Network Working Group B. Leiba

Network Working Group B. Leiba
Request for Comments: 5436 IBM T.J. Watson Research Center
Updates: 3834 M. Haardt
Category: Standards Track freenet.de GmbH
January 2009

Sieve Notification Mechanism: mailto

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.

Copyright Notice

Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (http://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.

Abstract

This document describes a profile of the Sieve extension for
notifications, to allow notifications to be sent by electronic mail.

Leiba & Haardt Standards Track [Page 1]

RFC 5436 Sieve Notification Mechanism: mailto January 2009

Table of Contents

1. Introduction ....................................................3
1.1. Overview ...................................................3
1.2. Conventions Used in This Document ..........................3
2. Definition ......................................................3
2.1. Notify Parameter "method" ..................................3
2.2. Test notify_method_capability ..............................3
2.3. Notify Tag ":from" .........................................3
2.4. Notify Tag ":importance" ...................................4
2.5. Notify Tag ":options" ......................................4
2.6. Notify Tag ":message" ......................................4
2.7. Other Definitions ..........................................4
2.7.1. The Auto-Submitted Header Field .....................6
3. Examples ........................................................7
4. Internationalization Considerations .............................8
5. Security Considerations .........................................9
6. IANA Considerations ............................................10
6.1. Registration of Notification Mechanism ....................10
6.2. New Registry for Auto-Submitted Header Field Keywords .....10
6.3. Initial Registration of Auto-Submitted Header
Field Keywords ............................................11
7. References .....................................................11
7.1. Normative References ......................................11
7.2. Informative References ....................................12

Leiba & Haardt Standards Track [Page 2]

RFC 5436 Sieve Notification Mechanism: mailto January 2009

1. Introduction

1.1. Overview

The [Notify] extension to the [Sieve] mail filtering language is a
framework for providing notifications by employing URIs to specify
the notification mechanism. This document defines how [mailto] URIs
are used to generate notifications by email.

1.2. Conventions Used in This Document

Conventions for notations are as in Section 1.1 of [Sieve], including
the use of [Kwds].

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 [Kwds].

2. Definition

The mailto mechanism results in the sending of a new email message (a
"notification message") to notify a recipient about a "triggering
message".

2.1. Notify Parameter "method"

The mailto notification mechanism uses standard mailto URIs as
specified in [mailto]. mailto URIs may contain header fields
consisting of a header name and value. These header fields are
called "URI headers" to distinguish them from "message headers".

2.2. Test notify_method_capability

The notify_method_capability test for "online" may return "yes" or
"no" only if the Sieve processor can determine with certainty whether
or not the recipients of the notification message are online and
logged in. Otherwise, the test returns "maybe" for this notification
method.

2.3. Notify Tag ":from"

The ":from" tag overrides the default sender of the notification
message. "Sender", here, refers to the value used in the [RFC5322]
"From" header. Implementations MAY also use this value in the
[RFC5321] "MAIL FROM" command (the "envelope sender"), or they may
prefer to establish a mailbox that receives bounces from notification
messages.

Leiba & Haardt Standards Track [Page 3]

RFC 5436 Sieve Notification Mechanism: mailto January 2009

2.4. Notify Tag ":importance"

The ":importance" tag has no special meaning for this notification
mechanism, and this specification puts no restriction on its use.
Implementations MAY use the value of ":importance" to set a priority
or importance indication on the notification message (perhaps a
visual indication, or perhaps making use of one of the non-standard
but commonly used message headers).

2.5. Notify Tag ":options"

This tag is not used by the mailto method.

2.6. Notify Tag ":message"

The value of this tag, if it is present, is used as the subject of
the notification message, and overrides all other mechanisms for
determining the subject (as described below). Its value SHOULD NOT
normally be truncated, though it may be sensible to truncate an
excessively long value.

2.7. Other Definitions

Because the receipt of an email message is generating another email
message, implementations MUST take steps to avoid mail loops. The
REQUIRED inclusion of an "Auto-Submitted:" field, as described in the
message composition guidelines, will also help in loop detection and
avoidance.

Implementations SHOULD NOT trigger notifications for messages
containing "Auto-Submitted:" header fields with any value other than
"No".

Implementations MUST allow messages with empty envelope senders to
trigger notifications.

Because this notification method uses a store-and-forward system for
delivery of the notification message, the Sieve processor should not
have a need to retry notifications. Therefore, implementations of
this method SHOULD use normal mechanisms for submitting SMTP messages
and for retrying the initial submission. Once the notification
message is submitted, implementations MUST NOT resubmit it, as this
is likely to result in multiple notifications, and increases the
danger of message loops.

Implementations SHOULD consider limiting notification messages. In
particular, they SHOULD NOT sent duplicate notifications to the same
address from the same script invocation. Batching of notifications

Leiba & Haardt Standards Track [Page 4]

RFC 5436 Sieve Notification Mechanism: mailto January 2009

within a short time to the same address might also be useful.
Different implementations, different administrative domains, and
different users may have different needs; configuration options are a
good idea here.

The overall notification message is composed using the following
guidelines (see [RFC5322] for references to message header fields):

o If the envelope sender of the triggering message is empty, the
envelope sender of the notification message MUST be empty as well,
to avoid message loops. Otherwise, the envelope sender of the
notification message SHOULD be set to the value of the ":from" tag
to the notify action, if one is specified, has email address
syntax, and is valid according to the implementation-specific
security checks (see Section 3.3 of [Notify]). If ":from" is not
specified or is not valid, the envelope sender of the notification
message SHOULD be set either to the envelope "to" field from the
triggering message, as used by Sieve, or to an email address
associated with the notification system, at the discretion of the
implementation. This MUST NOT be overridden by a "from" URI
header, and any such URI header MUST be ignored.

o The envelope recipient(s) of the notification message SHOULD be
set to the address(es) specified in the URI (including any URI
headers where the hname is "to" or "cc").

o The header field "Auto-Submitted: auto-notified" MUST be included
in the notification message (see Section 2.7.1). This is to
reduce the likelihood of message loops, by tagging this as an
automatically generated message. Among other results, it will
inform other notification systems not to generate further
notifications. mailto URI headers with hname "auto-submitted" are
considered unsafe and MUST be ignored.

o The "From:" header field of the notification message SHOULD be set
to the value of the ":from" tag to the notify action, if one is
specified, has email address syntax, and is valid according to the
implementation-specific security checks (see Section 3.3 of
[Notify]). If ":from" is not specified or is not valid, the
"From:" header field of the notification message SHOULD be set
either to the envelope "to" field from the triggering message, as
used by Sieve, or to an email address associated with the
notification system, at the discretion of the implementation.
This MUST NOT be overridden by a "from" URI header, and any such
URI header MUST be ignored.

Leiba & Haardt Standards Track [Page 5]

RFC 5436 Sieve Notification Mechanism: mailto January 2009

o The "To:" header field of the notification message SHOULD be set
to the address(es) specified in the URI (including any URI headers
where the hname is "to").

o The "Subject:" field of the notification message SHOULD contain
the value defined by the ":message" tag, as described in [Notify].
If there is no ":message" tag and there is a "subject" header on
the URI, then that value SHOULD be used. If the "subject" header
is also absent, the subject SHOULD be retained from the triggering
message. Note that Sieve [Variables] can be used to advantage
here, as shown in the example in Section 3.

o The "References:" field of the notification message MAY be set to
refer to the triggering message, and MAY include references from
the triggering message.

o If the mailto URI contains a "body" header, the value of that
header SHOULD be used as the body of the notification message. If
there is no "body" header, it is up to the implementation whether
to leave the body empty or to use an excerpt of the original
message.

o The "Received:" fields from the triggering message MAY be retained
in the notification message, as these could provide useful trace/
history/diagnostic information. The "Auto-Submitted" header field
MUST be placed above these (see Section 2.7.1). URI headers with
hname "received" are considered unsafe, and MUST be ignored.

o Other header fields of the notification message that are normally
related to an individual new message (such as "Message-ID" and
"Date") are generated for the notification message in the normal
manner, and MUST NOT be copied from the triggering message. Any
URI headers with those names MUST be ignored. Further, the "Date"
header serves as the notification timestamp defined in [Notify].

o All other header fields of the notification message either are as
specified by URI headers, or have implementation-specific values;
their values are not defined here. It is suggested that the
implementation capitalize the first letter of URI headers and add
a space character after the colon between the mail header name and
value when adding URI headers to the message, to be consistent
with common practice in email headers.

2.7.1. The Auto-Submitted Header Field

The header field "Auto-Submitted: auto-notified" MUST be included in
the notification message (see [RFC3834]). The "Auto-Submitted"
header field is considered a "trace field", similar to "Received"

Leiba & Haardt Standards Track [Page 6]

RFC 5436 Sieve Notification Mechanism: mailto January 2009

header fields (see [RFC5321]). If the implementation retains the
"Received" fields from the triggering message (see above), the "Auto-
Submitted" field MUST be placed above those "Received" fields,
serving as a boundary between the ones from the triggering message
and those that will be part of the notification message.

The header field "Auto-Submitted: auto-notified" MUST include one or
both of the following parameters:

o owner-email - specifies an email address, determined by the
implementation, of the owner of the Sieve script that generated
this notification. If specified, it might be used to identify or
contact the script's owner. The parameter attribute is "owner-
email", and the parameter value is a quoted string containing an
email address, as defined by "addr-spec" in [RFC5322]. Example:
Auto-Submitted: auto-notified; owner-email="[email protected]"

o owner-token - specifies an opaque token, determined by the
implementation, that the administrative domain of the owner of the
Sieve script that generated this notification can use to identify
the owner. This might be used to allow identification of the
owner while protecting the owner's privacy. The parameter
attribute is "owner-token", and the parameter value is as defined
by "token" in [RFC3834]. Example:
Auto-Submitted: auto-notified; owner-token=af3NN2pK5dDXI0W

See Section 5 for discussion of possible uses of these parameters.

3. Examples

Triggering message (received by [email protected]):

Return-Path: <[email protected]>
Received: from mail.example.com by mail.example.org
for <[email protected]>; Wed, 7 Dec 2005 05:08:02 -0500
Received: from hobbies.example.com by mail.example.com
for <[email protected]>; Wed, 7 Dec 2005 02:00:26 -0800
Message-ID: <[email protected]>
Date: Wed, 07 Dec 2005 10:59:19 +0100
Precedence: list
List-Id: Knitting Mailing List <knitting.example.com>
Sender: [email protected]
Errors-To: [email protected]
From: "Jeff Smith" <[email protected]>
To: "Knitting Mailing List" <[email protected]>
Subject: [Knitting] A new sweater

I just finished a great new sweater!

Leiba & Haardt Standards Track [Page 7]

RFC 5436 Sieve Notification Mechanism: mailto January 2009

Sieve script (run on behalf of [email protected]):

require ["enotify", "variables"];

if header :contains "list-id" "knitting.example.com" {
if header :matches "Subject" "[*] *" {
notify :message "From ${1} list: ${2}"
:importance "3"
"mailto:[email protected][email protected]";
}
}

Notification message:

Auto-Submitted: auto-notified; owner-email="[email protected]"
Received: from mail.example.com by mail.example.org
for <[email protected]>; Wed, 7 Dec 2005 05:08:02 -0500
Received: from hobbies.example.com by mail.example.com
for <[email protected]>; Wed, 7 Dec 2005 02:00:26 -0800
Date: Wed, 7 Dec 2005 05:08:55 -0500
Message-ID: <[email protected]>
From: [email protected]
To: [email protected], [email protected]
Subject: From Knitting list: A new sweater

Note that:

o Fields such as "Message-ID:" and "Date:" were generated afresh for
the notification message, and do not relate to the triggering
message.

o Additional "Received:" fields will be added to the notification
message in transit; the ones shown were copied from the triggering
message. New ones will be added above the Auto-Submitted: header
field.

o If this message should appear at the mail.example.org server
again, the server can use the presence of a "mail.example.org"
received line to recognize that. The Auto-Submitted header field
is also present to tell the server to avoid sending another
notification, and it includes an optional owner-email parameter
for identification.

4. Internationalization Considerations

This specification introduces no specific internationalization issues
that are not already addressed in [Sieve] and in [Notify].

Leiba & Haardt Standards Track [Page 8]

RFC 5436 Sieve Notification Mechanism: mailto January 2009

5. Security Considerations

Sending a notification is comparable with forwarding mail to the
notification recipient. Care must be taken when forwarding mail
automatically, to ensure that confidential information is not sent
into an insecure environment.

The automated sending of email messages exposes the system to mail
loops, which can cause operational problems. Implementations of this
specification MUST protect themselves against mail loops; see
Section 2.7 for discussion of this and some suggestions. Other
possible mitigations for mail loops involve types of service
limitations. For example, the number of notifications generated for
a single user might be limited to no more than, say, 30 in a
60-minute period. Of course, this technique presents its own
problems, in that the actual rate-limit must be selected carefully,
to allow most legitimate situations in the given environment. Even
with careful selection, it's inevitable that there will be false
positives -- and false negatives.

Ultimately, human intervention may be necessary to re-enable
notifications that have been disabled because a loop was detected, or
to terminate a very slow loop that's under the automatic-detection
radar. Administrative mechanisms MUST be available to handle these
sorts of situations.

Email addresses specified as recipients of notifications might not be
owned by the entity that owns the Sieve script. As a result, a
notification recipient could wind up as the target of unwanted
notifications, either through intent (using scripts to mount a mail-
bomb attack) or by accident (an address was mistyped or has been
reassigned). The situation is arguably no worse than any other in
which a recipient gets unwanted email, and some of the same
mechanisms can be used in this case. But those deploying this
extension have to be aware of the potential extra problems here,
where scripts might be created through means that do not adequately
validate email addresses, and such scripts might then be forgotten
and left to run indefinitely.

In particular, note that the Auto-Submitted header field is required
to include a value that a recipient can use when contacting the
source domain of the notification message (see Section 2.7.1). That
value will allow the domain to track down the script's owner and have
the script corrected or disabled. Domains that enable this extension
MUST be prepared to respond to such complaints, in order to limit the
damage caused by a faulty script.

Leiba & Haardt Standards Track [Page 9]

RFC 5436 Sieve Notification Mechanism: mailto January 2009

Problems can also show up if notification messages are sent to a
gateway into another service, such as SMS. Information from the
email message is often lost in the gateway translation; and in this
case, critical information needed to avoid loops, to contact the
script owner, and to resolve other problems might be lost.
Developers of email gateways should consider these issues, and try to
preserve as much information as possible, including what appears in
email trace headers and the Auto-Submitted header field.

Additional security considerations are discussed in [Sieve] and in
[Notify].

6. IANA Considerations

6.1. Registration of Notification Mechanism

The following template specifies the IANA registration of the Sieve
notification mechanism specified in this document:

To: [email protected]
Subject: Registration of new Sieve notification mechanism
Mechanism name: mailto
Mechanism URI: RFC2368
Mechanism-specific options: none
Permanent and readily available reference: RFC 5436
Person and email address to contact for further information:
Michael Haardt <[email protected]>

This information should be added to the list of Sieve notification
mechanisms available from http://www.iana.org.

6.2. New Registry for Auto-Submitted Header Field Keywords

Because [RFC3834] does not define a registry for new keywords used in
the Auto-Submitted header field, we define one here, which has been
created and is available from http://www.iana.org. Keywords are
registered using the "Specification Required" policy [IANA].

This defines the template to be used to register new keywords.
Initial entries to this registry follow in Section 6.3.

To: [email protected]
Subject: Registration of new auto-submitted header field keyword
Keyword value: [the text value of the field]
Description: [a brief explanation of the purpose of this value]
Parameters: [list any keyword-specific parameters, specify their
meanings, specify whether they are required or optional; use
"none" if there are none]

Leiba & Haardt Standards Track [Page 10]

RFC 5436 Sieve Notification Mechanism: mailto January 2009

Permanent and readily available reference: [identifies
the specification that defines the value being registered]
Contact: [name and email address to contact for further information]

6.3. Initial Registration of Auto-Submitted Header Field Keywords

The following are the initial keywords that have been registered in
the "Auto-Submitted Header Field Keywords" registry, available from
http://www.iana.org.

Keyword value: no
Description: Indicates that a message was NOT automatically
generated, but was created by a human. It is the equivalent to
the absence of an Auto-Submitted header altogether.
Parameters: none
Permanent and readily available reference: RFC3834
Contact: Keith Moore <[email protected]>

Keyword value: auto-generated
Description: Indicates that a message was generated by an automatic
process, and is not a direct response to another message.
Parameters: none
Permanent and readily available reference: RFC3834
Contact: Keith Moore <[email protected]>

Keyword value: auto-replied
Description: Indicates that a message was automatically generated as
a direct response to another message.
Parameters: none
Permanent and readily available reference: RFC3834
Contact: Keith Moore <[email protected]>

Keyword value: auto-notified
Description: Indicates that a message was generated by a Sieve
notification system.
Parameters: owner-email, owner-token. At least one is required;
both refer to the owner of the Sieve script that generated this
message. See the relevant RFC for details.
Permanent and readily available reference: RFC 5436
Contact: Michael Haardt <[email protected]>

7. References

7.1. Normative References

[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.

Leiba & Haardt Standards Track [Page 11]

RFC 5436 Sieve Notification Mechanism: mailto January 2009

[Kwds] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[Notify] Melnikov, A., Ed., Leiba, B., Ed., Segmuller, W., and T.
Martin, "Sieve Email Filtering: Extension for
Notifications", RFC 5435, January 2009.

[RFC3834] Moore, K., "Recommendations for Automatic Responses to
Electronic Mail", RFC 3834, August 2004.

[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.

[Sieve] Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An
Email Filtering Language", RFC 5228, January 2008.

[mailto] Hoffman, P., Masinter, L., and J. Zawinski, "The mailto
URL scheme", RFC 2368, July 1998.

7.2. Informative References

[RFC5321] Klensin, J., Ed., "Simple Mail Transfer Protocol",
RFC 5321, October 2008.

[Variables] Homme, K., "Sieve Extension: Variables", RFC 5229,
January 2008.

Authors' Addresses

Barry Leiba
IBM T.J. Watson Research Center
19 Skyline Drive
Hawthorne, NY 10532
US

Phone: +1 914 784 7941
EMail: [email protected]

Michael Haardt
freenet.de GmbH
Willstaetter Str. 13
Duesseldorf, NRW 40549
Germany

Phone: +49 241 53087 520
EMail: [email protected]

Leiba & Haardt Standards Track [Page 12]