dhc S. Krishnan
Internet-Draft Ericsson
Intended status: Standards Track T. Mrugalski
Expires: April 30, 2015 ISC
S. Jiang
Huawei Technologies Co., Ltd
October 27, 2014
Privacy considerations for DHCPv6
draft-krishnan-dhc-dhcpv6-privacy-00
Abstract
DHCPv6 is a protocol that is used to provide addressing and
configuration information to IPv6 hosts. This document discusses the
various identifiers used by DHCPv6 and the potential privacy issues.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Identifiers in DHCPv6 . . . . . . . . . . . . . . . . . . . . 3
3.1. DUID . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Client ID Option . . . . . . . . . . . . . . . . . . . . 4
3.3. IA_NA, IA_TA, IA_PD, IA Address and IA Prefix Options . . 4
3.4. Interface ID . . . . . . . . . . . . . . . . . . . . . . 5
3.5. Subscriber ID . . . . . . . . . . . . . . . . . . . . . . 5
3.6. Remote ID . . . . . . . . . . . . . . . . . . . . . . . . 5
3.7. Client FQDN Option . . . . . . . . . . . . . . . . . . . 6
3.8. Client Link-layer Address Option . . . . . . . . . . . . 6
3.9. Option Request Option . . . . . . . . . . . . . . . . . . 6
3.10. Vendor Class Option . . . . . . . . . . . . . . . . . . . 6
3.11. Civic Location Option . . . . . . . . . . . . . . . . . . 7
3.12. Coordinate-Based Location Option . . . . . . . . . . . . 7
3.13. Client System Architecture Type Option . . . . . . . . . 7
4. Existing Mechanisms That Affect Privacy . . . . . . . . . . . 7
4.1. Temporary addresses . . . . . . . . . . . . . . . . . . . 7
4.2. DNS Updates . . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Allocation strategies . . . . . . . . . . . . . . . . . . 8
5. Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Device type discovery (fingerprinting) . . . . . . . . . 9
5.2. Operating system discovery (fingerprinting) . . . . . . . 10
5.3. Finding location information . . . . . . . . . . . . . . 10
5.4. Finding previously visited networks . . . . . . . . . . . 10
5.5. Finding a stable identity . . . . . . . . . . . . . . . . 10
5.6. Pervasive monitoring . . . . . . . . . . . . . . . . . . 10
5.7. Finding client's IP address or hostname . . . . . . . . . 11
5.8. Correlation of activities over time . . . . . . . . . . . 11
5.9. Location tracking . . . . . . . . . . . . . . . . . . . . 11
5.10. Leasequery & bulk leasequery . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
DHCPv6 [RFC3315] is a protocol that is used to provide addressing and
configuration information to IPv6 hosts. The DHCPv6 protocol uses
several identifiers that could become a source for gleaning
additional information about the IPv6 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 DHCPv6 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 DHCPv6 client. The privacy of DHCPv6 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 DHCPv6 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 DHCPv6
There are several identifiers used in DHCPv6. This section provides
an introduction to the various options that will be used further in
the document.
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3.1. DUID
Each DHCPv6 client and server has a DHCPv6 Unique Identifier (DUID)
[RFC3315]. The DUID is designed to be unique across all DHCPv6
clients and servers, and to remain stable after it has been initially
generated. The DUID can be of different forms. Commonly used forms
are based on the link-layer address of one of the device's network
interfaces (with or without a timestamp), on the Universally Unique
IDentifier (UUID) [RFC6355]. The default type, recommended by
[RFC3315], is DUID-LLT that is based on link-layer address, which is
commonly implemented in most popular clients.
It is important to understand DUID lifecycle. Clients and servers
are expected to generate their DUID once (during first operation) and
store it in a non-volatile storage or use the same deterministic
algorithm to generate the same DUID value again. This means that
most implementations will use the available link-layer address during
its first boot. Even if the administrator enables privacy extensions
(see [RFC4941]) and its equivalent for link-layer address
randomization, it is likely that those privacy mechanisms were
disabled during the first device boot. Hence the original,
unobfuscated link-layer address will likely end up being announced as
client DUID, even if the link-layer address has changed (or even if
being changed on a periodic basis).
3.2. Client ID Option
The Client Identifier Option (OPTION_CLIENTID) [RFC3315] is used to
carry the DUID of a DHCPv6 client between a client and a 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). Client ID is an example of DUID. See
Section 3.1 for relevant discussion about DUIDs.
3.3. IA_NA, IA_TA, IA_PD, IA Address and IA Prefix Options
The Identity Association for Non-temporary Addresses (IA_NA) option
[RFC3315] is used to carry the parameters and any non-temporary
addresses associated with the given IA_NA. The Identity Association
for Temporary Addresses (IA_TA) option [RFC3315] is analogous to the
IA_NA option but for temporary addresses. The IA Address option
[RFC3315] is used to specify IPv6 addresses associated with an IA_NA
or an IA_TA and is encapsulated within the Options field of such an
IA_NA or IA_TA option. The Identity Association for Prefix
Delegation (IA_PD) [RFC3633] option is used to carry the prefixes
that are assigned to the requesting router. IA Prefix option
[RFC3633] is used to specify IPv6 prefixes associated with an IA_PD
and is encapsulated within the Options field of such an IA_PD option.
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To differentiate between instances of the same type of IA containers,
each IA_NA, IA_TA and IA_PD options have an IAID field that is unique
for each client/option type pair. It is up to the client to pick
unique IAID values. At least one popular implementation uses last
four octets of the link-layer address. In most cases, that means
that merely two bytes are missing for a full link-layer address
reconstruction. However, the first three octets in a typical link-
layer address are vendor identifier. That can be determined with
high level of certainty using other means, thus allowing full link-
layer address discovery.
3.4. Interface ID
A DHCPv6 relay includes the Interface ID [RFC3315] option to identify
the interface on which it received the client message that is being
relayed.
Although in principle Interface ID can be arbitrarily long with
completely random values, it is often a text string that includes the
relay agent name followed by interface name. This can be used for
fingerprinting the relay or determining client's point of attachment.
3.5. Subscriber ID
A DHCPv6 relay includes a Subscriber ID option [RFC4580] to associate
some provider-specific information with clients' DHCPv6 messages that
is independent of the physical network configuration.
In many deployments, the relay agent that inserts this option is
configured to use client's link-layer address as Subscriber ID.
3.6. Remote ID
A DHCPv6 relay includes a Remote ID option [RFC4649] to identify the
remote host end of the circuit.
The remote-id is vendor specific, for which the vendor is indicated
in the enterprise-number field. The remote-id field may encode the
information that identified the DHCPv6 clients:
o a "caller ID" telephone number for dial-up connection
o a "user name" prompted for by a Remote Access Server
o a remote caller ATM address o a "modem ID" of a cable data modem
o the remote IP address of a point-to-point link
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o an interface or port identifier
3.7. Client FQDN Option
The Client Fully Qualified Domain Name (FQDN) option [RFC4704] is
used by DHCPv6 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 DHCPv6 server for the IPv6-address-to-FQDN mapping. In most
case a client sends its hostname as a hint for the server. The
DHCPv6 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.
3.8. Client Link-layer Address Option
The Client link-layer address option [RFC6939] is used by first-hop
DHCPv6 relays to provide the client's link-layer address towards the
server.
DHCPv6 relay agents that receive messages originating from clients
may include the link-layer source address of the received DHCPv6
message in the Client Link-Layer Address option, in relayed DHCPv6
Relay-Forward messages.
3.9. Option Request Option
DHCPv6 clients include an Option Request option [RFC3315] in DHCPv6
messages to inform the server about options the client wants the
server to send to the client.
The content of an Option Request option are the option codes for an
option requested by the client. The client may additionally include
instances of those options that are identified in the Option Request
option, with data values as hints to the server about parameter
values the client would like to have returned.
3.10. Vendor Class Option
This Vendor Class option [RFC3315] is used by a DHCPv6 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 details of the
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hardware configuration, for example, the version of the operating
system the client is running or the amount of memory installed on the
client.
3.11. Civic Location Option
DHCPv6 servers use the Civic Location option [RFC4776] to delivery of
location information (the civic and postal addresses) from the DHCPv6
server to the DHCPv6 clients. It may refer to three locations: the
location of the DHCPv6 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.
3.12. Coordinate-Based Location Option
The GeoLoc options [RFC6225] is used by DHCPv6 server to provide the
coordinate- based geographic location information to the DHCPv6
clients. It enable a DHCPv6 client to obtain its location.
After the relevant DHCPv6 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.
3.13. Client System Architecture Type Option
The Client System Architecture Type option [RFC5970] is used by
DHCPv6 client to send a list of supported architecture types to the
DHCPv6 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 DHCPv6 mechanisms that one can use
to protect or enhance one's privacy.
4.1. Temporary addresses
[RFC3315] defines a mechanism for a client to request temporary
addresses. The idea behind temporary addresses is that a client can
request a temporary address for a specific purpose, use it, and then
never renew it. i.e. let it expire.
There are number of serious issues, both protocolar and
implementational, that make them nearly useless for their original
goal. First, [RFC3315] does not include T1 and T2 renewal timers in
IA_TA (a container for temporary addresses). However, it mentions
that temporary addresses can be renewed. Many client implementations
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renew those addresses during a renewal procedure initiated by other
resources (non-temporary addresses or prefixes), thus forfeiting
shortliveness. Second, [RFC4704] allows servers to update DNS for
assigned temporary addresses. Publishing client's IPv6 address in
DNS that is publicly available is a major privacy breach.
4.2. 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 IPv6 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
client across different network attachments even when its IPv6
address keeps changing.
4.3. Allocation strategies
A DHCPv6 server running in typical, stateful mode is given a task of
managing one or more pools of IPv6 resources (currently non-temporary
addresses, temporary addresses and/or prefixes, but more resource
types may be defined in the future). 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. IID or
MAC address. This has a property of being convenient for converting
IP address to/from other identifiers, especially if the identifier is
or contains MAC address. It is also convenient, as returning client
is very likely to get the same address, even if the server does not
store previous client's address. Those properties are convenient for
system administrators, so DHCPv6 server implementors are sometimes
requested to implement it. There is at least one implementation that
supports it. On the other hand, the downside of such allocation is
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that the client now discloses its identifier in its IPv6 address to
all services it connects to. 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
or prefix again. On the other hand, it is beneficial from privacy
perspective as addresses and prefixes generated that way are not
susceptible to correlation attacks, OS/vendor discovery attacks or
identity discovery attacks. Note that even though the address or
prefix 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.
5. Attacks
5.1. Device type discovery (fingerprinting)
The type of device used by the client can be guessed by the attacker
using the Vendor Class option, the Client Link-layer Address option,
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. See Section 3.4 of
[I-D.ietf-6man-ipv6-address-generation-privacy] for a discussion
about this type of attack.
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5.2. Operating system discovery (fingerprinting)
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 Option Request option. See Section 3.4 of
[I-D.ietf-6man-ipv6-address-generation-privacy] for a discussion
about this type of attack.
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 or
GeoLoc option. It can also be indirectly inferred using the Remote
ID Option (e.g. using a telephone number), the Interface 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).
5.4. Finding previously visited networks
When DHCPv6 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(es) in the IA
Address Option(s) inside the IA_NA, IA_TA. 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 DHCPv6 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 DHCPv6 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.
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5.7. Finding client's IP address or hostname
Many DHCPv6 deployments use DNS Updates [RFC4704] 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 DHCPv6
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.
5.8. Correlation of activities over time
As with other identifiers, an IPv6 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. See Section 3.1 of
[I-D.ietf-6man-ipv6-address-generation-privacy] for detailed
discussion.
5.9. Location tracking
If a stable identifier is used for assigning an address and such
mapping is discovered by an attacker (e.g. a server that uses IEEE-
identifier-based IID to generate IPv6 address), all scenarios
discussed in Section 3.2 of
[I-D.ietf-6man-ipv6-address-generation-privacy] apply. 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 prefix it is connecting from) and active (attacker
can send ICMPv6 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 DHCPv6 relay
agent or DHCPv6 client, to obtain location information directly from
the DHCP server(s) using the DHCPv6 Leasequery [RFC5007] 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.
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Furthermore, the attackers may use DHCPv6 bulk leasequery [RFC5460]
mechanism to obtain bulk information about DHCPv6 bindings, even
without knowing the target bindings.
6. Security Considerations
TBD
7. Privacy Considerations
This document at its entirety discusses privacy considerations in
DHCPv6. 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
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
10.2. Informative References
[I-D.ietf-6man-ipv6-address-generation-privacy]
Cooper, A., Gont, F., and D. Thaler, "Privacy
Considerations for IPv6 Address Generation Mechanisms",
draft-ietf-6man-ipv6-address-generation-privacy-02 (work
in progress), October 2014.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
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[RFC3633] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
Host Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[RFC4580] Volz, B., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6) Relay Agent Subscriber-ID Option", RFC 4580, June
2006.
[RFC4649] Volz, B., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6) Relay Agent Remote-ID Option", RFC 4649, August
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.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC5007] Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng,
"DHCPv6 Leasequery", RFC 5007, September 2007.
[RFC5460] Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460, February
2009.
[RFC5970] Huth, T., Freimann, J., Zimmer, V., and D. Thaler, "DHCPv6
Options for Network Boot", RFC 5970, September 2010.
[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.
[RFC6355] Narten, T. and J. Johnson, "Definition of the UUID-Based
DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355, August
2011.
[RFC6939] Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer
Address Option in DHCPv6", RFC 6939, May 2013.
[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.
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Authors' Addresses
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
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
Krishnan, et al. Expires April 30, 2015 [Page 14]
