In HTTP 502, what is meant by an invalid response?

Question

I'm trying to test a server to see if it behaves according to the definition below and for that, I need to construct and send an invalid response to the gateway server, from the upstream server. The HTTP spec says the following regarding the status code 502 - Bad Gateway.

The 502 (Bad Gateway) status code indicates that the server, while acting as a gateway or proxy, received an invalid response from an inbound server it accessed while attempting to fulfill the request.

But what exactly constitute an invalid response? Does a non-standard response code (say, some random number like 6789) count as an invalid response? Or a response with an invalid HTTP version?

Came across this: 502 HTTP Status Code, which says incomplete headers and response bodies can cause 502 errors. How does a server determine if the headers or the body is incomplete? And more importantly, is it possible to create such a response programmatically (I'm using Java)?

Answer 1

I managed to find the following in the HTTP spec (RFC7230 section 3.3.3).

If a message is received without Transfer-Encoding and with either multiple Content-Length header fields having differing field-values or a single Content-Length header field having an invalid value, then the message framing is invalid and the recipient MUST treat it as an unrecoverable error. If this is a request message, the server MUST respond with a 400 (Bad Request) status code and then close the connection. If this is a response message received by a proxy, the proxy MUST close the connection to the server, discard the received response, and send a 502 (Bad Gateway) response to the client. If this is a response message received by a user agent, the user agent MUST close the connection to the server and discard the received response.

Apart from this, I've also noticed that Nginx returns a 502 when acting as a reverse proxy/load balancer, when the upstream server is down.