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Frequently Asked Questions
What is the ACME protocol?
The ACME (Automatic Certificate Management Environment) protocol is a standardized communication protocol used to automate the issuance, renewal, and revocation of SSL/TLS certificates. Defined in RFC 8555, it enables certificate authorities like Let's Encrypt to verify domain ownership and issue certificates without manual intervention, typically completing the process in under 60 seconds.
How does the ACME protocol automate certificate renewal?
ACME automates renewal by running a client (such as Certbot or acme.sh) on your server that communicates with the certificate authority before expiration — typically 30 days out. The client completes a domain validation challenge (HTTP-01, DNS-01, or TLS-ALPN-01), receives the renewed certificate, and installs it automatically, eliminating manual renewal steps entirely.
What are the different ACME challenge types?
ACME supports three primary challenge types: HTTP-01, which places a token file at a well-known URL on port 80; DNS-01, which requires creating a specific TXT record in your domain's DNS; and TLS-ALPN-01, which validates via a self-signed certificate on port 443. DNS-01 is the only option that supports wildcard certificates.
Which ACME clients are most commonly used?
The most popular ACME clients include Certbot (maintained by the EFF), acme.sh (a lightweight shell script), Caddy (a web server with built-in ACME support), and Lego (written in Go). Certbot has over 300 million certificates issued and supports Apache, Nginx, and standalone modes. Choice depends on your server environment and automation needs.
Is ACME only used with Let's Encrypt?
No. While Let's Encrypt popularized ACME, other certificate authorities also support it, including ZeroSSL, Buypass Go, and Google Trust Services. Additionally, enterprise tools like Smallstep and HashiCorp Vault use ACME for internal PKI, allowing organizations to automate private certificate management across their infrastructure using the same protocol.
How do I set up ACME certificate automation on my server?
Install an ACME client like Certbot, register an account with your chosen certificate authority, and run the client with your domain name. For example: certbot --nginx -d example.com. The client handles validation, certificate installation, and configures a cron job or systemd timer for automatic renewal every 60–90 days.
What happens if ACME certificate renewal fails?
If renewal fails, most ACME clients retry automatically over several days before the certificate expires. Common failure causes include firewall rules blocking port 80, DNS misconfiguration, or rate limits (Let's Encrypt allows 50 certificates per domain per week). Monitoring tools like Certbot's built-in hooks or external services like UptimeRobot can alert you before expiration.
This is why we built CertPulse
CertPulse connects to your AWS, Azure, and GCP accounts, enumerates every certificate, monitors your external endpoints, and watches Certificate Transparency logs. One dashboard for every cert. Alerts when auto-renewal fails. Alerts when certs approach expiry. Alerts when someone issues a cert for your domain that you didn't request.
If you're looking for complete certificate visibility without maintaining scripts, we can get you there in about 5 minutes.