最近在测试CDN服务质量问题，测试过程中返回了一些不同的状态码，当然有一些常用的，也有一些不常用的。最奇葩的是在使用curl命令的时候出现000状态码，问了很多同事，对这个000的反应跟新事物是的。
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（临时跳转），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)
$ curl -w "%{http_code}
" http://example.invalid/ ; echo "Exit code: $?"
000
curl: (6) Could not resolve host: example.invalid
Exit code: 6
2.Connection refused (7)
$ curl -w "%{http_code}
" http://localhost:81/ ; echo "Exit code: $?"
000
curl: (7) Failed to connect to localhost port 81: Connection refused
Exit code: 7
Connection timed out (28)
$ curl -w "%{http_code}
" -m 5 http://10.255.255.1/ ; echo "Exit code: $?"
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
EOF
客户端请求：
$ curl -w "%{http_code}
" http://localhost:65535/ ; echo "Exit code: $?"
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。
参考文章：
wikipedia：https://en.wikipedia.org/wiki/List_of_HTTP_status_codes
http://superuser.com/questions/501690/curl-http-code-of-000