Certificates from Let's Encrypt [1] are free and popular – supposedly half of all digital certificates used on the web originate there. However, anyone running web servers with many subdomains has had the displeasure of having to manage a bag full of certificates. A single wildcard certificate from one of the usual certification bodies solves the problem in principle, but it is expensive.

This changed in March 2018: All those who have access to their domain name's DNS server can now also obtain wildcard certificates from Let's Encrypt.

If you have not yet installed the Let's Encrypt client, you can load it as follows:

git clone https://github.com/letsencrypt/letsencrypt

After that, web admins usually have to add some Python packages. That's easily taken care of with

letsencrypt/letsencrypt-auto --help

which will ask you for your sudo password when launched. Let's Encrypt uses a DNS-based challenge-response method for authentication.

Let's Dance

The following command lets me request a certificate for the namespace *.sensorenresidenz.de (just don't ask why I registered the domain):

sudo /usr/local/letsencrypt/letsencrypt-auto certonly --manual --preferred-challenges dns --server https://acme-v02.api.letsencrypt.org/directory --email charly@kuehnast.com --domains *.sensorenresidenz.de

After a second of recollection, the Let's Encrypt client answers. It wants me to create a TXT record named _acme-challenge.sensorenresidenz.de in the DNS with a given random string as content:

Please deploy a DNS TXT record under the name _acme-challenge.sensorenresidenz.de with the following value:
ST8ehm-bKS6wRAxZk5vYDzUO9-OEr3NeIpClFyaq-kA
Before continuing, verify the record is deployed.

After I have created the entry and checked that it is available, I press Enter. Let's Encrypt retrieves my request and verifies it. Then it creates a certificate (fullchain.pem ) and key (privkey.pem) and stores both under /etc/letsencrypt/live/Name – applause (Listing 1).

Congratulations! Your certificate and chain have been saved at:
  /etc/letsencrypt/live/sensorenresidenz.de/fullchain.pem
  Your key file has been saved at:
  /etc/letsencrypt/live/sensorenresidenz.de/privkey.pem
  Your cert will expire on 2018-08-07. To obtain a new or tweaked version of this certificate in the future, simply run letsencrypt-auto again. To non-interactively renew *all* of your certificates, run "letsencrypt-auto renew".

Integrating the certificate with the web server follows the usual steps. The browser is happy with the certificate that my servers now deliver (Figure 1).

Figure 1: Browsers – here Firefox – accept wildcard certificates from Let's Encrypt without any restrictions.

Wildcard certificates from Let's Encrypt expire after 90 days like all other certificates from the issuer. To renew them, you simply call letsencrypt-auto renew.