--- 1/draft-ietf-dmarc-rfc7601bis-04.txt 2019-01-21 00:13:09.887668546 -0800 +++ 2/draft-ietf-dmarc-rfc7601bis-05.txt 2019-01-21 00:13:09.991671075 -0800 @@ -1,136 +1,143 @@ -DMARC Working Group M. Kucherawy -Internet-Draft November 14, 2018 -Updates: 7601 (if approved) +Individual submission M. Kucherawy +Internet-Draft January 20, 2019 +Obsoletes: 7601 (if approved) Intended status: Standards Track -Expires: May 18, 2019 +Expires: July 24, 2019 Message Header Field for Indicating Message Authentication Status - draft-ietf-dmarc-rfc7601bis-04 + draft-ietf-dmarc-rfc7601bis-05 Abstract This document specifies a message header field called Authentication- Results for use with electronic mail messages to indicate the results of message authentication efforts. Any receiver-side software, such as mail filters or Mail User Agents (MUAs), can use this header field to relay that information in a convenient and meaningful way to users or to make sorting and filtering decisions. + This document obsoletes [RFC7601]. + Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." - This Internet-Draft will expire on May 18, 2019. + This Internet-Draft will expire on July 24, 2019. Copyright Notice - Copyright (c) 2018 IETF Trust and the persons identified as the + Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 6 1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 6 - 1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6 + 1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7 1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7 1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7 1.5.2. Internationalized Email . . . . . . . . . . . . . . . 7 1.5.3. Security . . . . . . . . . . . . . . . . . . . . . . . 7 1.5.4. Email Architecture . . . . . . . . . . . . . . . . . . 8 1.5.5. Other Terms . . . . . . . . . . . . . . . . . . . . . 9 1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 9 - 2. Definition and Format of the Header Field . . . . . . . . . . 9 - 2.1. General Description . . . . . . . . . . . . . . . . . . . 9 + 2. Definition and Format of the Header Field . . . . . . . . . . 10 + 2.1. General Description . . . . . . . . . . . . . . . . . . . 10 2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 10 2.3. Property Types (ptypes) and Properties . . . . . . . . . . 13 2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . . 14 - 2.5. Authentication Identifier Field . . . . . . . . . . . . . 14 + 2.5. Authentication Identifier Field . . . . . . . . . . . . . 15 2.6. Version Tokens . . . . . . . . . . . . . . . . . . . . . . 16 2.7. Defined Methods and Result Values . . . . . . . . . . . . 16 - 2.7.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 16 + 2.7.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 17 2.7.2. SPF and Sender ID . . . . . . . . . . . . . . . . . . 18 - 2.7.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 19 - 2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 20 - 2.7.5. Other Registered Codes . . . . . . . . . . . . . . . . 21 - 2.7.6. Extension Methods . . . . . . . . . . . . . . . . . . 21 - 2.7.7. Extension Result Codes . . . . . . . . . . . . . . . . 22 + 2.7.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 20 + 2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 21 + 2.7.5. Other Registered Codes . . . . . . . . . . . . . . . . 22 + 2.7.6. Extension Methods . . . . . . . . . . . . . . . . . . 22 + 2.7.7. Extension Result Codes . . . . . . . . . . . . . . . . 23 3. The "iprev" Authentication Method . . . . . . . . . . . . . . 23 4. Adding the Header Field to a Message . . . . . . . . . . . . . 24 - 4.1. Header Field Position and Interpretation . . . . . . . . . 25 - 4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 26 + 4.1. Header Field Position and Interpretation . . . . . . . . . 26 + 4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 27 5. Removing Existing Header Fields . . . . . . . . . . . . . . . 27 - 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 - 6.1. The Authentication-Results Header Field . . . . . . . . . 28 - 6.2. "Email Authentication Methods" Registry Description . . . 28 - 6.3. "Email Authentication Methods" Registry Update . . . . . . 28 - 6.3.1. 'header.a' for DKIM . . . . . . . . . . . . . . . . . 29 - 6.3.2. 'header.s' for DKIM . . . . . . . . . . . . . . . . . 29 - 6.4. "Email Authentication Property Types" Registry . . . . . . 29 - 6.5. "Email Authentication Result Names" Description . . . . . 29 - 6.6. "Email Authentication Result Names" Update . . . . . . . . 30 - 7. Security Considerations . . . . . . . . . . . . . . . . . . . 30 - 7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 30 - 7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 32 - 7.3. Header Field Position . . . . . . . . . . . . . . . . . . 32 - 7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 32 - 7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 32 - 7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 32 - 7.7. Attacks against Authentication Methods . . . . . . . . . . 33 - 7.8. Intentionally Malformed Header Fields . . . . . . . . . . 33 - 7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 33 - 7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 33 - 7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 34 - 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34 - 8.1. Normative References . . . . . . . . . . . . . . . . . . . 34 - 8.2. Informative References . . . . . . . . . . . . . . . . . . 36 - Appendix A. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 38 - Appendix B. Authentication-Results Examples . . . . . . . . . . . 38 - B.1. Trivial Case; Header Field Not Present . . . . . . . . . . 39 + 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 + 6.1. The Authentication-Results Header Field . . . . . . . . . 29 + 6.2. "Email Authentication Methods" Registry Description . . . 29 + 6.3. "Email Authentication Methods" Registry Update . . . . . . 30 + 6.3.1. 'header.a' for DKIM . . . . . . . . . . . . . . . . . 31 + 6.3.2. 'header.s' for DKIM . . . . . . . . . . . . . . . . . 32 + 6.4. "Email Authentication Property Types" Registry + Description . . . . . . . . . . . . . . . . . . . . . . . 32 + + 6.5. "Email Authentication Property Types" Registry Update . . 33 + 6.6. "Email Authentication Result Names" Registry + Description . . . . . . . . . . . . . . . . . . . . . . . 33 + 6.7. "Email Authentication Result Names" Registry Update . . . 33 + 6.8. SMTP Enhanced Status Codes . . . . . . . . . . . . . . . . 34 + 7. Security Considerations . . . . . . . . . . . . . . . . . . . 34 + 7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 34 + 7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 36 + 7.3. Header Field Position . . . . . . . . . . . . . . . . . . 36 + 7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 36 + 7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 37 + 7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 37 + 7.7. Attacks against Authentication Methods . . . . . . . . . . 37 + 7.8. Intentionally Malformed Header Fields . . . . . . . . . . 37 + 7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 37 + 7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 38 + 7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 38 + 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38 + 8.1. Normative References . . . . . . . . . . . . . . . . . . . 38 + 8.2. Informative References . . . . . . . . . . . . . . . . . . 39 + Appendix A. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 43 + Appendix B. Authentication-Results Examples . . . . . . . . . . . 43 + B.1. Trivial Case; Header Field Not Present . . . . . . . . . . 43 B.2. Nearly Trivial Case; Service Provided, but No - Authentication Done . . . . . . . . . . . . . . . . . . . 39 - B.3. Service Provided, Authentication Done . . . . . . . . . . 40 + Authentication Done . . . . . . . . . . . . . . . . . . . 44 + B.3. Service Provided, Authentication Done . . . . . . . . . . 45 B.4. Service Provided, Several Authentications Done, Single - MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 + MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 B.5. Service Provided, Several Authentications Done, - Different MTAs . . . . . . . . . . . . . . . . . . . . . . 42 - B.6. Service Provided, Multi-tiered Authentication Done . . . . 44 - B.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 45 + Different MTAs . . . . . . . . . . . . . . . . . . . . . . 47 + B.6. Service Provided, Multi-tiered Authentication Done . . . . 49 + B.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 50 Appendix C. Operational Considerations about Message - Authentication . . . . . . . . . . . . . . . . . . . 46 - Appendix D. Changes Since RFC7601 . . . . . . . . . . . . . . . . 47 - Appendix E. Acknowledgments . . . . . . . . . . . . . . . . . . . 47 - Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 48 + Authentication . . . . . . . . . . . . . . . . . . . 51 + Appendix D. Changes since RFC7601 . . . . . . . . . . . . . . . . 52 + Appendix E. Acknowledgments . . . . . . . . . . . . . . . . . . . 53 + Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 53 1. Introduction This document describes a header field called Authentication-Results for electronic mail messages that presents the results of a message authentication effort in a machine-readable format. The intent of the header field is to create a place to collect such data when message authentication mechanisms are in use so that a Mail User Agent (MUA) and downstream filters can make filtering decisions and/or provide a recommendation to the user as to the validity of the @@ -208,22 +215,22 @@ 2. Provide a common location within a message for this data; 3. Create an extensible framework for reporting new authentication methods as they emerge. In particular, the mere presence of this header field does not mean its contents are valid. Rather, the header field is reporting assertions made by one or more authentication schemes applied somewhere upstream. For an MUA or downstream filter to treat the assertions as actually valid, there must be an assessment of the - trust relationship among such agents, the validating MTA, and the - mechanism for conveying the information. + trust relationship among such agents, the validating MTA, the paths + between them, and the mechanism for conveying the information. 1.2. Trust Boundary This document makes several references to the "trust boundary" of an administrative management domain (ADMD). Given the diversity among existing mail environments, a precise definition of this term isn't possible. Simply put, a transfer from the producer of the header field to the consumer must occur within a context that permits the consumer to @@ -243,48 +250,60 @@ its data center. It is possible for a message to be evaluated inside a trust boundary but then depart and re-enter the trust boundary. An example might be a forwarded message such as a message/rfc822 attachment (see Multipurpose Internet Mail Extensions [MIME]) or one that is part of a multipart/digest. The details reported by this field cannot be trusted in that case. Thus, this field found within one of those media types is typically ignored. + Note that an MUA could be configured to retrieve messages from a + Receiver yet not be within the Receiver's ADMD. In this case, for + the purposes of this work, that MUA is considered to be within the + Receiver's ADMD if is configured to identify and ascribe value to + authentication results recorded by that ADMD. + 1.3. Processing Scope The content of this header field is meant to convey to message consumers that authentication work on the message was already done within its trust boundary, and those results are being presented. It is not intended to provide message parameters to consumers so that they can perform authentication protocols on their own. 1.4. Requirements - This document establishes no new requirements on existing protocols - or servers. + This document establishes no new requirements on existing protocols, + insofar as a non-participating service will continue to interoperate + with the deployed messaging infrastructure. In particular, this document establishes no requirement on MTAs to reject or filter arriving messages that do not pass authentication checks. The data conveyed by the specified header field's contents are for the information of MUAs and filters and are to be used at their discretion. + A participating ADMD does undertake some filtering and message + modification obligations described in Section 5. + 1.5. Definitions This section defines various terms used throughout this document. 1.5.1. Key Words 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 [KEYWORDS]. + document are to be interpreted as described in BCP 14 [RFC2119] + [RFC8174] when, and only when, they appear in all capitals, as shown + here. 1.5.2. Internationalized Email In this document, there are references to messages formatted to support Email Address Internationalization (EAI). Reference material for this can be found in [RFC6530], [RFC6531], and [RFC6532]. Generally speaking, these documents allow UTF-8 in most places that free-form text can be found and U-labels where domain names can be used, and this document extends Authentication-Results accordingly. @@ -300,32 +319,32 @@ o "Authorization" is the establishment of permission to use a resource or represent an identity. In this context, authorization indicates that a message from a particular ADMD arrived via a route the ADMD has explicitly approved. o "Authentication" is the assertion of validity of a piece of data about a message (such as the sender's identity) or the message in its entirety. As examples: SPF and Sender ID are authorization mechanisms in that - they express a result that shows whether or not the ADMD that - apparently sent the message has explicitly authorized the connecting - Simple Mail Transfer Protocol ([SMTP]) client to relay messages on - its behalf, but they do not actually validate any other property of - the message itself. By contrast, DKIM is agnostic as to the routing - of a message but uses cryptographic signatures to authenticate - agents, assign (some) responsibility for the message (which implies - authorization), and ensure that the listed portions of the message - were not modified in transit. Since the signatures are not tied to - SMTP connections, they can be added by either the ADMD of origin, - intermediate ADMDs (such as a mailing list server), other handling - agents, or any combination. + they express a result that shows whether the ADMD that apparently + sent the message has explicitly authorized the connecting Simple Mail + Transfer Protocol ([SMTP]) client to relay messages on its behalf, + but they do not actually validate any other property of the message + itself. By contrast, DKIM is agnostic as to the routing of a message + but uses cryptographic signatures to authenticate agents, assign + (some) responsibility for the message (which implies authorization), + and ensure that the listed portions of the message were not modified + in transit. Since the signatures are not tied to SMTP connections, + they can be added by either the ADMD of origin, intermediate ADMDs + (such as a mailing list server), other handling agents, or any + combination. Rather than create a separate header field for each class of solution, this specification groups them both into a single header field. 1.5.4. Email Architecture o A "border MTA" is an MTA that acts as a gateway between the general Internet and the users within an organizational boundary. (See also Section 1.2.) @@ -426,22 +444,23 @@ supporting data can include a "reason" string and one or more "property=value" statements indicating which message properties were evaluated to reach that conclusion. The header field can appear more than once in a single message, more than one result can be represented in a single header field, or a combination of these can be applied. 2.2. Formal Definition - Formally, the header field is specified as follows using Augmented - Backus-Naur Form ([ABNF]): + Formally, the header field is specified as shown below using + Augmented Backus-Naur Form ([ABNF]). Examples of valid header fields + with explanations of their semantics can be found in Appendix B. authres-header-field = "Authentication-Results:" authres-payload authres-payload = [CFWS] authserv-id [ CFWS authres-version ] ( no-result / 1*resinfo ) [CFWS] CRLF authserv-id = value ; see below for a description of this element @@ -499,40 +518,41 @@ ; indicates more specifically than "ptype" what the ; source of the evaluated property is; the exact meaning ; is specific to the method whose result is being reported ; and is defined more clearly below special-smtp-verb = "mailfrom" / "rcptto" ; special cases of [SMTP] commands that are made up ; of multiple words pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name ) - [CFWS] ; the value extracted from the message property defined ; by the "ptype.property" construction "local-part" is defined in Section 3.4.1 of [MAIL], as modified by [RFC6531]. "CFWS" is defined in Section 3.2.2 of [MAIL]. - "domain-name" is as defined in Section 3.5 of [DKIM] where the "d=" - tag is defined, with "sub-domain" as modified by [RFC6531]. - "Keyword" is defined in Section 4.1.2 of [SMTP]. It is further - constrained by the necessity of being registered in the IANA registry - relevant to the context in which it is used. See Section 2.7, + constrained by the necesity of being registered in the IANA registry + relevant to the context in which it it is used. See Section 2.7, and Section 2.3, and Section 6. - "value" is as defined in Section 5.1 of [MIME], with "quoted-string" - updated as specified in RFC 6532. + The "value" is as defined in Section 5.1 of [MIME], with "quoted- + string" updated as specified in [RFC6532]. + + The "domain-name" is as defined in Section 3.5 of [DKIM]. + + The "Keyword" used in "result" above is further constrained by the + necessity of being enumerated in Section 2.7. See Section 2.5 for a description of the authserv-id element. If the value portion of a "pvalue" construction identifies something intended to be an email identity, then it MUST use the right hand portion of that ABNF definition. The list of commands eligible for use with the "smtp" ptype can be found in Section 4.1 of [SMTP]. @@ -552,41 +572,43 @@ Examples of complete messages using this header field can be found in Appendix B. 2.3. Property Types (ptypes) and Properties The "ptype" in the ABNF above indicates the general type of property being described by the result being reported, upon which the reported result was based. Coupled with the "property", which is more specific, they indicate from where the reported data were extracted. - This can include part of the message header or body, some part of the - SMTP session, a secondary output of an authentication method (apart - from its pure result), or some other aspect of the message's - handling. + This can include a particular part of the message header or body, + some part of the SMTP session, a secondary output of an + authentication method (apart from its pure result), or some other + aspect of the message's handling. Combinations of ptypes and properties are registered and described in the "Email Authentication Methods" registry, coupled with the authentication methods with which they are used. This is further described in Section 6. Legal values of "ptype" are as defined in the IANA "Email Authentication Property Types" registry, created by [RFC7410]. The initial values and what they typically indicate are as follows, based on [RFC7001]: body: Information that was extracted from the body of the message. This might be an arbitrary string of bytes, a hash of a string of bytes, a Uniform Resource Identifier, or some other content of interest. The "property" is an indication of where within the message body the extracted content was found, and can indicate an - offset, identify a MIME part, etc. + offset, identify a MIME part, etc. (At the time of this revision, + no properties matching this ptype have been registered. + Accordingly, this ptype may be deprecated in the future.) header: Indicates information that was extracted from the header of the message. This might be the value of a header field or some portion of a header field. The "property" gives a more precise indication of the place in the header from which the extraction took place. policy: A local policy mechanism was applied that augments or overrides the result returned by the authentication mechanism. (See Section 2.4.) @@ -625,24 +647,24 @@ In this case, the property is "dkim-rules", indicating some local check by that name took place and that check returned a result of "unsigned-subject". These are arbitrary names selected by (and presumably used within) the ADMD making use of them, so they are not normally registered with IANA or otherwise specified apart from setting syntax restrictions that allow for easy parsing within the rest of the header field. This ptype existed in the original specification for this header - field, but without a complete description or example of intended use. - As a result, it has not seen any practical use to date that matches - its intended purpose. These added details are provided to guide - implementers toward proper use. + field ([RFC5451]), but without a complete description or example of + intended use. As a result, it has not seen any practical use to date + that matches its intended purpose. These added details are provided + to guide implementers toward proper use. 2.5. Authentication Identifier Field Every Authentication-Results header field has an authentication service identifier field (authserv-id above). Specifically, this is any string intended to identify the authentication service within the ADMD that conducted authentication checks on the message. This identifier is intended to be machine-readable and not necessarily meaningful to users. @@ -809,63 +831,64 @@ included with any [MAIL]-style comments removed; moreover, the local- part of the address and the "@" character are removed if it has not been authenticated in some way. 2.7.2. SPF and Sender ID SPF and Sender ID use the "spf" and "sender-id" method names, respectively. The result values for SPF are defined in Section 2.6 of [SPF], and those definitions are included here by reference: - +-----------+--------------------------------+ + +-----------+------------------------------+ | Code | Meaning | - +-----------+--------------------------------+ - | none | [RFC7208], Section 2.6.1 | - +-----------+--------------------------------+ - | pass | [RFC7208], Section 2.6.3 | - +-----------+--------------------------------+ - | fail | [RFC7208], Section 2.6.4 | - +-----------+--------------------------------+ - | softfail | [RFC7208], Section 2.6.5 | - +-----------+--------------------------------+ - | policy | RFC 7601, Section 2.4 | - +-----------+--------------------------------+ - | neutral | [RFC7208], Section 2.6.2 | - +-----------+--------------------------------+ - | temperror | [RFC7208], Section 2.6.6 | - +-----------+--------------------------------+ - | permerror | [RFC7208], Section 2.6.7 | - +-----------+--------------------------------+ + +-----------+------------------------------+ + | none | [SPF], Section 2.6.1 | + +-----------+------------------------------+ + | pass | [SPF], Section 2.6.3 | + +-----------+------------------------------+ + | fail | [SPF], Section 2.6.4 | + +-----------+------------------------------+ + | softfail | [SPF], Section 2.6.5 | + +-----------+------------------------------+ + | policy | [this document], Section 2.4 | + +-----------+------------------------------+ + | neutral | [SPF], Section 2.6.2 | + +-----------+------------------------------+ + | temperror | [SPF], Section 2.6.6 | + +-----------+------------------------------+ + | permerror | [SPF], Section 2.6.7 | + +-----------+------------------------------+ These result codes are used in the context of this specification to reflect the result returned by the component conducting SPF evaluation. For SPF, the ptype used is "smtp", and the property is either "mailfrom" or "helo", since those values are the ones SPF can evaluate. (If the SMTP client issued the EHLO command instead of HELO, the property used is "helo".) - Note that in an EAI-formatted message, the "mailfrom" value can be - expressed in UTF-8. + Note that in an EAI-formatted message, the local-part of the + "mailfrom" can be expressed in UTF-8 and the domain part can be + expressed as a U-label. The "sender-id" method is described in [SENDERID]. For this method, the ptype used is "header" and the property will be the name of the header field from which the Purported Responsible Address (see [PRA]) was extracted -- namely, one of "Resent-Sender", "Resent-From", "Sender", or "From". The results for Sender ID are listed and described in Section 4.2 of [SENDERID], but for the purposes of this specification, the SPF definitions enumerated above are used instead. Also, [SENDERID] - specifies result codes that use mixed case, but they are typically - used all lowercase in this context. + specifies result codes that use mixed case, but they are used all + lowercase in this context. For both methods, an additional result of "policy" is defined, which means the client was authorized to inject or relay mail on behalf of the sender's DNS domain according to the authentication method's algorithm, but local policy dictates that the result is unacceptable. For example, "policy" might be used if SPF returns a "pass" result, but a local policy check matches the sending DNS domain to one found in an explicit list of unacceptable DNS domains (e.g., spammers). If the retrieved sender policies used to evaluate SPF and Sender ID @@ -938,22 +961,24 @@ such as a permanent directory service lookup error. A later attempt is not likely to produce a final result. The result of AUTH is reported using a ptype of "smtp" and a property of either: o "auth", in which case the value is the authorization identity generated by the exchange initiated by the AUTH command; or o "mailfrom", in which case the value is the mailbox identified by - the AUTH parameter used with the MAIL FROM command. Note that in - an EAI-formatted message, these values can be expressed in UTF-8. + the AUTH parameter used with the MAIL FROM command. + + Note that in an EAI-formatted message, the local-part can be + expressed in UTF-8, and the domain can be expressed as a U-label. If both identities are available, both can be reported. For example, consider this command issued by a client that has completed session authentication with the AUTH command resulting in an authorized identity of "client@c.example": MAIL FROM: AUTH= This could result in a "resinfo" construction like so: @@ -972,22 +997,22 @@ o Authorized Third-Party Signatures (in [ATPS], represented by "dkim-atps"); o Author Domain Signing Practices (in [ADSP], represented by "dkim- adsp"); o Require-Recipient-Valid-Since (in [RRVS], represented by "rrvs"); o S/MIME (in [SMIME-REG], represented by "smime"). - Note that "vbr.mv" and "vbr.md", which are already registered, are - permitted to be UTF-8 in an EAI-formatted message. + Note that in an EAI-formatted message, "vbr.mv" and "vbr.md", which + are already registered, can be expressed as U-labels. 2.7.6. Extension Methods Additional authentication method identifiers (extension methods) may be defined in the future by later revisions or extensions to this specification. These method identifiers are registered with the Internet Assigned Numbers Authority (IANA) and, preferably, published in an RFC. See Section 6 for further details. Extension methods can be defined for the following reasons: @@ -1006,21 +1031,21 @@ an MUA developer to understand or relay ancillary 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: example.com; foo=pass bar.baz=blob (2 of 3 tests OK) Experimental method identifiers MUST only be used within ADMDs that have explicitly consented to use them. These method identifiers and - the parameters associated with them are not documented in RFCs. + the parameters associated with them are not documented formally. Therefore, they are subject to change at any time and not suitable for production use. Any MTA, MUA, or downstream filter intended for production use SHOULD ignore or delete any Authentication-Results header field that includes an experimental (unknown) method identifier. 2.7.7. Extension Result Codes Additional result codes (extension results) might be defined in the future by later revisions or extensions to this specification. Non- @@ -1175,21 +1200,22 @@ Furthermore, MUAs and downstream filters SHOULD NOT interpret this header field unless the authentication service identifier of the header field is used within the ADMD as configured by the user or administrator. MUAs and downstream filters MUST ignore any result reported using a "result" not specified in the IANA "Result Code" registry or a "ptype" not listed in the "Email Authentication Property Types" registry for such values as defined in Section 6. Moreover, such agents MUST ignore a result indicated for any "method" they do not - specifically support. + specifically support. The exception to this is experimental methods + as discussed in Section 2.7.6. An MUA SHOULD NOT reveal these results to end users, absent careful human factors design considerations and testing, for the presentation of trust-related materials. For example, an attacker could register examp1e.com (note the digit "1" (one)) and send signed mail to intended victims; a verifier would detect that the signature was valid and report a "pass" even though it's clear the DNS domain name was intended to mislead. See Section 7.2 for further discussion. As stated in Section 2.1, this header field MUST be treated as though @@ -1200,21 +1226,24 @@ Note that there are a few message handlers that are only capable of appending new header fields to a message. Strictly speaking, these handlers are not compliant with this specification. They can still add the header field to carry authentication details, but any signal about where in the handling chain the work was done may be lost. Consumers SHOULD be designed such that this can be tolerated, especially from a producer known to have this limitation. MUAs SHOULD ignore instances of this header field discovered within - message/rfc822 MIME attachments. + message/rfc822 MIME attachments. They are likely to contain the + results of authentication checks done in the past, possibly long ago, + and have no contemporary value. Due caution to this needs to be + taken when choosing to consume them. Further discussion of these topics can be found in Section 7 below. 4.2. Local Policy Enforcement Some sites have a local policy that considers any particular authentication policy's non-recoverable failure results (typically "fail" or similar) as justification for rejecting the message. In such cases, the border MTA SHOULD issue an SMTP rejection response to the message, rather than adding this header field and allowing the @@ -1235,22 +1264,23 @@ To mitigate the impact of forged header fields, any MTA conforming to this specification MUST delete any discovered instance of this header field that claims, by virtue of its authentication service identifier, to have been added within its trust boundary but that did not come directly from another trusted MTA. For example, an MTA for example.com receiving a message MUST delete or otherwise obscure any instance of this header field bearing an authentication service identifier indicating that the header field was added within example.com prior to adding its own header fields. This could mean - each MTA will have to be equipped with a list of internal MTAs known - to be compliant (and hence trustworthy). + each internal MTA will need to be configured with a list of other + known, trusted MTAs that are thus expected to be using that same + identifier. For messages that are EAI-formatted messages, this test is done after converting A-labels into U-labels. For simplicity and maximum security, a border MTA could remove all instances of this header field on mail crossing into its trust boundary. However, this may conflict with the desire to access authentication results performed by trusted external service providers. It may also invalidate signed messages whose signatures cover external instances of this header field. A more robust border @@ -1274,126 +1304,287 @@ could invalidate one or more signatures on the message if they covered the header field to be removed. This behavior can be desirable since there's little value in validating the signature on a message with forged header fields. However, signing agents MAY therefore elect to omit these header fields from signing to avoid this situation. An MTA SHOULD remove any instance of this header field bearing a version (express or implied) that it does not support. However, an MTA MUST remove such a header field if the [SMTP] connection relaying - the message is not from a trusted internal MTA. This means the MTA - needs to be able to understand versions of this header field at least - as late as the ones understood by the MUAs or other consumers within - its ADMD. + the message is not from a trusted internal MTA. (As discussed above, + this too can result in invalidation of signatures.) This means the + MTA needs to be able to understand versions of this header field at + least as late as the ones understood by the MUAs or other consumers + within its ADMD. 6. IANA Considerations IANA has registered the defined header field and created registries as described below. These registry actions were originally defined by [RFC5451] and updated by [RFC6577] and [RFC7001]. The created registries were further updated in [RFC7601] to make them more complete. - Each is listed below, though generally they are not changed by this - document. + Each registry has two related sections below. The first describes + the registry and its update procedures, which are unchanged from + [RFC7601]. The second enumerates changes to entries that are + relevant to this document. 6.1. The Authentication-Results Header Field The Authentication-Results header field was added to the IANA "Permanent Message Header Field Names" registry, per the procedure found in [IANA-HEADERS]. That entry will be updated to reference this document. The following is the registration template: Header field name: Authentication-Results Applicable protocol: mail ([MAIL]) Status: Standard Author/Change controller: IETF Specification document(s): [this document] Related information: none 6.2. "Email Authentication Methods" Registry Description - No changes are made to the description of this registry. + Names of message authentication methods supported by this + specification have been registered with IANA, with the exception of + experimental names as described in Section 2.7.6. Along with each + method is recorded the properties that accompany the method's result. + + The "Email Authentication Parameters" group, and within it the "Email + Authentication Methods" registry, were created by [RFC5451] for this + purpose. [RFC6577] added a "status" field for each entry. [RFC7001] + amended the rules governing that registry and also added a "version" + field to the registry. + + The reference for that registry will be updated to reference this + document. + + New entries are assigned only for values that have received Expert + Review, per [IANA-CONSIDERATIONS]. The designated expert shall be + appointed by the IESG. The designated expert has discretion to + request that a publication be referenced if a clear, concise + definition of the authentication method cannot be provided such that + interoperability is assured. Registrations should otherwise be + permitted. The designated expert can also handle requests to mark + any current registration as "deprecated". + + No two entries can have the same combination of method, ptype, and + property. + + An entry in this registry contains the following: + + Method: the name of the method. + + Definition: a reference to the document that created this entry, if + any (see below). + + ptype: a "ptype" value appropriate for use with that method. + + property: a "property" value matching that "ptype" also appropriate + for use with that method. + + Value: a brief description of the value to be supplied with that + method/ptype/property tuple. + + Status: the status of this entry, which is either: + + active: The entry is in current use. + + deprecated: The entry is no longer in current use. + + Version: a version number associated with the method (preferably + starting at "1"). + + The "Definition" field will typically refer to a permanent document, + or at least some descriptive text, where additional information about + the entry being added can be found. This might in turn reference the + document where the method is defined so that all of the semantics + around creating or interpreting an Authentication-Results header + field using this method, ptype, and property can be understood. 6.3. "Email Authentication Methods" Registry Update - The following two entries are added. + The following entries in this registry are to be updated to replace + [RFC7601] with this document: + + +------------+--------+----------------------------------+ + | Method | ptype | Property | + +------------+--------+----------------------------------+ + | auth | smtp | auth | + +------------+--------+----------------------------------+ + | auth | smtp | mailfrom | + +------------+--------+----------------------------------+ + | dkim | header | d | + +------------+--------+----------------------------------+ + | dkim | header | i | + +------------+--------+----------------------------------+ + | domainkeys | header | d | + +------------+--------+----------------------------------+ + | domainkeys | header | from | + +------------+--------+----------------------------------+ + | domainkeys | header | sender | + +------------+--------+----------------------------------+ + | iprev | policy | iprev | + +------------+--------+----------------------------------+ + | sender-id | header | name of header field used by PRA | + +------------+--------+----------------------------------+ + | spf | smtp | mailfrom | + +------------+--------+----------------------------------+ + | spf | smtp | helo | + +------------+--------+----------------------------------+ + + In addition, two new entries are added to this registry, as follows: 6.3.1. 'header.a' for DKIM Method: dkim Definition: [this document] ptype: header property: a Description: value of signature "a" tag Status: active Version: 1 6.3.2. 'header.s' for DKIM - "header.s" for DKIM: - Method: dkim Definition: [this document] ptype: header property: s Description: value of signature "s" tag Status: active Version: 1 -6.4. "Email Authentication Property Types" Registry +6.4. "Email Authentication Property Types" Registry Description [RFC7410] created the "Email Authentication Property Types" registry. - No changes are made to the description of this registry. However, it - should be noted that Section 2.3 contains slightly different language - than prior versions of this document, allowing a broader space from - which to extract meaningful identifiers and report them through this - mechanism. + Entries in this registry are subject to the Expert Review rules as + described in [IANA-CONSIDERATIONS]. Each entry in the registry + requires the following values: -6.5. "Email Authentication Result Names" Description + ptype: The name of the ptype being registered, which must fit within + the ABNF described in Section 2.2. - No changes are made to the description of this registry. + Definition: An optional reference to a defining specification. -6.6. "Email Authentication Result Names" Update + Description: A brief description of what sort of information this + "ptype" is meant to cover. - No changes are made to entries in this registry. + For new entries, the Designated Expert needs to assure that the + description provided for the new entry adequately describes the + intended use. An example would be helpful to include in the entry's + defining document, if any, although entries in the "Email + Authentication Methods" registry or the "Email Authentication Result + Names" registry might also serve as examples of intended use. + + As this is a complete restatement of the definition and rules for + this registry, IANA will update this registry to show Section 2.3 of + this document as the current definitions for the "body", "header", + "policy", and "smtp" entries of that registry. References to other + documents will be removed. + +6.5. "Email Authentication Property Types" Registry Update + + All current entries in this registry are to be updated to replace + [RFC7601] with this document. + +6.6. "Email Authentication Result Names" Registry Description + + Names of message authentication result codes supported by this + specification must be registered with IANA, with the exception of + experimental codes as described in Section 2.7.7. + + New entries are assigned only for values that have received Expert + Review, per [IANA-CONSIDERATIONS]. The designated expert shall be + appointed by the IESG. The designated expert has discretion to + request that a publication be referenced if a clear, concise + definition of the authentication result cannot be provided such that + interoperability is assured. Registrations should otherwise be + permitted. The designated expert can also handle requests to mark + any current registration as "deprecated". + + No two entries can have the same combination of method and code. + + An entry in this registry contains the following: + + Auth Method: an authentication method for which results are being + returned using the header field defined in this document. + + Code: a result code that can be returned for this authentication + method. + + Specification: either free form text explaining the meaning of this + method-code combination, or a reference to such a definition. + + Status: the status of this entry, which is either: + + active: The entry is in current use. + + deprecated: The entry is no longer in current use. + +6.7. "Email Authentication Result Names" Registry Update + + The following entries in this registry are to be updated to reflect + new Specifications as follows: + + o All "auth" method result codes ("fail", "none", "pass", + "permerror", "temperror") are now specified in Section 2.7.4 of + this document. + + o All "dkim" method result names ("fail", "neutral", "none", "pass", + "permerror", "policy", "temperror") are now specified in + Section 2.7.1 of this document. + + o All "iprev" method result names ("fail", "pass", "permerror", + "temperror") are now specified in Section 2.7.3 of this document. + + o The "sender-id" and "spf" method result names "fail", "neutral", + "none", "pass", "permerror", "policy", "softfail", and "temperror" + are now specified in Section 2.7.2 of this document. The + registrations for result name "hardfail" are not updated. + +6.8. SMTP Enhanced Status Codes + + The entry for X.7.25 in the "Enumerated Status Codes" sub-registry of + the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes + Registry" is to be updated to refer only to Section 3.3 of [AUTH-ESC] + as that is where that registration was done. 7. Security Considerations The following security considerations apply when adding or processing the Authentication-Results header field: 7.1. Forged Header Fields - An MUA or filter that accesses a mailbox whose messages are 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 DNS domain of a receiving ADMD as the authserv-id - token in the value of the header field and, with the rest of the - value, claim that the message was properly authenticated. The non- - conformant MTA would fail to strip the forged header field, and the - MUA could inappropriately trust it. + An MTA not applying the filtering discussed in Section 5 exposes MUAs + to false conclusions based on forged header fields. A malicious user + or agent could forge a header field using the DNS domain of a + receiving ADMD as the authserv-id token in the value of the header + field and, with the rest of the value, claim that the message was + properly authenticated. The non-conformant MTA would fail to strip + the forged header field, and the MUA could inappropriately trust it. For this reason, it is best not to 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 or administrator after verifying that the border MTA is compliant. It is acceptable to have an MUA aware of this specification but have an explicit list of hostnames whose Authentication-Results header fields are trustworthy; however, this list should initially be empty. @@ -1516,22 +1707,22 @@ If an attack becomes known against an authentication method, clearly then the agent verifying that method can be fooled into thinking an inauthentic message is authentic, and thus the value of this header field can be misleading. It follows that any attack against the authentication methods supported by this document is also a security consideration here. 7.8. Intentionally Malformed Header Fields - As with any other header field found in a message, it is possible for - an attacker to add an Authentication-Results header field that is + As with any other header field found in the message, it is possible + for an attacker to add an Authentication-Results header field that is extraordinarily large or otherwise malformed in an attempt to discover or exploit weaknesses in header field parsing code. Implementers must thoroughly verify all such header fields received from MTAs and be robust against intentionally as well as unintentionally malformed header fields. 7.9. Compromised Internal Hosts An internal MUA or MTA that has been compromised could generate mail with a forged From header field and a forged Authentication-Results @@ -1545,115 +1736,98 @@ legitimate MTAs will have to add this header field when legitimate internal-only messages are generated. This is also covered in Section 5. 7.10. Encapsulated Instances MIME messages can contain attachments of type "message/rfc822", which contain other messages. Such an encapsulated message can also contain an Authentication-Results header field. Although the processing of these is outside of the intended scope of this document - (see Section 1.3), some early guidance to MUA developers is + (see Section 1.3), some simple guidance to MUA developers is appropriate here. - Since MTAs are unlikely to strip Authentication-Results header fields - after mailbox delivery, MUAs are advised in Section 4.1 to ignore - such instances within MIME attachments. Moreover, when extracting a - message digest to separate mail store messages or other media, such - header fields should be removed so that they will never be - interpreted improperly by MUAs that might later consume them. + Since MTAs are generally unlikely to strip Authentication-Results + header fields during mailbox delivery, normative language exists in + Section 4.1 cautioning MUAs to ignore such instances within MIME + attachments, as might be included when a message is forwarded. + Moreover, when extracting a message digest to separate mail store + messages or other media, such header fields should be removed so that + they will never be interpreted improperly by MUAs that might later + consume them. + + There can be cases where these header fields included as part of + encapsulated messages might actually be of value, such as when they + are taken from messages within the same ADMD where they will be + consumed. Caution must be taken that the consumer fully understands + the semantics of what the header field is indicating and the + message's handling history before ascribing any value, positive or + negative, to such data. 7.11. Reverse Mapping Although Section 3 of this memo includes explicit support for the "iprev" method, its value as an authentication mechanism is limited. Implementers of both this specification and agents that use the data it relays are encouraged to become familiar with the issues raised by [DNSOP-REVERSE] when deciding whether or not to include support for "iprev". 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, . - [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, - . - [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, . - [KEYWORDS] - Bradner, S., "Key words for use in RFCs to Indicate - Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ - RFC2119, March 1997, - . - [MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008, . [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, . - [RFC5451] Kucherawy, M., "Message Header Field for Indicating - Message Authentication Status", RFC 5451, DOI 10.17487/ - RFC5451, April 2009, - . - - [RFC6008] Kucherawy, M., "Authentication-Results Registration for - Differentiating among Cryptographic Results", RFC 6008, - DOI 10.17487/RFC6008, September 2010, - . + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ + RFC2119, March 1997, + . [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", RFC 6530, DOI 10.17487/RFC6530, February 2012, . [RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized Email", RFC 6531, DOI 10.17487/RFC6531, February 2012, . [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized Email Headers", RFC 6532, DOI 10.17487/RFC6532, February 2012, . - [RFC6577] Kucherawy, M., "Authentication-Results Registration Update - for Sender Policy Framework (SPF) Results", RFC 6577, - DOI 10.17487/RFC6577, March 2012, - . - - [RFC7001] Kucherawy, M., "Message Header Field for Indicating - Message Authentication Status", RFC 7001, DOI 10.17487/ - RFC7001, September 2013, - . - [RFC7601] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 7601, DOI 10.17487/ RFC7601, August 2015, . - [RFC8301] Kitterman, S., "Cryptographic Algorithm and Key Usage - Update to DomainKeys Identified Mail (DKIM)", RFC 8301, - DOI 10.17487/RFC8301, January 2018, - . + [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC + 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, + May 2017, . [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008, . 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, @@ -1672,20 +1846,25 @@ [AUTH] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service Extension for Authentication", RFC 4954, DOI 10.17487/ RFC4954, July 2007, . [AUTH-ESC] Kucherawy, M., "Email Authentication Status Codes", RFC 7372, DOI 10.17487/RFC7372, September 2014, . + [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, + . + [DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, . [DNS] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, . [DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, @@ -1707,37 +1886,68 @@ [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format for Delivery Status Notifications", RFC 3464, DOI 10.17487/RFC3464, January 2003, . [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598, DOI 10.17487/RFC5598, July 2009, . + [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, + . + [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, . [POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, . [PRA] Lyon, J., "Purported Responsible Address in E-Mail Messages", RFC 4407, DOI 10.17487/RFC4407, April 2006, . + [RFC5451] Kucherawy, M., "Message Header Field for Indicating + Message Authentication Status", RFC 5451, DOI 10.17487/ + RFC5451, April 2009, + . + + [RFC6008] Kucherawy, M., "Authentication-Results Registration for + Differentiating among Cryptographic Results", RFC 6008, + DOI 10.17487/RFC6008, September 2010, + . + + [RFC6577] Kucherawy, M., "Authentication-Results Registration Update + for Sender Policy Framework (SPF) Results", RFC 6577, + DOI 10.17487/RFC6577, March 2012, + . + + [RFC7001] Kucherawy, M., "Message Header Field for Indicating + Message Authentication Status", RFC 7001, DOI 10.17487/ + RFC7001, September 2013, + . + [RFC7410] Kucherawy, M., "A Property Types Registry for the Authentication-Results Header Field", RFC 7410, DOI 10.17487/RFC7410, December 2014, . + [RFC8301] Kitterman, S., "Cryptographic Algorithm and Key Usage + Update to DomainKeys Identified Mail (DKIM)", RFC 8301, + DOI 10.17487/RFC8301, January 2018, + . + [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, . [SECURITY] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, DOI 10.17487/RFC3552, July 2003, . @@ -2092,45 +2302,46 @@ 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 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. + 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 cryptanalysis - (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. + 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 @@ -2150,33 +2361,35 @@ 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 RFC7601 +Appendix D. Changes since RFC7601 o Added IANA registration for DKIM "a" and "s" properties. o Include EAI guidance. - o Adjust some ABNF tokens and names for easier inclusion by other + o Adjust some ABNF tokes and names for easier inclusion by other documents. + o Made minor editorial adjustments. + Appendix E. Acknowledgments The author wishes to acknowledge the following individuals for their review and constructive criticism of this document: Seth Blank, Tim - Draegen, John Levine, Scott Kitterman, and Alessandro Vesely. + Draegen, Scott Kitterman, John Levine, and Alessandro Vesely. Author's Address Murray S. Kucherawy 270 Upland Drive San Francisco, CA 94127 United States Email: superuser@gmail.com