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rfc:rfc6698

Internet Engineering Task Force (IETF) P. Hoffman Request for Comments: 6698 VPN Consortium Category: Standards Track J. Schlyter ISSN: 2070-1721 Kirei AB

                                                           August 2012
       The DNS-Based Authentication of Named Entities (DANE)
           Transport Layer Security (TLS) Protocol: TLSA

Abstract

 Encrypted communication on the Internet often uses Transport Layer
 Security (TLS), which depends on third parties to certify the keys
 used.  This document improves on that situation by enabling the
 administrators of domain names to specify the keys used in that
 domain's TLS servers.  This requires matching improvements in TLS
 client software, but no change in TLS server software.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6698.

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Hoffman & Schlyter Standards Track [Page 1] RFC 6698 DNS-Based Authentication for TLS August 2012

Table of Contents

 1. Introduction ....................................................3
    1.1. Background and Motivation ..................................3
    1.2. Securing the Association of a Domain Name with a
         Server's Certificate .......................................4
    1.3. Method for Securing Certificate Associations ...............5
    1.4. Terminology ................................................6
 2. The TLSA Resource Record ........................................7
    2.1. TLSA RDATA Wire Format .....................................7
         2.1.1. The Certificate Usage Field .........................7
         2.1.2. The Selector Field ..................................8
         2.1.3. The Matching Type Field .............................9
         2.1.4. The Certificate Association Data Field ..............9
    2.2. TLSA RR Presentation Format ................................9
    2.3. TLSA RR Examples ..........................................10
 3. Domain Names for TLSA Certificate Associations .................10
 4. Use of TLSA Records in TLS .....................................11
    4.1. Usable Certificate Associations ...........................11
 5. TLSA and DANE Use Cases and Requirements .......................13
 6. Mandatory-to-Implement Features ................................15
 7. IANA Considerations ............................................15
    7.1. TLSA RRtype ...............................................15
    7.2. TLSA Certificate Usages ...................................15
    7.3. TLSA Selectors ............................................16
    7.4. TLSA Matching Types .......................................16
 8. Security Considerations ........................................16
    8.1. Comparing DANE to Public CAs ..............................18
         8.1.1. Risk of Key Compromise .............................19
         8.1.2. Impact of Key Compromise ...........................20
         8.1.3. Detection of Key Compromise ........................20
         8.1.4. Spoofing Hostnames .................................20
    8.2. DNS Caching ...............................................21
    8.3. External DNSSEC Validators ................................21
 9. Acknowledgements ...............................................22
 10. References ....................................................22
    10.1. Normative References .....................................22
    10.2. Informative References ...................................23
 Appendix A. Operational Considerations for Deploying TLSA
             Records ...............................................25
   A.1. Creating TLSA Records ......................................25
     A.1.1. Ambiguities and Corner Cases When TLS Clients
            Build Trust Chains .....................................26
     A.1.2. Choosing a Selector Type ...............................26
   A.2. Provisioning TLSA Records in DNS ...........................28
     A.2.1. Provisioning TLSA Records with Aliases .................28
   A.3. Securing the Last Hop ......................................30
   A.4. Handling Certificate Rollover ..............................31

Hoffman & Schlyter Standards Track [Page 2] RFC 6698 DNS-Based Authentication for TLS August 2012

 Appendix B. Pseudocode for Using TLSA .............................32
   B.1. Helper Functions ...........................................32
   B.2. Main TLSA Pseudocode .......................................33
 Appendix C. Examples ..............................................35

1. Introduction

1.1. Background and Motivation

 Applications that communicate over the Internet often need to prevent
 eavesdropping, tampering, or forgery of their communications.  The
 Transport Layer Security (TLS) protocol provides this kind of
 communications security over the Internet, using channel encryption.
 The security properties of encryption systems depend strongly on the
 keys that they use.  If secret keys are revealed, or if public keys
 can be replaced by fake keys (that is, a key not corresponding to the
 entity identified in the certificate), these systems provide little
 or no security.
 TLS uses certificates to bind keys and names.  A certificate combines
 a published key with other information such as the name of the
 service that uses the key, and this combination is digitally signed
 by another key.  Having a key in a certificate is only helpful if one
 trusts the other key that signed the certificate.  If that other key
 was itself revealed or substituted, then its signature is worthless
 in proving anything about the first key.
 On the Internet, this problem has been solved for years by entities
 called "Certification Authorities" (CAs).  CAs protect their secret
 key vigorously, while supplying their public key to the software
 vendors who build TLS clients.  They then sign certificates, and
 supply those to TLS servers.  TLS client software uses a set of these
 CA keys as "trust anchors" to validate the signatures on certificates
 that the client receives from TLS servers.  Client software typically
 allows any CA to usefully sign any other certificate.
 The public CA model upon which TLS has depended is fundamentally
 vulnerable because it allows any of these CAs to issue a certificate
 for any domain name.  A single trusted CA that betrays its trust,
 either voluntarily or by providing less-than-vigorous protection for
 its secrets and capabilities, can undermine the security offered by
 any certificates employed with TLS.  This problem arises because a
 compromised CA can issue a replacement certificate that contains a
 fake key.  Recent experiences with compromises of CAs or their
 trusted partners have led to very serious security problems, such as
 the governments of multiple countries attempting to wiretap and/or
 subvert major TLS-protected web sites trusted by millions of users.

Hoffman & Schlyter Standards Track [Page 3] RFC 6698 DNS-Based Authentication for TLS August 2012

 The DNS Security Extensions (DNSSEC) provide a similar model that
 involves trusted keys signing the information for untrusted keys.
 However, DNSSEC provides three significant improvements.  Keys are
 tied to names in the Domain Name System (DNS), rather than to
 arbitrary identifying strings; this is more convenient for Internet
 protocols.  Signed keys for any domain are accessible online through
 a straightforward query using the standard DNSSEC protocol, so there
 is no problem distributing the signed keys.  Most significantly, the
 keys associated with a domain name can only be signed by a key
 associated with the parent of that domain name; for example, the keys
 for "example.com" can only be signed by the keys for "com", and the
 keys for "com" can only be signed by the DNS root.  This prevents an
 untrustworthy signer from compromising anyone's keys except those in
 their own subdomains.  Like TLS, DNSSEC relies on public keys that
 come built into the DNSSEC client software, but these keys come only
 from a single root domain rather than from a multiplicity of CAs.
 DNS-Based Authentication of Named Entities (DANE) offers the option
 to use the DNSSEC infrastructure to store and sign keys and
 certificates that are used by TLS.  DANE is envisioned as a
 preferable basis for binding public keys to DNS names, because the
 entities that vouch for the binding of public key data to DNS names
 are the same entities responsible for managing the DNS names in
 question.  While the resulting system still has residual security
 vulnerabilities, it restricts the scope of assertions that can be
 made by any entity, consistent with the naming scope imposed by the
 DNS hierarchy.  As a result, DANE embodies the security "principle of
 least privilege" that is lacking in the current public CA model.

1.2. Securing the Association of a Domain Name with a Server's

    Certificate
 A TLS client begins a connection by exchanging messages with a TLS
 server.  For many application protocols, it looks up the server's
 name using the DNS to get an Internet Protocol (IP) address
 associated with the name.  It then begins a connection to a
 particular port at that address, and sends an initial message there.
 However, the client does not yet know whether an adversary is
 intercepting and/or altering its communication before it reaches the
 TLS server.  It does not even know whether the real TLS server
 associated with that domain name has ever received its initial
 messages.
 The first response from the server in TLS may contain a certificate.
 In order for the TLS client to authenticate that it is talking to the
 expected TLS server, the client must validate that this certificate
 is associated with the domain name used by the client to get to the
 server.  Currently, the client must extract the domain name from the

Hoffman & Schlyter Standards Track [Page 4] RFC 6698 DNS-Based Authentication for TLS August 2012

 certificate and must successfully validate the certificate, including
 chaining to a trust anchor.
 There is a different way to authenticate the association of the
 server's certificate with the intended domain name without trusting
 an external CA.  Given that the DNS administrator for a domain name
 is authorized to give identifying information about the zone, it
 makes sense to allow that administrator to also make an authoritative
 binding between the domain name and a certificate that might be used
 by a host at that domain name.  The easiest way to do this is to use
 the DNS, securing the binding with DNSSEC.
 There are many use cases for such functionality.  [RFC6394] lists the
 ones to which the DNS RRtype in this document apply.  [RFC6394] also
 lists many requirements, most of which this document is believed to
 meet.  Section 5 covers the applicability of this document to the use
 cases in detail.  The protocol in this document can generally be
 referred to as the "DANE TLSA" protocol.  ("TLSA" does not stand for
 anything; it is just the name of the RRtype.)
 This document applies to both TLS [RFC5246] and Datagram TLS (DTLS)
 [RFC6347].  In order to make the document more readable, it mostly
 only talks about "TLS", but in all cases, it means "TLS or DTLS".
 Although the references in this paragraph are to TLS and DTLS
 version 1.2, the DANE TLSA protocol can also be used with earlier
 versions of TLS and DTLS.
 This document only relates to securely associating certificates for
 TLS and DTLS with host names; retrieving certificates from DNS for
 other protocols is handled in other documents.  For example, keys for
 IPsec are covered in [RFC4025], and keys for Secure SHell (SSH) are
 covered in [RFC4255].

1.3. Method for Securing Certificate Associations

 A certificate association is formed from a piece of information
 identifying a certificate and the domain name where the server
 application runs.  The combination of a trust anchor and a domain
 name can also be a certificate association.
 A DNS query can return multiple certificate associations, such as in
 the case of a server that is changing from one certificate to another
 (described in more detail in Appendix A.4).
 This document only applies to PKIX [RFC5280] certificates, not
 certificates of other formats.

Hoffman & Schlyter Standards Track [Page 5] RFC 6698 DNS-Based Authentication for TLS August 2012

 This document defines a secure method to associate the certificate
 that is obtained from the TLS server with a domain name using DNS;
 the DNS information needs to be protected by DNSSEC.  Because the
 certificate association was retrieved based on a DNS query, the
 domain name in the query is by definition associated with the
 certificate.  Note that this document does not cover how to associate
 certificates with domain names for application protocols that depend
 on SRV, NAPTR, and similar DNS resource records.  It is expected that
 future documents will cover methods for making those associations,
 and those documents may or may not need to update this one.
 DNSSEC, which is defined in [RFC4033], [RFC4034], and [RFC4035], uses
 cryptographic keys and digital signatures to provide authentication
 of DNS data.  Information that is retrieved from the DNS and that is
 validated using DNSSEC is thereby proved to be the authoritative
 data.  The DNSSEC signature needs to be validated on all responses
 that use DNSSEC in order to assure the proof of origin of the data.
 This document does not specify how DNSSEC validation occurs because
 there are many different proposals for how a client might get
 validated DNSSEC results, such as from a DNSSEC-aware resolver that
 is coded in the application, from a trusted DNSSEC resolver on the
 machine on which the application is running, or from a trusted DNSSEC
 resolver with which the application is communicating over an
 authenticated and integrity-protected channel or network.  This is
 described in more detail in Section 7 of [RFC4033].
 This document only relates to getting the DNS information for the
 certificate association securely using DNSSEC; other secure DNS
 mechanisms are out of scope.

1.4. 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 RFC 2119 [RFC2119].
 This document also makes use of standard PKIX, DNSSEC, TLS, and DNS
 terminology.  See [RFC5280], [RFC4033], [RFC5246], and STD 13
 [RFC1034] [RFC1035], respectively, for these terms.  In addition,
 terms related to TLS-protected application services and DNS names are
 taken from [RFC6125].

Hoffman & Schlyter Standards Track [Page 6] RFC 6698 DNS-Based Authentication for TLS August 2012

2. The TLSA Resource Record

 The TLSA DNS resource record (RR) is used to associate a TLS server
 certificate or public key with the domain name where the record is
 found, thus forming a "TLSA certificate association".  The semantics
 of how the TLSA RR is interpreted are given later in this document.
 The type value for the TLSA RR type is defined in Section 7.1.
 The TLSA RR is class independent.
 The TLSA RR has no special Time to Live (TTL) requirements.

2.1. TLSA RDATA Wire Format

 The RDATA for a TLSA RR consists of a one-octet certificate usage
 field, a one-octet selector field, a one-octet matching type field,
 and the certificate association data field.
                      1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Cert. Usage  |   Selector    | Matching Type |               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               /
 /                                                               /
 /                 Certificate Association Data                  /
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.1.1. The Certificate Usage Field

 A one-octet value, called "certificate usage", specifies the provided
 association that will be used to match the certificate presented in
 the TLS handshake.  This value is defined in a new IANA registry (see
 Section 7.2) in order to make it easier to add additional certificate
 usages in the future.  The certificate usages defined in this
 document are:
    0 -- Certificate usage 0 is used to specify a CA certificate, or
    the public key of such a certificate, that MUST be found in any of
    the PKIX certification paths for the end entity certificate given
    by the server in TLS.  This certificate usage is sometimes
    referred to as "CA constraint" because it limits which CA can be
    used to issue certificates for a given service on a host.  The
    presented certificate MUST pass PKIX certification path
    validation, and a CA certificate that matches the TLSA record MUST
    be included as part of a valid certification path.  Because this
    certificate usage allows both trust anchors and CA certificates,

Hoffman & Schlyter Standards Track [Page 7] RFC 6698 DNS-Based Authentication for TLS August 2012

    the certificate might or might not have the basicConstraints
    extension present.
    1 -- Certificate usage 1 is used to specify an end entity
    certificate, or the public key of such a certificate, that MUST be
    matched with the end entity certificate given by the server in
    TLS.  This certificate usage is sometimes referred to as "service
    certificate constraint" because it limits which end entity
    certificate can be used by a given service on a host.  The target
    certificate MUST pass PKIX certification path validation and MUST
    match the TLSA record.
    2 -- Certificate usage 2 is used to specify a certificate, or the
    public key of such a certificate, that MUST be used as the trust
    anchor when validating the end entity certificate given by the
    server in TLS.  This certificate usage is sometimes referred to as
    "trust anchor assertion" and allows a domain name administrator to
    specify a new trust anchor -- for example, if the domain issues
    its own certificates under its own CA that is not expected to be
    in the end users' collection of trust anchors.  The target
    certificate MUST pass PKIX certification path validation, with any
    certificate matching the TLSA record considered to be a trust
    anchor for this certification path validation.
    3 -- Certificate usage 3 is used to specify a certificate, or the
    public key of such a certificate, that MUST match the end entity
    certificate given by the server in TLS.  This certificate usage is
    sometimes referred to as "domain-issued certificate" because it
    allows for a domain name administrator to issue certificates for a
    domain without involving a third-party CA.  The target certificate
    MUST match the TLSA record.  The difference between certificate
    usage 1 and certificate usage 3 is that certificate usage 1
    requires that the certificate pass PKIX validation, but PKIX
    validation is not tested for certificate usage 3.
 The certificate usages defined in this document explicitly only apply
 to PKIX-formatted certificates in DER encoding [X.690].  If TLS
 allows other formats later, or if extensions to this RRtype are made
 that accept other formats for certificates, those certificates will
 need their own certificate usage values.

2.1.2. The Selector Field

 A one-octet value, called "selector", specifies which part of the TLS
 certificate presented by the server will be matched against the
 association data.  This value is defined in a new IANA registry (see
 Section 7.3).  The selectors defined in this document are:

Hoffman & Schlyter Standards Track [Page 8] RFC 6698 DNS-Based Authentication for TLS August 2012

    0 -- Full certificate: the Certificate binary structure as defined
    in [RFC5280]
    1 -- SubjectPublicKeyInfo: DER-encoded binary structure as defined
    in [RFC5280]
 (Note that the use of "selector" in this document is completely
 unrelated to the use of "selector" in DomainKeys Identified Mail
 (DKIM) [RFC6376].)

2.1.3. The Matching Type Field

 A one-octet value, called "matching type", specifies how the
 certificate association is presented.  This value is defined in a new
 IANA registry (see Section 7.4).  The types defined in this document
 are:
    0 -- Exact match on selected content
    1 -- SHA-256 hash of selected content [RFC6234]
    2 -- SHA-512 hash of selected content [RFC6234]
 If the TLSA record's matching type is a hash, having the record use
 the same hash algorithm that was used in the signature in the
 certificate (if possible) will assist clients that support a small
 number of hash algorithms.

2.1.4. The Certificate Association Data Field

 This field specifies the "certificate association data" to be
 matched.  These bytes are either raw data (that is, the full
 certificate or its SubjectPublicKeyInfo, depending on the selector)
 for matching type 0, or the hash of the raw data for matching types 1
 and 2.  The data refers to the certificate in the association, not to
 the TLS ASN.1 Certificate object.

2.2. TLSA RR Presentation Format

 The presentation format of the RDATA portion (as defined in
 [RFC1035]) is as follows:
 o  The certificate usage field MUST be represented as an 8-bit
    unsigned integer.
 o  The selector field MUST be represented as an 8-bit unsigned
    integer.

Hoffman & Schlyter Standards Track [Page 9] RFC 6698 DNS-Based Authentication for TLS August 2012

 o  The matching type field MUST be represented as an 8-bit unsigned
    integer.
 o  The certificate association data field MUST be represented as a
    string of hexadecimal characters.  Whitespace is allowed within
    the string of hexadecimal characters, as described in [RFC1035].

2.3. TLSA RR Examples

 In the following examples, the domain name is formed using the rules
 in Section 3.
 An example of a hashed (SHA-256) association of a PKIX CA
 certificate:
 _443._tcp.www.example.com. IN TLSA (
    0 0 1 d2abde240d7cd3ee6b4b28c54df034b9
          7983a1d16e8a410e4561cb106618e971 )
 An example of a hashed (SHA-512) subject public key association of a
 PKIX end entity certificate:
 _443._tcp.www.example.com. IN TLSA (
    1 1 2 92003ba34942dc74152e2f2c408d29ec
          a5a520e7f2e06bb944f4dca346baf63c
          1b177615d466f6c4b71c216a50292bd5
          8c9ebdd2f74e38fe51ffd48c43326cbc )
 An example of a full certificate association of a PKIX end entity
 certificate:
 _443._tcp.www.example.com. IN TLSA (
    3 0 0 30820307308201efa003020102020... )

3. Domain Names for TLSA Certificate Associations

 Unless there is a protocol-specific specification that is different
 than this one, TLSA resource records are stored at a prefixed DNS
 domain name.  The prefix is prepared in the following manner:
 1.  The decimal representation of the port number on which a TLS-
     based service is assumed to exist is prepended with an underscore
     character ("_") to become the left-most label in the prepared
     domain name.  This number has no leading zeros.

Hoffman & Schlyter Standards Track [Page 10] RFC 6698 DNS-Based Authentication for TLS August 2012

 2.  The protocol name of the transport on which a TLS-based service
     is assumed to exist is prepended with an underscore character
     ("_") to become the second left-most label in the prepared domain
     name.  The transport names defined for this protocol are "tcp",
     "udp", and "sctp".
 3.  The base domain name is appended to the result of step 2 to
     complete the prepared domain name.  The base domain name is the
     fully qualified DNS domain name [RFC1035] of the TLS server, with
     the additional restriction that every label MUST meet the rules
     of [RFC0952].  The latter restriction means that, if the query is
     for an internationalized domain name, it MUST use the A-label
     form as defined in [RFC5890].
 For example, to request a TLSA resource record for an HTTP server
 running TLS on port 443 at "www.example.com",
 "_443._tcp.www.example.com" is used in the request.  To request a
 TLSA resource record for an SMTP server running the STARTTLS protocol
 on port 25 at "mail.example.com", "_25._tcp.mail.example.com" is
 used.

4. Use of TLSA Records in TLS

 Section 2.1 of this document defines the mandatory matching rules for
 the data from the TLSA certificate associations and the certificates
 received from the TLS server.
 The TLS session that is to be set up MUST be for the specific port
 number and transport name that was given in the TLSA query.
 Some specifications for applications that run over TLS, such as
 [RFC2818] for HTTP, require that the server's certificate have a
 domain name that matches the host name expected by the client.  Some
 specifications, such as [RFC6125], detail how to match the identity
 given in a PKIX certificate with those expected by the user.
 If a TLSA record has certificate usage 2, the corresponding TLS
 server SHOULD send the certificate that is referenced just like it
 currently sends intermediate certificates.

4.1. Usable Certificate Associations

 An implementation of this protocol makes a DNS query for TLSA
 records, validates these records using DNSSEC, and uses the resulting
 TLSA records and validation status to modify its responses to the TLS
 server.

Hoffman & Schlyter Standards Track [Page 11] RFC 6698 DNS-Based Authentication for TLS August 2012

 Determining whether a TLSA RRSet can be used MUST be based on the
 DNSSEC validation state (as defined in [RFC4033]).
 o  A TLSA RRSet whose DNSSEC validation state is secure MUST be used
    as a certificate association for TLS unless a local policy would
    prohibit the use of the specific certificate association in the
    secure TLSA RRSet.
 o  If the DNSSEC validation state on the response to the request for
    the TLSA RRSet is bogus, this MUST cause TLS not to be started or,
    if the TLS negotiation is already in progress, MUST cause the
    connection to be aborted.
 o  A TLSA RRSet whose DNSSEC validation state is indeterminate or
    insecure cannot be used for TLS and MUST be considered unusable.
 Clients that validate the DNSSEC signatures themselves MUST use
 standard DNSSEC validation procedures.  Clients that rely on another
 entity to perform the DNSSEC signature validation MUST use a secure
 mechanism between themselves and the validator.  Examples of secure
 transports to other hosts include TSIG [RFC2845], SIG(0) [RFC2931],
 and IPsec [RFC6071].  Note that it is not sufficient to use secure
 transport to a DNS resolver that does not do DNSSEC signature
 validation.  See Section 8.3 for more security considerations related
 to external validators.
 If a certificate association contains a certificate usage, selector,
 or matching type that is not understood by the TLS client, that
 certificate association MUST be considered unusable.  If the
 comparison data for a certificate is malformed, the certificate
 association MUST be considered unusable.
 If a certificate association contains a matching type or certificate
 association data that uses a cryptographic algorithm that is
 considered too weak for the TLS client's policy, the certificate
 association MUST be considered unusable.
 If an application receives zero usable certificate associations from
 a DNS request or from its cache, it processes TLS in the normal
 fashion without any input from the TLSA records.  If an application
 receives one or more usable certificate associations, it attempts to
 match each certificate association with the TLS server's end entity
 certificate until a successful match is found.  During the TLS
 handshake, if none of the certificate associations matches the
 certificate given by the TLS server, the TLS client MUST abort the
 handshake.

Hoffman & Schlyter Standards Track [Page 12] RFC 6698 DNS-Based Authentication for TLS August 2012

 An attacker who is able to divert a user to a server under his
 control is also likely to be able to block DNS requests from the user
 or DNS responses being sent to the user.  Thus, in order to achieve
 any security benefit from certificate usage 0 or 1, an application
 that sends a request for TLSA records needs to get either a valid
 signed response containing TLSA records or verification that the
 domain is insecure or indeterminate.  If a request for a TLSA record
 does not meet one of those two criteria but the application continues
 with the TLS handshake anyway, the application has gotten no benefit
 from TLSA and SHOULD NOT make any internal or external indication
 that TLSA was applied.  If an application has a configuration setting
 that has turned on TLSA use, or has any indication that TLSA is in
 use (regardless of whether or not this is configurable), that
 application either MUST NOT start a TLS connection or it MUST abort a
 TLS handshake if both of the two criteria above are not met.
 The application can perform the TLSA lookup before initiating the TLS
 handshake, or do it during the TLS handshake: the choice is up to the
 client.

5. TLSA and DANE Use Cases and Requirements

 The different types of certificate associations defined in TLSA are
 matched with various sections of [RFC6394].  The use cases from
 Section 3 of [RFC6394] are covered in this document as follows:
 3.1 CA Constraints -- Implemented using certificate usage 0.
 3.2 Certificate Constraints -- Implemented using certificate usage 1.
 3.3 Trust Anchor Assertion and Domain-Issued Certificates --
    Implemented using certificate usages 2 and 3, respectively.
 The requirements from Section 4 of [RFC6394] are covered in this
 document as follows:
 Multiple Ports -- The TLSA records for different application services
    running on a single host can be distinguished through the service
    name and port number prefixed to the host name (see Section 3).
 No Downgrade -- Section 4 specifies the conditions under which a
    client can process and act upon TLSA records.  Specifically, if
    the DNSSEC status for the TLSA resource record set is determined
    to be bogus, the TLS connection (if started) will fail.
 Encapsulation -- Encapsulation is covered in the TLSA response
    semantics.

Hoffman & Schlyter Standards Track [Page 13] RFC 6698 DNS-Based Authentication for TLS August 2012

 Predictability -- The appendices of this specification provide
    operational considerations and implementation guidance in order to
    enable application developers to form a consistent interpretation
    of the recommended client behavior.
 Opportunistic Security -- If a client conformant to this
    specification can reliably determine the presence of a TLSA
    record, it will attempt to use this information.  Conversely, if a
    client can reliably determine the absence of any TLSA record, it
    will fall back to processing TLS in the normal fashion.  This is
    discussed in Section 4.
 Combination -- Multiple TLSA records can be published for a given
    host name, thus enabling the client to construct multiple TLSA
    certificate associations that reflect different assertions.  No
    support is provided to combine two TLSA certificate associations
    in a single operation.
 Roll-over -- TLSA records are processed in the normal manner within
    the scope of the DNS protocol, including the TTL expiration of the
    records.  This ensures that clients will not latch onto assertions
    made by expired TLSA records, and will be able to transition from
    using one public key or certificate usage to another.
 Simple Key Management -- The SubjectPublicKeyInfo selector in the
    TLSA record provides a mode that enables a domain holder to only
    have to maintain a single long-lived public/private key pair
    without the need to manage certificates.  Appendix A outlines the
    usefulness and the potential downsides to using this mode.
 Minimal Dependencies -- This specification relies on DNSSEC to
    protect the origin authenticity and integrity of the TLSA resource
    record set.  Additionally, if DNSSEC validation is not performed
    on the system that wishes to use TLSA certificate bindings, this
    specification requires that the "last mile" be over a secure
    transport.  There are no other deployment dependencies for this
    approach.
 Minimal Options -- The operating modes map precisely to the DANE use
    cases and requirements.  DNSSEC use is mandatory in that this
    specification encourages applications to use only those TLSA
    records that are shown to be validated.
 Wildcards -- Wildcards are covered in a limited manner in the TLSA
    request syntax; see Appendix A.
 Redirection -- Redirection is covered in the TLSA request syntax; see
    Appendix A.

Hoffman & Schlyter Standards Track [Page 14] RFC 6698 DNS-Based Authentication for TLS August 2012

6. Mandatory-to-Implement Features

 TLS clients conforming to this specification MUST be able to
 correctly interpret TLSA records with certificate usages 0, 1, 2,
 and 3.  TLS clients conforming to this specification MUST be able to
 compare a certificate association with a certificate from the TLS
 handshake using selector types 0 and 1, and matching type 0 (no hash
 used) and matching type 1 (SHA-256), and SHOULD be able to make such
 comparisons with matching type 2 (SHA-512).

7. IANA Considerations

 IANA has made the assignments in this section.
 In the following sections, "RFC Required" was chosen for TLSA
 certificate usages and "Specification Required" for selectors and
 matching types because of the amount of detail that is likely to be
 needed for implementers to correctly implement new certificate usages
 as compared to new selectors and matching types.

7.1. TLSA RRtype

 This document uses a new DNS RR type, TLSA, whose value (52) was
 allocated by IANA from the Resource Record (RR) TYPEs subregistry of
 the Domain Name System (DNS) Parameters registry.

7.2. TLSA Certificate Usages

 This document creates a new registry, "TLSA Certificate Usages".  The
 registry policy is "RFC Required".  The initial entries in the
 registry are:
 Value    Short description                       Reference
 ----------------------------------------------------------
 0        CA constraint                           RFC 6698
 1        Service certificate constraint          RFC 6698
 2        Trust anchor assertion                  RFC 6698
 3        Domain-issued certificate               RFC 6698
 4-254    Unassigned
 255      Private use
 Applications to the registry can request specific values that have
 yet to be assigned.

Hoffman & Schlyter Standards Track [Page 15] RFC 6698 DNS-Based Authentication for TLS August 2012

7.3. TLSA Selectors

 This document creates a new registry, "TLSA Selectors".  The registry
 policy is "Specification Required".  The initial entries in the
 registry are:
 Value    Short description                       Reference
 ----------------------------------------------------------
 0        Full certificate                        RFC 6698
 1        SubjectPublicKeyInfo                    RFC 6698
 2-254    Unassigned
 255      Private use
 Applications to the registry can request specific values that have
 yet to be assigned.

7.4. TLSA Matching Types

 This document creates a new registry, "TLSA Matching Types".  The
 registry policy is "Specification Required".  The initial entries in
 the registry are:
 Value    Short description                       Reference
 ----------------------------------------------------------
 0        No hash used                            RFC 6698
 1        SHA-256                                 RFC 6234
 2        SHA-512                                 RFC 6234
 3-254    Unassigned
 255      Private use
 Applications to the registry can request specific values that have
 yet to be assigned.

8. Security Considerations

 The security of the DNS RRtype described in this document relies on
 the security of DNSSEC to verify that the TLSA record has not been
 altered.
 A rogue DNS administrator who changes the A, AAAA, and/or TLSA
 records for a domain name can cause the client to go to an
 unauthorized server that will appear authorized, unless the client
 performs PKIX certification path validation and rejects the
 certificate.  That administrator could probably get a certificate
 issued by some CA anyway, so this is not an additional threat.

Hoffman & Schlyter Standards Track [Page 16] RFC 6698 DNS-Based Authentication for TLS August 2012

 If the authentication mechanism for adding or changing TLSA data in a
 zone is weaker than the authentication mechanism for changing the A
 and/or AAAA records, a man-in-the-middle who can redirect traffic to
 his site may be able to impersonate the attacked host in TLS if he
 can use the weaker authentication mechanism.  A better design for
 authenticating DNS would be to have the same level of authentication
 used for all DNS additions and changes for a particular domain name.
 Secure Socket Layer (SSL) proxies can sometimes act as a man-in-the-
 middle for TLS clients.  In these scenarios, the clients add a new
 trust anchor whose private key is kept on the SSL proxy; the proxy
 intercepts TLS requests, creates a new TLS session with the intended
 host, and sets up a TLS session with the client using a certificate
 that chains to the trust anchor installed in the client by the proxy.
 In such environments, using TLSA records will prevent the SSL proxy
 from functioning as expected because the TLS client will get a
 certificate association from the DNS that will not match the
 certificate that the SSL proxy uses with the client.  The client,
 seeing the proxy's new certificate for the supposed destination, will
 not set up a TLS session.
 Client treatment of any information included in the trust anchor is a
 matter of local policy.  This specification does not mandate that
 such information be inspected or validated by the server's domain
 name administrator.
 If a server's certificate is revoked, or if an intermediate CA in a
 chain between the server and a trust anchor has its certificate
 revoked, a TLSA record with a certificate usage of 2 that matches the
 revoked certificate would in essence override the revocation because
 the client would treat that revoked certificate as a trust anchor and
 thus not check its revocation status.  Because of this, domain
 administrators need to be responsible for being sure that the keys or
 certificates used in TLSA records with a certificate usage of 2 are
 in fact able to be used as reliable trust anchors.
 Certificates that are delivered in TLSA with certificate usage 2
 fundamentally change the way the TLS server's end entity certificate
 is evaluated.  For example, the server's certificate might chain to
 an existing CA through an intermediate CA that has certain policy
 restrictions, and the certificate would not pass those restrictions
 and thus normally be rejected.  That intermediate CA could issue
 itself a new certificate without the policy restrictions and tell its
 customers to use that certificate with certificate usage 2.  This in
 essence allows an intermediate CA to become a trust anchor for
 certificates that the end user might have expected to chain to an
 existing trust anchor.

Hoffman & Schlyter Standards Track [Page 17] RFC 6698 DNS-Based Authentication for TLS August 2012

 If an administrator wishes to stop using a TLSA record, the
 administrator can simply remove it from the DNS.  Normal clients will
 stop using the TLSA record after the TTL has expired.  Replay attacks
 against the TLSA record are not possible after the expiration date on
 the RRsig of the TLSA record that was removed.
 Generators of TLSA records should be aware that the client's full
 trust of a certificate association retrieved from a TLSA record may
 be a matter of local policy.  While such trust is limited to the
 specific domain name, protocol, and port for which the TLSA query was
 made, local policy may decline to accept the certificate (for reasons
 such as weak cryptography), as is also the case with PKIX trust
 anchors.

8.1. Comparing DANE to Public CAs

 As stated above, the security of the DNS RRtype described in this
 document relies on the security of DNSSEC to verify that the TLSA
 record has not been altered.  This section describes where the
 security of public CAs and the security of TLSA are similar and
 different.  This section applies equally to other security-related
 DNS RRtypes such as keys for IPsec and SSH.
 DNSSEC forms certificates (the binding of an identity to a key) by
 combining a DNSKEY, DS, or DLV resource record with an associated
 RRSIG record.  These records then form a signing chain extending from
 the client's trust anchors to the RR of interest.
 Although the DNSSEC protocol does not enforce it, DNSKEYs are often
 marked with a SEP flag indicating whether the key is a Zone Signing
 Key (ZSK) or a Key Signing Key (KSK).  ZSKs protect records in the
 zone (including DS and DLV records), and KSKs protect ZSK DNSKEY
 records.  This allows KSKs to be stored offline.
 The TLSA RRtype allows keys from the DNSSEC PKI hierarchy to
 authenticate keys wrapped in PKIX certificates for a particular host
 name, protocol, and port.
 With the exception of the DLV RRtype, all of these certificates
 constrain the keys they identify to names that are within the zone
 signing the certificate.  In order for a domain's DLV resource
 records to be honored, the domain must be configured as a DLV domain,
 and the domain's DNSKEYs must be configured as trust anchors or be
 authentic [RFC5074].

Hoffman & Schlyter Standards Track [Page 18] RFC 6698 DNS-Based Authentication for TLS August 2012

8.1.1. Risk of Key Compromise

 The risk that a given certificate that has a valid signing chain is
 fake is related to the number of keys that can contribute to the
 validation of the certificate, the quality of protection each private
 key receives, the value of each key to an attacker, and the value of
 falsifying the certificate.
 DNSSEC allows any set of domains to be configured as trust anchors
 and/or DLVs, but most clients are likely to use the root zone as
 their only trust anchor.  Also, because a given DNSKEY can only sign
 resource records for that zone, the number of private keys capable of
 compromising a given TLSA resource record is limited to the number of
 zones between the TLSA resource record and the nearest trust anchor,
 plus any configured DLV domains.  Typically, this will be six keys,
 half of which will be KSKs.
 PKIX only describes how to validate a certificate based on a client-
 chosen set of trust anchors, but says nothing about how many trust
 anchors to use or how they should be constrained.  As currently
 deployed, most PKIX clients use a large number of trust anchors
 provided with the client or operating system software.  These trust
 anchors are selected carefully, but with a desire for broad
 interoperability.  The trust anchors and CA certificates for public
 CAs rarely have name constraints applied.
 A combination of technical protections, process controls, and
 personnel experience contribute to the quality of security that keys
 receive.
 o  The security surrounding DNSSEC DNSKEYs varies significantly.  The
    KSK/ZSK split allows the KSK to be stored offline and protected
    more carefully than the ZSK, but not all domains do so.  The
    security applied to a zone's DNSKEYs should be proportional to the
    value of the domain, but that is difficult to estimate.  For
    example, the root DNSKEY has protections and controls comparable
    to or exceeding those of public CAs.  On the other end of the
    spectrum, small domains might provide no more protection to their
    keys than they do to their other data.
 o  The security surrounding public CAs also varies.  However, due to
    financial incentives and standards imposed by clients for
    acceptance into their trust anchor stores, CAs generally employ
    security experts and protect their keys carefully, though highly
    public compromises have occurred.

Hoffman & Schlyter Standards Track [Page 19] RFC 6698 DNS-Based Authentication for TLS August 2012

8.1.2. Impact of Key Compromise

 The impact of a key compromise differs significantly between the two
 models.
 o  DNSKEYs are inherently limited in what they can sign, so a
    compromise of the DNSKEY for "example.com" provides no avenue of
    attack against "example.org".  Even the impact of a compromise of
    .com's DNSKEY, while considerable, would be limited to .com
    domains.  Only the compromise of the root DNSKEY would have the
    equivalent impact of an unconstrained public CA.
 o  Public CAs are not typically constrained in what names they can
    sign, and therefore a compromise of even one CA allows the
    attacker to generate a certificate for any name in the DNS.  A
    domain holder can get a certificate from any willing CA, or even
    multiple CAs simultaneously, making it impossible for a client to
    determine whether the certificate it is validating is legitimate
    or fraudulent.
 Because a TLSA certificate association is constrained to its
 associated name, protocol, and port, the PKIX certificate is
 similarly constrained, even if its public CAs signing the certificate
 (if any) are not.

8.1.3. Detection of Key Compromise

 If a key is compromised, rapid and reliable detection is important in
 order to limit the impact of the compromise.  In this regard, neither
 model prevents an attacker from near-invisibly attacking their
 victim, provided that the necessary keys are compromised.
 If a public CA is compromised, only the victim will see the
 fraudulent certificate, as there is typically no publicly accessible
 directory of all the certificates issued by a CA that can be
 inspected.  DNS resource records are typically published publicly.
 However, the attacker could also allow the uncompromised records to
 be published to the Internet as usual but provide a compromised DNS
 view to the victim to achieve the same effect.

8.1.4. Spoofing Hostnames

 Some CAs implement technical controls to ensure that certificates are
 not issued to domains with names similar to domains that are popular
 and prone to attack.  Of course, an attacker can attempt to
 circumvent this restriction by finding a CA willing to issue the
 certificate anyway.  However, by using DNSSEC and TLSA, the attacker
 can circumvent this check completely.

Hoffman & Schlyter Standards Track [Page 20] RFC 6698 DNS-Based Authentication for TLS August 2012

8.2. DNS Caching

 Implementations of this protocol rely heavily on the DNS, and are
 thus prone to security attacks based on the deliberate
 mis-association of TLSA records and DNS names.  Implementations need
 to be cautious in assuming the continuing validity of an association
 between a TLSA record and a DNS name.
 In particular, implementations SHOULD rely on their DNS resolver for
 confirmation of an association between a TLSA record and a DNS name,
 rather than caching the result of previous domain name lookups.  Many
 platforms already can cache domain name lookups locally when
 appropriate, and they SHOULD be configured to do so.  It is proper
 for these lookups to be cached, however, only when the TTL (Time To
 Live) information reported by the DNS makes it likely that the cached
 information will remain useful.
 If implementations cache the results of domain name lookups in order
 to achieve a performance improvement, they MUST observe the TTL
 information reported by DNS.  Implementations that fail to follow
 this rule could be spoofed or have access denied when a previously
 accessed server's TLSA record changes, such as during a certificate
 rollover.

8.3. External DNSSEC Validators

 Due to a lack of DNSSEC support in the most commonly deployed stub
 resolvers today, some ISPs have begun checking DNSSEC in the
 recursive resolvers they provide to their customers, setting the
 Authentic Data (AD) flag as appropriate.  DNSSEC-aware clients could
 use that data, ignoring the fact that the DNSSEC data has been
 validated externally.  Because there is typically no authentication
 of the recursive resolver or integrity protection of the data and AD
 flag between the client and the recursive resolver, this can be
 trivially spoofed by an attacker.
 However, even with secure communications between a host and the
 external validating resolver, there is a risk that the external
 validator could become compromised.  Nothing prevents a compromised
 external DNSSEC validator from claiming that all the records it
 provides are secure, even if the data is falsified, unless the client
 checks the DNSSEC data itself (rendering the external validator
 unnecessary).
 For this reason, DNSSEC validation is best performed on-host, even
 when a secure path to an external validator is available.

Hoffman & Schlyter Standards Track [Page 21] RFC 6698 DNS-Based Authentication for TLS August 2012

9. Acknowledgements

 Many of the ideas in this document have been discussed over many
 years.  More recently, the ideas have been discussed by the authors
 and others in a more focused fashion.  In particular, some of the
 ideas and words here originated with Paul Vixie, Dan Kaminsky, Jeff
 Hodges, Phillip Hallam-Baker, Simon Josefsson, Warren Kumari, Adam
 Langley, Ben Laurie, Ilari Liusvaara, Ondrej Mikle, Scott Schmit,
 Ondrej Sury, Richard Barnes, Jim Schaad, Stephen Farrell, Suresh
 Krishnaswamy, Peter Palfrader, Pieter Lexis, Wouter Wijngaards, John
 Gilmore, and Murray Kucherawy.
 This document has also been greatly helped by many active
 participants of the DANE Working Group.

10. References

10.1. Normative References

 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            STD 13, RFC 1034, November 1987.
 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, November 1987.
 [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.

Hoffman & Schlyter Standards Track [Page 22] RFC 6698 DNS-Based Authentication for TLS August 2012

 [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.

10.2. Informative References

 [RFC0952]  Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
            host table specification", RFC 952, October 1985.
 [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
            specifying the location of services (DNS SRV)", RFC 2782,
            February 2000.
 [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
 [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
            Wellington, "Secret Key Transaction Authentication for DNS
            (TSIG)", RFC 2845, May 2000.
 [RFC2931]  Eastlake 3rd, D., "DNS Request and Transaction Signatures
            ( SIG(0)s)", RFC 2931, September 2000.
 [RFC4025]  Richardson, M., "A Method for Storing IPsec Keying
            Material in DNS", RFC 4025, March 2005.
 [RFC4255]  Schlyter, J. and W. Griffin, "Using DNS to Securely
            Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
            January 2006.
 [RFC4641]  Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
            RFC 4641, September 2006.
 [RFC5074]  Weiler, S., "DNSSEC Lookaside Validation (DLV)", RFC 5074,
            November 2007.
 [RFC5890]  Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Definitions and Document Framework",
            RFC 5890, August 2010.
 [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
            Extensions: Extension Definitions", RFC 6066,
            January 2011.

Hoffman & Schlyter Standards Track [Page 23] RFC 6698 DNS-Based Authentication for TLS August 2012

 [RFC6071]  Frankel, S. and S. Krishnan, "IP Security (IPsec) and
            Internet Key Exchange (IKE) Document Roadmap", RFC 6071,
            February 2011.
 [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
            (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.
 [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
            "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376,
            September 2011.
 [RFC6394]  Barnes, R., "Use Cases and Requirements for DNS-Based
            Authentication of Named Entities (DANE)", RFC 6394,
            October 2011.
 [X.690]    "Recommendation ITU-T X.690 (2002) | ISO/IEC 8825-1:2002,
            Information technology - ASN.1 encoding rules:
            Specification of Basic Encoding Rules (BER), Canonical
            Encoding Rules (CER) and Distinguished Encoding Rules
            (DER)", July 2002.

Hoffman & Schlyter Standards Track [Page 24] RFC 6698 DNS-Based Authentication for TLS August 2012

Appendix A. Operational Considerations for Deploying TLSA Records

A.1. Creating TLSA Records

 When creating TLSA records, care must be taken to avoid
 misconfigurations.  Section 4 of this document states that a TLSA
 RRSet whose validation state is secure MUST be used.  This means that
 the existence of such a RRSet effectively disables other forms of
 name and path validation.  A misconfigured TLSA RRSet will
 effectively disable access to the TLS server for all conforming
 clients, and this document does not provide any means of making a
 gradual transition to using TLSA.
 When creating TLSA records with certificate usage 0 (CA certificate)
 or usage 2 (trust anchor), one needs to understand the implications
 when choosing between selector type 0 (Full certificate) and 1
 (SubjectPublicKeyInfo).  A careful choice is required because
 different methods for building trust chains are used by different TLS
 clients.  The following outlines the cases that one ought to be aware
 of and discusses the implications of the choice of selector type.
 Certificate usage 2 is not affected by the different types of chain
 building when the end entity certificate is the same as the trust
 anchor certificate.

A.1.1. Ambiguities and Corner Cases When TLS Clients Build Trust Chains

 TLS clients can implement their own chain-building code rather than
 rely on the chain presented by the TLS server.  This means that,
 except for the end entity certificate, any certificate presented in
 the suggested chain might or might not be present in the final chain
 built by the client.
 Certificates that the client can use to replace certificates from the
 original chain include:
 o  Client's trust anchors
 o  Certificates cached locally
 o  Certificates retrieved from a URI listed in an Authority
    Information Access X.509v3 extension
 CAs frequently reissue certificates with different validity periods,
 signature algorithms (such as a different hash algorithm in the
 signature algorithm), CA key pairs (such as for a cross-certificate),

Hoffman & Schlyter Standards Track [Page 25] RFC 6698 DNS-Based Authentication for TLS August 2012

 or PKIX extensions where the public key and subject remain the same.
 These reissued certificates are the certificates that the TLS client
 can use in place of an original certificate.
 Clients are known to exchange or remove certificates that could cause
 TLSA certificate associations that rely on the full certificate to
 fail.  For example:
 o  The client considers the signature algorithm of a certificate to
    no longer be sufficiently secure.
 o  The client might not have an associated root certificate in its
    trust store and instead uses a cross-certificate with an identical
    subject and public key.

A.1.2. Choosing a Selector Type

 In this section, "false-negative failure" means that a client will
 not accept the TLSA certificate association for a certificate
 designated by the DNS administrator.  Also, "false-positive
 acceptance" means that the client accepts a TLSA association for a
 certificate that is not designated by the DNS administrator.

A.1.2.1. Selector Type 0 (Full Certificate)

 The "Full certificate" selector provides the most precise
 specification of a TLSA certificate association, capturing all
 fields of the PKIX certificate.  For a DNS administrator, the best
 course to avoid false-negative failures in the client when using this
 selector is:
 1.  If a CA issued a replacement certificate, don't associate to CA
     certificates that have a signature algorithm with a hash that is
     considered weak by local policy.
 2.  Determine how common client applications process the TLSA
     certificate association using a fresh client installation -- that
     is, with the local certificate cache empty.

Hoffman & Schlyter Standards Track [Page 26] RFC 6698 DNS-Based Authentication for TLS August 2012

A.1.2.2. Selector Type 1 (SubjectPublicKeyInfo)

 A SubjectPublicKeyInfo selector gives greater flexibility in avoiding
 some false-negative failures caused by trust-chain-building
 algorithms used in clients.
 One specific use case ought to be noted: creating a TLSA certificate
 association to CA certificate I1 that directly signed end entity
 certificate S1 of the server.  The case can be illustrated by the
 following graph:
         +----+                      +----+
         | I1 |                      | I2 |
         +----+                      +----+
            |                           |
            v                           v
         +----+                      +----+
         | S1 |                      | S1 |
         +----+                      +----+
 Certificate chain sent by    A different validation path
 server in TLS handshake      built by the TLS client
 I2 is a reissued version of CA certificate I1 (that is, it has a
 different hash in its signature algorithm).
 In the above scenario, both certificates I1 and I2 that sign S1 need
 to have identical SubjectPublicKeyInfo fields because the key used to
 sign S1 is fixed.  An association to SubjectPublicKeyInfo (selector
 type 1) will always succeed in such a case, but an association with a
 full certificate (selector type 0) might not work due to a false-
 negative failure.
 The attack surface is a bit broader compared to the "Full
 certificate" selector: the DNS administrator might unintentionally
 specify an association that would lead to false-positive acceptance.
 o  The administrator must know or trust that the CA does not engage
    in bad practices, such as not sharing the key of I1 for unrelated
    CA certificates (which would lead to trust-chain redirection).  If
    possible, the administrator ought to review all CA certificates
    that have the same SubjectPublicKeyInfo field.
 o  The administrator ought to understand whether some PKIX extension
    may adversely affect security of the association.  If possible,
    administrators ought to review all CA certificates that share the
    SubjectPublicKeyInfo.

Hoffman & Schlyter Standards Track [Page 27] RFC 6698 DNS-Based Authentication for TLS August 2012

 o  The administrator ought to understand that any CA could, in the
    future, issue a certificate that contains the same
    SubjectPublicKeyInfo.  Therefore, new chains can crop up in the
    future without any warning.
 Using the SubjectPublicKeyInfo selector for association with a
 certificate in a chain above I1 needs to be decided on a case-by-case
 basis: there are too many possibilities based on the issuing CA's
 practices.  Unless the full implications of such an association are
 understood by the administrator, using selector type 0 is a better
 option from a security perspective.

A.2. Provisioning TLSA Records in DNS

A.2.1. Provisioning TLSA Records with Aliases

 The TLSA resource record is not special in the DNS; it acts exactly
 like any other RRtype where the queried name has one or more labels
 prefixed to the base name, such as the SRV RRtype [RFC2782].  This
 affects the way that the TLSA resource record is used when aliasing
 in the DNS.
 Note that the IETF sometimes adds new types of aliasing in the DNS.
 If that happens in the future, those aliases might affect TLSA
 records, hopefully in a good way.

A.2.1.1. Provisioning TLSA Records with CNAME Records

 Using CNAME to alias in DNS only aliases from the exact name given,
 not any zones below the given name.  For example, assume that a zone
 file has only the following:
 sub1.example.com.          IN CNAME sub2.example.com.
 In this case, a request for the A record at "bottom.sub1.example.com"
 would not return any records because the CNAME given only aliases the
 name given.  Assume, instead, the zone file has the following:
 sub3.example.com.          IN CNAME sub4.example.com.
 bottom.sub3.example.com.   IN CNAME bottom.sub4.example.com.
 In this case, a request for the A record at bottom.sub3.example.com
 would in fact return whatever value for the A record exists at
 bottom.sub4.example.com.

Hoffman & Schlyter Standards Track [Page 28] RFC 6698 DNS-Based Authentication for TLS August 2012

 Application implementations and full-service resolvers request DNS
 records using libraries that automatically follow CNAME (and DNAME)
 aliasing.  This allows hosts to put TLSA records in their own zones
 or to use CNAME to do redirection.
 If the owner of the original domain wants a TLSA record for the same,
 they simply enter it under the defined prefix:
 ; No TLSA record in target domain
 ;
 sub5.example.com.            IN CNAME sub6.example.com.
 _443._tcp.sub5.example.com.  IN TLSA 1 1 1 308202c5308201ab...
 sub6.example.com.            IN A 192.0.2.1
 sub6.example.com.            IN AAAA 2001:db8::1
 If the owner of the original domain wants to have the target domain
 host the TLSA record, the original domain uses a CNAME record:
 ; TLSA record for original domain has CNAME to target domain
 ;
 sub5.example.com.            IN CNAME sub6.example.com.
 _443._tcp.sub5.example.com.  IN CNAME _443._tcp.sub6.example.com.
 sub6.example.com.            IN A 192.0.2.1
 sub6.example.com.            IN AAAA 2001:db8::1
 _443._tcp.sub6.example.com.  IN TLSA 1 1 1 536a570ac49d9ba4...
 Note that it is acceptable for both the original domain and the
 target domain to have TLSA records, but the two records are
 unrelated.  Consider the following:
 ; TLSA record in both the original and target domain
 ;
 sub5.example.com.            IN CNAME sub6.example.com.
 _443._tcp.sub5.example.com.  IN TLSA 1 1 1 308202c5308201ab...
 sub6.example.com.            IN A 192.0.2.1
 sub6.example.com.            IN AAAA 2001:db8::1
 _443._tcp.sub6.example.com.  IN TLSA 1 1 1 ac49d9ba4570ac49...
 In this example, someone looking for the TLSA record for
 sub5.example.com would always get the record whose value starts with
 "308202c5308201ab"; the TLSA record whose value starts with
 "ac49d9ba4570ac49" would only be sought by someone who is looking for
 the TLSA record for sub6.example.com, and never for sub5.example.com.
 Note that deploying different certificates for multiple services
 located at a shared TLS listener often requires the use of TLS SNI
 (Server Name Indication) [RFC6066].

Hoffman & Schlyter Standards Track [Page 29] RFC 6698 DNS-Based Authentication for TLS August 2012

 Note that these methods use the normal method for DNS aliasing using
 CNAME: the DNS client requests the record type that they actually
 want.

A.2.1.2. Provisioning TLSA Records with DNAME Records

 Using DNAME records allows a zone owner to alias an entire subtree of
 names below the name that has the DNAME.  This allows the wholesale
 aliasing of prefixed records such as those used by TLSA, SRV, and so
 on without aliasing the name itself.  However, because DNAME can only
 be used for subtrees of a base name, it is rarely used to alias
 individual hosts that might also be running TLS.
 ; TLSA record in target domain, visible in original domain via DNAME
 ;
 sub5.example.com.            IN CNAME sub6.example.com.
 _tcp.sub5.example.com.       IN DNAME _tcp.sub6.example.com.
 sub6.example.com.            IN A 192.0.2.1
 sub6.example.com.            IN AAAA 2001:db8::1
 _443._tcp.sub6.example.com.  IN TLSA 1 1 1 536a570ac49d9ba4...

A.2.1.3. Provisioning TLSA Records with Wildcards

 Wildcards are generally not terribly useful for RRtypes that require
 prefixing because one can only wildcard at a layer below the host
 name.  For example, if one wants to have the same TLSA record for
 every TCP port for www.example.com, the result might be:
 This is possibly useful in some scenarios where the same service is
 offered on many ports or the same certificate and/or key is used for
 all services on a host.  Note that the domain being searched for is
 not necessarily related to the domain name found in the certificate,
 so a certificate with a wildcard in it is not searched for using a
 wildcard in the search request.

A.3. Securing the Last Hop

 As described in Section 4, an application processing TLSA records
 must know the DNSSEC validity of those records.  There are many ways
 for the application to determine this securely, and this
 specification does not mandate any single method.

Hoffman & Schlyter Standards Track [Page 30] RFC 6698 DNS-Based Authentication for TLS August 2012

 Some common methods for an application to know the DNSSEC validity of
 TLSA records include:
 o  The application can have its own DNS resolver and DNSSEC
    validation stack.
 o  The application can communicate through a trusted channel (such as
    requests to the operating system under which the application is
    running) to a local DNS resolver that does DNSSEC validation.
 o  The application can communicate through a secured channel (such as
    requests running over TLS, IPsec, TSIG, or SIG(0)) to a non-local
    DNS resolver that does DNSSEC validation.
 o  The application can communicate through a secured channel (such as
    requests running over TLS, IPsec, TSIG, or SIG(0)) to a non-local
    DNS resolver that does not do DNSSEC validation, but gets
    responses through a secured channel from a different DNS resolver
    that does DNSSEC validation.

A.4. Handling Certificate Rollover

 Certificate rollover is handled in much the same way as for rolling
 DNSSEC zone signing keys using the pre-publish key rollover method
 [RFC4641].  Suppose example.com has a single TLSA record for a TLS
 service on TCP port 990:
 _990._tcp.example.com IN TLSA 1 1 1 1CFC98A706BCF3683015...
 To start the rollover process, obtain or generate the new certificate
 or SubjectPublicKeyInfo to be used after the rollover and generate
 the new TLSA record.  Add that record alongside the old one:
 _990._tcp.example.com IN TLSA 1 1 1 1CFC98A706BCF3683015...
 _990._tcp.example.com IN TLSA 1 1 1 62D5414CD1CC657E3D30...
 After the new records have propagated to the authoritative
 nameservers and the TTL of the old record has expired, switch to the
 new certificate on the TLS server.  Once this has occurred, the old
 TLSA record can be removed:
 _990._tcp.example.com IN TLSA 1 1 1 62D5414CD1CC657E3D30...
 This completes the certificate rollover.

Hoffman & Schlyter Standards Track [Page 31] RFC 6698 DNS-Based Authentication for TLS August 2012

Appendix B. Pseudocode for Using TLSA

 This appendix describes, in pseudocode format, the interactions given
 earlier in this specification.  If the steps below disagree with the
 text earlier in the document, the steps earlier in the document ought
 to be considered correct and this text incorrect.
 Note that this pseudocode is more strict than the normative text.
 For instance, it forces an order on the evaluation of criteria, which
 is not mandatory from the normative text.

B.1. Helper Functions

 // implement the function for exiting
 function Finish (F) = {
   if (F == ABORT_TLS) {
     abort the TLS handshake or prevent TLS from starting
     exit
   }
   if (F == NO_TLSA) {
     fall back to non-TLSA certificate validation
     exit
   }
   if (F == ACCEPT) {
     accept the TLS connection
     exit
   }
   // unreachable
 }
 // implement the selector function
 function Select (S, X) = {
   // Full certificate
   if (S == 0) {
     return X in DER encoding
   }
   // SubjectPublicKeyInfo
   if (S == 1) {
     return X.SubjectPublicKeyInfo in DER encoding
   }
   // unreachable
 }

Hoffman & Schlyter Standards Track [Page 32] RFC 6698 DNS-Based Authentication for TLS August 2012

 // implement the matching function
 function Match (M, X, Y) {
   // Exact match on selected content
   if (M == 0) {
     return (X == Y)
   }
   // SHA-256 hash of selected content
   if (M == 1) {
     return (SHA-256(X) == Y)
   }
   // SHA-512 hash of selected content
   if (M == 2) {
     return (SHA-512(X) == Y)
   }
   // unreachable
 }

B.2. Main TLSA Pseudocode

 TLS connect using [transport] to [name] on [port] and receiving end
 entity cert C for the TLS server:
 (TLSArecords, ValState) = DNSSECValidatedLookup(
   domainname=_[port]._[transport].[name], RRtype=TLSA)
 // check for states that would change processing
 if (ValState == BOGUS) {
   Finish(ABORT_TLS)
 }
 if ((ValState == INDETERMINATE) or (ValState == INSECURE)) {
   Finish(NO_TLSA)
 }
 // if here, ValState must be SECURE
 for each R in TLSArecords {
   // unusable records include unknown certUsage, unknown
   // selectorType, unknown matchingType, erroneous RDATA, and
   // prohibited by local policy
   if (R is unusable) {
     remove R from TLSArecords
   }
 }
 if (length(TLSArecords) == 0) {
   Finish(NO_TLSA)
 }

Hoffman & Schlyter Standards Track [Page 33] RFC 6698 DNS-Based Authentication for TLS August 2012

 // A TLS client might have multiple trust anchors that it might use
 //    when validating the TLS server's end entity (EE) certificate.
 //    Also, there can be multiple PKIX certification paths for the
 //    certificates given by the server in TLS.  Thus, there are
 //    possibly many chains that might need to be tested during
 //    PKIX path validation.
 for each R in TLSArecords {
   // pass PKIX certificate validation and chain through a CA cert
   //    that comes from TLSA
   if (R.certUsage == 0) {
     for each PKIX certification path H {
       if (C passes PKIX certification path validation in H) {
         for each D in H {
           if ((D is a CA certificate) and
               Match(R.matchingType, Select(R.selectorType, D),
                     R.cert)) {
             Finish(ACCEPT)
           }
         }
       }
     }
   }
   // pass PKIX certificate validation and match EE cert from TLSA
   if (R.certUsage == 1) {
     for each PKIX certification path H {
       if ((C passes PKIX certificate validation in H) and
               Match(R.matchingType, Select(R.selectorType, C),
               R.cert)) {
           Finish(ACCEPT)
       }
     }
   }
   // pass PKIX certification validation using TLSA record as the
   //    trust anchor
   if (R.certUsage == 2) {
     // the following assert() is merely a formalization of the
     // "trust anchor" condition for a certificate D matching R
     assert(Match(R.matchingType, Select(R.selectorType, D), R.cert))

Hoffman & Schlyter Standards Track [Page 34] RFC 6698 DNS-Based Authentication for TLS August 2012

     for each PKIX certification path H that has certificate D
         matching R as the trust anchor {
       if (C passes PKIX validation in H) {
         Finish(ACCEPT);
       }
     }
   }
   // match the TLSA record and the TLS certificate
   if (R.certUsage == 3) {
     if Match(R.matchingType, Select(R.selectorType, C), R.cert)
       Finish(ACCEPT)
     }
   }
 }
 // if here, then none of the TLSA records ended in "Finish(ACCEPT)"
 //   so abort TLS
 Finish(ABORT_TLS)

Appendix C. Examples

 The following are examples of self-signed certificates that have been
 generated with various selectors and matching types.  They were
 generated with one piece of software, and validated by an individual
 using other tools.
 S = Selector
 M = Matching Type
 S M Association Data
 0 0 30820454308202BC020900AB58D24E77AD2AF6300D06092A86
     4886F70D0101050500306C310B3009060355040613024E4C31163014
     0603550408130D4E6F6F72642D486F6C6C616E643112301006035504
     071309416D7374657264616D310C300A060355040A13034F53333123
     30210603550403131A64616E652E6B6965762E70726163746963756D
     2E6F73332E6E6C301E170D3132303131363136353730335A170D3232
     303131333136353730335A306C310B3009060355040613024E4C3116
     30140603550408130D4E6F6F72642D486F6C6C616E64311230100603
     5504071309416D7374657264616D310C300A060355040A13034F5333
     312330210603550403131A64616E652E6B6965762E70726163746963
     756D2E6F73332E6E6C308201A2300D06092A864886F70D0101010500
     0382018F003082018A0282018100E62C84A5AFE59F0A2A6B250DEE68
     7AC8C5C604F57D26CEB2119140FFAC38C4B9CBBE8923082E7F81626B
     6AD5DEA0C8771C74E3CAA7F613054AEFA3673E48FFE47B3F7AF987DE
     281A68230B24B9DA1A98DCBE51195B60E42FD7517C328D983E26A827
     C877AB914EE4C1BFDEAD48BD25BE5F2C473BA9C1CBBDDDA0C374D0D5

Hoffman & Schlyter Standards Track [Page 35] RFC 6698 DNS-Based Authentication for TLS August 2012

     8C389CC3D6D8C20662E19CF768F32441B7F7D14AEA8966CE7C32A172
     2AB38623D008029A9E4702883F8B977A1A1E5292BF8AD72239D40393
     37B86A3AC60FA001290452177BF1798609A05A130F033457A5212629
     FBDDB8E70E2A9E6556873C4F7CA46AE4A8B178F05FB319005E1C1C7D
     4BD77DFA34035563C126AA2C3328B900E7990AC9787F01DA82F74C3D
     4B6674CCECE1FD4C6EF9E6644F4635EDEDA39D8B0E2F7C8E06DAE775
     6213BD3D60831175BE290442B4AFC5AE6F46B769855A067C1097E617
     962529E166F22AEE10DDB981B8CD6FF17D3D70723169038DBFBC1A44
     9C8D0D31BC683C5F3CE26148E42EC9BBD4D9F261569B25B53C1D7FC2
     DDFF6B4CAC050203010001300D06092A864886F70D01010505000382
     0181002B2ABE063E9C86AC4A1F7835372091079C8276A9C2C5D1EC57
     64DE523FDDABDEAB3FD34E6FE6CBA054580A6785A663595D90132B93
     D473929E81FA0887D2FFF78A81C7D014B97778AB6AC9E5E690F6F5A9
     E92BB5FBAB71B857AE69B6E18BDCCB0BA6FCD9D4B084A34F3635148C
     495D48FE635903B888EC1DEB2610548EDD48D63F86513A4562469831
     48C0D5DB82D73A4C350A42BB661D763430FC6C8E5F9D13EA1B76AA52
     A4C358E5EA04000F794618303AB6CEEA4E9A8E9C74D73C1B0B7BAF16
     DEDE7696B5E2F206F777100F5727E1684D4132F5E692F47AF6756EA8
     B421000BE031B5D8F0220E436B51FB154FE9595333C13A2403F9DE08
     E5DDC5A22FD6182E339593E26374450220BC14F3E40FF33F084526B0
     9C34250702E8A352B332CCCB0F9DE2CF2B338823B92AFC61C0B6B8AB
     DB5AF718ED8DDA97C298E46B82A01B14814868CFA4F2C36268BFFF4A
     591F42658BF75918902D3E426DFE1D5FF0FC6A212071F6DA8BD833FE
     2E560D87775E8EE9333C05B6FB8EB56589D910DB5EA903
 0 1 EFDDF0D915C7BDC5782C0881E1B2A95AD099FBDD06D7B1F779
     82D9364338D955
 0 2 81EE7F6C0ECC6B09B7785A9418F54432DE630DD54DC6EE9E3C
     49DE547708D236D4C413C3E97E44F969E635958AA410495844127C04
     883503E5B024CF7A8F6A94
 1 0 308201A2300D06092A864886F70D01010105000382018F0030
     82018A0282018100E62C84A5AFE59F0A2A6B250DEE687AC8C5C604F5
     7D26CEB2119140FFAC38C4B9CBBE8923082E7F81626B6AD5DEA0C877
     1C74E3CAA7F613054AEFA3673E48FFE47B3F7AF987DE281A68230B24
     B9DA1A98DCBE51195B60E42FD7517C328D983E26A827C877AB914EE4
     C1BFDEAD48BD25BE5F2C473BA9C1CBBDDDA0C374D0D58C389CC3D6D8
     C20662E19CF768F32441B7F7D14AEA8966CE7C32A1722AB38623D008
     029A9E4702883F8B977A1A1E5292BF8AD72239D4039337B86A3AC60F
     A001290452177BF1798609A05A130F033457A5212629FBDDB8E70E2A
     9E6556873C4F7CA46AE4A8B178F05FB319005E1C1C7D4BD77DFA3403
     5563C126AA2C3328B900E7990AC9787F01DA82F74C3D4B6674CCECE1
     FD4C6EF9E6644F4635EDEDA39D8B0E2F7C8E06DAE7756213BD3D6083
     1175BE290442B4AFC5AE6F46B769855A067C1097E617962529E166F2
     2AEE10DDB981B8CD6FF17D3D70723169038DBFBC1A449C8D0D31BC68
     3C5F3CE26148E42EC9BBD4D9F261569B25B53C1D7FC2DDFF6B4CAC05
     0203010001

Hoffman & Schlyter Standards Track [Page 36] RFC 6698 DNS-Based Authentication for TLS August 2012

 1 1 8755CDAA8FE24EF16CC0F2C918063185E433FAAF1415664911
     D9E30A924138C4
 1 2 D43165B4CDF8F8660AECCCC5344D9D9AE45FFD7E6AAB7AB9EE
     C169B58E11F227ED90C17330CC17B5CCEF0390066008C720CEC6AAE5
     33A934B3A2D7E232C94AB4

Authors' Addresses

 Paul Hoffman
 VPN Consortium
 EMail: paul.hoffman@vpnc.org
 Jakob Schlyter
 Kirei AB
 EMail: jakob@kirei.se

Hoffman & Schlyter Standards Track [Page 37]

/data/webs/external/dokuwiki/data/pages/rfc/rfc6698.txt · Last modified: 2012/08/07 05:32 by 127.0.0.1

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