List of HTTP header fields

List of HTTP header fields
HTTP
Persistence · Compression · HTTPS
Request methods
OPTIONS · GET · HEAD · POST · PUT · DELETE · TRACE · CONNECT
Header fields
Cookie · ETag · Location · Referer
DNT · X-Forwarded-For
Status codes
301 Moved permanently
302 Found
303 See Other
403 Forbidden
404 Not Found
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HTTP header fields are components of the message header of requests and responses in the Hypertext Transfer Protocol (HTTP). They define the operating parameters of an HTTP transaction.

The header fields are transmitted after the request or response line, 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 fields is indicated by an empty field, resulting in the transmission of two consecutive CR-LF pairs.

A core set of fields is standardized by the Internet Engineering Task Force (IETF) in RFC 2616 and other updates and extension documents (e.g., RFC 4229), and must be implemented by all HTTP-compliant protocol implementations. Additional field names and permissible values may be defined by each application.

A list of permanent[1] and provisional[2] http header fields is maintained by the IANA.

Many field values may contain a quality (q) key-value pair, specifying a statistical weight to use in content negotiation.[3]

Contents

Requests

Field name Description Example
Accept Content-Types that are acceptable Accept: text/plain
Accept-Charset Character sets that are acceptable Accept-Charset: utf-8
Accept-Encoding Acceptable encodings. See HTTP compression. Accept-Encoding: <compress | gzip | deflate | sdch | identity>
Accept-Language Acceptable languages for response Accept-Language: en-US
Authorization Authentication credentials for HTTP authentication Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
Cache-Control Used to specify directives that MUST be obeyed by all caching mechanisms along the request/response chain Cache-Control: no-cache
Connection What type of connection the user-agent would prefer Connection: close
Cookie an HTTP cookie previously sent by the server with Set-Cookie (below) Cookie: $Version=1; Skin=new;
Content-Length The length of the request body in octets (8-bit bytes) Content-Length: 348
Content-MD5 A Base64-encoded binary MD5 sum of the content of the request body Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==
Content-Type The mime type of the body of the request (used with POST and PUT requests) Content-Type: application/x-www-form-urlencoded
Date The date and time that the message was sent Date: Tue, 15 Nov 1994 08:12:31 GMT
Expect Indicates that particular server behaviors are required by the client Expect: 100-continue
From The email address of the user making the request From: user@example.com
Host The domain name of the server (for virtual hosting), mandatory since HTTP/1.1 Host: en.wikipedia.org
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"
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
If-None-Match Allows a 304 Not Modified to be returned if content is unchanged, see HTTP ETag If-None-Match: "737060cd8c284d8af7ad3082f209582d"
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"
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
Max-Forwards Limit the number of times the message can be forwarded through proxies or gateways. Max-Forwards: 10
Pragma Implementation-specific headers that may have various effects anywhere along the request-response chain. Pragma: no-cache
Proxy-Authorization Authorization credentials for connecting to a proxy. Proxy-Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
Range Request only part of an entity. Bytes are numbered from 0. Range: bytes=500-999
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” is misspelled in the RFC as well as in most implementations.) Referer: http://en.wikipedia.org/wiki/Main_Page
TE The transfer encodings the user agent is willing to accept: the same values as for the response header Transfer-Encoding can be used, plus the "trailers" value (related to the "chunked" transfer method) to notify the server it accepts to receive additional headers (the trailers) after the last, zero-sized, chunk. TE: trailers, deflate
Upgrade Ask the server to upgrade to another protocol. Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
User-Agent The user agent string of the user agent User-Agent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)
Via Informs the server of proxies through which the request was sent. Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
Warning A general warning about possible problems with the entity body. Warning: 199 Miscellaneous warning

Responses

Field name Description Example
Accept-Ranges What partial content range types this server supports Accept-Ranges: bytes
Age The age the object has been in a proxy cache in seconds Age: 12
Allow Valid actions for a specified resource. To be used for a 405 Method not allowed Allow: GET, HEAD
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
Connection Options that are desired for the connection[4] Connection: close
Content-Encoding The type of encoding used on the data. See HTTP compression. Content-Encoding: gzip
Content-Language The language the content is in Content-Language: da
Content-Length The length of the response body in octets (8-bit bytes) Content-Length: 348
Content-Location An alternate location for the returned data Content-Location: /index.htm
Content-MD5 A Base64-encoded binary MD5 sum of the content of the response Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==
Content-Disposition An opportunity to raise a "File Download" dialogue box for a known MIME type Content-Disposition: attachment; filename=fname.ext
Content-Range Where in a full body message this partial message belongs Content-Range: bytes 21010-47021/47022
Content-Type The mime type of this content Content-Type: text/html; charset=utf-8
Date The date and time that the message was sent Date: Tue, 15 Nov 1994 08:12:31 GMT
ETag An identifier for a specific version of a resource, often a message digest ETag: "737060cd8c284d8af7ad3082f209582d"
Expires Gives the date/time after which the response is considered stale Expires: Thu, 01 Dec 1994 16:00:00 GMT
Last-Modified The last modified date for the requested object, in RFC 2822 format Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
Link Used to express a typed relationship with another resource, where the relation type is defined by RFC 5988 Link: </feed>; rel="alternate"
Location Used in redirection, or when a new resource has been created. Location: http://www.w3.org/pub/WWW/People.html
P3P This header is supposed to set P3P policy, in the form of P3P:CP="your_compact_policy". However, P3P did not take off,[5] most browsers have never fully implemented it, a lot of websites set this header 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."
Pragma Implementation-specific headers that may have various effects anywhere along the request-response chain. Pragma: no-cache
Proxy-Authenticate Request authentication to access the proxy. Proxy-Authenticate: Basic
Refresh Used in redirection, or when a new resource has been created. This refresh redirects after 5 seconds. This is a proprietary, non-standard header extension introduced by Netscape and supported by most web browsers. Refresh: 5; url=http://www.w3.org/pub/WWW/People.html
Retry-After If an entity is temporarily unavailable, this instructs the client to try again after a specified period of time. Retry-After: 120
Server A name for the server Server: Apache/1.3.27 (Unix) (Red-Hat/Linux)
Set-Cookie an HTTP cookie Set-Cookie: UserID=JohnDoe; Max-Age=3600; Version=1
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
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
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
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: *
Via Informs the client of proxies through which the response was sent. Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
Warning A general warning about possible problems with the entity body. Warning: 199 Miscellaneous warning
WWW-Authenticate Indicates the authentication scheme that should be used to access the requested entity. WWW-Authenticate: Basic

Common non-standard request headers

Non-standard header fields are conventionally marked by prefixing the field name with X- .[6]

Field name Description Example
X-Requested-With[7] mainly used to identify Ajax requests. Most JavaScript frameworks send this header with value of XMLHttpRequest X-Requested-With: XMLHttpRequest
X-Do-Not-Track[8] Requests a web application to disable their tracking of a user. Note that, as of yet, this is largely ignored by web applications. It does however open the door to future legislation requiring web applications to comply with a user's request to not be tracked. Mozilla implements the DNT header with a similar purpose. X-Do-Not-Track: 1
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 (since Firefox 4.0 Beta 11). Safari and IE9 also have support for this header.[10] On March 7, 2011, a draft proposal was submitted to IETF.[11] DNT: 1

Common non-standard response headers

Non-standard header fields are conventionally marked by prefixing the field name with X- .

Field name Description Example
X-Frame-Options[12] Clickjacking protection: "deny" - no rendering within a frame, "sameorigin" - no rendering if origin mismatch X-Frame-Options: deny
X-XSS-Protection[13] Cross-site scripting (XSS) filter X-XSS-Protection: 1; mode=block
X-Content-Type-Options[14] the only defined value, "nosniff", prevents Internet Explorer from MIME-sniffing a response away from the declared content-type X-Content-Type-Options: nosniff
X-Forwarded-For[15] 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-Proto[16] a de facto standard for identifying the originating protocol of an HTTP request, since a reverse proxy (load balancer) communicates with a web server using HTTP X-Forwarded-Proto: https
X-Powered-By[17] specifies the technology (ASP.NET, PHP, JBoss, e.g.) supporting the web application (version details are often in X-Runtime, X-Version, or X-AspNet-Version) X-Powered-By: PHP/5.2.1

Effects of selected HTTP header fields

Avoiding caching

If a web server responds with Cache-Control: no-cache then a web browser or other caching system must not use the response to satisfy subsequent responses without first checking with the originating server. 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.

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 header field Cache-Control: no-store is intended to instruct a browser application to make a best effort not to write it to disk.

The Pragma: no-cache header field is an HTTP/1.0 header intended for use in requests. It is a means for the browser to tell the server and any intermediate caches that it wants a fresh version of the resource, not for the server to tell the browser not to cache the resource. Some user agents do pay attention to this header in responses, but the HTTP/1.1 RFC specifically warns against relying on this behavior.

See also

References

  1. ^ IANA Permanent Message Header Field Names
  2. ^ IANA Provisional Message Header Field Names
  3. ^ RFC 2616 §3.9
  4. ^ RFC 2616 §14.10
  5. ^ W3C P3P Work Suspended
  6. ^ Simtec Limited. "2. HTTP Headers". http://www.httpwatch.com/httpgallery/headers/. Retrieved 2010-09-10. 
  7. ^ Django Software Foundation. "Cross Site Request Forgery protection". http://docs.djangoproject.com/en/1.2/ref/contrib/csrf/. Retrieved 2009-09-10. 
  8. ^ "X-Do-Not-Track support in NoScript". http://hackademix.net/2010/12/28/x-do-not-track-support-in-noscript/. Retrieved 2010-12-28. 
  9. ^ "Try out the "Do Not Track" HTTP header". http://blog.sidstamm.com/2011/01/try-out-do-not-track-http-header.html. Retrieved 2011-01-31. 
  10. ^ "Web Tracking Protection: Minimum Standards and Opportunities to Innovate". http://blogs.msdn.com/b/ie/archive/2011/03/14/web-tracking-protection-minimum-standards-and-opportunities-to-innovate.aspx. Retrieved 2011-03-24. 
  11. ^ IETF Do Not Track: A Universal Third-Party Web Tracking Opt Out March 7, 2011
  12. ^ Eric Lawrence (2009-01-27). "IE8 Security Part VII: ClickJacking Defenses". http://blogs.msdn.com/ie/archive/2009/01/27/ie8-security-part-vii-clickjacking-defenses.aspx. Retrieved 2009-06-10. 
  13. ^ Eric Lawrence (2008-07-02). "IE8 Security Part IV: The XSS Filter". http://blogs.msdn.com/b/ie/archive/2008/07/02/ie8-security-part-iv-the-xss-filter.aspx. Retrieved 2010-09-30. 
  14. ^ Eric Lawrence (2008-09-03). "IE8 Security Part VI: Beta 2 Update". http://blogs.msdn.com/b/ie/archive/2008/09/02/ie8-security-part-vi-beta-2-update.aspx. Retrieved 2010-09-28. 
  15. ^ Amos Jeffries (2010-07-02). "SquidFaq/ConfiguringSquid - Squid Web Proxy Wiki". http://wiki.squid-cache.org/SquidFaq/ConfiguringSquid#head-3518b69c63e221cc3cd7885415e365ffaf3dd27f. Retrieved 2009-09-10. 
  16. ^ Dave Steinberg (2007-04-10). "How do I adjust my SSL site to work with GeekISP's loadbalancer?". http://www.geekisp.com/faq/6_65_en.html. Retrieved 2010-09-30. 
  17. ^ "Why does ASP.NET framework add the 'X-Powered-By:ASP.NET' HTTP Header in responses? - Stack Overflow". http://stackoverflow.com/questions/1288338/why-does-asp-net-framework-add-the-x-powered-byasp-net-http-header-in-response. Retrieved 2010-09-30. 

External links


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