title: what is difference between active & passive check in nagios
body: <p>What is exact difference between active and passive checks in Nagios ?</p>
  
  <p>I have checked on google found passive check is related to external services. Now what are these external services. And I need to configure passive then how to do ?</p>
  
  <p>Also unable to find relevant about Active checks in nagios</p>