This Nagios Mini-Course shows you how to configure Passive Checks from clients which are sent to a central Nagios Server. Passive checks use the NSCA or Nagios Service Check Adaptor which is an add-on for the Nagios Server. You can write custom scripts to evaluate applications and send the output to a central Nagios Server. Passive Checks are often used when a firewall prevents Nagios from obtaining information from the client.