Voir la source de Utilisateur:Fblondiaux/Brouillon

{{HTTP}}
'''HTTP header fields''' are components of the header section of [[HTTP request|request]] and response messages in the [[Hypertext Transfer Protocol]] (HTTP). They define the operating parameters of an HTTP transaction.

== General format ==

The header fields are transmitted after the request or response line, which is the first line of a message. Header fields are colon-separated name-value pairs in clear-text [[String (computer science)|string]] format, terminated by a [[carriage return]] (CR) and [[line feed]] (LF) character sequence. The end of the header section is indicated by an empty field, resulting in the transmission of two consecutive CR-LF pairs. In the past, long lines could be folded into multiple lines; continuation lines are indicated by the presence of a space (SP) or horizontal tab (HT) as the first character on the next line. This folding is now deprecated<ref>{{cite web|url=http://tools.ietf.org/html/rfc7230#section-3.2.4|title=Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing|publisher=ietf.org|date=|accessdate=2014-07-23}}</ref>.

== Field names ==

A core set of fields is standardized by the [[Internet Engineering Task Force]] (IETF) in RFCs 7230, 7231, 7232, 7233, 7234, and 7235. The [http://www.iana.org/assignments/message-headers/message-headers.xml#perm-headers permanent registry of header fields] and [http://www.iana.org/assignments/message-headers/message-headers.xml#prov-headers repository of provisional registrations] are maintained by the [[Internet Assigned Numbers Authority|IANA]]. Additional field names and permissible values may be defined by each application.

[[HTTP/2]] makes some restrictions on specific header fields (see below).

Non-standard header fields were conventionally marked by prefixing the field name with <code>X-</code> but this convention was deprecated in June 2012 because of the inconveniences it caused when non-standard fields became standard<ref>{{cite web|url=http://tools.ietf.org/html/rfc6648|title=RFC 6648|author=Internet Engineering Task Force|date=2012-06-01|accessdate=2012-11-12}}</ref>. An earlier restriction on use of <code>Downgraded-</code> was  lifted in March 2013<ref>{{cite web|url=http://www.iana.org/assignments/message-headers/message-headers.xml|title=Message Headers|publisher=Iana.org|date=2014-06-11|accessdate=2014-06-12}}</ref>.

== Field values ==

A few fields can contain comments (i.e. in User-Agent, Server, Via fields), which can be ignored by software<ref>{{cite web|url=http://tools.ietf.org/html/rfc7230#section-3.2.6|title=Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing|publisher=itef.org|date=|accessdate=2014-07-24}}</ref>.

Many field values may contain a quality (''q'') key-value pair, specifying a weight to use in [[content negotiation]]<ref>{{cite web|url=http://tools.ietf.org/html/rfc7231#section-5.3.1|title=Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content|publisher=ietf.org|date=|accessdate=2014-07-24}}</ref>.

== Size limits ==

The standard imposes no limits to the size of each header field name or value, or to the number of fields. However, most servers, clients, and proxy software impose some limits for practical and security reasons. For example, the Apache 2.3 server by default limits the size of each field to 8190 bytes, and there can be at most 100 header fields in a single request<ref>{{cite web|url=http://httpd.apache.org/docs/2.3/mod/core.html#limitrequestfieldsize|archiveurl=https://web.archive.org/web/20120509104709/https://httpd.apache.org/docs/2.3/mod/core.html#limitrequestfieldsize|archivedate=2012-05-09|title=core - Apache HTTP Server|publisher=Httpd.apache.org|date=|accessdate=2012-03-13}}</ref>.

== Request fields ==
{| class="wikitable sortable"
|+
! Header field name
! class="unsortable" | Description
! class="unsortable" | Example
! Status
|-
| Accept 
| Type de réponses qui sont acceptées. 
| <code>Accept: text/plain</code> 
| Permanent
|-
| Accept-Charset 
| Character sets that are acceptable. 
| <code>Accept-Charset: utf-8</code> 
| Permanent
|- id="Accept-Encoding"
| Accept-Encoding 
| Liste des encodages supportés.  
| <code>Accept-Encoding: gzip, deflate</code> 
| Permanent
|- id="Accept-Language"
| Accept-Language 
| List of acceptable human languages for response. See [[Content negotiation]]. 
| <code>Accept-Language: en-US</code> 
| Permanent
|- id="Accept-Datetime"
| Accept-Datetime 
| Acceptable version in time. 
| <code>Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT</code> 
| Provisional
|- id="access-control-request-header"
| {{nowrap|Access-Control-Request-Method,<br />Access-Control-Request-Headers}}<ref name="CORS">{{cite web|url=https://www.w3.org/TR/cors/|title=Cross-Origin Resource Sharing|accessdate=2017-07-24}}</ref>
| Initiates a request for [[cross-origin resource sharing]] with [[Utilisateur:Fblondiaux/Brouillon#origin-request-header|Origin]] (below). 
| <code>Access-Control-Request-Method: GET</code> 
| {{nowrap|Permanent: standard}}
|-
| Authorization 
| Authentication credentials for [[HTTP authentication]]{{dn|date=November 2017}}. 
| <code>Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==</code> 
| Permanent
|-
| [[Cache-Control]] 
| Used to specify directives that ''must'' be obeyed by all caching mechanisms along the request-response chain. 
| <code>Cache-Control: no-cache</code> 
| Permanent
|-
| Connection 
| Control options for the current connection and list of hop-by-hop request fields<ref name="rfc7230_connection">{{cite web|url=http://tools.ietf.org/html/rfc7230#section-6.1|title=Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing|publisher=[[IETF]]|date=June 2014|accessdate=2014-12-19}}</ref>.
Must not be used with HTTP/2<ref name="rfc7540_connection">{{cite web|url=https://tools.ietf.org/html/rfc7540#section-8.1.2.2|title=Hypertext Transfer Protocol Version 2 (HTTP/2)|publisher=[[IETF]]|date=May 2015|accessdate=2017-06-06}}</ref>.
| <code>Connection: keep-alive</code>
<code>[[HTTP/1.1 Upgrade header|Connection: Upgrade]]</code> 
| Permanent
|-
| Cookie 
| An [[HTTP cookie]] previously sent by the server with <tt>[[Utilisateur:Fblondiaux/Brouillon#innerlink set-cookie|Set-Cookie]]</tt> (below). 
| <code>Cookie: $Version=1; Skin=new;</code> 
| Permanent: standard
|- id="Content-Length"
| Content-Length 
| The length of the request body in [[Octet (computing)|octets]] (8-bit bytes). 
| <code>Content-Length: 348</code> 
| Permanent
|-
| Content-MD5 
| A [[Base64]]-encoded binary [[MD5]] sum of the content of the request body. 
| <code>Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==</code> 
| Obsolete<ref name="tools.ietf.org">{{cite web|url=http://tools.ietf.org/html/rfc7231#appendix-B|title=Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content|accessdate=2015-06-03}}</ref>
|-
| Content-Type 
| The [[MIME type]] of the body of the request (used with POST and PUT requests). 
| <code>Content-Type: application/x-www-form-urlencoded</code> 
| Permanent
|-
| Date 
| The date and time that the message was originated (in "HTTP-date" format as defined by [[rfc:7231#section-7.1.1.1|RFC 7231 Date/Time Formats]]). 
| <code>Date: Tue, 15 Nov 1994 08:12:31 GMT</code> 
| Permanent
|-
| Expect 
| Indicates that particular server behaviors are required by the client. 
| <code>Expect: 100-continue</code> 
| Permanent
|-
| Forwarded 
| Disclose original information of a client connecting to a web server through an HTTP proxy<ref>{{cite web|url=http://tools.ietf.org/html/rfc7239#section-1|title=Forwarded HTTP Extension: Introduction|publisher=[[IETF]]|date=June 2014|accessdate=2016-01-07}}
</ref>.
| <code>Forwarded: for=192.0.2.60;proto=http;by=203.0.113.43</code> <code>Forwarded: for=192.0.2.43, for=198.51.100.17</code> 
| Permanent
|-
| From 
| The email address of the user making the request. 
| <code>From: user@example.com</code> 
| Permanent
|-
| Host 
| The domain name of the server (for [[virtual hosting]]), and the [[List of TCP and UDP port numbers|TCP port]] number on which the server is listening. The [[Port (computer networking)|port]] number may be omitted if the port is the standard port for the service requested.
Mandatory since HTTP/1.1<ref>{{cite web|url=http://tools.ietf.org/html/rfc7230#section-5.4|title=Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing|publisher=[[IETF]]|date=June 2014|accessdate=2014-07-24}}</ref>.
If the request is generated directly in HTTP/2, it should not be used<ref name="rfc7540_Request_Pseudo_Header_Fields">{{cite web|url=https://tools.ietf.org/html/rfc7540#section-8.1.2.3|title=Hypertext Transfer Protocol Version 2 (HTTP/2)|publisher=[[IETF]]|date=May 2015|accessdate=2017-06-06}}</ref>.
| <code>Host: en.wikipedia.org:8080</code>
<code>Host: en.wikipedia.org</code>
| Permanent
|-
| If-Match 
| Only perform the action if the client supplied entity matches the same entity on the server. This is mainly for methods like PUT to only update a resource if it has not been modified since the user last updated it. 
| <code>If-Match: "737060cd8c284d8af7ad3082f209582d"</code> 
| Permanent
|-
| If-Modified-Since 
| Allows a ''304 Not Modified'' to be returned if content is unchanged. 
| <code>If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT</code> 
| Permanent
|-
| If-None-Match 
| Allows a ''304 Not Modified'' to be returned if content is unchanged, see [[HTTP ETag]]. 
| <code>If-None-Match: "737060cd8c284d8af7ad3082f209582d"</code> 
| Permanent
|-
| If-Range 
| If the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity. 
| <code>If-Range: "737060cd8c284d8af7ad3082f209582d"</code> 
| Permanent
|-
| style="white-space:nowrap" | If-Unmodified-Since 
| Only send the response if the entity has not been modified since a specific time. 
| <code>If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT</code> 
| Permanent
|-
| Max-Forwards 
| Limit the number of times the message can be forwarded through proxies or gateways. 
| <code>Max-Forwards: 10</code> 
| Permanent
|- id="origin-request-header"
| Origin<ref name="CORS" /> 
| Initiates a request for [[cross-origin resource sharing]] (asks server for [[Utilisateur:Fblondiaux/Brouillon#access-control-response-headers|Access-Control-*]] response fields). 
| <code>Origin: <nowiki>http://www.example-social-network.com</nowiki></code> 
| Permanent: standard
|-
| Pragma 
| Champ pouvant avoir de nombreux effets, utile pour HTTP/1.0, on le conserve entre autres pour la compatibilité.  
| <code>[[Utilisateur:Fblondiaux/Brouillon#Avoiding caching|Pragma: no-cache]]</code> 
| Permanent
|-
| Proxy-Authorization 
| Authorization credentials for connecting to a proxy. 
| <code>Proxy-Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==</code> 
| Permanent
|- id="range-request-header"
| {{anchor|Range}} Range 
| Request only part of an entity.  Bytes are numbered from 0.  See [[Byte serving]]. 
| <code>Range: bytes=500-999</code> 
| Permanent
|-
| [[HTTP referer|Referer]]{{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) 
| <code><nowiki>Referer: http://en.wikipedia.org/wiki/Main_Page</nowiki></code> 
| Permanent
|- id="te-request-header"
| 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 transfer encoding|chunked]]" transfer method) to notify the server it expects to receive additional fields in the trailer after the last, zero-sized, chunk. 
Only <code>trailers</code> is supported in HTTP/2<ref name="rfc7540_connection" />.
| <code>TE: trailers, [[deflate]]</code> 
| Permanent
|-
| User-Agent 
| The [[user agent string]] of the user agent. 
| <code>User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20100101 Firefox/12.0</code> 
| Permanent
|-
| [[Upgrade header|Upgrade]] 
| Ask the server to upgrade to another protocol.
Must not be used to upgrade to HTTP/2<ref name="rfc7540_connection" />.
| <code>Upgrade: HTTPS/1.3, IRC/6.9, RTA/x11, websocket</code> 
| Permanent
|-
| Via 
| Informs the server of proxies through which the request was sent. 
| <code>Via: 1.0 fred, 1.1 example.com (Apache/1.1)</code> 
| Permanent
|-
| Warning 
| A general warning about possible problems with the entity body. 
| <code>Warning: 199 Miscellaneous warning</code> 
| Permanent
|}

{{anchor|Common non-standard request headers}}

=== Common non-standard request fields ===

{| class="wikitable sortable" style="width: 100%; clear:right;"
! Field name
! class="unsortable" | Description
! class="unsortable" | Example
|- id="Requested-With"
| style="white-space:nowrap" | X-Requested-With 
| Mainly used to identify [[Ajax (programming)|Ajax]] requests. Most [[JavaScript framework|JavaScript frameworks]] send this field with value of <code>XMLHttpRequest</code> 
| <code> X-Requested-With: XMLHttpRequest</code>
|-
| [[Do Not Track|DNT]]<ref>{{cite web|url=http://blog.sidstamm.com/2011/01/try-out-do-not-track-http-header.html|title=Try out the "Do Not Track" HTTP header|accessdate=2011-01-31}}</ref> 
| Requests a web application to disable their tracking of a user. This is Mozilla's version of the X-Do-Not-Track header field (since [[Mozilla Firefox 4|Firefox 4.0]] Beta 11).  [[Safari (web browser)|Safari]] and [[Internet Explorer 9|IE9]] also have support for this field<ref>{{cite web|url=http://blogs.msdn.com/b/ie/archive/2011/03/14/web-tracking-protection-minimum-standards-and-opportunities-to-innovate.aspx|title=Web Tracking Protection: Minimum Standards and Opportunities to Innovate|accessdate=2011-03-24}}</ref>.  On March 7, 2011, a draft proposal was submitted to IETF.<ref>IETF [http://tools.ietf.org/html/draft-mayer-do-not-track-00 Do Not Track: A Universal Third-Party Web Tracking Opt Out] March 7, 2011</ref> The [[World Wide Web Consortium|W3C]] Tracking Protection Working Group is producing a specification<ref>W3C [http://www.w3.org/2011/tracking-protection/drafts/tracking-dnt.html Tracking Preference Expression (DNT)], January 26, 2012</ref>.
| <code>DNT: 1    (Do Not Track Enabled)</code>
<code>DNT: 0    (Do Not Track Disabled)</code>
|-
| style="white-space:nowrap" | [[X-Forwarded-For]]<ref>{{cite web|url=http://wiki.squid-cache.org/SquidFaq/ConfiguringSquid#head-3518b69c63e221cc3cd7885415e365ffaf3dd27f|title=SquidFaq/ConfiguringSquid - Squid Web Proxy Wiki|author=Amos Jeffries|date=2010-07-02|accessdate=2009-09-10}}</ref> 
| A [[De facto standard|''de facto'' standard]] for identifying the originating IP address of a client connecting to a web server through an HTTP proxy or load balancer. Superseded by ''Forwarded'' header. 
| <code> X-Forwarded-For: client1, proxy1, proxy2</code>
<code>X-Forwarded-For: 129.78.138.66, 129.78.64.103</code>
|-
| X-Forwarded-Host<ref>{{cite web|url=http://httpd.apache.org/docs/2.2/mod/mod_proxy.html#x-headers|title=mod_proxy - Apache HTTP Server Version 2.2|author=The Apache Software Foundation|accessdate=2014-11-12}}</ref> 
| A [[De facto standard|''de facto'' standard]] for identifying the original host requested by the client in the <code>Host</code> HTTP request header, since the host name and/or port of the reverse proxy (load balancer) may differ from the origin server handling the request. Superseded by ''Forwarded'' header. 
| <code>X-Forwarded-Host: en.wikipedia.org:8080</code>
<code>X-Forwarded-Host: en.wikipedia.org</code>
|-
| X-Forwarded-Proto<ref>{{cite web|url=http://www.geekisp.com/faq/6_65_en.html|title=How do I adjust my SSL site to work with GeekISP's loadbalancer?|author=Dave Steinberg|date=2007-04-10|accessdate=2010-09-30}}</ref> 
| A [[De facto standard|''de facto'' standard]] for identifying the originating protocol of an HTTP request, since a reverse proxy (or a load balancer) may communicate with a web server using HTTP even if the request to the reverse proxy is HTTPS.  An alternative form of the header (X-ProxyUser-Ip) is used by Google clients talking to Google servers. Superseded by ''Forwarded'' header. 
| <code>X-Forwarded-Proto: https</code>
|-
| Front-End-Https<ref>{{cite web|url=https://technet.microsoft.com/en-us/library/aa997519(v=exchg.65).aspx|title=Helping to Secure Communication: Client to Front-End Server|date=2006-07-27|accessdate=2012-04-23}}</ref> 
| Non-standard header field used by Microsoft applications and load-balancers 
| <code>Front-End-Https: on</code>
|-
| X-Http-Method-Override<ref>{{cite web|url=http://opensocial.github.io/spec/2.5.1/Core-API-Server.xml#rfc.section.2.1.1.1|title=OpenSocial Core API Server Specification 2.5.1|accessdate=2014-10-08}}</ref> 
| Requests a web application to override the method specified in the request (typically POST) with the method given in the header field (typically PUT or DELETE). This can be used when a user agent or firewall prevents PUT or DELETE methods from being sent directly (note that this is either a bug in the software component, which ought to be fixed, or an intentional configuration, in which case bypassing it may be the wrong thing to do). 
| <code>X-HTTP-Method-Override: DELETE</code>
|-
| X-ATT-DeviceId<ref>{{cite web|url=http://developer.att.com/developer/forward.jsp?passedItemId=5300270|title=ATT Device ID|accessdate=2012-01-14}}</ref> 
| Allows easier parsing of the MakeModel/Firmware that is usually found in the User-Agent String of AT&T Devices
| <code>X-Att-Deviceid: GT-P7320/P7320XXLPG</code>
|-
| X-Wap-Profile<ref>{{cite web|url=http://www.developershome.com/wap/detection/detection.asp?page=profileHeader|title=WAP Profile|accessdate=2012-01-14}}</ref> 
| Links to an XML file on the Internet with a full description and details about the device currently connecting. In the example to the right is an XML file for an AT&T Samsung Galaxy S2.
| <code>x-wap-profile: http://wap.samsungmobile.com/uaprof/SGH-I777.xml</code>
|-
| Proxy-Connection<ref>{{cite web|url=http://homepage.ntlworld.com/jonathan.deboynepollard/FGA/web-proxy-connection-header.html|title=The Proxy-Connection: header is a mistake in how some web browsers use HTTP.|accessdate=2014-01-02}}</ref> 
| Implemented as a misunderstanding of the HTTP specifications. Common because of mistakes in implementations of early HTTP versions. Has exactly the same functionality as standard Connection field.
Must not be used with HTTP/2<ref name="rfc7540_connection" />.
| <code>Proxy-Connection: keep-alive</code>
|-
| X-UIDH<ref>{{cite web|url=https://www.eff.org/deeplinks/2014/11/verizon-x-uidh|title=Verizon Injecting Perma-Cookies to Track Mobile Customers, Bypassing Privacy Controls|publisher=[[Electronic Frontier Foundation]]|accessdate=2014-01-19}}</ref><ref>{{cite web|url=http://lessonslearned.org/sniff|title=Checking known AT&T, Verizon, Sprint, Bell Canada & Vodacom Unique Identifier beacons|accessdate=2014-01-19}}</ref><ref>{{cite web|url=https://www.washingtonpost.com/business/technology/verizon-atandt-tracking-their-users-with-super-cookies/2014/11/03/7bbbf382-6395-11e4-bb14-4cfea1e742d5_story.html|title=Verizon, AT&T tracking their users with 'supercookies'|author=Craig Timberg|publisher=[[The Washington Post]]|accessdate=2014-01-19}}</ref> 
| Server-side [[Deep packet inspection|deep packet insertion]] of a unique ID identifying customers of [[Verizon Wireless]]; also known as "perma-cookie" or "supercookie" 
| <code>X-UIDH: ...</code>
|-
| X-Csrf-Token<ref>{{cite web|url=https://help.sap.com/saphelp_nw74/helpdata/en/b3/5c22518bc72214e10000000a44176d/content.htm|title=SAP Cross-Site Request Forgery Protection|publisher=[[SAP SE]]|accessdate=2015-01-20}}</ref> 
| Used to prevent [[cross-site request forgery]]. Alternative header names are: <code>X-CSRFToken</code><ref>{{cite web|url=https://docs.djangoproject.com/en/1.7/ref/contrib/csrf/|title=Django Cross Site Request Forgery protection|publisher=[[Django (web framework)]]|accessdate=2015-01-20}}</ref> and <code>X-XSRF-TOKEN</code><ref>{{cite web|url=https://docs.angularjs.org/api/ng/service/$http#cross-site-request-forgery-xsrf-protection|title=Angular Cross Site Request Forgery (XSRF) Protection|publisher=[[AngularJS]]|accessdate=2015-01-20}}</ref> 
| <code>X-Csrf-Token: i8XNjC4b8KVok4uw5RftR38Wgp2BFwql</code>
|-
|X-Request-ID,
X-Correlation-ID<ref name="w0">{{Cite web|url=https://stackoverflow.com/questions/25433258/what-is-the-x-request-id-http-header|title=What is the X-REQUEST-ID http header?|website=stackoverflow.com|access-date=2016-05-19}}</ref>
|Correlates HTTP requests between a client and server.
|<code class="nowrap">X-Request-ID: f058ebd6-02f7-4d3f-942e-904344e8cde5</code>
|}

== Response fields ==
{| class="wikitable sortable" style="width: 100%"
! Field name
! class="unsortable" | Description
! class="unsortable" | Example
! Status
|- id="access-Control-response-headers"
| {{nowrap|Access-Control-Allow-Origin,<br/>Access-Control-Allow-Credentials,<br />Access-Control-Expose-Headers,<br />Access-Control-Max-Age,<br />Access-Control-Allow-Methods,<br />Access-Control-Allow-Headers}}<ref name="CORS" />
| Specifying which web sites can participate in [[cross-origin resource sharing]] 
| <code>Access-Control-Allow-Origin: *</code> 
| {{nowrap|Permanent: standard}}
|- id="accept-patch-response-header"
| Accept-Patch<ref>{{cite web|url=http://tools.ietf.org/html/rfc5789#section-3.1|title=RFC 5789|accessdate=2014-12-24}}</ref> 
| Specifies which patch document formats this server supports 
| <code>Accept-Patch: text/example;charset=utf-8</code> 
| Permanent
|- id="accept-ranges-response-header"
| Accept-Ranges 
| What partial content range types this server supports via [[byte serving]]
| <code>Accept-Ranges: bytes</code> 
| Permanent
|-
| Age 
| The age the object has been in a [[proxy cache]] in seconds 
| <code>Age: 12</code> 
| Permanent
|-
| Allow 
| Valid methods for a specified resource. To be used for a ''405 Method not allowed'' 
| <code>Allow: GET, HEAD</code> 
| Permanent
|-
| Alt-Svc<ref>{{cite web|url=https://tools.ietf.org/html/rfc7838|title=HTTP Alternative Services|publisher=IETF|date=April 2016|accessdate=2016-04-19}}</ref> 
| A server uses "Alt-Svc" header (meaning Alternative Services) to indicate that its resources can also be accessed at a different network location (host or port) or using a different protocol

When using HTTP/2, servers should instead send an ALTSVC frame<ref>{{cite web|url=https://tools.ietf.org/html/rfc7838#section-3|title=HTTP Alternative Services, section 3|publisher=IETF|date=April 2016|accessdate=2017-06-08}}</ref>.
| <code>Alt-Svc: http/1.1="http2.example.com:8001"; ma=7200</code> 
| Permanent
|-
| [[Cache-Control]] 
| Tells all caching mechanisms from server to client whether they may cache this object. It is measured in seconds
| <code>Cache-Control: max-age=3600</code> 
| Permanent
|-
| Connection 
| Control options for the current connection and list of hop-by-hop response fields<ref name="rfc7230_connection" />. 
Must not be used with HTTP/2<ref name="rfc7540_connection" />.
| <code>Connection: close</code> 
| Permanent
|-
| Content-Disposition<ref>{{cite web|url=http://tools.ietf.org/html/rfc6266|title=RFC 6266|accessdate=2015-03-13}}</ref>  
| An opportunity to raise a "File Download" dialogue box for a known MIME type with binary format or suggest a filename for dynamic content. Quotes are necessary with special characters. 
| <code>Content-Disposition: attachment; filename="fname.ext"</code> 
| Permanent
|-
| Content-Encoding 
| The type of encoding used on the data. See [[HTTP compression]]. 
| <code>Content-Encoding: gzip</code> 
| Permanent
|-
| Content-Language 
| The natural language or languages of the intended audience for the enclosed content<ref>https://tools.ietf.org/html/rfc7231#section-3.1.3.2</ref> 
| <code>Content-Language: da</code> 
| Permanent
|- id="content-length-response-header"
| Content-Length 
| The length of the response body in [[Octet (computing)|octets]] (8-bit bytes)
| <code>Content-Length: 348</code> 
| Permanent
|-
| Content-Location 
| An alternate location for the returned data 
| <code>Content-Location: /index.htm</code> 
| Permanent
|-
| Content-MD5 
| A [[Base64]]-encoded binary [[MD5]] sum of the content of the response 
| <code>Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==</code> 
| Obsolete<ref name="tools.ietf.org" />
|- id="content-range-response-header"
| Content-Range 
| Where in a full body message this partial message belongs 
| <code>Content-Range: bytes 21010-47021/47022</code> 
| Permanent
|-
| Content-Type 
| The [[MIME type]] of this content 
| <code>Content-Type: text/html; charset=utf-8</code> 
| Permanent
|-
| Date 
| The date and time that the message was sent (in "HTTP-date" format as defined by RFC 7231) <ref>{{cite web|url=http://ronwilliams.io/blog/post/rfc7231-compliant-http-date-headers|title=RFC7231 Compliant HTTP Date Headers}}</ref> 
| <code>Date: Tue, 15 Nov 1994 08:12:31 GMT</code> 
| Permanent
|-
| [[HTTP ETag|ETag]] 
| An identifier for a specific version of a resource, often a [[message digest]] 
| <code>ETag: "737060cd8c284d8af7ad3082f209582d"</code> 
| Permanent
|- id="Expires"
| Expires 
| Gives the date/time after which the response is considered stale (in "HTTP-date" format as defined by RFC 7231) 
| <code>Expires: Thu, 01 Dec 1994 16:00:00 GMT</code> 
| Permanent: standard
|-
| Last-Modified 
| The last modified date for the requested object (in "HTTP-date" format as defined by RFC 7231) 
| <code>Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT</code> 
| Permanent
|-
| Link 
| Used to express a typed relationship with another resource, where the relation type is defined by RFC 5988 
| <code>Link: &#x3C;/feed&#x3E;; rel="alternate"</code><ref name="google_canonical">[https://support.google.com/webmasters/bin/answer.py?hl=en&answer=139394 Indicate the canonical version of a URL by responding with the Link rel="canonical" HTTP header] Retrieved: 2012-02-09</ref> 
| Permanent
|-
| [[HTTP location|Location]] 
| Used in [[URL redirection|redirection]], or when a new resource has been created. 
| <code>Location: <nowiki>http://www.w3.org/pub/WWW/People.html</nowiki></code> 
| Permanent
|-
| P3P 
| This field is supposed to set [[P3P]] policy, in the form of <code>P3P:CP="your_compact_policy"</code>. However, P3P did not take off<ref>W3C [http://www.w3.org/P3P P3P Work Suspended]</ref>, most browsers have never fully implemented it, a lot of websites set this field with fake policy text, that was enough to fool browsers the existence of P3P policy and grant permissions for [[HTTP cookie#Third-party cookie|third party cookies]]. 
| <code>P3P: CP="This is not a P3P policy! See <nowiki>http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=151657</nowiki> for more info."</code> 
| Permanent
|-
| Pragma 
| Implementation-specific fields that may have various effects anywhere along the request-response chain. 
| <code>Pragma: no-cache</code> 
| Permanent
|-
| Proxy-Authenticate 
| Request authentication to access the proxy. 
| <code>Proxy-Authenticate: Basic</code> 
| Permanent
|- id="PKP"
| Public-Key-Pins<ref>{{cite web|url=http://www.rfc-editor.org/rfc/rfc7469.txt|title=Public Key Pinning Extension for HTTP|publisher=IETF|accessdate=17 April 2015}}</ref> 
| [[HTTP Public Key Pinning]], announces hash of website's authentic [[Transport Layer Security|TLS]] certificate 
| <code>Public-Key-Pins: max-age=2592000; pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";</code> 
| Permanent
|-
| Retry-After 
| If an entity is temporarily unavailable, this instructs the client to try again later. Value could be a specified period of time (in seconds) or a HTTP-date<ref>{{cite web|url=http://tools.ietf.org/html/rfc7231#section-7.1.3|title=Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content|accessdate=2014-07-24}}</ref>.
|
* Example 1: <code>Retry-After: 120</code>
* Example 2: <code>Retry-After: Fri, 07 Nov 2014 23:59:59 GMT</code>
|
Permanent
|-
| Server 
| A name for the server 
| <code>Server: Apache/2.4.1 (Unix)</code> 
| Permanent
|-
| id="innerlink_set-cookie" | Set-Cookie 
| An [[HTTP cookie]] 
| <code>Set-Cookie: UserID=JohnDoe; Max-Age=3600; Version=1</code> 
| Permanent: standard
|-
| [[HTTP Strict Transport Security|Strict-Transport-Security]] 
| A HSTS Policy informing the HTTP client how long to cache the HTTPS only policy and whether this applies to subdomains. 
| <code>Strict-Transport-Security: max-age=16070400; includeSubDomains</code> 
| Permanent: standard
|- id="trailer-response-header"
| Trailer 
| The Trailer general field value indicates that the given set of header fields is present in the trailer of a message encoded with [[chunked transfer coding]]. 
| <code>Trailer: Max-Forwards</code> 
| Permanent
|- id="transfer-encoding-response-header"
| Transfer-Encoding 
| The form of encoding used to safely transfer the entity to the user. [http://www.iana.org/assignments/http-parameters Currently defined methods] are: [[Chunked transfer encoding|chunked]], compress, deflate, gzip, identity. 
Must not be used with HTTP/2<ref name="rfc7540_connection" />.
| <code>Transfer-Encoding: chunked</code> 
| Permanent
|-
|Tk
|Tracking Status header, value suggested to be sent in response to a DNT(do-not-track), possible values:
 "!" — under construction
 "?" — dynamic
 "G" — gateway to multiple parties
 "N" — not tracking
 "T" — tracking
 "C" — tracking with consent
 "P" — tracking only if consented
 "D" — disregarding DNT
 "U" — updated
|<code>Tk: ?</code>
|Permanent
|-
| [[Upgrade header|Upgrade]] 
| Ask the client to upgrade to another protocol. 
Must not be used to upgrade to HTTP/2<ref name="rfc7540_connection" />
| <code>Upgrade: HTTPS/1.3, IRC/6.9, RTA/x11, websocket</code> 
| Permanent
|-
| Vary 
| Indique comment gerer les futures réponses. Permet de savoir si le cache peut-être utilisé ou s'il faut réaliser réaliser un nouvelle requête. Tells downstream proxies how to match future request headers to decide whether the cached response can be used rather than requesting a fresh one from the origin server. 
|
* Example 1: <code>Vary: *</code>
* Example 2: <code>Vary: Accept-Language</code>
| Permanent
|-
| Via 
| Informs the client of proxies through which the response was sent. 
| <code>Via: 1.0 fred, 1.1 example.com (Apache/1.1)</code> 
| Permanent
|-
| Warning 
| A general warning about possible problems with the entity body. 
| <code>Warning: 199 Miscellaneous warning</code> 
| Permanent
|- id="www-authenticate-response-header"
| WWW-Authenticate 
| Indicates the authentication scheme that should be used to access the requested entity. 
| <code>WWW-Authenticate: Basic</code> 
| Permanent
|- id="Frame-Options"
| X-Frame-Options<ref>{{cite web|url=https://tools.ietf.org/html/rfc7034|title=HTTP Header Field X-Frame-Options|year=2013|publisher=IETF|accessdate=2014-06-12}}</ref> 
| [[Clickjacking]] protection: <tt>deny</tt> - no rendering within a frame, <tt>sameorigin</tt> - no rendering if origin mismatch, <tt>allow-from</tt> - allow from specified location, <tt>allowall</tt> - non-standard, allow from any location 
| <code> X-Frame-Options: deny</code> 
| Obsolete<ref>{{cite web|url=http://www.w3.org/TR/CSP11/#frame-ancestors-and-frame-options|title=Content Security Policy Level 2|accessdate=2014-08-02}}</ref>
|}

=== Common non-standard response fields ===

{| class="wikitable sortable" style="width: 100%; clear:right;"
! Field name
! class="unsortable" | Description
! class="unsortable" | Example
|-
| Content-Security-Policy,<br>
X-Content-Security-Policy,<br>
X-WebKit-CSP<ref>{{cite web|url=http://www.w3.org/TR/CSP/|title=Content Security Policy|publisher=W3C|year=2012|accessdate=28 April 2017}}</ref>
|[[Content Security Policy]] definition.
|<code class="nowrap">X-WebKit-CSP: default-src 'self'</code>
|- id="Refresh"
| [[HTTP refresh|Refresh]] 
| Used in redirection, or when a new resource has been created.  This refresh redirects after 5 seconds. Header extension introduced by Netscape and supported by most web browsers.  
| <code>Refresh: 5; url=<nowiki>http://www.w3.org/pub/WWW/People.html</nowiki></code>
|-
| Status 
| [[Common Gateway Interface|CGI]] header field specifying the [[HTTP status|status]] of the HTTP response. Normal HTTP responses use a separate "Status-Line" instead, defined by RFC 7230<ref>{{cite web|url=http://tools.ietf.org/html/rfc7230#section-3.1.2|title=Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing|accessdate=2014-07-24}}</ref>. 
| <code>Status: 200 OK </code>
|-
|{{nowrap|Upgrade-Insecure-Requests}}<ref>{{cite web|title=Upgrade Insecure Requests - W3C Candidate Recommendation|url=https://www.w3.org/TR/upgrade-insecure-requests/#preference|website=W3C|accessdate=14 January 2016|date=8 October 2015}}</ref> 
| Tells a server which (presumably in the middle of a HTTP → HTTPS migration) hosts mixed content that the client would prefer redirection to HTTPS and can handle <code>Content-Security-Policy: upgrade-insecure-requests</code> 
Must not be used with HTTP/2<ref name="rfc7540_connection" />
| <code class="nowrap">Upgrade-Insecure-Requests: 1</code>
|-
|X-Content-Duration<ref>{{cite web|url=https://developer.mozilla.org/en-US/docs/Web/HTTP/Configuring_servers_for_Ogg_media#Serve_X-Content-Duration_headers|title=Configuring servers for Ogg media|date=2014-05-26|accessdate=2015-01-03}}</ref> 
| Provide the duration of the audio or video in seconds; only supported by Gecko browsers 
| <code class="nowrap">X-Content-Duration: 42.666</code>
|-
| X-Content-Type-Options<ref>{{cite web|url=http://blogs.msdn.com/b/ie/archive/2008/09/02/ie8-security-part-vi-beta-2-update.aspx|title=IE8 Security Part VI: Beta 2 Update|author=Eric Lawrence|date=2008-09-03|accessdate=2010-09-28}}</ref> 
| The only defined value, "nosniff", prevents [[Internet Explorer]] from MIME-sniffing a response away from the declared content-type. This also applies to [[Google Chrome]], when downloading extensions<ref>{{cite web|url=https://code.google.com/chrome/extensions/hosting.html|title=Hosting - Google Chrome Extensions - Google Code|accessdate=2012-06-14}}</ref>. 
| <code class="nowrap">X-Content-Type-Options: nosniff</code><ref name="whatwg-fetch-xcto">{{cite web|url=https://fetch.spec.whatwg.org/#x-content-type-options-header|title=Fetch standard|last1=van Kesteren|first1=Anne|date=2016-08-26|website=WHATWG|access-date=2016-08-26|dead-url=no|archive-url=https://web.archive.org/web/20160826132911/https://fetch.spec.whatwg.org/#x-content-type-options-header|archive-date=2016-08-26}}</ref>
|- id="Powered-By"
| X-Powered-By<ref>{{cite web|url=https://stackoverflow.com/questions/1288338/why-does-asp-net-framework-add-the-x-powered-byasp-net-http-header-in-response|title=Why does ASP.NET framework add the 'X-Powered-By:ASP.NET' HTTP Header in responses? - Stack Overflow|accessdate=2010-09-30}}</ref> 
| Specifies the technology (e.g. ASP.NET, PHP, JBoss) supporting the web application (version details are often in <code class="nowrap">X-Runtime</code>, <code class="nowrap">X-Version</code>, or <code class="nowrap">X-AspNet-Version</code>) 
| <code class="nowrap">X-Powered-By: PHP/5.4.0</code>
|-
|X-Request-ID,<br>
X-Correlation-ID<ref name="w0" />
|Correlates HTTP requests between a client and server.
|<code class="nowrap">X-Request-ID: f058ebd6-02f7-4d3f-942e-904344e8cde5</code>
|-
| X-UA-Compatible<ref>{{cite web|url=http://msdn.microsoft.com/en-us/library/ie/cc288325%28v=vs.85%29.aspx#SetMode|title=Defining Document Compatibility: Specifying Document Compatibility Modes|date=2011-04-01|accessdate=2012-01-24}}</ref> 
| Recommends the preferred rendering engine (often a backward-compatibility mode) to use to display the content. Also used to activate [[Chrome Frame]] in Internet Explorer. 
| <code class="nowrap">X-UA-Compatible: IE=EmulateIE7</code><br>
<code class="nowrap">X-UA-Compatible: IE=edge</code><br>
<code class="nowrap">X-UA-Compatible: Chrome=1</code>
|-
| X-XSS-Protection<ref>{{cite web|url=http://blogs.msdn.com/b/ie/archive/2008/07/02/ie8-security-part-iv-the-xss-filter.aspx|title=IE8 Security Part IV: The XSS Filter|author=Eric Lawrence|date=2008-07-02|accessdate=2010-09-30}}</ref> 
| [[Cross-site scripting]] (XSS) filter 
|  <code class="nowrap">X-XSS-Protection: 1; mode=block</code>
|}

== Effects of selected fields ==

=== Avoiding caching ===
If a web server responds with <code>Cache-Control: no-cache</code> then a web browser or other [[HTTP cache|caching system]] (intermediate proxies) must not use the response to satisfy subsequent responses without first checking with the originating server (this process is called validation).  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 <code>Expires</code> 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. It just tells the browser and proxies to validate the cache content with the server before using it (this is done by using If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match attributes mentioned above). Sending a no-cache value thus instructs a browser or proxy to not use the cache contents merely based on "freshness criteria" of the cache content. Another common way to prevent old content from being shown to the user without validation is <code>Cache-Control: max-age=0</code>. This instructs the user agent that the content is stale and should be validated before use.

The header field <code>Cache-Control: no-store</code> is intended to instruct a browser application to make a best effort not to write it to disk (i.e not to cache it).

The request that a resource should not be cached is no guarantee that it will not be written to disk.  In particular, the HTTP/1.1 definition draws a distinction between history stores and caches.  If the user navigates back to a previous page a browser may still show you a page that has been stored on disk in the history store. This is correct behavior according to the specification.  Many user agents show different behavior in loading pages from the history store or cache depending on whether the protocol is HTTP or HTTPS.

The <code>Cache-Control: no-cache</code> HTTP/1.1 header field is also intended for use in requests made by the client. It is a means for the browser to tell the server and any intermediate caches that it wants a fresh version of the resource.  The <code>Pragma: no-cache</code> header field, defined in the HTTP/1.0 spec, has the same purpose.  It, however, is only defined for the request header. Its meaning in a response header is not specified<ref>{{cite web|url=http://tools.ietf.org/html/rfc7234#section-5.4|title=Hypertext Transfer Protocol (HTTP/1.1): Caching|publisher=ietf.org|date=|accessdate=2014-07-24}}</ref>. The behavior of <code>Pragma: no-cache</code> in a response is implementation specific. While some user agents do pay attention to this field in responses<ref name="Q234067">{{cite web|url=https://support.microsoft.com/en-us/kb/234067/|title=How to prevent caching in Internet Explorer|publisher=[[Microsoft]]|date=2011-09-22|accessdate=2015-04-15}}</ref>, the HTTP/1.1 RFC specifically warns against relying on this behavior.

== See also ==
* [[HTTP header injection]]
* [[HTTP ETag]]
* [[List of HTTP status codes]]

== References ==
{{reflist|colwidth=30em}}

== External links ==
* [http://www.iana.org/assignments/message-headers/message-headers.xml#perm-headers%7CMessage Headers: Permanent Message Header Field Names]
* RFC 7230: Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
* RFC 7231: Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
* RFC 7232: Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests
* RFC 7233: Hypertext Transfer Protocol (HTTP/1.1): Range Requests
* RFC 7234: Hypertext Transfer Protocol (HTTP/1.1): Caching
* RFC 7235: Hypertext Transfer Protocol (HTTP/1.1): Authentication
* RFC 7239: Forwarded HTTP Extension
* RFC 2965: IETF HTTP State Management Mechanism RFC
* [http://www.and.org/texts/server-http HTTP/1.1 headers from a web server point of view]
* [http://blogs.msdn.com/b/ieinternals/archive/2009/06/30/internet-explorer-custom-http-headers.aspx Internet Explorer and Custom HTTP Headers - EricLaw's IEInternals - Site Home - MSDN Blogs]

{{DEFAULTSORT:List Of Http Header Fields}}
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