eturned if content is unchanged.  |  ` If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
If-None-Match  |  Allows a _304 Not Modified_ to be returned if content is unchanged, see [ HTTP ETag ](/wiki/HTTP_ETag "HTTP ETag") .  |  ` If-None-Match: "737060cd8c284d8af7ad3082f209582d" ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
If-Range  |  If the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity.  |  ` If-Range: "737060cd8c284d8af7ad3082f209582d" ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
If-Unmodified-Since  |  Only send the response if the entity has not been modified since a specific time.  |  ` If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
Max-Forwards  |  Limit the number of times the message can be forwarded through proxies or gateways.  |  ` Max-Forwards: 10 ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
Origin  [12]  |  Initiates a request for [ cross-origin resource sharing ](/wiki/Cross-origin_resource_sharing "Cross-origin resource sharing") (asks server for  Access-Control-*  response fields).  |  ` Origin: http://www.example-social-network.com ` |  Permanent: standard  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 6454 ](https://datatracker.ietf.org/doc/html/rfc6454)  
Pragma  |  Implementation-specific fields that may have various effects anywhere along the request-response chain.  |  ` Pragma: no-cache  ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9111 ](https://datatracker.ietf.org/doc/html/rfc9111)  
Prefer  |  Allows client to request that certain behaviors be employed by a server while processing a request.  |  ` Prefer: return=representation ` |  Permanent  |  RFC 7240   
Proxy-Authorization  |  Authorization credentials for connecting to a proxy.  |  ` Proxy-Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ== ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
Range  |  Request only part of an entity. Bytes are numbered from 0. See [ Byte serving ](/wiki/Byte_serving "Byte serving") .  |  ` Range: bytes=500-999 ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
[ Referer ](/wiki/HTTP_referer "HTTP referer") [ _[ sic ](/wiki/Sic "Sic") _ ]  |  This is the address of the previous web page from which a link to the currently requested page was followed. (The word "referrer" has been misspelled in the RFC as well as in most implementations to the point that it has become standard usage and is considered correct terminology)  |  ` Referer: http://en.wikipedia.org/wiki/Main_Page ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
TE  |  The transfer encodings the user agent is willing to accept: the same values as for the response header field Transfer-Encoding can be used, plus the "trailers" value (related to the " [ chunked ](/wiki/Chunked_transfer_encoding "Chunked transfer encoding") " transfer method) to notify the server it expects to receive additional fields in the trailer after the last, zero-sized, chunk.  Only ` trailers ` is supported in HTTP/2.  [14]  |  ` TE: trailers, [ deflate ](/wiki/Deflate "Deflate") ` |  Permanent  |  [ RFC ](/wiki/RFC_\(identifier\) "RFC \(identifier\)") [ 9110 ](https://datatracker.ietf.org/doc/html/rfc9110)  
Trailer  |  The Trai