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Versions: (draft-dukhovni-dane-ops) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 7671

DANE                                                         V. Dukhovni
Internet-Draft                                              Unaffiliated
Updates: 6698 (if approved)                                  W. Hardaker
Intended status: Standards Track                                 Parsons
Expires: January 3, 2016                                    July 2, 2015


       Updates to and Operational Guidance for the DANE Protocol
                         draft-ietf-dane-ops-13

Abstract

   This document clarifies and updates the DNS-Based Authentication of
   Named Entities (DANE) TLSA specification (RFC6698) based on
   subsequent implementation experience.  It also contains guidance for
   implementers, operators and protocol developers who want to make use
   of DANE records.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 3, 2016.

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




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  DANE TLSA Record Overview . . . . . . . . . . . . . . . . . .   5
     2.1.  Example TLSA record . . . . . . . . . . . . . . . . . . .   5
   3.  DANE TLS Requirements . . . . . . . . . . . . . . . . . . . .   6
   4.  DANE Certificate Usage Selection Guidelines . . . . . . . . .   6
     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 . . . . .   8
   5.  Certificate-Usage-Specific DANE Updates and Guidelines  . . .   8
     5.1.  Certificate Usage DANE-EE(3)  . . . . . . . . . . . . . .   9
     5.2.  Certificate Usage DANE-TA(2)  . . . . . . . . . . . . . .  10
     5.3.  Certificate Usage PKIX-EE(1)  . . . . . . . . . . . . . .  13
     5.4.  Certificate Usage PKIX-TA(0)  . . . . . . . . . . . . . .  13
   6.  Service Provider and TLSA Publisher Synchronization . . . . .  14
   7.  TLSA Base Domain and CNAMEs . . . . . . . . . . . . . . . . .  16
   8.  TLSA Publisher Requirements . . . . . . . . . . . . . . . . .  17
     8.1.  Key rollover with fixed TLSA Parameters . . . . . . . . .  18
     8.2.  Switching to DANE-TA from DANE-EE . . . . . . . . . . . .  19
     8.3.  Switching to New TLSA Parameters  . . . . . . . . . . . .  20
     8.4.  TLSA Publisher Requirements Summary . . . . . . . . . . .  20
   9.  Digest Algorithm Agility  . . . . . . . . . . . . . . . . . .  21
   10. General DANE Guidelines . . . . . . . . . . . . . . . . . . .  22
     10.1.  DANE DNS Record Size Guidelines  . . . . . . . . . . . .  22
     10.2.  Certificate Name Check Conventions . . . . . . . . . . .  23
     10.3.  Design Considerations for Protocols Using DANE . . . . .  24
   11. Note on DNSSEC Security . . . . . . . . . . . . . . . . . . .  25
   12. Summary of Updates to RFC6698 . . . . . . . . . . . . . . . .  26
   13. Operational Considerations  . . . . . . . . . . . . . . . . .  26
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  27
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  28
     17.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   The Transport Layer Security (TLS) [RFC5246] and Datagram Transport
   Layer Security (DTLS) [RFC6347] 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.



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   By convention, "TLS" will be used throughout this document and,
   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 provides integrity protection and
   authentication, which protects the transport against man-in-the-
   middle (MITM) attacks.

   The DNS-Based Authentication of Named Entities (DANE) specification
   ([RFC6698]) introduces the DNS "TLSA" resource record 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 TLS transport endpoint.  DANE relies on the DNS
   Security Extensions (DNSSEC, [RFC4033]).  DNSSEC validated DANE TLSA
   records can be used to augment or replace the use of trusted public
   Certification Authorities (CAs).

   [RFC6698] defines three TLSA record fields with respectively 4, 2 and
   3 currently specified values.  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
   [I-D.ietf-dane-srv] and [I-D.ietf-dane-smtp-with-dane].

   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 Certification Authorities (CAs) to vouch for the
      authenticity of public keys associated with a particular party
      (the subject).




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   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 resource record.
      Example redirection records include MX, SRV, and CNAME.  The
      Service Provider frequently provides services for many customers
      and needs to carefully manage any TLS credentials offered to
      connecting applications to ensure name matching is handled easily
      by the applications.

   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, unless otherwise specified.  If the
      Customer Domain is not outsourcing their DNS service, the TLSA
      Publisher will be the customer themselves.  Otherwise, the TLSA
      Publisher may be the operator of the outsourced DNS service.

   public key:  The term "public key" is short-hand 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-
      addressed 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 which 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



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      "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].
   The DANE TLSA specification defines multiple TLSA RR types via
   combinations of numeric values of the first three fields of the TLSA
   record (i.e. the "TLSA parameters").  The numeric values of these
   parameters were later given symbolic names in [RFC7218].  These
   parameters are:

   The Certificate Usage field:  Section 2.1.1 of [RFC6698] specifies 4
      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 2 values:
      Cert(0), 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 3
      values: Full(0), SHA2-256(1), SHA2-512(2).  There is an additional
      private-use value: PrivMatch(255).  All other values are reserved
      for use by future specifications.

   In the matching type field, of the two digest algorithms, for now
   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:





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

   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.

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 "Server Name Indication"
   (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 servers does
   support SNI, but is not configured with a certificate chain that
   exactly matches the client's SNI extension, SHOULD respond with some
   other (default or closest match) certificate chain, since clients may
   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 pre-
   configured CA trust anchors that are mutually trusted by the
   operators of the TLS server and client.  With DANE-TA(2) and DANE-
   EE(3), no pre-configured CA trust anchors are required and the
   published DANE TLSA records are sufficient to verify the peer's
   certificate chain.

   Protocol designers need to carefully consider which set of DANE
   certificate usages to support.  Simultaneous support for all four
   usages is NOT RECOMMENDED for DANE clients.  Protocol designers are
   encouraged to specify use of either the PKIX-TA(0) and PKIX-EE(1)
   certificate usages, or the use of the DANE-TA(2) and DANE-EE(3)
   usages.  When all four usages are supported, an attacker capable of
   compromising the integrity of DNSSEC needs only to replace server's
   TLSA RRset with one that lists suitable DANE-EE(3) or DANE-TA(2)
   records, effectively bypassing an added verification via public CAs.
   In other words, when all four usages are supported, PKIX-TA(2) and




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   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 generally RECOMMENDED.  With DANE-TA(2) and
   DANE-EE(3) clients don't need to track a large changing list of X.509
   trust-anchors in order to successfully authenticate servers whose
   certificates are issued by a brand new or not widely trusted CA.

   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 usable secure TLSA records are
   present, unauthenticated TLS is used if possible, and otherwise
   perhaps even cleartext.  If however, usable secure TLSA records are
   published, authentication must succeed.  Also, outside the browser
   space, there is no pre-ordained 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 opportunistic security 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.  Use of fragile mechanisms (like public CA
   authentication for some unspecified set of trusted CAs) is not
   sufficiently reliable for an opportunistic security client to honor
   the server's request for authentication.  Opportunistic security
   needs to be unintrusive and to require few, if any, work-arounds for
   valid and yet mismatched peers.

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





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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 certification authority.  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 trust anchors, TLS
   clients SHOULD NOT perform CT checks with usage DANE-TA(2).

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

   The choice of preferred certificate usages may need to change as a
   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 trust anchors.  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.






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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 that validity period
   enforcement is 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.

   Similarly, 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.

   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., they 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, MX 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.

   TLSA records published for DANE servers should, as a best practice,
   be "DANE-EE(3) SPKI(1) SHA2-256(1)" records.  Since all DANE
   implementations are required to support SHA2-256, this record type
   works for all clients and need not change across certificate renewals
   with the same key.  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.



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   Another advantage of "DANE-EE(3) SPKI(1)" (with any suitable matching
   type) TLSA records is that they are compatible with the raw public
   key TLS extension specified in [RFC7250].  DANE clients that support
   this extension can use the TLSA record to authenticate servers that
   negotiate the use of raw public keys in place of X.509 certificate
   chains.  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 compatible 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 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).

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 trust-anchor 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 Certification Authority to create
   certificates for multiple TLS services, it may be simpler to publish
   the issuing authority as a trust anchor (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:

   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 tlsa201._dane.example.com.
   _443._tcp.www2.example.com.  IN CNAME tlsa201._dane.example.com.
   tlsa201._dane.example.com.   IN TLSA 2 0 1 e3b0c44298fc1c14...

   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 client domains (that can't all appear



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   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 matching type 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 trust anchor certificate, not just with the trust anchor
   public key.  In particular, when authenticating the peer certificate
   chain via such a TLSA record, the client SHOULD apply any relevant
   constraints from the trust anchor 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 trust anchor certificate content,
   and elements of the trust anchor certificate other than the public
   key become mutable.  This may, for example, enable a subsidiary CA to
   issue a chain that violates the trust anchor'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 certification 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 pre-configured with the trust anchor certificate.  In
   fact, client implementations are free to ignore all locally



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   configured trust anchors 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 trust anchor certificate
   nor the full public key.  If the TLS server's certificate chain does
   not contain the trust anchor 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 using a digest-based
   matching type (not Full(0)) MUST ensure that the corresponding server
   is configured to also include the trust anchor 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 trust-anchor certificate, is
   the trust-anchor certificate optional in the server's TLS certificate
   message.  Only in this case, the client MUST be able to verify the
   server's certificate chain via a trust-anchor provided via DNS or via
   the TLS handshake.

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 trust
   anchor via DNS.  In section 6.1.1 of [RFC5280] the definition of a
   trust anchor 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 trust anchor certificate, and cannot generally verify
   whether a particular certificate chain is "issued by" the trust
   anchor described in the TLSA record.




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   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 trust
   anchor'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 trust-anchor 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 whether 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, 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 certification authorities trusted to
   issue certificates for its TLS servers.  This TLSA record matches
   PKIX-verified trust chains which contain an issuer certificate (root
   or intermediate) that matches its association data field (typically a
   certificate or digest).

   PKIX-TA(0) also requires more complex coordination 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.




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   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,
   while no matching certificate is 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 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



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      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 certification authority, 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 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.
   ;
   ; A single certificate at the provider works for all Customer
   ; Domains due to the use of the DANE-EE(3) Certificate Usage.
   ;
   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 use the DANE-TA(2) Certificate Usage type to
   ; indicate a trust anchor.
   ;
   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 end-point, rather than the



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   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 or 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
   organization boundaries.

   For further information about combining DANE and SRV, please see
   [I-D.ietf-dane-srv].

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, since 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



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   hop-by-hop to its ultimate target host (noting at each step whether
   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, use the
   CNAME expanded domain also in SNI and certificate name checks).

8.  TLSA Publisher Requirements

   This section updates [RFC6698] by specifying a requirement on the
   TLSA Publisher to ensure that each combination of Certificate Usage,
   selector and matching type in the server's TLSA RRset MUST include 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 may 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



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   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 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 TTLs or longer before the 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 no longer active chain:

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

   ; Transitional TLSA RRset published at least 2*TTL seconds
   ; 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:




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   ; 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 from DANE-EE

   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 trust-
   anchor.

   The original "3 1 1" TLSA record supports [RFC7250] raw public keys,
   and clients may choose to negotiate their use.  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 2*TTL 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 agility 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 SHA2-256 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 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:

   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.



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   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 no longer deployed certificate
      chains.

   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 which 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 DANE matching type Full(0), as these do not employ a digest
   algorithm that might some day 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



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   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 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 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.








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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 TC bit 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 without
   the overhead of a containing X.509 certificate) are generally more
   compact, these are also best avoided as when significantly larger
   than their digests.  Rather, servers SHOULD publish digest-based TLSA
   Matching Types in their TLSA records.  Instead, the complete
   corresponding certificate SHOULD 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 the use of a digest-based matching type, such as
   SHA2-256(1) SHOULD be used.

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
   subjectAltName 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



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   MUST send the SNI extension with a HostName value of the base domain
   of the TLSA RRset.

   Except with 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.  The 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 host name are acceptable names to be found in the
   server certificate (see [I-D.ietf-dane-smtp-with-dane]).

   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 be signed by a trust anchor with the above
   certificate SHA2-256 digest.  Finally, one of the DNS names in the
   server certificate is required to be be either "mail.example.com" or
   "example.com" (this additional name is a concession to compatibility
   with prior practice, see [I-D.ietf-dane-smtp-with-dane] for details).

   [RFC6125] specifies the 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



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   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
   associated data, DANE clients cannot authenticate the service
   certificate chain.  When authenticated TLS is mandatory, the client
   SHOULD NOT connect to the associated server.

   If, on the other hand, the use of TLS is "opportunistic", then the
   client SHOULD generally use the server via an unauthenticated TLS
   connection, but if TLS encryption cannot be established, the client
   MUST NOT use the server.  Standards for DANE specific to the
   particular application protocol may modify the above requirements, as
   appropriate.

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 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 SHOULD use a registrar lock of 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 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 SHOULD ensure their
   registrar publishes a suitable domain transfer policy.

   DNSSEC signed RRsets cannot be securely revoked before they expire.
   Operators need to plan accordingly and not generate signatures with
   excessively long duration periods.  For domains publishing high-value
   keys, a signature lifetime of a few days is reasonable, and the zone
   can be re-signed daily.  For domains with less critical data, a



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   reasonable signature lifetime is a couple of weeks to a month, and
   the zone can be re-signed weekly.  Monitoring of the signature
   lifetime is important.  If the zone is not re-signed in a timely
   manner, one risks a major outage and the entire domain will become
   bogus.

12.  Summary of Updates to RFC6698

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

   o  Section 5.1 updates [RFC6698] to specify peer identity matching
      and certificate validity interval based solely on the basis of the
      TLSA RRset.  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
      trust-anchor certificate in their TLS server certificate message.
      This extends to the case of "2 1 0" TLSA records which publish a
      full public key.

   o  Section 5.3 and Section 5.4 explain that PKIX-EE(1) and PKIX-TA(0)
      are generally NOT RECOMMENDED.  This document notes that with
      usage PKIX-TA(0) clients may need to processes 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 time-to-live (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,



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   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 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.  See
   Section 4.4.1 of [RFC6781].

14.  Security Considerations

   Application protocols that cannot make use of 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 plaintext connection.  When TLS is not
   opportunistic, the client MUST NOT connect to the server.

   Thus, when TLSA records are used with opportunistic protocols where
   the 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 SHOULD implement these certificate
   usages as described in [RFC6698].

15.  IANA Considerations

   This specification requires no support from IANA.

16.  Acknowledgements

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

   Acknowledgments 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 drafts.  Thanks also to Samuel Dukhovni for
   editorial assistance.



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17.  References

17.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [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, May 2008.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [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, March 2011.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

   [RFC7218]  Gudmundsson, O., "Adding Acronyms to Simplify
              Conversations about DNS-Based Authentication of Named
              Entities (DANE)", RFC 7218, April 2014.





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   [RFC7250]  Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
              T. Kivinen, "Using Raw Public Keys in Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7250, June 2014.

17.2.  Informative References

   [I-D.ietf-dane-smtp-with-dane]
              Dukhovni, V. and W. Hardaker, "SMTP security via
              opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-16
              (work in progress), April 2015.

   [I-D.ietf-dane-srv]
              Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
              Based Authentication of Named Entities (DANE) TLSA Records
              with SRV Records", draft-ietf-dane-srv-14 (work in
              progress), April 2015.

   [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
              Operational Practices, Version 2", RFC 6781, December
              2012.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, June 2013.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, December 2014.

Authors' Addresses

   Viktor Dukhovni
   Unaffiliated

   Email: ietf-dane@dukhovni.org


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

   Email: ietf@hardakers.net








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