Use Nagios to keep tabs on your network.
Since remote exploits can often crash the service that is being broken into or cause its CPU use to skyrocket, you should monitor the services that are running on your network. Just looking for an open port (such as by using Nmap [Hack #42] ) isn’t enough. The machine may be able to respond to a TCP connect request, but the service may be unable to respond (or worse, could be replaced by a different program entirely!). One tool that can help you verify your services at a glance is Nagios (http://www.nagios.org).
Nagios is a network-monitoring application that monitors not only the services running on the hosts on your network, but also the resources on each host, such as CPU usage, disk space, memory usage, running processes, log files, and much more. In the advent of a problem it can notify you through email, pager, or any other method that you define, and you can check the status of your network at a glace by using the web GUI. Nagios is also easily extensible through its plug-in API.
To install Nagios, download the source distribution from the Nagios web site. Then, unpack the source distribution and go into the directory it creates:
tar xfz nagios-1.1.tar.gz$
Before running Nagios’s configure script, you should create a user and group for Nagios to run as (e.g., nagios). Then run the configure script with a command similar to this: