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{{Short description|None}}
{{
{{Use American English|date=June 2019}}
{{HTTP}}
'''HTTP header fields''' are a list of [[string (computer science)|string]]s sent and received by both the client program and server on every HTTP request and response. These [[Header (computing)|headers]] are usually invisible to the [[end-user]] and are only processed or [[computer data logging|logged]] by the server and client applications. They define how information sent/received through the connection are encoded (as in [[HTTP compression#Content-Encoding tokens|Content-Encoding]]), the session verification and identification of the client (as in [[browser cookies]], IP address, [[user-agent]]) or their anonymity thereof (VPN or proxy masking, user-agent spoofing), how the server should handle data (as in [[Do-Not-Track]] or [[Global Privacy Control]]), the age (the time it has resided in a shared [[web cache|cache]]) of the document being downloaded, amongst others.
==General format==
In HTTP version 1.x, header fields are transmitted after the request line (in case of a request HTTP message) or the response line (in case of a response HTTP message), which is the first line of a message. Header fields are colon-separated key-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 line, 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 was deprecated in RFC 7230.<ref>{{Cite ietf |date=June 2014 |rfc=7230 |title=Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing |section=3.2.4 |sectionname=Field Parsing}}</ref>
[[HTTP/2]]<ref name="rfc9113">{{cite ietf |rfc=9113 |title=HTTP/2 |date=June 2022}}</ref> and [[HTTP/3]] instead use a [[Communication protocol#Binary|binary protocol]], where headers are encoded in a single <code>HEADERS</code> and zero or more <code>CONTINUATION</code> frames using HPACK<ref name="rfc7541">{{cite journal|url=https://tools.ietf.org/html/rfc7541|title=HPACK: Header Compression for HTTP/2|publisher=[[IETF]]|date=May 2015|doi=10.17487/RFC7541 |access-date=2021-12-13|last1=Peon |first1=R. |last2=Ruellan |first2=H. |url-access=subscription}}</ref> (HTTP/2) or QPACK (HTTP/3), which both provide efficient header compression. The request or response line from HTTP/1 has also been replaced by several pseudo-header fields, each beginning with a colon (<code>:</code>).
A core set of fields is standardized by the [[Internet Engineering Task Force]] (IETF) in {{IETF RFC|9110|9111|leadout=and}}. The [https://www.iana.org/assignments/http-fields/http-fields.xhtml#field-names Field Names], [https://www.iana.org/assignments/message-headers/message-headers.xml#perm-headers Header Fields] and [https://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.
Header field names are case-insensitive.<ref>{{Cite ietf |date=June 2022 |rfc=9110 |title=HTTP Semantics |section=5.1 |sectionname=Field Names}}</ref> This is in contrast to HTTP method names (GET, POST, etc.), which are case-sensitive.<ref>{{Cite ietf |date=June 2022 |rfc=9110 |title=HTTP Semantics |section=9.1 |sectionname=Methods: Overview}}</ref>
[[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 journal|url=http://tools.ietf.org/html/rfc6648|title=RFC 6648|author=Internet Engineering Task Force|date=2012-06-01|doi=10.17487/RFC6648 |access-date=2012-11-12|url-access=subscription}}</ref> An earlier restriction on use of <code>Downgraded-</code> was lifted in March 2013.<ref>{{cite web|url=https://www.iana.org/assignments/message-headers/message-headers.xml |title=Message Headers |publisher=Iana.org |date=2014-06-11 |access-date=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 ietf |date=June 2022 |rfc=9110 |title=HTTP Semantics |section=5.6.5 |sectionname=Comments}}</ref>
Many field values may contain a quality (''q'') key-value pair separated by [[equals sign]], specifying a weight to use in [[content negotiation]].<ref>{{cite ietf |date=June 2022 |rfc=9110 |title=HTTP Semantics |section=12.4.2 |sectionname=Quality Values}}</ref> For example, a browser may indicate that it accepts information in German or English, with German as preferred by setting the ''q'' value for <code>de</code> higher than that of <code>en</code>, as follows:
<code>Accept-Language: de; q=1.0, en; q=0.5</code>
==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 8,190 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|archive-url=https://web.archive.org/web/20120509104709/https://httpd.apache.org/docs/2.3/mod/core.html#limitrequestfieldsize|archive-date=2012-05-09 |title=core - Apache HTTP Server |publisher=Httpd.apache.org |access-date=2012-03-13}}</ref>
==Request fields==
===Standard request fields===
{| class="wikitable sortable"
|+
|-
! Name
! class="unsortable" | Description
! class="unsortable" | Example
! Status
!Standard
|-
| A-IM || Acceptable instance-manipulations for the request.<ref name="rfc3229">{{cite IETF|rfc=3229}}</ref>||<code>A-IM: feed</code> || Permanent
|{{IETF RFC|3229}}
|-
|- id="accept-request-header"
| Accept || [[Media type|Media type(s)]] that is/are acceptable for the response. See [[Content negotiation]]. || <code>Accept: text/html</code> || Permanent
|{{IETF RFC|9110}}
|-
|
|{{IETF RFC|9110}}
|- id="Accept-Datetime"
| Accept-Datetime || Acceptable version in time. ||<code>Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT</code>|| Provisional
|{{IETF RFC|7089|}}
|- id="Accept-Encoding"
| Accept-Encoding || List of acceptable encodings. See [[HTTP compression]]. ||<code>Accept-Encoding: gzip, deflate</code>|| Permanent
|{{IETF RFC|9110}}
|- id="Accept-Language"
| Accept-Language || List of acceptable human languages for response. See [[Content negotiation]]. ||<code>Accept-Language: en-US</code>|| Permanent
|{{IETF RFC|9110}}
|- 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 |access-date=2017-07-24}}</ref>|| Initiates a request for [[cross-origin resource sharing]] with [[#origin-request-header|Origin]] (below). || <code>Access-Control-Request-Method: GET</code> || {{nowrap|Permanent: standard}}
|
|-
| Authorization || Authentication credentials for [[Basic access authentication|HTTP authentication]]. || <code>Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==</code> || Permanent
|{{IETF RFC|9110}}
|-
| [[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
|{{IETF RFC|9111}}
|-
|- id="connection-request-header"
| Connection || Control options for the current connection and list of hop-by-hop request fields.<ref name="rfc9110_connection">{{cite ietf
|rfc=9110
|title=HTTP Semantics
|section=7.6.1
|sectionname=Connection header
|date=June 2022
}}</ref>
Must not be used with HTTP/2.<ref name="rfc9113_connection">{{cite ietf
|rfc=9113
|title=HTTP/2
|section=8.2.2
|sectionname=Connection-Specific Header Fields
|date=June 2022
}}</ref>
|| <code>Connection: keep-alive</code>
<code>[[HTTP/1.1 Upgrade header|Connection: Upgrade]]</code>
|| Permanent
|{{IETF RFC|9110}}
|-
|- id="content-encoding-request-header"
| Content-Encoding || The type of encoding used on the data. See [[HTTP compression]]. || <code>Content-Encoding: gzip</code> || Permanent
|{{IETF RFC|9110}}
|- id="content-length-request-header"
| Content-Length || The length of the request body in [[Octet (computing)|octets]] (8-bit bytes). || <code>Content-Length: 348</code> || Permanent
|{{IETF RFC|9110}}
|-
| Content-MD5 || A [[Base64]]-encoded binary [[MD5]] sum of the content of the request body. || <code>Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==</code> || Obsolete<ref name="7231_appendixB">{{cite ietf |rfc=7231 |title=Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content |appendix=B |sectionname=Changes from RFC 2616 |date=June 2014}}</ref>
|{{IETF RFC|1544|1864|4021}}
|- id="content-type-request-header"
| Content-Type || The [[Media type]] of the body of the request (used with POST and PUT requests). || <code>Content-Type: application/x-www-form-urlencoded</code> || Permanent
|{{IETF RFC|9110}}
|-
| Cookie || An [[HTTP cookie]] previously sent by the server with <code>[[#innerlink set-cookie|Set-Cookie]]</code> (below). || <code>Cookie: $Version=1; Skin=new;</code> || Permanent: standard
|{{IETF RFC|2965|6265}}
|- id="Content-Length"
|-
| Date || The date and time at which the message was originated (in "HTTP-date" format as defined by [[rfc:9110#section-5.6.7|RFC 9110: HTTP Semantics, section 5.6.7 "Date/Time Formats"]]). || <code>Date: Tue, 15 Nov 1994 08:12:31 GMT</code> || Permanent
|{{IETF RFC|9110}}
|-
|
|{{IETF RFC|9110}}
|-
| Forwarded || Disclose original information of a client connecting to a web server through an HTTP proxy.<ref>{{cite journal
|url=http://tools.ietf.org/html/rfc7239#section-1
|title=Forwarded HTTP Extension: Introduction
|publisher=[[IETF]]
|date=June 2014
|doi=10.17487/RFC7239
|access-date=2016-01-07
|last1=Petersson
|first1=A.
|last2=Nilsson
|first2=M.
|url-access=subscription
}}
</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
|{{IETF RFC|7239|}}
|-
| From || The email address of the user making the request. || <code>From: user@example.com</code> || Permanent
|{{IETF RFC|9110}}
|-
|- id="host-request-header"
| 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 ietf
|rfc=9110
|title=HTTP Semantics
|section=7.2
|sectionname=Host and :authority
|date=June 2022
}}</ref>
If the request is generated directly in HTTP/2, it should not be used.<ref name="rfc9113_Request_Pseudo_Header_Fields">{{cite ietf
|rfc=9113
|title=HTTP/2
|section=8.3.1
|sectionname=Request Pseudo-Header Fields
|date=June 2022
}}</ref>
| colspan="1"| <code>Host: en.wikipedia.org:8080</code>
<code>Host: en.wikipedia.org</code>
|Permanent
|{{IETF RFC|9110|9113}}
|-
|HTTP2-Settings
|A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly one <code>HTTP2-Settings</code> header field. The <code>HTTP2-Settings</code> header field is a connection-specific header field that includes parameters that govern the HTTP/2 connection, provided in anticipation of the server accepting the request to upgrade.<ref>{{Cite web|url=https://www.iana.org/assignments/message-headers/message-headers.xml|title=Message Headers|website=www.iana.org|access-date=2018-11-26}}</ref><ref>{{Cite ietf |rfc=7540 |title=Hypertext Transfer Protocol Version 2 (HTTP/2) |section=3.2.1 |sectionname=HTTP2-Settings Header Field}}</ref>
|<code>HTTP2-Settings: token64</code>
|Obsolete
|{{IETF RFC|7540|9113}}
|-
| 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
|{{IETF RFC|9110}}
|-
| If-
|{{IETF RFC|9110}}
|-
| If-
|{{IETF RFC|9110}}
|-
| If-
|{{IETF RFC|9110}}
|-
|
|{{IETF RFC|9110}}
|-
| Max-Forwards || Limit the number of times the message can be forwarded through proxies or gateways. || <code>Max-Forwards: 10</code> || Permanent
|{{IETF RFC|9110}}
|- id="origin-request-header"
| Origin<ref name="CORS" /> || Initiates a request for [[cross-origin resource sharing]] (asks server for [[#access-control-response-headers|Access-Control-*]] response fields). || <code>Origin: <nowiki>http://www.example-social-network.com</nowiki></code> || Permanent: standard
|{{IETF RFC|6454}}
|-
| Pragma || Implementation-specific fields that may have various effects anywhere along the request-response chain. || <code>[[#Avoiding caching|Pragma: no-cache]]</code> || Outdated
|{{IETF RFC|9111}}
|-
|Prefer
|Allows client to request that certain behaviors be employed by a server while processing a request.
|<code>Prefer: return=representation</code>
|Permanent
|{{IETF RFC| 7240}}
|-
| Proxy-Authorization || Authorization credentials for connecting to a proxy. || <code>Proxy-Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==</code> || Permanent
|{{IETF RFC|9110}}
|- 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
|{{IETF RFC|9110}}
|-
| [[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
|{{IETF RFC|9110}}
|- 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="rfc9113_connection"/>
|| <code>TE: trailers, [[deflate]]</code> || Permanent
|{{IETF RFC|9110}}
|-
|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
|{{IETF RFC|9110}}
|-
|Transfer-Encoding
|The form of encoding used to safely transfer the entity to the user. [https://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="rfc9113_connection" />
|<code>Transfer-Encoding: chunked</code>
|Permanent
|{{IETF RFC|9110}}
|-
|- id="user-agent-request-header"
| [[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
|{{IETF RFC|9110}}
|-
| [[Upgrade header|Upgrade]] || Ask the server to upgrade to another protocol.
Must not be used in HTTP/2.<ref name="rfc9113_connection"/>
|| <code>Upgrade: h2c, HTTPS/1.3, IRC/6.9, RTA/x11, websocket</code> || Permanent
|{{IETF RFC|9110}}
|-
| 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
|{{IETF RFC|9110}}
|-
| Warning || A general warning about possible problems with the entity body. || <code>Warning: 199 Miscellaneous warning</code> || Obsolete<ref name="rfc9111_5.5">{{cite ietf |rfc=9111 |title=HTTP Caching |section=5.5 |sectionname=Warning header |date=June 2022}}</ref>
|{{IETF RFC|7234|9111}}
|}
{{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
|-
|{{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|access-date=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>
|| <code class="nowrap">Upgrade-Insecure-Requests: 1</code>
|- id="Requested-With"
| style=white-space:nowrap | X-Requested-With || Mainly used to identify [[Ajax (programming)|Ajax]] requests (most [[JavaScript framework]]s send this field with value of <code>XMLHttpRequest</code>); also identifies Android apps using WebView<ref>{{cite web | url=https://www.stoutner.com/the-x-requested-with-header/ | title=The "X-Requested-With" Header – Stoutner | date=October 31, 2022 }}</ref> || <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 |access-date=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 |access-date=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</code> (Do Not Track Enabled)
<code>DNT: 0</code> (Do Not Track Disabled)
|-
| 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 |access-date=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|access-date=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 |access-date=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 |access-date=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=https://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 |access-date=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 (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 |access-date=2012-01-14 |archive-url=https://web.archive.org/web/20120216021736/http://developer.att.com/developer/forward.jsp?passedItemId=5300270 |archive-date=2012-02-16}}</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 |access-date=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=https://jdebp.eu/FGA/web-proxy-connection-header.html |title=The Proxy-Connection: header is a mistake in how some web browsers use HTTP. |access-date=2018-01-16| last=de Boyne Pollard| first=Jonathan|year=2007 | archive-url=https://web.archive.org/web/20161023162007/https://jdebp.eu/FGA/web-proxy-connection-header.html | archive-date=2016-10-23 | url-status=dead}}</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="rfc9113_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]]|access-date=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|access-date=2014-01-19}}</ref><ref>{{cite news|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|newspaper=The Washington Post|access-date=2014-01-19}}</ref> || Server-side [[deep packet inspection]] 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]]|access-date=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)]]|access-date=2015-01-20|archive-url=https://web.archive.org/web/20150120134602/https://docs.djangoproject.com/en/1.7/ref/contrib/csrf/|archive-date=January 20, 2015|url-status=dead}}</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]]|access-date=2015-01-20}}</ref> || <code>X-Csrf-Token: i8XNjC4b8KVok4uw5RftR38Wgp2BFwql</code>
|-
|X-Request-ID,<ref name="stackoverflow2" group="stackoverflow2" /><ref>{{cite web|url=https://devcenter.heroku.com/articles/http-request-id|title=HTTP Request IDs|website=devcenter.heroku.com|access-date=2022-03-22}}</ref>
X-Correlation-ID,<ref>{{Cite news|url=https://blog.rapid7.com/2016/12/23/the-value-of-correlation-ids/|title=The Value of Correlation IDs|date=2016-12-23|work=Rapid7 Blog|access-date=2018-04-13|language=en}}</ref>
Correlation-ID<ref>{{Cite web|url=http://hilton.org.uk/blog/microservices-correlation-id|title=Correlation IDs for microservices architectures - Peter Hilton|last=Hilton|first=Peter|website=hilton.org.uk|date=July 12, 2017 |language=en|access-date=2018-04-13}}</ref>
|Correlates HTTP requests between a client and server. Superseded by the traceparent header
<ref>{{cite web|url=https://www.w3.org/TR/trace-context/|title=W3C Trace Context|website=w3c.org|access-date=2024-06-19}}</ref>
|<code class="nowrap">X-Request-ID: f058ebd6-02f7-4d3f-942e-904344e8cde5</code>
|-
|Save-Data<ref>{{cite web |url=https://wicg.github.io/savedata/#save-data-request-header-field |title=Save Data API Living Document Draft Community Group Report 2.1.1. Save-Data Request Header Field |author=<!--Not stated--> |date=30 June 2020 |website=Web Platform Incubator Community Group |access-date=2021-03-05}}</ref>
|The Save-Data client hint request header available in Chrome, Opera, and Yandex browsers lets developers deliver lighter, faster applications to users who opt-in to data saving mode in their browser.
|<code class="nowrap">Save-Data: on</code>
|-
|Sec-GPC<ref>{{cite web |url=https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Sec-GPC |title=Sec-GPC |author=((MDN contributors)) |date=3 Mar 2023 |website=MDN Web Docs |access-date=2023-03-12}}</ref>
|The Sec-GPC ([[Do Not Track#Global Privacy Controls|Global Privacy Control]]{{Broken anchor|date=2025-05-23|bot=User:Cewbot/log/20201008/configuration|target_link=Do Not Track#Global Privacy Controls|reason= The anchor (Global Privacy Controls) [[Special:Diff/1232726086|has been deleted]].|diff_id=1232726086}}) request header indicates whether the user consents to a website or service selling or sharing their personal information with third parties.
|<code class="nowrap">Sec-GPC: 1</code>
|}
==Response fields==
===Standard response fields===
{| class="wikitable sortable" style="width: 100%"
|-
! Field name
! class="unsortable" | Description
! class="unsortable" | Example
! Status
!Standard
|-
| Accept-
|{{IETF RFC|8942}}
|- 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}}
|{{IETF RFC|7480}}
|- id="accept-patch-response-header"
| Accept-Patch<ref>{{cite journal|url=http://tools.ietf.org/html/rfc5789#section-3.1 |title=RFC 5789 |year=2010 |doi=10.17487/RFC5789 |access-date=2014-12-24|last1=Dusseault |first1=L. |last2=Snell |first2=J. |s2cid=42062521 |url-access=subscription }}</ref> || Specifies which patch document formats this server supports || <code>Accept-Patch: text/example;charset=utf-8</code> || Permanent
|{{IETF RFC|5789}}
|- id="accept-ranges-response-header"
| Accept-Ranges || What partial content range types this server supports via [[byte serving]]|| <code>Accept-Ranges: bytes</code> || Permanent
|{{IETF RFC|9110}}
|-
| Age || The age the object has been in a [[proxy
|{{IETF RFC|9111}}
|-
| Allow || Valid
|{{IETF RFC|9110}}
|-
| Alt-Svc<ref>{{cite journal|url=https://tools.ietf.org/html/rfc7838|title=HTTP Alternative Services
|publisher=IETF|date=April 2016|doi=10.17487/RFC7838
|access-date=2016-04-19|last1=Nottingham
|first1=M.
|last2=McManus
|first2=P.
|last3=Reschke
|first3=J.
|doi-access=free
|url-access=subscription
}}</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 journal|url=https://tools.ietf.org/html/rfc7838#section-3|title=HTTP Alternative Services, section 3
|publisher=IETF|date=April 2016|doi=10.17487/RFC7838
|access-date=2017-06-08|last1=Nottingham
|first1=M.
|last2=McManus
|first2=P.
|last3=Reschke
|first3=J.
|doi-access=free
|url-access=subscription
}}</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
|{{IETF RFC|9111}}
|-
| Connection || Control options for the current connection and list of hop-by-hop response fields.<ref name=rfc9110_connection />
Must not be used with HTTP/2.<ref name="rfc9113_connection" />
|| <code>Connection: close</code> || Permanent
|{{IETF RFC|9110}}
|-
| Content-Disposition<ref>{{cite journal|url=http://tools.ietf.org/html/rfc6266 |title=RFC 6266 |year=2011 |doi=10.17487/RFC6266 |access-date=2015-03-13|last1=Reschke |first1=J. |doi-access=free |url-access=subscription }}</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
|{{IETF RFC|2616|4021|6266}}
|-
|- id="content-encoding-response-header"
| Content-Encoding || The type of encoding used on the data. See [[HTTP compression]]. || <code>Content-Encoding: gzip</code> || Permanent
|{{IETF RFC|9110}}
|-
| Content-Language || The natural language or languages of the intended audience for the enclosed content<ref>{{cite ietf |date=June 2022 |rfc=9110 |title=HTTP Semantics |section=8.5 |sectionname=Content-Language}}</ref> || <code>Content-Language: da</code> || Permanent
|{{IETF RFC|9110}}
|- 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
|{{IETF RFC|9110}}
|-
| Content-
|{{IETF RFC|9110}}
|-
| Content-
|{{IETF RFC|1544|1864|4021}}
|- 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
|{{IETF RFC|9110}}
|- id="content-type-response-header"
| Content-Type || The [[MIME type]] of this content || <code>Content-Type: text/html; charset=utf-8</code> || Permanent
|{{IETF RFC|9110}}
|-
| Date || The date and time that the message was sent (in "HTTP-date" format as defined by RFC 9110) || <code>Date: Tue, 15 Nov 1994 08:12:31 GMT</code> || Permanent
|{{IETF RFC|9110}}
|-
|
|{{IETF RFC|3229}}
|-
| [[HTTP ETag|ETag]] || An identifier for a specific version of a resource, often a [[
|{{IETF RFC|9110}}
|- id="Expires"
| Expires || Gives the date/time after which the response is considered stale (in "HTTP-date" format as defined by RFC 9110) || <code>Expires: Thu, 01 Dec 1994 16:00:00 GMT</code> || Permanent: standard
|{{IETF RFC|9111}}
|-
|
|{{IETF RFC|3229}}
|-
| Last-Modified || The last modified date for the requested object
|{{IETF RFC|9110}}
|-
| Link || Used to express a typed relationship with another resource, where the relation type is defined by
|{{IETF RFC|8288}}
|-
| [[HTTP ___location|Location]] || Used in [[URL redirection|redirection]], or when a new resource has been created.
| * Example 1: <code>Location: <nowiki>http://www.w3.org/pub/WWW/People.html</nowiki></code> * Example 2: <code>Location: <nowiki>/pub/WWW/People.html</nowiki></code>
|| Permanent
|{{IETF RFC|9110}}
|-
|
|
|-
| Pragma || Implementation-specific
|{{IETF RFC|9111}}
|-
|Preference-Applied
|Indicates which Prefer tokens were honored by the server and applied to the processing of the request.
|<code>Preference-Applied: return=representation</code>
|Permanent
|RFC 7240
|-
| Proxy-Authenticate || Request authentication to access the proxy. || <code>Proxy-Authenticate: Basic</code> || Permanent
|{{IETF RFC|9110}}
|- 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 | access-date=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
|{{IETF RFC|7469}}
|-
| Retry-After || If an entity is temporarily unavailable, this instructs the client to try again
|
* Example 1: <code>Retry-After: 120</code>
* Example 2: <code>Retry-After: Fri, 07 Nov 2014 23:59:59 GMT</code>
|
Permanent
|{{IETF RFC|9110}}
|-
| Server || A name for the server || <code>Server: Apache/
|{{IETF RFC|9110}}
|-
| id="innerlink_set-cookie" | Set-Cookie ||
|{{IETF RFC|6265}}
|-
| [[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
|{{IETF RFC|9110}}
|- id="transfer-encoding-response-header"
| Transfer-Encoding || The form of encoding used to safely transfer the entity to the user. [https://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="rfc9113_connection" />
|| <code>Transfer-Encoding: chunked</code> || Permanent
|{{IETF RFC|9110}}
|-
|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 in HTTP/2<ref name="rfc9113_connection" />
|| <code>Upgrade: h2c, HTTPS/1.3, IRC/6.9, RTA/x11, websocket</code> || Permanent
|{{IETF RFC|9110}}
|-
| Vary || 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
|{{IETF RFC|9110}}
|-
|
|{{IETF RFC|9110}}
|-
| Warning || A general warning about possible problems with the entity body. || <code>Warning: 199 Miscellaneous warning</code> || Obsolete<ref name="rfc9111_5.5" />
|{{IETF RFC|7234|9111}}
|- 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
|{{IETF RFC|9110}}
|- id="Frame-Options"
| X-Frame-Options<ref>{{cite journal|url=https://tools.ietf.org/html/rfc7034|title=HTTP Header Field X-Frame-Options |year=2013 |publisher=IETF |doi=10.17487/RFC7034 |access-date=2014-06-12|last1=Ross |first1=D. |last2=Gondrom |first2=T. |doi-access=free }}</ref> || [[Clickjacking]] protection: <code>deny</code> - no rendering within a frame, <code>sameorigin</code> - no rendering if origin mismatch, <code>allow-from</code> - allow from specified ___location, <code>allowall</code> - 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 |access-date=2014-08-02}}</ref>
|
|}
===Common non-standard
{| 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|access-date=28 April 2017}}</ref>
|[[Content Security Policy]] definition.
|<code class="nowrap">X-WebKit-CSP: default-src 'self'</code>
|-
| Expect-CT<ref>{{cite web|url=https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Expect-CT|title=Expect-CT|website=Mozilla Developer Network|language=en-US|access-date=2021-07-23}}</ref>
| Notify to prefer to enforce [[Certificate Transparency]].
|<code>Expect-CT: max-age=604800, enforce, report-uri="https://example.example/report"</code>
|-
| NEL<ref>{{cite web|url=https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/NEL/|title=NEL|website=Mozilla Developer Network|language=en-US|year=2021|access-date=2021-05-18}}</ref> || Used to configure network request logging. || {{code|2=yaml|1=NEL: { "report_to": "name_of_reporting_group", "max_age": 12345, "include_subdomains": false, "success_fraction": 0.0, "failure_fraction": 1.0 } }}
|-
| Permissions-Policy<ref>{{cite web|url=https://www.w3.org/TR/permissions-policy/|title=Permissions Policy | publisher=W3C|year=2020|access-date=2021-05-01}}</ref> || To allow or disable different features or APIs of the browser. || <code>Permissions-Policy: fullscreen=(), camera=(), microphone=(), geolocation=(), interest-cohort=()<ref>{{cite web|url=https://amifloced.org/|title=Am I FLoCed?|publisher=EFF|year=2021|access-date=2021-05-01}}</ref></code>
|- id="Refresh"
| Refresh || Tells the browser to [[Meta refresh|refresh]] the page or [[URL_redirection#Refresh_Meta_tag_and_HTTP_refresh_header|redirect]] to a different URL, after a given number of seconds (<code>0</code> meaning immediately); {{clarify |text=or when a new resource has been created|date=August 2024}}. Header introduced by Netscape in 1995 and became a de facto standard supported by most web browsers. Eventually standardized in the HTML Living Standard in 2017.<ref>{{cite web |url=https://github.com/whatwg/html/pull/2892 |title=Define the HTTP Refresh header by annevk · Pull Request #2892 · whatwg/html |date=2017-08-09 |website=GitHub |access-date=2021-04-17 |quote=}}</ref> || <code>Refresh: 5; url=<span class="plainlinks">http://www.w3.org/pub/WWW/People.html</span></code>
|-
| Report-To<ref>{{cite web|url=https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/report-to|title=CSP: report-to |website=Mozilla Developer Network|language=en-US|year=2021|access-date=2021-05-18}}</ref> || Instructs the user agent to store reporting endpoints for an origin. || {{code|2=yaml|1=Report-To: { "group": "csp-endpoint", "max_age": 10886400, "endpoints": [ { "url": "https-url-of-site-which-collects-reports" } ] } }}
|-
| 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 9110.<ref>
{{IETF RFC|9110|link=no}}: HTTP Semantics</ref>
| <code>Status: 200 OK </code>
|-
|Timing-Allow-Origin
|The <code>Timing-Allow-Origin</code> response header specifies origins that are allowed to see values of attributes retrieved via features of the [https://developer.mozilla.org/en-US/docs/Web/API/Resource_Timing_API Resource Timing API], which would otherwise be reported as zero due to cross-origin restrictions.<ref>{{Cite web|url=https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Timing-Allow-Origin|title=Timing-Allow-Origin|website=Mozilla Developer Network|language=en-US|access-date=2018-01-25}}</ref>
|<code>Timing-Allow-Origin: *</code>
<code>Timing-Allow-Origin: <origin>[, <origin>]*</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 |access-date=2015-01-03}}</ref> || Provide the duration of the audio or video in seconds. Not supported by current browsers – the header was only supported by Gecko browsers, from which support was removed in 2015.<ref>{{cite web|url=https://bugzilla.mozilla.org/show_bug.cgi?id=1160695|title=Clean up duration tracking and use mirroring for cross-thread access|website=Bugzilla@Mozilla|access-date=2024-02-09}}</ref> || <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 |access-date=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 |access-date=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 |url-status=live |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 name="stackoverflow1" group="stackoverflow1">{{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 |access-date=2022-03-20 |ref=stackoverflow1}}</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-Redirect-By<ref>{{cite web|url=https://webtechsurvey.com/response-header/x-redirect-by |title=X-Redirect-By HTTP response header |access-date=2021-05-29}}</ref>
|Specifies the component that is responsible for a particular redirect.
|<code class="nowrap">X-Redirect-By: WordPress</code><br><code class="nowrap">X-Redirect-By: Polylang</code>
|-
|X-Request-ID, X-Correlation-ID<ref name="stackoverflow2" group="stackoverflow2">{{Cite web |date= |title=What is the X-REQUEST-ID http header? |url=https://stackoverflow.com/questions/25433258/what-is-the-x-request-id-http-header |access-date=2022-03-20 |ref=stackoverflow2}}</ref>
|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 |access-date=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. In HTML Standard, only the <code>IE=edge</code> value is defined.<ref>{{cite web |url=https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-http-equiv-x-ua-compatible |title=HTML Living Standard 4.2.5.3 Pragma directives, X-UA-Compatible state |date=2021-03-12 |website=WHATWG |access-date=2021-03-14 |quote=For meta elements with an http-equiv attribute in the X-UA-Compatible state, the content attribute must have a value that is an ASCII case-insensitive match for the string<code>"IE=edge"</code>.}}</ref> || <code class="nowrap">X-UA-Compatible: IE=edge</code><br><code class="nowrap">X-UA-Compatible: IE=EmulateIE7</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 |access-date=2010-09-30}}</ref> || [[Cross-site scripting]] (XSS) filter || <code class="nowrap">X-XSS-Protection: 1; mode=block</code>
|}
==Effects of selected
===Avoiding caching===
If
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>
==
* [[HTTP header injection]]
* [[HTTP ETag]]
* [[List of HTTP status codes]]
==References==
{{
{{CCBYSASource
|sourcepath = https://stackoverflow.com/q/25433258
|sourcearticle = What is the X-REQUEST-ID http header?
|revision = 27174552
|author(s) = [https://stackoverflow.com/users/693140/stefan-k%c3%b6gl Stefan Kögl] at Stack Exchange }}
{{reflist|group=stackoverflow2}}
{{CCBYSASource
|sourcepath = https://stackoverflow.com/q/1288338
|sourcearticle = Why does ASP.NET framework add the 'X-Powered-By:ASP.NET' HTTP Header in responses?
|revision = 1288385
|author(s) = [https://stackoverflow.com/users/59301/adrian-grigore Adrian Grigore] at Stack Exchange }}
{{reflist|group=stackoverflow1}}
==External links==
* [https://www.iana.org/assignments/message-headers/message-headers.xml#perm-headers|Message Headers: Permanent Message Header Field Names]
*
* {{IETF RFC|9110|link=no}}: HTTP Semantics
* {{IETF RFC|9111|link=no}}: HTTP Caching
* {{IETF RFC|9112|link=no}}: HTTP/1.1
* {{IETF RFC|9113|link=no}}: HTTP/2
* {{IETF RFC|9114|link=no}}: HTTP/3
* {{IETF RFC|7239|link=no}}: Forwarded HTTP Extension
* {{IETF RFC|7240|link=no}}: Prefer Header for HTTP
* [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]
<references group="stackoverflow1" />
{{DEFAULTSORT:List Of Http Header Fields}}
[[Category:Hypertext Transfer Protocol headers| ]]
[[Category:Internet-related lists|HTTP header fields]]
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