[1] [CITE@en-US[Error 521 – CloudFlare Support]]
( ([TIME[2014-05-14 07:35:22 +09:00]] 版))
<https://support.cloudflare.com/hc/en-us/articles/200171916-Error-521>

[2] [CITE[proxy - How to tell httpd to preserve the proxied error message? - Server Fault]]
( ([TIME[2015-01-14 11:36:35 +09:00]] 版))
<http://serverfault.com/questions/449427/how-to-tell-httpd-to-preserve-the-proxied-error-message>