How do I monitor a server behind a firewall with Nagios?

Nagios comes with a client-side agent called NRPE that the Nagios server contacts on port 5666 to run remote service-checks, but what if the server is firewalled?? There is a solution, using a feature of Nagios called “passive checks”.  Its called that because Nagios will passively wait for the client servers to actively check-in with their condition. …

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