Tutorials
Install Nagios Core on Ubuntu

Install Nagios Core on Ubuntu

This guide installs Nagios Core 4.5.14 from source on Ubuntu 26.04 LTS (Resolute Raccoon).

Everything goes to /opt/nagios. To remove Nagios later, delete that directory and the systemd unit. All commands run as root, or via sudo.

ℹ️
Running Naemon instead? There is a separate guide for Naemon Core . Statusengine’s broker module supports both, but it is built against one or the other.

Prepare your system 

addgroup --system nagios
adduser --system nagios
adduser nagios nagios

Install dependencies 

apt-get update
apt-get install build-essential libgd-dev libpng-dev libssl-dev unzip

libssl-dev is not optional: configure stops with Cannot find ssl headers without it. Neither is unzip — the build unpacks the bundled web assets with it and aborts on Cannot continue without unzip!.

Download and install Nagios Core

ℹ️
This guide pins Nagios Core 4.5.14. Check whether a newer version is available before you start.
cd /tmp/
wget https://github.com/NagiosEnterprises/nagioscore/archive/nagios-4.5.14.tar.gz
tar xfv nagios-4.5.14.tar.gz
cd nagioscore-nagios-4.5.14/

./configure --prefix=/opt/nagios --with-nagios-user=nagios --with-nagios-group=nagios
make all
make install
make install-commandmode
make install-config
make install-devel

configure prints a summary before it finishes. The line that matters for Statusengine is the event broker:

        Nagios executable:  nagios
        Nagios user/group:  nagios,nagios
       Command user/group:  nagios,nagios
             Event Broker:  yes
        Install ${prefix}:  /opt/nagios
    Install ${includedir}:  /opt/nagios/include/nagios
                Lock file:  /run/nagios.lock
   Check result directory:  /opt/nagios/var/spool/checkresults

make install-devel is the step that puts the Nagios headers in /opt/nagios/include/nagios/. The broker module includes them as <nagios/nagios.h>, so without this target there is nothing to build it against later.

Start Nagios Core through systemd

🚫

Pitfall! If you are going to use Nagios with the Statusengine broker module , the Gearman Job Server has to be running before Nagios starts, so systemd needs to know about the dependency. Replace the After= line below with:

After=network.target local-fs.target gearman-job-server.service

If you have not installed it yet:

apt-get install gearman-job-server

Put the following in /etc/systemd/system/nagios.service:

[Unit]
Description=Nagios Core
Documentation=https://www.nagios.org/documentation
After=network.target local-fs.target

[Service]
Type=forking
PIDFile=/run/nagios.lock
ExecStartPre=/opt/nagios/bin/nagios -v /opt/nagios/etc/nagios.cfg
ExecStart=/opt/nagios/bin/nagios -d /opt/nagios/etc/nagios.cfg
ExecStop=/bin/kill -s TERM ${MAINPID}
ExecStopPost=/bin/rm -f /opt/nagios/var/rw/nagios.cmd
ExecReload=/bin/kill -s HUP ${MAINPID}

[Install]
WantedBy=multi-user.target
ℹ️

There is deliberately no User=nagios here, unlike the Naemon unit. Nagios writes its lock file to /run/nagios.lock, which only root may create; the daemon then drops to the nagios_user from nagios.cfg on its own, so the process you end up with runs as nagios either way.

PIDFile= points at that same lock file. Without it systemd has to guess which of the forked processes is the main one.

systemctl daemon-reload
systemctl start nagios

Check that it came up with systemctl status nagios:

● nagios.service - Nagios Core
     Loaded: loaded (/etc/systemd/system/nagios.service; disabled; preset: enabled)
     Active: active (running)
       Docs: https://www.nagios.org/documentation
    Process: 127 ExecStartPre=/opt/nagios/bin/nagios -v /opt/nagios/etc/nagios.cfg (code=exited, status=0/SUCCESS)
    Process: 129 ExecStart=/opt/nagios/bin/nagios -d /opt/nagios/etc/nagios.cfg (code=exited, status=0/SUCCESS)
   Main PID: 130 (nagios)
      Tasks: 26
     Memory: 25.3M
     CGroup: /system.slice/nagios.service
             ├─130 /opt/nagios/bin/nagios -d /opt/nagios/etc/nagios.cfg
             ├─133 /opt/nagios/bin/nagios --worker /opt/nagios/var/rw/nagios.qh
             ├─134 /opt/nagios/bin/nagios --worker /opt/nagios/var/rw/nagios.qh
             └─...

The line to look for in the pre-flight check is Things look okay - No serious problems were detected.

To start Nagios on boot:

systemctl enable nagios.service

Install monitoring plugins

Nagios installs a sample configuration with some basic checks, so you need the plugins for those checks to work. Unlike Naemon, Nagios does not look in /usr/lib/nagios/plugins by default, so resource.cfg has to point at it:

apt-get install monitoring-plugins
echo '$USER1$=/usr/lib/nagios/plugins' > /opt/nagios/etc/resource.cfg
systemctl restart nagios

The sample localhost host should go green within a minute. To confirm without a web interface, look at the status file:

# grep -A30 "hoststatus {" /opt/nagios/var/status.dat | grep -E "host_name|current_state|plugin_output"
	host_name=localhost
	current_state=0
	plugin_output=PING OK - Packet loss = 0%, RTA = 0.04 ms

Congratulations, you have installed Nagios Core.

ℹ️
You are now ready to install and load the Statusengine broker module . Build it with -Dnagios=true -Dnagios_include_dir=/opt/nagios/include — the headers make install-devel put there above.

Running Nagios in the foreground

For debugging it can be useful to run Nagios in the foreground instead. Exit with Ctrl+C.

root@ubuntu:/opt/nagios# sudo -u nagios /bin/bash
nagios@ubuntu:/opt/nagios$ /opt/nagios/bin/nagios /opt/nagios/etc/nagios.cfg

Nagios Core 4.5.14
Copyright (c) 2009-present Nagios Core Development Team and Community Contributors
Copyright (c) 1999-2009 Ethan Galstad
Last Modified: 2026-08-05
License: GPL

Website: https://www.nagios.org
Nagios 4.5.14 starting... (PID=11863)
Local time is Sat Sep 12 14:17:54 UTC 2026
wproc: Successfully registered manager as @wproc with query handler
wproc: Registry request: name=Core Worker 11864;pid=11864
wproc: Registry request: name=Core Worker 11865;pid=11865
wproc: Registry request: name=Core Worker 11866;pid=11866
Successfully launched command file worker with pid 11877
^C

Nagios is a registered trademark of Nagios Enterprises, LLC. This guide is not affiliated with or endorsed by Nagios Enterprises.

Install Naemon Core on UbuntuGearman Job Server: Too many open files