Certbot ACME Client

This free, open-source software tool automates the process of obtaining and renewing TLS and SSL certificates from the Let's Encrypt certificate authority, dramatically simplifying HTTPS deployment for millions of websites. Developed by the Electronic Frontier Foundation (EFF) and first released in 2014 (originally named letsencrypt), the tool has become the de facto standard for automated certificate management with over 31,000 stars as of 2026. Key features include: ACME protocol (full RFC 8555 Automatic Certificate Management Environment implementation, communicating with Let's Encrypt and other ACME v2 compatible certificate authorities), automatic domain validation (HTTP-01, TLS-ALPN-01, and DNS-01 challenge types for proving domain ownership without manual intervention), automatic certificate installation (configuring web servers including Apache, Nginx, and HAProxy with the obtained certificate, modifying server configuration automatically), automatic renewal (systemd timer or cron job automatically renewing certificates before expiration, with configurable pre and post hooks for reloading web servers), wildcard certificates (obtaining wildcard certificates via DNS-01 challenge with plugins for Cloudflare, Route 53, DigitalOcean, and Linode DNS APIs), ECC and RSA certificates (supporting both Elliptic Curve Cryptography and RSA key generation with configurable key sizes), certificate revocation (revoking compromised or unwanted certificates via ACME protocol), staging environment (testing certificate issuance against the Let's Encrypt staging environment without rate limits for CI/CD and testing), webroot and standalone modes (serving challenges via existing web server document root or built-in standalone HTTP server), and DNS plugin ecosystem (over 20 DNS provider plugins for automated DNS-01 challenge response).