Port 9100: Prometheus node_exporter

Port 9100 is the default port of the Prometheus node_exporter for Linux system metrics.

Port: 9100
Protocol: TCP
Service: Prometheus node_exporter
Exposure: Public: no, only for the Prometheus server

daniele@mikweb: ~/ports — less 9100.md

The node_exporter exposes CPU, memory, disk, filesystem and network metrics at /metrics on port 9100, which the Prometheus server scrapes regularly. Historically, network printers (HP JetDirect, raw printing) also use port 9100.

Not public. The metrics reveal kernel version, mounts, network interfaces and load. Allow only the Prometheus server's IP in the firewall, e.g. ufw allow from 10.0.0.5 to any port 9100 proto tcp.

Typical: target DOWN in Prometheus because the firewall blocks it or the exporter only listens on 127.0.0.1 (--web.listen-address).

Check

curl -s http://localhost:9100/metrics | head