Individual submission M. Kucherawy Internet-Draft Sendmail, Inc. Intended status: Standards Track July 26, 2007 Expires: January 27, 2008 Message Header Field for Indicating Sender Authentication Status draft-kucherawy-sender-auth-header-06 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 January 27, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Kucherawy Expires January 27, 2008 [Page 1] Internet-Draft Authentication-Results Header July 2007 Abstract This memo defines a new message header field for use with electronic mail messages to indicate the results of sender authentication efforts. Mail user agents (MUAs) may use this message header field to relay that information in a convenient way to users or to make sorting and filtering decisions. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Requirements . . . . . . . . . . . . . . . . . . . . . . . 4 1.3. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4 2. Definition and Format of the Header . . . . . . . . . . . . . 6 2.1. General Description . . . . . . . . . . . . . . . . . . . 6 2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 6 2.3. Authentication Identifier Fields . . . . . . . . . . . . . 8 2.4. Result Values . . . . . . . . . . . . . . . . . . . . . . 9 2.5. Definition Of Initial Methods . . . . . . . . . . . . . . 9 2.6. Extension Fields . . . . . . . . . . . . . . . . . . . . . 10 3. Adding The Header To A Message . . . . . . . . . . . . . . . . 11 3.1. Header Position and Interpretation . . . . . . . . . . . . 11 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.1. Removing The Header . . . . . . . . . . . . . . . . . . . 13 5. Conformance and Usage Requirements . . . . . . . . . . . . . . 14 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 6.1. The Authentication-Results: header . . . . . . . . . . . . 15 6.2. Method Registry . . . . . . . . . . . . . . . . . . . . . 15 7. Security Considerations . . . . . . . . . . . . . . . . . . . 16 7.1. Non-conformant MTAs . . . . . . . . . . . . . . . . . . . 16 7.2. Header Position . . . . . . . . . . . . . . . . . . . . . 16 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 8.1. Normative References . . . . . . . . . . . . . . . . . . . 17 8.2. Informative References . . . . . . . . . . . . . . . . . . 17 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 19 Appendix B. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 20 Appendix C. Authentication-Results Examples . . . . . . . . . . . 21 C.1. Trivial case; header field not present . . . . . . . . . . 21 C.2. Nearly-trivial case; service provided, but no authentication done . . . . . . . . . . . . . . . . . . . 22 C.3. Service provided, authentication done . . . . . . . . . . 22 C.4. Service provided, several authentications done, single MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 C.5. Service provided, several authentications done, different MTAs . . . . . . . . . . . . . . . . . . . . . . 24 Appendix D. Public Discussion . . . . . . . . . . . . . . . . . . 26 Kucherawy Expires January 27, 2008 [Page 2] Internet-Draft Authentication-Results Header July 2007 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 27 Intellectual Property and Copyright Statements . . . . . . . . . . 28 Kucherawy Expires January 27, 2008 [Page 3] Internet-Draft Authentication-Results Header July 2007 1. Introduction This memo defines a new message header field 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 a Mail User Agent (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 field, and discusses the implications of its presence or absence. [UPDATE PRIOR TO FINAL VERSION] At the time of publication of this draft, [AUTH], [DKIM], [DOMAINKEYS], [SENDERID] and [SPF] are the published e-mail authentication methods in common use. 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 field defined in this memo is expected to serve several purposes: 1. Convey to MUAs from filters and Mail Transfer Agents (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 no standard place for the described data to be collected or presented. 1.3. Definitions 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. Kucherawy Expires January 27, 2008 [Page 4] Internet-Draft Authentication-Results Header July 2007 A "border MTA" is an MTA which acts as a gateway between the general Internet and the users within an organizational boundary. A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA which actually enacts delivery of a message to a user's inbox or other final delivery. Generally it is assumed that the work of applying sender authentication schemes takes place at a border MTA or a delivery MTA. 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. See [I-D.DRAFT-CROCKER-EMAIL-ARCH] for further discussion on e-mail system architecture. Kucherawy Expires January 27, 2008 [Page 5] Internet-Draft Authentication-Results Header July 2007 2. Definition and Format of the Header This section gives a general overview of the format of the header field being defined, and then provides more formal specification. 2.1. General Description The new header field being defined here is called "Authentication- Results". It is a Structured Header Field as defined in [MAIL] and thus all of the related definitions in that document apply. This new header field MUST be added at the top of the message as it transits MTAs which do authentication checks so some idea of how far away the checks were done can be inferred. It therefore should be treated as a Trace Header Field as defined in [MAIL] and thus all of the related definitions in that document apply. The decommented value of the header field consists of an authentication identifier, 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. The header field MAY appear more than once in a single message, or more than one result MAY be represented in a single header field, or a combination of these MAY be applied. 2.2. Formal Definition Formally, the header field is specified as follows: header = "Authentication-Results:" [CFWS] authres-id *([CFWS] ";" methodspec propspec ) authres-id = dot-atom-text ; see below for a description of this element; ; "dot-atom-text" is defined in section 3.2.4 of [MAIL] methodspec = [CFWS] method [CFWS] "=" [CFWS] result [CFWS] ; indicates which authentication method was evaluated Kucherawy Expires January 27, 2008 [Page 6] Internet-Draft Authentication-Results Header July 2007 propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" value ; an indication of which property of the message ; was evaluated by the authentication scheme being ; applied to yield the reported result method = token [ "/" version ] ; 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 version = 1*( ALPHA / DIGIT ) 0*( "." 1*( ALPHA / DIGIT ) ) ; indicates which version of the method was applied result = "pass" / "hardfail" / "softfail" / "neutral" / "temperror" / "permerror" ; an indication of the results of the attempt to ; authenticate the sender ptype = "smtp" / "header" / "body" / "policy" ; indicates whether the property being evaluated was ; a parameter to an [SMTP] command, or was a value taken ; from a message header field, or was some property of ; the message body, or some other property evaluated by ; the receiving MTA property = token ; if "ptype" is "smtp", this indicates which [SMTP] ; command provided the value which was evaluated by the ; authentication scheme being applied; if "ptype" is ; "header", this indicates from which header field the ; value being evaluated was extracted; if "ptype" is ; "body", this indicates the offset into the body at which ; content of interest was detected; if "ptype" is "policy" ; then this indicates the name of the policy which caused ; this header field to be added (see below) value = [CFWS] token [CFWS] / mailbox ; the value extracted from the message property defined ; by the "ptype.property" construction; if the value ; identifies a mailbox, then it is a "mailbox" ; as defined in section 3.4 of [MAIL]; Kucherawy Expires January 27, 2008 [Page 7] Internet-Draft Authentication-Results Header July 2007 ; "mailbox" allows CFWS The "token" is as defined in Appendix A of [MIME]. See Section 2.3 for a description of the "authres-id" element. The list of commands eligible for use with the "smtp" ptype can be found in [SMTP] and subsequent amendments. "CFWS" is as defined in section 3.2.3 of [MAIL]. The "ptype" and "property" values used by each authentication method should be defined in the specification for that method (or its amendments). The "ptype" and "property" are case-insensitive. A "ptype" value of "policy" indicates a policy decision about the message not specific to a property of the message that could be extracted. For example, if a method would normally report a "ptype.property" of "header.From" and no From: header field was present, the method can use "policy" to indicate that no conclusion about the authenticity of the message could be reached. 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. 2.3. Authentication Identifier Fields Every Authentication-Results header field MUST have an authentication identifier field ("authres-id" above) which is a single result identifier. This is similar in syntax to a fully-qualified domain name. The authentication identifier field provides a unique identifier that refers to the authenticating service within a given mail administrative domain. The uniqueness of the identifier is guaranteed by the mail administrative domain that generates it and must pertain to exactly that one mail administrative domain. This identifier is intended to be machine-readable and not necessarily meaningful to users. MUAs may use this identifier to determine whether or not the data contained in an Authentication-Results header field can be trusted. The mail administrative domain's unique domain name MUST be used as Kucherawy Expires January 27, 2008 [Page 8] Internet-Draft Authentication-Results Header July 2007 the last portion of the identifier. Examples of valid authentication identifiers are mail.example.org, engineering.example.edu and ms1.newyork.example.com. 2.4. Result Values 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.) hardfail: The message failed the authentication tests. (This may require accessing an authentication policy of some kind published by the sending domain.) softfail: The message failed the authentication tests, and 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. 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. A later retry is unlikely to yield a final result. New methods not specified in this document MUST indicate which of these should be returned when exceptions such as syntax errors are detected. 2.5. Definition Of Initial Methods As they are currently existing specifications for sender authentication, it is appropriate to define an authentication method identifier for each of [AUTH], [DKIM], [DOMAINKEYS], [SENDERID] and Kucherawy Expires January 27, 2008 [Page 9] Internet-Draft Authentication-Results Header July 2007 [SPF]. Therefore, the authentication method identifiers "auth", "dkim", "domainkeys", "senderid" and "spf" respectively are hereby defined for MTAs applying those specifications for e-mail sender authentication. See Section 6 for details. 2.6. 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. See Section 6 for further details. 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 field, such as: Authentication-Results: mx.example.com; foo=pass bar.baz=blob (2 of 3 tests OK) Kucherawy Expires January 27, 2008 [Page 10] Internet-Draft Authentication-Results Header July 2007 3. 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 MUST NOT be used to determine the importance or strength of any given method over another. Instead, the MUA must interpret the result of each method based on its knowledge of what that method evaluates. The "method" MUST refer to an authentication method declared in the IANA registry. See Section 6 for further information. An MTA compliant with this specification MUST add this header field (after performing one or more sender authentication tests) 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 field once per test, or one header field indicating all of the results. An MTA MUST NOT add a result to an existing header. For security reasons, a border MTA conforming to this specification MUST delete any discovered instance of this header field which claims to have been added within its trust boundary. For example, a border MTA for example.com receiving a message from outside of its mail domain MUST delete any instance of this header field bearing an authentication identifier indicating the header field was added within example.com prior to adding its own header fields. However, care must be taken not to remove header fields added on messages that remain entirely within the originator's trust boundary (e.g. local- to-local mail). A "border MTA" in this discussion is considered to be the first MTA which is listed as a mail exchanger (MX) for the recipient domain. An MTA MAY add this header field containing only the authentication identifier portion to indicate explicitly that no sender authentication schemes were applied prior to delivery of this message. 3.1. Header Position and Interpretation In order to ensure non-ambiguous results and avoid the impact of false header fields, an MUA SHOULD NOT interpret this header field unless specifically instructed to do so by the user. That is, this interpretation should not be "on by default". Naturally then, users would not activate such a feature unless they are certain the header field will be added by the receiving MTA that accepts the mail that Kucherawy Expires January 27, 2008 [Page 11] Internet-Draft Authentication-Results Header July 2007 is ultimately read by the MUA, and instances of the header field appearing to be from within the trust domain but actually added by foreign MTAs will be removed before delivery. Furthermore, an MUA SHOULD NOT interpret this header field unless the hostname it bears appears to be one within its own trust domain as configured by the user. An MUA should not reveal these results to naive end users unless the results are accompanied by, at a minimum, some associated reputation data about the sender that was authenticated. As stated in Section 2.1, this header field SHOULD be treated as though it were a trace header field as defined in section 3.6 of [MAIL], and hence MUST not be reordered and MUST be prepended to 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. Kucherawy Expires January 27, 2008 [Page 12] Internet-Draft Authentication-Results Header July 2007 4. Discussion This section discusses various implementation issues not specifically related to security. Security issues are discussed in a later section. 4.1. Removing The Header As specified in Section 3, instances of this header field added by outside MTAs that appear to come from inside an MTA's trust boundary must be removed. In the case of messages signed using [DKIM] or other message signing methods that sign headers, this may invalidate one or more signature on the message if they included the header field to be removed at the time of signing. This behaviour is desirable since there's no value in validating the signature on a message with forged headers. However, signing agents MAY elect to omit these header fields from signing to avoid this situation. Kucherawy Expires January 27, 2008 [Page 13] Internet-Draft Authentication-Results Header July 2007 5. 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 field corresponding to this specification after doing so and prior to delivery. MTAs that are relaying mail rather than delivering it, i.e. are not part of an intended recipient's trust boundary, MAY perform sender authentication or even take actions based on the results found, but SHOULD NOT add an "Authentication-Results" header field if relaying (rather than rejecting or discarding at the gateway). Conversely, an MTA doing local delivery and some form of sender authentication MUST add this header field prior to delivery the message in order to be compliant. A minimal implementation that does at least one sender authentication check will add the header field defined by this memo prior to invoking local delivery procedures. This specification places no restrictions on the processing of the header field's contents by user agents or distribution lists. It is presented to those packages solely for their own information. Kucherawy Expires January 27, 2008 [Page 14] Internet-Draft Authentication-Results Header July 2007 6. IANA Considerations This specification introduces some new namespaces that will be registered with IANA. In all cases, new entries are assigned only for values that have been documented in a published RFC that has IETF Consensus, per [IANA-CONSIDERATIONS]. 6.1. The Authentication-Results: header Per [IANA-HEADERS], the "Authentication-Results:" header field is added to the IANA Permanent Message Header Field Registry. The following is the registration template: Header field name: Authentication-Results Applicable protocol: mail ([RFC2822]) Status: Standard Author/Change controller: IETF Specification document(s): [TBD] Related information: Requesting review of any proposed changes and additions to this field is recommended. 6.2. Method Registry As stated above, names of sender authentication methods supported by this specification must be registered with IANA, with the exception of experimental names as described above. Each method must register a name, the specification that defines it, one or more "ptype" values appropriate for use with that method, and which "property" value(s) should be reported by that method. The initial set of entries in this registry is as follows: +------------+---------+--------+----------------------------------+ | Method | defined | ptype | property | +------------+---------+--------+----------------------------------+ | auth | RFC2554 | smtp | auth | +------------+---------+--------+----------------------------------+ | dkim | RFC4871 | header | value of signature "i" tag | +------------+---------+--------+----------------------------------+ | domainkeys | RFC4870 | header | From or Sender | +------------+---------+--------+----------------------------------+ | senderid | RFC4406 | header | name of header field used by PRA | | | | smtp | from (envelope sender) | +------------+---------+--------+----------------------------------+ | spf | RFC4408 | smtp | from | +------------+---------+--------+----------------------------------+ Kucherawy Expires January 27, 2008 [Page 15] Internet-Draft Authentication-Results Header July 2007 7. Security Considerations The following security considerations apply when applying or processing the "Authentication-Results" header field: 7.1. Non-conformant MTAs An MUA that accesses a mailbox whose mail is handled by a non- conformant MTA, and understands Authentication-Results header fields, could potentially make false conclusions based on forged header fields. A malicious user or agent could forge a header field 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 field, and the MUA could inappropriately trust it. It is for this reason an MUA SHOULD NOT have processing of the "Authentication-Results" header field enabled by default; instead it SHOULD 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" header fields 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 field that can be verified by a posted public key. Another would be a means to interrogate the MTA that added the header field to see if it is actually providing any sender authentication services and saw the message in question, but this isn't especially palatable. In either case, a method needs to exist to verify that the host that appears to have added the header field (a) actually did so, and (b) is legitimately adding that header field for this delivery. 7.2. Header Position Despite the requirements of [MAIL], header fields can sometimes be reordered enroute by intermediate MTAs. The goal of requiring header field addition only at the top of a message is an acknowledgement that some MTAs do reorder header fields, 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 field defined here. Kucherawy Expires January 27, 2008 [Page 16] Internet-Draft Authentication-Results Header July 2007 8. References 8.1. Normative References [MAIL] Resnick, P., "Internet Message Format", RFC 2822, April 2001. [MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. 8.2. Informative References [AUTH] Myers, J., "SMTP Service Extension for Authentication", RFC 2554, March 1999. [DKIM] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures", RFC 4817, May 2007. [DOMAINKEYS] Delany, M., "Domain-based Email Authentication Using Public Keys Advertised in the DNS (DomainKeys)", RFC 4870, May 2007. [I-D.DRAFT-CROCKER-EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", I-D draft-crocker-email-arch, May 2007. [IANA-CONSIDERATIONS] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 2434, October 1998. [IANA-HEADERS] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", RFC 2434, September 2004. [SENDERID] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail", RFC 4406, April 2006. [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001. [SPF] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", Kucherawy Expires January 27, 2008 [Page 17] Internet-Draft Authentication-Results Header July 2007 RFC 4408, April 2006. Kucherawy Expires January 27, 2008 [Page 18] Internet-Draft Authentication-Results Header July 2007 Appendix A. Acknowledgements The author wishes to acknowledge the following for their review and constructive criticism of this proposal: Eric Allman, Mark Delany, Jim Fenton, Tony Hansen, Paul Hoffman, John Levine, Miles Libbey, S. Moonesamy, Michael Thomas. Kucherawy Expires January 27, 2008 [Page 19] Internet-Draft Authentication-Results Header July 2007 Appendix B. Legacy MUAs Implementors of this proposal should be aware that many MUAs are unlikely to be retrofit to support the new header field 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 in addition to the Authentication-Results header field. One suggestion is to include a Priority: header field, on messages that don't already have such a header field, containing 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 or strong authentication. Certainly some modern MUAs can filter based on the content of this header field, but as there is keen interest in having MUAs make some kind of graphical representation of this header field's meaning, other interim means of doing so may be necessary while this proposal is adopted. Kucherawy Expires January 27, 2008 [Page 20] Internet-Draft Authentication-Results Header July 2007 Appendix C. Authentication-Results Examples This section presents some examples of the use of this header field to indicate authentication results. C.1. Trivial case; header field 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 field 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 January 27, 2008 [Page 21] Internet-Draft Authentication-Results Header July 2007 C.2. Nearly-trivial case; service provided, but no authentication done A message that was delivered by an MTA that 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 field is present, indicating that the delivering MTA (which is named in the value of the header field) conforms to this specification. The absence of any method and result tokens indicates that no sender authentication was done. C.3. Service provided, authentication done A message that was delivered by an MTA that conforms to this standard and applied some sender authentication: Authentication-Results: mail-router.example.com; auth=pass (cram-md5) smtp.auth=sender@example.com 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 January 27, 2008 [Page 22] Internet-Draft Authentication-Results Header July 2007 The "Authentication-Results" header field is present, indicating that the delivering MTA (which is identified in the value of the header field) 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 field 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 that conforms to this specification and applied three different sender authentication checks: Authentication-Results: mail-router.example.com; auth=pass (cram-md5) smtp.mail=sender@example.com; spf=pass smtp.mail=sender@example.com Authentication-Results: mail-router.example.com; sender-id=pass header.from=sender@example.com 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 field is present, indicating the delivering MTA (which is identified in the value of the header field) conforms to this specification. Furthermore, the sender authenticated herself/himself to the MTA via a method specified in [AUTH], and both [SPF] and [SENDERID] checks were done and passed. The MUA could extract and relay this extra information if desired. Two "Authentication-Results" header fields are not required since the same host did all of the checking. The authenticating agent could have consolidated all the results into one header field. Kucherawy Expires January 27, 2008 [Page 23] Internet-Draft Authentication-Results Header July 2007 C.5. Service provided, several authentications done, different MTAs A message that was relayed inbound by two different MTAs that conform to this specification and applied multiple sender authentication checks: Authentication-Results: auth-checker.example.com; sender-id=pass header.from=sender@example.com; dkim=pass (good signature) header.i=sender@example.com 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; auth=pass (cram-md5) smtp.mail=sender@example.com; spf=fail smtp.mail=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 DKIM-Signature: a=rsa-sha1; s=gatsby; d=example.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 field 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, identifying itself as "auth-checker.example.com", reports that it did a [SENDERID] test and a [DKIM] test, both of which passed. Again, additional data about one of the tests is provided as a comment, which the MUA may choose to render. Kucherawy Expires January 27, 2008 [Page 24] Internet-Draft Authentication-Results Header July 2007 Since different hosts did the two sets of authentication checks, the header fields cannot be consolidated in this example. Kucherawy Expires January 27, 2008 [Page 25] Internet-Draft Authentication-Results Header July 2007 Appendix D. Public Discussion [REMOVE BEFORE PUBLICATION] 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 January 27, 2008 [Page 26] Internet-Draft Authentication-Results Header July 2007 Author's Address Murray S. Kucherawy Sendmail, Inc. 6475 Christie Ave., Suite 350 Emeryville, CA 94608 US Phone: +1 510 594 5400 Email: msk+ietf@sendmail.com Kucherawy Expires January 27, 2008 [Page 27] Internet-Draft Authentication-Results Header July 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. 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. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Kucherawy Expires January 27, 2008 [Page 28]