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An In-Depth Guide to HTTP Headers

HTTP headers allow clients and servers to provide additional context and data in requests and responses. Though often overlooked, properly leveraging headers can make a big difference in areas like web scraping and application security.

In this comprehensive guide, we‘ll dive deep into the world of HTTP headers – what they are, how they‘re used, and how you as a developer can optimize them for your needs.

What Exactly are HTTP Headers?

HTTP stands for HyperText Transfer Protocol. It‘s the underlying protocol that defines how data communication happens between clients and servers over the web.

HTTP headers allow the clients making requests – like browsers, cURL, Postman, web scrapers etc – to send additional information about the request itself or client‘s capabilities.

Similarly, servers can include additional metadata about the response through headers.

Headers are additional context about the HTTP request or response, sent as key-value pairs. According to the MDN Web Docs, some examples of how headers are used:

  • A server can indicate the data format returned through Content-Type header.
  • A client can indicate supported data formats with the Accept header.
  • Clients pass authorization tokens using Authorization header.
  • Origin requests are allowed through Access-Control-Allow-Origin.
  • Cookies are set via Set-Cookie header.

Simply put, headers allow richer interaction and metadata exchange between clients and origin servers. There are different categories of HTTP headers:

Request Headers

As the name suggests, these headers are sent by the client making the HTTP request. They indicate details about the source of the request, formats supported by client, authorization information, and more.

Some common request headers are:

  • User-Agent – Indicates client software details like browser name, version, operating system etc.
  • Accept – Indicates acceptable response formats for client like JSON, HTML etc.
  • Accept-Language – Indicates preferred languages in response.
  • Authorization – Contains authentication credentials if request needs authorization.
  • Cookie – Sends cookies stored by client to the server.
  • Referer – Indicates previous web page visited by client – i.e the referrer.

Response Headers

These headers are sent by the server in response to client requests. They contain details about the response itself like content format, status, cookies being set, redirection instructions etc.

Some common examples of response headers:

  • Content-Type – Indicates format of response body like JSON, HTML, XML etc.
  • Set-Cookie – Contains cookies being set by server in client.
  • Location – Used to redirect client‘s request to a different endpoint.
  • WWW-Authenticate – Indicates type of authentication required for a resource.
  • Server – Indicates server software like nginx, Apache, IIS etc.

General Headers

General headers contain contextual information that applies to both requests and responses, but not the body itself.

Some common general headers are:

  • Connection – Controls if network connection stays open after current request/response cycle.
  • Date – Indicates date and time when message was originated.
  • Cache-Control – Indicates caching policies to be followed by clients and proxies.
  • Pragma – Used to include implementation specific directives like caching policies.

Entity Headers

These headers contain metadata about the body of the resource itself, like content length, encodings etc.

Some examples of entity headers:

  • Content-Length – Indicates length of the resource body in bytes.
  • Content-Encoding – Indicates encoding algorithms like gzip, deflate etc used on resource.
  • Content-Language – Indicates languages used in the representation like en, en-US etc.

To summarize, here is a table comparing request and response headers:

Request HeadersResponse Headers

And a table of different header types:

Header TypeDescriptionExamples
RequestSent by client about requestUser-Agent, Accept
ResponseSent by server about responseContent-Type, Set-Cookie
GeneralApplies to req and resCache-Control, Connection
EntityAbout resource bodyContent-Length, Content-Encoding

Now that we understand what headers are, let‘s explore why they matter.

Why Optimize HTTP Headers for Web Scraping?

When writing scripts or programs like web scrapers to extract data from websites, properly structuring HTTP requests is crucial.

Optimizing headers for scrapers is important because:

Headers can avoid blocks – By mimicking headers of real users, scrapers appear organic and avoid getting blocked.

Retrieve data effectively – Headers like Accept signal the data format needed, so APIs return optimized responses.

Let‘s explore some common techniques to optimize scraper headers:

Rotate User-Agents – Setting a diverse and updated list of legitimate User-Agents makes scrapers appear like real users. Browsers like Chrome are updating User-Agents all the time, so rotating them is advised. Services like User-Agent Rotator can help here.

Accept JSON responses – APIs will return JSON formatted data if the scraper sets Accept header to application/json. JSON can be easier to parse and handle than HTML.

Distribute requests – Spreading requests over multiple IPs through proxies simulates real user traffic. Tools like Luminati provide Proxy-as-a-Service solutions.

Authorize requests – If the site needs authorization, properly formatting the Authorization header with tokens will grant access.

Referrer header – Populating the Referer header with the previous page URL mimics user browser behavior.

According to Imperva, over 50% of web traffic today is bots. Many are scrapers harvesting data. This has led websites to be increasingly suspicious of non-human traffic.

Mimicking organic users through headers is key to avoiding blocks. Paying attention to headers can help retrieve responses optimized for scraping needs.

Securing Web Applications with HTTP Headers

While HTTP headers help scrape data, they also enable securing web applications by configuring client-server interactions securely.

Headers set boundaries for the browser and strengthen defenses against common attacks like XSS, clickjacking etc.

Some important security headers are:

Content-Security-Policy (CSP)

CSP allows listing approved sources for content like scripts and images. This prevents XSS and code injection attacks.

For example, CSP can whitelist as the only approved source for scripts. Any other script sources will be blocked.


This header prevents clickjacking by not allowing the website to be iframed or embedded in frames.

It can be set to values like DENY to deny framing completely.

Strict-Transport-Security (HSTS)

HSTS enforces HTTPS for future requests to the domain, even if site is accessed over HTTP.

Once enabled, the browser will automatically convert HTTP urls to HTTPS.


This header enables Cross-Origin Resource Sharing (CORS) which allows cross-domain AJAX requests from frontends.

It needs to be set to the request origin like to enable CORS.

There are many other headers like X-XSS-Protection, Referrer-Policy etc that provide security enhancements. Tools like allow easy checking of headers implemented on a site.

According to OWASP, critical attacks like XSS and clickjacking account for over 50% of vulnerabilities. Headers provide an added layer of protection.


We‘ve explored the world of HTTP headers – what they are, different types, how they help in web scraping, and how they secure apps.

Key takeaways are:

  • Headers provide metadata and context to HTTP requests and responses.
  • Optimizing headers helps scrapers avoid blocks and get useful data.
  • Headers like CSP and HSTS bolster security and mitigate common attacks.
  • There are four types of headers – request, response, general and entity.

HTTP headers enable richer client-server communication. Both web scrapers and application developers should understand headers to use them effectively!

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