521 Web Server Is Down — Causes & Fix
HTTP 521 (Web Server Is Down) means Cloudflare cannot establish a connection to the origin server — the origin refused or dropped the connection.
- Applies to
- Any website or web server (HTTP standard)
- Time to fix
- 15 minutes – 2 hours
What it means
Causes include the origin being offline, a firewall blocking Cloudflare's IPs, or the web server service being stopped. This is a Cloudflare-specific status code (not part of the HTTP standard). Cloudflare returns it when it sits in front of a site and has a problem reaching or getting a valid response from the origin (your) server — so the fix is almost always at the origin, not with the visitor.
Symptoms
- Cloudflare "Error 521" page
- Site unreachable behind Cloudflare
How to fix it
- 1
Start/verify the origin
1. As the operator: confirm the web server is running and listening on the expected port.
- 2
Allow Cloudflare IPs
1. Ensure the origin firewall allows Cloudflare's IP ranges.
Frequently asked questions
What does Error 521 mean?
Cloudflare couldn't connect to your origin server — it is down or blocking Cloudflare. Start the server and allow Cloudflare's IPs.
Last updated July 20, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.