520 Web Server Returned an Unknown Error

Cloudflare received an empty, unknown or unexpected response from the origin server.

Unofficial code. 520 is used only by Cloudflare. It is not in the IANA registry or any HTTP standard, and other software may give the number a different meaning or none.

Code: 520
Reason phrase: Web Server Returned an Unknown Error
Class: 5xx Server error
Used by: Cloudflare
Cacheable by default: No

What it means

Server error: the request may be fine, but the server failed to carry it out.

Usual causes. The origin crashed, reset the connection, or sent headers Cloudflare could not read (often too large).

What to do. Site owners should check the origin's error logs at the time of the error and the size of its response headers.

How clients and crawlers treat it

Google Search. Googlebot sees the 5xx code Cloudflare sends and treats it like any server error: it slows down crawling, and URLs that keep failing are eventually dropped from the index. Source: Google Search Central, How HTTP status codes, and network and DNS errors affect Google Search.

Caching. Not cacheable by default (RFC 9110 section 15.1 does not list it). A cache stores it only when the response says so with Cache-Control or Expires.

Example response

HTTP/1.1 520 Web Server Returned an Unknown Error
Content-Type: text/plain; charset=utf-8

Web Server Returned an Unknown Error

Related codes

All status codes · All 5xx codes

Names and numbers from the IANA HTTP Status Code Registry (this code is not in it). Google Search behaviour as documented by Google Search Central.