You can easily set up a dedicated Debian system as a stand-alone Intrusion Detection System using snort and a web-based interface to analyse the intrusion detection alerts:

BASE is currently packaged for Debian in acidbase and ACID is packaged as acidlab^[77]. Both provide a graphical WWW interface to Snort's output.

Besides the base installation you will also need a web server (such as apache), a PHP interpreter and a relational database (such postgresql or mysql) where Snort will store its alerts.

This system should be set up with at least two interfaces: one interface connected to a management LAN (for accessing the results and maintaining the system), and one interface with no IP address attached to the network segment being analyzed. You should configure the web server to listen only on the interface connected to the management LAN.

You should configure both interfaces in the standard Debian /etc/network/interfaces configuration file. One (the management LAN) address can be configured as you would normally do. The other interface needs to be configured so that it is started up when the system boots, but with no interface address. You can use the following interface definition:

auto eth0
iface eth0 inet manual
      up ifconfig $IFACE 0.0.0.0 up
      up ip link set $IFACE promisc on
      down ip link set $IFACE promisc off
      down ifconfig $IFACE down

The above configures an interface to read all the traffic on the network in a stealth-type configuration. This prevents the NIDS system to be a direct target in a hostile network since the sensors have no IP address on the network. Notice, however, that there have been known bugs over time in sensors part of NIDS (for example see http://www.debian.org/security/2003/dsa-297 related to Snort) and remote buffer overflows might even be triggered by network packet processing.

You might also want to read the http://www.faqs.org/docs/Linux-HOWTO/Snort-Statistics-HOWTO.html and the documentation available at the http://www.snort.org/docs/.