502 HTTP状态码
问题描述：
根据RFC：
10.5.3 502 Bad Gateway
服务器在充当网关或代理时，在尝试完成请求时从其访问的上游服务器收到无效响应。
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.
无效回复是否也意味着没有回复（例如拒绝连接）？
Can invalid response also mean no response at all (e.g. connection refused)?
答
是。如果通过网关或代理访问，通常由连接断开或服务器端崩溃导致的空或不完整的标头或响应正文可能导致502错误。
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.
有关网络错误的更多信息
For more information about the network errors