What Are HTTP Headers: Things You Should Know As a Developer

Web scraping tools automate the collection of large amounts of publicly available data. But there’s a whole technical side of that process you don’t see. One key factor is an HTTP header. Properly using and optimizing headers can improve data accuracy and reduce the risk of blocks. In this article, we’ll cover what an HTTP header is, how it works and why it matters in the world of web scraping.

HTTP header defined

HTTP stands for HyperText Transfer Protocol and governs how communication is structured and transmitted across the internet. It dictates how web servers (like websites) and browsers (such as Chrome, Firefox, or Safari) should handle different requests.

An HTTP header allows clients and servers to exchange additional information within a request or response. These headers provide context that helps both sides understand how to process the request or response correctly.

Typically, a client sends a request containing headers, which include extra details for the server. The server then replies with the requested data, and certain request headers can even influence how the server formats or customizes its response.

How an HTTP header works: a simple explanation

HTTP headers work by attaching additional information to the messages exchanged between a client (such as a browser or scraper) and a web server.

When a client sends a request, request headers inform the server about:

• The type of content it can accept
• The preferred language
• The software or client being used

In response, the server returns response headers that provide information about:

• The content being sent
• The server itself
• How the client should handle the response

While headers don’t carry the main content (like the web page itself), they are essential for controlling how content is delivered, displayed, and secured.

HTTP headers list

To make sense of the HTTP headers list, HTTP headers are commonly grouped according to their context within a request or response.

1. General HTTP headers

General headers apply to both requests and responses but do not describe the content itself. They can appear in any HTTP message. Common examples include:

• Connection
• Cache-control
• Date

2. General HTTP headers

Entity headers provide information about the body of the resource. Each header conveys specific details, such as Content-Language or Content-Length.

Note: “Entity headers” is a legacy term. In newer HTTP specifications, they are often referred to as representation headers.

3. HTTP request headers

Next on our HTTP headers list is request headers. They’re sent from the client (such as a browser or scraping tool) to the server and provide key information about the origin of the request — like the browser or application being used, its version, and the operating system.

These headers play a critical role in how websites deliver content. Many sites adapt their layout, format, and features depending on the requester’s “user agent,” a collection of details describing the client’s software and hardware. Without this information, a site may not know how to render content correctly.

If a server doesn’t recognize or accept the user agent, it typically responds in one of two ways:

• It serves a simplified, default version of the page, or
• It rejects the request altogether.

4. HTTP response headers

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Final on our HTTP headers list is response headers. They’re sent by the server after receiving a request and provide details about the outcome of that request — such as connection type, encoding, or whether the operation succeeded. When something goes wrong, response headers include an HTTP status code indicating the type of issue. These codes are grouped into five categories:

• 1xx — Informational
• 2xx — Success
• 3xx — Redirection
• 4xx — Client Error
• 5xx — Server Error

Each category contains multiple, more specific status codes. You can find a full HTTP headers list of these codes in the official HTTP documentation.

Which HTTP header to choose for scraping

When scraping websites, the right HTTP header can mean the difference between smooth data extraction and getting blocked. Below is a HTTP header list with the most commonly used headers that help scrapers stay undetected and collect accurate data.

1. HTTP header User-agent. Identifies the browser, device, or application sending the request. Servers use it to customize responses or spot automated traffic.
2. HTTP header Accept-language. Indicates the client’s preferred languages so the server can deliver content in the most appropriate locale.
3. HTTP header Accept-encoding. Lists the compression formats the client supports (e.g., gzip, deflate) to enable faster, compressed data transfers.
4. HTTP header Accept. Specifies which content types (MIME types) the client is willing to receive in the response.
5. HTTP header Referer. Provides the URL of the page that initiated the request. Commonly used for analytics, routing, or access control.
6. HTTP header Host. Indicates the domain name the client wants to reach, crucial for servers hosting multiple websites on the same IP.
7. HTTP header Connection. Defines whether the connection should remain open after the response or close immediately.

HTTP header examples

Below is an example of a typical HTTP header. It shows how a browser communicates its preferences and identity to the server.

GET /URL/destination/to/get/ HTTP/1.1
Host: targetwebsite.com
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.1.5) 
Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729)
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
Keep-Alive: 300
Connection: keep-alive
Cookie: PHPSESSID=r2t5uvjq435r4q7ib3vtdjq120
Pragma: no-cache
Cache-Control: no-cache

The User-Agent header is one of the key factors that determines whether a request is accepted. Using common, legitimate user agents is essential for avoiding blocks during web scraping.

Several other HTTP headers are also affected by how a proxy forwards or modifies requests. Here are the headers most relevant to proxy handling:

1. Connection. Controls whether the network connection remains open after completing the current request.
2. Keep-alive. Lets the client set a maximum number of requests and a timeout for the connection. Only valid when Connection is set to “Keep-Alive.”
3. Proxy-authenticate. A response header indicating the authentication method required to access a resource through a proxy.
4. Proxy-authorization. A request header containing credentials to authenticate with a proxy server.
5. Trailer. Allows additional fields to be sent at the end of chunked responses, such as checksums, post-processing status, or digital signatures.
6. Transfer-encoding. Defines the encoding method used to safely deliver the message body between nodes, rather than the resource itself.

Benefits of using an HTTP header

Reduce the risk of getting blocked
Using an HTTP header correctly can help your scraper mimic real user behavior, lowering the chances of being blocked by the target server.

Improve data quality
Proper headers ensure that the server delivers content in the expected format, increasing the accuracy and completeness of the data you retrieve.

In short, HTTP headers directly affect both the type and quality of data collected. By customizing headers to resemble genuine user requests, your traffic is much less likely to trigger anti-scraping measures.

HTTP header for security

While scrapers often use an HTTP header to avoid blocks, web servers rely on them to enforce security. Security headers act as a contract between the server and the browser, defining how content should be handled to protect users.

Common security headers

• Content-Security-Policy (CSP) — Adds an extra layer of protection, preventing XSS and code injection by specifying trusted content sources.
• Permissions-Policy — Controls access to browser features like geolocation, camera, or fullscreen.
• X-Frame-Options — Shields visitors from clickjacking attacks.
• X-XSS-Protection — Previously used for reflective XSS protection; now deprecated in modern browsers.
• Referrer-Policy — Determines how much referrer information is sent with requests.
• X-Content-Type-Options — Ensures browsers follow the MIME types declared in the Content-Type header.

You can quickly check which security headers your website uses with online tools — simply enter your URL to see the current setup.

Other web scraping methods

Traditional scraping involves managing HTTP headers, rotating proxies, headless browsers, and robots.txt — it’s a complex and technical process.

With all-in-one scraping tools, you can skip all that. They usually offer these features, allowing you to focus on other strategic tasks rather than the scraping process:

• IP rotation
• AI-driven fingerprinting
• JavaScript rendering
• CAPTCHA handling
• Custom Parser

You can find such tools at ScrapingBee, Decodo, Webshare, or Oxylabs , though each requires its own configuration and management.

Understanding headers, proxies, and headless browsers can still help fine-tune scraping, but modern APIs handle most of the heavy lifting for you.