NodeExporterDown

Host or node_exporter is down. Free rule (MIT): the complete runbook and rule definition are below.


Severity	critical
Pending (`for:`)	5m (critical)
Domain	Hosts (node_exporter)
Requires	node_exporter 1.x (default collectors; systemd and hwmon noted per alert)
Rule file	`rules/node-exporter.rules.yml` (group `ftw.node-exporter.alerts`)

Meaning

Prometheus cannot scrape node_exporter on the host. Either the host is down, the exporter stopped, or the network path between Prometheus and the host is broken.

Impact

All host alerts for this instance are blind. If the host itself is down, everything running on it is unavailable.

Diagnosis

Check the scrape error in Prometheus: Status → Targets, or
```
up{instance="<instance>"}
```
Is the host reachable? ping <host> / curl -s http://<host>:9100/metrics | head.
On the host: systemctl status node_exporter (Kubernetes: kubectl -n monitoring get pods -o wide | grep node-exporter).

Mitigation

Restart node_exporter or the host.
Fix firewall/security-group rules for port 9100.
If the host was intentionally removed, remove it from service discovery / file_sd targets.

Escalation

critical: Page the on-call engineer immediately. If not acknowledged within 15 minutes, escalate to the secondary on-call. Open an incident channel when user impact is confirmed.

Owned by whoever operates the host (platform team for Kubernetes nodes, service owner for standalone VMs).

NodeFilesystemSpaceFillingUp: Filesystem is predicted to run out of space within 24 hours.
NodeFilesystemAlmostOutOfSpace: Filesystem has less than 10% space left.
NodeHighCPUUsage: Host CPU usage is above 90%.
NodeMemoryHighUtilization: Host memory utilisation is above 90%.
NodeFilesystemAlmostOutOfFiles: Filesystem has less than 10% inodes left.

Rule definition

From rules/node-exporter.rules.yml in the free repository (MIT). Unit tests for it are in tests/.

- alert: NodeExporterDown
  expr: up{job=~"node|node-exporter|node_exporter|prometheus-node-exporter"} == 0
  for: 5m
  labels:
    severity: critical
  annotations:
    summary: Host or node_exporter is down.
    description: node_exporter target {{ $labels.instance }} (job {{ $labels.job }}) has been unreachable for 5m.
    runbook_url: runbooks/node-exporter/NodeExporterDown.md