Arequest header is anHTTP header that can be used in an HTTP request to provide information about the request context, so that the server can tailor the response. For example, theAccept-* headers indicate the allowed and preferred formats of the response. Other headers can be used to supply authentication credentials (e.g.,Authorization), to control caching, or to get information about the user agent or referrer, etc.

Not all headers that can appear in a request are referred to asrequest headers by the specification. For example, theContent-Type header is referred to as arepresentation header.

In addition,CORS defines a subset of request headers assimple headers, request headers that are always considered authorized and are not explicitly listed in responses topreflight requests.

The HTTP message below shows a few request headers after aGET request:

GET /home.html HTTP/1.1Host: developer.mozilla.orgUser-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:50.0) Gecko/20100101 Firefox/50.0Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8Accept-Language: en-US,en;q=0.5Accept-Encoding: gzip, deflate, brReferer: https://developer.mozilla.org/testpage.htmlConnection: keep-aliveUpgrade-Insecure-Requests: 1If-Modified-Since: Mon, 18 Jul 2016 02:36:04 GMTIf-None-Match: "c561c68d0ba92bbeb8b0fff2a9199f722e3a621a"Cache-Control: max-age=0

See also

• List of all HTTP headers
• RFC 9110, section 6.3: Header Fields