Nagios Server : open-source integrated monitoring software
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. Nagios Install
# dnf install -y nagios nagios-selinux
Normally a nagios user group is created, but if a package installation script or trigger is skipped for any reason, you can force generate a system user from the configuration file with the following command:
# systemd-sysusers
2. Install the Nagios plugin
②Install
# dnf install nagios-plugins-all
The following nagios-plugins are available
# ls /usr/lib64/nagios/plugins/
check_breeze check_file_age check_ircd check_mysql_query check_oracle check_smtp check_ups
check_by_ssh check_flexlm check_jabber check_nagios check_overcr check_snmp check_users
check_clamd check_fping check_ldap check_nntp check_pgsql check_spop check_wave
check_cluster check_ftp check_ldaps check_nntps check_ping check_ssh eventhandlers
check_dhcp check_game check_load check_nt check_pop check_ssl_validity negate
check_dig check_hpjd check_log check_ntp check_procs check_ssmtp urlize
check_disk check_http check_mailq check_ntp_peer check_real check_swap utils.pm
check_disk_smb check_icmp check_mrtg check_ntp.pl check_rpc check_tcp utils.sh
check_dns check_ide_smart check_mrtgtraf check_ntp_time check_sensors check_time
check_dummy check_imap check_mysql check_nwstat check_simap check_udp
3. 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 for the user is nagiosadmin, which is defined as the preferred username in the /etc/nagios/cgi.cfg file.
# htpasswd -c /etc/nagios/htpasswd nagiosadmin
New password: [password]
Re-type new password: [again password]
Adding password for user nagiosadmin
Syntax check for configuration files
# nagios -v /etc/nagios/nagios.cfg
----------------------------------------------------------------------------------
Total Warnings: 0
Total Errors: 0
Things look okay - No serious problems were detected during the pre-flight check
Total Warnings: 0
Total Errors: 0
If it says "This is normal"
Enable Nagios service
# systemctl enable nagios --now
Check if the service is running
# systemctl status nagios
● nagios.service - Nagios Core 4.5.14
Loaded: loaded (/usr/lib/systemd/system/nagios.service; enabled; preset: disabled)
Drop-In: /usr/lib/systemd/system/service.d
└─10-timeout-abort.conf
Active: active (running) since Thu 2026-11-12 12:10:38 JST; 36s ago
Invocation: 95ab1c4ffbad480b90ae096073da459b
Docs: https://www.nagios.org/documentation
Process: 3296 ExecStartPre=/usr/bin/nagios -v /etc/nagios/nagios.cfg (code=exited, status=0/SUCCESS)
Process: 3297 ExecStart=/usr/bin/nagios -d /etc/nagios/nagios.cfg (code=exited, status=0/SUCCESS)
Main PID: 3299 (nagios)
Tasks: 8 (limit: 4564)
Memory: 6.9M (peak: 7.8M)
CPU: 46ms
CGroup: /system.slice/nagios.service
├─3299 /usr/bin/nagios -d /etc/nagios/nagios.cfg
├─3301 /usr/bin/nagios --worker /var/spool/nagios/cmd/nagios.qh
├─3302 /usr/bin/nagios --worker /var/spool/nagios/cmd/nagios.qh
├─3303 /usr/bin/nagios --worker /var/spool/nagios/cmd/nagios.qh
├─3304 /usr/bin/nagios --worker /var/spool/nagios/cmd/nagios.qh
├─3305 /usr/bin/nagios --worker /var/spool/nagios/cmd/nagios.qh
├─3306 /usr/bin/nagios --worker /var/spool/nagios/cmd/nagios.qh
└─3309 /usr/bin/nagios -d /etc/nagios/nagios.cfg
Nov 12 12:10:38 Lepard nagios[3299]: qh: help for the query handler registered
Nov 12 12:10:38 Lepard nagios[3299]: wproc: Successfully registered manager as @wproc with query handler
Nov 12 12:10:38 Lepard systemd[1]: Started nagios.service - Nagios Core 4.5.14.
Nov 12 12:10:38 Lepard nagios[3299]: wproc: Registry request: name=Core Worker 3301;pid=3301
Nov 12 12:10:38 Lepard nagios[3299]: wproc: Registry request: name=Core Worker 3302;pid=3302
Nov 12 12:10:38 Lepard nagios[3299]: wproc: Registry request: name=Core Worker 3304;pid=3304
Nov 12 12:10:38 Lepard nagios[3299]: wproc: Registry request: name=Core Worker 3305;pid=3305
Nov 12 12:10:38 Lepard nagios[3299]: wproc: Registry request: name=Core Worker 3303;pid=3303
5. Access the Nagios Web Interface
Access http://[FQDN]/nagios or http://[IP address]/nagios in any browser.
Username : nagoisadmin
Password : Password specified when the user was created above
[Sign in]
After successful login, the following dashboard will appear
Show host availability
Click on [Hosts] in the left menu
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.
※ If you have assigned SSH port 2244 with custom settings, change localhost.cfg as follows:
# vi /etc/nagios/objects/localhost.cfg
Per Line 143 :
check_command check_ssh
↓
check_command check_ssh!-p 2244
# systemctl restart nagios
6. Configuring the NRPE Plugin
To monitor the agent, install the following:
6.1 Install the nrpe plugin
# dnf install nrpe nagios-plugins-nrpe
Open the/etc/nagios/nrpe.cfg file and add the IP addresses of your localhost and Nagios monitoring server
# vi /etc/nagios/nrpe.cfg
Line 106 : Add
allowed_hosts=127.0.0.1,::1,192.168.11.83
Check Version
# /usr/bin/nrpe -V
NRPE - Nagios Remote Plugin Executor
Version: 4.2.0
Start the service and enable it
# systemctl enable nrpe
Created symlink '/etc/systemd/system/multi-user.target.wants/nrpe.service' → '/usr/lib/systemd/system/nrpe.service'.
# systemctl restart nrpe
Open the NRPE port in the firewall
NRPE uses TCP port 5666 by default. If you have a firewall in place, open this port to allow external checks from the Nagios monitoring server.
# firewall-cmd --add-port=5666/tcp --permanent
# firewall-cmd --reload
Check if port 5666/tcp is listening
# netstat -na | grep "5666"
tcp 0 0 0.0.0.0:5666 0.0.0.0:* LISTEN
Run the "check_nrpe" command to verify that the NRPE daemon is running properly
The version of NRPE currently installed is displayed
/usr/lib64/nagios/plugins/
# /usr/lib64/nagios/plugins/check_nrpe -H 127.0.0.1
NRPE v4.2.0
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 : Fedora45
IPアドレス : 192.168.11.85
7.1 Configuration File Editing
# vi /etc/nagios/nagios.cfg
Line 51 : Uncomment
cfg_dir=/usr/local/nagios/etc/servers
7.2 Directory Create
# mkdir /etc/nagios/servers
# chgrp nagios /etc/nagios/servers
# chmod 750 /etc/nagios/servers
7.3 Create new definition file
The name "korodes.cfg" can be arbitrary.
# vi /etc/nagios/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 : Fedora45
IPアドレス : 192.168.11.85
8.1 Configuration on the server to be monitored
8.1.1 Agent installed on monitored hosts
Install NRPE and common service plugins
# dnf -y install nagios-nrpe nagios-plugins-all
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
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 $ARG1$
8.1.3 nrpe server activation and startup
# systemctl enable --now nrpe
Created symlink '/etc/systemd/system/multi-user.target.wants/nrpe.service' → '/usr/lib/systemd/system/nrpe.service'.
# systemctl start nrpe
# systemctl status nrpe
● nrpe.service - Nagios Remote Plugin Executor
Loaded: loaded (/usr/lib/systemd/system/nrpe.service; enabled; preset: disabled)
Drop-In: /usr/lib/systemd/system/service.d
└─10-timeout-abort.conf
Active: active (running) since Thu 2026-11-12 14:08:17 JST; 57s ago
Invocation: df4bf3e9fcfd4f37b373e5be4491cc89
Docs: http://www.nagios.org/documentation
Main PID: 2334 (nrpe)
Tasks: 1 (limit: 4558)
Memory: 1.3M (peak: 1.6M)
CPU: 12ms
CGroup: /system.slice/nrpe.service
└─2334 /usr/bin/nrpe -c /etc/nagios/nrpe.cfg -f
Nov 12 14:08:17 Lion systemd[1]: Started nrpe.service - Nagios Remote Plugin Executor.
Nov 12 14:08:17 Lion nrpe[2334]: WARN: Limiting SSL/TLS version to v1.2 to support ADH
Nov 12 14:08:17 Lion nrpe[2334]: Starting up daemon
Nov 12 14:08:17 Lion nrpe[2334]: Server listening on 0.0.0.0 port 5666.
Nov 12 14:08:17 Lion nrpe[2334]: Server listening on :: port 5666.
Nov 12 14:08:17 Lion nrpe[2334]: Warning: Daemon is configured to accept command arguments from clients!
Nov 12 14:08:17 Lion nrpe[2334]: Listening for connections on port 5666
Nov 12 14:08:17 Lion nrpe[2334]: Allowing connections from: 127.0.0.1,::1,192.168.11.83
8.1.4 Open NRPE ports 5666/tcp and 5666/udp on firewall
# firewall-cmd --permanent --add-port=5666/tcp
# firewall-cmd --permanent --add-port=5666/udp
# firewall-cmd --reload
8.2 Nagios Server Side Configuration
8.2.1 commands.cfg edit
# vi /etc/nagios/objects/commands.cfg
# Add to the last line
define command {
command_name check_nrpe
command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -c $ARG1$
}
8.2.2 Edit and add korodes.cfg created when you added the host earlier
I'll add HTTP,SSH,FTP this time.
# vi /etc/nagios/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 Accsess http://[Nagios Server IP Address]/nagios
Newly added monitoring services are added as shown in the figure below
