DNS Certification Authority Authorization (CAA) uses the Internet's Domain Name System to allow the holder of a domain to specify which certificate authorities (CAs) are allowed to issue certificates for that domain. This is not intended to support additional cross-checking at the client end of Transport Layer Security (TLS) connections (rather, DNS-based Authentication of Named Entities – DANE – is intended to be used for that purpose), but as a check for CAs to carry out as part of their issuance procedures. CAA records are intended to allow CAs to avoid mis-issuing certificates in some circumstances, while DANE records are intended to allow relying applications (TLS clients) to avoid relying on mis-issued certificates in some circumstances.^[1]

DNS Certification Authority Authorization is specified by RFC 6844. It defines a new "CAA" DNS resource record type for name-value pairs that can carry a wide range of information to be used as part of the CA authorization process. It may also be possible for certificate evaluators to use CAA records to detect possible mis-issued certificates. However, the certificate evaluator should consider that the CAA records may have changed between the time the certificate was issued and the time the certificate is observed by the evaluator.^[1]

Structure of CAA resource record

Each CAA resource record contains a flags byte and a property. The flags byte contains flags which can influence the interpretation of the record. The property consists of a "tag" which allows selection between several kinds of CAA record, and a "value" string whose meaning depends on the choice of tag.

Currently one flag is defined: the issuer critical flag is represented by the most significant bit of the flag's byte. If the issuer-critical flag's value is 1 (that is, a flags byte equal to 128), this indicates that a CA which does not understand or does not implement the property tag in this record should refuse to issue a certificate for the domain.^[1] This is similar to the way critical extensions in X509 certificates work.

Besides the flag, three property tags are defined:

issue

This property authorizes the holder of the domain specified in the "value" field to issue certificates for the domain for which the property is published.

issuewild

This property acts like issue but allows wildcard certificates.

iodef

This property specifies a method for CAs to report to the domain holder when a certificate is issued. Not all CAs support this tag, so there is no guarantee that all certificate issuances will be reported.

Supporting servers

As of 2016^[update], CAA records are supported in the BIND DNS server (as of version 9.10.1B),^[2] the NSD authoritative DNS server (as of version 4.0.1),^[3] the Knot DNS server (since version 2.2.0).^[4] and PowerDNS (since version 4.0.0).^[5]

Mandatory checking

Originally the implementation of CAA was voluntary: CAs could decide whether they would check for the records or not. However, in March 2017 the CA/Browser Forum voted in favor of a rule that will make CAA mandatory for all certificate authorities.^[6] Starting September 2017 all certificate authorities have to implement CAA checking.

Examples

In the following examples, assume that we want to control certificate issuance for the domain example.com.

To signify that only the CA Let's Encrypt can issue certificates to the domain, as well as all of its subdomains, one may use the CAA record

example.com.	IN	CAA	0 issue "letsencrypt.org"

To disallow issuance for a specific subdomain, nocerts.example.com, one would allow issuance only to the empty issuer list,

nocerts.example.com.	IN	CAA	0 issue ";"

In this case, in a request to issue a certificate to nocerts.example.com, the relevant CA will stop its search for RAA records at the subdomain. To signify that the CA may report certificate issues or policy violations by email to caa@example.com, or through Real-time Inter-network Defense to caa.example.com, the iodef property may be applied as follows:

nocerts.example.com.	IN	CAA	0 issue ";"
nocerts.example.com.	IN	CAA	0 iodef "mailto:caa@example.com"
nocerts.example.com.	IN	CAA	0 iodef "https://caa.example.com"

If, in the original example, we would like to allow issuance to specific domains, but disallow issuance of any wildcard certificates, the issuewild property may be applied:

example.com.	IN	CAA	0 issue "letsencrypt.org"
example.com.	IN	CAA	0 issuewild ";"

The critical flag is intended for usage when future inclusions of new properties may impact issuance. If, for example, a future version of CAA would include the tag "future", then the record set

example.com.	IN	CAA	0 issue "letsencrypt.org"
example.com.	IN	CAA	128 future "Some value"

would result in no issuance, if the CA does not know how to parse the record, for instance if it has not yet updated its parsing engine to the new version.

References

See also

• DNS-based Authentication of Named Entities
• HTTP Public Key Pinning
• List of DNS record types