How to monitor an ubuntu Servers using Nagios core 4.x

In my previous article we had discovered how to install Nagios Server on Ubuntu 16 LTS 64 bit, now we need to understand how to monitor a remote host using Nagios server. Firstly, to be able to monitor remote host server we need to NRPE daemon over an (optionally) SSL-protected connection that will be contacted by check_nrpe executed on the nagios server to run the appropriate Nagios plugin.

Prerequisites

– Ubuntu 16.04 LTS – 64bit server
– A superuser Privilege
– A Nagios Server Monitor, you can take a look to this article : How To Install Nagios core 4 on Ubuntu 16 LTS

What we will do in this tutorial:

– Install NRPE service from a repository.
– Configure NRPE to allow Nagios server to execute checks.
– Add a host file configuration on the Nagios server.

Let’s start our tutorial.

1./ Install NRPE Daemon and Nagios Plugins

NRPE is available under default apt repositories of Ubuntu systems. Execute the following command to install it

# apt install nagios-nrpe-server nagios-plugins 

2./ Configure NRPE Daemon

To allow our Nagios server to execute checks plugins in our remote host:
– First we need to edit the /etc/nagios/nrpe.cfg file and add the IP address of the monitoring server to the allowed_hosts directive and modify dont_blame_nrpe value from 0 to 1 to allow command arguments.

 # vi /etc/nagios/nrpe.cfg 

allowed_hosts=127.0.0.1, 192.168.1.17 # add your nagios_ip_address 
dont_blame_nrpe=1

– Second,we need to add the following entry nrpe 5666/tcp # NRPE for the NRPE daemon to the /etc/services file.

 # vi /etc/services 
nrpe 5666/tcp # NRPE

finally, we need to restart our NRPE Daemon, open firewall rules and make some checks to verify if every things work properly.

# firewall-cmd --permanent --add-port=5666/tcp
# firewall-cmd --reload 

# systemctl restart nagios-nrpe-server 

 # netstat -at | grep nrpe
tcp 0 0 *:nrpe *:* LISTEN
tcp6 0 0 [::]:nrpe [::]:* LISTEN

3./Add Check Commands in NRPE

By default after the installation of NRPE daemon in the file /etc/nagios/nrpe.cfg it will be created a few commands like below.

command[check_load]=/usr/lib/nagios/plugins/check_load -w 15,10,5 -c 30,25,20
command[check_hda1]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /dev/hda1
command[check_zombie_procs]=/usr/lib/nagios/plugins/check_procs -w 5 -c 10 -s Z
command[check_total_procs]=/usr/lib/nagios/plugins/check_procs -w 150 -c 200

Here i’ll add more commands

command[check_users]=/usr/lib/nagios/plugins/check_users -w 5 -c 10
command[check_sda1]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /dev/sda1
command[check_procs]=/usr/lib/nagios/plugins/check_procs -w 150 -c 200 -s RSZDT
command[check_disk_root]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /
command[check_swap]=/usr/lib/nagios/plugins/check_swap -w 20% -c 10%

Each time we make any changes in configuration file we need to restart the nrpe service.

# systemctl restart nagios-nrpe-server 

4./ Add a remote host to the monitoring Nagios server

Let’s move to our monitoring nagios server.

i.)Create a Command Definition

First, to be able to use check_nrpe plugin we need to create a command definition in the commands.cfg file.

# vi /usr/local/nagios/etc/commands.cfg 

and add the following definition to the file:

define command{
command_name check_nrpe
command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
}

ii.)Organizing Nagios Configuration

-open nagios file configuration:

 # sudo vi /usr/local/nagios/etc/nagios.cfg

-uncomment this line

 cfg_dir=/usr/local/nagios/etc/servers 

-create servers directory where we’ll store the configuration file for each server that we will monitor

# sudo mkdir /usr/local/nagios/etc/servers
# chown -R nagios.nagios /usr/local/nagios

iii.)Configure Nagios Contacts

# sudo vi /usr/local/nagios/etc/objects/contacts.cfg 
email     nagios@localhost        ; # CHANGE THIS TO YOUR EMAIL ADDRESS

iiii.)Create host and service definitions

-Create a new remote host file configuration under the directory /usr/local/nagios/servers/ and add the host definition and the services definition that you want to be monitored.

# vi /usr/local/nagios/etc/servers/node01.cfg 

###############################################################################
###############################################################################
#
# HOST DEFINITION
#
###############################################################################
###############################################################################
define host{
use linux-server
host_name node01 #PUT HERE YOUR HOSTNAME REMOTE SERVER
alias node01 #PUT HERE THE ALIAS OF YOUR REMOTE SERVER
address 192.168.1.55 #PUT HERE THE IP ADDRESS OF YOUR REMOTE SERVER
max_check_attempts 5
check_period 24x7
notification_interval 30
notification_period 24x7
}
###############################################################################
###############################################################################
#
# HOST SERVICES DEFINITION
#
###############################################################################
###############################################################################
define service{
use generic-service
host_name node01
service_description PING
check_command check_ping!100.0,20%!500.0,60%
}
define service {
use generic-service
host_name node01
service_description SSH
check_command check_ssh
notifications_enabled 0
}
# for free disk SDA1
define service{
use generic-service
host_name node01
service_description SDA1 Disk
check_command check_nrpe!check_sda1
}
# for free Root Partition
define service{
use generic-service
host_name node01
service_description Root Partition
check_command check_nrpe!check_disk_root
}
# for current users
define service{
use generic-service
host_name node01
service_description Current Users
check_command check_nrpe!check_users
}
# for total processes
define service{
use generic-service
host_name node01
service_description Total Processes
check_command check_nrpe!check_procs
}
# for current load
define service{
use generic-service
host_name node01
service_description Current Load
check_command check_nrpe!check_load
}

-Verify nagios configuration files using this command

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

-Restart nagios

 # systemctl restart nagios

-Finally open the nagios web interface to check if the remote host is added properly.

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