How to use Nagios with Munin

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. Most network services are monitored for performance, availability, or both. This section will cover installation and configuration of Nagios 3 for availability monitoring alongside Munin for performance monitoring.

The examples in this section will use two servers with hostnames server01 and server02. Server01 will be configured with Nagios 3 to monitor services on both itself and server02. Server01 will also be set up with the Munin package to gather information from the network. Using the munin-node package, server02 will be configured to send information to server01.

Install Nagios 3

On server01

First, on server01, install the nagios3 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

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 run local checks on remote hosts. There are other ways of accomplishing this, including through other Nagios plugins.

Configuration overview

There are a few 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: is located on the remote host and 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 many checks Nagios can be configured to run for any particular host. In this example, Nagios will be configured to check disk space, DNS, and a MySQL host group. The DNS check will be on server02, and the MySQL host group will include both server01 and server02.

Note:
See these additional guides for details on setting up Apache, Domain Name Service (DNS), 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 servers, 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, e.g., multiple HTTP.
Contact: the 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 (i.e. with one or two hosts), get things configured the way you like, and then expand.

Configure Nagios

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 host group 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:

```bash
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` host group.
    
Restart Nagios to start checking the MySQL servers.

```bash
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 nagios3:

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.

Install Munin

Before installing Munin on server01 Apache2 will need to be installed. The default configuration is fine for running a Munin server. For more information see setting up Apache.

On server01

First, on server01 install munin by running the following command in a terminal:

sudo apt install munin

On server02

Now on server02 install the munin-node package:

sudo apt install munin-node

Configure Munin on server01

On server01 edit the /etc/munin/munin.conf to add the IP address for server02:

## First our "normal" host.
[server02]
       address 172.18.100.101

Note:
Replace server02 and 172.18.100.101 with the actual hostname and IP address of your server.

Configure munin-node on server02

To configure munin-node on server02, edit /etc/munin/munin-node.conf to allow access by server01:

allow ^172\.18\.100\.100$

Note:
Replace ^172\.18\.100\.100$ with IP address for your Munin server.

Now restart munin-node on server02 for the changes to take effect:

sudo systemctl restart munin-node.service

Finally, in a browser go to http://server01/munin, and you should see links to some graphs displaying information from the standard munin-plugins for disk, network, processes, and system.

Note:
Since this is a new install it may take some time for the graphs to display anything useful.

Additional plugins

The munin-plugins-extra package contains performance checks and additional services such as DNS, DHCP, Samba, etc. To install the package, from a terminal enter:

sudo apt install munin-plugins-extra

Be sure to install the package on both the server and node machines.

How to use Nagios with Munin

Install Nagios 3

On server01

On server02

Configuration overview

Configure Nagios

Install Munin

On server01

On server02

Configure Munin on server01

Configure munin-node on server02

Additional plugins

Further reading