200 – OK，服务器成功返回网页
– Standard response for successful HTTP requests.
301 – Moved Permanently（永久跳转），请求的网页已永久跳转到新位置。
– This and all future requests should be directed to the given.
403 – Forbidden（禁止访问）,服务器拒绝请求
– forbidden request (matches a deny filter) => HTTP 403
– The request was a legal request, but the server is refusing to respond to it.
404 – Not Found,服务器找不到请求的页面。
– The requested resource could not be found but may be available again in the future.
500 – Internal Server Error（内部服务器错误）
– internal error in haproxy => HTTP 500
– A generic error message, given when no more specific message is suitable.
502 – Bad Gateway（坏的网关）,一般是网关服务器请求后端服务时，后端服务没有按照http协议正确返回结果。
– the server returned an invalid or incomplete response => HTTP 502
– The server was acting as a gateway or proxy and received an invalid response from the upstream server.
503 – Service Unavailable（服务当前不可用）,可能因为超载或停机维护。
– no server was available to handle the request => HTTP 503
– The server is currently unavailable (because it is overloaded or down for maintenance).
504 – Gateway Timeout（网关超时）,一般是网关服务器请求后端服务时，后端服务没有在特定的时间内完成服务。
– the server failed to reply in time => HTTP 504
– The server was acting as a gateway or proxy and did not receive a timely response from the upstream server