Having a tool that can quickly, and regularly, take snapshots of your network landscape is critical to keeping tabs on your PCs. Of course you can shell out some budget dollars for a proprietary tool, but why bother when you can fire up a Linux machine and use the trusty Nmap tool for the job.

Nmap is a command line tool that rapidly scans a network gathering information about machines and ports. It is easy to use and flexible, making it perfect for the job of asset scanning. In this tutorial you will see how to set up a system that will regularly scan your network and create a report that can then be used to keep inventory of your networked machines.

Installing

Before we get to the actual scanning we need to install a couple of applications. Since I am using a Ubuntu system, we’ll run the installation using apt-get. With some simple modifications, you can do the same on a fedora system. The two applications to install are: nmap and ndiff. We use ndiff to compare the results of scans. To install these applications open up a terminal window and issue the following command:

sudo apt-get install nmap ndiff

You will have to accept dependencies, at which point the two applications will install. Upon completion of the installation, you are ready to scan.

Using nmap

Nmap is actually a fairly powerful tool. If you issue the command man nmap you will see just how powerful this tool is. You can also see how many arguments you can use with Nmap as well as what each argument does. Fortunately I will show you a simple command you can issue to make this a bit easier.

I am going to illustrate how these tools work together by running an nmap scan on a small internal network. I will then scan the network after making a change to one machine and see if ndiff catches the change.

The command for the scan is:

sudo nmap -n -PN 192.168.1.1/24 -O > network_scan

I will then run that same scan after making the change with one alteration:

sudo nmap -n -PN 192.168.1.1/24 -O > network2_scan


The above commands will output to the files network_scan, and network2_scan.

Once you have the two files you will compare them using the ndiff command like so:

ndiff -b network_scan -o network2_scan

The two options used are:

• b – Baseline.
• o – Observed.

You can think of Baseline as your control group.

Figure 1

The results of the command are shown in Figure 1.

The results show exactly what occurred in my network change. I shut down the machine associated with IP address 192.168.1.37.

Of course you could also get a much clearer picture of your network by combing through the results of the initial scan, but if you are looking for how your network topography has changed from scan to scan, using ndiff is the best way.

To see the full usage of both nmap and ndiff, take a look at the man pages. I will warn you, they are fairly complex. But this tutorial should give you a solid understanding of how the basics of the tools work.

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The Hostname resolving that is supported as well is more or less instantly. Users can enter a hostname, IP or IP range and start the scan. The Port Scanning will take a while depending on the number if IPs that are to be scanned. A HTML report is generated at the end that is displaying the amount of discovered hosts and the open TCP and UDP ports.

Several port scanning options are available. It is for instance possible to select a distinct port range that should be scanned, timeouts and host discovery requests. SuperScan is not a pretty application but one that does the job very well. It takes a while to look through all the tabs provided in the software program to get accustomed to it.

SuperScan does offer a wide range of networking tools besides the port scanning. It provides access to networking tools like Ping, Whois Lookups and Get Requests and even Windows host enumeration.

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