HTTP 520 Web Server Returned an Unknown Error

What does HTTP 520 mean?

Cloudflare received an empty, unknown or unexpected response from the origin server. A Cloudflare-specific catch-all: the origin reset the connection, sent headers that are too large or returned an empty reply.

5xx Server error: The request looked valid, but the server failed to fulfil it. The fix is on the server side.

Common causes of a 520 error

• Origin crashed or closed the connection.
• Response headers larger than Cloudflare allows (32 KB).
• Firewall on the origin blocking Cloudflare IPs.

How to fix 520 Web Server Returned an Unknown Error

• Check origin server logs at the time of the error.
• Allow Cloudflare’s IP ranges in the origin firewall.

520 and SEO

Treated like a 5xx error by search engines.

Example response

HTTP/1.1 520 Web Server Returned an Unknown Error

Related status codes

• 502 Bad Gateway — A gateway or proxy got an invalid response from the upstream server.
• 521 Web Server Is Down — Cloudflare could not connect to the origin server — the connection was refused.
• 522 Connection Timed Out — Cloudflare timed out while connecting to the origin server.
• 524 A Timeout Occurred — Cloudflare connected to the origin, but it did not send a response within 100 seconds.

Check which status code a URL returns with the HTTP header checker.