Nagios Server
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 -y install autoconf gcc libc6 make wget unzip libgd-dev libssl-dev
2. Installing Nagios Core
①Download and Extract and Compile
# wget https://go.nagios.org/get-core/4-5-12
# 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
Install 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-2.5.tar.gz
②Install
# tar -xvf nagios-plugins-2.5.tar.gz
# cd nagios-plugins-2.5
# ./configure --with-nagios-user=nagios --with-nagios-group=nagios
# make
# make install
The following nagios-plugins are available:
# ls /usr/local/nagios/libexec
check_apt check_file_age check_load check_ntp_time check_smtp check_users
check_breeze check_flexlm check_log check_nwstat check_spop check_wave
check_by_ssh check_ftp check_mailq check_oracle check_ssh negate
check_clamd check_http check_mrtg check_overcr check_ssl_validity remove_perfdata
check_cluster check_icmp check_mrtgtraf check_ping check_ssmtp urlize
check_dhcp check_ide_smart check_nagios check_pop check_swap utils.pm
check_dig check_ifoperstatus check_nntp check_procs check_tcp utils.sh
check_disk check_ifstatus check_nntps check_real check_time
check_disk_smb check_imap check_nt check_rpc check_udp
check_dns check_ircd check_ntp check_sensors check_ups
check_dummy check_jabber check_ntp_peer check_simap check_uptime
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
5. Starting Nagios
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
Reboot system
# shutdown -r now
Confirmation of service execution
# systemctl status nagios
● nagios.service - Nagios Core 4.5.12
Loaded: loaded (/usr/lib/systemd/system/nagios.service; enabled; preset: enabled)
Active: active (running) since Fri 2026-05-22 09:35:05 JST; 1min 56s ago
Invocation: d8ddc96f75a747bd912f592385c95af4
Docs: https://www.nagios.org/documentation
Main PID: 1199 (nagios)
Tasks: 6 (limit: 4593)
Memory: 7.4M (peak: 8.8M)
CPU: 147ms
CGroup: /system.slice/nagios.service
├─1199 /usr/local/nagios/bin/nagios -d /usr/local/nagios/etc/nagios.cfg
├─1206 /usr/local/nagios/bin/nagios --worker /usr/local/nagios/var/rw/nagios.qh
├─1207 /usr/local/nagios/bin/nagios --worker /usr/local/nagios/var/rw/nagios.qh
├─1208 /usr/local/nagios/bin/nagios --worker /usr/local/nagios/var/rw/nagios.qh
├─1209 /usr/local/nagios/bin/nagios --worker /usr/local/nagios/var/rw/nagios.qh
└─1219 /usr/local/nagios/bin/nagios -d /usr/local/nagios/etc/nagios.cfg
May 22 09:35:05 Lepard nagios[1199]: qh: Socket '/usr/local/nagios/var/rw/nagios.qh' successfully initiali>
May 22 09:35:05 Lepard nagios[1199]: qh: core query handler registered
May 22 09:35:05 Lepard nagios[1199]: qh: echo service query handler registered
May 22 09:35:05 Lepard nagios[1199]: qh: help for the query handler registered
May 22 09:35:05 Lepard nagios[1199]: wproc: Successfully registered manager as @wproc with query handler
May 22 09:35:05 Lepard nagios[1199]: wproc: Registry request: name=Core Worker 1209;pid=1209
May 22 09:35:05 Lepard nagios[1199]: wproc: Registry request: name=Core Worker 1208;pid=1208
May 22 09:35:05 Lepard nagios[1199]: wproc: Registry request: name=Core Worker 1207;pid=1207
May 22 09:35:05 Lepard nagios[1199]: wproc: Registry request: name=Core Worker 1206;pid=1206
May 22 09:35:05 Lepard nagios[1199]: Successfully launched command file worker with pid 1219
6. 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
Currently, 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 : The current monitoring service details are displayed.
7. Nagios Agent Configuration
To monitor the agent. Install the following
• NRPE Agent to run plugins
① nstalling Nagios NRPE Agent
NRPE agent available in default Debian repository
# apt-cache policy nagios-nrpe-server
nagios-nrpe-server:
Installed: (none)
Candidate: 4.1.3-1
Version table:
4.1.3-1 500
500 http://deb.debian.org/debian trixie/main amd64 Packages
# apt -y install nagios-nrpe-server
➁Version Check
# nrpe -V
NRPE - Nagios Remote Plugin Executor
Version: 4.1.3
➂Start and activate the service
# systemctl enable --now nagios-nrpe-server
Synchronizing state of nagios-nrpe-server.service with SysV service script with /usr/lib/systemd/systemd-sysv-install.
Executing: /usr/lib/systemd/systemd-sysv-install enable nagios-nrpe-server
④Check the NRPE agent status
# systemctl status nagios-nrpe-server
● nagios-nrpe-server.service - Nagios Remote Plugin Executor
Loaded: loaded (/usr/lib/systemd/system/nagios-nrpe-server.service; enabled; preset: enabled)
Active: active (running) since Fri 2026-05-22 10:20:56 JST; 2min 10s ago
Invocation: b594ed48f48d4e3ca7c392fa7f6190f4
Docs: http://www.nagios.org/documentation
Main PID: 2217 (nrpe)
Tasks: 1 (limit: 4593)
Memory: 1.4M (peak: 1.9M)
CPU: 12ms
CGroup: /system.slice/nagios-nrpe-server.service
└─2217 /usr/sbin/nrpe -c /etc/nagios/nrpe.cfg -f
May 22 10:20:56 Lepard systemd[1]: Started nagios-nrpe-server.service - Nagios Remote Plugin Executor.
May 22 10:20:56 Lepard (nrpe)[2217]: nagios-nrpe-server.service: Referenced but unset environment variable>
May 22 10:20:56 Lepard nrpe[2217]: Starting up daemon
May 22 10:20:56 Lepard nrpe[2217]: Server listening on 0.0.0.0 port 5666.
May 22 10:20:56 Lepard nrpe[2217]: Server listening on :: port 5666.
May 22 10:20:56 Lepard nrpe[2217]: Listening for connections on port 5666
May 22 10:20:56 Lepard nrpe[2217]: Allowing connections from: 127.0.0.1,::1
⑤Open the NRPE port in the firewall
By default, NRPE uses TCP port 5666.
If a firewall is in use, open this port to allow external checks from the Nagios monitoring server.
# 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=21947,fd=4))
LISTEN 0 5 [::]:5666 [::]:* users:(("nrpe",pid=21947,fd=5))
8. Adding a monitored host
Add the following server to the host and configure it as a target for simple ping death/activity monitoring
OS : Debian13.1
IP address : 192.168.11.85
①Configuration File Editing
# vi /usr/local/nagios/etc/nagios.cfg
Line 51 : Uncomment
cfg_dir=/usr/local/nagios/etc/servers
➁ Directory Create
# mkdir /usr/local/nagios/etc/servers
# chgrp nagios /usr/local/nagios/etc/servers
# chmod 750 /usr/local/nagios/etc/servers
➂ Create new definition file
The name "korodes.cfg" can be arbitrary.
# 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%
}
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.
9. Adding Monitoring Services
Add monitoring services to the hosts added above.
OS : Debian13.1
IP address : 192.168.11.85
9.1 Configuration on the server to be monitored
① Agent installed on monitored hosts
Install nrpe and typical service plugins from EPEL
# apt -y install nagios-nrpe-server
➁ 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
Add part of the plugin
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 $ARG1$
➂ nrpe server activation and startup
# systemctl enable nagios-nrpe-server
Synchronizing state of nagios-nrpe-server.service with SysV service script with /usr/lib/systemd/systemd-sysv-install.
Executing: /usr/lib/systemd/systemd-sysv-install enable nagios-nrpe-server
# systemctl start nagios-nrpe-server
# systemctl status nagios-nrpe-server
● nagios-nrpe-server.service - Nagios Remote Plugin Executor
Loaded: loaded (/usr/lib/systemd/system/nagios-nrpe-server.service; enabled; preset: enabled)
Active: active (running) since Fri 2026-05-22 11:20:05 JST; 1min 14s ago
Invocation: 982ef56c2dd648f6801a48eeded6d671
Docs: http://www.nagios.org/documentation
Main PID: 3735 (nrpe)
Tasks: 1 (limit: 4553)
Memory: 1.4M (peak: 1.8M)
CPU: 21ms
CGroup: /system.slice/nagios-nrpe-server.service
mq3735 /usr/sbin/nrpe -c /etc/nagios/nrpe.cfg -f
May 22 11:20:05 Lion systemd[1]: Started nagios-nrpe-server.service - Nagios Remote Plugin Executor.
May 22 11:20:05 Lion (nrpe)[3735]: nagios-nrpe-server.service: Referenced but unset environment variable evaluates to >
May 22 11:20:05 Lion nrpe[3735]: Starting up daemon
May 22 11:20:05 Lion nrpe[3735]: Server listening on 0.0.0.0 port 5666.
May 22 11:20:05 Lion nrpe[3735]: Server listening on :: port 5666.
May 22 11:20:05 Lion nrpe[3735]: Listening for connections on port 5666
May 22 11:20:05 Lion nrpe[3735]: Allowing connections from: 127.0.0.1,::1,192.168.11.83
④ Open NRPE ports 5666/tcp and 5666/udp on firewall
# ufw allow 5666/tcp
# ufw allow 5666/udp
# ufw reload
9.2 Nagios Server Side Configuration
① nrpe plugin installation
# apt -y install nagios-nrpe-plugin
➁ Copy to nagios plugin directory
# cp /usr/lib/nagios/plugins/check_nrpe /usr/local/nagios/libexec/
➂ 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 $ARG1$
}
④ Edit and add korodes.cfg created when you added the host earlier
I'll add HTTP,SSH,FTP this time.
# 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
}
⑤ Restart Nagios
# systemctl restart nagios.service
⑥ Access http://[Nagios Server IP Address]/nagios
Newly added monitoring services are added as shown in the figure below.
