HTTP headers

HTTP headers let the client and the server pass additional information with a message in a request or response. In HTTP/1.X, a header is a case-insensitive name followed by a colon, then optional whitespace which will be ignored, and finally by its value (for example: Allow: POST). In HTTP/2 and above, headers are displayed in lowercase when viewed in developer tools (accept: */*), and prefixed with a colon for a special group of pseudo-headers (:status: 200). You can find more information on the syntax in each protocol version in the HTTP messages page.

Custom proprietary headers have historically been used with an X- prefix, but this convention was deprecated in 2012 because of the inconveniences it caused when nonstandard fields became standard in RFC 6648; others are listed in the IANA HTTP Field Name Registry, whose original content was defined in RFC 4229. The IANA registry lists headers, including information about their status.

Headers can be grouped according to their contexts:

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.
Representation headers: Contain information about the body of the resource, like its MIME type, or encoding/compression applied.
Payload headers: Contain representation-independent information about payload data, including content length and the encoding used for transport.

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 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.
No-Vary-Search Experimental: Specifies a set of rules that define how a URL's query parameters will affect cache matching. These rules dictate whether the same URL with different URL parameters should be saved as separate browser cache entries.

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 uploading a new resource when one already exists.
If-Modified-Since: Makes the request conditional, and expects the resource 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 resource 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.

Content negotiation

For more details, refer to the Content negotiation article.

Accept: Informs the server about the types of data that can be sent back.
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).
Accept-Patch: A request content negotiation response header that advertises which media type the server is able to understand in a PATCH request.
Accept-Post: A request content negotiation response header that advertises which media type the server is able to understand in a POST request.

Controls

Expect: Indicates expectations that need to be fulfilled by the server to properly handle the request.
Max-Forwards: When using TRACE, indicates the maximum number of hops the request can do before being reflected to the sender.

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.

CORS

For more information, refer to the CORS documentation.

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-Allow-Origin: Indicates whether the response can be shared.
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.

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.

Integrity digests

Content-Digest Experimental: Provides a digest of the stream of octets framed in an HTTP message (the message content) dependent on Content-Encoding and Content-Range.
Repr-Digest Experimental: Provides a digest of the selected representation of the target resource before transmission. Unlike the Content-Digest, the digest does not consider Content-Encoding or Content-Range.
Want-Content-Digest Experimental: States the wish for a Content-Digest header. It is the Content- analogue of Want-Repr-Digest.
Want-Repr-Digest Experimental: States the wish for a Repr-Digest header. It is the Repr- analogue of Want-Content-Digest.

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.
Via: Added by proxies, both forward and reverse proxies, and can appear in the request headers and the response headers.

Range requests

HTTP range requests allow the client to request a portion of a resource from the server. Range requests are useful for applications like media players that support random access, data tools that know they need only part of a large file, and download managers that let the user pause and resume a download.

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.

Redirects

Location: Indicates the URL to redirect a page to.
Refresh: Directs the browser to reload the page or redirect to another. Takes the same value as the meta element with http-equiv="refresh".

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.

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.

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. See also CORP explainer article.
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 Deprecated: Lets sites opt in to reporting and enforcement of Certificate Transparency to detect use of misissued certificates for that site.
Permissions-Policy: Provides a mechanism to allow and deny the use of browser features in a website's own frame, and in <iframe>s that it embeds.
Reporting-Endpoints Experimental: Response header that allows website owners to specify one or more endpoints used to receive errors such as CSP violation reports, Cross-Origin-Opener-Policy reports, or other generic violations.
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-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 (now obsolete), Adobe Acrobat, Microsoft Silverlight (now obsolete), 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.

Fetch metadata request headers

Fetch metadata request headers provide information about the context from which the request originated. A server can use them to make decisions about whether a request should be allowed, based on where the request came from and how the resource will be used.

Sec-Fetch-Site: 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: Indicates the request's mode to a server. It is a Structured Header whose value is a token with possible values cors, navigate, no-cors, same-origin, and websocket.
Sec-Fetch-User: 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: Indicates the request's destination. 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, and xslt.

The following request headers are not strictly "fetch metadata request headers", but similarly provide information about the context of how a resource will be used. A server might use them to modify its caching behavior, or the information that is returned:

Sec-Purpose: Indicates the purpose of the request, when the purpose is something other than immediate use by the user-agent. The header currently has one possible value, prefetch, which indicates that the resource is being fetched preemptively for a possible future navigation.
Service-Worker-Navigation-Preload: A request header sent in preemptive request to fetch() a resource during service worker boot. The value, which is set with NavigationPreloadManager.setHeaderValue(), can be used to inform a server that a different resource should be returned than in a normal fetch() operation.

Server-sent events

Reporting-Endpoints: Response header used to specify server endpoints where the browser should send warning and error reports when using the Reporting API.
Report-To Deprecated Non-standard: Response header used to specify server endpoints where the browser should send warning and error reports when using the Reporting API.

Transfer coding

Transfer-Encoding: Specifies the form of encoding used to safely transfer the resource 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

Headers used by the WebSockets API in the WebSocket handshake:

Sec-WebSocket-Accept: Response header that indicates that the server is willing to upgrade to a WebSocket connection.
Sec-WebSocket-Extensions: In requests, this header indicates the WebSocket extensions supported by the client in preferred order. In responses, it indicates the extension selected by the server from the client's preferences.
Sec-WebSocket-Key: Request header containing a key that verifies that the client explicitly intends to open a WebSocket.
Sec-WebSocket-Protocol: In requests, this header indicates the sub-protocols supported by the client in preferred order. In responses, it indicates the the sub-protocol selected by the server from the client's preferences.
Sec-WebSocket-Version: In requests, this header indicates the version of the WebSocket protocol used by the client. In responses, it is sent only if the requested protocol version is not supported by the server, and lists the versions that the server supports.

Other

Alt-Svc: Used to list alternate ways to reach this service.
Alt-Used: Used to identify the alternative service in use.
Date: Contains the date and time at which the message was originated.
Link: This entity-header field provides a means for serializing one or more links in HTTP headers. It is semantically equivalent to the HTML <link> element.
Retry-After: Indicates how long the user agent should wait before making a follow-up request.
Server-Timing: Communicates one or more metrics and descriptions for the given request-response cycle.
Service-Worker-Allowed: Used to remove the path restriction by including this header in the response of the Service Worker script.
SourceMap: Links to a source map so that debuggers can step through original source code instead of generated or transformed code.
Upgrade: This HTTP/1.1 (only) header can be used to upgrade an already established client/server connection to a different protocol (over the same transport protocol). For example, it can be used by a client to upgrade a connection from HTTP 1.1 to HTTP 2.0, or an HTTP or HTTPS connection into a WebSocket.
Priority: Provides a hint from about the priority of a particular resource request on a particular connection. The value can be sent in a request to indicate the client priority, or in a response if the server chooses to reprioritize the request.

Experimental headers

Attribution reporting headers

The Attribution Reporting API enables developers to measure conversions — for example when a user clicks an ad embedded on one site and then proceeds to purchase the item over on the vendor's site — and then access reports on those conversions. It does this without relying on third-party tracking cookies, instead relying on various headers to register sources and triggers that are matched to indicate a conversion.

Attribution-Reporting-Eligible: Used to indicate that the response corresponding to the current request is eligible to take part in attribution reporting, by registering either an attribution source or trigger.
Attribution-Reporting-Register-Source: Included as part of a response to a request that included an Attribution-Reporting-Eligible header, this is used to register an attribution source.
Attribution-Reporting-Register-Trigger: Included as part of a response to a request that included an Attribution-Reporting-Eligible header, this is used to register an attribution trigger.

Client hints

HTTP Client hints are a set of request headers that provide useful information about the client such as device type and network conditions, and allow servers to optimize what is served for those conditions.

Servers proactively requests the client hint headers they are interested in from the client using Accept-CH. The client may then choose to include the requested headers in subsequent requests.

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.
Critical-CH Experimental: Servers use Critical-CH along with Accept-CH to specify that accepted client hints are also critical client hints.

The different categories of client hints are listed below.

User agent client hints

The UA client hints are request headers that provide information about the user agent, the platform/architecture it is running on, and user preferences set on the user agent or platform:

Sec-CH-UA Experimental: User agent's branding and version.
Sec-CH-UA-Arch Experimental: User agent's underlying platform architecture.
Sec-CH-UA-Bitness Experimental: User agent's underlying CPU architecture bitness (for example "64" bit).
Sec-CH-UA-Form-Factor Experimental: User agent's form-factors, describing how the user interacts with the user-agent.
Sec-CH-UA-Full-Version Deprecated: User agent's full version string.
Sec-CH-UA-Full-Version-List Experimental: Full version for each brand in the user agent's brand list.
Sec-CH-UA-Mobile Experimental: User agent is running on a mobile device or, more generally, prefers a "mobile" user experience.
Sec-CH-UA-Model Experimental: User agent's device model.
Sec-CH-UA-Platform Experimental: User agent's underlying operation system/platform.
Sec-CH-UA-Platform-Version Experimental: User agent's underlying operation system version.
Sec-CH-UA-WoW64 Experimental: Whether or not the user agent binary is running in 32-bit mode on 64-bit Windows.
Sec-CH-Prefers-Color-Scheme Experimental: User's preference of dark or light color scheme.
Sec-CH-Prefers-Reduced-Motion Experimental: User's preference to see fewer animations and content layout shifts.
Sec-CH-Prefers-Reduced-Transparency Experimental: Request header indicates the user agent's preference for reduced transparency.

Note: User-agent client hints are not available inside fenced frames because they rely on permissions policy delegation, which could be used to leak data.

Device client hints

Content-DPR Deprecated Non-standard: Response header used to confirm the image device to pixel ratio (DPR) in requests where the screen DPR client hint was used to select an image resource.
Device-Memory: Approximate amount of available client RAM memory. This is part of the Device Memory API.
DPR Deprecated Non-standard: Request header that provides the client device pixel ratio (the number of physical device pixels for each CSS pixel).
Viewport-Width Deprecated Non-standard: Request header provides the client's layout viewport width in CSS pixels.
Width Deprecated Non-standard: Request header indicates the desired resource width in physical pixels (the intrinsic size of an image).

Network client hints

Network client hints allow a server to choose what information is sent based on the user choice and network bandwidth and latency.

Downlink Experimental: Approximate bandwidth of the client's connection to the server, in Mbps. This is part of the Network Information API.
ECT Experimental: The effective connection type ("network profile") that best matches the connection's latency and bandwidth. This is part of the Network Information API.
RTT Experimental: Application layer round trip time (RTT) in milliseconds, which includes the server processing time. This is part of the Network Information API.
Save-Data Experimental: A string on that indicates the user agent's preference for reduced data usage.

Privacy

DNT Deprecated Non-standard: Request header that indicates the user's tracking preference (Do Not Track). Deprecated in favor of Global Privacy Control (GPC), which is communicated to servers using the Sec-GPC header, and accessible to clients via navigator.globalPrivacyControl.
Tk Deprecated Non-standard: Response header that indicates the tracking status that applied to the corresponding request. Used in conjunction with DNT.
Sec-GPC Non-standard Experimental: Indicates whether the user consents to a website or service selling or sharing their personal information with third parties.

Security

Origin-Isolation Experimental: Provides a mechanism to allow web applications to isolate their origins.

Server-sent events

NEL Experimental: Defines a mechanism that enables developers to declare a network error reporting policy.

Topics API

The Topics API provides a mechanism for developers to implement use cases such as interest-based advertising (IBA). See the Topics API documentation for more information.

Observe-Browsing-Topics Experimental Non-standard: Response header used to mark topics of interest inferred from a calling site's URL as observed in the response to a request generated by a feature that enables the Topics API.
Sec-Browsing-Topics Experimental Non-standard: Request header that sends the selected topics for the current user along with the associated request, which are used by an ad tech platform to choose a personalized ad to display.

Other

Accept-Signature Experimental: 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.
Early-Data Experimental: Indicates that the request has been conveyed in TLS early data.
Origin-Agent-Cluster Experimental: Response header used to indicate that the associated Document should be placed in an origin-keyed agent cluster. This isolation allows user agents to allocate implementation-specific resources for agent clusters, such as processes or threads, more efficiently.
Set-Login Experimental: Response header sent by a federated identity provider (IdP) to set its login status, meaning whether any users are logged into the IdP on the current browser or not. This is stored by the browser and used by the FedCM API.
Signature Experimental: 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 Experimental: The Signed-Headers header field identifies an ordered list of response header fields to include in a signature.
Speculation-Rules Experimental: Provides a list of URLs pointing to text resources containing speculation rule JSON definitions. When the response is an HTML document, these rules will be added to the document's speculation rule set.
Supports-Loading-Mode Experimental: Set by a navigation target to opt-in to using various higher-risk loading modes. For example, cross-origin, same-site prerendering requires a Supports-Loading-Mode value of credentialed-prerender.

Non-standard headers

X-Forwarded-For Non-standard: Identifies the originating IP addresses of a client connecting to a web server through an HTTP proxy or a load balancer.
X-Forwarded-Host Non-standard: Identifies the original host requested that a client used to connect to your proxy or load balancer.
X-Forwarded-Proto Non-standard: Identifies the protocol (HTTP or HTTPS) that a client used to connect to your proxy or load balancer.
X-DNS-Prefetch-Control Non-standard: 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-Robots-Tag Non-standard: The X-Robots-Tag HTTP header is 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="…">.

Deprecated headers

Pragma Deprecated: 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 Deprecated: General warning information about possible problems.

Contributing

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