[1] Error 521 – CloudFlare Support ( (2014-05-14 07:35:22 +09:00 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 ( (2015-01-14 11:36:35 +09:00 版)) http://serverfault.com/questions/449427/how-to-tell-httpd-to-preserve-the-proxied-error-message