8. References 8.1. Normative References [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <https://www.rfc-editor.org/info/rfc5234>. [IANA-HEADERS] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, September 2004, <https://www.rfc-editor.org/info/rfc3864>. [MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008, <https://www.rfc-editor.org/info/rfc5322>. [MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, <https://www.rfc-editor.org/info/rfc2045>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", RFC 6530, DOI 10.17487/RFC6530, February 2012, <https://www.rfc-editor.org/info/rfc6530>. [RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized Email", RFC 6531, DOI 10.17487/RFC6531, February 2012, <https://www.rfc-editor.org/info/rfc6531>. [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized Email Headers", RFC 6532, DOI 10.17487/RFC6532, February 2012, <https://www.rfc-editor.org/info/rfc6532>. [RFC7601] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 7601, DOI 10.17487/RFC7601, August 2015, <https://www.rfc-editor.org/info/rfc7601>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008, <https://www.rfc-editor.org/info/rfc5321>. 8.2. Informative References [ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine, "DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617, August 2009, <https://www.rfc-editor.org/info/rfc5617>. [AR-VBR] Kucherawy, M., "Authentication-Results Registration for Vouch by Reference Results", RFC 6212, DOI 10.17487/RFC6212, April 2011, <https://www.rfc-editor.org/info/rfc6212>. [ATPS] Kucherawy, M., "DomainKeys Identified Mail (DKIM) Authorized Third-Party Signatures", RFC 6541, DOI 10.17487/RFC6541, February 2012, <https://www.rfc-editor.org/info/rfc6541>.
[AUTH] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service Extension for Authentication", RFC 4954, DOI 10.17487/RFC4954, July 2007, <https://www.rfc-editor.org/info/rfc4954>. [AUTH-ESC] Kucherawy, M., "Email Authentication Status Codes", RFC 7372, DOI 10.17487/RFC7372, September 2014, <https://www.rfc-editor.org/info/rfc7372>. [DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011, <https://www.rfc-editor.org/info/rfc6376>. [DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, <https://www.rfc-editor.org/info/rfc7489>. [DNS] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, <https://www.rfc-editor.org/info/rfc1035>. [DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, "DNS Extensions to Support IP Version 6", STD 88, RFC 3596, DOI 10.17487/RFC3596, October 2003, <https://www.rfc-editor.org/info/rfc3596>. [DNSOP-REVERSE] Senie, D. and A. Sullivan, "Considerations for the use of DNS Reverse Mapping", Work in Progress, draft-ietf-dnsop-reverse-mapping-considerations-06, March 2008. [DOMAINKEYS] Delany, M., "Domain-Based Email Authentication Using Public Keys Advertised in the DNS (DomainKeys)", RFC 4870, DOI 10.17487/RFC4870, May 2007, <https://www.rfc-editor.org/info/rfc4870>. [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format for Delivery Status Notifications", RFC 3464, DOI 10.17487/RFC3464, January 2003, <https://www.rfc-editor.org/info/rfc3464>.
[EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598, DOI 10.17487/RFC5598, July 2009, <https://www.rfc-editor.org/info/rfc5598>. [IANA-CONSIDERATIONS] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>. [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, <https://www.rfc-editor.org/info/rfc3501>. [POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, <https://www.rfc-editor.org/info/rfc1939>. [RFC5451] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 5451, DOI 10.17487/RFC5451, April 2009, <https://www.rfc-editor.org/info/rfc5451>. [RFC6008] Kucherawy, M., "Authentication-Results Registration for Differentiating among Cryptographic Results", RFC 6008, DOI 10.17487/RFC6008, September 2010, <https://www.rfc-editor.org/info/rfc6008>. [RFC6577] Kucherawy, M., "Authentication-Results Registration Update for Sender Policy Framework (SPF) Results", RFC 6577, DOI 10.17487/RFC6577, March 2012, <https://www.rfc-editor.org/info/rfc6577>. [RFC7001] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 7001, DOI 10.17487/RFC7001, September 2013, <https://www.rfc-editor.org/info/rfc7001>. [RFC7410] Kucherawy, M., "A Property Types Registry for the Authentication-Results Header Field", RFC 7410, DOI 10.17487/RFC7410, December 2014, <https://www.rfc-editor.org/info/rfc7410>. [RFC8301] Kitterman, S., "Cryptographic Algorithm and Key Usage Update to DomainKeys Identified Mail (DKIM)", RFC 8301, DOI 10.17487/RFC8301, January 2018, <https://www.rfc-editor.org/info/rfc8301>.
[RRVS] Mills, W. and M. Kucherawy, "The Require-Recipient-Valid- Since Header Field and SMTP Service Extension", RFC 7293, DOI 10.17487/RFC7293, July 2014, <https://www.rfc-editor.org/info/rfc7293>. [SECURITY] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, DOI 10.17487/RFC3552, July 2003, <https://www.rfc-editor.org/info/rfc3552>. [SENDERID] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail", RFC 4406, DOI 10.17487/RFC4406, April 2006, <https://www.rfc-editor.org/info/rfc4406>. [SMIME-REG] Melnikov, A., "Authentication-Results Registration for S/MIME Signature Verification", RFC 7281, DOI 10.17487/RFC7281, June 2014, <https://www.rfc-editor.org/info/rfc7281>. [SPF] Kitterman, S., "Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1", RFC 7208, DOI 10.17487/RFC7208, April 2014, <https://www.rfc-editor.org/info/rfc7208>. [VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009, <https://www.rfc-editor.org/info/rfc5518>.
Appendix A. Legacy MUAs Implementers of this specification should be aware that many MUAs are unlikely to be retrofitted to support the Authentication-Results header field and its semantics. In the interests of convenience and quicker adoption, 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. Some modern MUAs can already filter based on the content of this header field. However, there is keen interest in having MUAs make some kind of graphical representation of this header field's meaning to end users. Until this capability is added (i.e., while this specification and its successors continue to be adopted), other interim means of conveying authentication results may be necessary. Appendix B. Authentication-Results Examples This section presents some examples of the use of this header field to indicate authentication results. B.1. Trivial Case: Header Field Not Present The trivial case: Received: from mail-router.example.com (mail-router.example.com [192.0.2.1]) by server.example.org (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 From: email@example.com Date: Fri, Feb 15 2002 16:54:30 -0800 To: firstname.lastname@example.org Message-Id: <email@example.com> Subject: here's a sample Hello! Goodbye! Example 1: Header Field Not Present
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 (1) the message authentication services were not available at the time of delivery, (2) no service is provided, or (3) the MTA is not in compliance with this specification. B.2. Nearly Trivial Case: Service Provided, but No Authentication Done A message that was delivered by an MTA that conforms to this specification but provides no actual message authentication service: Authentication-Results: example.org 1; none Received: from mail-router.example.com (mail-router.example.com [192.0.2.1]) by server.example.org (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 From: firstname.lastname@example.org Date: Fri, Feb 15 2002 16:54:30 -0800 To: email@example.com Message-Id: <firstname.lastname@example.org> Subject: here's a sample Hello! Goodbye! Example 2: Header Present but No Authentication Done The Authentication-Results header field is present, showing that the delivering MTA conforms to this specification. It used its DNS domain name as the authserv-id. The presence of "none" (and the absence of any method or result tokens) indicates that no message authentication was done. The version number of the specification to which the field's content conforms is explicitly provided.
B.3. Service Provided, Authentication Done A message that was delivered by an MTA that conforms to this specification and applied some message authentication: Authentication-Results: example.com; spf=pass smtp.mailfrom=example.net Received: from dialup-1-2-3-4.example.net (dialup-1-2-3-4.example.net [192.0.2.200]) by mail-router.example.com (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 From: email@example.com Date: Fri, Feb 15 2002 16:54:30 -0800 To: firstname.lastname@example.org Message-Id: <email@example.com> Subject: here's a sample Hello! Goodbye! Example 3: Header Reporting Results The Authentication-Results header field is present, indicating that the border MTA conforms to this specification. The authserv-id is once again the DNS domain name. Furthermore, the message was authenticated by that MTA via the method specified in [SPF]. Note that since that method cannot authenticate the local-part, it has been omitted from the result's value. The MUA could extract and relay this extra information if desired.
B.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 message authentication checks: Authentication-Results: example.com; auth=pass (cram-md5) firstname.lastname@example.org; spf=pass smtp.mailfrom=example.net Authentication-Results: example.com; iprev=pass policy.iprev=192.0.2.200 Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6) (dialup-1-2-3-4.example.net [192.0.2.200]) by mail-router.example.com (8.11.6/8.11.6) with ESMTPA id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 Date: Fri, Feb 15 2002 16:54:30 -0800 To: email@example.com From: firstname.lastname@example.org Message-Id: <email@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 that the delivering MTA conforms to this specification. Once again, the receiving DNS domain name is used as the authserv-id. Furthermore, the sender authenticated themselves to the MTA via a method specified in [AUTH], and both SPF and "iprev" 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. This example illustrates a scenario in which a remote user on a dial-up connection (example.net) sends mail to a border MTA (example.com) using SMTP authentication to prove identity. The dial-up provider has been explicitly authorized to relay mail as example.net, producing a "pass" result from the SPF check.
B.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 message authentication checks: Authentication-Results: example.com; dkim=pass (good signature) header.d=example.com Received: from mail-router.example.com (mail-router.example.com [192.0.2.1]) by auth-checker.example.com (8.11.6/8.11.6) with ESMTP id i7PK0sH7021929; Fri, Feb 15 2002 17:19:22 -0800 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=example.com; t=1188964191; c=simple/simple; h=From:Date:To:Subject: Message-Id:Authentication-Results; bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70; b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= Authentication-Results: example.com; auth=pass (cram-md5) firstname.lastname@example.org; spf=fail smtp.mailfrom=example.com Received: from dialup-1-2-3-4.example.net (dialup-1-2-3-4.example.net [192.0.2.200]) by mail-router.example.com (8.11.6/8.11.6) with ESMTPA id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 From: email@example.com Date: Fri, Feb 15 2002 16:54:30 -0800 To: firstname.lastname@example.org Message-Id: <email@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. Once again, the authserv-id used is the recipient's DNS domain name. The header field is present twice because two different MTAs in the chain of delivery did authentication tests. The first MTA, mail-router.example.com, reports that SMTP AUTH and SPF were both used and that the former passed while the latter failed. In the SMTP AUTH case, additional information 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 DKIM test (which passed). Again, additional data about one of the tests are provided as a comment, which the MUA may choose to render. Also noteworthy here is the fact that there is a DKIM signature added by example.com that assured the integrity of the lower Authentication- Results field. Since different hosts did the two sets of authentication checks, the header fields cannot be consolidated in this example. This example illustrates more typical transmission of a message into example.com from a user on a dial-up connection example.net. The user appears to be legitimate, as they had a valid password allowing authentication at the border MTA using SMTP AUTH. The SPF test failed, since example.com has not granted example.net's dial-up network authority to relay mail on its behalf. The DKIM test passed because the sending user had a private key matching one of example.com's published public keys and mail-router.example.com used it to sign the message.
B.6. Service Provided, Multi-tiered Authentication Done A message that had authentication done at various stages, one of which was outside the receiving ADMD: Authentication-Results: example.com; dkim=pass reason="good signature" firstname.lastname@example.org; dkim=fail reason="bad signature" email@example.com Received: from mail-router.example.net (mail-router.example.net [192.0.2.250]) by chicago.example.com (8.11.6/8.11.6) for <firstname.lastname@example.org> with ESMTP id i7PK0sH7021929; Fri, Feb 15 2002 17:19:22 -0800 DKIM-Signature: v=1; a=rsa-sha256; s=furble; d=mail-router.example.net; t=1188964198; c=relaxed/simple; h=From:Date:To:Message-Id:Subject:Authentication-Results; bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=; b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3= Authentication-Results: example.net; dkim=pass (good signature) email@example.com Received: from smtp.newyork.example.com (smtp.newyork.example.com [192.0.2.220]) by mail-router.example.net (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=newyork.example.com; t=1188964191; c=simple/simple; h=From:Date:To:Message-Id:Subject; bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=; b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= From: firstname.lastname@example.org Date: Fri, Feb 15 2002 16:54:30 -0800 To: email@example.com Message-Id: <firstname.lastname@example.org> Subject: here's a sample Example 6: Headers Reporting Results from Multiple MTAs in Different ADMDs In this example, we see multi-tiered authentication with an extended trust boundary.
The message was sent from someone at example.com's New York office (newyork.example.com) to a mailing list managed at an intermediary. The message was signed at the origin using DKIM. The message was sent to a mailing list service provider called "example.net", which is used by example.com. There, email@example.com is expanded to a long list of recipients, one of whom is at the Chicago office. In this example, we will assume that the trust boundary for chicago.example.com includes the mailing list server at example.net. The mailing list server there first authenticated the message and affixed an Authentication-Results header field indicating such using its DNS domain name for the authserv-id. It then altered the message by affixing some footer text to the body, including some administrivia such as unsubscription instructions. Finally, the mailing list server affixes a second DKIM signature and begins distribution of the message. The border MTA for chicago.example.com explicitly trusts results from mail-router.example.net, so that header field is not removed. It performs evaluation of both signatures and determines that the first (most recent) is a "pass" but, because of the aforementioned modifications, the second is a "fail". However, the first signature included the Authentication-Results header added at mail-router.example.net that validated the second signature. Thus, indirectly, it can be determined that the authentications claimed by both signatures are indeed valid. Note that two styles of presenting metadata about the result are in use here. In one case, the "reason=" clause is present, which is intended for easy extraction by parsers; in the other case, the CFWS production of the ABNF is used to include such data as a header field comment. The latter can be harder for parsers to extract given the varied supported syntaxes of mail header fields. B.7. Comment-Heavy Example The formal syntax permits comments within the content in a number of places. For the sake of illustration, this example is also legal: Authentication-Results: foo.example.net (foobar) 1 (baz); dkim (Because I like it) / 1 (One yay) = (wait for it) fail policy (A dot can go here) . (like that) expired (this surprised me) = (as I wasn't expecting it) 1362471462 Example 7: A Very Comment-Heavy but Perfectly Legal Example
Appendix C. Operational Considerations about Message Authentication Implementation of the Authentication-Results header field is predicated on the idea that authentication (and presumably in the future, reputation) work is typically done by border MTAs rather than MUAs or intermediate MTAs; the latter merely make use of the results determined by the former. Certainly this is not mandatory for participation in electronic mail or message authentication, but this header field and its deployment to date are based on that model. The assumption satisfies several common ADMD requirements: 1. Service operators prefer to resolve the handling of problem messages as close to the border of the ADMD as possible. This enables, for example, rejection of messages at the SMTP level rather than generating a DSN internally. Thus, doing any of the authentication or reputation work exclusively at the MUA or intermediate MTA renders this desire unattainable. 2. Border MTAs are more likely to have direct access to external sources of authentication or reputation information, since modern MUAs inside of an ADMD are more likely to be heavily firewalled. Thus, some MUAs might not even be able to complete the task of performing authentication or reputation evaluations without complex proxy configurations or similar burdens. 3. MUAs rely upon the upstream MTAs within their trust boundaries to make correct (as much as is possible) evaluations about the message's envelope, header, and content. Thus, MUAs don't need to know how to do the work that upstream MTAs do; they only need the results of that work. 4. Evaluations about the quality of a message, from simple token matching (e.g., a list of preferred DNS domains) to cryptographic verification (e.g., public/private key work), do have a cost and thus need to be minimized. To that end, performing those tests at the border MTA is far preferred to doing that work at each MUA that handles a message. If an ADMD's environment adheres to common messaging protocols, a reputation query or an authentication check performed by a border MTA would return the same result as the same query performed by an MUA. By contrast, in an environment where the MUA does the work, a message arriving for multiple recipients would thus cause authentication or reputation evaluation to be done more than once for the same message (i.e., at each MUA), causing needless amplification of resource use and creating a possible denial-of-service attack vector.
5. Minimizing change is good. As new authentication and reputation methods emerge, the list of methods supported by this header field would presumably be extended. If MUAs simply consume the contents of this header field rather than actually attempt to do authentication and/or reputation work, then MUAs only need to learn to parse this header field once; emergence of new methods requires only a configuration change at the MUAs and software changes at the MTAs (which are presumably fewer in number). When choosing to implement these functions in MTAs vs. MUAs, the issues of individual flexibility, infrastructure inertia, and scale of effort must be considered. It is typically easier to change a single MUA than an MTA because the modification affects fewer users and can be pursued with less care. However, changing many MUAs is more effort than changing a smaller number of MTAs. 6. For decisions affecting message delivery and display, assessment based on authentication and reputation is best performed close to the time of message transit, as a message makes its journey toward a user's inbox, not afterwards. DKIM keys, IP address reputations, etc., can change over time or even become invalid, and users can take a long time to read a message once delivered. The value of this work thus degrades, perhaps quickly, once the delivery process has completed. This seriously diminishes the value of this work when done elsewhere than at MTAs. Many operational choices are possible within an ADMD, including the venue for performing authentication and/or reputation assessment. The current specification does not dictate any of those choices. Rather, it facilitates those cases in which information produced by one stage of analysis needs to be transported with the message to the next stage. Appendix D. Changes since RFC 7601 o Added IANA registration for DKIM "a" and "s" properties. o Included EAI guidance. o Adjusted some ABNF tokens and names for easier inclusion by other documents. o Made minor editorial adjustments. o Deprecated entries from RFCs that are now Historic. o Erratum 4671 resolved. o Erratum 5435 resolved.
Acknowledgments The author wishes to acknowledge the following individuals for their review and constructive criticism of this document: Kurt Andersen, Seth Blank, Tim Draegen, Scott Kitterman, John Levine, and Alessandro Vesely. Author's Address Murray S. Kucherawy 270 Upland Drive San Francisco, CA 94127 United States of America Email: firstname.lastname@example.org