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Versions: (draft-hallambaker-donotissue) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 6844

Internet Engineering Task Force                          P. Hallam-Baker
Internet-Draft                                         Comodo Group Inc.
Intended status: Standards Track                            R. Stradling
Expires: February 24, 2013                                Comodo CA Ltd.
                                                         August 23, 2012


    DNS Certification Authority Authorization (CAA) Resource Record
                         draft-ietf-pkix-caa-12

Abstract

   The Certification Authority Authorization (CAA) DNS Resource Record
   allows a DNS domain name holder to specify one or more Certification
   Authorities (CAs) authorized to issue certificates for that domain.
   CAA resource records allow a public Certification Authority to
   implement additional controls to reduce the risk of unintended
   certificate mis-issue.  This document defines the syntax of the CAA
   record and rules for processing CAA records by certificate issuers.

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 February 24, 2013.

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



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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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
     2.2.  Defined Terms  . . . . . . . . . . . . . . . . . . . . . .  4
   3.  The CAA RR type  . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Certification Authority Processing . . . . . . . . . . . . . .  7
     4.1.  Use of DNS Security  . . . . . . . . . . . . . . . . . . .  8
   5.  Mechanism  . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  8
       5.1.1.  Canonical Presentation Format  . . . . . . . . . . . . 10
     5.2.  CAA issue Property . . . . . . . . . . . . . . . . . . . . 10
     5.3.  CAA issuewild Property . . . . . . . . . . . . . . . . . . 12
     5.4.  CAA iodef Property . . . . . . . . . . . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
     6.1.  Non-Compliance by Certification Authority  . . . . . . . . 13
     6.2.  Mis-Issue by Authorized Certification Authority  . . . . . 13
     6.3.  Suppression or spoofing of CAA records . . . . . . . . . . 13
     6.4.  Denial of Service  . . . . . . . . . . . . . . . . . . . . 14
     6.5.  Abuse of the Critical Flag . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Registration of the CAA Resource Record Type . . . . . . . 14
     7.2.  Certification Authority Authorization Properties . . . . . 15
     7.3.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . 15
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     8.2.  Informative  References  . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17

















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1.  Introduction

   The Certification Authority Authorization (CAA) DNS Resource Record
   allows a DNS domain name holder to specify the Certification
   Authorities authorized to issue certificates for that domain.
   Publication of CAA resource records allow a public Certification
   Authority (CA) to implement additional controls to reduce the risk of
   unintended certificate mis-issue.

   Like the TLSA record defined in DNS-Based Authentication of Named
   Entities (DANE) [DANE], CAA records are used as a part of a mechanism
   for checking PKIX certificate data.  The distinction between the two
   specifications is that CAA records specify a authorization control to
   be performed by a certificate issuer before issue of a certificate
   and TLSA records specify a verification control to be performed by a
   Relying Party after the certificate is issued.

   Conformance with a published CAA record is a necessary but not
   sufficient condition for issueance of a certificate.  Before issuing
   a certificate, a PKIX CA is required to validate the request
   according to the policies set out in its Certificate Policy.  In the
   case of a public CA that validates certificate requests as a third
   party, the certificate will be typically issued under a public trust
   anchor certificate embedded in one or more relevant Relying
   Applications.

   Criteria for inclusion of embedded trust anchor certificates in
   applications are outside the scope of this document.  Typically such
   criteria require the CA to publish a Certificate Practices Statement
   (CPS) that specifies how the requirements of the Certificate Policy
   (CP) are achieved.  It is also common for a CA to engage an
   independent third party auditor to prepare an annual audit statement
   of its performance against its CPS.

   A set of CAA records describes only current grants of authority to
   issue certificates for the corresponding DNS domain.  Since a
   certificate is typically valid for at least a year, it is possible
   that a certificate that is not conformant with the CAA records
   currently published was conformant with the CAA records published at
   the time that the certificate was issued.  Relying Applications MUST
   NOT use CAA records as part of certificate validation.

   CAA Records MAY be used by Certificate Evaluators as a possible
   indicator of a security policy violation.  Such use SHOULD take
   account of the possibility that published CAA records changed between
   the time a certificate was issued and the time at which the
   certificate was observed by the Certificate Evaluator.




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2.  Definitions

2.1.  Requirements Language

   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].

2.2.  Defined Terms

   The following terms are used in this document:

   Authorization Entry:  An authorization assertion that grants or
      denies a specific set of permissions to a specific group of
      entities.

   Certificate:  An X.509 Certificate, as specified in [RFC5280].

   Certificate Evaluator:  A party other than a Relying Party that
      evaluates the trustworthiness of certificates issued by
      Certification Authorities.

   Certification Authority (CA):  An Issuer that issues Certificates in
      accordance with a specified Certificate Policy.

   Certificate Policy (CP):  Specifies the criteria that a Certification
      Authority undertakes to meet in its issue of certificates.  See
      [RFC3647].

   Certification Practices Statement (CPS):  Specifies the means by
      which the criteria of the Certificate Policy are met.  In most
      cases this will be the document against which the operations of
      the Certification Authority are audited.  See [RFC3647].

   Domain:  The set of resources associated with a DNS Domain Name.

   Domain Name:  A DNS Domain name as specified in [STD13].

   Domain Name System (DNS):  The Internet naming system specified in
      [STD13].

   DNS Security (DNSSEC):  Extensions to the DNS that provide
      authentication services as specified in [RFC4033], [RFC4034],
      [RFC4035], [RFC5155] and revisions.







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   Issuer:  An entity that issues Certificates.  See [RFC5280].

   Property:  The tag-value portion of a CAA Resource Record.

   Property Tag:  The tag portion of a CAA Resource Record.

   Property Value:  The value portion of a CAA Resource Record.

   Public Key Infrastructure X.509 (PKIX):  Standards and specifications
      issued by the IETF that apply the [X.509] certificate standards
      specified by the ITU to Internet applications as specified in
      [RFC5280] and related documents.

   Resource Record (RR):  A set of attributes bound to a Domain Name as
      defined in [RFC1035].

   Relying Party:  A party that makes use of an application whose
      operation depends on use of a Certificate for making a security
      decision.  See [RFC5280].

   Relying Application:  An application whose operation depends on use
      of a Certificate for making a security decision.


3.  The CAA RR type

   A CAA RR consists of a flags byte and a tag-value pair referred to as
   a property.  Multiple properties MAY be associated with the same
   domain name by publishing multiple CAA RRs at that domain name.  The
   following flag is defined:

   Issuer Critical:  If set (=1), indicates that the corresponding
      property entry tag MUST be understood if the semantics of the CAA
      record are to be correctly interpreted by an issuer.

      Issuers MUST NOT issue certificates for a domain if the relevant
      CAA Resource Record set contains unknown property entry tags that
      have the Critical bit set.

   The following property tags are defined:

   issue <Issuer Domain Name> [; <tag=value> ]* :  The issue property
      entry authorizes the holder of the domain name <Issuer Domain
      Name> or a party acting under the explicit authority of the holder
      of that domain name to issue certificates for the domain in which
      the property is published.





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   iodef <URL> :  Specifies a URL to which an issuer MAY report
      certificate issue requests that are inconsistent with the issuer's
      Certification Practices or Certificate Policy, or that a
      certificate evaluator may use to report observation of a possible
      policy violation.  The IODEF format is used [RFC5070].

   The following example is a DNS zone file (see [RFC1035]) that informs
   CAs that certificates are not to be issued except by the holder of
   the domain name 'ca.example.net' or an authorized agent thereof.
   This policy applies to all subordinate domains under example.com.

   $ORIGIN example.com
   .       CAA 0 issue "ca.example.net"

   If the domain name holder specifies one or more iodef properties, a
   certificate issuer MAY report invalid certificate requests to that
   address.  In the following example the domain name holder specifies
   that reports may be made by means of email with the IODEF data as an
   attachment, a Web service [RFC6546] or both:

   $ORIGIN example.com
   .       CAA 0 issue "ca.example.net"
   .       CAA 0 iodef "mailto:security@example.com"
   .       CAA 0 iodef "http://iodef.example.com/"

   A certificate issuer MAY specify additional parameters that allow
   customers to specify additional parameters governing certificate
   issuance.  This might be the Certificate Policy under which the
   certificate is to be issued, the authentication process to be used
   might be specified or an account number specified by the CA to enable
   these parameters to be retrieved.

   For example, the CA 'ca.example.net' has requested its customer
   'example.com' to specify the CA's account number '230123' in each of
   the customer's CAA records.

   $ORIGIN example.com
   .       CAA 0 issue "ca.example.net; account=230123"

   The syntax of additional parameters is a sequence of tag-value pairs
   as defined in section [RFC5234].  The semantics of such parameters is
   left to site policy and is outside the scope of this document.

   The critical flag is intended to permit future versions CAA to
   introduce new semantics that MUST be understood for correct
   processing of the record, preventing conforming CAs that do not
   recognize the new semantics from issuing certificates for the
   indicated domains.



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   In the following example, the property 'tbs' is flagged as critical.
   Neither the example.net CA, nor any other issuer is authorized to
   issue under either policy unless the processing rules for the 'tbs'
   property tag are understood.

   $ORIGIN example.com
   .       CAA 0 issue "ca.example.net; policy=ev"
   .       CAA 128 tbs "Unknown"

   Note that the above restrictions only apply to issue of certificates.
   Since the validity of an end entity certificate is typically a year
   or more, it is quite possible that the CAA records published at a
   domain will change between the time a certificate was issued and
   validation by a relying party.


4.  Certification Authority Processing

   Before issuing a certificate, a compliant CA MUST check for
   publication of an relevant CAA Resource Record set.  If such a record
   set exists, a CA MUST NOT issue a certificate unless the CA
   determines that either (1) the certificate request is consistent with
   the applicable CAA Resource Record set or (2) that an exception
   specified in the relevant Certificate Policy or Certification
   Practices Statement Applies.

   A certificate request MAY specify more than one domain name and MAY
   specify wildcard domains.  Issuers MUST verify authorization for all
   the domains and wildcard domains specified in the request.

   Given a request for a specific domain X, or a request for a wildcard
   domain *.X, the relevant record set R(X) is determined as follows:

   Let CAA(X) be the record set returned in response to performing a CAA
   record query on the label X, P(X) be the DNS label immediately above
   X in the DNS hierarchy and A(X) be the target of a CNAME or DNAME
   alias record specified at the label X.

   o  If CAA(X) is not empty, R(X) = CAA (X), otherwise

   o  If A(X) is not null, and R(A(X)) is not empty, then R(X) =
      R(A(X)), otherwise

   o  If X is not a top level domain then R(X) = R(P(X), otherwise

   o  R(X) is empty.

   For example, if a certificate is requested for X.Y.Z the issuer will



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   search for the relevant CAA record set in the following order:

      X.Y.Z

      Alias (X.Y.Z)

      Y.Z

      Alias (Y.Z)

      Z

      Alias (Z)

      Return Empty

4.1.  Use of DNS Security

   Use of DNSSEC to authenticate CAA RRs is strongly RECOMMENDED but not
   required.  An issuer MUST NOT issue certificates if doing so would
   conflict with the relevant CAA Resource Record set, irrespective of
   whether the corresponding DNS records are signed.

   DNSSEC provides a proof of non-existence for both DNS domains and RR
   set within domains.  DNSSEC verification thus enables an issuer to
   determine if the answer to a CAA record query is empty because the RR
   set is empty or if it is non-empty but the response has been
   supressed.

   Use of DNSSEC allows an issuer to acquire and archive a proof that
   they were authorized to issue certificates for the domain.
   Verification of such archives MAY be an audit requirement to verify
   CAA record processing compliance.  Publication of such archives MAY
   be a transparency requirement to verify CAA record processing
   compliance.


5.  Mechanism

5.1.  Syntax

   A CAA RR contains a single property entry consisting of a tag value
   pair.  Each tag represents a property of the CAA record.  The value
   of a CAA property is that specified in the corresponding value field.

   A domain name MAY have multiple CAA RRs associated with it and a
   given property MAY be specified more than once.




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   The CAA data field contains one property entry.  A property entry
   consists of the following data fields:

   +0-1-2-3-4-5-6-7-|0-1-2-3-4-5-6-7-|
   | Flags          | Tag Length = n |
   +----------------+----------------+...+---------------+
   | Tag char 0     | Tag Char 1     |...| Tag Char n-1  |
   +----------------+----------------+...+---------------+
   +----------------+----------------+.....+----------------+
   | Value byte 0   | Value byte 1   |.....| Value byte m-1 |
   +----------------+----------------+.....+----------------+

   Where n is the length specified in the Tag length field and m is the
   remaining octets in the Value field (m = d - n - 2) where d is the
   length of the RDATA section.

   The data fields are defined as follows:

   Flags:  One octet containing the following fields:

      Bit 0: Issuer Critical Flag  If the value is set (1), the critical
         flag is asserted and the property MUST be understood if the CAA
         record is to be correctly processed by a certificate issuer.

         A Certification Authority MUST NOT issue certificates for any
         Domain that contains a CAA critical property for an unknown or
         unsupported property tag that for which the issuer critical
         flag is set.

      Note that according to the conventions set out in [RFC1035], Bit 0
      is the Most Significant Bit and Bit 7 is the Least Significant
      Bit. Thus the Flags value 1 means that bit 7 is set while a value
      of 128 means that bit 0 is set according to this convention.

      All other bit positions are reserved for future use.

      To ensure compatibility with future extensions to CAA, DNS records
      compliant with this version of the CAA specification MUST clear
      (set to "0") all reserved flags bits.  Applications that interpret
      CAA records MUST ignore the value of all reserved flag bits.

   Tag Length:  A single octet containing an unsigned integer specifying
      the tag length in octets.  The tag length MUST be at least 1 and
      SHOULD be no more than 15.







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   Tag:  The property identifier, a sequence of ASCII characters.

      Tag values MAY contain ASCII characters 'a' through 'z', 'A'
      through 'Z' and the numbers 0 through 9.  Tag values SHOULD NOT
      contain any other characters.  Matching of tag values is case
      insensitive.

      Tag values submitted for registration by IANA MUST NOT contain any
      characters other than the (lowercase) ASCII characters 'a' through
      'z' and the numbers 0 through 9.

   Value:  A sequence of octets representing the property value.
      Property values are encoded as binary values and MAY employ sub-
      formats.

      The length of the value field is specified implicitly as the
      remaining length of the enclosing Resource Record data field.

5.1.1.  Canonical Presentation Format

   The canonical presentation format of the CAA record is as follows:

   CAA <flags> <tag> <value>

   Where:

   Flags:  Is an unsigned integer between 0 and 255.

   Tag:  Is a non-zero sequence of ASCII letter and numbers in lower
      case.

   Value:  Is the US-ASCII text Encoding of the value field

5.2.  CAA issue Property

   The issue property tag is used to request that certificate issuers
   perform CAA issue restriction processing for the domain and to grant
   authorization to specific certificate issuers.

   The CAA issue property value has the following sub-syntax (specified
   in ABNF as per [RFC5234]).










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   Property = space [domain] space [";" space * (parameter space)

   domain = label *("." label)
   label = (ALPHA / DIGIT) * ( ["-"] (ALPHA / DIGIT ))

   space = *(SP / HTAB)

   parameter =  / space tag "=" value

   tag = 1* (ALPHA / DIGIT)

   value = *VCHAR | DQUOTE *(%x20-21 / %x23-7E) DQUOTE


   For consistency with other aspects of DNS administration, Domain name
   values are specified in Letter-Digit-Hyphen Label (LDH-Label) form.

   A CAA record with an issue parameter tag that does not specify a
   domain name is a request that certificate issuers perform CAA issue
   restriction processing for the corresponding domain without granting
   authorization to any certificate issuer.

   This form of issue restriction would be appropriate to specify that
   no certificates are to be issued for the domain in question.

   For example, the following CAA record set requests that no
   certificates be issued for the domain 'nocerts.example.com' by any
   certificate issuer.

   nocerts.example.com       CAA 0 issue ";"

   A CAA record with an issue parameter tag that specifies a domain name
   is a request that certificate issuers perform CAA issue restriction
   processing for the corresponding domain and grants authorization to
   the certificate issuer specified by the domain name.

   For example, the following CAA record set requests that no
   certificates be issued for the domain 'certs.example.com' by any
   certificate issuer other than the example.net certificate issuer.

   certs.example.com       CAA 0 issue "example.net"

   CAA authorizations are additive. thus the result of specifying both
   the empty issuer and a specified issuer is the same as specifying
   just the specified issuer alone.

   An issuer MAY choose to specify issuer-parameters that further
   constrain the issue of certificates by that issuer.  For example



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   specifying that certificates are to be subject to specific validation
   polices, billed to certain accounts or issued under specific trust
   anchors.

   The semantics of issuer-parameters are determined by the issuer
   alone.

5.3.  CAA issuewild Property

   The issuewild property has the same syntax and semantics as the issue
   property except that issuewild properties only grant authorization to
   issue certificates that specify a wildcard domain and issuewild
   properties take precedence over issue properties when specified.
   Specifically:

      issuewild properties MUST be ignored when processing a request for
      a domain that is not a wildcard domain.

      If at least one issuewild property is specified in the relevant
      CAA record set, all issue properties MUST be ignored when
      processing a request for a domain that is a wildcard domain.

5.4.  CAA iodef Property

   The iodef property specifies a means of reporting certificate issue
   requests or cases of certificate issue for the corresponding domain,
   that violate the security policy of the issuer or the domain name
   holder.

   The Incident Object Description Exchange Format (IODEF) [RFC5070] is
   used to present the incident report in machine readable form.

   The iodef property takes a URL as its parameter.  The URL scheme type
   determines the method used for reporting:

   mailto:  The IODEF incident report is reported as a MIME email
      attachment to an SMTP email that is submitted to the mail address
      specified.  The mail message sent SHOULD contain a brief text
      message to alert the recipient to the nature of the attachment.

   http or https:  The IODEF report is submitted as a web service
      request to the HTTP address specified using the protocol specified
      in [RFC6546].








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6.  Security Considerations

   CAA Records assert a security policy that the holder of a domain name
   wishes to be observed by certificate issuers.  The effectiveness of
   CAA records as an access control mechanism is thus dependent on
   observance of CAA constraints by issuers.

   The objective of the CAA record properties described in this document
   is to reduce the risk of certificate mis-issue rather than avoid
   reliance on a certificate that has ben mis-issued.  DANE [DANE]
   describes a mechanism for avoiding reliance on mis-issued
   certificates.

6.1.  Non-Compliance by Certification Authority

   CAA records offer CAs a cost-effective means of mitigating the risk
   of certificate mis-issue: The cost of implementing CAA checks is very
   small and the potential costs of a mis-issue event include the
   removal of an embedded trust anchor.

6.2.  Mis-Issue by Authorized Certification Authority

   Use of CAA records does not prevent mis-issue by an authorized
   Certification Authority. , i.e., a CA that is authorized to issue
   certificates for the domain in question by CAA records..

   Domain name holders SHOULD verify that the CAs they authorize to
   issue certificates for their domains employ appropriate controls to
   ensure that certificates are issued only to authorized parties within
   their organization.

   Such controls are most appropriately determined by the domain name
   holder and the authorized CA(s) directly and are thus out of scope of
   this document.

6.3.  Suppression or spoofing of CAA records

   Suppression of the CAA record or insertion of a bogus CAA record
   could enable an attacker to obtain a certificate from an issuer that
   was not authorized to issue for that domain name.

   Where possible, Issuers SHOULD perform DNSSEC validation to detect
   missing or modified CAA record sets.

   In cases where DNSSEC is not deployed in a corresponding domain, an
   issuer SHOULD attempt to mitigate this risk by employing appropriate
   DNS security controls.  For example all portions of the DNS lookup
   process SHOULD be performed against the authoritative name server.



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   Data cached by third parties MUST NOT be relied on but MAY be used to
   support additional anti-spoofing or anti-suppression controls.

6.4.  Denial of Service

   Introduction of a malformed or malicious CAA RR could in theory
   enable a Denial of Service attack.

   This specific threat is not considered to add significantly to the
   risk of running an insecure DNS service.

   An attacker could, in principle, perform a Denial of Service attack
   against an issuer by requesting a certificate with a maliciously long
   DNS name.  In practice, the DNS protocol imposes a maximum name
   length and CAA processing does not exacerbate the existing need to
   mitigate Denial of Service attacks to any meaningful degree.

6.5.  Abuse of the Critical Flag

   A Certification Authority could make use of the critical flag to
   trick customers into publishing records which prevent competing
   Certification Authorities from issuing certificates even though the
   customer intends to authorize multiple providers.

   In practice, such an attack would be of minimal effect since any
   competent competitor that found itself unable to issue certificates
   due to lack of support for a property marked critical SHOULD
   investigate the cause and report the reason to the customer who will
   thus discover that they had been deceived.


7.  IANA Considerations

7.1.  Registration of the CAA Resource Record Type

   [Note to IANA, the CAA resource record has already been assigned.  On
   issue of this draft as an RFC, the record should be updated to
   reflect this document as the authoritative specification and this
   paragraph (but not the following ones deleted]

   IANA has assigned Resource Record Type 257 for the CAA Resource
   Record Type and added the line depicted below to the registry named
   Resource Record (RR) TYPEs and QTYPEs as defined in BCP 42 [RFC6195]
   and located at http://www.iana.org/assignments/dns-parameters.

RR Name      Value and meaning                                Reference
-----------  ---------------------------------------------    ---------
CAA          257 Certification Authority Restriction          [RFC-THIS]



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7.2.  Certification Authority Authorization Properties

   [Note to IANA, this is a new registry that needs to be created and
   this paragraph but not the following ones deleted.]

   IANA has created the Certification Authority Authorization Properties
   registry with the following initial values:


 Tag          Meaning                            Reference
 -----------  ---------------------------------- ---------
 issue        Authorization Entry by Domain      [RFC-THIS]
 issuewild    Authorization Entry by Domain      [RFC-THIS]
 iodef        Report incident by IODEF report    [RFC-THIS]
 auth         Reserved                           <xref target="HB2011"/>
 path         Reserved                           <xref target="HB2011"/>
 policy       Reserved                           <xref target="HB2011"/>

   Although [HB2011]" has expired, deployed clients implement the CAA
   properties specified in the draft and re-use of these property tags
   for a different purtpose could cause unexpected behavior.

   Addition of tag identifiers requires a public specification and
   expert review as set out in [RFC6195].

   The tag space is designed to be sufficiently large that exhausting
   the possible tag space need not be a concern.  The scope of expert
   review SHOULD be limited to the question of whether the specification
   provided is sufficiently clear to permit implementation and to avoid
   unnecessary duplication of functionality.

7.3.  Acknowledgements

   The authors would like to thank the following people who contributed
   to the design and documentation of this work item: Chris Evans,
   Stephen Farrell, Jeff Hodges, Paul Hoffman, Stephen Kent, Adam
   Langley, Ben Laurie, Chris Palmer, Scott Schmit, Sean Turner and Ben
   Wilson.


8.  References

8.1.  Normative References

   [DANE]     P. Hoffman., J. Schlyter, "draft-ietf-dane-protocol-23:
              Replace with reference to RFC before issue.", 2012.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",



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              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.

   [RFC5070]  Danyliw, R., Meijer, J., and Y. Demchenko, "The Incident
              Object Description Exchange Format", RFC 5070,
              December 2007.

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, March 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 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.

   [RFC6195]  Eastlake, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6195, March 2011.

   [RFC6546]  Trammell, B., "Transport of Real-time Inter-network
              Defense (RID) Messages over HTTP/TLS", RFC 6546,
              April 2012.

   [STD13]    Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

              Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.



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   [X.509]    International Telecommunication Union, "ITU-T
              Recommendation X.509 (11/2008): Information technology -
              Open systems interconnection - The Directory: Public-key
              and attribute certificate frameworks", ITU-T
              Recommendation X.509, November 2008.

8.2.  Informative  References

   [HB2011]   Hallam-Baker, P., Stradling, R., and B. Laurie, "DNS
              Certification Authority Authorization (CAA) Resource
              Record", draft-hallambaker-donotissue-04 (work in
              progress), May 2011.

   [RFC3647]  Chokhani, S., Ford, W., Sabett, R., Merrill, C., and S.
              Wu, "Internet X.509 Public Key Infrastructure Certificate
              Policy and Certification Practices Framework", RFC 3647,
              November 2003.


Authors' Addresses

   Phillip Hallam-Baker
   Comodo Group Inc.

   Email: philliph@comodo.com


   Rob Stradling
   Comodo CA Ltd.

   Email: rob.stradling@comodo.com




















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