wdiff draft-ietf-lamps-rfc6844bis-04.txt draft-ietf-lamps-rfc6844bis-05.txt

Network Working Group P. Hallam-Baker
Internet-Draft Comodo Group, Inc
Obsoletes: 6844 (if approved) R. Stradling
Intended status: Standards Track Sectigo
Expires: June 6, August 8, 2019 J. Hoffman-Andrews
Let's Encrypt
December 03, 2018
February 04, 2019

DNS Certification Authority Authorization (CAA) Resource Record
draft-ietf-lamps-rfc6844bis-04
draft-ietf-lamps-rfc6844bis-05

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. domain
name. 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.

This document obsoletes RFC 6844.

Status of This Memo

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

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This Internet-Draft will expire on June 6, August 8, 2019.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 3
2.2. Defined Terms . . . . . . . . . . . . . . . . . . . . . . 4
3. The CAA RR Type Relevant Resource Record Set . . . . . . . . . . . . . . . . 5
4. Mechanism . . . . . . . 5
4. Certification Authority Processing . . . . . . . . . . . . . 7
4.1. Use of DNS Security . . . . . . 6
4.1. Syntax . . . . . . . . . . . . . 8
5. Mechanism . . . . . . . . . . . . 6
4.1.1. Canonical Presentation Format . . . . . . . . . . . . 7
4.2. CAA issue Property . . 9
5.1. Syntax . . . . . . . . . . . . . . . . . 8
4.3. CAA issuewild Property . . . . . . . . 9
5.1.1. Canonical Presentation Format . . . . . . . . . 9
4.4. CAA iodef Property . . . 10
5.2. CAA issue Property . . . . . . . . . . . . . . . . 10
4.5. Critical Flag . . . . 11
5.3. CAA issuewild Property . . . . . . . . . . . . . . . . . 12
5.4. CAA iodef Property . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . 13
6. . 11
5.1. Use of DNS Security Considerations . . . . . . . . . . . . . . . . . . . 13
6.1. 12
5.2. Non-Compliance by Certification Authority . . . . . . . . 13
6.2. 12
5.3. Mis-Issue by Authorized Certification Authority . . . . . 13
6.3. 12
5.4. Suppression or Spoofing of CAA Records . . . . . . . . . 14
6.4. 12
5.5. Denial of Service . . . . . . . . . . . . . . . . . . . . 14
6.5. 13
5.6. Abuse of the Critical Flag . . . . . . . . . . . . . . . 14
7. 13
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 15
7.1. 13
6.1. Blocked Queries or Responses . . . . . . . . . . . . . . 15
7.2. 14
6.2. Rejected Queries and Malformed Responses . . . . . . . . 15
7.3. 14
6.3. Delegation to Private Nameservers . . . . . . . . . . . . 15
7.4. 14
6.4. Bogus DNSSEC Responses . . . . . . . . . . . . . . . . . 15
8. 14
7. Differences versus RFC6844 . . . . . . . . . . . . . . . . . 16
9. 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. 15
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
11. 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
11.1. 16
10.1. Normative References . . . . . . . . . . . . . . . . . . 17
11.2. 16
10.2. Informative References . . . . . . . . . . . . . . . . . 18 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 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. domain
name. Publication of CAA Resource Records allows a public
Certification Authority 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) [RFC6698], CAA records are used as a part of a
mechanism for checking PKIX [RFC6698] certificate data. The
distinction between the two specifications is that CAA records
specify 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 after the
certificate is issued.

Conformance with a published CAA record is a necessary but not
sufficient condition for 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 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, such
criteria require the CA to publish a Certification 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. domain name. Since a
certificate is typically
certificates are valid for at least a year, period of time, 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 parties 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.

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]. [RFC8174].

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 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: A DNS Domain Name.

Domain Name: A DNS The label assigned to a node in the Domain Name as specified in [RFC1034]. System.

Domain Name System (DNS): The Internet naming system specified in
[RFC1034] and [RFC1035].

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

Fully-Qualified Domain Name: A Domain Name that includes the labels
of all superior nodes in the Domain Name System.

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 particular entry in the DNS including the
owner name, class, type, time to live, and data, as defined in
[RFC1034] and [RFC2181].

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

Relevant Resource Record Set (Relevant RRSet): A set of CAA Resource
Records resulting from applying the algorithm in Section 4 to a
specific Domain Name or Wildcard Domain Name.

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

Wildcard Domain Name: A Domain Name consisting of a certificate for making single asterisk
character followed by a security decision.

3. The CAA RR Type

A CAA RR consists of single full stop character ("*.") followed by
a flags byte and Fully-Qualified Domain Name.

3. Relevant Resource Record Set

Before issuing a tag-value pair referred to as certificate, 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 to '1', indicates that the corresponding
property tag compliant CA MUST be understood if the semantics check for
publication of the CAA record
are to be correctly interpreted by a Relevant RRSet. If such an issuer.

Issuers RRSet exists, a CA MUST
NOT issue certificates for a domain if certificate unless the relevant CA determines that either (1) the
certificate request is consistent with the applicable CAA Resource
Record set contains unknown property tags that have the
Critical bit set.

The following property tags are defined:

issue <Issuer Domain Name> [; <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 (2) an exception specified in which the
property is published.

issuewild <Issuer Domain Name> [; <name>=<value> ]* : The issuewild
property entry authorizes the holder of relevant Certificate
Policy or Certification Practices Statement applies. If the domain name <Issuer Relevant
RRSet for a Domain Name> 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 may use to report observation of a possible policy
violation. The Incident Object Description Exchange Format (IODEF)
format is used [RFC7970].

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 name-value pairs
as defined in 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 of 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 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 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 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 Wildcard Domain Name contains no Property
Tags that restrict issuance (for instance, if it contains only iodef
Property Tags, or (2) an exception specified
in only Property Tags unrecognized by the relevant Certificate Policy or Certification Practices
Statement applies. CA), CAA
does not restrict issuance.

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

The search for a CAA Resource Record set RRSet climbs the DNS name tree from the
specified label up to but not including the DNS root '.' until a CAA Resource Record set
RRSet is found.

Given a request for a specific domain name Domain Name X, or a request for a
wildcard domain name
Wildcard Domain Name *.X, the relevant record set Relevant Resource Record Set
RelevantCAASet(X) is determined as follows:

Let CAA(X) be the record set RRSet returned by performing a CAA record query for
the domain name Domain Name X, according to the lookup algorithm specified in RFC
1034 section 4.3.2 (in particular chasing aliases). Let Parent(X) be
the domain name Domain Name produced by removing the leftmost label of X.

RelevantCAASet(domain):
for domain is not ".":
if CAA(domain) is not Empty:
return CAA(domain)
domain = Parent(domain)
return Empty

For example, processing CAA for the domain name Domain Name "X.Y.Z" where there
are no CAA records at any level in the tree RelevantCAASet would have
the following steps:

CAA("X.Y.Z.") = Empty; domain = Parent("X.Y.Z.") = "Y.Z."
CAA("Y.Z.") = Empty; domain = Parent("Y.Z.") = "Z."
CAA("Z.") = Empty; domain = Parent("Z.") = "."
return Empty

Processing CAA for the domain name Domain Name "A.B.C" where there is a CAA
record "issue example.com" at "B.C" would terminate early upon
finding the CAA record:

CAA("A.B.C.") = Empty; domain = Parent("A.B.C.") = "B.C."
CAA("B.C.") = "issue example.com"
return "issue example.com"

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
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 there is a transparency requirement to verify CAA
record processing
compliance.

5. "issue example.com" at "B.C" would terminate early upon
finding the CAA record:

CAA("A.B.C.") = Empty; domain = Parent("A.B.C.") = "B.C."
CAA("B.C.") = "issue example.com"
return "issue example.com"

4. Mechanism

5.1.

4.1. Syntax

A CAA RR Resource Record contains a single property entry Property 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 tag-
value field. pair. A domain name Domain Name MAY have multiple CAA RRs associated with
it and a given property Property Tag MAY be specified more than once. once across
those RRs.

The RDATA section for a CAA data field Resource Record contains one property entry. Property. A property entry
Property consists of the following data fields: following:

+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 field. They are related by (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 field:

Bit 0, Issuer Critical Flag: If the value is set to '1', the critical
flag is asserted and the property MUST be understood if the CAA
record Property
is to be correctly processed by a certificate issuer. critical. A Certification Authority MUST NOT issue certificates
for any Domain Name where the Relevant RRSet for that Domain Name
contains a CAA critical property Property for an unknown or unsupported
property tag that for which the issuer critical flag is set.
Property Tag.

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.

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

Tag values

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

Tag values

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

Value: A sequence of octets representing the property value. Property values 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. RDATA section.

4.1.1. 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 US-ASCII letters and numbers in lower
case.

Value: The value field, expressed as a contiguous set of characters
without interior spaces, or as a quoted string. See the the
<character-string> format specified in [RFC1035], Section 5.1, but
note that the value field contains no length byte and is not limited
to 255 characters.

5.2.

4.2. CAA issue Property

The

If the issue property tag Property Tag is used to present in the Relevant RRSet for a
Domain Name, it is a request that certificate issuers
perform Issuers

1. Perform CAA issue restriction processing for the domain Domain Name, and to grant

2. Grant authorization to specific certificate issuers. issue certificates containing that Domain
Name to the holder of the issuer-domain-name or a party acting
under the explicit authority of the holder of the issuer-domain-
name.

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

issuevalue

issue-value = *WSP [domain [issuer-domain-name *WSP] [";" *WSP [parameters *WSP]]

domain

issuer-domain-name = label *("." label)
label = (ALPHA / DIGIT) *( *("-") (ALPHA / DIGIT))

parameters = (parameter *WSP ";" *WSP parameters) / parameter
parameter = tag *WSP "=" *WSP value
tag = (ALPHA / DIGIT) *( *("-") (ALPHA / DIGIT))
value = *(%x21-3A / %x3C-7E)

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

For consistency with other aspects of issue restriction would be appropriate to specify that
no certificates DNS administration, Domain Name
values are to be issued for the domain specified in question.

For example, the letter-digit-hyphen Label (LDH-Label) form.

The following CAA resource record set requests that no certificates be issued
for the domain 'nocerts.example.com' Domain Name 'certs.example.com' by any
certificate issuer.

nocerts.example.com Issuer other than
ca1.example.net or ca2.example.org.

certs.example.com CAA 0 issue ";"

A "ca1.example.net"
certs.example.com CAA record with 0 issue "ca2.example.org"
Because the presence of an issue parameter tag that specifies a domain name
is Property Tag in the Relevant RRSet
for a request that certificate issuers perform CAA Domain Name restricts issuance, Domain Name owners can use an
issue restriction
processing for the corresponding domain and grants authorization Property Tag with no issuer-domain-name to
the certificate issuer specified by the domain name. request no issuance.

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

certs.example.com Issuer.

nocerts.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 issue property tag Property Tag where the issuevalue issue-value does not match the ABNF
grammar MUST be treated the same as one specifying the an empty issuer. issuer-
domain-name. For example, the following malformed CAA resource record set RRSet forbids
issuance:

malformed.example.com CAA 0 issue "%%%%%"

A non-empty

CAA record set that contains no issue property tags is
authorization to any certificate issuer to issue for authorizations are additive; thus, the
corresponding domain, provided that it is a non-wildcard domain, result of specifying both
an empty issuer-domain-name and
no records in a non-empty issuer-domain-name is the CAA record set otherwise prohibit issuance.
same as specifying just the non-empty issuer-domain-name.

An issuer Issuer MAY choose to specify issuer-parameters parameters that further constrain the
issue of certificates by that issuer, Issuer, for example, specifying that
certificates are to be subject to specific validation polices, billed
to certain accounts, or issued under specific trust anchors.

For example, if ca1.example.net has requested its customer
accountable.example.com to specify their account number "230123" in
each of the customer's CAA records using the (CA-defined) "account"
parameter, it would look like this:

accountable.example.com CAA 0 issue "ca1.example.net; account=230123"

The semantics of issuer-parameters parameters to the issue Property Tag are determined
by the issuer Issuer alone.

5.3.

4.3. CAA issuewild Property

The issuewild property Property Tag has the same syntax and semantics as the
issue
property Property Tag except that issuewild properties it only grant grants authorization to issue
certificates that specify a wildcard domain Wildcard Domain Name 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
Domain Name (that is, not a wildcard domain. Wildcard Domain Name).

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

A non-empty CAA record set Wildcard Domain Name.

For example, the following RRSet requests that contains no _only_ ca1.example.net
issue certificates for "wild.example.com" or "sub.wild.example.com",
and that _only_ ca2.example.org issue certificates for
"*.wild.example.com" or "*.sub.wild.example.com).

wild.example.com CAA 0 issue "ca1.example.net"
wild.example.com CAA 0 issuewild
property tags is authorization to "ca2.example.org"

The following RRSet requests that _only_ ca1.example.net issue
certificates for "wild2.example.com", "*.wild2.example.com" or
"*.sub.wild2.example.com".

wild2.example.com CAA 0 issue "ca1.example.net"

The following RRSet requests that _only_ ca2.example.org issue
certificates for "*.wild3.example.com" or "*.sub.wild3.example.com".
It does not permit any certificate issuer Issuer to issue for
the corresponding wildcard domain, provided "wild3.example.com" or
"sub.wild3.example.com".

wild3.example.com CAA 0 issuewild "ca2.example.org"
wild3.example.com CAA 0 issue ";"

The following RRSet requests that no records in the _only_ ca2.example.org issue
certificates for "*.wild3.example.com" or "*.sub.wild3.example.com".
It permits any Issuer to issue for "wild3.example.com" or
"sub.wild3.example.com".

wild3.example.com CAA record set otherwise prohibit issuance.

5.4. 0 issuewild "ca2.example.org"

4.4. CAA iodef Property

The iodef property Property specifies a means of reporting certificate issue
requests or cases of certificate issue for domains for which the corresponding domain
that
Property appears in the Relevant RRSet, when those requests or
issuances violate the security policy of the issuer Issuer or the domain name Domain
Name holder.

The Incident Object Description Exchange Format (IODEF) [RFC7970] is
used to present the incident report in machine-readable form.

The iodef property Property Tag takes a URL as its parameter. Property Value. 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 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].

The following RRSet specifies that reports may be made by means of
email with the IODEF data as an attachment, a Web service [RFC6546],
or both:

report.example.com CAA 0 issue "ca1.example.net"
report.example.com CAA 0 iodef "mailto:security@example.com"
report.example.com CAA 0 iodef "http://iodef.example.com/"

4.5. Critical Flag

The critical flag is intended to permit future versions of CAA to
introduce new semantics that MUST be understood for correct
processing of the record, preventing conforming CAs that do not
recognize the mail address
specified. The mail message sent SHOULD contain a brief text message
to alert new semantics from issuing certificates for the recipient to
indicated Domain Names.

In the nature of following example, the attachment.

http or https: The IODEF report Property with a Property Tag of 'tbs'
is submitted flagged as a Web service request critical. Neither the ca1.example.net CA nor any other
Issuer is authorized to issue for "new.example.com" (or any other
domains for which this is the HTTP address specified using Relevant RRSet) unless the protocol specified in
[RFC6546].

6. Issuer has
implemented the processing rules for the 'tbs' Property Tag.

new.example.com CAA 0 issue "ca1.example.net"
new.example.com CAA 128 tbs "Unknown"

5. Security Considerations

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

The objective of the CAA record properties described in this document
is to reduce the risk properties described in this document
is to reduce the risk of certificate mis-issue rather than avoid
reliance on a certificate that has been mis-issued. DANE [RFC6698]
describes a mechanism for avoiding reliance on mis-issued
certificates.

5.1. 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 RRSet, irrespective of whether the
corresponding DNS records are signed.

DNSSEC provides a proof of non-existence for both DNS Domain Names
and RRSets within Domain Names. DNSSEC verification thus enables an
Issuer to determine if the answer to a CAA record query is empty
because the RRSet is empty or if it is non-empty but the response has
been suppressed.

Use of DNSSEC allows an Issuer to acquire and archive a proof that
they were authorized to issue certificates for the Domain Name.
Verification of such archives MAY be an audit requirement to verify
CAA record processing compliance. Publication of certificate mis-issue rather than avoid
reliance on a certificate that has been mis-issued. DANE [RFC6698]
describes such archives MAY
be a mechanism for avoiding reliance on mis-issued
certificates.

6.1. transparency requirement to verify CAA record processing
compliance.

5.2. 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.

5.3. 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 Domain Name in question by CAA records.

Domain name Name holders SHOULD verify that the CAs they authorize to
issue certificates for their domains Domain Names 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 Domain Name
holder and the authorized CA(s) directly and are thus out of scope of
this document.

6.3.

5.4. 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 Issuer that
was not authorized to issue for that domain name. 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 for a corresponding domain, Domain
Name, an
issuer 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. 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.

5.5. Denial of Service

Introduction of a malformed or malicious CAA RR could in theory
enable a 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 DoS attack against an
issuer
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 DoS
attacks to any meaningful degree.

6.5.

5.6. Abuse of the Critical Flag

A Certification Authority could make use of the critical flag to
trick customers into publishing records 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 Property marked critical SHOULD
investigate the cause and report the reason to the customer. The
customer will thus discover that they had been deceived.

6. Deployment Considerations

7.1.

A CA implementing CAA may find that they receive errors looking up
CAA records. The following are some common causes of such errors, so
that CAs may provide guidance to their subscribers on fixing the
underlying problems.

6.1. Blocked Queries or Responses

Some middleboxes, in particular anti-DDoS appliances, may be
configured to drop DNS packets of unknown types, or may start
dropping such packets when they consider themselves under attack.
This generally manifests as a timed-out DNS query, or a SERVFAIL at a
local recursive resolver.

For deployability of CAA and future DNS record types, middleboxes
SHOULD block DNS packets by volume and size rather than by query
type.

7.2.

6.2. Rejected Queries and Malformed Responses

Some authoritative nameservers respond with REJECTED or NOTIMP when
queried for a resource record Resource Record type they do not recognize. At least
one authoritative resolver produces a malformed response (with the QR
bit set to 0) when queried for unknown resource record Resource Record types. Per
RFC 1034, the correct response for unknown resource record Resource Record types is
NOERROR.

7.3.

6.3. Delegation to Private Nameservers

Some domain Domain Name administrators make the contents of a subdomain
unresolvable on the public internet Internet by delegating that subdomain to a
nameserver whose IP address is private. A CA processing CAA records
for such subdomains will receive SERVFAIL from its recursive
resolver. The CA MAY interpret that as preventing issuance. Domain
Name administrators wishing to issue certificates for private domains Domain
Names SHOULD use split-horizon DNS with a publicly available
nameserver, so that CAs can receive a valid, empty CAA response for
those domains.

7.4. Domain Names.

6.4. Bogus DNSSEC Responses

Queries for CAA resource records Resource Records are different from most DNS RR
types, because a signed, empty response to a query for CAA RRs is
meaningfully different from a bogus response. A signed, empty
response indicates that there is definitely no CAA policy set at a
given label. A bogus response may mean either a misconfigured zone,
or an attacker tampering with records. DNSSEC implementations may
have bugs with signatures on empty responses that go unnoticed,
because for more common resource record Resource Record types like A and AAAA, the
difference to an end user between empty and bogus is irrelevant; they
both mean a site is unavailable.

In particular, at least two authoritative resolvers that implement
live signing had bugs when returning empty resource record Resource Record sets for
DNSSEC-signed zones, in combination with mixed-case queries. Mixed-
case queries, also known as DNS 0x20, are used by some recursive
resolvers to increase resilience against DNS poisoning attacks.
DNSSEC-signing authoritative resolvers are expected to copy the same
capitalization from the query into their ANSWER section, but sign the
response as if they had use all lowercase. In particular, PowerDNS
versions prior to 4.0.4 had this bug.

7. Differences versus RFC6844

This document obsoletes RFC6844. The most important change is to the
Certification Authority Processing section. RFC6844 specified an
algorithm that performed DNS tree-climbing not only on the domain
name Domain
Name being processed, but also on all CNAMEs and DNAMEs encountered
along the way. This made the processing algorithm very inefficient
when used on domains Domain Names that utilize many CNAMEs, and would have
made it difficult for hosting providers to set CAA policies on their
own
domains Domain Names without setting potentially unwanted CAA policies on
their customers' domains. Domain Names. This document specifies a simplified
processing algorithm that only performs tree climbing on the domain Domain
Name being processed, and leaves processing of CNAMEs and DNAMEs up
to the CA's recursive resolver.

This document also includes a "Deployment Considerations" section
detailing experience gained with practical deployment of CAA
enforcement among CAs in the WebPKI.

This document clarifies the ABNF grammar for issue and issuewild tags
and resolves some inconsistencies with the document text. In
particular, it specifies that parameters are separated with hyphens.
semicolons. It also allows hyphens in property names. Property Tags.

This document also clarifies processing of a CAA RRset that is not
empty, but contains no issue or issuewild tags.

This document removes the section titled "The CAA RR Type," merging
it with "Mechanism" because the definitions were mainly duplicates.
It moves the "Use of DNS Security" section into Security
Considerations. It renames "Certification Authority Processing" to
"Relevant Resource Record Set," and emphasizes the use of that term
to more clearly define which domains are affected by a given RRset.

8. IANA Considerations

IANA is requested to add [[[ RFC Editor: Please replace with this RFC
]]] as a reference for the Certification Authority Restriction Flags
and Certification Authority Restriction Properties registries.

10.

9. Acknowledgements

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

11.

10. References

11.1.

10.1. Normative References

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.

[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<https://www.rfc-editor.org/info/rfc4034>.

[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.

[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
<https://www.rfc-editor.org/info/rfc5155>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.

[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, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.

[RFC6546] Trammell, B., "Transport of Real-time Inter-network
Defense (RID) Messages over HTTP/TLS", RFC 6546,
DOI 10.17487/RFC6546, April 2012,
<https://www.rfc-editor.org/info/rfc6546>.

[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <https://www.rfc-editor.org/info/rfc6698>.

[RFC7970] Danyliw, R., "The Incident Object Description Exchange
Format Version 2", RFC 7970, DOI 10.17487/RFC7970,
November 2016, <https://www.rfc-editor.org/info/rfc7970>.

11.2.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2. Informative References

[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,
DOI 10.17487/RFC3647, November 2003,
<https://www.rfc-editor.org/info/rfc3647>.

Authors' Addresses

Phillip Hallam-Baker
Comodo Group, Inc

Email: philliph@comodo.com phill@hallambaker.com

Rob Stradling
Sectigo Ltd.

Email: rob@sectigo.com
Jacob Hoffman-Andrews
Let's Encrypt

Email: jsha@letsencrypt.org