List of HTTP header fields

In the Hypertext Transfer Protocol (HTTP), HTTP header fields contain the operating parameters of an HTTP request or response. With the request or response line (first line of message), they form the message header.

The header fields define various characteristics of the data transfer that is requested or the data that is provided in the message body. Header fields start with the field name, terminated with a colon character, followed by the field value. Field names and values may be any application-specific strings, but 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 are commonly understood by all compliant protocol implementations.

Many parameters may be followed by a quality or 'q' value, giving a weight to use in content negotiation. The HTTP message header is separated from the message body by a blank line.

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	Accept-Encoding: <compress | gzip | deflate | 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-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
Referrer	This is the address of the previous web page from which a link to the currently requested page was followed.	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/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/4.0)
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	Cache-Control: max-age=3600
Content-Encoding	The type of encoding used on the data	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-Disposition	An opportunity to raise a "File Download" dialogue box for a known MIME type	Content-Disposition: attachment; filename=fname.ext
Content-MD5	A Base64-encoded binary MD5 sum of the content of the response	Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==
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, see ETag	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, 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
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 headers

Non-standard headers can be used, with the convention of prefixing the header 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	X-Content-Type-Options: nosniff
X-Requested-With	mainly used to identify Ajax requests. Most JavaScript frameworks send this header with value of XMLHttpRequest	X-Requested-With: XMLHttpRequest
X-Forwarded-For	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	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	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
X-Do-Not-Track	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	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). IE9 also has support for this header.. On March 7, 2011, the draft proposal is submitted to IETF.	DNT: 1

Effects of selected HTTP header fields

Avoiding caching

If the 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 is part of HTTP/1.1, and is ignored by some caches and browsers. To make sure that an object is not cached, it is therefore sensible to also set the Expires HTTP 1.0 header to a value earlier than the response date (e.g. -1). This is interpreted as an instruction not to cache by HTTP/1.0 caches and browsers.

The fact that a resource is not to 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 you use navigation buttons to go back to a previous page a browser may still show you a page that has been stored on disk in the history store, even if it has been instructed not to cache that page. This is correct behaviour according to the specification. Many user agents (including both Firefox and IE) will show different behaviour with regards to loading something from history store and loading something from its cache depending on whether the protocol is http or https.

In the rare event that you specifically do not want a resource to be stored to disk anywhere - perhaps the resource is highly sensitive and you don't want it to appear in backups or to be written to insecure permanent storage - you can use the header Cache-Control: no-store. This does not guarantee that the resource will not be written, but instructs the browser to make a best effort not to write it, or in the worst case, that it does not remain on disk.

The Pragma: no-cache header 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 (e.g. some versions of IE, but only when using https), but the HTTP/1.1 RFC specifically warns against relying on this behaviour.

Compression

If Internet Explorer (tested up to version 8) is set to use HTTP/1.0 rather than HTTP/1.1, it will not send an Accept-Encoding: header. It will, as a result, ignore any Content-Encoding headers that it sees, and will not decompress compressed content. Enabling HTTP/1.1 in Internet Explorer sends the Accept-Encoding: header, and decompresses the response correctly.

References

• RFC 4229: HTTP Header Field Registrations. December 2005 (contains a more complete list of HTTP headers)

External links

• IETF HTTP/1.1 RFC
• IETF HTTP State Management Mechanism RFC
• HTTP/1.1: Header Field Definitions
• HTTP/1.1 headers from a web server point of view
• HTTP Request Header Viewer
• HTTP Response Header Viewer - Retrieves the HTTP response headers of any domain.
• HTTP Header Viewer with Google App Engine
• Internet Explorer and Custom HTTP Headers - EricLaw's IEInternals - Site Home - MSDN Blogs