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This header field is part of HTTP version 1.1, and is ignored by some caches and browsers. It may be simulated by setting the Expires HTTP version 1.0 header field value to a time earlier than the response time. Notice that no-cache is not instructing the browser or proxies about whether or not to cache the content.
The response provided by a cache is stale (the content's age exceeds a maximum age set by a Cache-Control header or heuristically chosen lifetime). 111 Revalidation Failed The cache was unable to validate the response, due to an inability to reach the origin server. 112 Disconnected Operation
The HTTP Location header field is returned in responses from an HTTP server under two circumstances: To ask a web browser to load a different web page (URL redirection). In this circumstance, the Location header should be sent with an HTTP status code of 3xx. It is passed as part of the response by a web server when the requested URI has:
The same is usually true even for programmers writing CGI scripts, though some servers allow scripts to add custom headers (e.g. by enabling "non-parsed-headers"). Many web servers will generate a 3xx status code if a script outputs a "Location:" header line. For example, in PHP, one can use the "header" function:
Each response header field has a defined meaning which can be further refined by the semantics of the request method or response status code. HTTP/1.1 example of request / response transaction Below is a sample HTTP transaction between an HTTP/1.1 client and an HTTP/1.1 server running on www.example.com , port 80.
Cache the response and ETag, assuming there is an ETag and that the response was not aborted. For a subsequent request that would've included the If-None-Match header, do not send this header with perhaps a random 20% probability.
In computing, the User-Agent header is an HTTP header intended to identify the user agent responsible for making a given HTTP request. Whereas the character sequence User-Agent comprises the name of the header itself, the header value that a given user agent uses to identify itself is colloquially known as its user agent string .
The request/response message consists of the following: Request line, such as GET /logo.gif HTTP/1.1 or Status line, such as HTTP/1.1 200 OK, Headers; An empty line; Optional HTTP message body data; The request/status line and headers must all end with <CR><LF> (that is, a carriage return followed by a line feed).