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RFC 7754

Technical Considerations for Internet Service Blocking and Filtering

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Internet Architecture Board (IAB)                              R. Barnes
Request for Comments: 7754                                     A. Cooper
Category: Informational                                       O. Kolkman
ISSN: 2070-1721                                                D. Thaler
                                                             E. Nordmark
                                                              March 2016

  Technical Considerations for Internet Service Blocking and Filtering


The Internet is structured to be an open communications medium. This openness is one of the key underpinnings of Internet innovation, but it can also allow communications that may be viewed as undesirable by certain parties. Thus, as the Internet has grown, so have mechanisms to limit the extent and impact of abusive or objectionable communications. Recently, there has been an increasing emphasis on "blocking" and "filtering", the active prevention of such communications. This document examines several technical approaches to Internet blocking and filtering in terms of their alignment with the overall Internet architecture. When it is possible to do so, the approach to blocking and filtering that is most coherent with the Internet architecture is to inform endpoints about potentially undesirable services, so that the communicants can avoid engaging in abusive or objectionable communications. We observe that certain filtering and blocking approaches can cause unintended consequences to third parties, and we discuss the limits of efficacy of various approaches. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Architecture Board (IAB) and represents information that the IAB has deemed valuable to provide for permanent record. It represents the consensus of the Internet Architecture Board (IAB). Documents approved for publication by the IAB are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at
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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Filtering Examples . . . . . . . . . . . . . . . . . . . . . 5 3. Characteristics of Blocking Systems . . . . . . . . . . . . . 7 3.1. The Party Who Sets Blocking Policies . . . . . . . . . . 8 3.2. Purposes of Blocking . . . . . . . . . . . . . . . . . . 8 3.2.1. Blacklist vs. Whitelist Model . . . . . . . . . . . . 9 3.3. Intended Targets of Blocking . . . . . . . . . . . . . . 9 3.4. Components Used for Blocking . . . . . . . . . . . . . . 10 4. Evaluation of Blocking Design Patterns . . . . . . . . . . . 11 4.1. Criteria for Evaluation . . . . . . . . . . . . . . . . . 11 4.1.1. Scope: What set of hosts and users are affected? . . 12 4.1.2. Granularity: How specific is the blocking? Will blocking one service also block others? . . . . . . . 12 4.1.3. Efficacy: How easy is it for a resource or service to avoid being blocked? . . . . . . . . . . . . . . . . 13 4.1.4. Security: How does the blocking impact existing trust infrastructures? . . . . . . . . . . . . . . . . . . 14 4.2. Network-Based Blocking . . . . . . . . . . . . . . . . . 15 4.2.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . 16 4.2.2. Granularity . . . . . . . . . . . . . . . . . . . . . 17 4.2.3. Efficacy and Security . . . . . . . . . . . . . . . . 17 4.2.4. Summary . . . . . . . . . . . . . . . . . . . . . . . 20 4.3. Rendezvous-Based Blocking . . . . . . . . . . . . . . . . 20 4.3.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . 21 4.3.2. Granularity . . . . . . . . . . . . . . . . . . . . . 21 4.3.3. Efficacy . . . . . . . . . . . . . . . . . . . . . . 21 4.3.4. Security and Other Implications . . . . . . . . . . . 22 4.3.5. Examples . . . . . . . . . . . . . . . . . . . . . . 22 4.3.6. Summary . . . . . . . . . . . . . . . . . . . . . . . 23 4.4. Endpoint-Based Blocking . . . . . . . . . . . . . . . . . 24 4.4.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . 24 4.4.2. Granularity . . . . . . . . . . . . . . . . . . . . . 24 4.4.3. Efficacy . . . . . . . . . . . . . . . . . . . . . . 25 4.4.4. Security . . . . . . . . . . . . . . . . . . . . . . 25 4.4.5. Server Endpoints . . . . . . . . . . . . . . . . . . 25 4.4.6. Summary . . . . . . . . . . . . . . . . . . . . . . . 26 5. Security Considerations . . . . . . . . . . . . . . . . . . . 26 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 27 7. Informative References . . . . . . . . . . . . . . . . . . . 28 IAB Members at the Time of Approval . . . . . . . . . . . . . . . 32 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 33 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
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1. Introduction

The original design goal of the Internet was to enable communications between hosts. As this goal was met and people started using the Internet to communicate, however, it became apparent that some hosts were engaging in communications that were viewed as undesirable by certain parties. The most famous early example of undesirable communications was the Morris worm [Morris], which used the Internet to infect many hosts in 1988. As the Internet has evolved into a rich communications medium, so too have mechanisms to restrict communications viewed as undesirable, ranging from acceptable use policies enforced through informal channels to technical blocking mechanisms. Efforts to restrict or deny access to Internet resources and services have evolved over time. As noted in [RFC4084], some Internet service providers perform filtering to restrict which applications their customers may use and which traffic they allow on their networks. These restrictions are often imposed with customer consent, where customers may be enterprises or individuals. However, governments, service providers, and enterprises are increasingly seeking to block or filter access to certain content, traffic, or services without the knowledge or agreement of affected users. Where these organizations do not directly control networks themselves, they commonly aim to make use of intermediary systems to implement the blocking or filtering. While blocking and filtering remain highly contentious in many cases, the desire to restrict communications or access to content will likely continue to exist. The difference between "blocking" and "filtering" is a matter of scale and perspective. "Blocking" often refers to preventing access to resources in the aggregate, while "filtering" refers to preventing access to specific resources within an aggregate. Both blocking and filtering can be implemented at the level of "services" (web hosting or video streaming, for example) or at the level of particular "content." For the analysis presented in this document, the distinction between blocking and filtering does not create meaningfully different conclusions. Hence, in the remainder of this document, we will treat the terms as being generally equivalent and applicable to restrictions on both content and services. This document aims to clarify the technical implications and trade- offs of various blocking strategies and to identify the potential for different strategies to potentially cause harmful side effects ("collateral damage") for Internet users and the overall Internet architecture. This analysis is limited to technical blocking
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   mechanisms.  The scope of the analyzed blocking is limited to
   intentional blocking, not accidental blocking due to misconfiguration
   or as an unintentional side effect of something else.

   Filtering may be considered legal, illegal, ethical, or unethical in
   different places, at different times, and by different parties.  This
   document is intended for those who are conducting filtering or are
   considering conducting filtering and want to understand the
   implications of their decisions with respect to the Internet
   architecture and the trade-offs that come with each type of filtering
   strategy.  This document does not present formulas on how to make
   those trade-offs; it is likely that filtering decisions require
   knowledge of context-specific details.  Whether particular forms of
   filtering are lawful in particular jurisdictions raises complicated
   legal questions that are outside the scope of this document.  For
   similar reasons, questions about the ethics of particular forms of
   filtering are also out of scope.

2. Filtering Examples

Blocking systems have evolved alongside the Internet technologies they seek to restrict. Looking back at the history of the Internet, there have been several such systems deployed by different parties and for different purposes. Firewalls: Firewalls of various sorts are very commonly employed at many points in today's Internet [RFC2979]. They can be deployed either on end hosts (under user or administrator control) or in the network, typically at network boundaries. While the Internet Security Glossary [RFC4949] contains an extended definition of a firewall, informally, most people would tend to think of a firewall as simply "something that blocks unwanted traffic" (see [RFC4948] for a discussion on many types of unwanted traffic). While there are many sorts of firewalls, there are several specific types of firewall functionality worth noting. o Stateless Packet Filtering: Stateless packet filters block according to content-neutral rules, e.g., blocking all inbound connections or outbound connections on certain ports, protocols, or network-layer addresses. For example, blocking outbound connections to port 25. o Stateful Packet Filtering: More advanced configurations require keeping state used to enforce flow-based policies, e.g., blocking inbound traffic for flows that have not been established.
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   o  Deep Packet Inspection: Yet more advanced configurations perform
      deep packet inspection and filter or block based on the content
      carried.  Many firewalls include web filtering capabilities (see

   Web Filtering: HTTP and HTTPS are common targets for blocking and
   filtering, typically targeted at specific URIs.  Some enterprises use
   HTTP blocking to block non-work-appropriate web sites, and several
   nations require HTTP and HTTPS filtering by their ISPs in order to
   block content deemed illegal.  HTTPS is a challenge for these
   systems, because the URI in an HTTPS request is carried inside the
   encrypted channel.  To block access to content made accessible via
   HTTPS, filtering systems thus must either block based on network- and
   transport-layer headers (IP address and/or port), or else obtain a
   trust anchor certificate that is trusted by endpoints (and thus act
   as a man in the middle).  These filtering systems often take the form
   of "portals" or "enterprise proxies" presenting their own,
   dynamically generated HTTPS certificates.  (See further discussion in
   Section 5.)

   Spam Filtering: Spam filtering is one of the oldest forms of content
   filtering.  Spam filters evaluate messages based on a variety of
   criteria and information sources to decide whether a given message is
   spam.  For example, DNS Blacklists use the reverse DNS to flag
   whether an IP address is a known spam source [RFC5782].  Spam filters
   can be installed on user devices (e.g., in a mail client), operated
   by a mail domain on behalf of users, or outsourced to a third party
   that acts as an intermediate MX proxy.

   Domain Name Seizure: A number of approaches are used to block or
   modify resolution of a domain name.  One approach is to make use of
   ICANN's Uniform Dispute Resolution Policy (URDP) for the purposes of
   dealing with fraudulent use of a name.  Other authorities may require
   that domains be blocked within their jurisdictions.  Substantial
   research has been performed on the value and efficacy of such
   seizures [Takedown08] [BlackLists14].

   The precise method of how domain names are seized will vary from
   place to place.  One approach in use is for queries to be redirected
   to resolve to IP addresses of the authority that hosts information
   about the seizure.  The effectiveness of domain seizures will
   similarly vary based on the method.  In some cases, the person whose
   name was seized will simply use a new name.  In other cases, the
   block may only be effective within a region or when specific name
   service infrastructure is used.
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   Seizures can also have overbroad effects, since access to content is
   blocked not only within the jurisdiction of the seizure, but
   globally, even when it may be affirmatively legal elsewhere
   [RojaDirecta].  When domain redirection is effected via redirections
   at intermediate resolvers rather than at authoritative servers, it
   directly contradicts end-to-end assumptions in the DNS security
   architecture [RFC4033], potentially causing validation failures by
   validating end-nodes.

   Safe Browsing: Modern web browsers provide some measures to prevent
   users from accessing malicious web sites.  For instance, before
   loading a URI, current versions of Google Chrome and Firefox use the
   Google Safe Browsing service to determine whether or not a given URI
   is safe to load [SafeBrowsing].  The DNS can also be used to store
   third party information that mark domains as safe or unsafe

   Manipulation of routing and addressing data: Governments have
   recently intervened in the management of IP addressing and routing
   information in order to maintain control over a specific set of DNS
   servers.  As part of an internationally coordinated response to the
   DNSChanger malware, a Dutch court ordered the RIPE NCC to freeze the
   accounts of several resource holders as a means to limit the resource
   holders' ability to use certain address blocks [GhostClickRIPE] (also
   see Section 4.3).  These actions have led to concerns that the number
   resource certification system and related secure routing technologies
   developed by the IETF's SIDR working group might be subject to
   government manipulation as well [RFC6480], potentially for the
   purpose of denying targeted networks access to the Internet.

   Ingress filtering: Network service providers use ingress filtering
   [RFC2827] [RFC3704] as a means to prevent source address spoofing
   which is used as a part of other attacks.

   Data loss prevention (DLP): Enterprise and other networks are
   concerned with potential leaking of confidential information, whether
   accidental or intentional.  Some of the tools used for this are
   similar to the main subject of this document of blocking and
   filtering.  In particular, enterprise proxies might be part of a DLP

3. Characteristics of Blocking Systems

At a generic level, blocking systems can be characterized by four attributes: the party who sets the blocking policy, the purpose of the blocking, the intended target of the blocking, and the Internet component(s) used as the basis of the blocking system.
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3.1. The Party Who Sets Blocking Policies

Parties that institute blocking policies include governments, courts, enterprises, network operators, reputation trackers, application providers, and individual end users. A government might create laws based on cultural norms and/or their elected mandate. Enterprises might use cultural, industry, or legal norms to guide their policies. There can be several steps of translation and transformation from the original intended purpose -- first to laws, then to (government) regulation, followed by high-level policies in, e.g., network operators, and from those policies to filtering architecture and implementation. Each of those steps is a potential source of unintended consequences as discussed in this document. In some cases, the policy setting entity is the same as the entity that enforces the policy. For example, a network operator might install a firewall in its own networking equipment, or a web application provider might block responses between its web server and certain clients. In other cases, the policy setting entity is different from the entity that enforces the policy. Such policy might be imposed upon the enforcing entity, such as in the case of blocking initiated by governments, or the enforcing entity might explicitly choose to use policy set by others, such as in the case of a reputation system used by a spam filter or safe browsing service. Because a policy might be enforced by others, it is best if it can be expressed in a form that is independent of the enforcing technology.

3.2. Purposes of Blocking

There are a variety of motivations to filter: o Preventing or responding to security threats. Network operators, enterprises, application providers, and end users often block communications that are believed to be associated with security threats or network attacks. o Restricting objectionable content or services. Certain communications may be viewed as undesirable, harmful, or illegal by particular governments, enterprises, or users. Governments may seek to block communications that are deemed to be defamation, hate speech, obscenity, intellectual property infringement, or otherwise objectionable. Enterprises may seek to restrict employees from accessing content that is not deemed to be work appropriate. Parents may restrict their children from accessing content or services targeted for adults.
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   o  Restricting access based on business arrangements.  Some networks
      are designed so as to only provide access to certain content or
      services ("walled gardens"), or to only provide limited access
      until end users pay for full Internet services (captive portals
      provided by hotspot operators, for example).

3.2.1. Blacklist vs. Whitelist Model

Note that the purpose for which blocking occurs often dictates whether the blocking system operates on a blacklist model, where communications are allowed by default but a subset are blocked, or a whitelist model, where communications are blocked by default with only a subset allowed. Captive portals, walled gardens, and sandboxes used for security or network endpoint assessment usually require a whitelist model since the scope of communications allowed is narrow. Blocking for other purposes often uses a blacklist model since only individual content or traffic is intended to be blocked.

3.3. Intended Targets of Blocking

Blocking systems are instituted so as to target particular content, services, endpoints, or some combination of these. For example, a "content" filtering system used by an enterprise might block access to specific URIs whose content is deemed by the enterprise to be inappropriate for the workplace. This is distinct from a "service" filtering system that blocks all web traffic (perhaps as part of a parental control system on an end-user device) and also distinct from an "endpoint" filtering system in which a web application blocks traffic from specific endpoints that are suspected of malicious activity. As discussed in Section 4, the design of a blocking system may affect content, services, or endpoints other than those that are the intended targets. For example, when domain name seizures described above are intended to address specific web pages associated with illegal activity, by removing the domains from use, they affect all services made available by the hosts associated with those names, including mail services and web services that may be unrelated to the illegal activity. Depending on where the block is imposed within the DNS hierarchy, entirely unrelated organizations may be impacted.
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3.4. Components Used for Blocking

Broadly speaking, the process of delivering an Internet service involves three different components: 1. Endpoints: The actual content of the service is typically an application-layer protocol between two or more Internet hosts. In many protocols, there are two endpoints, a client and a server. 2. Network services: The endpoints communicate by way of a collection of IP networks that use routing protocols to determine how to deliver packets between the endpoints. 3. Rendezvous services: Service endpoints are typically identified by identifiers that are more "human-friendly" than IP addresses. Rendezvous services allow one endpoint to figure out how to contact another endpoint based on an identifier. An example of a rendezvous service is the domain name system. Distributed Hash Tables (DHTs) have also been used as rendezvous services. Consider, for example, an HTTP transaction fetching the content of the URI <>. The client endpoint is an end host running a browser. The client uses the DNS as a rendezvous service when it performs a AAAA query to obtain the IP address for the server name "". The client then establishes a connection to the server, and sends the actual HTTP request. The server endpoint then responds to the HTTP request. As another example, in the SIP protocol, the two endpoints communicating are IP phones, and the rendezvous service is provided by an application-layer SIP proxy as well as the DNS. Blocking access to Internet content, services, or endpoints is done by controlling one or more of the components involved in the provision of the communications involved in accessing the content, services, or endpoints. In the HTTP example above, the successful completion of the HTTP request could have been prevented in several ways: o [Endpoint] Preventing the client from making the request o [Endpoint] Preventing the server from responding to the request o [Endpoint] Preventing the client from making the DNS request needed to resolve o [Network] Preventing the request from reaching the server
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   o  [Network] Preventing the response from reaching the client

   o  [Network] Preventing the client from reaching the DNS servers

   o  [Network] Preventing the DNS responses from reaching the client

   o  [Rendezvous] Preventing the DNS servers from providing the client
      the correct IP address of the server

   Those who desire to block communications will typically have access
   to only one or two components; therefore their choices for how to
   perform blocking will be limited.  End users and application
   providers can usually only control their own software and hardware,
   which means that they are limited to endpoint-based filtering.  Some
   network operators offer filtering services that their customers can
   activate individually, in which case end users might have network-
   based filtering systems available to them.  Network operators can
   control their own networks and the rendezvous services for which they
   provide infrastructure support (e.g., DNS resolvers) or to which they
   may have access (e.g., SIP proxies), but not usually endpoints.
   Enterprises usually have access to their own networks and endpoints
   for filtering purposes.  Governments might make arrangements with the
   operators or owners of any of the three components that exist within
   their jurisdictions to perform filtering.

   In the next section, blocking systems designed according to each of
   the three patterns -- network services, rendezvous services, and
   endpoints -- are evaluated for their technical and architectural
   implications.  The analysis is as agnostic as possible as to who sets
   the blocking policy (government, end user, network operator,
   application provider, or enterprise), but in some cases the way in
   which a particular blocking design pattern is used might differ,
   depending on the who desires a block.  For example, a network-based
   firewall provided by an ISP that parents can elect to use for
   parental control purposes will likely function differently from one
   that all ISPs in a particular jurisdiction are required to use by the
   local government, even though in both cases the same component
   (network) forms the basis of the blocking system.

(page 11 continued on part 2)

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