List of HTTP header fields
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.
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.
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.
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.
|Header field name||Description||Example||Status|
|Accept-Charset||Character sets that are acceptable||
|Accept-Encoding||List of acceptable encodings. See HTTP compression.||
|Accept-Datetime||Acceptable version in time||
|Authorization||Authentication credentials for HTTP authentication||
|Connection||Control options for the current connection and list of hop-by-hop request fields||
|Expect||Indicates that particular server behaviors are required by the client||
|Forwarded||Disclose original information of a client connecting to a web server through an HTTP proxy||
|From||The email address of the user making the request||
|If-Modified-Since||Allows a 304 Not Modified to be returned if content is unchanged||
|If-Range||If the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity||
|If-Unmodified-Since||Only send the response if the entity has not been modified since a specific time.||
|Via||Informs the server of proxies through which the request was sent.||
|Warning||A general warning about possible problems with the entity body.||
Common non-standard request fields
|Access-Control-Allow-Origin||Specifying which web sites can participate in cross-origin resource sharing||
|Accept-Patch||Specifies which patch document formats this server supports||
|Accept-Ranges||What partial content range types this server supports via byte serving||
|Age||The age the object has been in a proxy cache in seconds||
|Allow||Valid actions for a specified resource. To be used for a 405 Method not allowed||
|Cache-Control||Tells all caching mechanisms from server to client whether they may cache this object. It is measured in seconds||
|Connection||Control options for the current connection and list of hop-by-hop response fields||
|Content-Disposition||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-Encoding||The type of encoding used on the data. See HTTP compression.||
|Content-Length||The length of the response body in octets (8-bit bytes)||
|Content-Location||An alternate location for the returned data||
|Content-Range||Where in a full body message this partial message belongs||
|Content-Type||The MIME type of this content||
|Date||The date and time that the message was sent (in "HTTP-date" format as defined by RFC 7231)||
|ETag||An identifier for a specific version of a resource, often a message digest||
|Expires||Gives the date/time after which the response is considered stale (in "HTTP-date" format as defined by RFC 7231)||
|Last-Modified||The last modified date for the requested object (in "HTTP-date" format as defined by RFC 7231)||
|Link||Used to express a typed relationship with another resource, where the relation type is defined by RFC 5988||
|Location||Used in redirection, or when a new resource has been created.||
|Pragma||Implementation-specific fields that may have various effects anywhere along the request-response chain.||
|Proxy-Authenticate||Request authentication to access the proxy.||
|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.||
|Server||A name for the server||
|Set-Cookie||An HTTP cookie||
|Status||CGI header field specifying the status of the HTTP response. Normal HTTP responses use a separate "Status-Line" instead, defined by RFC 7230.||
||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.||
|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.||
|Transfer-Encoding||The form of encoding used to safely transfer the entity to the user. Currently defined methods are: chunked, compress, deflate, gzip, identity.||
|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
|Upgrade||Ask the client to upgrade to another protocol.||
|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.||
|Via||Informs the client of proxies through which the response was sent.||
|Warning||A general warning about possible problems with the entity body.||
|WWW-Authenticate||Indicates the authentication scheme that should be used to access the requested entity.||
Common non-standard response fields
|Content-Security-Policy, X-Content-Security-Policy, X-WebKit-CSP||
Effects of selected fields
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.
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. The behavior of
Pragma: no-cache in a response is implementation specific. While some user agents do pay attention to this field in responses, the HTTP/1.1 RFC specifically warns against relying on this behavior.
- "RFC 5789". Retrieved 2014-12-24.
- "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing". IETF. June 2014. Retrieved 2014-12-19.
- "RFC 6266". Retrieved 2015-03-13.
- Indicate the canonical version of a URL by responding with the Link rel="canonical" HTTP header Retrieved: 2012-02-09
- "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content". Retrieved 2014-07-24.
- "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing". Retrieved 2014-07-24.
- "Hypertext Transfer Protocol (HTTP/1.1): Caching". ietf.org. Retrieved 2014-07-24.
- "How to prevent caching in Internet Explorer". Microsoft. 2011-09-22. Retrieved 2015-04-15.
- Headers: Permanent Message Header Field Names
- RFC 7230: Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
- RFC 7231: Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
- RFC 7232: Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests
- RFC 7233: Hypertext Transfer Protocol (HTTP/1.1): Range Requests
- RFC 7234: Hypertext Transfer Protocol (HTTP/1.1): Caching
- RFC 7235: Hypertext Transfer Protocol (HTTP/1.1): Authentication
- RFC 2965: IETF HTTP State Management Mechanism RFC
- HTTP/1.1 headers from a web server point of view
- Internet Explorer and Custom HTTP Headers - EricLaw's IEInternals - Site Home - MSDN Blogs