Internet Engineering Task Force (IETF) M. Kucherawy Request for Comments: 7001 September 2013 Obsoletes: 5451, 6577 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. Status of This Memo This is an Internet Standards Track document. 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 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7001. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents 1. Introduction ....................................................3 1.1. Purpose ....................................................4 1.2. Trust Boundary .............................................5 1.3. Processing Scope ...........................................6 1.4. Requirements ...............................................6 1.5. Definitions ................................................6 1.5.1. Key Words ...........................................6 1.5.2. Security ............................................6 1.5.3. Email Architecture ..................................7 1.5.4. Other Terms .........................................8 1.6. Trust Environment ..........................................8 2. Definition and Format of the Header Field .......................9 2.1. General Description ........................................9 2.2. Formal Definition ..........................................9 2.3. The "policy" ptype ........................................12 2.4. Authentication Identifier Field ...........................13 2.5. Version Tokens ............................................14 2.6. Defined Methods and Result Values .........................14 2.6.1. DKIM and DomainKeys ................................14 2.6.2. SPF and Sender ID ..................................15 2.6.3. "iprev" ............................................17 2.6.4. SMTP AUTH ..........................................17 2.6.5. Other Registered Codes .............................18 2.6.6. Extension Methods ..................................18 2.6.7. Extension Result Codes .............................19 3. The "iprev" Authentication Method ..............................19 4. Adding the Header Field to a Message ...........................20 4.1. Header Field Position and Interpretation ..................22 4.2. Local Policy Enforcement ..................................23 5. Removing Existing Header Fields ................................23 6. IANA Considerations ............................................24 6.1. The Authentication-Results Header Field ...................25 6.2. "Email Authentication Methods" Registry ...................25 6.3. "Email Authentication Result Names" Registry ..............26 7. Security Considerations ........................................26 7.1. Forged Header Fields ......................................26 7.2. Misleading Results ........................................28 7.3. Header Field Position .....................................28 7.4. Reverse IP Query Denial-of-Service Attacks ................28 7.5. Mitigation of Backscatter .................................29 7.6. Internal MTA Lists ........................................29 7.7. Attacks against Authentication Methods ....................29 7.8. Intentionally Malformed Header Fields .....................29 7.9. Compromised Internal Hosts ................................29 7.10. Encapsulated Instances ...................................30 7.11. Reverse Mapping ..........................................30
8. References .....................................................30 8.1. Normative References ......................................30 8.2. Informative References ....................................31 Appendix A. Acknowledgements .....................................33 Appendix B. Legacy MUAs ..........................................33 Appendix C. Authentication-Results Examples ......................33 C.1. Trivial Case; Header Field Not Present ....................34 C.2. Nearly Trivial Case; Service Provided, but No Authentication Done .......................................34 C.3. Service Provided, Authentication Done .....................35 C.4. Service Provided, Several Authentications Done, Single MTA .......................................................36 C.5. Service Provided, Several Authentications Done, Different MTAs ............................................37 C.6. Service Provided, Multi-Tiered Authentication Done ........38 C.7. Comment-Heavy Example .....................................40 Appendix D. Operational Considerations about Message Authentication .......................................40 Appendix E. Changes since RFC 5451 ...............................42 1. Introduction This document describes a header field called Authentication-Results for electronic mail messages that presents the results of a message authentication effort in a machine-readable format. The intent of the header field is to create a place to collect such data when message authentication mechanisms are in use so that a Mail User Agent (MUA) and downstream filters can make filtering decisions and/or provide a recommendation to the user as to the validity of the message's origin and possibly the safety and integrity of its content. This document revises the original definition found in [RFC5451] based upon various authentication protocols in current use and incorporates errata logged since the publication of the original specification. 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, domain-level email authentication methods in common use: o Author Domain Signing Practices ([ADSP]) o SMTP Service Extension for Authentication ([AUTH]) o DomainKeys Identified Mail Signatures ([DKIM]) o Sender Policy Framework ([SPF]) o Vouch By Reference ([VBR]) o reverse IP address name validation ("iprev", defined in Section 3) In addition, the following are non-standard methods recognized by this specification that are no longer common: o DomainKeys ([DOMAINKEYS]) (Historic) o Sender ID ([SENDERID]) (Experimental) This specification is not intended to be restricted to domain-based authentication schemes, but the existing schemes in that family have proven to be a good starting point for implementations. The goal 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. 1.1. Purpose 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 this data; 3. Create an extensible framework for reporting new authentication methods as they emerge. In particular, the mere presence of this header field does not mean its contents are valid. Rather, the header field is reporting assertions made by one or more authentication schemes (supposedly) applied somewhere upstream. For an MUA or downstream filter to treat the assertions as actually valid, there must be an assessment of the trust relationship among such agents, the validating MTA, and the mechanism for conveying the information. 1.2. Trust Boundary This document makes several references to the "trust boundary" of an administrative management domain (ADMD). Given the diversity among existing mail environments, a precise definition of this term isn't possible. Simply put, a transfer from the producer of the header field to the consumer must occur within a context that permits the consumer to 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, this field found within one of those media types is typically ignored.
1.3. Processing Scope The content of this header field is meant to convey to message consumers that authentication work on the message was already done within its trust boundary, and those results are being presented. It is not intended to provide message parameters to consumers so that they can perform authentication protocols on their own. 1.4. Requirements This document establishes no new requirements on existing protocols or servers. 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. 1.5. Definitions This section defines various terms used throughout this document. 1.5.1. Key Words The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [KEYWORDS]. 1.5.2. Security "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 and Sender ID are authorization mechanisms in that they express a result that shows whether or not the ADMD that apparently sent the message has explicitly authorized the connecting Simple Mail Transfer Protocol ([SMTP]) client to relay messages on its behalf, but they do not actually validate any other property of the message itself. By contrast, DKIM is agnostic as to the routing of a message but uses cryptographic signatures to authenticate agents, assign (some) responsibility for the message (which implies authorization), and ensure that the listed portions of the message were not modified in transit. Since the signatures are not tied to SMTP connections, they can be added by either the ADMD of origin, intermediate ADMDs (such as a mailing list server), other handling agents, or any combination. Rather than create a separate header field for each class of solution, this proposal groups them both into a single header field. 1.5.3. Email Architecture o A "border MTA" is an MTA that acts as a gateway between the general Internet and the users within an organizational boundary. (See also Section 1.2.) 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. o An "intermediate MTA" is any MTA that is not a delivery MTA and is also not the first MTA to handle the message. 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 D 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. 1.5.4. Other Terms 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. 1.6. Trust Environment 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 Administrative Management Domain (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 are present 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 more formal specification. 2.1. General Description 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 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(s) 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. The header field can appear more than once in a single message, more than one result can be represented in a single header field, or a combination of these can be applied. 2.2. Formal Definition Formally, the header field is specified as follows using Augmented Backus-Naur Form ([ABNF]): authres-header = "Authentication-Results:" [CFWS] authserv-id [ CFWS authres-version ] ( no-result / 1*resinfo ) [CFWS] CRLF 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 resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ] *( CFWS propspec ) 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 method = Keyword [ [CFWS] "/" [CFWS] method-version ] ; a method indicates which method's result is ; represented by "result", and 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 = "smtp" / "header" / "body" / "policy" ; indicates whether the property being evaluated was ; a parameter to an [SMTP] command, 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
property = special-smtp-verb / Keyword ; if "ptype" is "smtp", this indicates which [SMTP] ; command provided the value that was evaluated by the ; authentication scheme being applied; if "ptype" is ; "header", this indicates from which header field the ; value being evaluated was extracted; if "ptype" is ; "body", this indicates where in the message body ; a value being evaluated can be found (e.g., a specific ; offset into the message or a reference to a MIME part); ; if "ptype" is "policy", then this indicates the name ; of the policy that caused this header field to be ; added (see below) special-smtp-verb = "mailfrom" / "rcptto" ; special cases of [SMTP] commands that are made up ; of multiple words pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name ) [CFWS] ; the value extracted from the message property defined ; by the "ptype.property" construction "local-part" is defined in Section 3.4.1 of [MAIL], and "CFWS" is defined in Section 3.2.2 of [MAIL]. "Keyword" is defined in Section 4.1.2 of [SMTP]. The "value" is as defined in Section 5.1 of [MIME]. The "domain-name" is as defined in Section 3.5 of [DKIM]. The "Keyword" used in "result" above is further constrained by the necessity of being enumerated in Section 2.6. See Section 2.4 for a description of the authserv-id element. If the value portion of a "pvalue" construction identifies something intended to be an e-mail 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 has a result to report.
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 2.3 for details. Examples of complete messages using this header field can be found in Appendix C. 2.3. The "policy" ptype A special ptype value of "policy" is 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-Result field: ... dkim=fail policy.dkim-rules=unsigned-subject ... In this case, the property is "dkim-rules", indicating some local check by that name took place and that check returned a result of "unsigned-subject". These are arbitrary names selected by (and presumably used within) the ADMD making use of them, so they are not normally registered with IANA or otherwise specified apart from setting syntax restrictions that allow for easy parsing within the rest of the header field. This ptype existed in the original specification for this header field, but without a complete description or example of intended use. As a result, it has not seen any practical use to date that matches its intended purpose. These added details are provided to guide implementers toward proper use.
2.4. Authentication Identifier Field Every Authentication-Results header field has an authentication service identifier field (authserv-id above). Specifically, this is any string intended to identify the authentication service within the ADMD that conducted authentication checks on the message. This identifier is intended to be machine-readable and not necessarily meaningful to users. 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 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; these 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 re-assessment of the header field's contents. Examples of valid authentication identifiers are "example.com", "mail.example.org", "ms1.newyork.example.com", and "example-auth". 2.5. Version Tokens 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. 2.6. Defined Methods and Result Values 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. 2.6.1. DKIM and DomainKeys DKIM is represented by the "dkim" method and is defined in [DKIM]. DomainKeys is defined in [DOMAINKEYS] and is represented by the "domainkeys" method.
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. Both DKIM and DomainKeys use the same result set, 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] 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 the message will be treated as if it had not been signed. 2.6.2. SPF and Sender ID SPF and Sender ID use the "spf" and "sender-id" method names, respectively. The result values for SPF are defined in Section 2.5 of [SPF], and those definitions are included here by reference:
+-----------+----------------------------+ | Code | Meaning | +-----------+----------------------------+ | none | [SPF], Section 2.5.1 | +-----------+----------------------------+ | pass | [SPF], Section 2.5.3 | +-----------+----------------------------+ | fail | [SPF], Section 2.5.4 | +-----------+----------------------------+ | softfail | [SPF], Section 2.5.5 | +-----------+----------------------------+ | policy | [RFC7001], Section 2.6.2 | +-----------+----------------------------+ | neutral | [SPF], Section 2.5.2 | +-----------+----------------------------+ | temperror | [SPF], Section 2.5.6 | +-----------+----------------------------+ | permerror | [SPF], Section 2.5.7 | +-----------+----------------------------+ These result codes are used in the context of this specification to reflect the result returned by the component conducting SPF evaluation. Similarly, the results for Sender ID are listed and described in Section 4.2 of [SENDERID], which in turn uses the SPF definitions. Note that both of those documents specify result codes that use mixed case, but they are typically used all lowercase in this context. In both cases, an additional result of "policy" is defined, which means the client was authorized to inject or relay mail on behalf of the sender's DNS domain according to the authentication method's algorithm, but local policy dictates that the result is unacceptable. For example, "policy" might be used if SPF returns a "pass" result, but a local policy check matches the sending DNS domain to one found in an explicit list of unacceptable DNS domains (e.g., spammers). If the retrieved sender policies used to evaluate SPF and Sender ID 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 these mechanisms SHOULD NOT include the local- part.
2.6.3. "iprev" 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. For discussion of the format of DNS replies, see "Domain Names - Implementation and Specification" ([DNS]). 2.6.4. SMTP AUTH SMTP AUTH (defined in [AUTH]) is represented by the "auth" method, and 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, yet continued to send the message about which a result is being reported. 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. An agent making use of the data provided by this header field SHOULD consider "fail" and "temperror" to be synonymous in terms of message authentication, i.e., the client did not authenticate in either case. 2.6.5. Other Registered Codes Result codes were also registered in other RFCs for Vouch By Reference (in [AR-VBR], represented by "vbr"), Authorized Third-Party Signatures (in [ATPS], represented by "dkim-atps"), and the DKIM- related Author Domain Signing Practices (in [ADSP], represented by "dkim-adsp"). 2.6.6. Extension Methods Additional authentication method identifiers (extension methods) may be defined in the future by later revisions or extensions to this specification. These method identifiers are registered with the Internet Assigned Numbers Authority (IANA) and, preferably, published in an RFC. See Section 6 for further details. Extension methods can be defined for the following reasons: 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".
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 was used to perform an evaluation, such information could be relayed as a comment in the header field, such as: Authentication-Results: example.com; foo=pass bar.baz=blob (2 of 3 tests OK) Experimental method identifiers MUST only be used within ADMDs that have explicitly consented to use them. These method identifiers and the parameters associated with them are not documented in RFCs. Therefore, they are subject to change at any time and not suitable for production use. Any MTA, MUA, or downstream filter intended for production use SHOULD ignore or delete any Authentication-Results header field that includes an experimental (unknown) method identifier. 2.6.7. Extension Result Codes Additional result codes (extension results) might be defined in the future by later revisions or extensions to this specification. Result codes MUST be registered with the Internet Assigned Numbers Authority (IANA) and preferably published in an RFC. See Section 6 for further details. Extension 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
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. 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 5.5 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 acknowledgement that the method remains common and provides the means to relay the results of that test. 4. Adding the Header Field to a Message 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 2.6.6, and each "result" MUST refer to a result code declared in the IANA registry or an extension result code as defined in Section 2.6.7. 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 got 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 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 and Sender ID differ in that they can yield a result based on more than one identifier; specifically, SPF can evaluate the RFC5321.HELO parameter or the RFC5321.MailFrom parameter, and Sender ID can evaluate the RFC5321.MailFrom parameter or the Purported Responsible Address (PRA) identity. When generating this field to report those results, only the parameter that yielded the result is included. 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 has 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. 4.1. Header Field Position and Interpretation 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 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 instances of the header field appearing 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 it bears appears to be one used within its own 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 corresponding 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. 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 "one") and send signed mail to intended victims; a verifier would detect that the signature was valid and report a "pass" even though it's clear the DNS domain name was intended to mislead. See Section 7.2 for further discussion. As stated in Section 2.1, this header field MUST be treated as though 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. Further discussion of these topics can be found in Section 7 below. 4.2. Local Policy Enforcement Some sites have a local policy that considers any particular authentication policy's non-recoverable failure results (typically "fail" or similar) as justification for rejecting the message. In such cases, the border MTA SHOULD issue an SMTP rejection response to the message, rather than adding this header field and allowing the 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 2.6). Such rejections at the SMTP protocol level are not possible if local policy is enforced at the MUA and not the MTA. 5. Removing Existing Header Fields For security reasons, any MTA conforming to this specification MUST delete any discovered instance of this header field that claims, by virtue of its authentication service identifier, to have been added within its trust boundary but that did not come directly from another trusted MTA. For example, an MTA for example.com receiving a message MUST delete or otherwise obscure any instance of this header field bearing an authentication service identifier indicating that the header field was added within example.com prior to adding its own header fields. This could mean each MTA will have to be equipped with a list of internal MTAs known to be compliant (and hence trustworthy).
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 C. 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. An MTA SHOULD remove any instance of this header field bearing a version (express or implied) that it does not support. However, an MTA MUST remove such a header field if the [SMTP] connection relaying the message is not from a trusted internal MTA. This means the MTA needs to be able to understand versions of this header field at least as late as the ones understood by the MUAs or other consumers within its ADMD. 6. IANA Considerations IANA has registered the defined header field and created two tables as described below. These registry actions were originally defined by [RFC5451] and are repeated here to provide a single, current reference.
6.1. The Authentication-Results Header Field [RFC5451] added the Authentication-Results header field 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 7001 Related information: Requesting review of any proposed changes and additions to this field is recommended. 6.2. "Email Authentication Methods" Registry Names of message authentication methods supported by this specification are to be registered with IANA, with the exception of experimental names as described in Section 2.6.6. A registry was created by [RFC5451] for this purpose. This document changes the rules governing that registry. 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". Each method must register a name, the specification that defines it, a version number associated with the method being registered (preferably starting at "1"), zero or more "ptype" values appropriate for use with that method, which "property" value(s) should be reported by that method, and a description of the "value" to be used with each. All existing registry entries that reference [RFC5451] have been updated to reference this document, except where entries have already been deprecated. IANA has also added a "version" field to all existing registry entries. All current methods are recorded as version "1".
6.3. "Email Authentication Result Names" Registry 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 2.6.7. A registry was created by [RFC5451] for this purpose. This document changes the rules governing that registry. 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". All existing registry entries that reference [RFC5451] have been updated to reference this document. The definitions for the SPF and Sender ID authentication methods are updated using the references found in Section 2.6.2.