How to install and configure Nagios Core 3

Note:
Nagios Core 3 has been deprecated and is now replaced by Nagios Core 4. The nagios3 package was last supported in Bionic, so subsequent releases should use nagios4 instead.

The monitoring of essential servers and services is an important part of system administration. This guide walks through how to install and configure Nagios Core 3 for availability monitoring.

The example in this guide uses two servers with hostnames: server01 and server02.

Server01 will be configured with Nagios to monitor services on itself and on server02, while server02 will be configured to send data to server01.

Install `nagios3` on `server01`

First, on server01, install the nagios3 package by entering the following command into your terminal:

sudo apt install nagios3 nagios-nrpe-plugin

You will be asked to enter a password for the nagiosadmin user. The user’s credentials are stored in /etc/nagios3/htpasswd.users. To change the nagiosadmin password, or add more users to the Nagios CGI scripts, use the htpasswd that is part of the apache2-utils package.

For example, to change the password for the nagiosadmin user, enter:

sudo htpasswd /etc/nagios3/htpasswd.users nagiosadmin

To add a user:

sudo htpasswd /etc/nagios3/htpasswd.users steve

Install `nagios-nrpe-server` on `server02`

Next, on server02 install the nagios-nrpe-server package. From a terminal on server02 enter:

sudo apt install nagios-nrpe-server

Note:
NRPE allows you to execute local checks on remote hosts. There are other ways of accomplishing this through other Nagios plugins, as well as other checks.

Configuration overview

There are a couple of directories containing Nagios configuration and check files.

/etc/nagios3: Contains configuration files for the operation of the Nagios daemon, CGI files, hosts, etc.
/etc/nagios-plugins: Contains configuration files for the service checks.
/etc/nagios: On the remote host, contains the nagios-nrpe-server configuration files.
/usr/lib/nagios/plugins/: Where the check binaries are stored. To see the options of a check use the -h option. For example: /usr/lib/nagios/plugins/check_dhcp -h

There are multiple checks Nagios can be configured to execute for any given host. For this example, Nagios will be configured to check disk space, DNS, and a MySQL hostgroup. The DNS check will be on server02, and the MySQL hostgroup will include both server01 and server02.

Note:
See these guides for details on setting up Apache, Domain Name Service, and MySQL.

Additionally, there are some terms that once explained will hopefully make understanding Nagios configuration easier:

Host: A server, workstation, network device, etc. that is being monitored.
Host Group: A group of similar hosts. For example, you could group all web servers, file server, etc.
Service: The service being monitored on the host, such as HTTP, DNS, NFS, etc.
Service Group: Allows you to group multiple services together. This is useful for grouping multiple HTTP for example.
Contact: Person to be notified when an event takes place. Nagios can be configured to send emails, SMS messages, etc.

By default, Nagios is configured to check HTTP, disk space, SSH, current users, processes, and load on the localhost. Nagios will also ping check the gateway.

Large Nagios installations can be quite complex to configure. It is usually best to start small, with one or two hosts, to get things configured the way you want before expanding.

Configure Nagios

Create host config file for server02

First, create a host configuration file for server02. Unless otherwise specified, run all these commands on server01. In a terminal enter:

sudo cp /etc/nagios3/conf.d/localhost_nagios2.cfg \
/etc/nagios3/conf.d/server02.cfg
    
> **Note**:
> In all command examples, replace "`server01`", "`server02`", `172.18.100.100`, and `172.18.100.101` with the host names and IP addresses of your servers.

### Edit the host config file    

Next, edit `/etc/nagios3/conf.d/server02.cfg`:
 
```text    
define host{
        use                     generic-host  ; Name of host template to use
        host_name               server02
        alias                   Server 02
        address                 172.18.100.101
}
        
# check DNS service.
define service {
        use                             generic-service
        host_name                       server02
        service_description             DNS
        check_command                   check_dns!172.18.100.101
}

Restart the Nagios daemon to enable the new configuration:

sudo systemctl restart nagio3.service

Add service definition

Now add a service definition for the MySQL check by adding the following to /etc/nagios3/conf.d/services_nagios2.cfg:

# check MySQL servers.
define service {
        hostgroup_name        mysql-servers
        service_description   MySQL
        check_command         check_mysql_cmdlinecred!nagios!secret!$HOSTADDRESS
        use                   generic-service
        notification_interval 0 ; set > 0 if you want to be renotified
}

A mysql-servers hostgroup now needs to be defined. Edit /etc/nagios3/conf.d/hostgroups_nagios2.cfg and add the following:

# MySQL hostgroup.
define hostgroup {
        hostgroup_name  mysql-servers
                alias           MySQL servers
                members         localhost, server02
        }

The Nagios check needs to authenticate to MySQL. To add a nagios user to MySQL enter:

mysql -u root -p -e "create user nagios identified by 'secret';"

Note:
The nagios user will need to be added to all hosts in the mysql-servers hostgroup.

Restart nagios to start checking the MySQL servers.

sudo systemctl restart nagios3.service

Configure NRPE

Lastly configure NRPE to check the disk space on server02.

On server01 add the service check to /etc/nagios3/conf.d/server02.cfg:

# NRPE disk check.
define service {
        use                     generic-service
        host_name               server02
        service_description     nrpe-disk
        check_command           check_nrpe_1arg!check_all_disks!172.18.100.101
}

Now on server02 edit /etc/nagios/nrpe.cfg changing:

allowed_hosts=172.18.100.100

And below, in the command definition area, add:

command[check_all_disks]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -e

Finally, restart nagios-nrpe-server:

sudo systemctl restart nagios-nrpe-server.service

Also, on server01 restart Nagios:

sudo systemctl restart nagios3.service

You should now be able to see the host and service checks in the Nagios CGI files. To access them, point a browser to http://server01/nagios3. You will then be prompted for the nagiosadmin username and password.