Nagios Core is appropriate for monitoring services and states on all sorts of hosts and one of its primary advantages is that the configuration can be as simple or as complex as required. Many Nagios Core users will only ever use the software as a way to send PING requests to a few hosts on their local network, or possibly the Internet, and use it to send an administrator an e-mail or pager message if they don't get any replies. Nagios Core is vastly capable of monitoring more complex systems than this, scaling from simple LAN configurations to being the cornerstone for monitoring for an entire network operations team.
However, for both simple and complex configurations of Nagios Core, the most basic building blocks of configuration are hosts, services, and contacts. These are the three things that administrators of even very simple networking setups will end up editing and probably creating. If you're a beginner to Nagios Core, you might have changed a hostname here and there or copied a stanza in a configuration to get it to do what you want; in this first chapter, we're going to look at what these configurations do in a bit more depth than that.
In a Nagios Core configuration, hosts, services, and contacts are three fundamental types of objects:
Hosts usually correspond to a computer, whether it's the machine running Nagios Core itself, a virtual machine hosted upon it, or a machine reachable via a network. Conceptually, however, a host can monitor any kind of network entity, such as the endpoint of a VPN.
Services usually correspond to an arrangement for Nagios Core to check something about a host; this can either be something as simple as whether the monitoring server can get PING replies from the host or something more complicated as whether the value of an SNMP OID is within acceptable bounds.
Contacts define a means to notify someone when events take place on the services on our hosts, such as not being able to get a PING response or send a test e-mail message.
In this chapter, we'll add all three of these configurations, we'll learn how to group their definitions together to make the configuration more readable, and to work with hosts in groups rather than having to edit each one individually. We'll also set up a custom time period for notifications so that hardworking system administrators like us don't end up getting paged at midnight unnecessarily!
