Individual submission M. Kucherawy Internet-Draft Sendmail, Inc. Intended status: Informational February 23, 2007 Expires: August 27, 2007 Message Header for Indicating Sender Authentication Status draft-kucherawy-sender-auth-header-04 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. This document may not be modified, and derivative works of it may not be created. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 27, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Kucherawy Expires August 27, 2007 [Page 1] Internet-Draft Authentication-Results Header February 2007 Abstract This memo defines a new message header for use with electronic mail messages to indicate the results of sender authentication efforts to mail user agents (MUAs) in order to equip them to relay that information in a convenient way to users or to make sorting and filtering decisions. Kucherawy Expires August 27, 2007 [Page 2] Internet-Draft Authentication-Results Header February 2007 1. Introduction This memo defines a new message header for electronic mail messages which presents the results of a sender authentication effort in a machine-readable format. The intent is to create a place to collect such data when sender authentication mechanisms are in use so that an MUA can provide a recommendation to the user as to the trustworthiness of the message's origin and content. This memo defines both the format of this new header, and discusses the implications of its presence or absence. [REMOVE OR REWORD PRIOR TO FINAL VERSION] At the time of publication of this draft, only [AUTH] is a published sender authentication standard. However, several more are in the Internet Draft stage. As various methods emerge, it is necessary to prepare for their appearance and encourage convergence in the area of interfacing these filters to MUAs. 1.1. Purpose The header defined in this memo is expected to serve several purposes: 1. Convey to MUAs from filters and MTAs the results of various sender authentication checks being applied; 2. Provide a common location for the presentation of this data; 3. Create an extensible framework for specifying new authentication methods as such emerge; 4. Convey the results of sender authentication tests to later filtering agents within the same "trust domain", as such agents might apply more or less stringent checks based on sender authentication results. 1.2. Requirements This memo establishes no new requirements on existing protocols or servers, as there is currently not a standard place for the described data to be collected or presented. 1.3. Definitions This document occasionally uses terms that appear in capital letters. When the terms "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "SHOULD NOT", and "MAY" appear capitalized, they are being used to indicate Kucherawy Expires August 27, 2007 [Page 3] Internet-Draft Authentication-Results Header February 2007 particular requirements of this specification. A discussion of the meanings of these terms appears in RFC2119. Generally it is assumed that the work of applying sender authentication schemes takes place at a border MTA, that is, an MTA which acts as a gateway between the general Internet and the users within an organizational boundary. This specification is written with that assumption in mind. However, there are some sites at which the entire mail infrastructure consists of a single host. In such cases, such terms as "border MTA" and "delivery MTA" may well apply to the same machine or even the very same agent. Kucherawy Expires August 27, 2007 [Page 4] Internet-Draft Authentication-Results Header February 2007 2. Definition and Format of the Header The new header being defined here is called "Authentication-Results". It qualifies in [MAIL] [3] terms as a Structured Header Field, and thus all of the related definitions in that document apply. The decommented value of the header consists of a hostname, some whitespace, a "property=value" statement indicating which property was selected to determine who sent the message and what value was extracted from that property, followed by zero or more authentication method names and a result associated with each, returned by the code that implements the method. As it is currently a matter of some debate, the header MAY appear more than once in a single message, or more than one result MAY be represented in a single header, or a combination of these MAY be applied. Formally, the header is specified as follows: header = "Authentication-Results:" CFWS hostname CFWS headerspec *(CFWS ";" CFWS method CFWS "=" CFWS result) CFWS hostname = domain ; as defined in section 3.4.1 of [MAIL] method = token ; a method indicates which method's result is ; is represented by "value", and is one of the methods ; explicitly defined as valid in this document ; or is an extension method as defined below result = "pass" / "fail" / "softfail" / "neutral" / "temperror" / "permerror" ; an indication of the results of the attempt to ; authenticate the sender token = ; as defined in Appendix A of [MIME] [4] token = ; as defined in Appendix A of CFWS = ; as defined in section 3.2.3 of [MAIL] Kucherawy Expires August 27, 2007 [Page 5] Internet-Draft Authentication-Results Header February 2007 headerspec = ptype CWFS "." CWFS property CWFS "=" CFWS value ; an indication of which property of the message ; was evaluated by the authentication scheme being ; applied to yield the reported result ptype = "smtp" / "header" ; indicates whether the property being evaluated was ; a parameter to an [SMTP] [5] ptype = "smtp" / "header" ; indicates whether the property being evaluated was ; a parameter to an property = token ; if "ptype" is "smtp", this indicates which [SMTP] ; command provided the value which was evaluated by the ; authentication scheme being applied, or if "ptype" is ; "header", this indicates from which header the value ; being evaluated was extracted value = token / mailbox ; the value extracted from the message property defined ; by the "ptype.property" construction; if the value is ; intended ; to identify a mailbox, then it is a "mailbox" ; as defined in section 3.4 of [MAIL] The "ptype" and "property" used by each authentication method should be defined in the specification for that method (or its amendments). The "ptype" and "property" are case-insensitive. If the parsed "ptype.property" construction clearly identifies a mailbox (in particular, smtp.mail, smtp.rcpt, header.from, header.sender), then the "value" MUST be a "mailbox". Other properties (e.g. smtp.helo) may be evaluated, but the property MUST still be expressed as a "token" for simplified parsing. The six possible values of the "result" are: pass: The message passed the authentication tests. (This may require accessing an authentication policy of some kind published by the sending domain.) Kucherawy Expires August 27, 2007 [Page 6] Internet-Draft Authentication-Results Header February 2007 fail: The message failed the authentication tests. (This may require accessing an authentication policy of some kind published by the sending domain.) softfail: The authentication method has either an explicit (published by the sending domain) or implicit policy, but the policy being used doesn't require successful authentication of all messages from that domain, and the message failed the authentication tests. neutral: The authentication method completed without errors, but was unable to reach either a positive or negative result about the message. temperror: A temporary (recoverable) error occurred attempting to authenticate the sender; either the process couldn't be completed locally, or (for methods requiring a policy to be accessed) there was a temporary failure retrieving the sending domain's policy. A later retry may produce a more final result. permerror: A permanent (unrecoverable) error occurred attempting to authenticate the sender; either the process couldn't be completed locally, or (for methods requiring a policy to be accessed) there was a permanent failure retrieving the sending domain's policy. Kucherawy Expires August 27, 2007 [Page 7] Internet-Draft Authentication-Results Header February 2007 3. Definition Of The 'auth' Method As it is currently an existing standard for sender authentication, it is appropriate to define an authentication method identifier for [AUTH] [1]. The authentication method identifier "auth" is thus defined for MTAs applying that standard for sender authentication. Kucherawy Expires August 27, 2007 [Page 8] Internet-Draft Authentication-Results Header February 2007 4. Adding The Header To A Message This specification makes no attempt to evaluate the relative strengths of various sender authentication methods that may become available. As such, the order of the presented authentication methods and results are not relevant since ultimately the importance on any given method over another is the decision of the MUA that is interpreting the value of the header. The "method" MUST refer to an authentication method declared in this memo, or in a subsequent one, or to an authentication method name assigned by IANA. If an MTA applies any authentication test, it MUST add this header to indicate at which host the test was done, which test got applied, and what the result was. If an MTA applies more than one such test, it MUST either add this header once per test, or one header indicating all of the results. An MTA MUST NOT add a result to an existing header. An MTA adding this header in either form MUST use its own hostname only. It MUST be a fully-qualified domain name. For security reasons, an MTA conforming to this specification MUST remove any discovered instance of this header for which the "hostname" is its own, i.e. headers which claim to be from the MTA but were added before the mail arrived at the MTA for processing. A border MTA SHOULD also delete any discovered instance of this header which claims to have been added within its trust boundary. For example, a border MTA at mx.example.com MUST delete any instance of this header claiming to come from mx.example.com and SHOULD delete any instance of this header claiming to come from any host in example.com prior to adding its own headers. This applies in both directions so that hosts outside the domain cannot claim results MUAs inside the domain might trust. An MTA compliant with this specification MUST add this header after performing one or more sender authentication tests. An MTA MAY add this header containing only the "hostname" portion to explicitly indicate that no sender authentication schemes were applied prior to delivery of this message. 4.1. Header Position and Interpretation In order to ensure non-ambiguous results and avoid the impact of false headers, an MUA SHOULD NOT interpret this header unless specifically instructed to do so by the user. That is, this should not be "on by default". Naturally then, users would not activate Kucherawy Expires August 27, 2007 [Page 9] Internet-Draft Authentication-Results Header February 2007 such a feature unless they are certain the header will be added by the receiving MTA that accepts the mail which is ultimately read by the MUA, and instances of the header added by foreign MTAs will be removed before delivery. Furthermore, an MUA SHOULD NOT interpret this header unless the hostname it bears appears to be one within its own trust domain as configured by the user. An MTA adding a header MUST add the header at the top of the message so that there is generally some indication upon delivery of where in the chain of handling MTAs the sender authentcation was done. Further discussion of this can be found in the Security Considerations section below. 4.2. Extension Fields Additional authentication method identifiers may be defined in the future by later revisions or extensions to this specification. Extension identifiers beginning with "x-" will never be defined as standard fields; such names are reserved for experimental use. Method identifiers NOT beginning with "x-" MUST be registered with the Internet Assigned Numbers Authority (IANA) and published in an RFC. Extension identifiers may be defined for the following reasons: 1. To allow additional information from emergent authentication systems to be communicated to MUAs. The names of such identifiers should reflect the name of the method being defined, but should not be needlessly long. 2. To allow the creation of "sub-identifiers" which indicate different levels of authentication and differentiate between their relative strengths, e.g. "auth1-weak" and "auth1-strong". Authentication method implementors are encouraged to provide adequate information, via [MAIL] comments if necessary, to allow an MUA developer to understand or relay ancilliary details of authentication results. For example, if it might be of interest to relay what data was used to perform an evaluation, such information could be relayed as a comment in the header, such as: Authentication-Results: mx.example.com; foo=pass (2 of 3 tests OK) Kucherawy Expires August 27, 2007 [Page 10] Internet-Draft Authentication-Results Header February 2007 5. Discussion This section discusses various implementation issues not specifically related to security. Security issues are discussed in a later section. 5.1. Legacy MUAs Implementors of this proposal should be aware that many MUAs are unlikely to be retrofit to support the new header and its semantics. In the interests of convenience and quicker adaptation, a delivery MTA might want to consider adding things that are processed by existing MUAs as well as the header defined by this specification. One suggestion is to provide a Priority: header with a value that reflects the strength of the authentication that was accomplished, e.g. "low" for weak or no authentication, "normal" or "high" for good authentication. Certainly some modern MUAs can filter based on the content of this header, but as there is keen interest in having MUAs make some kind of graphical representation of this header's meaning, other interim means of doing so may be necessary while this proposal is adopted. 5.2. Trusting The Header Rather than the coding rigor of doing a digital signature on the content of this header, it may be sufficient to establish a maximum [SMTP] path length between the addition of the header and final delivery. The path length is defined as the count of Received headers above the Authentication-Results header. If more than that maximum path length is traversed between insertion of the header and delivery, the value of the header should no longer be trusted to be valid. This requires a configurable MUA setting to define this maximum path length. The setting would need to take into account possible additional hops should the final delivery server be unavailable. Although this approach likely bears a much shorter time to implement, it is a less general solution than the proposed one and would probably require additional user education above the norm and thus lead to more confusion on deployment. The MUA SHOULD trust this header if it is trusting such headers and the path length after the addition of the header is less than three. Kucherawy Expires August 27, 2007 [Page 11] Internet-Draft Authentication-Results Header February 2007 6. Conformance and Usage Requirements An MTA or gateway conforms to this specification if it applies one or more sender authentication mechanisms and inserts a header corresponding to this standard after doing so and prior to delivery. MTAs that are relaying mail rather than delivering it MAY perform sender authentication or even take actions based on the results found, but MUST NOT add a "Authentication-Results" header if relaying rather than rejecting or discarding at the gateway. Conversely, an MTA doing local delivery MUST add this header prior to delivery the message in order to be compliant. A minimal implementation which does at least one sender authentication check will add the header defined by this memo prior to invoking local delivery procedures. This specification places no restrictions on the processing of the header's contents by user agents or distribution lists. It is presented to those packages solely for their own information. Kucherawy Expires August 27, 2007 [Page 12] Internet-Draft Authentication-Results Header February 2007 7. IANA Considerations Following the policies outlined in [IANA-CONSIDERATIONS] [2] names of sender authentication methods supported by this specification must be registered with IANA under the IETF Consensus method, with the exception of experimental names as described above. Kucherawy Expires August 27, 2007 [Page 13] Internet-Draft Authentication-Results Header February 2007 8. Security Considerations The following security considerations apply when applying or processing the "Authentication-Results" header: 8.1. Non-conformant MTAs An MUA that is aware of this specification which accesses a mailbox whose mail is handled by a non-conformant MTA is in a position to make false conclusions based on forged headers. A malicious user or agent could forge a header using the destination MX for a receiving domain as the hostname token in the value of the header, and with the rest of the value claim that the sender was properly authenticated. The non-conformant MTA would fail to strip the forged header, and the MUA could trust it. It is for this reason an MUA SHOULD NOT have processing of the "Authentication-Results" header enabled by default; instead it must be ignored, at least for the purposes of enacting filtering decisions, unless specifically enabled by the user after verifying that the MTA is compliant. It is acceptable to have an MUA aware of this standard, but have an explicit list of hostnames whose "Authentication-Results" headers are trustworthy, however this list SHOULD initially be empty. Proposed alternate solutions to this problem are nascent. Possibly the simplest is a digital signature on the header which can be verified by a posted public key. Another would be a means to interrogate the MTA that added the header to see if it is actually providing any sender authentication services and saw the message in question. In either case, a method needs to exist to verify that the host which appears to have added the header (a) actually did so, and (b) is legitimately adding that header for this delivery. 8.2. Header Position Headers can sometimes be reordered enroute by intermediate MTAs. The goal of requiring header addition only at the top of a message is an acknowledgement that some MTAs do reorder headers, but most do not. Thus, in the general case, there will be some indication of which MTAs (if any) handled the message after the addition of the header defined here. However, this is not universally true. In an environment involving an MTA that does reorder headers, the above-mentioned hop count test for header validity may not be possible. Kucherawy Expires August 27, 2007 [Page 14] Internet-Draft Authentication-Results Header February 2007 9. References [1] Myers, J., "SMTP Service Extension for Authentication", RFC 2554, March 1999. [2] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, October 1998. [3] Resnick, P., "Internet Message Format", RFC 2822, April 2001. [4] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [5] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. Kucherawy Expires August 27, 2007 [Page 15] Internet-Draft Authentication-Results Header February 2007 Appendix A. Acknowledgements The author wishes to acknowledge the following for their review and constructive criticism of this proposal: Mark Delany and Miles Libbey of Yahoo! Inc., Jim Fenton of Cisco, and Rand Wacker and Eric Allman of Sendmail, Inc. Kucherawy Expires August 27, 2007 [Page 16] Internet-Draft Authentication-Results Header February 2007 Appendix B. Public Discussion Public discussion of this proposed specification is handled via the mail-vet-discuss@mipassoc.org mailing list. The list is open. Access to subscription forms and to list archives can be found at http://mipassoc.org/mailman/listinfo/mail-vet-discuss. Kucherawy Expires August 27, 2007 [Page 17] Internet-Draft Authentication-Results Header February 2007 Appendix C. Authentication-Results Examples This section presents some examples of the use of this header to indicate authentication results. It makes use of some authentication methods which are still in the draft phase, namely "domainkeys", "sender-id" and "spf". These are not IANA-registered or otherwise formally defined, but they are presented here under the assumption that they eventually will be. C.1. Trivial case; header not present The trivial case: From: sender@example.com Received: from mail-router.example.com (mail-router.example.com [1.2.3.4]) by server.sendmail.com (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@sendmail.com Message-Id: <12345.abc@example.com> Subject: here's a sample Hello! Goodbye! Example 1: Trivial case The "Authentication-Results" header is completely absent. The MUA may make no conclusion about the validity of the message. This could be the case because the sender authentication services were not available at the time of delivery, or no service is provided, or the MTA is not in compliance with this specification. Kucherawy Expires August 27, 2007 [Page 18] Internet-Draft Authentication-Results Header February 2007 C.2. Nearly-trivial case; service provided, but no authentication done A message that was delivered by an MTA which conforms to this standard but provides no actual sender authentication service: Authentication-Results: mail-router.example.com From: sender@example.com Received: from mail-router.example.com (mail-router.example.com [1.2.3.4]) by server.sendmail.com (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@sendmail.com Message-Id: <12345.abc@example.com> Subject: here's a sample Hello! Goodbye! Example 2: Header present but no authentication done The "Authentication-Results" header is present, indicating the delivering MTA (which is named in the value of the header) conforms to this specification. The absence of any method and result tokens indicates no sender authentication was done. C.3. Service provided, authentication done A message that was delivered by an MTA which conforms to this standard and applied some sender authentication: Authentication-Results: mail-router.example.com smtp.auth=sender@example.com; auth=pass (cram-md5) From: sender@example.com Received: from dialup-1-2-3-4.example-isp.com (dialup-1-2-3-4.example-isp.com [1.2.3.4]) by mail-router.example.com (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@sendmail.com Message-Id: <12345.abc@example.com> Subject: here's a sample Hello! Goodbye! Example 3: Header reporting results Kucherawy Expires August 27, 2007 [Page 19] Internet-Draft Authentication-Results Header February 2007 The "Authentication-Results" header is present, indicating the delivering MTA (which is named in the value of the header) conforms to this specification. Furthermore, the sender authenticated herself/himself to the MTA via a method specified in [AUTH]. The actual method is identified in a header comment after the method's result is indicated. The MUA could extract and relay this extra information if desired. C.4. Service provided, several authentications done, single MTA A message that was relayed inbound via a single MTA which conforms to this standard and applied two different sender authentication checks: Authentication-Results: mail-router.example.com smtp.mail=sender@example.com; auth=pass (cram-md5); spf=pass Authentication-Results: mail-router.example.com header.from=sender@example.com; sender-id=pass From: sender@example.com Received: from mail-router.example.com (mail-router.example.com [1.2.3.4]) by dialup-1-2-3-4.example-isp.com (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@sendmail.com Message-Id: <12345.abc@example.com> Subject: here's a sample Hello! Goodbye! Example 4: Headers reporting results from one MTA The "Authentication-Results" header is present, indicating the delivering MTA (which is named in the value of the header) conforms to this specification. Furthermore, the sender authenticated herself/himself to the MTA via a method specified in [AUTH], and both SPF and Sender-ID checks were done and passed. The MUA could extract and relay this extra information if desired. Two "Authentication-Results" headers are required because the methods applied did not all base their results on the same property of the message. Kucherawy Expires August 27, 2007 [Page 20] Internet-Draft Authentication-Results Header February 2007 C.5. Service provided, several authentications done, different MTAs A message that was relayed inbound by two different MTAs which conform to this standard and applied multiple sender authentication checks: Authentication-Results: auth-checker.example.com header.from=sender@example.com; sender-id=pass; domainkeys=pass (good signature) Received: from mail-router.example.com (mail-router.example.com [10.11.12.13]) by auth-checker.example.com (8.11.6/8.11.6) with ESMTP id i7PK0sH7021929; Fri, Feb 15 2002 17:19:22 -0800 Authentication-Results: mail-router.example.com smtp.mail=sender@example.com; auth=pass (cram-md5); spf=fail Received: from dialup-1-2-3-4.example-isp.com (dialup-1-2-3-4.example-isp.com [1.2.3.4]) by mail-router.example.com (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 DomainKey-Signature: a=rsa-sha1; s=gatsby; d=sendmail.com; c=simple; q=dns; b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= From: sender@example.com Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@sendmail.com Message-Id: <12345.abc@example.com> Subject: here's a sample Hello! Goodbye! Example 5: Headers reporting results from multiple MTAs The "Authentication-Results" header is present, indicating conformance to this specification. It is present twice because two different MTAs in the chain of delivery did authentication tests. The first, "mail-router.example.com" reports that [AUTH] and SPF were both used and [AUTH] passed but SPF failed. In the [AUTH] case, additional data is provided in the comment field, which the MUA can choose to render if desired. The second MTA, "auth- checker.example.com", reports that it did a Sender-ID test and a DomainKeys test, both of which which passed. Again, additional data about one of the tests is provided as a comment, which the MUA may choose to render. Kucherawy Expires August 27, 2007 [Page 21] Internet-Draft Authentication-Results Header February 2007 Author's Address Murray S. Kucherawy Sendmail, Inc. 6425 Christie Ave., Suite 400 Emeryville, CA 94608 US Phone: +1 510 594 5400 Email: msk+ietf@sendmail.com Kucherawy Expires August 27, 2007 [Page 22] Internet-Draft Authentication-Results Header February 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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, THE IETF TRUST 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. 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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. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Kucherawy Expires August 27, 2007 [Page 23]