Open source integrated monitoring software "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
# apt -y install nagios4
Apache Restart
# a2enmod cgi
# systemctl restart nagios4 apache2
3. Install Nagios Plugin
①Install
# apt -y install nagios-plugins
The following nagios-plugins are available:
# ls /usr/lib/nagios/plugins/
check_apt check_fping check_load check_nwstat check_ssh
check_breeze check_ftp check_log check_oracle check_ssmtp
check_by_ssh check_game check_mailq check_overcr check_swap
check_clamd check_host check_mrtg check_ping check_tcp
check_cluster check_hpjd check_mrtgtraf check_pop check_time
check_curl check_http check_mssql check_procs check_udp
check_dbi check_icmp check_mysql check_radius check_ups
check_dhcp check_ide_smart check_mysql_query check_real check_users
check_dig check_ifoperstatus check_nagios check_rpc check_wave
check_disk check_ifstatus check_nntp check_rta_multi negate
check_disk_smb check_imap check_nntps check_sensors urlize
check_dns check_ircd check_nt check_simap utils.pm
check_dummy check_jabber check_ntp check_smtp utils.sh
check_file_age check_ldap check_ntp_peer check_snmp
check_flexlm check_ldaps check_ntp_time check_spop
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.
# htdigest /etc/nagios4/htdigest.users "Nagios4" nagiosadmin
New password: [password]
Re-type new password: [again password]
Adding password for user nagiosadmin
Allow HTTP service port in firewall
# ufw allow http
# ufw reload
Start and Enable Nagios Services
# systemctl enable nagios4 --now
Created symlink '/etc/systemd/system/multi-user.target.wants/nagios4.service' → '/usr/lib/systemd/system/nagios4.service'.
Confirmation of service execution
# systemctl status nagios4
● nagios4.service - nagios4
Loaded: loaded (/usr/lib/systemd/system/nagios4.service; enabled; preset: enabled)
Active: active (running) since Mon 2026-10-19 13:41:43 JST; 7min ago
Invocation: cfa131779dc9434cbd5c011aabc18f3c
Docs: man:nagios4
Process: 11351 ExecStartPre=sh -c nagiospipe=$$(sed -n "s/^command_file=\(.*\)/\1/p" ${NAGIOSCFG}); [ >
Main PID: 11356 (nagios4)
Tasks: 6 (limit: 3938)
Memory: 2.5M (peak: 4M)
CPU: 186ms
CGroup: /system.slice/nagios4.service
├─11356 /usr/sbin/nagios4 /etc/nagios4/nagios.cfg
├─11357 /usr/sbin/nagios4 --worker /var/lib/nagios4/rw/nagios.qh
├─11358 /usr/sbin/nagios4 --worker /var/lib/nagios4/rw/nagios.qh
├─11359 /usr/sbin/nagios4 --worker /var/lib/nagios4/rw/nagios.qh
├─11360 /usr/sbin/nagios4 --worker /var/lib/nagios4/rw/nagios.qh
└─11362 /usr/sbin/nagios4 /etc/nagios4/nagios.cfg
Oct 19 13:41:43 Lepard nagios4[11356]: wproc: Registry request: name=Core Worker 11357;pid=11357
Oct 19 13:41:43 Lepard nagios4[11356]: wproc: Registry request: name=Core Worker 11357;pid=11357
Oct 19 13:41:43 Lepard nagios4[11356]: wproc: Registry request: name=Core Worker 11358;pid=11358
Oct 19 13:41:43 Lepard nagios4[11356]: wproc: Registry request: name=Core Worker 11359;pid=11359
Oct 19 13:41:43 Lepard nagios4[11356]: wproc: Registry request: name=Core Worker 11360;pid=11360
Oct 19 13:41:43 Lepard nagios4[11356]: wproc: Registry request: name=Core Worker 11358;pid=11358
Oct 19 13:41:43 Lepard nagios4[11356]: wproc: Registry request: name=Core Worker 11359;pid=11359
Oct 19 13:41:43 Lepard nagios4[11356]: wproc: Registry request: name=Core Worker 11360;pid=11360
Oct 19 13:41:43 Lepard nagios4[11356]: Successfully launched command file worker with pid 11362
Oct 19 13:41:43 Lepard nagios4[11356]: Successfully launched command file worker with pid 11362
5. Access the Nagios Web Interface
If you go to http://[FQDN]/nagios/ or http://[IP address]/nagios4 in any browser, you will see the following dashboard.
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 the SSH port to custom configuration 2244, change localhost.cfg as follows
# vi /etc/nagios4/objects/localhost.cfg
Per Line143:
check_command check_ssh
↓
check_command check_ssh!-p 2244
# systemctl restart nagios4
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 Ubuntu repository
# apt-cache policy nagios-nrpe-server
nagios-nrpe-server:
Installed: (none)
Candidate: 4.1.3-1ubuntu3
Version table:
4.1.3-1ubuntu3 500
500 http://archive.ubuntu.com/ubuntu stonking/universe amd64 Packages
# apt install -y nagios-nrpe-server
Version Check
# nrpe -V
NRPE - Nagios Remote Plugin Executor
Version: 4.1.3
Start and activate the service
# systemctl enable --now 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 (/usr/lib/systemd/system/nagios-nrpe-server.service; enabled; preset: enabled)
Active: active (running) since Mon 2026-10-19 14:22:50 JST; 1min 54s ago
Invocation: aa3f5dcbe4aa4f59a4a493eb0d3074e5
Docs: http://www.nagios.org/documentation
Main PID: 6213 (nrpe)
Tasks: 1 (limit: 3938)
Memory: 1.3M (peak: 2.2M)
CPU: 10ms
CGroup: /system.slice/nagios-nrpe-server.service
└─6213 /usr/sbin/nrpe -c /etc/nagios/nrpe.cfg -f
Oct 19 14:22:50 Lepard systemd[1]: Started nagios-nrpe-server.service - Nagios Remote Plugin Executor.
Oct 19 14:22:50 Lepard (nrpe)[6213]: nagios-nrpe-server.service: Referenced but unset environment variable>
Oct 19 14:22:50 Lepard nrpe[6213]: Starting up daemon
Oct 19 14:22:50 Lepard nrpe[6213]: Server listening on 0.0.0.0 port 5666.
Oct 19 14:22:50 Lepard nrpe[6213]: Server listening on :: port 5666.
Oct 19 14:22:50 Lepard nrpe[6213]: Listening for connections on port 5666
Oct 19 14:22:50 Lepard nrpe[6213]: 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=6213,fd=4))
LISTEN 0 5 [::]:5666 [::]:* users:(("nrpe",pid=6213,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 : Ubuntu26.10
IP address : 192.168.11.85
7.1 Configuration File Editing
# vi /etc/nagios4/nagios.cfg
Line 62 : Uncomment
cfg_dir=/etc/nagios4/servers
7.2 Directory Create
# mkdir /etc/nagios4/servers
# chgrp nagios /etc/nagios4/servers
# chmod 750 /etc/nagios4/servers
7.3 Create new definition file
The name "korodes.cfg" can be arbitrary.
# vi /etc/nagios4/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 nagios4.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 : Ubuntu26.10
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
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 nagios-nrpe-server
# systemctl start nagios-nrpe-server
# /etc/init.d/nagios-nrpe-server status
● 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 Mon 2026-10-19 14:33:56 JST; 6min ago
Invocation: 1337774dc6424a0c885b9947e5dadc5a
Docs: http://www.nagios.org/documentation
Main PID: 5491 (nrpe)
Tasks: 1 (limit: 3939)
Memory: 1.3M (peak: 2.2M)
CPU: 31ms
CGroup: /system.slice/nagios-nrpe-server.service
mq5491 /usr/sbin/nrpe -c /etc/nagios/nrpe.cfg -f
Oct 19 14:33:56 Lion systemd[1]: Started nagios-nrpe-server.service - Nagios Remote Plugin Executor.
Oct 19 14:33:56 Lion (nrpe)[5491]: nagios-nrpe-server.service: Referenced but unset environment variable evaluates to an empty string: NRPE_OPTS
Oct 19 14:33:56 Lion nrpe[5491]: Starting up daemon
Oct 19 14:33:56 Lion nrpe[5491]: Server listening on 0.0.0.0 port 5666.
Oct 19 14:33:56 Lion nrpe[5491]: Server listening on :: port 5666.
Oct 19 14:33:56 Lion nrpe[5491]: Listening for connections on port 5666
Oct 19 14:33:56 Lion nrpe[5491]: 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 Edit korodes.cfg created when you added the host earlier
I'll add HTTP,SSH,FTP this time.
# vi /etc/nagios4/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
}
8.2.2 Restart Nagios
# systemctl restart nagios4.service
8.2.3 Accsess http://[Nagios Server IP Address]/nagios
Newly added monitoring services are added as shown in the figure below.
