Internet DRAFT - draft-jiang-dhc-dhcp-privacy
draft-jiang-dhc-dhcp-privacy
dhc S. Jiang
Internet-Draft Huawei Technologies Co., Ltd
Intended status: Standards Track S. Krishnan
Expires: April 30, 2015 Ericsson
T. Mrugalski
ISC
October 27, 2014
Privacy considerations for DHCP
draft-jiang-dhc-dhcp-privacy-00
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
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This Internet-Draft will expire on April 30, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
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the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Identifiers in DHCP . . . . . . . . . . . . . . . . . . . . . 3
3.1. Client ID Option . . . . . . . . . . . . . . . . . . . . 3
3.2. Address Fields & Options . . . . . . . . . . . . . . . . 4
3.3. Subscriber-ID Option . . . . . . . . . . . . . . . . . . 4
3.4. Relay Agent Information Option and Sub-options . . . . . 4
3.5. Client FQDN Option . . . . . . . . . . . . . . . . . . . 5
3.6. Parameter Request List Option . . . . . . . . . . . . . . 5
3.7. Vendor Class and Vendor-Identifying Vendor Class Options 5
3.8. Civic Location Option . . . . . . . . . . . . . . . . . . 6
3.9. Coordinate-Based Location Option . . . . . . . . . . . . 6
3.10. Client System Architecture Type Option . . . . . . . . . 6
4. Existing Mechanisms That Affect Privacy . . . . . . . . . . . 6
4.1. DNS Updates . . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Allocation strategies . . . . . . . . . . . . . . . . . . 7
5. Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Device type discovery . . . . . . . . . . . . . . . . . . 8
5.2. Operating system discovery . . . . . . . . . . . . . . . 8
5.3. Finding location information . . . . . . . . . . . . . . 8
5.4. Finding previously visited networks . . . . . . . . . . . 9
5.5. Finding a stable identity . . . . . . . . . . . . . . . . 9
5.6. Pervasive monitoring . . . . . . . . . . . . . . . . . . 9
5.7. Finding client's IP address or hostname . . . . . . . . . 9
5.8. Correlation of activities over time . . . . . . . . . . . 9
5.9. Location tracking . . . . . . . . . . . . . . . . . . . . 9
5.10. Leasequery & bulk leasequery . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Dynamic Host Configuration Protocol (DHCP) [RFC2131] 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 additional information about the IPv4
host. This information may include device type, operating system
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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 [RFC6973].
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.
Editor notes: for now, the document is mainly considering the privacy
of DHCP client. The privacy of DHCP server and relay agent are
considered less important because they are open for public services.
However, this may be a subject to change if further study shows
opposite result.
2. Terminology
This section clarifies the terminology used throughout this document.
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 that does not change or a hostname. Stable identifier may
or may not be globally unique.
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 [RFC2119]. 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
[RFC2119] key words.
3. Identifiers in DHCP
There are several identifiers used in DHCP. This section provides an
introduction to the various options that will be used further in the
document.
3.1. Client ID Option
The Client Identifier Option [RFC2131] is used to pass an explicit
client identifier to a DHCP server. There is an analogous Server
Identifier Option but it is not as interesting in the privacy context
(unless a host can be convinced to start acting as a server).
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
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'chaddr' field, or another type of identifier, such as a DNS name.
It is recommended that client identifiers be generated by using the
permanent link-layer address of the network interface that the client
is trying to configure. [RFC4361] 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.
3.2. Address Fields & Options
The 'yiaddr' field [RFC2131] in DHCP message is used to allocate
address from the server to the client.
The DHCPv4 specification [RFC2131] 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.
The 'requested IP address' option [RFC2131] is used by client to
suggest that a particular IP address be assigned.
3.3. Subscriber-ID Option
A DHCP relay includes a Subscriber-ID option [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.
3.4. Relay Agent Information Option and Sub-options
A DHCP relay agent includes a Relay Agent Information [RFC3046] 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.
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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.
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.
The link-selection sub-option [RFC3527] 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.
3.5. Client FQDN Option
The Client Fully Qualified Domain Name (FQDN) option [RFC4702] 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.
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.
3.6. Parameter Request List Option
The Parameter Request List option [RFC2131] 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.
3.7. Vendor Class and Vendor-Identifying Vendor Class Options
The Vendor Class option [RFC2131] and the Vendor-Identifying Vendor
Class option [RFC3925] is used by a 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
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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.
3.8. Civic Location Option
DHCP servers use the Civic Location Option [RFC4776] 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.
3.9. Coordinate-Based Location Option
The GeoConf and GeoLoc options [RFC6225] 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.
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.
3.10. Client System Architecture Type Option
The Client System Architecture Type Option [RFC4578] 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.
4. Existing Mechanisms That Affect Privacy
This section describes available DHCP mechanisms that one can use to
protect or enhance one's privacy.
4.1. DNS Updates
DNS Updates [RFC4704] defines a mechanism that allows both clients
and server to insert into DNS domain information about clients. Both
forward (AAAA) 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 by used to correlate the
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client across different network attachments even when its IP
addresses keep changing.
4.2. Allocation strategies
A DHCP server running in typical, stateful mode is given a task of
managing one or more pools of IP address resources. When a client
requests a resource, server must pick a resource out of configured
pool. Depending on the server's implementation, various allocation
strategies are possible. Choices in this regard may have privacy
implications.
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 resources very predictable. Also, since the resources allocated
tend to be clustered at the beginning of available pool, it makes
scanning attacks much easier.
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.
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.
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
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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.
Other allocation strategies may be implemented.
However, giving the limited resource of IPv4 public address pool,
allocation mechanism in IPv4 may not provide much protection, while
in IPv6, the network has very large address space to distribute the
address allocation.
5. Attacks
5.1. Device type discovery
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 OUI
(Organizationally Unique Identifier) that represents the device's
vendor. That knowledge can be used for device-specific vulnerability
exploitation attacks.
5.2. Operating system discovery
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.
5.3. Finding location information
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 server initiated message 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
(e.g. using a telephone number), 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).
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5.4. Finding previously visited networks
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.
5.5. Finding a stable identity
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.
5.6. Pervasive monitoring
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.
5.7. Finding client's IP address or hostname
Many DHCP deployments use DNS Updates [RFC4702] that put client's
information (current IP address, client's hostname). Client ID is
also disclosed, able it in not easily accessible form (SHA-256 digest
of the client-id). Although SHA-256 is irreversible, so DHCID 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.
5.8. Correlation of activities over time
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.
5.9. Location tracking
If a stable identifier is used for assigning an address and such
mapping is discovered by an attacker. In particular both passive (a
service that the client connects to can log client's address and draw
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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).
5.10. Leasequery & bulk leasequery
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 [RFC4388], [RFC6148]
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 DHCP bulk leasequery [RFC6926]
mechanism to obtain bulk information about DHCP bindings, even
without knowing the target bindings.
6. Security Considerations
TBD
7. Privacy Considerations
This document at its entirety discusses privacy considerations in
DHCP. As such, no separate section about this is needed.
8. IANA Considerations
This draft does not request any IANA action.
9. Acknowledgements
The authors would like to thanks the valuable comments made by
Stephen Farrell, Ted Lemon, Ines Robles, Russ White, Christian
Schaefer and other members of DHC WG.
This document was produced using the xml2rfc tool [RFC2629].
10. References
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10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 2001.
[RFC3527] Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy,
"Link Selection sub-option for the Relay Agent Information
Option for DHCPv4", RFC 3527, April 2003.
[RFC3925] Littlefield, J., "Vendor-Identifying Vendor Options for
Dynamic Host Configuration Protocol version 4 (DHCPv4)",
RFC 3925, October 2004.
[RFC3993] Johnson, R., Palaniappan, T., and M. Stapp, "Subscriber-ID
Suboption for the Dynamic Host Configuration Protocol
(DHCP) Relay Agent Option", RFC 3993, March 2005.
[RFC4361] Lemon, T. and B. Sommerfeld, "Node-specific Client
Identifiers for Dynamic Host Configuration Protocol
Version Four (DHCPv4)", RFC 4361, February 2006.
[RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration
Protocol (DHCP) Leasequery", RFC 4388, February 2006.
[RFC4702] Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host
Configuration Protocol (DHCP) Client Fully Qualified
Domain Name (FQDN) Option", RFC 4702, October 2006.
[RFC4704] Volz, B., "The Dynamic Host Configuration Protocol for
IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
Option", RFC 4704, October 2006.
[RFC4776] Schulzrinne, H., "Dynamic Host Configuration Protocol
(DHCPv4 and DHCPv6) Option for Civic Addresses
Configuration Information", RFC 4776, November 2006.
[RFC6148] Kurapati, P., Desetti, R., and B. Joshi, "DHCPv4 Lease
Query by Relay Agent Remote ID", RFC 6148, February 2011.
[RFC6225] Polk, J., Linsner, M., Thomson, M., and B. Aboba, "Dynamic
Host Configuration Protocol Options for Coordinate-Based
Location Configuration Information", RFC 6225, July 2011.
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[RFC6926] Kinnear, K., Stapp, M., Desetti, R., Joshi, B., Russell,
N., Kurapati, P., and B. Volz, "DHCPv4 Bulk Leasequery",
RFC 6926, April 2013.
10.2. Informative References
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC4578] Johnston, M. and S. Venaas, "Dynamic Host Configuration
Protocol (DHCP) Options for the Intel Preboot eXecution
Environment (PXE)", RFC 4578, November 2006.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, July
2013.
Authors' Addresses
Sheng Jiang
Huawei Technologies Co., Ltd
Q14, Huawei Campus, No.156 Beiqing Road
Hai-Dian District, Beijing, 100095
P.R. 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
USA
Phone: +1 650 423 1345
Email: tomasz.mrugalski@gmail.com
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