Suggestions for Improving Linux Server Security: Beyond User Permissions and Groups?

Hi all,

I found a hobby in trying to secure my Linux server, maybe even beyond reasonable means.

Currently, my system is heavily locked down with user permissions. Every file has a group owner, and every server application has its own user. Each user will only have access to files it is explicitly added to.

My server is only accessible from LAN or VPN (though I've been interested in hosting publicly accessible stuff). I have TLS certs for most everything they can use it (albeit they're self signed certs, which some people don't like), and ssh is only via ssh keys that are passphrase protected.

What are some suggestions for things I can do to further improve my security? It doesn't have to be super useful, as this is also fun for me.

Some things in mind:

• 2 factor auth for SSH (and maybe all shell sessions if I can)
• look into firejail, nsjail, etc.
• look into access control lists
• network namespace and vlan to prevent server applications from accessing the internal network when they don't need to
• considering containerization, but so far, I find it not worth foregoing the benefits I get of a single package manager for the entire server

Other questions:

• Is there a way for me to be "notified" if shell access of any form is gained by someone? Or somehow block all shell access that is not 2FA'd?
• my system currently secures files on the device. But all applications can see all process PIDs. Do I need to protect against this?

threat model

• attacker gains shell access
• attacker influences server application to perform unauthorized actions
• not in my threat model: physical access