject" form, noting that it was rejected by the recipient. In
the following script, message A is rejected and returned to the
sender.
Example: if header "from" contains "coyote@znic.net" {
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reject "I am not taking mail from you, and I don't want
your birdseed, either!";
}
A reject message MUST takes the form of a failed DSN as specified by
[DSN]. The human-readable portion of the message, the first
component of the DSN, contains the human readable message describing
the error, although it SHOULD contain additional text alerting the
original sender that mail was refused by a filter. This part of the
DSN might appear as follows:
------------------------------------------------------------
Message was refused by recipient's mail filtering program.
Reason given was as follows:
I am not taking mail from you, and I don't want your
birdseed, either!
------------------------------------------------------------
The action-value field as defined in the DSN specification MUST be
"failed".
A rejected message may not be filed, forwarded, or kept. A message
that triggers a "reject" action is never allowed to be kept by the
user, and the "reject" overrides all other actions.
A message may only be rejected once.
4.2. Action fileinto
Syntax: fileinto <folder>
The "fileinto" action drops the message into a named folder.
Implementations SHOULD support fileinto, but may not be able to in
cases where the filtering agent is not able to write to the users'
folders (such as a [POP3] implementation running inside the mail
server where the folders are stored on the users' local disks).
In the following script, message A is filed into folder
"INBOX.harassment".
Example: if header ["to"] contains "coyote" {
fileinto "INBOX.harassment";
}
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4.3. Action forward
Syntax: forward <address>
The "forward" action is used to forward the message to another user
at the supplied address, as a mail forwarding feature does. The
"forward" action makes no changes to the message body or headers, and
only modifies the envelope recipient.
A simple script can be used for forwarding:
Example: forward "bart@frobnitzm.edu";
The forward command performs an MTA-style forward--that is, what you
get from a .forward file using sendmail under UNIX. The address on
the SMTP envelope is replaced with the one on the forward command and
the message is sent back out. (This is not an MUA-style forward,
which creates a new message with a different sender and message ID,
wrapping the old message in a new one.)
OPEN: At least one person rejects to this definition, claiming that
a sendmail-style forward is inherently broken.
4.4. Action keep
Syntax: 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
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.
Example: if size under 1M keep; else discard;
4.6. Action stop
Syntax: stop
The "stop" action ends all processing. If no actions have been
executed, then the keep action is taken.
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Example: [There needs to be an example here.]
4.7. Action discard
Syntax: discard
Discard drops the message. In the following script, any mail from
"idiot@frobnitzm.edu" is thrown out.
Example: if header ["from"] contains ["idiot@frobnitzm.edu"]
discard;
Discard takes no arguments.
While an important part of this language, "discard" has the potential
to create serious problems for users: A student leaving themselves
logged in to a machine in a 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.
5. Tests
Tests are used in conditionals to decide which part(s) of the
conditional to execute.
5.1. Test allof
Syntax: allof ( <test> , <test> , ... <test> )
The allof test preforms 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.
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5.2. Test anyof
Syntax: anyof ( <test> , <test> , ... <test> )
The anyof test preforms a logical OR on the tests supplied to it.
Example: anyof (false, false) => false
anyof (false, true) => true
anyof (true, true) => true
5.3. Test exists
Syntax: exists <header-name-list>
The "exists" test is true if the headers listed in the
<header-name-list> 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;
}
5.4. Test false
Syntax: false
The "false" test always evaluates to false.
5.5. Test header
Syntax: header <header-name-list> <match-keyword> <key-list>
The "header" test evaluates to true if the any header name matches
any key. How the match is done is described by the second argument,
which is one of the string comparison arguments discussed in section
2.6. The first argument to header, the header-name-list, is a list
of headers to get values from to be searched. The key-list is a list
of keys.
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If a header listed in the header-name-list argument exists, it
contains the null key (""). However, if the named header is not
present, it does not contain the null key. So if a message contained
the header
X-Caffeine: C8H10N4O2
these tests on that header evaluate as follows:
header ["X-Y-Z"] is [""] => false
header ["X-Y-Z"] contains [""] => true
5.6. Test not
Syntax: not <test>
The "not" test takes some other test as an argument, and yields the
opposite result.
5.7. Test size
Syntax: size <":over" / ":under"> <limit [quantifier]>
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.
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.
The size of a message is defined to be the number of octets from the
initial header until the last character in the message body.
6. Errors in Processing a Script
In any programming language, errors are inevitable. Users are
expected to make errors, and even if a script works correctly today,
it may fail tomorrow due to quotas, mailboxes being removed or
renamed, or some piece of hardware being down. It is imperative that
mail get through.
Implementations SHOULD check a script before it is run in order to
ensure that it is valid. Implementations SHOULD NOT try and recover
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from a script with errors, and should instead file mail into the
user's primary mailbox.
Users MUST be notified of errors in processing a script. The method
by which users are notified is implementation defined, but a mail
message clearly describing the error is suggested if a preferable
alternative cannot be found.
In an implementation that allows for a script to be checked when it
is turned over to the server, the script can be checked for errors
before it is submitted. Implementations SHOULD notify the user of
the error and refuse to accept a syntactically invalid script or one
that makes use of extensions that the server does not report.
Implementations MUST allow mail to be filed without filtering in case
of a syntax error in the script. Implementations MUST avoid sending
multiple messages describing the same error.
Implementations are REQUIRED to notify users of errors in filtering
scripts. If there are errors in the script being used, mail SHOULD
be filed into the user's main mailbox. Implementations MUST NOT
discard mail unless a command explicitly demands it.
7. Extensibility
New control structures, 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 string that uniquely
identifies that extension. If a new version of an extension changes
the functionality of a previously defined extension, it MUST use a
different name. The purpose of such a string is for the "support"
test, which mandates that script requires the use of that extension.
Additionally, in a situation where there is a 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 that the server does not support.
7.1. Capability String
Capability strings are typically short strings describing what
capabilities are supported by the server. The following capability
strings are defined by this document:
fileinto The string "fileinto" indicates the implementation
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supports filing into mailboxes.
comparator- The string "comparator-elbonia" is provided if the
implementation supports the "elbonia" comparator.
Therefore, all implementations have at least the
"comparator-i;octet" capability.
7.2. Registry
In order to provide a standard set of extensions, a registry is
provided by IANA. Capability names may be registered on a first-
come, first-served basis. Extensions designed for interoperable use
should be defined as standards track or IESG approved experimental
RFCs.
To: XXX@XXX.XXX
Subject: Registration of new Sieve extension
Capability name:
Capability keyword:
Capability arguments:
Standards Track/IESG-approved experimental RFC number:
Person and email address to contact for further information:
7.3. Capability Transport
As the range of mail systems that this draft is intended to apply to
is quite large, 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
following properties.
(1) The implementation can advertise the complete set of extensions
that it supports.
OPEN: There needs to be a more complete description here.
8. Transmission
The MIME type for a SIEVE script is "application/sieve". Scripts are
encoded in UTF-8 during transmission.
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9. Acknowledgments
I am very thankful to Chris Newman for his support and his ABNF
syntax checker, to John Myers and Steve Hole for outlining the
requirements for the original drafts, and to Rob Earhart for an early
implementation and a great deal of help. I am also indebted to all
of the readers of the ietf-mta-filters@imc.org mailing list.
10. Formal Grammar
The grammar used in this section is the same as the ABNF described in
[ABNF].
In the case of alternative or optional rules in which a later rule
overlaps an earlier rule, the rule which is listed earlier MUST take
priority. (This shouldn't happen. Please let me know if it does.)
argument = string / string-list / number / tag / test
block = "{" [WSP] commands [WSP] "}"
;; C-style block
CHAR-NOT-DOT = (%x01-2d / %x2f-%xff)
;; all the characters that aren't "."
command = identifier WSP *(argument WSP) [WSP] ";"
commands = *([WSP] command [WSP])
comment = "#" *VCHAR CRLF
identifier = (ALPHA / "_") *(ALPHA DIGIT "_")
multi-line = "text:" [WSP] CRLF
*((1*CHAR-NOT-DOT *CHAR CRLF) / ("." 1*CHAR-NOT-DOT *CHAR CRLF) /
(".." *CHAR CRLF) / CRLF)
"." CRLF
;; Note when used,
;; a leading ".." on a line is mapped to ".".
number = 1*DIGIT [QUANTIFIER]
;; quantifier is a multiplier (or bit shift)
QUANTIFIER = "K" / "M" / "G"
;; K == 2^10; M == 2^20; G = 2^30
quoted-string = DQUOTE *CHAR DQUOTE
;; \" inside a string maps to "
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;; \\ inside a string maps to \
;; All other characters map to themselves.
;; Note that newlines and other weird characters
;; are all allowed strings.
string = quoted-string / multi-line
string-list = "[" [WSP] *(string [WSP] "," [WSP]) string [WSP] "]" / string
;; if there is only a single string, the brackets are optional
tag = ":" identifier
test = identifier *(WSP argument) [WSP test-list]
test-list = [WSP] "(" [WSP] *(test [WSP] "," [WSP])
test [WSP] ")" [WSP]
WSP = 1*(SP / CRLF / HTAB) / comment
;; just whitespace. anyplace this is allowed, a comment is
;; as well
11. Security Considerations
Users must get their mail. It is imperative that whatever method
implementations use to store the user-defined filtering scripts be
secure.
It is equally important that implementations sanity-check the user's
scripts, and not allow users to create on-demand mailbombs. For
instance, an implementation that allows a user to reject or forward
multiple times to a single message might also allow a user to create
a mailbomb triggered by mail from a specific user.
Therefore, an implementation SHOULD only allow one "reject" per
message processed, and MAY limit the number of forward actions taken.
An implementation MUST refuse to forward a message to itself. [OPEN:
What do you do when a site limit prevents you from this? Say I do
three replies; which ones take effect when the limit is 1? 2? 0?]
Several commands, such as "discard", "forward", and "fileinto" allow
for actions to be taken that are potentially very dangerous.
12. Author's Address
Tim Showalter
Carnegie Mellon University
5000 Forbes Avenue
Pittsburgh, PA 15213
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E-Mail: tjs+@andrew.cmu.edu
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Appendices
Appendix A. References
[ABNF] Crocker, D., "Augmented BNF for Syntax Specifications: ABNF",
Internet Mail Consortium, RFC 2234, November 1997.
[DSN] Moore, K., and G. Vaudreuil, "An Extensible Message Format for
Delivery Status Notifications", RFC 1894, January 1996.
[FLAMES] Borenstein, Nathaniel S., and Chris A. 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.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, Harvard University, March 1997.
[IMAP] Crispin, M., "Internet Mail Access Protocol - version 4rev1",
RFC 2060, University of Washington, December 1996.
[IMAIL] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, University of Delaware, August 1982.
[MIME] Freed, N., and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies", RFC
2045, Innosoft and First Virtual, November 1996.
[SMTP] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
USC/Information Sciences Institute, August 1982.
[UTF-8] Yergeau, F. "UTF-8, a transformation format of Unicode and
ISO 10646", RFC 2044, Alis Technologies, October 1996.
Appendix B. Full Copyright Statement
Copyright (C) The Internet Society 1998. All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
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included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document will expire before January 31, 1999.
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