System:

• CentOS release 5.5 (Final)
• Nagios 3.2
• Apache 2.2

Packages:

• nagios-3.2.3-3.el5.rf
• nagios-plugins-nrpe-2.12-1.el5.rf
• nagios-plugins-1.4.15-2.el5.rf
• nagios-devel-3.2.3-3.el5.rf

Others:

• httpd-2.2.3-43.el5.centos.3

Installation:

	# yum install nagios nagios-plugins nagios-plugins-nrpe nagios-devel

Setup the password file:

	# htpasswd -c /etc/nagios/htpasswd.users nagiosadmin
	New password: type_your_password
	Re-type new password: re-type_your_password
	Adding password for user nagiosadmin

And/or you may create a "guest" account. But this is up to you.

	# htpasswd /etc/nagios/htpasswd.users guest
	New password: type_your_password
	Re-type new password: re-type_your_password
	Adding password for user guest

NOTE: "-c" option is the create option. Since you already created the file, make sure any other accounts you add are not with the create option. You'll wipe the file out if you do.

Setup the CGI file:

	# vi /etc/nagios/cgi.cfg

	# AUTHENTICATION USAGE
	use_authentication=1

	# SYSTEM/PROCESS INFORMATION ACCESS
	authorized_for_system_information=nagiosadmin

	# CONFIGURATION INFORMATION ACCESS
	authorized_for_configuration_information=nagiosadmin

	# SYSTEM/PROCESS COMMAND ACCESS
	authorized_for_system_commands=nagiosadmin

	# GLOBAL HOST/SERVICE VIEW ACCESS
	authorized_for_all_services=nagiosadmin
	authorized_for_all_hosts=nagiosadmin

	# GLOBAL HOST/SERVICE COMMAND ACCESS
	authorized_for_all_service_commands=nagiosadmin
	authorized_for_all_host_commands=nagiosadmin

Save the file after editing.

Starting nagios:

	# nagios -v /etc/nagios/nagios.cfg

	Nagios Core 3.2.3
	Copyright (c) 2009-2010 Nagios Core Development Team and Community Contributors
	Copyright (c) 1999-2009 Ethan Galstad
	Last Modified: 10-03-2010
	License: GPL

	Website: http://www.nagios.org
	Reading configuration data...
	   Read main config file okay...
	Processing object config file '/etc/nagios/objects/commands.cfg'...
	Processing object config file '/etc/nagios/objects/contacts.cfg'...
	Processing object config file '/etc/nagios/objects/timeperiods.cfg'...
	Processing object config file '/etc/nagios/objects/templates.cfg'...
	Processing object config file '/etc/nagios/objects/localhost.cfg'...
	...
	...
	...
	Things look okay - No serious problems were detected during the pre-flight check

	# service nagios start

Start httpd:

	# service httpd start

Browse to:

	http://localhost/nagios

After you supply the credentials, you should see the nagios web interface. You can browse through the links to the left, but the majority of them won't work because nothing is configured yet.

Coming: How To Configure Nagios On CentOS

Reference:
Nagios
HowTos/Nagios - CentOS Wiki