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Install Nagios Core on Ubuntu Focal 20.04

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Prepare your system

In this how to, we are going to install all files to /opt/nagios.

If you want to delete Nagios Core, just remove this folder and the systemd config.

All commands needs to run as user root or via sudo.

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

Install dependencies

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

Download and install Nagios Core

Check for new versions

In this how to I use Nagios 4.4.6. Check if there is a new version available! 

cd /tmp/
wget https://github.com/NagiosEnterprises/nagioscore/archive/nagios-4.4.6.tar.gz
tar xfv nagios-4.4.6.tar.gz
cd nagioscore-nagios-4.4.6/

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

Start Nagios Core (through systemd - recommended)

Copy the following to the file /lib/systemd/system/nagios.service using your favorite editor.

Pitfall!

If you want to use Nagios with the Statusengine Broker Module, it is required that the Gearman Job Server is running, BEFORE your Nagios gets started!
So you need to make sure that systemd will start the Gearman Job Server first.
To configure this, replace the AFTER= line in the following systemd configuration with this: 

After=network.target gearman-job-server.service
If not already done, install the Gearman Job Server now! 
apt-get install gearman-job-server

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

[Service]
Type=forking
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

systemctl daemon-reload
systemctl start nagios

Now you can check if your Nagios Core is running using systemctl status nagios:

root@ubuntu-s-2vcpu-2gb-intel-fra1-01:/tmp/nagioscore-nagios-4.4.6# systemctl status nagios
● nagios.service - Nagios Core
     Loaded: loaded (/lib/systemd/system/nagios.service; disabled; vendor preset: enabled)
     Active: active (running) since Sun 2021-02-28 09:23:39 UTC; 4s ago
       Docs: https://www.nagios.org/documentation
    Process: 58178 ExecStartPre=/opt/nagios/bin/nagios -v /opt/nagios/etc/nagios.cfg (code=exited, status=0/SUCCESS)
    Process: 58179 ExecStart=/opt/nagios/bin/nagios -d /opt/nagios/etc/nagios.cfg (code=exited, status=0/SUCCESS)
   Main PID: 58180 (nagios)
      Tasks: 6 (limit: 2344)
     Memory: 2.5M
     CGroup: /system.slice/nagios.service
             ├─58180 /opt/nagios/bin/nagios -d /opt/nagios/etc/nagios.cfg
             ├─58181 /opt/nagios/bin/nagios --worker /opt/nagios/var/rw/nagios.qh
             ├─58182 /opt/nagios/bin/nagios --worker /opt/nagios/var/rw/nagios.qh
             ├─58183 /opt/nagios/bin/nagios --worker /opt/nagios/var/rw/nagios.qh
             ├─58184 /opt/nagios/bin/nagios --worker /opt/nagios/var/rw/nagios.qh
             └─58200 /opt/nagios/bin/nagios -d /opt/nagios/etc/nagios.cfg

Feb 28 09:23:39 ubuntu-s-2vcpu-2gb-intel-fra1-01 nagios[58180]: qh: core query handler registered
Feb 28 09:23:39 ubuntu-s-2vcpu-2gb-intel-fra1-01 nagios[58180]: qh: echo service query handler registered
Feb 28 09:23:39 ubuntu-s-2vcpu-2gb-intel-fra1-01 nagios[58180]: qh: help for the query handler registered
Feb 28 09:23:39 ubuntu-s-2vcpu-2gb-intel-fra1-01 nagios[58180]: wproc: Successfully registered manager as @wproc with query handler
Feb 28 09:23:39 ubuntu-s-2vcpu-2gb-intel-fra1-01 nagios[58180]: wproc: Registry request: name=Core Worker 58182;pid=58182
Feb 28 09:23:39 ubuntu-s-2vcpu-2gb-intel-fra1-01 nagios[58180]: wproc: Registry request: name=Core Worker 58181;pid=58181
Feb 28 09:23:39 ubuntu-s-2vcpu-2gb-intel-fra1-01 nagios[58180]: wproc: Registry request: name=Core Worker 58183;pid=58183
Feb 28 09:23:39 ubuntu-s-2vcpu-2gb-intel-fra1-01 nagios[58180]: wproc: Registry request: name=Core Worker 58184;pid=58184
Feb 28 09:23:40 ubuntu-s-2vcpu-2gb-intel-fra1-01 nagios[58180]: Successfully launched command file worker with pid 58200
Feb 28 09:23:40 ubuntu-s-2vcpu-2gb-intel-fra1-01 nagios[58180]: HOST ALERT: localhost;DOWN;SOFT;1;(No output on stdout) stderr: execvp(/opt/nagios/libexec/check_ping, ...) failed. errno is 2: No such file or directory
root@ubuntu-s-2vcpu-2gb-intel-fra1-01:/tmp/nagioscore-nagios-4.4.6#

To make sure that you Nagios will start automatically on boot, you need to enable the systemd configuration:

systemctl enable nagios.service

Install Monitoring Plugins

By default, Nagios will install a sample config with some basic checks. So you need to install the monitoring plugins to get them to work.

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


Congrats! You installed Nagios Core!

Load Statusengine Broker Module

Now you are ready to install and load the Statusengine Broker Module.

Start Nagios in foreground (expert or debugging)

For some reasons it can be useful to run Nagios in foreground.

You can do this with this command /opt/nagios/bin/nagios /opt/nagios/etc/nagios.cfg

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

Nagios Core 4.4.6
Copyright (c) 2009-present Nagios Core Development Team and Community Contributors
Copyright (c) 1999-2009 Ethan Galstad
Last Modified: 2020-04-28
License: GPL

Website: https://www.nagios.org
Nagios 4.4.6 starting... (PID=58340)
Local time is Sun Feb 28 09:25:12 UTC 2021
wproc: Successfully registered manager as @wproc with query handler
wproc: Registry request: name=Core Worker 58343;pid=58343
wproc: Registry request: name=Core Worker 58341;pid=58341
wproc: Registry request: name=Core Worker 58342;pid=58342
wproc: Registry request: name=Core Worker 58344;pid=58344
Successfully launched command file worker with pid 58345
^C
nagios@focal:/opt/nagios$

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