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<pre>Internet Engineering Task Force (IETF) S. Jiang
Request for Comments: 7819 Huawei Technologies Co., Ltd
Category: Informational S. Krishnan
ISSN: 2070-1721 Ericsson
T. Mrugalski
ISC
April 2016
<span class="h1">Privacy Considerations for DHCP</span>
Abstract
DHCP is a protocol that is used to provide addressing and
configuration information to IPv4 hosts. This document discusses the
various identifiers used by DHCP and the potential privacy issues.
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 Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="http://www.rfc-editor.org/info/rfc7819">http://www.rfc-editor.org/info/rfc7819</a>.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
<span class="grey">Jiang, et al. Informational [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc7819">RFC 7819</a> DHCP Privacy Considerations April 2016</span>
Table of Contents
<a href="#section-1">1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-2">2</a>. Requirements Language and Terminology . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-3">3</a>. DHCP Options Carrying Identifiers . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-3.1">3.1</a>. Client Identifier Option . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-3.2">3.2</a>. Address Fields and Options . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-3.3">3.3</a>. Client FQDN Option . . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-3.4">3.4</a>. Parameter Request List Option . . . . . . . . . . . . . . <a href="#page-5">5</a>
3.5. Vendor Class and Vendor-Identifying Vendor Class Options 5
<a href="#section-3.6">3.6</a>. Civic Location Option . . . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
<a href="#section-3.7">3.7</a>. Coordinate-Based Location Option . . . . . . . . . . . . <a href="#page-6">6</a>
<a href="#section-3.8">3.8</a>. Client System Architecture Type Option . . . . . . . . . <a href="#page-6">6</a>
<a href="#section-3.9">3.9</a>. Relay Agent Information Option and Suboptions . . . . . . <a href="#page-6">6</a>
<a href="#section-4">4</a>. Existing Mechanisms That Affect Privacy . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-4.1">4.1</a>. DNS Updates . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-4.2">4.2</a>. Allocation Strategies . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-5">5</a>. Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-5.1">5.1</a>. Device Type Discovery . . . . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-5.2">5.2</a>. Operating System Discovery . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-5.3">5.3</a>. Finding Location Information . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-5.4">5.4</a>. Finding Previously Visited Networks . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-5.5">5.5</a>. Finding a Stable Identity . . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-5.6">5.6</a>. Pervasive Monitoring . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-5.7">5.7</a>. Finding Client's IP Address or Hostname . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-5.8">5.8</a>. Correlation of Activities over Time . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-5.9">5.9</a>. Location Tracking . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-5.10">5.10</a>. Leasequery and Bulk Leasequery . . . . . . . . . . . . . <a href="#page-11">11</a>
<a href="#section-6">6</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
<a href="#section-7">7</a>. Privacy Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
<a href="#section-8">8</a>. References . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
<a href="#section-8.1">8.1</a>. Normative References . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
<a href="#section-8.2">8.2</a>. Informative References . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<span class="grey">Jiang, et al. Informational [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc7819">RFC 7819</a> DHCP Privacy Considerations April 2016</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
The Dynamic Host Configuration Protocol (DHCP) [<a href="./rfc2131" title=""Dynamic Host Configuration Protocol"">RFC2131</a>] is used to
provide addressing and configuration information to IPv4 hosts. DHCP
uses several identifiers that could become a source for gleaning
information about the IPv4 host. This information may include device
type, operating system information, location(s) that the device may
have previously visited, etc. This document discusses the various
identifiers used by DHCP and the potential privacy issues [<a href="./rfc6973" title=""Privacy Considerations for Internet Protocols"">RFC6973</a>].
In particular, it takes into consideration the problem of pervasive
monitoring [<a href="./rfc7258" title=""Pervasive Monitoring Is an Attack"">RFC7258</a>].
Future works may propose protocol changes to fix the privacy issues
that have been analyzed in this document. Those changes are out of
scope for this document.
The primary focus of this document is around privacy considerations
for clients to support client mobility and connection to random
networks. The privacy of DHCP servers and relay agents is considered
less important as they are typically open for public services. And,
it is generally assumed that communication from relay agent to server
is protected from casual snooping, as that communication occurs in
the provider's backbone. Nevertheless, the topics involving relay
agents and servers are explored to some degree. However, future work
may want to further explore the privacy of DHCP servers and relay
agents.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Requirements Language and Terminology</span>
Naming conventions from [<a href="./rfc2131" title=""Dynamic Host Configuration Protocol"">RFC2131</a>] and related documents are used
throughout this document.
In addition, the following terminology is used:
Stable identifier - Any property disclosed by a DHCP client that
does not change over time or changes very infrequently and is
unique for said client in a given context. Examples include
MAC address, client-id, and a hostname. Some identifiers may
be considered stable only under certain conditions; for
example, one client implementation may keep its client-id
stored in stable storage, while another may generate it on
the fly and use a different one after each boot. Stable
identifiers may or may not be globally unique.
<span class="grey">Jiang, et al. Informational [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc7819">RFC 7819</a> DHCP Privacy Considerations April 2016</span>
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. DHCP Options Carrying Identifiers</span>
In DHCP, there are a few options that contain identification
information or that can be used to extract identification information
about the client. This section enumerates various options and the
identifiers that they convey and that can be used to disclose client
identification. They are targets of various attacks that are
analyzed in <a href="#section-5">Section 5</a>.
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. Client Identifier Option</span>
The Client Identifier option [<a href="./rfc2131" title=""Dynamic Host Configuration Protocol"">RFC2131</a>] is used to pass an explicit
client identifier to a DHCP server.
The client identifier is an opaque key that must be unique to that
client within the subnet to which the client is attached. It
typically remains stable after it has been initially generated. It
may contain a hardware address, identical to the contents of the
'chaddr' field, or another type of identifier, such as a DNS name.
<a href="./rfc3315#section-9.2">Section 9.2 of [RFC3315]</a> specifies DUID-LLT (Link-layer plus time) as
the recommended DUID (DHCP Unique Identifier) type in DHCPv6.
<a href="./rfc4361#section-6.1">Section 6.1 of [RFC4361]</a> introduces this concept to DHCP. Those two
documents recommend that client identifiers be generated by using the
permanent link-layer address of the network interface that the client
is trying to configure. [<a href="./rfc4361" title=""Node-specific Client Identifiers for Dynamic Host Configuration Protocol Version Four (DHCPv4)"">RFC4361</a>] updates the recommendation for a
Client Identifier as follows: "[it] consists of a type field whose
value is normally 255, followed by a four-byte IA_ID field, followed
by the DUID for the client as defined in <a href="./rfc3315#section-9">RFC 3315, section 9</a>". This
does not change the lifecycle of client identifiers. Clients are
expected to generate their client identifiers once (during first
operation) and store them in non-volatile storage or use the same
deterministic algorithm to generate the same client identifier values
again.
This means that typically an implementation will use the available
link-layer address during its first boot. Even if the administrator
enables link-layer address randomization, it is likely that it was
not yet enabled during the first device boot. Hence the original,
unobfuscated link-layer address will likely end up being announced as
the client identifier, even if the link-layer address has changed (or
even if it is being changed on a periodic basis). The exposure of
the original link-layer address in the client identifier will also
undermine other privacy extensions such as [<a href="./rfc4941" title=""Privacy Extensions for Stateless Address Autoconfiguration in IPv6"">RFC4941</a>].
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. Address Fields and Options</span>
The 'yiaddr' field [<a href="./rfc2131" title=""Dynamic Host Configuration Protocol"">RFC2131</a>] in a DHCP message is used to convey an
allocated address from the server to the client.
<span class="grey">Jiang, et al. Informational [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc7819">RFC 7819</a> DHCP Privacy Considerations April 2016</span>
The DHCP specification [<a href="./rfc2131" title=""Dynamic Host Configuration Protocol"">RFC2131</a>] provides a way to specify the client
link-layer address in the DHCP message header. A DHCP message header
has 'htype' and 'chaddr' fields to specify the client link-layer
address type and the link-layer address, respectively. The 'chaddr'
field is used both as a hardware address for transmission of reply
messages and as a client identifier.
The 'requested IP address' option [<a href="./rfc2131" title=""Dynamic Host Configuration Protocol"">RFC2131</a>] is used by a client to
suggest that a particular IP address be assigned.
<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>. Client FQDN Option</span>
The Client Fully Qualified Domain Name (FQDN) option [<a href="./rfc4702" title=""The Dynamic Host Configuration Protocol (DHCP) Client Fully Qualified Domain Name (FQDN) Option"">RFC4702</a>] is
used by DHCP clients and servers to exchange information about the
client's FQDN and about who has the responsibility for updating the
DNS with the associated A and PTR RRs.
A client can use this option to convey all or part of its domain name
to a DHCP server for the IP-address-to-FQDN mapping. In most cases,
a client sends its hostname as a hint for the server. The DHCP
server may be configured to modify the supplied name or to substitute
a different name. The server should send its notion of the complete
FQDN for the client in the Domain Name field.
<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>. Parameter Request List Option</span>
The Parameter Request List option [<a href="./rfc2131" title=""Dynamic Host Configuration Protocol"">RFC2131</a>] is used to inform the
server about options the client wants the server to send to the
client. The contents of a Parameter Request List option are the
option codes of the options requested by the client.
<span class="h3"><a class="selflink" id="section-3.5" href="#section-3.5">3.5</a>. Vendor Class and Vendor-Identifying Vendor Class Options</span>
The Vendor Class option [<a href="./rfc2131" title=""Dynamic Host Configuration Protocol"">RFC2131</a>], the Vendor-Identifying Vendor
Class option, and the Vendor-Identifying Vendor Information option
[<a href="./rfc3925" title=""Vendor-Identifying Vendor Options for Dynamic Host Configuration Protocol version 4 (DHCPv4)"">RFC3925</a>] are used by the DHCP client to identify the vendor that
manufactured the hardware on which the client is running.
The information contained in the data area of this option is
contained in one or more opaque fields that identify the details of
the hardware configuration of the host on which the client is running
or of industry consortium compliance -- for example, the version of
the operating system the client is running or the amount of memory
installed on the client.
<span class="grey">Jiang, et al. Informational [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc7819">RFC 7819</a> DHCP Privacy Considerations April 2016</span>
<span class="h3"><a class="selflink" id="section-3.6" href="#section-3.6">3.6</a>. Civic Location Option</span>
DHCP servers use the Civic Location Option [<a href="./rfc4776" title=""Dynamic Host Configuration Protocol (DHCPv4 and DHCPv6) Option for Civic Addresses Configuration Information"">RFC4776</a>] to deliver
location information (the civic and postal addresses) to DHCP
clients. It may refer to three locations: the location of the DHCP
server, the location of the network element believed to be closest to
the client, or the location of the client, identified by the "what"
element within the option.
<span class="h3"><a class="selflink" id="section-3.7" href="#section-3.7">3.7</a>. Coordinate-Based Location Option</span>
The GeoConf and GeoLoc options [<a href="./rfc6225" title=""Dynamic Host Configuration Protocol Options for Coordinate-Based Location Configuration Information"">RFC6225</a>] are used by a DHCP server to
provide coordinate-based geographic location information to DHCP
clients. They enable a DHCP client to obtain its geographic
location.
<span class="h3"><a class="selflink" id="section-3.8" href="#section-3.8">3.8</a>. Client System Architecture Type Option</span>
The Client System Architecture Type Option [<a href="./rfc4578" title=""Dynamic Host Configuration Protocol (DHCP) Options for the Intel Preboot eXecution Environment (PXE)"">RFC4578</a>] is used by a
DHCP client to send a list of supported architecture types to the
DHCP server. It is used by clients that must be booted using the
network rather than from local storage, so the server can decide
which boot file should be provided to the client.
<span class="h3"><a class="selflink" id="section-3.9" href="#section-3.9">3.9</a>. Relay Agent Information Option and Suboptions</span>
A DHCP relay agent includes a Relay Agent Information option[RFC3046]
to identify the remote host end of the circuit. It contains a
"circuit ID" suboption for the incoming circuit, which is an agent-
local identifier of the circuit from which a DHCP client-to-server
packet was received, and a "remote ID" suboption that provides a
trusted identifier for the remote high-speed modem.
Possible encoding of the "circuit ID" suboption includes: router
interface number, switching hub port number, remote access server
port number, frame relay Data Link Connection Identifier (DLCI), ATM
virtual circuit number, cable data virtual circuit number, etc.
Possible encoding of the "remote ID" suboption includes: a "caller
ID" telephone number for dial-up connection, a "user name" prompted
for by a remote access server, a remote caller's ATM address, a
"modem ID" of a cable data modem, the remote IP address of a point-
to-point link, a remote X.25 address for X.25 connections, etc.
The link-selection suboption [<a href="./rfc3527" title=""Link Selection sub-option for the Relay Agent Information Option for DHCPv4"">RFC3527</a>] is used by any DHCP relay
agent that desires to specify a subnet/link for a DHCP client request
that it is relaying but needs the subnet/link specification to be
different from the IP address the DHCP server should use when
<span class="grey">Jiang, et al. Informational [Page 6]</span></pre>
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<span class="grey"><a href="./rfc7819">RFC 7819</a> DHCP Privacy Considerations April 2016</span>
communicating with the relay agent. It contains an IP address that
can identify the client's subnet/link. Also, assuming there is
knowledge of the network topology, it also reveals client location.
A DHCP relay includes a Subscriber-ID option [<a href="./rfc3993" title=""Subscriber-ID Suboption for the Dynamic Host Configuration Protocol (DHCP) Relay Agent Option"">RFC3993</a>] to associate
some provider-specific information with clients' DHCP messages that
is independent of the physical network configuration through which
the subscriber is connected. The "subscriber-id" assigned by the
provider is intended to be stable as customers connect through
different paths and as network changes occur. The Subscriber-ID is
an ASCII string that is assigned and configured by the network
provider.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Existing Mechanisms That Affect Privacy</span>
This section describes deployed DHCP mechanisms that affect privacy.
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. DNS Updates</span>
The Client FQDN (Fully Qualified Domain Name) Option [<a href="./rfc4702" title=""The Dynamic Host Configuration Protocol (DHCP) Client Fully Qualified Domain Name (FQDN) Option"">RFC4702</a>] used
along with DNS Updates [<a href="./rfc2136" title=""Dynamic Updates in the Domain Name System (DNS UPDATE)"">RFC2136</a>] defines a mechanism that allows both
clients and server to insert into the DNS domain information about
clients. Both forward (A) and reverse (PTR) resource records can be
updated. This allows other nodes to conveniently refer to a host,
despite the fact that its IP address may be changing.
This mechanism exposes two important pieces of information: current
address (which can be mapped to current location) and client's
hostname. The stable hostname can then be used to correlate the
client across different network attachments even when its IP
addresses keep changing.
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. Allocation Strategies</span>
A DHCP server running in typical, stateful mode is given a task of
managing one or more pools of IP addresses. When a client requests
an address, the server must pick an address out of a configured pool.
Depending on the server's implementation, various allocation
strategies are possible. Choices in this regard may have privacy
implications. Note that the constraints in DHCP and DHCPv6 are
radically different, but servers that allow allocation strategy
configuration may allow configuring them in both DHCP and DHCPv6.
Not every allocation strategy is equally suitable for DHCP and for
DHCPv6.
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Iterative allocation: A server may choose to allocate addresses one
by one. That strategy has the benefit of being very fast, thus
being favored in deployments that prefer performance. However, it
makes the allocated addresses very predictable. Also, since the
addresses allocated tend to be clustered at the beginning of an
available pool, it makes scanning attacks much easier.
Identifier-based allocation: Some server implementations may choose
to allocate an address that is based on one of the available
identifiers, e.g., client identifier or MAC address. It is also
convenient, as a returning client is very likely to get the same
address. Those properties are convenient for system
administrators, so DHCP server implementers are often requested to
implement it. The downside of such an allocation is that the
client has a very stable IP address. That means that correlation
of activities over time, location tracking, address scanning, and
OS/vendor discovery apply. This is certainly an issue in DHCPv6,
but due to a much smaller address space it is almost never a
problem in DHCP.
Hash allocation: This is an extension of identifier-based
allocation. Instead of using the identifier directly, it is
hashed first. If the hash is implemented correctly, it removes
the flaw of disclosing the identifier, a property that eliminates
susceptibility to address scanning and OS/vendor discovery. If
the hash is poorly implemented (e.g., it can be reversed), it
introduces no improvement over identifier-based allocation.
Random allocation: A server can pick a resource randomly out of an
available pool. This allocation scheme essentially prevents
returning clients from getting the same address again. On the
other hand, it is beneficial from a privacy perspective as
addresses generated that way are not susceptible to correlation
attacks, OS/vendor discovery attacks, or identity discovery
attacks. Note that even though the address itself may be
resilient to a given attack, the client may still be susceptible
if additional information is disclosed in another way, e.g., the
client's address may be randomized, but it still can leak its MAC
address in the Client Identifier option.
Other allocation strategies may be implemented.
Given the limited size of most IPv4 public address pools, allocation
mechanisms in IPv4 may not provide much privacy protection or leak
much useful information, if misused.
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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-9" ></span>
<span class="grey"><a href="./rfc7819">RFC 7819</a> DHCP Privacy Considerations April 2016</span>
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Attacks</span>
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. Device Type Discovery</span>
The type of device used by the client can be guessed by the attacker
using the Vendor Class Option, the 'chaddr' field, and by parsing the
Client ID Option. All of those options may contain an
Organizationally Unique Identifier (OUI) that represents the device's
vendor. That knowledge can be used for device-specific vulnerability
exploitation attacks.
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. Operating System Discovery</span>
The operating system running on a client can be guessed using the
Vendor Class option, the Client System Architecture Type option, or
by using fingerprinting techniques on the combination of options
requested using the Parameter Request List option.
<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>. Finding Location Information</span>
The location information can be obtained by the attacker by many
means. The most direct way to obtain this information is by looking
into a message originating from the server that contains the Civic
Location, GeoConf, or GeoLoc options. It can also be indirectly
inferred using the Relay Agent Information option, with the remote ID
suboption, the circuit ID option (e.g., if an access circuit on an
Access Node corresponds to a civic location), or the Subscriber ID
Option (if the attacker has access to subscriber information).
<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>. Finding Previously Visited Networks</span>
When DHCP clients connect to a network, they attempt to obtain the
same address they had used before they attached to the network. They
do this by putting the previously assigned address in the requested
IP address option. By observing these addresses, an attacker can
identify the network the client had previously visited.
<span class="h3"><a class="selflink" id="section-5.5" href="#section-5.5">5.5</a>. Finding a Stable Identity</span>
An attacker might use a stable identity gleaned from DHCP messages to
correlate activities of a given client on unrelated networks. The
Client FQDN option, the Subscriber ID option, and the Client ID
option can serve as long-lived identifiers of DHCP clients. The
Client FQDN option can also provide an identity that can easily be
correlated with web server activity logs.
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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc7819">RFC 7819</a> DHCP Privacy Considerations April 2016</span>
<span class="h3"><a class="selflink" id="section-5.6" href="#section-5.6">5.6</a>. Pervasive Monitoring</span>
Pervasive monitoring [<a href="./rfc7258" title=""Pervasive Monitoring Is an Attack"">RFC7258</a>] is widespread (and often covert)
surveillance through intrusive gathering of protocol artifacts,
including application content, or protocol metadata such as headers.
An operator who controls a nontrivial number of access points or
network segments may use obtained information about a single client
and observe the client's habits. Although users may not expect true
privacy from their operators, the information that is set up to be
monitored by users' service operators may also be gathered by an
adversary who monitors a wide range of networks and develops
correlations from that information.
<span class="h3"><a class="selflink" id="section-5.7" href="#section-5.7">5.7</a>. Finding Client's IP Address or Hostname</span>
Many DHCP deployments use DNS Updates [<a href="./rfc4702" title=""The Dynamic Host Configuration Protocol (DHCP) Client Fully Qualified Domain Name (FQDN) Option"">RFC4702</a>] that put a client's
information (current IP address, client's hostname) into the DNS,
where it is easily accessible by anyone interested. Client ID is
also disclosed, albeit not in an easily accessible form (SHA-256
digest of the client-id). As SHA-256 is considered irreversible,
DHCP client ID can't be converted back to client-id. However,
SHA-256 digest can be used as a unique identifier that is accessible
by any host.
<span class="h3"><a class="selflink" id="section-5.8" href="#section-5.8">5.8</a>. Correlation of Activities over Time</span>
As with other identifiers, an IP address can be used to correlate the
activities of a host for at least as long as the lifetime of the
address. If that address was generated from some other, stable
identifier and that generation scheme can be deduced by an attacker,
the duration of the correlation attack extends to that of the
identifier. In many cases, its lifetime is equal to the lifetime of
the device itself.
<span class="h3"><a class="selflink" id="section-5.9" href="#section-5.9">5.9</a>. Location Tracking</span>
If a stable identifier is used for assigning an address and such
mapping is discovered by an attacker, it can be used for tracking a
user. In particular, both passive (a service that the client
connects to can log the client's address and draw conclusions
regarding its location and movement patterns based on the addresses
it is connecting from) and active (an attacker can send ICMP echo
requests or other probe packets to networks of suspected client
locations) methods can be used. To give a specific example, by
accessing a social portal from
tomek-laptop.coffee.somecity.com.example,
tomek-laptop.mycompany.com.example, and
tomek-laptop.myisp.example.com, the portal administrator can draw
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<span class="grey"><a href="./rfc7819">RFC 7819</a> DHCP Privacy Considerations April 2016</span>
conclusions about tomek-laptop's owner's current location and his
habits.
<span class="h3"><a class="selflink" id="section-5.10" href="#section-5.10">5.10</a>. Leasequery and Bulk Leasequery</span>
Attackers may pretend to be an access concentrator, either as a DHCP
relay agent or as a DHCP client, to obtain location information
directly from the DHCP server(s) using the DHCP leasequery [<a href="./rfc4388" title=""Dynamic Host Configuration Protocol (DHCP) Leasequery"">RFC4388</a>]
mechanism.
Location information is information needed by the access concentrator
to forward traffic to a broadband-accessible host. This information
includes knowledge of the host hardware address, the port or virtual
circuit that leads to the host, and/or the hardware address of the
intervening subscriber modem.
Furthermore, the attackers may use the DHCP bulk leasequery [<a href="./rfc6926" title=""DHCPv4 Bulk Leasequery"">RFC6926</a>]
mechanism to obtain bulk information about DHCP bindings, even
without knowing the target bindings.
Additionally, active leasequery [<a href="./rfc7724" title=""Active DHCPv4 Lease Query"">RFC7724</a>] is a mechanism for
subscribing to DHCP lease update changes in near real-time. The
intent of this mechanism is to update an operator's database;
however, if the mechanism is misused, an attacker could defeat the
server's authentication mechanisms and subscribe to all updates. He
then could continue receiving updates, without any need for local
presence.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Security Considerations</span>
In current practice, the client privacy and client authentication are
mutually exclusive. The client authentication procedure reveals
additional client information in the certificates and identifiers.
Full privacy for the clients may mean the clients are also anonymous
to the server and the network.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Privacy Considerations</span>
This document in its entirety discusses privacy considerations in
DHCP. As such, no dedicated discussion is needed.
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<span class="grey"><a href="./rfc7819">RFC 7819</a> DHCP Privacy Considerations April 2016</span>
<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. References</span>
<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>. Normative References</span>
[<a id="ref-RFC2131">RFC2131</a>] Droms, R., "Dynamic Host Configuration Protocol",
<a href="./rfc2131">RFC 2131</a>, DOI 10.17487/RFC2131, March 1997,
<<a href="http://www.rfc-editor.org/info/rfc2131">http://www.rfc-editor.org/info/rfc2131</a>>.
[<a id="ref-RFC2136">RFC2136</a>] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
<a href="./rfc2136">RFC 2136</a>, DOI 10.17487/RFC2136, April 1997,
<<a href="http://www.rfc-editor.org/info/rfc2136">http://www.rfc-editor.org/info/rfc2136</a>>.
[<a id="ref-RFC6973">RFC6973</a>] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", <a href="./rfc6973">RFC 6973</a>,
DOI 10.17487/RFC6973, July 2013,
<<a href="http://www.rfc-editor.org/info/rfc6973">http://www.rfc-editor.org/info/rfc6973</a>>.
[<a id="ref-RFC7258">RFC7258</a>] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", <a href="https://www.rfc-editor.org/bcp/bcp188">BCP 188</a>, <a href="./rfc7258">RFC 7258</a>, DOI 10.17487/RFC7258, May
2014, <<a href="http://www.rfc-editor.org/info/rfc7258">http://www.rfc-editor.org/info/rfc7258</a>>.
<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>. Informative References</span>
[<a id="ref-RFC3046">RFC3046</a>] Patrick, M., "DHCP Relay Agent Information Option",
<a href="./rfc3046">RFC 3046</a>, DOI 10.17487/RFC3046, January 2001,
<<a href="http://www.rfc-editor.org/info/rfc3046">http://www.rfc-editor.org/info/rfc3046</a>>.
[<a id="ref-RFC3315">RFC3315</a>] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", <a href="./rfc3315">RFC 3315</a>, DOI 10.17487/RFC3315, July
2003, <<a href="http://www.rfc-editor.org/info/rfc3315">http://www.rfc-editor.org/info/rfc3315</a>>.
[<a id="ref-RFC3527">RFC3527</a>] Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy,
"Link Selection sub-option for the Relay Agent Information
Option for DHCPv4", <a href="./rfc3527">RFC 3527</a>, DOI 10.17487/RFC3527, April
2003, <<a href="http://www.rfc-editor.org/info/rfc3527">http://www.rfc-editor.org/info/rfc3527</a>>.
[<a id="ref-RFC3925">RFC3925</a>] Littlefield, J., "Vendor-Identifying Vendor Options for
Dynamic Host Configuration Protocol version 4 (DHCPv4)",
<a href="./rfc3925">RFC 3925</a>, DOI 10.17487/RFC3925, October 2004,
<<a href="http://www.rfc-editor.org/info/rfc3925">http://www.rfc-editor.org/info/rfc3925</a>>.
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[<a id="ref-RFC3993">RFC3993</a>] Johnson, R., Palaniappan, T., and M. Stapp, "Subscriber-ID
Suboption for the Dynamic Host Configuration Protocol
(DHCP) Relay Agent Option", <a href="./rfc3993">RFC 3993</a>,
DOI 10.17487/RFC3993, March 2005,
<<a href="http://www.rfc-editor.org/info/rfc3993">http://www.rfc-editor.org/info/rfc3993</a>>.
[<a id="ref-RFC4361">RFC4361</a>] Lemon, T. and B. Sommerfeld, "Node-specific Client
Identifiers for Dynamic Host Configuration Protocol
Version Four (DHCPv4)", <a href="./rfc4361">RFC 4361</a>, DOI 10.17487/RFC4361,
February 2006, <<a href="http://www.rfc-editor.org/info/rfc4361">http://www.rfc-editor.org/info/rfc4361</a>>.
[<a id="ref-RFC4388">RFC4388</a>] Woundy, R. and K. Kinnear, "Dynamic Host Configuration
Protocol (DHCP) Leasequery", <a href="./rfc4388">RFC 4388</a>,
DOI 10.17487/RFC4388, February 2006,
<<a href="http://www.rfc-editor.org/info/rfc4388">http://www.rfc-editor.org/info/rfc4388</a>>.
[<a id="ref-RFC4578">RFC4578</a>] Johnston, M. and S. Venaas, Ed., "Dynamic Host
Configuration Protocol (DHCP) Options for the Intel
Preboot eXecution Environment (PXE)", <a href="./rfc4578">RFC 4578</a>,
DOI 10.17487/RFC4578, November 2006,
<<a href="http://www.rfc-editor.org/info/rfc4578">http://www.rfc-editor.org/info/rfc4578</a>>.
[<a id="ref-RFC4702">RFC4702</a>] Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host
Configuration Protocol (DHCP) Client Fully Qualified
Domain Name (FQDN) Option", <a href="./rfc4702">RFC 4702</a>,
DOI 10.17487/RFC4702, October 2006,
<<a href="http://www.rfc-editor.org/info/rfc4702">http://www.rfc-editor.org/info/rfc4702</a>>.
[<a id="ref-RFC4776">RFC4776</a>] Schulzrinne, H., "Dynamic Host Configuration Protocol
(DHCPv4 and DHCPv6) Option for Civic Addresses
Configuration Information", <a href="./rfc4776">RFC 4776</a>,
DOI 10.17487/RFC4776, November 2006,
<<a href="http://www.rfc-editor.org/info/rfc4776">http://www.rfc-editor.org/info/rfc4776</a>>.
[<a id="ref-RFC4941">RFC4941</a>] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", <a href="./rfc4941">RFC 4941</a>, DOI 10.17487/RFC4941, September 2007,
<<a href="http://www.rfc-editor.org/info/rfc4941">http://www.rfc-editor.org/info/rfc4941</a>>.
[<a id="ref-RFC6225">RFC6225</a>] Polk, J., Linsner, M., Thomson, M., and B. Aboba, Ed.,
"Dynamic Host Configuration Protocol Options for
Coordinate-Based Location Configuration Information",
<a href="./rfc6225">RFC 6225</a>, DOI 10.17487/RFC6225, July 2011,
<<a href="http://www.rfc-editor.org/info/rfc6225">http://www.rfc-editor.org/info/rfc6225</a>>.
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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc7819">RFC 7819</a> DHCP Privacy Considerations April 2016</span>
[<a id="ref-RFC6926">RFC6926</a>] Kinnear, K., Stapp, M., Desetti, R., Joshi, B., Russell,
N., Kurapati, P., and B. Volz, "DHCPv4 Bulk Leasequery",
<a href="./rfc6926">RFC 6926</a>, DOI 10.17487/RFC6926, April 2013,
<<a href="http://www.rfc-editor.org/info/rfc6926">http://www.rfc-editor.org/info/rfc6926</a>>.
[<a id="ref-RFC7724">RFC7724</a>] Kinnear, K., Stapp, M., Volz, B., and N. Russell, "Active
DHCPv4 Lease Query", <a href="./rfc7724">RFC 7724</a>, DOI 10.17487/RFC7724,
December 2015, <<a href="http://www.rfc-editor.org/info/rfc7724">http://www.rfc-editor.org/info/rfc7724</a>>.
Acknowledgements
The authors would like to thank the valuable comments made by Stephen
Farrell, Ted Lemon, Ines Robles, Russ White, Christian Huitema,
Bernie Volz, Jinmei Tatuya, Marcin Siodelski, Christian Schaefer,
Robert Sparks, Peter Yee, and other members of DHC WG.
Authors' Addresses
Sheng Jiang
Huawei Technologies Co., Ltd
Q14, Huawei Campus, No.156 Beiqing Road
Hai-Dian District, Beijing 100095
China
Email: jiangsheng@huawei.com
Suresh Krishnan
Ericsson
8400 Decarie Blvd.
Town of Mount Royal, QC
Canada
Phone: +1 514 345 7900 x42871
Email: suresh.krishnan@ericsson.com
Tomek Mrugalski
Internet Systems Consortium, Inc.
950 Charter Street
Redwood City, CA 94063
United States
Email: tomasz.mrugalski@gmail.com
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