Write Nagios Plugin Using Bash Script

By
anismaj

Nagios is a popular open source computer system and network monitoring software application. It watches hosts and services, alerting users when things go wrong and again when they get better.

It was originally designed to run under Linux, but also runs well on other Unix variants. It is free software, licensed under the terms of the GNU General Public License version 2 as published by the Free Software Foundation.

This tool is designed as a client/server model application needs two components to work – Nagios server and Nagios client or agent (NRPE for linux or NSClient++ for Windows). Nagios server is the master host how connects to Nagios client and uses a Nagios plugin to get information from Nagios client.

nrpe1
Nagios plugin is a script which can execute on the Nagios client machine (perl, php, python, bash script).

On this artilce we will expand on this idea and create Nagios plugins using Bash. These plugins will be running on nagios client VPS, and be executed via NRPE.

Prerequisites:

Before to start, if you still didn’t install Nagios Core , check the following articles.

And also you need to install nrpe agent in the linux using the following command for ubuntu.

apt-get install -y nagios-nrpe-server
useradd nrpe && update-rc.d nagios-nrpe-server defaults

Good Practices To Develop Nagios Plugin

Before we show you how to develop nagios plugin using bash script let’s give you the best practices to develop monitoring plugin.

Return codes

A plugin have to send a return code. This interpreted code is the result of the plugin execution. We call this result “status”. This is two summary tables about return codes for hosts and services :

Hosts:

Plugin return code Host status
0 UP
1 DOWN
Other Maintains last known state

Services:

Return code Service status
0 OK
1 WARNING
2 CRITICAL
3 UNKNOWN
Other CRITICAL : unknown return code

Plugin Output

The output message helps the user to understand the information. There are two types of output for plugin:

  • OUTPUT: displayed on monitoring screen in a real-time hosts and services. Its size is limited to 255 characters.
  • LONGOUTPUT: displayed in details page of host and service. Its size is limited to 8192 characters.

The plugin can provide performance data which are optional. However, if you want to have a graph showing the evolution of the result, it is mandatory that the plugin generates performance data.
Performance data are described after the “|” (pipe). This feature is available through the keystroke AltGR 6.
The performance data should be displayed as :
‘label’=value[UOM];[warn] ;[crit];[min];[max]

  • UOM: measure unit (octets, bits/s, volts, …)
  • warn: WARNING threshold
  • crit: CRITICAL threshold
  • min: minimal value of control
  • max: maximal value of control

Plugin Options

A well written plugin should have –help as a way to get verbose help.

There are a few reserved options that should not be used for other purposes:

  • -V version (–version) ;
  • -h help (–help) ;
  • -t timeout (–timeout) ;
  • -w warning threshold (–warning) ;
  • -c critical threshold (–critical) ;
  • -H hostname (–hostname) ;
  • -v verbose (–verbose).

In addition to the reserved options above, some other standard options are:

  • -C SNMP community (–community) ;
  • -a authentication password (–authentication) ;
  • -l login name (–logname) ;
  • -p port or password (–port or –passwd/–password)monitors operational ;
  • -u url or username (–url or –username).

Language used

You can write your script using any language like perl , c , php, python and bash. But you shloud use a language that:

  • easy-to-learn
  • easy and generally know by the administrators
  • many free library available on the web
  • often installed on Unix or linux system by default
  • easy to manage system start-up command and recovered results
  • advanced treatment characters strings
  • relatively efficient

Create Bash Script

For this article, we present plugin in bash.

For our example, we will create a script that checks the stats for any service like mysql, apache etc.

#!/bin/bash
# Nagios Plugin Bash Script - check_service.sh
# This script checks if program is running
# Check for missing parameters
if [[ -z "$1" ]] 
then
        echo "Missing parameters! Syntax: ./check_service.sh service_name"
        exit 3
fi
if ps ax | grep -v grep | grep $1 > /dev/null
then
    echo "OK, $SERVICE service is running"
        exit 0
else
    echo "CRITICAL , $SERVICE service is not running"
        exit 2
fi

We will save this script in /usr/lib/nagios/plugins/check_service.sh and make it executable:

chmod +x /usr/lib/nagios/plugins/check_service.sh

To make this script work properly when run via “check_nrpe” from Nagios Server you must add this line to /etc/sudoers file on the Host Client side! This is because of the sudo command used in script.

nagios  ALL=(ALL)     NOPASSWD:/usr/sbin/lsof,/usr/lib/nagios/plugins/check_service.sh

And you need to configure  /etc/nagios/nrpe.cfg and add this line

command[check_service]=/usr/lib/nagios/plugins/check_service.sh

Now let’s test our plugin locally from our Nagios Client machine:

# /usr/lib/nagios/plugins/check_service.sh
Missing parameters! Syntax: ./check_service.sh service_name

# /usr/lib/nagios/plugins/check_service.sh mysqld
CRITICAL - mysqld service is not running

# /usr/lib/nagios/plugins/check_service.sh apache2
OK - apache2 service is running

Now you need to define new command in your nagios server command file /etc/nagios/objects/commands.cfg

define command{
        command_name    check_service
        command_line    $USER1$/check_nrpe -H $HOSTADDRESS$ -c check_service 
        }

And then you must add a new service check on Nagios Server side.

define service {
        use                             generic-service
        host_name                       unixmen
        service_description             service check
        check_command                   check_service
        }

In order to verify your configuration, run Nagios with the -v command line option like so:

/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg

The last step is to restart the nagios service using this command

service nagios restart

That’s all. Thank you!!!