9090 Prometheus TCP
Reference for TCP port 9090, commonly associated with Prometheus.
What it's for
Port 9090 is the default port for Prometheus's web UI and API — the widely used open-source metrics collection and time-series monitoring system that's become close to a default choice in Kubernetes and cloud-native environments. Prometheus scrapes metrics from configured targets (applications, exporters, infrastructure) at regular intervals and stores them for querying and alerting.
Protocol basics
Prometheus exposes its query interface, graphing UI, and configuration status over HTTP on port 9090. Separately (and this trips people up), the targets being monitored expose their own metrics endpoints on whatever port their application or exporter uses — Prometheus's own port 9090 is specifically where you interact with the Prometheus server itself, not where individual monitored services expose their metrics. Like other observability tooling, exposing the Prometheus UI without authentication can leak operational details about your infrastructure, so it's typically kept on an internal network or behind an authenticating reverse proxy.