List of HTTP headers
Encyclopedia
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 and provisional 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.
Responses
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
The
Hypertext Transfer Protocol
The Hypertext Transfer Protocol is a networking protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web....
(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
String (computer science)
In formal languages, which are used in mathematical logic and theoretical computer science, a string is a finite sequence of symbols that are chosen from a set or alphabet....
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
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
(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 and provisional http header fields is maintained by the IANA
IANA
IANA is an initialism that may stand for a number of things:*the Internet Assigned Numbers Authority, an organisation that oversees IP address, Top-level domain and Internet protocol code point allocations*the Iranian Agriculture News Agency...
.
Many field values may contain a quality (q) key-value pair, specifying a statistical weight to use in content negotiation.
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 Http compression HTTP compression is a capability that can be built into web servers and web clients to make better use of available bandwidth , and provide faster transmission speeds between both... . |
Accept-Encoding: <compress Compress Compress is a UNIX compression program based on the LZC compression method, which is an LZW implementation using variable size pointers as in LZ78.- Description of program :Files compressed by compress are typically given the extension .Z... > gzip Gzip Gzip is any of several software applications used for file compression and decompression. The term usually refers to the GNU Project's implementation, "gzip" standing for GNU zip. It is based on the DEFLATE algorithm, which is a combination of Lempel-Ziv and Huffman coding... | deflate DEFLATE Deflate is a lossless data compression algorithm that uses a combination of the LZ77 algorithm and Huffman coding. It was originally defined by Phil Katz for version 2 of his PKZIP archiving tool and was later specified in RFC 1951.... | sdch Shared Dictionary Compression Over HTTP Shared Dictionary Compression over HTTP is a data compression method aimed to reduce redundancy across HTTP responses proposed by Google... | 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 HTTP cookie A cookie, also known as an HTTP cookie, web cookie, or browser cookie, is used for an origin website to send state information to a user's browser and for the browser to return the state information to the origin site... |
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 Octet (computing) An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:... (8-bit bytes) |
Content-Length: 348 |
| Content-MD5 | A Base64 Base64 Base64 is a group of similar encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation... -encoded binary MD5 MD5 The MD5 Message-Digest Algorithm is a widely used cryptographic hash function that produces a 128-bit hash value. Specified in RFC 1321, MD5 has been employed in a wide variety of security applications, and is also commonly used to check data integrity... 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 Virtual hosting Virtual hosting is a method for hosting multiple domain names on a server using a single IP address. This allows one server to share its resources, such as memory and processor cycles, in order to use its resources more efficiently.... ), 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 HTTP ETag An ETag, or entity tag, is part of HTTP, the protocol for the World Wide Web. It is one of several mechanisms that HTTP provides for cache validation, and which allows a client to make conditional requests. This allows caches to be more efficient, and saves bandwidth, as a web server does not... |
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.) | |
| 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 Chunked transfer encoding Chunked transfer encoding is a data transfer mechanism in version 1.1 of the Hypertext Transfer Protocol in which a web server serves content in a series of chunks. It uses the Transfer-Encoding HTTP response header in place of the Content-Length header, which the protocol would otherwise require... " 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 | Connection: close |
| Content-Encoding | The type of encoding used on the data. See HTTP compression Http compression HTTP compression is a capability that can be built into web servers and web clients to make better use of available bandwidth , and provide faster transmission speeds between both... . |
Content-Encoding: gzip |
| Content-Language | The language the content is in | Content-Language: da |
| Content-Length | The length of the response body in octets Octet (computing) An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:... (8-bit bytes) |
Content-Length: 348 |
| Content-Location | An alternate location for the returned data | Content-Location: /index.htm |
| Content-MD5 | A Base64 Base64 Base64 is a group of similar encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation... -encoded binary MD5 MD5 The MD5 Message-Digest Algorithm is a widely used cryptographic hash function that produces a 128-bit hash value. Specified in RFC 1321, MD5 has been employed in a wide variety of security applications, and is also commonly used to check data integrity... 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 HTTP ETag An ETag, or entity tag, is part of HTTP, the protocol for the World Wide Web. It is one of several mechanisms that HTTP provides for cache validation, and which allows a client to make conditional requests. This allows caches to be more efficient, and saves bandwidth, as a web server does not... |
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 HTTP location The HTTP Location header is returned in responses from an HTTP server under two circumstances:1. To ask a web browser to load a different web page. It is passed as part of the response by a web server when the requested URI has:* Moved temporarily, or... |
Used in redirection, or when a new resource has been created. | Location: |
| P3P | This header is supposed to set P3P P3P The Platform for Privacy Preferences Project, or P3P, is a protocol allowing websites to declare their intended use of information they collect about browsing users... 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 |
| 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= |
| 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 HTTP cookie A cookie, also known as an HTTP cookie, web cookie, or browser cookie, is used for an origin website to send state information to a user's browser and for the browser to return the state information to the origin site... |
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 Chunked transfer encoding Chunked transfer encoding is a data transfer mechanism in version 1.1 of the Hypertext Transfer Protocol in which a web server serves content in a series of chunks. It uses the Transfer-Encoding HTTP response header in place of the Content-Length header, which the protocol would otherwise require... . |
Trailer: Max-Forwards |
| Transfer-Encoding | The form of encoding used to safely transfer the entity to the user. Currently defined methods are: chunked Chunked transfer encoding Chunked transfer encoding is a data transfer mechanism in version 1.1 of the Hypertext Transfer Protocol in which a web server serves content in a series of chunks. It uses the Transfer-Encoding HTTP response header in place of the Content-Length header, which the protocol would otherwise require... , 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 withX- .
| Field name | Description | Example |
|---|---|---|
| X-Requested-With | mainly used to identify Ajax Ajax (programming) Ajax is a group of interrelated web development methods used on the client-side to create asynchronous web applications... requests. Most JavaScript frameworks send this header with value of XMLHttpRequest |
X-Requested-With: XMLHttpRequest |
| 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 Mozilla Firefox 4 Mozilla Firefox 4 is a version of the Firefox web browser, released on 22 March 2011. The first beta was made available on 6 July 2010; Release Candidate 2 was released on 18 March 2011. It was codenamed Tumucumaque, and has been confirmed as Firefox's last large release cycle... Beta 11). Safari Safari (web browser) Safari is a web browser developed by Apple Inc. and included with the Mac OS X and iOS operating systems. First released as a public beta on January 7, 2003 on the company's Mac OS X operating system, it became Apple's default browser beginning with Mac OS X v10.3 "Panther". Safari is also the... and IE9 Internet Explorer 9 Windows Internet Explorer 9 is the current version of the Internet Explorer web browser from Microsoft. It was released to the public on March 14, 2011 at 21:00 PDT. Internet Explorer 9 supports several CSS 3 properties, embedded ICC v2 or v4 color profiles support via Windows Color System, and... also have support for this header. On March 7, 2011, a draft proposal was submitted to IETF. |
DNT: 1 |
| X-Forwarded-For X-Forwarded-For The X-Forwarded-For HTTP header field is 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. This is an HTTP request header which was introduced by the Squid caching proxy server's developers... |
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 |
Common non-standard response headers
Non-standard header fields are conventionally marked by prefixing the field name withX- .
| Field name | Description | Example |
|---|---|---|
| X-Frame-Options | Clickjacking Clickjacking Clickjacking is a malicious technique of tricking Web users into revealing confidential information or taking control of their computer while clicking on seemingly innocuous web pages... protection: "deny" - no rendering within a frame, "sameorigin" - no rendering if origin mismatch |
X-Frame-Options: deny |
| X-XSS-Protection | Cross-site scripting Cross-site scripting Cross-site scripting is a type of computer security vulnerability typically found in Web applications that enables attackers to inject client-side script into Web pages viewed by other users. A cross-site scripting vulnerability may be used by attackers to bypass access controls such as the same... (XSS) filter |
X-XSS-Protection: 1; mode=block |
| X-Content-Type-Options | the only defined value, "nosniff", prevents Internet Explorer Internet Explorer Windows Internet Explorer is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems, starting in 1995. It was first released as part of the add-on package Plus! for Windows 95 that year... from MIME-sniffing a response away from the declared content-type |
X-Content-Type-Options: nosniff |
| 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 |
Avoiding caching
If a web server responds withCache-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.External links
- RFC 4229: HTTP Header Field Registrations. December 2005 (contains a more complete list of HTTP headers)
- RFC 2616: IETF HTTP/1.1 RFC
- RFC 2965: 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
- crwlr.net - HTTP Header index
- HTTP Header with Privacyinfo - Display your HTTP request and response headers