Nmap a free tool that can be used to conduct various sorts of scans on networks. Normally, when people think of Nmap, they assume it’s used to conduct some sort of nefarious network reconnaissance in preparation for an attack. But as with all powerful tools, Nmap can be used for far more than breaking into networks.

Nmap can provide much more information if you run it as root. When run as root, it can use special packets to determine the operating system of the remote machine by using the -O flag. Additionally, you can do half-open TCP scanning by using the -sS flag. When doing a half-open scan, Nmap sends a SYN packet to the remote host and waits to receive the ACK from it; if it receives an ACK, it knows that the port is open. This is different from a normal three-way TCP handshake, where the client sends a SYN packet and then sends an ACK back to the server once it has received the initial server ACK. Attackers typically use this option to avoid having their scans logged on the remote machine

Nmap Port Scanner Run Command

UNIX> nmap -sS -O 192.168.0.61

Starting nmap V. 3.00 ( www.insecure.org/nmap/ )
Interesting ports on test.server (192.168.0.61):
(The 1578 ports scanned but not shown below are in state: filtered)
Port State Service
7/tcp open echo
9/tcp open discard
13/tcp open daytime
19/tcp open chargen
21/tcp open ftp
22/tcp open ssh
23/tcp open telnet
25/tcp open smtp
37/tcp open time
79/tcp open finger
111/tcp open sunrpc
512/tcp open exec
513/tcp open login
514/tcp open shell
587/tcp open submission
7100/tcp open font-service
32771/tcp open sometimes-rpc5
32772/tcp open sometimes-rpc7
32773/tcp open sometimes-rpc9
32774/tcp open sometimes-rpc11
32775/tcp open sometimes-rpc13
32776/tcp open sometimes-rpc15
32777/tcp open sometimes-rpc17
Remote operating system guess: Solaris 9 Beta through Release on SPARC
Uptime 44.051 days (since Sat Nov 1 16:41:50 2003)

Nmap run completed — 1 IP address (1 host up) scanned in 166 seconds

With OS detection enabled, Nmap has confirmed that the operating system is Solaris, but now you also know that it’s probably Version 9 running on a SPARC processor.