Short how-to w/o any explanation. You HAVE TO know what you are doing at this level of using linux server! Do NOT experiment on a production server!

Starting on a plain instalation.

$ nano /etc/apt/sources.list

deb http://archive.ubuntu.com/ubuntu intrepid main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu intrepid-security main restricted universe multiverse

$ apt-get update
$ apt-get install mc

Update locales if necessary: http://www.evil.cz/clanek/setting-locale-failed-ubuntu-810-server-after-...

Edit /etc/hostname and set server name as myserver.domain.tld.

$ shutdown -r now

$ apt-get install nagios3

---- web interface not necessary ---

$ cd /root
$ mkdir install
$ cd install

$ apt-get install wget
$ apt-get install zip

Visit http://www.nagiosql.org

$ wget http://www.nagiosql.org/images/fbfiles/files/nagiosql_2.zip
$ unzip nagiosql_2.zip
$ cd nagiosql_2.0.2-1_all

$ apt-get install php5 php5-common php5-mysql php-html-template-it mysql-client mysql-client-5.0 pwgen dbconfig-common libapache2-mod-php5 apache2-mpm-prefork libdbi-perl libdbd-mysql-perl php-pear php5-cli

$ apt-get install mysql-server

$ dpkg -i nagiosql_2.0.2-1_all.deb

Configuring db: enter mysql root pwd, 2x pwd for nagiosql program, admin user name, 2x admin pwd.

$ /etc/init.d/apache2 restart

$ apt-get install nsca

$ apt-get install nagiosgrapher nagios-images nagios-plugins nagios-plugins-basic nagios-plugins-extra nagios-plugins-standard

$ touch /etc/nagios3/htpasswd.users
nagiosadmin:__________________

Follow
http://www.debianhelp.co.uk/nagios.htm