Note:
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Note:
Nagios Core 3 has been deprecated and is now replaced by Nagios Core 4. Thenagios3
package was last supported in Bionic, so subsequent releases should usenagios4
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 thenagios-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:
Thenagios
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.
Further reading
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 about Nagios, see the Nagios website.
-
The Nagios Core Documentation and Nagios Core 3 Documentation may also be useful.
-
They also provide a list of books related to Nagios and network monitoring.
-
The Nagios Ubuntu Wiki page also has more details.