TCP SYN (Stealth) Scan (-sS) | Nmap Network Scanning
SYN scan has long been called the stealth scan because it is subtler than TCP connect scan (discussed next), which was the most common scan type before Nmap was released. Despite that moniker, don't count on a default SYN scan slipping undetected through sensitive networks. Widely deployed intrusion detection systems and even personal firewalls ...