500 = Internal Server Error
- A generic error message, given when an unexpected condition was encountered and no more specific message is suitable.
501 = Not Implemented
- The server either does not recognize the request method, or it lacks the ability to fulfil the request. Usually this implies future availability (e.g., a new feature of a web-service API).
502 = Bad Gateway
- The server was acting as a gateway or proxy and received an invalid response from the upstream server.
503 = Service Unavailable - The server is currently unavailable (because it is overloaded or down for maintenance). Generally, this is a temporary state.
504 = Gateway Timeout - The server was acting as a gateway or proxy and did not receive a timely response from the upstream server.
505 = HTTP Version Not Supported
- The server does not support the HTTP protocol version used in the request.
506 = Variant Also Negotiates (RFC 2295)
- Transparent content negotiation for the request results in a circular reference.
507 = Insufficient Storage (WebDAV; RFC 4918)
- The server is unable to store the representation needed to complete the request.
508 = Loop Detected (WebDAV; RFC 5842)
- The server detected an infinite loop while processing the request (sent in lieu of 208 Already Reported).
509 = Bandwidth Limit Exceeded (Apache Web Server/cPanel)
- The server has exceeded the bandwidth specified by the server administrator; this is often used by shared hosting providers to limit the bandwidth of customers.
510 = Not Extended (RFC 2774) - Further extensions to the request are required for the server to fulfil it.
511 = Network Authentication Required (RFC 6585) - The client needs to authenticate to gain network access. Intended for use by intercepting proxies used to control access to the network (e.g., "captive portals" used to require agreement to Terms of Service before granting full Internet access via a Wi-Fi hotspot).
520 = Unknown Error - The 520 error is used as a "catch-all response for when the origin server returns something unexpected", listing connection resets, large headers, and empty or invalid responses as common triggers.
521 = Web Server Is Down - The origin server has refused the connection from Cloudflare.
522 = Connection Timed Out - Cloudflare could not negotiate a TCP handshake with the origin server.
523 = Origin Is Unreachable - Cloudflare could not reach the origin server; for example, if the DNS records for the origin server are incorrect.
524 = A Timeout Occurred - Cloudflare was able to complete a TCP connection to the origin server, but did not receive a timely HTTP response.
525 = SSL Handshake Failed - Cloudflare could not negotiate a SSL/TLS handshake with the origin server.
526 = Invalid SSL Certificate - Cloudflare could not validate the SSL/TLS certificate that the origin server presented.
527 = Railgun Error - Error 527 indicates that the request timed out or failed after the WAN connection had been established.
530 = Site is frozen
- Used by the Pantheon web platform to indicate a site that has been frozen due to inactivity. - or - indicates that the requested host name could not be resolved on the Cloudflare network to an origin server.
598 = (Informal convention) Network read timeout error - Used by some HTTP proxies to signal a network read timeout behind the proxy to a client in front of the proxy.