So far, we have used plugins that are part of the standard Nagios Plugins package. It provides plugins for monitoring typical servers. The IT setup often consists of large variety of hardware and software that has to be monitored. There are many devices and services that should be monitored. In many cases, standard plugins are enough to properly monitor them, such as monitoring using PING, SSH, or HTTP.
There are, however, many applications that require more sophisticated checks, such as applications communicating over a custom protocol that can be checked using
check_tcp by specifying handshake to perform and expected response. In addition, many services require more sophisticated checks, such as verifying that OpenVPN server performs a proper handshake, which cannot easily be done using
check_tcp. A check that it is listening can be done, but it could simply be another service running at the same port.