Basic nagios setup
the first two steps are only required if you are still on debian 4 (like ec2... you will need to upgrade soon, security updates for etch will be abandoned next spring, I believe.)
first, install backports http://wiki.debian.org/Backports
next, set your default version http://www.debian.org/doc/manuals/apt-howto/ch-apt-get.en.html#s-default-version
now, no matter what version of debian, run
apt-get install nagios3
if dpkg-configure didn't set a passwd in /etc/nagios3/htpasswd.users, make a password for nagiosadmin
login to the web interface (http://hostname/nagios3/) as user nagiosadmin and the password you set. should work.
edit /etc/nagios3/conf.d/contacts_nagios2.cfg for who to page
edit conf.d/hostgroups_nagios2.cfg for what groups you have and who is in what group
edit conf.d/services_nagios2.cfg for what programs to run against what groups of servers.
on all clients you must
apt-get install nagios-nrpe-plugin
or use check_by_ssh http://blogs.techrepublic.com.com/opensource/?p=321
then open the firewall so that