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

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.

See HTTPD - Apache2 Web Server for details on setting up Apache, Domain Name Service (DNS) for DNS, and MySQL 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

    1. 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
      

      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.

    2. 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
      }
      
    3. Restart the nagios daemon to enable the new configuration:

      sudo systemctl restart nagio3.service
      
    1. 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
      }
      
    2. 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
              }
      
    3. 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';"
      

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

    4. Restart nagios to start checking the MySQL servers.

      sudo systemctl restart nagios3.service
      
    1. 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
      }
      
    2. 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
      
    3. Finally, restart nagios-nrpe-server:

      sudo systemctl restart nagios-nrpe-server.service
      
    4. 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.