dhc S. Jiang
Internet-Draft Huawei Technologies Co., Ltd
Intended status: Informational S. Krishnan
Expires: August 24, 2016 Ericsson
T. Mrugalski
ISC
February 21, 2016
Privacy considerations for DHCP
draft-ietf-dhc-dhcp-privacy-05
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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Copyright (c) 2016 IETF Trust and the persons identified as the
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language and Terminology . . . . . . . . . . . . 3
3. DHCP Options Carrying Identifiers . . . . . . . . . . . . . . 3
3.1. Client Identifier Option . . . . . . . . . . . . . . . . 4
3.2. Address Fields & Options . . . . . . . . . . . . . . . . 4
3.3. Client FQDN Option . . . . . . . . . . . . . . . . . . . 5
3.4. Parameter Request List Option . . . . . . . . . . . . . . 5
3.5. Vendor Class and Vendor-Identifying Vendor Class Options 5
3.6. Civic Location Option . . . . . . . . . . . . . . . . . . 5
3.7. Coordinate-Based Location Option . . . . . . . . . . . . 6
3.8. Client System Architecture Type Option . . . . . . . . . 6
3.9. Relay Agent Information Option and Sub-options . . . . . 6
4. Existing Mechanisms That Affect Privacy . . . . . . . . . . . 7
4.1. DNS Updates . . . . . . . . . . . . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . 9
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 . . . . . . . . . 10
5.8. Correlation of activities over time . . . . . . . . . . . 10
5.9. Location tracking . . . . . . . . . . . . . . . . . . . . 10
5.10. Leasequery & bulk leasequery . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
10.1. Normative References . . . . . . . . . . . . . . . . . . 11
10.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
Dynamic Host Configuration Protocol (DHCP) [RFC2131] is a protocol
that is used to provide addressing and configuration information to
IPv4 hosts. DHCP uses several identifiers that could become a source
for gleaning information about the IPv4 host. This information may
include device type, operating system information, location(s) that
the device may have previously visited, etc. This document discusses
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the various identifiers used by DHCP 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. These changes are out of
scope for this document.
The primary focus of this document is around privacy considerations
for clients to support client mobility and connection to random
networks. The privacy of DHCP servers and relay agents 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.
2. Requirements Language and Terminology
Naming convention from [RFC2131] and related is used throughout this
document.
In addition the following terminology is used:
Stable identifier - Any property disclosed by a DHCP client that
does not change over time or changes very infrequently and is
unique for said client in a given context. Examples include
MAC address, client-id, and a hostname. Some identifiers may
be considered stable only under certain conditions, for
example one client implementation may keep its client-id
stored in stable storage while another may generate it on the
fly and use a different one after each boot. Stable
identifiers may or may not be globally unique.
3. DHCP Options Carrying Identifiers
In DHCP, there are a few options that contain identification
information or that can be used to extract identification information
about the client. This section enumerates various options and the
identifiers conveyed in them, which can be used to disclose client
identification. They are targets of various attacks that are
analyzed in Section 5.
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3.1. Client Identifier Option
The Client Identifier Option [RFC2131] is used to pass an explicit
client identifier to a DHCP 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
'chaddr' field, or another type of identifier, such as a DNS name.
[RFC3315] in Section 9.2 specifies DUID-LLT (Link-layer + time) as
the recommended DUID (DHCP Unique Identifier) type. [RFC4361],
Section 6.1 introduces this concept to DHCP. Those two documents
recommend that client identifiers be generated by using the permanent
link-layer address of the network interface that the client is trying
to configure. [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 non-volatile storage or use the same deterministic
algorithm to generate the same Client Identifier values 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 address will likely end up being announced as
the client identifier, even if the link-layer address has changed (or
even if it is being changed on a periodic basis). The exposure of
the original link-layer address in the client identifier will also
undermine other privacy extensions such as [RFC4941].
3.2. Address Fields & Options
The 'yiaddr' field [RFC2131] in DHCP message is used to convey an
allocated address from the server to the client.
The DHCP specification [RFC2131] provides a way to specify the client
link-layer address in the DHCP message header. A DHCP message header
has 'htype' and 'chaddr' fields to specify the client link-layer
address type and the link-layer address, respectively. The 'chaddr'
field is used both as a hardware address for transmission of reply
messages and as a client identifier.
The 'requested IP address' option [RFC2131] is used by a client to
suggest that a particular IP address be assigned.
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3.3. 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 A and PTR
RRs.
A client can use this option to convey all or part of its domain name
to a DHCP server for the IP-address-to-FQDN mapping. In most 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.
3.4. 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 options requested by the client.
3.5. Vendor Class and Vendor-Identifying Vendor Class Options
The Vendor Class option [RFC2131], the Vendor-Identifying Vendor
Class option, and the Vendor-Identifying Vendor Information option
[RFC3925] are used by the DHCP client to identify the vendor that
manufactured the hardware on which the client is running.
The information contained in the data area of this option is
contained in one or more opaque fields that identify the details of
the hardware configuration of the host on which the client is
running, or of industry consortium compliance, for example, the
version of the operating system the client is running or the amount
of memory installed on the client.
3.6. Civic Location Option
DHCP servers use the Civic Location Option [RFC4776] to deliver
location information (the civic and postal addresses) to DHCP
clients. It may refer to three locations: the location of the DHCP
server, the location of the network element believed to be closest to
the client, or the location of the client, identified by the "what"
element within the option.
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3.7. Coordinate-Based Location Option
The GeoConf and GeoLoc options [RFC6225] are used by a DHCP server to
provide coordinate-based geographic location information to DHCP
clients. They enable a DHCP client to obtain its geographic
location.
3.8. Client System Architecture Type Option
The Client System Architecture Type Option [RFC4578] is used by a
DHCP client to send a list of supported architecture types to the
DHCP server. It is used by clients that must be booted using the
network rather than from local storage, so the server can decide
which boot file should be provided to the client.
3.9. Relay Agent Information Option and Sub-options
A DHCP relay agent includes a Relay Agent Information option[RFC3046]
to identify the remote host end of the circuit. It contains a
"circuit ID" 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.
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. Also, assuming network
topology knowledge, it also reveals client location.
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
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different paths, and as network changes occur. The Subscriber-ID is
an ASCII string, which is assigned and configured by the network
provider.
4. Existing Mechanisms That Affect Privacy
This section describes deployed DHCP mechanisms that affect privacy.
4.1. DNS Updates
The Client FQDN (Fully Qualified Domain Name) Option [RFC4702] used
along with DNS Updates [RFC2136] defines a mechanism that allows both
clients and server to insert into the DNS domain information about
clients. Both forward (A) and reverse (PTR) resource records can be
updated. This allows other nodes to conveniently refer to a host,
despite the fact that its IP address may be changing.
This mechanism exposes two important pieces of information: current
address (which can be mapped to current location) and client's
hostname. The stable hostname can then be used to correlate the
client across different network attachments even when its IP
addresses keep changing.
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. When a client requests an
address, the server must pick an address out of a configured pool.
Depending on the server's implementation, various allocation
strategies are possible. Choices in this regard may have privacy
implications. Note that the constraints in DHCP and DHCPv6 are
radically different, but servers that allow allocation strategy
configuration may allow configuring them in both DHCP and DHCPv6.
Not every allocation strategy is equally suitable for DHCP and for
DHCPv6.
Iterative allocation - a server may choose to allocate addresses one
by one. That strategy has the benefit of being very fast, thus being
favored in deployments that prefer performance. However, it makes
the allocated addresses very predictable. Also, since the addresses
allocated tend to be clustered at the beginning of an available pool,
it makes scanning attacks much easier.
Identifier-based allocation - some server implementations may choose
to allocate an address that is based on one of the available
identifiers, e.g., client identifier or MAC address. It is also
convenient, as a returning client is very likely to get the same
address. Those properties are convenient for system administrators,
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so DHCP server implementers are often requested to implement it. 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 a much smaller
address space is almost never a problem in DHCP.
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., it can be reversed), it introduces no improvement over
identifier-based allocation.
Random allocation - a server can pick a resource randomly out of an
available pool. This allocation scheme essentially prevents
returning clients from getting the same address again. On the other
hand, it is beneficial from a privacy perspective as addresses
generated that way are not susceptible to correlation attacks, OS/
vendor discovery attacks, or identity discovery attacks. Note that
even though the address itself may be resilient to a given attack,
the client may still be susceptible if additional information is
disclosed other way, e.g., the client's address may be randomized,
but it still can leak its MAC address in the client-id option.
Other allocation strategies may be implemented.
Given the limited size of most IPv4 public address pools, allocation
mechanisms in IPv4 may not provide much privacy protection or leak
much useful information, if misused.
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 an
Organizationally Unique Identifier (OUI) 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
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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 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).
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
option can serve as long-lived identifiers of DHCP clients. The
Client FQDN option can also provide an identity that can easily be
correlated with web server activity logs.
5.6. Pervasive monitoring
Pervasive Monitoring [RFC7258] is widespread (and often covert)
surveillance through intrusive gathering of protocol artefacts,
including application content, or protocol metadata such as headers.
An operator who controls a non-trivial number of access points or
network segments, may use obtained information about a single client
and observe the client's habits. Although users may not expect true
privacy from their operators, the information that is set up to be
monitored by users' service operators may also be gathered by an
adversary who monitors a wide range of networks and develops
correlations from that information.
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5.7. Finding client's IP address or hostname
Many DHCP deployments use DNS Updates [RFC4702] that put a client's
information (current IP address, client's hostname) into the DNS,
where it is easily accessible by anyone interested. Client ID is
also disclosed, albeit in not easily accessible form (SHA-256 digest
of the client-id). As SHA-256 is considered irreversible, DHCP
client ID can't be converted back to client-id. However, SHA-256
digest can be used as an 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 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.
5.9. Location tracking
If a stable identifier is used for assigning an address and such
mapping is discovered by an attacker, it can be used for tracking a
user. In particular both passive (a service that the client connects
to can log the client's address and draw conclusions regarding its
location and movement patterns based on the addresses it is
connecting from) and active (an attacker can send ICMP echo requests
or other probe packets to networks of suspected client locations)
methods can be used. To give 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.
5.10. Leasequery & bulk leasequery
Attackers may pretend to be an access concentrator, either as a DHCP
relay agent or as a DHCP client, to obtain location information
directly from the DHCP server(s) using the DHCP leasequery [RFC4388]
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 the DHCP bulk leasequery [RFC6926]
mechanism to obtain bulk information about DHCP bindings, even
without knowing the target bindings.
Additionally, active leasequery [RFC7724] is a mechanism for
subscribing to DHCP lease update changes in near real-time. The
intent of this mechanism is to update an operator's database, 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
DHCP. 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 the valuable comments made by Stephen
Farrell, Ted Lemon, Ines Robles, Russ White, Christian Huitema,
Bernie Volz, Jinmei Tatuya, Marcin Siodelski, Christian Schaefer,
Robert Sparks, Peter Yee, and other members of DHC WG.
This document was produced using the xml2rfc tool [RFC7749].
10. References
10.1. Normative References
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<http://www.rfc-editor.org/info/rfc2131>.
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[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>.
[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
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, DOI 10.17487/RFC3046, January 2001,
<http://www.rfc-editor.org/info/rfc3046>.
[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>.
[RFC3527] Kinnear, K., Stapp, M., Johnson, R., and J. Kumarasamy,
"Link Selection sub-option for the Relay Agent Information
Option for DHCPv4", RFC 3527, DOI 10.17487/RFC3527, April
2003, <http://www.rfc-editor.org/info/rfc3527>.
[RFC3925] Littlefield, J., "Vendor-Identifying Vendor Options for
Dynamic Host Configuration Protocol version 4 (DHCPv4)",
RFC 3925, DOI 10.17487/RFC3925, October 2004,
<http://www.rfc-editor.org/info/rfc3925>.
[RFC3993] Johnson, R., Palaniappan, T., and M. Stapp, "Subscriber-ID
Suboption for the Dynamic Host Configuration Protocol
(DHCP) Relay Agent Option", RFC 3993,
DOI 10.17487/RFC3993, March 2005,
<http://www.rfc-editor.org/info/rfc3993>.
[RFC4361] Lemon, T. and B. Sommerfeld, "Node-specific Client
Identifiers for Dynamic Host Configuration Protocol
Version Four (DHCPv4)", RFC 4361, DOI 10.17487/RFC4361,
February 2006, <http://www.rfc-editor.org/info/rfc4361>.
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[RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration
Protocol (DHCP) Leasequery", RFC 4388,
DOI 10.17487/RFC4388, February 2006,
<http://www.rfc-editor.org/info/rfc4388>.
[RFC4578] Johnston, M. and S. Venaas, Ed., "Dynamic Host
Configuration Protocol (DHCP) Options for the Intel
Preboot eXecution Environment (PXE)", RFC 4578,
DOI 10.17487/RFC4578, November 2006,
<http://www.rfc-editor.org/info/rfc4578>.
[RFC4702] Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host
Configuration Protocol (DHCP) Client Fully Qualified
Domain Name (FQDN) Option", RFC 4702,
DOI 10.17487/RFC4702, October 2006,
<http://www.rfc-editor.org/info/rfc4702>.
[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>.
[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>.
[RFC6926] Kinnear, K., Stapp, M., Desetti, R., Joshi, B., Russell,
N., Kurapati, P., and B. Volz, "DHCPv4 Bulk Leasequery",
RFC 6926, DOI 10.17487/RFC6926, April 2013,
<http://www.rfc-editor.org/info/rfc6926>.
[RFC7724] Kinnear, K., Stapp, M., Volz, B., and N. Russell, "Active
DHCPv4 Lease Query", RFC 7724, DOI 10.17487/RFC7724,
December 2015, <http://www.rfc-editor.org/info/rfc7724>.
[RFC7749] Reschke, J., "The "xml2rfc" Version 2 Vocabulary",
RFC 7749, DOI 10.17487/RFC7749, February 2016,
<http://www.rfc-editor.org/info/rfc7749>.
Jiang, et al. Expires August 24, 2016 [Page 13]
Internet-Draft DHCP Privacy considerations February 2016
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
Email: tomasz.mrugalski@gmail.com
Jiang, et al. Expires August 24, 2016 [Page 14]
