wdiff rfc6844.original rfc6844.txt

Internet Engineering Task Force (IETF) P. Hallam-Baker
Internet-Draft
Request for Comments: 6844 Comodo Group Group, Inc.
Intended status:
Category: Standards Track R. Stradling
Expires: April 21, 2013
ISSN: 2070-1721 Comodo CA CA, Ltd.
October 18, 2012
January 2013

DNS Certification Authority Authorization (CAA) Resource Record
draft-ietf-pkix-caa-15

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 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 This Memo

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provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents an Internet Standards Track document.

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(IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

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This Internet-Draft will expire on April 21, 2013.
http://www.rfc-editor.org/info/rfc6844.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 ....................................................2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 .....................................................3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 ......................................3
2.2. Defined Terms . . . . . . . . . . . . . . . . . . . . . . 4 ..............................................3
3. The CAA RR type . . . . . . . . . . . . . . . . . . . . . . . 5 Type .................................................5
4. Certification Authority Processing . . . . . . . . . . . . . . 7 ..............................7
4.1. Use of DNS Security . . . . . . . . . . . . . . . . . . . 8 ........................................8
5. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . 9 .......................................................8
5.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 9 .....................................................8
5.1.1. Canonical Presentation Format . . . . . . . . . . . . 10 ......................10
5.2. CAA issue Property . . . . . . . . . . . . . . . . . . . . 11 ........................................10
5.3. CAA issuewild Property . . . . . . . . . . . . . . . . . . 12 ....................................12
5.4. CAA iodef Property . . . . . . . . . . . . . . . . . . . . 12 ........................................12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 ........................................13
6.1. Non-Compliance by Certification Authority . . . . . . . . 13 .................13
6.2. Mis-Issue by Authorized Certification Authority . . . . . 13 ...........13
6.3. Suppression or spoofing Spoofing of CAA records . . . . . . . . . . 14 Records ....................13
6.4. Denial of Service . . . . . . . . . . . . . . . . . . . . 14 .........................................14
6.5. Abuse of the Critical Flag . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7.1. Registration of the CAA Resource Record Type . . . . . . . 15
7.2. Certification Authority Authorization Properties . . . . . 15
7.3. Certification Authority Authorization Flags . . . . . . . 16
7.4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 16
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Normative References . . . . . . . . . . . . . . . . . . . 16
8.2. Informative References . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18 of the Critical Flag ................................14
7. IANA Considerations ............................................14
7.1. Registration of the CAA Resource Record Type ..............14
7.2. Certification Authority Restriction Properties ............15
7.3. Certification Authority Restriction Flags .................15
8. Acknowledgements ...............................................16
9. References .....................................................16
9.1. Normative References ......................................16
9.2. Informative References ....................................17

1. Introduction

The Certification Authority Authorization (CAA) DNS Resource Record
allows a DNS domain name holder to specify the Certification
Authorities (CAs) authorized to issue certificates for that domain.
Publication of CAA resource records allow Resource Records allows 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], [RFC6698], 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 an
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 relying party after the certificate is
issued.

Conformance with a published CAA record is a necessary but not
sufficient condition for issueance issuance 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 be 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 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 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 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.

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 relying party that
evaluates the trustworthiness of certificates issued by
Certification Authorities.

Certification Authority (CA): An Issuer issuer that issues Certificates 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
cases, this will be the document against which the operations of
the Certification Authority are audited. See [RFC3647].

Domain: A DNS Domain Name.

Domain Name: A DNS Domain name 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] [RFC5155], and revisions.

Issuer: An entity that issues Certificates. 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 particular entry in the DNS including the
owner name, class, type, time to live, and data, as defined in
[STD13] and [RFC2181].

Resource Record Set(RRSet): Set (RRSet): A set of Resource Records or a
particularowner
particular owner name, classand class, and type. The time to live on all
RRs with an RRSet is always the same same, but the data may be
different among RRs in the RRSet.

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

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

3. The CAA RR type 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), to '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> <name>=<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.

issue

issuewild <Issuer Domain Name> [; <tag=value> <name>=<value> ]* : The issue issuewild
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 wildcard certificates for the
domain in which the property is published.

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
Certificate Evaluator may use to report observation of a possible
policy violation. The IODEF Incident Object Description Exchange Format
(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 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] [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 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 name-value pairs
as defined in section Section 5.2. 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.

In the following example, the property 'tbs' is flagged as critical.
Neither the example.net CA, 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. at certificate issue.
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 a 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. 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.

The search for a CAA record climbs the DNS name tree from the
specified label up to but not including the DNS root '.'.

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 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 top-level domain, then R(X) = R(P(X), R(P(X)), otherwise

o R(X) is empty.

For example, if a certificate is requested for X.Y.Z the issuer will
search for the relevant CAA record set in the following order:

X.Y.Z

Alias (X.Y.Z)

Y.Z
Alias (Y.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.
suppressed.

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 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.

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 char 1 |...| Tag Char 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: 0, Issuer Critical Flag Flag: If the value is set (1), to '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 bit 0
is the Most Significant Bit and Bit bit 7 is the Least Significant
Bit. Thus 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.

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

Tag values MAY contain ASCII US-ASCII characters 'a' through 'z', 'A'
through 'Z' '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 US-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:

CAA <flags> <tag> <value>

Where:

Flags: Is an unsigned integer between 0 and 255.

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

Value: Is the <character-string> encoding of the value field as
specified in [RFC1035] section [RFC1035], Section 5.1.

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]).

Property

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

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

space = *(SP / HTAB)

parameter = tag "=" value

tag = 1* (ALPHA 1*(ALPHA / DIGIT)

value = *VCHAR

For consistency with other aspects of DNS administration, Domain domain name
values are specified in Letter-Digit-Hyphen 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 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 issuer, for example,
specifying that certificates are to be subject to specific validation
polices, billed to certain accounts 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, 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 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 Web service
request to the HTTP address specified using the protocol specified
in [RFC6546].

6. Security Considerations

CAA Records 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 been mis-issued. DANE [DANE] [RFC6698]
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 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. , Authority, i.e., a CA that is authorized to issue
certificates for the domain in question by CAA records.. 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 Spoofing of CAA records 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 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 example, all portions of the DNS lookup
process SHOULD be performed against the authoritative name server.
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 Denial-of-Service (DoS) 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 DoS 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 DoS
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 that 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. 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
"Resource Record (RR) TYPEs 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] [RFC6844]

7.2. Certification Authority Authorization Restriction 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 "Certification Authority Authorization Properties Restriction Properties"
registry with the following initial values:

Tag Meaning Reference
----------- ---------------------------------- -------------------------------------- ---------
issue Authorization Entry by Domain [RFC-THIS] [RFC6844]
issuewild Authorization Entry by Wildcard Domain [RFC-THIS] [RFC6844]
iodef Report incident by IODEF report [RFC-THIS] [RFC6844]
auth Reserved <xref target="HB2011"/> [HB2011]
path Reserved <xref target="HB2011"/> [HB2011]
policy Reserved <xref target="HB2011"/> [HB2011]

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

Addition of tag identifiers requires a public specification and
expert review
Expert Review as set out in [RFC6195] [RFC6195], Section 3.1.1.. 3.1.1.

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 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. Certification Authority Authorization Restriction Flags

IANA has created the Certification "Certification Authority Authorization Flags Restriction Flags"
registry with the following initial values:

Flag Meaning Reference
----------- ---------------------------------- ---------
0 Issuer Critical Flag [RFC-THIS] [RFC6844]
1-7 Reserved> [RFC-THIS] [RFC6844]

Assignment of new flags follows the 'RFC Required' RFC Required policy set out in
[RFC5226] section
[RFC5226], Section 4.1.

7.4.

8. 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, James Manager, Chris Palmer, Scott Schmit, Sean Turner
Turner, and Ben Wilson.

9. References

8.1.

9.1. Normative References

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

[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.

[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 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.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 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.

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

[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.

9.2. Informative References

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

Email:

EMail: philliph@comodo.com

Rob Stradling
Comodo CA CA, Ltd.

Email:

EMail: rob.stradling@comodo.com