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Configure Nagios Core for Log Monitoring

No matter how well we set up our infrastructure, eventually something will go wrong that needs to be fixed. When that happens, the first step is to check the logs for errors and warnings. In this hands-on lab, we will create a Nagios service that monitors the logs for errors and warnings using the `check_log` plugin.

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Lab platform
Lab Info
Level
Intermediate
Last updated
Aug 30, 2025
Duration
1h 0m

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Table of Contents
  1. Challenge

    Define Two Command Objects that Check the Logs **/var/log/messages** for Errors and Warnings in **/usr/local/nagios/etc/objects/commands.cfg**
    1. Add the following text to the commands.cfg file:

      define command {
      
              command_name    check_log_error
              command_line    /usr/local/nagios/libexec/check_log -F /var/log/messages -O /home/nagios/check_log.error -q "error"
      
      }
      
      
      define command {
      
              command_name    check_log_warning
              command_line    /usr/local/nagios/libexec/check_log -F /var/log/messages -O /home/nagios/check_log.warning -q "warning"
      
      }
      
      
  2. Challenge

    Define Two Services that Use the Commands
    1. Add the following text to the localhost.cfg file:

      define service {
      
              use                     local-service
              host_name               localhost
              service_description     Check Log Error
              check_command           check_log_error
      
      }
      
      define service {
      
              use                     local-service
              host_name               localhost
              service_description     Check Log Warnings
              check_command           check_log_warning
      
      }
      
  3. Challenge

    Give the `nagios` User Read Permissions for `/var/log/messages`
    1. Run the following command:
    sudo setfacl -R -m u:nagios:r-- /var/log/messages
    
  4. Challenge

    Restart and Log In to the Nagios Server
    1. Restart the Nagios server.
    sudo systemctl restart nagios
    
    1. Navigate to http://NAGIOS_SERVER_IP/nagios, and log in with the following credentials:
      • User Name: nagiosadmin
      • Password: BlaBla321
    2. Navigate to Services.
    3. Reschedule the checks for the two services to run 10-20 seconds from the current server time.
    4. Verify that there are no errors under Status Information.
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