Linux · Firewalls (nftables & firewalld)

You're testing a new firewall over SSH on a remote server. What's the safe way to avoid permanently locking yourself out?

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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?

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?

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?

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