How ACME HTTP-01 and DNS-01 Challenges Work Internally
Automated TLS certificate issuance looks simple from the outside. An ACME client requests a certificate, a certificate authority creates a challenge, the client proves control of the requested domain, and the CA issues the certificate. Once the process has been automated, certificate renewal can happen without an administrator manually generating a CSR, uploading validation files, or downloading…
The article discusses the internal workings of ACME HTTP-01 and DNS-01 challenges, which are used for automated TLS certificate issuance. While it may appear simple from the outside, the process involves several steps and challenges to ensure the requester truly controls the requested domain. ACME provides two commonly used challenge mechanisms: HTTP-01 and DNS-01, each proving domain control through different infrastructure paths.
HTTP-01 places a challenge response into the HTTP serving path, while DNS-01 places a cryptographically derived value into the DNS hierarchy. This difference affects which systems participate in validation, which credentials are required, and where certificate issuance can fail. The article emphasizes the importance of understanding ACME as a domain-control verification protocol, rather than just a certificate request process.
The certificate request creates a relationship between an identifier, an ACME account, an authorization, a challenge, and a validation result, with the CA performing the validation itself.
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