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List of HTTP header fields

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HTTP
Request methods
Header fields
Response status codes
Security access control methods
Security vulnerabilities

HTTP header fields are a list of strings sent and received by both the client program and server on every HTTP request and response. These headers are usually invisible to the end-user and are only processed or logged by the server and client applications. They define how information sent/received through the connection are encoded (as in 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 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 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 IETF|date=|access-|first2=H. }}</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 The request or response line from HTTP/1 has also been replaced by several pseudo-header fields, each beginning with a colon (:).

Field names

A core set of fields is standardized by the Internet Engineering Task Force (IETF) in RFC 9110 and 9111. The Field Names, Header Fields and Repository of Provisional Registrations are maintained by the IANA. Additional field names and permissible values may be defined by each application.

Header field names are case-insensitive.[1] This is in contrast to HTTP method names (GET, POST, etc.), which are case-sensitive.[2]

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

Non-standard header fields were conventionally marked by prefixing the field name with X- but this convention was deprecated in June 2012 because of the inconveniences it caused when non-standard fields became standard.[3] An earlier restriction on use of Downgraded- was lifted in March 2013.[4]

Field values

A few fields can contain comments (i.e. in User-Agent, Server, Via fields), which can be ignored by software.[5]

Many field values may contain a quality (q) key-value pair separated by equals sign, specifying a weight to use in content negotiation.[6] For example, a browser may indicate that it accepts information in German or English, with German as preferred by setting the q value for de higher than that of en, as follows:

Accept-Language: de; q=1.0, en; q=0.5

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 |title=core - Apache HTTP Server |publisher=Httpd.apache.org |access-date=2012-03-13}}</ref>

Request fields

Standard request fields

Name Description

Common non-standard request fields

Field name Description Example
Upgrade-Insecure-Requests[7] 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 Content-Security-Policy: upgrade-insecure-requests Upgrade-Insecure-Requests: 1
X-Requested-With Mainly used to identify Ajax requests (most JavaScript frameworks send this field with value of XMLHttpRequest); also identifies Android apps using WebView[8] X-Requested-With: XMLHttpRequest
DNT[9] 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 Firefox 4.0 Beta 11). Safari and IE9 also have support for this field.[10] On March 7, 2011, a draft proposal was submitted to IETF.[11] The W3C Tracking Protection Working Group is producing a specification.[12] DNT: 1 (Do Not Track Enabled)

DNT: 0 (Do Not Track Disabled)

X-Forwarded-For[13] A 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. X-Forwarded-For: client1, proxy1, proxy2

X-Forwarded-For: 129.78.138.66, 129.78.64.103

X-Forwarded-Host[14] A de facto standard for identifying the original host requested by the client in the Host 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. X-Forwarded-Host: en.wikipedia.org:8080

X-Forwarded-Host: en.wikipedia.org

X-Forwarded-Proto[15] A 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. X-Forwarded-Proto: https
Front-End-Https[16] Non-standard header field used by Microsoft applications and load-balancers Front-End-Https: on
X-Http-Method-Override[17] 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). X-HTTP-Method-Override: DELETE
X-ATT-DeviceId[18] Allows easier parsing of the MakeModel/Firmware that is usually found in the User-Agent String of AT&T Devices X-Att-Deviceid: GT-P7320/P7320XXLPG
X-Wap-Profile[19] 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. x-wap-profile: http://wap.samsungmobile.com/uaprof/SGH-I777.xml
Proxy-Connection[20] 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.[21]

Proxy-Connection: keep-alive
X-UIDH[22][23][24] Server-side deep packet inspection of a unique ID identifying customers of Verizon Wireless; also known as "perma-cookie" or "supercookie" X-UIDH: ...
X-Csrf-Token[25] Used to prevent cross-site request forgery. Alternative header names are: X-CSRFToken[26] and X-XSRF-TOKEN[27] X-Csrf-Token: i8XNjC4b8KVok4uw5RftR38Wgp2BFwql
X-Request-ID,[stackoverflow2 1][28]

X-Correlation-ID,[29] Correlation-ID[30]

Correlates HTTP requests between a client and server. Superseded by the traceparent header

[31]

X-Request-ID: f058ebd6-02f7-4d3f-942e-904344e8cde5
Save-Data[32] 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. Save-Data: on
Sec-GPC[33] The Sec-GPC (Global Privacy Control) request header indicates whether the user consents to a website or service selling or sharing their personal information with third parties. Sec-GPC: 1

Response fields

Standard response fields

Field name Description Example Status Standard
Accept-CH Requests HTTP Client Hints Accept-CH: UA, Platform Experimental RFC 8942
Access-Control-Allow-Origin,
Access-Control-Allow-Credentials,
Access-Control-Expose-Headers,
Access-Control-Max-Age,
Access-Control-Allow-Methods,
Access-Control-Allow-Headers[34] 		Specifying which web sites can participate in cross-origin resource sharing Access-Control-Allow-Origin: * Permanent: standard RFC 7480
Accept-Patch[35] Specifies which patch document formats this server supports Accept-Patch: text/example;charset=utf-8 Permanent RFC 5789
Accept-Ranges What partial content range types this server supports via byte serving Accept-Ranges: bytes Permanent RFC 9110
Age The age the object has been in a proxy cache in seconds Age: 12 Permanent RFC 9111
Allow Valid methods for a specified resource. To be used for a 405 Method not allowed Allow: GET, HEAD Permanent RFC 9110
Alt-Svc[36] 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.[37]

Alt-Svc: http/1.1="http2.example.com:8001"; ma=7200 Permanent
Cache-Control Tells all caching mechanisms from server to client whether they may cache this object. It is measured in seconds Cache-Control: max-age=3600 Permanent RFC 9111
Connection Control options for the current connection and list of hop-by-hop response fields.[38]

Must not be used with HTTP/2.[21]

Connection: close Permanent RFC 9110
Content-Disposition[39] 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. Content-Disposition: attachment; filename="fname.ext" Permanent RFC 2616, 4021, 6266
Content-Encoding The type of encoding used on the data. See HTTP compression. Content-Encoding: gzip Permanent RFC 9110
Content-Language The natural language or languages of the intended audience for the enclosed content[40] Content-Language: da Permanent RFC 9110
Content-Length The length of the response body in octets (8-bit bytes) Content-Length: 348 Permanent RFC 9110
Content-Location An alternate location for the returned data Content-Location: /index.htm Permanent RFC 9110
Content-MD5 A Base64-encoded binary MD5 sum of the content of the response Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ== Obsolete[41] RFC 1544, 1864, 4021
Content-Range Where in a full body message this partial message belongs Content-Range: bytes 21010-47021/47022 Permanent RFC 9110
Content-Type The MIME type of this content Content-Type: text/html; charset=utf-8 Permanent RFC 9110
Date The date and time that the message was sent (in "HTTP-date" format as defined by RFC 9110) Date: Tue, 15 Nov 1994 08:12:31 GMT Permanent RFC 9110
Delta-Base Specifies the delta-encoding entity tag of the response.[42] Delta-Base: "abc" Permanent RFC 3229
ETag An identifier for a specific version of a resource, often a message digest ETag: "737060cd8c284d8af7ad3082f209582d" Permanent RFC 9110
Expires Gives the date/time after which the response is considered stale (in "HTTP-date" format as defined by RFC 9110) Expires: Thu, 01 Dec 1994 16:00:00 GMT Permanent: standard RFC 9111
IM Instance-manipulations applied to the response.[42] IM: feed Permanent RFC 3229
Last-Modified The last modified date for the requested object (in "HTTP-date" format as defined by RFC 9110) Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT Permanent RFC 9110
Link Used to express a typed relationship with another resource, where the relation type is defined by RFC 8288 Link: </feed>; rel="alternate"[43] Permanent RFC 8288
Location Used in redirection, or when a new resource has been created.
  • Example 1: Location: http://www.w3.org/pub/WWW/People.html
  • Example 2: Location: /pub/WWW/People.html
 Permanent RFC 9110
P3P This field is supposed to set P3P policy, in the form of P3P:CP="your_compact_policy". However, P3P did not take off,[44] 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 third party cookies. P3P: CP="This is not a P3P policy! See https://en.wikipedia.org/wiki/Special:CentralAutoLogin/P3P for more info." Permanent
Pragma Implementation-specific fields that may have various effects anywhere along the request-response chain. Pragma: no-cache Permanent RFC 9111
Preference-Applied Indicates which Prefer tokens were honored by the server and applied to the processing of the request. Preference-Applied: return=representation Permanent RFC 7240
Proxy-Authenticate Request authentication to access the proxy. Proxy-Authenticate: Basic Permanent RFC 9110
Public-Key-Pins[45] HTTP Public Key Pinning, announces hash of website's authentic TLS certificate Public-Key-Pins: max-age=2592000; pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g="; Permanent RFC 7469
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.[46]
  • Example 1: Retry-After: 120
  • Example 2: Retry-After: Fri, 07 Nov 2014 23:59:59 GMT

Permanent

RFC 9110
Server A name for the server Server: Apache/2.4.1 (Unix) Permanent RFC 9110
An HTTP cookie Set-Cookie: CookieName=CookieValue; Max-Age=3600; Version=1 Permanent: standard RFC 6265
Strict-Transport-Security A HSTS Policy informing the HTTP client how long to cache the HTTPS only policy and whether this applies to subdomains. Strict-Transport-Security: max-age=16070400; includeSubDomains Permanent: standard
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. Trailer: Max-Forwards Permanent RFC 9110
Transfer-Encoding The form of encoding used to safely transfer the entity to the user. Currently defined methods are: chunked, compress, deflate, gzip, identity.

Must not be used with HTTP/2.[21]

Transfer-Encoding: chunked Permanent 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
Tk: ? Permanent
Upgrade Ask the client to upgrade to another protocol.

Must not be used in HTTP/2[21]

Upgrade: h2c, HTTPS/1.3, IRC/6.9, RTA/x11, websocket Permanent 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: Vary: *
  • Example 2: Vary: Accept-Language
 Permanent RFC 9110
Via Informs the client of proxies through which the response was sent. Via: 1.0 fred, 1.1 example.com (Apache/1.1) Permanent RFC 9110
Warning A general warning about possible problems with the entity body. Warning: 199 Miscellaneous warning Obsolete[47] RFC 7234, 9111
WWW-Authenticate Indicates the authentication scheme that should be used to access the requested entity. WWW-Authenticate: Basic Permanent RFC 9110
X-Frame-Options[48] Clickjacking protection: deny - no rendering within a frame, sameorigin - no rendering if origin mismatch, allow-from - allow from specified location, allowall - non-standard, allow from any location X-Frame-Options: deny Obsolete[49]

Common non-standard response fields

Field name Description Example
Content-Security-Policy,
X-Content-Security-Policy,
X-WebKit-CSP[50] 		Content Security Policy definition. X-WebKit-CSP: default-src 'self'
Expect-CT[51] Notify to prefer to enforce Certificate Transparency. Expect-CT: max-age=604800, enforce, report-uri="https://example.example/report"
NEL[52] Used to configure network request logging. NEL: { "report_to": "name_of_reporting_group", "max_age": 12345, "include_subdomains": false, "success_fraction": 0.0, "failure_fraction": 1.0 }
Permissions-Policy[53] To allow or disable different features or APIs of the browser. Permissions-Policy: fullscreen=(), camera=(), microphone=(), geolocation=(), interest-cohort=()[54]
Refresh Tells the browser to refresh the page or redirect to a different URL, after a given number of seconds (0 meaning immediately); or when a new resource has been created[clarification needed]. 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.[55] Refresh: 5; url=http://www.w3.org/pub/WWW/People.html
Report-To[56] Instructs the user agent to store reporting endpoints for an origin. Report-To: { "group": "csp-endpoint", "max_age": 10886400, "endpoints": [ { "url": "https-url-of-site-which-collects-reports" } ] }
Status CGI header field specifying the status of the HTTP response. Normal HTTP responses use a separate "Status-Line" instead, defined by RFC 9110.[57] Status: 200 OK
Timing-Allow-Origin The Timing-Allow-Origin response header specifies origins that are allowed to see values of attributes retrieved via features of the Resource Timing API, which would otherwise be reported as zero due to cross-origin restrictions.[58] Timing-Allow-Origin: *

Timing-Allow-Origin: <origin>[, <origin>]*

X-Content-Duration[59] 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.[60] X-Content-Duration: 42.666
X-Content-Type-Options[61] 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.[62] X-Content-Type-Options: nosniff[63]
X-Powered-By[stackoverflow1 1] Specifies the technology (e.g. ASP.NET, PHP, JBoss) supporting the web application (version details are often in X-Runtime, X-Version, or X-AspNet-Version) X-Powered-By: PHP/5.4.0
X-Redirect-By[64] Specifies the component that is responsible for a particular redirect. X-Redirect-By: WordPress
X-Redirect-By: Polylang
X-Request-ID, X-Correlation-ID[stackoverflow2 1] Correlates HTTP requests between a client and server. X-Request-ID: f058ebd6-02f7-4d3f-942e-904344e8cde5
X-UA-Compatible[65] 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 IE=edge value is defined.[66] X-UA-Compatible: IE=edge
X-UA-Compatible: IE=EmulateIE7
X-UA-Compatible: Chrome=1
X-XSS-Protection[67] Cross-site scripting (XSS) filter X-XSS-Protection: 1; mode=block

Effects of selected fields

Avoiding caching

If a web server responds with Cache-Control: no-cache then a web browser or other caching system (intermediate proxies) must not use the response to satisfy subsequent requests 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 Expires HTTP version 1.0 header field value to a time earlier than the response time. Notice that no-cache is not instructing the browser or proxies about whether or not to cache the content. 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 Cache-Control: max-age=0. This instructs the user agent that the content is stale and should be validated before use.

The header field Cache-Control: no-store 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 Cache-Control: no-cache 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 Pragma: no-cache 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.[68] The behavior of Pragma: no-cache in a response is implementation specific. While some user agents do pay attention to this field in responses,[69] the HTTP/1.1 RFC specifically warns against relying on this behavior.

See also

References

  1. ^ "Field Names". HTTP Semantics. June 2022. sec. 5.1. doi:10.17487/RFC9110. RFC 9110.
  2. ^ "Methods: Overview". HTTP Semantics. June 2022. sec. 9.1. doi:10.17487/RFC9110. RFC 9110.
  3. ^ Internet Engineering Task Force (June 1, 2012). "RFC 6648". doi:10.17487/RFC6648. Retrieved November 12, 2012. {{cite journal}}: Cite journal requires |journal= (help)
  4. ^ "Message Headers". Iana.org. June 11, 2014. Retrieved June 12, 2014.
  5. ^ "Comments". HTTP Semantics. June 2022. sec. 5.6.5. doi:10.17487/RFC9110. RFC 9110.
  6. ^ "Quality Values". HTTP Semantics. June 2022. sec. 12.4.2. doi:10.17487/RFC9110. RFC 9110.
  7. ^ "Upgrade Insecure Requests - W3C Candidate Recommendation". W3C. October 8, 2015. Retrieved January 14, 2016.
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  9. ^ "Try out the "Do Not Track" HTTP header". Retrieved January 31, 2011.
  10. ^ "Web Tracking Protection: Minimum Standards and Opportunities to Innovate". Retrieved March 24, 2011.
  11. ^ IETF Do Not Track: A Universal Third-Party Web Tracking Opt Out March 7, 2011
  12. ^ W3C Tracking Preference Expression (DNT), January 26, 2012
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  36. ^ Nottingham, M.; McManus, P.; Reschke, J. (April 2016). "HTTP Alternative Services". IETF. doi:10.17487/RFC7838. Retrieved April 19, 2016. {{cite journal}}: Cite journal requires |journal= (help)
  37. ^ Nottingham, M.; McManus, P.; Reschke, J. (April 2016). "HTTP Alternative Services, section 3". IETF. doi:10.17487/RFC7838. Retrieved June 8, 2017. {{cite journal}}: Cite journal requires |journal= (help)
  38. ^ Cite error: The named reference rfc9110_connection was invoked but never defined (see the help page).
  39. ^ Reschke, J. (2011). "RFC 6266". doi:10.17487/RFC6266. Retrieved March 13, 2015. {{cite journal}}: Cite journal requires |journal= (help)
  40. ^ "Content-Language". HTTP Semantics. June 2022. sec. 8.5. doi:10.17487/RFC9110. RFC 9110.
  41. ^ Cite error: The named reference 7231_appendixB was invoked but never defined (see the help page).
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  43. ^ Indicate the canonical version of a URL by responding with the Link rel="canonical" HTTP header Retrieved: 2012-02-09
  44. ^ W3C P3P Work Suspended
  45. ^ "Public Key Pinning Extension for HTTP". IETF. Retrieved April 17, 2015.
  46. ^ "Retry-After". HTTP Semantics. June 2022. sec. 10.2.3. doi:10.17487/RFC9110. RFC 9110.
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  48. ^ Ross, D.; Gondrom, T. (2013). "HTTP Header Field X-Frame-Options". IETF. doi:10.17487/RFC7034. Retrieved June 12, 2014. {{cite journal}}: Cite journal requires |journal= (help)
  49. ^ "Content Security Policy Level 2". Retrieved August 2, 2014.
  50. ^ "Content Security Policy". W3C. 2012. Retrieved April 28, 2017.
  51. ^ "Expect-CT". Mozilla Developer Network. Retrieved July 23, 2021.
  52. ^ "NEL". Mozilla Developer Network. 2021. Retrieved May 18, 2021.
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  56. ^ "CSP: report-to". Mozilla Developer Network. 2021. Retrieved May 18, 2021.
  57. ^ RFC 9110: HTTP Semantics
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  59. ^ "Configuring servers for Ogg media". May 26, 2014. Retrieved January 3, 2015.
  60. ^ "Clean up duration tracking and use mirroring for cross-thread access". Bugzilla@Mozilla. Retrieved February 9, 2024.
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  67. ^ Eric Lawrence (July 2, 2008). "IE8 Security Part IV: The XSS Filter". Retrieved September 30, 2010.
  68. ^ "Pragme". HTTP Caching. June 2022. sec. 5.4. doi:10.17487/RFC9111. RFC 9111.
  69. ^ "How to prevent caching in Internet Explorer". Microsoft. September 22, 2011. Retrieved April 15, 2015.

As of this edit, this article uses content from "What is the X-REQUEST-ID http header?", authored by Stefan Kögl at Stack Exchange, which is licensed in a way that permits reuse under the Creative Commons Attribution-ShareAlike 3.0 Unported License, but not under the GFDL. All relevant terms must be followed.

  1. ^ a b "What is the X-REQUEST-ID http header?". Retrieved March 20, 2022.

As of this edit, this article uses content from "Why does ASP.NET framework add the 'X-Powered-By:ASP.NET' HTTP Header in responses?", authored by Adrian Grigore at Stack Exchange, which is licensed in a way that permits reuse under the Creative Commons Attribution-ShareAlike 3.0 Unported License, but not under the GFDL. All relevant terms must be followed.

  1. ^ "Why does ASP.NET framework add the 'X-Powered-By:ASP.NET' HTTP Header in responses? - Stack Overflow". Retrieved March 20, 2022.


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