1. Introduction

So our OpenBSD-based network now includes redundant firewalls, domain name servers, a mail gateway and a web proxy cache. All the services provided by these machines are particularly critical and can't afford even minimal downtime. Redundancy may give us the time to recover a failure before having angry users trying to knock down our door, but it doesn't free us from the responsibility to detect and solve ongoing problems.

To put it short, it's time to think about monitoring our network! And the following are the perfect ingredients for implementing a full-featured, secure and reliable network monitoring system:

the operating system for the security paranoid, with only two remote holes in the default install, in a heck of a long time!;
the industry standard in Open Source monitoring;
the secure, efficient and extensible server that provides HTTP services in sync with the current HTTP standards.

My pick goes to Nagios for its ease of use, flexibility and extensibility. It also features a very clean and straightforward design, as it is structured into three basic building blocks:

Furthermore, these basic components can be easily extended with external modules, making it easy for Nagios to meet even your most demanding needs! Therefore, after the installation and configuration of the Nagios' core components, we will take a brief look at some of its most popular and useful addons:

A good knowledge of OpenBSD is assumed, since we won't delve into system management topics such as base configuration or packages/ports installation.