List of HTTP header fields

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HTTP header fields are components of the header section of request and response messages in the Hypertext Transfer Protocol (HTTP). They define the operating parameters of an HTTP transaction.

General format[edit]

The header fields are transmitted after the request or response line, which is 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 section is indicated by an empty field, resulting in the transmission of two consecutive CR-LF pairs. Historically, long lines could be folded into multiple lines; continuation lines are indicated by the presence of a space (SP) or horizontal tab (HT) as the first character on the next line. This folding is now deprecated.

Field names[edit]

A core set of fields is standardized by the Internet Engineering Task Force (IETF) in RFCs 7230, 7231, 7232, 7233, 7234, and 7235. The permanent registry of header fields and repository of provisional registrations are maintained by the IANA. Additional field names and permissible values may be defined by each application.

Non-standard header fields were conventionally marked by prefixing the field name with X- but this convention was deprecated in June 2012 because of the inconveniences it caused when non-standard fields became standard. An earlier restriction on use of Downgraded- was lifted in March 2013.

Field values[edit]

A few fields can contain comments (i.e. in User-Agent, Server, Via fields), which can be ignored by software.

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

Size limits[edit]

The standard imposes no limits to the size of each header field name or value, or to the number of fields. 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 the size of each field to 8190 bytes, and there can be at most 100 header fields in a single request.

Request fields[edit]

Header field name Description Example Status
Accept-Charset Character sets that are acceptable Accept-Charset: utf-8 Permanent
Accept-Encoding List of acceptable encodings. See HTTP compression. Accept-Encoding: gzip, deflate Permanent
Accept-Datetime Acceptable version in time Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT Provisional
Authorization Authentication credentials for HTTP authentication Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ== Permanent
Connection Control options for the current connection and list of hop-by-hop request fields Connection: keep-alive

Connection: Upgrade

Permanent
Expect Indicates that particular server behaviors are required by the client Expect: 100-continue Permanent
Forwarded Disclose original information of a client connecting to a web server through an HTTP proxy Forwarded: for=192.0.2.60;proto=http;by=203.0.113.43 Forwarded: for=192.0.2.43, for=198.51.100.17 Permanent
From The email address of the user making the request From: user@example.com Permanent
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 Permanent
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" Permanent
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 Permanent
Via Informs the server of proxies through which the request was sent. Via: 1.0 fred, 1.1 example.com (Apache/1.1) Permanent
Warning A general warning about possible problems with the entity body. Warning: 199 Miscellaneous warning Permanent

Common non-standard request fields[edit]

Field name Example
X-Forwarded-For X-Forwarded-For: client1, proxy1, proxy2

X-Forwarded-For: 129.78.138.66, 129.78.64.103

X-Forwarded-Host X-Forwarded-Host: en.wikipedia.org:80

X-Forwarded-Host: en.wikipedia.org

Front-End-Https Front-End-Https: on
X-ATT-DeviceId X-Att-Deviceid: GT-P7320/P7320XXLPG
X-Wap-Profile https://www.example-social-network.com.

Response fields[edit]

Field name Description Example Status
Access-Control-Allow-Origin Specifying which web sites can participate in cross-origin resource sharing Access-Control-Allow-Origin: * Provisional
Accept-Patch[1] Specifies which patch document formats this server supports Accept-Patch: text/example;charset=utf-8 Permanent
Accept-Ranges What partial content range types this server supports via byte serving Accept-Ranges: bytes Permanent
Age The age the object has been in a proxy cache in seconds Age: 12 Permanent
Allow Valid actions for a specified resource. To be used for a 405 Method not allowed Allow: GET, HEAD Permanent
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 Permanent
Connection Control options for the current connection and list of hop-by-hop response fields[2] Connection: close Permanent
Content-Disposition[3] 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" Permanent
Content-Encoding The type of encoding used on the data. See HTTP compression. Content-Encoding: gzip Permanent
Content-Length The length of the response body in octets (8-bit bytes) Content-Length: 348 Permanent
Content-Location An alternate location for the returned data Content-Location: /index.htm Permanent
Content-Range Where in a full body message this partial message belongs Content-Range: bytes 21010-47021/47022 Permanent
Content-Type The MIME type of this content Content-Type: text/html; charset=utf-8 Permanent
Date The date and time that the message was sent (in "HTTP-date" format as defined by RFC 7231) Date: Tue, 15 Nov 1994 08:12:31 GMT Permanent
ETag An identifier for a specific version of a resource, often a message digest ETag: "737060cd8c284d8af7ad3082f209582d" Permanent
Expires Gives the date/time after which the response is considered stale (in "HTTP-date" format as defined by RFC 7231) Expires: Thu, 01 Dec 1994 16:00:00 GMT Permanent: standard
Last-Modified The last modified date for the requested object (in "HTTP-date" format as defined by RFC 7231) Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT Permanent
Link Used to express a typed relationship with another resource, where the relation type is defined by RFC 5988 Link: </feed>; rel="alternate"[4] Permanent
Location Used in redirection, or when a new resource has been created. Location: http://www.w3.org/pub/WWW/People.html Permanent
Pragma Implementation-specific fields that may have various effects anywhere along the request-response chain. Pragma: no-cache Permanent
Proxy-Authenticate Request authentication to access the proxy. Proxy-Authenticate: Basic Permanent
Retry-After If an entity is temporarily unavailable, this instructs the client to try again later. Value could be a specified period of time (in seconds) or a HTTP-date.[5]
  • Example 1: Retry-After: 120
  • Example 2: Retry-After: Fri, 07 Nov 2014 23:59:59 GMT

Permanent

Server A name for the server Server: Apache/2.4.1 (Unix) Permanent
An HTTP cookie Set-Cookie: UserID=JohnDoe; Max-Age=3600; Version=1 Permanent: standard
Status CGI header field specifying the status of the HTTP response. Normal HTTP responses use a separate "Status-Line" instead, defined by RFC 7230.[6] Status: 200 OK Not listed as a registered field name
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 Permanent: standard
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 Permanent
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 Permanent
TSV Tracking Status Value, value suggested to be sent in response to a DNT(do-not-track), possible values:
"!" — under construction
"?" — dynamic
"G" — gateway to multiple parties
"N" — not tracking
"T" — tracking
"C" — tracking with consent
"P" — tracking only if consented
"D" — disregarding DNT
"U" — updated
TSV: ? Permanent
Upgrade Ask the client to upgrade to another protocol. Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11 Permanent
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.
  • Example 1: Vary: *
  • Example 2: Vary: Accept-Language
Permanent
Via Informs the client of proxies through which the response was sent. Via: 1.0 fred, 1.1 example.com (Apache/1.1) Permanent
Warning A general warning about possible problems with the entity body. Warning: 199 Miscellaneous warning Permanent
WWW-Authenticate Indicates the authentication scheme that should be used to access the requested entity. WWW-Authenticate: Basic Permanent

Common non-standard response fields[edit]

Field name Example
Content-Security-Policy, X-Content-Security-Policy, X-WebKit-CSP X-WebKit-CSP: default-src 'self'

Effects of selected fields[edit]

Avoiding caching[edit]

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 called 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 Cache-Control: no-cache HTTP/1.1 header field is also intended for use in requests made by the client. It is a means for the browser to tell the server and any intermediate caches that it wants a fresh version of the resource. The Pragma: no-cache header field, defined in the HTTP/1.0 spec, has the same purpose. It, however, is only defined for the request header. Its meaning in a response header is not specified.[7] The behavior of Pragma: no-cache in a response is implementation specific. While some user agents do pay attention to this field in responses,[8] the HTTP/1.1 RFC specifically warns against relying on this behavior.

See also[edit]

References[edit]

External links[edit]