Using nmap to Scan for Open Ports

nmap (Network Mapper) is the standard tool for discovering open ports and services on a server or network.

Install nmap

apt install nmap -y    # Ubuntu/Debian
dnf install nmap -y    # CentOS/RHEL

Basic Port Scan

nmap YOUR_SERVER_IP

Scans the 1,000 most common ports and shows which are open.

Scan All 65,535 Ports

nmap -p- YOUR_SERVER_IP

Scan Specific Ports

nmap -p 22,80,443,3306 YOUR_SERVER_IP

Detect Service Versions

nmap -sV YOUR_SERVER_IP

Shows the software and version running on each open port (e.g., nginx 1.24.0).

OS Detection

nmap -O YOUR_SERVER_IP

UDP Scan

nmap -sU -p 53,67,123,161 YOUR_SERVER_IP

Output Formats

nmap -oN output.txt YOUR_SERVER_IP     # Normal text
nmap -oX output.xml YOUR_SERVER_IP     # XML format

Verify Your Own Firewall Rules

Run nmap against your server from a different machine (or use an online nmap tool) to verify your firewall is blocking the ports you expect:

nmap -p 22,80,443,3306,5432 YOUR_SERVER_IP

Ports with status filtered are being blocked by the firewall. Ports with status closed are not blocked but nothing is listening.