502 / Bad Gateway

“服务器作为网关或者代理工作的服务器尝试执行请求时,从上游服务器接收到无效的响应。“ — wikipedia

“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.“ — ietf

502 / code reference(s)
Rails HTTP Status Symbol
:bad_gateway
← return