> For the complete documentation index, see [llms.txt](https://0xgosu.gitbook.io/monolith-framework/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://0xgosu.gitbook.io/monolith-framework/examples/auto-tls.md).

# AutoTLS (ACME / Let's Encrypt)

**Automatic Let's Encrypt certificates for the embedded NATS server.** The framework obtains a certificate over ACME on startup and renews it in the background — no certificate files to provision, and no restart or reload at renewal time.

**Unlike every other example, this one talks to a real certificate authority.** It needs a publicly resolvable domain pointing at the host and inbound port 80, so it cannot complete inside a dev container or CI. Running it with no configuration is safe: it prints what it would do and exits without contacting any CA.

```go
app, err := mono.NewMonoApplication(
    mono.WithNATSHost("0.0.0.0"),
    mono.WithNATSPort(4222),
    mono.WithNATSAutoTLS(types.AutoTLSConfig{
        Domains:   []string{"nats.example.com"},
        Email:     "ops@example.com",
        CacheDir:  "/var/lib/mono/acme",
        AcceptTOS: true,
    }),
)
```

## Key points

* **Client-to-server connections only**: the certificate is installed on the NATS client listener. Route (cluster), gateway, leafnode, websocket and MQTT listeners each have their own separate TLS configuration and are untouched, so traffic between cluster nodes stays plaintext unless you configure it yourself. Routes are peer-to-peer and conventionally use mutual TLS, which needs a client certificate autocert does not issue — use an internal CA and a `cluster { tls { ... } }` block in a [NATS config file](/monolith-framework/examples/nats-config.md) instead.
* **http-01 only**: the framework serves the challenge from its own listener on port 80, for the whole life of the process — every renewal re-runs the challenge. tls-alpn-01 would need the listener on port 443, and autocert does not implement dns-01, so wildcard domains are rejected.
* **`CacheDir` is required and must persist**: it holds the ACME account key and the issued certificates. Losing it on every restart forces reissuance and will exhaust Let's Encrypt's limit of 5 duplicate certificates per week. There is deliberately no default, so persistence is an explicit decision.
* **Clients must dial by hostname over `tls://`**: the certificate is selected from the TLS SNI extension, which `nats.go` takes from the URL host. Dialling by IP presents no SNI and is refused, and `Domains` doubles as the allowlist, so any other name is refused too.
* **Enabling AutoTLS makes TLS mandatory**: plaintext clients are rejected once a certificate is configured. Turning it on is a breaking change for an existing deployment.
* **Startup is fail-fast**: the certificate is obtained before the framework reports ready, so a misconfigured domain or an unreachable port 80 fails startup instead of surfacing as a handshake error hours later.
* **Staging first**: this example defaults to the Let's Encrypt staging endpoint, whose rate limits are far more forgiving while DNS and firewall rules are still being sorted out. Staging issues from an untrusted root, so clients must be told to trust it. Drop `DirectoryURL` to use production once a staging run succeeds.

## Prerequisites

1. A public DNS A record for the domain, pointing at the host.
2. Inbound TCP port 80 reachable from the internet, for the http-01 challenge.
3. Permission to bind port 80 — root, or `CAP_NET_BIND_SERVICE`.
4. A cache directory that survives restarts.

## Run

```bash
# Dry run: prints the configuration and prerequisites, contacts nothing.
cd examples/auto-tls && go run .

# For real, against Let's Encrypt staging.
MONO_ACME_DOMAIN=nats.example.com \
MONO_ACME_EMAIL=ops@example.com \
MONO_ACME_CACHE_DIR=/var/lib/mono/acme \
  go run ./examples/auto-tls
```

There is no `make run-example-N` target for this example, because it cannot complete without public DNS — the same reason `event-emitter` and `nats-config` have none.

Once started, the echo service is reachable over TLS:

```go
nc, err := nats.Connect("tls://nats.example.com:4222")
reply, err := nc.Request("services.echo.say", []byte("hello"), 5*time.Second)
// reply.Data == "echo: hello"
```

## Related Documentation

* [Foundation Spec — AutoTLS Design](https://github.com/go-monolith/mono/tree/main/docs/spec/foundation.md)
* [Implementation Spec](https://github.com/go-monolith/mono/tree/main/internal/nats/autotls.spec.md)
* [NATS Config File Example](/monolith-framework/examples/nats-config.md)
* [SECURITY.md](https://github.com/go-monolith/mono/tree/main/SECURITY.md)


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