Internet Engineering Task Force (IETF) V. Dukhovni

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.

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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

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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].

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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").

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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.

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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.

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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

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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).

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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.

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; 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.

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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

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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).

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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.

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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.

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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,

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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.

6. Service Provider and TLSA Publisher Synchronization

Whenever possible, the TLSA Publisher and the Service Provider should
be the same entity. Otherwise, they need to coordinate changes to
ensure that TLSA records published by the TLSA Publisher don't fall
out of sync with the server certificate used by the Service Provider.
Such coordination is difficult, and service outages will result when
coordination fails.

Publishing the TLSA record in the Service Provider's zone avoids the
complexity of bilateral coordination of server certificate
configuration and TLSA record management. Even when the TLSA RRset
has to be published in the Customer Domain's DNS zone (perhaps the
client application does not "chase" CNAMEs to the TLSA base domain),
it is possible to employ CNAME records to delegate the content of the
TLSA RRset to a domain operated by the Service Provider.

Only certificate usages DANE-EE(3) and DANE-TA(2) work well with TLSA
CNAMEs across organizational boundaries. With PKIX-TA(0) or
PKIX-EE(1), the Service Provider would need to obtain certificates in
the name of the Customer Domain from a suitable public CA (securely
impersonate the customer), or the customer would need to provision
the relevant private keys and certificates at the Service Provider's
systems.

Certificate Usage DANE-EE(3): In this case, the Service Provider can
publish a single TLSA RRset that matches the server certificate or
public key digest. The same RRset works for all Customer Domains
because name checks do not apply with DANE-EE(3) TLSA records (see
Section 5.1). A Customer Domain can create a CNAME record
pointing to the TLSA RRset published by the Service Provider.

Certificate Usage DANE-TA(2): When the Service Provider operates a
private CA, the Service Provider is free to issue a certificate
bearing any customer's domain name. Without DANE, such a
certificate would not pass trust verification, but with DANE, the
customer's TLSA RRset that is aliased to the provider's TLSA RRset
can delegate authority to the provider's CA for the corresponding
service. The Service Provider can generate appropriate

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certificates for each customer and use the SNI information
provided by clients to select the right certificate chain to
present to each client.

Below are example DNS records (assumed "secure" and shown without the
associated DNSSEC information, such as record signatures) that
illustrate both of the above models in the case of an HTTPS service
whose clients all support DANE TLS. These examples work even with
clients that don't "chase" CNAMEs when constructing the TLSA base
domain (see Section 7 below).

; The hosted web service is redirected via a CNAME alias.
; The associated TLSA RRset is also redirected via a CNAME alias.
;
; Certificate usage DANE-EE(3) makes it possible to deploy
; a single provider certificate for all Customer Domains.
;
www1.example.com. IN CNAME w1.example.net.
_443._tcp.www1.example.com. IN CNAME _443._tcp.w1.example.net.
_443._tcp.w1.example.net. IN TLSA 3 1 1 (
8A9A70596E869BED72C69D97A8895DFA
D86F300A343FECEFF19E89C27C896BC9 )

;
; A CA at the provider can also issue certificates for each Customer
; Domain and employ the DANE-TA(2) certificate usage to
; designate the provider's CA as a TA.
;
www2.example.com. IN CNAME w2.example.net.
_443._tcp.www2.example.com. IN CNAME _443._tcp.w2.example.net.
_443._tcp.w2.example.net. IN TLSA 2 0 1 (
C164B2C3F36D068D42A6138E446152F5
68615F28C69BD96A73E354CAC88ED00C )

With protocols that support explicit transport redirection via DNS MX
records, SRV records, or other similar records, the TLSA base domain
is based on the redirected transport endpoint rather than the origin
domain. With SMTP, for example, when an email service is hosted by a
Service Provider, the Customer Domain's MX hostnames will point at
the Service Provider's SMTP hosts. When the Customer Domain's DNS
zone is signed, the MX hostnames can be securely used as the base

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domains for TLSA records that are published and managed by the
Service Provider. For example (without the required DNSSEC
information, such as record signatures):

; Hosted SMTP service.
;
example.com. IN MX 0 mx1.example.net.
example.com. IN MX 0 mx2.example.net.
_25._tcp.mx1.example.net. IN TLSA 3 1 1 (
8A9A70596E869BED72C69D97A8895DFA
D86F300A343FECEFF19E89C27C896BC9 )
_25._tcp.mx2.example.net. IN TLSA 3 1 1 (
C164B2C3F36D068D42A6138E446152F5
68615F28C69BD96A73E354CAC88ED00C )

If redirection to the Service Provider's domain (via MX records, SRV
records, or any similar mechanism) is not possible and aliasing of
the TLSA record is not an option, then more complex coordination
between the Customer Domain and Service Provider will be required.
Either the Customer Domain periodically provides private keys and a
corresponding certificate chain to the provider (after making
appropriate changes in its TLSA records), or the Service Provider
periodically generates the keys and certificates and needs to wait
for matching TLSA records to be published by its Customer Domains
before deploying newly generated keys and certificate chains.
Section 7 below describes an approach that employs CNAME "chasing" to
avoid the difficulties of coordinating key management across
organizational boundaries.

For further information about combining DANE and SRV, please see
[RFC7673].

7. TLSA Base Domain and CNAMEs

When the application protocol does not support service location
indirection via MX, SRV, or similar DNS records, the service may be
redirected via a CNAME. A CNAME is a more blunt instrument for this
purpose because, unlike an MX or SRV record, it remaps the entire
origin domain to the target domain for all protocols.

The complexity of coordinating key management is largely eliminated
when DANE TLSA records are found in the Service Provider's domain, as
discussed in Section 6. Therefore, DANE TLS clients connecting to a
server whose domain name is a CNAME alias SHOULD follow the CNAME
"hop by hop" to its ultimate target host (noting at each step whether
or not the CNAME is DNSSEC validated). If at each stage of CNAME
expansion the DNSSEC validation status is "secure", the final target
name SHOULD be the preferred base domain for TLSA lookups.

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Implementations failing to find a TLSA record using a base name of
the final target of a CNAME expansion SHOULD issue a TLSA query using
the original destination name. That is, the preferred TLSA base
domain SHOULD be derived from the fully expanded name and, failing
that, SHOULD be the initial domain name.

When the TLSA base domain is the result of "secure" CNAME expansion,
the resulting domain name MUST be used as the HostName in the
client's SNI extension and MUST be the primary reference identifier
for peer certificate matching with certificate usages other than
DANE-EE(3).

Protocol-specific TLSA specifications may provide additional guidance
or restrictions when following CNAME expansions.

Though CNAMEs are illegal on the right-hand side of most indirection
records, such as MX and SRV records, they are supported by some
implementations. For example, if the MX or SRV host is a CNAME
alias, some implementations may "chase" the CNAME. If they do, they
SHOULD use the target hostname as the preferred TLSA base domain as
described above (and, if the TLSA records are found there, also use
the CNAME-expanded domain in SNI and certificate name checks).

8. TLSA Publisher Requirements

This section updates [RFC6698] by specifying that the TLSA Publisher
MUST ensure that each combination of certificate usage, selector, and
matching type in the server's TLSA RRset includes at least one record
that matches the server's current certificate chain. TLSA records
that match recently retired or yet-to-be-deployed certificate chains
will be present during key rollover. Such past or future records
MUST NOT at any time be the only records published for any given
combination of usage, selector, and matching type. The TLSA record
update process described below ensures that this requirement is met.

While a server is to be considered authenticated when its certificate
chain is matched by any of the published TLSA records, not all
clients support all combinations of TLSA record parameters. Some
clients may not support some digest algorithms; others may either not
support or exclusively support the PKIX certificate usages. Some
clients may prefer to negotiate [RFC7250] raw public keys, which are
only compatible with TLSA records whose certificate usage is
DANE-EE(3) with selector SPKI(1). The only other TLSA record type
that is potentially compatible with raw public keys is "DANE-EE(3)
Cert(0) Full(0)", but support for raw public keys with that TLSA
record type is not expected to be broadly implemented.

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A consequence of the above uncertainty as to which TLSA parameters
are supported by any given client is that servers need to ensure that
each and every parameter combination that appears in the TLSA RRset
is, on its own, sufficient to match the server's current certificate
chain. In particular, when deploying new keys or new parameter
combinations, some care is required to not generate parameter
combinations that only match past or future certificate chains (or
raw public keys). The rest of this section explains how to update
the TLSA RRset in a manner that ensures that the above requirement
is met.

8.1. Key Rollover with Fixed TLSA Parameters

The simplest case is key rollover while retaining the same set of
published parameter combinations. In this case, TLSA records
matching the existing server certificate chain (or raw public keys)
are first augmented with corresponding records matching the future
keys, at least two Times to Live (TTLs) or longer before the new
chain is deployed. This allows the obsolete RRset to age out of
client caches before the new chain is used in TLS handshakes. Once
sufficient time has elapsed and all clients performing DNS lookups
are retrieving the updated TLSA records, the server administrator may
deploy the new certificate chain, verify that it works, and then
remove any obsolete records matching the chain that is no longer
active:

; Initial TLSA RRset.
;
_443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...

; Transitional TLSA RRset published at least two TTLs before
; the actual key change.
;
_443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...
_443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b...

; Final TLSA RRset after the key change.
;
_443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b...

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The next case to consider is adding or switching to a new combination
of TLSA parameters. In this case, publish the new parameter
combinations for the server's existing certificate chain first, and
only then deploy new keys if desired:

; Initial TLSA RRset.
;
_443._tcp.www.example.org. IN TLSA 1 1 1 01d09d19c2139a46...

; New TLSA RRset, same key re-published as DANE-EE(3).
;
_443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...

8.2. Switching to DANE-TA(2) from DANE-EE(3)

This section explains how to migrate to a new certificate chain and
TLSA record with usage DANE-TA(2) from a self-signed server
certificate and a "DANE-EE(3) SPKI(1) SHA2-256(1)" TLSA record. This
example assumes that a new private key is generated in conjunction
with transitioning to a new certificate issued by the desired TA.

The original "3 1 1" TLSA record supports [RFC7250] raw public keys,
and clients may choose to negotiate their use. The use of raw public
keys rules out the possibility of certificate chain verification.
Therefore, the transitional TLSA record for the planned DANE-TA(2)
certificate chain is a "3 1 1" record that works even when raw public
keys are used. The TLSA RRset is updated to use DANE-TA(2) only
after the new chain is deployed and the "3 1 1" record matching the
original key is dropped.

This process follows the requirement that each combination of
parameters present in the RRset is always sufficient to validate the
server. It avoids publishing a transitional TLSA RRset in which
"3 1 1" matches only the current key and "2 0 1" matches only the
future certificate chain, because these might not work reliably
during the initial deployment of the new keys.

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; Initial TLSA RRset.
;
_443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...

; Transitional TLSA RRset, published at least two TTLs before the
; actual key change. The new keys are issued by a DANE-TA(2) CA
; but are initially specified via a DANE-EE(3) association.
;
_443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...
_443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b...

; The final TLSA RRset after the key change. Now that the old
; self-signed EE key is out of the picture, publish the issuing
; TA of the new chain.
;
_443._tcp.www.example.org. IN TLSA 2 0 1 c57bce38455d9e3d...

8.3. Switching to New TLSA Parameters

When employing a new digest algorithm in the TLSA RRset, for
compatibility with digest algorithm agility as specified in Section 9
below, administrators SHOULD publish the new digest algorithm with
each combination of certificate usage and selector for each
associated key or chain used with any other digest algorithm. When
removing an algorithm, remove it entirely. Each digest algorithm
employed SHOULD match the same set of chains (or raw public keys).

; Initial TLSA RRset with "DANE-EE(3) SHA2-256(1)" associations
; for two keys.
;
_443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...
_443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b...

; New TLSA RRset, also with SHA2-512(2) associations
; for each key.
;
_443._tcp.www.example.org. IN TLSA 3 1 1 01d09d19c2139a46...
_443._tcp.www.example.org. IN TLSA 3 1 2 d9947c35089310bc...
_443._tcp.www.example.org. IN TLSA 3 1 1 7aa7a5359173d05b...
_443._tcp.www.example.org. IN TLSA 3 1 2 89a7486a4b6ae714...

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8.4. TLSA Publisher Requirements: Summary

In summary, server operators updating TLSA records should make one
change at a time. The individual safe changes are as follows:

o Pre-publish new certificate associations that employ the same TLSA
parameters (usage, selector, and matching type) as existing TLSA
records, but match certificate chains that will be deployed in the
near future.

o Wait for stale TLSA RRsets to expire from DNS caches before
configuring servers to use the new certificate chain.

o Remove TLSA records matching any certificate chains that are no
longer deployed.

o Publish TLSA RRsets in which all parameter combinations
(certificate usage, selector, and matching type) present in the
RRset match the same set of current and planned certificate
chains.

The above steps are intended to ensure that at all times, and for
each combination of usage, selector, and matching type, at least one
TLSA record corresponds to the server's current certificate chain.
Each combination of certificate usage, selector, and matching type in
a server's TLSA RRset SHOULD NOT at any time (including unexpired
RRsets in client caches) match only some combination of future or
past certificate chains. As a result, no matter what combinations of
usage, selector, and matching type may be supported by a given
client, they will be sufficient to authenticate the server.

9. Digest Algorithm Agility

While [RFC6698] specifies multiple digest algorithms, it does not
specify a protocol by which the client and TLSA record publisher can
agree on the strongest shared algorithm. Such a protocol would allow
the client and server to avoid exposure to deprecated weaker
algorithms that are published for compatibility with less capable
clients but that SHOULD be avoided when possible. Such a protocol is
specified below.

This section defines a protocol for avoiding deprecated digest
algorithms when these are published in a peer's TLSA RRset alongside
stronger digest algorithms. Note that this protocol never avoids RRs
with a DANE matching type of Full(0), as these do not employ a digest
algorithm that might someday be weakened by cryptanalysis.

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Client implementations SHOULD implement a default order of digest
algorithms by strength. This order SHOULD be configurable by the
administrator or user of the client software. If possible, a
configurable mapping from numeric DANE TLSA matching types to
underlying digest algorithms provided by the cryptographic library
SHOULD be implemented to allow new matching types to be used with
software that predates their introduction. Configurable ordering of
digest algorithms SHOULD be extensible to any new digest algorithms.

To make digest algorithm agility possible, all published DANE TLSA
RRsets MUST conform to the requirements of Section 8. Clients SHOULD
use digest algorithm agility when processing the peer's DANE TLSA
records. Algorithm agility is to be applied after first discarding
any unusable or malformed records (unsupported digest algorithm, or
incorrect digest length). For each usage and selector, the client
SHOULD process only any usable records with a matching type of
Full(0) and the usable records whose digest algorithm is considered
by the client to be the strongest among usable records with the given
usage and selector.

Example: a client implements digest algorithm agility and prefers
SHA2-512(2) over SHA2-256(1), while the server publishes an RRset
that employs both digest algorithms as well as a Full(0) record.

_25._tcp.mail.example.com. IN TLSA 3 1 1 (
3FE246A848798236DD2AB78D39F0651D
6B6E7CA8E2984012EB0A2E1AC8A87B72 )
_25._tcp.mail.example.com. IN TLSA 3 1 2 (
D4F5AF015B46C5057B841C7E7BAB759C
BF029526D29520C5BE6A32C67475439E
54AB3A945D80C743347C9BD4DADC9D8D
57FAB78EAA835362F3CA07CCC19A3214 )
_25._tcp.mail.example.com. IN TLSA 3 1 0 (
3059301306072A8648CE3D020106082A
8648CE3D0301070342000471CB1F504F
9E4B33971376C005445DACD33CD79A28
81C3DED1981F18E7AAA76609DD0E4EF2
8265C82703030AD60C5DBA6FB8A9397A
C0FCF06D424C885D484887 )

In this case, the client SHOULD accept a server public key that
matches either the "3 1 0" record or the "3 1 2" record, but it
SHOULD NOT accept keys that match only the weaker "3 1 1" record.

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10. General DANE Guidelines

These guidelines provide guidance for using or designing protocols
for DANE.

10.1. DANE DNS Record Size Guidelines

Selecting a combination of TLSA parameters to use requires careful
thought. One important consideration to take into account is the
size of the resulting TLSA record after its parameters are selected.

10.1.1. UDP and TCP Considerations

Deployments SHOULD avoid TLSA record sizes that cause UDP
fragmentation.

Although DNS over TCP would provide the ability to more easily
transfer larger DNS records between clients and servers, it is not
universally deployed and is still prohibited by some firewalls.
Clients that request DNS records via UDP typically only use TCP upon
receipt of a truncated response in the DNS response message sent over
UDP. Setting the Truncation (TC) bit (Section 4.1.1 of [RFC1035])
alone will be insufficient if the response containing the TC bit is
itself fragmented.

10.1.2. Packet Size Considerations for TLSA Parameters

Server operators SHOULD NOT publish TLSA records using both a TLSA
selector of Cert(0) and a TLSA matching type of Full(0), as even a
single certificate is generally too large to be reliably delivered
via DNS over UDP. Furthermore, two TLSA records containing full
certificates will need to be published simultaneously during a
certificate rollover, as discussed in Section 8.1.

While TLSA records using a TLSA selector of SPKI(1) and a TLSA
matching type of Full(0) (which publish the bare public keys, i.e.,
without the overhead of encapsulating the keys in an X.509
certificate) are generally more compact, these are also best avoided
when significantly larger than their digests. Rather, servers SHOULD
publish digest-based TLSA matching types in their TLSA records, in
which case the complete corresponding certificate MUST be transmitted
to the client in-band during the TLS handshake. The certificate (or
raw public key) can be easily verified using the digest value.

In summary, the use of a TLSA matching type of Full(0) is
NOT RECOMMENDED, and a digest-based matching type, such as
SHA2-256(1), SHOULD be used instead.

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10.2. Certificate Name Check Conventions

Certificates presented by a TLS server will generally contain a
subjectAltName (SAN) extension or a Common Name (CN) element within
the subject Distinguished Name (DN). The TLS server's DNS domain
name is normally published within these elements, ideally within the
SAN extension. (The use of the CN field for this purpose is
deprecated.)

When a server hosts multiple domains at the same transport endpoint,
the server's ability to respond with the right certificate chain is
predicated on correct SNI information from the client. DANE clients
MUST send the SNI extension with a HostName value of the base domain
of the TLSA RRset.

With the exception of TLSA certificate usage DANE-EE(3), where name
checks are not applicable (see Section 5.1), DANE clients MUST verify
that the client has reached the correct server by checking that the
server name is listed in the server certificate's SAN or CN (when
still supported). The primary server name used for this comparison
MUST be the TLSA base domain; however, additional acceptable names
may be specified by protocol-specific DANE standards. For example,
with SMTP, both the destination domain name and the MX hostname are
acceptable names to be found in the server certificate (see
[RFC7672]).

It is the responsibility of the service operator, in coordination
with the TLSA Publisher, to ensure that at least one of the TLSA
records published for the service will match the server's certificate
chain (either the default chain or the certificate that was selected
based on the SNI information provided by the client).

Given the DNSSEC-validated DNS records below:

example.com. IN MX 0 mail.example.com.
mail.example.com. IN A 192.0.2.1
_25._tcp.mail.example.com. IN TLSA 2 0 1 (
E8B54E0B4BAA815B06D3462D65FBC7C0
CF556ECCF9F5303EBFBB77D022F834C0 )

The TLSA base domain is "mail.example.com" and is required to be the
HostName in the client's SNI extension. The server certificate chain
is required to be signed by a TA with the above certificate SHA2-256
digest. Finally, one of the DNS names in the server certificate is
required to be either "mail.example.com" or "example.com" (this
additional name is a concession to compatibility with prior practice;
see [RFC7672] for details).

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[RFC6125] specifies the semantics of wildcards in server certificates
for various application protocols. DANE does not change how
wildcards are treated by any given application.

10.3. Design Considerations for Protocols Using DANE

When a TLS client goes to the trouble of authenticating a certificate
chain presented by a TLS server, it will typically not continue to
use that server in the event of authentication failure, or else
authentication serves no purpose. Some clients may, at times,
operate in an "audit" mode, where authentication failure is reported
to the user or in logs as a potential problem, but the connection
proceeds despite the failure. Nevertheless, servers publishing TLSA
records MUST be configured to allow correctly configured clients to
successfully authenticate their TLS certificate chains.

A service with DNSSEC-validated TLSA records implicitly promises TLS
support. When all the TLSA records for a service are found
"unusable" due to unsupported parameter combinations or malformed
certificate association data, DANE clients cannot authenticate the
service certificate chain. When authenticated TLS is mandatory, the
client MUST NOT connect to the associated server.

If, on the other hand, the use of TLS and DANE is "opportunistic"
[RFC7435], then when all TLSA records are unusable, the client SHOULD
connect to the server via an unauthenticated TLS connection, and if
TLS encryption cannot be established, the client MUST NOT connect to
the server.

Standards for opportunistic DANE TLS specific to a particular
application protocol may modify the above requirements. The key
consideration is whether or not mandating the use of
(unauthenticated) TLS even with unusable TLSA records is asking for
more security than one can realistically expect. If expecting TLS
support when unusable TLSA records are published is realistic for the
application in question, then the application MUST avoid cleartext.
If not realistic, then mandating TLS would cause clients (even in the
absence of active attacks) to run into problems with various peers
that do not interoperate "securely enough". That would create strong
incentives to just disable Opportunistic Security and stick with
cleartext.

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11. Note on DNSSEC Security

Clearly, the security of the DANE TLSA PKI rests on the security of
the underlying DNSSEC infrastructure. While this document is not a
guide to DNSSEC security, a few comments may be helpful to TLSA
implementers.

With the existing public CA Web PKI, name constraints are rarely
used, and a public root CA can issue certificates for any domain of
its choice. With DNSSEC, under the Registry/Registrar/Registrant
model, the situation is different: only the registrar of record can
update a domain's DS record [RFC4034] in the registry parent zone (in
some cases, however, the registry is the sole registrar). With many
Generic Top-Level Domains (gTLDs) for which multiple registrars
compete to provide domains in a single registry, it is important to
make sure that rogue registrars cannot easily initiate an
unauthorized domain transfer and thus take over DNSSEC for the
domain. DNS operators are advised to set a registrar lock on their
domains to offer some protection against this possibility.

When the registrar is also the DNS operator for the domain, one needs
to consider whether or not the registrar will allow orderly migration
of the domain to another registrar or DNS operator in a way that will
maintain DNSSEC integrity. TLSA Publishers are advised to seek out a
DNS hosting registrar that makes it possible to transfer domains to
another hosting provider without disabling DNSSEC.

DNSSEC-signed RRsets cannot be securely revoked before they expire.
Operators need to plan accordingly and not generate signatures of
excessively long duration. For domains publishing high-value keys, a
signature lifetime (length of the "signature validity period" as
described in Section 8.1 of [RFC4033]) of a few days is reasonable,
and the zone can be re-signed daily. For domains with less critical
data, a reasonable signature lifetime is a couple of weeks to a
month, and the zone can be re-signed weekly.

Short signature lifetimes require tighter synchronization of primary
and secondary nameservers, to make sure that secondary servers never
serve records with expired signatures. They also limit the maximum
time for which a primary server that signs the zone can be down.
Therefore, short signature lifetimes are more appropriate for sites
with dedicated operations staff, who can restore service quickly in
case of a problem.

Monitoring is important. If a DNS zone is not re-signed in a timely
manner, a major outage is likely, as the entire domain and all its
sub-domains become "bogus".

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12. Summary of Updates to RFC 6698

o Section 3 updates [RFC6698] to specify a requirement for clients
to support at least TLS 1.0 and to support SNI.

o Section 4 explains that application support for all four
certificate usages is NOT RECOMMENDED. The recommended design is
to support just DANE-EE(3) and DANE-TA(2).

o Section 5.1 updates [RFC6698] to specify that peer identity
matching and validity period enforcement are based solely on the
TLSA RRset properties. It also specifies DANE authentication of
raw public keys [RFC7250] via TLSA records with certificate usage
DANE-EE(3) and selector SPKI(1).

o Section 5.2 updates [RFC6698] to require that servers publishing
digest TLSA records with a usage of DANE-TA(2) MUST include the
TA certificate in their TLS server certificate message. This
extends to the case of "2 1 0" TLSA records that publish a full
public key.

o Section 5.4 observes that with usage PKIX-TA(0), clients may need
to process extended trust chains beyond the first trusted issuer
when that issuer is not self-signed.

o Section 7 recommends that DANE application protocols specify that,
when possible, securely CNAME-expanded names be used to derive the
TLSA base domain.

o Section 8 specifies a strategy for managing TLSA records that
interoperates with DANE clients regardless of what subset of the
possible TLSA record types (combinations of TLSA parameters) is
supported by the client.

o Section 9 specifies a digest algorithm agility protocol.

o Section 10.1 recommends against the use of Full(0) TLSA records,
as digest records are generally much more compact.

13. Operational Considerations

The DNS TTL of TLSA records needs to be chosen with care. When an
unplanned change in the server's certificate chain and TLSA RRset is
required, such as when keys are compromised or lost, clients that
cache stale TLSA records will fail to validate the certificate chain
of the updated server. Publish TLSA RRsets with TTLs that are short
enough to limit unplanned service disruption to an acceptable
duration.

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The signature lifetime (length of the signature validity period) for
TLSA records SHOULD NOT be too long. Signed DNSSEC records can be
replayed by an MITM attacker, provided the signatures have not yet
expired. Shorter signature validity periods allow for faster
invalidation of compromised keys. Zone refresh and expiration times
for secondary nameservers often imply a lower bound on the signature
validity period (Section 11). See Section 4.4.1 of [RFC6781].

14. Security Considerations

Application protocols that cannot use the existing public CA Web PKI
may choose to not implement certain TLSA record types defined in
[RFC6698]. If such records are published despite not being supported
by the application protocol, they are treated as "unusable". When
TLS is opportunistic, the client MAY proceed to use the server with
mandatory unauthenticated TLS. This is stronger than opportunistic
TLS without DANE, since in that case the client may also proceed with
a cleartext connection. When TLS is not opportunistic, the client
MUST NOT connect to the server.

Thus, when TLSA records are used with opportunistic protocols where
PKIX-TA(0) and PKIX-EE(1) do not apply, the recommended protocol
design is for servers to not publish such TLSA records, and for
opportunistic TLS clients to use them to only enforce the use of
(albeit unauthenticated) TLS but otherwise treat them as unusable.
Of course, when PKIX-TA(0) and PKIX-EE(1) are supported by the
application protocol, clients MUST implement these certificate usages
as described in [RFC6698] and this document.

15. References

15.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.

[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<http://www.rfc-editor.org/info/rfc4034>.

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[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<http://www.rfc-editor.org/info/rfc4035>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.

[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.

[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.

[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125,
March 2011, <http://www.rfc-editor.org/info/rfc6125>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.

[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698,
August 2012, <http://www.rfc-editor.org/info/rfc6698>.

[RFC7218] Gudmundsson, O., "Adding Acronyms to Simplify
Conversations about DNS-Based Authentication of Named
Entities (DANE)", RFC 7218, DOI 10.17487/RFC7218,
April 2014, <http://www.rfc-editor.org/info/rfc7218>.

[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <http://www.rfc-editor.org/info/rfc7250>.

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15.2. Informative References

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.

[RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
Operational Practices, Version 2", RFC 6781,
DOI 10.17487/RFC6781, December 2012,
<http://www.rfc-editor.org/info/rfc6781>.

[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<http://www.rfc-editor.org/info/rfc6962>.

[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <http://www.rfc-editor.org/info/rfc7435>.

[RFC7672] Dukhovni, V. and W. Hardaker, "SMTP Security via
Opportunistic DNS-Based Authentication of Named Entities
(DANE) Transport Layer Security (TLS)", RFC 7672,
DOI 10.17487/RFC7672, October 2015,
<http://www.rfc-editor.org/info/rfc7672>.

[RFC7673] Finch, T., Miller, M., and P. Saint-Andre, "Using
DNS-Based Authentication of Named Entities (DANE) TLSA
Records with SRV Records", RFC 7673, DOI 10.17487/RFC7673,
October 2015, <http://www.rfc-editor.org/info/rfc7673>.

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Acknowledgements

The authors would like to thank Phil Pennock for his comments and
advice on this document.

Acknowledgements from Viktor: Thanks to Tony Finch, who finally
prodded me into participating in DANE working group discussions.
Thanks to Paul Hoffman, who motivated me to produce this document and
provided feedback on early draft versions of it. Thanks also to
Samuel Dukhovni for editorial assistance.

Authors' Addresses

Viktor Dukhovni
Two Sigma

Email: [email protected]

Wes Hardaker
Parsons
P.O. Box 382
Davis, CA 95617
United States

Email: [email protected]

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