What ports are listening

In this guide: What ports are listening. Audit with ss/lsof.

Recommendation: keep a fallback access path (second SSH session, SSH key, provider console). Apply security changes in small steps and verify access after each change.

If something goes wrong: check the service is running, listening on the expected port, and that your firewall allows the connection. For web services, `nginx -t` and `journalctl -u nginx` are good starting points. If a process listens on `0.0.0.0`, it’s reachable externally. For internal services prefer `127.0.0.1`.

After security changes, always re-check login and access rights. If something breaks, rollback should be quick and obvious (fallback session/console).

Below you’ll find a quick checklist, verification commands, and common pitfalls. This helps you not only “do it”, but also confirm what a correct outcome looks like.

Quick checklist

• Keep a fallback access path (second SSH session/provider console).
• Do not expose unnecessary ports. Publish only what you need.
• Verify permissions on keys/configs (a frequent cause of issues).
• Make one small change at a time and verify the result immediately.
• Keep notes of what you changed (file/command/time).

Verify the result

# Verify / sanity checks
sudo sshd -t
sudo ufw status verbose || true
sudo fail2ban-client status sshd || true
sudo ss -lntup | head -n 80

Common pitfalls

• Disabling passwords/root before verifying key login.
• Overly aggressive firewall rules (locking yourself out).

sudo ss -lntup

sudo lsof -iTCP -sTCP:LISTEN -n -P | head -n 50