Configure Nagios 4 with Nginx on Ubuntu 20.04

Nagios is an open source software for system and network monitoring. It remotely monitors specified hosts and services, alerting the administrator in case critical services in a network, such as SMTP, HTTP, SSH, FTP and others fail. With Nagios, you can also monitor attributes such as CPU load, memory, disk usage, logged in users, running processes, etc.

This article describes how to install Nagios 4 on Ubuntu 20.04 with Nginx webserver rather than the default Apache webserver.

System Requirements

• Ubuntu Server 20.04
• 2gb RAM, 2 cores
• 40gb storage
• Nginx

Step 1: Update the System

Make sure that all Ubuntu OS packages installed on the server are up to date. You can do that by running the following commands:

sudo apt update
sudo apt upgrade -y

Step 2: Install required packages

Now we need to install the required packages with the following command:

sudo apt install wget unzip curl openssl build-essential libgd-dev libssl-dev php7.4 php7.4-fpm fcgiwrap php7.4-gd -y

Step 3: Download and install Nagios

• Download Nagios Core setup files

wget https://assets.nagios.com/downloads/nagioscore/releases/nagios-4.4.6.tar.gz

• Extract the downloaded files

sudo tar -zxvf nagios-4.4.6.tar.gz

• Navigate to the setup directory

cd nagios-4.4.6 

• Run the Nagios Core configure script.

sudo ./configure

• Compile the main program and CGIs.

sudo make all

• Now lets create a nagios user and group, and also add the www-data user to the nagios group.

sudo make install-groups-users
sudo usermod -a -G nagios www-data

• Install Nagios

sudo make all

• Initialize all the installation configuration scripts.

sudo make install-init

• Install Command Mode

This installs and configures the external command file.

sudo make install-commandmode

• Install Configuration Files

sudo make install-config

Step 4: Configure Nginx

In order to access the Nagios admin page, we need to configure Nginx to serve it via HTTP/HTTPS.

• Create a file called nagios in /etc/nginx/sites-available using the command below.

sudo nano /etc/nginx/sites-available/nagios

• Paste the following content and replace the server_name with your actual domain

server {
    server_name     nagios.your-domain.com;
    root            /usr/local/nagios/share;
    listen          80;
    index           index.php index.html index.htm;
    access_log      /var/log/nginx/nagios.access.log;
    error_log       /var/log/nginx/nagios.error.log;
    auth_basic            "Nagios Access";
    auth_basic_user_file  /usr/local/nagios/etc/htpasswd.users;
    # Fixes frames not working
    add_header X-Frame-Options "ALLOW";
    location ~ \.php$ {
        try_files       $uri = 404;
        fastcgi_index   index.php;
        fastcgi_pass    unix:/run/php/php7.4-fpm.sock;
        include         fastcgi.conf;
    }
    location ~ \.cgi$ {
        root            /usr/local/nagios/sbin;
        rewrite         ^/nagios/cgi-bin/(.*)\.cgi /$1.cgi break;
        fastcgi_param   AUTH_USER $remote_user;
        fastcgi_param   REMOTE_USER $remote_user;
        include         fastcgi.conf;
        fastcgi_pass    unix:/run/fcgiwrap.socket;
    }
    # Fixes the fact some links are expected to resolve to /nagios, see here.
    location /nagios {
        alias /usr/local/nagios/share;
    }
}

• Save and exit
• Now lets symlink /etc/nginx/sites-available/nagios to /etc/nginx/sites-enabled/nagios

sudo ln -s /etc/nginx/sites-available/nagios /etc/nginx/sites-enabled/nagios

• Now lets restart nginx

sudo systemctl reload nginx 

Step 5: Add a User for Nagios

Create a user and set the password when prompted.

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

Step 6: Install Nagios Plugins

Nagios Core needs plugins to operate properly. The following steps will walk you through installing Nagios Plugins.

• Download the Nagios core plugin

cd ~/
wget https://nagios-plugins.org/download/nagios-plugins-2.3.3.tar.gz

• Extract the downloaded plugin

sudo tar -zxvf nagios-plugins-2.3.3.tar.gz

• Navigate to the plugins directory

cd nagios-plugins-2.3.3

• Run the plugin configure script.

sudo ./configure --with-nagios-user=nagios --with-nagios-group=nagios

• Compile Nagios core plugins

sudo make

• Install the plugins

sudo make install

Step 7: Verify Nagios Configuration

Use the command below to verify that nagios is running correctly.

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

If nagios is running correctly then it should give the output below

• Now lets start and enable nagios to run at system startup

sudo systemctl start nagios
sudo systemctl enable nagios

Step 8: Access the Web Interface

Firstly, lets edit the url path in /usr/local/nagios/etc/cgi.cfg

from url html path=/nagios to url html path=/

• Restart nagios

sudo systemctl reload nagios

• Open your web browser and access the Nagios web interface via the url http://nagios.your-domain.com

That should load the admin login interface as shown below

To log in, use nagiosadmin as your username and the password you set during the user account creation. You can now access the dashboard and begin configuring Nagios.

NOTE: If you set a username other than nagiosadmin then you need to open the cgi.cfg file in /usr/local/nagios/etc and edit accordingly.

authorized_for_system_information=nagiosadmin, username
authorized_for_configuration_information=nagiosadmin, username
authorized_for_system_commands=nagiosadmin, username
authorized_for_all_services=nagiosadmin, username
authorized_for_all_hosts=nagiosadmin, username
authorized_for_all_service_commands=nagiosadmin, username
authorized_for_all_host_commands=nagiosadmin, username

Use * to authorize any user who has authenticated to the webserver.

• Now Click on Hosts to see the servers that are being monitored.

Step 9: Adding Hosts to Nagios Server For Monitoring

In order to add hosts to our Nagios server, we have to do the following steps:

Create Nagios Host Object Definition

• To create new object definitions, you can create a sub-directory to place your files as below:

sudo mkdir /usr/local/nagios/etc/objects/servers

• Open main Nagios configuration file and insert the line, cfg_dir=/usr/local/nagios/etc/objects/servers under the Object Configuration File(s) section.

sudo nano /usr/local/nagios/etc/nagios.cfg

• Next create your host object definition file

sudo nano /usr/local/nagios/etc/objects/servers/hosts.cfg

• define you hosts details such as hostname, IP address and the host template to use.

                                                                      # REMOTE HOST DEFINITION                                                                                                                            
define host {
    use                     servers
    host_name               server1
    alias                   my_first_server
    address                 192.168.20.21
}
define host {
    use                     servers
    host_name               server2
    alias                   my_second_server
    address                 192.168.20.29
}
# REMOTE HOST GROUP DEFINITION     
                                           
define hostgroup {
    hostgroup_name         my-linux-hosts
    alias                  Linux Hosts
    members                server1,server2
}

Create Host and Service Template Configuration

• Lets create a custom host and service definition template as shown below.

sudo nano /usr/local/nagios/etc/objects/servers/hosts-service-template.cfg

• Paste the below content and edit accordingly.

# Host Template Definition 
define host{
name                        servers
notifications_enabled        1
event_handler_enabled        1
flap_detection_enabled       1
process_perf_data            1
retain_status_information    1
retain_nonstatus_information 1
       check_command                check-host-alive
       check_interval               5
       max_check_attempts           2
       notification_interval        0
       notification_period          24x7
       notification_options         d,u,r
       contact_groups               server-admins
register                     0
}
# Service Template definition 
define service{
name                         hosts-service
active_checks_enabled        1
passive_checks_enabled       1
parallelize_check            1
obsess_over_service          1
check_freshness              0
notifications_enabled        1
event_handler_enabled        1
flap_detection_enabled       1
process_perf_data            1
retain_status_information    1
retain_nonstatus_information 1
       notification_interval        0
       is_volatile                  0
       check_period                 24x7
       check_interval               5
       retry_interval               1
       max_check_attempts           2
       notification_period          24x7
       notification_options         w,u,c,r
       contact_groups               server-admins
register                     0
}

• Save and exit the config

Create Contacts and Contact Group

We defined our contact group as server-admins. Hence, we need to create that contact group.

sudo nano /usr/local/nagios/etc/objects/servers/server-admins-contacts.cfg

• Paste the following content and edit accordingly.

# Define Your Contacts Here 
define contact {
   contact_name                   your-name       ; Short name of user 
   use                            generic-contact   ; Inherit default values from generic-contact template (defined above) 
   alias                          your-full-name        ; Full name of user 
   email                          your-name@your-email.com ; <<***** CHANGE THIS TO YOUR EMAIL ADDRESS ****** 
}
# Create Contact Group 
define contactgroup {
   contactgroup_name             server-admins
   alias                         server Administrators
   members                       your-name
}

Verify Nagios Configuration

Run the following command to check if the configuration file is okay.

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

You should get the output below

• Restart Nagios

sudo systemctl reload nagios

Now visit Nagios web interface and verify that your new server has been added for monitoring: http://nagios.your-domain.com

Click on Hosts on the left pane and you should see the localhost and your remote hosts.

Conclusion

We have successfully installed Nagios Core on Ubuntu 20.04 LTS using nginx as a webserver and added hosts for monitoring.