Question 1

What is the maximum size of an HTTP header?

Accepted Answer

The official HTTP protocols (HTTP/1.1, HTTP/2, HTTP/3) do not define a maximum size limit for headers. However, practical limits are strictly enforced by web server software to prevent memory exhaustion attacks. Apache limits the total size of request headers to 8,192 bytes (8 KB) by default, while Nginx typically limits them to between 4 KB and 8 KB. If your headers, particularly the Cookie header, exceed this size, the server will reject the request with a 431 Request Header Fields Too Large status code.

Question 2

Can HTTP headers carry malware or viruses?

Accepted Answer

No, HTTP headers themselves are purely text-based metadata and cannot execute as code, meaning they cannot independently carry or execute malware. However, headers can be used maliciously to facilitate attacks. For example, a hacker might inject a malicious SQL command into a User-Agent header, hoping the server blindly logs that text into a vulnerable database (SQL Injection). Additionally, a missing or misconfigured Content-Security-Policy header allows malware (like malicious JavaScript) embedded in the HTML body to execute successfully in the user's browser.

Question 3

Why do some HTTP headers start with "X-"?

Accepted Answer

Historically, the "X-" prefix was used to designate non-standard, experimental, or custom headers that were not part of the official HTTP specification (e.g., X-Powered-By or X-Forwarded-For). The "X" stood for "eXperimental." However, in 2012, the Internet Engineering Task Force (IETF) published RFC 6648, which officially deprecated the practice of adding "X-" to new headers. They realized that experimental headers often became permanent industry standards, leaving developers stuck with awkwardly named "X-" headers forever. Today, custom headers should simply be given clear, descriptive names without the prefix.

Question 4

How do HTTP headers affect Search Engine Optimization (SEO)?

Accepted Answer

HTTP headers have a profound and direct impact on SEO. Search engine bots (like Googlebot) heavily rely on headers to understand how to index a site. The status code in the header determines if a page is indexed (200 OK), removed (404 Not Found), or transferred to a new URL (301 Moved Permanently). Furthermore, caching headers (Cache-Control) dictate how fast your website loads, and page speed is a major ranking factor in Google's algorithm. Finally, the Canonical link header can be used to tell search engines which version of a duplicate page is the primary one, preventing duplicate content penalties.

Question 5

What is the difference between HTTP/1.1 headers and HTTP/2 headers?

Accepted Answer

Logically and functionally, the headers are the same; a Content-Type header serves the exact same purpose in both protocols. The difference lies in how they are transmitted. HTTP/1.1 transmits headers as plain text, which is highly inefficient because the same large headers (like User-Agent and Cookie) are sent repeatedly with every request. HTTP/2 introduced a compression algorithm called HPACK, which converts headers into a binary format and uses a dynamic dictionary to only transmit the differences between headers on subsequent requests. This drastically reduces bandwidth usage and latency.

Question 6

Why am I getting a CORS error when my API works perfectly in Postman?

Accepted Answer

This is the most common point of confusion for backend developers. Postman, cURL, and server-to-server scripts do not enforce the Same-Origin Policy. They are not web browsers, so they do not care what domain the data comes from, and they do not send preflight OPTIONS requests. Web browsers (Chrome, Safari, Firefox) are the only clients that enforce CORS to protect end-users. If your API lacks the proper Access-Control-Allow-Origin headers, it will work perfectly in Postman but will immediately fail when accessed via JavaScript in a web browser.

Question 7

What does the "Content-Type: application/x-www-form-urlencoded" header mean?

Accepted Answer

This header tells the server that the data contained in the body of the HTTP request is formatted exactly like a URL query string. It is the default format used by standard HTML <form> elements when a user clicks submit. The data is structured as key-value pairs separated by ampersands, with spaces and special characters encoded (e.g., name=John+Doe&age=35). It is highly efficient for simple text data, but cannot be used to upload binary files like images; file uploads require the multipart/form-data content type instead.

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