GIT – 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-textstring 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. Long lines can be folded into multiple lines; continuation lines are indicated by presence of space (SP) or horizontal tab (HT) as first character on next line. Few fields can also contain comments (i.e. in. User-Agent, Server, Via fields), which can be ignored by software.

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.

The permanent of headers and repository of provisional registrations are maintained by the IANA.

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

There is no limits to size of each header field name or value, or number of headers in standard itself. However, most servers, clients and proxy software impose some limits for practical and reasons. For example, Apache 2.3 server by default limits each header size to 8190 bytes, and there can be at most 100 headers in single request.

Contents

  • 1 Requests
    • 1.1 Common non-standard request headers
  • 2 Responses
    • 2.1 Common non-standard response headers
  • 3 Effects of selected HTTP header fields
    • 3.1 Avoiding caching
  • 4 See also
  • 5 References
  • 6 External links

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: gzip, deflate
Accept-Language Acceptable languages for response Accept-Language: en-US
Accept-Datetime Acceptable version in time Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT
Authorization Authentication credentials for HTTP authentication Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
-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: keep-alive
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: [email protected]
Host The domain name of the server (for virtual hosting), and the TCP port number on which the server is listening. The portnumber may be omitted if the port is the standard port for the service requested.[5] Mandatory since HTTP/1.1. Although domain name are specified as case-insensitive,[6][7] it is not specified whether the contents of the Host field should be interpreted in a case-insensitive manner[8] and in practice some implementations of virtual hosting interpret the contents of the Host field in a case-sensitive manner.[citation needed] Host: en.wikipedia.org:80Host: 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 expects 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 (X11; x86_64; rv:12.0) Gecko/20100101 Firefox/12.0
Via Informs the server of proxies through which the request was sent. Via: 1.0 fred, 1.1 example.com (Apache/1.1)
Warning A general warning about possible problems with the entity body. Warning: 199 Miscellaneous warning

Common non-standard request headers

Non-standard header fields were conventionally marked by prefixing the field name with X- .. However, this convention became deprecated in June 2012 due to the inconveniences it caused when non-standard headers then became standard . For example, X-Gzip and Gzip are now both supported headers for compressed HTTP requests and responses.

Field name Description Example
X-Requested-With mainly used to identify Ajax requests. Most JavaScript frameworks send this header with value ofXMLHttpRequest X-Requested-With: XMLHttpRequest
DNT[ Requests a web application to 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. On March 7, 2011, a draft proposal was submitted to IETF. The W3C Tracking Protection Working Group is producing a specification. DNT: 1 (Do Not Track Enabled)DNT: 0 (Do Not Track Disabled)
X-Forwarded-For 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, proxy2X-Forwarded-For: 129.78.138.66, 129.78.64.103
X-Forwarded-Proto 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 X-Forwarded-Proto: https
Front-End-Https Non-standard header used by Microsoft applications and load-balancers Front-End-Https: on
X-ATT-DeviceId Allows easier parsing of the MakeModel/Firmware that is usually found in the User-Agent String of AT&T Devices x-att-deviceid: MakeModel/Firmware
X-Wap-Profile 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 Galaxy S2. x-wap-profile:http://wap.samsungmobile.com/uaprof/SGH-I777.xml
Proxy-Connection 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 header. Proxy-Connection: keep-alive

Responses

Field name Description Example
Access-Control-Allow-Origin Specifying which web sites can participate in cross-origin resource sharing Access-Control-Allow-Origin: *
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[21] 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 with binary format or suggest a filename for dynamic content. Quotes are necessary with special characters. 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"[25]
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 ofP3P:CP="your_compact_policy". However, P3P did not take off,[26] 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 (seconds). Retry-After: 120
Server A name for the server Server: Apache/2.4.1 (Unix)
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 example.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 response headers

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

Field name Description Example
X-Frame-Options Clickjacking protection: “deny” – no rendering within a frame, “sameorigin” – no rendering if origin mismatch X-Frame-Options: deny
X-XSS-Protection Cross-site scripting (XSS) filter X-XSS-Protection: 1; mode=block
X-Content-Type-Options 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. X-Content-Type-Options: nosniff
X-Powered-By specifies the technology (e.g. ASP.NET, PHP, JBoss) supporting the web application (version details are often in X-RuntimeX-Version, or X-AspNet-Version) X-Powered-By: PHP/5.4.0
X-UA-Compatible Recommends the preferred rendering engine (often a backward-compatibility mode) to use to display the content. Also used to activateChrome Frame in Internet Explorer. X-UA-Compatible: IE=EmulateIE7
X-UA-Compatible: IE=edge
X-UA-Compatible: Chrome=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.

Source : Wikipedia.org

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