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Copyright © The IETF Trust (2007).
DomainKeys Identified Mail (DKIM) defines a domain-level authentication framework for email using public-key cryptography and key server technology to permit verification of the source and contents of messages by either Mail Transport Agents (MTAs) or Mail User Agents (MUAs). The primary DKIM protocol is described in [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.).
This document describes the records that senders may use to advertise how they sign their outgoing mail, and how verifiers should access and interpret those results.
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.).
Need to consider handling of multiple responses to a DNS query for the SSP record.
2. Language and Terminology
2.1. Terms Imported from DKIM Signatures Specification
2.2. Valid Signature
2.3. Originator Address
2.4. Originator Domain
2.5. Alleged Signer
2.6. Alleged Originator
2.7. Sender Signing Practices
2.8. Originator Signature
2.10. Third-Party Signature
2.11. Verifier Acceptable Third-Party Signature
3. Operation Overview
4. Detailed Description
4.1. DNS Representation
4.2. Publication of SSP Records
4.3. Record Syntax
4.4. Sender Signing Practices Check Procedure
5. IANA Considerations
6. Security Considerations
6.1. Fraudulent Sender Address
6.2. DNS Attacks
7.1. Normative References
7.2. Informative References
Appendix A. Acknowledgements
Appendix B. Change Log
B.1. Changes since -ietf-dkim-ssp-00
B.2. Changes since -allman-ssp-02
B.3. Changes since -allman-ssp-01
B.4. 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 must be expected that most messages will remain unsigned. However, some domains may choose to sign all of their outgoing mail, for example, to protect their brand name. It is highly desirable for such domains to be able to advertise that fact to verifiers, and that messages claiming to be from them that do not have a valid signature are likely to be forgeries. This is the topic for sender signing practices.
In the absence of a valid DKIM signature on behalf of the "From" address [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.), message verifiers implementing this specification MUST determine whether messages from that sender are expected to be signed, and what signatures are acceptable. In particular, whether a domain signs all outbound email must be made available to the verifier. Without such a mechanism, the benefit of message signing techniques such as DKIM is limited since unsigned messages will always need to be considered to be potentially legitimate. This determination is referred to as a Sender Signing Practices check.
Conceivably, such expressions might be imagined to be extended in the future to include information about what hashing algorithms a domain uses, what kind of messages might be sent (e.g., bulk vs. personal vs. transactional), etc. Such concerns are out of scope of this standard, because they can be expressed in the key record ("Selector") with which the signature is verified. In contrast, this specification focuses on information which is relevant in the absence of a valid signature. Expressions of signing practice which require outside auditing are similarly out of scope for this specification because they fall under the purview of reputation and accreditation.
The detailed requirements for Sender Signing Practices are given in [I‑D.ietf‑dkim‑ssp‑requirements] (Thomas, M., “Requirements for a DKIM Signing Practices Protocol,” April 2007.), which the protocol described in this document attempts to satisfy. 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.), which should be read as a prerequisite to this document.
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.). 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.).
The "Originator Address" is the email address in the From header field of a message [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.), or if and only if the From header field contains multiple addresses, the first address in the From header field.
NON-NORMATIVE RATIONALE: The alternative option when there are multiple addresses in the From header field is to use the value of the Sender header field. This would be closer to the semantics indicated in [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.) than using the first address in the From header field. However, the large number of deployed Mail User Agents that do not display the Sender header field value argues against that. Multiple addresses in the From header field are rare in real life.
Even when there is only one address in the From header field, this address is chosen as the reference address for SSP lookups because it represents the author of the message and is more widely displayed by Mail User Agents as a result. The Sender header field frequently has other meanings.
The "Originator Domain" is everything to the right of the "@" in the Originator Address (excluding the "@" itself).
An "Alleged Signer" is the identity of the signer claimed in a DKIM-Signature header field in a message received by a Verifier; it is "alleged" because it has not yet been verified.
An "Alleged Originator" is the Originator Address of a message received by a Verifier; it is "alleged" because it has not yet been verified.
"Sender Signing Practices" (or just "practices") consist of a machine-readable record published by the domain of the Alleged Originator which includes information about whether or not that domain signs all of their email, and whether signatures from third parties are sanctioned by the Alleged Originator.
An "Originator Signature" is any Valid Signature where the signing address (listed in the "i=" tag if present, otherwise its default value, consisting of the null address, representing an unknown user, followed by "@", followed by the value of the "d=" tag) matches the Originator Address. If the signing address does not include a local-part, then only the domains must match; otherwise, the two addresses must be identical.
Messages that do not contain a valid Originator Signature and which are inconsistent with a Sender Signing Practices check (e.g., are received without a Valid Signature and the sender's signing practices indicate all messages from the domain are signed) are referred to as "Suspicious". The handling of such messages is at the discretion of the Verifier or final recipient. "Suspicious" applies only to the DKIM evaluation of the message; a Verifier may decide the message should be accepted on the basis of other information beyond the scope of this document. Conversely, messages not deemed Suspicious may be rejected for other reasons.
A "Third-Party Signature" is a Valid Signature which is not an Originator Signature.
A Verifier Acceptable Third-Party Signature is a Third-Party Signature that the Verifier is willing to accept as meaningful for the message under consideration. The Verifier may use any criteria it deems appropriate for making this determination.
Sender Signing Practices checks MUST be based on the Originator Address. If the message contains a valid Originator Signature, no Sender Signing Practices check need be performed: the Verifier SHOULD NOT look up the Sender Signing Practices and the message MUST NOT be considered Suspicious.
Verifiers checking messages that do not have at least one valid Originator Signature MUST perform a Sender Signing Practices check on the domain specified by the Originator Address as described in Section 4.4 (Sender Signing Practices Check Procedure).
A Sender Signing Practices check produces one of four possible results:
NON-NORMATIVE RATIONALE: Third-party signatures, since they can potentially represent any domain, are considered more likely to be abused by attackers seeking to spoof a specific address. It may therefore be desirable for verifiers to apply other criteria outside the scope of this specification in deciding to accept a given third-party signature. For example, a list of known mailing list domains used by addresses served by the verifier might be specifically considered acceptable third-party signers.
If the Sender Signing Practices record for the domain does not exist but the domain does exist, Verifier systems MUST assume that some messages from this domain are not signed and the message MUST NOT be considered Suspicious.
Sender Signing Practices records are published using the DNS TXT resource record type.
NON-NORMATIVE DISCUSSION: There has been considerable discussion on the DKIM WG mailing list regarding the relative advantages of TXT and a new resource record (RR) type. Many DNS server and resolver implementations are incapable of quickly and easily supporting new resource record types. For this reason, support of TXT records is required whether a new RR type is defined or not. However, without a "flag day" on which SSP TXT record support is to be withdrawn, such support is likely to continue indefinitely. As a result, this specification defines no new RR type for SSP.
Another alternative proposed by P. Hallam-Baker is the publication of both a TXT record and, when implementations permit, a new RR, referred to as XPTR, which gives the location from which SSP and other policy information relating to a give domain can be retrieved. This has the advantage of supporting a variety of policies in a scalable manner, with better handling of wildcards and centralized publication of policy records, with caching advantages. However, the above implementation issues also apply to XPTR, and an additional lookup is required to retrieve SSP via the XPTR method. At the time of publication of this draft, consensus on this proposal was unclear.
The RDATA for SSP resource records is textual in format, with specific syntax and semantics relating to their role in describing sender signing practices. 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 (Record Syntax) MUST be ignored (considered equivalent to a NODATA result) for purposes of message disposition, although they MAY cause the logging of warning messages via an appropriate system logging mechanism.
SSP records for a domain are published at a location in the domain's DNS hierarchy prefixed by _ssp._domainkey; e.g., the SSP record for example.com would be a TXT record that is published at _ssp._domainkey.example.com.
Sender Signing Practices are intended to apply to all mail sent from the domain of an Alleged Originator, and to the greatest extent possible, to all subdomains of that domain. There are several cases that need to be considered in that regard:
Normally, a domain expressing Sender Signing Practices will want to do so for both itself and all of its "descendents" (child domains and hosts, at all lower levels). Domains wishing to do so MUST publish SSP records as follows:
Publish an SSP record for the domain itself
Publish an SSP record for any existing subdomain
Note that since the lookup algorithm described below references the immediate parent of the alleged originating domain, it is not necessary to publish SSP records for every single-level label within the domain. This has been done to relieve domain administrators of the burden of publishing an SSP record for every other record in the domain, which would be otherwise required.
Wildcards within a domain, including but not limited to wildcard MX records, pose a particular problem. While referencing the immediate parent domain allows the discovery of an SSP record corresponding to an unintended immediate-child subdomain, wildcard records apply at multiple levels. For example, if there is a wildcard MX record for example.com, the domain foo.bar.example.com can receive mail through the named mail exchanger. Conversely, the existence of the record makes it impossible to tell whether foo.bar.example.com is a legitimate name since a query for that name will not return an NXDOMAIN error. For that reason, SSP coverage for subdomains of domains containing a wildcard record is incomplete.
NON-NORMATIVE NOTE: Complete SSP coverage of domains containing (or where any parent contains) wildcards generally cannot be guaranteed.
SSP records follow 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.). The SSP record syntax is a tag-list as defined in that section, including the restriction on duplicate tags, the use of white space, and case sensitivity.
Tags used in SSP records are as follows. Unrecognized tags MUST be ignored. In the ABNF below, the FWS token is inherited from [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.) with the exclusion of obs-FWS. The ALPHA and DIGIT tokens are imported from [RFC4234] (Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” October 2005.).
- Outbound signing practices for the domain (plain-text; REQUIRED). Possible values are as follows:
- The domain may sign some or all email.
- All mail from the domain is signed; unsigned email MUST be considered Suspicious. The domain may send messages through agents that may modify and re-sign messages, so email signed with a Verifier Acceptable Third-Party Signature SHOULD NOT be considered Suspicious.
- All mail from the domain is signed; messages lacking a valid Originator Signature MUST be considered Suspicious. The domain does not expect to send messages through agents that may modify and re-sign messages.
- NON-NORMATIVE RATIONALE: Strict practices may be used by entities which send only transactional email to individual addresses and which are willing to accept the consequence of having some mail which is re-signed appear suspicious in return for additional control over their addresses. Strict practices may also be used by entities which do not send (and therefore do not sign) any email.
- ABNF:ssp-dkim-tag = "dkim" [FWS] "=" [FWS] ("unknown" / "all" / "strict")
- Non-compliant message handling request for the domain (plain-text; OPTIONAL). Possible values are as follows:
- Messages which are Suspicious from this domain SHOULD be processed by the verifier, although the SSP failure MAY be considered in subsequent evaluation of the message. This is the default value.
- Messages which are Suspicious from this domain MAY be rejected, bounced, or otherwise not delivered at the option of the verifier.
- NON-NORMATIVE EXPLANATION: The "deny" practice is intended for use by domains that value security over deliverability. For example, a domain used by a financial institution to send transactional email, which signs all of its messages, might consider their concern about phishing messages purporting to come from their domain to be higher than their concern about some some legitimate messages not being delivered. The "handling=deny" practice allows them to express that concern in a way that can be acted upon by verifiers, if they so choose.
- ABNF:ssp-handling-tag = "handling" [FWS] "=" [FWS] ("process" / "deny")
- Flags, represented as a colon-separated list of names (plain-text; OPTIONAL, default is that no flags are set). Flag values are:
- The domain is testing signing practices, and the Verifier SHOULD NOT consider a message suspicious based on the record.
- The signing practices apply only to the named domain, and not to subdomains.
- ABNF:ssp-t-tag = %x75 [FWS] "=" [FWS] ssp-t-tag-flag 0*( [FWS] ":" [FWS] ssp-t-tag-flag ) ssp-t-tag-flag = "y" / "s" / hyphenated-word ; for future extension hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ]
- Unrecognized flags MUST be ignored.
Verifiers MUST produce a result that is semantically equivalent to applying the following steps in the order listed. In practice, several of these steps can be performed in parallel in order to improve performance.
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. The choice of MX for this purpose is because this record type is thought to be the most common for likely domains, and will therefore result in a result which can be more readily cached than a negative result.
If any of the queries involved in the Sender Signing Practices Check result in a SERVFAIL error response, the verifier MAY either queue the message or return an SMTP error indicating a temporary failure.
IANA is requested to create a "DKIM selector name" registry and to reserve the selector name "_ssp" to avoid confusion between DKIM key records and SSP records.
Security considerations in the Sender Signing Practices are mostly related to attempts on the part of malicious senders to represent themselves as other senders, often in an attempt to defraud either the recipient or the Alleged Originator.
Additional security considerations regarding Sender Signing Practices may be found in the DKIM threat analysis (Fenton, J., “Analysis of Threats Motivating DomainKeys Identified Mail (DKIM),” September 2006.) [RFC4686].
[[Assuming 3rd party signature is based on Sender header field]] If the Sender Signing Practices sanction third-party signing, an attacker can create a message with a From header field of an arbitrary sender and a legitimately signed Sender header field
An attacker might attack the DNS infrastructure in an attempt to impersonate SSP records. 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. Domains concerned about this should use DNSSEC (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.) [RFC4033].
Because SSP operates within the framework of the legacy e-mail system, the default result in the absence of an SSP record is that the domain does not sign all of its messages. Therefore, a DNS attack which is successful in suppressing the SSP response to the verifier is sufficient to cause the verifier to see unsigned messages as non-suspicious, even when that is not intended by the alleged originating domain.
|[RFC1035]||Mockapetris, P., “Domain names - implementation and specification,” STD 13, RFC 1035, November 1987.|
|[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.|
|[RFC4234]||Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” RFC 4234, October 2005 (TXT, HTML, XML).|
|[RFC4871]||Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” RFC 4871, May 2007.|
|[I-D.ietf-dkim-ssp-requirements]||Thomas, M., “Requirements for a DKIM Signing Practices Protocol,” draft-ietf-dkim-ssp-requirements-05 (work in progress), April 2007.|
|[RFC4033]||Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” RFC 4033, March 2005.|
|[RFC4686]||Fenton, J., “Analysis of Threats Motivating DomainKeys Identified Mail (DKIM),” RFC 4686, September 2006.|
The authors wish to thank many members of the ietf-dkim mailing list, in particular Arvel Hathcock and Eliot Lear, for valuable suggestions and constructive criticism of earlier versions of this draft.
From a "diff" perspective, the changes are extensive. Semantically, the changes are:
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