draft-ietf-lamps-rfc6844bis-06.txt   draft-ietf-lamps-rfc6844bis-07.txt 
Network Working Group P. Hallam-Baker Network Working Group P. Hallam-Baker
Internet-Draft Internet-Draft
Obsoletes: 6844 (if approved) R. Stradling Obsoletes: 6844 (if approved) R. Stradling
Intended status: Standards Track Sectigo Intended status: Standards Track Sectigo
Expires: November 10, 2019 J. Hoffman-Andrews Expires: December 1, 2019 J. Hoffman-Andrews
Let's Encrypt Let's Encrypt
May 09, 2019 May 30, 2019
DNS Certification Authority Authorization (CAA) Resource Record DNS Certification Authority Authorization (CAA) Resource Record
draft-ietf-lamps-rfc6844bis-06 draft-ietf-lamps-rfc6844bis-07
Abstract Abstract
The Certification Authority Authorization (CAA) DNS Resource Record The Certification Authority Authorization (CAA) DNS Resource Record
allows a DNS domain name holder to specify one or more Certification allows a DNS domain name holder to specify one or more Certification
Authorities (CAs) authorized to issue certificates for that domain Authorities (CAs) authorized to issue certificates for that domain
name. CAA Resource Records allow a public Certification Authority to name. CAA Resource Records allow a public Certification Authority to
implement additional controls to reduce the risk of unintended implement additional controls to reduce the risk of unintended
certificate mis-issue. This document defines the syntax of the CAA certificate mis-issue. This document defines the syntax of the CAA
record and rules for processing CAA records by certificate issuers. record and rules for processing CAA records by certificate issuers.
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 10, 2019. This Internet-Draft will expire on December 1, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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Domain Name: The label assigned to a node in the Domain Name System. Domain Name: The label assigned to a node in the Domain Name System.
Domain Name System (DNS): The Internet naming system specified in Domain Name System (DNS): The Internet naming system specified in
[RFC1034] and [RFC1035]. [RFC1034] and [RFC1035].
DNS Security (DNSSEC): Extensions to the DNS that provide DNS Security (DNSSEC): Extensions to the DNS that provide
authentication services as specified in [RFC4033], [RFC4034], authentication services as specified in [RFC4033], [RFC4034],
[RFC4035], [RFC5155], and revisions. [RFC4035], [RFC5155], and revisions.
Fully-Qualified Domain Name: A Domain Name that includes the labels Fully-Qualified Domain Name (FQDN): A Domain Name that includes the
of all superior nodes in the Domain Name System. labels of all superior nodes in the Domain Name System.
Issuer: An entity that issues certificates. See [RFC5280]. Issuer: An entity that issues certificates. See [RFC5280].
Property: The tag-value portion of a CAA Resource Record. Property: The tag-value portion of a CAA Resource Record.
Property Tag: The tag 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. Property Value: The value portion of a CAA Resource Record.
Resource Record (RR): A particular entry in the DNS including the Resource Record (RR): A particular entry in the DNS including the
owner name, class, type, time to live, and data, as defined in owner name, class, type, time to live, and data, as defined in
[RFC1034] and [RFC2181]. [RFC1034] and [RFC2181].
Resource Record Set (RRSet): A set of Resource Records of a Resource Record Set (RRSet): A set of Resource Records of a
particular owner name, class, and type. The time to live on all RRs 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 within an RRSet is always the same, but the data may be different
RRs in the RRSet. among RRs in the RRSet.
Relevant Resource Record Set (Relevant RRSet): A set of CAA Resource Relevant Resource Record Set (Relevant RRSet): A set of CAA Resource
Records resulting from applying the algorithm in Section 4 to a Records resulting from applying the algorithm in Section 3 to a
specific Domain Name or Wildcard Domain Name. specific Fully-Qualified Domain Name or Wildcard Domain Name.
Relying Party: A party that makes use of an application whose Relying Party: A party that makes use of an application whose
operation depends on use of a certificate for making a security operation depends on use of a certificate for making a security
decision. See [RFC5280]. decision. See [RFC5280].
Wildcard Domain Name: A Domain Name consisting of a single asterisk Wildcard Domain Name: A Domain Name consisting of a single asterisk
character followed by a single full stop character ("*.") followed by character followed by a single full stop character ("*.") followed by
a Fully-Qualified Domain Name. a Fully-Qualified Domain Name.
3. Relevant Resource Record Set 3. Relevant Resource Record Set
Before issuing a certificate, a compliant CA MUST check for Before issuing a certificate, a compliant CA MUST check for
publication of a Relevant RRSet. If such an RRSet exists, a CA MUST publication of a Relevant RRSet. If such an RRSet exists, a CA MUST
NOT issue a certificate unless the CA determines that either (1) the NOT issue a certificate unless the CA determines that either (1) the
certificate request is consistent with the applicable CAA Resource certificate request is consistent with the applicable CAA Resource
Record set or (2) an exception specified in the relevant Certificate Record set or (2) an exception specified in the relevant Certificate
Policy or Certification Practices Statement applies. If the Relevant Policy or Certification Practices Statement applies. If the Relevant
RRSet for a Domain Name or Wildcard Domain Name contains no Property RRSet for a Fully-Qualified Domain Name or Wildcard Domain Name
Tags that restrict issuance (for instance, if it contains only iodef contains no Property Tags that restrict issuance (for instance, if it
Property Tags, or only Property Tags unrecognized by the CA), CAA contains only iodef Property Tags, or only Property Tags unrecognized
does not restrict issuance. by the CA), CAA does not restrict issuance.
A certificate request MAY specify more than one Domain Name and MAY A certificate request MAY specify more than one Fully-Qualified
specify Wildcard Domain Names. Issuers MUST verify authorization for Domain Name and MAY specify Wildcard Domain Names. Issuers MUST
all the Domain Names and Wildcard Domain Names specified in the verify authorization for all the Fully-Qualified Domain Names and
request. Wildcard Domain Names specified in the request.
The search for a CAA RRSet climbs the DNS name tree from the The search for a CAA RRSet climbs the DNS name tree from the
specified label up to but not including the DNS root '.' until a CAA specified label up to but not including the DNS root '.' until a CAA
RRSet is found. RRSet is found.
Given a request for a specific Domain Name X, or a request for a Given a request for a specific Fully-Qualified Domain Name X, or a
Wildcard Domain Name *.X, the Relevant Resource Record Set request for a Wildcard Domain Name *.X, the Relevant Resource Record
RelevantCAASet(X) is determined as follows: Set RelevantCAASet(X) is determined as follows (in pseudocode):
Let CAA(X) be the RRSet returned by performing a CAA record query for Let CAA(X) be the RRSet returned by performing a CAA record query for
the Domain Name X, according to the lookup algorithm specified in RFC the Fully-Qualified Domain Name X, according to the lookup algorithm
1034 section 4.3.2 (in particular chasing aliases). Let Parent(X) be specified in RFC 1034 section 4.3.2 (in particular chasing aliases).
the Domain Name produced by removing the leftmost label of X. Let Parent(X) be the Fully-Qualified Domain Name produced by removing
the leftmost label of X.
RelevantCAASet(domain): RelevantCAASet(domain):
for domain is not ".": while domain is not ".":
if CAA(domain) is not Empty: if CAA(domain) is not Empty:
return CAA(domain) return CAA(domain)
domain = Parent(domain) domain = Parent(domain)
return Empty return Empty
For example, processing CAA for the Domain Name "X.Y.Z" where there For example, processing CAA for the Fully-Qualified Domain Name
are no CAA records at any level in the tree RelevantCAASet would have "X.Y.Z" where there are no CAA records at any level in the tree
the following steps: RelevantCAASet would have the following steps:
CAA("X.Y.Z.") = Empty; domain = Parent("X.Y.Z.") = "Y.Z." CAA("X.Y.Z.") = Empty; domain = Parent("X.Y.Z.") = "Y.Z."
CAA("Y.Z.") = Empty; domain = Parent("Y.Z.") = "Z." CAA("Y.Z.") = Empty; domain = Parent("Y.Z.") = "Z."
CAA("Z.") = Empty; domain = Parent("Z.") = "." CAA("Z.") = Empty; domain = Parent("Z.") = "."
return Empty return Empty
Processing CAA for the Domain Name "A.B.C" where there is a CAA Processing CAA for the Fully-Qualified Domain Name "A.B.C" where
record "issue example.com" at "B.C" would terminate early upon there is a CAA record "issue example.com" at "B.C" would terminate
finding the CAA record: early upon finding the CAA record:
CAA("A.B.C.") = Empty; domain = Parent("A.B.C.") = "B.C." CAA("A.B.C.") = Empty; domain = Parent("A.B.C.") = "B.C."
CAA("B.C.") = "issue example.com" CAA("B.C.") = "issue example.com"
return "issue example.com" return "issue example.com"
4. Mechanism 4. Mechanism
4.1. Syntax 4.1. Syntax
A CAA Resource Record contains a single Property consisting of a tag- A CAA Resource Record contains a single Property consisting of a tag-
value pair. A Domain Name MAY have multiple CAA RRs associated with value pair. A Fully-Qualified Domain Name MAY have multiple CAA RRs
it and a given Property Tag MAY be specified more than once across associated with it and a given Property Tag MAY be specified more
those RRs. than once across those RRs.
The RDATA section for a CAA Resource Record contains one Property. A The RDATA section for a CAA Resource Record contains one Property. A
Property consists of the following: Property consists of the following:
+0-1-2-3-4-5-6-7-|0-1-2-3-4-5-6-7-| +0-1-2-3-4-5-6-7-|0-1-2-3-4-5-6-7-|
| Flags | Tag Length = n | | Flags | Tag Length = n |
+----------------|----------------+...+---------------+ +----------------|----------------+...+---------------+
| Tag char 0 | Tag char 1 |...| Tag char n-1 | | Tag char 0 | Tag char 1 |...| Tag char n-1 |
+----------------|----------------+...+---------------+ +----------------|----------------+...+---------------+
+----------------|----------------+.....+----------------+ +----------------|----------------+.....+----------------+
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Where n is the length specified in the Tag length field and m is the Where n is the length specified in the Tag length field and m is the
remaining octets in the Value field. They are related by (m = d - n remaining octets in the Value field. They are related by (m = d - n
- 2) where d is the length of the RDATA section. - 2) where d is the length of the RDATA section.
The fields are defined as follows: The fields are defined as follows:
Flags: One octet containing the following field: Flags: One octet containing the following field:
Bit 0, Issuer Critical Flag: If the value is set to '1', the Property Bit 0, Issuer Critical Flag: If the value is set to '1', the Property
is critical. A Certification Authority MUST NOT issue certificates is critical. A Certification Authority MUST NOT issue certificates
for any Domain Name where the Relevant RRSet for that Domain Name for any FQDN the Relevant RRSet for that FQDN contains a CAA critical
contains a CAA critical Property for an unknown or unsupported Property for an unknown or unsupported Property Tag.
Property Tag.
Note that according to the conventions set out in [RFC1035], bit 0 is 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. 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 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. means that bit 0 is set according to this convention.
All other bit positions are reserved for future use. All other bit positions are reserved for future use.
To ensure compatibility with future extensions to CAA, DNS records To ensure compatibility with future extensions to CAA, DNS records
compliant with this version of the CAA specification MUST clear (set compliant with this version of the CAA specification MUST clear (set
to "0") all reserved flags bits. Applications that interpret CAA to "0") all reserved flags bits. Applications that interpret CAA
records MUST ignore the value of all reserved flag bits. records MUST ignore the value of all reserved flag bits.
Tag Length: A single octet containing an unsigned integer specifying Tag Length: A single octet containing an unsigned integer specifying
the tag length in octets. The tag length MUST be at least 1 and the tag length in octets. The tag length MUST be at least 1.
SHOULD be no more than 15.
Tag: The Property identifier, a sequence of US-ASCII characters. Tag: The Property identifier, a sequence of US-ASCII characters.
Tags MAY contain US-ASCII characters 'a' through 'z', 'A' through Tags MAY contain US-ASCII characters 'a' through 'z', 'A' through
'Z', and the numbers 0 through 9. Tags SHOULD NOT contain any other 'Z', and the numbers 0 through 9. Tags MUST NOT contain any other
characters. Matching of tags is case insensitive. characters. Matching of tags is case insensitive.
Tags submitted for registration by IANA MUST NOT contain any Tags submitted for registration by IANA MUST NOT contain any
characters other than the (lowercase) US-ASCII characters 'a' through characters other than the (lowercase) US-ASCII characters 'a' through
'z' and the numbers 0 through 9. 'z' and the numbers 0 through 9.
Value: A sequence of octets representing the Property Value. Value: A sequence of octets representing the Property Value.
Property Values are encoded as binary values and MAY employ sub- Property Values are encoded as binary values and MAY employ sub-
formats. formats.
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formats. formats.
The length of the value field is specified implicitly as the The length of the value field is specified implicitly as the
remaining length of the enclosing RDATA section. remaining length of the enclosing RDATA section.
4.1.1. Canonical Presentation Format 4.1.1. Canonical Presentation Format
The canonical presentation format of the CAA record is: The canonical presentation format of the CAA record is:
CAA <flags> <tag> <value> CAA <flags> <tag> <value>
Where: Where:
Flags: Is an unsigned integer between 0 and 255. Flags: Is an unsigned integer between 0 and 255.
Tag: Is a non-zero sequence of US-ASCII letters and numbers in lower Tag: Is a non-zero-length sequence of US-ASCII letters and numbers in
case. lower case.
Value: The value field, expressed as a contiguous set of characters Value: The value field, expressed as a contiguous set of characters
without interior spaces, or as a quoted string. See the the without interior spaces, or as a quoted string. See the <character-
<character-string> format specified in [RFC1035], Section 5.1, but string> format specified in [RFC1035], Section 5.1, but note that the
note that the value field contains no length byte and is not limited value field contains no length byte and is not limited to 255
to 255 characters. characters.
4.2. CAA issue Property 4.2. CAA issue Property
If the issue Property Tag is present in the Relevant RRSet for a If the issue Property Tag is present in the Relevant RRSet for a
Domain Name, it is a request that Issuers Fully-Qualified Domain Name, it is a request that Issuers
1. Perform CAA issue restriction processing for the Domain Name, and 1. Perform CAA issue restriction processing for the FQDN, and
2. Grant authorization to issue certificates containing that Domain 2. Grant authorization to issue certificates containing that FQDN to
Name to the holder of the issuer-domain-name or a party acting the holder of the issuer-domain-name or a party acting under the
under the explicit authority of the holder of the issuer-domain- explicit authority of the holder of the issuer-domain-name.
name.
The CAA issue Property Value has the following sub-syntax (specified The CAA issue Property Value has the following sub-syntax (specified
in ABNF as per [RFC5234]). in ABNF as per [RFC5234]).
issue-value = *WSP [issuer-domain-name *WSP] [";" *WSP [parameters *WSP]] issue-value = *WSP [issuer-domain-name *WSP] [";" *WSP [parameters *WSP]]
issuer-domain-name = label *("." label) issuer-domain-name = label *("." label)
label = (ALPHA / DIGIT) *( *("-") (ALPHA / DIGIT)) label = (ALPHA / DIGIT) *( *("-") (ALPHA / DIGIT))
parameters = (parameter *WSP ";" *WSP parameters) / parameter parameters = (parameter *WSP ";" *WSP parameters) / parameter
parameter = tag *WSP "=" *WSP value parameter = tag *WSP "=" *WSP value
tag = (ALPHA / DIGIT) *( *("-") (ALPHA / DIGIT)) tag = (ALPHA / DIGIT) *( *("-") (ALPHA / DIGIT))
value = *(%x21-3A / %x3C-7E) value = *(%x21-3A / %x3C-7E)
For consistency with other aspects of DNS administration, Domain Name For consistency with other aspects of DNS administration, FQDN values
values are specified in letter-digit-hyphen Label (LDH-Label) form. are specified in letter-digit-hyphen Label (LDH-Label) form.
The following CAA record set requests that no certificates be issued The following CAA record set requests that no certificates be issued
for the Domain Name 'certs.example.com' by any Issuer other than for the FQDN 'certs.example.com' by any Issuer other than
ca1.example.net or ca2.example.org. ca1.example.net or ca2.example.org.
certs.example.com CAA 0 issue "ca1.example.net" certs.example.com CAA 0 issue "ca1.example.net"
certs.example.com CAA 0 issue "ca2.example.org" certs.example.com CAA 0 issue "ca2.example.org"
Because the presence of an issue Property Tag in the Relevant RRSet Because the presence of an issue Property Tag in the Relevant RRSet
for a Domain Name restricts issuance, Domain Name owners can use an for an FQDN restricts issuance, FQDN owners can use an issue Property
issue Property Tag with no issuer-domain-name to request no issuance. Tag with no issuer-domain-name to request no issuance.
For example, the following RRSet requests that no certificates be For example, the following RRSet requests that no certificates be
issued for the Domain Name 'nocerts.example.com' by any Issuer. issued for the FQDN 'nocerts.example.com' by any Issuer.
nocerts.example.com CAA 0 issue ";" nocerts.example.com CAA 0 issue ";"
An issue Property Tag where the issue-value does not match the ABNF 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- grammar MUST be treated the same as one specifying an empty issuer-
domain-name. For example, the following malformed CAA RRSet forbids domain-name. For example, the following malformed CAA RRSet forbids
issuance: issuance:
malformed.example.com CAA 0 issue "%%%%%" malformed.example.com CAA 0 issue "%%%%%"
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4.3. CAA issuewild Property 4.3. CAA issuewild Property
The issuewild Property Tag has the same syntax and semantics as the The issuewild Property Tag has the same syntax and semantics as the
issue Property Tag except that it only grants authorization to issue issue Property Tag except that it only grants authorization to issue
certificates that specify a Wildcard Domain Name and issuewild certificates that specify a Wildcard Domain Name and issuewild
properties take precedence over issue properties when specified. properties take precedence over issue properties when specified.
Specifically: Specifically:
issuewild properties MUST be ignored when processing a request for a issuewild properties MUST be ignored when processing a request for a
Domain Name (that is, not a Wildcard Domain Name). Fully-Qualified Domain Name that is not a Wildcard Domain Name.
If at least one issuewild Property is specified in the Relevant RRSet If at least one issuewild Property is specified in the Relevant RRSet
for a Wildcard Domain Name, all issue properties MUST be ignored when for a Wildcard Domain Name, all issue properties MUST be ignored when
processing a request for that Wildcard Domain Name. processing a request for that Wildcard Domain Name.
For example, the following RRSet requests that _only_ ca1.example.net For example, the following RRSet requests that _only_ ca1.example.net
issue certificates for "wild.example.com" or "sub.wild.example.com", issue certificates for "wild.example.com" or "sub.wild.example.com",
and that _only_ ca2.example.org issue certificates for and that _only_ ca2.example.org issue certificates for
"*.wild.example.com" or "*.sub.wild.example.com). "*.wild.example.com" or "*.sub.wild.example.com). Note that this
presumes there are no CAA RRs for sub.wild.example.com.
wild.example.com CAA 0 issue "ca1.example.net" wild.example.com CAA 0 issue "ca1.example.net"
wild.example.com CAA 0 issuewild "ca2.example.org" wild.example.com CAA 0 issuewild "ca2.example.org"
The following RRSet requests that _only_ ca1.example.net issue The following RRSet requests that _only_ ca1.example.net issue
certificates for "wild2.example.com", "*.wild2.example.com" or certificates for "wild2.example.com", "*.wild2.example.com" or
"*.sub.wild2.example.com". "*.sub.wild2.example.com".
wild2.example.com CAA 0 issue "ca1.example.net" wild2.example.com CAA 0 issue "ca1.example.net"
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It permits any Issuer to issue for "wild3.example.com" or It permits any Issuer to issue for "wild3.example.com" or
"sub.wild3.example.com". "sub.wild3.example.com".
wild3.example.com CAA 0 issuewild "ca2.example.org" wild3.example.com CAA 0 issuewild "ca2.example.org"
4.4. CAA iodef Property 4.4. CAA iodef Property
The iodef Property specifies a means of reporting certificate issue The iodef Property specifies a means of reporting certificate issue
requests or cases of certificate issue for domains for which the requests or cases of certificate issue for domains for which the
Property appears in the Relevant RRSet, when those requests or Property appears in the Relevant RRSet, when those requests or
issuances violate the security policy of the Issuer or the Domain issuances violate the security policy of the Issuer or the FQDN
Name holder. holder.
The Incident Object Description Exchange Format (IODEF) [RFC7970] is The Incident Object Description Exchange Format (IODEF) [RFC7970] is
used to present the incident report in machine-readable form. used to present the incident report in machine-readable form.
The iodef Property Tag takes a URL as its Property Value. The URL The iodef Property Tag takes a URL as its Property Value. The URL
scheme type determines the method used for reporting: scheme type determines the method used for reporting:
mailto: The IODEF incident report is reported as a MIME email mailto: The IODEF incident report is reported as a MIME email
attachment to an SMTP email that is submitted to the mail address attachment to an SMTP email that is submitted to the mail address
specified. The mail message sent SHOULD contain a brief text message specified. The mail message sent SHOULD contain a brief text message
to alert the recipient to the nature of the attachment. to alert the recipient to the nature of the attachment.
http or https: The IODEF report is submitted as a Web service request http or https: The IODEF report is submitted as a Web service request
to the HTTP address specified using the protocol specified in to the HTTP address specified using the protocol specified in
[RFC6546]. [RFC6546].
These are the only supported URL schemes.
The following RRSet specifies that reports may be made by means of The following RRSet specifies that reports may be made by means of
email with the IODEF data as an attachment, a Web service [RFC6546], email with the IODEF data as an attachment, a Web service [RFC6546],
or both: or both:
report.example.com CAA 0 issue "ca1.example.net" 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 "mailto:security@example.com"
report.example.com CAA 0 iodef "http://iodef.example.com/" report.example.com CAA 0 iodef "http://iodef.example.com/"
4.5. Critical Flag 4.5. Critical Flag
The critical flag is intended to permit future versions of CAA to The critical flag is intended to permit future versions of CAA to
introduce new semantics that MUST be understood for correct introduce new semantics that MUST be understood for correct
processing of the record, preventing conforming CAs that do not processing of the record, preventing conforming CAs that do not
recognize the new semantics from issuing certificates for the recognize the new semantics from issuing certificates for the
indicated Domain Names. indicated FQDNs.
In the following example, the Property with a Property Tag of 'tbs' In the following example, the Property with a Property Tag of 'tbs'
is flagged as critical. Neither the ca1.example.net CA nor any other 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 Issuer is authorized to issue for "new.example.com" (or any other
domains for which this is the Relevant RRSet) unless the Issuer has domains for which this is the Relevant RRSet) unless the Issuer has
implemented the processing rules for the 'tbs' Property Tag. implemented the processing rules for the 'tbs' Property Tag.
new.example.com CAA 0 issue "ca1.example.net" new.example.com CAA 0 issue "ca1.example.net"
new.example.com CAA 128 tbs "Unknown" new.example.com CAA 128 tbs "Unknown"
5. Security Considerations 5. Security Considerations
CAA records assert a security policy that the holder of a Domain Name CAA records assert a security policy that the holder of an FDQN
wishes to be observed by Issuers. The effectiveness of CAA records wishes to be observed by Issuers. The effectiveness of CAA records
as an access control mechanism is thus dependent on observance of CAA as an access control mechanism is thus dependent on observance of CAA
constraints by Issuers. constraints by Issuers.
The objective of the CAA record properties described in this document The objective of the CAA record properties described in this document
is to reduce the risk of certificate mis-issue rather than avoid is to reduce the risk of certificate mis-issue rather than avoid
reliance on a certificate that has been mis-issued. DANE [RFC6698] reliance on a certificate that has been mis-issued. DANE [RFC6698]
describes a mechanism for avoiding reliance on mis-issued describes a mechanism for avoiding reliance on mis-issued
certificates. certificates.
5.1. Use of DNS Security 5.1. Use of DNS Security
Use of DNSSEC to authenticate CAA RRs is strongly RECOMMENDED but not Use of DNSSEC to authenticate CAA RRs is strongly RECOMMENDED but not
required. An Issuer MUST NOT issue certificates if doing so would required. An Issuer MUST NOT issue certificates if doing so would
conflict with the Relevant RRSet, irrespective of whether the conflict with the Relevant RRSet, irrespective of whether the
corresponding DNS records are signed. corresponding DNS records are signed.
DNSSEC provides a proof of non-existence for both DNS Domain Names DNSSEC provides a proof of non-existence for both DNS Fully-Qualified
and RRSets within Domain Names. DNSSEC verification thus enables an Domain Names and RRSets within FQDNs. DNSSEC verification thus
Issuer to determine if the answer to a CAA record query is empty enables an Issuer to determine if the answer to a CAA record query is
because the RRSet is empty or if it is non-empty but the response has empty because the RRSet is empty or if it is non-empty but the
been suppressed. response has been suppressed.
Use of DNSSEC allows an Issuer to acquire and archive a proof that Use of DNSSEC allows an Issuer to acquire and archive a proof that
they were authorized to issue certificates for the Domain Name. they were authorized to issue certificates for the FQDN.
Verification of such archives MAY be an audit requirement to verify Verification of such archives may be an audit requirement to verify
CAA record processing compliance. Publication of such archives MAY CAA record processing compliance. Publication of such archives may
be a transparency requirement to verify CAA record processing be a transparency requirement to verify CAA record processing
compliance. compliance.
5.2. Non-Compliance by Certification Authority 5.2. Non-Compliance by Certification Authority
CAA records offer CAs a cost-effective means of mitigating the risk CAA records offer CAs a cost-effective means of mitigating the risk
of certificate mis-issue: the cost of implementing CAA checks is very of certificate mis-issue: the cost of implementing CAA checks is very
small and the potential costs of a mis-issue event include the small and the potential costs of a mis-issue event include the
removal of an embedded trust anchor. removal of an embedded trust anchor.
5.3. Mis-Issue by Authorized Certification Authority 5.3. Mis-Issue by Authorized Certification Authority
Use of CAA records does not prevent mis-issue by an authorized Use of CAA records does not prevent mis-issue by an authorized
Certification Authority, i.e., a CA that is authorized to issue Certification Authority, i.e., a CA that is authorized to issue
certificates for the Domain Name in question by CAA records. certificates for the FQDN in question by CAA records.
Domain Name holders SHOULD verify that the CAs they authorize to FQDN holders SHOULD verify that the CAs they authorize to issue
issue certificates for their Domain Names employ appropriate controls certificates for their FQDNs employ appropriate controls to ensure
to ensure that certificates are issued only to authorized parties that certificates are issued only to authorized parties within their
within their organization. organization.
Such controls are most appropriately determined by the Domain Name Such controls are most appropriately determined by the FQDN holder
holder and the authorized CA(s) directly and are thus out of scope of and the authorized CA(s) directly and are thus out of scope of this
this document. document.
5.4. Suppression or Spoofing of CAA Records 5.4. Suppression or Spoofing of CAA Records
Suppression of the CAA record or insertion of a bogus CAA record Suppression of the CAA record or insertion of a bogus CAA record
could enable an attacker to obtain a certificate from an Issuer that could enable an attacker to obtain a certificate from an Issuer that
was not authorized to issue for that Domain Name. was not authorized to issue for an affected FQDN.
Where possible, Issuers SHOULD perform DNSSEC validation to detect Where possible, Issuers SHOULD perform DNSSEC validation to detect
missing or modified CAA record sets. missing or modified CAA record sets.
In cases where DNSSEC is not deployed for a corresponding Domain In cases where DNSSEC is not deployed for a corresponding FQDN, an
Name, an Issuer SHOULD attempt to mitigate this risk by employing Issuer SHOULD attempt to mitigate this risk by employing appropriate
appropriate DNS security controls. For example, all portions of the DNS security controls. For example, all portions of the DNS lookup
DNS lookup process SHOULD be performed against the authoritative name process SHOULD be performed against the authoritative name server.
server. Data cached by third parties MUST NOT be relied on but MAY Data cached by third parties MUST NOT be relied on as the sole source
be used to support additional anti-spoofing or anti-suppression of DNS CAA information but MAY be used to support additional anti-
controls. spoofing or anti-suppression controls.
5.5. Denial of Service 5.5. Denial of Service
Introduction of a malformed or malicious CAA RR could in theory Introduction of a malformed or malicious CAA RR could in theory
enable a Denial-of-Service (DoS) attack. 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 This specific threat is not considered to add significantly to the
risk of running an insecure DNS service. risk of running an insecure DNS service.
An attacker could, in principle, perform a DoS attack against an An attacker could, in principle, perform a DoS attack against an
Issuer by requesting a certificate with a maliciously long DNS name. Issuer by requesting a certificate with a maliciously long DNS name.
In practice, the DNS protocol imposes a maximum name length and CAA In practice, the DNS protocol imposes a maximum name length and CAA
processing does not exacerbate the existing need to mitigate DoS processing does not exacerbate the existing need to mitigate DoS
attacks to any meaningful degree. attacks to any meaningful degree.
5.6. Abuse of the Critical Flag 5.6. Abuse of the Critical Flag
A Certification Authority could make use of the critical flag to A Certification Authority could make use of the critical flag to
trick customers into publishing records that prevent competing trick customers into publishing records that prevent competing
Certification Authorities from issuing certificates even though the Certification Authorities from issuing certificates even though the
customer intends to authorize multiple providers. 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 In practice, such an attack would be of minimal effect since any
competent competitor that found itself unable to issue certificates competent competitor that found itself unable to issue certificates
due to lack of support for a Property marked critical SHOULD due to lack of support for a Property marked critical should
investigate the cause and report the reason to the customer. The investigate the cause and report the reason to the customer. The
customer will thus discover that they had been deceived. customer will thus discover that they had been deceived.
6. Deployment Considerations 6. Deployment Considerations
A CA implementing CAA may find that they receive errors looking up A CA implementing CAA may find that they receive errors looking up
CAA records. The following are some common causes of such errors, so CAA records. The following are some common causes of such errors, so
that CAs may provide guidance to their subscribers on fixing the that CAs may provide guidance to their subscribers on fixing the
underlying problems. underlying problems.
skipping to change at page 14, line 24 skipping to change at page 14, line 24
Some authoritative nameservers respond with REJECTED or NOTIMP when Some authoritative nameservers respond with REJECTED or NOTIMP when
queried for a Resource Record type they do not recognize. At least queried for a Resource Record type they do not recognize. At least
one authoritative resolver produces a malformed response (with the QR one authoritative resolver produces a malformed response (with the QR
bit set to 0) when queried for unknown Resource Record types. Per bit set to 0) when queried for unknown Resource Record types. Per
RFC 1034, the correct response for unknown Resource Record types is RFC 1034, the correct response for unknown Resource Record types is
NOERROR. NOERROR.
6.3. Delegation to Private Nameservers 6.3. Delegation to Private Nameservers
Some Domain Name administrators make the contents of a subdomain Some FQDN administrators make the contents of a subdomain
unresolvable on the public Internet by delegating that subdomain to a unresolvable on the public Internet by delegating that subdomain to a
nameserver whose IP address is private. A CA processing CAA records nameserver whose IP address is private. A CA processing CAA records
for such subdomains will receive SERVFAIL from its recursive for such subdomains will receive SERVFAIL from its recursive
resolver. The CA MAY interpret that as preventing issuance. Domain resolver. The CA MAY interpret that as preventing issuance. FQDN
Name administrators wishing to issue certificates for private Domain administrators wishing to issue certificates for private FQDNs SHOULD
Names SHOULD use split-horizon DNS with a publicly available use split-horizon DNS with a publicly available nameserver, so that
nameserver, so that CAs can receive a valid, empty CAA response for CAs can receive a valid, empty CAA response for those FQDNs.
those Domain Names.
6.4. Bogus DNSSEC Responses 6.4. Bogus DNSSEC Responses
Queries for CAA Resource Records are different from most DNS RR Queries for CAA Resource Records are different from most DNS RR
types, because a signed, empty response to a query for CAA RRs is types, because a signed, empty response to a query for CAA RRs is
meaningfully different from a bogus response. A signed, empty meaningfully different from a bogus response. A signed, empty
response indicates that there is definitely no CAA policy set at a response indicates that there is definitely no CAA policy set at a
given label. A bogus response may mean either a misconfigured zone, given label. A bogus response may mean either a misconfigured zone,
or an attacker tampering with records. DNSSEC implementations may or an attacker tampering with records. DNSSEC implementations may
have bugs with signatures on empty responses that go unnoticed, have bugs with signatures on empty responses that go unnoticed,
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difference to an end user between empty and bogus is irrelevant; they difference to an end user between empty and bogus is irrelevant; they
both mean a site is unavailable. both mean a site is unavailable.
In particular, at least two authoritative resolvers that implement In particular, at least two authoritative resolvers that implement
live signing had bugs when returning empty Resource Record sets for live signing had bugs when returning empty Resource Record sets for
DNSSEC-signed zones, in combination with mixed-case queries. Mixed- DNSSEC-signed zones, in combination with mixed-case queries. Mixed-
case queries, also known as DNS 0x20, are used by some recursive case queries, also known as DNS 0x20, are used by some recursive
resolvers to increase resilience against DNS poisoning attacks. resolvers to increase resilience against DNS poisoning attacks.
DNSSEC-signing authoritative resolvers are expected to copy the same DNSSEC-signing authoritative resolvers are expected to copy the same
capitalization from the query into their ANSWER section, but sign the capitalization from the query into their ANSWER section, but sign the
response as if they had use all lowercase. In particular, PowerDNS response as if they had used all lowercase. In particular, PowerDNS
versions prior to 4.0.4 had this bug. versions prior to 4.0.4 had this bug.
7. Differences versus RFC6844 7. Differences versus RFC6844
This document obsoletes RFC6844. The most important change is to the This document obsoletes RFC6844. The most important change is to the
Certification Authority Processing section. RFC6844 specified an Certification Authority Processing section. RFC6844 specified an
algorithm that performed DNS tree-climbing not only on the Domain algorithm that performed DNS tree-climbing not only on the FQDN being
Name being processed, but also on all CNAMEs and DNAMEs encountered processed, but also on all CNAMEs and DNAMEs encountered along the
along the way. This made the processing algorithm very inefficient way. This made the processing algorithm very inefficient when used
when used on Domain Names that utilize many CNAMEs, and would have on FQDNs that utilize many CNAMEs, and would have made it difficult
made it difficult for hosting providers to set CAA policies on their for hosting providers to set CAA policies on their own FQDNs without
own Domain Names without setting potentially unwanted CAA policies on setting potentially unwanted CAA policies on their customers' FQDNs.
their customers' Domain Names. This document specifies a simplified This document specifies a simplified processing algorithm that only
processing algorithm that only performs tree climbing on the Domain performs tree climbing on the FQDN being processed, and leaves
Name being processed, and leaves processing of CNAMEs and DNAMEs up processing of CNAMEs and DNAMEs up to the CA's recursive resolver.
to the CA's recursive resolver.
This document also includes a "Deployment Considerations" section This document also includes a "Deployment Considerations" section
detailing experience gained with practical deployment of CAA detailing experience gained with practical deployment of CAA
enforcement among CAs in the WebPKI. enforcement among CAs in the WebPKI.
This document clarifies the ABNF grammar for issue and issuewild tags This document clarifies the ABNF grammar for the issue and issuewild
and resolves some inconsistencies with the document text. In tags and resolves some inconsistencies with the document text. In
particular, it specifies that parameters are separated with particular, it specifies that parameters are separated with
semicolons. It also allows hyphens in Property Tags. semicolons. It also allows hyphens in Property Tags.
This document also clarifies processing of a CAA RRset that is not This document also clarifies processing of a CAA RRset that is not
empty, but contains no issue or issuewild tags. empty, but contains no issue or issuewild tags.
This document removes the section titled "The CAA RR Type," merging This document removes the section titled "The CAA RR Type," merging
it with "Mechanism" because the definitions were mainly duplicates. it with "Mechanism" because the definitions were mainly duplicates.
It moves the "Use of DNS Security" section into Security It moves the "Use of DNS Security" section into Security
Considerations. It renames "Certification Authority Processing" to Considerations. It renames "Certification Authority Processing" to
"Relevant Resource Record Set," and emphasizes the use of that term "Relevant Resource Record Set," and emphasizes the use of that term
to more clearly define which domains are affected by a given RRset. to more clearly define which domains are affected by a given RRset.
8. IANA Considerations 8. IANA Considerations
IANA is requested to add [[[ RFC Editor: Please replace with this RFC IANA is requested to add [[[ RFC Editor: Please replace with this RFC
]]] as a reference for the Certification Authority Restriction Flags ]]] as a reference for the Certification Authority Restriction Flags
and Certification Authority Restriction Properties registries. and Certification Authority Restriction Properties registries, and
update references to [RFC6844] within those registries to refer to
[[[ RFC Editor: Please replace with this RFC ]]]. IANA is also
requested to update the CAA TYPE in the DNS Parameters registry with
a reference to [[[ RFC Editor: Please replace with this RFC ]]].
9. Acknowledgements 9. Acknowledgements
The authors would like to thank the following people who contributed The authors would like to thank the following people who contributed
to the design and documentation of this work item: Corey Bonnell, to the design and documentation of this work item: Corey Bonnell,
Chris Evans, Stephen Farrell, Jeff Hodges, Paul Hoffman, Tim Chris Evans, Stephen Farrell, Jeff Hodges, Paul Hoffman, Tim
Hollebeek, Stephen Kent, Adam Langley, Ben Laurie, James Manger, Hollebeek, Stephen Kent, Adam Langley, Ben Laurie, James Manger,
Chris Palmer, Scott Schmit, Sean Turner, and Ben Wilson. Chris Palmer, Scott Schmit, Sean Turner, and Ben Wilson.
10. References 10. References
skipping to change at page 17, line 26 skipping to change at page 17, line 26
[RFC6546] Trammell, B., "Transport of Real-time Inter-network [RFC6546] Trammell, B., "Transport of Real-time Inter-network
Defense (RID) Messages over HTTP/TLS", RFC 6546, Defense (RID) Messages over HTTP/TLS", RFC 6546,
DOI 10.17487/RFC6546, April 2012, DOI 10.17487/RFC6546, April 2012,
<https://www.rfc-editor.org/info/rfc6546>. <https://www.rfc-editor.org/info/rfc6546>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS) of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <https://www.rfc-editor.org/info/rfc6698>. 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 [RFC7970] Danyliw, R., "The Incident Object Description Exchange
Format Version 2", RFC 7970, DOI 10.17487/RFC7970, Format Version 2", RFC 7970, DOI 10.17487/RFC7970,
November 2016, <https://www.rfc-editor.org/info/rfc7970>. November 2016, <https://www.rfc-editor.org/info/rfc7970>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
10.2. Informative References 10.2. Informative References
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