How Http Headers Work
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 ,
HTTP Headers for Dummies – Tuts+ Code
Read Time:21 minsLanguages:
Whether you’re a programmer or not, you have seen it everywhere on the web. Even your first Hello World PHP script sent HTTP headers without you realizing it. In this article, we are going to learn about the basics of HTTP headers and how we can use them in our web applications.
What Are HTTP Headers?
HTTP stands for “Hypertext Transfer Protocol”. The entire World Wide Web uses this protocol. It was established in the early 1990s. Almost everything you see in your browser is transmitted to your computer over HTTP. For example, when you opened this article page, your browser probably sent over 40 HTTP requests and received HTTP responses for each.
HTTP headers are the core part of these HTTP requests and responses, and they carry information about the client browser, the requested page, the server, and more.
When you type a URL in your address bar, your browser sends an HTTP request, and it may look like this:
GET /tutorials/other/top-20-mysql-best-practices/ HTTP/1. 1
User-Agent: Mozilla/5. 0 (Windows; U; Windows NT 6. 1; en-US; rv:1. 9. 1. 5) Gecko/20091102 Firefox/3. 5. 5 ( CLR 3. 30729)
Accept: text/html, application/xhtml+xml, application/xml;q=0. 9, */*;q=0. 8
Accept-Language: en-us, en;q=0. 5
Accept-Encoding: gzip, deflate
Accept-Charset: ISO-8859-1, utf-8;q=0. 7, *;q=0. 7
The first line is the “Request Line”, which contains some basic information on the request. And the rest are the HTTP headers.
After that request, your browser receives an HTTP response that may look like this:
HTTP/1. x 200 OK
Date: Sat, 28 Nov 2009 04:36:25 GMT
X-Powered-By: W3 Total Cache/0. 8
Expires: Sat, 28 Nov 2009 05:36:25 GMT
Cache-Control: max-age=3600, public
Content-Type: text/html; charset=UTF-8
Last-Modified: Sat, 28 Nov 2009 03:50:37 GMT
X-Pingback: Content-Encoding: gzip
Vary: Accept-Encoding, Cookie, User-Agent
The first line is the “Status Line”, followed by “HTTP Headers”, until the blank line. After that, the “content” starts (in this case, the HTML output).
When you look at the source code of a web page in your browser, you will only see the HTML portion and not the HTTP headers, even though they actually have been transmitted together, as you can see above.
Now, let’s start reviewing the structure in more detail.
How to See HTTP Headers
I used Firefox Firebug to analyze HTTP headers, but you can use the Developer Tools in Firefox, Chrome, or any modern web browser to view HTTP headers.
getallheaders() gets the request headers. You can also use the $_SERVER array.
headers_list() gets the response headers.
Further in the article, we will see some code examples in PHP.
HTTP Request Structure
The first line of the HTTP request is called the request line and consists of three parts:
The “method” indicates what kind of request this is. The most common methods are GET, POST, and HEAD.
The “path” is generally the part of the URL that comes after the host (domain). For example, when requesting “, the path portion is “/tutorials/top-20-mysql-best-practices–net-7855”.
The protocol part contains HTTP and the version, which is usually 1. 1 in modern browsers.
The remainder of the request contains HTTP headers as Name: Value pairs on each line. These contain various information about the HTTP request and your browser. For example, the User-Agent line provides information on the browser version and the Operating System you are using. Accept-Encoding tells the server if your browser can accept compressed output like gzip.
You may have noticed that the cookie data is also transmitted inside an HTTP header. And if there was a referring URL, that would have been in the header too.
Most of these headers are optional. This HTTP request could have been as small as this:
And you would still get a valid response from the web server.
The three most commonly used request methods are GET, POST, and HEAD. You’re probably already familiar with the first two from writing HTML forms.
GET: Retrieve a Document
For example, when loading an Envato Tuts+ article, the very first line of the HTTP request looks like so:
GET /tutorials/other/top-20-mysql-best-practices/ HTTP/1. 1…
Once the HTML loads, the browser will start sending GET requests for images that may look like this:
GET /wp-content/themes/tuts_theme/images/ HTTP/1. 1…
Web forms can be set to use the GET method. Here’s an example.
When that form is submitted, the HTTP request begins like this:
GET / HTTP/1. 1…
You can see that each form input was added to the query string.
POST: Send Data to the Server
Even though you can send data to the server using GET and the query string, in many cases POST will be preferable. Sending large amounts of data using GET is not practical and has limitations.
POST requests are most commonly sent by web forms. Let’s change the previous form example to a POST method.