Port 9090 — Prometheus & Cockpit

Port 9090 is shared by two entirely unrelated tools: Prometheus (the industry-standard metrics monitoring system) and Cockpit (a browser-based Linux server management panel). You'd rarely run both on the same machine — Prometheus is for DevOps infrastructure monitoring, Cockpit for sysadmins managing Linux servers. But knowing which one you have is step one when you hit localhost:9090.

Open port 9090: localhost:9090

Prometheus: plain HTTP, no login by default | Cockpit: HTTPS (https://localhost:9090), system user login

Prometheus on Port 9090

Prometheus is a time-series metrics database and monitoring system. It works by pulling (scraping) metrics from instrumented applications and infrastructure exporters at regular intervals, then storing them for querying and alerting. Port 9090 is Prometheus's own web interface and API — the same port you'd use to run PromQL queries directly, check scrape target health, and view alerting rules.

In a typical monitoring stack, Prometheus runs at 9090 and Grafana at 3000 — Grafana connects to Prometheus as a data source and renders the dashboards, while Prometheus does the data collection and storage.

Prometheus Install and Config

# Docker
docker run -d \
  --name prometheus \
  -p 9090:9090 \
  -v ./prometheus.yml:/etc/prometheus/prometheus.yml \
  -v prom_data:/prometheus \
  prom/prometheus

# prometheus.yml — minimal config
global:
  scrape_interval: 15s

scrape_configs:
  - job_name: 'prometheus'          # Prometheus scrapes itself
    static_configs:
      - targets: ['localhost:9090']

  - job_name: 'node'                # Linux host metrics
    static_configs:
      - targets: ['node-exporter:9100']

  - job_name: 'my-app'              # Custom application
    static_configs:
      - targets: ['app:8080']
    metrics_path: '/metrics'

PromQL — Querying Metrics

PromQL (Prometheus Query Language) is how you query collected metrics. At localhost:9090, the Graph tab gives a PromQL query interface. Common queries:

# CPU usage percentage (all cores, last 5 min)
100 - (avg by(instance) (rate(node_cpu_seconds_total{mode="idle"}[5m])) * 100)

# HTTP request rate per second
rate(http_requests_total[1m])

# Memory available in GB
node_memory_MemAvailable_bytes / 1024^3

# 95th percentile request duration
histogram_quantile(0.95, rate(http_request_duration_seconds_bucket[5m]))

Common Prometheus Exporters and Their Ports

Exporter	Port	Metrics
node_exporter	9100	Linux CPU, memory, disk, network
cAdvisor	8080	Docker container metrics
blackbox_exporter	9115	HTTP, TCP, DNS probe checks
mysqld_exporter	9104	MySQL query and connection stats
postgres_exporter	9187	PostgreSQL metrics
redis_exporter	9121	Redis memory and commands

Cockpit on Port 9090

Cockpit is a web-based Linux server management console included by default in RHEL, Fedora, CentOS Stream, and available on Ubuntu. It provides a browser interface for server tasks: service management (start/stop systemd units), log viewing, storage and networking configuration, terminal access, software updates, and container management. Unlike Prometheus, Cockpit uses HTTPS on port 9090 and requires Linux system user credentials to log in.

# Enable and start Cockpit (RHEL/Fedora/CentOS)
sudo systemctl enable --now cockpit.socket

# Install on Ubuntu/Debian
sudo apt install cockpit
sudo systemctl enable --now cockpit.socket

# Access at:
# https://[server-ip]:9090
# Login with your Linux user account (e.g., root or sudo-capable user)

Cockpit is particularly useful on headless servers — instead of SSH + memorizing commands, you get a browser GUI for common maintenance tasks. The terminal tab gives a full shell when you need it.

Resolving Prometheus / Cockpit Conflict

If you need both on the same server, change one of them. Change Prometheus's port with the --web.listen-address flag:

# Prometheus on 9091 instead
docker run -d -p 9091:9090 prom/prometheus \
  --config.file=/etc/prometheus/prometheus.yml \
  --web.listen-address=:9090

Or change Cockpit's port in /etc/cockpit/cockpit.conf:

[WebService]
Port = 9091

Troubleshooting

Problem	Fix
Prometheus targets showing "DOWN"	Check the exporter is running, the port is correct, and no firewall blocks it
PromQL returns no data	Check scrape interval and lookback window — `rate(metric[5m])` needs at least 5 minutes of data
Cockpit HTTPS cert warning	Expected — Cockpit uses a self-signed cert by default. Install a real cert via `cockpit-certificates` package.
Port 9090 conflict	`sudo lsof -i :9090` to identify which process is running — then change one service's port