Nagios is also known as Nagios Core. It is a free and open source application that monitors systems, networks and infrastructure. Nagios offers monitoring and alerting services for servers. In this article you will learn that how to install nagios server and monitor your hosts.
Prerequisites
You must login on the server as root user or superuser privileges.
A LAMP stack required. Follow this article if you want to install it: Install LAMP in CentOS 7 and RHEL7
Install Dependencies
yum install gcc glibc glibc-common gd gd-devel make net-snmp openssl-devel xinetd unzip -y
Create Nagios User and Group
you must create a user and group for Nagios process. Create a “nagios” user and “nagcmd” group, then add the user to the group with these commands:
useradd nagios groupadd nagcmd usermod -a -G nagcmd nagios
Install Nagios Core
Download the latest stable release of Nagios Core from the given link copy nagios core link address and download it to your server.
Donwload Nagios Core using command wget
cd wget https://assets.nagios.com/downloads/nagioscore/releases/nagios-4.4.3.tar.gz
Extract archive and switch to Nagios directory
tar -xvf nagios-4.4.3.tar.gz cd nagios-4.4.3
Configure Nagios Script using below command
./configure --with-command-group=nagcmd make all
Now run following commands to install Nagios, init scripts, and sample configuration files
make install make install-commandmode make install-init make install-config make install-webconf make install-daemoninit
If you want to issue external commands from web interface to Nagios, you need to add the web server user “apache” to “nagcmd” group
usermod -G nagcmd apache
Now create nagios user “nagiosadmin” and restart the apache services.
htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
These login details will be used to access the Nagios web interface.
Install Nagios Plugins
Download the latest stable release of Nagios Plugin from the given link copy nagios plugin link address and download it to your server.
Donwload Nagios plugin using command wget
cd wget http://nagios-plugins.org/download/nagios-plugins-2.2.1.tar.gz
Extract archive and switch to Nagios Plugin directory
tar -xvf nagios-plugins-2.2.1.tar.gz cd nagios-plugins-2.2.1
Configure Nagios Plugin Script using below command
./configure --with-nagios-user=nagios --with-nagios-group=nagios --with-openssl
Now compile Nagios Plugins
make
Now Install Nagios Plugin
make install
Install NRPE
Use following commands
cd wget http://downloads.sourceforge.net/project/nagios/nrpe-2.x/nrpe-2.15/nrpe-2.15.tar.gz
Extract NRPE archive and switch to NRPE directory
tar xvf nrpe-2.15.tar.gz cd nrpe-2.15
Configure NRPE
./configure --enable-command-args --with-nagios-user=nagios --with-nagios-group=nagios --with-ssl=/usr/bin/openssl --with-ssl-lib=/usr/lib/x86_64-linux-gnu
Now compile and install NRPE
make all make install make install-xinetd make install-daemon-config
Add your server IP address in xinetd startup script
vi /etc/xinetd.d/nrpe
Modify the only_from line and add the IP address of the your Nagios server to the end, in my case it is 192.168.130.138
only_from = 127.0.0.1 192.168.130.138
Save and exit.
Only Nagios server’s IP must be allowed to communicate with NRPE.
Now restart xinetd service to start NRPE
systemctl restart xinetd
Now Nagios Server is installed and we need to configure it.
Configure Nagios
Create the directory which will store the configuration file for each server that will monitor be monitor by Nagios.
mkdir /usr/local/nagios/etc/servers
Now open the main Nagios configuration file
vi /usr/local/nagios/etc/nagios.cfg
Then add below line
cfg_dir=/usr/local/nagios/etc/servers
save and exit
Configure Nagios Contacts
If you want, you can also configure nagios contacts as per your requirements using below command
vi /usr/local/nagios/etc/objects/contacts.cfg
Add a new NRPE command to our Nagios configuration
vi /usr/local/nagios/etc/objects/commands.cfg
Add the following lines at the end of the file:
define command{
command_name check_nrpe
command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
}
Save and exit.
Now you will be able to use check_nrpe command in Nagios service definitions.
Nagios is ready to use, Lets restart the services and add them to start on server boot.
systemctl daemon-reload systemctl restart httpd.service systemctl start nagios.service systemctl enable nagios.service
Access Nagios Web Interface
Open your web browser and browse below link to login your Nagios server. You must enter the login credentials that you created earlier. We will use the username “nagiosadmin”
http://nagios_server_ip/nagios
Now you will see default Nagios home page. Under “current Status” left navigation bar Click on the “Hosts” option, to see hosts which are monitoring by the Nagios.
Here you can see Nagios is monitoring only “localhost” it means itself.
Monitor a CentOS 7 Host with NRPE
In this section you will learn that how to add a new host to Nagios server, so it will be monitored. You need to repeat this section for each CentOS or RHEL server you want to monitor.
Login to the server you want to monitor and install the EPEL repository
yum install epel-release -y
Then install Nagios Plugins and NRPE:
yum install nrpe nagios-plugins-all -y
Now modify NRPE configuration file
vi /etc/nagios/nrpe.cfg
Find the line allowed_hosts and add the IP address of your Nagios server, In my case Nagios server address is 192.168.130.138
allowed_hosts=127.0.0.1,::1,192.168.130.138
Save and exit.
this configuration will accept requests from your Nagios server
Now Restart NRPE to apply changes
systemctl start nrpe.service systemctl enable nrpe.service
All configurations done on host machine, Now we have to add these hosts to Nagios server configuration so it can start monitoring them.
Add Host to Nagios Configuration
On your Nagios server, create a new configuration file for each remote host that you want to monitor in /usr/local/nagios/etc/servers/ direcotry. I will create host1.cfg file for my fisrt host.
vi /usr/local/nagios/etc/servers/host1.cfg
Add the following host definition in host file, replace the host_name value with your remote hostname (In my case it is host1), the alias value with a description of the host, and the address value with the IP address of the remote host (My host1 address is 192.168.130.222)
define host {
use linux-server
host_name host1
alias Apache server host1
address 192.168.130.222
max_check_attempts 5
check_period 24x7
notification_interval 30
notification_period 24x7
}
With above configurations, nagios will only monitor if the host is up or down.
save and exit
Now restart Nagios server to apply new changes.
systemctl reload nagios.service
Once done, access your Nagios web interface go to Under “current Status” left navigation bar Click on the “Hosts” option, and check all hosts which are monitoring by the Nagios. now you will see that host1 is showing up there.
