needs on u\linux box, an apache webserver, username nagios group nagios
need tcp/IP configured//running CGI's included you need to get Thomas Boutell's gc library 1.6.3v or higher.

-there is ssh keys addon to get info, this is cpu intensive when you get into the 100's of servers
-there is the NRPE check addon that allows you to execute plugins on the remote servers, that send
-NSCA this is passive checks running on the remote server that send siinfo to a deamon on the server.

hundred of addons are a
Nagios X1 or Nagios core.
Nagios x1 costs $1300. Nagios core is free
Nagios x1 support is $325  Nagios core annual support is $2,495
so Nagios core is better, we don't need support.

nagios requirements:

-need a linux box with a C compiler.
-need tcp/IP configured
-a web server
-for CGI you need thomas Boutell's gd library installed
-nagios site says you can be up and running with basics in 20 minutes

Three ways to get monitoring info

-NRPE addon this allows you to monitor remote resources like cpu load, disk usage, memory, etc there are
 100's of plugins that already exist to monitor just about anything. If you can write a script to monitor
 something it can become a nagios plug-in.

-ssh keys and the check_by_ssh plugin this method becomes taxing on the load when you get into the
 100's of nodes. for our site it is fine.

-NSCA passive check. a passive check is from an external application, I am not sure if that also means
 from an external host. I think it can be an application on the nagios server as well as a remote host, it jus
 isn't generated from nagios. It is then passed to the external command file on nagios which is grabbed by nag
 and integrated into the monitoring if it is added to do so.

-NDOUtils this is an addon that will allow you to configure and store all info in a mysql database. it is a go
 option for a site with 100's and 100's of hosts to monitor, also good for use with PHP for more advanced
 web presentations. probably not necessary for 35-100 machines.

-nagios runs as a deamen and it listens to input.

-active vs passive checks active checks are run on a schedulabale basis from the nagios server. Passive checks
 are run from external applications . active checks have two ways they run, one is through the check_interval
 option where you can schedule when checks are done, and the second is the on-demand check that occurs everyti
 there is a change in the host state.

-distributed monitoring across many servers is important to reduce overhead when you are monitoring thousands 

-redundant monitoring, you can set up two servers to run failover nodes on different networks if need be.

-there are 2,418 addons available at for the nagios monitoring.

-it is clear that you can make nagios as detailed or as generic as you want. It is possible to configure
 for everything known to man, as you can create your own plug-ins til the cows come home.