Nagios, 2nd Edition

6.7 Generic Network Plugins

Sometimes no plugin can be found that is precisely geared to the service to be monitored. For such cases, two generic plugins are available: check_tcp and check_udp. Both of them test whether a service is active on the target port for the protocol in question. Although this does not yet guarantee that the service running on the port really is the one in question, in an environment that one adminstrator looks after and configures, this can be sufficiently guaranteed in other ways.

Both plugins send a string to the server and evaluate the reply. This is at its most simple for text-based protocols such as POP or IMAP: these two "specific" plugins, which are tailor-made for these two mail services (see 6.3.2 POP and IMAP ...