HTTP 426 Upgrade Required vs 521 Web Server Is Down
HTTP 426 (Upgrade Required) is a 4xx Client Error response, while 521 (Web Server Is Down) is a 5xx Server Error response. 426 indicates that the server refuses to perform the request using the current protocol but might after the client upgrades to a different protocol. In contrast, 521 means that cloudflare-specific. The origin server refused the connection.
Description
The server refuses to perform the request using the current protocol but might after the client upgrades to a different protocol.
When You See It
When a server requires HTTPS or a newer protocol version.
How to Fix
Switch to the protocol specified in the Upgrade response header.
Description
Cloudflare-specific. The origin server refused the connection.
When You See It
When Cloudflare can't establish a TCP connection to the origin server.
How to Fix
Check that the origin server is running and the firewall allows Cloudflare IPs.
Key Differences
426 is a 4xx Client Error response, while 521 is a 5xx Server Error response.
HTTP 426: The server refuses to perform the request using the current protocol but might after the client upgrades to a different protocol.
HTTP 521: Cloudflare-specific. The origin server refused the connection.
You encounter 426 when when a server requires HTTPS or a newer protocol version.
You encounter 521 when when Cloudflare can't establish a TCP connection to the origin server.
When to Use Which
For 426 (Upgrade Required): Switch to the protocol specified in the Upgrade response header. For 521 (Web Server Is Down): Check that the origin server is running and the firewall allows Cloudflare IPs.