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Debian12.14 : Nagios Server

2026.05.29

Nagios Server installed

Nagios is an open source monitoring solution running on Linux.
Nagios is designed to periodically check critical network, application, or server parameters.
These parameters include microprocessor load, number of running processes, log files, disk and memory usage, as well as many other services such as SMTP (Simple Mail Transfer Protocol), HTTP (Hypertext Transfer Protocol), POP3 (Post Office Protocol 3), etc. can be checked.

These parameters require a microprocessor load, a database such as PHP or MySQL to run Nagois, and a web server such as Apache or Nginx. We will proceed under the assumption that all of these have already been configured

1. Install necessary packages

# apt update
# apt install -y autoconf gcc libc6 make wget unzip libgd-dev

2. Installing Nagios Core

①Download/Install

# wget https://go.nagios.org/get-core/4-5-12

# apt install libssl-dev

# tar zxvf 4-5-12
# cd nagios-4.5.12
# ./configure
# make all

➁Create Nagios Users and Groups

# make install-groups-users
groupadd -r nagios
useradd -g nagios nagios

➂Add Apache User to Nagios Group

# usermod -aG nagios www-data

④Install Nagios Core

# make install

⑤Install Nagios init script

# make install-init

/usr/bin/install -c -m 755 -d -o root -g root /lib/systemd/system
/usr/bin/install -c -m 755 -o root -g root startup/default-service /lib/systemd/system/nagios.service

⑥Install external command files and permissions

# make install-commandmode

/usr/bin/install -c -m 775 -o nagios -g nagios -d /usr/local/nagios/var/rw
chmod g+s /usr/local/nagios/var/rw

*** External command directory configured ***

⑦Install Nagios Configuration Files

# make install-config

*** Config files installed ***

Remember, these are *SAMPLE* config files.  You'll need to read
the documentation for more information on how to actually define
services, hosts, etc. to fit your particular needs

⑧Install Apache configuration files for Nagios

# make install-webconf

/usr/bin/install -c -m 644 sample-config/httpd.conf /etc/apache2/sites-available/nagios.conf
if [ 1 -eq 1 ]; then \
        ln -s /etc/apache2/sites-available/nagios.conf /etc/apache2/sites-enabled/nagios.conf; \
fi

*** Nagios/Apache conf file installed ***

# a2enmod cgi
# systemctl restart apache2

3. Install Nagios Plugin

①Download the latest code from the Nagios Plugins page

# wget https://nagios-plugins.org/download/nagios-plugins-2.4.9.tar.gz

②Install

# tar -xvf nagios-plugins-2.4.9.tar.gz
# cd nagios-plugins-2.4.9
# ./configure --with-nagios-user=nagios --with-nagios-group=nagios
# make
# make install

③Create necessary directories

# mkdir -p /usr/local/nagios/var/spool/checkresults
# chown -R nagios:nagios /usr/local/nagios/var/spool/checkresults

4. Creating a Nagios Web User

Create a user account to access the Nagios Web Dashboard. This user account is used for authentication.
The default name of the user is nagiosadmin, which is defined as the preferred user name in the /usr/local/nagios/etc/cgi.cfg file.

# htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
New password:  [password]
Re-type new password:  [again password]
Adding password for user nagiosadmin

Set ownership and permissions

# chown www-data:www-data /usr/local/nagios/etc/htpasswd.users
# chmod 640 /usr/local/nagios/etc/htpasswd.users

Restart Apache

# systemctl restart apache2

Allow HTTP service port in firewall

# ufw allow http
# ufw reload

Start and Enable Nagios Services

# systemctl enable nagios --now
Created symlink /etc/systemd/system/multi-user.target.wants/nagios.service → /lib/systemd/system/nagios.service.

Confirmation of service execution

# systemctl status nagios

● nagios.service - Nagios Core 4.5.12
     Loaded: loaded (/lib/systemd/system/nagios.service; enabled; preset: enabled)
     Active: active (running) since Fri 2026-05-29 15:22:49 JST; 36s ago
       Docs: https://www.nagios.org/documentation
    Process: 44724 ExecStartPre=/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg (code=exited, s>
    Process: 44725 ExecStart=/usr/local/nagios/bin/nagios -d /usr/local/nagios/etc/nagios.cfg (code=exited, stat>
   Main PID: 44726 (nagios)
      Tasks: 6 (limit: 4554)
     Memory: 5.7M
        CPU: 201ms
     CGroup: /system.slice/nagios.service
             tq44726 /usr/local/nagios/bin/nagios -d /usr/local/nagios/etc/nagios.cfg
             tq44727 /usr/local/nagios/bin/nagios --worker /usr/local/nagios/var/rw/nagios.qh
             tq44728 /usr/local/nagios/bin/nagios --worker /usr/local/nagios/var/rw/nagios.qh
             tq44729 /usr/local/nagios/bin/nagios --worker /usr/local/nagios/var/rw/nagios.qh
             tq44730 /usr/local/nagios/bin/nagios --worker /usr/local/nagios/var/rw/nagios.qh
             mq44731 /usr/local/nagios/bin/nagios -d /usr/local/nagios/etc/nagios.cfg

May 29 15:22:49 Lepard nagios[44726]: qh: Socket '/usr/local/nagios/var/rw/nagios.qh' successfully initialized
May 29 15:22:49 Lepard nagios[44726]: qh: core query handler registered
May 29 15:22:49 Lepard nagios[44726]: qh: echo service query handler registered
May 29 15:22:49 Lepard nagios[44726]: qh: help for the query handler registered
May 29 15:22:49 Lepard nagios[44726]: wproc: Successfully registered manager as @wproc with query handler

5. Access the Nagios Web Interface

Access http://[IP_Address]/nagios/ with any browser
Username : nagoisadmin
Password : Password specified when the user was created above

After successful login, the following dashboard will appear

Show host availability
Click on [Hosts] in the left menu
Only the Nagios server is displayed

Click [Tactical Overview] on the left menu to view the monitoring data

Click [Current Status] [Services] on the left menu

6. Nagios Agent Configuration

To monitor the agent. Install the following

NRPE Agent to run plugins

6.1 Installing Nagios NRPE Agent
NRPE agent available in default Debian repository

# apt-cache policy nagios-nrpe-server
nagios-nrpe-server:
  Installed: (none)
  Candidate: 4.1.0-1+b1
  Version table:
     4.1.0-1+b1 500
        500 http://deb.debian.org/debian bookworm/main amd64 Packages

# apt install -y nagios-nrpe-server

Version Check

# nrpe -V
NRPE - Nagios Remote Plugin Executor
Version: 4.1.0

Start and activate the service

# systemctl enable --now nagios-nrpe-server

Synchronizing state of nagios-nrpe-server.service with SysV service script with /lib/systemd/systemd-sysv-install.
Executing: /lib/systemd/systemd-sysv-install enable nagios-nrpe-server

Check the status of the NRPE agent

# systemctl status nagios-nrpe-server

● nagios-nrpe-server.service - Nagios Remote Plugin Executor
     Loaded: loaded (/lib/systemd/system/nagios-nrpe-server.service; enabled; preset: enabled)
     Active: active (running) since Fri 2026-05-29 15:35:09 JST; 1min 36s ago
       Docs: http://www.nagios.org/documentation
   Main PID: 44952 (nrpe)
      Tasks: 1 (limit: 4554)
     Memory: 1.2M
        CPU: 6ms
     CGroup: /system.slice/nagios-nrpe-server.service
             mq44952 /usr/sbin/nrpe -c /etc/nagios/nrpe.cfg -f

May 29 15:35:09 Lepard systemd[1]: Started nagios-nrpe-server.service - Nagios Remote Plugin Executor.
May 29 15:35:09 Lepard nrpe[44952]: Starting up daemon
May 29 15:35:09 Lepard nrpe[44952]: Server listening on 0.0.0.0 port 5666.
May 29 15:35:09 Lepard nrpe[44952]: Server listening on :: port 5666.
May 29 15:35:09 Lepard nrpe[44952]: Listening for connections on port 5666
May 29 15:35:09 Lepard nrpe[44952]: Allowing connections from: 127.0.0.1,::1

Allow services to pass through the firewall
NRPE uses TCP port 5666 by default.

# ufw allow 5666/tcp
# ufw reload

Check if port 5666/tcp is listening

# ss -altnp | grep 5666

LISTEN 0      5            0.0.0.0:5666       0.0.0.0:*    users:(("nrpe",pid=44952,fd=4))                                                                                                                                                                                                                                                         
LISTEN 0      5               [::]:5666          [::]:*    users:(("nrpe",pid=44952,fd=5))

7. Adding a monitored host

Add the following server to the host and configure it as a target for simple ping death/activity monitoring
OS : Debian12.11
IP address : 192.168.11.85

7.1 Configuration File Editing

# vi /usr/local/nagios/etc/nagios.cfg
Line 51 : Uncomment
cfg_dir=/usr/local/nagios/etc/servers

7.2 Directory Create

# mkdir /usr/local/nagios/etc/servers
# chgrp nagios /usr/local/nagios/etc/servers
# chmod 750 /usr/local/nagios/etc/servers

7.3 Create new definition file
The "korodes" in korodes.cfg can be any name.

# vi /usr/local/nagios/etc/servers/korodes.cfg

define host {
    use                     linux-server
    host_name               korodes
    alias                   korodes
    address                 192.168.11.85
}
define service {
    use                     generic-service
    host_name               korodes
    service_description     PING
    check_command           check_ping!100.0,20%!500.0,60%
}

7.4 Restart Nagios server

# systemctl restart nagios.service

Log in to the Nagios server (http://[server IP address]/nagios) and look at Hosts, and the host is added.

8. Adding Monitoring Services

Add monitoring services to the hosts added above.
OS : Debian12.11
IP address : 192.168.11.85

8.1  Configuration on the server to be monitored

8.1.1 Agent installed on monitored hosts

# apt install -y nagios-nrpe-server

8.1.2 Configuration File Editing

# vi /etc/nagios/nrpe.cfg

Line 106 : Add a host to allow connections (specify Nagios server)
allowed_hosts=127.0.0.1,::1,192.168.11.83

Line 122 : Allow command arguments
dont_blame_nrpe=1

Line 300-304 : Comment and add
# command[check_users]=/usr/lib/nagios/plugins/check_users -w 5 -c 10
# command[check_load]=/usr/lib/nagios/plugins/check_load -r -w .15,.10,.05 -c .30,.25,.20
# command[check_hda1]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /dev/hda1
# command[check_zombie_procs]=/usr/lib/nagios/plugins/check_procs -w 5 -c 10 -s Z
# command[check_total_procs]=/usr/lib/nagios/plugins/check_procs -w 150 -c 200

The following has been added
command[check_by_ssh]=/usr/lib/nagios/plugins/check_by_ssh $ARG1$
command[check_dhcp]=/usr/lib/nagios/plugins/check_dhcp $ARG1$
command[check_disk]=/usr/lib/nagios/plugins/check_disk $ARG1$
command[check_file_age]=/usr/lib/nagios/plugins/check_file_age $ARG1$
command[check_ftp]=/usr/lib/nagios/plugins/check_ftp $ARG1$
command[check_http]=/usr/lib/nagios/plugins/check_http $ARG1$
command[check_imap]=/usr/lib/nagios/plugins/check_imap $ARG1$
command[check_load]=/usr/lib/nagios/plugins/check_load $ARG1$
command[check_log]=/usr/lib/nagios/plugins/check_log $ARG1$
command[check_mysql]=/usr/lib/nagios/plugins/check_mysql $ARG1$
command[check_ntp]=/usr/lib/nagios/plugins/check_ntp $ARG1$
command[check_ntp_peer]=/usr/lib/nagios/plugins/check_ntp_peer $ARG1$
command[check_ntp_time]=/usr/lib/nagios/plugins/check_ntp_time $ARG1$
command[check_ping]=/usr/lib/nagios/plugins/check_ping $ARG1$
command[check_pop]=/usr/lib/nagios/plugins/check_pop $ARG1$
command[check_spop]=/usr/lib/nagios/plugins/check_spop $ARG1$
command[check_procs]=/usr/lib/nagios/plugins/check_procs $ARG1$
command[check_smtp]=/usr/lib/nagios/plugins/check_smtp $ARG1$
command[check_ssmtp]=/usr/lib/nagios/plugins/check_ssmtp $ARG1$
command[check_ssh]=/usr/lib/nagios/plugins/check_ssh $ARG1$
command[check_swap]=/usr/lib/nagios/plugins/check_swap $ARG1$
command[check_tcp]=/usr/lib/nagios/plugins/check_tcp $ARG1$
command[check_udp]=/usr/lib/nagios/plugins/check_udp $ARG1$
command[check_ups]=/usr/lib/nagios/plugins/check_ups $ARG1$
command[check_users]=/usr/lib/nagios/plugins/check_users $ARG1$

8.1.3 nrpe server activation and startup

# /etc/init.d/nagios-nrpe-server status

● nagios-nrpe-server.service - Nagios Remote Plugin Executor
     Loaded: loaded (/lib/systemd/system/nagios-nrpe-server.service; enabled; preset: enabled)
     Active: active (running) since Fri 2026-05-29 15:45:15 JST; 3min 10s ago
       Docs: http://www.nagios.org/documentation
   Main PID: 3199 (nrpe)
      Tasks: 1 (limit: 2255)
     Memory: 1.3M
        CPU: 15ms
     CGroup: /system.slice/nagios-nrpe-server.service
             └─3199 /usr/sbin/nrpe -c /etc/nagios/nrpe.cfg -f

May 29 15:45:15 Lion systemd[1]: Started nagios-nrpe-server.service - Nagios Remote Plugin Executor.
May 29 15:45:15 Lion nrpe[3199]: Starting up daemon
May 29 15:45:15 Lion nrpe[3199]: Server listening on 0.0.0.0 port 5666.
May 29 15:45:15 Lion nrpe[3199]: Server listening on :: port 5666.
May 29 15:45:15 Lion nrpe[3199]: Listening for connections on port 5666
May 29 15:45:15 Lion nrpe[3199]: Allowing connections from: 127.0.0.1,::1

8.1.4 Open NRPE ports 5666/tcp and 5666/udp on firewall

# ufw allow 5666/tcp
# ufw allow 5666/udp
# ufw reload
8.2  Nagios Server Side Configuration

8.2.1 nrpe plugin installation

# apt install nagios-nrpe-plugin

8.2.2 Copy to nagios plugin directory

# cp /usr/lib/nagios/plugins/check_nrpe /usr/local/nagios/libexec/

8.2.3 commands.cfg edit

# vi /usr/local/nagios/etc/objects/commands.cfg

Add to the last line
define command {
    command_name    check_nrpe
    command_line    $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
}

8.2.4 Edit and add korodes.cfg created when you added the host earlier
This time, let's add HTTP, SSH, and FTP.

# vi /usr/local/nagios/etc/servers/korodes.cfg

Add the following:
# 'HTTP' command definition
define service{
        use                              local-service
        host_name                        korodes
        service_description              HTTP
        check_command                    check_http
        }
# 'SSH' command definition
define service{
        use                              local-service
        host_name                        korodes
        service_description              SSH
        check_command                    check_ssh
       }
# 'FTP' command definition
define service{
        use                              local-service
        host_name                        korodes
        service_description              FTP
        check_command                    check_ftp
        }

8.2.5 Restart Nagios

# systemctl restart nagios.service

8.2.6 Access http://[Nagios Server IP Address]/nagios
Newly added monitoring services are added as shown in the figure below.

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