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HTTP headers – MDN Web Docs
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:
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:
Upgrade (see also Protocol upgrade mechanism).
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.
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.
Defines the authentication method that should be used to access a resource.
Contains the credentials to authenticate a user-agent with a server.
Defines the authentication method that should be used to access a resource behind a proxy server.
Contains the credentials to authenticate a user agent with a proxy server.
The time, in seconds, that the object has been in a proxy cache.
Directives for caching mechanisms in both requests and responses.
Clears browsing data (e. g. cookies, storage, cache) associated with the requesting website.
The date/time after which the response is considered stale.
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.
General warning information about possible problems.
Client hintsHTTP 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.
Servers can advertise support for Client Hints using the Accept-CH header field or an equivalent HTML element with -equiv attribute.
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.
The different categories of client hints are listed client hints
Response header used to confirm the image device to pixel ratio in requests where the DPR client hint was used to select an image resource.
Approximate amount of available client RAM memory. This is part of the Device Memory API.
Client device pixel ratio (DPR), which is the number of physical device pixels corresponding to every CSS pixel.
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).
The Width request header field is a number that indicates the desired resource width in physical pixels (i. intrinsic size of an image).
Network client hintsNetwork client hints allow a server to choose what information is sent based on the user choice and network bandwidth and latency.
Approximate bandwidth of the client’s connection to the server, in Mbps. This is part of the Network Information API.
The effective connection type (“network profile”) that best matches the connection’s latency and bandwidth. This is part of the Network Information API.
Application layer round trip time (RTT) in miliseconds, which includes the server processing time. This is part of the Network Information API.
A boolean that indicates the user agent’s preference for reduced data usage.
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.
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.
Makes the request conditional, and applies the method only if the stored resource matches one of the given ETags.
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.
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.
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.
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.
Controls whether the network connection stays open after the current transaction finishes.
Controls how long a persistent connection should stay open.
Content negotiationContent negotiation headers.
Informs the server about the types of data that can be sent back.
The encoding algorithm, usually a compression algorithm, that can be used on the resource sent back.
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).
Indicates expectations that need to be fulfilled by the server to properly handle the request.
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
The size of the resource, in decimal number of bytes.
Indicates the media type of the resource.
Used to specify the compression algorithm.
Describes the human language(s) intended for the audience, so that it allows a user to differentiate according to the users’ own preferred language.
Indicates an alternate location for the returned data.
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.
Identifies the originating IP addresses of a client connecting to a web server through an HTTP proxy or a load balancer.
Identifies the original host requested that a client used to connect to your proxy or load balancer.
Identifies the protocol (HTTP or HTTPS) that a client used to connect to your proxy or load balancer.
Added by proxies, both forward and reverse proxies, and can appear in the request headers and the response headers.
Indicates the URL to redirect a page to.
Contains an Internet email address for a human user who controls the requesting user agent.
Specifies the domain name of the server (for virtual hosting), and (optionally) the TCP port number on which the server is listening.
The address of the previous web page from which a link to the currently requested page was followed.
Governs which referrer information sent in the Referer header should be included with requests made.
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.
Lists the set of HTTP request methods supported by a resource.
Contains information about the software used by the origin server to handle the request.
Indicates if the server supports range requests, and if so in which unit the range can be expressed.
Indicates the part of a document that the server should return.
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.
Indicates where in a full body message a partial message belongs.
Allows a server to declare an embedder policy for a given document.
Prevents other domains from opening/controlling a window.
Prevents other domains from reading the response of the resources to which this header is applied.
Controls resources the user agent is allowed to load for a given page.
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.
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.
Provides a mechanism to allow and deny the use of browser features in its own frame, and in iframes that it embeds.
Provides a mechanism to allow web applications to isolate their origins.
Force communication using HTTPS instead of HTTP.
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.
Disables MIME sniffing and forces browser to use the type given in Content-Type.
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).
Indicates whether a browser should be allowed to render a page in a ,
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HTTP header and HTTP body format with examples. – Clean …
This post is for advanced learners.
Hyper Text Transfer Protocol communication allows two types of messages to be transferred between the Client and the server, HTTP Request and the HTTP Response. The request is sent by the client/browser to the server and the response is sent by the server to the browser. Both the message have a common format, they both contain a HTTP Header and a HTTP Body.
The HTTP Header contains information about the HTTP Body and the Request/Response.
Information about the body is related to the content of the Body such as the length of the content inside the body.
The information about Request/Response is the general information about the request/response and it is not specific to the content of the body, example at what time the Request was made or the browser used to make the request.
The properties in header are specified as name-value pair which are separated from each other by a colon ‘:’. The format is given below.
What are the types of HTTP Headers?
In HTTP 1. 1, header can further divided into 3 parts.
Nothing to do with the HTTP body’s content being transferred but it contains general information about the communication such as the date and time on which the request/response was generated. This header is common to both Request and Response. Example
Date:Tue, 17 Nov 2015 16:39:15 GMT
Request Header is present when you make a request to the server and the response header is present when the server sends a response back to the client/browser.
Request Header contains information about the request such as the URL that you have requested, the method(GET, POST, HEAD), the browser used to generate the request and other info. Example
User-Agent:”Mozilla/5. 0 (Windows NT 10. 0; WOW64; rv:41. 0) Gecko/20100101 Firefox/41. 0″
The term browser is also called user-Agent. So even a simple request to a page involves sending the information about your browser and the operating system you are using. You can see from the above Header field that i am using Windows 10 and Firefox 41. 0 browser.
Response Header is received by the browser from the server after the user sends a request for a particular page or resource and it contains information such as the encoding used in the content, the server software that is used on the server machine to generate the response and other information. Example
Most of the sites usually hide their server information in order to make it hard for hackers to know which software is being used on the server.
This header contains information about the actual message or the HTTP body that is being sent. Information such as content length, the language of the content, encoding, expiration date and other important stuff. Example
HTTP Headers: Understanding HTTP and APIs Part 2 – Steve’s …
In the second part of our short course we will look at headers.
HTTP headers re used to convey additional information between the client and the server.
Although they are optional they make up the most of the request and are almost always present.
When you request a web page using a web browser the headers are inserted automatically by the web browser, and you don’t see them.
Similarly the response headers are inserted by the web server and are not seen by the user.
There are extensions available for both Firefox and Chrome that let you view headers and also command line tools like curl.
We will look at some example request and response headers later.
Request and Response Header Structure
Request and response headers share a common structure.
They consist of a header name + colon + header value. Example
An header can have multiple values, in which case they are separated using a comma.
Field names are case insensitive according to the RFC 2616 but field values should be treated as case sensitive
There are many headers, and it is not important to be familiar with them all.
There is a really good list of headers with explanation on tutorialspoint
You should note that only necessary headers are sent all other headers are assumed by the web server and client to be their default.
For example the connection header is not normally sent as the default behaviour is keep-alive and this is assumed by the server.
Common Request Headers
The original HTTP protocol used non persistent connections.
This meant that the client
Made a request
Got a Response
Closed the Connection
If you consider a TCP/IP connection to be the same as a telephone connection. This means that you:
Dial the number and get an answer (connection established.
Say something and get an acknowledgement
Because it takes time and resources to establish the connection in the first place it makes no sense to drop it so quickly.
Therefore in HTTP 1. 0 the client can tell the server that it will keep the connection open by using the connection: keep-alive header.
In HTTP v1. 1 the default behaviour was changed and persistent connections become the default mode.
Now the client can tell the server that it will close the connection by using the header connection: close.
This header is not normally sent as the default assumed by the server is keep-alive.
Almost all websites, including this one, use shared hosting.
With shared hosting the web server is configured as a virtual host and all virtual hosts will be assigned to a single IP address.
The hosts header tells the web server which server to refer the request to e. g.
This gives information about the client making the request as shown below:
Accept Request Headers
These headers are used for content negotiation and are sent by the client (browser) to the server, and tells the server what formats the client can understand.
The Accept header is used to tell the server what media types the client prefers e. g. Text, audio etc.
For normal web pages common values are text/plain and text/html.
For JSON encoded data the header
Other accept headers are
Example Headers from live session:
Common Response Headers
The screen shot below shows the response headers using the curl command requesting a web page from
The first line of the response is mandatory and consists of the protocol ( HTTP/1. 1), response code (200)and description (OK).
All subsequent lines are optional
The headers shown are:
CONTENT-Type -This is Text/html which is a web page. It also includes the character set which is UTF-8.
Connection – Is keep-alive which means that the connection is held open.
Keep-Alive -This setting as shown is a timeout which says that the server will keep the connection open for 15 seconds then close it. See here for more details.
All of the other headers are self explanatory.
Here is another screen shot (partial) of a site returning JSON data, notice the Content type.
For normal users headers are of no interest as they are hidden and created automatically either by the browser (request headers) or web server (response headers).
However web developers and IOT developers will need to be able to set these headers manually.
The most common tool used for this is the command line curl utility but extensions are available for Google Chrome and Firefox that also let you set request headers.
Just do a search for chrome headers extension or Firefox headers add-on.
HTTP headers convey addition information between the client and web server and are used on the request and response.
Although they are inserted automatically by the web browser and web server it is important for web developers, IOT developers and engineers to have a basic understanding of them as it is sometimes necessary to manually add headers when making API requests using command line tools like curl.
It is also sometimes necessary to process response headers in IOT applications.
Frequently Asked Questions about what is a http header
What is HTTP header and body?
The HTTP Header contains information about the HTTP Body and the Request/Response. Information about the body is related to the content of the Body such as the length of the content inside the body. … The properties in header are specified as name-value pair which are separated from each other by a colon ‘:’ .Nov 17, 2015
When should I use HTTP headers?
HTTP headers re used to convey additional information between the client and the server. Although they are optional they make up the most of the http request and are almost always present. When you request a web page using a web browser the headers are inserted automatically by the web browser, and you don’t see them.Jan 25, 2021
How do I read HTTP headers?
To view the request or response HTTP headers in Google Chrome, take the following steps :In Chrome, visit a URL, right click , select Inspect to open the developer tools.Select Network tab.Reload the page, select any HTTP request on the left panel, and the HTTP headers will be displayed on the right panel.Jan 21, 2016