In this chapter, we will cover:
- Creating a network host hierarchy
- Using the network map
- Choosing icons for hosts
- Establishing a host dependency
- Establishing a service dependency
- Monitoring individual nodes in a cluster
- Using the network map as an overlay
While Nagios Core is still very useful when configured to monitor only a simple list of hosts and services, it includes some optional directives that allow you to define some structural and functional properties of the monitored network—specifically, how hosts and services interrelate. Describing this structure in the configuration calls for some additional intelligent behavior in the monitoring and notification that Nagios Core performs.
There are two main approaches to working with network structure in Nagios Core:
- Host parent definitions allow an administrator to define the hierarchy of connectivity to monitored hosts from the "point of view" of the Nagios Core server. An example might be a server with the monitored address in another subnet linked to the Nagios Core server by a router. If the router enters the
DOWNstate, this triggers Nagios Core's host reachability logic to automatically determine which hosts will become inaccessible and flags these asUNREACHABLErather thanDOWN, allowing refined notification behavior. - Host and service dependencies allow us to formalize relationships between hosts or services, usually for the purposes of suppressing unnecessary notifications. An example might be a service that tests a login to a mail service that itself requires a database service to work properly. If Nagios Core finds that the database and login services are both down, a service dependency allows you to suppress the notification about the login service; the administrator would, therefore, only be notified about the database service being down, which is more likely to be the actual problem.
There is some overlap of functionality here, but the general pattern is that host parent definitions describe the structure of your network from the vantage point of your monitoring server and host and service dependencies describe the way it functions independent of the monitoring server. We will define both parent definitions and dependencies in this chapter, with the primary goal of filtering and improving the notifications that Nagios Core sends in response to failed checks, which can assist greatly in diagnosing problems.
We'll also look at another, more subtle benefit of establishing host parent definitions in making the network map of the Nagios Core web interface useful and, once a basic hierarchy is set up, we'll show how to customize the map's appearance (including defining icons for hosts) to make it generally useful as a network weather map.