5xx Server Error

Server error responses

The server failed to fulfil an apparently valid request. The fault is on the server side.

5xx codes are worth reading precisely, because they localize the failure. 500 is the application, 502 means an intermediary got a bad response from upstream, 503 means the service is deliberately unavailable, and 504 means an upstream was too slow.

The unofficial 52x range from Cloudflare narrows this further, distinguishing a refused connection (521) from an unanswered handshake (522), a timeout (524), and TLS problems (525 and 526).

Codes in this class

• 500 Internal Server Error: The server hit an unexpected condition and cannot say anything more specific about it.
• 501 Not Implemented: The server does not support the functionality required to fulfil the request, usually the method itself.
• 502 Bad Gateway: A server acting as a gateway or proxy received an invalid response from the upstream server it contacted.
• 503 Service Unavailable: The server is temporarily unable to handle the request, typically because it is overloaded or down for maintenance.
• 504 Gateway Timeout: A gateway or proxy did not receive a timely response from the upstream server it needed to complete the request.
• 505 HTTP Version Not Supported: The server does not support the major HTTP version used in the request.
• 506 Variant Also Negotiates: A content negotiation misconfiguration: the chosen variant is itself a negotiable resource, creating a circular reference.
• 507 Insufficient Storage: The server cannot store the representation needed to complete the request.
• 508 Loop Detected: The server terminated the operation because it encountered an infinite loop while processing it.
• 509 Bandwidth Limit Exceeded: Shared hosting code indicating the account exceeded its allotted bandwidth for the billing period.
• 510 Not Extended: Obsolete. The server required further extensions to the request before it could fulfil it.
• 511 Network Authentication Required: The client must authenticate with the network itself, typically a captive portal on public Wi-Fi.
• 520 Web Server Returned an Unknown Error: Cloudflare's catch-all: the origin returned something empty, malformed, or otherwise unintelligible.
• 521 Web Server Is Down: Cloudflare could not open a connection to the origin because it refused the connection.
• 522 Connection Timed Out: Cloudflare's TCP handshake with the origin did not complete within the allowed time.
• 523 Origin Is Unreachable: Cloudflare could not reach the origin at all, usually a DNS or routing failure.
• 524 A Timeout Occurred: Cloudflare connected to the origin but the origin did not send a complete response within the time limit.
• 525 SSL Handshake Failed: The TLS handshake between Cloudflare and the origin failed.
• 526 Invalid SSL Certificate: The origin presented a TLS certificate that could not be validated.
• 527 Railgun Error: Deprecated Cloudflare code for a connection failure between the edge and a Railgun listener.
• 529 Site Is Overloaded: Non-standard code used by a few platforms to indicate the site is overloaded and shedding requests.
• 530 Site Is Frozen / Cloudflare Error: Cloudflare pairs 530 with a specific 1xxx error code, and some platforms use it for a site frozen by the provider.
• 561 Unauthorized (Load Balancer): AWS load balancer code logged when an identity provider returned an error during authentication.
• 598 Network Read Timeout Error: Non-standard proxy code indicating a network read timeout behind the proxy.
• 599 Network Connect Timeout Error: Non-standard proxy code indicating the proxy could not establish a connection to the upstream in time.