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rfc:rfc4407

Network Working Group J. Lyon Request for Comments: 4407 Microsoft Corp. Category: Experimental April 2006

          Purported Responsible Address in E-Mail Messages

Status of This Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

IESG Note

 The following documents  (RFC 4405, RFC 4406, RFC 4407, and RFC 4408)
 are published simultaneously as Experimental RFCs, although there is
 no general technical consensus and efforts to reconcile the two
 approaches have failed.  As such, these documents have not received
 full IETF review and are published "AS-IS" to document the different
 approaches as they were considered in the MARID working group.
 The IESG takes no position about which approach is to be preferred
 and cautions the reader that there are serious open issues for each
 approach and concerns about using them in tandem.  The IESG believes
 that documenting the different approaches does less harm than not
 documenting them.
 Note that the Sender ID experiment may use DNS records that may have
 been created for the current SPF experiment or earlier versions in
 this set of experiments.  Depending on the content of the record,
 this may mean that Sender-ID heuristics would be applied incorrectly
 to a message.  Depending on the actions associated by the recipient
 with those heuristics, the message may not be delivered or may be
 discarded on receipt.
 Participants relying on Sender ID experiment DNS records are warned
 that they may lose valid messages in this set of circumstances.
 Participants publishing SPF experiment DNS records should consider
 the advice given in section 3.4 of RFC 4406 and may wish to publish
 both v=spf1 and spf2.0 records to avoid the conflict.

Lyon Experimental [Page 1] RFC 4407 Purported Responsible Address April 2006

 Participants in the Sender-ID experiment need to be aware that the
 way Resent-* header fields are used will result in failure to receive
 legitimate email when interacting with standards-compliant systems
 (specifically automatic forwarders which comply with the standards by
 not adding Resent-* headers, and systems which comply with RFC 822
 but have not yet implemented RFC 2822 Resent-* semantics).  It would
 be inappropriate to advance Sender-ID on the standards track without
 resolving this interoperability problem.
 The community is invited to observe the success or failure of the two
 approaches during the two years following publication, in order that
 a community consensus can be reached in the future.

Abstract

 This document defines an algorithm by which, given an e-mail message,
 one can extract the identity of the party that appears to have most
 proximately caused that message to be delivered.  This identity is
 called the Purported Responsible Address (PRA).

Table of Contents

 1. Introduction ....................................................2
    1.1. Conventions Used in This Document ..........................3
 2. Determining the Purported Responsible Address ...................3
 3. Security Considerations .........................................5
 4. Acknowledgements ................................................5
 5. References ......................................................5
    5.1. Normative References .......................................5
    5.2. Informative References .....................................5

1. Introduction

 Most e-mail flows relatively directly from a sender to a recipient,
 with a small number of Mail Transfer Agents (MTAs) in between.  Some
 messages, however, are resent by forwarding agents, mailing list
 servers, and other such software.  These messages effectively result
 in two or more mail transactions: one from the sender to the
 forwarding agent, and another from the agent to the destination.
 In some cases, messages travel through more than one of these agents.
 This can occur, for example, when one mailing list is subscribed to
 another, or when the address subscribed to a mailing list is a
 forwarding service.
 Further complicating the situation, in some cases the party that
 introduces a message is not the author of the message.  For example,
 many news web sites have a "Mail this article" function that the

Lyon Experimental [Page 2] RFC 4407 Purported Responsible Address April 2006

 public can use to e-mail a copy of the article to a friend.  In this
 case, the mail is "from" the person who pressed the button, but is
 physically sent by the operator of the web site.
 This document defines a new identity associated with an e-mail
 message, called the Purported Responsible Address (PRA), which is
 determined by inspecting the header of the message.  The PRA is
 designed to be the entity that (according to the header) most
 recently caused the message to be delivered.
 Note that the results of this algorithm are only as truthful as the
 headers contained in the message; if a message contains fraudulent or
 incorrect headers, this algorithm will yield an incorrect result.
 For this reason, the result of the algorithm is called the "Purported
 Responsible Address" -- "purported" because it tells you what a
 message claims about where it came from, but not necessarily where it
 actually came from.
 This document does not prescribe any particular uses for the
 Purported Responsible Address.  However, [RFC4406] describes a method
 of determining whether a particular MTA is authorized to send mail on
 behalf of the domain contained in the PRA.

1.1. 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 [RFC2119].

2. Determining the Purported Responsible Address

 The PRA of a message is determined by the following algorithm:
 1. Select the first non-empty Resent-Sender header in the message.
    If no such header is found, continue with step 2.  If it is
    preceded by a non-empty Resent-From header and one or more
    Received or Return-Path headers occur after said Resent-From
    header and before the Resent-Sender header, continue with step 2.
    Otherwise, proceed to step 5.
 2. Select the first non-empty Resent-From header in the message.  If
    a Resent-From header is found, proceed to step 5.  Otherwise,
    continue with step 3.
 3. Select all the non-empty Sender headers in the message.  If there
    are no such headers, continue with step 4.  If there is exactly
    one such header, proceed to step 5.  If there is more than one
    such header, proceed to step 6.

Lyon Experimental [Page 3] RFC 4407 Purported Responsible Address April 2006

 4. Select all the non-empty From headers in the message.  If there is
    exactly one such header, continue with step 5.  Otherwise, proceed
    to step 6.
 5. A previous step has selected a single header from the message.  If
    that header is malformed (e.g., it appears to contain multiple
    mailboxes, or the single mailbox is hopelessly malformed, or the
    single mailbox does not contain a domain name), continue with step
    6.  Otherwise, return that single mailbox as the Purported
    Responsible Address.
 6. The message is ill-formed, and it is impossible to determine a
    Purported Responsible Address.
 For the purposes of this algorithm, a header field is "non-empty" if
 and only if it contains any non-whitespace characters.  Header fields
 that are otherwise relevant but contain only whitespace are ignored
 and treated as if they were not present.
 Note that steps 1 and 2 above extract the Resent-Sender or Resent-
 From header from the first resent block (as defined by section 3.6.6
 of [RFC2822]) if any.  Steps 3 and 4 above extract the Sender or From
 header if there are no resent blocks.
 Note that what constitutes a hopelessly malformed header or a
 hopelessly malformed mailbox in step 5 above is a matter for local
 policy.  Such local policy will never cause two implementations to
 return different PRAs.  However, it may cause one implementation to
 return a PRA where another implementation does not.  This will occur
 only when dealing with a message containing headers of questionable
 legality.
 Although the algorithm specifies how messages that are not in strict
 conformance with the provisions of RFC 2822 should be treated for the
 purposes of determining the PRA, this should not be taken as
 requiring or recommending that any systems accept such messages when
 they otherwise would not have done so.  However, if a liberal
 implementation accepts such messages and desires to know their PRAs,
 it MUST use the algorithm specified here.
 Where messages conform to RFC 822 rather than RFC 2822, it is
 possible for the algorithm to give unexpected results.  An RFC822
 message should not normally contain more than one set of resent
 headers; however, the placement of those headers is not specified,
 nor are they required to be contiguous.  It is therefore possible
 that the Resent-From header will be selected even though a Resent-
 Sender header is present.  Such cases are expected to be rare or
 non-existent in practice.

Lyon Experimental [Page 4] RFC 4407 Purported Responsible Address April 2006

3. Security Considerations

 The PRA, as described by this document, is extracted from message
 headers that have historically not been verified.  Thus, anyone using
 the PRA for any purpose MUST be aware that the headers from which it
 is derived might be fraudulent, malicious, malformed, and/or
 incorrect.  [RFC4406] describes one mechanism for validating the PRA.
 A message's PRA will often be extracted from a header field that is
 not normally displayed by existing mail user agent software.  If the
 PRA is used as part of a mechanism to authenticate the message's
 origin, the message SHOULD NOT be displayed with an indication of its
 authenticity (positive or negative) without the PRA header field also
 being displayed.

4. Acknowledgements

 The PRA concept was first published in [CallerID].  It has been
 refined using valuable suggestions from members of the MARID working
 group.

5. References

5.1. Normative References

 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2822]   Resnick, P., "Internet Message Format", RFC 2822, April
             2001.

5.2. Informative References

 [CallerID]  Microsoft Corporation, Caller ID for E-Mail Technical
             Specification,
             http://www.microsoft.com/mscorp/safety/technologies/
             senderid/resources.mspx
 [RFC4406]   Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
             RFC 4406, April 2006.

Lyon Experimental [Page 5] RFC 4407 Purported Responsible Address April 2006

Author's Address

 Jim Lyon
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052
 USA
 EMail: jimlyon@microsoft.com

Lyon Experimental [Page 6] RFC 4407 Purported Responsible Address April 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at
 ietf-ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Lyon Experimental [Page 7]

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