Nagios is an event monitoring system created by Ethan Galstad and first released in 2002, which offers monitoring and alerting services for servers, switches, applications, and services. It alerts users when things go wrong and alerts them again when the problem has been resolved. There are 2 types of Nagios: Nagios XI for the enterprise version and Nagios Core for the free version. This article will explain how to install Nagios Core on Ubuntu.

Problem

How to install Nagios on Ubuntu?

Solution

Here are the steps to install Nagios on Ubuntu, and these steps work on Ubuntu 24.04 and below and I think it should also work on Debian.

1. Download the packages

Install the packages needed to install Nagios using the command below:

sudo apt-get install autoconf gcc libc6 make wget unzip apache2 php libapache2-mod-php libgd-dev libssl-dev

2. Create a user and a group

After that, create a user and group for Nagios using the commands:

sudo useradd nagios
sudo groupadd nagcmd
sudo usermod -a -G nagcmd nagios
sudo usermod -a -G nagcmd www-data

3. Download Nagios

Use the commands below to download Nagios, where at the time of this writing (February 2025), the latest version of Nagios is version 4.5.9:

cd /tmp
wget https://github.com/NagiosEnterprises/nagioscore/archive/refs/heads/master.zip -O nagios.zip
unzip nagios.zip
cd nagioscore-master/

4. Install Nagios

By default, Linux will create a Nagios folder in the /usr/local folder to save Nagios configuration files. So, use the following commands to install Nagios:

sudo ./configure --with-command-group=nagcmd --with-httpd-conf=/etc/apache2/sites-enabled

After that, run the following commands:

sudo make all
sudo make install
sudo make install-init
sudo make install-daemoninit
sudo make install-config
sudo make install-commandmode
sudo make install-webconf
sudo a2enmod rewrite
sudo a2enmod cgi

5. Create the password

Create a password for the user Nagios to access the Nagios application. Nagiosadmin is usually a popular username for Nagios, but you can create another.

sudo htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin

6. Download Nagios Plugins

Plugins are compiled executables or scripts (Perl, shell, Python, PHP, Ruby, etc.) that can be run from a command line to check the status of a host or service. Nagios Core uses the results from plugins to determine the current status of hosts and services on your network. As of this writing (February 2025), the latest version of Nagios plugins is version 2.4.12. You can check the latest version of Nagios plugins on this site. Run the following commands to download Nagios plugins:

cd /tmp
wget https://github.com/nagios-plugins/nagios-plugins/archive/refs/heads/master.zip -O nagios-plugins.zip
unzip nagios-plugins.zip 
cd nagios-plugins-master/

7. Install Nagios Plugins

After that, install Nagios plugins using the following commands:

./tools/setup
sudo ./configure --with-nagios-user=nagios --with-nagios-group=nagios
sudo make
sudo make install

8. Check the configuration

After installing Nagios and Nagios plugins, run the following command to check the configuration of Nagios:

sudo /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg

and make sure there is no error like in the image below:

9. Turn on the services

Turn on the services using the commands below:

sudo systemctl start nagios.service
sudo systemctl enable nagios
sudo systemctl restart apache2.service

10. Check the application

Open your browser, and type in your browser:

http://your_ip_address_server/nagios

And there should be a display like the image below:

If you don’t see the image like the above image in your browser, maybe the Firewall/IPTables is still on your server. Run the following commands:

sudo ufw allow Apache
sudo ufw reload

Back to your browser again, and it should work now. Insert the username (nagiosadmin) and the password for Nagios. If the username and the password are right, the Nagios application will appear like this:

If you want to know which hosts are being monitored by Nagios, click Hosts, and Nagios will display the hosts that are being monitored:

You can see from the picture above, Nagios only monitors the Nagios server or localhost. If you want to know which services are being monitored by Nagios, click Services then Nagios will display the services that are being monitored:

From the picture above, Nagios monitors 8 services for the Nagios server or localhost.

Note

If you have a domain/subdomain and want to use that domain/subdomain for the Nagios application, create a virtual host on your web server. For example, I have the domain sysadminpedia.com and want to use the subdomain nagios.sysadminpedia.com for the Nagios application. So, I created the script below in the file /etc/apache2/sites-enabled/nagios.sysadminpedia.com.conf:

<VirtualHost *:80>
    ServerName nagios.sysadminpedia.com
    ServerAdmin sysadmin@nagios.sysadminpedia.com
    DocumentRoot /usr/local/nagios/share
    <Directory /usr/local/nagios/share>
         Options -Indexes +FollowSymLinks
         AllowOverride All
    </Directory>

    ErrorLog /var/log/apache2/nagios.sysadminpedia.com-error.log
    CustomLog /var/log/apache2/nagios.sysadminpedia.com-access.log combined
</VirtualHost>

Restart the webserver, open your browser, and type your domain/subdomain for Nagios, and it should be like the image below:

References

en.wikipedia.org
assets.nagios.com
techoverflow.net

SUDO stands for “SuperUser DO” and it is a program for Unix-like computer operating systems that enables users to run programs with the security privileges of another user, by default, the superuser. With sudo, a normal user can install or delete an application, change the server network, or even reboot or shut down the server.

Problem

How to make a Linux user have the sudo function?

Solution

This article will explain how to make a Linux user have the sudo function on RockyLinux/AlmaLinux/CentOS, Ubuntu/Debian, and OpenSUSE distros. For example, you want to add the user john to these distros and want that user to be able to use the sudo function. As far as I know, there are two methods to do it:

1. Change the sudoers file

Open the /etc/sudoers file or use the command below:

visudo

Add to the file the user name as in the image below:

After that, save the file and then try to add a new user using the user john, if there is a display like the image below:

Then select number 1, and the user should successfully add a new user as in the image above.

2. Add the user to the sudo group

Add the user to the sudo group, where the name of this sudo group can vary in each distro. To see the name of the sudo group, look in the sudoers file and look for a sentence similar to ‘Allows people in group to execute any command‘. For example, in RockyLinux and OpenSUSE, the name of the sudo group is wheel, sudo in Ubuntu, and don’t forget to make sure to uncomment the section as in the image below:

Then type the command below so that a user can use sudo:

RockyLinux & OpenSUSE

usermod -aG wheel john

Ubuntu/Debian

usermod -aG sudo john

Note

The two methods above can provide the sudo feature to a user on Linux so that the user can run commands that can only be executed by root if the user uses the sudo command by writing down the password. However, if you want the bob user not to have to enter a password when running the sudo command, then in the sudoers file, type the script below:

bob             ALL=(ALL)       NOPASSWD: ALL

Use the command below if you want the robin user to only be able to perform reboot commands using sudo, but not other commands using sudo:

robin           ALL=(ALL)     /usr/sbin/reboot

References

en.wikipedia.org
askubuntu.com
phoenixnap.com
hostinger.com

The previous article explained how to open and close ports in RockyLinux/AlmaLinux/CentOS. This article will explain how to open and close a port in Ubuntu.

Problem

How to open and close a port in Ubuntu?

Solution

A. Check the firewall

By default, Ubuntu and Debian use the UFW or Uncomplicated Firewall application as the default firewall, and it is installed automatically when you install Ubuntu/Debian. If the firewall is not installed on your Ubuntu/Debian distro, use the command below:

sudo apt install ufw

To see whether ufw is running or not, use the command below:

sudo ufw status

From the image above, you can see that the application is not yet active. To enable it, type the command below:

sudo ufw enable

If you want to see the complete current status of the firewall, use the command below:

sudo ufw status verbose

By default, the firewall only opens the OpenSSH service, which you can view by using the command below:

sudo ufw app list

B. Open the port

To open a port, for example, port 43210, use the command below:

sudo ufw allow 43210

To open a port range, for example, from port numbers 45000 to 45010 with the TCP protocol, use the command below:

sudo ufw allow 45000:45010/tcp

C. Open the service

You can see from the image above that port 43210 has been opened on your Ubuntu server. You can also use the service name when opening a port. For example, if you want to open the SMTP service on your Ubuntu server, then use the command below:

sudo ufw allow smtp

D. Open the port from a certain IP

If you want to open a port from a certain IP, for example, you  only allow IP 192.168.56.1 to access port 22 on this server, then use the command below:

sudo ufw allow from 192.168.56.1 to any port 22

To allow the 192.168.56.0 subnet to the SMTP service, use the command below:

sudo ufw allow from 192.168.56.0/24 to any port 25

E. Close the port

To close port 25, use the command below:

sudo ufw deny 25

F. Delete the port

You can also close a port and delete the port that has been opened, for example, port 43210, using the syntax below:

sudo ufw delete number

Note

You can remove all the rules in ufw by using the command below:

sudo ufw reset

After that, enable the ufw by using the command below:

sudo ufw enable

By default, if you open a port, it will automatically open in IPv4 and IPv6, and likewise, if you close the port. To see the UFW settings, open the /etc/default/ufw file.

References

cyberciti.biz
phoenixnap.com
digitalocean.com
help.ubuntu.com
askubuntu.com

The previous article explained how the install gcloud on RockyLinux/AlmaLinux/CentOS. This article will explain how to install gcloud on Ubuntu.

Problem

How to install gcloud on Ubuntu?

Solution

Here are the steps to install gcloud on Ubuntu/Debian:

A. Install gcloud

As far as I know, there are 3 methods to install gcloud on Ubuntu/Debian and the methods recommend using a user other than root.

1. Using the script

Before you download the script, install the packages using the command below:

sudo apt update
sudo apt-get install curl tar

Use the below command to download and install the script:

curl https://sdk.cloud.google.com | bash

Then you will see a display like the one below:

Wait until it’s finished, and you will see a display like the one below:

From the image above, you are asked to create a new SSH connection so that the effect can be seen, and type the command below:

gcloud version

However, you can use the command below:

source /home/sysadmin/.bashrc

So you don’t need to create a new SSH connection to run the gcloud version command, which results in the image below:

2. Using the repository

Type the following commands to install gcloud on the Ubuntu/Debian distro:

sudo apt update
echo 'deb [signed-by=/usr/share/keyrings/cloud.google.gpg] https://packages.cloud.google.com/apt cloud-sdk main' | sudo tee -a 
sudo apt-get -y install apt-transport-https ca-certificates gnupg
curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | sudo apt-key --keyring /usr/share/keyrings/cloud.google.gpg add -
sudo apt update
sudo apt-get install -y google-cloud-sdk

3. Using the snap

Run the below command to install gcloud:

sudo snap install google-cloud-sdk --classic 

B. Connect to GCP

After you install gcloud on your server, type the command below:

gcloud init 

Then there will be a display like the image below:

You can open the link in a browser by clicking the Ctrl+Click button located in the red box. If you are having trouble doing so, copy what is included in the red box and paste it into your browser. This will allow you to view a display similar to the one that is shown below:

When you click on your Google account, that will allow you to access GCP, and a display similar to the one shown below will appear:

After you click the Continue button, the screen below will show:

When you click the Allow button, the screen below will show:

Click the Copy button, and paste it into the CLI on your server as in the image below:

Select the project and configure the zone as in the image above. After that, the gcloud configuration is complete, like in the image below:

C. Test gcloud

Now, try gcloud to access your GCP. I try to list my virtual machine in GCP using the below command:

gcloud compute instances list

Then the display below will appear:

If you get a display like the image above, then you have successfully used your GCloud to access your GCP.

Note

If you have many projects on your GCP, you can choose one of these projects as the starting point for your gcloud on GCP. You can switch projects using the command:

gcloud config set project PROJECT_ID

Change PROJECT_ID to the project ID you want to switch to.

References

cloud.google.com
liquidweb.com
bacancytechnology.com
attuneops.io
tecadmin.net