Can I pick the ports?
No. The list is fixed, and there is no way to ask for a different port. The check is rate-limited.
TCP
Common port check
Tries a fixed list of ports from this Worker: 22, 53, 80, 443, 3306, 5432, 8080, 8443. Each attempt waits about 1.5 seconds. There is no way to ask for a different port.
“No connection” means the handshake did not finish before the timeout. A firewall that drops packets looks the same as a closed port. Private and reserved addresses are refused.