Table of Contents
What’s a CAA record?
A Certification Authority Authorization (CAA) record is used to specify which certificate authorities (CAs) are allowed to issue certificates for a domain.
CAA records allow domain owners to declare which certificate authorities are allowed to issue a certificate for a domain. They also provide a means of indicating notification rules in case someone requests a certificate from an unauthorized certificate authority. If no CAA record is present, any CA is allowed to issue a certificate for the domain. If a CAA record is present, only the CAs listed in the record(s) are allowed to issue certificates for that hostname.
CAA records can set policy for the entire domain or for specific hostnames. CAA records are also inherited by subdomains. For example, a CAA record set on
example.com also applies to any subdomain, like
subdomain.example.com (unless overridden). CAA records can control the issuance of single-name certificates, wildcard certificates, or both.
The DNS CAA record is specified by RFC 6844.
CAA record format
The structure of a CAA record follows the standard top-level format definition defined in RFC 1035. The RDATA section is composed of the following elements:
|flag||An unsigned integer between 0-255.|
|It is currently used to represent the critical flag, that has a specific meaning per RFC.|
|tag||An ASCII string that represents the identifier of the property represented by the record.|
|value||The value associated with the tag.|
The CAA record consists of a flags byte and a tag-value pair referred to as a ‘property’. Multiple properties may be associated with the same domain name by publishing multiple CAA RRs at that domain name.
The canonical representation is:
CAA <flags> <tag> <value>
The RFC currently defines three available tags:
issue: explicitly authorizes a single certificate authority to issue a certificate (any type) for the hostname.
issuewild: explicitly authorizes a single certificate authority to issue a wildcard certificate (and only wildcard) for the hostname.
iodef: specifies a URL to which a certificate authority may report policy violations.
issuewild tags take precedence over
issue tags when specified. Once there’s one CAA record with the
issuewild tag in place, regardless of its value, wildcard certificate requests will be rejected unless there’s a specific CAA record with the
issuewild tag for that CA and the requested hostname.
In DNSimple, the CAA record is represented by the following customizable elements:
|Name||The host name for the record, without the domain name. This is generally referred to as “subdomain”. We automatically append the domain name.|
|TTL||The time-to-live in seconds. This is the amount of time the record is allowed to be cached by a resolver.|
|Tag||An ASCII string that represents the identifier of the property represented by the record.|
|Value||The value associated with the tag.|
We don’t allow configuration of the bit flag.
CAA record usage
Each CAA record contains only one tag-value pair. The tag must be one of the available tags. For example, if you want to limit the issuance of SSL certificates for
example.com to the Let’s Encrypt certificate authority, add the following CAA record:
example.com. CAA 0 issue "letsencrypt.org"
To allow both Let’s Encrypt and Comodo, add 2 CAA records, one for each CA:
example.com. CAA 0 issue "comodoca.com" example.com. CAA 0 issue "letsencrypt.org"
To allow Let’s Encrypt and Comodo only for wildcard, use
example.com. CAA 0 issue "letsencrypt.org" example.com. CAA 0 issuewild "comodoca.com"
The presence of issuewild overrides the
issue. Let’s Encrypt is not allowed to issue wildcard certificates.
To be notified of policy violations, add a record with the
iodef tag that contains the email address to notify:
example.com. CAA 0 iodef "mailto:email@example.com"
The records are inherited by child hostnames, which are offshoots of the parent hostname. Let’s look at an example of subdomain configuration:
example.com. CAA 0 issue "letsencrypt.org" alpha.example.com. CAA 0 issue "comodoca.com" beta.example.com. CAA 0 issue "letsencrypt.org" beta.example.com. CAA 0 issue "comodoca.com"
In the example above, Let’s Encrypt is the default CA for the example.com domain. However, only Comodo can issue a certificate for
alpha.example.com. Both Comodo and Let’s Encrypt can issue certificates for
beta.example.com. And what about
foo.example.com? Because no record exists for
foo.example.com, but there’s a record for
example.com, in this case only Let’s Encrypt is allowed to issue for
Comodo Wildcard Certificates
Customers who purchase a Comodo wildcard certificate from us need to make sure they have an
issuewild CAA record, because they add an additional single-name to the certificate to cover the non-wildcard name. For example, buying a certificate for
*.example.com issues a certificate with both
*.example.com in the certificate names. This means you need to configure your CAA records:
example.com. CAA 0 issue "comodoca.com" example.com. CAA 0 issuewild "comodoca.com"
The above rules also apply to subdomain wildcard certificates.
Querying CAA records
The CAA record is a relatively new resource record (RR), and not all tools support it. A notable example is
dig – it doesn’t support the standard syntax for querying CAA records. To query the CAA record for a domain with
dig, you must specify the RR type (257) directly.
$ dig google.com type257 ; <<>> DiG 9.8.3-P1 <<>> google.com type257 ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 64266 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION: ;google.com. IN TYPE257 ;; ANSWER SECTION: google.com. 86399 IN TYPE257 \# 19 0005697373756573796D616E7465632E636F6D ;; Query time: 51 msec ;; SERVER: 18.104.22.168#53(22.214.171.124) ;; WHEN: Thu Dec 29 21:07:18 2016 ;; MSG SIZE rcvd: 59
dig also doesn’t support CAA inheritance, and the output of the query is the binary-encoded record (although in newer versions of
dig, support for parsing the record data is present).
To test the development of our CAA implementation, we developed a simple utility called
dnscaa. It’s a Go package that allows you to fetch CAA records, and it comes with a handy CLI.
$ digcaa google.com 1 records found google.com. 86399 IN CAA 0 issue "symantec.com"
Manage CAA records
From the DNSimple record editor you can add, remove, and update CAA records.