DANE Updates: The DNS-Based Authentication of Named Entities (DANE) Protocol: Updates and Operational Guidance

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
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................4
   2. DANE TLSA Record Overview .......................................5
      2.1. Example TLSA Record ........................................6
   3. DANE TLS Requirements ...........................................6
   4. DANE Certificate Usage Selection Guidelines .....................7
      4.1. Opportunistic Security and PKIX Usages .....................7
      4.2. Interaction with Certificate Transparency ..................8
      4.3. Switching from/to PKIX-TA/EE to/from DANE-TA/EE ............9
   5. Certificate-Usage-Specific DANE Updates and Guidelines ..........9
      5.1. Certificate Usage DANE-EE(3) ...............................9
      5.2. Certificate Usage DANE-TA(2) ..............................11
      5.3. Certificate Usage PKIX-EE(1) ..............................15
      5.4. Certificate Usage PKIX-TA(0) ..............................15
   6. Service Provider and TLSA Publisher Synchronization ............16
   7. TLSA Base Domain and CNAMEs ....................................18
   8. TLSA Publisher Requirements ....................................19
      8.1. Key Rollover with Fixed TLSA Parameters ...................20
      8.2. Switching to DANE-TA(2) from DANE-EE(3) ...................21
      8.3. Switching to New TLSA Parameters ..........................22
      8.4. TLSA Publisher Requirements: Summary ......................23
   9. Digest Algorithm Agility .......................................23
   10. General DANE Guidelines .......................................25
      10.1. DANE DNS Record Size Guidelines ..........................25
      10.2. Certificate Name Check Conventions .......................26
      10.3. Design Considerations for Protocols Using DANE ...........27
   11. Note on DNSSEC Security .......................................28
   12. Summary of Updates to RFC 6698 ................................29
   13. Operational Considerations ....................................29
   14. Security Considerations .......................................30
   15. References ....................................................30
      15.1. Normative References .....................................30
      15.2. Informative References ...................................32
   Acknowledgements ..................................................33
   Authors' Addresses ................................................33

1.  Introduction

   The DNS-Based Authentication of Named Entities (DANE) specification
   [RFC6698] introduces the DNS "TLSA" resource record (RR) type ("TLSA"
   is not an acronym).  TLSA records associate a certificate or a public
   key of an end-entity or a trusted issuing authority with the
   corresponding Transport Layer Security (TLS) [RFC5246] or Datagram
   Transport Layer Security (DTLS) [RFC6347] transport endpoint.  DANE
   relies on the DNS Security Extensions (DNSSEC) [RFC4033].  DANE TLSA
   records validated by DNSSEC can be used to augment or replace the use
   of trusted public Certification Authorities (CAs).

   The TLS and DTLS protocols provide secured TCP and UDP communication,
   respectively, over IP.  In the context of this document, channel
   security is assumed to be provided by TLS or DTLS.  By convention,
   "TLS" will be used throughout this document; unless otherwise
   specified, the text applies equally well to DTLS over UDP.  Used
   without authentication, TLS provides protection only against
   eavesdropping through its use of encryption.  With authentication,
   TLS also protects the transport against man-in-the-middle (MITM)
   attacks.

   [RFC6698] defines three TLSA record fields: the first with four
   possible values, the second with two, and the third with three.
   These yield 24 distinct combinations of TLSA record types.  This
   document recommends a smaller set of best-practice combinations of
   these fields to simplify protocol design, implementation, and
   deployment.

   This document explains and recommends DANE-specific strategies to
   simplify "virtual hosting", where a single Service Provider transport
   endpoint simultaneously supports multiple hosted Customer Domains.

   Other related documents that build on [RFC6698] are [RFC7673] and
   [RFC7672].

   Section 12 summarizes the normative updates this document makes to
   [RFC6698].

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   The following terms are used throughout this document:

   Web PKI:  The Public Key Infrastructure (PKI) model employed by
      browsers to authenticate web servers.  This employs a set of
      trusted public CAs to vouch for the authenticity of public keys
      associated with a particular party (the subject).

   Service Provider:  A company or organization that offers to host a
      service on behalf of the owner of a Customer Domain.  The original
      domain name associated with the service often remains under the
      control of the customer.  Connecting applications may be directed
      to the Service Provider via a redirection RR.  Example redirection
      records include MX, SRV, and CNAME.  The Service Provider
      frequently provides services for many customers and needs to
      ensure that the TLS credentials presented to connecting
      applications authenticate it as a valid server for the requested
      domain.

   Customer Domain:  As described above, a TLS client may be interacting
      with a service that is hosted by a third party.  This document
      refers to the domain name used to locate the service (prior to any
      redirection) as the "Customer Domain".

   TLSA Publisher:  The entity responsible for publishing a TLSA record
      within a DNS zone.  This zone will be assumed DNSSEC-signed and
      validatable to a trust anchor (TA), unless otherwise specified.
      If the Customer Domain is not outsourcing its DNS service, the
      TLSA Publisher will be the customer itself.  Otherwise, the TLSA
      Publisher may be the operator of the outsourced DNS service.

   Public key:  The term "public key" is shorthand for the
      subjectPublicKeyInfo component of a PKIX [RFC5280] certificate.

   SNI:  The Server Name Indication (SNI) TLS protocol extension allows
      a TLS client to request a connection to a particular service name
      of a TLS server ([RFC6066], Section 3).  Without this TLS
      extension, a TLS server has no choice but to offer a certificate
      with a default list of server names, making it difficult to host
      multiple Customer Domains at the same IP-address-based TLS service
      endpoint (i.e., provide "secure virtual hosting").

   TLSA parameters:  In [RFC6698], the TLSA record is defined to consist
      of four fields.  The first three of these are numeric parameters
      that specify the meaning of the data in the fourth and final
      field.  This document refers to the first three fields as "TLSA
      parameters", or sometimes just "parameters" when obvious from
      context.

   TLSA base domain:  Per Section 3 of [RFC6698], TLSA records are
      stored at a DNS domain name that is a combination of a port and
      protocol prefix and a "base domain".  In [RFC6698], the "base
      domain" is the fully qualified domain name of the TLS server.
      This document modifies the TLSA record lookup strategy to prefer
      the fully CNAME-expanded name of the TLS server, provided that
      expansion is "secure" (DNSSEC validated) at each stage of the
      expansion, and TLSA records are published for this fully expanded
      name.  Thus, the "TLSA base domain" is either the fully
      CNAME-expanded TLS server name or otherwise the initial fully
      qualified TLS server name, whichever is used in combination with a
      port and protocol prefix to obtain the TLSA RRset.

2.  DANE TLSA Record Overview

   DANE TLSA [RFC6698] specifies a protocol for publishing TLS server
   certificate associations via DNSSEC [RFC4033] [RFC4034] [RFC4035].
   DANE TLSA records consist of four fields.  The record type is
   determined by the values of the first three fields, which this
   document refers to as the "TLSA parameters" to distinguish them from
   the fourth and last field.  The numeric values of these parameters
   were given symbolic names in [RFC7218].  The four fields are as
   follows:

   The Certificate Usage field:  Section 2.1.1 of [RFC6698] specifies
      four values: PKIX-TA(0), PKIX-EE(1), DANE-TA(2), and DANE-EE(3).
      There is an additional private-use value: PrivCert(255), which,
      given its private scope, shall not be considered further in this
      document.  All other values are reserved for use by future
      specifications.

   The Selector field:  Section 2.1.2 of [RFC6698] specifies two values:
      Cert(0) and SPKI(1).  There is an additional private-use value:
      PrivSel(255).  All other values are reserved for use by future
      specifications.

   The Matching Type field:  Section 2.1.3 of [RFC6698] specifies three
      values: Full(0), SHA2-256(1), and SHA2-512(2).  There is an
      additional private-use value: PrivMatch(255).  All other values
      are reserved for use by future specifications.

   The Certificate Association Data field:  See Section 2.1.4 of
      [RFC6698].  This field stores the full value or digest of the
      certificate or subject public key as determined by the matching
      type and selector, respectively.

   In the Matching Type field, of the two digest algorithms --
   SHA2-256(1) and SHA2-512(2) -- as of the time of this writing, only
   SHA2-256(1) is mandatory to implement.  Clients SHOULD implement
   SHA2-512(2), but servers SHOULD NOT exclusively publish SHA2-512(2)
   digests.  The digest algorithm agility protocol defined in Section 9
   SHOULD be used by clients to decide how to process TLSA RRsets that
   employ multiple digest algorithms.  Server operators MUST publish
   TLSA RRsets that are compatible (see Section 8) with digest algorithm
   agility (Section 9).

2.1.  Example TLSA Record

   In the example TLSA record below, the TLSA certificate usage is
   DANE-TA(2), the selector is Cert(0), and the matching type is
   SHA2-256(1).  The last field is the Certificate Association Data
   field, which in this case contains the SHA2-256 digest of the server
   certificate.

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

3.  DANE TLS Requirements

   [RFC6698] does not discuss what versions of TLS are required when
   using DANE records.  This document specifies that TLS clients that
   support DANE/TLSA MUST support at least TLS 1.0 and SHOULD support
   TLS 1.2 or later.

   TLS clients using DANE MUST support the SNI extension of TLS
   [RFC6066].  Servers MAY support SNI and respond with a matching
   certificate chain but MAY also ignore SNI and respond with a default
   certificate chain.  When a server supports SNI but is not configured
   with a certificate chain that exactly matches the client's SNI
   extension, the server SHOULD respond with another certificate chain
   (a default or closest match).  This is because clients might support
   more than one server name but can only put a single name in the SNI
   extension.

4.  DANE Certificate Usage Selection Guidelines

   As mentioned in Section 2, the TLSA Certificate Usage field takes one
   of four possible values.  With PKIX-TA(0) and PKIX-EE(1), the
   validation of peer certificate chains requires additional
   preconfigured CA TAs that are mutually trusted by the operators of
   the TLS server and client.  With DANE-TA(2) and DANE-EE(3), no
   preconfigured CA TAs are required and the published DANE TLSA records
   are sufficient to verify the peer's certificate chain.

   Standards for application protocols that employ DANE TLSA can specify
   more specific guidance than [RFC6698] or this document.  Such
   application-specific standards need to carefully consider which set
   of DANE certificate usages to support.  Simultaneous support for all
   four usages is NOT RECOMMENDED for DANE clients.  When all four
   usages are supported, an attacker capable of compromising the
   integrity of DNSSEC needs only to replace the server's TLSA RRset
   with one that lists suitable DANE-EE(3) or DANE-TA(2) records,
   effectively bypassing any added verification via public CAs.  In
   other words, when all four usages are supported, PKIX-TA(0) and
   PKIX-EE(1) offer only illusory incremental security over DANE-TA(2)
   and DANE-EE(3).

   Designs in which clients support just the DANE-TA(2) and DANE-EE(3)
   certificate usages are RECOMMENDED.  With DANE-TA(2) and DANE-EE(3),
   clients don't need to track a large changing list of X.509 TAs in
   order to successfully authenticate servers whose certificates are
   issued by a CA that is brand new or not widely trusted.

   The DNSSEC TLSA records for servers MAY include both sets of usages
   if the server needs to support a mixture of clients, some supporting
   one pair of usages and some the other.

4.1.  Opportunistic Security and PKIX Usages

   When the client's protocol design is based on "Opportunistic
   Security" (OS) [RFC7435] and the use of authentication is based on
   the presence of server TLSA records, it is especially important to
   avoid the PKIX-EE(1) and PKIX-TA(0) certificate usages.

   When authenticated TLS is used opportunistically based on the
   presence of DANE TLSA records and no secure TLSA records are present,
   unauthenticated TLS is used if possible, and if TLS is not possible,
   perhaps even cleartext.  However, if usable secure TLSA records are
   published, then authentication MUST succeed.  Also, outside the
   browser space, there is no preordained canon of trusted CAs, and in
   any case there is no security advantage in using PKIX-TA(0) or
   PKIX-EE(1) when the DANE-TA(2) and DANE-EE(3) usages are also
   supported (as an attacker who can compromise DNS can replace the
   former with the latter).

   Authentication via the PKIX-TA(0) and PKIX-EE(1) certificate usages
   is more brittle; the client and server need to happen to agree on a
   mutually trusted CA, but with OS the client is just trying to protect
   the communication channel at the request of the server and would
   otherwise be willing to use cleartext or unauthenticated TLS.  The
   use of fragile mechanisms (like public CA authentication for some
   unspecified set of trusted CAs) is not sufficiently reliable for an
   OS client to honor the server's request for authentication.  OS needs
   to be non-intrusive and to require few, if any, workarounds for valid
   but mismatched peers.

   Because the PKIX-TA(0) and PKIX-EE(1) usages offer no more security
   and are more prone to failure, they are a poor fit for OS and
   SHOULD NOT be used in that context.

4.2.  Interaction with Certificate Transparency

   Certificate Transparency (CT) [RFC6962] defines an experimental
   approach that could be used to mitigate the risk of rogue or
   compromised public CAs issuing unauthorized certificates.  This
   section clarifies the interaction of the experimental CT and DANE.
   This section may need to be revised in light of any future Standards
   Track version of CT.

   When a server is authenticated via a DANE TLSA RR with TLSA
   certificate usage DANE-EE(3), the domain owner has directly specified
   the certificate associated with the given service without reference
   to any public CA.  Therefore, when a TLS client authenticates the TLS
   server via a TLSA record with usage DANE-EE(3), CT checks SHOULD NOT
   be performed.  Publication of the server certificate or public key
   (digest) in a TLSA record in a DNSSEC-signed zone by the domain owner
   assures the TLS client that the certificate is not an unauthorized
   certificate issued by a rogue CA without the domain owner's consent.

   When a server is authenticated via a DANE TLSA record with TLSA usage
   DANE-TA(2) and the server certificate does not chain to a known
   public root CA, CT cannot apply (CT logs only accept chains that
   start with a known public root).  Since TLSA certificate usage
   DANE-TA(2) is generally intended to support non-public TAs, TLS
   clients SHOULD NOT perform CT checks with usage DANE-TA(2).

   With certificate usages PKIX-TA(0) and PKIX-EE(1), CT applies just as
   it would without DANE.  TLSA records of this type only constrain
   which CAs are acceptable in PKIX validation.  All checks used in the
   absence of DANE still apply when validating certificate chains with
   DANE PKIX-TA(0) and PKIX-EE(1) constraints.

4.3.  Switching from/to PKIX-TA/EE to/from DANE-TA/EE

   The choice of preferred certificate usages may need to change as an
   application protocol evolves.  When transitioning between PKIX-TA/
   PKIX-EE and DANE-TA/DANE-EE, clients begin to enable support for the
   new certificate usage values.  If the new preferred certificate
   usages are PKIX-TA/EE, this requires installing and managing the
   appropriate set of CA TAs.  During this time, servers will publish
   both types of TLSA records.  At some later time, when the vast
   majority of servers have published the new preferred TLSA records,
   clients can stop supporting the legacy certificate usages.
   Similarly, servers can stop publishing legacy TLSA records once the
   vast majority of clients support the new certificate usages.

5.  Certificate-Usage-Specific DANE Updates and Guidelines

   The four certificate usage values from the TLSA record -- DANE-EE(3),
   DANE-TA(2), PKIX-EE(1), and PKIX-TA(0) -- are discussed below.

5.1.  Certificate Usage DANE-EE(3)

   In this section, the meaning of DANE-EE(3) is updated from [RFC6698]
   to specify that peer identity matching and validity period
   enforcement are based solely on the TLSA RRset properties.  This
   document also extends [RFC6698] to cover the use of DANE
   authentication of raw public keys [RFC7250] via TLSA records with
   certificate usage DANE-EE(3) and selector SPKI(1).

   Authentication via certificate usage DANE-EE(3) TLSA records involves
   simply checking that the server's leaf certificate matches the TLSA
   record.  In particular, the binding of the server public key to its
   name is based entirely on the TLSA record association.  The server
   MUST be considered authenticated even if none of the names in the
   certificate match the client's reference identity for the server.
   This simplifies the operation of servers that host multiple Customer
   Domains, as a single certificate can be associated with multiple
   domains without having to match each of the corresponding reference
   identifiers.

   ; Multiple Customer Domains hosted by an example.net
   ; Service Provider:
   ;
   www.example.com.              IN CNAME ex-com.example.net.
   www.example.org.              IN CNAME ex-org.example.net.
   ;
   ; In the provider's DNS zone, a single certificate and TLSA
   ; record support multiple Customer Domains, greatly simplifying
   ; "virtual hosting".
   ;
   ex-com.example.net.           IN A 192.0.2.1
   ex-org.example.net.           IN A 192.0.2.1
   _443._tcp.ex-com.example.net. IN CNAME tlsa._dane.example.net.
   _443._tcp.ex-org.example.net. IN CNAME tlsa._dane.example.net.
   tlsa._dane.example.net.       IN TLSA 3 1 1 e3b0c44298fc1c14...

   Also, with DANE-EE(3), the expiration date of the server certificate
   MUST be ignored.  The validity period of the TLSA record key binding
   is determined by the validity period of the TLSA record DNSSEC
   signatures.  Validity is reaffirmed on an ongoing basis by continuing
   to publish the TLSA record and signing the zone in which the record
   is contained, rather than via dates "set in stone" in the
   certificate.  The expiration becomes a reminder to the administrator
   that it is likely time to rotate the key, but missing the date no
   longer causes an outage.  When keys are rotated (for whatever
   reason), it is important to follow the procedures outlined in
   Section 8.

   If a server uses just DANE-EE(3) TLSA records and all its clients are
   DANE clients, the server need not employ SNI (i.e., it may ignore the
   client's SNI message) even when the server is known via multiple
   domain names that would otherwise require separate certificates.  It
   is instead sufficient for the TLSA RRsets for all the domain names in
   question to match the server's default certificate.  For application
   protocols where the server name is obtained indirectly via SRV
   records, MX records, or similar records, it is simplest to publish a
   single hostname as the target server name for all the hosted domains.

   In organizations where it is practical to make coordinated changes in
   DNS TLSA records before server key rotation, it is generally best to
   publish end-entity DANE-EE(3) certificate associations in preference
   to other choices of certificate usage.  DANE-EE(3) TLSA records
   support multiple server names without SNI, don't suddenly stop
   working when leaf or intermediate certificates expire, and don't fail
   when a server operator neglects to include all the required issuer
   certificates in the server certificate chain.

   More specifically, it is RECOMMENDED that at most sites TLSA records
   published for DANE servers be "DANE-EE(3) SPKI(1) SHA2-256(1)"
   records.  Selector SPKI(1) is chosen because it is compatible with
   raw public keys [RFC7250] and the resulting TLSA record need not
   change across certificate renewals with the same key.  Matching type
   SHA2-256(1) is chosen because all DANE implementations are required
   to support SHA2-256.  This TLSA record type easily supports hosting
   arrangements with a single certificate matching all hosted domains.
   It is also the easiest to implement correctly in the client.

   Clients that support raw public keys can use DANE TLSA records with
   certificate usage DANE-EE(3) and selector SPKI(1) to authenticate
   servers that negotiate the use of raw public keys.  Provided the
   server adheres to the requirements of Section 8, the fact that raw
   public keys are not compatible with any other TLSA record types will
   not get in the way of successful authentication.  Clients that employ
   DANE to authenticate the peer server SHOULD NOT negotiate the use of
   raw public keys unless the server's TLSA RRset includes "DANE-EE(3)
   SPKI(1)" TLSA records.

   While it is, in principle, also possible to authenticate raw public
   keys via "DANE-EE(3) Cert(0) Full(0)" records by extracting the
   public key from the certificate in DNS, extracting just the public
   key from a "3 0 0" TLSA record requires extra logic on clients that
   not all implementations are expected to provide.  Servers that wish
   to support [RFC7250] raw public keys need to publish TLSA records
   with a certificate usage of DANE-EE(3) and a selector of SPKI(1).

   While DANE-EE(3) TLSA records are expected to be by far the most
   prevalent, as explained in Section 5.2, DANE-TA(2) records are a
   valid alternative for sites with many DANE services.  Note, however,
   that virtual hosting is more complex with DANE-TA(2).  Also, with
   DANE-TA(2), server operators MUST ensure that the server is
   configured with a sufficiently complete certificate chain and need to
   remember to replace certificates prior to their expiration dates.

5.2.  Certificate Usage DANE-TA(2)

   This section updates [RFC6698] by specifying a new operational
   requirement for servers publishing TLSA records with a usage of
   DANE-TA(2): such servers MUST include the TA certificate in their TLS
   server certificate message unless all such TLSA records are "2 0 0"
   records that publish the server certificate in full.

   Some domains may prefer to avoid the operational complexity of
   publishing unique TLSA RRs for each TLS service.  If the domain
   employs a common issuing CA to create certificates for multiple TLS
   services, it may be simpler to publish the issuing authority as a TA
   for the certificate chains of all relevant services.  The TLSA query
   domain (TLSA base domain with port and protocol prefix labels) for
   each service issued by the same TA may then be set to a CNAME alias
   that points to a common TLSA RRset that matches the TA.  For example:

   ; Two servers, each with its own certificate, that share
   ; a common issuer (TA).
   ;
   www1.example.com.            IN A 192.0.2.1
   www2.example.com.            IN A 192.0.2.2
   _443._tcp.www1.example.com.  IN CNAME tlsa._dane.example.com.
   _443._tcp.www2.example.com.  IN CNAME tlsa._dane.example.com.
   tlsa._dane.example.com.      IN TLSA 2 0 1 e3b0c44298fc1c14...

   The above configuration simplifies server key rotation, because while
   the servers continue to receive new certificates from a CA matched by
   the shared (target of the CNAMEs) TLSA record, server certificates
   can be updated without making any DNS changes.  As the list of active
   issuing CAs changes, the shared TLSA record will be updated (much
   less frequently) by the administrators who manage the CAs.  Those
   administrators still need to perform TLSA record updates with care,
   as described in Section 8.

   With usage DANE-TA(2), the server certificates will need to have
   names that match one of the client's reference identifiers (see
   [RFC6125]).  When hosting multiple unrelated Customer Domains (that
   can't all appear in a single certificate), such a server SHOULD
   employ SNI to select the appropriate certificate to present to the
   client.

5.2.1.  Recommended Record Combinations

   TLSA records with a matching type of Full(0) are NOT RECOMMENDED.
   While these potentially obviate the need to transmit the TA
   certificate in the TLS server certificate message, client
   implementations may not be able to augment the server certificate
   chain with the data obtained from DNS, especially when the TLSA
   record supplies a bare key (selector SPKI(1)).  Since the server will
   need to transmit the TA certificate in any case, server operators
   SHOULD publish TLSA records with a matching type other than Full(0)
   and avoid potential DNS interoperability issues with large TLSA
   records containing full certificates or keys (see Section 10.1.1).

   TLSA Publishers employing DANE-TA(2) records SHOULD publish records
   with a selector of Cert(0).  Such TLSA records are associated with
   the whole TA certificate, not just with the TA public key.  In
   particular, when authenticating the peer certificate chain via such a
   TLSA record, the client SHOULD apply any relevant constraints from
   the TA certificate, such as, for example, path length constraints.

   While a selector of SPKI(1) may also be employed, the resulting TLSA
   record will not specify the full TA certificate content, and elements
   of the TA certificate other than the public key become mutable.  This
   may, for example, enable a subsidiary CA to issue a chain that
   violates the TA's path length or name constraints.

5.2.2.  Trust Anchor Digests and Server Certificate Chain

   With DANE-TA(2), a complication arises when the TA certificate is
   omitted from the server's certificate chain, perhaps on the basis of
   Section 7.4.2 of [RFC5246]:

      The sender's certificate MUST come first in the list.  Each
      following certificate MUST directly certify the one preceding it.
      Because certificate validation requires that root keys be
      distributed independently, the self-signed certificate that
      specifies the root certificate authority MAY be omitted from the
      chain, under the assumption that the remote end must already
      possess it in order to validate it in any case.

   With TLSA certificate usage DANE-TA(2), there is no expectation that
   the client is preconfigured with the TA certificate.  In fact, client
   implementations are free to ignore all locally configured TAs when
   processing usage DANE-TA(2) TLSA records and may rely exclusively on
   the certificates provided in the server's certificate chain.  But,
   with a digest in the TLSA record, the TLSA record contains neither
   the full TA certificate nor the full public key.  If the TLS server's
   certificate chain does not contain the TA certificate, DANE clients
   will be unable to authenticate the server.

   TLSA Publishers that publish TLSA certificate usage DANE-TA(2)
   associations with a selector of SPKI(1) or with a digest-based
   matching type (not Full(0)) MUST ensure that the corresponding server
   is configured to also include the TA certificate in its TLS handshake
   certificate chain, even if that certificate is a self-signed root CA
   and would have been optional in the context of the existing public
   CA PKI.

   Only when the server TLSA record includes a "DANE-TA(2) Cert(0)
   Full(0)" TLSA record containing a full TA certificate is the TA
   certificate optional in the server's TLS certificate message.  This
   is also the only type of DANE-TA(2) record for which the client MUST
   be able to verify the server's certificate chain even if the TA
   certificate appears only in DNS and is absent from the TLS handshake
   server certificate message.

5.2.3.  Trust Anchor Public Keys

   TLSA records with TLSA certificate usage DANE-TA(2), selector
   SPKI(1), and a matching type of Full(0) publish the full public key
   of a TA via DNS.  In Section 6.1.1 of [RFC5280], the definition of a
   TA consists of the following four parts:

   1.  the trusted issuer name,

   2.  the trusted public key algorithm,

   3.  the trusted public key, and

   4.  optionally, the trusted public key parameters associated with the
       public key.

   Items 2-4 are precisely the contents of the subjectPublicKeyInfo
   published in the TLSA record.  The issuer name is not included in the
   subjectPublicKeyInfo.

   With TLSA certificate usage DANE-TA(2), the client may not have the
   associated TA certificate and cannot generally verify whether or not
   a particular certificate chain is "issued by" the TA described in the
   TLSA record.

   When the server certificate chain includes a CA certificate whose
   public key matches the TLSA record, the client can match that CA as
   the intended issuer.  Otherwise, the client can only check that the
   topmost certificate in the server's chain is "signed by" the TA's
   public key in the TLSA record.  Such a check may be difficult to
   implement and cannot be expected to be supported by all clients.

   Thus, servers cannot rely on "DANE-TA(2) SPKI(1) Full(0)" TLSA
   records to be sufficient to authenticate chains issued by the
   associated public key in the absence of a corresponding certificate
   in the server's TLS certificate message.  Servers employing "2 1 0"
   TLSA records MUST include the corresponding TA certificate in their
   certificate chain.

   If none of the server's certificate chain elements match a public key
   specified in a TLSA record, and at least one "DANE-TA(2) SPKI(1)
   Full(0)" TLSA record is available, it is RECOMMENDED that clients
   check to see whether or not the topmost certificate in the chain is
   signed by the provided public key and has not expired, and in that
   case consider the server authenticated, provided the rest of the
   chain passes validation, including leaf certificate name checks.

5.3.  Certificate Usage PKIX-EE(1)

   This certificate usage is similar to DANE-EE(3); but, in addition,
   PKIX verification is required.  Therefore, name checks, certificate
   expiration, CT, etc. apply as they would without DANE.

5.4.  Certificate Usage PKIX-TA(0)

   This section updates [RFC6698] by specifying new client
   implementation requirements.  Clients that trust intermediate
   certificates MUST be prepared to construct longer PKIX chains than
   would be required for PKIX alone.

   TLSA certificate usage PKIX-TA(0) allows a domain to publish
   constraints on the set of PKIX CAs trusted to issue certificates for
   its TLS servers.  A PKIX-TA(0) TLSA record matches PKIX-verified
   trust chains that contain an issuer certificate (root or
   intermediate) that matches its Certificate Association Data field
   (typically a certificate or digest).

   PKIX-TA(0) requires more complex coordination (than with DANE-TA(2)
   or DANE-EE(3)) between the Customer Domain and the Service Provider
   in hosting arrangements.  Thus, this certificate usage is
   NOT RECOMMENDED when the Service Provider is not also the TLSA
   Publisher (at the TLSA base domain obtained via CNAMEs, SRV records,
   or MX records).

   TLSA Publishers who publish TLSA records for a particular public root
   CA will expect that clients will only accept chains anchored at that
   root.  It is possible, however, that the client's trusted certificate
   store includes some intermediate CAs, either with or without the
   corresponding root CA.  When a client constructs a trust chain
   leading from a trusted intermediate CA to the server leaf
   certificate, such a "truncated" chain might not contain the trusted
   root published in the server's TLSA record.

   If the omitted root is also trusted, the client may erroneously
   reject the server chain if it fails to determine that the shorter
   chain it constructed extends to a longer trusted chain that matches
   the TLSA record.  Thus, when matching a usage PKIX-TA(0) TLSA record,
   so long as no matching certificate has yet been found, a client MUST
   continue extending the chain even after any locally trusted
   certificate is found.  If no TLSA records have matched any of the
   elements of the chain and the trusted certificate found is not
   self-issued, the client MUST attempt to build a longer chain in case
   a certificate closer to the root matches the server's TLSA record.

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
      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
   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.

   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.

   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...

   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.

   ; 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...

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.

   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.

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.

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).

   [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.

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".

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.

   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>.

   [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>.

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>.

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: ietf-dane@dukhovni.org


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

   Email: ietf@hardakers.net