Nagios Cheatsheet

1. Introduction:

Nagios is a powerful open-source monitoring tool that provides comprehensive monitoring of systems, networks, and infrastructure. It is known for its robustness, flexibility, and extensive plugin system.

2. Installation:

Installing Nagios Core:

 
sudo apt-get update
sudo apt-get install -y build-essential libgd2-xpm-dev openssl libssl-dev xinetd apache2-utils unzip
wget https://assets.nagios.com/downloads/nagioscore/releases/nagios-4.4.6.tar.gz
tar -xzf nagios-4.4.6.tar.gz
cd nagios-4.4.6/
./configure --with-httpd-conf=/etc/apache2/sites-enabled
make all
sudo make install
sudo make install-commandmode
sudo make install-init
sudo make install-config
sudo make install-webconf

Starting Nagios:

 
sudo systemctl start nagios
sudo systemctl enable nagios

3. Configuration:

Basic Configuration:
- Nagios configuration files are typically located in /usr/local/nagios/etc/.

Defining a Host:

 
define host {
  use             linux-server
  host_name       myserver
  alias           My Linux Server
  address         192.168.1.1
  }

Defining a Service:

 
define service {
  use                     generic-service
  host_name               myserver
  service_description     HTTP
  check_command           check_http
  }

4. Nagios Plugins:

Installing Plugins:

 
wget https://nagios-plugins.org/download/nagios-plugins-2.3.3.tar.gz
tar -xzf nagios-plugins-2.3.3.tar.gz
cd nagios-plugins-2.3.3/
./configure
make
sudo make install

Common Plugins:
- check_ping: Monitors network connectivity.
- check_http: Monitors HTTP servers.
- check_disk: Monitors disk usage.

5. Notifications:

Setting Up Email Notifications:

Configure email settings in /usr/local/nagios/etc/objects/contacts.cfg:

 
define contact {
  contact_name                    nagiosadmin
  use                             generic-contact
  alias                           Nagios Admin
  email                           nagios@yourdomain.com
}

Notification Commands:
- Use commands like notify-host-by-email and notify-service-by-email to define how notifications are sent.

6. Web Interface:

Accessing Nagios Web Interface:
- Nagios web interface is usually accessible at http://<your-server-ip>/nagios.
- Default credentials: nagiosadmin and the password set during installation.
Customizing the Interface:
- Modify the theme and layout by editing files in /usr/local/nagios/share.

7. Monitoring Remote Hosts:

NRPE (Nagios Remote Plugin Executor):

Installing NRPE:

 
sudo apt-get install nagios-nrpe-server nagios-plugins
sudo systemctl start nagios-nrpe-server

Configuring NRPE:

Edit /etc/nagios/nrpe.cfg to define allowed hosts and monitored services.

 
allowed_hosts=127.0.0.1,192.168.1.100
command[check_disk]=/usr/lib/nagios/plugins/check_disk -w 20% -c 10% -p /dev/sda1

Monitoring with NRPE:

Add a service in Nagios to monitor a remote host using NRPE.

 
define service {
  use                     generic-service
  host_name               remotehost
  service_description     Disk Usage
  check_command           check_nrpe!check_disk
}

8. Nagios XI:

Introduction to Nagios XI:
- Nagios XI is the commercial version of Nagios Core, providing additional features like a more user-friendly interface, reporting, and advanced monitoring capabilities.
Differences from Nagios Core:
- Built-in wizards, easier configuration, and more extensive support.

9. Advanced Nagios Concepts:

Passive Checks:
- Useful for monitoring systems where Nagios cannot initiate checks, but the system can send results to Nagios.
Distributed Monitoring:
- Implement distributed monitoring by setting up multiple Nagios servers and configuring them to send data to a central Nagios server.

10. Securing Nagios:

Enabling HTTPS:
- Configure Apache to serve Nagios over HTTPS.
```
 
sudo a2enmod ssl
sudo service apache2 restart
```
- Update Nagios configuration in /etc/apache2/sites-available/nagios.conf to use SSL certificates.
User Authentication:
- Use .htpasswd files to manage user access to the Nagios web interface.

11. Troubleshooting Nagios:

Common Issues:
- Service Check Failing: Ensure plugins are executable and paths are correct.
- Email Notifications Not Working: Verify the mail server configuration and check the maillog for errors.
Debugging:
- Use the Nagios log file at /usr/local/nagios/var/nagios.log to troubleshoot issues.
- Run checks manually to verify plugin output.
```
 
/usr/local/nagios/libexec/check_http -I 127.0.0.1
```

12. Nagios and Docker:

Running Nagios in Docker:

 
docker run --name nagios -p 0.0.0.0:8080:80 jasonrivers/nagios

Customizing Dockerized Nagios:

Mount volumes to add custom configurations and plugins.

 
docker run --name nagios -v /path/to/nagios.cfg:/usr/local/nagios/etc/nagios.cfg jasonrivers/nagios

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