The query handler allows two-way communication with Nagios internal processes and external applications. It is designed to be extensible, and future versions of Nagios may provide more functionality using the query handlers.
The query handler communicates using Unix domain sockets (refer to http://en.wikipedia.org/wiki/Unix_domain_socket for more details). These are meant for communication between processes on the same machine. Unix domain sockets use filesystem as names for remote addresses. The location (address) of the Nagios query handler is similar to the Nagios external command pipe—it is called
nagios.qh and by default resides in the same directory as the external commands pipe. For example,