]> DKIM Author Signing Practices (ASP) Sendmail, Inc.
6475 Christie Ave, Suite 350 Emeryville 94608 CA +1 510 594 5501 eric+dkim@sendmail.org
Cisco Systems, Inc.
MS SJ-9/2 170 W. Tasman Drive San Jose 95134-1706 CA +1 408 526 5914 fenton@cisco.com
Yahoo! Inc.
701 First Avenue Sunnyvale 94089 CA +1 408 349 6831 markd+dkim@yahoo-inc.com
Taughannock Networks
PO Box 727 Trumansburg 14886 NY standards@taugh.com +1 831 480 2300 http://www.taugh.com
Applications / Email email, e-mail, rfc2822, rfc 2822, rfc822, rfc 822, rfc2821, rfc 2821, rfc821, rfc 821, rfc4871, rfc 4871, DKIM, domain keys, ASP, architecture, mta, user, delivery, smtp, submission, Internet, mailfrom, author, return address 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 . This document describes the records that authors' domains can use to advertise their practices for signing their outgoing mail, and how other hosts can access those records.
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 name. It is highly desirable for such domains to be able to advertise that fact to other hosts. This is the topic of Author Signing Practices (ASP). Hosts implementing this specification can inquire what Author Signing Practices a domain advertises. This inquiry is called an Author Signing Practices check. The detailed requirements for Author 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 . Briefly, A "Signer" is the agent that signs a message, as defined in section 2.1 of . A "Selector" specifies which of the keys published by a signing domain is to be queried, as defined in section 3.1 of .
A "Valid Signature" is any signature on a message which correctly verifies using the procedure described in section 6.1 of .
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 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 verified.
"Author 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 signing address (listed in the "i=" tag if present, otherwise its default value, consisting of the value of the "d=" tag) matches an Author Address. If a message has a Valid Signature, with the DKIM-Signature field containing i=a@domain.example, then domain.example is asserting that it takes responsibility for the message. If the message's From: field contains the address b@domain.example and an ASP 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, its failure to satisfy ASP could be problematic.
Domain owners can publish Author Signing Practices via a query mechanism such as the Domain Name System; specific details are given in . A host looks up Author Signing Practices based on the Author Address(es). Hosts can look up the Author Signing Practices of the domain(s) specified by the Author Address(es) as described in .
Depending on the Author Domain(s) and the signatures in a message, a recipient gets varying amounts of useful information from each ASP lookup. If a message has no Valid Signature, the ASP result is directly relevant to the message. If a message has a Valid Signature from an Author Domain, ASP provides no benefit relative to that domain since the message is already known to be compliant with any possible ASP for that domain. If a message has a Valid Signature from a domain other than an Author Domain, the receiver can use both the Signature and the ASP result in its evaluation of the message.
An Author Signing Practices lookup for an Author Address produces one of four possible results: Messages from this domain might or might not have an author signature. This is the default if the domain exists in the DNS but no record is found. All messages from this domain are signed. All messages from this domain are signed and discardable. The domain does not exist.
Author Signing Practices records are published using the DNS TXT resource record type. NON-NORMATIVE DISCUSSION [to be removed before publication]: 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 ASP resource records is textual in format, with specific syntax and semantics relating to their role in describing Author Signing Practices. The "Tag=Value List" syntax described in section 3.2 of 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 ASP, 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 delimeters between the strings. ASP records for a domain are published at a location in the domain's DNS hierarchy prefixed by _asp._domainkey.; e.g., the ASP record for example.com would be a TXT record that is published at _asp._domainkey.example.com. A domain MUST NOT publish more than one ASP record; the semantics of an ASP lookup that returns multiple ASP records for a single domain are undefined. (Note that example.com and mail.example.com are different domains.)
Author Signing Practices are intended to apply to all mail sent from a domain, and to the greatest extent possible, to all subdomains of that domain. There are several cases that need to be considered in that regard: The domain itself Subdomains which might or might not be used for email Host names which might or might not be used for email Other named resource records in the domain Multi-level examples of the above, e.g., a.b.example.com Non-existent cases, i.e., a subdomain or hostname that does not actually exist within the domain A domain publishing Author Signing Practices can if desired do so for both itself and all of its "descendants" (child domains and hosts, at all lower levels). Domains wishing to do so publish ASP records as follows: Publish an ASP record for the domain itself. Publish an ASP 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 ASP records for every single-level label within the domain. This has been done to relieve domain administrators of the burden of publishing an ASP record for every other record in the domain, which would be otherwise needed. 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 ASP 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, ASP coverage for subdomains of domains containing a wildcard record is incomplete. NON-NORMATIVE NOTE [to be removed before publication]: Complete ASP coverage of domains containing (or where any parent contains) wildcards generally cannot be guaranteed.
ASP records use the "tag=value" syntax described in section 3.2 of . Tags used in ASP 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 . 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: asp-dkim-tag = "dkim" *WSP "=" *WSP ("unknown" / "all" / "discardable")
Flags, represented as a colon-separated list of names (plain-text; OPTIONAL, default is that no flags are set). Flag values are: The signing practices apply only to the named domain, and not to subdomains.
ABNF: asp-t-tag = "t" *WSP "=" *WSP { asp-t-tag-flag 0*( *WSP ":" *WSP asp-t-tag-flag ) asp-t-tag-flag = "s" / hyphenated-word ; for future extension hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ]
Unrecognized flags MUST be ignored.
To look up the Author Signing Practices of a domain, ASP Checkers doing an ASP lookup MUST produce a result that is semantically equivalent to applying the following steps in the order listed below. In practice, several of 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 ASP record" is one that is both syntactically and semantically correct; that is, it matches the ABNF for a "tag-list" and includes a defined "dkim=" tag. The host makes a DNS query for a TXT record corresponding to the domain prefixed by "_asp._domainkey.". If the result of this query is a NOERROR response with an answer which contains a syntactically-valid ASP record, the Author Signing Practices are described by the contents of the record and the algorithm terminates. The host MUST query DNS for an MX record corresponding to the domain (with no prefix). This query is made only to check the existence of the domain name and MAY be done in parallel with the query made in step 1. If the result of this query is an NXDOMAIN error, the domain does not exist and the algorithm terminates. NON-NORMATIVE DISCUSSION [to be removed before publication]: 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. The host MUST query DNS for a TXT record for the immediate parent domain, prefixed with "_asp._domainkey." If the result of this query is a NOERROR response with an answer that is a syntactically-valid ASP response, and the response does not contain an "s" flag, the Author Signing Practices are described by the contents of the record. If any of the queries involved in the Author Signing Practices Check result in a SERVFAIL error response, the host MAY either queue the message or return an SMTP error indicating a temporary failure.
These examples are intended to illustrate typical uses of ASP. They are not intended to be exhaustive, nor to apply to every domain 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 ASP 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, 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 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.
In some cases, a domain might sign all 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. (This latter kind of mail is sometimes loosely called "forgeries".) In that case, it might be appropriate to publish an ASP 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. As a special case, if a domain sends no mail at all, it can safely publish a "discardable" ASP record, since any mail with an author address in the domain is 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.
ASP introduces some new namespaces that have been registered with IANA. In all cases, new values are assigned only for values that have been documented in a published RFC that has IETF Consensus . INFORMATIVE NOTE [ to be removed before publication ]: RFC 4871 defines a selector as a sub-domain, importing the term from RFC 2822. A sub-domain starts with a letter or digit, hence names such as _asp that start with an underscore cannot collide with valid selectors.
An ASP record provides for a list of specification tags. IANA has established the ASP Specification Tag Registry for specification tags that can be used in ASP fields. The initial entries in the registry comprise:
+------+-----------------+ | TYPE | REFERENCE | +------+-----------------+ | dkim | (this document) | | t | (this document) | +------+-----------------+ ASP Specification Tag Registry Initial Values
The dkim= tag spec, defined in , provides for a value specifying Outbound Signing Practices. IANA has established the ASP 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) | +-------------+-----------------+ ASP Outbound Signing Practices Registry Initial Values
The t= tag spec, defined in , provides for a value specifying Flags. IANA has established the ASP Flags Registry for ASP Flags. The initial entries in the registry comprise:
+------+-----------------+ | TYPE | REFERENCE | +------+-----------------+ | s | (this document) | +------+-----------------+ ASP Flags Registry Initial Values
Security considerations in the Author Signing Practices 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 Sender Signing Practices are found in the DKIM threat analysis.
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 the name-recognition that recipients will have, for these legitimate email authors, by using their 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. ASP 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. ASP Checkers perform up to three DNS lookups per Alleged Author Domain. Since these lookups are driven by domain names in email message headers of possibly fraudulent email, legitimate ASP Checkers can become participants in traffic multiplication attacks.
An attacker might attack the DNS infrastructure in an attempt to impersonate ASP records, in an attempt 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. Because SSP operates within the framework of the legacy e-mail system, the default result in the absence of an ASP record is that the domain does not sign all of its messages. It is therefore important that the ASP clients distinguish a DNS failure such as SERVFAIL from other DNS errors so that appropriate actions can be taken.
Wildcards within a domain publishing ASP records, including but not limited to wildcard "MX" records, pose a particular problem. While referencing the immediate parent domain allows the discovery of an ASP 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 ASP coverage of domains containing (or where any parent contains) wildcards generally cannot be provided by standard DNS servers.
&rfc2119; &rfc2434; &rfc2822; &rfc4033; &rfc4234; &rfc4686; &rfc4871; &rfc5016;
This document greatly benefited from comments by Steve Atkins, Jon Callas, Dave Crocker, JD Falk, Arvel Hathcock, Ellen Siegel, Michael Thomas, and Wietse Venema.
Merge authors list Restore white space in ABNF Add i= matching with warning, change Author Domain to Author Address where appropriate. Lookup procedure consistently refers to one ASP record per lookup. Update security section w/ language from W. Venema Simplify imports of terms from other RFCs. Clarify usage examples. Add IANA considerations.