Linux · Firewalls (nftables & firewalld)

A host is listed in neither hosts.allow nor hosts.deny. Many admins assume that means blocked — what actually happens, and how do you build a secure setup?

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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?

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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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