How Do Ad Blocking Browser Extensions Work?

Partial of below content were copied from answers using Perplexity AI

Ad blocking browser extensions work by intercepting and filtering web content before it’s rendered in your browser. Here’s a detailed explanation of how they function:

Filter lists and rules

Ad blockers rely on filter lists, which are collections of predefined rules that determine what content should be blocked or hidden on web pages. These lists contain patterns and rules that match known ad servers, tracking scripts, and other unwanted elements.

Content interception process

When you visit a website, the ad blocker extension activates before the page is fully loaded. It performs the following steps:

• HTTPS Request Blocking: The extension listens to outgoing HTTPS requests from your browser. It compares these requests against its filter lists and blocks any that match known ad platforms or tracking services.
• URL Filtering: As the page loads, the ad blocker checks URLs of various elements against its filter lists. If a match is found, the content from that URL is blocked.
• Content Filtering: The extension analyzes the HTML, CSS, and JavaScript of the page, looking for patterns that indicate ads or unwanted content.
• CSS Injection: Ad blockers may inject custom CSS rules to hide elements that couldn’t be blocked at the network level.
• JavaScript Injection: Some ad blockers inject their own JavaScript code to counteract advertising scripts and prevent them from functioning.

What happens behind scene?

Ad blocking browser extensions utilize both content scripts and background scripts to effectively block ads and unwanted content. Let’s delve deeper into how these scripts work together.

Background scripts

Background scripts run continuously in the extension’s background page, separate from any particular web page. They play a crucial role in ad blocking:

• Filter List Management: Background scripts are responsible for downloading, parsing, and updating filter lists. These lists contain rules for blocking ads and are typically updated periodically to stay current with new ad servers and patterns.
• Request Interception: Background scripts use the browser’s webRequest API to intercept and analyze network requests before they are sent. This allows the ad blocker to block requests to known ad servers or tracking domains at the network level, preventing the ads from loading in the first place.
• Communication Hub: The background script acts as a central communication point, receiving messages from content scripts and popup interfaces, and coordinating the extension’s overall behavior.
• Rule Matching: When a request is intercepted, the background script quickly checks it against the loaded filter lists to determine if it should be blocked.

Content scripts

Content scripts are injected into web pages and can manipulate the DOM (Document Object Model) directly. They are essential for handling ad blocking tasks that can’t be accomplished through network request blocking alone:

• Element Hiding: Content scripts can inject CSS rules or modify the page’s existing CSS to hide ad elements that have already loaded. This is useful for ads that are served from the same domain as the main content and can’t be blocked at the network level.
• DOM Scanning: These scripts can scan the page’s DOM structure to identify and remove ad-related elements based on specific patterns or rules.
• Script Injection: Content scripts can inject additional JavaScript into the page to neutralize ad-related scripts or prevent them from executing.
• Cosmetic Filtering: They apply cosmetic filters to remove empty spaces left by blocked ads, improving the page’s appearance.

Interaction between background and content scripts

The background and content scripts work together to provide comprehensive ad blocking:

• Message Passing: Content scripts communicate with the background script via message passing, sending information about the current page and receiving instructions on what to block or modify.
• Dynamic Rule Application: The background script can send updated blocking rules to content scripts in real-time, allowing for dynamic ad blocking that adapts to changes on the page.
• Performance Optimization: By dividing tasks between background and content scripts, ad blockers can optimize performance. Network-level blocking happens in the background script, while page-specific modifications occur in the content script.

This combination of background and content scripts allows ad blocking extensions to provide a comprehensive and efficient ad-blocking experience, handling both network-level blocking and page-specific content manipulation.