Technical Considerations for Internet Service Filtering
draft-iab-filtering-considerations-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7754.
|
|
|---|---|---|---|
| Authors | Richard Barnes , Alissa Cooper , Olaf Kolkman | ||
| Last updated | 2012-10-22 | ||
| RFC stream | Internet Architecture Board (IAB) | ||
| Formats | |||
| Additional resources | |||
| Stream | IAB state | (None) | |
| Consensus boilerplate | Unknown | ||
| IAB shepherd | (None) |
draft-iab-filtering-considerations-01
Network Working Group R. Barnes
Internet-Draft BBN Technologies
Intended status: Informational A. Cooper
Expires: April 25, 2013 CDT
O. Kolkman
NLnet Labs
October 22, 2012
Technical Considerations for Internet Service Filtering
draft-iab-filtering-considerations-01.txt
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 allegedly illegal
communications. Recently, there has been an increasing emphasis on
"blocking" or "filtering," the active prevention of abusive or
allegedly illegal communications. This document examines several
technical approaches to Internet content blocking in terms of their
alignment with the overall Internet architecture. In general, the
approach to content 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
illegal communications.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2013.
Copyright Notice
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Copyright (c) 2012 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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Architectural Principles . . . . . . . . . . . . . . . . . . . 4
2.1. End-to-End Connectivity and "Transparency" . . . . . . . . 4
2.2. Layering . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Distribution and Mobility . . . . . . . . . . . . . . . . 5
2.4. Locality and Autonomy . . . . . . . . . . . . . . . . . . 6
3. Examples of Blocking . . . . . . . . . . . . . . . . . . . . . 7
4. Blocking Design Patterns . . . . . . . . . . . . . . . . . . . 8
4.1. Intermediary-Based Blocking . . . . . . . . . . . . . . . 9
4.2. Server-Based Blocking . . . . . . . . . . . . . . . . . . 12
4.3. Endpoint-Based Blocking . . . . . . . . . . . . . . . . . 15
5. Summary of Trade-offs . . . . . . . . . . . . . . . . . . . . 16
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. Informative References . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
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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 arguably undesirable communications. The most
famous early example of undesirable communications was the Morris
worm, which used the Internet to infect many hosts in 1988. As the
Internet has evolved into a rich communications medium, so have
mechanisms to restrict undesirable communications.
Efforts to restrict or deny access to Internet resources have evolved
over time. As noted in [RFC4084], some Internet service providers
impose restrictions on 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. Increasingly, however, both governmental
and private sector entities are seeking to block access to certain
content, traffic, or communications without the knowledge or
agreement of affected users. Where these entities do not directly
control networks, they aim to make use of intermediary systems to
effectuate the blocking.
Entities may seek to block Internet content for a diversity of
reasons, including defending against security threats, restricting
access to content thought to be objectionable, and preventing illegal
activity. While blocking remains highly contentious in many cases,
the desire to restrict access to content will likely continue to
exist.
This document aims to clarify the technical implications and trade-
offs of various blocking strategies and to identify the potential for
different strategies to come into conflict with the Internet's
architecture or cause harmful side effects ("collateral damage").
The strategies broadly fall into three categories:
1. Control by intermediaries (intermediary-based filtering)
2. Manipulation of authoritative data (server-based filtering)
3. Reputation and authentication systems (endpoint-based filtering)
Examples of blocking or attempted blocking using the DNS, HTTP
proxies, domain name seizures, spam filters, and RPKI manipulation
are used to illustrate each category's properties.
Whether particular forms of blocking are lawful in particular
jurisdictions raises complicated legal questions that are outside the
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scope of this document.
2. Architectural Principles
To understand the implications of different blocking strategies, it
is important to understand the key principles that have informed the
design of the Internet. While much of this ground has been well trod
before, this section highlights four architectural principles that
have a direct impact on the viability of content blocking: end-to-end
connectivity and "transparency", layering, distribution and mobility,
and locality and autonomy.
2.1. End-to-End Connectivity and "Transparency"
The end-to-end principle is "the core architectural guideline of the
Internet" [RFC3724]. Adherence to the principle of vesting endpoints
with the functionality to accomplish end-to-end tasks results in a
"transparent" network in which packets are not filtered or
transformed en route [RFC2775]. This transparency in turn is a key
requirement for providing end-to-end security features on the
network. Modern security mechanisms that rely on trusted hosts
communicating via a secure channel without intermediary interference
enable the network to support e-commerce, confidential communication,
and other similar uses.
The end-to-end principle is fundamental for Internet security, and
the foundation on which Internet security protocols are built.
Protocols such as TLS and IPsec [RFC5246][RFC4301] are designed to
ensure that each endpoint of the communication knows the identity of
the other endpoint, 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 that the
user's banking information is communicated to the bank and nobody
else.
Some blocking strategies require intermediaries to insert themselves
within the end-to-end communications path, potentially breaking
security properties of Internet protocols. In these cases it can be
difficult or impossible for endpoints to distinguish between
attackers and the entities conducting blocking.
A similar notion to the end-to-end principle is the notion of
"transparency," that is, the idea that the network should provide a
generic connectivity service between endpoints, with minimal
interaction by intermediaries aside from routing packets from source
to destination. In "Reflections on Internet Transparency" [RFC4924],
the IAB assessed the relevance of this principle and concluded that
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"far from having lessened in relevance, technical implications of
intentionally or inadvertently impeding network transparency play a
critical role in the Internet's ability to support innovation and
global communication."
2.2. Layering
Internet applications are built out of a collection of loosely-
coupled components or "layers." Different layers serve different
purposes, 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
entities. For example, in many networks, physical and link-layer
connectivity is provided by an "access provider", while IP routing is
performed by an "Internet service provider" -- and application-layer
services are provided by a completely separate entity (e.g., a web
server). Upper-layer protocols and applications rely on combinations
of lower-layer functions in order to work. As a consequence of the
end-to-end principle, 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 higher layers. For example,
manipulating routing or naming functions to restrict access to a
narrow set of resources via specific applications will likely affect
all applications that depend on those functions.
In a similar manner, the physical distance between a host and a
service provider at a particular layer grows as one moves up the
stack. A host must be physically connected to a link-layer access
provider network, and its distance from its ISP is limited by the
length of the link that connects it, but Internet applications can be
delivered to the host from anywhere in the world.
Thus, as one considers changes at each layer of the stack, changes at
higher layers become more specific in terms of application, but more
broad in terms of the size and geography of hosts impacted. Changes
to an access network will only affect a relatively small, well-
defined set of users (namely, those connected to the access network),
but can affect all applications for those users. Changes to an
application service can affect users across the entire Internet, but
only for that specific application.
2.3. Distribution and Mobility
The Internet is designed as a distributed system both geographically
and topologically. Resources can be made globally accessible
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regardless of their physical location or connectivity providers used.
Resources are also highly mobile -- moving content from one physical
or logical address to another can often be easily accomplished.
This distribution and mobility underlies a large part of the
resiliency of the Internet. Internet routing can survive major
outages such as cuts in undersea fibers because the distributed
routing system of the Internet allows individual networks to
collaborate to route traffic. Application services are commonly
protected using distributed servers. For example, even though the
2010 earthquake in Haiti destroyed almost all of the Internet
infrastructure in the country, the Haitian top-level domain name
(.ht) had no interruption in service because it was also accessible
via servers in the United States, Canada, and France.
Undesirable communications also benefit from this resiliency --
resources that are blocked or restricted in one part of the Internet
can be reconstituted in another part of the Internet. If a web site
is prevented from using a domain name or set of IP addresses, the web
site can simply move to another domain name or network.
2.4. Locality and Autonomy
The basic unit of Internet routing is an "Autonomous System" -- a
network that manages its own routing internally. The concept of
autonomy is present in many aspects of the Internet, as is the
related concept of locality, the idea that local changes should not
have a broader impact on the network.
These concepts are critical to the stability and scalability of the
Internet. With millions of individual actors engineering different
parts of the network, there would be chaos if every change had impact
across the entire Internet.
Locality implies that the impact of technical changes made to realize
blocking will only be within a defined scope. As discussed above,
this scope might be narrow in one dimension (set of users or set of
applications affected) but broad in another. Changes made to
effectuate blocking are often targeted at a particular locality, but
result in blocking outside of the intended scope. For example, web
filtering systems in India and China have been shown to cause
"collateral damage" by blocking users in Oman and the US from
accessing web sites in Germany and Korea
[IN-OM-filtering][CCS-GFC-collateral-damage].
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3. Examples of Blocking
As noted above, systems to restrict or block Internet communications
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, with varying degrees of
effectiveness.
o Firewalls: Firewalls are a very common form of service blocking,
employed at many points in today's Internet. Typically, firewalls
block according to content-neutral rules, e.g., blocking all
inbound connections or outbound connections on certain ports.
Firewalls can be deployed either on end hosts (under user
control), or at network boundaries.
o Web Filtering: HTTP and HTTPS are common targets for blocking and
filtering, typically targeted at specific URLs. 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 illegal content. HTTPS is a challenge for these
systems, because the URL in an HTTPS request is carried inside the
secure channel. To block access to content made accessible via
HTTPS, filtering systems thus must either block based only on IP
address, 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." These portals present their own HTTPS certificates that
are invalid for any given domain according to normal validation
rules, but may still be trusted if the user install a security
exception.
o Spam Filtering: Spam filtering is one of the oldest forms of
service blocking, in the sense that it denies spammers access to
recipients' mailboxes. Spam filters evaulate messages based on a
variety of criteria and information sources to decide whether a
given message is spam. For example, DNS Reverse Black Lists use
the reverse DNS to flag whether an IP address is a known spam
source [RFC5782]. Spam filters are typically either installed on
user devices (e.g., in a mail client) or operated by a mail domain
on behalf of users.
o Domain name seizure: In recent years, US law enforcement
authorities have been issuing legal orders to domain name
registries to seize domain names associated with the distribution
of counterfeit goods and other allegedly illegal activity
[US-ICE]. When domain names are seized, DNS queries for the
seized names are typically redirected to resolve to U.S.
government IP addresses that host information about the seizure,
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either by an authoritative server or by an intermediate resolver.
The effectiveness of domain seizures is limited by the mobility
principle, since the application using the seized name can simply
use another name. Seizures can come into conflict with the
locality principle, since 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 the
DNS security architecture [RFC4033].
o Safe Browsing: Modern web browsers provide some measures to
prevent users from accessing malicious web sites. For instance,
before loading a URL, current versions of Google Chrome and
Firefox web browsers use the Google Safe Browsing service to
determine whether or not a given URL is safe to load
[SafeBrowsing]. The DNS can also be used to mark domains as safe
or unsafe [RFC5782].
o 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]. These actions have led to concerns that the
resource certification system and related secure routing
technologies developed by the IETF SIDR working group might be
subject to government manipulation as well [RFC6480], potentially
for the purpose of denying targeted networks access to the
Internet.
4. Blocking Design Patterns
Considering a typical end-to-end Internet communcation, there are
three logical points at which blocking mechanisms can be put in
place: the middle and either end. Mechanisms based in the middle
usually involve an intermediary device in the network that observes
Internet traffic and decides which communications to block. At the
service end of a communication, authoritative databases (such as the
DNS) and servers can be manipulated to deny or alter service
delivery. At the user end of a communication, authentication and
reputation systems enable user devices (and users) to make decisions
about which communications should be blocked.
In this section, we discuss these three "blocking design patterns"
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and how they align with the Internet architectural principles
outlined above. In general, the third pattern -- informing user
devices of which services should be blocked -- is the most consistent
with the Internet architecture.
4.1. Intermediary-Based Blocking
A common goal for blocking systems is for the system to be able to
block communications without the consent or cooperation of either
endpoint to the communication. Such systems are thus implemented
using intermediary devices in the network, such as firewalls or
filtering systems. These systems inspect user traffic as it passes
through the network, decide based on the content of a given
communication whether it should be blocked, and then block or allow
the communication as desired.
Common examples of intermediary-based filtering are firewalls and
network-based web-filtering systems. For example, web filtering
devices usually inspect HTTP requests to determine the URL being
requested, compare that URL to a list of black-listed or white-listed
URLs, and allow the request to proceed only if it is permitted by
policy (or at least not forbidden). Firewalls perform a similar
function for other classes of traffic in addition to HTTP.
It should be noted that these "intermediaries" 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 users. 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 authorize mail servers on the local network to
filter spam on their behalf. These cases are effectively equivalent
to the "Endpoint-Based Blocking" scenarios discussed below, since the
endpoint has authorized the intermediary to block on its behalf. The
challenges discussed in this section arise mostly for scenarios where
endpoints are not assumed to cooperate with filtering (i.e., they
might have incentives to circumvent filtering).
Accomplishing blocking by using intermediaries conflicts with the
end-to-end and transparency principles noted above. The very goal of
blocking in this way is to impede transparency for particular content
or communications. For this reason, intermediary-based approaches to
blocking run into several technical issues that limit their viability
in practice. In particular, many issues arise from the fact that an
intermediary needs to have access to a sufficient amount of traffic
to make its blocking determinations.
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The first challenge to obtaining this traffic is simply gaining
access to the constituent packets. The Internet is designed to
deliver packets from source to destination -- not to any particular
point along the way. In practice, inter-network routing is often
asymmetric, and for sufficiently complex local networks, intra-
network traffic flows can be asymmetric as well.
This asymmetry means that an intermediary will often see only one
half of a given communication (if it sees any of it at all), limiting
its ability to make decisions based on the content of the
communication. For example, a URL-based filter cannot make blocking
decisions if it only has access to HTTP responses (not requests).
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 often reduce network performance relative to
asymmetric routing.
Once an intermediary 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 (URLs). The
communicating endpoints can deny the intermediary access to these by
using encryption (see below), but IP addresses must be visible, even
if packets are protected with IPsec. However, blocking based on IP
addresses is the simplest form of filtering to circumvent, because a
filtered site need only change a single DNS record to move all of its
services to a new IP address. Indeed, in the face of IP-based
blocking in some networks, services such as The Pirate Bay are now
using cloud hosting services so that their IP addresses are difficult
for intermediaries to predict [BT-TPB][TPB-cloud].
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. Since security
protocols are 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.
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 and port numbers. 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.
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However, as the Telex project recently demonstrated, if an endpoint
cooperates with a server, it can create a TLS tunnel that is
indistinguishable from legitimate traffic [Telex]. For example, if a
banking website operated a Telex server, then a blocking system would
be unable to distinguish legitimate encrypted banking traffic from
Telex-tunneled traffic to that server (potentially carrying content
that the blocking system would have blocked).
Thus, in principle it is impossible to block tunneled traffic through
an intermediary device without blocking all secure traffic. (The
only limitation in practice is the requirement for special software
on the client.) In most cases, blocking all secure traffic is an
unacceptable consequence of blocking, since security is often
required for services such as online commerce, enterprise VPNs, and
management of critical infrastructure. If governments or network
operators were to force these services to use insecure protocols so
as to effectuate blocking, they would expose their users to the
various attacks that the security protocols were put in place to
prevent.
Some network 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 intermediary-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 tunnelling 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,
intermediary-based blocking becomes less effective over time.
Intermediary-based blocking is a key contributor to the arms race
that has led to the development of these kinds of tools, the result
of which is to create unecessary layers of complexity in the
Internet. Before content-based blocking became common, the next best
option for intermediaries was port blocking, the widespread use of
which has driven more applications and services to use ports (80 most
commonly) that are unlikely to be blocked. In turn, intermediaries
shifted to finer-grained content blocking over port 80, content
providers shifted to encrypted channels, and intermediaries began
seeking to identify those channels. Because the premise of
intermediary-based blocking is that endpoints have incentives to
circumvent it, this cat-and-mouse game is an invetiable by-product of
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this form of blocking.
In sum, blocking via intermediaries is only effective in a fairly
constrained set of circumstances. First, the routing structure of
the network needs to be such that the intermediary 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), which may be resource-
prohibitive, especially if tunnel endpoints begin to change
frequently. If the network is sufficiently complex, or the risk of
tunneling too high, then intermediary-based blocking is unlikely to
be effective, and in any case this type of blocking drives the
development of increasingly complex layers of circumvention.
4.2. Server-Based Blocking
[TO DO: Likely to distinguish server-based filtering from
infrastructure-based (DNS, RPKI) filtering in a future version.]
Internet services are driven by physical devices such as web servers,
DNS servers, certificate authorities, WHOIS databases, and Internet
Route Registries. These devices control the structure and
availability of Internet applications by providing data elements that
are used by application code. For example, changing an A or AAAA
record on a DNS server will change the IP address that is bound to a
given domain name; applications trying to communicate with the host
at that name will then communicate with the host at the new address.
As physical objects, the servers that underlie Internet applications
exist within the jurisdiction of governments, and their operators are
thus subject to certain local laws. It is thus possible for laws to
be structured to facilitate blocking of Internet services operated
within a jurisdiction, either via direct government action or by
allowing private actors to demand blocking (e.g., through lawsuits).
The "seizure" of domain names discussed above is an example of this
type of blocking. Even though some of the affected domain names
belonged to non-US entities (e.g., RojaDirecta is Spanish), they were
subject to blocking by the US government because certain servers were
operated in the US. 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 government-operated name
servers. Users of services 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. The action of the Dutch police
against the RIPE NCC is of a similar character, limiting the ability
of certain ISPs to manage their Internet services by controlling
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their WHOIS information.
Blocking services by disabling or manipulating servers does respect
the end-to-end principle, since the affected server is one end of the
blocked communication. However, its interactions with layering,
resource mobility, and autonomy can limit its effectiveness and cause
undesirable consequences.
The layered architecture of the Internet means that there are several
points at which access to a service can be blocked. The service can
be denied Internet access (via control of routers), DNS services (DNS
servers), or application-layer services (application servers, e.g.,
web servers). Blocking via these channels, however, can be both
amplified and limited by the global nature of the Internet.
On the one hand, the global nature of Internet resources amplifies
blocking actions, in the sense that it increases the risk of
overblocking -- collateral damage to legitimate use of a resource. A
given network or domain name might host both legitimate services and
services that governments desire to block. A service hosted under a
domain name and operated in a jurisdiction where it is considered
undesirable might be considered legitimate in another jurisdiction; a
blocking action in the host jurisdiction would deny legitimate
services in the other.
On the other hand, the distributed and mobile nature of Internet
resources limits the effectiveness of blocking actions. Because an
Internet service can be reached from anywhere on the Internet, a
service that is blocked in one jurisdiction can often be moved or re-
instantiated in another jurisdiction. Likewise, services that rely
on blocked resources can often be rapidly re-configured to use non-
blocked resources. For example, 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
jursidictions to prevent spam blocking from being effective.
The efficacy of server-based blocking is further limited by the
autonomy principle discussed above. 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 to avoid
the blocking. For example, 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 due to a route leak, but service was
restored in approximately two hours because network operators around
the world re-configured their routers to ignore the blocking routes
[RenesysPK]. In the context of SIDR and secure routing, a similar
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re-configuration could be done if a resource certificate were to be
revoked in order to block routing to a given network.
In the DNS context, similar work-arounds are available. If a domain
name were 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 un-blocked results for the affected name. Indeed
these techniques are commonly used in practice to provide service to
domains that have been disrupted, such as the .ht domain during the
2010 earthquake in Haiti [EarthquakeHT]. While the point of these
measures was to counter the effects of a natural disaster rather than
to counter filtering, the same technical means can also counter the
effects of filtering based on modifications to the authoritative
server for a domain.
Resources such as the DNS, the RPKI, and the Internet Route
Registries are 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; the RPKI about
which entities have been allocated IP addresses. To offer
specialized Internet services and applications, different entities
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).
Server-based blocking also has a variety of other implications that
may reduce the stability, accessibility, and usability of the global
Internet. Server-side blocking may encourage the development of
parallel or "underground" server-side infrastructure, for example.
These considerations are further discussed in ISOC's whitepaper on
DNS filtering [ISOCFiltering], but they also apply to other global
Internet resources.
In summary, server-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 often have harmful
side effects due to the global nature of Internet resources. The
global mobility of Internet resources, together with the autonomy of
the networks that comprise the Internet, can mean that the effects of
server-based blocking can be quickly be negated. To adapt a quote by
John Gilmore, "The Internet treats blocking as damage and routes
around it".
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4.3. Endpoint-Based Blocking
Internet users and their devices make thousands of decisions every
day 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
blocking systems in the Internet; modern blocking 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 blocking systems (browsers, MTAs) share a single reputation
service.
This approach to blocking is consistent with the Internet
architectural principles discussed above, dealing well with the end-
to-end principle, layering, mobility, and locality/autonomy.
Much like server-based blocking, 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 intermediary-based
blocking runs into. Endpoint-based blocking also lacks some of the
limitations of server-based blocking: While server-based blocking can
only see and affect the portion of an application that happens at a
given server (e.g., DNS name resolution), endpoint-based blocking has
visibility into the entire application, across all layers and
transactions. This visibility provides endpoint-based blocking
systems with a much richer set of information on which to make
blocking decisions.
In particular, endpoint-based blocking deals well with adversary
mobility. If a blocked service relocates resources or uses different
resources, a server-based blocking approach may not be able to affect
the new resources. An intermediary-based blocking system may not
even be able to tell whether the new resources are being used, if the
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
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through a reputation system.
Finally, in an endpoint-based blocking system, blocking actions are
performed autonomously, by individual endpoints or their delegates.
The effects of blocking are thus local in scope, minimizing the
effects on other users or other, legitimate services.
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.
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.
5. Summary of Trade-offs
Intermediary-based blocking is a relatively low-cost blocking
solution in some cases, but a poor fit with the Internet
architecture, especially the end-to-end principle. It thus suffers
from several limitations.
o Examples: Firewalls, web filtering systems.
o A single intermediary device can be used to block access by many
users to many services.
o Intermediary blocking can be done without the cooperation of
either endpoint to a communication (although having that
cooperation makes it more likely to be effective).
o Intermediaries often lack sufficient information to make blocking
decisions, due to routing asymmetry or encryption.
o Intermediary blocking sometimes involves breaking end-to-end
security assurances.
o Tunneling through blocking is difficult to prevent without
preventing legitimate secure services.
Server-based blocking can provide rapid effects for resources under
the control of the blocking entity, but its ultimate effectiveness is
limited by the global, autonomous nature of Internet resources and
networks, and it may create undesirable collateral damage to Internet
services.
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o Examples: Domain name seizures, WHOIS account freezing, RPKI
certificate revocation.
o Internet services that depend on specific resources can be blocked
by disabling those resources.
o Blocked resources can often be easily relocated or reinstantiated
in a location where they will not be blocked.
o Resources used by undesirable services are often also used by
legitimate services, resulting in collateral damage.
o Autonomy of Internet networks and users allows them to "route
around" blocking.
Endpoint-based blocking matches well with the overall design of the
Internet.
o Examples: Safe browsing, spam filtering, enterprise HTTPS proxies
(explicitly trusted by clients).
o Endpoints block services by deciding whether or not to engage in a
given communication.
o Blocking system has full visibility into all layers involved in a
communication.
o Adversary mobility can be quickly observed so that blocking
systems can be updated to account for it.
o Requires cooperation of endpoints.
Because it agrees so well with Internet architectural principles,
endpoint-based blocking is the form of Internet service blocking that
is least harmful to the Internet. It is likely to be the most
effective long-term technical filtering mechanism in many cases.
While this document has focused on technical mechanisms used to
filter Internet content, a variety of non-technical mechanisms may
also be available depending on the particular context and goals of
the public or private entity seeking to restrict access to content.
For example, purveyors of illegal online content can be pursued
through international cooperation, by using the criminal justice
system, and by targeting the funding that supports their activities
through collaboration with financial services companies
[click-trajectories]. Thus even in cases where endpoint-based
filtering is not viewed as a viable means of restricting access to
content, entities seeking to filter may find other strategies for
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achieving their goals that do not involve interfering with the
architecture or operation of the Internet.
In reality, the various approaches discussed above are all applied
for different reasons. Often, the choice of a filtering solution is
constrained by practical limitations on which parts of the network
are under the control of the entity implementing filtering, and which
parts of the network are trusted to cooperate. For example, an ISP
that is subject to filtering requirements might implement an
intermediary-based filtering approach because it cannot be sure that
endpoints will cooperate in filtering. As discussed above,
government agencies tasked with disabling certain foreign web sites
have done so by manipulating servers that are within their own
jurisdictions, since those are the servers they can access. An
enterprise with filtering requirements might require install a
certain filtering software package on enterprise-owned PCs.
It is therefore realistic to expect that certain entities will
continue to attempt to conduct intermediary- or server-based
filtering since they may not have control over the endpoints they
wish to affect or because the endpoints do not have incentives to
consent to the filtering. In some cases, an approach that combines
one of these with endpoint-based filtering can help strike a better
balance. For example, a filtering system might make it possible for
some endpoints to cooperate or "opt in" to filtering, rather than
deploying a purely intermediary-based solution.
Those with a desire to filter should take into account the
limitations discussed in this document and wholistically assess the
space of technical and non-technical solutions at their disposal and
the likely effectiveness of each combination of approaches.
6. 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
typically 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 website, so that it can act as a man-in-the-middle on TLS
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connections passing through the firewall.
Modifications such as these obviously make the firewall itself a
point of weakness. If an attacker can gain control of the firewall
or compromise the key pair used by the firewall to sign certificates,
he will have access to the plaintext of all TLS sessions for all
users behind that firewall, in a way that is undetectable to users.
When blocking systems are unable to inspect and block secure
protocols, it is tempting to simply block those protocols. For
example, a web blocking system that is unable to hijack 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 protocols.
Security protocols can, of course, also be used a mechanism 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.
7. Informative References
[BT-TPB] Meyer, D., "BT blocks The Pirate Bay", June 2012, <http://
www.zdnet.com/bt-blocks-the-pirate-bay-4010026434/>.
[CCS-GFC-collateral-damage]
"The Collateral Damage of Internet Censorship by DNS
Injection", July 2012, <http://conferences.sigcomm.org/
sigcomm/2012/paper/ccr-paper266.pdf>.
[EarthquakeHT]
Raj Upadhaya, G., ".ht: Recovering DNS from the Quake",
March 2010, <http://www.apricot.net/apricot2010/__data/
assets/pdf_file/0019/19018/
Lightning-Talk_03_Gaurab-Upadhaya-dotht-apricot-
lightning.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/
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ripe-ncc-blocks-registration-in-ripe-registry-following-
order-from-dutch-police>.
[IN-OM-filtering]
Citizen Lab, "Routing Gone Wild", 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>.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2775] Carpenter, B., "Internet Transparency", RFC 2775,
February 2000.
[RFC3724] Kempf, J., Austein, R., and IAB, "The Rise of the Middle
and the Future of End-to-End: Reflections on the Evolution
of the Internet Architecture", RFC 3724, March 2004.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4084] Klensin, J., "Terminology for Describing Internet
Connectivity", BCP 104, RFC 4084, May 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4924] Aboba, B. and E. Davies, "Reflections on Internet
Transparency", RFC 4924, July 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782,
February 2010.
[RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", RFC 6480, February 2012.
[RenesysPK]
Brown, M., "Pakistan hijacks YouTube", February 2008, <htt
p://www.renesys.com/blog/2008/02/
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pakistan_hijacks_youtube_1.shtml>.
[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>.
[SafeBrowsing]
Google, "Safe Browsing API", 2012,
<https://developers.google.com/safe-browsing/>.
[TPB-cloud]
"The Pirate Cloud", October 2012,
<http://thepiratebay.se/blog/224>.
[Telex] Wustrow, E., Wolchok, S., Goldberg, I., and J. Halderman,
"Telex: Anticensorship in the Network Infrastructure",
August 2011, <https://telex.cc/>.
[Tor] "Tor Project: Anonymity Online", 2012,
<https://www.torproject.org/>.
[US-ICE] U.S. Immigration and Customs Enforcement, "Operation in
Our Sites", 2011, <http://www.ice.gov/doclib/news/library/
factsheets/pdf/operation-in-our-sites.pdf>.
[click-trajectories]
Levchenko, K., Pitsillidis, A., Chacra, N., Enright, B.,
Felegyhazi, M., Grier, C., Halvorson, T., Kreibich, C.,
Liu, H., McCoy, D., Weaver, N., Paxson, V., Voelker, G.,
and S. Savage, "Click Trajectories: End-to-End Analysis of
the Spam Value Chain", 2011,
<http://cseweb.ucsd.edu/~savage/papers/Oakland11.pdf>.
Authors' Addresses
Richard Barnes
BBN Technologies
1300 N. 17th St
Arlington, VA 22209
USA
Phone: +1 703 284 1340
Email: rbarnes@bbn.com
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Alissa Cooper
CDT
1634 Eye St. NW, Suite 1100
Washington, DC 20006
USA
Email: acooper@cdt.org
Olaf Kolkman
NLnet Labs
Email: olaf@nlnetlabs.nl
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