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
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
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
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.
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)
[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
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
Section 12 summarizes the normative updates this document makes to
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
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
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
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
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
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
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
_25._tcp.mail.example.com. IN TLSA 2 0 1 (
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
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)
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
; 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
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
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
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
Items 2-4 are precisely the contents of the subjectPublicKeyInfo
published in the TLSA record. The issuer name is not included in the
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
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
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.