Internet Engineering Task Force (IETF) V. Dukhovni Request for Comments: 7671 Two Sigma Updates: 6698 W. Hardaker Category: Standards Track Parsons ISSN: 2070-1721 October 2015 The DNS-Based Authentication of Named Entities (DANE) Protocol: Updates and Operational Guidance Abstract This document clarifies and updates the DNS-Based Authentication of Named Entities (DANE) TLSA specification (RFC 6698), based on subsequent implementation experience. It also contains guidance for implementers, operators, and protocol developers who want to use DANE records. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7671. Copyright Notice Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents 1. Introduction ....................................................3 1.1. Terminology ................................................4 2. DANE TLSA Record Overview .......................................5 2.1. Example TLSA Record ........................................6 3. DANE TLS Requirements ...........................................6 4. DANE Certificate Usage Selection Guidelines .....................7 4.1. Opportunistic Security and PKIX Usages .....................7 4.2. Interaction with Certificate Transparency ..................8 4.3. Switching from/to PKIX-TA/EE to/from DANE-TA/EE ............9 5. Certificate-Usage-Specific DANE Updates and Guidelines ..........9 5.1. Certificate Usage DANE-EE(3) ...............................9 5.2. Certificate Usage DANE-TA(2) ..............................11 5.3. Certificate Usage PKIX-EE(1) ..............................15 5.4. Certificate Usage PKIX-TA(0) ..............................15 6. Service Provider and TLSA Publisher Synchronization ............16 7. TLSA Base Domain and CNAMEs ....................................18 8. TLSA Publisher Requirements ....................................19 8.1. Key Rollover with Fixed TLSA Parameters ...................20 8.2. Switching to DANE-TA(2) from DANE-EE(3) ...................21 8.3. Switching to New TLSA Parameters ..........................22 8.4. TLSA Publisher Requirements: Summary ......................23 9. Digest Algorithm Agility .......................................23 10. General DANE Guidelines .......................................25 10.1. DANE DNS Record Size Guidelines ..........................25 10.2. Certificate Name Check Conventions .......................26 10.3. Design Considerations for Protocols Using DANE ...........27 11. Note on DNSSEC Security .......................................28 12. Summary of Updates to RFC 6698 ................................29 13. Operational Considerations ....................................29 14. Security Considerations .......................................30 15. References ....................................................30 15.1. Normative References .....................................30 15.2. Informative References ...................................32 Acknowledgements ..................................................33 Authors' Addresses ................................................33
1. Introduction The DNS-Based Authentication of Named Entities (DANE) specification [RFC6698] introduces the DNS "TLSA" resource record (RR) type ("TLSA" is not an acronym). TLSA records associate a certificate or a public key of an end-entity or a trusted issuing authority with the corresponding Transport Layer Security (TLS) [RFC5246] or Datagram Transport Layer Security (DTLS) [RFC6347] transport endpoint. DANE relies on the DNS Security Extensions (DNSSEC) [RFC4033]. DANE TLSA records validated by DNSSEC can be used to augment or replace the use of trusted public Certification Authorities (CAs). The TLS and DTLS protocols provide secured TCP and UDP communication, respectively, over IP. In the context of this document, channel security is assumed to be provided by TLS or DTLS. By convention, "TLS" will be used throughout this document; unless otherwise specified, the text applies equally well to DTLS over UDP. Used without authentication, TLS provides protection only against eavesdropping through its use of encryption. With authentication, TLS also protects the transport against man-in-the-middle (MITM) attacks. [RFC6698] defines three TLSA record fields: the first with four possible values, the second with two, and the third with three. These yield 24 distinct combinations of TLSA record types. This document recommends a smaller set of best-practice combinations of these fields to simplify protocol design, implementation, and deployment. This document explains and recommends DANE-specific strategies to simplify "virtual hosting", where a single Service Provider transport endpoint simultaneously supports multiple hosted Customer Domains. Other related documents that build on [RFC6698] are [RFC7673] and [RFC7672]. Section 12 summarizes the normative updates this document makes to [RFC6698].
1.1. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. The following terms are used throughout this document: Web PKI: The Public Key Infrastructure (PKI) model employed by browsers to authenticate web servers. This employs a set of trusted public CAs to vouch for the authenticity of public keys associated with a particular party (the subject). Service Provider: A company or organization that offers to host a service on behalf of the owner of a Customer Domain. The original domain name associated with the service often remains under the control of the customer. Connecting applications may be directed to the Service Provider via a redirection RR. Example redirection records include MX, SRV, and CNAME. The Service Provider frequently provides services for many customers and needs to ensure that the TLS credentials presented to connecting applications authenticate it as a valid server for the requested domain. Customer Domain: As described above, a TLS client may be interacting with a service that is hosted by a third party. This document refers to the domain name used to locate the service (prior to any redirection) as the "Customer Domain". TLSA Publisher: The entity responsible for publishing a TLSA record within a DNS zone. This zone will be assumed DNSSEC-signed and validatable to a trust anchor (TA), unless otherwise specified. If the Customer Domain is not outsourcing its DNS service, the TLSA Publisher will be the customer itself. Otherwise, the TLSA Publisher may be the operator of the outsourced DNS service. Public key: The term "public key" is shorthand for the subjectPublicKeyInfo component of a PKIX [RFC5280] certificate. SNI: The Server Name Indication (SNI) TLS protocol extension allows a TLS client to request a connection to a particular service name of a TLS server ([RFC6066], Section 3). Without this TLS extension, a TLS server has no choice but to offer a certificate with a default list of server names, making it difficult to host multiple Customer Domains at the same IP-address-based TLS service endpoint (i.e., provide "secure virtual hosting").
TLSA parameters: In [RFC6698], the TLSA record is defined to consist of four fields. The first three of these are numeric parameters that specify the meaning of the data in the fourth and final field. This document refers to the first three fields as "TLSA parameters", or sometimes just "parameters" when obvious from context. TLSA base domain: Per Section 3 of [RFC6698], TLSA records are stored at a DNS domain name that is a combination of a port and protocol prefix and a "base domain". In [RFC6698], the "base domain" is the fully qualified domain name of the TLS server. This document modifies the TLSA record lookup strategy to prefer the fully CNAME-expanded name of the TLS server, provided that expansion is "secure" (DNSSEC validated) at each stage of the expansion, and TLSA records are published for this fully expanded name. Thus, the "TLSA base domain" is either the fully CNAME-expanded TLS server name or otherwise the initial fully qualified TLS server name, whichever is used in combination with a port and protocol prefix to obtain the TLSA RRset. 2. DANE TLSA Record Overview DANE TLSA [RFC6698] specifies a protocol for publishing TLS server certificate associations via DNSSEC [RFC4033] [RFC4034] [RFC4035]. DANE TLSA records consist of four fields. The record type is determined by the values of the first three fields, which this document refers to as the "TLSA parameters" to distinguish them from the fourth and last field. The numeric values of these parameters were given symbolic names in [RFC7218]. The four fields are as follows: The Certificate Usage field: Section 2.1.1 of [RFC6698] specifies four values: PKIX-TA(0), PKIX-EE(1), DANE-TA(2), and DANE-EE(3). There is an additional private-use value: PrivCert(255), which, given its private scope, shall not be considered further in this document. All other values are reserved for use by future specifications. The Selector field: Section 2.1.2 of [RFC6698] specifies two values: Cert(0) and SPKI(1). There is an additional private-use value: PrivSel(255). All other values are reserved for use by future specifications. The Matching Type field: Section 2.1.3 of [RFC6698] specifies three values: Full(0), SHA2-256(1), and SHA2-512(2). There is an additional private-use value: PrivMatch(255). All other values are reserved for use by future specifications.
The Certificate Association Data field: See Section 2.1.4 of [RFC6698]. This field stores the full value or digest of the certificate or subject public key as determined by the matching type and selector, respectively. In the Matching Type field, of the two digest algorithms -- SHA2-256(1) and SHA2-512(2) -- as of the time of this writing, only SHA2-256(1) is mandatory to implement. Clients SHOULD implement SHA2-512(2), but servers SHOULD NOT exclusively publish SHA2-512(2) digests. The digest algorithm agility protocol defined in Section 9 SHOULD be used by clients to decide how to process TLSA RRsets that employ multiple digest algorithms. Server operators MUST publish TLSA RRsets that are compatible (see Section 8) with digest algorithm agility (Section 9). 2.1. Example TLSA Record In the example TLSA record below, the TLSA certificate usage is DANE-TA(2), the selector is Cert(0), and the matching type is SHA2-256(1). The last field is the Certificate Association Data field, which in this case contains the SHA2-256 digest of the server certificate. _25._tcp.mail.example.com. IN TLSA 2 0 1 ( E8B54E0B4BAA815B06D3462D65FBC7C0 CF556ECCF9F5303EBFBB77D022F834C0 ) 3. DANE TLS Requirements [RFC6698] does not discuss what versions of TLS are required when using DANE records. This document specifies that TLS clients that support DANE/TLSA MUST support at least TLS 1.0 and SHOULD support TLS 1.2 or later. TLS clients using DANE MUST support the SNI extension of TLS [RFC6066]. Servers MAY support SNI and respond with a matching certificate chain but MAY also ignore SNI and respond with a default certificate chain. When a server supports SNI but is not configured with a certificate chain that exactly matches the client's SNI extension, the server SHOULD respond with another certificate chain (a default or closest match). This is because clients might support more than one server name but can only put a single name in the SNI extension.
4. DANE Certificate Usage Selection Guidelines As mentioned in Section 2, the TLSA Certificate Usage field takes one of four possible values. With PKIX-TA(0) and PKIX-EE(1), the validation of peer certificate chains requires additional preconfigured CA TAs that are mutually trusted by the operators of the TLS server and client. With DANE-TA(2) and DANE-EE(3), no preconfigured CA TAs are required and the published DANE TLSA records are sufficient to verify the peer's certificate chain. Standards for application protocols that employ DANE TLSA can specify more specific guidance than [RFC6698] or this document. Such application-specific standards need to carefully consider which set of DANE certificate usages to support. Simultaneous support for all four usages is NOT RECOMMENDED for DANE clients. When all four usages are supported, an attacker capable of compromising the integrity of DNSSEC needs only to replace the server's TLSA RRset with one that lists suitable DANE-EE(3) or DANE-TA(2) records, effectively bypassing any added verification via public CAs. In other words, when all four usages are supported, PKIX-TA(0) and PKIX-EE(1) offer only illusory incremental security over DANE-TA(2) and DANE-EE(3). Designs in which clients support just the DANE-TA(2) and DANE-EE(3) certificate usages are RECOMMENDED. With DANE-TA(2) and DANE-EE(3), clients don't need to track a large changing list of X.509 TAs in order to successfully authenticate servers whose certificates are issued by a CA that is brand new or not widely trusted. The DNSSEC TLSA records for servers MAY include both sets of usages if the server needs to support a mixture of clients, some supporting one pair of usages and some the other. 4.1. Opportunistic Security and PKIX Usages When the client's protocol design is based on "Opportunistic Security" (OS) [RFC7435] and the use of authentication is based on the presence of server TLSA records, it is especially important to avoid the PKIX-EE(1) and PKIX-TA(0) certificate usages. When authenticated TLS is used opportunistically based on the presence of DANE TLSA records and no secure TLSA records are present, unauthenticated TLS is used if possible, and if TLS is not possible, perhaps even cleartext. However, if usable secure TLSA records are published, then authentication MUST succeed. Also, outside the browser space, there is no preordained canon of trusted CAs, and in any case there is no security advantage in using PKIX-TA(0) or
PKIX-EE(1) when the DANE-TA(2) and DANE-EE(3) usages are also supported (as an attacker who can compromise DNS can replace the former with the latter). Authentication via the PKIX-TA(0) and PKIX-EE(1) certificate usages is more brittle; the client and server need to happen to agree on a mutually trusted CA, but with OS the client is just trying to protect the communication channel at the request of the server and would otherwise be willing to use cleartext or unauthenticated TLS. The use of fragile mechanisms (like public CA authentication for some unspecified set of trusted CAs) is not sufficiently reliable for an OS client to honor the server's request for authentication. OS needs to be non-intrusive and to require few, if any, workarounds for valid but mismatched peers. Because the PKIX-TA(0) and PKIX-EE(1) usages offer no more security and are more prone to failure, they are a poor fit for OS and SHOULD NOT be used in that context. 4.2. Interaction with Certificate Transparency Certificate Transparency (CT) [RFC6962] defines an experimental approach that could be used to mitigate the risk of rogue or compromised public CAs issuing unauthorized certificates. This section clarifies the interaction of the experimental CT and DANE. This section may need to be revised in light of any future Standards Track version of CT. When a server is authenticated via a DANE TLSA RR with TLSA certificate usage DANE-EE(3), the domain owner has directly specified the certificate associated with the given service without reference to any public CA. Therefore, when a TLS client authenticates the TLS server via a TLSA record with usage DANE-EE(3), CT checks SHOULD NOT be performed. Publication of the server certificate or public key (digest) in a TLSA record in a DNSSEC-signed zone by the domain owner assures the TLS client that the certificate is not an unauthorized certificate issued by a rogue CA without the domain owner's consent. When a server is authenticated via a DANE TLSA record with TLSA usage DANE-TA(2) and the server certificate does not chain to a known public root CA, CT cannot apply (CT logs only accept chains that start with a known public root). Since TLSA certificate usage DANE-TA(2) is generally intended to support non-public TAs, TLS clients SHOULD NOT perform CT checks with usage DANE-TA(2).
With certificate usages PKIX-TA(0) and PKIX-EE(1), CT applies just as it would without DANE. TLSA records of this type only constrain which CAs are acceptable in PKIX validation. All checks used in the absence of DANE still apply when validating certificate chains with DANE PKIX-TA(0) and PKIX-EE(1) constraints. 4.3. Switching from/to PKIX-TA/EE to/from DANE-TA/EE The choice of preferred certificate usages may need to change as an application protocol evolves. When transitioning between PKIX-TA/ PKIX-EE and DANE-TA/DANE-EE, clients begin to enable support for the new certificate usage values. If the new preferred certificate usages are PKIX-TA/EE, this requires installing and managing the appropriate set of CA TAs. During this time, servers will publish both types of TLSA records. At some later time, when the vast majority of servers have published the new preferred TLSA records, clients can stop supporting the legacy certificate usages. Similarly, servers can stop publishing legacy TLSA records once the vast majority of clients support the new certificate usages. 5. Certificate-Usage-Specific DANE Updates and Guidelines The four certificate usage values from the TLSA record -- DANE-EE(3), DANE-TA(2), PKIX-EE(1), and PKIX-TA(0) -- are discussed below. 5.1. Certificate Usage DANE-EE(3) In this section, the meaning of DANE-EE(3) is updated from [RFC6698] to specify that peer identity matching and validity period enforcement are based solely on the TLSA RRset properties. This document also extends [RFC6698] to cover the use of DANE authentication of raw public keys [RFC7250] via TLSA records with certificate usage DANE-EE(3) and selector SPKI(1). Authentication via certificate usage DANE-EE(3) TLSA records involves simply checking that the server's leaf certificate matches the TLSA record. In particular, the binding of the server public key to its name is based entirely on the TLSA record association. The server MUST be considered authenticated even if none of the names in the certificate match the client's reference identity for the server. This simplifies the operation of servers that host multiple Customer Domains, as a single certificate can be associated with multiple domains without having to match each of the corresponding reference identifiers.
; Multiple Customer Domains hosted by an example.net ; Service Provider: ; www.example.com. IN CNAME ex-com.example.net. www.example.org. IN CNAME ex-org.example.net. ; ; In the provider's DNS zone, a single certificate and TLSA ; record support multiple Customer Domains, greatly simplifying ; "virtual hosting". ; ex-com.example.net. IN A 192.0.2.1 ex-org.example.net. IN A 192.0.2.1 _443._tcp.ex-com.example.net. IN CNAME tlsa._dane.example.net. _443._tcp.ex-org.example.net. IN CNAME tlsa._dane.example.net. tlsa._dane.example.net. IN TLSA 3 1 1 e3b0c44298fc1c14... Also, with DANE-EE(3), the expiration date of the server certificate MUST be ignored. The validity period of the TLSA record key binding is determined by the validity period of the TLSA record DNSSEC signatures. Validity is reaffirmed on an ongoing basis by continuing to publish the TLSA record and signing the zone in which the record is contained, rather than via dates "set in stone" in the certificate. The expiration becomes a reminder to the administrator that it is likely time to rotate the key, but missing the date no longer causes an outage. When keys are rotated (for whatever reason), it is important to follow the procedures outlined in Section 8. If a server uses just DANE-EE(3) TLSA records and all its clients are DANE clients, the server need not employ SNI (i.e., it may ignore the client's SNI message) even when the server is known via multiple domain names that would otherwise require separate certificates. It is instead sufficient for the TLSA RRsets for all the domain names in question to match the server's default certificate. For application protocols where the server name is obtained indirectly via SRV records, MX records, or similar records, it is simplest to publish a single hostname as the target server name for all the hosted domains. In organizations where it is practical to make coordinated changes in DNS TLSA records before server key rotation, it is generally best to publish end-entity DANE-EE(3) certificate associations in preference to other choices of certificate usage. DANE-EE(3) TLSA records support multiple server names without SNI, don't suddenly stop working when leaf or intermediate certificates expire, and don't fail when a server operator neglects to include all the required issuer certificates in the server certificate chain.
More specifically, it is RECOMMENDED that at most sites TLSA records published for DANE servers be "DANE-EE(3) SPKI(1) SHA2-256(1)" records. Selector SPKI(1) is chosen because it is compatible with raw public keys [RFC7250] and the resulting TLSA record need not change across certificate renewals with the same key. Matching type SHA2-256(1) is chosen because all DANE implementations are required to support SHA2-256. This TLSA record type easily supports hosting arrangements with a single certificate matching all hosted domains. It is also the easiest to implement correctly in the client. Clients that support raw public keys can use DANE TLSA records with certificate usage DANE-EE(3) and selector SPKI(1) to authenticate servers that negotiate the use of raw public keys. Provided the server adheres to the requirements of Section 8, the fact that raw public keys are not compatible with any other TLSA record types will not get in the way of successful authentication. Clients that employ DANE to authenticate the peer server SHOULD NOT negotiate the use of raw public keys unless the server's TLSA RRset includes "DANE-EE(3) SPKI(1)" TLSA records. While it is, in principle, also possible to authenticate raw public keys via "DANE-EE(3) Cert(0) Full(0)" records by extracting the public key from the certificate in DNS, extracting just the public key from a "3 0 0" TLSA record requires extra logic on clients that not all implementations are expected to provide. Servers that wish to support [RFC7250] raw public keys need to publish TLSA records with a certificate usage of DANE-EE(3) and a selector of SPKI(1). While DANE-EE(3) TLSA records are expected to be by far the most prevalent, as explained in Section 5.2, DANE-TA(2) records are a valid alternative for sites with many DANE services. Note, however, that virtual hosting is more complex with DANE-TA(2). Also, with DANE-TA(2), server operators MUST ensure that the server is configured with a sufficiently complete certificate chain and need to remember to replace certificates prior to their expiration dates. 5.2. Certificate Usage DANE-TA(2) This section updates [RFC6698] by specifying a new operational requirement for servers publishing TLSA records with a usage of DANE-TA(2): such servers MUST include the TA certificate in their TLS server certificate message unless all such TLSA records are "2 0 0" records that publish the server certificate in full. Some domains may prefer to avoid the operational complexity of publishing unique TLSA RRs for each TLS service. If the domain employs a common issuing CA to create certificates for multiple TLS services, it may be simpler to publish the issuing authority as a TA
for the certificate chains of all relevant services. The TLSA query domain (TLSA base domain with port and protocol prefix labels) for each service issued by the same TA may then be set to a CNAME alias that points to a common TLSA RRset that matches the TA. For example: ; Two servers, each with its own certificate, that share ; a common issuer (TA). ; www1.example.com. IN A 192.0.2.1 www2.example.com. IN A 192.0.2.2 _443._tcp.www1.example.com. IN CNAME tlsa._dane.example.com. _443._tcp.www2.example.com. IN CNAME tlsa._dane.example.com. tlsa._dane.example.com. IN TLSA 2 0 1 e3b0c44298fc1c14... The above configuration simplifies server key rotation, because while the servers continue to receive new certificates from a CA matched by the shared (target of the CNAMEs) TLSA record, server certificates can be updated without making any DNS changes. As the list of active issuing CAs changes, the shared TLSA record will be updated (much less frequently) by the administrators who manage the CAs. Those administrators still need to perform TLSA record updates with care, as described in Section 8. With usage DANE-TA(2), the server certificates will need to have names that match one of the client's reference identifiers (see [RFC6125]). When hosting multiple unrelated Customer Domains (that can't all appear in a single certificate), such a server SHOULD employ SNI to select the appropriate certificate to present to the client. 5.2.1. Recommended Record Combinations TLSA records with a matching type of Full(0) are NOT RECOMMENDED. While these potentially obviate the need to transmit the TA certificate in the TLS server certificate message, client implementations may not be able to augment the server certificate chain with the data obtained from DNS, especially when the TLSA record supplies a bare key (selector SPKI(1)). Since the server will need to transmit the TA certificate in any case, server operators SHOULD publish TLSA records with a matching type other than Full(0) and avoid potential DNS interoperability issues with large TLSA records containing full certificates or keys (see Section 10.1.1).
TLSA Publishers employing DANE-TA(2) records SHOULD publish records with a selector of Cert(0). Such TLSA records are associated with the whole TA certificate, not just with the TA public key. In particular, when authenticating the peer certificate chain via such a TLSA record, the client SHOULD apply any relevant constraints from the TA certificate, such as, for example, path length constraints. While a selector of SPKI(1) may also be employed, the resulting TLSA record will not specify the full TA certificate content, and elements of the TA certificate other than the public key become mutable. This may, for example, enable a subsidiary CA to issue a chain that violates the TA's path length or name constraints. 5.2.2. Trust Anchor Digests and Server Certificate Chain With DANE-TA(2), a complication arises when the TA certificate is omitted from the server's certificate chain, perhaps on the basis of Section 7.4.2 of [RFC5246]: The sender's certificate MUST come first in the list. Each following certificate MUST directly certify the one preceding it. Because certificate validation requires that root keys be distributed independently, the self-signed certificate that specifies the root certificate authority MAY be omitted from the chain, under the assumption that the remote end must already possess it in order to validate it in any case. With TLSA certificate usage DANE-TA(2), there is no expectation that the client is preconfigured with the TA certificate. In fact, client implementations are free to ignore all locally configured TAs when processing usage DANE-TA(2) TLSA records and may rely exclusively on the certificates provided in the server's certificate chain. But, with a digest in the TLSA record, the TLSA record contains neither the full TA certificate nor the full public key. If the TLS server's certificate chain does not contain the TA certificate, DANE clients will be unable to authenticate the server. TLSA Publishers that publish TLSA certificate usage DANE-TA(2) associations with a selector of SPKI(1) or with a digest-based matching type (not Full(0)) MUST ensure that the corresponding server is configured to also include the TA certificate in its TLS handshake certificate chain, even if that certificate is a self-signed root CA and would have been optional in the context of the existing public CA PKI.
Only when the server TLSA record includes a "DANE-TA(2) Cert(0) Full(0)" TLSA record containing a full TA certificate is the TA certificate optional in the server's TLS certificate message. This is also the only type of DANE-TA(2) record for which the client MUST be able to verify the server's certificate chain even if the TA certificate appears only in DNS and is absent from the TLS handshake server certificate message. 5.2.3. Trust Anchor Public Keys TLSA records with TLSA certificate usage DANE-TA(2), selector SPKI(1), and a matching type of Full(0) publish the full public key of a TA via DNS. In Section 6.1.1 of [RFC5280], the definition of a TA consists of the following four parts: 1. the trusted issuer name, 2. the trusted public key algorithm, 3. the trusted public key, and 4. optionally, the trusted public key parameters associated with the public key. Items 2-4 are precisely the contents of the subjectPublicKeyInfo published in the TLSA record. The issuer name is not included in the subjectPublicKeyInfo. With TLSA certificate usage DANE-TA(2), the client may not have the associated TA certificate and cannot generally verify whether or not a particular certificate chain is "issued by" the TA described in the TLSA record. When the server certificate chain includes a CA certificate whose public key matches the TLSA record, the client can match that CA as the intended issuer. Otherwise, the client can only check that the topmost certificate in the server's chain is "signed by" the TA's public key in the TLSA record. Such a check may be difficult to implement and cannot be expected to be supported by all clients. Thus, servers cannot rely on "DANE-TA(2) SPKI(1) Full(0)" TLSA records to be sufficient to authenticate chains issued by the associated public key in the absence of a corresponding certificate in the server's TLS certificate message. Servers employing "2 1 0" TLSA records MUST include the corresponding TA certificate in their certificate chain.
If none of the server's certificate chain elements match a public key specified in a TLSA record, and at least one "DANE-TA(2) SPKI(1) Full(0)" TLSA record is available, it is RECOMMENDED that clients check to see whether or not the topmost certificate in the chain is signed by the provided public key and has not expired, and in that case consider the server authenticated, provided the rest of the chain passes validation, including leaf certificate name checks. 5.3. Certificate Usage PKIX-EE(1) This certificate usage is similar to DANE-EE(3); but, in addition, PKIX verification is required. Therefore, name checks, certificate expiration, CT, etc. apply as they would without DANE. 5.4. Certificate Usage PKIX-TA(0) This section updates [RFC6698] by specifying new client implementation requirements. Clients that trust intermediate certificates MUST be prepared to construct longer PKIX chains than would be required for PKIX alone. TLSA certificate usage PKIX-TA(0) allows a domain to publish constraints on the set of PKIX CAs trusted to issue certificates for its TLS servers. A PKIX-TA(0) TLSA record matches PKIX-verified trust chains that contain an issuer certificate (root or intermediate) that matches its Certificate Association Data field (typically a certificate or digest). PKIX-TA(0) requires more complex coordination (than with DANE-TA(2) or DANE-EE(3)) between the Customer Domain and the Service Provider in hosting arrangements. Thus, this certificate usage is NOT RECOMMENDED when the Service Provider is not also the TLSA Publisher (at the TLSA base domain obtained via CNAMEs, SRV records, or MX records). TLSA Publishers who publish TLSA records for a particular public root CA will expect that clients will only accept chains anchored at that root. It is possible, however, that the client's trusted certificate store includes some intermediate CAs, either with or without the corresponding root CA. When a client constructs a trust chain leading from a trusted intermediate CA to the server leaf certificate, such a "truncated" chain might not contain the trusted root published in the server's TLSA record. If the omitted root is also trusted, the client may erroneously reject the server chain if it fails to determine that the shorter chain it constructed extends to a longer trusted chain that matches the TLSA record. Thus, when matching a usage PKIX-TA(0) TLSA record,
so long as no matching certificate has yet been found, a client MUST continue extending the chain even after any locally trusted certificate is found. If no TLSA records have matched any of the elements of the chain and the trusted certificate found is not self-issued, the client MUST attempt to build a longer chain in case a certificate closer to the root matches the server's TLSA record.