Step by step install nagios3 on ubuntu 14.04 (trusty) it's better to run all command in super user mode (su) or if you wish you could use "sudo" instead.

1. Install apache2 : #apt-get -y install apache2
2. Open file security.conf : #vi /etc/apache2/conf-enabled/security.conf
3. Change line #26 ServerTokens Prod and line #37 ServerSignature Off
4. Open file dir.conf : #vi /etc/apache2/mods-enabled/dir.conf
5. Change line #2 : add file name that it can access only with directory's name
   DirectoryIndex index.html index.htm
6. Open file apache2.conf : ~#vi /etc/apache2/apache2.conf
7. Change line #70 : add server name
   ServerName www.mynagios.com
8. Open file 000-default.conff : ~#vi /etc/apache2/sites-enabled/000-default.conf
9. Change line #11 : change admin email address (you don't have to change it if you run it locally)
   ServerAdmin [email protected]
10. Restart apache2 web server : #/etc/init.d/apache2 restart