HTTP Status Code 520 – Web Server Is Returning an Unknown Error

The HTTP status code 520 is an error response provided by Cloudflare when the origin server returns an unexpected or unknown error. This error typically occurs when Cloudflare, acting as a reverse proxy, receives a response from the origin server that is unexpected or doesn’t fall into any of the standard HTTP status code categories.

Examples

Example 1: Misconfigured Server

In this scenario, the origin server is misconfigured and returns a response that does not conform to HTTP protocols. This results in Cloudflare being unable to interpret the response, leading to a 520 error being returned to the client.


GET /example HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0


Response:


HTTP/1.1 520 Web Server Is Returning an Unknown Error
Content-Type: text/html
Connection: keep-alive
Date: Mon, 01 Nov 2023 12:00:00 GMT
Server: Cloudflare


Explanation: In this example, the client attempts to access a resource, but due to server misconfiguration, Cloudflare is unable to process the server’s response correctly, resulting in a 520 error.

Example 2: Application Error

Consider a web application that crashes due to an unhandled exception or error, leading to an invalid or malformed response being generated by the server.


GET /api/data HTTP/1.1
Host: api.example.com
User-Agent: Mozilla/5.0


Response:


HTTP/1.1 520 Web Server Is Returning an Unknown Error
Content-Type: text/html
Connection: keep-alive
Date: Mon, 01 Nov 2023 12:05:00 GMT
Server: Cloudflare


Explanation: This example illustrates how an application error can cause the server to return a response that Cloudflare cannot interpret, leading to a 520 error being sent back to the client.

Example 3 Scenario

# Client sends a request example.
GET /example HTTP/1.1
Host: www.example.com

# Server Response
HTTP/1.1 520 520 Web Server Is Returning an Unknown Error
Date: Wed, 09 Oct 2024 23:12:48 GMT
Server: ExampleServer/1.0
Content-Type: application/json

{
    "error": "Description of the error for 520"
}

Example 4 Scenario

# Client sends another example request.
POST /another-example HTTP/1.1
Host: www.example.com

# Server Response
HTTP/1.1 520 520 Web Server Is Returning an Unknown Error
Date: Wed, 09 Oct 2024 23:12:48 GMT
Server: ExampleServer/1.0
Content-Type: application/json

{
    "error": "Detailed message for 520"
}

Summary

The 520 status code indicates that Cloudflare encountered an unknown error from the origin server. This typically arises from various issues, such as server misconfigurations, application errors, or a response format that Cloudflare cannot interpret. To resolve a 520 error, it’s essential to investigate and correct any underlying issues on the origin server, ensuring that it returns valid and expected HTTP responses. Administrators should check server logs, configuration settings, and application code to identify and rectify the cause of the error.