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Port Lookup

What usually runs on port 5432, 6443 or 9090 — and what to check if it’s open.

● Runs 100% in your browser — nothing you type leaves this page.

93 ports and ranges

Defaults only: any service can be moved to another port, and any program can take a free one. ss and lsof show what really listens.

The address bar updates as you go, so you can bookmark or share the result.

How to use it

  1. Type a port number (6443) or a name (redis, kubernetes, docker swarm).
  2. Read what usually listens there and whether it belongs on the internet: can face the internet, keep it internal, or never expose.
  3. Copy the commands on the right to see what really uses that port on your machine, or to test it from another one.

The link updates as you type, so you can share a lookup like /tools/port-lookup/#6379.

What you get

These are defaults. Any service can be moved to another port, and any program can take a free one — only ss or lsof on the machine tells you what is really there.

Common commands

Terminal window
sudo ss -tulpn                       # everything listening, with the process
sudo ss -tulpn 'sport = :5432'       # only port 5432
sudo lsof -i :80                     # which process holds port 80
nc -zv db.internal 5432              # can I reach it from here?
sudo ufw allow from 10.0.0.0/24 to any port 5432 proto tcp   # allow one network only
sudo firewall-cmd --list-ports       # open ports on RHEL / Rocky (firewalld)

FAQ

“Address already in use” — how do I find out what took the port?

sudo ss -tulpn 'sport = :80' shows the process that listens on it. Stop it, move it, or choose another port. The full walkthrough, with the classic Apache-vs-nginx case, is in Address already in use.

Why does it say “Connection refused” when the service is running?

Usually it listens only on 127.0.0.1, or on another port than you think. ss -tlnp shows the address next to the port: 127.0.0.1:5432 is only reachable from the same machine, 0.0.0.0:5432 from everywhere a firewall allows. If it times out instead, a firewall drops the packets — the difference is explained in “Connection timed out” vs “Connection refused”. See also PostgreSQL connection refused, and Docker container can’t connect to localhost for the container version of the same problem.

Is a service safe if it has a password?

Safer, not safe. A password protects against strangers, but not against brute force or a bug in the service itself. Keep databases, caches and admin APIs on private addresses, and allow only the machines that need them in the firewall. Reach the rest through a VPN or an SSH tunnel: ssh -L 5432:localhost:5432 db-server.

What is the difference between TCP and UDP here?

Most services use TCP. DNS, NTP, syslog, SNMP, VPNs like WireGuard and HTTP/3 use UDP. nc -zv tests TCP; for UDP, nc -zvu can’t tell you much, because a UDP service may simply not answer.

Where can I practice checking ports?

In the lesson Ports: ss, lsof and curl, where two web servers want the same port.

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