1. Set up your nagios.conf file under /etc/apache2/conf.d
2. Add the following lines to the nagios share <Directory>

      AuthName   LDAP_Auth_Test
      AuthBasicProvider ldap
      AuthzLDAPAuthoritative off
      AuthLDAPBindDN uid=your user name,dc=your domain,dc=com
      AuthLDAPBindPassword  password for your user name
      AuthLDAPURL ldap://ldap.com:389/dc=your value
   ,dc=your value,dc=com?uid?
   

3. Change the permissions on the nagios.conf file to 400 with the Apache user as the owner. The reason is for security and the only user that needs to read the file is Apache. Don’t worry, root will be able to modify the file.
4. Vi /etc/sysconfig/apache2 and add the following to the APACHE_MODULES section: authnz_ldap ldap
5. Start Apache and you are finished. When someone connects to the application, they will receive the authentication text box, so they can enter their userid and password before entering the application.

NOTE: Users are only authenticating against the application. Once they leave the web application, they will be required to authenticate against your proxy server. That is considering you are running Nagios internally with apache on the same host.

Enjoy and I hope this helps.