The section.io platform connects back to the origin infrastructure for requests that are not in cache.
There are several scenarios that determine the HTTP response code that is sent to the browser:
The origin servers respond with an HTTP status code - In this scenario we will pass the HTTP status code back to the browser
section.io is unable to TCP connect to the origin servers - In this case the origin servers might be down or there may be a firewall product blocking section.io connections.
There is no HTTP status code response from the origin servers in this case so our Origin Proxy layer sends an HTTP 502 response back to the browser.
You can confirm that scenario 2 has occurred by looking at the HTTP Logs for the Origin Proxy layer (last_proxy-access-log) and look for the field: upstream_bytes_received when this is a 0 value it means there was no TCP connection established to the origin infrastructure.