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Internet Area DKIM Working Group email, e-mail, rfc2822, rfc 2822, rfc822, rfc 822, rfc2821, rfc 2821, rfc821, rfc 821, rfc4871, rfc 4871, DKIM, domain keys, domainkeys, ADSP, ADSP, SSP, architecture, mta, user, delivery, smtp, submission, email, e-mail, smtp, Internet, mailfrom, mail from, author, return address, sender signing, signing practices, key domain, author key domain, author address, author domain DomainKeys Identified Mail (DKIM) as described in , defines a domain-level authentication framework for email to permit verification of the source and contents of messages. This document specifies an adjunct mechanism to aid in assessing messages that lack valid DKIM signatures for domains used in the author's address. It defines a record that can advertise the extent to which a domain signs outgoing mail that is publicly exchanged on SMTP port 25, as described in . Also, how other hosts can access those records. Advertisements, defined by this document, may also increase DKIM signature expectations for messages received by Mail User Agents (MUAs) or for messages which might have been exchanged over protocols other than SMTP. In some circumstances, author domains may wish to have accommodations for protocol failures or for mixed public protocol messaging not to be made. In addition, DKIM's identity parameters related to the author address are decisive only when a corresponding DKIM key local-part template precludes an author address. DKIM in conjunction with ADSP is to provide methods for detecting the spoofing of known domains, but not for making strong assertions about the identity of the message author.

DomainKeys Identified Mail (DKIM) defines a mechanism by which email messages can be cryptographically signed, permitting a Key Domain to claim responsibility for the introduction of a message. Receiving hosts can verify the signature by querying the Key Domain to retrieve the appropriate public key, and thereby confirming that a message has been attested to by a party in possession of the private key and in control of a portion of the Key Domain. However, the legacy of the Internet is such that not all messages will be signed or will retain a valid signature, and that absence of a valid signature on a message is not an a priori indication of forgery. In fact, during early phases of deployment, it is likely that most messages will remain unsigned. However, some domains might decide to sign all of their outgoing mail, for example, to better protect their brand name. It is desirable for such domains to be able to advertise that fact to other hosts. This is the premise of Author Domain Signing Practices (ADSP). Receiving hosts implementing this specification ensure greater safety by first inquiring into the validity of the SMTP domain before attempting a series of transactions related to DKIM validations. The transactions pertaining to this document determine Author Domain Signing Practices advertised by the Author Domains. This determination is called ADSP Discovery. The detailed requirements for Author Domain Signing Practices are given in . This document refers extensively to and assumes the reader is familiar with it. 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

Some terminology used herein is derived directly from . In several cases, references in that document to Sender have been changed to Author here, to emphasize the relationship to the Author address(es) in the From: header field described in . In addition, requires that a valid signature having a restrictive local-part template, the key's g= tag and value, must match against an undefined signing address. The signing address could be understood to be the email address associated with the signature's on behalf of identity which would be the i= tag and value or its default, but the benefit of this check is limited since DKIM signatures are not normally displayed. This document seeks to clearly define the signing address used in conjunction with a restrictive key's local-part template as being the Author Address. Briefly, The local-part is the part of an address preceding the @ sign, as defined in and used in .

A Valid Signature is any message signature which correctly verifies using procedures described in section 6.1 of .

A Valid Author Signature is any message signature which correctly verifies using procedures described in section 6.1 of , and where the local-part template, the key g= tag and value, and the Key Domain, match against the Author Address.

The Key Domain is the domain listed in the d= tag of a Valid Signature.

The Author Key Domain is the domain listed in the d= tag of a Valid Author Signature that is at or above the Author Domain. The Author Key Domain must match all of its domain components with that of the Author Domain. When a referenced Key contains a t=s tag and value, the Author Key Domain will contain the entire Author Domain for the signature to be valid.

An Author Address is an email address in the From header field of a message . If the From header field contains multiple addresses, the message has multiple Author Addresses.

An Author Domain is determined by the entire right-hand-side of the Author Address (the portion that is to the right of the @, excluding the @ itself).

Author Domain Signing Practices (or just practices) consist of a machine-readable record published at the _adsp. subdomain of the Author Domain. The ADSP record includes statements about outgoing mail practices for messages containing the Author Domain.

Domain owners can publish Author Domain Signing Practices via a distribution service, such as the Domain Name System; specific details related to the use of DNS are given in . Hosts can obtain Author Domain Signing Practices of the domain(s) specified by the Author Domain as described in . If a message has multiple Author Addresses, ADSP Discovery SHOULD be performed independently. This standard will not cover the consolidation of combined ADSP Discovery results.

A receiving host might obtain varying amounts of useful information through ADSP Discovery. Such as: When a message has a Valid Author Signature, the ADSP Discovery result is of no benefit since the message is compliant with any possible ADSP assertion. When a message has a Valid Signature that is not also a Valid Author Signature, the ADSP Discovery result, in conjunction with the Key Domain of the Valid Signature, is directly relevant to message assessment. When a message is without a Valid Signature, the ADSP Discovery result at the Author Domain is directly relevant to message assessment.

Author Domain Signing Practices Discovery at an Author Domain provide four possible results: Message contains an Author Domain that does not advertise practices. Message contains an Author Domain that advertises practices indicating they do not ensure messages are initially signed by an Author Key Domain. Message contains an Author Domain that advertises practices indicating they ensure messages are initially signed by an Author Key Domain. Message contains an Author Domain that advertises practices indicating they ensure messages are initially signed, and they recommend dismissing messages not signed by an Author Key Domain.

Author Signing Practices records are published using the DNS TXT resource record type. There has been considerable discussion on the DKIM WG mailing list regarding the relative advantages of TXT and a new resource record (RR) type. Read the archive for details. The RDATA for ADSP resource records is textual in format, with specific syntax and semantics relating to their role in describing Author Domain Signing Practices. The Tag=Value List syntax described in section 3.2 of is to be used. Records not in compliance with that syntax or with the syntax of individual tags described in Section 4.3 MUST be ignored, although they MAY cause the logging of warning messages via an appropriate system logging mechanism. If the RDATA contains multiple character strings, the strings are to be logically concatenated with no delimiters placed between the strings. The ADSP record for an Author Domain is published at a _adsp. subdomain directly below the Author Domain; e.g., the ADSP record for example.com would be a TXT record that is published at _adsp.example.com. A domain MUST NOT publish more than one ADSP record; the semantics of an ADSP transaction returning multiple ADSP records for a single domain are undefined. (Note that example.com and mail.example.com are different domains.)

Author Domain Signing Practices are intended to apply to all mail containing the Author Domain. As a defensive strategy against subdomain spoofing, ADSP records can be placed at domains that might appear to support SMTP. Wildcards within a domain that is also publishing ADSP records will not pose a problem. This is discussed in more detail in .

ADSP records use the "tag=value" syntax described in section 3.2 of . Terms used to describe signing practices employ a metaphor of a door to avoid connotations that might differ from definitions given in this document. Tags used in ADSP records are described below. Unrecognized tags MUST be ignored. In the ABNF below, the WSP token is imported from . The ALPHA and DIGIT tokens are imported from . practices (plain-text; REQUIRED). Possible values are as follows: (Default) The Author Domain practice permits unsigned outbound mail. The Author Domain practice always initially signs mail containing the Author Domain by an Author Key Domain. The Author Domain practice always initially signs mail containing the Author Domain by an Author Key Domain. Furthermore, when a message is received without a Valid Author Signature, receiving hosts are requested to dismiss such messages.

ABNF: adsp-dkim-tag = %x64.6b.69.6d *WSP "=" *WSP ("OPEN" / "CLOSED" / "LOCKED")

Hosts performing ADSP Discovery should first exclude SMTP clients with a demonstrated history of abuse. Also, the transactions needed for ADSP Discovery or DKIM signature validation should be subsequent to some confirmation that the Author Domain might support SMTP. In addition, hosts may consider some domains to be exempt, such as Top Level Domains (TLDs) listed in , for example .invalid. does not represent a comprehensive list of all possible exempted domains which might also include .local, therefore, appending to a list of exempted domains may be required. For the purposes of this section, a valid ADSP record is one that is both syntactically and semantically correct; in particular, it matches the ABNF for a tag-list and includes a defined dkim= tag. ADSP Discovery. The host SHOULD query DNS for a TXT record corresponding to the Author Domain prefixed by _adsp. (note the trailing dot). The results returned by this operation would be the value of the dkim tag or the value of MISSING when none exist. Rather than placing ADSP records below the _domainkey. prefix used by DKIM, _adsp. prefixed to the Author Domain reduces the number of DNS entities needed when ADSP records are desired at every address record. Delegation of a domain at or below _domainkey. and at _adsp. may be required when consolidating control of DNS entries related to DKIM. When any of the DNS transactions involved in ADSP Discovery result in a temporary error condition, the algorithm terminates without returning a result; possible actions include queuing the message or returning an SMTP error indicating a temporary failure. Within a DNS transaction, as defined by section 5.2.2 and section 3, when a CNAME is returned, the alias name is to be processed as if it were the initial name. section 10.3 makes an exception for Exchange host names returned by MX records. An Exchange host name must not return a CNAME.

ADSP introduces the _adsp name into currently unregistered name space. Although domain names beginning with an underscore will not collide with host names, service names for SRV records and labels for TXT records, defined by other protocols, reference underscore prefixed names to designate specific use. If at the time of publication no registry has been established or is planned for underscore prefixed names, this section may be removed.

Security considerations in the Author Domain Signing Practices mostly relate to attempts on the part of malicious senders to represent themselves as sending messages from the Author Domain for whom they are not authorized to use in their message. This is often done in an attempt to defraud recipients of the message. DKIM keys with a restrictive local-part template in the g= tag and value are likely to be employed for untrusted systems that are beyond the direct control of the Author Key Domain. As a result, additional care should be taken when a restrictive local-part template does not match against the Author Address. Signatures, where a restrictive local-part key g= tag and value and the Key Domain do not match against the Author Addresses, should be considered invalid. Signatures with restrictive local-part keys where the signing address is not that of the Author Address will be deceptive when marked as valid. Recipients are often unaware of the signature's on behalf of identity that is not normally displayed. In addition, these keys are prone to theft since they are also likely to be used by less secure mobile users, for example. Signatures with DKIM keys lacking a restrictive local-part template are only safely added when the Author Key Domain is able to directly evaluate signed header fields and content. Recognition of directly controlled signing improves security in several ways: Discerns potentially deceptive signatures independent of ADSP Discovery. Permits the accurate indication of on whose behalf the signature was added, even when not on behalf of the Author Address. Permits the on behalf of identity to be derived from an account instead of being omitted when the Author Key Domain is unable or unwilling to affirm the identity of the Author Address. Permits the identity to track either the author or the account used. This ability can be useful to third-parties who are attempting to isolate bot-net 0wned systems. In response to a growing portion of the IP address space being blocked, bot-nets increasingly send their mail through a provider's outbound server after obtaining access to valid accounts. Additional security considerations regarding Author Domain Signing Practices are found in the DKIM threat analysis.

Email recipients often have a core set of Author Domains that they trust. Common examples include those of financial institutions with which they have an existing relationship and Internet web commerce sites with which they conduct business. DKIM validation and ADSP Discovery results will not provide any benefit unless receiving hosts either treat the messages differently during delivery, or provide some indicator to the end recipient. Such email annotation is out of scope for this document. Bad actors often seek to exploit the name-recognition of a trusted Author Domain. This might be done by just spoofing display-names, or by placing user local-parts above subdomains or cousin domains in the From: header field. This problem is made worse by popular MUAs that do not display actual email addresses. As a result, there is no empirical evidence showing to what extent unauthorized use of a domain name contributes to recipient deception, nor that its elimination will provide a significant effect. Nevertheless, the automated accrual of behavioural feedback that ignores invalid identifiers better ensures that systematic confidence is retained for trusted Author Key Domains. Training recipients to use automated folder placement could also help reduce deceptions that utilize domain look-alike and subdomain based tactics. In addition, automated recognition facilitates optimized processing by receiver-side message filtering engines that attempt to curb unauthorized uses of domain names, organizations' names, and their logos elsewhere within the message. These attacks and their mitigation are also outside the scope of this document. The ADSP Discovery algorithm performs one DNS transaction per Author Domain. Since this transaction, as well as the ones needed to validate the DKIM signature, are driven by domain names in email message headers of possibly fraudulent email, receiving hosts attempting ADSP Discovery and DKIM validation can become participants in traffic multiplication attacks. These attacks often target servers consolidating and distributing behavioral information about bad-actor activities. Such attacks may dramatically impact the cost of offering the protective service. As a result, a reduction in number of those offering consolidated behavioral information places the remaining providers in greater jeopardy of receiving a larger portion of the abuse.

An attack might be waged against DNS infrastructure in an attempt to disable services dependent upon DNS. Such attacks could be made worse when receiving hosts employ ADSP Discovery and DKIM validation. A goal to First, do no harm is increasingly difficult to achieve when confronting massive bot-nets. For this reason, SMTP should consider eventually making MX records mandatory for the acceptance of public exchanges. The ADSP Discovery process is not expected to impact the likelihood of an attacker being successful at poisoning local DNS resolvers. In addition, such DNS security issues are addressed by DNSSEC. Although a steady attack may not cause a denial of service, it can consume significant resources related to in the cloud consolidation and distribution of behavioral information. A typical strategy used by bad actors employing bot-nets is to rapidly transition from an active to a dormant state. The duration of activity experienced by an SMTP server is often brief, and is then followed by a fairly long dormant period. This tactic proves challenging for defensive strategies attempted by individual hosts. There is some evidence that there may be tens of millions of bot-net controlled systems in the active state, while hundreds of millions appear dormant to individual SMTP servers. Consolidating and distributing behavioral information offers receiving hosts a defensive tactic that can minimize the effectiveness of the blitzkrieg or fast-flux tactic. Unfortunately, often part of a bad-actor's strategy is to inundate behavioral repositories with virtual identifiers. For DKIM, the signature's identity, i= tag and value, and key selector, s= tag and value, can be synthesized since wildcard domain support is possible, unlike for the Key Domain d= tag and value, or the location of the ADSP record. Because ADSP operates within the framework of the legacy e-mail system, the default result in the absence of an ADSP record is for the Author Domain to be considered OPEN where not all messages are expected to be signed by a Author Key Domain. It is therefore important that the ADSP clients distinguish a DNS failure such as SERVFAIL from other DNS errors, so that appropriate actions can be taken. It is likely that DKIM's and ADSP's combined roles will be to prevent deception when used in conjunction with automated folder placement of domains considered trustworthy. To ensure message reception remains viable for crucial systems when DNS fails, the IP addresses of crucial SMTP clients should be white-listed. This will allow ADSP and DKIM to be selectively bypassed during such events.

With the exception of wildcard MX records, wildcards within a domain that also publish ADSP records do not pose a significant problem. Although referencing SMTP related records will not provide NXDOMAIN results when a domain contains a wildcard, SMTP discovery records, such as MX or A records, still offer evidence of SMTP support. Whether AAAA records, absent MX or A records, can be considered evidence of SMTP support has not withstood widespread use of AAAA only servers. Complete ADSP coverage for all subdomains of a domain remains possible. However, ADSP records would need to be published at every subdomain containing A records, in addition to subdomains containing MX records. When SMTP adopts an MX record mandate for the acceptance of public exchanges, only then could ADSP records be limited to subdomains containing the MX records. This strategy would also shelter domains not publishing MX records from the additional transactions associated with any number of Author Addresses and DKIM signatures that might be generated per message.

&rfc1034; &rfc2119; &rfc2181; &rfc2606; &rfc2821; &rfc2822; &rfc4033; &rfc4034; &rfc4686; &rfc4871; &rfc5234; &rfc2782; &rfc5016;

These examples are intended to illustrate typical uses of ADSP. They are not intended to be exhaustive, nor to apply to every domain's or mail system's individual situation. Administrators are advised to consider the ways that mail processing can modify messages in a manner that will invalidate existing DKIM signatures, such as mailing lists, courtesy forwarders, and other paths that could add or modify headers, or modify the message body. In that case, if these modifications invalidate DKIM signatures, receiving hosts will consider the mail not to have a Valid Author Signature, even though one was present when the mail was originally sent.

A common mail system configuration handles all of a domain's users' incoming and outgoing mail through a single MTA or group of MTAs. In that case, the MTA(s) can be configured to sign outgoing mail with an Author Signature. In this situation, it might be appropriate to publish a CLOSED ADSP record for the Author Domain, depending on whether users also send mail through other paths that do not apply an Author Key Domain signature. Such paths could include MTAs at hotels or hotspot networks used by travelling users, or web sites that provide mail an article features.

Another common configuration uses a domain solely for bulk or broadcast mail, with no individual human users, again, typically sending all the mail through a single MTA or group of MTAs that can apply an Author Key Domain signature. In this case, before publishing a CLOSED ADSP record, the domain's management should be confident that all of its outgoing mail will be sent through signing MTAs. Because it lacks individual users, the domain is unlikely to participate in mailing lists, but could still send mail through other paths that might invalidate signatures. Domain owners often use specialist mailing providers to send their bulk mail. In that case, the mailing provider needs access to a suitable signing key in order to apply an Author Key Domain signature. One possible method would be for the Author Key Domain owner to exchange keys with the mailing provider. Another would be for the Author Key Domain to delegate a subdomain below the _domainkey. label to the mailing provider. For example, bigbank.example.com might delegate esp-00._domainkey.bigbank.example.com to such a provider. As a result, the provider could generate keys and DKIM DNS records itself and thereby provide Author Key Domain signatures.

In some cases, a domain might sign all its outgoing mail with an Author Key Domain signature, but prefer that receiving host systems dismiss mail without a valid Author Key Domain signature to avoid confusion with mail sent from fraudulent sources that are unable to apply an Author Key Domain signature. (This latter kind of mail is sometimes loosely called "forgeries".) In that case, it might be appropriate to publish a "LOCKED" ADSP record. Note that a domain SHOULD NOT publish a LOCKED ADSP record when it wishes to maximize the likelihood that its mail is delivered, since it could cause some fraction of the mail to become rejected or discarded. As a special case, if a domain sends no mail at all, it can safely publish a LOCKED ADSP record, since any mail with this Author Domain would be a forgery.

Another common use case is for a third party to enter into an agreement whereby that third party will send bulk or other mail on behalf of a designated Author Domain, using that domain in the RFC2822 From: or other headers. Due to the many and varied complexities of such agreements, third party signing is not addressed in this specification.

This document was based upon the draft-ietf-dkim-ssp-003. Dave Crocker, Frank Ellermann, and Charles Lindsey inputs were valuable. However, inclusion of their names should not be misconstrued as an endorsement of this draft. This draft is an individual submission intended to illustrate a comprehensive solution that might help foreclose protracted debate when there is otherwise general agreement.

Conditioned Author Signing Practices Discovery Procedure SMTP verification step to be made only when an MX record had not been found.

Modified the Author Signing Practices Discovery Procedure to better conform with terms in RFC2821. In addition, a note now covers the issue of CNAMEs.

Modified the abstract to include the language recommended by Dave Crocker, clarified the relationship this document has with DKIM, SMTP and other protocols, and clarified the extent of DKIM identity parameter. The general language describing the intent was taken from the WG charter. Included non retention of a valid signature and offered an admonishment to first validate from domain in the introduction. Added a separate definition for Valid Author Signatures by including the requirement the local-part template must match against the author addresses. Made a few minor changes to the Author Key Domain definition. Included the phrase "related to the use of DNS" to the operation Overview as well as expanding upon the term ADSP Discovery in several places. Modified ADSP Usage to Discovery Results Usage. The information provided was reorganized from least to most useful. Modified the terms in ADSP Discovery Results to be consistent with advertised practices to align more closely with Dave Crocker's Abstract. The Record syntax now mentions the terms used are a metaphor for a door, and the terms modified to be in closer alignment with the abstract. The ADSP Discovery procedure now warns about unlimited application of this process, and suggests some domains may require exemption, and introduces the term MISSING when no ADSP record is discovered. The IANA considerations where shortened based upon the assumption a registry may not be established for underscore prefixed TXT records. Change the beginning of the security section to clarify the domain and not the author identity is assured by DKIM and ADSP. Changed the wording related to the key "g=" parameter to be more consistent throughout the document. Mention in the threat model annotation is required, but is out of scope. Modified the paragraph that describes exploitation of trust to be about the domain and not the author identity. Mention that the target of an attack is often directed to behavioral information services. Add paragraph describing the typical nature of bot-net behaviour, and how the DKIM "i=" represents a significant vulnerability for the accrual of behavioral information. Add a sentence to highlight benefits using automatic folder placement. Expanded the DNS wildcard section to generally describe what might be involved when validating the domain's support of SMTP.

Clarify the definition of signing address when used in conjunction with restrictive local-part templates by adding a paragraph to Section 2.1. Modified the list in Section 3.2 to include the case where the Author Domain has no ADSP record. Give examples in Section 4.2.2 regarding possibly exempt TLDs. Expand upon the recognition of direct versus indirect control of the DKIM signing process, and how this relates to the use of the "on behalf of" identity by adding two paragraphs to Section 6. Made several minor grammatical changes.