One document matched: draft-ietf-dhc-dhcp-privacy-01.xml
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<rfc category="info" docName="draft-ietf-dhc-dhcp-privacy-01"
ipr="trust200902">
<front>
<title abbrev="DHCP Privacy considerations">Privacy considerations for
DHCPv4</title>
<author fullname="Sheng Jiang" initials="S." surname="Jiang">
<organization>Huawei Technologies Co., Ltd</organization>
<address>
<postal>
<street>Q14, Huawei Campus, No.156 Beiqing Road</street>
<city>Hai-Dian District, Beijing, 100095</city>
<country>P.R. China</country>
</postal>
<email>jiangsheng@huawei.com</email>
</address>
</author>
<author fullname="Suresh Krishnan" initials="S." surname="Krishnan">
<organization>Ericsson</organization>
<address>
<postal>
<street>8400 Decarie Blvd.</street>
<city>Town of Mount Royal</city>
<region>QC</region>
<country>Canada</country>
</postal>
<phone>+1 514 345 7900 x42871</phone>
<email>suresh.krishnan@ericsson.com</email>
</address>
</author>
<author fullname="Tomek Mrugalski" initials="T." surname="Mrugalski">
<organization abbrev="ISC">Internet Systems Consortium,
Inc.</organization>
<address>
<postal>
<street>950 Charter Street</street>
<city>Redwood City</city>
<region>CA</region>
<code>94063</code>
<country>USA</country>
</postal>
<email>tomasz.mrugalski@gmail.com</email>
</address>
</author>
<date />
<area>Internet</area>
<workgroup>dhc</workgroup>
<keyword>DHCP(v4) Privacy</keyword>
<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>Dynamic Host Configuration Protocol (DHCP) <xref
target="RFC2131"></xref> is a protocol that is used to provide
addressing and configuration information to IPv4 hosts. The DHCP
protocol 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 <xref
target="RFC6973"></xref>. In particular, it also takes into consideration
the problem of pervasive monitoring <xref target="RFC7258"/>.</t>
<t>Future works may propose protocol changes to fix the privacy issues
that have been analyzed in this document. It is out of scope for this
document.</t>
<t>The primary focus of this document is around privacy considerations for
clients to support client mobility and connection to random networks. The
privacy or DHCP servers and relay agents are considered less important as
they are typically open for public services. And, it is generally assumed
that relay agent to server communication 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 privacy of DHCP servers and relay agents.</t>
</section>
<section title="Requirements Language and Terminology">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119"></xref>. When these words are not in ALL CAPS (such as
"should" or "Should"), they have their usual English meanings, and are
not to be interpreted as <xref target="RFC2119"></xref> key words.</t>
<t>In addition the following terminology is used:
<list hangIndent="8" style="hanging">
<t hangText="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 may include
MAC address, client-id or 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 other may generate it on the fly and use a different one after
each boot. Stable identifier may or may not be globally unique.</t>
</list></t>
</section>
<section title="Identifiers in DHCP">
<t>There are several identifiers used in DHCP. This section provides an
introduction to the various options that will be used further in the
document.</t>
<section title="Client ID Option">
<t>The Client Identifier Option <xref target="RFC2131"></xref> is used
to pass an explicit client identifier to a DHCP server.</t>
<t>The client identifier is an opaque key, which 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. <xref
target="RFC3315"/> in Section 9.2 specifies DUID-LLT (Link-layer + time)
as the recommended DUID type. <xref target="RFC4361"/>, Section 6.1
introduces this concept to DHCPv4. Those two document recommend that
client identifiers be generated by using the
permanent link-layer address of the network interface that the client
is trying to configure. <xref target="RFC4361"></xref> updates the
recommendation of Client Identifiers to be "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 RFC 3315, section
9". This does not change the lifecycle of the Client Identifiers.
Clients are expected to generate their Client Identifiers once (during
first operation) and store it in a non-volatile storage or use the
same deterministic algorithm to generate the same Client Identifier
values again.</t>
</section>
<section title="Address Fields & Options">
<t>The 'yiaddr' field <xref target="RFC2131"></xref> in DHCP message
is used to allocate address from the server to the client.</t>
<t>The DHCPv4 specification <xref target="RFC2131"></xref> provides a
way to specify the client link-layer address in the DHCPv4 message
header. A DHCPv4 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.</t>
<t>The 'requested IP address' option <xref target="RFC2131"></xref> is
used by client to suggest that a particular IP address be
assigned.</t>
</section>
<section title="Client FQDN Option">
<t>The Client Fully Qualified Domain Name (FQDN) option <xref
target="RFC4702"></xref> is used by DHCP clients and servers to
exchange information about the client's fully qualified domain name
and about who has the responsibility for updating the DNS with the
associated AAAA and PTR RRs.</t>
<t>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 case
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.</t>
</section>
<section title="Parameter Request List Option">
<t>The Parameter Request List option <xref target="RFC2131"></xref> is
used to inform the server about options the client wants the server to
send to the client. The content of a Parameter Request List option are
the option codes for an option requested by the client.</t>
</section>
<section title="Vendor Class and Vendor-Identifying Vendor Class Options">
<t>The Vendor Class option <xref target="RFC2131"></xref>, the
Vendor-Identifying Vendor Class option and Vendor-Identifying Vendor
Information option <xref target="RFC3925"></xref> are used by the DHCP
client to identify the vendor that manufactured the hardware on which
the client is running.</t>
<t>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.</t>
</section>
<section title="Civic Location Option">
<t>DHCP servers use the Civic Location Option <xref
target="RFC4776"></xref> to delivery of the location information (the
civic and postal addresses) to the 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.</t>
</section>
<section title="Coordinate-Based Location Option">
<t>The GeoConf and GeoLoc options <xref target="RFC6225"></xref> is
used by DHCP server to provide the coordinate-based geographic
location information to the DHCP clients. It enables a DHCP client to
obtain its geographic location.</t>
<t>After the relevant DHCP exchanges have taken place, the location
information is stored on the end device rather than somewhere else,
where retrieving it might be difficult in practice.</t>
</section>
<section title="Client System Architecture Type Option">
<t>The Client System Architecture Type Option <xref
target="RFC4578"></xref> is used by DHCP client to send a list of
supported architecture types to the DHCP server. It is used to provide
configuration information for a node that must be booted using the
network rather than from local storage.</t>
<t><!--Client Network Interface Identifier Option and Client Machine
Identifier Option seems not in use. Together with the Client System
Architecture Type Option, they are used for the Intel Preboot eXecution
Environment (PXE). Client Network Interface Identifier Option provide
information about the level of UNDI support.--></t>
</section>
<section title="Relay Agent Information Option and Sub-options">
<t>A DHCP relay agent includes a Relay Agent Information <xref
target="RFC3046"></xref> to identify the remote host end of the
circuit. It contains a "circuit ID" sub-option 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"
sub-option which provides a trusted identifier for the remote
high-speed modem.</t>
<t>Possible encoding of "circuit ID" sub-option includes: router
interface number, switching hub port number, remote access server port
number, frame relay DLCI, ATM virtual circuit number, cable data
virtual circuit number, etc.</t>
<t>Possible encoding of the "remote ID" sub-option includes: a "caller
ID" telephone number for dial-up connection, a "user name" prompted
for by a remote access server, a remote caller 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.</t>
<t>The link-selection sub-option <xref target="RFC3527"></xref> 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 communicating with the relay agent. It contains an IP
address, which can identify the client's subnet/link. Also, assuming
network topology knowledge, it also reveals client location.</t>
<t>A DHCP relay includes a Subscriber-ID option <xref target="RFC3993"/>
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, which is assigned and configured by the network
provider.</t>
</section>
</section>
<section title="Existing Mechanisms That Affect Privacy">
<t>This section describes available DHCP mechanisms that one can use to
protect or enhance one's privacy.</t>
<section title="DNS Updates">
<t>DNS Updates <xref target="RFC4702"></xref> defines a mechanism that
allows both clients and server to insert into 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.</t>
<t>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.</t>
</section>
<section title="Allocation strategies">
<t>A DHCP server running in typical, stateful mode is given a task of
managing one or more pools of IP address. When a client
requests an address, the server must pick an address out of 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 DHCPv4 and DHCPv6 are
radically different, but servers that allow allocation strategy
configuration may allow configuring them in both DHCPv4 and DHCPv6.
Not every allocation strategy is equally suitable for DHCPv4 and for
DHCPv6.</t>
<t>Iterative allocation - a server may choose to allocate addresses one
by one. That strategy has the benefit of being very fast, thus can be
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 available pool, it
makes scanning attacks much easier.</t>
<t>Identifier-based allocation - a server may choose to allocate an
address that is based on one of available identifiers, e.g. client
identifier or MAC address. It is also convenient, as returning client
is very likely to get the same address. Those properties are
convenient for system administrators, so DHCP server implementors are
often requested to implement it. On the other hand, the downside of
such 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 much smaller address space is almost never
a problem in DHCPv4.</t>
<t>Hash allocation - it's an extension of identifier based allocation.
Instead of using the identifier directly, it is being 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. can be reverted), it introduces no improvement over
identifier-based allocation.</t>
<t>Random allocation - a server can pick a resource randomly out of
available pool. That strategy works well in scenarios where pool
utilization is small, as the likelihood of collision (resulting in the
server needing to repeat randomization) is small. With the pool
allocation increasing, the collision is disproportionally large, due
to birthday paradox. With high pool utilization (e.g. when 90% of
available resources being allocated already), the server will use most
computational resources to repeatedly pick a random resource, which
will degrade its performance. This allocation scheme essentially
prevents returning clients from getting the same address again. On the
other hand, it is beneficial from 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
other way, e.g. client's address can be randomized, but it still can
leak its MAC address in client-id option.</t>
<t>Other allocation strategies may be implemented.</t>
<t>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.</t>
</section>
</section>
<section title="Attacks">
<section title="Device type discovery">
<t>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.</t>
</section>
<section title="Operating system discovery">
<t>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.</t>
</section>
<section title="Finding location information">
<t>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 sub-option, 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 info).</t>
</section>
<section title="Finding previously visited networks">
<t>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.</t>
</section>
<section title="Finding a stable identity">
<t>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 options
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.</t>
</section>
<section title="Pervasive monitoring">
<t>This is an enhancement, or a combination of most aforementioned
mechanisms. Operator who controls non-trivial number of access points
or network segments, may use obtained information about a single
client and observer client's habits.</t>
</section>
<section title="Finding client's IP address or hostname">
<t>Many DHCP deployments use DNS Updates <xref
target="RFC4702"></xref> that put client's information (current IP
address, client's hostname) into DNS, where it is easily accessible
by anyone interested. Client ID is also disclosed, albeit in
not easily accessible form (SHA-256 digest of the client-id). As
SHA-256 is considered irreversible, DHCID can't be converted back to
client-id. However, SHA-256 digest can be used as an unique identifier
that is accessible by any host.</t>
</section>
<section title="Correlation of activities over time">
<t>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 deducted by an attacker,
the duration of correlation attack extends to that identifier. In many
cases, its lifetime is equal to the lifetime of the device itself.</t>
</section>
<section title="Location tracking">
<t>If a stable identifier is used for assigning an address and such
mapping is discovered by an attacker, e.g. a hostname being put into
DNS, it can be used for tracking user. In particular both passive (a
service that the client connects to can log client's address and draw
conclusions regarding its location and movement patterns based on
address it is connecting from) and active (attacker can send ICMP echo
requests or other probe packets to networks of suspected client
locations) methods can be used. To give 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 conclusions about tomek-laptop's owner
current location and his habits.</t>
</section>
<section title="Leasequery & bulk leasequery">
<t>Attackers may pretend as an access concentrator, either DHCP relay
agent or DHCP client, to obtain location information directly from the
DHCP server(s) using the DHCP leasequery <xref
target="RFC4388"></xref> mechanism.</t>
<t>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.</t>
<t>Furthermore, the attackers may use DHCP bulk leasequery <xref
target="RFC6926"></xref> mechanism to obtain bulk information about
DHCP bindings, even without knowing the target bindings.</t>
<t>Additionally, active leasequery <xref
target="I-D.ietf-dhc-dhcpv4-active-leasequery" /> is a mechanism for
subscribing to DHCPv4 lease update changes in near real-time. The
intent of this mechanism is to update operator's database, but if
misused, an attacker could defeat server's authentication mechanisms and
subscribe to all updates. He then could continue receiving updates,
without any need for local presence.</t>
</section>
</section>
<section anchor="security" title="Security Considerations">
<t>In current practice, the client privacy and the client authentication
are mutually exclusive. The client authentication procedure reveals
additional client information in their certificates/identifiers. Full
privacy for the clients may mean the clients are also anonymous for the
server and the network.</t>
</section>
<section anchor="privacy-consider" title="Privacy Considerations">
<t>This document at its entirety discusses privacy considerations in
DHCP. As such, no dedicated section about this is needed.</t>
</section>
<section title="IANA Considerations">
<t>This draft does not request any IANA action.</t>
</section>
<section anchor="acks" title="Acknowledgements">
<t>The authors would like to thanks the valuable comments made by
Stephen Farrell, Ted Lemon, Ines Robles, Russ White, Christian Huitema,
Bernie Volz and other members of DHC WG.</t>
<t>This document was produced using the xml2rfc tool <xref
target="RFC2629"></xref>.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include='reference.RFC.2119'?>
<?rfc include='reference.RFC.2131'?>
<?rfc include='reference.RFC.6973'?>
<?rfc include='reference.RFC.7258'?>
</references>
<references title="Informative References">
<?rfc include='reference.RFC.2629'?>
<?rfc include='reference.RFC.3046'?>
<?rfc include='reference.RFC.3315'?>
<?rfc include='reference.RFC.3527'?>
<?rfc include='reference.RFC.3925'?>
<?rfc include='reference.RFC.3993'?>
<?rfc include='reference.RFC.4361'?>
<?rfc include='reference.RFC.4388'?>
<?rfc include='reference.RFC.4578'?>
<?rfc include='reference.RFC.4702'?>
<?rfc include='reference.RFC.4776'?>
<?rfc include='reference.RFC.6225'?>
<?rfc include='reference.RFC.6926'?>
<?rfc include='reference.I-D.ietf-dhc-dhcpv4-active-leasequery'?>
</references>
</back>
</rfc>
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