Nmap or (Network Mapper) is a free open source tool that is used by many network administrators for security analysis created by insecure.org. Nmap can be used in may different ways, including checking a range of IP addresses to see what hosts are on the network. It can also be used as a post scanner to tell you which ports are open on a specific host computer. Nmap has been made to run on most popular operating systems such as Windows, MAC OS, and Linux.

In this article we will discuss some of the command line aspect of namp, there is also a GUI version called zenmap but I still prefer nmap and the command line utility over the GUI.

Once installed nmap can be launched simply by typing nmap from command line, but it will not run properly unless you add a few attributes to it. The syntax for nmap is:

nmap <scan type> <options> <destination>

You can however leave out the scan type and options and just type nmap <destination>, the great thing is that nmap acutally gives you a server you can scan and play with. scanme.nmap.org is a web site setup that you are free to scan for the purposes of learning nmap. So we are going to go ahead and scan that server with no attributes set.

nmap scanme.nmap.org
Starting Nmap 4.76 ( http://nmap.org ) at 2009-08-22 19:17 EDT
Strange read error from 64.13.134.52 (104 – ‘Connection reset by peer’): Operation now in progress
Strange read error from 64.13.134.52 (104 – ‘Connection reset by peer’): Operation now in progress
Interesting ports on scanme.nmap.org (64.13.134.52):
PORT      STATE  SERVICE
1/tcp     open   tcpmux
3/tcp     open   compressnet
4/tcp     open   unknown
6/tcp     open   unknown
7/tcp     open   echo
9/tcp     open   discard
13/tcp    open   daytime
17/tcp    open   qotd
19/tcp    open   chargen
20/tcp    open   ftp-data
21/tcp    open   ftp
22/tcp    open   ssh
23/tcp    open   telnet
24/tcp    open   priv-mail
25/tcp    closed smtp

So as you can see from above, which is only a little bit of what actually displayed, we see which ports are considered interesting and we look at them all. There are four different statuses the ports can be in open, filtered, closed, unfiltered. Open is just that the port is open and an application on the host machine is listening on that port for connections. Filtered means that the port is being blocked by a firewall or router ACL list so nmap can’t tell whether the port is open or closed. Closed is also just that, they don’t have any applications listening on them at that time but could be opened up at any time by an application. Unfiltered ports are ports that will respond to the probing that nmap does but can’t tell from that whether they are open or closed.

Well that is a quick and easy overview of nmap, please stay tuned as I plan to write much more on nmap.