Stefan's Blog

How I run Certbot (as non-root and automated)

I have previously noted that I get all my TLS certificates from Let’s Encrypt, but since my usage of the client deviates quite a bit from the standard, I figured I should take a few minutes to describe my setup.

Prelude: Configuration management

My system configuration is formatted as holograms that can be compiled with holo-build and applied with Holo. This article is about my hologram-letsencrypt, so I’ll be linking to its source code a few times. If you don’t know about Holo, don’t worry; the source code should be clear enough as to instruct you how to port this setup to other configuration management schemes.

Choosing the challenge method

In the ACME protocol, you need to prove your control over the domain name for which you are requesting a certificate. The two most common methods are the DNS challenge (where you need to place a certain TXT record in the domain’s DNS) and the webroot challenge (where you need to configure your webserver to serve a certain file at a certain defined URL on that domain). Since I want a method that works regardless of DNS providers (I have a lot of these, some without automatable DNS zone API), I went with the webroot challenge.

This means that every server that wants to get TLS certificates needs to run an HTTP server on port 80 to respond to the webroot challenge. I chose nginx, which is my go-to choice for HTTP servers, and configured it to

The latter implies that all my web domains exclusively use HTTPS, which is the case on all my servers since I don’t care about ad servers or other garbage.

The webserver is activated as a systemd unit called nginx-for-letsencrypt.service. When there are actual websites on that server, another nginx will be listening on port 443 to serve these. This separation is useful to break a circular dependency: The nginx that listens on port 443 needs the TLS certificates to start, but Certbot can only run if nginx is running on port 80. If both nginx were the same, manual intervention would be required to resolve this dependency cycle when setting up a new server or a new website. It’s also nice because the nginx on port 443 can be left out entirely when not needed, i. e. when the server needs TLS certificates for services that are not HTTPS (e. g. XMPP or mail).

Side-note: This is also the reason why I haven’t looked at webservers with integrated Let’s Encrypt support, such as Caddy. If you only serve HTTPS, then Caddy is probably a good idea, but my method allows me to issue certificates for all TLS-enabled protocols using the same procedure.

Certbot

I started to use Certbot back in 2015 when it was still called letsencrypt. There were no other mature ACME clients at this point, so I played it safe by going with the reference implementation. The big drawback of Certbot is that it supposedly only runs with root privileges. I found this to be very untrue. I’m running it under user/group letsencrypt. All that it takes is to chown letsencrypt:letsencrypt a few things:

Now I can invoke Certbot like this:

sudo -u letsencrypt -g letsencrypt \
  certbot certonly --quiet --non-interactive --agree-tos \
  --keep-until-expiring --webroot -w /srv/letsencrypt/ -d "$domain"

That’s a mouthful, so let’s take this apart:

Finding out which domains to handle

Now we basically just need to call Certbot once for every domain that we need a TLS certificate for. (You can also issue certificates for multiple domain names at once, but I like to keep things neatly separated.) I use the fact that every TLS certificate and private key must be mentioned in the configuration file of the server that provides the TLS-secured endpoint. For example, an nginx configuration will have a statement like this:

ssl_certificate /etc/letsencrypt/live/www.example.org/fullchain.pem;

With some grep and cut, you can easily extract the domain names from the nginx.conf. This is what collector scripts do in my setup. I have one for each service with TLS support, e. g.  for nginx or for Prosody.

A small shell script, fittingly called letsencrypt-allofthem, runs all of these collectors and then invokes Certbot once for each of the collected domains. That script also has hooks that allow to reload or restart services afterwards, so that the new certificates and keys can be loaded before the old ones in memory expire.

End-to-end example: Deploying a TLS-enabled static website in just 2 minutes

I have some further automation for static websites. I just dump the HTML files etc. into a directory /data/static-web/www.example.org. Now only two steps are left: sudo holo apply renders an nginx configuration like this:

$ cat /etc/nginx/sites-enabled/static-web.conf
server {
    server_name www.example.org;
    include     /etc/nginx/server-baseline-https.inc;

    # CSP includes unsafe-inline to allow <style> tags in hand-written HTML
    add_header  Content-Security-Policy "default-src 'self' 'unsafe-inline'" always;

    ssl_certificate     /etc/letsencrypt/live/www.example.org/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/www.example.org/privkey.pem;
    ssl_trusted_certificate /etc/letsencrypt/live/www.example.org/chain.pem;

    location / {
        root    /data/static-web/www.example.org;
        index   index.html index.htm;
        charset utf-8;
    }
}

The second step is sudo letsencrypt-allofthem, which provisions the TLS certificate and key for that domain, and because that also reloads nginx at the end, the new configuration takes effect immediately.

Created: Fri, 24 Feb 2017 16:49:59 UTC