If you have decided to use Let's Encrypt certificates,
it is necessary to install the certbot utility
or an equivalent utility implementing the
Automated Certificate Management Environment (ACME).
Install certbot using the package manager as
demonstrated in
Example 9.3, “Installing certbot on Debian/Ubuntu” and
Example 9.4, “Installing certbot on Fedora/RHEL/CentOS”.
Example 9.3. Installing certbot on Debian/Ubuntu
$ sudo apt-get install -t jessie-backports ssl-cert certbot