Wildcard certificates are really useful, especially in cases where you are using a load balancer like HAProxy that targets multiple backends serving separate subdomains.

This article will describe:

  • How to install certbot (correctly) and run it in manual certonly mode to only get a certificate without any additional config magic.
  • Receive a wildcard DNS certificate with Subject Alt Names, allowing multiple domains in the same certificate.
  • Smash it together in a way so that HAProxy can use it.

Reservations:

  • Installing certbot correctly requires snap, which is gross. In the future I’ll explore using acme.sh instead.
  • There might be better ways to prep the certificate for HAProxy than smashing it together manually or via a script, I don’t know.
  • In order to receive wildcard certificates, authentication must be done over DNS records (not HTTP).

Step 1: Install certbot⌗

In order to get the latest version of certbot and any plugins required, you must install via snap. Regardless of what you think about snap, this is important because the version differences to, for example, the Ubuntu 22 repos, is immense.

First, if you already have apt packages or similar system packages installed providing certbot or any certbot plugins, remove them. Ensure you have snap installed and running, then do:

snap install certbot

In this case, we will be DNS authenticating against Cloudflare, so we also need the plugin for that:

snap set certbot trust-plugin-with-root=ok # required
snap install certbot-dns-cloudflare

Create a symlink to /usr/bin:

ln -s /snap/bin/certbot /usr/bin/certbot

Verify with certbot --version

Step 2: Set up configuration⌗

Ensure you have a file like /root/.secrets/cloudflare.ini that is only readable by root and contains something like:

dns_cloudflare_email = "[email protected]"
dns_cloudflare_api_key = "786sdf6f87fs87ssdffd"

You can set up an API key via the Cloudflare web GUI (google it).

Step 3: Request certificate⌗

Now that certbot has access to modifying your DNS records, you can run something like:

certbot certonly --dry-run --dns-cloudflare \
--dns-cloudflare-credentials /root/.secrets/cloudflare.ini \
-d "my.domain,*.my.domain" \
--agree-tos --email [email protected] --preferred-challenges dns-01 -v

This will use your Cloudflare credentials and the --dns-cloudflare plugin to make DNS changes on your behalf, validating your ownership of the domain. The certificate will be issued to both my.domain and *.my.domain, meaning that it will also work for any subdomains.

In order to actually receive a certificate, you must remove --dry-run.

Step 4: Smash certificate⌗

Once you have successfully gotten a certificate, you’ll see something like:

Successfully received certificate.
Certificate is saved at: /etc/letsencrypt/live/my.domain/fullchain.pem
Key is saved at:         /etc/letsencrypt/live/my.domain/privkey.pem
This certificate expires on 2024-06-01.

In order for HAProxy to use this certificate, both the full chain and the private key must be contained in one single PEM file. This is easily achieved by doing:

cat /etc/letsencrypt/live/my.domain/fullchain.pem \
/etc/letsencrypt/live/my.domain/privkey.pem > \
/etc/ssl/haproxy/my_cert.pem

Step 5: Configure HAProxy⌗

You’ll only need to do this once, if you keep using the same path for the certificate:

bind *:443 ssl crt /etc/ssl/haproxy/

HAProxy is smart enough to figure out which certificate to use, so that should be enough. For general TLS configuration, see the HAProxy SSL/TLS configuration tutorial

Whenever the certificate has changed on disk, you’ll want to do:

systemctl reload haproxy