Typically the job of a web browser is to download and display content-- establishing a network connection, sending HTTP requests, retrieving the web page, and downloading and running all of its content. These operations pose non-trivial challenges, and as such, web-browsers are among the most complicated software that most of us routinely use. However, there’s a whole separate (higher level!) challenge around selectively not running (filtering) content.
Today, different browsers offer many different mechanisms for selectively filtering content. This post is a survey of how these mechanisms work, and the subtle and sometimes not so subtle differences between them.
Examples and Motivations
Different users have shown an interest in myriad different types of Content Blocking, and not all users have similar goals.
Certain types of blockers are over a decade old and extremely commonly used (e.g. popup-blockers) while others are less often used or only of interest to a small niche audience. Just reading the comments on this blog, it’s clear that some users want to be able to block cookies, plugins or ActiveX controls, certain types of content (e.g. malware, adult content), privacy-impactful “trackers” (e.g. “web beacons”), advertisements, file downloads, or content they consider “annoying” (e.g. popups, flashing content). Individual consumers may have many different reasons for wanting to block particular content: faster performance, improved security, increased reliability and stability, enhanced privacy, increased battery life, preference about user-experience, legal or supervisory requirements (e.g. parental controls) lower bandwidth charges, as well as many others.
However, on the other end of the internet connection, a website provider may or may not want content blocked, for any of any number of reasons: revenue (direct or indirect), site analytics and understanding customers and markets, predictability and reliability of the user experience, malicious intent, and many others.
In some scenarios, site publishers and developers are just fine with content blocking and modification. For instance, a site owner whose legitimate site was compromised to serve malware probably wants that malware content blocked to keep his visitors safe until the site can be cleaned. Accessibility tools are crucial for some people to use the web and websites. Some sites and networks may offer users a way for to opt-out of analytics or other tracking.
Blocking at the Network Level
There are several common ways to block content at the network level—the most common are by using the HOSTS file, or by filtering content with a proxy. There are a number of other, less-common network-level approaches, including using a router to block particular content (most Linksys routers can be configured to block Java, ActiveX installers, and cookies, for example). Large organizations or networks with restricted bandwidth, for instance, may block content at the gateway:
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