HTTP Headers

Understanding HTTP Headers: All You Need to Know

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HTTP headers are a key part of the web. They’re essential for allowing us to interact with websites, and understanding them is vital if we want to make full use of the internet.

HTTP stands for Hypertext Transfer Protocol – it’s the protocol that computers use to communicate over the internet. When you enter an address into your browser, such as, it sends out a request message which contains information about where it would like data from. The response includes something called ‘headers’, which provide more details about how the website should be displayed on your screen. They contain instructions on how to process the content being sent back by the server, including things like caching rules or security protocols.

Headers help ensure that our experience of visiting a website is consistent across different devices and browsers; without them, many features wouldn’t work properly. Read on to find out all you need to know about how these important components function – and why they matter when it comes to accessing online resources efficiently and securely.

What Are The Http Headers?

HTTP Headers are a set of key-value pairs that form part of an HTTP request and response. They are sent between the web server and the browser to provide information about the type of request, who is sending it, what content is being requested, etc. The most common headers include the host http request header, user agent header, access control header and content length header.

The host header specifies which domain or IP address is making the request and can be used by servers with multiple websites hosted on them to identify which site should respond to the request. The User Agent header lets the server know which browser is making the request so that it can serve up a version of its website optimized for that specific browser. Access Control allows browsers to limit how resources on their sites are accessed while Content Length tells the server how much data was sent in the body of a POST or PUT request.

All these headers combined help both parties understand each other’s requests better and ensure smooth communication over the internet. To summarize, they provide vital pieces of information necessary for successful communication over HTTP protocol.

Importance Of Understanding HTTP Headers

The importance of understanding HTTP Headers lies in the fact they help maintain security while allowing user-friendly interaction with websites and applications. Security headers have been designed to protect against various malicious attacks like Cross-site scripting (XSS) and clickjacking; these are largely dependent on correctly configuring all necessary HTTP headers. Additionally, when using an HTTP proxy server –which allows internet traffic from multiple devices to be routed through it–it’s important to understand how different requests will behave when using specific HTTP headers so as to configure them accordingly.

Understanding HTTP headers is an essential skill for any web developer or system administrator. It is a fundamental part of managing websites and ensuring optimal performance, security, and reliability. Without knowledge of how to use request and response headers correctly, many important tasks would be difficult – if not impossible – to complete properly.

How To Troubleshoot HTTP Headers?

When troubleshooting HTTP headers, the first step is to identify any proxy or load balancer that may be present. This should include inspecting the server logs for information about the currently requested page as well as examining web browsers for forwarded header attributes retrieved from other locations.

Additionally, it’s essential to check for strict transport security settings and requesting user agent information in order to ensure all caching mechanisms are correctly configured. Furthermore, identifying irregularities between a browser request and responses sent back by the server can help pinpoint issues with a particular system configuration. To further assist in debugging, there are several tools available that provide detailed insight into HTTP headers. These will enable quick identification of any problems present and allow users to take corrective action accordingly.

List Of Tools For Troubleshooting Http Headers

Charles Proxy

It supports both request and response HTTP Messages and comes equipped with many features such as throttling, external view of internal networks, SSL proxying, repeating requests, etc. In addition to identifying the main Header section containing fields like Content-Type or User-Agent, Charles also shows other HTTP headers like Transfer-Encoding or Keep-Alive which aren’t visible from the browser.

Fiddler Web Debugger by Telerik

This tool provides similar functionality as Charles but has additional profiling capabilities like performance diagnostics and session recording. With Fiddler you can inspect all parts of an HTTP message including full body messages and header fields in real time capture mode.


Makes it easy to construct complex HTTP requests without having to worry about syntax or structure. All content types are supported (JSON/XML) along with their respective encodings. The intuitive interface allows users to create custom headers easily while allowing them to compare responses between different endpoints side by side.

Examples of HTTP Headers

Here’s an example of an HTTP request header sent by a client:

GET /example.html HTTP/1.1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.110 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Accept-Encoding: gzip, deflate, br
Connection: keep-alive

Let’s break this down and explain what each of the headers does:

  1. GET /example.html HTTP/1.1: This line specifies the HTTP method, the requested resource (in this case, “/example.html”), and the HTTP version being used.
  2. Host: This header specifies the hostname of the server being requested. It is required for HTTP/1.1 requests.
  3. User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.110 Safari/537.36: This header provides information about the client making the request. In this example, it identifies the client as running Windows 10 and using the Chrome web browser.
  4. Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9: This header specifies the MIME types that the client is willing to accept in the response. In this example, the client will accept HTML, XHTML, XML, and several image formats.
  5. Accept-Encoding: gzip, deflate, br: This header specifies the encoding schemes that the client is willing to accept in the response. In this example, the client will accept gzip, deflate, and Brotli compression.
  6. Connection: keep-alive: This header specifies that the client would like to keep the TCP connection open after the request has been fulfilled, so that it can be reused for future requests.

here’s an example of HTTP response headers that the server might send in response to the request we looked at earlier:

HTTP/1.1 200 OK
Date: Wed, 24 Mar 2023 00:00:00 GMT
Server: Apache/2.4.38 (Debian)
Last-Modified: Tue, 23 Mar 2023 12:00:00 GMT
ETag: "123abc456def789ghi"
Content-Length: 1234
Content-Type: text/html; charset=UTF-8

Let’s break this down and explain what each of the headers means:

  1. HTTP/1.1 200 OK: This line specifies the HTTP version, the status code (in this case, 200 OK), and a brief message about the status.
  2. Date: Wed, 24 Mar 2023 00:00:00 GMT: This header specifies the date and time that the response was generated.
  3. Server: Apache/2.4.38 (Debian): This header specifies the name and version of the server software being used.
  4. Last-Modified: Tue, 23 Mar 2023 12:00:00 GMT: This header specifies the date and time that the requested resource was last modified.
  5. ETag: “123abc456def789ghi”: This header provides a unique identifier for the requested resource, which can be used for caching purposes.
  6. Content-Length: 1234: This header specifies the length of the response body, in bytes.
  7. Content-Type: text/html; charset=UTF-8: This header specifies the MIME type and character encoding of the response body.

HTTP Header Categories

HTTP Header Categories are an important concept in web development, as it helps to categorize the different types of HTTP headers and their associated functions. In this section, we will explore the different categories of HTTP headers and their usage.



Indicates the authentication method the server is expecting the client to use.

Authorization Header

It is used to authenticate a user and provide a token that can be used to access the requested resources.


Indicate the authentication scheme that should be used by the client to gain access.


Identifies the client to a proxy server requesting access.



The age of the response message in seconds.


Specifies the caching behavior of the response message


Instructs the browser to clear specific types of data for a given website


Specifies the date and time after which the response message is considered stale


Provides backward compatibility with HTTP/1.0 caches that do not implement Cache-Control.


Carries additional information about the status or transformation of a message that may not be reflected in the message.

Client Hints


Indicates the client’s willingness to receive information about client hints from the server.


Specifies the duration for which the server can use the client hints provided in the Accept-CH header.


Indicates that the client requires certain headers to be present in the response for security reasons.


Indicates the client’s preference for reduced motion graphics.


Sends user-agent information from the client to the server.


Specifies the architecture of the client’s CPU.


Specifies the bitness of the client’s CPU.


Sends the full version of the client’s user-agent.


Sends a list of client’s user-agent full version strings.


Indicates whether the client is a mobile device or not.


Specifies the model of the client’s device.


Specifies the platform on which the client is running.


Specifies the version of the platform on which the client is running.


Indicates the device pixel ratio for the rendered content.


Indicates the approximate amount of device memory in gigabytes.


Indicates the device pixel ratio of the output device.


Specifies the width of the layout viewport in CSS pixels.


Indicates the width of the viewport in device-independent pixels.


Indicates the effective network bandwidth estimate in megabits per second.


Indicates the effective connection type, which represents the network conditions.


Indicates the estimated round-trip time between the client and the server.


Indicates whether the client has requested a reduced data usage mode.



Indicates the last time the resource was modified by the server


A unique identifier for the current version of the resource.


Used in conditional requests to only perform the action if the provided ETag matches the current ETag of the resource.


Used in conditional requests to only perform the action if the provided ETag does not match the current ETag of the resource.


Used in conditional requests to only perform the action if the resource has been modified since the provided timestamp.


Used in conditional requests to only perform the action if the resource has not been modified since the provided timestamp.


Indicates the request headers that should be taken into account when caching the response.

Connection management


This header specifies whether the connection will be kept open or closed after the current request/response has been completed.


This header allows the client and server to negotiate how long the connection should be kept open after the response has been sent.

Content negotiation


This header specifies the media types that are acceptable for the response.


This header specifies the encodings that the client can understand and that can be applied to the response.


This header specifies the natural languages that are preferred for the response.



Specifies certain expectations the client has for a server response.


Limits the number of times a request may be forwarded.



Contains data sent by a server to a client, which is then sent back by the client with subsequent requests.


Sets a cookie on the client’s browser for use with subsequent requests.



Specifies which origins are allowed to access a resource.


Indicates whether the response can be exposed when the credentials flag is true.


Lists the headers that a server will allow a client to use in a request.


Lists the HTTP methods that a server will allow a client to use in a request.


Indicates which headers are safe to expose to the response of a CORS API request.


Specifies the time (in seconds) that a preflight request can be cached.


Specifies the origin of the request.


Allows for timing information to be shared across origins.



Indicates whether a response body should be displayed as an attachment or inline.

Message body information


Specifies the size (in bytes) of a response body.


Specifies the MIME type of a response.


Specifies the encoding used to compress a response.


Specifies the natural language used in a response.


Specifies the URL of the resource returned in a response.



Provides information about the originating client and other proxies in a chain of proxies.


Provides information about the originating client IP address in a chain of proxies.


Provides information about the original host requested by the client in a chain of proxies.


Specifies the protocol used by the client to access the resource in a chain of proxies.


Provides information about intermediate proxies through which the request was sent.



Specifies the URL to which a client should be redirected.


Specifies how long a client should wait before refreshing a page.

Request context


Specifies the email address of the user making a request.


Specifies the host name and port number of the server to which a client is connected.


Specifies how much information the browser should include in the Referer header.

Response context


Lists the HTTP methods that a server will allow a client to use.


Provides information about the server that generated the response.

Range requests


Indicates whether a server supports byte-range requests.


Requests a specific range of data from a resource.


Specifies the range of a resource that a client is requesting.


Indicates the range of bytes returned in a response.



Controls whether a resource can be embedded in a cross-origin context.


Controls whether a resource can open a new browsing context in a cross-origin context.


Controls which resources can be accessed from a cross-origin context.

Content-Security-Policy (CSP)

Specifies which resources are allowed to be loaded on a page.


Similar to Content-Security-Policy, but only generates reports.


Requires a certificate transparency check to be performed.


Specifies which features a page can use.

Strict-Transport-Security (HSTS)

Forces the use of HTTPS for a period of time.


Requests that a client be redirected to HTTPS if it initially made an insecure request.


Prevents MIME-type sniffing and forces a browser to use the declared MIME type.

X-Frame-Options (XFO)

Specifies whether a page can be displayed in a frame or iframe.


Enables the browser’s built-in XSS protection.

Fetch metadata request headers


Indicates the context in which a request was made.


Indicates the type of request that is being made.


Indicates whether a request was made by a user gesture.


Indicates the type of resource that a request is intended for.


Requests that a service worker preloads certain resources.

Server-sent events


Notifies the server that the client wants to receive network error logging information.

Transfer coding


Specifies the encoding used to transfer a message body to the recipient.


Specifies that a trailer field is present in the message body.



Contains a value that confirms that the server has accepted the WebSocket upgrade request.



Specifies alternative services that a client can use to connect to a resource.


Specifies the date and time when the response was generated.


Allows a client to send data in the first request to save time.


Enables a page to request a large allocation of memory.


Specifies relationships between the current document and other resources.


Specifies when a client should try a request again after a server returns a 503 (service unavailable) status code.


Provides performance timing information about the server’s handling of a request.


Maps a minified file back to its original source code.


Requests that the server upgrade the protocol being used.


Controls whether a browser should prefetch DNS requests for links on a page.