HTTP headers allow the client and the server to pass additional information with the request or the response. An HTTP header consists of its case-insensitive name followed by a colon ':', then by its value (without line breaks). Leading white space before the value is ignored.

Custom proprietary headers can be added using the 'X-' prefix, but this convention was deprecated in June 2012, because of the inconveniences it caused when non-standard fields became standard in RFC 6648; others are listed in an IANA registry, whose original content was defined in RFC 4229. IANA also maintains a registry of proposed new HTTP message headers.

Headers can be grouped according to their contexts:

  • General header: Headers applying to both requests and responses but with no relation to the data eventually transmitted in the body.
  • Request header: Headers containing more information about the resource to be fetched or about the client itself.
  • Response header: Headers with additional information about the response, like its location or about the server itself (name and version etc.).
  • Entity header: Headers containing more information about the body of the entity, like its content length or its MIME-type.

Headers can also be grouped according to how proxies handle them:

End-to-end headers
These headers must be transmitted to the final recipient of the message; that is, the server for a request or the client for a response. Intermediate proxies must retransmit end-to-end headers unmodified and caches must store them.
Hop-by-hop headers
These headers are meaningful only for a single transport-level connection and must not be retransmitted by proxies or cached. Such headers are: Connection, Keep-Alive, Proxy-Authenticate, Proxy-Authorization, TE, Trailer, Transfer-Encoding and Upgrade. Note that only hop-by-hop headers may be set using the Connection general header.

The following list summarizes HTTP headers by their usage category. For an alphabetical list, see the navigation on the left side.

Authentication

WWW-Authenticate
Defines the authentication method that should be used to gain access to a resource.
Authorization
Contains the credentials to authenticate a user agent with a server.
Proxy-Authenticate
Defines the authentication method that should be used to gain access to a resource behind a Proxy server.
Proxy-Authorization
Contains the credentials to authenticate a user agent with a proxy server.

Caching

Age
The time in seconds the object has been in a proxy cache.
Cache-Control
Specifies directives for caching mechanisms in both requests and responses.
Clear-Site-Data
Clears browsing data (e.g. cookies, storage, cache) associated with the requesting website.
Expires
The date/time after which the response is considered stale.
Pragma
Implementation-specific header that may have various effects anywhere along the request-response chain. Used for backwards compatibility with HTTP/1.0 caches where the Cache-Control header is not yet present.
Warning
A general warning field containing information about possible problems.

Client hints

HTTP Client hints are a work in progress. Actual documentation can be found on the website of the HTTP working group.

Accept-CH
Servers can advertise support for Client Hints using the Accept-CH header field or an equivalent HTML meta element with http-equiv attribute ([HTML5]).
Accept-CH-Lifetime
Servers can ask the client to remember the set of Client Hints that the server supports for a specified period of time, to enable delivery of Client Hints on subsequent requests to the server’s origin ([RFC6454]).
Early-Data
Indicates that the request has been conveyed in early data.
Content-DPR
The “Content-DPR” response header field is a number that indicates the ratio between physical pixels over CSS px of the selected image response.
DPR
The “DPR” request header field is a number that indicates the client’s current Device Pixel Ratio (DPR), which is the ratio of physical pixels over CSS px (Section 5.2 of [CSSVAL]) of the layout viewport (Section 9.1.1 of [CSS2]) on the device.
Save-Data
The SaveData [CLIENT-HINTS] request header field consists of one or more tokens that indicate user agent's preference for reduced data usage
Viewport-Width

The “Viewport-Width” request header field is a number that indicates the layout viewport width in CSS px. The provided CSS px value is a number rounded to the smallest following integer (i.e. ceiling value).

If Viewport-Width occurs in a message more than once, the last value overrides all previous occurrences.

Width

The “Width” request header field is a number that indicates the desired resource width in physical px (i.e. intrinsic size of an image). The provided physical px value is a number rounded to the smallest following integer (i.e. ceiling value).

If the desired resource width is not known at the time of the request or the resource does not have a display width, the Width header field can be omitted. If Width occurs in a message more than once, the last value overrides all previous occurrences

Conditionals

Last-Modified
It is a validator, the last modification date of the resource, used to compare several versions of the same resource. It is less accurate than ETag, but easier to calculate in some environments. Conditional requests using If-Modified-Since and If-Unmodified-Since use this value to change the behavior of the request.
ETag
It is a validator, a unique string identifying the version of the resource. Conditional requests using If-Match and If-None-Match use this value to change the behavior of the request.
If-Match
Makes the request conditional and applies the method only if the stored resource matches one of the given ETags.
If-None-Match
Makes the request conditional and applies the method only if the stored resource doesn't match any of the given ETags. This is used to update caches (for safe requests), or to prevent to upload a new resource when one is already existing.
If-Modified-Since
Makes the request conditional and expects the entity to be transmitted only if it has been modified after the given date. This is used to transmit data only when the cache is out of date.
If-Unmodified-Since
Makes the request conditional and expects the entity to be transmitted only if it has not been modified after the given date. This is used to ensure the coherence of a new fragment of a specific range with previous ones, or to implement an optimistic concurrency control system when modifying existing documents.
Vary
Determines how to match future request headers to decide whether a cached response can be used rather than requesting a fresh one from the origin server.

Connection management

Connection
Controls whether the network connection stays open after the current transaction finishes.
Keep-Alive
Controls how long a persistent connection should stay open.

Content negotiation

Accept
Informs the server about the types of data that can be sent back. It is MIME-type.
Accept-Charset
Informs the server about which character set the client is able to understand.
Accept-Encoding
Informs the server about the encoding algorithm, usually a compression algorithm, that can be used on the resource sent back.
Accept-Language
Informs the server about the language the server is expected to send back. This is a hint and is not necessarily under the full control of the user: the server should always pay attention not to override an explicit user choice (like selecting a language in a drop down list).

Controls

Expect
Indicates expectations that need to be fulfilled by the server in order to properly handle the request.
Max-Forwards
...

Cookies

Cookie
Contains stored HTTP cookies previously sent by the server with the Set-Cookie header.
Set-Cookie
Send cookies from the server to the user agent.
Cookie2
Used to contain an HTTP cookie, previously sent by the server with the Set-Cookie2 header, but has been obsoleted by the specification. Use Cookie instead.
Set-Cookie2
Used to send cookies from the server to the user agent, but has been obsoleted by the specification. Use Set-Cookie instead.

CORS

Learn more about CORS here.

Access-Control-Allow-Origin
Indicates whether the response can be shared.
Access-Control-Allow-Credentials
Indicates whether the response to the request can be exposed when the credentials flag is true.
Access-Control-Allow-Headers
Used in response to a preflight request to indicate which HTTP headers can be used when making the actual request.
Access-Control-Allow-Methods
Specifies the method or methods allowed when accessing the resource in response to a preflight request.
Access-Control-Expose-Headers
Indicates which headers can be exposed as part of the response by listing their names.
Access-Control-Max-Age
Indicates how long the results of a preflight request can be cached.
Access-Control-Request-Headers
Used when issuing a preflight request to let the server know which HTTP headers will be used when the actual request is made.
Access-Control-Request-Method
Used when issuing a preflight request to let the server know which HTTP method will be used when the actual request is made.
Cross-Origin-Resource-Policy
The Cross-Origin-Resource-Policy header prevents other domains from loading the resources.
Origin
Indicates where a fetch originates from.
Timing-Allow-Origin
Specifies origins that are allowed to see values of attributes retrieved via features of the Resource Timing API, which would otherwise be reported as zero due to cross-origin restrictions.
X-Permitted-Cross-Domain-Policies
Specifies if a cross-domain policy-file (XML) is allowed. The file may define a policy to grant web clients, such as Adobe Flash Player or Adobe Acrobat (e.g. PDF), permission to handle data across domains.

Do Not Track

DNT
Used for expressing the user's tracking preference.
Tk
Indicates the tracking status that applied to the corresponding request.

Downloads

Content-Disposition
Is a response header if the resource transmitted should be displayed inline (default behavior when the header is not present), or it should be handled like a download and the browser should present a 'Save As' window.

Message body information

Content-Length
Indicates the size of the entity-body, in decimal number of octets, sent to the recipient.
Content-Type
Indicates the media type of the resource.
Content-Encoding
Used to specify the compression algorithm.
Content-Language
Describes the language(s) intended for the audience, so that it allows a user to differentiate according to the users' own preferred language.
Content-Location
Indicates an alternate location for the returned data.

Proxies

Forwarded
Contains information from the client-facing side of proxy servers that is altered or lost when a proxy is involved in the path of the request.
X-Forwarded-For
Identifies the originating IP addresses of a client connecting to a web server through an HTTP proxy or a load balancer.
X-Forwarded-Host
Identifies the original host requested that a client used to connect to your proxy or load balancer.
X-Forwarded-Proto
Identifies the protocol (HTTP or HTTPS) that a client used to connect to your proxy or load balancer.
Via
Added by proxies, both forward and reverse proxies, and can appear in the request headers and the response headers.

Redirects

Location
Indicates the URL to redirect a page to.

Request context

From
Contains an Internet email address for a human user who controls the requesting user agent.
Host
Specifies the domain name of the server (for virtual hosting), and (optionally) the TCP port number on which the server is listening.
Referer
The address of the previous web page from which a link to the currently requested page was followed.
Referrer-Policy
Governs which referrer information sent in the Referer header should be included with requests made.
User-Agent
Contains a characteristic string that allows the network protocol peers to identify the application type, operating system, software vendor or software version of the requesting software user agent. See also the Firefox user agent string reference.

Response context

Allow
Lists the set of HTTP request methods support by a resource.
Server
Contains information about the software used by the origin server to handle the request.

Range requests

Accept-Ranges
Indicates if the server supports range requests, and if so in which unit the range can be expressed.
Range
Indicates the part of a document that the server should return.
If-Range
Creates a conditional range request that is only fulfilled if the given etag or date matches the remote resource. Used to prevent downloading two ranges from incompatible version of the resource.
Content-Range
Indicates where in a full body message a partial message belongs.

Security

Content-Security-Policy (CSP)
Controls resources the user agent is allowed to load for a given page.
Content-Security-Policy-Report-Only
Allows web developers to experiment with policies by monitoring, but not enforcing, their effects. These violation reports consist of JSON documents sent via an HTTP POST request to the specified URI.
Expect-CT
Allows sites to opt in to reporting and/or enforcement of Certificate Transparency requirements, which prevents the use of misissued certificates for that site from going unnoticed. When a site enables the Expect-CT header, they are requesting that Chrome check that any certificate for that site appears in public CT logs.
Feature-Policy
Provides a mechanism to allow and deny the use of browser features in its own frame, and in iframes that it embeds.
Public-Key-Pins (HPKP)
Associates a specific cryptographic public key with a certain web server to decrease the risk of MITM attacks with forged certificates.
Public-Key-Pins-Report-Only
Sends reports to the report-uri specified in the header and does still allow clients to connect to the server even if the pinning is violated.
Strict-Transport-Security (HSTS)
Force communication using HTTPS instead of HTTP.
Upgrade-Insecure-Requests
Sends a signal to the server expressing the client’s preference for an encrypted and authenticated response, and that it can successfully handle the upgrade-insecure-requests directive.
X-Content-Type-Options
Disables MIME sniffing and forces browser to use the type given in Content-Type.
X-Download-Options
Indicates that the browser (Internet Explorer) should not display the option to "Open" a file that has been downloaded from an application, to prevent phishing attacks as the file otherwise would gain access to execute in the context of the application.
X-Frame-Options (XFO)
Indicates whether a browser should be allowed to render a page in a <frame>, <iframe><embed> or <object>.
X-Powered-By
May be set by hosting environments or other frameworks and contains information about them while not providing any usefulness to the application or its visitors. Unset this header to avoid exposing potential vulnerabilities.
X-XSS-Protection
Enables cross-site scripting filtering.
 

Server-sent events

 

Last-Event-ID
...
Ping-From
...
Ping-To
...
Report-To
Used to specify a server endpoint for the browser to send warning and error reports to.

Transfer coding

Transfer-Encoding
Specifies the form of encoding used to safely transfer the entity to the user.
TE
Specifies the transfer encodings the user agent is willing to accept.
Trailer
Allows the sender to include additional fields at the end of chunked message.

WebSockets

Sec-WebSocket-Key
...
Sec-WebSocket-Extensions
...
Sec-WebSocket-Accept
...
Sec-WebSocket-Protocol
...
Sec-WebSocket-Version
...

Other

Accept-Push-Policy
A client can express the desired push policy for a request by sending an Accept-Push-Policy header field in the request.
Accept-Signature 
A client can send the Accept-Signature header field to indicate intention to take advantage of any available signatures and to indicate what kinds of signatures it supports.
Alt-Svc
Used to list alternate ways to reach this service.
Date
Contains the date and time at which the message was originated.
Expect-CT
Allows sites to opt in to reporting and/or enforcement of Certificate Transparency requirements.
Large-Allocation
Tells the browser that the page being loaded is going to want to perform a large allocation.
Link
The Link entity-header field provides a means for serialising one or more links in HTTP headers. It is semantically equivalent to the HTML <link> element.
Push-Policy 
A Push-Policy defines the server behaviour regarding push when processing a request.
Retry-After
Indicates how long the user agent should wait before making a follow-up request.
Signature 
The Signature header field conveys a list of signatures for an exchange, each one accompanied by information about how to determine the authority of and refresh that signature.
Signed-Headers 
The Signed-Headers header field identifies an ordered list of response header fields to include in a signature.
Server-Timing
Communicates one or more metrics and descriptions for the given request-response cycle.
SourceMap
Links generated code to a source map.
Upgrade
The relevant RFC document for the Upgrade header field is RFC 7230, section 6.7. The standard establishes rules for upgrading or changing to a different protocol on the current client, server, transport protocol connection. For example, this header standard allows a client to change from HTTP 1.1 to HTTP 2.0, assuming the server decides to acknowledge and implement the Upgrade header field. Neither party is required to accept the terms specified in the Upgrade header field. It can be used in both client and server headers. If the Upgrade header field is specified, then the sender MUST also send the Connection header field with the upgrade option specified. For details on the Connection header field please see section 6.1 of the aforementioned RFC.
X-DNS-Prefetch-Control
Controls DNS prefetching, a feature by which browsers proactively perform domain name resolution on both links that the user may choose to follow as well as URLs for items referenced by the document, including images, CSS, JavaScript, and so forth.
X-Firefox-Spdy
...
X-Pingback
...
X-Requested-With
...
X-Robots-Tag
Used to indicate how a web page is to be indexed within public search engine results. The header is effectively equivalent to <meta name="robots" content="...">.
X-UA-Compatible
Used by Internet Explorer to signal which document mode to use.

Contributing

You can help by writing new entries or improving the existing ones.

See also