How to install a free SSL certificate on a Linode running with Debain 8 ?

Debian 8 is end-of-life as of June 30th 2020.

If you're looking to improve security of your website and Linode, I'd recommend you consider updating to a supported version of Debian.

Now, upgrades are only supported from the previous major version, so to get to the most current (v11), you'd have to go 8 -> 9 -> 10 -> 11. There is a Linode guide available here.

However, it would probably be easier to build yourself a new Linode based on Debian 11 and transfer your applications/websites across.

To answer your original question, there are several ACME-compatible clients compatible with providers such as Let's Encrypt and ZeroSSL.

• Certbot - guide - Debian 9/10
• acme.sh - guide