Caddy

Caddy is an extensible HTTPS server platform written in Go. It serves HTTP/1.1, HTTP/2, and HTTP/3, manages TLS certificates automatically through ACME issuers like ZeroSSL and Let's Encrypt, and runs anywhere with no external dependencies.

What Caddy is

At its core, Caddy is a Go program that loads a JSON configuration document and runs zero or more "apps" (top-level modules). Two apps ship in the standard distribution:

• http — a configurable HTTP server with routing, matchers, and a middleware-style handler chain (modules/caddyhttp/app.go).
• tls — TLS certificate management built on top of CertMagic (modules/caddytls/tls.go).

Two more apps ride along in the default build: pki for an internal certificate authority (modules/caddypki/pki.go) and events for a small pub/sub event system (modules/caddyevents/app.go).

Most users never touch the JSON. They write a Caddyfile and let the caddyfile adapter convert it to JSON before Caddy loads it (caddyconfig/httpcaddyfile/). Other adapters (NGINX, YAML, JSON 5, TOML) live as separate plugins.

Who uses it

Caddy is used as a public-facing web server, a reverse proxy, an API gateway, a static file server, and an embedded HTTP platform inside other Go programs. It is in production at scale — the project notes cover trillions of requests and millions of managed certificates.

The repo also exposes a developer-facing surface:

• A library API (github.com/caddyserver/caddy/v2) that other Go programs can import to embed Caddy or build custom binaries.
• A plugin system via xcaddy for compiling Caddy with third-party modules.
• An admin HTTP API for live configuration, defaulting to localhost:2019.

Quick links

• Architecture — apps, modules, and the request lifecycle
• Getting started — build, run, and serve a hello-world site
• Glossary — Caddy-specific terms (App, Module, Provisioner, Replacer, etc.)
• How to contribute — workflow, tests, and conventions
• Apps — http, tls, pki, events
• Systems — module registry, admin API, listeners, replacer, storage, logging
• Features — automatic HTTPS, reverse proxy, file server, Caddyfile
• Reference — config, dependencies, environment variables

Repository at a glance

Path	Role
caddy.go, admin.go, modules.go, context.go, replacer.go, logging.go, storage.go, listeners.go	Core platform (the caddy package)
cmd/	CLI surface (caddy run, caddy reload, caddy adapt, …)
cmd/caddy/main.go	Default binary entry point
caddyconfig/	Config adapter framework + Caddyfile parser + HTTP Caddyfile adapter
modules/caddyhttp/	HTTP app, matchers, handlers, routes
modules/caddytls/	TLS app, issuers, loaders, connection policies
modules/caddypki/	Internal certificate authority + ACME server
modules/caddyevents/	Event app
modules/logging/	Log encoders and writers
modules/metrics/	Prometheus admin endpoint
modules/filestorage/	File-based storage backend
modules/standard/imports.go	Default module registry — controls what ships in the standard binary
caddytest/	Integration test harness and fixtures