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Versions: 00 01

DANE                                                         V. Dukhovni
Internet-Draft                                              Unaffiliated
Intended status: Best Current Practice                       W. Hardaker
Expires: January 16, 2014                                        Parsons
                                                           July 15, 2013


           DANE TLSA implementation and operational guidance
                       draft-dukhovni-dane-ops-01

Abstract

   This memo provides operational guidance to server operators to help
   ensure that clients will be able to authenticate a server's
   certificate chain via published TLSA records.  Guidance is also
   provided to clients for selecting reliable TLSA record parameters to
   use for server authentication.  Finally, guidance is given to
   protocol designers who wish to make use of TLSA records to secure
   protocols using a TLS and TLSA combination.

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 16, 2014.

Copyright Notice

   Copyright (c) 2013 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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  DANE TLSA record overview . . . . . . . . . . . . . . . . . .   4
     2.1.  Example TLSA record . . . . . . . . . . . . . . . . . . .   5
   3.  General DANE Guidelines . . . . . . . . . . . . . . . . . . .   6
     3.1.  TLS Requirements  . . . . . . . . . . . . . . . . . . . .   6
     3.2.  DANE DNS Record Size Guidelines . . . . . . . . . . . . .   6
     3.3.  Certificate Name Check Conventions  . . . . . . . . . . .   7
     3.4.  Service Provider and TLSA Publisher Synchronization . . .   7
     3.5.  TLSA Base Domain and CNAMEs . . . . . . . . . . . . . . .   8
     3.6.  TLSA Base Name Priorities . . . . . . . . . . . . . . . .   9
     3.7.  Interaction with Certificate Transparency . . . . . . . .   9
     3.8.  Design Considerations for Protocols Using DANE  . . . . .  10
     3.9.  TLSA Records and Trust Anchor Digests . . . . . . . . . .  12
     3.10. Trust anchor public keys  . . . . . . . . . . . . . . . .  13
   4.  Type Specific DANE Guidelines . . . . . . . . . . . . . . . .  14
     4.1.  Type 3 Guidelines . . . . . . . . . . . . . . . . . . . .  14
     4.2.  Type 2 Guidelines . . . . . . . . . . . . . . . . . . . .  14
     4.3.  Type 1 Guidelines . . . . . . . . . . . . . . . . . . . .  14
     4.4.  Type 0 Guidelines . . . . . . . . . . . . . . . . . . . .  14
   5.  Note on DNSSEC security . . . . . . . . . . . . . . . . . . .  15
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The Domain Name System Security Extensions (DNSSEC) add data origin
   authentication and data integrity to the Domain Name System.  DNSSEC
   is defined in [RFC4033], [RFC4034] and [RFC4035].

   In the context of this memo, channel security is assumed to be
   provided by TLS or DTLS.  The Transport Layer Security (TLS) and
   Datagram Transport Layer Security (DTLS) protocols provide secured
   TCP and UDP communication over the Internet Protocol.  By convention,
   "TLS" will be used through this document and, unless otherwise
   specified, the text applies equally as well to the DTLS protocol.
   Used without authentication, TLS provides protection only against
   eavesdropping.  With authentication, TLS also provides protection



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   against man-in-the-middle (MITM) attacks.  Since the publication of
   the TLS 1.0 specification in [RFC2246], two updates to the protocol
   have been published: TLS 1.1 [RFC4346] and TLS 1.2 [RFC5246].  The
   DTLS protocol was later documented in [RFC6347].

   As described in the introduction of [RFC6698], TLS authentication via
   the existing public Certificate Authority (CA) Public Key
   Infrastructure (PKI) suffers from an over-abundance of trusted
   certificate authorities capable of issuing certificates for any
   domain of their choice.  DNS-Based Authentication of Named Entities
   (DANE) leverages the DNSSEC infrastructure to publish trusted keys
   and certificates for use with TLS via a new TLSA record type.  DNSSEC
   validated DANE TLSA records have created a new PKI designed to
   augment or replace the trust model of the existing public CA PKI.

   When a TLS client goes to the trouble of authenticating a certificate
   presented by a TLS server, it should not continue to use the server
   in case of authentication failure or else authentication serves no
   purpose.  Consequently, if a client cannot reliably authenticate
   correctly configured, legitimate servers via a particular combination
   of TLSA parameters, then the client should treat that combination of
   parameters as unusable.  Otherwise, the client risks routinely
   dropping connections to legitimate servers.  Servers publishing TLSA
   records MUST be configured to allow correctly configured clients to
   successfully authenticate the server's TLS certificate.

   If a TLSA record is found as unusable because of a parameter
   combination, it is protocol specific as to whether the connection
   should be established anyway without security, with only TLS
   encryption and not authentication, or to refuse to connect entirely.

1.1.  Terminology

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

   This memo is being discussed on the dane@ietf.org mailing list.

   The following terms are used throughout this document:

   Service Provider:  A company or organization that offers to host a
      service on behalf of a Client Domain.  The original domain name
      associated with the service is typically still within the control
      of the client, however, and the service provider is frequently
      referred to by a redirection resource record.  Example redirection
      records include MX, SRV, and CNAME.  Many times the Service
      Provider provides services for many customers and must carefully



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      manage TLS credentials offered to their clients to ensure name
      matching is handled easily by clients.

   Client Domain:  Clients that make use of a Service Provider to
      outsource their services will be referred to as "Client Domains".

   TLSA Publisher:  The entity responsible for publishing a TLSA record
      within a DNS zone.  This zone will be considered DNSSEC signed,
      unless otherwise specified.  If the Client Domain is not
      outsourcing their DNS service, the TLSA Publisher will be the
      client themselves.  Otherwise the TLSA Publisher may be the
      outsourced DNS service instead.

   public key:  The term "public key" will be an informal short-hand for
      the subjectPublicKeyInfo from a PKIX certificate.

   SNI:  The "Server Name Indication", or SNI, describes the process by
      which a TLS client requests to connect to a particular service
      name for a TLS server ([RFC3546]).  Without this TLS extension, a
      TLS server has no choice but to offer a PKIX certificate with a
      default server name.  Service Providers that are expected to host
      services for many clients need to present the correct certificate
      for the correct client, and the SNI extension provides a hint to
      the server which certificate should be transmitted to the client.

2.  DANE TLSA record overview

   [RFC6698] specifies a protocol for publishing TLS server certificate
   associations via DNSSEC.  The DANE TLSA specification defines
   multiple TLSA RR types via combinations of the following 3
   parameters:

   o  The TLSA Certificate Usage field.  Section 2.1.1 of [RFC6698]
      specifies 4 values ranging from 0 to 3.

   o  The selector field.  Section 2.1.2 of [RFC6698] specifies 2 values
      ranging from 0 to 1.

   o  The matching type field.  Section 2.1.3 of [RFC6698] specifies 3
      values ranging from 0 to 2.

   We may consider the TLSA Certificate Usage values 0 through 3 to be a
   combination of two one-bit flags.  The low-bit chooses between
   referencing trust-anchor (TA) and end-entity (EE) certificates.  The
   high bit chooses between public PKI issued and domain-issued
   certificates:





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   o  When the low bit is set (TLSA Certificate Usages 1 and 3) the TLSA
      record matches an EE (server) certificate.

   o  When the low bit is not set (TLSA Certificate Usages 0 and 2) the
      TLSA record matches a trust-anchor (a certificate authority) that
      issued a certificate somewhere in the certificate chain that
      authenticates the final end-entity certificate.

   o  When the high bit is set (TLSA Certificate Usages 2 and 3) the
      server certificate chain is domain-issued and may be verified
      without reference to the existing public certificate authority
      PKI.  Trust is entirely placed on the content of the TLSA records
      obtained via DNSSEC.

   o  When the high bit is not set (TLSA Certificate Usages 0 and 1) the
      TLSA record publishes a server policy stating that its certificate
      chain must pass PKIX validation [RFC5280], with the DANE TLSA
      record used to constrain the server certificate chain to contain
      the referenced CA or EE certificate.

   The selector field specifies whether the TLSA RR matches the whole
   certificate or just its subjectPublicKeyInfo (i.e. an ASN.1 DER
   encoding of the certificate's algorithm id, any parameters and the
   public key data).  A selector field of "0" specifies the whole
   certificate.  A selector field of "1" specifies just the public key.

   The matching type field specifies how the TLSA RR Certificate
   Association Data field is to be compared with the certificate or
   public key.  A value of "0" means exact match, the DER encoding of
   the certificate or public key is given in the TLSA RR.  A "1" value
   means a SHA-256 digest and "2" means a SHA-512 digest.  Of these,
   only SHA-256 is mandatory to implement.  Clients SHOULD implement
   SHA-512, but servers SHOULD NOT exclusively publish SHA-512 digests.
   Unless a "second preimage" attack is found against SHA-256, servers
   should only publish SHA-256 digests.

2.1.  Example TLSA record

   In the example TLSA record below:

   _25._tcp.mail.example.com. IN TLSA 3 0 1 (
                                 E8B54E0B4BAA815B06D3462D65FBC7C0
                                 CF556ECCF9F5303EBFBB77D022F834C0 )

   The TLSA Certificate Usage is "3", the selector is "0" and the
   matching type is "1".  The rest of the record is the certificate
   association data field, which is in this case the SHA-256 digest of
   the server certificate.



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3.  General DANE Guidelines

   These guidelines provide guidance for using or designing protocols
   for DANE, regardless of what type the TLSA record will actually
   contain.

3.1.  TLS Requirements

   TLS clients that support DANE/TLSA MUST support at least TLS 1.0 and
   SHOULD support TLS 1.2.  TLS clients and servers using DANE SHOULD
   support the "Server Name Indication" extension of TLS.

3.2.  DANE DNS Record Size Guidelines

   Selecting a combination of TLSA parameters to use requires careful
   thought.  One important consideration is the size of the resulting
   TLSA record based on the parameters chosen.

3.2.1.  UDP and TCP Considerations

   Deployments SHOULD avoid TLSA record sizes that cause UDP
   fragmentation.

   Although DNS over TCP would provide the ability to transfer larger
   DNS records between clients and servers, it is not yet widely
   deployed or permitted through many firewalls.  TCP must be expected
   to be deployed on all the DNS servers and DNS clients for it to be a
   truly viable large-record solution.

3.2.2.  Packet Size Considerations for TLSA Parameters

   Server operators SHOULD NOT publish "TLSA * 0 0" records, as even a
   single certificate is generally too large to be reliably delivered
   via DNS without TCP being widely available.  Furthermore, two full
   certificates may need to be published in the TLSA RRset for
   certificate rollover.

   While "TLSA * 1 0" records, which publish full public keys without
   the full X.509 wrapping, are generally more compact, these too should
   be used with caution.  Servers SHOULD publish digests within TLSA
   records instead.  The complete certificate should, instead, be
   transmitted to the client in band during the TLS handshake.









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3.3.  Certificate Name Check Conventions

   Certificates presented by a TLS server will contain either a Common
   Name (CN) or subjectAltName (or both), according to [RFC5280].  The
   server's hostname should be published within these fields, ideally
   within the subjectAltName.  This section discusses what must be done
   to match an expected name against the name found within a
   certificate, if required.

   The TLSA Publisher for TLSA records for a given service MUST ensure
   that at least one of these TLSA records will match the server's
   certificate chain.  If SNI is not employed for a TLS connection, the
   TLSA record must match the server's default certificate.  If the SNI
   extension is sent by the client with a host_name (see [RFC3546]
   Section 3.1) equal to the base domain of the TLSA RRset, at least one
   TLSA record must match the certificate presented by the server for
   that host_name.

   When, for example, the TLSA RRset is published at

        _25._tcp.mail.example.com

   the TLSA base domain is mail.example.com.  At least one of the TLSA
   records in the _25._tcp.mail.example.com RRset MUST match the server
   certificate chain, provided the client TLS hanshake included the SNI
   extension with a host_name of "mail.example.com".

   Note: Except with TLSA Certificate Usage "3", where name checks are
   not applicable (see Section 4.1), DANE aware clients SHOULD use the
   base domain of the TLSA RRset to verify that the client has reached
   the correct server by checking that the TLSA base domain is matched
   by one of the subjectAltName ([RFC5280]) in the server certificate.
   The commonName from the certificate subject DN MAY be used only when
   no subjectAltNames of type 'dns' are present.  Additional acceptable
   names may be specified by protocol specific DANE RFCs.  For example,
   with SMTP both the destination domain name and the MX host name are
   acceptable in the server certificate.

   Since the server's ability to respond with the right certificate
   chain requires the TLS client to provide the correct SNI information,
   DANE PKI aware clients SHOULD send the SNI extension with a host_name
   value of the base domain of the TLSA RRset (otherwise they risk
   failure to authenticate the server).

3.4.  Service Provider and TLSA Publisher Synchronization

   Complications arise when the TLSA Publisher is not the same entity as
   the Service Provider.  In this situation, the TLSA Publisher and the



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   Service Provider must cooperate to ensure that TLSA records don't
   fall out of sync with the server certificate configuration.

   Ideally, the TLSA Publisher and the Service Provider should be the
   same entity.  If a TLSA record must be published in the client's base
   domain, CNAME records can be easily used to point at the real TLSA
   record in the Service Provider's zone assuming certificate usage 3
   TLSA records are published by the Service Provider (see Section 3.5).
   Having the master TLSA record in the Service Provider's zone avoids
   the complexity of bilateral coordination of server certificate
   configuration and TLSA record management.

   For example, with SMTP, the customer's MX records can be pointed at
   the Service Provider's MX hosts.  When the customer's DNS zone is
   signed, the MX records can be securely used as the base names for
   TLSA records managed by the Service Provider.

3.5.  TLSA Base Domain and CNAMEs

   When the 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 origin domain to the target
   domain for all protocols.  Also Unlike MX or SRV records, CNAME
   records may chain (though clients will generally impose
   implementation dependent maximum nesting depths).

   When CNAMEs are employed, the best place to seek DANE TLSA records is
   in the Service Provider's domain, as discussed in Section 3.4.
   Therefore, DANE PKI 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 the CNAME is DNSSEC
   validated) and use the resulting target host as the base domain for
   TLSA lookups.  Standards defining how to use DANE anchored TLS for
   each application protocol are expected to specify where to locate
   TLSA RRs when the destination is referred to by a CNAME.

   If CNAMEs were not followed, Client Domains would need to publish
   TLSA records that match the Service Provider's certificate chain or
   always use an entity that was both the Service Provider and the TLSA
   publisher.  Having the TLSA base domain be different than the Service
   Provider's domain imposes a difficult key management burden on the
   Client Domain and the Service Provider.

   It is possible to publish CNAMEs in the Client Domain pointing to the
   Service Provider's TLSA RRset if the TLSA certificate usage field is
   set to 3.  Otherwise, a client that used the alias name (from the
   hosted domain rather than the Service Provider's domain) as the base



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   domain to obtain the TLSA RRset would look for the hosted domain in
   the server certificate when performing name checks, and would
   generally fail to authenticate the server except in the rare cases
   when the server's certificate does include the Client Domain.  SNI
   SHOULD be used to help perform the right certificate selection by the
   server, although this imposes a management burden on the TLS server
   that could be avoided by ensuring the TLSA base domain is within the
   Service Provider's control in the first place.

   Example CNAME record for a TLSA domain:

    ; TLSA RRs aliased to Service Provider, but the base domain is
    ; the hosted domain.  Likely to fail name check unless Service
    ; Provider usage is "3".
    ;
    _25._tcp.mail.example.com. IN CNAME _25._tcp.mail.example.net.
        _25._tcp.mail.example.net. IN TLSA 3 1 1 ...

   Note: when the TLSA RRset query domain (base domain plus port and
   protocol prefixes) resolves to a DNSSEC validated CNAME that points
   to a DNSSEC signed zone with the actual TLSA records, as the above
   example indicates, it has no effect on the value of the base domain,
   which remains the original domain to which the client prefixed the
   port and protocol.  In the example above, the base domain is
   "mail.example.com" and not "mail.example.net".

   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.  In this case, if the MX or SRV host is a CNAME
   alias the client MAY "chase" the CNAME and SHOULD use the target
   hostname as the base domain for TLSA records as well as the host_name
   in SNI, provided the CNAME RR is found to be "secure" at each step in
   the CNAME expansion.

3.6.  TLSA Base Name Priorities

   There are multiple steps within a chaining DNS lookup process that
   TLSA base names can be pulled from.  This section will discuss what
   the preferred selection points are.  TBD.

   1.  Final Domain Name

   2.  Redirect Name

   3.  Initial Name

3.7.  Interaction with Certificate Transparency




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   [RFC6962] Certificate Transparency or CT for short, defines an
   approach to mitigate the risk of rogue or compromised public CAs
   issuing unauthorized certificates.  This section clarifies the
   interaction of CT and DANE.  CT is a protocol and auditing system
   that applies only to public CAs, and only when they are free to issue
   unauthorized certificates for a domain.  If the CA is not a public
   CA, or DANE TLSA RRs constrain the end-entity certificate to a fixed
   public key, there is no role for CT, and clients SHOULD NOT apply CT
   checks.

   When a server is authenticated via a DANE TLSA RR with TLSA
   Certificate Usage "1" or "3" (that is an end-entity certificate
   association), the domain owner has unambiguously specified the
   certificate associated with the given service.  Even if a rogue CA
   were able to issue an unauthorized end-entity certificate that binds
   a public key to a name in that domain, barring "second preimage"
   attacks on the hashing algorithms in use, any such certificate would
   not match the TLSA record and would be rejected.  Therefore, when a
   TLS client authenticates the TLS server via a TLSA certificate
   association with usage "1" or "3", CT checks SHOULD NOT be performed.
   Publication of the server certificate or public key (digest) in a
   DNSSEC signed zone by the domain owner assures the 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 RR with TLSA usage "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
   root).  Since TLSA Certificate Usage "2" is generally intended to
   support non-PKIX trust anchors, clients SHOULD NOT perform CT checks
   with usage "2" using unknown root CAs.  A server operator that wants
   CT checks SHOULD publish TLSA RRs with usage "0", or can obviate them
   with usage "1" or "3".

   CT checks remain applicable with TLSA Certificate Usage "0" when the
   client supports both DANE and CT and the trusted PKIX root issuer is
   a known public root.

3.8.  Design Considerations for Protocols Using DANE

3.8.1.  Design Considerations for non-PKIX Protocols

   For some application protocols, the existing public CA PKI may not be
   viable.  For these (non-PKIX) protocols, servers SHOULD NOT suggest
   publishing TLSA records with TLSA Certificate Usage "0" or "1", as
   clients cannot be expected to perform [RFC5280] PKIX validation or
   [RFC6125] identity verification.




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   Protocols designed for non-PKIX use SHOULD choose to treat any TLSA
   records with TLSA Certificate Usage "0" or "1" as unusable.  After
   verifying that the only available TLSA Certificate Usage types are
   "0" or "1", protocol definitions MAY instruct clients to either
   refuse to initiate a connection or to connect via unauthenticated
   mandatory TLS if no alternative authentication mechanisms are
   available.

   If non-PKIX protocols do allow for publication of TLSA records with
   TLSA Certificate Usage "0" or "1", clients SHOULD make use of the
   TLSA verification to the fullest extent possible.

3.8.1.1.  TLSA Certificate Usage 1

   With non-PKIX protocols, clients using TLSA Certificate Usage "1"
   records MAY ignore the PKIX validation requirement, and authenticate
   the server per the content of the TLSA record alone.  Since servers
   will hopefully rely on SNI to select the correct certificate for
   presentation, the client SHOULD use the SNI extension to signal the
   base domain of the TLSA RRset.

3.8.1.2.  TLSA Certificate Usage 0

   With TLSA Certificate Usage "0" in non-PKIX protocols, the usability
   of the TLSA records depends on its matching type.

   If the matching type is "0", the TLSA record contains the full
   certificate or full public key of the trusted certificate authority.
   In this case the client has all the information it needs to match the
   server trust-chain to the TLSA record.  The client MAY ignore the
   PKIX validation requirement and authenticate the server via its DANE
   TLSA records alone (sending SNI with the base domain as usual).  The
   client SHOULD use the base domain of the TLSA record(s) in
   certificate name checks.

   If the matching type is not "0", the TLSA record contains only a
   digest of the trust certificate authority certificate or public key.
   The full certificate may not be included in the server's certificate
   chain and the client may not be able to match the server trust chain
   against the TLSA record when a non-PKIX protocol is being used, as
   the client won't have a default CA trust list.  See Section 3.9.1 for
   a more complete discussion of this case.  The client cannot reliably
   authenticate the server in this case and SHOULD treat the TLSA record
   as unusable.

   If the client is configured with a set of trusted CAs that are
   believed to be sufficiently complete to authenticate all the servers
   it expects to communicate with, then it MAY elect to honor



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   certificate usage "0" TLSA records that publish digests of the
   trusted CA certificate or public key.

3.9.  TLSA Records and Trust Anchor Digests

   With TLSA records that match the EE certificate, the TLS client has
   no difficulty matching the TLS record against the server certificate,
   as this certificate is always present in the TLS server certificate
   chain.  The TLS client can, if necessary, extract the public key from
   the server certificate, and can compute the appropriate digest.

   With DANE TLSA records that match the digest of a TA certificate or
   public key, a complication arises when the TA certificate is omitted
   from the server's certificate chain.  This can happen when the trust-
   anchor is a root certificate authority, as stated in 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.

   This means that TLSA records that match a TA certificate or public
   key digest are not entirely sufficient to validate the peer
   certificate chain.  If no matching certificate is found in the
   server's certificate chain, the chain may be signed by an omitted
   root CA whose digest matches the TLSA record.  We will consider each
   trust-anchor TLSA Certificate Usage in turn.

3.9.1.  Trust Anchor Digests With TLSA Certificate Usage 0

   In this case, from the server's perspective, the omission of the root
   CA seems reasonable, since in addition to authentication via DANE
   TLSA records, the client is expected to perform [RFC5280] PKIX
   validation of the server's trust chain and thus to already have a
   copy of the omitted root certificate.

   From the client's perspective the situation is more nuanced.  Despite
   the server's indicated preference for PKIX validation, the client may
   not possess (or may not fully trust) a complete set of public root
   CAs.  This is especially likely in protocols where the existing
   public CA PKI is not applicable, as described in Section 3.8.1.  If
   it is likely that a client lacks a sufficiently complete list of
   trusted CAs, and that a non-negligible number of DNS servers publish
   TLSA Certificate Usage 0 TLSA records with digests of omitted root



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   CAs, then such a client SHOULD treat such TLSA records as "unusable".
   Simply ignoring PKIX validation is not an option, since the client
   will also be unable to match the TLSA record without position of the
   root certificate.  The client MAY choose fall back to unauthenticated
   TLS, if PKIX is also not an option (see [I-D.ietf-dane-srv]) or
   refuse to initiate a connection.

3.9.2.  Trust Anchor Digests With TLSA Certificate Usage 2

   With TLSA Certificate Usage "2", there is no expectation that the
   client is pre-configured with the trust anchor certificate.  With
   TLSA Certificate Usage "2" clients are expecting to rely on the TLSA
   records alone.  But, with a matching type other than "0" the TLSA
   records contain 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, clients will be unable to
   authenticate the server.

   TLSA Publishers that publish TLSA Certificate Usage "2" with a non-
   zero matching type MUST ensure that the corresponding server is
   configured to include the associated 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.

   Since servers are expected to always provide usage "2" trust anchor
   certificates (either via DNS or else via the TLS hanshake), clients
   SHOULD fully support this TLSA Certificate Usage.  Clients MAY choose
   to treat it as unusable if experience proves that servers don't
   consistently live up to their obligations.

3.10.  Trust anchor public keys

   TLSA records with TLSA Certificate Usage "0" or "2", selector "1" and
   a matching type of "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.





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

   With TLSA Certificate Usage "0", when the client is able to perform
   PKIX validation, the client can construct a complete PKIX trust chain
   as it will have access to the trust anchor name.  So in that case,
   the client can verify that the server certificate chain is issued by
   a trust anchor that matches the TLSA record.

   With TLSA Certificate Usage "2", the client may not have the missing
   trust anchor certificate, and cannot generally verify whether a
   particular certificate chain is "issued by" the trust anchor
   described in the TLSA record.  If 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" by the trust anchor public key in the TLSA
   record.

   Since trust chain validation via bare public keys rather than trusted
   CA certificates may be difficult to implement using existing TLS
   libraries, servers SHOULD include the trust anchor certificate in
   their certificate chain when the TLSA Certificate Usage is "2".

   If none of the server's certificate chain elements match a public key
   specified in full (selector = 0, match type = 0) in a TLSA record,
   clients SHOULD attempt to check whether the topmost certificate in
   the chain is signed by the provided public key, and if so consider
   the server trust chain valid, with authentication complete if name
   checks are also successful.

4.  Type Specific DANE Guidelines

4.1.  Type 3 Guidelines

4.2.  Type 2 Guidelines

4.3.  Type 1 Guidelines

4.4.  Type 0 Guidelines










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   TLSA Certificate Usage "0" allows a domain to publish constraints on
   the set of certificate authorities trusted to issue certificates for
   its TLS servers.  It is expected that clients will only accept trust
   chains which contain a match for one of the published TLSA records.
   This is simple for TLSA Certificate Usage "1" where the PKIX trust
   chain always contains the leaf server certificate.  The situation for
   TLSA Certificate Usage "0" is more subtle.

   TLSA Publishers may publish TLSA records for a particular public root
   CA, expecting that clients will then only accept chains anchored at
   that root.  It is possible, however, that the client's set of trusted
   certificates 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 chain may omit any trusted roots published in the
   server's TLSA records.

   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 records.  This means that a client SHOULD not always stop
   extending the chain when the first locally trusted certificate is
   found.  If no TLSA records have matched any of the elements of the
   chain, it MUST attempt to build a longer chain if the trusted
   certificate found is not self-issued, in the hope that a certificate
   closer to the root may in fact match the server's TLSA records.

5.  Note on DNSSEC security

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

   With the existing public CA PKI, name constraints are rarely used and
   public root CAs can issue certificates for any domain of its choice.
   With DNSSEC, 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
   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 of 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



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   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 should plan accordingly and not generate signatures with
   excessively long duration.  For domains publishing high-value keys, a
   signature lifetime of a few days is reasonable, and the zone should
   be resigned every day.  For more domains with less critical data, a
   reasonable signature lifetime is a couple of weeks to a month, and
   the zone should be resigned every week.  Monitoring of the signature
   lifetime is important.  If the zone is not resigned in a timely
   manner, one risks a major outage with the entire domain becoming
   invalid.

6.  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 memo and provided
   feedback on early drafts.

7.  Security Considerations

   Application protocols that cannot make use of the existing public CA
   PKI (so called non-PKIX protocols), may choose to not implement
   certain PKIX-dependent TLSA record types defined in [RFC6698], or may
   choose to make a best-effort use of such records.  In neither case is
   security compromised, since by assumption PKIX verification is simply
   not an option for these protocols.  When the TLS server is
   authenticated based on the TLSA records alone, the client is as well
   authenticated as possible, treating the TLSA records as unusable
   would lead to weaker security.

   Therefore, when TLSA records are used with protocols where PKIX does
   not apply, the recommended trade-off is for servers to not publish
   PKIX-dependent TLSA records, and for clients to use them as best they
   can, but otherwise treat them unusable.  Of course when PKIX
   validation is an option clients SHOULD perform PKIX validation per
   [RFC6698].









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8.  References

8.1.  Normative References

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

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

   [RFC3546]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 3546, June 2003.

   [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.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [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.

   [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.






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

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

8.2.  Informative References

   [I-D.ietf-dane-srv]
              Finch, T., "Using DNS-Based Authentication of Named
              Entities (DANE) TLSA records with SRV and MX records.",
              draft-ietf-dane-srv-02 (work in progress), February 2013.

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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