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Network Working Group                                            J. LyonRequest for Comments: 4407                               Microsoft Corp.Category: Experimental                                        April 2006Purported Responsible Address in E-Mail MessagesStatus 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, andRFC 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 insection 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 withRFC 822   but have not yet implementedRFC 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 Contents1. Introduction ....................................................21.1. Conventions Used in This Document ..........................32. Determining the Purported Responsible Address ...................33. Security Considerations .........................................54. Acknowledgements ................................................55. References ......................................................55.1. Normative References .......................................55.2. Informative References .....................................51.  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 theLyon                          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 bysection 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 ofRFC 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 toRFC 822 rather thanRFC 2822, it is   possible for the algorithm to give unexpected results.  AnRFC822   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 20063.  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.  References5.1.  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 2006Author's Address   Jim Lyon   Microsoft Corporation   One Microsoft Way   Redmond, WA 98052   USA   EMail: jimlyon@microsoft.comLyon                          Experimental                      [Page 6]

RFC 4407             Purported Responsible Address            April 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 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 inBCP 78 andBCP 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 athttp://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]