What is Nagios for?
Nagios Core is an open source tool for IT infrastructure monitoring. It can check the availability of servers, network services, applications, processes, disks, CPU, RAM, certificates, ports, databases and network devices. The results allow you to detect failures, overloads and problem symptoms faster.
Servers and services
Availability of hosts, system services, processes, disks, ports and business applications.
Network and devices
Routers, switches, VPNs, firewalls, delays, packet loss and basic SNMP parameters.
Alerts and escalations
Notifications are intended to reach the right people with clear priority and response instructions.
Checks, sensors and automated responses need configuration
Optional Nagios Core event handlers run configured commands on state changes. Each action needs testing, restricted privileges, an execution record and agreed conditions. Monitoring should not delete backups outside an approved retention policy.
- Check free space and alert on capacity; backup cleanup stays within the approved backup process.
- Monitoring CPU, RAM, processes, services and ports requires suitable checks or plugins; temperature and humidity also require available sensors and integration.
- Identification of hosts whose failure may disrupt system operation or cut off access to the service.
- Data visualisation in tables, graphs and panels that are also readable by management staff.
Nagios alerts and SMS: notifications about problems
SMS and e-mail alerts are especially useful in extensive infrastructure. The administrator receives information about abnormal conditions and can respond before the problem causes a failure. Nagios may also indicate the need to update software, plug-ins or network services. Checking update availability requires a suitable plugin and access to the monitored system. Running Nagios alone does not automatically assess or install every patch.
WARNING
Early warning, e.g. increasing disk usage, slowing performance or approaching certificate expiry.
CRITICAL
Condition requiring quick response: service unavailable, overload, backup error or outage.
Escalation
Notifications can be sent to the on-call team, tickets, SIEM/SOAR systems or people responsible for the service.
What is Nagios and how does IT monitoring work?
Nagios Core runs active checks and receives passive results. Service states are OK, WARNING, CRITICAL or UNKNOWN; host states are UP, DOWN or UNREACHABLE. Interpretation and notifications depend on checks, retries, dependencies and contact configuration.
- Active checks query hosts, ports, services and applications from the central Nagios server.
- Passive checks allow agents to report status from monitored machines.
- Nagios Core gives you full open source flexibility, and Nagios XI adds an interface, wizards, and business reports.
Nagios Core or Nagios XI
Nagios Core offers great flexibility, but requires more administrative knowledge and work in configuration files. Nagios XI offers a more convenient interface, reporting and wizards, but is a commercial solution. The choice should depend on the team's competences, the scale of the environment, reporting requirements and budget.
Nagios Core
A good choice for a technical team that wants control, custom plugins, and a low licensing cost.
Nagios XI
Better where fast service, business reporting and less dependence on manual configuration are important.
Alternatives
In larger environments, it is worth combining infrastructure monitoring with logs, SIEM, SOAR and the incident process.
When to consider Nagios IT monitoring
Monitoring helps detect critical-service problems and plan capacity. The tool does not replace an on-call team: round-the-clock notifications need an agreed recipient, response hours and escalation. Select Nagios according to scope, skills and integrations.
- Multiple servers or an extensive network that cannot be checked manually several times a day.
- SLAs in which every minute of service unavailability represents a cost or contractual risk.
- There is no 24/7 IT service, but you need 24/7 alerts via SMS, e-mail or ticket.
- Capacity planning: disks, CPU, memory, workload and resource usage trends.
- Linux, Windows, physical, virtual, cloud and network environments monitored by one process.
A practical decision map before implementing Nagios
First, you need to answer what the company wants to detect and who should respond. Only then are plugins, thresholds and notification channels selected.
- Scope: indicate critical services, owners, dependencies, working hours, RTO/RPO and the impact of failure on the business.
- Controls: establish hosts, ports, processes, certificates, disks, backups, queues, databases and network parameters.
- Alerts: define WARNING/CRITICAL thresholds, service windows, noise suppression and notification channels.
- Response: Assign shifts, escalations, checklists, incident log and post-incident reporting.
The most common errors in IT monitoring
Monitoring without process quickly turns into noise. Too sensitive thresholds generate hundreds of alerts that no one reads. The lack of a service owner means that the alert goes to several people and no one makes a decision. The lack of recovery tests means that the system sees a failure, but the organisation is unable to get back to work.
Alert fatigue
Too many notifications without priority lowers the team's vigilance and delays the response.
No owners
Each critical service should have a business and technical owner.
Monitoring without backup
Detecting a failure is not enough if there is no tested recovery plan.
No security context
Some infrastructure alerts may be a symptom of a security incident.
How to configure SMS notifications in Nagios?
Nagios sends notifications through commands and scripts. These can connect it to an SMS gateway, email, a ticketing system via webhook or an on-call management tool. In Nagios Core, configuration usually requires editing files, and Nagios XI simplifies this through a graphical interface.
- Define the contacts, contact groups, notification times and status levels you want to trigger SMS.
- Define an SMS integration command and assign it to contacts using service_notification_commands or host_notification_commands. Test delivery, service recovery, escalation and failure of the notification channel itself.
- Combine SMS with tickets so that the alert has an owner, priority, response history and resolution status.
Does Nagios monitor cloud applications and virtual machines?
Yes. Thanks to plugins and agents, Nagios can monitor VMware environments, containers, Linux and Windows systems, web applications, databases and selected cloud metrics from AWS, Microsoft Azure or other platforms. In multi-cloud environments, it can act as an alert aggregator.
Virtual machines
Monitoring of the guest system, resources, services and dependencies via NRPE, NCPA agents or plug-ins.
Cloud and SaaS
Selected cloud service metrics, certificates, endpoint availability and integration with native tools.
Network and SNMP
Routers, switches, printers, firewalls, VPN, packet loss, delays and interface status.
How many servers can one Nagios installation monitor?
There is no single guaranteed host capacity. Measure load for service count, plugin runtime, intervals, data recording and required check latency. A load test informs distributed monitoring and resource headroom.
- Choose check_interval, retry_interval and retry count for the service and load. Five minutes can be an example setting, not a universal standard or detection guarantee.
- Large environments benefit from distributed nodes, reporting servers, and database optimisation.
- Monitor critical services first and then add less important metrics.
Can a professional IT company manage Nagios for the organisation?
Yes. In an IT outsourcing model, an external team can implement Nagios, maintain configuration, respond to alerts, tune thresholds, run reports and document incidents. This is especially important for companies that do not have their own network administrator but do not want to risk the downtime of critical services.
Implementation
Scope of monitoring, installation, agents, plugins, SNMP, thresholds, dashboard and notification tests.
Maintenance
Alert handling, escalations, SLA reports, threshold reviews and configuration updates.
Security
Combining monitoring with backup, logs, incidents, access and infrastructure audit.
Phased action plan for monitoring implementation
Start with minimum critical-service checks, an owner and tested escalation. Urgent gaps must not wait for the next phase. 30/60/90-day reviews assess coverage, alert quality and response readiness.
30-day review: coverage
Confirm critical services, dependencies, contacts, thresholds and notification effectiveness.
60-day review: quality
Assess plugins, agents, access security, false positives and escalation.
90-day review: maintenance
Review availability reports, response exercises, updates and remaining monitoring gaps.
How Virtline helps implement IT monitoring
Virtline implements monitoring as an element of infrastructure support, not as a single panel. We establish the scope, configure controls, thresholds and notifications, document reactions and can take over the ongoing handling of alerts in the IT outsourcing model.
Monitoring project
We select the scope, metrics, plugins, thresholds and reporting method for the real environment.
Alert handling
We respond to incidents, conduct escalations and maintain documentation of activities.
Operational security
We combine monitoring with backup, access, logs and incident procedures.
Services to verify next
IT monitoring provides the greatest value when combined with maintenance, backup and security.
IT monitoring
Design, implementation and maintenance of infrastructure and service monitoring.
SOC / SIEM / SOAR
Correlation of logs, security alerts and incident response processes.
Contact
We'll figure out what to monitor first and how to avoid ownerless alerts.
What to monitor and how to respond to alerts
Monitoring should not be a wall of alerts. It should describe services, dependencies, owners and response priorities so IT can react before users report an outage. Virtline also organises thresholds, notification channels, reports and accountability; call +48 12 200 27 10 to define the response process.
- Availability of servers, network devices, applications, websites and critical ports.
- Backup jobs, storage capacity, certificates, disk health and service dependencies.
- Alert owners, escalation paths, maintenance windows and business priority.
- Reports showing recurring failures, response time and risks that require budget decisions.
- Operational experience in infrastructure maintenance and alert handling.
- Lessons from business environments as a benchmark for thresholds and escalation.
- Deliverable: monitoring scope, thresholds, runbooks and an operating-stability plan.
Use cases for IT monitoring
The best monitoring links technical signals to business continuity. It helps detect small failures before they become downtime.
IT monitoring implementation
Monitoring architecture, thresholds, alerts, dashboards and response workflow.
SOC/SIEM/SOAR
Security event monitoring and correlation for incidents.
IT outsourcing
Ongoing service desk, maintenance, reports and escalation.
Monitoring rollout checklist
Start with critical services instead of trying to monitor everything on day one.
- List business-critical services and technical dependencies.
- Define thresholds, notification channels and escalation owners.
- Test alerts, false positives, maintenance windows and incident handover.
- Review reports, threshold tuning and recurring alerts monthly. Before disabling a check, assess the risk of losing visibility into a critical service.
