The primary configuration file is /etc/nagios/nagios.cfg
Debian uses include directives in this file to logically divide up the various configuration sections
cfg_file=/etc/nagios/checkcommands.cfg cfg_dir=/etc/nagios-plugins/config/ cfg_file=/etc/nagios/misccommands.cfg cfg_file=/etc/nagios/contactgroups.cfg cfg_file=/etc/nagios/contacts.cfg cfg_file=/etc/nagios/dependencies.cfg cfg_file=/etc/nagios/escalations.cfg cfg_file=/etc/nagios/hostgroups.cfg cfg_file=/etc/nagios/hosts.cfg cfg_file=/etc/nagios/services.cfg cfg_file=/etc/nagios/timeperiods.cfg