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Keywords: reputation, domain, security, messaging, dkim, spf, authentication
Internet Engineering Task Force (IETF) N. Borenstein
Request for Comments: 7073 Mimecast
Category: Standards Track M. Kucherawy
ISSN: 2070-1721 November 2013
A Reputation Response Set for Email Identifiers
Abstract
This document defines a response set for describing assertions a
reputation service provider can make about email identifiers, for use
in generating reputons.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7073.
Copyright Notice
Copyright (c) 2013 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. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 7073 Email Identifiers Response Set November 2013
Table of Contents
1. Introduction ....................................................2
2. Terminology and Definitions .....................................2
2.1. Key Words ..................................................2
2.2. Email Definitions ..........................................2
2.3. Other Definitions ..........................................3
3. Discussion ......................................................3
3.1. Assertions .................................................3
3.2. Response Set Extensions ....................................4
3.3. Identifiers ................................................4
3.4. Query Extensions ...........................................5
4. IANA Considerations .............................................5
4.1. Registration of 'email-id' Reputation Application ..........5
5. Security Considerations .........................................6
6. References ......................................................7
6.1. Normative References .......................................7
6.2. Informative References .....................................7
Appendix A. Positive vs. Negative Assertions .......................8
Appendix B. Acknowledgments ........................................8
1. Introduction
This document specifies a response set for describing the reputation
of an email identifier. A "response set" in this context is defined
in [RFC7070] and is used to describe assertions a reputation service
provider can make about email identifiers as well as metadata that
can be included in such a reply beyond the base set specified there.
An atomic reputation response is called a "reputon", defined in
[RFC7071]. That document also defines a media type to contain a
reputon for transport, and creates a registry for reputation
applications and the interesting parameters of each.
2. Terminology and Definitions
This section defines terms used in the rest of the document.
2.1. Key Words
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 [KEYWORDS].
2.2. Email Definitions
Commonly used definitions describing entities in the email
architecture are defined and discussed in [EMAIL-ARCH].
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RFC 7073 Email Identifiers Response Set November 2013
2.3. Other Definitions
Other terms of importance in this document are defined in [RFC7070],
the base document for the reputation services work.
3. Discussion
The expression of reputation about an email identifier requires
extensions of the base set defined in [RFC7070]. This document
defines and registers some common assertions about an entity found in
a piece of [MAIL].
3.1. Assertions
The "email-id" reputation application recognizes the following
assertions:
abusive: The subject identifier is associated with sending or
handling email of a personally abusive, threatening, or otherwise
harassing nature
fraud: The subject identifier is associated with the sending or
handling of fraudulent email, such as "phishing" (some good
discussion on this topic can be found in [IODEF-PHISHING])
invalid-recipients: The subject identifier is associated with
delivery attempts to nonexistent recipients
malware: The subject identifier is associated with the sending or
handling of malware via email
spam: The subject identifier is associated with the sending or
handling of unwanted bulk email
For all assertions, the "rating" scale is linear: a value of 0.0
means there is no data to support the assertion, a value of 1.0 means
all accumulated data support the assertion, and the intervening
values have a linear relationship (i.e., a score of "x" is twice as
strong of an assertion as a value of "x/2").
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RFC 7073 Email Identifiers Response Set November 2013
3.2. Response Set Extensions
The "email-id" reputation application recognizes the following
OPTIONAL extensions to the basic response set defined in [RFC7071]:
email-id-identity: A token indicating the source of the identifier;
that is, where the subject identifier was found in the message.
This MUST be one of:
dkim: The signing domain, i.e., the value of the "d=" tag, found
on a valid DomainKeys Identified Mail [DKIM] signature in
the message
ipv4: The IPv4 address of the client
ipv6: The IPv6 address of the client
rfc5321.helo: The RFC5321.HELO value used by the client (see
[SMTP])
rfc5321.mailfrom: The RFC5321.MailFrom value of the envelope of
the message (see [SMTP])
rfc5322.from: The RFC5322.From field of the message (see [MAIL])
spf: The domain name portion of the identifier (RFC5321.MailFrom
or RFC5321.HELO) verified by [SPF]
sources: A token relating a count of the number of sources of data
that contributed to the reported reputation. This is in contrast
to the "sample-size" parameter, which indicates the total number
of reports across all reporting sources.
A reply that does not contain the "identity" or "sources" extensions
is making a non-specific statement about how the reputation returned
was developed. A client can use or ignore such a reply at its
discretion.
3.3. Identifiers
In evaluating an email message on the basis of reputation, there can
be more than one identifier in the message needing to be validated.
For example, a message may have different email addresses in the
RFC5321.MailFrom parameter and the RFC5322.From header field. The
RFC5321.Helo identifier will obviously be different. Consequently,
the software evaluating the email message may need to query for the
reputation of more than one identifier.
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RFC 7073 Email Identifiers Response Set November 2013
The purpose of including the identity in the reply is to expose to
the client the context in which the identifier was extracted from the
message under evaluation. In particular, several of the items listed
are extracted verbatim from the message and have not been subjected
to any kind of validation, while a domain name present in a valid
DKIM signature has some more reliable semantics associated with it.
Computing or using reputation information about unauthenticated
identifiers has substantially reduced value, but can sometimes be
useful when combined. For example, a reply that indicates a message
contained one of these low-value identifiers with a high "spam"
rating might not be worthy of notice, but a reply that indicates a
message contained several of them could be grounds for suspicion.
A client interested in checking these weaker identifiers would issue
a query about each of them using the same assertion (e.g., "spam"),
and then collate the results to determine which ones and how many of
them came back with ratings indicating content of concern, and take
action accordingly. For stronger identifiers, decisions can
typically be made based on a few or even just one of them.
3.4. Query Extensions
A query within this application can include the OPTIONAL query
parameter "identity" to indicate which specific identity is of
interest to the query. Legal values are the same as those listed in
Section 3.2.
4. IANA Considerations
This memo presents one action for IANA, namely the registration of
the reputation application "email-id".
4.1. Registration of 'email-id' Reputation Application
This section registers the "email-id" reputation application, as per
the IANA Considerations section of [RFC7071]. The registration
parameters are as follows:
o Application symbolic name: email-id
o Short description: Evaluates DNS domain names or IP addresses
found in email identifiers
o Defining document: [RFC7073]
o Status: current
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RFC 7073 Email Identifiers Response Set November 2013
o Subject: A string appropriate to the identifier of interest (see
Section 3.2 of this document)
o Application-specific query parameters:
identity: (current) as defined in Section 3.4 of this document
o Application-specific assertions:
abusive: (current) as defined in Section 3.1 of this document
fraud: (current) as defined in Section 3.1 of this document
invalid-recipients: (current) as defined in Section 3.1 of this
document
malware: (current) as defined in Section 3.1 of this document
spam: (current) as defined in Section 3.1 of this document
o Application-specific response set extensions:
identity: (current) as defined in Section 3.2 of this document
5. Security Considerations
This document is primarily an IANA action and doesn't describe any
protocols or protocol elements that might introduce new security
concerns.
Security considerations relevant to email and email authentication
can be found in most of the documents listed in the References
sections below. Information specific to use of reputation services
can be found in [CONSIDERATIONS].
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RFC 7073 Email Identifiers Response Set November 2013
6. References
6.1. Normative References
[DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, September 2011.
[EMAIL-ARCH]
Crocker, D., "Internet Mail Architecture", RFC 5598, July
2009.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC7070] Borenstein, N., Kucherawy, M., and A. Sullivan, "An
Architecture for Reputation Reporting", RFC 7070, November
2013.
[RFC7071] Borenstein, N. and M. Kucherawy, "A Media Type for
Reputation Interchange", RFC 7071, November 2013.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[SPF] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
for Authorizing Use of Domains in E-Mail, Version 1", RFC
4408, April 2006.
6.2. Informative References
[CONSIDERATIONS]
Kucherawy, M., "Operational Considerations Regarding
Reputation Services", Work in Progress, May 2013.
[IODEF-PHISHING]
Cain, P. and D. Jevans, "Extensions to the IODEF-Document
Class for Reporting Phishing", RFC 5901, July 2010.
[MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
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RFC 7073 Email Identifiers Response Set November 2013
Appendix A. Positive vs. Negative Assertions
[CONSIDERATIONS] some current theories about reputation, namely that
it will possibly have more impact to develop positive reputations and
focus on giving preferential treatment to content or sources that
earn those. However, the assertions defined in this document are all
clearly negative in nature.
In effect, this document is recording current use of reputation and
of this framework in particular. It is expected that, in the future,
the application being registered here will be augmented, and other
applications registered, that focus more on positive assertions
rather than negative ones.
Appendix B. Acknowledgments
The authors wish to acknowledge the contributions of the following to
this specification: Scott Hollenbeck, Scott Kitterman, Peter Koch,
John Levine, Danny McPherson, S. Moonesamy, Doug Otis, and David F.
Skoll.
Authors' Addresses
Nathaniel Borenstein
Mimecast
203 Crescent St., Suite 303
Waltham, MA 02453
USA
Phone: +1 781 996 5340
EMail: nsb@guppylake.com
Murray S. Kucherawy
270 Upland Drive
San Francisco, CA 94127
USA
EMail: superuser@gmail.com
Borenstein & Kucherawy Standards Track [Page 8]