使用curl命令的时候出现000状态码.
一、生产环境常见的HTTP CODE
生产环境常见的HTTP状态码列表（List of HTTP status codes）为：
2XX成功： 200 - OK，服务器成功返回网页 - Standard response for successful HTTP requests.
3XX重定向： 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. 302 - Moved Temporarily（永久跳转），请求的网页已临时跳转到新位置。
4XX客户端错误： 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.
5XX服务器错误： 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.
二、curl奇怪的返回值000。
经过测试发现在curl的时候出现000的情况有如下几种：
1.Failed DNS resolution (6)
?" 000 curl: (6) Could not resolve host: example.invalid Exit code: 6
2.Connection refused (7)
?" 000 curl: (7) Failed to connect to localhost port 81: Connection refused Exit code: 7
Connection timed out (28)
?" 000 curl: (28) Connection timed out after 5001 milliseconds Exit code: 28
Server actually returns 000 for some reason (0)
开启一个监听端口：
nc -l -p 65535 & <<EOF
HTTP/1.1 000 Fake Status Code
Content-Length: 0
Connection: close
客户端请求：
?" 000 Exit code: 0
*注释：一般情况下遇到000，默认考虑为200，正常。
在测试过程中打印了000时的错误信息，错误为：Connection timed out。使用curl时，参数：-sS ,然后将curl的错误 2>> /tmp/err.log。
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