Excerpt: Certificate Authority Authorization records limit which certificate authorities may issue TLS certificates for your domain. This guide shows how a scheduled web cron job can detect missing, conflicting, or unauthorized CAA changes before they weaken issuance policy or block a legitimate renewal.
CAA is a DNS-based control that tells public certificate authorities which organizations are allowed to issue certificates for a domain. It can also define separate policy for wildcard certificates and provide an incident-reporting address. Because certificate authorities check CAA during issuance, one small DNS mistake can either broaden authorization unexpectedly or prevent an urgent renewal.
What a CAA monitor should verify
- Record presence: protected domains and subdomains still publish the expected policy.
- Authorized issuers: every issue and issuewild value belongs to the approved certificate authority set.
- Wildcard policy: issuewild is explicit when wildcard issuance should differ from normal issuance.
- Critical flags: unknown critical tags are not accidentally introduced.
- Reporting destination: iodef addresses remain valid and monitored.
- Resolver agreement: authoritative servers and public resolvers return consistent answers.
- Inheritance: child domains inherit or override policy exactly as intended.
CAA monitoring belongs beside DNS record change monitoring, but it needs policy-aware comparisons. A generic DNS diff can tell you that text changed; a CAA monitor explains whether issuance became broader, narrower, or invalid.
Understand tags, flags, and inheritance
A CAA record contains a flag byte, a tag, and a value. The issue tag authorizes normal certificate issuance. The issuewild tag controls wildcard issuance. The iodef tag provides a location for incident reports. A value may include issuer-specific parameters, so compare normalized components rather than a raw string only.
Certificate authorities walk up the DNS hierarchy when a name has no CAA record. That means a policy at the registrable domain can cover many subdomains, while a record on one child can override inherited policy. Your monitor should calculate the effective policy for every certificate name, not just query the apex once.
Build a policy inventory
Create a small configuration that lists production hostnames, whether wildcard certificates are permitted, the approved issuers, required parameters, expected inheritance source, and incident-reporting destination. Treat this as the desired state.
| Hostname class | Expected policy | Primary risk |
|---|---|---|
| Apex and www | Approved issue issuer | Unauthorized issuer added |
| Wildcard names | Explicit issuewild decision | Wildcard unintentionally permitted |
| Delegated subdomain | Documented local override | Parent policy assumed incorrectly |
| Inactive certificate name | Deny issuance if appropriate | Forgotten name remains issuable |
Connect this inventory to certificate transparency monitoring. CAA expresses who should be able to issue; transparency logs show what was actually issued.
Create a validation endpoint
Build a server-side endpoint that queries the authoritative nameservers and at least two validating recursive resolvers. It should normalize results, calculate effective policy, compare that result with the inventory, and return a non-success HTTP status for critical differences.
load certificate names and desired CAA policy
for each name:
query authoritative servers
find the closest effective CAA record set
normalize tags, issuer domains, and parameters
compare issue and issuewild authorization
validate iodef destinations
compare public resolver results
classify changes as broader, narrower, or equivalent
return 200 only when required policy matchesKeep registrar or DNS-provider credentials inside the application. The public monitoring endpoint should expose only health and non-sensitive diagnostics. Protect it from abuse using the principles in secure web cron endpoints.
Normalize before comparing
DNS tooling may change case, spacing, quoting, and record order without changing meaning. Lowercase issuer domains, trim permitted whitespace, sort independent records, and parse semicolon-separated parameters. Preserve flag values and unknown tags exactly because they can alter processing.
Store both the normalized effective policy and the raw authoritative responses. The normalized form prevents formatting-only alerts; the raw data helps an operator diagnose a real mismatch.
Detect policy broadening and narrowing
Not every change has the same severity. Adding an unapproved issuer broadens authority and should be critical. Removing the only approved issuer narrows policy and can break renewal. Changing only record order is equivalent. A useful alert states the direction of change.
- Critical: an unknown issuer is newly authorized.
- Critical: the expected issuer no longer appears.
- Critical: wildcard issuance becomes allowed unexpectedly.
- Warning: iodef is missing or points to an unmonitored address.
- Warning: one authoritative nameserver serves a stale policy.
- Informational: formatting or order changed with no semantic effect.
Query every authoritative server
A partially completed DNS deployment can leave nameservers disagreeing. Query each authority directly and compare its SOA serial and CAA answer. A recursive resolver may cache the healthy answer and conceal the stale server until later.
Use DNSSEC validation monitoring to confirm that signed CAA answers have a valid chain of trust. A correct-looking CAA value does not help if validators classify the response as bogus.
Test expected issuance behavior safely
Do not request a real production certificate on every run. Instead, keep automated issuance checks in a controlled staging name when your certificate authority offers a test environment. For production, compare DNS policy with the issuer configured in your ACME client and run renewal dry-runs on a separate schedule.
Monitor certificate lifetime independently with SSL expiration alerts. CAA health does not prove that renewal ran, and a valid certificate does not prove that future issuance will succeed.
Handle intentional changes
DNS and certificate migrations often require a temporary overlap where both old and new issuers are authorized. Define a change window with an owner, approved issuers, and automatic expiration. Alert if the temporary issuer remains after the window closes.
Pair the rollout with TLS protocol and cipher monitoring so issuance policy and deployed endpoint security are reviewed together.
Avoid false positives
- Respect TTLs during documented propagation windows.
- Retry one timeout before treating it as a policy failure.
- Require two consecutive resolver disagreements before escalation.
- Compare semantic issuer sets rather than record order.
- Do not assume every subdomain inherits from the apex; calculate the closest record set.
- Expire temporary exceptions automatically.
Schedule and alerting
Run authoritative and recursive checks every 15 to 60 minutes for important domains and immediately after DNS, registrar, ACME, or certificate-provider changes. A daily inventory can verify that every active certificate name has an owner and an intentional effective policy.
Alerts should include the queried name, inheritance source, authoritative server, previous issuer set, current issuer set, wildcard decision, TTL, and first-seen time. Link to the change request or runbook without placing secrets in the alert.
For a broader domain safety program, combine this check with domain expiration monitoring.
Implementation checklist
- Inventory every production certificate name and wildcard.
- Define approved issuers and parameters as desired state.
- Calculate effective inherited policy for each name.
- Query every authoritative server and two recursive resolvers.
- Normalize formatting while preserving meaningful flags and tags.
- Classify changes as broader, narrower, or equivalent.
- Test alerts with a safe staging record.
- Schedule the endpoint and prevent overlapping runs.
Final takeaway
CAA is small DNS data with significant operational impact. Scheduled policy-aware monitoring can detect unauthorized issuers, renewal-blocking mistakes, wildcard drift, and incomplete DNS rollouts before the next certificate request depends on them.