HTTP Header Fields
HTTP deader fields provide required information about the request or response, or about the object sent in the message body. There are following four types of HTTP message headers:
- General-header: These header fields have general applicability for both request and response messages.
- Client Request-header: These header fields are applicability only for request messages.
- Server Response-header: These header fields are applicability only for response messages.
- Entity-header: These header fields define metainformation about the entity-body or, if no body is present
General Headers
Cache-control
The Cache-Control general-header field is used to specify directives that MUST be obeyed by all caching system. Following is the syntax:
Cache-Control : cache-request-directive|cache-response-directive
An HTTP clients or servers can use the Cache-control general header to specify parameters for the cache or to request certain kinds of documents from the cache. The caching directives are specified in a comma-separated list. For example:
Cache-control: no-cache
There are following important cache request directives which can be used by the client in its HTTP request:
| S.N. | Cache Request Directive and Description |
|---|---|
| 1 | no-cache A cache must not use the response to satisfy a subsequent request without successful revalidation with the origin server. |
| 2 | no-store The cache should not store anything about the client request or server response. |
| 3 | max-age = seconds Indicates that the client is willing to accept a response whose age is no greater than the specified time in seconds. |
| 4 | max-stale [ = seconds ] Indicates that the client is willing to accept a response that has exceeded its expiration time. If seconds are given, it must not be expired by more than that time. |
| 5 | min-fresh = seconds Indicates that the client is willing to accept a response whose freshness lifetime is no less than its current age plus the specified time in seconds. |
| 6 | no-transform Do not convert the entity-body. |
| 7 | only-if-cached Do not retrieve new data. The cache can send a document only if it is in the cache, and should not contact the origin-server to see if a newer copy exists. |
There are following important cache response directives which can be used by the server in its HTTP response:
| S.N. | Cache Request Directive and Description |
|---|---|
| 1 | public Indicates that the response may be cached by any cache. |
| 2 | private Indicates that all or part of the response message is intended for a single user and must not be cached by a shared cache. |
| 3 | no-cache A cache must not use the response to satisfy a subsequent request without successful revalidation with the origin server. |
| 4 | no-store The cache should not store anything about the client request or server response. |
| 5 | no-transform Do not convert the entity-body. |
| 6 | must-revalidate The cache must verify the status of stale documents before using it and expired one should not be used. |
| 7 | proxy-revalidate The proxy-revalidate directive has the same meaning as the must- revalidate directive, except that it does not apply to non-shared user agent caches. |
| 8 | max-age = seconds Indicates that the client is willing to accept a response whose age is no greater than the specified time in seconds. |
| 9 | s-maxage = seconds The maximum age specified by this directive overrides the maximum age specified by either the max-age directive or the Expires header. The s-maxage directive is always ignored by a private cache. |
Connection
The Connection general-header field allows the sender to specify options that are desired for that particular connection and must not be communicated by proxies over further connections. Following is the simple syntax of using connection header:
Connection : "Connection"
HTTP/1.1 defines the "close" connection option for the sender to signal that the connection will be closed after completion of the response. For example:
Connection: close
By default, HTTP 1.1 uses persistent connections, where the connection does not automatically close after a transaction. HTTP 1.0, on the other hand, does not have persistent connections by default. If a 1.0 client wishes to use persistent connections, it uses the keep-alive parameter as follows:
Connection: keep-alive
Date
All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception. HTTP applications are allowed to use any of the following three representations of date/time stamps:
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123 Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036 Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
Here first format is the most preferred one.
Pragma
The Pragma general-header field is used to include implementation- specific directives that might apply to any recipient along the request/response chain. For example:
Pragma: no-cache
The only directive defined in HTTP/1.0 is the no-cache directive and is maintained in HTTP 1.1 for backward compatibility. No new Pragma directives will be defined in the future.
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. Following is the syntax of Trailer header field:
Trailer : field-name
Message header fields listed in the Trailer header field must not include the following header fields:
- Transfer-Encoding
- Content-Length
- Trailer
Transfer-Encoding
The Transfer-Encoding general-header field indicates what type of transformation has been applied to the message body in order to safely transfer it between the sender and the recipient. This is not the same as content-encoding because transfer-encodings are a property of the message, not of the entity-body. Following is the syntax of Transfer-Encoding header field:
Transfer-Encoding: chunked
All transfer-coding values are case-insensitive.
Upgrade
The Upgrade general-header allows the client to specify what additional communication protocols it supports and would like to use if the server finds it appropriate to switch protocols. For example:
Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
The Upgrade header field is intended to provide a simple mechanism for transition from HTTP/1.1 to some other, incompatible protocol
Via
The Via general-header must be used by gateways and proxies to indicate the intermediate protocols and recipients. For example, a request message could be sent from an HTTP/1.0 user agent to an internal proxy code-named "fred", which uses HTTP/1.1 to forward the request to a public proxy at nowhere.com, which completes the request by forwarding it to the origin server at www.ics.uci.edu. The request received by www.ics.uci.edu would then have the following Via header field:
Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
The Upgrade header field is intended to provide a simple mechanism for transition from HTTP/1.1 to some other, incompatible protocol
Warning
The Warning general-header is used to carry additional information about the status or transformation of a message which might not be reflected in the message. A response may carry more than one Warning header.
Warning : warn-code SP warn-agent SP warn-text SP warn-date
Client Request Headers
Accept
The Accept request-header field can be used to specify certain media types which are acceptable for the response. Following is the general syntax:
Accept: type/subtype [q=qvalue]
Multiple media types can be listed separated by commas and the optional qvalue represents an acceptable quality level for accept types on a scale of 0 to 1. Following is an example:
Accept: text/plain; q=0.5, text/html, text/x-dvi; q=0.8, text/x-c
This would be interpreted as text/html and text/x-c are the preferred media types, but if they do not exist, then send the text/x-dvi entity, and if that does not exist, send the text/plain entity.
Accept-Charset
The Accept-Charset request-header field can be used to indicate what character sets are acceptable for the response. Following is the general syntax:
Accept-Charset: character_set [q=qvalue]
Multiple character sets can be listed separated by commas and the optional qvalue represents an acceptable quality level for nonpreferred character sets on a scale of 0 to 1. Following is an example:
Accept-Charset: iso-8859-5, unicode-1-1; q=0.8
The special value "*", if present in the Accept-Charset field, matches every character set and if no Accept-Charset header is present, the default is that any character set is acceptable.
Accept-Encoding
The Accept-Encoding request-header field is similar to Accept, but restricts the content-codings that are acceptable in the response. Following is the general syntax:
Accept-Encoding: encoding types
Following are examples:
Accept-Encoding: compress, gzip Accept-Encoding: Accept-Encoding: * Accept-Encoding: compress;q=0.5, gzip;q=1.0 Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
Accept-Language
The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are preferred as a response to the request. Following is the general syntax:
Accept-Language: language [q=qvalue]
Multiple languages can be listed separated by commas and the optional qvalue represents an acceptable quality level for nonpreferred languages on a scale of 0 to 1. Following is an example:
Accept-Language: da, en-gb;q=0.8, en;q=0.7
Authorization
The Authorization request-header field value consists of credentials containing the authentication information of the user agent for the realm of the resource being requested. Following is the general syntax:
Authorization : credentials
The HTTP/1.0 specification defines the BASIC authorization scheme, where the authorization parameter is the string of username:password encoded in base 64. Following is an example:
Authorization: BASIC Z3Vlc3Q6Z3Vlc3QxMjM=
The value decodes into is guest:guest123 where guest is user ID andguest123 is the password.
Cookie
The Cookie request-header field value contains a name/value pair of information stored for that URL. Following is the general syntax:
Cookie: name=value
Multiple cookies can be specified separated by semicolons as follows:
Cookie: name1=value1;name2=value2;name3=value3
Expect
The Expect request-header field is used to indicate that particular server behaviors are required by the client. Following is the general syntax:
Expect : 100-continue | expectation-extension
If a server receives a request containing an Expect field that includes an expectation-extension that it does not support, it must respond with a 417 (Expectation Failed) status.
From
The From request-header field contains an Internet e-mail address for the human user who controls the requesting user agent. Following is a simple example:
From: webmaster@w3.org
This header field may be used for logging purposes and as a means for identifying the source of invalid or unwanted requests.
Host
The Host request-header field is used to specify the Internet host and port number of the resource being requested. Following is the general syntax:
Host : "Host" ":" host [ ":" port ] ;
A host without any trailing port information implies the default port, which is 80. For example, a request on the origin server forhttp://www.w3.org/pub/WWW/ would be:
GET /pub/WWW/ HTTP/1.1 Host: www.w3.org
If-Match
The If-Match request-header field is used with a method to make it conditional. This header request the server to perform the requested method only if given value in this tag matches the given entity tags represented byETag. Following is the general syntax:
If-Match : entity-tag
An asterisk (*) matches any entity, and the transaction continues only if the entity exists. Following are possible examples:
If-Match: "xyzzy" If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-Match: *
If none of the entity tags match, or if "*" is given and no current entity exists, the server must not perform the requested method, and must return a 412 (Precondition Failed) response.
If-Modified-Since
The If-Modified-Since request-header field is used with a method to make it conditional. If the requested URL has not been modified since the time specified in this field, an entity will not be returned from the server; instead, a 304 (not modified) response will be returned without any message-body. Following is the general syntax:
If-Modified-Since : HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If none of the entity tags match, or if "*" is given and no current entity exists, the server must not perform the requested method, and must return a 412 (Precondition Failed) response.
If-None-Match
The If-None-Match request-header field is used with a method to make it conditional. This header request the server to perform the requested method only if one of the given value in this tag matches the given entity tags represented by ETag. Following is the general syntax:
If-None-Match : entity-tag
An asterisk (*) matches any entity, and the transaction continues only if the entity does not exist. Following are possible examples:
If-None-Match: "xyzzy" If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-None-Match: *
If-Range
The If-Range request-header field can be used with a conditional GET to request only the portion of the entity that is missing, if it has not been changed, and the entire entity if it has changed. Following is the general syntax:
If-Range : entity-tag | HTTP-date
Either an entity tag or a date can be used to identify the partial entity already received. For example:
If-Range: Sat, 29 Oct 1994 19:43:31 GMT
Here if the document has not been modified since the given date, the server returns the byte range given by the Range header otherwise, it returns all of the new document.
If-Unmodified-Since
The If-Unmodified-Since request-header field is used with a method to make it conditional. Following is the general syntax:
If-Unmodified-Since : HTTP-date
If the requested resource has not been modified since the time specified in this field, the server should perform the requested operation as if the If-Unmodified-Since header were not present. For example:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
If the request normally would result in anything other than a 2xx or 412 status, the If-Unmodified-Since header should be ignored.
Max-Forwards
The Max-Forwards request-header field provides a mechanism with the TRACE and OPTIONS methods to limit the number of proxies or gateways that can forward the request to the next inbound server. Following is the general syntax:
Max-Forwards : n
The Max-Forwards value is a decimal integer indicating the remaining number of times this request message may be forwarded. This is useful for debugging with the TRACE method, avoiding infinite loops. For example:
Max-Forwards : 5
The Max-Forwards header field may be ignored for all other methods defined in HTTP specification.
Proxy-Authorization
The Proxy-Authorization request-header field allows the client to identify itself (or its user) to a proxy which requires authentication. Following is the general syntax:
Proxy-Authorization : credentials
The Proxy-Authorization field value consists of credentials containing the authentication information of the user agent for the proxy and/or realm of the resource being requested.
The Range request-header field specifies the partial range(s) of the content requested from the document. Following is the general syntax:
Range: bytes-unit=first-byte-pos "-" [last-byte-pos]
The first-byte-pos value in a byte-range-spec gives the byte-offset of the first byte in a range. The last-byte-pos value gives the byte-offset of the last byte in the range; that is, the byte positions specified are inclusive. You can specify a byte-unit as bytes Byte offsets start at zero. Following are a simple examples:
- The first 500 bytes
Range: bytes=0-499
- The second 500 bytes
Range: bytes=500-999
- The final 500 bytes
Range: bytes=-500
- The first and last bytes only
Range: bytes=0-0,-1
Multiple ranges can be listed, separated by commas. If the first digit in the comma-separated byte range(s) is missing, the range is assumed to count from the end of the document. If the second digit is missing, the range is byte n to the end of the document.
Referer
The Referer request-header field allows the client to specify the address (URI) of the resource from which the URL has been requested. Following is the general syntax:
Referer : absoluteURI | relativeURI
Following is a simple example:
Referer: http://www.tutorialspoint.org/http/index.htm
If the field value is a relative URI, it should be interpreted relative to theRequest-URI.
TE
The TE request-header field indicates what extension transfer-coding it is willing to accept in the response and whether or not it is willing to accept trailer fields in a chunked transfer-coding. Following is the general syntax:
TE : t-codings
The presence of the keyword "trailers" indicates that the client is willing to accept trailer fields in a chunked transfer-coding and it is specified either of the ways:
TE: deflate TE: TE: trailers, deflate;q=0.5
If the TE field-value is empty or if no TE field is present, the only transfer-coding is chunked. A message with no transfer-coding is always acceptable.
User-Agent
The User-Agent request-header field contains information about the user agent originating the request. Following is the general syntax:
User-Agent : product | comment
Example:
User-Agent: Mozilla/4.0 (compatible; MSIE5.01; Windows NT)
Server Response Headers
Accept-Ranges
The Accept-Ranges response-header field allows the server to indicate its acceptance of range requests for a resource. Following is the general syntax:
Accept-Ranges : range-unit | none
For example a server that accept byte-range requests may send
Accept-Ranges: bytes
Servers that do not accept any kind of range request for a resource may send:
Accept-Ranges: none
This will advise the client not to attempt a range request.
Age
The Age response-header field conveys the sender's estimate of the amount of time since the response (or its revalidation) was generated at the origin server. Following is the general syntax:
Age : delta-seconds
Age values are non-negative decimal integers, representing time in seconds. Following is a simple example:
Age: 1030
An HTTP/1.1 server that includes a cache must include an Age header field in every response generated from its own cache.
ETag
The ETag response-header field provides the current value of the entity tag for the requested variant. Following is the general syntax:
ETag : entity-tag
Following are simple examples:
ETag: "xyzzy" ETag: W/"xyzzy" ETag: ""
Location
The Location response-header field is used to redirect the recipient to a location other than the Request-URI for completion. Following is the general syntax:
Location : absoluteURI
Following is a simple example:
Location: http://www.tutorialspoint.org/http/index.htm
The Content-Location header field differs from Location in that the Content-Location identifies the original location of the entity enclosed in the request.
Proxy-Authenticate
The Proxy-Authenticate response-header field must be included as part of a 407 (Proxy Authentication Required) response. Following is the general syntax:
Proxy-Authenticate : challenge
Retry-After
The Retry-After response-header field can be used with a 503 (Service Unavailable) response to indicate how long the service is expected to be unavailable to the requesting client. Following is the general syntax:
Retry-After : HTTP-date | delta-seconds
Following are two simple examples:
Retry-After: Fri, 31 Dec 1999 23:59:59 GMT Retry-After: 120
In the latter example, the delay is 2 minutes.
Server
The Server response-header field contains information about the software used by the origin server to handle the request. Following is the general syntax:
Server : product | comment
Following is a simple example:
Server: Apache/2.2.14 (Win32)
If the response is being forwarded through a proxy, the proxy application must not modify the Server response-header.
Set-Cookie
The Set-Cookie response-header field contains a name/value pair of information to retain for this URL. Following is the general syntax:
Set-Cookie: NAME=VALUE; OPTIONS
Set-Cookie response header comprises the token Set-Cookie:, followed by a comma-separated list of one or more cookies. Here are possible values you can specify as options:
| S.N. | Options and Description |
|---|---|
| 1 | Comment=comment This option can be used to specify any comment associated with the cookie. |
| 2 | Domain=domain The Domain attribute specifies the domain for which the cookie is valid. |
| 3 | Expires=Date-time The date the cookie will expire. If this is blank, the cookie will expire when the visitor quits the browser |
| 4 | Path=path The Path attribute specifies the subset of URLs to which this cookie applies. |
| 5 | Secure This instructs the user agent to return the cookie only under a secure connection. |
Following is an example of a simple cookie header generated by the server:
Set-Cookie: name1=value1,name2=value2; Expires=Wed, 09 Jun 2021 10:18:14 GMT
Vary
The Vary response-header field specifies that the entity has multiple sources and may therefore vary according to specified list of request header(s). Following is the general syntax:
Vary : field-name
You can specify multiple headers separated by commas and a value of asterisk "*" signals that unspecified parameters not limited to the request-headers. Following is a simple example:
Vary: Accept-Language, Accept-Encoding
Here field names are case-insensitive.
WWW-Authenticate
The WWW-Authenticate response-header field must be included in 401 (Unauthorized) response messages. The field value consists of at least one challenge that indicates the authentication scheme(s) and parameters applicable to the Request-URI. Following is the general syntax:
WWW-Authenticate : challenge
WWW- Authenticate field value as it might contain more than one challenge, or if more than one WWW-Authenticate header field is provided, the contents of a challenge itself can contain a comma-separated list of authentication parameters. Following is a simple example:
WWW-Authenticate: BASIC realm="Admin"
Entity Headers
Allow
The Allow entity-header field lists the set of methods supported by the resource identified by the Request-URI. Following is the general syntax:
Allow : Method
You can specify multiple method separated by commas. Following is a simple example:
Allow: GET, HEAD, PUT
This field cannot prevent a client from trying other methods.
Content-Encoding
The Content-Encoding entity-header field is used as a modifier to the media-type. Following is the general syntax:
Content-Encoding : content-coding
The content-coding is a characteristic of the entity identified by the Request-URI. Following is a simple example:
Content-Encoding: gzip
If the content-coding of an entity in a request message is not acceptable to the origin server, the server should respond with a status code of 415 (Unsupported Media Type).
Content-Language
The Content-Language entity-header field describes the natural language(s) of the intended audience for the enclosed entity. Following is the general syntax:
Content-Language : language-tag
Multiple languages may be listed for content that is intended for multiple audiences. Following is a simple example:
Content-Language: mi, en
The primary purpose of Content-Language is to allow a user to identify and differentiate entities according to the user's own preferred language.
Content-Length
The Content-Length entity-header field indicates the size of the entity-body, in decimal number of OCTETs, sent to the recipient or, in the case of the HEAD method, the size of the entity-body that would have been sent had the request been a GET. Following is the general syntax:
Content-Length : DIGITS
Following is a simple example:
Content-Length: 3495
Any Content-Length greater than or equal to zero is a valid value.
Content-Location
The Content-Location entity-header field may be used to supply the resource location for the entity enclosed in the message when that entity is accessible from a location separate from the requested resource's URI. Following is the general syntax:
Content-Location: absoluteURI | relativeURI
Following is a simple example:
Content-Location: http://www.tutorialspoint.org/http/index.htm
The value of Content-Location also defines the base URI for the entity.
Content-MD5
The Content-MD5 entity-header field may be used to supply an MD5 digest of the entity, for checking the integrity of the message upon receipt. Following is the general syntax:
Content-MD5 : md5-digest using base64 of 128 bit MD5 digest as per RFC 1864
Following is a simple example:
Content-MD5 : 8c2d46911f3f5a326455f0ed7a8ed3b3
The MD5 digest is computed based on the content of the entity-body, including any content-coding that has been applied, but not including any transfer-encoding applied to the message-body.
Content-Range
The Content-Range entity-header field is sent with a partial entity-body to specify where in the full entity-body the partial body should be applied. Following is the general syntax:
Content-Range : bytes-unit SP first-byte-pos "-" last-byte-pos
Examples of byte-content-range-spec values, assuming that the entity contains a total of 1234 bytes:
- The first 500 bytes: Content-Range : bytes 0-499/1234 - The second 500 bytes: Content-Range : bytes 500-999/1234 - All except for the first 500 bytes: Content-Range : bytes 500-1233/1234 - The last 500 bytes: Content-Range : bytes 734-1233/1234
When an HTTP message includes the content of a single range, this content is transmitted with a Content-Range header, and a Content-Length header showing the number of bytes actually transferred. For example,
HTTP/1.1 206 Partial content Date: Wed, 15 Nov 1995 06:25:24 GMT Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT Content-Range: bytes 21010-47021/47022 Content-Length: 26012 Content-Type: image/gif
Content-Type
The Content-Type entity-header field indicates the media type of the entity-body sent to the recipient or, in the case of the HEAD method, the media type that would have been sent had the request been a GET. Following is the general syntax:
Content-Type : media-type
Following is an example:
Content-Type: text/html; charset=ISO-8859-4
Expires
The Expires entity-header field gives the date/time after which the response is considered stale. Following is the general syntax:
Expires : HTTP-date
Following is an example:
Expires: Thu, 01 Dec 1994 16:00:00 GMT
Last-Modified
The Last-Modified entity-header field indicates the date and time at which the origin server believes the variant was last modified. Following is the general syntax:
Last-Modified: HTTP-date
Following is an example:
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
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