Internet Engineering Task Force (IETF) M. Kucherawy Request for Comments: 8601 May 2019 Obsoletes: 7601 Category: Standards Track ISSN: 2070-1721
Message Header Field for Indicating Message Authentication Status
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 is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8601.
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Copyright Notice
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 (https://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 ...........................................7 1.4. Requirements ...............................................7 1.5. Definitions ................................................7 1.5.1. Key Words ...........................................7 1.5.2. Internationalized Email .............................7 1.5.3. Security ............................................8 1.5.4. Email Architecture ..................................8 1.5.5. Other Terms .........................................9 1.6. Trust Environment .........................................10 2. Definition and Format of the Header Field ......................10 2.1. General Description .......................................10 2.2. Formal Definition .........................................11 2.3. Property Types (ptypes) and Properties ....................13 2.4. The "policy" ptype ........................................15 2.5. Authentication Service Identifier Field ...................15 2.6. Version Tokens ............................................17 2.7. Defined Methods and Result Values .........................17 2.7.1. DKIM ...............................................17 2.7.2. SPF ................................................19 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 ...........................25 4.1. Header Field Position and Interpretation ..................26 4.2. Local Policy Enforcement ..................................27
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5. Removing Existing Header Fields ................................28 6. IANA Considerations ............................................29 6.1. The Authentication-Results Header Field ...................29 6.2. "Email Authentication Methods" Registry Description .......30 6.3. "Email Authentication Methods" Registry Update ............31 6.3.1. "header.a" for DKIM ................................32 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 .......34 6.8. SMTP Enhanced Status Codes ................................34 7. Security Considerations ........................................35 7.1. Forged Header Fields ......................................35 7.2. Misleading Results ........................................36 7.3. Header Field Position .....................................37 7.4. Reverse IP Query Denial-of-Service Attacks ................37 7.5. Mitigation of Backscatter .................................37 7.6. Internal MTA Lists ........................................37 7.7. Attacks against Authentication Methods ....................38 7.8. Intentionally Malformed Header Fields .....................38 7.9. Compromised Internal Hosts ................................38 7.10. Encapsulated Instances ...................................38 7.11. Reverse Mapping ..........................................39 8. References .....................................................39 8.1. Normative References ......................................39 8.2. Informative References ....................................40 Appendix A. Legacy MUAs ...........................................44 Appendix B. Authentication-Results Examples .......................44 B.1. Trivial Case: Header Field Not Present .....................44 B.2. Nearly Trivial Case: Service Provided, but No Authentication Done ........................................45 B.3. Service Provided, Authentication Done ......................46 B.4. Service Provided, Several Authentications Done, Single MTA ........................................................47 B.5. Service Provided, Several Authentications Done, Different MTAs .......................................................48 B.6. Service Provided, Multi-tiered Authentication Done .........50 B.7. Comment-Heavy Example ......................................51 Appendix C. Operational Considerations about Message Authentication ........................................52 Appendix D. Changes since RFC 7601 ................................53 Acknowledgments ...................................................54 Author's Address ..................................................54
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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 message's origin and possibly the safety and integrity of its content.
End users are not expected to be direct consumers of this header field. This header field is intended for consumption by programs that will then use such data or render it in a human-usable form.
This document specifies the format of this header field and discusses the implications of its presence or absence. However, it does not discuss how the data contained in the header field ought to be used, such as what filtering decisions are appropriate or how an MUA might render those results, as these are local policy and/or user interface design questions that are not appropriate for this document.
At the time of publication of this document, the following are published email authentication methods:
o SMTP Service Extension for Authentication [AUTH]
o DomainKeys Identified Mail Signatures [DKIM]
o Domain-based Message Authentication, Reporting, and Conformance [DMARC]
o Sender Policy Framework [SPF]
o reverse IP address name validation ("iprev", defined in Section 3)
o Require-Recipient-Valid-Since Header Field and SMTP Service Extension [RRVS]
o S/MIME Signature Verification [SMIME-REG]
o Vouch By Reference [VBR]
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The following Historic specifications were previously supported by this framework but have since become obsolete:
o Author Domain Signing Practices [ADSP] (Historic)
o DomainKeys [DOMAINKEYS] (Historic)
Note that at the time of publication of this document the Sender ID specification [SENDERID] (Experimental) is no longer supported by this framework. Discussion regarding moving it to Historic status is underway.
There exist registries for tokens used within this header field that refer to the specifications listed above. Section 6 describes the registries and their contents and specifies the process by which entries are added or updated. It also updates the existing contents to match the current states of these specifications.
The goal of this work is to give current and future authentication schemes a common framework within which to deliver their results to downstream agents and discourage the creation of unique header fields for each.
Although SPF defined a header field called "Received-SPF" and the historic DomainKeys defined one called "DomainKey-Status" for this purpose, those header fields are specific to the conveyance of their respective results only and thus are insufficient to satisfy the requirements enumerated below. In addition, many SPF implementations have adopted the header field specified here at least as an option, and DomainKeys has been obsoleted by DKIM.
The header field defined in this document is expected to serve several purposes:
1. Convey the results of various message authentication checks, which are applied by upstream filters and Mail Transfer Agents (MTAs) and then passed to MUAs and downstream filters within the same "trust domain". Such agents might wish to render those results to end users or to use those data to apply more or less stringent content checks based on authentication results.
2. Provide a common location within a message for such data.
3. Create an extensible framework for reporting new authentication methods as they emerge.
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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, the paths between them, and the mechanism for conveying the information.
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 treat assertions by the producer as being reliable and accurate (trustworthy). How this trust is obtained is outside the scope of this document. It is entirely a local matter.
Thus, this document defines a "trust boundary" as the delineation between "external" and "internal" entities. Services that are internal -- within the trust boundary -- are provided by the ADMD's infrastructure for its users. Those that are external are outside of the authority of the ADMD. By this definition, hosts that are within a trust boundary are subject to the ADMD's authority and policies, independent of their physical placement or their physical operation. For example, a host within a trust boundary might actually be operated by a remote service provider and reside physically within 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, if found within one of those media types, this field 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 it is configured to identify and ascribe value to authentication results recorded by that ADMD.
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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.
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 as described in Section 5.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
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.
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"Guidelines for Writing RFC Text on Security Considerations" [SECURITY] discusses authentication and authorization and the conflation of the two concepts. The use of those terms within the context of recent message security work has given rise to slightly different definitions, and this document reflects those current usages, as follows:
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 is an authorization mechanism in that it expresses a result that shows whether the ADMD that apparently sent the message has explicitly authorized the connecting Simple Mail Transfer Protocol (SMTP) client [SMTP] to relay messages on its behalf, but it does 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 the ADMD of origin, intermediate ADMDs (such as a mailing list server), other handling agents, or any combination of these.
Rather than create a separate header field for each class of solution, this specification groups them both into a single header field.
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.)
o A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that actually enacts delivery of a message to a user's inbox or other final delivery.
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o An "intermediate MTA" is any MTA that is not a delivery MTA and is also not the first MTA to handle the message.
o A Message Submission Agent (MSA) is an agent that accepts a message from an MUA, introducing it to the mail-handling stream.
The following diagram illustrates the flow of mail among these defined components. See "Internet Mail Architecture" [EMAIL-ARCH] for further discussion on general email system architecture, which includes detailed descriptions of these components, and Appendix C of this document for discussion about the common aspects of email authentication in current environments.
+-----+ +-----+ +------------+ | MUA |-->| MSA |-->| Border MTA | +-----+ +-----+ +------------+ | | V +----------+ | Internet | +----------+ | | V +-----+ +-----+ +------------------+ +------------+ | MUA |<--| MDA |<--| Intermediate MTA |<--| Border MTA | +-----+ +-----+ +------------------+ +------------+
Generally, it is assumed that the work of applying message 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" might well apply to the same machine or even the very same agent. It is also possible that some message authentication tests could take place on an intermediate MTA. Although this document doesn't specifically describe such cases, they are not meant to be excluded.
In this document, the term "producer" refers to any component that adds this header field to messages it is handling, and "consumer" refers to any component that identifies, extracts, and parses the header field to use as part of a handling decision.
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This header field permits one or more message validation mechanisms to communicate output to one or more separate assessment mechanisms. These mechanisms operate within a unified trust boundary that defines an ADMD. An ADMD contains one or more entities that perform validation and generate the header field and one or more that consume it for some type of assessment. The field often contains no integrity or validation mechanism of its own, so its presence must be trusted implicitly. Hence, valid use of the header field requires removing any occurrences of it that claim to be associated with the ADMD when the message enters the ADMD. This ensures that later occurrences have been added within the trust boundary of the ADMD.
The authserv-id token defined in Section 2.2 can be used to reference an entire ADMD or a specific validation engine within an ADMD. Although the labeling scheme is left as an operational choice, some guidance for selecting a token is provided in later sections of this document.
2. Definition and Format of the Header Field
This section gives a general overview of the format of the header field being defined and then provides a formal specification.
The header field specified here is called "Authentication-Results". It is a structured header field as defined in "Internet Message Format" [MAIL], and thus all of the related definitions in that document apply.
This header field is added at the top of the message as it transits MTAs that do authentication checks, so some idea of how far away the checks were done can be inferred. It is therefore considered to be a trace field as defined in [MAIL], and thus all of the related definitions in that document apply.
The value of the header field (after removing comments) consists of an authentication service identifier, an optional version, and then a series of statements and supporting data. The statements are of the form "method=result" and indicate which authentication method or methods were applied and their respective results. For each such statement, the supporting data can include a "reason" string and one or more "property=value" statements indicating which message properties were evaluated to reach that conclusion.
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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.
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.
authserv-id = value ; see below for a description of this element
authres-version = 1*DIGIT [CFWS] ; indicates which version of this specification is in use; ; this specification is version "1", and the absence of a ; version implies this version of the specification
no-result = [CFWS] ";" [CFWS] "none" ; the special case of "none" is used to indicate that no ; message authentication was performed
methodspec = [CFWS] method [CFWS] "=" [CFWS] result ; indicates which authentication method was evaluated ; and what its output was
reasonspec = "reason" [CFWS] "=" [CFWS] value ; a free-form comment on the reason the given result ; was returned
propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue ; an indication of which properties of the message ; were evaluated by the authentication scheme being ; applied to yield the reported result
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method = Keyword [ [CFWS] "/" [CFWS] method-version ] ; a method indicates which method's result is ; represented by "result"; it is one of the methods ; explicitly defined as valid in this document ; or is an extension method as defined below
method-version = 1*DIGIT [CFWS] ; indicates which version of the method specification is ; in use, corresponding to the matching entry in the IANA ; "Email Authentication Methods" registry; a value of "1" ; is assumed if this version string is absent
result = Keyword ; indicates the results of the attempt to authenticate ; the message; see below for details
ptype = Keyword ; indicates whether the property being evaluated was ; a parameter to an SMTP command [SMTP], was a value taken ; from a message header field, was some property of ; the message body, or was some other property evaluated by ; the receiving MTA; expected to be one of the "property ; types" explicitly defined as valid, or an extension ; ptype, as defined below
property = special-smtp-verb / Keyword ; 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 [SMTP] 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].
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"Keyword" is defined in Section 4.1.2 of [SMTP]. It is further constrained by the necessity of being registered in the Internet Assigned Numbers Authority (IANA) registry relevant to the context in which it is used. See Sections 2.3, 2.7, and 6.
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].
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].
The "propspec" may be omitted if, for example, the method was unable to extract any properties to do its evaluation yet still has a result to report. It may also be omitted if the agent generating this result wishes not to reveal such properties to downstream agents.
Where an SMTP command name is being reported as a "property", the agent generating the header field represents that command by converting it to lowercase and dropping any spaces (e.g., "MAIL FROM" becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.).
A "ptype" value of "policy" indicates a policy decision about the message not specific to a property of the message that could be extracted. See Section 2.4 for details.
Examples of complete messages using this header field can be found in Appendix B.
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, it indicates from where the reported "pvalue" was extracted. 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.
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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. (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.)
smtp: Indicates information that was extracted from an SMTP command that was used to relay the message. The "property" indicates which SMTP command included the extracted content as a parameter.
Results reported using unknown ptypes MUST NOT be used in making handling decisions. They can be safely ignored by consumers.
Entries in the "Email Authentication Methods" registry can define properties that deviate from these definitions when appropriate. Such deviations need to be clear in the registry and/or in the defining document. See Section 2.7.1 for an example.
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A special ptype value of "policy" is also defined. This ptype is provided to indicate that some local policy mechanism was applied that augments or even replaces (i.e., overrides) the result returned by the authentication mechanism. The property and value in this case identify the local policy that was applied and the result it returned.
For example, a DKIM signature is not required to include the Subject header field in the set of fields that are signed. An ADMD receiving such a message might decide that such a signature is unacceptable, even if it passes, because the content of the Subject header field could be altered post-signing without invalidating the signature. Such an ADMD could replace the DKIM "pass" result with a "policy" result and then also include the following in the corresponding Authentication-Results field:
In this case, the property is "dkim-rules", indicating that 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 [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.
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.
Note that in an EAI-formatted message, this identifier may be expressed in UTF-8.
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Since agents consuming this field will use this identifier to determine whether its contents are of interest (and are safe to use), the uniqueness of the identifier MUST be guaranteed by the ADMD that generates it and MUST pertain to that ADMD. MUAs or downstream filters SHOULD use this identifier to determine whether or not the data contained in an Authentication-Results header field ought to be used or ignored.
For simplicity and scalability, the authentication service identifier SHOULD be a common token used throughout the ADMD. Common practice is to use the DNS domain name used by or within that ADMD, sometimes called the "organizational domain", but this is not strictly necessary.
For tracing and debugging purposes, the authentication service identifier can instead be the specific hostname of the MTA performing the authentication check whose result is being reported. Moreover, some implementations define a substructure to the identifier; such structures are outside of the scope of this specification.
Note, however, that using a local, relative identifier like a flat hostname, rather than a hierarchical and globally unique ADMD identifier like a DNS domain name, makes configuration more difficult for large sites. The hierarchical identifier permits aggregating related, trusted systems together under a single, parent identifier, which in turn permits assessing the trust relationship with a single reference. The alternative is a flat namespace requiring individually listing each trusted system. Since consumers will use the identifier to determine whether to use the contents of the header field:
o Changes to the identifier impose a large, centralized administrative burden.
o Ongoing administrative changes require constantly updating this centralized table, making it difficult to ensure that an MUA or downstream filter will have access to accurate information for assessing the usability of the header field's content. In particular, consumers of the header field will need to know not only the current identifier(s) in use but previous ones as well to account for delivery latency or later reassessment of the header field's content.
Examples of valid authentication service identifiers are "example.com", "mail.example.org", "ms1.newyork.example.com", and "example-auth".
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The grammar above provides for the optional inclusion of versions on both the header field itself (attached to the authserv-id token) and on each of the methods being reported. The method version refers to the method itself, which is specified in the documents describing those methods, while the authserv-id version refers to this document and thus the syntax of this header field.
The purpose of including these is to avoid misinterpretation of the results. That is, if a parser finds a version after an authserv-id that it does not explicitly know, it can immediately discontinue trying to parse, since what follows might not be in an expected format. For a method version, the parser SHOULD ignore a method result if the version is not supported in case the semantics of the result have a different meaning than what is expected. For example, if a hypothetical DKIM version 2 yielded a "pass" result for different reasons than version 1 does, a consumer of this field might not want to use the altered semantics. Allowing versions in the syntax is a way to indicate this and let the consumer of the header field decide.
Each individual authentication method returns one of a set of specific result values. The subsections below provide references to the documents defining the authentication methods specifically supported by this document, and their corresponding result values. Verifiers SHOULD use these values as described below. New methods not specified in this document, but intended to be supported by the header field defined here, MUST include a similar result table either in their defining documents or in supplementary ones.
DKIM is represented by the "dkim" method and is defined in [DKIM].
A signature is "acceptable to the ADMD" if it passes local policy checks (or there are no specific local policy checks). For example, an ADMD policy might require that the signature(s) on the message be added using the DNS domain present in the From header field of the message, thus making third-party signatures unacceptable even if they verify.
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The DKIM result set is as follows:
none: The message was not signed.
pass: The message was signed, the signature or signatures were acceptable to the ADMD, and the signature(s) passed verification tests.
fail: The message was signed and the signature or signatures were acceptable to the ADMD, but they failed the verification test(s).
policy: The message was signed, but some aspect of the signature or signatures was not acceptable to the ADMD.
neutral: The message was signed, but the signature or signatures contained syntax errors or were not otherwise able to be processed. This result is also used for other failures not covered elsewhere in this list.
temperror: The message could not be verified due to some error that is likely transient in nature, such as a temporary inability to retrieve a public key. A later attempt may produce a final result.
permerror: The message could not be verified due to some error that is unrecoverable, such as a required header field being absent. A later attempt is unlikely to produce a final result.
DKIM results are reported using a ptype of "header". The property, however, represents one of the tags found in the DKIM-Signature header field rather than a distinct header field. For example, the ptype-property combination "header.d" refers to the content of the "d" (signing domain) tag from within the signature header field, and not a distinct header field called "d".
Note that in an EAI-formatted message, the values of the "d" and "i" properties can be expressed in UTF-8.
In addition to previous registrations, this document registers the DKIM tags "a" (cryptographic algorithm used to sign the message) and "s" (selector) as reportable properties. These can be used to aid receivers during post-verification processing. In particular, [RFC8301] obsoleted use of the "rsa-sha1" algorithm in DKIM, so it is important to be able to distinguish such signatures from those using preferred algorithms.
The ability to report different DKIM results for a message with multiple signatures is described in [RFC6008].
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[DKIM] advises that if a message fails verification, it is to be treated as an unsigned message. A report of "fail" here permits the receiver of the report to decide how to handle the failure. A report of "neutral" or "none" preempts that choice, ensuring that the message will be treated as if it had not been signed.
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 local-part of the "mailfrom" can be expressed in UTF-8 and the domain part can be expressed as a U-label.
For this method, 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
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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 do not contain explicit provisions for authenticating the local-part (see Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported along with results for this mechanism SHOULD NOT include the local-part or the following "@" character.
The result values used by the "iprev" method, defined in Section 3, are as follows:
pass: The DNS evaluation succeeded, i.e., the "reverse" and "forward" lookup results were returned and were in agreement.
fail: The DNS evaluation failed. In particular, the "reverse" and "forward" lookups each produced results, but they were not in agreement, or the "forward" query completed but produced no result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR) in a reply containing no answers, was returned.
temperror: The DNS evaluation could not be completed due to some error that is likely transient in nature, such as a temporary DNS error, e.g., a DNS RCODE of 2, commonly known as SERVFAIL, or other error condition resulted. A later attempt may produce a final result.
permerror: The DNS evaluation could not be completed because no PTR data are published for the connecting IP address, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR) in a reply containing no answers, was returned. This prevented completion of the evaluation. A later attempt is unlikely to produce a final result.
There is no "none" for this method, since any TCP connection delivering email has an IP address associated with it, so some kind of evaluation will always be possible.
The result is reported using a ptype of "policy" (as this is not part of any established protocol) and a property of "iprev".
For discussion of the format of DNS replies, see "Domain names - implementation and specification" [DNS].
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SMTP AUTH (defined in [AUTH]) is represented by the "auth" method. Its result values are as follows:
none: SMTP authentication was not attempted.
pass: The SMTP client authenticated to the server reporting the result using the protocol described in [AUTH].
fail: The SMTP client attempted to authenticate to the server using the protocol described in [AUTH] but was not successful (such as providing a valid identity but an incorrect password).
temperror: The SMTP client attempted to authenticate using the protocol described in [AUTH] but was not able to complete the attempt due to some error that is likely transient in nature, such as a temporary directory service lookup error. A later attempt may produce a final result.
permerror: The SMTP client attempted to authenticate using the protocol described in [AUTH] but was not able to complete the attempt due to some error that is likely not transient in nature, 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, 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:<alice@a.example> AUTH=<bob@b.example>
This could result in a "resinfo" construction like so:
RFC 8601 Authentication-Results Header Field May 2019
Note that in all cases other than "pass", the message was sent by an unauthenticated client. All non-"pass" cases SHOULD thus be treated as equivalent with respect to this method.
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 IANA and, preferably, published in an RFC. See Section 6 for further details.
Extension methods can be defined for the following reasons:
1. To allow additional information from new authentication systems to be communicated to MUAs or downstream filters. The names of such identifiers ought to reflect the name of the method being defined but ought not be needlessly long.
2. To allow the creation of "sub-identifiers" that indicate different levels of authentication and differentiate between their relative strengths, e.g., "auth1-weak" and "auth1-strong".
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Authentication method implementers are encouraged to provide adequate information, via message header field comments if necessary, to allow an MUA developer to understand or relay ancillary details of authentication results. For example, if it might be of interest to relay what data were 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 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.
Additional result codes (extension results) might be defined in the future by later revisions or extensions to this specification. Non-experimental result codes MUST be registered with IANA (and, preferably, published in an RFC). See Section 6 for further details.
Experimental results MUST only be used within ADMDs that have explicitly consented to use them. These results and the parameters associated with them are not formally documented. 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 extension result.
3. The "iprev" Authentication Method
This section defines an additional authentication method called "iprev".
"iprev" is an attempt to verify that a client appears to be valid based on some DNS queries, which is to say that the IP address is explicitly associated with a domain name. Upon receiving a session initiation of some kind from a client, the IP address of the client peer is queried for matching names (i.e., a number-to-name translation, also known as a "reverse lookup" or a "PTR" record query). Once that result is acquired, a lookup of each of the names (i.e., a name-to-number translation, or an "A" or "AAAA" record
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query) thus retrieved is done. The response to this second check will typically result in at least one mapping back to the client's IP address.
Expressed as an algorithm: If the client peer's IP address is I, the list of names to which I maps (after a "PTR" query) is the set N, and the union of IP addresses to which each member of N maps (after corresponding "A" and "AAAA" queries) is L, then this test is successful if I is an element of L.
Often an MTA receiving a connection that fails this test will simply reject the connection using the enhanced status code defined in [AUTH-ESC]. If an operator instead wishes to make this information available to downstream agents as a factor in handling decisions, it records a result in accordance with Section 2.7.3.
The response to a "PTR" query could contain multiple names. To prevent heavy DNS loads, agents performing these queries MUST be implemented such that the number of names evaluated by generation of corresponding "A" or "AAAA" queries is limited so as not to be unduly taxing to the DNS infrastructure, though it MAY be configurable by an administrator. As an example, Section 4.6.4 of [SPF] chose a limit of 10 for its implementation of this algorithm.
"DNS Extensions to Support IP Version 6" [DNS-IP6] discusses the query formats for the IPv6 case.
There is some contention regarding the wisdom and reliability of this test. For example, in some regions, it can be difficult for this test ever to pass because the practice of arranging to match the forward and reverse DNS is infrequently observed. Therefore, the precise implementation details of how a verifier performs an "iprev" test are not specified here. The verifier MAY report a successful or failed "iprev" test at its discretion having done some kind of check of the validity of the connection's identity using DNS. It is incumbent upon an agent making use of the reported "iprev" result to understand what exactly that particular verifier is attempting to report.
Extensive discussion of reverse DNS mapping and its implications can be found in "Considerations for the use of DNS Reverse Mapping" [DNSOP-REVERSE]. In particular, it recommends that applications avoid using this test as a means of authentication or security. Its presence in this document is not an endorsement but is merely acknowledgment that the method remains common and provides the means to relay the results of that test.
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This specification makes no attempt to evaluate the relative strengths of various message authentication methods that may become available. The methods listed are an order-independent set; their sequence does not indicate relative strength or importance of one method over another. Instead, the MUA or downstream filter consuming this header field is to interpret the result of each method based on its own knowledge of what that method evaluates.
Each "method" MUST refer to an authentication method declared in the IANA registry or an extension method as described in Section 2.7.6, and each "result" MUST refer to a result code declared in the IANA registry or an extension result code as defined in Section 2.7.7. See Section 6 for further information about the registered methods and result codes.
An MTA compliant with this specification adds this header field (after performing one or more message authentication tests) to indicate which MTA or ADMD performed the test, which test was applied, and what the result was. If an MTA applies more than one such test, it adds this header field either once per test or once indicating all of the results. An MTA MUST NOT add a result to an existing header field.
An MTA MAY add this header field containing only the authentication service identifier portion and the "none" token (see Section 2.2) to indicate explicitly that no message authentication schemes were applied prior to delivery of this message.
An MTA adding this header field has to take steps to identify it as legitimate to the MUAs or downstream filters that will ultimately consume its content. One process to do so is described in Section 5. Further measures may be necessary in some environments. Some possible solutions are enumerated in Section 7.1. This document does not mandate any specific solution to this issue, as each environment has its own facilities and limitations.
Most known message authentication methods focus on a particular identifier to evaluate. SPF differs in that it can yield a result based on more than one identifier; specifically, SPF can evaluate the RFC5321.HELO parameter or the RFC5321.MailFrom parameter. When generating this field to report those results, only the parameter that yielded the result is included.
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For MTAs that add this header field, adding header fields in order (at the top), per Section 3.6 of [MAIL], is particularly important. Moreover, this header field SHOULD be inserted above any other trace header fields such MTAs might prepend. This placement allows easy detection of header fields that can be trusted.
End users making direct use of this header field might inadvertently trust information that has not been properly vetted. If, for example, a basic SPF result were to be relayed that claims an authenticated addr-spec, the local-part of that addr-spec has actually not been authenticated. Thus, an MTA adding this header field SHOULD NOT include any data that have not been authenticated by the method(s) being applied. Moreover, MUAs SHOULD NOT render to users such information if it is presented by a method known not to authenticate it.
In order to ensure non-ambiguous results and avoid the impact of false header fields, MUAs and downstream filters SHOULD NOT interpret this header field unless specifically configured to do so by the user or administrator. That is, this interpretation should not be "on by default". Naturally then, users or administrators ought not activate such a feature unless (1) they are certain the header field will be validly added by an agent within the ADMD that accepts the mail that is ultimately read by the MUA, and (2) instances of the header field that appear to originate within the ADMD but are actually added by foreign MTAs will be removed before delivery.
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. 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
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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 it were a trace header field as defined in Section 3.6.7 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 message authentication was done.
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. They are likely to contain the results of authentication checks done in the past, possibly long ago, and have no contemporary value. Due caution therefore needs to be taken when choosing to consume them.
Further discussion of these topics can be found in Section 7 below.
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 message to proceed toward delivery. This is more desirable than allowing the message to reach an internal host's MTA or spam filter, thus possibly generating a local rejection such as a Delivery Status Notification (DSN) [DSN] to a forged originator. Such generated rejections are colloquially known as "backscatter".
The same MAY also be done for local policy decisions overriding the results of the authentication methods (e.g., the "policy" result codes described in Section 2.7).
Such rejections at the SMTP protocol level are not possible if local policy is enforced at the MUA and not the MTA.
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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 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.
In the case of 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 MTA could allow a specific list of authenticating MTAs whose information is to be admitted, removing the header field originating from all others.
As stated in Section 1.2, a formal definition of "trust boundary" is deliberately not made here. It is entirely possible that a border MTA for example.com will explicitly trust authentication results asserted by upstream host example.net even though they exist in completely disjoint administrative boundaries. In that case, the border MTA MAY elect not to delete those results; moreover, the upstream host doing some authentication work could apply a signing technology such as [DKIM] on its own results to assure downstream hosts of their authenticity. An example of this is provided in Appendix B.
Similarly, in the case of messages signed using [DKIM] or other message-signing methods that sign header fields, this removal action 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.
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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 [SMTP] relaying 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.
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 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.
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 has been 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): RFC 8601 Related information: none
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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 are 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 has been 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.
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Status: the status of this entry, which is one of the following:
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.
Notably, the DomainKeys and Sender ID entries are not updated to refer to this revised specification, as they are considered obsolete. Accordingly, IANA has changed the "Status" field of the "sender-id" entry in this table to "deprecated".
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Finally, two new entries have been added to this registry, as follows:
[RFC7410] created the "Email Authentication Property Types" registry.
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:
ptype: the name of the ptype being registered, which must fit within the ABNF described in Section 2.2.
Definition: an optional reference to a defining specification.
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Description: a brief description of what sort of information this "ptype" is meant to cover.
For new entries, the designated expert needs to ensure 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 has updated 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.
All current entries in this registry have been 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.
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Status: the status of this entry, which is one of the following:
active: The entry is in current use.
deprecated: The entry is no longer in current use.
6.7. "Email Authentication Result Names" Registry Update
For the following entries in this registry, the new "Specification" field has been set as follows:
o All "auth" method result codes ("fail", "none", "pass", "permerror", and "temperror") are now specified in Section 2.7.4 of this document.
o All "dkim" method result names ("fail", "neutral", "none", "pass", "permerror", "policy", and "temperror") are now specified in Section 2.7.1 of this document.
o All "iprev" method result names ("fail", "pass", "permerror", and "temperror") are now specified in Section 2.7.3 of this document.
o The "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 registration for result name "hardfail" is not updated.
The following entries in this registry have been updated with a new "Status" field set to "deprecated", and with no change to the "Specification" field as they reference historic protocols:
o All "domainkeys" method result names ("fail", "neutral", "none", "pass", "permerror", "policy", and "temperror").
o All "sender-id" method result names ("fail", "neutral", "none", "pass", "permerror", "policy", "softfail", and "temperror").
The entry for X.7.25 in the "Enumerated Status Codes" subregistry of the "Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry" has been updated to refer only to Section 3.3 of [AUTH-ESC], as that is where that registration was done.
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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.
Proposed alternative solutions to this problem were made some time ago and are listed below. To date, they have not been developed due to lack of demand but are documented here should the information be useful at some point in the future:
1. Possibly the simplest is a digital signature protecting the header field, such as using [DKIM], that can be verified by an MUA by using a posted public key. Although one of the main purposes of this document is to relieve the burden of doing message authentication work at the MUA, this only requires that the MUA learn a single authentication scheme even if a number of them are in use at the border MTA. Note that [DKIM] requires that the From header field be signed, although in this application, the signing agent (a trusted MTA) likely cannot authenticate that value, so the fact that it is signed should be ignored. Where the authserv-id is the ADMD's domain name, the authserv-id matching this valid internal signature's "d=" DKIM value is sufficient.
2. Another would be a means to interrogate the MTA that added the header field to see if it is actually providing any message authentication services and saw the message in question, but this isn't especially palatable given the work required to craft and implement such a scheme.
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3. Yet another might be a method to interrogate the internal MTAs that apparently handled the message (based on Received header fields) to determine whether any of them conform to Section 5 of this memo. This, too, has potentially high barriers to entry.
4. Extensions to [IMAP], [SMTP], and [POP3] could be defined to allow an MUA or filtering agent to acquire the authserv-id in use within an ADMD, thus allowing it to identify which Authentication-Results header fields it can trust.
5. On the presumption that internal MTAs are fully compliant with Section 3.6 of [MAIL] and the compliant internal MTAs are using their own hostnames or the ADMD's DNS domain name as the authserv-id token, this header field should always appear above a Received header added by a trusted MTA. This can be used as a test for header field validity.
Support for some of these is being considered for future work.
In any case, a mechanism needs to exist for an MUA or filter 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. Given the variety of messaging environments deployed today, consensus appears to be that specifying a particular mechanism for doing so is not appropriate for this document.
Mitigation of the forged header field attack can also be accomplished by moving the authentication results data into metadata associated with the message. In particular, an SMTP extension [SMTP] could be established to communicate authentication results from the border MTA to intermediate and delivery MTAs; the latter of these could arrange to store the authentication results as metadata retrieved and rendered along with the message by an IMAP client [IMAP] aware of a similar extension in that protocol. The delivery MTA would be told to trust data via this extension only from MTAs it trusts, and border MTAs would not accept data via this extension from any source. There is no vector in such an arrangement for forgery of authentication data by an outside agent.
Until some form of service for querying the reputation of a sending agent is widely deployed, the existence of this header field indicating a "pass" does not render the message trustworthy. It is possible for an arriving piece of spam or other undesirable mail to pass checks by several of the methods enumerated above (e.g., a piece of spam signed using [DKIM] by the originator of the spam, which
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might be a spammer or a compromised system). In particular, this issue is not resolved by forged header field removal (discussed above).
Hence, MUAs and downstream filters must take some care with use of this header even after possibly malicious headers are scrubbed.
Despite the requirements of [MAIL], header fields can sometimes be reordered en route by intermediate MTAs. The goal of requiring header field addition only at the top of a message is an acknowledgment 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.
Section 4.6.4 of [SPF] observes that limits are necessary on recursive evaluations of SPF records in order to avoid abuse of or attacks on the DNS when verifying arriving client connections. A verifier wishing to do this check and report this information needs to take care not to go to unbounded lengths to resolve "A" and "PTR" queries. MUAs or other filters making use of an "iprev" result specified by this document need to be aware of the algorithm used by the verifier reporting the result and, especially, its limitations.
Failing to follow the instructions of Section 4.2 can result in a denial-of-service attack caused by the generation of DSN messages [DSN] (or equivalent) to addresses that did not send the messages being rejected.
Section 5 describes a procedure for scrubbing header fields that may contain forged authentication results about a message. A compliant installation will have to include, at each MTA, a list of other MTAs known to be compliant and trustworthy. Failing to keep this list current as internal infrastructure changes may expose an ADMD to attack.
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RFC 8601 Authentication-Results Header Field May 2019
If an attack against an authentication method becomes known, 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.
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.
An internal MUA or MTA that has been compromised could generate mail with a forged From header field and a forged Authentication-Results header field that endorses it. Although it is clearly a larger concern to have compromised internal machines than it is to prove the value of this header field, this risk can be mitigated by arranging that internal MTAs will remove this header field if it claims to have been added by a trusted border MTA (as described above), yet the SMTP connection [SMTP] is not coming from an internal machine known to be running an authorized MTA. However, in such a configuration, legitimate MTAs will have to add this header field when legitimate internal-only messages are generated. This is also covered in Section 5.
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 simple guidance to MUA developers is appropriate here.
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
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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 to ensure 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.
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".
[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>.
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[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>.
[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.
[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>.
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.
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: sender@example.com Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@example.org Message-Id: <12345.abc@example.com> Subject: here's a sample
Hello! Goodbye!
Example 1: Header Field Not Present
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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: sender@example.com Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@example.org 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, 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.
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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: sender@example.net Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@example.com Message-Id: <12345.abc@example.net> 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.
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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) smtp.auth=sender@example.net; 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: receiver@example.com From: sender@example.net Message-Id: <12345.abc@example.net> 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.
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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) smtp.auth=sender@example.com; 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: sender@example.com Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@example.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. 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.
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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.
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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" header.i=@mail-router.example.net; dkim=fail reason="bad signature" header.i=@newyork.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 <recipient@chicago.example.com> 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) header.i=@newyork.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: sender@newyork.example.com Date: Fri, Feb 15 2002 16:54:30 -0800 To: meetings@example.net Message-Id: <12345.abc@newyork.example.com> 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.
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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, meetings@example.net 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.
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
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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.
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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.
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.
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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