HTTP/5xx (Server Error)
521 Web Server Is Down
Cloudflare-specific: the origin server refused the connection from Cloudflare.
❓ Why does this happen?
The web server process is stopped, or a firewall blocks Cloudflare's IP ranges.
✅ How to fix it
Start the web server (systemctl start nginx) and allow-list Cloudflare IP ranges in your firewall.
Return or handle 521 in your stack
// Note: 521 is normally generated by Cloudflare, not your app.
// Emitting it yourself is only useful for testing client error handling.
// app/api/resource/route.ts
export function GET() {
return Response.json(
{ error: "Web Server Is Down", status: 521 },
{
status: 521
}
);
}Live 521 mock endpoint
Point your app, tests, or retry logic at this free public URL to reproduce a real 521 response (CORS enabled, optional ?delay=ms up to 5000).
https://castov.com/api/mock/521Frequently Asked Questions
What does HTTP Status 521 mean?
Why am I getting a 521 status code?
How do I fix a 521 Web Server Is Down error?
How can I test how my app handles a 521 response?
Other 5xx status codes
Browse the full HTTP status codes directory or troubleshoot terminal errors in the Error Fix Encyclopedia.