Tools - nagios

Installation

First, on server01 install the nagios package. In a terminal enter:

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 additional 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

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: houses 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 a plethora of 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 ??? for details on setting up Apache, ??? for DNS, and ??? for 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, one or two hosts, get things configured the way you like then expand.

Configuration

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 the above and following command examples, replace “server01”, “server02” 172.18.100.100, and 172.18.100.101 with the host names and IP addresses of your servers.

Next, edit /etc/nagios3/conf.d/server02.cfg:

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

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 adding:

# 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 all hosts in the mysql-servers hostgroup.

Restart nagios to start checking the MySQL servers.

sudo systemctl restart nagios3.service

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.

References

This section has just scratched the surface of Nagios’ features. The nagios-plugins-extra and nagios-snmp-plugins contain many more service checks.

For more information see Nagios website.
Specifically the Nagios Online Documentation site.
There is also a list of books related to Nagios and network monitoring:
The Nagios Ubuntu Wiki page also has more details.

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