draft-ietf-dhc-dhcpv6-privacy-04.txt

dhc                                                          S. Krishnan
Internet-Draft                                                  Ericsson
Intended status: Informational                              T. Mrugalski
Expires: August 19, 2016                                             ISC
                                                                S. Jiang
                                            Huawei Technologies Co., Ltd
                                                       February 16, 2016


                   Privacy considerations for DHCPv6
                    draft-ietf-dhc-dhcpv6-privacy-04

Abstract

   DHCPv6 is a protocol that is used to provide addressing and
   configuration information to IPv6 hosts.  This document describes the
   privacy issues associated with the use of DHCPv6 by the Internet
   users.  It is intended to be an analysis of the present situation and
   does not propose any solutions.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 19, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) 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



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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Identifiers in DHCPv6 options and fields  . . . . . . . . . .   4
     3.1.  Source IPv6 address . . . . . . . . . . . . . . . . . . .   4
     3.2.  DUID  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Client Identifier Option  . . . . . . . . . . . . . . . .   5
     3.4.  IA_NA, IA_TA, IA_PD, IA Address and IA Prefix Options . .   5
     3.5.  Client FQDN Option  . . . . . . . . . . . . . . . . . . .   5
     3.6.  Client Link-layer Address Option  . . . . . . . . . . . .   6
     3.7.  Option Request Option . . . . . . . . . . . . . . . . . .   6
     3.8.  Vendor Class and Vendor-specific Information Options  . .   6
     3.9.  Civic Location Option . . . . . . . . . . . . . . . . . .   7
     3.10. Coordinate-Based Location Option  . . . . . . . . . . . .   7
     3.11. Client System Architecture Type Option  . . . . . . . . .   7
     3.12. Relay Agent Options . . . . . . . . . . . . . . . . . . .   7
       3.12.1.  Subscriber ID Option . . . . . . . . . . . . . . . .   8
       3.12.2.  Interface ID Option  . . . . . . . . . . . . . . . .   8
       3.12.3.  Remote ID Option . . . . . . . . . . . . . . . . . .   8
       3.12.4.  Relay-ID Option  . . . . . . . . . . . . . . . . . .   8
   4.  Existing Mechanisms That Affect Privacy . . . . . . . . . . .   9
     4.1.  Temporary addresses . . . . . . . . . . . . . . . . . . .   9
     4.2.  DNS Updates . . . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Allocation strategies . . . . . . . . . . . . . . . . . .   9
   5.  Attacks . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Device type discovery (fingerprinting)  . . . . . . . . .  11
     5.2.  Operating system discovery (fingerprinting) . . . . . . .  11
     5.3.  Finding location information  . . . . . . . . . . . . . .  11
     5.4.  Finding previously visited networks . . . . . . . . . . .  12
     5.5.  Finding a stable identity . . . . . . . . . . . . . . . .  12
     5.6.  Pervasive monitoring  . . . . . . . . . . . . . . . . . .  12
     5.7.  Finding client's IP address or hostname . . . . . . . . .  13
     5.8.  Correlation of activities over time . . . . . . . . . . .  13
     5.9.  Location tracking . . . . . . . . . . . . . . . . . . . .  13
     5.10. Leasequery & bulk leasequery  . . . . . . . . . . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  14
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     10.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17



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1.  Introduction

   DHCPv6 [RFC3315] is a protocol that is used to provide addressing and
   configuration information to IPv6 hosts.  DHCPv6 uses several
   identifiers that could become a source for gleaning 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].  In particular,
   it also takes into consideration the problem of pervasive monitoring
   [RFC7258].

   Future works may propose protocol changes to fix the privacy issues
   that have been analyzed in this document.  Protocol 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 DHCPv6 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
   DHCPv6 servers and relay agents.

2.  Terminology

   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.

   Naming convention from [RFC3315] and related is used throughout this
   document.  In addition the following terminology is used:

   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, 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.



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3.  Identifiers in DHCPv6 options and fields

   In DHCPv6, there are many options that include identification
   information or that can be used to extract identification information
   about the client.  This section enumerates various options or fields
   and the identifiers conveyed in them, which can be used to disclose
   client identification.  The attacks that are enabled by such
   disclosures are detailed in Section 5.

3.1.  Source IPv6 address

   Although IPv6 link-local address is technically not a part of DHCPv6,
   it appears in the DHCPv6 transmissions, so it is mentioned here for
   completeness.

   If the client does not use privacy extensions (see [RFC4941]) or
   similar solutions and its IPv6 link-local address is based on
   physical link-layer address, this information is disclosed to the
   DHCPv6 server and to anyone who manages to intercept this
   transmission.

   There are multiple cases where IPv6 link-local addresses are used in
   DHCPv6.  Initial client transmissions are always sent from the IPv6
   link-local addresses even when the server unicast option (see
   Sections 22.12 and 18 of [RFC3315] for details) is enabled.  If there
   are relay agents, they forward client's traffic wrapped in Relay-
   forward and store original source IPv6 address in peer-address field.

3.2.  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, defined in
   Section 9.2 of [RFC3315] is DUID-LLT that is based on link-layer
   address.  It 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 link-layer address
   randomization, it is likely that it was not yet enabled during the
   first device boot.  Hence the original, unobfuscated link-layer



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   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).  The exposure of the original link-layer address in
   DUID will also undermine other privacy extensions such as [RFC4941].

3.3.  Client Identifier 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).  See Section 3.2 for relevant discussion
   about DUIDs.

3.4.  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.

   To differentiate between instances of the same type of IA containers
   for a client, each IA_NA, IA_TA and IA_PD options have an IAID field
   with a unique value for a given IA type.  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.

3.5.  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.




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   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.

3.6.  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.

3.7.  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
   options 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.

3.8.  Vendor Class and Vendor-specific Information Options

   The Vendor Class option, defined in Section 22.16 of [RFC3315], is
   used by a DHCPv6 client to identify the vendor that manufactured the
   hardware on which the client is running.

   The Vendor-specific Information option, defined in Section 22.17 of
   [RFC3315], includes enterprise number, which identifies the client's
   vendor and often includes a number of additional parameters that are
   specific to a given vendor.  That may include any type of information
   the vendor deems useful.  It should be noted that this information
   may be present (and different) in both directions: client to server
   and server to client communications.

   The information contained in the data area of this option is
   contained in one or more opaque fields that identify details of the
   hardware configuration, for example, the version of the operating




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   system the client is running or the amount of memory installed on the
   client.

3.9.  Civic Location Option

   DHCPv6 servers use the Civic Location option [RFC4776] to deliver
   location information (the civic and postal addresses) from the DHCPv6
   server to 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.

3.10.  Coordinate-Based Location Option

   The GeoLoc options [RFC6225] are used by DHCPv6 server to provide
   coordinate-based geographic location information to DHCPv6 clients.
   They enable a DHCPv6 client to obtain its location.

3.11.  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 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.

3.12.  Relay Agent Options

   A DHCPv6 relay agent may include a number of options.  Those option
   contain information that can be used to identify the client.  Those
   options are almost exclusively exchanged between the relay agent and
   the server, thus never leaving the operators network.  In particular,
   they're almost never present in the last wireless hop in case of WiFi
   networks.  The only exception to that rule is somewhat infrequently
   used Relay Supplied Options option [RFC6422].  This fact implies that
   the threat model related relay options is slightly different.
   Traffic sniffing at the last hop and related class of attacks
   typically do not apply.  On the other hand, all attacks that involve
   operator's intfrastructure (either willing or coerced cooperation or
   infrastructure being compromised) usually apply.

   The following subsections describe various options inserted by the
   relay agents.








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3.12.1.  Subscriber ID Option

   A DHCPv6 relay may include 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.

3.12.2.  Interface ID Option

   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 sometimes 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.

3.12.3.  Remote ID Option

   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 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

   o  an interface or port identifier

3.12.4.  Relay-ID Option

   Relay agent may include Relay-ID [RFC5460], which contains a unique
   relay agent identifier.  While its intended use is to provide
   additional information for the server, so it would be able to respond
   to leasequeries later, this information can be also used to identify
   client's location within the network.




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4.  Existing Mechanisms That Affect Privacy

   This section describes deployed DHCPv6 mechanisms that can affect
   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 a number of serious issues, both related to protocol and
   its implementations, that make temporary addresses 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, in section 18.1.3 it explicitly mentions that temporary
   addresses can be renewed.  Client implementations may mistakenly
   renew temporary addresses if they are not careful (i.e., by including
   the IA_TA with the same IAID in Renew or Rebind requests, rather than
   a new IAID - see [RFC3315] Section 22.5), thus forfeiting short
   liveness.  [RFC4704] does not explicitly prohibit servers to update
   DNS for assigned temporary addresses and there are implementations
   that can be configured to do that.  However, this is not advised as
   publishing a client's IPv6 address in DNS that is publicly available
   is a major privacy breach.

4.2.  DNS Updates

   The Client FQDN Option[RFC4704] used along with DNS Update [RFC2136]
   defines a mechanism that allows both clients and server to insert
   into the 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



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   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 being
   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 an available pool, it makes
   scanning attacks much easier.

   Identifier-based allocation - some server implementations use a fixed
   identifier for a specific client, seemingly taken from the client's
   MAC address when available or some lower bits of client's source IPv6
   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 a returning client
   is very likely to get the same address, even if the server does not
   retain 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.  The downside of such allocation is
   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 attacks apply.

   Hash allocation - it's 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., can be reversed), it introduces no improvement over
   identifier-based allocation.  Even a well implemented hash does not
   mitigate the threat of correlation over time.

   Random allocation - a server can pick a resource pseudo-randomly out
   of an available pool.  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.,
   the client's address may be randomized, but it still can leak its MAC
   address in the client-id option.



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   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, Vendor-specific Information 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.

5.2.  Operating system discovery (fingerprinting)

   The operating system running on a client can be guessed using the
   Vendor Class option, the Vendor-specific Information 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 physical 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 or GeoLoc option.  It can also be indirectly inferred
   using the Remote ID option, 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).

   Another way to discover client's physical location is to use
   geolocation services.  Those services typically map IP prefixes into
   geographical locations.  Those services are usually based on known
   locations of the subnet, so they may reveal client's location as
   precise as they can locate a network it is connected to.  They
   usually are not able to discover specific physical location within a
   network.  That is not awlays true and it depends on the quality of
   the apriori information available in the geolocation services
   databases.  It should be noted that this threat is general to the
   DHCPv6 mechanism.  Regardless of the allocation strategy used by the
   DHCPv6 server implementation, the addresses assigned will always



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   belong to the subnet the server is configured to manage.  Cases of
   using ULA (Unique Local Addresses) assigned by the DHCPv6 server are
   out of scope for this document.

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).  [RFC3315] does not exclude IA_TA in such a case,
   so it is possible that a client implementation includes an address
   contained in an IA_TA for the Confirm message.  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
   option 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.

   It should be noted that in general case, the MAC addresses as such
   are not available in the DHCPv6 packets.  Therefore they cannot be
   used directly in a reliable way.  However, they may become indirectly
   available using other mechanisms: client-id contains link-local
   address if DUID-LL or DUID-LLT types are used, source IPv6 address
   may use EUI-64 that contains MAC address, some access technologies
   may specify MAC address in dedicated options (e.g., cable modems use
   MAC addresses in DOCSIS options).  Relay agents may insert additional
   information that are used to help the server to identify the client.
   This could be Remote-Id option, Subscriber-Id option, client link-
   layer address option or vendor specific information options.  Options
   inserted by relay agents usually traverse only relay-server path, so
   they typically can't be eavesdropped by intercepting client's
   transmissions.  This depends on the actual deployment model and used
   access technologies.

5.6.  Pervasive monitoring

   Pervasive Monitoring (PM) is widespread (and often covert)
   surveillance through intrusive gathering of protocol artefacts,
   including application content, or protocol metadata such as headers.
   Active or passive wiretaps and traffic analysis, (e.g., correlation,
   timing or measuring packet sizes), or subverting the cryptographic
   keys used to secure protocols can also be used as part of pervasive



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   monitoring.  PM is distinguished by being indiscriminate and very
   large scale, rather than by introducing new types of technical
   compromise.

   See [RFC7258] for a discussion about PM.

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) into the 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.

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 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.  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 the
   client's address and draw conclusions regarding its location and
   movement patterns based on the prefix it is connecting from) and
   active (an attacker can send ICMPv6 echo requests or other probe
   packets to networks of suspected client locations) 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's current location and his habits.






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5.10.  Leasequery & bulk leasequery

   Attackers may masquerade to be an access concentrator, either as a
   DHCPv6 relay agent or as a DHCPv6 client, to obtain location
   information directly from the DHCPv6 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.

   Furthermore, the attackers may use the DHCPv6 bulk leasequery
   [RFC5460] mechanism to obtain bulk information about DHCPv6 bindings,
   even without knowing the target bindings.

   Additionally, active leasequery [RFC7653] is a mechanism for
   subscribing to DHCPv6 lease update changes in near real-time.  The
   intent of this mechanism is to update an operator's database, but if
   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.

6.  Security Considerations

   In current practice, the client privacy and 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
   to the server and the network.

7.  Privacy Considerations

   This document in its entirety discusses privacy considerations in
   DHCPv6.  As such, no dedicated discussion is needed.

8.  IANA Considerations

   This draft does not request any IANA action.

9.  Acknowledgements

   The authors would like to thank Stephen Farrell, Ted Lemon, Ines
   Robles, Russ White, Christian Schaefer, Jinmei Tatuya, Bernie Volz,
   Marcin Siodelski, Christian Huitema, Brian Haberman, Robert Sparks,
   Peter Yee and other members of DHC WG for their valuable comments.




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   This document was produced using the xml2rfc tool [RFC7749].

10.  References

10.1.  Normative 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-08 (work
              in progress), September 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <http://www.rfc-editor.org/info/rfc3315>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <http://www.rfc-editor.org/info/rfc6973>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <http://www.rfc-editor.org/info/rfc7258>.

10.2.  Informative References

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <http://www.rfc-editor.org/info/rfc2136>.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              DOI 10.17487/RFC3633, December 2003,
              <http://www.rfc-editor.org/info/rfc3633>.

   [RFC4580]  Volz, B., "Dynamic Host Configuration Protocol for IPv6
              (DHCPv6) Relay Agent Subscriber-ID Option", RFC 4580,
              DOI 10.17487/RFC4580, June 2006,
              <http://www.rfc-editor.org/info/rfc4580>.



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   [RFC4649]  Volz, B., "Dynamic Host Configuration Protocol for IPv6
              (DHCPv6) Relay Agent Remote-ID Option", RFC 4649,
              DOI 10.17487/RFC4649, August 2006,
              <http://www.rfc-editor.org/info/rfc4649>.

   [RFC4704]  Volz, B., "The Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
              Option", RFC 4704, DOI 10.17487/RFC4704, October 2006,
              <http://www.rfc-editor.org/info/rfc4704>.

   [RFC4776]  Schulzrinne, H., "Dynamic Host Configuration Protocol
              (DHCPv4 and DHCPv6) Option for Civic Addresses
              Configuration Information", RFC 4776,
              DOI 10.17487/RFC4776, November 2006,
              <http://www.rfc-editor.org/info/rfc4776>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,
              <http://www.rfc-editor.org/info/rfc4941>.

   [RFC5007]  Brzozowski, J., Kinnear, K., Volz, B., and S. Zeng,
              "DHCPv6 Leasequery", RFC 5007, DOI 10.17487/RFC5007,
              September 2007, <http://www.rfc-editor.org/info/rfc5007>.

   [RFC5460]  Stapp, M., "DHCPv6 Bulk Leasequery", RFC 5460,
              DOI 10.17487/RFC5460, February 2009,
              <http://www.rfc-editor.org/info/rfc5460>.

   [RFC5970]  Huth, T., Freimann, J., Zimmer, V., and D. Thaler, "DHCPv6
              Options for Network Boot", RFC 5970, DOI 10.17487/RFC5970,
              September 2010, <http://www.rfc-editor.org/info/rfc5970>.

   [RFC6225]  Polk, J., Linsner, M., Thomson, M., and B. Aboba, Ed.,
              "Dynamic Host Configuration Protocol Options for
              Coordinate-Based Location Configuration Information",
              RFC 6225, DOI 10.17487/RFC6225, July 2011,
              <http://www.rfc-editor.org/info/rfc6225>.

   [RFC6355]  Narten, T. and J. Johnson, "Definition of the UUID-Based
              DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355,
              DOI 10.17487/RFC6355, August 2011,
              <http://www.rfc-editor.org/info/rfc6355>.

   [RFC6422]  Lemon, T. and Q. Wu, "Relay-Supplied DHCP Options",
              RFC 6422, DOI 10.17487/RFC6422, December 2011,
              <http://www.rfc-editor.org/info/rfc6422>.




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   [RFC6939]  Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer
              Address Option in DHCPv6", RFC 6939, DOI 10.17487/RFC6939,
              May 2013, <http://www.rfc-editor.org/info/rfc6939>.

   [RFC7653]  Raghuvanshi, D., Kinnear, K., and D. Kukrety, "DHCPv6
              Active Leasequery", RFC 7653, DOI 10.17487/RFC7653,
              October 2015, <http://www.rfc-editor.org/info/rfc7653>.

   [RFC7749]  Reschke, J., "The "xml2rfc" Version 2 Vocabulary",
              RFC 7749, DOI 10.17487/RFC7749, February 2016,
              <http://www.rfc-editor.org/info/rfc7749>.

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

   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











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