DNS Certification Authority Authorization

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.

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.

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. This is similar to the way critical extensions in X509 certificates work.

Besides the flag, three property tags are defined:

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

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