502 HTTP status code

Question

According to RFC:

10.5.3 502 Bad Gateway
The server, acting as a gateway or proxy, received an incorrect response from the upstream server, which it accessed when trying to fulfill the request.

Can an invalid response also mean no response at all (for example, the connection was refused)?

Answer 1

Yes. Empty or incomplete headers or response body, usually caused by damaged connections or server-side failure, can cause 502 access errors through a gateway or proxy.

Additional Network Error Information

https://en.wikipedia.org/wiki/List_of_HTTP_status_codes