The query handler communicates using Unix domain sockets (for more details, visit ).
These are meant for communication between processes on same machine.
The format of the response varies by the service that implements it.
Many commands return an answer or send notifications after it is invoked.
For example, /var/nagios/rw/is the path to the query handler's Unix domain socket for an installation performed according to the steps given in .
The core service allows the querying of information about Nagios processes and scheduled jobs.
The nerd service allows you to subscribe to events and can be used to receive real-time updates about changes on the Nagios host and/or service status.
It allows two-way communication between Nagios internal processes and external applications, and is designed to be extensible.
The future versions of Nagios may provide more functionality using the query handlers.
Another difference is that you cannot open Unix domain sockets as a file and/or send commands to the socket using shell commands such as echo, which is possible with named pipes such as the Nagios external commands pipe.