nginx返回400，414错误
如果一个请求头的大小大于一个buffer的大小，就会返回400错误
如果一个请求行的大小大于一个buffer的大小，就会返回414错误
通过large_client_header_buffers来设置最大buffer值
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生产环境常见的HTTP状态码列表
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.