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. 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.[1] Few fields can also contain comments (i.e. in. User-Agent, Server, Via fields), which can be ignored by software.[2]
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 registry 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.[3]
There are no limits to the size of each header field name or value, or the number of headers, in the standard itself. 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 each header size to 8190 bytes, and there can be at most 100 headers in single request.[4]
Requests
| Description | Example |
|---|
| Accept | Content-Types that are acceptable for the response | 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 human 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== |
| 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: 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 port number 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 |
| Origin | Initiates a request for cross-origin resource sharing (asks server for an 'Access-Control-Allow-Origin' response header) . | Origin: http://www.example-social-network.com |
| 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== |
| 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_Pag e |
| 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; Linux 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- .[9] However, this convention became deprecated in June 2012 due to the inconveniences it caused when non-standard headers then became standard.[10] For example, X-Gzip and Gzip are now both supported headers for compressed HTTP requests and responses.
| Description | Example |
|---|
| X-Requested-With | mainly used to identify Ajax requests. Most JavaScript frameworks send this header with value of XMLHttpRequest | X-Requested-With: XMLHttpRequest |
| DNT[11] | 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.[12] On March 7, 2011, a draft proposal was submitted to IETF.[13] The W3C Tracking Protection Working Group is producing a specification.[14] | DNT: 1 (Do Not Track Enabled)DNT: 0 (Do Not Track Disabled)
|
| 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, proxy2X-Forwarded-For: 129.78.138.66, 129.78.64.103
|
| X-Forwarded-Proto[16] | 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 | X-Forwarded-Proto: https |
| Front-End-Https[17] | Non-standard header used by Microsoft applications and load-balancers | Front-End-Https: on |
| 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: MakeModel/Firmware |
| 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/S GH-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 header. | Proxy-Connection: keep-alive |
Responses
| Description | Example |
|---|
| Access-Control-Allow-Origin | Specifying which web sites can participate in cross-origin resource sharing | Access-Control-Allow-Origin: * |
| 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-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[22][23][24] | 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-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 of P3P: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/account s/bin/answer.py?hl=en&answer=1516 57 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 |
| Status | The HTTP status of the response | Status: 200 OK |
| 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 |
| 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- .
| Description | Example |
|---|
| X-Frame-Options[27] | Clickjacking protection: "deny" - no rendering within a frame, "sameorigin" - no rendering if origin mismatch | X-Frame-Options: deny |
| X-XSS-Protection[28] | Cross-site scripting (XSS) filter | X-XSS-Protection: 1; mode=block |
| X-Content-Security-Policy, X-WebKit-CSP[29] | Content Security Policy definition. | X-WebKit-CSP: default-src 'self' |
| X-Content-Type-Options[30] | 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.[31] | X-Content-Type-Options: nosniff |
| X-Powered-By[32] | 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[33] | 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 |
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 responses without first checking with the originating server (this process is call 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 Pragma: no-cache and Cache-Control: no-cache header field is an HTTP/1.0 header intended for use in requests header also. 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. Pragma: no-cache is implementation specific, and 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
External links
