Introducing Active and passive checks
In nagios 2 kinds of service checks can be enabled i.e. Active checks & Passive checks.
These checks can be enabled for different services depending upon the needs.
Active checks - In an active check Nagios server send the request to client and at regular interval of time specified within the configuration and get’s the result.
In an active check command initiated from Nagios server.
These checks can be enabled for those services which are needed to be monitored at regular intervals of time like - Disk, Memory , CPU etc.
Passive checks - These checks are enabled at client side i.e the command is triggered at client end and the result is communicated to Nagios server.
The command can be triggered via crontab or any other timing mechanism.
These checks are enabled where a specific service needs to be checked which changes its state at a particular time, like once in a day or week, etc. Hence, enabling such checks prevents Nagios from checking the service every time.