Measuring the Internet 24 March 2017

Internet Society Perspectives on Internet Content Blocking: An Overview

* This paper is also available in:  Arabic – French – Portuguese – Russian – SpanishJapanese


The use of Internet blocking by governments to prevent access to illegal content is a worldwide and growing trend. There are many reasons why policy makers choose to block access to some content, such as online gambling, intellectual property, child protection, and national security. However, apart from issues relating to child pornography, there is little international consensus on what constitutes appropriate content from a public policy perspective.

The goal of this paper is to provide a technical assessment of different methods of blocking Internet content, including how well each method works and what are the pitfalls and problems associated with each. We make no attempt to assess the legality or policy motivations of blocking Internet content[1].

Our conclusion, based on technical analyses, is that using Internet blocking to address illegal content or activities is generally inefficient, often ineffective and generally causes unintended damages to Internet users.

From a technical point of view, we recommend that policy makers think twice when considering the use of Internet blocking tools to solve public policy issues. If they do and choose to pursue alternative approaches, this will be an important win for a global, open, interoperable and trusted Internet.


The Internet’s evolution into a worldwide societal phenomenon has much to credit to the content and services that have taken advantage of the network’s unique architecture. Entire economies depend on cross-border content flows. Daily innovations have the potential to disrupt entire industries. The Internet is now a critical part of democratic processes and policy discussions. Personal relationships are created and broken online.

The trend is not slowing down. According to estimates[2], Global Internet traffic in 2020 will be equivalent to 95 times the volume of the entire global Internet in 2005. The number of devices connected to IP networks will be three times as high as the global population in 2020.

Yet, the Internet also contains content that policy makers, legislators, and regulators around the world want to block. From blocking foreign gambling websites in Europe and North America to blocking political speech in China, the use of Internet content blocking techniques to prevent access to content considered illegal under certain national laws is a worldwide phenomenon. Public policy motivations to block Internet content are diverse, ranging from combating intellectual property infringement, child abuse material and illegal online activities, to protecting national security.

The objective of this paper is neither to assess such motivations nor to qualify whether a certain type of blocking is good or bad from an ethical, legal, economic, political or social perspective. Instead, we will provide a technical assessment of the benefits and drawbacks of the most common blocking techniques used to prevent access to content deemed illegal. The aim is to help readers understand what each technique can, and cannot, block, along with the side effects, pitfalls, trade-offs, and associated costs.

Our conclusion is that the use of Internet blocking to address illegal content is generally inefficient, often ineffective, and prone to cause unintended collateral damages to Internet users, summarized further in the table below.

Sidebar: Filtering, Blocking, or Censorship?

When describing Internet filtering, terms such as “filtering,” “blocking,” “shut down,” and “censorship” all come up (along with several others). From the point of view of the user, the term chosen is less important than the effect: some part of the Internet is inaccessible. For policy makers and digital activists, choosing a particular term is usually more driven by semantic overtones than technical correctness. The word “censorship” carries a strong negative connotation, while “filtering” seems a more gentle and harmless operation, like removing unwanted seeds from a glass of orange juice. We have chosen to use “blocking” as a simple and straightforward term throughout this paper.

From a technical point of view, we call on policy makers to think twice about the use of such measures and invite them to prioritize their responses focusing primarily on alternative measures that focus on addressing the issue at the source (see more detailed recommendations at the end of this paper, including guidance on how to minimize the negative effects of such measures.).

It should further be noted that this paper is not focusing on blocking measures when implemented for regular network management or security reasons (e.g. addressing spam, malware). In such cases, some of the same tools we describe in this paper can often be effective to achieve the intended aims.

The table below summarizes the major drawbacks associated with Internet content blocking based on public policy considerations:

Motivations for Blocking Content

In this paper, we focus on blocking based on public policy considerations and its effects on the Internet and Internet users (see side-bar for other motivations for content blocking).

Blocking based on public policy considerations is used by national authorities to restrict access to information (or related services) that is either illegal in a particular jurisdiction, is considered a threat to public order, or is objectionable for a particular audience.

Other Types of Motivations for Blocking Content

In this paper, we focus on blocking based on public policy considerations, but there are two other common reasons that network blocking is put into place. The first is preventing or responding to network security threats. This type of blocking is very common. For example, most enterprises attempt to block malware from entering their networks. Many Internet Service Providers (ISPs) are putting in blocks for malicious traffic exiting their networks, such as from hijacked IoT devices (e.g. web cams). Email filtering is extremely common, and includes blocking unwanted bulk email as well as malicious email such as phishing messages. These types of blocking are not discussed in this paper.

A second reason for blocking is managing network usage. A growing area of Internet content blocking is based on network, bandwidth, or time management requirements, rather than particular types of content. For example, employers may wish to restrict access to social networking sites for their employees while still offering Internet access at the desktop. ISPs may block or permit, throttle or accelerate certain content based on contracted services. Network usage management is rarely a public policy issue, except when it steps into the area of anti-competitive behavior. Readers interested in Network Neutrality will find references in For Further Reading, page 26.

For example, there’s a common desire in most countries to block access by children to obscene material, or access by anyone to child abuse material. Depending on the local legal environment, content may also be blocked if it violates intellectual property laws, is considered a threat to national security, or is prohibited for cultural or political reasons.

One of the challenges leading national authorities to use Internet content blocking measures is that different actors delivering the source’s content to consumers may be in different countries, with different laws covering what is and is not “illegal content”. Moreover, the global environment of the Internet makes stopping the source of illegal content more complicated than simply shutting down a local server. For example, the person providing the content, the servers hosting the content, and finally the domain name pointing to the content may in three different countries, all beyond the jurisdiction of an individual national authority. This highlights the importance of cooperation across jurisdictions and the need for close coordination with non-governmental stakeholders.

Overview of Content Blocking Techniques

Each technique has both technical and policy limitations and consequences that need to be considered when any type of content blocking is being proposed. The goal of this paper is to provide a common way to evaluate their efficacy and side effects. Readers interested in a more technical discussion of content blocking will find references to IETF technical documents in For Further Reading, page 26.

This paper will assess the following types of content blocking:

  • IP and Protocol-based blocking
  • Deep Packet Inspection-based blocking
  • URL-based blocking
  • Platform-based blocking (especially search engines)
  • DNS-based blocking

We chose these five types of blocking because they target the elements of a typical end-user cycle of finding and retrieving information, including the use of a search engine and viewing information with a web browser or similar tool. This cycle is very familiar to policy makers, themselves Internet users, and these are the operations that most blocking based on public policy considerations tries to disrupt.

In the diagram to the right, we show the steps that a typical Internet user might take to find information, as well as the kinds of blocks that have been used to disrupt this cycle when blocking based on public policy considerations is implemented. In our diagram, an Internet user searches for some type of content using a search engine (step 1), a common starting point. The search engine returns a set of results (step 2), and the user selects one and clicks on the result (step 3). One type of blocking, Platform-based Blocking, is used to disrupt this part of the cycle by blocking some results coming back from the search engine.

The user’s computer tries to find the server hosting the data in the Internet’s DNS (steps 4 and 5). A second type of blocking, DNS-based Blocking, is used to disrupt this part of the cycle.

Then, the user’s web browser tries to connect to the server (step 6). This part of the cycle can be blocked using three other types of blocking: IP and Protocol-based Blocking, URL-based blocking, and Deep Packet Inspection-based blocking.

Of course, the Internet is much more than search and web browsers, and many of the techniques discussed below are effective at blocking more than web pages. For example, use of VPN services to encrypt and hide traffic can often be blocked using a combination of Deep Packet Inspection-based blocking and IP/Protocol-based blocking.

These types of blocks may be applied very specifically (such as a particular document on a particular web site) or very generically (such as “material on an issue” or “Voice over IP services”).

Where Does Content Blocking Occur?

Many of the content blocking techniques discussed here can be used at different points, as shown in the table below.

Note that in the case of blocking based on public policy considerations, the majority of measures are being applied at the first two levels (national, carrier, and ISP levels).

The diagram below summarizes some of the main locations where blocking can occur, and which types of blocking can occur at each point.

Sidebar: Endpoint Content Blocking

This paper focuses on Internet content blocking based on public policy considerations.

Yet, it is important to note that one of the most effective ways to block undesired content is through the use of software installed on the user’s device, commonly called the “endpoint” because it is the last point of the connection between the user and the Internet. Most computer users make use of endpoint software to block malware (viruses, Trojan horses, and phishing), whether installed personally or by an organizational IT group.

Endpoint content blocking software is also used by organizations to block content for other reasons. For example, libraries often install this type of software on public computers to block the viewing of pornography by patrons, and parents may use it to block unwanted content from their children.

Endpoint content blocking may use many of the techniques described in this paper, including content scanning, URL categorization, IP address blocking, and DNS interception. Generally, the blocking and analysis occurs on the actual endpoint. However, vendors of this software are increasing also using cloud-based tools including content scanning and DNS-based blocking, in cooperation with a small amount of endpoint software. In these newer solutions, some or all of the Internet content may pass through a cloud-based service. The advantage of moving the decision-making to the cloud is that endpoints do not have to be constantly updated, and the performance impact of evaluating content is moved from the user’s computer or smart phone to an easily scaled cloud of computers. When traffic is routed through a third party, though, this also creates privacy issues by making the content available to the third party and, if poorly implemented, security issues arise well.

Content Blocking Types Evaluated

The five common content blocking types are distinct in what they block and how they operate.

Below, the content blocking techniques are discussed in greater detail and are evaluated against four specific criteria[3] :

  1. Which sets of users and Internet services are affected by this technique? What sets are unaffected?
  2. How specific is the technique in preventing access to particular content? How much collateral damage (unintended blockage) is created by this blocking technique?
  3. How effective is this technique in blocking content? What types of users and content providers are able to circumvent this technique?
  4. What are common side-effects of this technique? What technical issues are caused by this technique? What nontechnical issues, such as impact on trust and fundamental rights, are raised in using this technique?

IP and Protocol-Based Blocking

IP-based blocking places barriers in the network, such as firewalls, that block all traffic to a set of IP addresses. Protocol-based blocking uses other low-level network identifiers, such as a TCP/ IP port number that can identify a particular application on a server or a type of application protocol. These simplest approaches to blocking content don’t actually directly block contentthey block traffic to known IP addresses or TCP/IP ports or protocols associated with some content or an application. IP and protocol-based blocking may also be done by software on user’s computers, typically for network security purposes.

For example, if the goal was to block all content hosted in the mythical country of Elbonia, IP blocking could be used if the set of all IP addresses hosting content in Elbonia were known. Similarly, if the goal was to block all VPN services (which are used to encrypt traffic and hide both the destination and the content), protocol-based blocking could be used to stop VPN services using well-known protocols or TCP/IP port numbers.

A variation on IP blocking is throttling of traffic. In this scenario, not all traffic is blocked, only a certain percentage. Users may perceive the service as very slow, or as simply going “up and down.” This can be used to discourage users from using a service by making it seem unreliable, or encourage the use of alternative services, without revealing that blocking is occurring. (This can also be done for network and bandwidth management reasons at both the ISP or enterprise level.)

Both IP and Protocol-based blocking use devices that sit between the end-user and the content, and thus requires the blocking party (such as the user’s ISP) to have complete control over the connection between the end-user and the Internet. A user who is not “behind” the blocking device, or who uses technology such as a VPN that conceals the true destination of their traffic, will not be affected by this type of blocking.

Generally, IP blocking is a poor filtering technique that is not very effective, is difficult to maintain effectively, has a high level of unintended additional blockage, and is easily evaded by publishers who move content to new servers (with new IP addresses).

IP blocking also does not work when information providers use content delivery networks (CDNs), since the IP addresses of the information are highly dynamic and constantly changing.[4] CDNs also use the same IP address for many different customers and types of content, causing a high level of unintended service interruption.

IP and protocol blocking work better when used to block specific applications, rather than specific content. For example, VPN traffic may be blocked by TCP/IP port and protocol blocks, combined with IP address blocks of known public VPN services. This is a common and highly effective technique.

IP blocking is also most effective when the content is hosted in a particular server in a specific data center, or a very specific set of files are of concern. IP-based blocking is not very effective for larger hosting services distributed across many data centers or which use content distribution networks (CDNs) to speed access.

Deep Packet Inspection-Based Blocking

Deep Packet Inspection (DPI)-based blocking uses devices between the end user and the rest of the Internet that filter based on specific content, patterns, or application types. This type of network blocking is computationally very intensive and thus costly, because all content must be evaluated against blocking rules. DPI blocking may also be done by software on user’s computers, typically for network security purposes.

DPI blocking requires some type of signature or information about the content to be effective. This may be keywords, traffic characteristics (such as packet sizes or transmission rates), filenames, or other content-specific information. DPI blocking is used very effectively to block or throttle certain applications (such as peer-to-peer file sharing or Voice over IP [VoIP] traffic) and data file types (such as multimedia files).

Sidebar: Encryption, Proxies, and Blocking Challenges

Several of the techniques discussed in this paper, including Deep Packet Inspection (DPI)-based blocking and URL-based blocking, have a very real limitation: they must be able to see the traffic being evaluated. Web servers that offer encryption or users who add encryption to their communications (typically through application-specific encryption technology, such as TLS/SSL) cannot be reliably blocked by in-the-network devices. Many of the other techniques are also easily evaded when user have access to VPN technology that encrypts communications and hides the true destination and type of traffic. Although researchers and vendors have developed some ways of identifying some types of traffic through inference and analysis, these techniques often are simply guessing at what type of traffic they are seeing.

In recent research, 49% of US web traffic (by volume) was encrypted in February, 2016. This traffic would be effectively invisible to URL-based blocking and DPI tools that look at content, because the only visible information would be the domain name of the server hosting the information. To compensate for this “going dark,” some network blocking uses active devices (called proxies) that intercept and decrypt the traffic between the user and the web server, breaking the end-to
end encryption model of TLS/SSL.

When proxies are used, these cause significant security and privacy concerns. By breaking the TLS/SSL model, the blocking party gains access to all encrypted data and can inadvertently enable third-parties to do the same. The proxy could also change the content. If the blocking party has control over the user’s system (for example, a corporate-managed device would be highly controlled), the proxy may be very transparent. Generally, however, the presence of a proxy would be obvious to the end user, at least for encrypted (TLS/SSL) traffic (e.g. the user may get an alert that the certificate is not from a trusted authority). In addition, new industry and IETF standards (such as HTTP Strict Transport Security [RFC6797], HTTP Public Key Pinning [RFC 7469], and DANE [RFC 6698]) and new security features in modern Internet browsers make it more difficult to proxy (and decrypt) TLS/SSL traffic without the knowledge and cooperation of the end user.

Proxies installed for content blocking reasons may also introduce performance bottlenecks into the flow of network traffic, making services slow or unreliable.

DPI blocking is very commonly used in enterprises for data leakage protection systems, anti-spam and anti-malware (anti-virus) products, and traffic prioritization (such as boosting the priority of enterprise videoconferencing) network management. However, it can also be used for more policy-based blocking purposes. For example, use of VoIP services not provided by the national telecommunications carrier are often regulated or restricted, and DPI blocking is effective at enforcing those restrictions.

DPI blocking uses devices that can see and control all traffic between the end-user and the content, so the blocking party (such as the user’s ISP) must have complete control over an end-user’s connection to the Internet. When the traffic is encrypted, as it often is, DPI blocking systems may no longer be effective. These are discussed in greater detail in the sidebar “Encryption, Proxies, and Blocking Challenges” to the right.

DPI blocking is generally an effective technique at blocking certain types of content that can be identified using signatures or other rules (such as “block all Voice over IP traffic”). DPI blocking has been much less successful with other types of content, such as particular multimedia files or documents with particular keywords in them. Because DPI blocking examines all traffic to end users, it is also quite invasive of end user privacy.

The overall efficacy of DPI blocking varies widely depending both on the goals and the specific DPI tools being used. Generally, DPI tools are most effective in network management and security enforcement, and are not well-suited for policy-based blocking.

URL-Based Blocking

URL-based blocking is a very popular blocking method, and may occur both on the individual computer, or in a network device between the computer and the rest of the Internet. URL blocking works with web-based applications, and is not used for blocking non-web applications (such as VoIP). With URL blocking, a filter intercepts the flow of web (HTTP) traffic and checks the URL, which appears in the HTTP request, against a local database or on-line service. Based on the response, the URL filter will allow or block the connection to the web server requested.

Generally, URLs are managed by category (such as “sports sites”) and an entire category is blocked, throttled, or allowed[5]. In the case of a national policy requiring URL blocking, the on-line service and blocking policy would likely be managed by the government. The URL filter can simply stop the traffic, or it can redirect the user to another web page, showing a policy statement or noting that the traffic was blocked. URL blocking in the network can be enforced by proxies, as well as firewalls and routers.

URL blocking requires the blocking party (such as the user’s ISP) to have the ability to intercept and control traffic between the end-user and the Internet. URL blocking is usually expensive, because the filtering device generally has to be in-line between the user and the Internet, and thus requires a high level of resources to give acceptable performance.

URL blocking is generally considered to be very effective at identifying content that may be on different servers or services because the URL doesn’t change even if the server changes IP addresses. In a few cases, URL blocking may fail to fully block the traffic when the URLs are very complicated or change frequently. This can happen because an information publisher has deliberately decided to actively evade URL filter blocking, or it can be a side effect of some advanced publishing systems such as those used for large on-line publications.

URL blocking usually is effective at high-level URLs, such as a particular web page, but is not as effective when deep links (such as individual bits of content within a web page) are considered. Depending on how the user navigated to the particular content, URL blocking may or may not be able to block all access—if the user has a “deep link” not covered by the URL filter, the content will be allowed. For example, the Playboy web site includes both URLs, but also embedded content using the “” domain name. A URL filter that didn’t also include “” URLs would not block the video content.

All types of URL blocking are highly dependent on the quality of the filter, and a poorly designed or overly broad filter may block unintended traffic or have other negative effects on the user experience, such as affecting the loading or formatting of web pages when some component is being blocked.

As with Deep Packet Inspection types of blocking, URL blocking requires some type of proxy to see the full URL when traffic is encrypted with HTTPS (TLS/SSL). See sidebar “Encryption, Proxies, and Blocking Challenges”, page 15, for more information on the effects on end-user privacy. For encrypted traffic, URL blocking can only see the IP address of the server, and not the full URL, resulting in a much higher level of unintended blocking. Because proxies are expensive and intrusive to the user experience, URL blocking does not work well as a tool for policybased blocking.

Platform-Based Blocking (Especially Search Engines)

In some cases, national authorities will work with major information service providers to block information within their geographic region without blocking the entire platform. The most common examples of platform filtering are through the major search engine providers and social media platforms. Recently, it has also been reported that mobile application stores (such as the Apple Store and Google Play) are working with national authorities to block downloads of specific applications in their country.

Sidebar: Blocking On Other Platforms

While search engine blocking is the most common type of platform blocking, other platforms with
enormous users communities are often considered for this technique. Common examples of these types of platforms include Facebook (which has over 1.5 billion active users each month) and YouTube (with over a billion unique users). Attempts to use network-based or URL-based techniques to block individual content elements, such as a particular news article, are very difficult. Because they don’t want to be seen as blocking all of Facebook (for example), national authorities have proposed working with major platform providers to filter out specific types of content they deem illegal.

Very little is known about the effectiveness, scope, or side effects of other kinds of platform blocking, as this technique has not been widely and reliably observed on platforms other than search engines. While the major platforms, such as Facebook, YouTube, and Twitter, will universally block certain types of content (such as malware and pornographic material) and provide customized content feeds to their users, information on national-specific blockages is not available.

Platform-based blocking is a technique that requires the assistance of the platform owner, such as a search engine operator like Google or Microsoft. In this technique, queries from a particular set of Internet users to a search engine will receive a different set of results from the rest of the Internet— filtering out pointers to content that are, in some way, objectionable. In some cases the definition of what is to be blocked is based on local regulation and government requirements, but it may also be due to concerns by the search engine operator. For example, a search engine may block pointers to malware or content considered inappropriate according to its own terms of service.

Because search engine blocking requires the cooperation of the search engine provider, this limits its use to two very specific scenarios: country-level rules (blocking content based on country-specific or regionspecific rules) and age-based rules (blocking material inappropriate for young people).

Search engine blocking only affects users who choose a particular search engine, and only when the users are identified as being from a particular set with filter rules. In age-based blocking, such as SafeSearch[6] (offered by major search engines and content providers), an explicit opt-in is required.

Since search engine blocking only filters out pointers to content, and not actual content, it is an extremely ineffective technique, and can have the unintended consequence of drawing increased attention to the blocked content. The presence of multiple search engines, as well as alternative methods of finding content, make this type of blocking very difficult to enforce.

Although search engine blocking seems like it does very little towards blocking content, the technique is extremely popular at the national level, and governments around the world are known to demand that major search engines implement filters according to their regulations, such as infringement of copyright or particular types of speech prohibited by national law. For example, Google reported in 2015 that it had received 8,398 requests from 74 national courts to remove 36,834 results from its search results[7]. Copyright infringement requests made by individuals are also very popular: in June 2016, Google reported that 6,937 copyright owners had requested over 86 million search results to be removed from Google results during that month[8] .

Search engine blocking is also used by individuals as part of the so-called “right to be forgotten,” with over a million URLs globally requested to be blocked in the last two years (May 2014 to June 2016).

DNS-Based Content Blocking

DNS-based content blocking avoids one of the problems with other techniques: the cost and performance impact of filtering all network traffic. Instead, DNS-based content blocking focuses on examining and controlling DNS queries.

With DNS-based content blocking, a specialized DNS resolver (see Sidebar: DNS Overview) has two functions: in addition to performing DNS lookups, the resolver checks names against a block list. When a user’s computer tries to use a blocked name, the special server returns incorrect information, such as the IP address of a server displaying a notice that the content has been blocked. Or, the server may claim that the name does not exist. The effect is that the user is impeded from easy access to content using certain domain names.

As with all network-based blocking, DNS-based content blocking is only effective when the organization doing the blocking has complete control over the network connection of the end user. If the user can select a different connection, or use a different set of DNS servers, the technique does not affect them. For example, when Turkey blocked some DNS queries in 2012, users changed their systems to use Google’s popular public DNS servers and avoid the blockage. Turkish authorities responded by hijacking all traffic to the Google DNS service, which caused significant collateral damage. DNS-based content blocking requires firewalls or other devices that can intercept and redirect all DNS queries to the specialized blocking-aware DNS servers or it will not be very effective.

Sidebar: DNS Overview

The DNS is a conceptually simple system that allows a string of labels (such as “www,” “isoc,” and “org”) separated by dots (the domain name) to be looked up in a database distributed across multiple DNS servers. The domain name lookup results in an answer (for example, an IP address or a website), or the answer that the name does not exist.

The most common type of DNS lookup is for IP (Internet Protocol) addresses. This is the type of lookup that occurs each time a user types a URL into a web browser, for example. Normally, the individual application (such as the web browser) does not perform the full lookup, which involves several steps. Instead, the application uses an intermediate system called a “resolver” (because it resolves DNS name lookups), which navigates the DNS distributed database to retrieve the information requested.

In DNS-based content blocking, the normal operation of the resolver is changed.

The effectiveness of DNS-based content blocking is similar to IP-based blocking. It is slightly more effective because the list of domain names is easier to keep updated and is more accurate than a list of IP addresses for most types of content blocking. However, it is slightly less effective because changing domain names is simpler than changing IP addresses, which makes it easier for both end users and information publishers to evade this type of block.

An alternative form of DNS-based content blocking is when domain names are taken down, or removed from the DNS altogether. This method is more difficult to circumvent and the collateral damage is somewhat limited. In many cases it depends on the efficacy of cross-border cooperation, when a request or a court order comes from a jurisdiction different from where the registry or registrar operates.

DNS-based content blocking has similar drawbacks to blocking based on IP address: both prohibited and non-prohibited content may be on the same server using the same name (such as “”), yet all would be blocked. In addition, the modification of DNS responses may cause other technical problems that interrupt other valid services[9].

DNS-based content blocking also depends on the user playing by the normal rules of the Internet and using the standard DNS service to translate names to IP addresses. Users who have complete control over their own computers and some technical expertise can reconfigure them to evade the standard DNS service and use alternatives, or simply have a list of name-to-address translations stored locally.

In DNS-based blocking, the blocking device has a list of DNS names to block.

Because most Internet connections require a translation from a DNS name to an IP address, blocking the query and returning a false answer can discourage users from trying to retrieve blocked content or connect to blocked services by other means (e.g. directly typing the IP address).

Content Blocking Summarized


Understanding the different blocking techniques, their effects and side effects, is important both for policy makers considering the use of such measures and for Internet advocates and others wishing to influence content blocking practices.

All blocking techniques are prone to two main drawbacks:

1. They do not solve the problem
Blocking techniques do not remove content from the Internet, nor do they stop the illegal activity or prosecute culprits; they simply put a curtain in front of the content. The underlying content remains in place.

2. They inflict collateral damage
Every blocking technique suffers from over-blocking and under-blocking: blocking more than is intended and, at the same time, less than intended. They also cause other damage to the Internet by putting users at risk (as they attempt to evade blocks), reducing transparency and trust in the Internet, driving services underground, and intruding on user privacy. These are costs that must be considered at the same time that blocking is discussed.

Sidebar: Circumventing Content Blocking

Policy makers should keep in mind an important point when considering blocking Internet content: all of the technical blocking techniques can be bypassed by a sufficiently motivated user. In many cases, only minimal work is needed to evade the block.

If traffic to a host or domain name is blocked, tools such as VPNs can be used to hide the traffic. If the traffic content is being inspected, then it can be encrypted so that it does not
trigger the block. If the content is taken down, other users may reload it on other servers. If the domain name used is removed, end users can still access the host if they know the IP address, or a new domain name can be selected as a replacement. If a search engine removes results, there are always other search engines.

End users are not the only ones who can and do evade blocks. Information publishers also have many approaches to duck various blocking techniques. If a publisher works hard enough to distribute and disseminate content, no block technique can stop them.


The Internet Society believes the most appropriate way to counteract illegal content and activities on the Internet is to attack them at their source. Using filters to block access to online content is inefficient, likely to be ineffective, and is prone to generate collateral damage affecting innocent Internet users.

We suggest two main strategies for policy makers concerned about illegal content on the Internet:

1. Attack the issue at the source: The least damaging approach for the Internet is to “attack” illegal content and activities at their source. Removing illegal content from its source, and undertaking enforcement against the perpetrators avoids the negative effects of blocking, and is more effective at removing illegal content[10]. Cooperation across jurisdictions and stakeholders is a prerequisite for success, as illegal content online extends beyond national borders and national law.

2. Prioritize and use alternative approaches: Depending on circumstances, different approaches can be quite effective. For example

  • Effective cooperation among service providers, law enforcement and national authorities may provide additional means to help the victims of illegal content, and to take enforcement action against the perpetrators[11].
  • Creating an environment of trust where users receive information on what is legal and what is not can improve self-policing.
  • In some cases (e.g. parental control), empowering user to use filters on their own devices, with their consent, can be effective and least damaging to the Internet.
  • On a voluntary or legal basis, some websites (e.g. gambling websites) could use geolocation to prevent access from countries where their services are not allowed.

Minimizing Negative Effects

All content blocking techniques have serious deficiencies, especially in the context of blocking based on public policy considerations. All techniques behave poorly and can be evaded. For this reason, and the reasons stated before, we advise against content blocking.

Nonetheless, these techniques are still used. Recognizing this reality, we offer the following specific guidelines to lessen the negative impact:

a. Rule out all non-blocking options: First, and foremost, exhaust all practical options to have content addressed at the source, or any other alternative means to blocking. Blocking content should not be pursued simply because it is easier.

b. Be transparent: There should be transparency about the blocking as well as the underlying objective and policies. National authorities should make sure that affected users have the opportunity to raise concerns about negative impacts on their rights, interests and opportunities.

c. Consider your responsibility towards the Internet: The blocking party should be aware that they share a responsibility towards the system as a whole to not harm the stability, security and resilience of the Internet. Blocking techniques adversely impact the way the Internet is collectively managed and functions. Sometimes the damage is direct, and sometimes, it is indirect. For instance, users working around the block may cause problems or threaten their personal security.

e. Think globally, act locally: Local blocking and filtering can have global effects. But generally, blocking content as locally as possible will minimize the global impact. Ideally, blocking at the user’s end-point is most efficacious and minimizes collateral damage.

f. Involve stakeholders: Policy development and implementation should involve a broad set of stakeholders including technological, economic, consumer rights and other specialists to ensure the appropriate steps are taken to minimize negative side-effects.

g. Keep it temporary: Any blocking measures should be temporary. They should be removed as soon as the reason for blocking ceases to exist. It is quite common for illegal content to be moved to evade blocking measures, yet the measures often remain in place long after the content has moved.

h. Follow due legal process: Any blocking order of unlawful content must be supported by law, independently reviewed, and narrowly targeted to achieve a legitimate aim. The least restrictive means available to deal with illegal activity should be prioritized. Internet Service Providers or other Internet intermediaries should not become de-facto law enforcement agents: they should not be required to determine when conduct or content is illegal.


A content delivery network or content distribution network (CDN) is a globally distributed network of proxy servers deployed in multiple data centers. The goal of a CDN is to serve content to end-users with high availability and high performance. CDNs serve a large fraction of the Internet content today, including web objects (text, graphics and scripts), downloadable objects (media files, software, documents), applications (e-commerce, portals), live streaming media, on-demand streaming media, and social networks.

In the context of this paper, we use “content” generally to describe information found on the Internet. This content might be a full document or just a paragraph of some text, an image, a video, or even just audio (such as a podcast). Content could be on web pages viewed in a browser, or it could be accessible through more specialized tools such as a custom application.

The Domain Name System (DNS) is a hierarchical decentralized naming system for computers, services, or other resources connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities. Most prominently, it translates more readily memorized domain names to the numerical IP addresses needed for locating and identifying computer services and devices with the underlying network protocols. By providing a worldwide, distributed directory service, the Domain Name System is an essential component of the functionality of the Internet, that has been in use since 1985.

Deep Packet Inspection (DPI) is a form of computer network packet filtering that examines the data part (and possibly also the header) of a packet as it passes an inspection point, searching for protocol non-compliance, viruses, spam, intrusions, or defined criteria to decide whether the packet may pass or if it needs to be treated in another way, including discarding the packet.

In the context of this paper, we use “illegal” to describe content that is prohibited in a national context no matter what the reason. This could be content that is illegal because it is a copyright violation (or some other type of intellectual property), such as a pirated movie. It could be content that is illegal because it is objectionable for moral reasons, such as obscenity or child pornography. It could be content that it is illegal because national authorities wish to suppress it or find it offensive, such as a cartoon depicting the president of the country in an unfavorable way. Content that is illegal in one jurisdiction may be completely legal in another. Content that is illegal in one context (such as indecent comedy, when viewed by children) may be completely legal in another (such as when viewed by adults), even within the same jurisdiction.

IP address
An IP address (abbreviation of Internet Protocol address) is an identifier assigned to each computer and other devices (e.g., printer, router, mobile device, etc.) connected to the Internet. It is used to locate and identify the node in communications with other nodes on the network.

False Negative
A false negative occurs when content is not blocked, but it should have been. For example, if illegal pharmacies are being blocked, a brand new illegal pharmacy might not be blocked if the server had not been added to the block list yet. This would be called a false negative.

False Positive
A false positive occurs when some content is blocked which was not intended to be blocked. For example, if pornography is being blocked, information about cooking of chicken breasts might be blocked if the block used a poorly constructed keyword search. This would be considered a false positive.

Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), both frequently referred to as “SSL”, are cryptographic protocols that provide communications security over a computer network. Several versions of the protocols find widespread use in applications such as web browsing, email, Internet faxing, instant messaging, and voice-over-IP (VoIP). Websites use TLS to secure all communications between their servers and web browsers. The Transport Layer Security protocol aims primarily to provide privacy and data integrity between two communicating computer applications.

Uniform Resource Locator (URL), commonly informally termed a web address, is a reference to a web resource that specifies its location in the network and a mechanism for retrieving it. URLs occur most commonly to reference web pages (https), but are also used for file transfer (ftp), email (mailto), database access (JDBC), and many other applications. Most web browsers display the URL of a web page above the page in an address bar. A typical URL could have the form, which indicates a protocol (https), a hostname (, and a file name (index. html).

A virtual private network (VPN) extends a private network across a public network, such as the Internet. It enables users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network. Applications running across the VPN may therefore benefit from the functionality, security, and management of the private network.

For Further Reading

The following publications may be of interest to readers looking for additional information on this topic.

Internet Engineering Task Force Technical Documents
Policy, Survey, and Background Documents


The Internet Society gratefully acknowledges the assistance of Joel Snyder of Opus One in preparing this paper.

The report was supervised by Nicolas Seidler and Andrei Robachevsky from the Internet Society.

The paper benefited from the reviews, comment and support from a set of Internet Society staff: Constance Bommelaer, Sally Wentworth, Olaf Kolkman, Carl Gahnberg, Christine Runnegar, Konstantinos Komaitis, Lia Kiessling, Joyce Dogniez, Kevin Craemer, Bastiaan Quast, Kevin Chege, Dan York, Raquel Gatto.

Special thanks to the Internet Society Communications team for shaping the visual aspect of this paper and promoting its release: James Wood, Beth Gombala, Lia Kiessling, Allesandra Desantillana.

Last but not least, the paper was significantly improved thanks to the input of a variety of Internet Society Chapter members, organizational members, individual members, as well as input by current and former Internet Society Board of Trustees.


[1] Readers interested in legal assessments of content blocking could visit the following resources:

[2]  Cisco® Visual Networking Index:

[3] These criteria are taken from Internet RFC 7754, “Technical Considerations for Internet Service Blocking and Filtering.”

[4] A content distribution network is a large, geographically distributed network of servers that speed the delivery of web content to Internet users. Large CDNs have hundreds of thousands of servers in many countries to give faster access to their customers’ content. CDNs store copies of their customers’ text, image, audio, and video content in their own servers around the “edges” of the Internet, so that user requests can be served by a nearby CDN edge server rather than the customer’s centralized servers

[5] URL filtering categories are established by security service providers and are often based on a combination of human analysis of web pages combined with some automated scanning of web page content. Most security service providers offer URL filtering databases for the purposes of managing corporate network traffic, but they can be used in other contexts, such as those discussed in this paper.

[6] SafeSearch is a feature of major search engines, including Google Search, Microsoft Bing, and Yahoo!, that blocks results containing “inappropriate or explicit images” from search results.



[9] Readers interested in more details may wish to refer to Internet Society’s “Perspectives on DNS Filtering” report at

[10] When the national authority is in the same jurisdiction as the consumer of content, removing illegal content at the source seems an easy way around the complexities and overhead of cross- border actions. We acknowledge that removing the content at the source is challenging in the context of a cross-border Internet, where providers and consumers of content may be located in different jurisdictions, subject to different laws. Yet, we consider this should not be a reason not to identify more efficient solutions that do not harm the Internet.

[11] For example, partnerships with the finance industry can be used to identify and limit illegal transactions.

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    Internet Society Perspectives on Internet Content Blocking: An Overview

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    Internet Society — обзор перспектив блокировки интернет-контента

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    Internet Society — Perspectivas sobre o bloqueio de conteúdo na Internet: visão geral

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    آراء مجتمع اإلنترنت حظر المحتوى على اإلنترنت: نظرة عامة

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    Internet Society Perspectives on Internet Content Blocking: An Overview

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    Internet Society — обзор перспектив блокировки интернет-контента

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    Internet Society — Perspectivas sobre o bloqueio de conteúdo na Internet: visão geral

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    آراء مجتمع اإلنترنت حظر المحتوى على اإلنترنت: نظرة عامة


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