Shan DevOpsNavigating the Nagios Web Interface
After installing Nagios on the Master server and NRPE on the Slaves, the Nagios Web UI becomes your mission control. It allows you to monitor hosts and services in real time, visualize network topologies, and quickly triage outages.
Tactical Status & Map View
Tactical Status Overview
Provides a high-level, bird's-eye view of all hosts and services. It instantly displays whether hosts are UP, DOWN, or UNREACHABLE.
Map Page (Network View)
A visual representation of your monitored infrastructure. It shows how hosts are physically or logically connected, displaying host groups and parent/child relationships to help pinpoint network partitions.
Map Color Codes
- Green UP / OK: Everything is healthy.
- Yellow WARNING: Threshold reached, investigate soon.
- Red DOWN / CRITICAL: Complete failure.
- Gray UNREACHABLE / PENDING: Awaiting check or network blocked.
The Host Page
The Host Page displays a detailed, tabular list of all monitored servers and devices with real-time status information.
| Column | Description / Meaning |
|---|---|
| Host Name / Alias | The exact registered name of the host and an optional friendly alias. |
| Status | Current state: UP (green), DOWN (red), or UNREACHABLE (gray). |
| Last Check | The exact timestamp of the last active or passive check performed. |
| Duration / Uptime | How long the host has continuously been in its current state. |
| Attempt / Hard State | Current attempt vs max attempts (e.g., 3/5). Shows if the state is HARD (confirmed) or SOFT (still verifying). |
| Last State Change | The timestamp indicating when the host status last flipped (e.g., UP to DOWN). |
| Notifications | Details on the last notification sent to contacts and total notifications fired. |
| Scheduled Downtime | Shows a maintenance icon if the host is in planned downtime (suppressing alerts). |
| Flap Detection | Indicates if the host is "flapping" (frequently bouncing between UP and DOWN states). |
The Service Page
While a host might be UP, its web server or database might be failing. The Service Page displays granular metrics (CPU, Disk, HTTP, SQL) grouped by host.
Service States
- OK The service is running smoothly within normal parameters.
- WARNING A soft threshold has been crossed (e.g., Disk space at 85%).
- CRITICAL A hard threshold has been crossed, or the service is completely dead (e.g., Disk space at 99%).
- UNKNOWN The plugin failed to execute correctly, or returned an invalid response.
Host Groups & The Problems Page
Host & Service Groups
Instead of viewing 500 flat servers, you can organize multiple hosts under one group (e.g., Web_Servers, DB_Cluster). This simplifies dashboards, makes reporting easier, and allows you to apply common checks or notifications to an entire fleet at once.
The Problems Page
When an outage occurs, you don't want to scroll through pages of healthy servers to find the broken one. The Problems page acts as an active incident filter, displaying only hosts and services currently in a non-OK state (WARNING, CRITICAL, UNKNOWN, DOWN, or UNREACHABLE).