Checking open ports

When a service is not reachable, start by checking what is actually listening on the server.

Listening TCP and UDP sockets

sudo ss -tulpn

Filter by port

sudo ss -tulpn | grep ':80'
sudo ss -tulpn | grep ':443'

Check firewall status

sudo ufw status verbose

Check a service

systemctl status nginx --no-pager
journalctl -u nginx -n 100 --no-pager