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 documentare 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:¶

Certificate:

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

Certificate Evaluator:

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

Certification Authority (CA):

An Issuer that issues certificates in accordance with a specified Certificate Policy.¶

Certificate Policy (CP):

Specifies the criteria that a CA undertakes to meet in its issue of certificates. See[RFC3647].¶

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].¶

Domain Name:

The label assigned to a node in the Domain Name 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 any revisions to these specifications.¶

Fully Qualified Domain Name (FQDN):

A domain name that includes the labels of all superior nodes in the DNS.¶

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.¶

Relevant Resource Record Set (Relevant RRset):

A set of CAA Resource Records resulting from applying the algorithm inSection 3 to a specific FQDN or Wildcard Domain Name.¶

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].¶

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 RRs of a particular owner name, class, and type. The time to live on all RRs within an RRset is always the same, but the data may be different among RRs in the RRset.¶

Wildcard Domain Name:

A domain name consisting of a single asterisk character followed by a single "full stop" character ("*.") followed by an FQDN.¶

4.1.Syntax

A CAA RR contains a single Property consisting of a tag‑valuepair. An FQDNMAY have multiple CAA RRs associated with it, and agiven Property TagMAY be specified more than once across those RRs.¶

The RDATA section for a CAA RR contains one Property. A Propertyconsists 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 thenumber 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", the Property is critical. A CAMUST NOT issue certificates for any FQDN if the Relevant 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 0is the Most Significant Bit and bit 7 is the Least SignificantBit. Thus, according to those conventions, the Flags value 1 means that bit 7 isset, 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 recordscompliant with this version of the CAA specificationMUST clear(set to "0") all reserved flag bits. Applications that interpretCAA recordsMUST ignore the value of all reserved flag bits.¶

Tag Length:

A single octet containing an unsigned integer specifyingthe tag length in octets. The tag lengthMUST be at least 1.¶

Tag:

The Property identifier -- a sequence of ASCII characters.¶

TagsMAY contain ASCII characters "a" through "z", "A"through "Z", and the numbers 0 through 9. TagsMUST NOTcontain any other characters. Matching of tags is caseinsensitive.¶

Tags submitted for registration by IANAMUST NOT contain anycharacters other than the (lowercase) ASCII characters "a"through "z" and the numbers 0 through 9.¶

Value:

A sequence of octets representing the Property Value.Property Values are encoded as binary values andMAY employsub‑formats.¶

The length of the Value field is specified implicitly as theremaining length of the enclosing RDATA section.¶

   CAA <flags> <tag> <value>

4.2.CAA issue Property

If the issue Property Tag is present in the Relevant RRset for anFQDN, 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 (specifiedin 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) / parameterparameter = tag *WSP "=" *WSP valuetag = (ALPHA / DIGIT) *( *("-") (ALPHA / DIGIT))value = *(%x21-3A / %x3C-7E)

For consistency with other aspects of DNS administration, FQDNvalues are specified in letter-digit-hyphen Label (LDH-Label) form.¶

The following CAA RRset requests that nocertificates be issued for the FQDN "certs.example.com" by anyIssuer 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 RRsetfor an FQDN restricts issuance, FQDN owners can use an issueProperty Tag with no issuer-domain-name to request no issuance.¶

For example, the following RRset requests that nocertificates be issued for the FQDN "nocerts.example.com" by anyIssuer.¶

nocerts.example.com       CAA 0 issue ";"

An issue Property Tag where the issue-value does not match the ABNFgrammarMUST be treated the same as one specifying an empty issuer‑domain-name. Forexample, the following malformed CAA RRset forbids issuance:¶

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

CAA authorizations are additive; thus, the result of specifying bothan empty issuer-domain-name and a non-empty issuer-domain-name is thesame as specifying just the non-empty issuer-domain-name.¶

An IssuerMAY choose to specify parameters that furtherconstrain the issue of certificates by that Issuer -- for example,specifying that certificates are to be subject to specific validationpolicies, billed to certain accounts, or issued under specific trustanchors.¶

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

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

The semantics of parameters to the issue Property Tag are determined bythe Issuer alone.¶

4.3.CAA issuewild Property

The issuewild Property Tag has the same syntax and semantics as the issueProperty Tag except that it only grants authorization toissue certificates that specify a Wildcard Domain Name and each issuewildProperty takes precedence over each issue Property when specified.Specifically:¶

Each issuewild PropertyMUST be ignored when processing a request foran FQDN that is not a Wildcard Domain Name.¶

If at least one issuewild Property is specified in the RelevantRRset for a Wildcard Domain Name, each issue PropertyMUSTbe ignored when processing a request for that Wildcard Domain Name.¶

For example, the following RRset requests thatonlyca1.example.net issue certificates for "wild.example.com" or"sub.wild.example.com", and thatonly ca2.example.org issue certificates for"*.wild.example.com" or "*.sub.wild.example.com". Note that this presumesthat 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 requests thatonly ca1.example.net issuecertificates 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 thatonly ca2.example.org issuecertificates for "*.wild3.example.com" or "*.sub.wild3.example.com". Itdoes 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 requests thatonly ca2.example.org issuecertificates for "*.wild3.example.com" or "*.sub.wild3.example.com". Itpermits 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 issuerequests or cases of certificate issue for domains for which the Propertyappears in the Relevant RRset, when those requests or issuancesviolate the security policy of the Issuer or the FQDN holder.¶

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

The iodef Property Tag takes a URL as its Property Value. The URL scheme typedetermines the method used for reporting:¶

mailto:

The IODEF report is reported as a MIME email attachment to an SMTP email that is submitted to the mail address specified. The mail message sentSHOULD 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 specifiesthat reports may be made by means of email with the IODEF data as anattachment, 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 "https://iodef.example.com/"

4.5.Critical Flag

The critical flag is intended to permit future versions of CAA tointroduce new semantics thatMUST be understood for correctprocessing of the record, preventing conforming CAs that do notrecognize the new semantics from issuing certificates for theindicated FQDNs.¶

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 Issuer is authorized toissue for "new.example.com" (or any other domains for which this isthe Relevant RRset) unless the Issuer has implemented theprocessing rules for the "tbs" Property Tag.¶

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

5.1.Use of DNS Security

The use of DNSSEC to authenticate CAA RRs is stronglyRECOMMENDED but notrequired. An IssuerMUST NOT issue certificates if doing so wouldconflict with the Relevant RRset, irrespective ofwhether the corresponding DNS records are signed.¶

DNSSEC provides a proof of non-existence for both DNS FQDNs andRRsets within FQDNs. DNSSEC verification thus enables an Issuer todetermine whether the answer to a CAA record query (1) is empty becausethe RRset is empty or (2) is non-empty but the response has beensuppressed.¶

The use of DNSSEC allows an Issuer to acquire and archive a proof thatthey were authorized to issue certificates for the FQDN.Verification of such archives may be an audit requirement to verifyCAA record-processing compliance. Publication of such archives maybe a transparency requirement to verify CAA record-processingcompliance.¶

5.2.Non-compliance by Certification Authority

CAA records offer CAs a cost-effective means of mitigating the riskof certificate mis-issue: the cost of implementing CAA checks is verysmall, and the potential costs of a mis-issue event include theremoval 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 authorizedCA, i.e., a CA that is authorized to issuecertificates for the FQDN in question by CAA records.¶

FQDN holdersSHOULD verify that the CAs they authorize toissue certificates for their FQDNs employ appropriate controls toensure that certificates are issued only to authorized parties withintheir organization.¶

Such controls are most appropriately determined by the FQDNholder and the authorized CA(s) directly and are thus outside the scope ofthis document.¶

5.4.Suppression or Spoofing of CAA Records

Suppression of a CAA record or insertion of a bogus CAA recordcould enable an attacker to obtain a certificate from an Issuer thatwas not authorized to issue for an affected FQDN.¶

Where possible, IssuersSHOULD perform DNSSEC validation to detectmissing or modified CAA RRsets.¶

In cases where DNSSEC is not deployed for a corresponding FQDN, anIssuerSHOULD attempt to mitigate this risk by employing appropriateDNS security controls. For example, all portions of the DNS lookupprocessSHOULD be performed against the authoritative nameserver.Data cached by third partiesMUST NOT be relied on as the sole source of DNS CAAinformation butMAY be used tosupport 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 ofauthoritative DNS records or by spoofing inflight DNS responses.¶

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

An attacker could, in principle, perform a DoS attack against anIssuer by requesting a certificate with a maliciously long DNS name.In practice, the DNS protocol imposes a maximum name length, and CAAprocessing does not exacerbate the existing need to mitigate DoSattacks to any meaningful degree.¶

5.6.Abuse of the Critical Flag

A CA could make use of the critical flag totrick customers into publishing records that prevent competing CAsfrom issuing certificates even though thecustomer intends to authorize multiple providers. This could happen if thecustomers were setting CAA records based on data provided by the CA rather thangenerating those records themselves.¶

In practice, such an attack would be of minimal effect, since anycompetent competitor that found itself unable to issue certificatesdue to lack of support for a Property marked critical shouldinvestigate the cause and report the reason to the customer. Thecustomer will thus discover that they had been deceived.¶

6.1.Blocked Queries or Responses

Some middleboxes -- in particular, anti-DDoS appliances -- may be configured todrop DNS packets of unknown types, or they may start dropping such packets whenthey consider themselves under attack. This generally manifests as a timed-outDNS 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 queriedfor an RR type they do not recognize. At least one authoritativeresolver produces a malformed response (with the QR (Query/Response) bit set to "0") when queriedfor unknown RR types. Per[RFC1034], the correct response RCODE forunknown RR types is 0 ("No error condition").¶

6.3.Delegation to Private Nameservers

Some FQDN administrators make the contents of a subdomain unresolvable on thepublic Internet by delegating that subdomain to a nameserver whose IP address isprivate. A CA processing CAA records for such subdomains will receiveSERVFAIL from its recursive resolver. The CAMAY interpret that as preventingissuance. FQDN administrators wishing to issue certificates for privateFQDNsSHOULD use split-horizon DNS with a publicly available nameserver, sothat 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, becausea signed, empty response to a query for CAA RRs is meaningfully differentfrom a bogus response. A signed, empty response indicates that there isdefinitely no CAA policy set at a given label. A bogus response may meaneither a misconfigured zone or an attacker tampering with records. DNSSECimplementations may have bugs with signatures on empty responses that gounnoticed, because for more common RR types like A and AAAA,the difference to an end user between empty and bogus is irrelevant; theyboth mean a site is unavailable.¶

In particular, at least two authoritative resolvers that implement live signinghad bugs when returning empty RRsets for DNSSEC-signed zones, incombination with mixed-case queries. Mixed‑case queries, also known as DNS 0x20,are used by some recursive resolvers to increase resilience against DNSpoisoning attacks. DNSSEC-signing authoritative resolvers are expected to copythe same capitalization from the query into their ANSWER section but also to sign theresponse as if they had used all lowercase. In particular, PowerDNS versionsprior to 4.0.4 had this bug.¶