It’s actually not that difficult. ISPConfig3 already sets the option there. We just need to provide all the necessary tools.
Refer to Let’s Encrypt in Debian for Wheezy or manual setup.
For Debian 8 Jessie, enable Backports repo to install
echo "deb http://ftp.debian.org/debian jessie-backports main" | sudo tee /etc/apt/sources.list.d/debian-backports.list sudo apt update sudo apt install certbot -t jessie-backports
For Debian 9 Stretch, you just have to install it.
sudo apt install certbot
According Perfect Guide tutorial, we have to make a directory. I don’t know if this is a necessary thing to do. I’m doing it just for the sake of completeness.
sudo mkdir /opt/certbot sudo ln -s /usr/bin/certbot /opt/certbot/certbot-auto