Common Nmap Flags

This article explains common Nmap flags used for port scanning, including TCP SYN Scan, TCP Connect Scan, UDP Scan, and Port Specification. It also covers how to utilize these flags to detect open ports and customize scan speed/aggressiveness with options for OS detection, script scanning, and timing templates.

Typically, if one wants to detect port somehow dropped by cloud service providers like AWS, the flag -sS or SYN stealth scan shall be enough.

Further info can be collected once the port has been confirmed open.

  1. -sS (TCP SYN Scan):
  • This flag instructs Nmap to perform a TCP SYN scan, also known as a half-open scan. It sends SYN packets to the target ports and analyzes the responses to determine which ports are open, closed, or filtered.
  1. -sT (TCP Connect Scan):
  • This flag tells Nmap to perform a TCP connect scan, in which Nmap completes the full TCP three-way handshake to determine the state of the target ports.
  1. -sU (UDP Scan):
  • This flag enables Nmap to perform a UDP scan, used to identify open UDP ports on the target system. UDP scans can be slower than TCP scans due to the stateless nature of the UDP protocol.
  1. -p (Port Specification):
  • The -p flag allows you to specify which ports to scan. You can specify individual ports, ranges of ports, or combination of both. For example, -p 1-1000 scans ports 1 through 1000.
  1. -A (Aggressive Scan):
  • The -A flag enables OS detection, version detection, script scanning, and traceroute. It’s a comprehensive option that provides detailed information about the target.
  1. -O (Enable OS Detection):
  • This flag instructs Nmap to attempt to determine the operating system running on the target host based on various characteristics observed during the scan.
  1. -v (Verbose Output):
  • The -v flag increases the verbosity of Nmap’s output, providing more detailed information about the scan process.
  1. -T (Timing Template):
  • The -T flag allows you to specify the timing template for the scan. Options range from 0 (paranoid) to 5 (insane), affecting the speed and aggressiveness of the scan.
  1. -O (Output to File):
  • The -o flag allows you to specify the output format and destination for the scan results. For example, -oN scan_results.txt saves the output in normal format to a file named scan_results.txt.

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