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Network Working Group T. Showalter
Internet Draft: Sieve Carnegie Mellon
Document: draft-showalter-sieve-02.txt October 1997
Expire in six months (12/1/97)
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
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), ftp.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
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.
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
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
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. Numbers
2.5. Strings
2.5.2. String lists
2.5.3. Headers
2.5.4. Addresses
2.6. String Comparison
2.6.1. Match Keyword
2.6.2. Comparators
2.7. Evaluation
3. Conditionals and Control Structures
3.1. If
3.2. Require
4. Actions
4.1. Action bounce
4.2. Action fileinto
4.3. Action forward
4.4. Action keep
4.5. Action reply
4.6. Action stop
4.7. Action discard
5. Tests
5.1. Test all-of
5.2. Test any-of
5.3. Test exists
5.4. Test false
5.5. Test header
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5.6. Test not
5.7. Test size
5.8. Test support
5.9. Test true
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
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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 "fmh" in an
address "tjs+fmh@andrew.cmu.edu") is not handled, and will be moved
to an extension for those systems that offer it.
A previous version included a "valid" test. I have tentatively
removed it because Randy had noted it was too fuzzy to implement, and
that's probably true.
A vacation command has been requested for an extension. It 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).
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An "include" command is not included, but has been suggested for an
extension.
0.2.2. Known Bugs
The formal grammar.
The bounce command needs to be rechecked against the DSN
specification.
The error-handling clauses of this specification may not be
completely sensible, and may conflict.
My knowledge of email is not comprehensive, and as a result, there
might be a better way to express some of the concepts in here.
Please let me know if there is a good way to clean up the wording.
0.3. Open Issues
The support and require tests cause some serious problems with
control structures. To some extent, this is solved by separating the
block construct out from the conditionals themselves. This has been
done in this draft (flames welcome, but it seems to be cleaner to
me).
Comma is mandatory in any-of/all-of but forbidden in a list of
strings; it should be required in both. This needs to be fixed. I'm
clinging to the status quo trying to fix the rest of the problems at
the moment.
Should there be a way to specify headers transmitted by reply?
Perhaps a separate command, since there are probably sites that are
going to be really paranoid about what headers get sent.
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 bounced 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.
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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.
Finally, I suspect that there are a lot of problems relating to what
filtering for the masses will do to mailing lists, especially what
will happen the first time someone rolls their own vacation program
consisting of a reply command. Should it be an error to reply to a
message that is not addressed to you (specifically)?
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1. Introduction
This draft is offered to provide a standard 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 and support of multiple folders,
but should work with a wide variety of systems that support 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 finally 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 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.
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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", "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. Each command is
either an action or a conditional. Each conditional contains a test;
depending on the results of the test, one set of commands in a
control structure is taken.
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;
}
2.4. 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,
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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.
2.5. 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 (such as
in globs, defined in section 2.6 on string comparison). 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.
Note that a comment may occur in between the "text:" and the CRLF,
but not within the string itself.
Example: if any-of (header ("from") contains
("bart" "homer" "smithers" "burns" "lisa"),
header ("subject") contains ("URGENT")) {
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keep;
} else {
reply text: # multi-line message here:
You are not one of the people I regularly
correspond with. I have deleted your message
due to the large volume of email I regularly
receive. If you feel that you need to speak
with me directly, and cannot find your answer
in my web pages, please send mail with the
word "URGENT" in the subject line. Thank you
for your time.
.
;
}
2.5.2. 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")'.
2.5.3. 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.
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2.5.4. 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.
2.6. 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 three allowed match keywords describing the allowed match
in this draft; they are "is", "contains", and "matches".
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.
The "matches" is a UNIX-style "glob" match; it specifies that the key
is not substring, but contains certain special characters that match
characters that are not themselves. These characters are
* Match zero or more characters
? Match any single character
\ Escape next character
Escaped special characters do not take on the meanings listed above.
The value "frobnitzm" matches the keys "*nit*", "f*b*m", and "fr?b*",
but not "nit" or "frob". The null key matches only the null value.
The "contains" and "matches" versions necessitate that one string
supplied as an argument is a key, and the other is a value. Commands
that utilize these comparisons, generally of the form "<value>
<match-keyword> <key>", must be sure to differentiate which is which.
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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 draft
borrows that registry.
All implementations MUST support the octet comparator, which simply
compares one octet with the next. If left unspecified, the
comparator is octet.
If an implementation supports a comparator "elbonia", it MUST provide
the capability "comparator-elbonia" for support and require commands.
Some comparators may not be usable with substring matches; that is,
they may only work with "is". [OPEN: Not sure what to do about
this.] It is an error to try and use a comparator with "matches" or
"contains" that is not compatible with it.
OPEN: Are there any other comparators that SHOULD or MUST be
supported?
2.7. Evaluation
If evaluation of the script fails to file the message into any
mailbox, as in the following script, the message is filed into INBOX.
With any of the short messages offered above, the following script
produces no actions.
Example: if size over 500K discard;
In cases like this, the "keep" action is taken. The "keep" action is
defined to be the action that is taken in a situation where the user
does no filtering. For instance, under an IMAP-based system, this
implies filing into INBOX.
Implementations define the specific meanings of actions.
Implementations MAY impose restrictions on the actions taken, such as
only honoring one "reply", "bounce", or "forward" per message.
In this case, which is honored? I'm tempted to say random, but
restrict it to those commands that send mail back out (fileinto as
many mailboxes as you want).
Precedence is not important in any of the commands in this base
specification. However, as an extension might make order of
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operation important, all arguments to rules MUST be evaluated in
left-to-right order. Those operations that can implement short-
circuit evaluation (such as "all-of" and "any-of") MUST do so.
3. Conditionals and Control Structures
In order for a script to do more than one set of actions, control
structures are needed.
3.1. If
Syntax: if <test> <command> [else <command>]
The "if" control structure is borrowed from any number of programming
languages. It is evaluated in the usual way if the test is true then
the first command (or set of commands) supplied is evaluated. If the
test is false and an else keyword follows the if block, the second
command is evaluated. The commands may be command blocks. [OPEN:
This allows C-style dangling statements; I construe this as a
feature.]
In the following example, both Message A and B are dropped.
Example: if header "from" contains "coyote" {
discard;
} else if header ("subject") contains ("$$$") {
discard;
} else fileinto "INBOX";
Only one command or block of commands in an if ... else if ... else
chain is executed.
In the script below, when run over message A, forwards the message to
acm@frobnitzm.edu; message B, to postmaster@frobnitzm.edu; any other
message is forwarded to field@frobnitzm.edu.
Example: if header ("From") contains ("coyote") {
forward "acm@frobnitzm.edu";
} else if header "Subject" contains "$$$" {
forward "postmaster@frobnitzm.edu";
} else
forward "field@frobnitzm.edu";
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3.2. Require
Syntax: require <extension-name>
Require SHOULD be declared in a user script before an extension is
used. It instructs the evaluator that the extension named
extension-name, supplied as a string, MUST be present in order to
allow further processing. If the string specifies an extension that
the evaluating mechanism supports, then processing continues.
Otherwise, an error has been encountered, and the script should not
be evaluated.
Require is intended to indicate that a script needs an extension not
described in this document, or a feature that is not mandatory.
The following example will fail on any server that does not implement
the extension known as DWIM.
Example: require "dwim";
if header ("subject") contains-nocase ("the secret
message") {
dwim blurdybloop body;
} stop
OPEN: I have serious concerns with require; it makes it impossible to
separate parsing from evaluation, and introduces some awkward
cases. For instance, a script "if size under 1 { require "foo";
do_foo; } else {... }" Even if the test will never happen, this
require will prevent the script from working. Just support
seems to make more sense.
4. Actions
This document supplies six actions that may be taken on a message:
keep, fileinto, forward, bounce, discard, and stop.
4.1. Action bounce
Syntax: bounce <reason-string>
The "bounce" action resends the message to the sender, wrapping it in
a "bounce" form, noting that it was rejected by the recipient. In
the following script, message A is bounced to the sender.
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Example: if header "from" contains "coyote@znic.net" {
bounce "I am not taking mail from you, and I don't want
your birdseed, either!";
}
A bounce message SHOULD 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 SHOULD be
"failed".
OPEN: This section is probably incomplete. I am not sure that the
right answer is to make it easy to refuse messages, but
secretly keep a copy. Should bounce prevent all other
actions from taking affect?
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).
As such, a server supporting fileinto MUST provide the "fileinto"
capability for the support and require tests.
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In the following script, message A is filed into folder
"INBOX.harassment".
Example: if header ("to") contains "coyote" {
fileinto "INBOX.harassment";
}
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.)
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;
4.5. Action reply
Syntax: reply <message>
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The "reply" action is used to generate a form letter reply to the
original sender. Message is a string to be sent as a reply message.
In the following example, any message larger than 500K (524288
octets) would be replied to with a message explaining that it was
rejected; otherwise, the message would be filed into INBOX (by
default).
Example: if size over 500K {
reply text:
Your message was unnecessarily large. I reject all large
messages; if you need to send me a large message, please
contact me first and arrange outer means.
.
; 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.
In the following script, if the mail is from the address
"boss@frobnitzm.edu" it is forwarded to "pleeb@frobnitzm.edu";
otherwise the mail receives a reply, and is thrown out.
Example: if header ("from") matches ("boss@frobnitzm.edu") {
forward "pleeb@xanadu.wv.us";
stop;
} reply text:
I'm on vacation and not taking any messages; try after
Sunday. I have thrown your message out. Please resend
it later.
.
; discard;
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;
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While an important part of this language, "discard" has the potential
to create serious problems for users. For instance, 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 all-of
Syntax: all-of ( <test> , <test> , ... <test> )
The all-of test preforms a logical AND on the tests supplied to it.
Example: all-of (false, false) => false
all-of (false, true) => false
all-of (true, true) => true
5.2. Test any-of
Syntax: any-of ( <test> , <test> , ... <test> )
The any-of test preforms a logical OR on the tests supplied to it.
Example: any-of (false, false) => false
any-of (false, true) => true
any-of (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 test
exists ("From" "To" "Cc")
is equivalent to
header ("From" "To" "Cc") contains ""
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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.
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-Caffeine") is ("") => false
header ("X-Caffeine") matches ("") => false
header ("X-Caffeine") contains ("") => true
5.6. Test not
Syntax: not <test>
The "not" test takes some other test as an argument, and yields the
opposite result.
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5.7. Test size
Syntax: size <"over" / "under"> <limit [quantifier]>
The "size" test deals with the size of a message. The test is true
only if the first argument is "over" and the size of the message is
strictly greater than the number of octets specified as limit. If
the first argument is "under", then the test is true only if the
message size is strictly less than the number of octets specified as
limit. In either case, if the message size is exactly the limit, the
test 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.
5.8. Test support
Syntax: support <extension-name>
The "support" test evaluates to true if the extension named by
<extension-name> is supported. In the following script, all mail is
filed into INBOX unless the "black-magic" extension is supported.
Otherwise, behavior is defined by the black-magic extension.
Example: if support "black-magic" {
black-magic ("zork@frobnitzm.edu");
}
5.9. Test true
Syntax: true
The "true" test is always true.
6. Errors in Processing a Script
In any programming language, errors are inevitable. Users are
expected to make errors, and changes in the environment, such as a
change in a user's rights on a mailbox, can cause a script to fail.
It is imperative that mail be allowed to 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
from a script with errors, and should file mail into the user's
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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 "require"
and "support" conditionals, 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:
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fileinto The string "fileinto" indicates the implementation
supports filing into mailboxes.
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.
9. Acknowledgments
I am very thankful to Chris Newman for his support and his ABNF
syntax checker. I am also indebted to all of the readers of the
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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.)
action = bounce / discard / fileinto / forward / keep / reply / stop
address = string
;; any legal [IMAIL] address.
any-of = "any-of" test-list
all-of = "all-of" test-list
block = "{" [WSP] commands [WSP] "}"
;; C-style block
bounce = "bounce" WSP string
;; string is the reason contained in the bounce message.
control-structure = if
command = ( action ";" ) / block / control-structure
commands = *([WSP] command [WSP])
comment = "#" *VCHAR CRLF
comparator = "octet"
;; octet is the only comparator mandated by this specification
;; but others may be defined by the ACAP registry.
discard = "discard"
exists = "exists" WSP string
false = "false"
fileinto = "fileinto" WSP string
;; string is a mailbox; semantics are defined by the
;; underlying mail system
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forward = "forward" WSP address
if = "if" WSP test WSP command [ "else" command ]
;; Commands are typically blocks.
header = "header" WSP string-list WSP match-keyword WSP string-list
keep = "keep"
match-keyword = ("contains" / "matches" / "is") ["-" comparator]
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 ".".
CHAR-NOT-DOT = (%x01-2d / %x2f-%xff)
;; all the characters that aren't "."
not = "not" WSP test
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 "
;; \\ inside a string maps to \
;; All other characters map to themselves.
;; Note that newlines and other weird characters
;; are all allowed strings.
reply = "reply" WSP multi-line
size = "size" WSP ( "over" / "under" ) WSP number
SPACE = %d32
stop = "stop"
string = quoted-string / multi-line
string-list = "(" [WSP] *(string WSP) string [WSP] ")" / string
;; if there is only a single string, the parens are optional
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test = [WSP] (any-of / all-of / exists / false / header /
not / size) [WSP]
test-list = [WSP] "(" [WSP] *(test [WSP] "," [WSP])
test [WSP] ")" [WSP]
true = "true"
WSP = 1*(SPACE / 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 bounce, forward, or
reply 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 "bounce" per
message processed, and MAY limit the number of forward and reply
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
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, Work in Progress.
[DSN] Moore, K., and G. Vaudreuil, "An Extensible Message Format for
Delivery Status Notifications", RFC 1894, January, 1996.
[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.
This document will expire before December 1, 1997.
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