Sieve relational: Sieve Email Filtering: Relational Extension
Lets Sieve mail filters compare values with greater-than and less-than, and count headers or addresses.
Segmuller, W. and B. Leiba, "Sieve Email Filtering: Relational Extension", RFC 5231, DOI 10.17487/RFC5231, January 2008, <https://www.rfc-editor.org/info/rfc5231>.
@misc{rfc5231,
series = {Request for Comments},
number = 5231,
howpublished = {RFC 5231},
publisher = {RFC Editor},
doi = {10.17487/RFC5231},
url = {https://www.rfc-editor.org/info/rfc5231},
author = {W. Segmuller and B. Leiba},
title = {{Sieve Email Filtering: Relational Extension}},
pagetotal = 9,
year = 2008,
month = jan,
} RFC text
Contents
Network Working Group W. Segmuller
Request for Comments: 5231 B. Leiba
Obsoletes: 3431 IBM T.J. Watson Research Center
Category: Standards Track January 2008
Sieve Email Filtering: Relational Extension
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document describes the RELATIONAL extension to the Sieve mail
filtering language defined in RFC 3028. This extension extends
existing conditional tests in Sieve to allow relational operators.
In addition to testing their content, it also allows for testing of
the number of entities in header and envelope fields.
This document obsoletes RFC 3431.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . . 2
3. Comparators . . . . . . . . . . . . . . . . . . . . . . . . . . 2
4. Match Types . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Match Type VALUE . . . . . . . . . . . . . . . . . . . . . 3
4.2. Match Type COUNT . . . . . . . . . . . . . . . . . . . . . 3
5. Interaction with Other Sieve Actions . . . . . . . . . . . . . 4
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
7. Extended Example . . . . . . . . . . . . . . . . . . . . . . . 6
8. Changes since RFC 3431 . . . . . . . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
10. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
11. Normative References . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The RELATIONAL extension to the Sieve mail filtering language [Sieve]
provides relational operators on the address, envelope, and header
tests. This extension also provides a way of counting the entities
in a message header or address field.
With this extension, the Sieve script may now determine if a field is
greater than or less than a value instead of just equivalent. One
use is for the x-priority field: move messages with a priority
greater than 3 to the "work on later" folder. Mail could also be
sorted by the from address. Those userids that start with 'a'-'m' go
to one folder, and the rest go to another folder.
The Sieve script can also determine the number of fields in the
header, or the number of addresses in a recipient field, for example,
whether there are more than 5 addresses in the to and cc fields.
The capability string associated with the extension defined in this
document is "relational".
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119.
Conventions for notations are as in [Sieve] section 1.1, including
the use of [Kwds] and the use of [ABNF].
3. Comparators
This document does not define any comparators or exempt any
comparators from the require clause. Any comparator used must be
treated as defined in [Sieve].
The "i;ascii-numeric" comparator, as defined in [RFC4790], MUST be
supported for any implementation of this extension. The comparator
"i;ascii-numeric" MUST support at least 32-bit unsigned integers.
Larger integers MAY be supported. Note: the "i;ascii-numeric"
comparator does not support negative numbers.
4. Match Types
This document defines two new match types. They are the VALUE match
type and the COUNT match type.
The syntax is:
MATCH-TYPE =/ COUNT / VALUE
COUNT = ":count" relational-match
VALUE = ":value" relational-match
relational-match = DQUOTE
("gt" / "ge" / "lt" / "le" / "eq" / "ne") DQUOTE
; "gt" means "greater than", the C operator ">".
; "ge" means "greater than or equal", the C operator ">=".
; "lt" means "less than", the C operator "<".
; "le" means "less than or equal", the C operator "<=".
; "eq" means "equal to", the C operator "==".
; "ne" means "not equal to", the C operator "!=".
4.1. Match Type VALUE
The VALUE match type does a relational comparison between strings.
The VALUE match type may be used with any comparator that returns
sort information.
A value from the message is considered the left side of the relation.
A value from the test expression, the key-list for address, envelope,
and header tests, is the right side of the relation.
If there are multiple values on either side or both sides, the test
is considered true if any pair is true.
4.2. Match Type COUNT
The COUNT match type first determines the number of the specified
entities in the message and does a relational comparison of the
number of entities, as defined below, to the values specified in the
test expression.
The COUNT match type SHOULD only be used with numeric comparators.
The Address Test counts the number of addresses (the number of
"mailbox" elements, as defined in [RFC2822]) in the specified fields.
Group names are ignored, but the contained mailboxes are counted.
The Envelope Test counts the number of addresses in the specified
envelope parts. The envelope "to" will always have only one entry,
which is the address of the user for whom the Sieve script is
running. Using this test, there is no way a Sieve script can
determine if the message was actually sent to someone else. The
envelope "from" will be 0 if the MAIL FROM is empty, or 1 if MAIL
FROM is not empty.
The Header Test counts the total number of instances of the specified
fields. This does not count individual addresses in the "to", "cc",
and other recipient fields.
In all cases, if more than one field name is specified, the counts
for all specified fields are added together to obtain the number for
comparison. Thus, specifying ["to", "cc"] in an address COUNT test
compares the total number of "to" and "cc" addresses; if separate
counts are desired, they must be done in two comparisons, perhaps
joined by "allof" or "anyof".
5. Interaction with Other Sieve Actions
This specification adds two match types. The VALUE match type only
works with comparators that return sort information. The COUNT match
type only makes sense with numeric comparators.
There is no interaction with any other Sieve operations, nor with any
known extensions. In particular, this specification has no effect on
implicit KEEP, nor on any explicit message actions.
6. Example
Using the message:
received: ...
received: ...
subject: example
to: foo@example.com, baz@example.com
cc: qux@example.com
The test:
address :count "ge" :comparator "i;ascii-numeric"
["to", "cc"] ["3"]
would evaluate to true, and the test
anyof ( address :count "ge" :comparator "i;ascii-numeric"
["to"] ["3"],
address :count "ge" :comparator "i;ascii-numeric"
["cc"] ["3"] )
would evaluate to false.
To check the number of received fields in the header, the following
test may be used:
header :count "ge" :comparator "i;ascii-numeric"
["received"] ["3"]
This would evaluate to false. But
header :count "ge" :comparator "i;ascii-numeric"
["received", "subject"] ["3"]
would evaluate to true.
The test:
header :count "ge" :comparator "i;ascii-numeric"
["to", "cc"] ["3"]
will always evaluate to false on an RFC 2822 compliant message
[RFC2822], since a message can have at most one "to" field and at
most one "cc" field. This test counts the number of fields, not the
number of addresses.
7. Extended Example
require ["relational", "comparator-i;ascii-numeric", "fileinto"];
if header :value "lt" :comparator "i;ascii-numeric"
["x-priority"] ["3"]
{
fileinto "Priority";
}
elsif address :count "gt" :comparator "i;ascii-numeric"
["to"] ["5"]
{
# everything with more than 5 recipients in the "to" field
# is considered SPAM
fileinto "SPAM";
}
elsif address :value "gt" :all :comparator "i;ascii-casemap"
["from"] ["M"]
{
fileinto "From N-Z";
} else {
fileinto "From A-M";
}
if allof ( address :count "eq" :comparator "i;ascii-numeric"
["to", "cc"] ["1"] ,
address :all :comparator "i;ascii-casemap"
["to", "cc"] ["me@foo.example.com"] )
{
fileinto "Only me";
}
8. Changes since RFC 3431
Apart from several minor editorial/wording changes, the following
list describes the notable changes to this specification since RFC
3431.
o Updated references, including changing the comparator reference
from the Application Configuration Access Protocol (ACAP) to the
"Internet Application Protocol Collation Registry" document
[RFC4790].
o Updated and corrected the examples.
o Added definition comments to ABNF for "gt", "lt", etc.
o Clarified what RFC 2822 elements are counted in the COUNT test.
o Removed the requirement to strip white space from header fields
before comparing; a more general version of this requirement has
been added to the Sieve base spec.
9. IANA Considerations
The following template specifies the IANA registration of the
relational Sieve extension specified in this document:
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name: relational
Description: Extends existing conditional tests in Sieve language
to allow relational operators
RFC number: RFC 5231
Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>
10. Security Considerations
An implementation MUST ensure that the test for envelope "to" only
reflects the delivery to the current user. Using this test, it MUST
not be possible for a user to determine if this message was delivered
to someone else.
Additional security considerations are discussed in [Sieve].
11. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[Kwds] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC4790] Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
Application Protocol Collation Registry", RFC 4790,
March 2007.
[Sieve] Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email
Filtering Language", RFC 5228, January 2008.
Authors' Addresses
Wolfgang Segmuller
IBM T.J. Watson Research Center
19 Skyline Drive
Hawthorne, NY 10532
US
Phone: +1 914 784 7408
EMail: werewolf@us.ibm.com
Barry Leiba
IBM T.J. Watson Research Center
19 Skyline Drive
Hawthorne, NY 10532
US
Phone: +1 914 784 7941
EMail: leiba@watson.ibm.com
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AI ELI5
On this page
§1What the Extension Adds
RFC 5231 defines the RELATIONAL extension to the Sieve mail filtering language. It gives the address, envelope and header tests relational operators, so a script can say that a field is greater than or less than a value instead of only equal to it.
The RFC gives two example uses. Messages whose x-priority field is greater than 3 can be moved to a 'work on later' folder, and mail can be sorted by the sender's userid, with names starting 'a' to 'm' going to one folder and the rest to another. A script can also count things, such as whether the to and cc fields hold more than 5 addresses.
The extension is switched on in a script with the capability string relational.
Before, a mail rule could only ask 'is it exactly this?'. Now it can also ask 'is it more than this?' or 'how many are there?'.
§4The VALUE and COUNT Match Types
The RFC defines two new match types. Each is a tag, :value or :count, followed by a quoted relational operator.
gt: greater than, like the C operator>ge: greater than or equallt: less thanle: less than or equaleq: equal tone: not equal to
For a relation, the value from the message is the left side. The value from the test's key list is the right side.
There are six words for comparing, like 'bigger', 'smaller' and 'the same'. You say which one you want, then what to compare against.
- gtGreater than — Like the C operator >.
- geGreater than or equal — Like the C operator >=.
- ltLess than — Like the C operator <.
- leLess than or equal — Like the C operator <=.
- eqEqual to — Like the C operator ==.
- neNot equal to — Like the C operator !=.
§4.1How VALUE Compares Strings
The VALUE match type does a relational comparison between strings. It may be used with any comparator that returns sort information.
If there are several values on either side or on both sides, the test is true if any pair satisfies the relation.
If you give the rule a few things to check against, it passes when at least one of them works out.
§4.2How COUNT Counts
The COUNT match type first works out how many of the named entities the message has. It then compares that number with the values in the test. It SHOULD only be used with numeric comparators.
- The
addresstest counts themailboxelements defined in RFC 2822. Group names are ignored, but the mailboxes inside them are counted. - The
envelopetest counts addresses in the named envelope parts. Envelopetoalways has one entry, the user the script runs for. Envelopefromcounts 0 if MAIL FROM is empty and 1 otherwise. - The
headertest counts the total number of instances of the named fields. It does not count the individual addresses insideto,ccand similar fields.
When several field names are listed, their counts are added together. To compare counts separately, use two comparisons joined by allof or anyof.
COUNT is like counting how many apples are in a basket and then asking if that number is big enough. If you name two baskets, you count both together.
§3Comparators and Other Sieve Actions
The extension defines no comparators of its own. Any comparator used is treated as defined in the base Sieve specification. The i;ascii-numeric comparator from RFC 4790 MUST be supported by any implementation of this extension, and it MUST handle at least 32-bit unsigned integers. Larger integers MAY be supported, and the comparator does not support negative numbers.
There is no interaction with other Sieve operations or known extensions. In particular it has no effect on the implicit KEEP or on any explicit message action.
Numbers are compared using a standard number-comparing rule that every mail filter with this feature has to know. It does not change what the filter does with your mail after it decides.
§6Worked Examples
The RFC uses a message with two received fields, a subject, a to field with two addresses and a cc field with one. Counting addresses across to and cc with ge 3 is true, because there are three. Counting to alone, or cc alone, against 3 is false.
Counting header instances works differently. received alone has 2 instances, so ge 3 is false, but received plus subject gives 3, so it is true. A header count over to and cc against 3 is always false on a compliant RFC 2822 message, since a message can have at most one of each field: it counts fields, not addresses.
The extended example in section 7 files mail with x-priority below 3 into Priority, mail with more than 5 to addresses into SPAM, and splits the rest by whether the sender sorts after 'M'. A final test files mail into Only me when the to and cc fields hold exactly one address and it is the user's own.
The examples show that counting 'how many addresses' and counting 'how many lines of a type' give different answers, so you have to pick the right one.
address :count "ge" :comparator "i;ascii-numeric"
["to", "cc"] ["3"]§10Security Considerations
An implementation MUST make sure the test for envelope to reflects only the delivery to the current user. A user must not be able to use this test to find out whether the message was also delivered to someone else. Further security considerations are in the base Sieve specification.
A rule must not be able to peek at who else got the same message. It only sees your own copy.
§8Changes Since RFC 3431
Apart from editorial changes, the notable changes are updated references, including moving the comparator reference from ACAP to RFC 4790, and updated and corrected examples. The ABNF gained definition comments for gt, lt and the rest, and the text now clarifies which RFC 2822 elements the COUNT test counts. The requirement to strip white space from header fields before comparing was removed, because a more general version was added to the base Sieve specification.
This is a tidied-up second edition: clearer wording, fixed examples and newer references.