5xx Server Error Status Codes
These status codes indicate that the server is aware it has encountered an error or is otherwise incapable of performing the request.
TL;DR
5xx errors mean the server made a mistake. Unlike 4xx errors, the client's request was valid, but the server failed to fulfill it due to an error on the server side.
Understanding 5xx Errors
Server error responses indicate that the server is aware it has encountered an error or is otherwise incapable of performing the request. These errors are the server's fault, not the client's.
Complete List of 5xx Status Codes
500
Internal Server Error
Generic server error when no specific error code is appropriate.
501
Not Implemented
The server does not support the functionality required to fulfill the request.
502
Bad Gateway
The server received an invalid response from the upstream server.
503
Service Unavailable
The server is temporarily unable to handle the request.
504
Gateway Timeout
The server 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
Internal content-negotiation configuration error causing a circular reference.
507
Insufficient Storage
The server (WebDAV) cannot store the representation needed to complete the request.
508
Loop Detected
The server detected an infinite loop while processing a WebDAV request.
510
Not Extended
Further extensions to the request are required for the server to fulfill it.
511
Network Authentication Required
The client needs to authenticate to gain network access.
Cloudflare 5xx Error Codes (520–530)
Cloudflare returns its own non-standard 5xx codes when it cannot get a valid response from your origin server. They are not part of the official HTTP spec, but they are common and each points to a specific origin or connection problem.
520
Web Server Returned an Unknown Error
Cloudflare received an empty or unexpected response from the origin.
521
Web Server Is Down
Cloudflare cannot open a connection to the origin server.
522
Connection Timed Out
The TCP connection between Cloudflare and the origin timed out.
523
Origin Is Unreachable
Cloudflare cannot route to the origin, usually a DNS or routing problem.
524
A Timeout Occurred
Connected to the origin, but it did not respond within 100 seconds.
525
SSL Handshake Failed
The TLS handshake between Cloudflare and the origin failed.
526
Invalid SSL Certificate
Cloudflare could not validate the origin's SSL certificate in strict mode.
530
Origin DNS Error
Shown with a paired 1xxx code, usually an origin DNS resolution failure.