404 Checker
HTTP Status Codes
9 Codes Documented
OTHER — Other & Proprietary
Non-standard status codes deployed by major edge networks and platforms (Cloudflare 520–526, Nginx 499, LinkedIn 999).
HTTP
499
Client Closed Request
Proprietary
An Nginx-specific proprietary code logged when a client closes the connection before the server finishes sending the response.
HTTP
520
Web Server Returned an Unknown Error
Proprietary
A Cloudflare-specific status code used as a catch-all when the origin web server returns an empty, corrupted, or unexpected response to the Cloudflare edge proxy.
HTTP
521
Web Server Is Down
Proprietary
A Cloudflare status code returned when the origin web server actively refuses TCP connection attempts from Cloudflare edge nodes on port 80 or 443.
HTTP
522
Connection Timed Out
Proprietary
A Cloudflare status code indicating that Cloudflare was unable to complete the initial TCP handshake with the origin server before timing out.
HTTP
523
Origin Is Unreachable
Proprietary
A Cloudflare status code returned when Cloudflare cannot reach the origin web server due to network routing failures or unreachable IP addresses.
HTTP
524
A Timeout Occurred
Proprietary
A Cloudflare status code returned when Cloudflare completed a TCP handshake with the origin server, but the origin did not reply with an HTTP response within the 100-second timeout window.
HTTP
525
SSL Handshake Failed
Proprietary
A Cloudflare status code indicating that Cloudflare and the origin server could not agree on a mutual SSL/TLS protocol or cipher suite during the secure handshake.
HTTP
526
Invalid SSL Certificate
Proprietary
A Cloudflare status code returned when Cloudflare cannot validate the origin server's SSL certificate (often when Cloudflare Full Strict mode is enabled and the origin cert is self-signed or expired).
HTTP
999
Request Denied
Proprietary
A non-standard proprietary status code used by certain platforms (notably LinkedIn) to blanket-deny automated requests, scrapers, or unverified crawler user agents.
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