The server was acting as a gateway or proxy and received an invalid response from the upstream server.
The server was acting as a gateway or proxy and did not receive a timely response from the upstream server.
The server does not support the HTTP protocol version used in the request.
The server is unable to store the representation needed to complete the request.
A generic error message, given when an unexpected condition was encountered and no more specific message is suitable.
The server detected an infinite loop while processing the request.
The client needs to authenticate to gain network access. Intended for use by intercepting proxies used to control access to the network.
Further extensions to the request are required for the server to fulfil it.
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).
The server is currently unavailable (because it is overloaded or down for maintenance). Generally, this is a temporary state.
Transparent content negotiation for the request results in a circular reference.
The server failed to fulfil a request.
Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has encountered an error or is otherwise incapable of performing the request. Except when responding to a HEAD request, the server should include an entity containing an explanation of the error situation, and indicate whether it is a temporary or permanent condition. Likewise, user agents should display any included entity to the user. These response codes are applicable to any request method.