HTTP headers let the client and the server pass additional information with an HTTP request or response. An HTTP header consists of its case-insensitive name followed by a colon ( : ), then by its value. Whitespace before the value is ignored.

Custom proprietary headers have historically been used with an X- prefix, but this convention was deprecated in June 2012 because of the inconveniences it caused when nonstandard 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 headers.

Headers can be grouped according to their contexts:

General headers apply to both requests and responses, but with no relation to the data transmitted in the body.

Request headers contain more information about the resource to be fetched, or about the client requesting the resource.

Response headers hold additional information about the response, like its location or about the server providing it.

Entity headers contain information about the body of the resource, like its content length or 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: the server for a request, or the client for a response. Intermediate proxies must retransmit these 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. Note that only hop-by-hop headers may be set using the Connection general header.

Authentication

WWW-Authenticate Defines the authentication method that should be used to access 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 access 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, that the object has been in a proxy cache. Cache-Control 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 General warning 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 A number that indicates the ratio between physical pixels over CSS pixels of the selected image response. DPR A number that indicates the client’s current Device Pixel Ratio (DPR), which is the ratio of physical pixels over CSS pixels (Section 5.2 of [CSSVAL] ) of the layout viewport (Section 9.1.1 of [CSS2] ) on the device. Device-Memory Technically a part of Device Memory API, this header represents an approximate amount of RAM client has. Save-Data A boolean that indicates the user agent's preference for reduced data usage. Viewport-Width A number that indicates the layout viewport width in CSS pixels. The provided pixel 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 pixels (i.e. intrinsic size of an image). The provided pixel 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 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 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 already exists. 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 ensures 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 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.

Accept Informs the server about the types of data that can be sent back. Accept-Charset Which character encodings the client understands. Accept-Encoding The encoding algorithm, usually a compression algorithm, that can be used on the resource sent back. Accept-Language Informs the server about the human 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 from a dropdown).

Controls

Expect Indicates expectations that need to be fulfilled by the server 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 Contains an HTTP cookie previously sent by the server with the Set-Cookie2 header, but has been obsoleted. Use Cookie instead. Set-Cookie2 Sends cookies from the server to the user-agent, but has been obsoleted. 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 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. 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.

Do Not Track

DNT Expresses the user's tracking preference. Tk Indicates the tracking status of the corresponding response.

Downloads

Content-Disposition Indicates if the resource transmitted should be displayed inline (default behavior without the header), or if it should be handled like a download and the browser should present a “Save As” dialog.

Message body information

Content-Length The size of the resource, in decimal number of bytes. Content-Type Indicates the media type of the resource. Content-Encoding Used to specify the compression algorithm. Content-Language Describes the human 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 supported 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

Cross-Origin-Embedder-Policy (COEP) Allows a server to declare an embedder policy for a given document.

Cross-Origin-Opener-Policy (COOP) Prevents other domains from opening/controlling a window.

Cross-Origin-Resource-Policy (CORP) Prevents other domains from reading the response of the resources to which this header is applied. 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. 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 The X-Download-Options HTTP header 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. (Note: related MS Edge bug). X-Frame-Options (XFO) Indicates whether a browser should be allowed to render a page in a <frame> , <iframe> , <embed> or <object> . X-Permitted-Cross-Domain-Policies Specifies if a cross-domain policy file ( crossdomain.xml ) is allowed. The file may define a policy to grant clients, such as Adobe's Flash Player, Adobe Acrobat, Microsoft Silverlight, or Apache Flex, permission to handle data across domains that would otherwise be restricted due to the Same-Origin Policy. See the Cross-domain Policy File Specification for more information. 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.

HTTP Public Key Pinning (HPKP)

HTTP Public Key Pinning has been deprecated and removed in favor of Certificate Transparency and Expect-CT .

Public-Key-Pins 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.

Fetch metadata request headers

Sec-Fetch-Site It is a request header that indicates the relationship between a request initiator's origin and its target's origin. It is a Structured Header whose value is a token with possible values cross-site , same-origin , same-site , and none . Sec-Fetch-Mode It is a request header that indicates the request's mode to a server. It is a Structured Header whose value is a token with possible values cors , navigate , nested-navigate , no-cors , same-origin , and websocket . Sec-Fetch-User It is a request header that indicates whether or not a navigation request was triggered by user activation. It is a Structured Header whose value is a boolean so possible values are ?0 for false and ?1 for true. Sec-Fetch-Dest It is a request header that indicates the request's destination to a server. It is a Structured Header whose value is a token with possible values audio , audioworklet , document , embed , empty , font , image , manifest , object , paintworklet , report , script , serviceworker , sharedworker , style , track , video , worker , xslt , and nested-document .

Server-sent events

Last-Event-ID ... NEL Defines a mechanism that enables developers to declare a network error reporting policy. 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

Other

Contributing

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See also