The Libravatar mirrors are setup using DNS round-robin which makes it a little challenging to automatically provision Let's Encrypt certificates.

In order to be able to use Certbot's webroot plugin, I need to be able to simultaneously host a randomly-named file into the webroot of each mirror. The reason is that the verifier will connect to seccdn.libravatar.org, but there's no way to know which of the DNS entries it will hit. I could copy the file over to all of the mirrors, but that would be annoying since some of the mirrors are run by volunteers and I don't have direct access to them.

Thankfully, Scott Helme has shared his elegant solution: proxy the .well-known/acme-challenge/ directory from all of the mirrors to a single validation host. Here's the exact configuration I ended up with.

DNS Configuration

In order to serve the certbot validation files separately from the main service, I created a new hostname, acme.libravatar.org, pointing to the main Libravatar server:

CNAME acme libravatar.org.

Mirror Configuration

On each mirror, I created a new Apache vhost on port 80 to proxy the acme challenge files by putting the following in the existing port 443 vhost config (/etc/apache2/sites-available/libravatar-seccdn.conf):

<VirtualHost *:80>
    ServerName __SECCDNSERVERNAME__
    ServerAdmin __WEBMASTEREMAIL__

    ProxyPass /.well-known/acme-challenge/ http://acme.libravatar.org/.well-known/acme-challenge/
    ProxyPassReverse /.well-known/acme-challenge/ http://acme.libravatar.org/.well-known/acme-challenge/
</VirtualHost>

Then I enabled the right modules and restarted Apache:

a2enmod proxy
a2enmod proxy_http
systemctl restart apache2.service

Finally, I added a cronjob in /etc/cron.daily/commit-new-seccdn-cert to commit the new cert to etckeeper automatically:

#!/bin/sh
cd /etc/libravatar
/usr/bin/git commit --quiet -m "New seccdn cert" seccdn.crt seccdn.pem seccdn-chain.pem > /dev/null || true

Main Configuration

On the main server, I created a new webroot:

mkdir -p /var/www/acme/.well-known

and a new vhost in /etc/apache2/sites-available/acme.conf:

<VirtualHost *:80>
    ServerName acme.libravatar.org
    ServerAdmin [email protected]
    DocumentRoot /var/www/acme
    <Directory /var/www/acme>
        Options -Indexes
    </Directory>
</VirtualHost>

before enabling it and restarting Apache:

a2ensite acme
systemctl restart apache2.service

Registering a new TLS certificate

With all of this in place, I was able to register the cert easily using the webroot plugin on the main server:

certbot certonly --webroot -w /var/www/acme -d seccdn.libravatar.org

The resulting certificate will then be automatically renewed before it expires.