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DomainKeys Identified Mail (DKIM) 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 do not contain a DKIM signature for the domain used in the author's address. It defines a record that can advertise whether a domain signs its outgoing mail, and how other hosts can access that record.
2. Language and Terminology
2.1. Terms Imported from DKIM Signatures Specification
2.2. Valid Signature
2.3. Author Address
2.4. Author Domain
2.5. Alleged Author
2.6. Author Domain Signing Practices
2.7. Author Signature
3. Operation Overview
3.1. ADSP Applicability
3.2. ADSP Usage
3.3. ADSP Results
4. Detailed Description
4.1. DNS Representation
4.2. Publication of ADSP Records
4.3. ADSP Lookup Procedure
5. IANA Considerations
5.1. ADSP Specification Tag Registry
5.2. ADSP Outbound Signing Practices Registry
6. Security Considerations
6.1. ADSP Threat Model
6.2. DNS Attacks
6.3. DNS Wildcards
7.1. References - Normative
7.2. References - Informative
Appendix A. Usage Examples
A.1. Single Location Domains
A.2. Bulk Mailing Domains
A.3. Bulk Mailing Domains with Discardable Mail
A.4. Third Party Senders
A.5. Domains with Independent Users and Liberal Use Policies
A.6. Non-email Domains
Appendix B. Acknowledgements
Appendix C. Change Log
C.1. Changes since -ietf-dkim-04
C.2. Changes since -ietf-dkim-03
C.3. Changes since -ietf-dkim-02
C.4. Changes since -ietf-dkim-ssp-01
C.5. Changes since -ietf-dkim-ssp-00
C.6. Changes since -allman-ssp-02
C.7. Changes since -allman-ssp-01
C.8. Changes since -allman-ssp-00
§ Authors' Addresses
§ Intellectual Property and Copyright Statements
DomainKeys Identified Mail (DKIM) defines a mechanism by which email messages can be cryptographically signed, permitting a signing domain to claim responsibility for the introduction of a message into the mail stream. Message recipients can verify the signature by querying the signer's domain directly to retrieve the appropriate public key, and thereby confirm that the message was attested to by a party in possession of the private key for the signing domain.
However, the legacy of the Internet is such that not all messages will be signed, and the absence of a signature on a message is not an a priori indication of forgery. In fact, during early phases of deployment it is very likely that most messages will remain unsigned. However, some domains might decide to sign all of their outgoing mail, for example, to protect their brand names. It is desirable for such domains to be able to advertise that fact to other hosts. This is the topic of Author Domain Signing Practices (ADSP).
Hosts implementing this specification can inquire what Author Signing Practices a domain advertises. This inquiry is called an Author Signing Practices check.
The basic requirements for ADSP are given in [RFC5016] (Thomas, M., “Requirements for a DomainKeys Identified Mail (DKIM) Signing Practices Protocol,” October 2007.). This document refers extensively to [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.) and assumes the reader is familiar with it.
- Requirements Notation:
- 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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.)
Some terminology used herein is derived directly from [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.). 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 [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.). Briefly,
A "Valid Signature" is any signature on a message which correctly verifies using the procedure described in section 6.1 of [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.).
An "Author Address" is an email address in the From header field of a message [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.). If the From header field contains multiple addresses, the message has multiple Author Addresses.
An "Author Domain" is everything to the right of the "@" in an Author Address (excluding the "@" itself).
An "Alleged Author" is an Author Address of a message; it is "alleged" because it has not yet been checked.
"Author Domain Signing Practices" (or just "practices") consist of a machine-readable record published by the domain of an Alleged Author which includes statements about the domain's practices with respect to mail it sends with its domain in the From: line.
An "Author Signature" is any Valid Signature where the identity of the user or agent on behalf of which the message is signed (listed in the i= tag or its default value from the d= tag) matches an Author Address in the message. When the identity of the user or agent includes a Local-part, the identities match if the Local-parts are the same string, and the domains are the same string. Otherwise, the identities match if the domains are the same string. Following [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.), Local-part comparisons are case sensitive, domain comparisons are case insensitive.
For example, if a message has a Valid Signature, with the DKIM-Signature field containing email@example.com, then domain.example is asserting that it takes responsibility for the message. If the message's From: field contains the address firstname.lastname@example.org and an ADSP query produces a dkim=all or dkim=discardable result, that would mean that the message does not have a valid Author Signature. Even though the message is signed by the same domain, it fails to satisfy ADSP.
Domain owners can publish ADSP information via a query mechanism such as the Domain Name System; specific details are given in Section 4.1 (DNS Representation).
Hosts can look up the ADSP information of the domain(s) specified by the Author Address(es) as described in Section 4.3 (ADSP Lookup Procedure). If a message has multiple Author Addresses the ADSP lookups SHOULD be performed independently on each address. This standard does not address the process a host might use to combine the lookup results.
ADSP as defined in this document is bound to DNS. For this reason, ADSP is applicable only to Author Domains with appropriate DNS records (see Note below). The handling of other Author Domains is outside the scope of this document. However, attackers may use such domain names in a deliberate attempt to sidestep an organization's ADSP policy statements. It is up to the ADSP checker implementation to return an appropriate error result for Author Domains outside the scope of ADSP.
- The results from DNS queries that are intended to validate a domain name unavoidably approximate the set of Author Domains that can appear in legitimate email. For example, a DNS A record could belong to a device that does not even have an email implementation. It is up to the checker to decide what degree of approximation is acceptable.
Depending on the Author Domain(s) and the signatures in a message, a recipient gets varying amounts of useful information from each ADSP lookup.
An ADSP lookup for an Author Address produces one of four possible results:
ADSP records are published using the DNS TXT resource record type.
The RDATA for ADSP resource records is textual in format, with specific syntax and semantics relating to their role in describing ADSP. The "Tag=Value List" syntax described in section 3.2 of [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.) is used. Records not in compliance with that syntax or the syntax of individual tags described in Section 4.3 MUST be ignored (considered equivalent to a NODATA result) for purposes of ADSP, 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 logically concatenated with no delimiters between the strings.
The ADSP record for a domain is published at a location in the domain's DNS hierarchy prefixed by _adsp._domainkey.; e.g., the ADSP record for example.com would be a TXT record that is published at _adsp._domainkey.example.com. A domain MUST NOT publish more than one ADSP record; the semantics of an ADSP lookup that returns multiple ADSP records for a single domain are undefined. (Note that example.com and mail.example.com are different domains.)
ADSP records MUST NOT be published at any location other than the_adsp._domainkey subdomain of the domain for which they are expressing signing practices. In particular, domains MUST NOT publish ADSP records with wildcard names. Wildcards within a domain publishing ADSP records pose a particular problem, as discussed in more detail in Section 6.3 (DNS Wildcards).
ADSP is intended to apply to all mail sent using the domain name string of an Alleged Author.
ADSP records use the "tag=value" syntax described in section 3.2 of [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.). Every ADSP record MUST start with an outbound signing practices tag, so the first four characters of the record are lower case "dkim".
Tags used in ADSP records are described below. Unrecognized tags MUST be ignored. In the ABNF below, the WSP token, and the ALPHA and DIGIT tokens are imported from [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.).
- Outbound signing practices for the domain (plain-text; REQUIRED). Possible values are as follows:
- The domain might sign some or all email.
- All mail from the domain is signed with an Author Signature.
- All mail from the domain is signed with an Author Signature. Furthermore, if a message arrives without a valid Author Signature due to modification in transit, submission via a path without access to a signing key, or other reason, the domain encourages the recipient(s) to discard it.
ABNF:adsp-dkim-tag = %x64.6b.69.6d *WSP "=" *WSP ("unknown" / "all" / "discardable")
Hosts doing an ADSP lookup MUST produce a result that is semantically equivalent to applying the following steps in the order listed below. In practice, these steps can be performed in parallel in order to improve performance. However, implementations SHOULD avoid doing unnecessary DNS lookups.
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 valid dkim tag.
- Check Domain Scope:
- An ADSP checker implementation MUST determine whether a given Author Domain is within scope for ADSP. Given the background in Section 3.1 (ADSP Applicability) the checker MUST decide which degree of approximation is acceptable. The checker MUST return an appropriate error result for Author Domains that are outside the scope of ADSP.
- The host MUST perform a DNS query for a record corresponding to the Author Domain (with no prefix). The type of the query can be of any type, since this step is only to determine if the domain itself exists in DNS. This query MAY be done in parallel with the query to fetch the Named ADSP Record. If the result of this query is that the Author domain does not exist in the DNS (often called an NXDOMAIN error), the algorithm MUST terminate with an error indicating that the domain is out of scope.
NON-NORMATIVE DISCUSSION: Any resource record type could be used for this query since the existence of a resource record of any type will prevent an NXDOMAIN error. MX is a reasonable choice for this purpose because this record type is thought to be the most common for domains used in e-mail, and will therefore produce a result which can be more readily cached than a negative result.
- If the domain does exist, the checker MAY make more extensive checks to verify the existence of the domain, such as the ones described in Section 5 of [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.). If those checks indicate that the Author domain does not exist for mail, e.g., the domain has no MX, A, or AAAA record, the checker SHOULD terminate with an error indicating that the domain is out of scope.
- Fetch Named ADSP Record:
- The host MUST query DNS for a TXT record corresponding to the Author Domain prefixed by _adsp._domainkey. (note the trailing dot).
- If the result of this query is a NOERROR response with an answer which is a valid ADSP record, use that record, and the algorithm terminates.
- If a query results in a SERVFAIL error response, the algorithm terminates without returning a result; possible actions include queuing the message or returning an SMTP error indicating a temporary failure.
ADSP adds the following namespaces to the IANA registry. In all cases, new values are assigned only for values that have been documented in a published RFC that has IETF Consensus [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).
An ADSP record provides for a list of specification tags. IANA has established the ADSP Specification Tag Registry for specification tags that can be used in ADSP fields.
The initial entry in the registry is:
+------+-----------------+ | TYPE | REFERENCE | +------+-----------------+ | dkim | (this document) | +------+-----------------+
ADSP Specification Tag Registry Initial Values
The dkim= tag spec, defined in Section 4.2.1 (Record Syntax), provides for a value specifying Outbound Signing Practices. IANA has established the ADSP Outbound Signing Practices Registry for Outbound Signing Practices.
The initial entries in the registry comprise:
+-------------+-----------------+ | TYPE | REFERENCE | +-------------+-----------------+ | unknown | (this document) | | all | (this document) | | discardable | (this document) | +-------------+-----------------+
ADSP Outbound Signing Practices Registry Initial Values
Security considerations in the ADSP are mostly related to attempts on the part of malicious senders to represent themselves as authors for whom they are not authorized to send mail, often in an attempt to defraud either the recipient or an Alleged Author.
Additional security considerations regarding Author Domain Signing Practices are found in the DKIM threat analysis (Fenton, J., “Analysis of Threats Motivating DomainKeys Identified Mail (DKIM),” September 2006.) [RFC4686].
Email recipients often have a core set of content authors that they already trust. Common examples include financial institutions with which they have an existing relationship and Internet web transaction sites with which they conduct business.
Email abuse often seeks to exploit a legitimate email author's name-recognition among recipients, by using the author's domain name in the From: header field. Especially since many popular MUAs do not display the author's email address, there is no empirical evidence of the extent that this particular unauthorized use of a domain name contributes to recipient deception or that eliminating it will have significant effect.
However, closing this exploit could facilitate some types of optimized processing by receive-side message filtering engines, since it could permit them to maintain higher-confidence assertions about From: header field uses of a domain, when the occurrence is authorized.
Unauthorized uses of domain names occur elsewhere in messages, as do unauthorized uses of organizations' names. These attacks are outside the scope of this specification.
ADSP does not provide any benefit--nor, indeed, have any effect at all--unless an external system acts upon the verdict, either by treating the message differently during the delivery process or by showing some indicator to the end recipient. Such a system is out of scope for this specification.
ADSP checkers may perform multiple DNS lookups per Alleged Author Domain. Since these lookups are driven by domain names in email message headers of possibly fraudulent email, legitimate ADSP checkers can become participants in traffic multiplication attacks.
An attacker might attack the DNS infrastructure in an attempt to impersonate ADSP records to influence a receiver's decision on how it will handle mail. However, such an attacker is more likely to attack at a higher level, e.g., redirecting A or MX record lookups in order to capture traffic that was legitimately intended for the target domain. These DNS security issues are addressed by DNSSEC (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.) [RFC4033].
Because ADSP operates within the framework of the legacy e-mail system, the default result in the absence of an ADSP record is that the domain does not sign all of its messages. 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.
DNS wildcards (described in [RFC4592] (Lewis, E., “The Role of Wildcards in the Domain Name System,” July 2006.)) that exist in the DNS hierarchy at or above the domain being checked interfere with the ability to verify the scope of the ADSP check described in Section 4.3 (ADSP Lookup Procedure). For example, a wildcard record for *.example.com makes all subdomains such as foo.example.com exist in the DNS. Domains that intend to make active use of ADSP by publishing a practice other than Unknown are advised to avoid the use of wildcards elsewhere in their hierarchy.
If a domain contains wildcards, then any name that matches the wildcard can appear to be a valid mail domain eligible for ADSP. But the _adsp._domainkey. prefix on ADSP records does not allow publication of wildcard records that cover ADSP records without also covering non-ADSP records, nor of wildcard records that cover non-ADSP records without also covering ADSP records. Hence a domain MUST NOT publish wildcard ADSP records.
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).|
|[RFC2822]||Resnick, P., “Internet Message Format,” RFC 2822, April 2001 (TXT).|
|[RFC4033]||Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” RFC 4033, March 2005 (TXT).|
|[RFC4592]||Lewis, E., “The Role of Wildcards in the Domain Name System,” RFC 4592, July 2006 (TXT).|
|[RFC4871]||Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” RFC 4871, May 2007 (TXT).|
|[RFC5226]||Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” BCP 26, RFC 5226, May 2008 (TXT).|
|[RFC5234]||Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” STD 68, RFC 5234, January 2008 (TXT).|
|[RFC2821]||Klensin, J., “Simple Mail Transfer Protocol,” RFC 2821, April 2001 (TXT).|
|[RFC4686]||Fenton, J., “Analysis of Threats Motivating DomainKeys Identified Mail (DKIM),” RFC 4686, September 2006 (TXT).|
|[RFC5016]||Thomas, M., “Requirements for a DomainKeys Identified Mail (DKIM) Signing Practices Protocol,” RFC 5016, October 2007 (TXT).|
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.
Domain managers are advised to consider the ways that mail processing can modify messages in ways that will invalidate an existing DKIM signature, such as mailing lists, courtesy forwarders, and other paths that could add or modify headers, or modify the message body. In that case, if the modifications invalidate the DKIM signature, recipient hosts will consider the mail not to have an Author Signature, even though the signature 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 an ADSP record for the domain containing "all", depending on whether the users also send mail through other paths that do not apply an Author Signature. Such paths could include MTAs at hotels or hotspot networks used by travelling users, web sites that provide "mail an article" features, user messages sent through mailing lists, or third party mail clients that support multiple user identities.
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 Signature. In this case, the domain's management can be confident that all of its outgoing mail will be sent through the signing MTA. Lacking 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 Signature. One possible route would be for the domain owner to generate the key and give it to the mailing provider. Another would be for the domain to delegate a subdomain to the mailing provider, for example, bigbank.example might delegate email.bigbank.example to such a provider. In that case, the provider can generate the keys and DKIM DNS records itself and use the subdomain in the Author address in the mail.
Regardless of the DNS and key management strategy chosen, whoever maintains the DKIM records for the domain could also install an ADSP record containing "all".
In some cases, a domain might sign all of its outgoing mail with an Author Signature, but prefer that recipient systems discard mail without a valid Author Signature to avoid confusion from mail sent from sources that do not apply an Author Signature. (In the case of domains with tightly controlled outgoing mail, this latter kind of mail is sometimes loosely called "forgeries".) In that case, it might be appropriate to publish an ADSP record containing "discardable". Note that a domain SHOULD NOT publish a "discardable" record if it wishes to maximize the likelihood that mail from the domain is delivered, since it could cause some fraction of the mail the domain sends to be discarded.
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 or 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.
When a domain has independent users and its usage policy does not explicitly restrict them to sending mail only from designated mail servers (e.g. many ISP domains and even some corporate domains), then it is only appropriate to publish an ADSP record containing "unknown". Publishing either "all" or "discardable" will likely result in significant breakage because independent users are likely to send mail from the external paths enumerated in Appendix A.1 (Single Location Domains).
If a domain sends no mail at all, it can safely publish a "discardable" ADSP record, since any mail with an author address in the domain is a forgery.
This document greatly benefited from comments by Steve Atkins, Jon Callas, Dave Crocker, JD Falk, Arvel Hathcock, Ellen Siegel, Michael Thomas, and Wietse Venema.
NOTE TO RFC EDITOR: This section may be removed upon publication of this document as an RFC.
From a "diff" perspective, the changes are extensive. Semantically, the changes are:
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