Error 521 through a proxy: the site’s server refused Cloudflare

Proxy error · Last revised Oct 19, 2026

Check that it is Cloudflare’s 521

curl -s -D - -o /dev/null -x "http://USERNAME:[email protected]:8000" https://the-site-you-need.com/page

Look for server: cloudflare and a cf-ray header beside the 521. Without them, the code came from somewhere else: read your proxy provider’s own error list.

What to do

1. Load the page without the proxy. A 521 there too means the site is down for everyone. Nothing on your side fixes it.
2. Try another page on the same site. If the home page loads and one section does not, that section’s server is down; the rest of your job can go on.
3. Wait, then retry slowly. A server restart or a deployment often takes a few minutes. Retry after one, then after five, rather than in a loop that spends traffic for nothing.
4. Do not change proxies over it. A 521 depends on whether the site’s server accepts Cloudflare, not on who asked Cloudflare. A new IP, country or proxy type gets the same answer.

If the site is yours, Cloudflare’s steps are to make sure the origin is running, check its error logs, “Allow all Cloudflare IP ranges” in its firewall, and confirm it listens on the port the SSL/TLS mode needs (80 for Flexible, 443 for Full).

Related

When the site’s server is up but slow to accept Cloudflare, the code is 522; when it answers with something broken, 520. Every code by who sends it: proxy errors.