DNS Certification Authority Authorization (CAA) is an Internet security mechanism which uses resource records in the Domain Name System (DNS) to allow domain name holders to specify which certificate authorities are authorized to issue certificates for that domain, and which types of certificates they are able to issue.^[1] In September 2017, the CA/Browser Forum will made it mandatory for publicly trusted certificate authorities to check a domain's CAA records before issuing a certificate.^[2] CAA is specified by RFC 6844 which defines a new "CAA" DNS resource record type containing name-value pairs of the certificate authorities and whether they are able to issue wildcard certificates or only specific-subdomain certificates.^[1] Unlike DNS-based Authentication of Named Entities (DANE), it is a purely administrative control, and does not prevent a malicious certificate authority from issuing fraudulent certificates. Third parties monitoring certificate authority behaviour might check newly issued certificates against the domain's CAA records, but must be aware that a domain's CAA records may have changed between the time the certificate was issued and the time the third-party makes checks them.

Record structure

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.^[1]

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 X.509 certificates work.

Besides the flag, three property tags are defined:^[1]

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.

Support

As of 2018^[update], CAA records are supported by BIND (since version 9.10.1B),^[3] Knot DNS (since version 2.2.0),^[4][5] ldns (since version 1.6.17),^[4] NSD (as of version 4.0.1)^[4][6], OpenDNSSEC,^[4] PowerDNS (since version 4.0.0),^[4] Simple DNS Plus (since version 6.0),^[4] tinydns^[4] and Windows Server 2016.^[4] Many hosted DNS providers also support CAA records, including Amazon Route 53,^[7] Cloudflare, DNS Made Easy and Google Cloud DNS.^[4]

Mandatory checking

Initially the implementation of CAA was voluntary. However, in March 2017 the CA/Browser Forum voted in favor of making CAA checking mandatory for all certificate authorities.^[2] All publicly trusted certificate authorities have until September 2017 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 Let's Encrypt certificate authority 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 an 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 CAA records at the subdomain. To signify that the CA may report certificate issues or policy violations by email to caa@example.com, or through RID messages 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