NULL Port Scan

NULL packet is a basic data packet which has none of its flags turned on.

Atatcker => NULL => Target

case 1: port open
Target => RST => Attacker
case 2: port closed
No response or error message

What happens in case 1 ie, when the port is open?
Whenthe port receives the NULL packet it does not know what to do with it. So it will send a RST packet to reset the connection.When we receive the RST packet from the target we can be sure that the port is open.

pros and cons :
Stealth : No connection so good stealth
Not accurate in all OS
Can be blocked using a good firewall

Null scan using Nmap :
nmap -SN -P -1-200 www.target.com