nmap
SYN scan (half-open scan)
sudo nmap -sS -v localhost # -sS means SYN
specific ports
sudo nmap -sS -p 80,443 localhost
sudo nmap -sS -p- localhost
operating system
sudo nmap -nS -O localhost
scan network
sudo nmap -sS -O 192.168.1.0/24
UDP scan
sudo nmap -sU ...