HTTP 521 Web Server Is Down vs 523 Origin Is Unreachable
Both HTTP 521 (Web Server Is Down) and 523 (Origin Is Unreachable) belong to the 5xx Server Error category. 521 indicates that cloudflare-specific. The origin server refused the connection. Meanwhile, 523 means that cloudflare-specific. Cloudflare could not reach the origin server (DNS resolution failed or no route).
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.
Description
Cloudflare-specific. Cloudflare could not reach the origin server (DNS resolution failed or no route).
When You See It
When Cloudflare's DNS for the origin fails to resolve.
How to Fix
Verify DNS records in Cloudflare dashboard. Check origin server's IP address.
Key Differences
HTTP 521: Cloudflare-specific. The origin server refused the connection.
HTTP 523: Cloudflare-specific. Cloudflare could not reach the origin server (DNS resolution failed or no route).
You encounter 521 when when Cloudflare can't establish a TCP connection to the origin server.
You encounter 523 when when Cloudflare's DNS for the origin fails to resolve.
When to Use Which
For 521 (Web Server Is Down): Check that the origin server is running and the firewall allows Cloudflare IPs. For 523 (Origin Is Unreachable): Verify DNS records in Cloudflare dashboard. Check origin server's IP address.