Automatically monitoring your home network

The Watcher

Article from Issue 169/2014
Author(s):

To discover possibly undesirable arrivals and departures on their networks, a Perl daemon periodically stores the data from Nmap scans and passes them on to Nagios via a built-in web interface.

The practical Nmap network scanner is used not only by the bad boys in exciting thrillers to detect intrusion targets [1], it also tells admins what devices are actually reachable on their home networks. If you regularly launch Nmap on your subnets and compare the output, you can keep track of newly added or removed devices and proactively ward off nasty surprises.

The fact that nmap has a -oX option that tells it to output the results in XML format is something I was unaware of until I read an Nmap manual that was recently released as a Kindle book [2]. Because an Nmap scan across multiple networks can take a few minutes, I got the idea of building a daemon that finds all the nodes once an hour, keeps the data in memory, and sends it via a built-in web server to requesting clients, such as a Nagios script.

Lean Script Defines the Search Space

The script in Listing 1 [3] does this, mainly by resorting to the NmapServer module (which is loaded in line 7 and discussed later on) and its start() method. Before the call, it defines the IP range of the home network that nmap needs to scan in the constructor – in this case the subnets are 192.168.14.x and 192.168.27.x.

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