Passive checks are sent to Nagios via the external command pipe. As it is a named pipe on a specific machine, the main problem is that all passive check results need to be sent from this machine.

Very often, checks need to be carried out on one or more remote hosts. This requires a mechanism to pass results from the machines that perform the tests to the computers running the Nagios daemon, which will process the results. This is why NSCA was developed. It is a client-server application that allows the passing of service and host check results over the network. This protocol allows the use of encryption, so the results are sent securely.

NSCA allows the sending of results directly to the Nagios external command pipe. It consists of two parts: the server and the client. The part responsible for receiving check results and passing them to Nagios is the server. The server listens on a specific TCP port for NSCA clients passing information. It accepts and authenticates incoming connections...