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Linux Server Firewall Test Shows How UFW Hides Ports from Nmap Scans

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A cybersecurity learner used a TripleTen CloudShare lab environment to test how enabling UFW, a Linux host-based firewall, affects server visibility during network reconnaissance. Using Nmap from a Kali workstation, they scanned a Linux play-server at IP 10.170.0.22 before and after configuring the firewall. Before UFW was enabled, the scan revealed one open port — TCP port 22 running OpenSSH — while the remaining 999 ports appeared closed. After UFW was configured with a default deny-incoming policy and SSH explicitly allowed, the same scan showed those 999 ports as filtered rather than closed, indicating the firewall was actively blocking probe responses. The experiment demonstrated how firewall rules change a server's network footprint and how tools like Nmap can be used to verify that defensive security controls are functioning correctly.

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