Top 50 Nagios Interview Question and Answers

Nagios is a widely used monitoring tool in IT infrastructure for keeping track of systems, applications, and networks. If you’re preparing for a Nagios-related role, this comprehensive list of interview questions and answers will help you ace your interview.

1. What is Nagios?

Nagios is an open-source IT infrastructure monitoring tool that helps organizations identify and resolve IT infrastructure problems before they affect critical business processes.

2. What are the key features of Nagios?

• Monitoring of network services (HTTP, SMTP, POP3, etc.)

• Monitoring of host resources (CPU load, memory usage, disk usage, etc.)

• Plugin support for custom checks

• Alerts via email, SMS, or other methods

• Performance graphs

3. What are the main components of Nagios?

• Nagios Core: The central engine responsible for scheduling checks and alerting.

• Plugins: Scripts or programs used to perform checks.

• NRPE (Nagios Remote Plugin Executor): A way to execute checks on remote hosts.

• Web Interface: A GUI for viewing alerts and logs.

4. What is NRPE in Nagios?

NRPE stands for Nagios Remote Plugin Executor. It allows Nagios to execute plugins on remote servers for monitoring purposes.

5. How does Nagios monitor remote hosts?

Nagios uses agents like NRPE, SSH, or SNMP to monitor remote hosts.

6. How do you install Nagios?

• Download the source code for Nagios Core and Plugins.

• Install prerequisites like Apache, PHP, and GCC.

• Compile and install Nagios Core and Plugins.

• Configure the Nagios configuration files.

• Set up the web interface and start Nagios services.

7. Where are the main configuration files of Nagios located?

The main configuration files are located in /usr/local/nagios/etc/. The primary files include:

• nagios.cfg

• objects/ (contains host, service, and contact definitions)

8. What is a Nagios plugin?

A plugin is an external script or binary that Nagios uses to perform checks. Plugins return status codes (0 for OK, 1 for Warning, 2 for Critical, 3 for Unknown).

9. How do you define a host in Nagios?

A host is defined in the hosts.cfg file with the following parameters:

define host {
    use             generic-host
    host_name       server1
    alias           Web Server
    address         192.168.1.1
    max_check_attempts  3
    check_period    24x7
    notification_interval 30
    notification_period 24x7
}

10. What is the purpose of a service definition in Nagios?

Services represent specific checks to monitor, such as disk usage, CPU load, or HTTP availability. Services are linked to hosts in the configuration.

11. What are contact groups in Nagios?

Contact groups are used to group contacts for sending alerts. Example:

define contactgroup {
    contactgroup_name    admins
    alias                Nagios Administrators
    members              nagiosadmin
}

12. How does Nagios handle alerts?

Nagios sends alerts when a check transitions from an “OK” state to a “Warning,” “Critical,” or “Unknown” state. Alerts are configured to go to specific contacts or groups.

13. What is the use of a time period in Nagios?

Time periods define when checks or notifications should occur. Example:

define timeperiod {
    timeperiod_name 24x7
    alias           24 Hours A Day, 7 Days A Week
    sunday          00:00-24:00
    monday          00:00-24:00
    ...
}

14. What is passive check-in Nagios?

Passive checks are performed externally and results are sent to Nagios. It’s useful for situations where Nagios can’t directly monitor the service.

15. How do you handle flapping in Nagios?

Flapping occurs when a host or service changes state too frequently. Nagios detects flapping and suppresses notifications until stability is restored.

16. What should you do if Nagios is not sending email notifications?

• Check email settings in commands.cfg.

• Verify the mail server configuration.

• Check the logs for errors.

• Ensure that the contact definition has a valid email address.

17. What is the purpose of the Nagios log file?

The Nagios log file (/usr/local/nagios/var/nagios.log) records all events, such as alerts, state changes, and scheduled checks.

18. How do you troubleshoot a plugin failure in Nagios?

• Manually execute the plugin script to check for errors.

• Verify permissions and dependencies.

• Check the Nagios configuration files for correct paths and parameters.

19. What is the difference between Nagios Core and Nagios XI?

• Nagios Core: Open-source and free.

• Nagios XI: Commercial version with additional features like reporting, dashboards, and enhanced UI.

20. What is a Nagios event handler?

An event handler is a script or command that runs automatically when a service or host state changes.

21. How does Nagios integrate with third-party tools?

Nagios can integrate with tools like Grafana, PagerDuty, and Slack using plugins, APIs, and event handlers.

22. What are macros in Nagios?

Macros are variables used in commands and notifications. Example:

• $HOSTNAME$: Host name

• $SERVICESTATE$: Current service state

23. What are templates in Nagios?

Templates allow you to define common settings that can be reused across multiple hosts or services, reducing redundancy in configuration files.

24. How do you check Nagios configuration for errors?

Use the following command:

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

This checks the Nagios configuration file for syntax errors and inconsistencies.

25. What is a distributed monitoring setup in Nagios?

In distributed monitoring, multiple Nagios servers monitor different parts of the network. Results are forwarded to a central server for unified reporting.

26. How do you configure SNMP monitoring in Nagios?

• Install the required SNMP plugins.

• Define a host and service in the configuration files with SNMP-specific checks.

27. What is the Nagios CGI?

CGIs (Common Gateway Interfaces) are scripts that provide dynamic content on the Nagios web interface, such as the status of hosts and services.

28. What is Nagios Fusion?

Nagios Fusion is a commercial product that aggregates data from multiple Nagios Core or Nagios XI instances into a single dashboard.

29. How do you monitor a Windows server using Nagios?

Use the NSClient++ agent on the Windows server and configure Nagios to communicate with it for remote checks.

30. What is a retained process status in Nagios?

Retained process status ensures that the state of Nagios (e.g., active checks, notifications) is saved and restored after a restart.

31. How do you integrate Nagios with Grafana?

Use a plugin or export data from Nagios into a time-series database like InfluxDB, which Grafana can read.

32. What are the different states in Nagios?

• OK (0): Everything is working as expected.

• Warning (1): A potential issue is detected.

• Critical (2): A significant problem is detected.

• Unknown (3): The plugin cannot determine the state.

33. How do you schedule downtime in Nagios?

Scheduled downtime is configured via the web interface or configuration files to suppress alerts during maintenance periods.

34. What is the importance of performance data in Nagios?

Performance data provides metrics that can be used to generate graphs and analyze trends.

35. What is the use of check intervals in Nagios?

Check intervals define how often a host or service check is performed.

36. What are event logs in Nagios?

Event logs record all events, such as state changes and alerts, for auditing and troubleshooting.

37. How do you monitor disk usage in Nagios?

Use the check_disk plugin with appropriate thresholds. Example:

check_disk -w 20% -c 10% -p /dev/sda1

38. What are adaptive monitoring techniques in Nagios?

Adaptive monitoring adjusts thresholds or check intervals dynamically based on the state of a host or service.

39. What is the role of “retain_state_information” in Nagios?

This parameter ensures that Nagios retains the state of hosts and services between restarts.

40. What are freshness checks in Nagios?

Freshness checks ensure that passive checks are updated regularly. If a check result is not received within a defined time, Nagios considers it stale and triggers an active check.

41. How do you handle plugin timeouts in Nagios?

Increase the timeout value in the nagios.cfg file using the service_check_timeout directive.

42. What is the role of “external commands” in Nagios?

External commands allow users or scripts to interact with Nagios dynamically, such as scheduling downtime or acknowledging alerts.

43. What is the difference between active and passive checks in Nagios?

• Active Checks: Initiated by Nagios on a regular schedule.

• Passive Checks: Results are sent to Nagios by external scripts or processes.

44. How do you monitor a MySQL database in Nagios?

Use the check_mysql plugin to monitor database metrics like connections, query performance, and uptime.

45. What is the purpose of the “max_check_attempts” directive?

This directive specifies how many times Nagios retries a check before marking it as a failure.

46. What is the Nagios Core Worker?

Nagios Core Workers are processes responsible for executing checks and processing results.

47. How do you create custom Nagios plugins?

Write a script in a language like Bash, Python, or Perl that performs the desired check and returns an appropriate status code.

48. What is “service escalation” in Nagios?

Service escalation defines how notifications are handled as the severity of an issue increases.

49. How do you optimize Nagios for large environments?

• Use distributed monitoring.

• Increase hardware resources.

• Optimize check intervals and thresholds.

50. What is the role of “object inheritance” in Nagios?

Object inheritance allows you to create objects based on existing ones, simplifying configuration by reusing common settings.