5xx · Server Error
521Web Server Is Down
Cloudflare：无法连接到源站。
What is HTTP 521?
521 Web Server Is Down 表示 Cloudflare 无法连接到源站。可能是源站离线、端口关闭或防火墙阻止了 Cloudflare IP。
Example
Origin server offline → Cloudflare 521Check a URL in real-time
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