HTTP 511 Network Authentication Required vs 521 Web Server Is Down
Both HTTP 511 (Network Authentication Required) and 521 (Web Server Is Down) belong to the 5xx Server Error category. 511 indicates that the client needs to authenticate to gain network access. Used by captive portals (hotel/airport WiFi). Meanwhile, 521 means that cloudflare-specific. The origin server refused the connection.
Description
The client needs to authenticate to gain network access. Used by captive portals (hotel/airport WiFi).
When You See It
When connecting to WiFi that requires login (airport, hotel, coffee shop).
How to Fix
Open a browser and complete the captive portal login process.
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
HTTP 511: The client needs to authenticate to gain network access. Used by captive portals (hotel/airport WiFi).
HTTP 521: Cloudflare-specific. The origin server refused the connection.
You encounter 511 when when connecting to WiFi that requires login (airport, hotel, coffee shop).
You encounter 521 when when Cloudflare can't establish a TCP connection to the origin server.
When to Use Which
For 511 (Network Authentication Required): Open a browser and complete the captive portal login process. For 521 (Web Server Is Down): Check that the origin server is running and the firewall allows Cloudflare IPs.