HTTP Status Code 521 – Web Server Is Down
The HTTP status code 521 Web Server Is Down is a non-standard status code used by the web infrastructure company Cloudflare. This status code indicates that the origin web server refused the connection from Cloudflare. It typically means that the server is unreachable for some reason, such as it is turned off, not responding to requests, or the IP address is incorrect.
Example 1: Server Unreachable
This example demonstrates a scenario where the web server is physically turned off or is not running any services to accept requests:
GET / HTTP/1.1
Host: example.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64)
Accept: text/html
HTTP/1.1 521 Web Server Is Down
Content-Type: text/html
Date: Mon, 30 Oct 2023 12:00:00 GMT
<html>
<head><title>521 Web Server Is Down</title></head>
<body><h1>Web Server Is Down</h1><p>The server is currently unreachable.</p></body>
</html>
In this example, a client tries to access the home page of a website hosted on a server that is down. As a result, Cloudflare returns a 521 status code indicating that the web server cannot be reached.
Example 2: Incorrect DNS Configuration
In this scenario, the domain is incorrectly configured to point to an IP address where no web server is listening:
GET /about HTTP/1.1
Host: example.org
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7)
Accept: text/html
HTTP/1.1 521 Web Server Is Down
Content-Type: text/html
Date: Tue, 31 Oct 2023 15:30:00 GMT
<html>
<head><title>521 Web Server Is Down</title></head>
<body><h1>Web Server Is Down</h1><p>The DNS configuration is incorrect.</p></body>
</html>
This example illustrates a case where the DNS records are misconfigured, causing requests to be sent to an incorrect server IP where no web server exists. Cloudflare returns a 521 status code indicating that it cannot connect to the server.
Example 3 Scenario
# Client sends a request example.
GET /example HTTP/1.1
Host: www.example.com
# Server Response
HTTP/1.1 521 521 Web Server Is Down
Date: Wed, 09 Oct 2024 23:13:04 GMT
Server: ExampleServer/1.0
Content-Type: application/json
{
"error": "Description of the error for 521"
}
Example 4: Another Scenario
# Client sends another example request.
POST /another-example HTTP/1.1
Host: www.example.com
# Server Response
HTTP/1.1 521 521 Web Server Is Down
Date: Wed, 09 Oct 2024 23:13:04 GMT
Server: ExampleServer/1.0
Content-Type: application/json
{
"error": "Detailed message for 521"
}
Summary
In summary, the HTTP status code 521 is a Cloudflare-specific response indicating that the web server is unreachable. This can occur due to various reasons, such as the server being offline, network issues, or incorrect DNS settings. This status code helps to inform users and administrators that the issue lies with the server’s availability or configuration rather than the client or Cloudflare itself.
