[1] Error 520 – CloudFlare Support ( ( 版)) <https://support.cloudflare.com/hc/en-us/articles/200171936-Error-520>
[2] 520 (Origin Error) in AJAX calls · Issue #57 · johnbillion/query-monitor ( ( 版)) <https://github.com/johnbillion/query-monitor/issues/57>
[3] Getting a lot of HTTP 520 errors. What to do with them > – Customer Feedback for Microsoft Azure ( ( 版)) <http://feedback.azure.com/forums/223579-azure-preview-portal/suggestions/5965429-getting-a-lot-of-http-520-errors-what-to-do-with>
[4] Understanding HTTP 520 Error – Proxy Publisher Failure | Code Web ( ( 版)) <http://codeweb.in/understanding-http-520-error-proxy-publisher-failure/>