What Is a Firewall and How Does It Work?

From packet filtering to stateful inspection and NGFWs: a practical model of network perimeter protection.

Inspect a rule without changing it

First inspect the active rules and service state. On Linux with nftables, this command reads the configuration:

sudo nft list ruleset

Then test the intended public port from another network. An open port does not prove secure access; review authentication, source restrictions, and logs.

Check it with Net Utils

Use the relevant Net Utils tool and compare the structured output with your expected configuration. Public network data changes, so confirm important conclusions with a repeat test.