List of HTTP header fields

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

General format[edit]

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 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. Historically, 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.[1]

Field names[edit]

A core set of fields is standardized by the Internet Engineering Task Force (IETF) in RFC 7231. An official registry of these fields as well as those of supplementary specifications is maintained by the IANA. Additional field names and permissible values may be defined by each application.

The permanent registry of header fields and repository of provisional registrations are maintained by the IANA.

Non-standard header fields were conventionally marked by prefixing the field name with X- .[2] However, this convention became deprecated in June 2012 due to the inconveniences it caused when non-standard fields became standard.[3]

A prior restriction on use of Downgraded- has also since been lifted.[4]

Field values[edit]

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, specifying a weight to use in content negotiation.[6]

Size limits[edit]

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.[7]

Request fields[edit]

Header field name Description Example Status
Accept Content-Types that are acceptable for the response Accept: text/plain Permanent
Accept-Charset Character sets that are acceptable Accept-Charset: utf-8 Permanent
Accept-Encoding List of acceptable encodings. See HTTP compression. Accept-Encoding: gzip, deflate Permanent
Accept-Language List of acceptable human languages for response Accept-Language: en-US Permanent
Accept-Datetime Acceptable version in time Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT Provisional
Authorization Authentication credentials for HTTP authentication Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ== Permanent
Cache-Control Used to specify directives that must be obeyed by all caching mechanisms along the request-response chain Cache-Control: no-cache Permanent
Connection What type of connection the user-agent would prefer Connection: keep-alive Permanent
Cookie An HTTP cookie previously sent by the server with Set-Cookie (below) Cookie: $Version=1; Skin=new; Permanent: standard
Content-Length The length of the request body in octets (8-bit bytes) Content-Length: 348 Permanent
Content-MD5 A Base64-encoded binary MD5 sum of the content of the request body Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ== Permanent
Content-Type The MIME type of the body of the request (used with POST and PUT requests) Content-Type: application/x-www-form-urlencoded Permanent
Date The date and time that the message was sent (in "HTTP-date" format as defined by RFC 7231) Date: Tue, 15 Nov 1994 08:12:31 GMT Permanent
Expect Indicates that particular server behaviors are required by the client Expect: 100-continue Permanent
From The email address of the user making the request From: user@example.com Permanent
Host The domain name of the server (for virtual hosting), and the TCP port number on which the server is listening. The port number may be omitted if the port is the standard port for the service requested.

[8] Mandatory since HTTP/1.1.

Host: en.wikipedia.org:80

Host: en.wikipedia.org

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. If-Match: "737060cd8c284d8af7ad3082f209582d" Permanent
If-Modified-Since Allows a 304 Not Modified to be returned if content is unchanged If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT Permanent
If-None-Match Allows a 304 Not Modified to be returned if content is unchanged, see HTTP ETag If-None-Match: "737060cd8c284d8af7ad3082f209582d" Permanent
If-Range If the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity If-Range: "737060cd8c284d8af7ad3082f209582d" Permanent
If-Unmodified-Since Only send the response if the entity has not been modified since a specific time. If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT Permanent
Max-Forwards Limit the number of times the message can be forwarded through proxies or gateways. Max-Forwards: 10 Permanent
Origin Initiates a request for cross-origin resource sharing (asks server for an 'Access-Control-Allow-Origin' response field) . Origin: http://www.example-social-network.com Permanent: standard
Pragma Implementation-specific fields that may have various effects anywhere along the request-response chain. Pragma: no-cache Permanent
Proxy-Authorization Authorization credentials for connecting to a proxy. Proxy-Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ== Permanent
Range Request only part of an entity. Bytes are numbered from 0. See Byte serving. Range: bytes=500-999 Permanent
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) Referer: http://en.wikipedia.org/wiki/Main_Page Permanent
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 method) to notify the server it expects to receive additional fields in the trailer after the last, zero-sized, chunk. TE: trailers, deflate Permanent
User-Agent The user agent string of the user agent User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20100101 Firefox/21.0 Permanent
Upgrade Ask the server to upgrade to another protocol. Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11 Permanent
Via Informs the server of proxies through which the request was sent. Via: 1.0 fred, 1.1 example.com (Apache/1.1) Permanent
Warning A general warning about possible problems with the entity body. Warning: 199 Miscellaneous warning Permanent

Common non-standard request fields[edit]

Field name Description Example
X-Requested-With mainly used to identify Ajax requests. Most JavaScript frameworks send this field with value of XMLHttpRequest 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 X-Forwarded-For: client1, proxy1, proxy2

X-Forwarded-For: 129.78.138.66, 129.78.64.103

X-Forwarded-Proto[14] a de facto standard for identifying the originating protocol of an HTTP request, since a reverse proxy (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. X-Forwarded-Proto: https
Front-End-Https[15] Non-standard header field used by Microsoft applications and load-balancers Front-End-Https: on
X-Http-Method-Override[16] Requests a web application override the method specified in the request (typically POST) with the method given in the header field (typically PUT or DELETE). Can be used when a user agent or firewall prevents PUT or DELETE methods from being sent directly (note that this 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[17] 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[18] 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[19] 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. Proxy-Connection: keep-alive

Response fields[edit]

Field name Description Example Status
Access-Control-Allow-Origin Specifying which web sites can participate in cross-origin resource sharing Access-Control-Allow-Origin: * Provisional
Accept-Ranges What partial content range types this server supports Accept-Ranges: bytes Permanent
Age The age the object has been in a proxy cache in seconds Age: 12 Permanent
Allow Valid actions for a specified resource. To be used for a 405 Method not allowed Allow: GET, HEAD 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
Connection Options that are desired for the connection[20] Connection: close Permanent
Content-Encoding The type of encoding used on the data. See HTTP compression. Content-Encoding: gzip Permanent
Content-Language The language the content is in Content-Language: da Permanent
Content-Length The length of the response body in octets (8-bit bytes) Content-Length: 348 Permanent
Content-Location An alternate location for the returned data Content-Location: /index.htm Permanent
Content-MD5 A Base64-encoded binary MD5 sum of the content of the response Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ== Permanent
Content-Disposition[21] 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
Content-Range Where in a full body message this partial message belongs Content-Range: bytes 21010-47021/47022 Permanent
Content-Type The MIME type of this content Content-Type: text/html; charset=utf-8 Permanent
Date The date and time that the message was sent (in "HTTP-date" format as defined by RFC 7231) Date: Tue, 15 Nov 1994 08:12:31 GMT Permanent
ETag An identifier for a specific version of a resource, often a message digest ETag: "737060cd8c284d8af7ad3082f209582d" Permanent
Expires Gives the date/time after which the response is considered stale Expires: Thu, 01 Dec 1994 16:00:00 GMT Permanent: standard
Last-Modified The last modified date for the requested object (in "HTTP-date" format as defined by RFC 7231) Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT Permanent
Link Used to express a typed relationship with another resource, where the relation type is defined by RFC 5988 Link: </feed>; rel="alternate"[22] Permanent
Location Used in redirection, or when a new resource has been created. Location: http://www.w3.org/pub/WWW/People.html Permanent
P3P This field is supposed to set P3P policy, in the form of P3P:CP="your_compact_policy". However, P3P did not take off,[23] 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 http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=151657 for more info." Permanent
Pragma Implementation-specific fields that may have various effects anywhere along the request-response chain. Pragma: no-cache Permanent
Proxy-Authenticate Request authentication to access the proxy. Proxy-Authenticate: Basic Permanent
Refresh Used in redirection, or when a new resource has been created. This refresh redirects after 5 seconds. Refresh: 5; url=http://www.w3.org/pub/WWW/People.html Proprietary and non-standard: a header extension introduced by Netscape and supported by most web browsers.
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.[24]
  • Example 1: Retry-After: 120
  • Example 2: Retry-After: Fri, 07 Nov 2014 23:59:59 GMT

Permanent

Server A name for the server Server: Apache/2.4.1 (Unix) Permanent
An HTTP cookie Set-Cookie: UserID=JohnDoe; Max-Age=3600; Version=1 Permanent: standard
Status CGI header field specifying the status of the HTTP response. Normal HTTP responses use a separate "Status-Line" instead, defined by RFC 7230.[25] Status: 200 OK Not listed as a registered field name
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
Transfer-Encoding The form of encoding used to safely transfer the entity to the user. Currently defined methods are: chunked, compress, deflate, gzip, identity. Transfer-Encoding: chunked Permanent
Upgrade Ask the client to upgrade to another protocol. Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11 Permanent
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. Vary: * Permanent
Via Informs the client of proxies through which the response was sent. Via: 1.0 fred, 1.1 example.com (Apache/1.1) Permanent
Warning A general warning about possible problems with the entity body. Warning: 199 Miscellaneous warning Permanent
WWW-Authenticate Indicates the authentication scheme that should be used to access the requested entity. WWW-Authenticate: Basic Permanent
X-Frame-Options[26] 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[27] X-Frame-Options: deny Obsolete[28]

Common non-standard response fields[edit]

Field name Description Example
Public-Key-Pins[29] Man-in-the-middle attack mitigation, announces hash of website's authentic TLS certificate Public-Key-Pins: max-age=2592000; pin-sha256="E9CZ9INDbd+2eRQozYqqbQ2yXLVKB9+xcprMF+44U1g=";
X-XSS-Protection[30] Cross-site scripting (XSS) filter X-XSS-Protection: 1; mode=block
Content-Security-Policy, X-Content-Security-Policy, X-WebKit-CSP[31] Content Security Policy definition. X-WebKit-CSP: default-src 'self'
X-Content-Type-Options[32] 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.[33] X-Content-Type-Options: nosniff
X-Powered-By[34] 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-UA-Compatible[35] 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. X-UA-Compatible: IE=EmulateIE7
X-UA-Compatible: IE=edge
X-UA-Compatible: Chrome=1

Effects of selected fields[edit]

Avoiding caching[edit]

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

See also[edit]

References[edit]

  1. ^ "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing". ietf.org. Retrieved 2014-07-23. 
  2. ^ Simtec Limited. "2. HTTP Headers". Retrieved 2010-09-10. 
  3. ^ Internet Engineering Task Force (2012-06-01). "RFC 6648". Retrieved 2012-11-12. 
  4. ^ "Message Headers". Iana.org. 2014-06-11. Retrieved 2014-06-12. 
  5. ^ "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing". itef.org. Retrieved 2014-07-24. 
  6. ^ "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content". ietf.org. Retrieved 2014-07-24. 
  7. ^ "core - Apache HTTP Server". Httpd.apache.org. Archived from the original on 2012-05-09. Retrieved 2012-03-13. 
  8. ^ "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing". IETF. June 2014. Retrieved 2014-07-24. 
  9. ^ "Try out the "Do Not Track" HTTP header". Retrieved 2011-01-31. 
  10. ^ "Web Tracking Protection: Minimum Standards and Opportunities to Innovate". Retrieved 2011-03-24. 
  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
  13. ^ Amos Jeffries (2010-07-02). "SquidFaq/ConfiguringSquid - Squid Web Proxy Wiki". Retrieved 2009-09-10. 
  14. ^ Dave Steinberg (2007-04-10). "How do I adjust my SSL site to work with GeekISP's loadbalancer?". Retrieved 2010-09-30. 
  15. ^ "Helping to Secure Communication: Client to Front-End Server". 2006-07-27. Retrieved 2012-04-23. 
  16. ^ "OpenSocial Core API Server Specification 2.5.1". Retrieved 2014-10-08. 
  17. ^ "ATT Device ID". Retrieved 2012-01-14. 
  18. ^ "WAP Profile". Retrieved 2012-01-14. 
  19. ^ "The Proxy-Connection: header is a mistake in how some web browsers use HTTP.". Retrieved 2014-01-02. 
  20. ^ "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing". ietf.org. Retrieved 2014-07-24. 
  21. ^ "RFC 2183". Retrieved 2012-05-17. 
  22. ^ Indicate the canonical version of a URL by responding with the Link rel="canonical" HTTP header Retrieved: 2012-02-09
  23. ^ W3C P3P Work Suspended
  24. ^ "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content". Retrieved 2014-07-24. 
  25. ^ "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing". Retrieved 2014-07-24. 
  26. ^ "HTTP Header Field X-Frame-Options". IETF. 2013. Retrieved 2014-06-12. 
  27. ^ "The mess with X-Frame-Options: ALLOWALL". IPSec.pl. Retrieved 1 March 2013. 
  28. ^ "Content Security Policy Level 2". Retrieved 2014-08-02. 
  29. ^ "Public key pins, a new safeguard for HTTPS websites". IPSec.pl. 2014. Retrieved 2014-06-12. 
  30. ^ Eric Lawrence (2008-07-02). "IE8 Security Part IV: The XSS Filter". Retrieved 2010-09-30. 
  31. ^ "Content Security Policy". W3C. 2012. Retrieved 2013. 
  32. ^ Eric Lawrence (2008-09-03). "IE8 Security Part VI: Beta 2 Update". Retrieved 2010-09-28. 
  33. ^ "Hosting - Google Chrome Extensions - Google Code". Retrieved 2012-06-14. 
  34. ^ "Why does ASP.NET framework add the 'X-Powered-By:ASP.NET' HTTP Header in responses? - Stack Overflow". Retrieved 2010-09-30. 
  35. ^ "Defining Document Compatibility: Specifying Document Compatibility Modes". 2011-04-01. Retrieved 2012-01-24. 
  36. ^ "Hypertext Transfer Protocol (HTTP/1.1): Caching". ietf.org. Retrieved 2014-07-24. 

External links[edit]