Internet Engineering Task Force (IETF) P. Hallam-Baker
Request for Comments: 8644 Venture Cryptography
Obsoletes: 6844 R. Stradling
Category: Standards Track Sectigo
ISSN: 2070-1721 J. Hoffman-Andrews
Let's Encrypt
August
September 2019
DNS Certification Authority Authorization (CAA) Resource Record
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
name. CAA Resource Records allow a public CA 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. CAs.
This document obsoletes RFC 6844.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8644.
Copyright Notice
Copyright (c) 2019 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
(https://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2.2. Defined Terms . . . . . . . . . . . . . . . . . . . . . . 4 3
3. Relevant Resource Record Set . . . . . . . . . . . . . . . . 5
4. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.1. Canonical Presentation Format . . . . . . . . . . . . 7
4.2. CAA issue Property . . . . . . . . . . . . . . . . . . . 8
4.3. CAA issuewild Property . . . . . . . . . . . . . . . . . 9
4.4. CAA iodef Property . . . . . . . . . . . . . . . . . . . 10
4.5. Critical Flag . . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5.1. Use of DNS Security . . . . . . . . . . . . . . . . . . . 12 11
5.2. Non-compliance by Certification Authority . . . . . . . . 12
5.3. Mis-Issue by Authorized Certification Authority . . . . . 12
5.4. Suppression or Spoofing of CAA Records . . . . . . . . . 12
5.5. Denial of Service . . . . . . . . . . . . . . . . . . . . 13
5.6. Abuse of the Critical Flag . . . . . . . . . . . . . . . 13
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 13
6.1. Blocked Queries or Responses . . . . . . . . . . . . . . 14 13
6.2. Rejected Queries and Malformed Responses . . . . . . . . 14
6.3. Delegation to Private Nameservers . . . . . . . . . . . . 14
6.4. Bogus DNSSEC Responses . . . . . . . . . . . . . . . . . 14
7. Differences from RFC 6844 . . . . . . . . . . . . . . . . . . 15 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 16 15
9.2. Informative References . . . . . . . . . . . . . . . . . 17
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 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
name. Publication of CAA Resource Records allows a public 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) [RFC6698], CAA records are used as a part of a
mechanism for checking PKIX [RFC6698] certificate data. The
distinction between the two specifications CAA and TLSA is that CAA records specify an
authorization control to be performed by a certificate
issuer CA before issuing 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 the issuance of a certificate.
Criteria for the 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 name. Since
certificates are valid for a 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 parties 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 into
account 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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.2. Defined Terms
The following terms are used in this document:
o Certificate: An X.509 Certificate, as specified in [RFC5280].
o Certificate Evaluator: A party other than a Relying Party that
evaluates the trustworthiness of certificates issued by
Certification Authorities.
o Certification Authority (CA): An Issuer that issues certificates
in accordance with a specified Certificate Policy.
o Certificate Policy (CP): Specifies the criteria that a CA
undertakes to meet in its issue of certificates. See [RFC3647].
o Certification Practices Statement (CPS): Specifies the means by
which the criteria of the CP are met. In most cases, this will be
the document against which the operations of the CA are audited.
See [RFC3647].
o Domain Name: The label assigned to a node in the Domain Name
System.
o Domain Name System (DNS): The Internet naming system specified in
[RFC1034] and [RFC1035].
o DNS Security (DNSSEC): Extensions to the DNS that provide
authentication services as specified in [RFC4033], [RFC4034],
[RFC4035], [RFC5155], and revisions. any revisions to these specifications.
o Fully Qualified Domain Name (FQDN): A domain name that includes
the labels of all superior nodes in the DNS.
o Issuer: An entity that issues certificates. See [RFC5280].
o Property: The tag-value portion of a CAA Resource Record.
o Property Tag: The tag portion of a CAA Resource Record.
o Property Value: The value portion of a CAA Resource Record.
o Relevant Resource Record Set (Relevant RRSet): RRset): A set of CAA
Resource Records resulting from applying the algorithm in
Section 3 to a specific Fully Qualified Domain Name FQDN or Wildcard Domain Name.
o Relying Party: A party that makes use of an application whose
operation depends on the use of a certificate for making a
security decision. See [RFC5280].
o 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].
o Resource Record Set (RRSet): (RRset): A set of RRs of a particular owner
name, class, and type. The time to live on all RRs within an
RRSet
RRset is always the same, but the data may be different among RRs
in the RRSet. RRset.
o Wildcard Domain Name: A domain name consisting of a single
asterisk character followed by a single "full stop" character
("*.") followed by a Fully Qualified Domain Name. an FQDN.
3. Relevant Resource Record Set
Before issuing a certificate, a compliant CA MUST check for
publication of a Relevant RRSet. RRset. If such an RRSet RRset exists, a CA
MUST NOT issue a certificate unless the CA determines that either
(1) the certificate request is consistent with the applicable CAA
RRset or (2) an exception specified in the relevant CP or CPS
applies. If the Relevant RRSet RRset for a Fully Qualified Domain Name an FQDN or Wildcard Domain Name
contains no Property Tags that restrict issuance (for instance, if it
contains only iodef Property Tags or only Property Tags unrecognized
by the CA), CAA does not restrict issuance.
A certificate request MAY specify more than one Fully Qualified
Domain Name FQDN and MAY specify
Wildcard Domain Names. Issuers MUST verify authorization for all the Fully Qualified Domain Names
FQDNs and Wildcard Domain Names specified in the request.
The search for a CAA RRSet RRset climbs the DNS name tree from the
specified label up to, but not including, the DNS root '.' "." until a
CAA RRSet RRset is found.
Given a request for a specific Fully Qualified Domain Name FQDN X or a request for a Wildcard
Domain Name *.X, the Relevant RRset RelevantCAASet(X) is determined
as follows (in pseudocode):
Let CAA(X) be the RRSet RRset returned by performing a CAA record query
for the Fully Qualified Domain Name FQDN X, according to the lookup algorithm specified in
RFC 1034, Section 4.3.2 (in particular, chasing aliases). Let
Parent(X) be the Fully Qualified Domain
Name FQDN produced by removing the leftmost label of
X.
RelevantCAASet(domain):
while domain is not ".":
if CAA(domain) is not Empty:
return CAA(domain)
domain = Parent(domain)
return Empty
For example, processing CAA for the Fully Qualified Domain Name FQDN "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 Fully Qualified Domain Name FQDN "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. Mechanism
4.1. Syntax
A CAA RR contains a single Property consisting of a tag-value pair.
A Fully Qualified Domain Name
An FQDN MAY have multiple CAA RRs associated with it, and a given
Property Tag MAY be specified more than once across those RRs.
The RDATA section for a CAA RR contains one Property. A Property
consists of the 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
number of remaining octets in the Value field. They are related by
(m = d - n - 2) where d is the length of the RDATA section.
The fields are defined as follows:
Flags: One octet containing the following field:
Bit 0, Issuer Critical Flag: If the value is set to '1', "1", the
Property is critical. A CA MUST NOT issue certificates for any
FQDN if the Relevant RRSet RRset for that FQDN contains a CAA
critical Property for an unknown or unsupported 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, according to those conventions, the Flags value 1 means that
bit 7 is set, while a value of 128 means that bit 0 is set.
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 flag 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.
Tag: The Property identifier -- a sequence of ASCII characters.
Tags MAY contain ASCII characters 'a' "a" through 'z', 'A' "z", "A" through 'Z', "Z",
and the numbers 0 through 9. Tags MUST NOT contain any other
characters. Matching of tags is case insensitive.
Tags submitted for registration by IANA MUST NOT contain any
characters other than the (lowercase) ASCII characters 'a' "a" through
'z'
"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 Value field is specified implicitly as the
remaining length of the enclosing RDATA section.
4.1.1. Canonical Presentation Format
The canonical presentation format of the CAA record is:
CAA <flags> <tag> <value>
Where:
Flags: An unsigned integer between 0 and 255.
Tag: A non-zero-length sequence of ASCII letters and numbers in
lowercase.
Value: The value Value field, expressed as either (1) a contiguous set of
characters without interior spaces or (2) a quoted string. See
the <character-string> format specified in [RFC1035], Section 5.1,
but note that the value Value field contains no length byte and is not
limited to 255 characters.
4.2. CAA issue Property
If the issue Property Tag is present in the Relevant RRSet RRset for a
Fully Qualified Domain Name, an
FQDN, it is a request that Issuers:
1. Perform CAA issue restriction processing for the FQDN, and
2. Grant authorization to issue certificates containing that FQDN 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 has the following sub-syntax (specified
in ABNF as per [RFC5234]).
issue-value = *WSP [issuer-domain-name *WSP]
[";" *WSP [parameters *WSP]]
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, FQDN values
are specified in letter-digit-hyphen Label (LDH-Label) form.
The following CAA record set RRset requests that no certificates be issued for
the FQDN 'certs.example.com' "certs.example.com" by any Issuer other than ca1.example.net
or ca2.example.org.
certs.example.com CAA 0 issue "ca1.example.net"
certs.example.com CAA 0 issue "ca2.example.org"
Because the presence of an issue Property Tag in the Relevant RRSet RRset
for an FQDN restricts issuance, FQDN owners can use an issue Property
Tag with no issuer-domain-name to request no issuance.
For example, the following RRSet RRset requests that no certificates be
issued for the FQDN 'nocerts.example.com' "nocerts.example.com" by any Issuer.
nocerts.example.com CAA 0 issue ";"
An issue Property Tag where the issue-value does not match the ABNF
grammar MUST be treated the same as one specifying an empty
issuer-domain-name. For example, the following malformed CAA RRSet RRset
forbids issuance:
malformed.example.com CAA 0 issue "%%%%%"
CAA authorizations are additive; thus, the result of specifying both
an empty issuer-domain-name and a non-empty issuer-domain-name is the
same as specifying just the non-empty issuer-domain-name.
An Issuer MAY choose to specify parameters that further constrain the
issue of certificates by that Issuer -- for example, specifying that
certificates are to be subject to specific validation policies,
billed to certain accounts, or issued under specific trust anchors.
For example, if ca1.example.net has requested that its customer
accountable.example.com 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 parameters to the issue Property Tag are determined
by the Issuer alone.
4.3. CAA issuewild Property
The issuewild Property Tag has the same syntax and semantics as the
issue Property Tag except that it only grants authorization to issue
certificates that specify a Wildcard Domain Name and each issuewild
properties take
Property takes precedence over each issue properties Property when specified.
Specifically:
Each issuewild properties Property MUST be ignored when processing a request for a
Fully Qualified Domain Name
an FQDN that is not a Wildcard Domain Name.
If at least one issuewild Property is specified in the Relevant RRSet RRset
for a Wildcard Domain Name, all each issue properties Property MUST be ignored when
processing a request for that Wildcard Domain Name.
For example, the following RRSet RRset requests that _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". Note that this
presumes that there are no CAA RRs for sub.wild.example.com.
wild.example.com CAA 0 issue "ca1.example.net"
wild.example.com CAA 0 issuewild "ca2.example.org"
The following RRSet 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 RRset requests that _only_ ca2.example.org issue
certificates for "*.wild3.example.com" or "*.sub.wild3.example.com".
It does not permit any Issuer to issue for "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 RRset requests that _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 0 issuewild "ca2.example.org"
4.4. CAA iodef Property
The iodef Property specifies a means of reporting certificate issue
requests or cases of certificate issue for domains for which the
Property appears in the Relevant RRSet, RRset, when those requests or
issuances violate the security policy of the Issuer or the FQDN
holder.
The Incident Object Description Exchange Format (IODEF) [RFC7970] is
used to present the incident report in machine-readable form.
The iodef Property Tag takes a URL as its 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 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].
These are the only supported URL schemes.
The following RRSet 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 new semantics from issuing certificates for the
indicated FQDNs.
In the following example, the Property with a Property Tag of 'tbs' "tbs"
is flagged as 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 Relevant RRSet) RRset) unless the Issuer has
implemented the processing rules for the 'tbs' "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 an FQDN
wishes to be observed by 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 been mis-issued. DANE [RFC6698]
describes a mechanism for avoiding reliance on mis-issued
certificates.
5.1. Use of DNS Security
The 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, RRset, irrespective of whether the
corresponding DNS records are signed.
DNSSEC provides a proof of non-existence for both DNS Fully Qualified
Domain Names FQDNs and RRSets
RRsets within FQDNs. DNSSEC verification thus enables an Issuer to
determine whether the answer to a CAA record query (1) is empty
because the RRSet RRset is empty or (2) is non-empty but the response has
been suppressed.
The use of DNSSEC allows an Issuer to acquire and archive a proof
that they were authorized to issue certificates for the FQDN.
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.
5.2. 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.
5.3. Mis-Issue by Authorized Certification Authority
The use of CAA records does not prevent mis-issue by an authorized
CA, i.e., a CA that is authorized to issue certificates for the FQDN
in question by CAA records.
FQDN holders SHOULD verify that the CAs they authorize to issue
certificates for their FQDNs employ appropriate controls to ensure
that certificates are issued only to authorized parties within their
organization.
Such controls are most appropriately determined by the FQDN holder
and the authorized CA(s) directly and are thus outside the scope of
this document.
5.4. Suppression or Spoofing of CAA Records
Suppression of a 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 an affected FQDN.
Where possible, Issuers SHOULD perform DNSSEC validation to detect
missing or modified CAA record sets. RRsets.
In cases where DNSSEC is not deployed for a corresponding FQDN, 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 nameserver.
Data cached by third parties MUST NOT be relied on as the sole source
of DNS CAA information but MAY be used to support additional
anti-spoofing or anti-suppression controls.
5.5. Denial of Service
Introduction of a malformed or malicious CAA RR could, in theory,
enable a Denial-of-Service (DoS) attack. This could happen by
modification of authoritative DNS records or by spoofing inflight DNS
responses.
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 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.
5.6. Abuse of the Critical Flag
A CA could make use of the critical flag to trick customers into
publishing records that prevent competing CAs from issuing
certificates even though the customer intends to authorize multiple
providers. This could happen if the customers were setting CAA
records based on data provided by the CA rather than generating those
records themselves.
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 will thus discover that they had been deceived.
6. Deployment Considerations
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 they may start
dropping such packets when they consider themselves under attack.
This generally manifests as a timed-out DNS query or as a SERVFAIL at
a local recursive resolver.
6.2. Rejected Queries and Malformed Responses
Some authoritative nameservers respond with REJECTED or NOTIMP when
queried for an RR type they do not recognize. At least one
authoritative resolver produces a malformed response (with the QR
(Query/Response) bit set to "0") when queried for unknown RR types.
Per RFC 1034, the correct response RCODE for unknown RR types is NOERROR. 0
("No error condition").
6.3. Delegation to Private Nameservers
Some FQDN administrators make the contents of a subdomain
unresolvable on the public 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. FQDN
administrators wishing to issue certificates for private FQDNs SHOULD
use split-horizon DNS with a publicly available nameserver, so that
CAs can receive a valid, empty CAA response for those FQDNs.
6.4. Bogus DNSSEC Responses
Queries for CAA RRs 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 RR 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 RRsets 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 also to sign the
response as if they had used all lowercase. In particular, PowerDNS
versions prior to 4.0.4 had this bug.
7. Differences from RFC 6844
This document obsoletes RFC 6844. The most important change is to
the "Certification Authority Processing" section (now called
"Relevant Resource Record Set," as noted below). RFC 6844 specified
an algorithm that performed DNS tree-climbing not only on the FQDN
being processed but also on all CNAMEs and DNAMEs encountered along
the way. This made the processing algorithm very inefficient when
used on FQDNs that utilize many CNAMEs and would have made it
difficult for hosting providers to set CAA policies on their own
FQDNs without setting potentially unwanted CAA policies on their
customers' FQDNs. This document specifies a simplified processing
algorithm that only performs tree-climbing on the FQDN being
processed, and it leaves the 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 the issue and issuewild
tags and resolves some inconsistencies with the document text. In
particular, it specifies that parameters are separated with
semicolons. It also allows hyphens in Property Tags.
This document also clarifies the processing of a CAA RRset that is
not empty but that does not contain any 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 has added this document as a reference for the "Certification
Authority Restriction Flags" and "Certification Authority Restriction
Properties" registries and updated references to [RFC6844] within
those registries to refer instead to this document. IANA has also
updated the CAA TYPE in the "Resource Record (RR) TYPEs" subregistry
of the "Domain Name System (DNS) Parameters" registry with a
reference to this document.
9. References
9.1. 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>.
[RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification
Authority Authorization (CAA) Resource Record", RFC 6844,
DOI 10.17487/RFC6844, January 2013,
<https://www.rfc-editor.org/info/rfc6844>.
[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>.
[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>.
9.2. 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>.
Acknowledgements
The authors would like to thank the following people who contributed
to the design and documentation of this work item: Corey Bonnell,
Chris Evans, Stephen Farrell, Jeff Hodges, Paul Hoffman, Tim
Hollebeek, Stephen Kent, Adam Langley, Ben Laurie, James Manger,
Chris Palmer, Scott Schmit, Sean Turner, and Ben Wilson.
Authors' Addresses
Phillip Hallam-Baker
Venture Cryptography
Email: phill@hallambaker.com
Rob Stradling
Sectigo Ltd.
Email: rob@sectigo.com
Jacob Hoffman-Andrews
Let's Encrypt
Email: jsha@letsencrypt.org