Configure popular ACME clients to use a private CA with the ACME protocol

The Automated Certificate Management Environment (ACME) protocol radically simplifies TLS deployment. With ACME, endpoints can obtain TLS certificates on their own, automatically. step-ca works with any ACME-compliant (specifically, ACMEv2; RFC8555) client.

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Here are the most common configuration parameters for any ACME client:

Most ACME clients connect to Let’s Encrypt’s CA by default. To connect to a private CA, you need to point the client your ACME Directory URL.

A single instance of step-ca can have multiple ACME provisioners, each with their own ACME Directory URL. The URL follows this form:

https://{ca-host}/acme/{provisioner-name}/directory

For example, an ACME provisioner named ACME on the host ca.internal has the directory URL:

https://ca.internal/acme/ACME/directory

You'll need to select the ACME challenge type.

Communication between an ACME client and server uses HTTPS. Many clients will validate the server’s TLS certificate using the public root certificates in your system’s default trust store. Some clients will let you pass a CA certificate bundle into the client.

Clients will validate the server’s HTTPS certificate using the public root certificates in your system’s default trust store. When you’re connecting to Let’s Encrypt, it’s a public certificate authority and its root certificate is already in your system’s default trust store. Your internal root certificate isn’t, so HTTPS connections from ACME clients to step-ca will fail.

There are two ways to address this challenge. Either:

step provides a helper command to do the latter:

If you are using your certificate authority for TLS in production, explicitly configuring your ACME client to only trust your root certificate is a better option. You will see how this method works with an example below. You can find several other examples here.

If you are simulating Let’s Encrypt in pre-production, installing your root certificate is a more realistic simulation of production. Once your root certificate is installed, no additional client configuration is necessary.

With most ACME clients, you can configure how often you want to renew your certificates. Choose a renewal period that is two-thirds of the entire certificate's lifetime, so that you'll have enough time to fix any renewal issues before it's too late.

certbot is the granddaddy of all ACME clients. Built and supported by the EFF, it's the standard-bearer for production-grade command-line ACME.

To get a certificate from step-ca using certbot you need to:

For example:

sudo is required in certbot's standalone mode so it can listen on port 80 to complete the http-01 challenge. If you already have a webserver running you can use webroot mode instead. With the appropriate plugin certbot also supports the dns-01 challenge for most popular DNS providers. Deeper integrations with nginx and apache can even configure your server to use HTTPS automatically (we'll set this up ourselves later). All of this works with step-ca.

You can renew all of the certificates you've installed using cerbot by running:

You can automate renewal with a simple cron entry:

The certbot packages for some Linux distributions will create a cron entry or systemd timer like this for you. This entry won't work with step-ca because it doesn't set the REQUESTS_CA_BUNDLE environment variable. You'll need to manually tweak it to do so.

More subtly, certbot's default renewal job is tuned for Let's Encrypt's 90 day certificate lifetimes: it's run every 12 hours, with actual renewals occurring for certificates within 30 days of expiry. By default, step-ca issues certificates with much shorter 24 hour lifetimes. The cron entry above accounts for this by running certbot renew every 15 minutes. You'll also want to configure your domain to only renew certificates when they're within a few hours of expiry by adding a line like:

to the top of your renewal configuration (e.g., in /etc/letsencrypt/renewal/foo.internal.conf).

acme.sh is another popular command-line ACME client. It's written completely in shell (bash, dash, and sh compatible) with very few dependencies.

To get a certificate from step-ca using acme.sh you need to:

For example:

Like certbot, acme.sh can solve the http-01 challenge in standalone mode and webroot mode. It can also solve the dns-01 challenge for many DNS providers.

Renewals are slightly easier since acme.sh remembers to use the right root certificate. It can also remember how long you'd like to wait before renewing a certificate. Unfortunately, the duration is specified in days (via the --days flag) which is too coarse for step-ca's default 24 hour certificate lifetimes. So the easiest way to schedule renewals with acme.sh is to force them at a reasonable frequency, like every 8 hours, via cron:

win-acme (wacs.exe) is a popular ACME client for Windows.

To use win-acme with step-ca, you'll need to do the following:

Caddy is an HTTP/2 web server with automatic HTTPS powered by an integrated ACME client. In addition to serving static websites, Caddy is commonly used as a TLS-terminating API gateway proxy.

Caddy comes with its own ACME server and by default it will generate an internal CA and issue certificates to itself. But, you can configure Caddy to use a local step-ca instance to obtain certificates.

Here's a Caddyfile global config block. Add this to the top of your Caddyfile to get certificates from ca.internal for all configured domains:

Here's a Caddyfile that will use ca.internal only to get a certificate for foo.internal:

Replace <step path> with the output of the step path command.

Now run caddy to start serving HTTPS!

Check your work with curl:

Caddy will automatically renew its certificates after ⅔ of the validity period elapses.

[NGINX][https://nginx.com/] doesn’t support ACME natively, but there are two options:

See njs-acme for full documentation.

Here’s an example nginx.conf that runs NGINX in a common configuration where it terminates TLS and proxies to a back-end server listening on local loopback:

With this code, you are telling NGINX to listen on port 443 using TLS, with a certificate and private key stored on disk. Other resources provide a more thorough explanation of NGINX's various TLS configuration options.

We can start an HTTP server using python and check our work with curl:

NGINX only reads certificates once, only at startup. When you renew the certificate on disk, NGINX won’t notice. After each renewal you’ll need to run the following command:

You can use the --exec flag on the step ca renew command to do this automatically:

If you’re using certbot, check out the --post-hook flag to do the same thing. If you’re using acme.sh, check out the --reloadcmd flag.

Apache httpd has integrated ACME support, via mod_md. Or you can deploy certificates to Apache using an external ACME client, such as certbot.

Here’s an example Apache configuration, using certificates issued by step-ca through certbot:

Start Apache and check your work with curl:

Apache needs to be signaled after certificates are renewed by running the following command:

Publish Lab’s acme-client is an excellent ACMEv2 client written in Node.js. Take a look at an example of how easy it is to obtain a certificate and serve HTTPS in JavaScript: https://gist.github.com/mmalone/f3c33a2381ffa3d67e86c6d5ad3042c9

Most importantly, to make things work:

To install dependencies and start the server run:

Then check your work with curl:

This server supports optional client authentication using certificates and checks if the client authenticated in the handler:

lego is an ACME client library written in Go. You can use it to obtain a certificate from step-ca programmatically. You can find an example of this code here: https://gist.github.com/mmalone/abce3c30df96972ed47f3298543be345

Essentially, the steps involved are:

Fetch the required dependencies and start the server:

Then test with curl:

The server is configured to verify client certificates if they are sent. That means the server is configured to support mutual TLS. The handler checks whether a client certificate was provided, and responds with a personalized greeting if one was.

You can get a client certificate from step-ca using an OAuth/OIDC provisioner:

And test mutual TLS out with curl:

With a few tweaks to this code you can implement robust access control.

There are other good options for programmatic ACME in Go. The certmagic package builds on lego and offers higher level, easier to use abstractions. The x/crypto/acme package is lower level and offers more control, but it currently implements a pre-standardization draft version of ACME that doesn’t work with step-ca.

certbot is written in Python and exposes its acme module as a standalone package. You can find an example of obtaining a certificate and serving HTTPS in Python here: https://gist.github.com/mmalone/12f5422b2ec68e64e9d11eae0c6ca47d

Make sure that you:

To install dependencies and start the server, run:

Then check your work with curl:

Like the Go example above, this server also supports mutual TLS and checks if the client authenticated in the handler:

Traefik is a modern reverse-proxy with integrated support for ACME. It's designed primarily to handle ingress for a compute cluster, dynamically routing traffic to microservices and web applications.

It's easy to get a certificate from step-ca in Traefik v2, using the tls-alpn-01 ACME challenge type.

Most importantly, Traefik will need to trust your root CA certificate. Either use the LEGO_CA_CERTIFICATES environment variable to provide the full path to your root_ca.crt when running traefik, or install your root certificate in your system's default trust store by running step certificate install root_ca.crt.

In your Traefik static configuration, you'll need to add a certificatesResolvers block:

Then, when you add routers to your dynamic configuration for HTTPS traffic, you need to set tls and tls.certresolver:

If you're running Traefik inside a Docker container, you can get your root CA certificate and add it to the container's trust store by running the following:

To get a certificate from step-ca to Traefik v1 you need to:

Here’s an example traefik.toml file that configures Traefik to terminate TLS and proxy to a service listening on localhost:

Start Traefik by running:

Start an HTTP server for Traefik to proxy to, and test with curl:

Certify The Web is a popular ACME Certificate Manager for Windows. It provides a full UI for managing thousands of certificates, supports a wide range of built in deployment tasks and integrates with many DNS API providers. Commercial licensing and support is also available.

To use with step-ca: