dhc S. Jiang Internet-Draft Huawei Technologies Co., Ltd Intended status: Informational S. Krishnan Expires: July 21, 2016 Ericsson T. Mrugalski ISC January 18, 2016 Privacy considerations for DHCPv4 draft-ietf-dhc-dhcp-privacy-03 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 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any 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 July 21, 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 Provisions Relating to IETF Documents (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 include Simplified BSD License text as described in Section 4.e of Jiang, et al. Expires July 21, 2016 [Page 1] Internet-Draft DHCP Privacy considerations January 2016 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 . . . . . . . . . . . . . . . 9 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. The DHCP protocol uses several identifiers that could become a source for gleaning information about the IPv4 host. This information may include device type, operating system information, location(s) that the device may have previously visited, etc. This Jiang, et al. Expires July 21, 2016 [Page 2] Internet-Draft DHCP Privacy considerations January 2016 document discusses 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. It is 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 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. 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 may include MAC address, client-id or a hostname. Some identifiers may be considered stable only under certain conditions, for example one client implementation may keep its client-id stored in stable storage while other may generate it on the fly and use a different one after each boot. Stable identifier may or may not be globally unique. 3. DHCP Options Carrying Identifiers In DHCP, there are a few options which contain identification information or can be used to extract the identification information about the client. This section enumerates various options and identifiers conveyed in them, which can be used to disclose client identification. They are targets of various attacks that would be analyzed in Section 5. Jiang, et al. Expires July 21, 2016 [Page 3] Internet-Draft DHCP Privacy considerations January 2016 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 DHCPv4. Those two document recommend that client identifiers be generated by using the permanent link-layer address of the network interface that the client is trying to configure. [RFC4361] updates the recommendation of Client Identifiers to be "consists of a type field whose value is normally 255, followed by a four-byte IA_ID field, followed by the DUID for the client as defined in RFC 3315, section 9". This does not change the lifecycle of the Client Identifiers. Clients are expected to generate their Client Identifiers once (during first operation) and store it in a non-volatile storage or use the same deterministic algorithm to generate the same Client Identifier values again. 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 disabled during the first device boot. Hence the original, unobfuscated link-layer address will likely end up being announced as client identifier, 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 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 allocated address from the server to the client. The DHCPv4 specification [RFC2131] provides a way to specify the client link-layer address in the DHCPv4 message header. A DHCPv4 message header has 'htype' and 'chaddr' fields to specify the client link-layer address type and the link-layer address, respectively. The 'chaddr' field is used both as a hardware address for transmission of reply messages and as a client identifier. The 'requested IP address' option [RFC2131] is used by client to suggest that a particular IP address be assigned. Jiang, et al. Expires July 21, 2016 [Page 4] Internet-Draft DHCP Privacy considerations January 2016 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 an option 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 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 of the location information (the civic and postal addresses) to the DHCP clients. It may refer to three locations: the location of the DHCP server, the location of the network element believed to be closest to the client, or the location of the client, identified by the "what" element within the option. Jiang, et al. Expires July 21, 2016 [Page 5] Internet-Draft DHCP Privacy considerations January 2016 3.7. Coordinate-Based Location Option The GeoConf and GeoLoc options [RFC6225] is used by DHCP server to provide the coordinate-based geographic location information to the DHCP clients. It enables a DHCP client to obtain its geographic location. 3.8. Client System Architecture Type Option The Client System Architecture Type Option [RFC4578] is used by DHCP client to send a list of supported architecture types to the DHCP server. It is used 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 [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 Jiang, et al. Expires July 21, 2016 [Page 6] Internet-Draft DHCP Privacy considerations January 2016 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 configured pool. Depending on the server's implementation, various allocation strategies are possible. Choices in this regard may have privacy implications. Note that the constraints in DHCPv4 and DHCPv6 are radically different, but servers that allow allocation strategy configuration may allow configuring them in both DHCPv4 and DHCPv6. Not every allocation strategy is equally suitable for DHCPv4 and for DHCPv6. Iterative allocation - a server may choose to allocate addresses one by one. That strategy has the benefit of being very fast, thus can be favored in deployments that prefer performance. However, it makes the allocated addresses very predictable. Also, since the addresses allocated tend to be clustered at the beginning of available pool, it makes scanning attacks much easier. Identifier-based allocation - some server implementations may choose to allocate an address that is based on one of available identifiers, e.g. client identifier or MAC address. It is also convenient, as returning client is very likely to get the same address. Those properties are convenient for system administrators, so DHCP server Jiang, et al. Expires July 21, 2016 [Page 7] Internet-Draft DHCP Privacy considerations January 2016 implementors 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 much smaller address space is almost never a problem in DHCPv4. Hash allocation - it's an extension of identifier based allocation. Instead of using the identifier directly, it is being hashed first. If the hash is implemented correctly, it removes the flaw of disclosing the identifier, a property that eliminates susceptibility to address scanning and OS/vendor discovery. If the hash is poorly implemented (e.g. can be reverted), it introduces no improvement over identifier-based allocation. Random allocation - a server can pick a resource randomly out of available pool. That strategy works well in scenarios where pool utilization is small, as the likelihood of collision (resulting in the server needing to repeat randomization) is small. With the pool allocation increasing, the collision is disproportionally large, due to birthday paradox. With high pool utilization (e.g. when 90% of available resources being allocated already), the server will use most computational resources to repeatedly pick a random resource, which will degrade its performance. This allocation scheme essentially prevents returning clients from getting the same address again. On the other hand, it is beneficial from privacy perspective as addresses generated that way are not susceptible to correlation attacks, OS/vendor discovery attacks or identity discovery attacks. Note that even though the address itself may be resilient to a given attack, the client may still be susceptible if additional information is disclosed other way, e.g. client's address can be randomized, but it still can leak its MAC address in client-id option. Other allocation strategies may be implemented. 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 Jiang, et al. Expires July 21, 2016 [Page 8] Internet-Draft DHCP Privacy considerations January 2016 vendor. That knowledge can be used for device-specific vulnerability exploitation attacks. 5.2. Operating system discovery The operating system running on a client can be guessed using the Vendor Class option, the Client System Architecture Type option, or by using fingerprinting techniques on the combination of options requested using the Parameter Request List option. 5.3. Finding location information The location information can be obtained by the attacker by many means. The most direct way to obtain this information is by looking into a 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 options can serve as long lived identifiers of DHCP clients. The Client FQDN option can also provide an identity that can easily be correlated with web server activity logs. 5.6. Pervasive monitoring This is an enhancement, or a combination of most aforementioned mechanisms. Operator who controls non-trivial number of access points or network segments, may use obtained information about a single client and observer client's habits. Jiang, et al. Expires July 21, 2016 [Page 9] Internet-Draft DHCP Privacy considerations January 2016 5.7. Finding client's IP address or hostname Many DHCP deployments use DNS Updates [RFC4702] that put client's information (current IP address, client's hostname) into DNS, where it is easily accessible by anyone interested. Client ID is also disclosed, albeit in not easily accessible form (SHA-256 digest of the client-id). As SHA-256 is considered irreversible, DHCID can't be converted back to client-id. However, SHA-256 digest can be used as an unique identifier that is accessible by any host. 5.8. Correlation of activities over time As with other identifiers, an IP address can be used to correlate the activities of a host for at least as long as the lifetime of the address. If that address was generated from some other, stable identifier and that generation scheme can be deducted by an attacker, the duration of correlation attack extends to that identifier. In many cases, its lifetime is equal to the lifetime of the device itself. 5.9. Location tracking If a stable identifier is used for assigning an address and such mapping is discovered by an attacker, e.g. a hostname being put into DNS, it can be used for tracking user. In particular both passive (a service that the client connects to can log client's address and draw conclusions regarding its location and movement patterns based on address it is connecting from) and active (attacker can send ICMP echo requests or other probe packets to networks of suspected client locations) methods can be used. To give specific example, by accessing a social portal from tomek- laptop.coffee.somecity.com.example, tomek- laptop.mycompany.com.example and tomek-laptop.myisp.example.com, the portal administrator can draw conclusions about tomek-laptop's owner current location and his habits. 5.10. Leasequery & bulk leasequery Attackers may pretend as an access concentrator, either DHCP relay agent or DHCP client, to obtain location information directly from the DHCP server(s) using the DHCP leasequery [RFC4388] 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. Jiang, et al. Expires July 21, 2016 [Page 10] Internet-Draft DHCP Privacy considerations January 2016 Furthermore, the attackers may use 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 DHCPv4 lease update changes in near real-time. The intent of this mechanism is to update operator's database, but if misused, an attacker could defeat server's authentication mechanisms and subscribe to all updates. He then could continue receiving updates, without any need for local presence. 6. Security Considerations In current practice, the client privacy and the client authentication are mutually exclusive. The client authentication procedure reveals additional client information in their certificates/identifiers. Full privacy for the clients may mean the clients are also anonymous for the server and the network. 7. Privacy Considerations This document at 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 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, DOI 10.17487/RFC2119, March 1997, . Jiang, et al. Expires July 21, 2016 [Page 11] Internet-Draft DHCP Privacy considerations January 2016 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, March 1997, . [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, . [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, . [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014, . 10.2. Informative References [RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, DOI 10.17487/RFC2629, June 1999, . [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, DOI 10.17487/RFC3046, January 2001, . [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, . [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, . [RFC3925] Littlefield, J., "Vendor-Identifying Vendor Options for Dynamic Host Configuration Protocol version 4 (DHCPv4)", RFC 3925, DOI 10.17487/RFC3925, October 2004, . Jiang, et al. Expires July 21, 2016 [Page 12] Internet-Draft DHCP Privacy considerations January 2016 [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, . [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, . [RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration Protocol (DHCP) Leasequery", RFC 4388, DOI 10.17487/RFC4388, February 2006, . [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, . [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, . [RFC4776] Schulzrinne, H., "Dynamic Host Configuration Protocol (DHCPv4 and DHCPv6) Option for Civic Addresses Configuration Information", RFC 4776, DOI 10.17487/RFC4776, November 2006, . [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007, . [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, . Jiang, et al. Expires July 21, 2016 [Page 13] Internet-Draft DHCP Privacy considerations January 2016 [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, . [RFC7724] Kinnear, K., Stapp, M., Volz, B., and N. Russell, "Active DHCPv4 Lease Query", RFC 7724, DOI 10.17487/RFC7724, December 2015, . 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 July 21, 2016 [Page 14]