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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. Showalter [Page 1] Internet DRAFT Sieve October 24, 1997 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 Showalter [Page 2] Internet DRAFT Sieve October 24, 1997 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 Showalter [Page 3] Internet DRAFT Sieve October 24, 1997 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). Showalter [Page 4] Internet DRAFT Sieve October 24, 1997 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. Showalter [Page 5] Internet DRAFT Sieve October 24, 1997 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)? Showalter [Page 6] Internet DRAFT Sieve October 24, 1997 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. Showalter [Page 7] Internet DRAFT Sieve October 24, 1997 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 ----------------------------------------------------------- Showalter [Page 8] Internet DRAFT Sieve October 24, 1997 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, Showalter [Page 9] Internet DRAFT Sieve October 24, 1997 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")) { Showalter [Page 10] Internet DRAFT Sieve October 24, 1997 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. Showalter [Page 11] Internet DRAFT Sieve October 24, 1997 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. Showalter [Page 12] Internet DRAFT Sieve October 24, 1997 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 Showalter [Page 13] Internet DRAFT Sieve October 24, 1997 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"; Showalter [Page 14] Internet DRAFT Sieve October 24, 1997 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. Showalter [Page 15] Internet DRAFT Sieve October 24, 1997 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. Showalter [Page 16] Internet DRAFT Sieve October 24, 1997 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> Showalter [Page 17] Internet DRAFT Sieve October 24, 1997 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; Showalter [Page 18] Internet DRAFT Sieve October 24, 1997 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 "" Showalter [Page 19] Internet DRAFT Sieve October 24, 1997 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. Showalter [Page 20] Internet DRAFT Sieve October 24, 1997 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 Showalter [Page 21] Internet DRAFT Sieve October 24, 1997 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: Showalter [Page 22] Internet DRAFT Sieve October 24, 1997 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 Showalter [Page 23] Internet DRAFT Sieve October 24, 1997 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 Showalter [Page 24] Internet DRAFT Sieve October 24, 1997 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 Showalter [Page 25] Internet DRAFT Sieve October 24, 1997 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 Showalter [Page 26] Internet DRAFT Sieve October 24, 1997 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. Showalter [Page 27]