Nagios

Warning: I do not like Nagios. The fact is I do not like frameworks. They only impose structure, usually too much structure and policies and mostly only that, with very little features. You loose true flexibility, which comes only from Turing-complete programming languages. So you better use libraries and not frameworks. Frameworks are for non-programmers or mediocre programmers. Whenever you need to do something not supported by the framework, you will end up fighting it. No framework designer can foretell all users needs and no framework can be as programmable and generic as a programming language. Anyway, here it is how to setup this monitoring framework, if you really have to. If you are not a programmer you may find it useful. But if you are a programmer and have time, you better use sysstat to collect data and cook up you own monitoring solution.

Installation
# yaourt -S nagios nagios-plugins
# htpasswd -c /etc/nagios/htpasswd.users nagiosadmin

# cp /etc/nagios/cgi.cfg.sample /etc/nagios/cgi.cfg
# cp /etc/nagios/resource.cfg.sample /etc/nagios/resource.cfg
# cp /etc/nagios/nagios.cfg.sample /etc/nagios/nagios.cfg
# cp /etc/nagios/objects/commands.cfg.sample /etc/nagios/objects/commands.cfg
# cp /etc/nagios/objects/contacts.cfg.sample /etc/nagios/objects/contacts.cfg
# cp /etc/nagios/objects/localhost.cfg.sample /etc/nagios/objects/localhost.cfg
# cp /etc/nagios/objects/templates.cfg.sample /etc/nagios/objects/templates.cfg
# cp /etc/nagios/objects/timeperiods.cfg.sample /etc/nagios/objects/timeperiods.cfg

# cat >>/etc/httpd/conf/httpd.conf

# Nagios
Include "conf/extra/nagios.conf"

# PHP
Include "conf/extra/php5_module.conf"
^D
# cp /etc/webapps/nagios/apache.example.conf /etc/httpd/conf/extra/nagios.conf

# usermod -G nagios -a http

# pacman -Sy apache php-apache gd
# sed -ri '/^(open_basedir = )/ s,$,:/etc/webapps:/usr/share/nagios,' /etc/php/php.ini

Add LoadModule php5_module modules/libphp5.so to /etc/httpd/conf/httpd.conf

# systemctl start httpd
# systemctl status nagios

Go to http://localhost/nagios
login as nagiosadmin

Edit /etc/nagios/objects/contacts.cfg and change 30@localhost with your email address. Log file is /var/nagios/nagios.log

How to create a plugin


I want to create a plugin file named check_something. Any plugin should return the following exit codes:

OK—0—service works properly
WARNING—1—service is in warning state
CRITICAL—exit code 2—service is in critical state
UNKNOWN—exit code 3—service is in unknown state

If an error/exception happens, you better return UNKNOWN, because this is usually the best thing to suggest to nagios in this case. Returning WARNING on error/exception is not appropriate. E.g. if the plugin user chooses to disable notifications for WARNINGS, he will not know that the plugin is actually not working.

Before exiting you should print a line with the format:


SOMETHING OK/WARNING/CRITICAL/UNKNOWN: ...

Standard plugins are installed into /usr/share/nagios/libexec/. You can put yours there too, but I have decided to use a separate directory /usr/local/share/nagios/libexec/ In /etc/nagios/objects/commands.cfg you have to choose a command_name (unique nickname) for your plugin and define the full path of the executable:

define command{
         command_name check_something
         command_line /usr/local/share/nagios/libexec/check_something
}

This way you register the plugin with nagios. Then to add this check to a host, e.g. localhost, edit file /etc/nagios/objects/localhost.cfg and add there a new definition of a service, e.g.:

define service{
         use local-service
         host_name localhost
         service_description My Plugin
         check_period 24x7
         contact_groups admins
         notification_options c,r
         check_command check_something
}


Here the value of check_command must match the command_name defined above. A command is just a command: you can even differently named commands  that call the same plugin of another command but with different options. Or you can define a command with some parameters. Use the placeholders $ARG1$, $ARG2$, ... in the command_line directive above and check_something!arg1!arg2!... is the syntax to pass parameters.


Plugins can be implemented in any language. For most tasks bash is appropriate, especially if there is no much processing to do, since it makes easy to interface with all unix commands. Here is a template for a Nagios plugin written using bash:

#!/bin/bash

# Plugin description.


## Processing parameter code.
OPTIND=1


# Define and initialize vars storing parameters
warnlvl=value1
critlvl=value2

...


help() {

  cat <
usage: $(basename $0) [-w warnlvl] [-c critlvl] [file]
-w warnlvl  description [value1]
-c critlvl  description [value2]
file        description [value3]
HELP
}


while getopts "h?a:b:c" opt; do
  case "$opt" in
  h|\?)
    help
    exit 0
    ;;
  w)
    # Example check for a numeric parameter
    if [[ $OPTARG != *[!0-9]* ]]; then
      warnlvl=$OPTARG
    else
      help
    fi
    ;;
  c)
    critlvl=$OPTARG
    ;;
  *)
    help
    exit 1
    ;;
  esac
done
shift $((OPTIND-1))

[ "$1" = "--" ] && shift

# Process $1 as the file name
if [ -z "$1" ]; then
  file=default_file_name

# Gather an integer lvl describing the state of something...

if [ $lvl -gt $critlvl]; then
  echo "CHECKNAME CRITICAL: description $lvl > $critlvl"
elif [ $lvl -gt $warnlvl]; then
  echo "CHECKNAME WARNING: description $lvl > $warnlvl"
else
  echo "CHECKNAME OK: description $lvl"
fi
Rif.
https://wiki.archlinux.org/index.php/Nagios
http://users.telenet.be/mydotcom/howto/nagios
http://www.ibm.com/developerworks/aix/library/au-nagios