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According to the RFC:
10.5.3 502 Bad Gateway
The server, while acting as a gateway or proxy, received an invalid response from the upstream server it accessed in attempting to fulfill the request.
Can invalid response also mean no response at all (e.g. connection refused)?
Yes. Empty or incomplete headers or response body typically caused by broken connections or server side crash can cause 502 errors if accessed via a gateway or proxy.
For more information about the network errors
https://en.wikipedia.org/wiki/List_of_HTTP_status_codes