How to configure passive checks.
Passive checks are enabled at client side as these are initiated at client end.
A passive check script is enabled at client host with the command which is used for respective service check.
This script can also be enabled via crontab to enable check at the same time when the service will change its state.
Within the script Nagios code needs to be mentioned and enabled within IF ELSE condition as per the service state.
The associated alerts code can be easily found within Nagios documentation or via Nagios exchange website