💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc7754.txt captured on 2023-12-28 at 16:46:02.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
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
Abstract
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
http://www.rfc-editor.org/info/rfc7754.
Barnes, et al. Informational [Page 1]
RFC 7754 Blocking and Filtering Considerations March 2016
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
(http://trustee.ietf.org/license-info) 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.
Barnes, et al. Informational [Page 2]
RFC 7754 Blocking and Filtering Considerations March 2016
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
Barnes, et al. Informational [Page 3]
RFC 7754 Blocking and Filtering Considerations March 2016
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
Barnes, et al. Informational [Page 4]
RFC 7754 Blocking and Filtering Considerations March 2016
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.
Barnes, et al. Informational [Page 5]
RFC 7754 Blocking and Filtering Considerations March 2016
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
below).
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.
Barnes, et al. Informational [Page 6]
RFC 7754 Blocking and Filtering Considerations March 2016
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
[RFC5782].
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
solution.
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.
Barnes, et al. Informational [Page 7]
RFC 7754 Blocking and Filtering Considerations March 2016
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.
Barnes, et al. Informational [Page 8]
RFC 7754 Blocking and Filtering Considerations March 2016
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.
Barnes, et al. Informational [Page 9]
RFC 7754 Blocking and Filtering Considerations March 2016
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 <http://example.com/index.html>. 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 "example.com". 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 example.com
o [Network] Preventing the request from reaching the server
Barnes, et al. Informational [Page 10]
RFC 7754 Blocking and Filtering Considerations March 2016
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.
4. Evaluation of Blocking Design Patterns
4.1. Criteria for Evaluation
To evaluate the technical implications of each of the blocking design
patterns, we compare them based on four criteria: scope, granularity,
efficacy, and security.
Barnes, et al. Informational [Page 11]
RFC 7754 Blocking and Filtering Considerations March 2016
4.1.1. Scope: What set of hosts and users are affected?
The Internet is comprised of many distinct autonomous networks and
applications, which means that the impact of a blocking system will
only be within a defined topological scope. For example, blocking
within an access network will only affect a well-defined set of users
(namely, those connected to the access network). Blocking performed
by an application provider can affect users across the entire
Internet.
Blocking systems are generally viewed as less objectionable if the
scope of their impact is as narrow as possible while still being
effective, and as long as the impact of the blocking is within the
administrative realm of the policy setting entity. As mentioned
previously, enterprise blocking systems are commonly deployed, and
will generally have impact on enterprise users. However, design
flaws in blocking systems may cause the effects of blocking to be
overbroad. For example, at least one service provider blocking
content in accordance with a regulation has ended up blocking content
for downstream service providers because it filtered routes to
particular systems and did not distribute the original information to
downstream service providers in other jurisdictions
[IN-OM-filtering]. Other service providers have accidentally leaked
such black hole routes beyond the jurisdiction [NW08]. A substantial
amount of work has gone into BGP security to avoid such attacks, but
deployment of such systems lags.
4.1.2. Granularity: How specific is the blocking? Will blocking one
service also block others?
Internet applications are built out of a collection of loosely
coupled components or "layers". Different layers serve different
purposes and rely on or offer different functions such as routing,
transport, and naming (see [RFC1122], especially Section 1.1.3). The
functions at these layers are developed autonomously and almost
always operated by different parties. For example, in many networks,
physical and link-layer connectivity is provided by an "access
provider", IP routing is performed by an "Internet service provider,"
and application-layer services are provided by completely separate
entities (e.g., web servers). Upper-layer protocols and applications
rely on combinations of lower-layer functions in order to work.
Functionality at higher layers tends to be more specialized, so that
many different specialized applications can make use of the same
generic underlying network functions.
As a result of this structure, actions taken at one layer can affect
functionality or applications at other layers. For example,
manipulating routing or naming functions to restrict access to a
Barnes, et al. Informational [Page 12]
RFC 7754 Blocking and Filtering Considerations March 2016
narrow set of resources via specific applications will likely affect
all applications that depend on those functions. As with the scope
criteria, blocking systems are generally viewed as less objectionable
when they are highly granular and do not cause collateral damage to
content or services unrelated to the target of the blocking
[RFC4924].
Even within the application layer, the granularity of blocking can
vary depending on how targeted the blocking system is designed to be.
Blocking all traffic associated with a particular application
protocol is less granular than blocking only traffic associated with
a subset of application instances that make use of that protocol.
Sophisticated heuristics that make use of information about the
application protocol, lower-layer protocols, payload signatures,
source and destination addresses, inter-packet timing, packet sizes,
and other characteristics are sometimes used to narrow the subset of
traffic to be blocked.
4.1.3. Efficacy: How easy is it for a resource or service to avoid
being blocked?
Although blocking a resource or service might have some immediate
effect, efficacy must be evaluated in terms of whether it is easy to
circumvent. Simply doing a one-time policy is often unlikely to have
lasting efficacy (e.g., see [CleanFeed] and [BlackLists14]).
Experience has shown that, in general, blacklisting requires
continual maintenance of the blacklist itself, both to add new
entries for unwanted traffic and deleting entries when offending
content is removed. Experience also shows that, depending on the
nature of the block, it may be difficult to determine when to
unblock. For instance, if a host is blocked because it has been
compromised and used as a source of attack, it may not be plainly
evident when that site has been fixed.
For blacklist-style blocking, the distributed and mobile nature of
Internet resources limits the effectiveness of blocking actions. A
service that is blocked in one jurisdiction can often be moved or re-
instantiated in another jurisdiction (see, for example,
[Malicious-Resolution]). Likewise, services that rely on blocked
resources can often be rapidly reconfigured to use non-blocked
resources. If a web site is prevented from using a domain name or
set of IP addresses, the content can simply be moved to another
domain name or network, or use alternate syntaxes to express the same
resource name (see the discussion of false negatives in [RFC6943]).
Barnes, et al. Informational [Page 13]
RFC 7754 Blocking and Filtering Considerations March 2016
In a process known as "snowshoe spamming," a spam originator uses
addresses in many different networks as sources for spam. This
technique is already widely used to spread spam generation across a
variety of resources and jurisdictions to prevent spam blocking from
being effective.
In the presence of either blacklist or whitelist systems, there are
several ways in which a user or application can try to circumvent the
filters.
The users may choose to use different sets of protocols or otherwise
alter their traffic characteristics to circumvent the filters. In
some cases, applications may shift their traffic to port 80 or 443
when other ports are blocked. Or, services may be tunneled within
other services, proxied by a collaborating external host (e.g., an
anonymous redirector), or simply run over an alternate port (e.g.,
port 8080 vs port 80 for HTTP). Another means of circumvention is
alteration of the service behavior to use a dynamic port negotiation
phase, in order to avoid use of a constant port address.
One of the primary motivations for arguing that HTTP/2 should be
encrypted by default was that unencrypted HTTP 1.1 traffic was
sometimes blocked or improperly processed. Users or applications
shifting their traffic to encrypted HTTP has the effect of
circumventing filters that depend on the HTTP plaintext payload.
If voice communication based on SIP [RFC3261] is blocked, users are
likely to use applications which use proprietary protocols that allow
them to talk to each other.
Some filtering systems are only capable of identifying IPv4 traffic
and therefore, by shifting to IPv6, users may be able to evade
filtering. Using IPv6 with header options, using multiple layers of
tunnels, or using encrypted tunnels can also make it more challenging
for blocking systems to find transport ports within packets, making
port-based blocking more difficult. Thus, distribution and mobility
can hamper efforts to block communications in a number of ways.
4.1.4. Security: How does the blocking impact existing trust
infrastructures?
Modern security mechanisms rely on trusted hosts communicating via a
secure channel without intermediary interference. Protocols such as
Transport Layer Security (TLS) [RFC5246] and IPsec [RFC4301] are
designed to ensure that each endpoint of the communication knows the
identity of the other endpoint(s) and that only the endpoints of the
communication can access the secured contents of the communication.
For example, when a user connects to a bank's web site, TLS ensures
Barnes, et al. Informational [Page 14]
RFC 7754 Blocking and Filtering Considerations March 2016
that the user's banking information is securely communicated to the
bank and nobody else, ensuring the data remains confidential while in
transit.
Some blocking strategies require intermediaries to insert themselves
within the end-to-end communications path, potentially breaking
security properties of Internet protocols [RFC4924]. In these cases,
it can be difficult or impossible for endpoints to distinguish
between attackers and "authorized" parties conducting blocking. For
example, an enterprise firewall administrator could gain access to
users' personal bank accounts when users on the enterprise network
connect to bank web sites.
Finally, one needs to evaluate whether a blocking mechanism can be
used by an end user to efficiently locate blocked resources that can
then be accessed via other mechanisms that circumvent the blocking
mechanism. For example, Clayton [CleanFeed] showed how special
treatment in one blocking system could be detected by end users in
order to efficiently locate illegal web sites, which was thus
counterproductive to the policy objective of the blocking mechanism.
4.2. Network-Based Blocking
Being able to block access to resources without the consent or
cooperation of either endpoint is viewed as a desirable feature by
some that deploy blocking systems. Systems that have this property
are often implemented using intermediary devices in the network, such
as firewalls or filtering systems. These systems inspect traffic as
it passes through the network, decide based on the characteristics or
content of a given communication whether it should be blocked, and
then block or allow the communication as desired. For example, web
filtering devices usually inspect HTTP requests to determine the URI
being requested, compare that URI to a list of blacklisted or
whitelisted URIs, and allow the request to proceed only if it is
permitted by policy. Firewalls perform a similar function for other
classes of traffic in addition to HTTP. Some blocking systems focus
on specific application-layer traffic, while others, such as router
Access Control Lists (ACLs), filter traffic based on lower-layer
criteria (transport protocol and source or destination addresses or
ports).
Intermediary systems used for blocking are often not far from the
edge of the network. For example, many enterprise networks operate
firewalls that block certain web sites, as do some residential ISPs.
In some cases, this filtering is done with the consent or cooperation
of the affected endpoints. PCs within an enterprise, for example,
might be configured to trust an enterprise proxy, a residential ISP
might offer a "safe browsing" service, or mail clients might
Barnes, et al. Informational [Page 15]
RFC 7754 Blocking and Filtering Considerations March 2016
authorize mail servers on the local network to filter spam on their
behalf. These cases share some of the properties of the "Endpoint-
Based Blocking" scenarios discussed in Section 4.4 below, since the
endpoint has made an informed decision to authorize the intermediary
to block on its behalf and is therefore unlikely to attempt to
circumvent the blocking. From an architectural perspective, however,
they may create many of the same problems as network-based filtering
conducted without consent.
4.2.1. Scope
In the case of government-initiated blocking, network operators
subject to a specific jurisdiction may be required to block or
filter. Thus, it is possible for laws to be structured to result in
blocking by imposing obligations on the operators of networks within
a jurisdiction, either via direct government action or by allowing
private actors to demand blocking (e.g., through lawsuits).
Regardless of who is responsible for a blocking policy, enforcement
can be done using Stateless Packet Filtering, Stateful Packet
Filtering, or Deep Packet Inspection as defined in Section 2. While
network-based Stateless Packet Filtering has granularity issues
discussed in Section 4.2.2, network-based Stateful Packet Filtering
and Deep Packet Inspection approaches often run into several
technical issues that limit their viability in practice. For
example, many issues arise from the fact that an intermediary needs
to have access to a sufficient amount of traffic to make its blocking
determinations.
For residential or consumer networks with many egress points, the
first step to obtaining this traffic is simply gaining access to the
constituent packets. The Internet is designed to deliver packets
independently from source to destination -- not to any particular
point along the way. Thus, the sequence of packets from the sender
can only be reliably reconstructed at the intended receiver. In
addition, inter-network routing is often asymmetric, and for
sufficiently complex local networks, intra-network traffic flows can
be asymmetric as well [asymmetry]. Thus, packets in the reverse
direction use a different sent of paths than the forward direction.
This asymmetry means that an intermediary in a network with many
egress points may, depending on topology and configuration, see only
one half of a given communication, which may limit the scope of the
communications that it can filter. For example, a filter aimed at
requests destined for particular URIs cannot make accurate blocking
decisions based on the URI if it is only in the data path for HTTP
responses and not requests, since the URI is not included in the
responses. Asymmetry may be surmountable given a filtering system
Barnes, et al. Informational [Page 16]
RFC 7754 Blocking and Filtering Considerations March 2016
with enough distributed, interconnected filtering nodes that can
coordinate information about flows belonging to the same
communication or transaction, but depending on the size of the
network this may imply significant complexity in the filtering
system. Routing can sometimes be forced to be symmetric within a
given network using routing configuration, NAT, or Layer 2 mechanisms
(e.g., MPLS), but these mechanisms are frequently brittle, complex,
and costly -- and can sometimes result in reduced network performance
relative to asymmetric routing. Enterprise networks may also be less
susceptible to these problems if they route all traffic through a
small number of egress points.
4.2.2. Granularity
Once an intermediary in a network has access to traffic, it must
identify which packets must be filtered. This decision is usually
based on some combination of information at the network layer (e.g.,
IP addresses), transport layer (ports), or application layer (URIs or
other content). Deep Packet Inspection type blocking based on
application-layer attributes can be potentially more granular and
less likely to cause collateral damage than blocking all traffic
associated with a particular address, which can impact unrelated
occupants of the same address. However, more narrowly focused
targeting may be more complex, less efficient, or easier to
circumvent than filtering that sweeps more broadly, and those who
seek to block must balance these attributes against each other when
choosing a blocking system.
4.2.3. Efficacy and Security
Regardless of the layer at which blocking occurs, it may be open to
circumvention, particularly in cases where network endpoints have not
authorized the blocking. The communicating endpoints can deny the
intermediary access to attributes at any layer by using encryption
(see below). IP addresses must be visible, even if packets are
protected with IPsec, but blocking based on IP addresses can be
trivial to circumvent. A filtered site may be able to quickly change
its IP address using only a few simple steps: changing a single DNS
record and provisioning the new address on its server or moving its
services to the new address [BT-TPB].
Indeed, Poort, et al. [Poort] found that "any behavioural change in
response to blocking access to The Pirate Bay has had no lasting net
impact on the overall number of downloaders from illegal sources, as
new consumers have started downloading from illegal sources and
people learn to circumvent the blocking while new illegal sources may
be launched, causing file sharing to increase again", and that these
Barnes, et al. Informational [Page 17]
RFC 7754 Blocking and Filtering Considerations March 2016
results "are in line with a tendency found in the literature that any
effects of legal action against file sharing often fade out after a
period of typically six months."
If application content is encrypted with a security protocol such as
IPsec or TLS, then the intermediary will require the ability to
decrypt the packets to examine application content, or resort to
statistical methods to guess what the content is. Since security
protocols are generally designed to provide end-to-end security
(i.e., to prevent intermediaries from examining content), the
intermediary would need to masquerade as one of the endpoints,
breaking the authentication in the security protocol, reducing the
security of the users and services affected, and interfering with
legitimate private communication. Besides, various techniques that
use public databases with whitelisted keys (e.g., DANE [RFC6698])
enable users to detect these sort of intermediaries. Those users are
then likely to act as if the service is blocked.
If the intermediary is unable to decrypt the security protocol, then
its blocking determinations for secure sessions can only be based on
unprotected attributes, such as IP addresses, protocol IDs, port
numbers, packet sizes, and packet timing. Some blocking systems
today still attempt to block based on these attributes, for example
by blocking TLS traffic to known proxies that could be used to tunnel
through the blocking system.
However, as the Telex project [Telex] recently demonstrated, if an
endpoint cooperates with a relay in the network (e.g., a Telex
station), it can create a TLS tunnel that is indistinguishable from
legitimate traffic. For example, if an ISP used by a banking web
site were to operate a Telex station at one of its routers, then a
blocking system would be unable to distinguish legitimate encrypted
banking traffic from Telex-tunneled traffic (potentially carrying
content that would have been filtered).
Thus, in principle in a blacklist system it is impossible to block
tunneled traffic through an intermediary device without blocking all
secure traffic from that system. (The only limitation in practice is
the requirement for special software on the client.) Those who
require that secure traffic be blocked from such sites risk blocking
content that would be valuable to their users, perhaps impeding
substantial economic activity. Conversely, those who are hosting a
myriad of content have an incentive to see that law abiding content
does not end up being blocked.
Barnes, et al. Informational [Page 18]
RFC 7754 Blocking and Filtering Considerations March 2016
Governments and network operators should, however, take care not to
encourage the use of insecure communications in the naming of
security, as doing so will invariably expose their users to the
various attacks that the security protocols were put in place to
prevent.
Some operators may assume that only blocking access to resources
available via unsecure channels is sufficient for their purposes --
i.e., that the size of the user base that will be willing to use
secure tunnels and/or special software to circumvent the blocking is
low enough to make blocking via intermediaries worthwhile. Under
that assumption, one might decide that there is no need to control
secure traffic and thus that network-based blocking is an attractive
option.
However, the longer such blocking systems are in place, the more
likely it is that efficient and easy-to-use tunneling tools will
become available. The proliferation of the Tor network, for example,
and its increasingly sophisticated blocking-avoidance techniques
demonstrate that there is energy behind this trend [Tor]. Thus,
network-based blocking becomes less effective over time.
Network-based blocking is a key contributor to the arms race that has
led to the development of such tools, the result of which is to
create unnecessary layers of complexity in the Internet. Before
content-based blocking became common, the next best option for
network operators was port blocking, the widespread use of which has
driven more applications and services to use ports (80 and 443 most
commonly) that are unlikely to be blocked. In turn, network
operators shifted to finer-grained content blocking over port 80,
content providers shifted to encrypted channels, and operators began
seeking to identify those channels (although doing so can be
resource-prohibitive, especially if tunnel endpoints begin to change
frequently). Because the premise of network-based blocking is that
endpoints have incentives to circumvent it, this cat-and-mouse game
is an inevitable by-product of this form of blocking.
One reason above all stands as an enormous challenge to network-based
blocking: the Internet was designed with the premise that people will
want to connect and communicate. IP will run on anything up to and
including carrier pigeons [RFC1149]. It often runs atop TLS and has
been made to run on other protocols that themselves run atop IP.
Because of this fundamental layering approach, nearly any authorized
avenue of communication can be used as a transport. This same
"problem" permits communications to succeed in the most challenging
of environments.
Barnes, et al. Informational [Page 19]
RFC 7754 Blocking and Filtering Considerations March 2016
4.2.4. Summary
In sum, network-based blocking is only effective in a fairly
constrained set of circumstances. First, the traffic needs to flow
through the network in such a way that the intermediary device has
access to any communications it intends to block. Second, the
blocking system needs an out-of-band mechanism to mitigate the risk
of secure protocols being used to avoid blocking (e.g., human
analysts identifying IP addresses of tunnel endpoints). If the
network is sufficiently complex, or the risk of tunneling too high,
then network-based blocking is unlikely to be effective, and in any
case this type of blocking drives the development of increasingly
complex layers of circumvention. Network-based blocking can be done
without the cooperation of either endpoint to a communication, but it
has the serious drawback of breaking end-to-end security assurances
in some cases. The fact that network-based blocking is premised on
this lack of cooperation results in arms races that increase the
complexity of both application design and network design.
4.3. Rendezvous-Based Blocking
Internet applications often require or rely on support from common,
global rendezvous services, including the DNS, certificate
authorities, search engines, WHOIS databases, and Internet Route
Registries. These services control or register the structure and
availability of Internet applications by providing data elements that
are used by application code. Some applications also have their own
specialized rendezvous services. For example, to establish an end-
to-end SIP call, the end-nodes (terminals) rely on presence and
session information supplied by SIP servers.
Global rendezvous services are comprised of generic technical
databases intended to record certain facts about the network. The
DNS, for example, stores information about which servers provide
services for a given name, and the Resource Public Key Infrastructure
(RPKI) stores information about which organizations have been
allocated IP addresses. To offer specialized Internet services and
applications, different people rely on these generic records in
different ways. Thus, the effects of changes to the databases can be
much more difficult to predict than, for example, the effect of
shutting down a web server (which fulfills the specific purpose of
serving web content).
Although rendezvous services are discussed as a single category, the
precise characteristics and implications of blocking each kind of
rendezvous service are slightly different. This section provides
examples to highlight these differences.
Barnes, et al. Informational [Page 20]
RFC 7754 Blocking and Filtering Considerations March 2016
4.3.1. Scope
In the case of government-initiated blocking, the operators of
servers used to provide rendezvous service that are subject to a
specific jurisdiction may be required to block or filter. Thus, it
is possible for laws to be structured to result in blocking by
imposing obligations on the operators of rendezvous services within a
jurisdiction, either via direct government action or by allowing
private actors to demand blocking (e.g., through lawsuits).
The scope of blocking conducted by others will depend on which
servers they can access. For example, network operators and
enterprises may be capable of conducting blocking using their own DNS
resolvers or application proxies within their networks, but not
authoritative servers controlled by others.
However, if a service is hosted and operated within a jurisdiction
where it is considered legitimate, then blocking access at a global
rendezvous service (e.g., one within a jurisdiction where it is
considered illegitimate) might deny services in jurisdictions where
they are considered legitimate. This type of collateral damage is
lessened when blocking is done at a local rendezvous server that only
has local impact, rather than at a global rendezvous server with
global impact.
4.3.2. Granularity
Blocking at a global rendezvous service can be overbroad if the
resources blocked support multiple services, since blocking service
can cause collateral damage to legitimate uses of other services.
For example, a given address or domain name might host both
legitimate services as well as services that some would desire to
block.
4.3.3. Efficacy
The distributed nature of the Internet limits the efficacy of
blocking based on rendezvous services. If the Internet community
realizes that a blocking decision has been made and wishes to counter
it, then local networks can "patch" the authoritative data that a
global rendezvous service provides to avoid the blocking (although
the development of DNSSEC and the RPKI are causing this to change by
requiring updates to be authorized). In the DNS case, registrants
whose names get blocked can relocate their resources to different
names.
Barnes, et al. Informational [Page 21]
RFC 7754 Blocking and Filtering Considerations March 2016
Endpoints can also choose not to use a particular rendezvous service.
They might switch to a competitor or use an alternate mechanism (for
example, IP literals in URIs to circumvent DNS filtering).
4.3.4. Security and Other Implications
Blocking of global rendezvous services also has a variety of other
implications that may reduce the stability, accessibility, and
usability of the global Internet. Infrastructure-based blocking may
erode the trust in the general Internet and encourage the development
of parallel or "underground" infrastructures causing forms of
Internet fragmentation, for example. This risk may become more acute
as the introduction of security infrastructures and mechanisms such
as DNSSEC and RPKI "hardens" the authoritative data -- including
blocked names or routes -- that the existing infrastructure services
provide. Those seeking to circumvent the blocks may opt to use less-
secure but unblocked parallel services. As applied to the DNS, these
considerations are further discussed in RFC 2826 [RFC2826], in the
advisory [SAC-056] from ICANN's Security and Stability Advisory
Committee (SSAC), and in the Internet Society's whitepaper on DNS
filtering [ISOCFiltering], but they also apply to other global
Internet resources.
4.3.5. Examples
Below we provide a few specific examples for routing, DNS, and WHOIS
services. These examples demonstrate that for these types of
rendezvous services (services that are often considered a global
commons), jurisdiction-specific legal and ethical motivations for
blocking can both have collateral effects in other jurisdictions and
be circumvented because of the distributed nature of the Internet.
In 2008, Pakistan Telecom attempted to deny access to YouTube within
Pakistan by announcing bogus routes for YouTube address space to
peers in Pakistan. YouTube was temporarily denied service on a
global basis as a result of a route leak beyond the Pakistani ISP's
scope, but service was restored in approximately two hours because
network operators around the world reconfigured their routers to
ignore the bogus routes [RenesysPK]. In the context of SIDR and
secure routing, a similar reconfiguration could theoretically be done
if a resource certificate were to be revoked in order to block
routing to a given network.
In the DNS realm, one of the recent cases of U.S. law enforcement
seizing domain names involved RojaDirecta, a Spanish web site. Even
though several of the affected domain names belonged to Spanish
organizations, they were subject to blocking by the U.S. government
because certain servers were operated in the United States.
Barnes, et al. Informational [Page 22]
RFC 7754 Blocking and Filtering Considerations March 2016
Government officials required the operators of the parent zones of a
target name (e.g., "com" for "example.com") to direct queries for
that name to a set of U.S.-government-operated name servers. Users
of other services (e.g., email) under a target name would thus be
unable to locate the servers providing services for that name,
denying them the ability to access these services.
Similar workarounds as those that were used in the Pakistan Telecom
case are also available in the DNS case. If a domain name is blocked
by changing authoritative records, network operators can restore
service simply by extending TTLs on cached pre-blocking records in
recursive resolvers, or by statically configuring resolvers to return
unblocked results for the affected name. However, depending on the
availability of valid signature data, these types of workarounds will
not work with DNSSEC-signed data.
The action of the Dutch authorities against the RIPE NCC, where RIPE
was ordered to freeze the accounts of Internet resource holders, is
of a similar character. By controlling the account holders' WHOIS
information, this type of action limited the ability of the ISPs in
question to manage their Internet resources. This example is
slightly different from the others because it does not immediately
impact the ability of ISPs to provide connectivity. While ISPs use
(and trust) the WHOIS databases to build route filters or use the
databases for trouble-shooting information, the use of the WHOIS
databases for those purposes is voluntary. Thus, seizure of this
sort may not have any immediate effect on network connectivity, but
it may impact overall trust in the common infrastructure. It is
similar to the other examples in that action in one jurisdiction can
have broader effects, and in that the global system may encourage
networks to develop their own autonomous solutions.
4.3.6. Summary
In summary, rendezvous-based blocking can sometimes be used to
immediately block a target service by removing some of the resources
it depends on. However, such blocking actions can have harmful side
effects due to the global nature of Internet resources and the fact
that many different application-layer services rely on generic,
global databases for rendezvous purposes. The fact that Internet
resources can quickly shift between network locations, names, and
addresses, together with the autonomy of the networks that comprise
the Internet, can mean that the effects of rendezvous-based blocking
can be negated on short order in some cases. For some applications,
rendezvous services are optional to use, not mandatory. Hence, they
are only effective when the endpoint or the endpoint's network
chooses to use them; they can be routed around by choosing not to use
Barnes, et al. Informational [Page 23]
RFC 7754 Blocking and Filtering Considerations March 2016
the rendezvous service or migrating to an alternative one. To adapt
a quote by John Gilmore, "The Internet treats blocking as damage and
routes around it".
4.4. Endpoint-Based Blocking
Internet users and their devices constantly make decisions as to
whether to engage in particular Internet communications. Users
decide whether to click on links in suspect email messages; browsers
advise users on sites that have suspicious characteristics; spam
filters evaluate the validity of senders and messages. If the
hardware and software making these decisions can be instructed not to
engage in certain communications, then the communications are
effectively blocked because they never happen.
There are several systems in place today that advise user systems
about which communications they should engage in. As discussed
above, several modern browsers consult with "Safe Browsing" services
before loading a web site in order to determine whether the site
could potentially be harmful. Spam filtering is one of the oldest
types of filtering in the Internet; modern filtering systems
typically make use of one or more "reputation" or "blacklist"
databases in order to make decisions about whether a given message or
sender should be blocked. These systems typically have the property
that many filtering systems (browsers, Mail Transfer Agents (MTAs))
share a single reputation service. Even the absence of provisioned
PTR records for an IP address may result in email messages not being
accepted.
4.4.1. Scope
In an endpoint-based blocking system, blocking actions are performed
autonomously, by individual endpoints or their delegates. The
effects of blocking are thus usually local in scope, minimizing the
effects on other users or other, legitimate services.
4.4.2. Granularity
Endpoint-based blocking avoids some of the limitations of rendezvous-
based blocking: while rendezvous-based blocking can only see and
affect the rendezvous service at hand (e.g., DNS name resolution),
endpoint-based blocking can potentially see into the entire
application, across all layers and transactions. This visibility can
provide endpoint-based blocking systems with a much richer set of
information for making narrow blocking decisions. Support for narrow
granularity depends on how the application protocol client and server
are designed, however. A typical endpoint-based firewall application
Barnes, et al. Informational [Page 24]
RFC 7754 Blocking and Filtering Considerations March 2016
may have less ability to make fine-grained decisions than an
application that does its own blocking (see [RFC7288] for further
discussion).
4.4.3. Efficacy
Endpoint-based blocking deals well with mobile adversaries. If a
blocked service relocates resources or uses different resources, a
rendezvous- or network-based blocking approach may not be able to
affect the new resources (at least not immediately). A network-based
blocking system may not even be able to tell whether the new
resources are being used, if the previously blocked service uses
secure protocols. By contrast, endpoint-based blocking systems can
detect when a blocked service's resources have changed (because of
their full visibility into transactions) and adjust blocking as
quickly as new blocking data can be sent out through a reputation
system.
The primary challenge to endpoint-based blocking is that it requires
the cooperation of endpoints. Where this cooperation is willing,
this is a fairly low barrier, requiring only reconfiguration or
software update. Where cooperation is unwilling, it can be
challenging to enforce cooperation for large numbers of endpoints.
That challenge is exacerbated when the endpoints are a diverse set of
static, mobile, or visiting endpoints. If cooperation can be
achieved, endpoint-based blocking can be much more effective than
other approaches because it is so coherent with the Internet's
architectural principles.
4.4.4. Security
Endpoint-based blocking is performed at one end of an Internet
communication, and thus avoids the problems related to end-to-end
security mechanisms that network-based blocking runs into and the
challenges to global trust infrastructures that rendezvous-based
blocking creates.
4.4.5. Server Endpoints
In this discussion of endpoint-based blocking, the focus has been on
the consuming side of the end-to-end communication, mostly the client
side of a client-server type connection. However, similar
considerations apply to the content-producing side of end-to-end
communications, regardless of whether that endpoint is a server in a
client-server connection or a peer in a peer-to-peer type of
connection.
Barnes, et al. Informational [Page 25]
RFC 7754 Blocking and Filtering Considerations March 2016
For instance, for blocking of web content, narrow targeting can be
achieved through whitelisting methods like password authentication,
whereby passwords are available only to authorized clients. For
example, a web site might only make adult content available to users
who provide credit card information, which is assumed to be a proxy
for age.
The fact that content-producing endpoints often do not take it upon
themselves to block particular forms of content in response to
requests from governments or other parties can sometimes motivate
those latter parties to engage in blocking elsewhere within the
Internet.
If a service is to be blocked, the best way of doing that is to
disable the service at the server endpoint.
4.4.6. Summary
Out of the three design patterns, endpoint-based blocking is the
least likely to cause collateral damage to Internet services or the
overall Internet architecture. Endpoint-based blocking systems can
potentially see into all layers involved in a communication, allowing
blocking to be narrowly targeted and can minimize unintended
consequences. Adversary mobility can be accounted for as soon as
reputation systems are updated with new adversary information. One
potential drawback of endpoint-based blocking is that it requires the
endpoint's cooperation; implementing blocking at an endpoint when it
is not in the endpoint's interest is therefore difficult to
accomplish because the endpoint's user can disable the blocking or
switch to a different endpoint.
5. Security Considerations
The primary security concern related to Internet service blocking is
the effect that it has on the end-to-end security model of many
Internet security protocols. When blocking is enforced by an
intermediary with respect to a given communication, the blocking
system may need to obtain access to confidentiality-protected data to
make blocking decisions. Mechanisms for obtaining such access often
require the blocking system to defeat the authentication mechanisms
built into security protocols.
For example, some enterprise firewalls will dynamically create TLS
certificates under a trust anchor recognized by endpoints subject to
blocking. These certificates allow the firewall to authenticate as
any web site, so that it can act as a man-in-the-middle on TLS
Barnes, et al. Informational [Page 26]
RFC 7754 Blocking and Filtering Considerations March 2016
connections passing through the firewall. This is not unlike an
external attacker using compromised certificates to intercept TLS
connections.
Modifications such as these obviously make the firewall itself an
attack surface. If an attacker can gain control of the firewall or
compromise the key pair used by the firewall to sign certificates,
the attacker will have access to the unencrypted data of all current
and recorded TLS sessions for all users behind that firewall, in a
way that is undetectable to users. Besides, if the compromised key-
pairs can be extracted from the firewall, all users, not only those
behind the firewall, that rely on that public key are vulnerable.
We must also consider the possibility that a legitimate administrator
of such a firewall could gain access to privacy-sensitive
information, such as the bank accounts or health records of users who
access such secure sites through the firewall. These privacy
considerations motivate legitimate use of secure end-to-end protocols
that often make it difficult to enforce granular blocking policies.
When blocking systems are unable to inspect and surgically block
secure protocols, it is tempting to completely block those protocols.
For example, a web blocking system that is unable to inspect HTTPS
connections might simply block any attempted HTTPS connection.
However, since Internet security protocols are commonly used for
critical services such as online commerce and banking, blocking these
protocols would block access to these services as well, or worse,
force them to be conducted over insecure communication.
Security protocols can, of course, also be used as mechanisms for
blocking services. For example, if a blocking system can insert
invalid credentials for one party in an authentication protocol, then
the other end will typically terminate the connection based on the
authentication failure. However, it is typically much simpler to
simply block secure protocols than to exploit those protocols for
service blocking.
6. Conclusion
Filtering will continue to occur on the Internet. We conclude that,
whenever possible, filtering should be done on the endpoint.
Cooperative endpoints are most likely to have sufficient contextual
knowledge to effectively target blocking; hence, such blocking
minimizes unintended consequences. It is realistic to expect that at
times filtering will not be done on the endpoints. In these cases,
promptly informing the endpoint that blocking has occurred provides
necessary transparency to redress any errors, particularly as they
relate to any collateral damage introduced by errant filters.
Barnes, et al. Informational [Page 27]
RFC 7754 Blocking and Filtering Considerations March 2016
Blacklist approaches are often a game of "cat and mouse", where those
with the content move it around to avoid blocking. Or, the content
may even be naturally mirrored or cached at other legitimate sites
such as the Internet Archive Wayback Machine [Wayback]. At the same
time, whitelists provide similar risks because sites that had
"acceptable" content may become targets for "unacceptable content",
and similarly, access to perfectly inoffensive and perhaps useful or
productive content is unnecessarily blocked.
From a technical perspective, there are no perfect or even good
solutions -- there is only least bad. On that front, we posit that a
hybrid approach that combines endpoint-based filtering with network
filtering may prove least damaging. An endpoint may choose to
participate in a filtering regime in exchange for the network
providing broader unfiltered access.
Finally, we note that where filtering is occurring to address content
that is generally agreed to be inappropriate or illegal, strong
cooperation among service providers and governments may provide
additional means to identify both the victims and the perpetrators
through non-filtering mechanisms, such as partnerships with the
finance industry to identify and limit illegal transactions.
7. Informative References
[asymmetry]
John, W., Dusi, M., and K. Claffy, "Estimating routing
symmetry on single links by passive flow measurements",
Proceedings of the 6th International Wireless
Communications and Mobile Computing Conference, IWCMC '10,
DOI 10.1145/1815396.1815506, 2010,
<http://www.caida.org/publications/papers/2010/
estimating_routing_symmetry/
estimating_routing_symmetry.pdf>.
[BlackLists14]
Chachra, N., McCoy, D., Savage, S., and G. Voelker,
"Empirically Characterizing Domain Abuse and the Revenue
Impact of Blacklisting", Workshop on the Economics of
Information Security 2014,
<http://www.econinfosec.org/archive/weis2014/papers/
Chachra-WEIS2014.pdf>.
[BT-TPB] Meyer, D., "BT blocks The Pirate Bay", June 2012,
<http://www.zdnet.com/
bt-blocks-the-pirate-bay-4010026434/>.
Barnes, et al. Informational [Page 28]
RFC 7754 Blocking and Filtering Considerations March 2016
[CleanFeed]
Clayton, R., "Failures in a Hybrid Content Blocking
System", Fifth Privacy Enhancing Technologies Workshop,
PET 2005, DOI 10.1007/11767831_6, 2005,
<http://www.cl.cam.ac.uk/~rnc1/cleanfeed.pdf>.
[GhostClickRIPE]
RIPE NCC, "RIPE NCC Blocks Registration in RIPE Registry
Following Order from Dutch Police", 2012,
<http://www.ripe.net/internet-coordination/news/
about-ripe-ncc-and-ripe/ripe-ncc-blocks-registration-in-
ripe-registry-following-order-from-dutch-police>.
[IN-OM-filtering]
Citizen Lab, "Routing Gone Wild: Documenting upstream
filtering in Oman via India", July 2012,
<https://citizenlab.org/2012/07/routing-gone-wild/>.
[ISOCFiltering]
Internet Society, "DNS: Finding Solutions to Illegal
On-line Activities", 2012,
<http://www.internetsociety.org/what-we-do/issues/dns/
finding-solutions-illegal-line-activities>.
[Malicious-Resolution]
Dagon, D., Provos, N., Lee, C., and W. Lee, "Corrupted DNS
Resolution Paths: The Rise of a Malicious Resolution
Authority", 2008,
<http://www.citi.umich.edu/u/provos/papers/
ndss08_dns.pdf>.
[Morris] Kehoe, B., "The Robert Morris Internet Worm", 1992,
<http://groups.csail.mit.edu/mac/classes/6.805/articles/
morris-worm.html>.
[NW08] Marsan, C., "YouTube/Pakistan incident: Could something
similar whack your site?", Network World, March 2008,
<http://www.networkworld.com/article/2284273/software/
youtube-pakistan-incident--could-something-similar-whack-
your-site-.html>.
[Poort] Poort, J., Leenheer, J., van der Ham, J., and C. Dumitru,
"Baywatch: Two approaches to measure the effects of
blocking access to The Pirate Bay", Telecommunications
Policy 38:383-392, DOI 10.1016/j.telpol.2013.12.008, 2014,
<http://staff.science.uva.nl/~vdham/research/
publications/1401-Baywatch.pdf>.
Barnes, et al. Informational [Page 29]
RFC 7754 Blocking and Filtering Considerations March 2016
[RenesysPK]
Brown, M., "Pakistan hijacks YouTube", February 2008,
<http://research.dyn.com/2008/02/
pakistan-hijacks-youtube-1/>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<http://www.rfc-editor.org/info/rfc1122>.
[RFC1149] Waitzman, D., "Standard for the transmission of IP
datagrams on avian carriers", RFC 1149,
DOI 10.17487/RFC1149, April 1990,
<http://www.rfc-editor.org/info/rfc1149>.
[RFC2826] Internet Architecture Board, "IAB Technical Comment on the
Unique DNS Root", RFC 2826, DOI 10.17487/RFC2826, May
2000, <http://www.rfc-editor.org/info/rfc2826>.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
May 2000, <http://www.rfc-editor.org/info/rfc2827>.
[RFC2979] Freed, N., "Behavior of and Requirements for Internet
Firewalls", RFC 2979, DOI 10.17487/RFC2979, October 2000,
<http://www.rfc-editor.org/info/rfc2979>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
2004, <http://www.rfc-editor.org/info/rfc3704>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC4084] Klensin, J., "Terminology for Describing Internet
Connectivity", BCP 104, RFC 4084, DOI 10.17487/RFC4084,
May 2005, <http://www.rfc-editor.org/info/rfc4084>.
Barnes, et al. Informational [Page 30]
RFC 7754 Blocking and Filtering Considerations March 2016
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.
[RFC4924] Aboba, B., Ed. and E. Davies, "Reflections on Internet
Transparency", RFC 4924, DOI 10.17487/RFC4924, July 2007,
<http://www.rfc-editor.org/info/rfc4924>.
[RFC4948] Andersson, L., Davies, E., and L. Zhang, "Report from the
IAB workshop on Unwanted Traffic March 9-10, 2006",
RFC 4948, DOI 10.17487/RFC4948, August 2007,
<http://www.rfc-editor.org/info/rfc4948>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
DOI 10.17487/RFC5782, February 2010,
<http://www.rfc-editor.org/info/rfc5782>.
[RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
February 2012, <http://www.rfc-editor.org/info/rfc6480>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <http://www.rfc-editor.org/info/rfc6698>.
[RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for
Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
2013, <http://www.rfc-editor.org/info/rfc6943>.
[RFC7288] Thaler, D., "Reflections on Host Firewalls", RFC 7288,
DOI 10.17487/RFC7288, June 2014,
<http://www.rfc-editor.org/info/rfc7288>.
Barnes, et al. Informational [Page 31]
RFC 7754 Blocking and Filtering Considerations March 2016
[RojaDirecta]
Masnick, M., "Homeland Security Seizes Spanish Domain Name
That Had Already Been Declared Legal", 2011,
<http://www.techdirt.com/articles/20110201/10252412910/
homeland-security-seizes-spanish-domain-name-that-had-
already-been-declared-legal.shtml>.
[SAC-056] ICANN SSAC, "SSAC Advisory on Impacts of Content Blocking
via the Domain Name System", October 2012,
<http://www.icann.org/en/groups/ssac/documents/
sac-056-en.pdf>.
[SafeBrowsing]
Google, "Safe Browsing API", 2012,
<https://developers.google.com/safe-browsing/>.
[Takedown08]
Moore, T. and R. Clayton, "The Impact of Incentives on
Notice and Take-down", Workshop on the Economics of
Information Security 2008,
<http://www.econinfosec.org/archive/weis2008/papers/
MooreImpact.pdf>.
[Telex] Wustrow, E., Wolchok, S., Goldberg, I., and J. Halderman,
"Telex: Anticensorship in the Network Infrastructure",
<https://telex.cc/>.
[Tor] "Tor Project: Anonymity Online",
<https://www.torproject.org/>.
[Wayback] "Internet Archive: Wayback Machine",
<http://archive.org/web/>.
IAB Members at the Time of Approval
Jari Arkko
Mary Barnes
Marc Blanchet
Ralph Droms
Ted Hardie
Joe Hildebrand
Russ Housley
Erik Nordmark
Robert Sparks
Andrew Sullivan
Dave Thaler
Brian Trammell
Suzanne Woolf
Barnes, et al. Informational [Page 32]
RFC 7754 Blocking and Filtering Considerations March 2016
Acknowledgments
Thanks to the many reviewers who provided helpful comments,
especially Bill Herrin, Eliot Lear, Patrik Faltstrom, Pekka Savola,
and Russ White. NLnet Labs is also acknowledged as Olaf Kolkman's
employer during most of this document's development.
Authors' Addresses
Richard Barnes
Mozilla
Suite 300
650 Castro Street
Mountain View, CA 94041
United States
Email: rlb@ipv.sx
Alissa Cooper
Cisco
707 Tasman Drive
Milpitas, CA 95035
United States
Email: alcoop@cisco.com
Olaf Kolkman
Internet Society
Email: kolkman@isoc.org
Dave Thaler
Microsoft
One Microsoft Way
Redmond, WA 98052
United States
Email: dthaler@microsoft.com
Erik Nordmark
Arista
Email: nordmark@arista.com
Barnes, et al. Informational [Page 33]