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CAA is a type of DNS record that allows site owners to specify which Certificate Authorities (CAs) are allowed to issue certificates containing their domain names. It was first standardized in 2013, and the version we use today was standardized in 2019 by RFC 8659 and RFC 8657. By default, every public CA is allowed to issue certificates for any domain name in the public DNS, provided they validate control of that domain name. That means that if there’s a bug in any one of the many public CAs’ validation processes, every domain name is potentially affected. CAA provides a way for domain holders to reduce that risk.
Using CAA
If you don’t care about CAA, you generally don’t have to do anything (but see CAA errors below). If you would like to use CAA to restrict which Certificate Authorities are allowed to issue certificates for your domain, you will need to use a DNS provider that supports setting CAA records. Check SSLMate’s CAA page for a list of such providers. If your provider is listed, you can use SSLMate’s CAA Record Generator to generate a set of CAA records listing the CAs that you would like to allow.
Where to put the record
Generally, you want to set CAA records on your registered domain (such as “example.org” or “mysite.co.uk”). This way they apply to both that domain and any subdomains you create under it, such as “community.example.org”.
Note that the CA will always respect the CAA record closest to the domain name it is issuing a certificate for. So if you’re requesting a cert for “www.community.example.org”, the CA will check “www.community.example.org”, then “community.example.org”, then “example.org”, stopping at the first CAA record it finds.
This means that you can override CAA for subdomains. For example, suppose that you host “example.org” yourself, but have “api.example.org” on a cloud provider. You could use a CAA record on “example.org” to say that only Let’s Encrypt can issue for that domain and all of its subdomains, but also use a CAA record on “api.example.org” to override that and allow the cloud provider to issue certificates for that one subdomain.
Note also that CAA checking follows CNAME redirects, just like all other DNS requests. If “community.example.org” is a CNAME to “example.forum.com”, the CA will respect any CAA records that are set on “example.forum.com”. It is not allowed for a domain name with a CNAME record to have any other records, so there cannot be conflicts between CAA records on the original name and CAA records on the target of the redirect.
What to put in the record
All CAA records follow the same basic format:
CAA <flags> <tag> <value>
The flags are just an integer, and should almost always just be the integer 0, indicating that no flags have been set. If you like, you can set the flags to the integer 128, indicating that the “critical bit” has been set, and that CAs should immediately halt and not issue a certificate if they don’t recognize the contents of the tag field.
The tag is a string indicating which kind of CAA record this is: either issue or issuewild in most cases. More on these below.
Finally, the value is a string containing at most one CA identifier (such as “letsencrypt.org”) and some optional semicolon-separated parameters, also discussed below.
The issue and issuewild properties
Records with the issue tag simply control whether a CA can issue certificates for this domain and its subdomains. Generally this is the only record you need, as it controls both normal (e.g. “example.org”) and wildcard (e.g. “*.example.org”) issuance in the absence of any other records. You control which CA can issue for this domain by putting that CA’s identifying domain name in the value portion of the CAA record.
Records with the issuewild tag control whether a CA can issue wildcard certificates (e.g. “*.example.org”). You only need to use issuewild records if you want different permissions for wildcard and non-wildcard issuance.
Note that you can have multiple records with the same property type and they are additive: if any one of those records allows the CA to issue, then it is allowed.
Let’s Encrypt’s identifying domain name for CAA is letsencrypt.org. This is
officially documented in Section 4.2.1 of our CP/CPS.
The validationmethods parameter
This parameter can be placed after the CA’s identifying domain name to control which validation methods that CA can use to confirm control over the domain. This can be used to restrict validation to methods that you trust more. For example, if you want to restrict the CA to only using the TLS-ALPN-01 method, you could append ;validationmethods=tls-alpn-01 to your CAA record value.
Let’s Encrypt recognizes the following validation method strings:
http-01dns-01tls-alpn-01
The accounturi parameter
This parameter can be placed after the CA’s identifying domain name to control which ACME Accounts can request issuance for the domain. This can be used to ensure that someone who temporarily hijacks your domain, but doesn’t have access to your ACME Account key, can’t issue malicious certificates.
Let’s Encrypt’s account URIs look like https://acme-v02.api.letsencrypt.org/acme/acct/1234567890, where the numbers at the end are your Account ID.
Examples
A simple CAA record which allows Let’s Encrypt to issue for “example.org” might look like this:
example.org CAA 0 issue "letsencrypt.org"
A more complex CAA record set might look like this:
example.org CAA 0 issue "myca.org;validationmethods=dns-01"
example.org CAA 0 issuewild "myca.org"
example.org CAA 128 issue "otherca.com;accounturi=https://otherca.com/acct/123456"
In this example, MyCA can issue for “example.org”, but only using the DNS-01 validation method. It can also issue wildcard certificates, using any validation method. Finally, OtherCA can also issue certificates, but only if the request comes from account number 123456, and only if OtherCA recognizes and knows how to correctly handle the accounturi restriction.
CAA errors
Since Let’s Encrypt checks CAA records before every certificate we issue, sometimes we get errors even for domains that haven’t set any CAA records. When we get an error, there’s no way to tell whether we are allowed to issue for the affected domain, since there could be CAA records present that forbid issuance, but are not visible because of the error.
If you receive CAA-related errors, try a few more times against our staging environment to see if they are temporary or permanent. If they are permanent, you will need to file a support issue with your DNS provider, or switch providers. If you’re not sure who your DNS provider is, ask your hosting provider.
Some DNS providers that are unfamiliar with CAA initially reply to problem reports with “We do not support CAA records.” Your DNS provider does not need to specifically support CAA records; it only needs to reply with a NOERROR response for unknown query types (including CAA). Returning other opcodes, including NOTIMP, for unrecognized qtypes is a violation of RFC 1035, and needs to be fixed.
SERVFAIL
One of the most common errors that people encounter is SERVFAIL. Most often this indicates a failure of DNSSEC validation. If you get a SERVFAIL error, your first step should be to use a DNSSEC debugger like dnsviz.net. If that doesn’t work, it’s possible that your nameservers generate incorrect signatures only when the response is empty. And CAA responses are most commonly empty. For instance, PowerDNS had this bug in version 4.0.3 and below.
If you don’t have DNSSEC enabled and get a SERVFAIL, the second most likely reason is that your authoritative nameserver returned NOTIMP, which as described above is an RFC 1035 violation; it should instead return NOERROR with an empty response. If this is the case, file a bug or a support ticket with your DNS provider.
Lastly, SERVFAILs may be caused by outages at your authoritative nameservers. Check the NS records for your nameservers and ensure that each server is available.
Timeout
Sometimes CAA queries time out. That is, the authoritative name server never replies with an answer at all, even after multiple retries. Most commonly this happens when your nameserver has a misconfigured firewall in front of it that drops DNS queries with unknown qtypes. File a support ticket with your DNS provider and ask them if they have such a firewall configured.