RE: Secure server against attacks and viruses
Very good post.
What about updates? 🖥️
Center for Internet Security offer guides on securing you box further
Ive personally used Lynis on a few tests very useful for a quick audit check.
Diving deeper you could run some Security Scanners such as Nessus or OpenVAS frequently to make sure theres no silly holes. Alienvault is also a good Free IDS