Linux · Firewalls (nftables & firewalld)

You want to run a custom service on port 900 versus port 2000. Why does one of those choices require root and the other does not?

This is one card from the KnowCard library. The full way to learn it — spaced-repetition review, progress tracking, and AI explanations — lives in the KnowCard app, free to start on the web. iOS & Android are coming soon.

Get started — it’s free

Already have an account? Sign in →

More in Firewalls (nftables & firewalld)

On a fresh openSUSE install the firewall is already blocking almost all outgoing traffic, and the built-in YaST firewall module is missing your Ethernet interface. What is the recommended way to fix rules?

Where does a Linux system map friendly names like ssh, http, and https to their numeric IP port numbers?

A colleague says every IP service confirms receipt of packets. Which common protocols contradict that, and why does it matter for firewall rules?

You want a quick list of which ports your local machine is actually listening on, including the owning process. Which netstat invocation gives that?

You need to find which process is holding a specific network port, say port 22. Which command targets that directly?

From an outside host you want to discover which services and OS a server is exposing. Which tool and flags do that, and what does each flag add?