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NMAP Basics

Nmap is a free open source software available for both Linux and Windows. Basically its a port scanning software.


Basic Scan Types
The two basic scan types used in nmap are TCP connect() scanning [-sT] and SYN scanning (also known as half-open,or stealth) [-sS].

TCP Connect() scanning [-sT]
These scans are named as connect() scanning because UNIX has a system call named connect() to start any TCP connection to a remote machine.If the connection succeeds to a particular port of remote machine i.e it connects , then that port is listed OPEN for that remote machine, if connection fails , then remote machine showed either offline or port is closed. It runs simple process of scannig all the ports of the remote machine and lists which posts are open and which are closed.

This is an easy way and gives a clear picture of the process but it has disadvantage too ,if the reomte machine is having advanced firewalls and intrusion detection systems, it will flag a warning that all ports are scanning, even modern servers log all the information of the TCP connection that is made to it. So it stores source ip aaddress from which scanning is made.
Thus this method has major drawback thats why stealth scanning is developed.\





next article on syn stealth scan..

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