FAQ Can I monitor an open or closed port
Open port
Yes, monitoring an open TCP port is possible with the TCP or TCPS type check. You can specify the port in the URL directly like this: tcp://updown.io:80 (by default it'll check port 80 for TCP and 443 for TCPS).
Updown will simply open the TCP connection and consider the check UP if it is accepted. For the TCPS check you'll also get the TLS certificate validation on this port (similar to an HTTPS checks), but only the TCP connection will be measured for uptime. So a valid TCP connect with an invalid/expired certificate will be considered UP (similar to what we do on HTTPS), and you'll receive a warning about the certificate.
Banner check
You can optionally fill in the "contains" field with some string you would like to make sure is received upon connection (commonly called the "banner"). For example OpenSSH_9.6 on port 22 to make sure not only the port is open but the service is correct and responsive too (it may also help detecting some tanpering attemps).
Closed port
NEW Since August 2026 it's also possible to check for a closed port in updown.io. This is useful if you want to make sure a firewall rule is working for example to prevent public access to some ressource. This way you are alerted in case the port suddenly becomes accessible from updown servers. To do that, you simply need to specify <closed> in the "contains" field. Whether the error is a quick closed port or a slower timeout, the check will be considered as UP.
Unrelated error like name resolution would still surface as an alert.
