CAA DNS Record Synthesizer
Synthesize RFC 8659-compliant Certificate Authority Authorization (CAA) DNS records with wildcard security controls and iodef alerts.
Policy Configuration
When an unauthorized issuance attempt is blocked by a CA, telemetry will be sent to this endpoint.
DNS Record Output
example.com. 3600 IN CAA 0 issue "letsencrypt.org" example.com. 3600 IN CAA 0 issuewild ";" example.com. 3600 IN CAA 0 iodef "mailto:security@example.com"
3
example.com
3600s
Configured
100% Client-Side In-Memory Execution: All CAA DNS zone syntax compiling, RFC 8659 flag calculations, and parameter parsing execute natively inside your browser. No domain names, private ACME account endpoints, or incident email addresses are ever transmitted or stored on external servers.
The CAA Standard: Securing the PKI Trust Hierarchy with DNS Validation
Public Key Infrastructure (PKI) underpins all modern HTTPS transport encryption. However, by default, the web operates on an open-trust model: any publicly trusted Certificate Authority (CA) among hundreds worldwide can issue a valid certificate for any domain name, provided domain ownership is validated. If a single CA suffers an infrastructure breach, misconfiguration, or government coercion, rogue certificates can be minted to execute undetectable man-in-the-middle (MITM) attacks.
Certificate Authority Authorization (CAA), codified in RFC 6844 and updated in RFC 8659, solves this vulnerability. CAA enables domain administrators to publish explicit DNS records dictating exactly which CAs possess authorization to issue certificates for their hostnames. As of September 2017, the CA/Browser Forum mandates that every publicly trusted CA must perform automated CAA DNS record checks prior to issuing any certificate.
Attack Surface Elimination
Restricting certificate issuance to only your chosen vendors (e.g., Let's Encrypt or DigiCert) immediately strips hundreds of untrusted or obscure CAs of signing authority.
Wildcard Scope Control
Use the separate issuewild tag to prohibit wildcard certificates completely or restrict wildcard provisioning to dedicated high-security teams.
Real-Time Breach Auditing
The iodef tag forces conforming CAs to transmit immediate violation telemetry via email or webhooks when unauthorized issuance attempts occur.
Technical Anatomy: Flags, Tags, and Parameter Directives
A CAA DNS Resource Record (DNS type 257) consists of three mandatory fields: an integer flag byte, a tag string, and a character string value:
| Field Component | Accepted Values | RFC Specification Role | Operational Behavior |
|---|---|---|---|
| Flags (Byte) | 0 or 128 | Issuer Compliance Requirement | Flag 0 is non-critical. Flag 128 sets the Issuer Critical bit; if the CA cannot parse the directive, it must refuse issuance. |
| issue | "ca-domain.com" or ";" | Direct Hostname Authorization | Authorizes the named CA to issue certificates for single hostnames and subdomains. A value of ";" prohibits all issuance. |
| issuewild | "ca-domain.com" or ";" | Wildcard Scope Authorization | Explicitly controls wildcard certificates (*.domain.com). Overrides standard "issue" tags for wildcard issuance only. |
| iodef | mailto: or https:// | Incident Telemetry Endpoint | Specifies where conforming CAs submit incident reports when an unlisted entity attempts to generate a certificate. |
| accounturi | https://acme.../acct/ID | Account Binding (RFC 8657) | Restricts certificate issuance exclusively to your specific ACME account ID with the authorized CA. |
Implementation Checklist: Enterprise DNS Deployment & Pitfalls
To successfully integrate CAA records into corporate DNS architectures without causing certificate renewal outages, observe these core implementation practices:
Critical Best Practices
- • Account Binding for Zero-Trust: Use
accounturiparameters with Let's Encrypt. This prevents other tenants who pass DNS challenges on your shared IPs from issuing certificates. - • Subdomain Inheritance Awareness: Remember that subdomains climb up the DNS tree until finding a CAA record. An apex CAA record protects all subdomains unless a subdomain specifies its own.
- • Include Multi-Cloud Fallbacks: If you use Cloudflare Universal SSL in front of AWS ALB, you must add both
cloudflare.comandamazon.com. - • Set Realistic TTLs: Use a 3600-second TTL during initial deployment so policy adjustments take effect promptly during maintenance windows.
Operational Pitfalls
- • Accidental Auto-Renewal Lockouts: Omitting automated renewal agents (like Certbot or ACME clients) causes automated certificate renewals to silently fail at expiration. Before deploying strict restrictive policies, cross-examine your active TLS certificates with our SSL certificate checker to ensure the signing Certificate Authority matches your authorized issuer list.
- • DNS Server SERVFAIL Responses: If an authoritative DNS nameserver does not support RFC 3597 unknown record types and returns SERVFAIL for CAA queries, compliant CAs must treat this as a failure and deny issuance. After adding your synthesized records, query your zone using our DNS record inspector to verify that your authoritative nameservers answer cleanly without protocol errors.
- • CNAME Tree Lookup Clashes: When a domain aliases via CNAME to another provider, the CA follows the CNAME target. Ensure your CNAME targets do not carry restrictive CAA records that block your primary authority.
- • Trailing Quotes in Zone Files: When pasting BIND zone files, ensure quotation marks around the CA canonical domain are correctly escaped.
Frequently Asked Questions (FAQ)
What is a CAA (Certificate Authority Authorization) DNS record?
A CAA (Certificate Authority Authorization) record is a type of DNS Resource Record defined in RFC 8659. It allows domain owners to explicitly specify which Certificate Authorities (CAs) are legally permitted to issue SSL/TLS certificates for their hostnames. Mandatory CA/Browser Forum rules require all publicly trusted CAs to verify CAA records before signing any certificate.
What happens if a domain has no CAA records configured?
If a domain publishes no CAA records, any publicly trusted Certificate Authority in the world is permitted to issue a certificate for that domain, provided the applicant satisfies baseline domain validation checks. This leaves your domain vulnerable to compromised or rogue CAs.
What is the difference between "issue" and "issuewild" tags?
The "issue" tag authorizes a CA to issue both single-domain and wildcard certificates unless an "issuewild" tag is present. The "issuewild" tag explicitly overrides the "issue" tag specifically for wildcard certificates (*.domain.com). Setting "issuewild ';'" prohibits all wildcard certificates while allowing normal FQDN certificates.
How does the "iodef" tag report security incidents?
The "iodef" (Incident Object Description Exchange Format) tag specifies an email address (mailto:admin@domain.com) or an HTTPS endpoint where CAs must dispatch telemetry reports whenever an unauthorized entity attempts to request a certificate for your domain.
What does the Critical Flag (value 128) mean in a CAA record?
By default, the CAA flag is 0 (non-critical). If flag 128 (critical bit) is set, a CA MUST understand and comply with the exact tag and parameter extensions. If the CA does not support or recognize an extension marked as critical, it is legally obliged to abort the issuance.
Do subdomains inherit parent domain CAA records?
Yes. When checking CAA compliance, CAs traverse up the DNS tree. If api.sub.example.com does not have its own CAA record, the CA evaluates sub.example.com, and finally example.com. Once a record set is discovered, the search halts without further traversal.
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