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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
503 Service Unavailable The server is undergoing maintenance or is temporarily overloaded and so cannot process the request. A "Retry-After" header field may specify when the client may reattempt its request. [1]: §21.5.4 504 Server Time-out
421 Service not available, closing transmission channel (This may be a reply to any command if the service knows it must shut down) 432 4.7.12 A password transition is needed [ 3 ] 450 Requested mail action not taken: mailbox unavailable (e.g., mailbox busy or temporarily blocked for policy reasons)
The HTTP Upgrade mechanism is used to establish HTTP/2 starting from plain HTTP. [4] The client starts an HTTP/1.1 connection and sends an Upgrade: h2c header. If the server supports HTTP/2, it replies with HTTP 101 Switching Protocol status code. The HTTP Upgrade mechanism is used only for cleartext HTTP2 (h2c).
Under HTTP 1.0, connections should always be closed by the server after sending the response. [1]Since at least late 1995, [2] developers of popular products (browsers, web servers, etc.) using HTTP/1.0, started to add an unofficial extension (to the protocol) named "keep-alive" in order to allow the reuse of a connection for multiple requests/responses.
It is defined in the HTTP/1.1 specification by the World Wide Web Consortium (W3C). According to the specification, the 502 status code indicates that the server, while acting as a gateway or proxy, received an invalid response from an upstream server.
HTTP's use of three-digit codes is similar to the use of such codes in earlier protocols such as FTP and NNTP. At the HTTP level, a 404 response code is followed by a human-readable "reason phrase". At the HTTP level, a 404 response code is followed by a human-readable "reason phrase".
HTTP 403 is an HTTP status code meaning access to the requested resource is forbidden. The server understood the request, but will not fulfill it, if it was correct. The server understood the request, but will not fulfill it, if it was correct.