Nagios

notification_options specifies which notification types for host states should be sent, separated by comma; there should be one or more of the following:

  d—host DOWN state
  u—host UNREACHABLE state
  r—host recovery (UP state)
  f—host starts and stops fl apping
  s—notify when scheduled downtime starts or ends

Service_notification_options Specifies the service states that the user should be notified about, separated by commas; this can
be one or more of the following:

  w—service WARNING state
  u—service UNKNOWN state
  c—service CRITICAL state
  r—service recovery (OK state)
  f—service starts and stops fl apping
  n—person will not receive any service notification

Nagios Web Interface

The default username for accessing the webinterface is: nagiosadmin
The password of nagiosadmin can be changed using "htpasswd /etc/nagios/passwd nagiosadmin" command
New user can be added using the same htpasswd command
After new user is added, he has to added in /etc/nagios/cgi.cfg file for various level of access.

Nagios Plugins

Nagios performs checks by running an external command, and uses the return code, along with output from the command, as information on whether the check worked or not. It is the command's responsibility to verify if a host or service is working at
the time the command is invoked.

Nagios requires that all plugins follow a specific, easy-to-follow behavior in order for them to work smoothly. These rules are common for both host checks and service checks. It requires that each command returns specific result codes, which are:

 Exit code Status      Description
 0         OK          Working correctly
 1         WARNING     Working, but needs attention (for example, low resources)
 2         CRITICAL    Not working correctly or requires attention
 3         UNKNOWN     Plugin was unable to determine the status for the host or service

Standard Nagios plugins usually accept the following parameters:

 Option Description
 -h,    --help Provide help
 -V,    --version Print the exact version of the plugin
 -v,    --verbose Make the plugin report more detailed information on what it is doing
 -t,    --timeout Timeout (seconds); after this time plugin will report CRITICAL status
 -w,    --warning Plugin-specifi c limits for the WARNING status
 -c,    --critical Plugin-specifi c limits for the CRITICAL status
 -H,    --hostname Host name, IP address or unix socket to communicate with
 -4,    --use-ipv4 Use IPv4 for network connectivity
 -6,    --use-ipv6 Use IPv6 for network connectivity

Commands that verify various daemons also have a common set of options. Many of the networking-related plugins use the following options in addition to the preceding standard ones

 Option Description
 -p,     --port TCP or UDP port to connect to
 -w,     --warning Response time that will issue a WARNING status (seconds)
 -c,     --critical Response time that will issue a CRITICAL status (seconds)
 -s,     --send String that will be sent to the server
 -e,     --expect String that should be sent back from the server (option might be
         passed several times; see --all for details)

Monitot Remote Host using NRPE:

ON THE CLIENT
01. Install Nagios Plugins and NRPE on the client

  yum install nrpe nagios-plugins-all

Nagios How to on Centos
Nagios Made Easy
Monitor Remote Linux Host using Nagios 3.0
Nagios on Ubuntu

NRPE- Nagios Remote Plug-in Executor