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 from 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:

OpenBSD

the operating system for the security paranoid, with only two remote holes in the default install, in a heck of a long time!;

Nagios

the Industry Standard in IT infrastructure monitoring;

Apache

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:

• a daemon process, running periodic checks on specific hosts and services and managing notifications when problems arise;
• an optional web interface, to access current status information, historical logs and reports via a simple web browser;
• a set of external plugins, i.e. the (possibly custom) scripts executed by the daemon process to actually perform the checks and send out notifications.

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:

• NRPE, the Nagios Remote Plugin Executor, which allows you to execute local plugins on remote hosts;
• NSCA, the Nagios Service Check Acceptor, which processes passive service check results submitted by clients to the Nagios server;
• NagVis, the Nagios Visualization Addon, which allows you to deeply customize how Nagios data is displayed;

If you're intereseted in a more "modern-looking" interface and some extra features, you might want to have a look at Icinga, a fork of Nagios which is also available as an OpenBSD package or port.

This document assumes a good knowledge of OpenBSD, since it doesn't cover basic system management topics such as OS installation, base configuration or packages/ports installation.