[1] Error 521 – CloudFlare Support ( ( 版)) <https://support.cloudflare.com/hc/en-us/articles/200171916-Error-521>
[2] proxy - How to tell httpd to preserve the proxied error message? - Server Fault ( ( 版)) <http://serverfault.com/questions/449427/how-to-tell-httpd-to-preserve-the-proxied-error-message>