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rfc:rfc7724

Internet Engineering Task Force (IETF) K. Kinnear Request for Comments: 7724 M. Stapp Updates: 6926 B. Volz Category: Standards Track Cisco Systems ISSN: 2070-1721 N. Russell

                                                               Staples
                                                         December 2015
                     Active DHCPv4 Lease Query

Abstract

 The Dynamic Host Configuration Protocol for IPv4 (DHCPv4) has been
 extended with a Leasequery capability that allows a requestor to
 request information about DHCPv4 bindings (RFC 4388).  That mechanism
 is limited to queries for individual bindings.  In some situations,
 individual binding queries may not be efficient, or even possible.
 In addition, continuous update of an external requestor with
 Leasequery data is sometimes desired.  This document expands on the
 DHCPv4 Leasequery protocol, and allows for active transfer of near
 real-time DHCPv4 binding information data via TCP.  This document
 updates RFC 6926, "DHCPv4 Bulk Leasequery".

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7724.

Kinnear, et al. Standards Track [Page 1] RFC 7724 Active DHCPv4 Lease Query December 2015

Copyright Notice

 Copyright (c) 2015 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
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Kinnear, et al. Standards Track [Page 2] RFC 7724 Active DHCPv4 Lease Query December 2015

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
 3.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
 4.  Interaction Between Active Leasequery and Bulk Leasequery . .   8
 5.  Message and Option Definitions  . . . . . . . . . . . . . . .   9
   5.1.  Message Framing for TCP . . . . . . . . . . . . . . . . .   9
   5.2.  New or Changed Options  . . . . . . . . . . . . . . . . .   9
     5.2.1.  dhcp-message-type . . . . . . . . . . . . . . . . . .  10
     5.2.2.  dhcp-status-code  . . . . . . . . . . . . . . . . . .  10
   5.3.  Connection and Transmission Parameters  . . . . . . . . .  11
 6.  Information Communicated by Active Leasequery . . . . . . . .  11
 7.  Requestor Behavior  . . . . . . . . . . . . . . . . . . . . .  12
   7.1.  General Processing  . . . . . . . . . . . . . . . . . . .  12
   7.2.  Initiating a Connection . . . . . . . . . . . . . . . . .  13
   7.3.  Forming an Active Leasequery  . . . . . . . . . . . . . .  14
   7.4.  Processing Active Replies . . . . . . . . . . . . . . . .  15
     7.4.1.  Processing Replies from a Request Containing a
             query-start-time  . . . . . . . . . . . . . . . . . .  17
   7.5.  Closing Connections . . . . . . . . . . . . . . . . . . .  19
 8.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . .  19
   8.1.  Accepting Connections . . . . . . . . . . . . . . . . . .  19
     8.1.1.  Update to RFC 6926  . . . . . . . . . . . . . . . . .  21
   8.2.  Replying to an Active Leasequery  . . . . . . . . . . . .  21
   8.3.  Multiple or Parallel Queries  . . . . . . . . . . . . . .  23
   8.4.  Closing Connections . . . . . . . . . . . . . . . . . . .  24
 9.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
 11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
   11.1.  Normative References . . . . . . . . . . . . . . . . . .  26
   11.2.  Informative References . . . . . . . . . . . . . . . . .  27
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  27
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1. Introduction

 The DHCPv4 Leasequery capability [RFC4388] extends the basic DHCPv4
 capability [RFC2131] [RFC2132] to allow an external entity to query a
 DHCPv4 server to recover lease state information about a particular
 IPv4 address or client in near real-time.
 Continuous update of an external requestor with Leasequery data is
 sometimes desired.  These requestors need to keep up with the current
 binding activity of the DHCPv4 server.  Keeping up with these binding
 activities is termed "active" leasequery.

Kinnear, et al. Standards Track [Page 3] RFC 7724 Active DHCPv4 Lease Query December 2015

 The DHCPv4 Bulk Leasequery [RFC6926] capability can be used to
 recover useful information from a DHCPv4 server when some external
 entity starts up.  This entity could be one that is directly involved
 in the DHCPv4 client-server transactions (e.g., a relay agent), or it
 could be an external process that needs information present in the
 DHCPv4 server's lease state database.
 The Active Leasequery capability documented here is designed to allow
 an entity not directly involved in DHCPv4 client-server transactions
 to nevertheless keep current with the state of the DHCPv4 lease state
 information in real-time.
 This document updates DHCPv4 Bulk Leasequery [RFC6926] in that it
 specifies the DHCPv4 server must close the TCP connection if it
 receives a DHCPv4 message that is not allowed over the TCP connection
 (for example, DHCPDISCOVER, DHCPLEASEQUERY).  See Section 8.1.1.

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].
 This document uses the following terms:
 o  "Active Leasequery"
    Keeping up to date in real-time (or near real-time) with DHCPv4
    binding activity.
 o  "binding"
    The information that a DHCPv4 server keeps regarding the
    relationship between a DHCPv4 client and an IPv4 address.  This
    includes the identity of the DHCPv4 client and the expiration
    time, if any, of any lease that client has on a particular IPv4
    address.
 o  "Bulk Leasequery"
    Requesting and receiving the information about all or some of the
    existing DHCPv4 binding information in an efficient manner, as
    defined by [RFC6926].

Kinnear, et al. Standards Track [Page 4] RFC 7724 Active DHCPv4 Lease Query December 2015

 o  "blocked TCP connection"
    A TCP connection is considered blocked if the underlying TCP
    transport will not accept new messages to be sent without blocking
    the thread that is attempting to send the message.
 o  "catch-up information"
    If a DHCPv4 Active Leasequery requestor sends in a query-start-
    time option in a DHCPACTIVELEASEQUERY message, the DHCPv4 server
    will attempt to send the requestor the information that changed
    since the time specified in the query-start-time option.  The
    binding information sent to satisfy this request is the catch-up
    information.
 o  "catch-up phase"
    The period while the catch-up information is being sent is the
    catch-up phase.
 o  "clock skew"
    The difference between the absolute time on a DHCPv4 server and
    the absolute time on the system where a requestor of an Active or
    Bulk Leasequery is executing is termed the "clock skew" for that
    Active or Bulk Leasequery connection.  It is not absolutely
    constant but is likely to vary only slowly.  While it is easy to
    think that this can be calculated precisely after one packet is
    received by a requestor from a DHCPv4 server, a more accurate
    value is derived from continuously examining the instantaneous
    value developed from each packet received from a DHCPv4 server and
    using it to make small adjustments to the existing value held in
    the requestor.
 o  "DHCPv4 client"
    A DHCPv4 client is an IPv4 node using DHCP to obtain configuration
    parameters such as a network address.
 o  "DHCPv4 relay agent"
    A DHCPv4 relay agent is a third-party agent that transfers BOOTP
    and DHCPv4 messages between clients and servers residing on
    different subnets, per [RFC951] and [RFC1542].

Kinnear, et al. Standards Track [Page 5] RFC 7724 Active DHCPv4 Lease Query December 2015

 o  "DHCPv4 server"
    A DHCPv4 server is an IPv4 node that returns configuration
    parameters to DHCPv4 clients.
 o  "insecure mode"
    When operating in insecure mode, the TCP connection between the
    requestor and DHCPv4 server is not protected in any way.  In
    addition, the identity of the requestor is not validated by the
    server nor is the identity of the server validated by the
    requestor.
 o  "MAC address"
    In the context of a DHCP message, a Media Access Control (MAC)
    address consists of the fields: hardware type "htype", hardware
    length "hlen", and client hardware address "chaddr".
 o  "requestor"
    The node that sends LEASEQUERY messages to one or more servers to
    retrieve information on the bindings for a client.
 o  "secure mode"
    When operating in secure mode, the TCP connection between the
    requestor and the DHCPv4 server is protected by TLS [RFC5246].  In
    addition, the requestor uses the certificates exchanged between it
    and the DHCPv4 server while setting up the TLS connection to
    validate the identity of the server.  The DHCPv4 server also uses
    these certificates to validate the identity of the requestor.

3. Protocol Overview

 The Active Leasequery mechanism is modeled on the existing individual
 Leasequery protocol in [RFC4388] as well as related work on DHCPv4
 Bulk Leasequery [RFC6926]; most differences arise from the long-term
 nature of the TCP [RFC7414] connection required for Active
 Leasequery.  In addition, a DHCPv4 server that supports Active
 Leasequery must support Bulk Leasequery [RFC6926] as well.  See
 Section 8.
 An Active Leasequery requestor opens a TCP connection to a DHCPv4
 Server, using the DHCPv4 port 67.  Note that this implies that the
 Leasequery requestor has the server IPv4 address(es) available via
 configuration or some other means, and that it has unicast IP

Kinnear, et al. Standards Track [Page 6] RFC 7724 Active DHCPv4 Lease Query December 2015

 reachability to the DHCPv4 server.  The message framing for TCP is
 discussed in Section 5.1.  No relaying for Active Leasequery is
 specified.
 After establishing a connection, the requestor sends an
 DHCPACTIVELEASEQUERY message over the connection.  In response, the
 server sends updates to the requestor using DHCPLEASEACTIVE and
 DHCPLEASEUNASSIGNED messages that are extensions of these messages as
 defined in [RFC4388] and [RFC6926].  This response procedure is
 similar to the procedure specified in [RFC6926], except that in the
 case of Active Leasequery the server sends updates whenever some
 activity occurs to change the binding state -- thus the need for the
 long-lived connection.  Additionally, the Active Leasequery server
 should provide a mechanism to control which data is allowed to be
 included in the messages sent to the requestor.  See Section 8.2.
 Since [RFC6926] did not specify what to do with an unknown message
 type received over the DHCP TCP connection, system administrators
 SHOULD NOT allow a DHCPACTIVELEASEQUERY message to be sent over a
 DHCP TCP connection to a DHCPv4 server that does not support Active
 Leasequery.
 Active Leasequery is designed to provide continuous updates of DHCPv4
 binding activity to an external entity.
 Active Leasequery has features that allow this external entity to
 lose its connection and then reconnect and receive the latest
 information concerning any IPv4 bindings changed while it was not
 connected.
 These capabilities are designed to allow the Active Leasequery
 requestor to efficiently become current with respect to the lease
 state database after it has been restarted or the machine on which it
 is running has been reinitialized.  It is easy to define a protocol
 that works when the requestor is always connected to the DHCPv4
 server.  Since that isn't sufficiently robust, much of the mechanism
 in this document is designed to deal efficiently with situations that
 occur when the Active Leasequery requestor becomes disconnected from
 the DHCPv4 server from which it is receiving updates and then becomes
 reconnected to that server.
 Central to this approach is the concept that, if the Active
 Leasequery requestor loses service, it is allowed to specify the time
 of its most recent update in a subsequent Active Leasequery request,
 and the DHCPv4 server will determine whether or not data was missed
 while the Active Leasequery requestor was not connected.

Kinnear, et al. Standards Track [Page 7] RFC 7724 Active DHCPv4 Lease Query December 2015

 The DHCP server processing the Active Leasequery request MAY limit
 the amount of data saved, and methods exist for the DHCPv4 server to
 inform the Active Leasequery requestor that more data was missed than
 could be saved.  In this situation, the Active Leasequery requestor
 would issue a Bulk Leasequery [RFC6926] to recover information not
 available through an Active Leasequery.
 DHCPv4 servers are not required to keep any data corresponding to
 data missed on an Active Leasequery connection, but will typically
 choose to keep data corresponding to some recent activity available
 for subsequent queries by a DHCPv4 Active Leasequery requestor whose
 connection was temporarily interrupted.
 An Active Leasequery requestor would typically use Bulk Leasequery to
 initialize its database with all current data when that database
 contains no binding information.  In addition, it would use Bulk
 Leasequery to recover missed information in the event that its
 connection with the DHCPv4 server was lost for a longer time than the
 DHCPv4 server would keep track of the specific changes to the IPv4
 binding information.
 The messages sent by the server in response to an Active Leasequery
 request should be identical to the messages sent by the server to a
 Bulk Leasequery request regarding the way the data is encoded into
 the Active Leasequery responses.  In addition, the actions taken by
 the Active Leasequery requestor to interpret the responses to an
 Active Leasequery request should be identical to the way that the
 requestor interprets the responses to a Bulk Leasequery request.
 Thus, the handling of time, clock skew, data source, and other items
 discussed in the Bulk Leasequery specification [RFC6926] are to be
 followed when implementing Active Leasequery, with the exception that
 a server responding to an Active Leasequery request SHOULD be able to
 be configured to prevent specific data items from being included in
 the response to the requestor even if they were requested by
 inclusion in the dhcp-parameter-request-list option.

4. Interaction between Active Leasequery and Bulk Leasequery

 Active Leasequery is an extension of the Bulk Leasequery protocol
 [RFC6926].  The contents of messages returned to an Active Leasequery
 requestor are identical to those defined for the Bulk Leasequery
 protocol.
 Applications that employ Active Leasequery to keep a database up to
 date with respect to the DHCPv4 server's lease state database should
 use an initial Bulk Leasequery to bring their database into

Kinnear, et al. Standards Track [Page 8] RFC 7724 Active DHCPv4 Lease Query December 2015

 equivalence with that of the DHCPv4 server, and then use Active
 Leasequery to keep that database current with respect to the DHCPv4
 server's lease state database.
 There are several differences between the Active and Bulk Leasequery
 protocols.  Active Leasequery defines only one qualifier (the query-
 start-time) and no query types, while Bulk Leasequery defines several
 query types and qualifiers.  An Active Leasequery connection sends
 all available updates to the requestor.
 An Active Leasequery connection does not ever "complete", though the
 DHCPv4 server can close the connection for a variety of reasons
 associated with some sort of exception condition.

5. Message and Option Definitions

5.1. Message Framing for TCP

 The use of TCP for the Active Leasequery protocol permits one or more
 DHCPv4 messages to be sent in response to a single Active Leasequery
 request.  The receiver needs to be able to determine how large each
 message is.  The same framing technique used for Bulk Leasequery
 [RFC6926] is used for Active Leasequery.
 When using TLS to secure a connection [RFC5246], the message framing
 for TLS uses the same format as that used for TCP.  One DHCP message
 is carried in one TLS record.

5.2. New or Changed Options

 The existing messages DHCPLEASEUNASSIGNED and DHCPLEASEACTIVE are
 used as the value of the dhcp-message-type option to indicate an IPv4
 address that is currently not leased or is currently leased to a
 DHCPv4 client, respectively.
 All of the message types and options defined for Bulk Leasequery
 [RFC6926] are also used by Active Leasequery.  In addition, new
 message types and option types are defined for Active Leasequery, as
 described below.

Kinnear, et al. Standards Track [Page 9] RFC 7724 Active DHCPv4 Lease Query December 2015

5.2.1. dhcp-message-type

 The message type option (option 53) from [RFC2132] requires
 additional values.  The values of these message types are shown below
 in an extension of the table from Section 9.6 of [RFC2132]:
                   +-------+----------------------+
                   | Value | Message Type         |
                   +-------+----------------------+
                   | 16    | DHCPACTIVELEASEQUERY |
                   | 17    | DHCPLEASEQUERYSTATUS |
                   | 18    | DHCPTLS              |
                   +-------+----------------------+

5.2.2. dhcp-status-code

 The dhcp-status-code option defined in [RFC6926] allows greater
 detail to be returned regarding the status of a DHCP request.  While
 specified in the Bulk Leasequery document, this DHCPv4 option is also
 used in Active Leasequery.
 This option has two possible scopes when used with Active Leasequery,
 depending on the context in which it appears.  It refers to the
 information in a single leasequery reply if the value of the dhcp-
 message-type is DHCPLEASEACTIVE, DHCPLEASEUNASSIGNED, or DHCPTLS.  It
 refers to the message stream related to an entire request if the
 value of the dhcp-message-type is DHCPLEASEQUERYSTATUS.
 Additional status codes defined for support of Active Leasequery are:
 +----------------------+-------------+------------------------------+
 | Name                 | Status-Code | Description                  |
 +----------------------+-------------+------------------------------+
 | DataMissing          | 5           | Indicates that IPv4 binding  |
 |                      |             | information requested is not |
 |                      |             | available.                   |
 | ConnectionActive     | 6           | Indicates that this          |
 |                      |             | connection remains active.   |
 | CatchUpComplete      | 7           | Indicates that this Active   |
 |                      |             | Leasequery connection has    |
 |                      |             | completed sending all of the |
 |                      |             | saved data requested.        |
 | TLSConnectionRefused | 8           | Indicates that a TLS         |
 |                      |             | connection is not allowed.   |
 +----------------------+-------------+------------------------------+

Kinnear, et al. Standards Track [Page 10] RFC 7724 Active DHCPv4 Lease Query December 2015

 A dhcp-status-code option MAY appear in the options field of a DHCP
 message.  If the dhcp-status-code option does not appear, it is
 assumed that the operation was successful.  The dhcp-status-code
 option SHOULD NOT appear in a message that is successful unless it is
 needed to convey some text message along with the Success status
 code.

5.3. Connection and Transmission Parameters

 Active Leasequery uses the same port configuration as DHCPv4 Bulk
 Leasequery [RFC6926].  It also uses other transmission parameters
 (BULK_LQ_DATA_TIMEOUT and BULK_LQ_MAX_CONNS) as defined in [RFC6926].
 This section presents a table of values used to control Active
 Leasequery behavior, including recommended defaults.  Implementations
 MAY make these values configurable.  However, configuring too-small
 timeout values may lead to harmful behavior both to this application
 as well as to other traffic in the network.  As a result, timeout
 values smaller than the default values SHOULD NOT be used.
 +------------------------+---------+-------------------------------+
 | Parameter              | Default | Description                   |
 +------------------------+---------+-------------------------------+
 | ACTIVE_LQ_RCV_TIMEOUT  | 120 s   | Active Leasequery receive     |
 |                        |         | timeout                       |
 | ACTIVE_LQ_SEND_TIMEOUT | 120 s   | Active Leasequery send        |
 |                        |         | timeout                       |
 | ACTIVE_LQ_IDLE_TIMEOUT | 60 s    | Active Leasequery idle        |
 |                        |         | timeout                       |
 +------------------------+---------+-------------------------------+

6. Information Communicated by Active Leasequery

 While the information communicated by a Bulk Leasequery [RFC6926] is
 taken directly from the DHCPv4 server's lease state database, the
 information communicated by an Active Leasequery is real-time
 information.  As such, it is the information that is currently
 associated with a particular binding in the DHCPv4 server's lease
 state database.
 This is of significance, because if the Active Leasequery requestor
 runs slowly or the requestor disconnects from the DHCPv4 server and
 then reconnects with a query-start-time (signaling a catch-up
 operation), the information communicated to the Active Leasequery
 requestor is only the most current information from the DHCPv4
 server's lease state database.

Kinnear, et al. Standards Track [Page 11] RFC 7724 Active DHCPv4 Lease Query December 2015

 The requestor of an Active Leasequery MUST NOT assume that every
 lease state change is communicated across an Active Leasequery
 connection.  Even if the Active Leasequery requestor remains
 connected, the DHCPv4 server is only required to transmit information
 about a binding that is current when the packet is created and handed
 off to the TCP stack to send to the requestor.
 If the TCP connection blocks and the DHCPv4 server is waiting to send
 information down the connection, when the connection becomes
 available to be written, the DHCPv4 server MAY create the packet to
 send at this time.  The current state of the binding will be sent,
 and any transition in state or other information that occurred while
 the TCP connection was blocked will be lost.
 Thus, the Active Leasequery protocol does not allow the requestor to
 build a complete history of every activity on every lease.  An
 effective history of the important state changes for a lease can be
 created if the parameters of the DHCPv4 server are tuned to take into
 account the requirements of an Active Leasequery requestor.  For
 instance, the period after the expiration or release of a binding
 could be configured long enough (say, several minutes, well more than
 the receive timeout), so that an Active Leasequery requestor would
 never miss any changes in the binding.

7. Requestor Behavior

7.1. General Processing

 A requestor attempts to establish a TCP connection to a DHCPv4 server
 in order to initiate a Leasequery exchange.  If the attempt fails,
 the Requestor MAY retry.  Retries should not be more frequent than
 one every ACTIVE_LQ_IDLE_TIMEOUT.  See Section 5.3.
 If an Active Leasequery is terminated prematurely by a
 DHCPLEASEQUERYDONE with a dhcp-message status-code of QueryTerminated
 or by the failure of the connection over which it was being
 submitted, the requestor MAY retry the request after the creation of
 a new connection.  Retries should not be more frequent than one every
 ACTIVE_LQ_IDLE_TIMEOUT.  See Section 5.3.
 Messages from the DHCPv4 server come as multiple responses to a
 single DHCPACTIVELEASEQUERY message.  Thus, each DHCPACTIVELEASEQUERY
 or DHCPBULKLEASEQUERY request must have an xid (transaction-id)
 unique on the connection on which it is sent (see Section 7.3), and
 all of the messages that come as a response to it contain the same
 xid as the request.

Kinnear, et al. Standards Track [Page 12] RFC 7724 Active DHCPv4 Lease Query December 2015

 Only one DHCPACTIVELEASEQUERY is allowed on any one TCP connection at
 a time.  Parallel DHCPACTIVELEASEQUERY requests on the same TCP
 connection are not allowed.

7.2. Initiating a Connection

 A requestor SHOULD be able to operate in either insecure or secure
 mode.  See Section 9.  This MAY be a feature that is administratively
 controlled.
 When operating in insecure mode, the requestor sends a
 DHCPACTIVELEASEQUERY request after the establishment of a TCP
 connection.
 When operating in secure mode, the requestor MUST attempt to
 negotiate a TLS [RFC5246] connection over the TCP connection.  If
 this negotiation fails, the requestor MUST close the TCP connection.
 The recommendations in [RFC7525] apply when negotiating this
 connection.
 A requestor requests the establishment of a TLS connection by sending
 the DHCPTLS message to the DHCPv4 server as the first message over
 the TCP connection.  The DHCPTLS message SHOULD be sent without any
 options.  This message indicates to the DHCPv4 server that a TLS
 connection over this TCP connection is desired.  There are four
 possibilities after the requestor sends the DHCPTLS message to the
 DHCPV4 server:
 1.  No response from the DHCPv4 server.
 2.  The DHCPv4 server closes the TCP connection after it receives the
     DHCPTLS message.
 3.  DHCPv4 server responds with a DHCPTLS message with a dhcp-status-
     code of TLSConnectionRefused.
 4.  DHCPv4 server responds with DHCPTLS message with no dhcp-status-
     code, indicating success.
 In any of the first three possibilities, the DHCPv4 server can be
 assumed to not support TLS.  In this case, the requestor MUST close
 the connection.
 In the final possibility, where the DHCPv4 server has responded with
 a DHCPTLS message with no dhcp-status-code in response to the
 requestor's DHCPTLS message, the requestor SHOULD initiate the
 exchange of the messages involved in a TLS handshake [RFC5246].

Kinnear, et al. Standards Track [Page 13] RFC 7724 Active DHCPv4 Lease Query December 2015

 During the TLS handshake, the requestor MUST validate the DHCPv4
 server's digital certificates.
 If the handshake exchange yields a functioning TLS connection, then
 the requestor SHOULD transmit a DHCPACTIVELEASEQUERY message over
 that TLS connection and use that TLS connection for all further
 interactions in which it engages with the DHCPv4 server over this TCP
 connection.
 If the handshake exchange does not yield a functioning TLS
 connection, then the requestor MUST close the TCP connection.

7.3. Forming an Active Leasequery

 The Active Leasequery is designed to create a long-lived connection
 between the requestor and the DHCPv4 server processing the active
 query.  The DHCPv4 server SHOULD send binding information back across
 this connection with minimal delay after it learns of the binding
 information.  It will learn about the bindings either because it
 makes the bindings itself or because it has received information
 about a binding from another server.
 An Active Leasequery is a DHCPv4 request with a dhcp-message-type of
 DHCPACTIVELEASEQUERY.  The DHCPv4 request MUST NOT have a ciaddr, a
 chaddr, or a dhcp-client-identifier.  The DHCPv4 request MUST have an
 xid (transaction-id) unique on the connection on which it is sent.
 The DHCPv4 request SHOULD have a dhcp-parameter-request-list to
 inform the DHCPv4 server which DHCPv4 options are of interest to the
 requestor sending the DHCPACTIVELEASEQUERY message.
 An important capability of the Active Leasequery is that the
 requestor can specify that some recent data be sent immediately to
 the requestor in parallel with the transmission of the ongoing
 binding information in more or less real time.  This capability is
 used in order to allow an Active Leasequery requestor to recover
 missed information in the event that it temporarily loses
 connectivity with the DHCPv4 server processing a previous Active
 Leasequery.
 This capability is enabled by the transmission of a 4-octet base-time
 option with each Leasequery reply sent as the result of a previous
 Active Leasequery.  The requestor SHOULD keep track of the highest
 base-time received from a particular DHCPv4 server over an Active
 Leasequery connection, and in the event that the requestor finds it
 necessary (for whatever reason) to reestablish an Active Leasequery
 connection to that DHCPv4 server, the requestor should place this

Kinnear, et al. Standards Track [Page 14] RFC 7724 Active DHCPv4 Lease Query December 2015

 highest base-time value into a query-start-time option in the new
 DHCPACTIVELEASEQUERY request.  (See Sections 6.2.5 and 7.2 of
 [RFC6926] for information on the query-start-time option.)
 Note that until all of the recent data (catch-up data) has been
 received, the requestor MUST NOT keep track of the base-time received
 in Leasequery reply messages to use later in a subsequent Bulk
 Leasequery or Active Leasequery request.
 If the requestor doesn't wish to request an update of information
 missed when it was not connected to the DHCPv4 server, then it does
 not include the query-start-time option in the DHCPACTIVELEASEQUERY
 request.
 If the TCP connection becomes blocked or stops being writable while
 the requestor is sending its query, the requestor SHOULD terminate
 the connection after BULK_LQ_DATA_TIMEOUT.  We make this
 recommendation to allow requestors to control the period of time they
 are willing to wait before abandoning a connection, independent of
 notifications from the TCP implementations they may be using.

7.4. Processing Active Replies

 The Requestor attempts to read a DHCPv4 leasequery reply message from
 the TCP connection.
 Note that the connection resulting from accepting a
 DHCPACTIVELEASEQUERY request may be long-lived and may not have data
 transferring continuously during its lifetime.  Therefore, the DHCPv4
 server SHOULD send a DHCPLEASEQUERYSTATUS message with a dhcp-status-
 code of ConnectionActive every ACTIVE_LQ_IDLE_TIMEOUT seconds
 (default 60) in order for the requestor to know that the connection
 remains alive.  This approach is followed only when the connection is
 idle (i.e., the server has no binding data to send).  During normal
 binding data exchange, receiving DHCPLEASEACTIVE or
 DHCPLEASEUNASSIGNED messages by the requestor itself signifies that
 the connection is active.  Note that the default for
 ACTIVE_LQ_RCV_TIMEOUT is 120 seconds, twice the value of the
 ACTIVE_LQ_IDLE_TIMEOUT's default of 60 seconds, which drives the
 DHCPv4 server to send messages.  Thus, ACTIVE_LQ_RCV_TIMEOUT controls
 how sensitive the requestor is to be to delays by the DHCPv4 server
 in sending updates or DHCPLEASEQUERYSTATUS messages.
 If the stream of replies becomes blocked with no messages being
 received, the Requestor SHOULD terminate the connection after
 ACTIVE_LQ_RCV_TIMEOUT, and MAY begin retry processing if configured
 to do so.

Kinnear, et al. Standards Track [Page 15] RFC 7724 Active DHCPv4 Lease Query December 2015

 A successful query that is returning binding data MUST include a non-
 zero ciaddr.  It may also include a non-zero chaddr, htype, and hlen
 as well as additional options.  If there are additional bindings to
 be returned, they will be carried in additional Active Leasequery
 messages.
 Any requestor of an Active Leasequery operation MUST be prepared to
 receive multiple copies of the binding information for a particular
 IPv4 address.  See the Bulk Leasequery document [RFC6926] for
 information on how to deal with this situation.
 A single Active Leasequery can and usually will result in a large
 number of replies.  The Requestor MUST be prepared to receive more
 than one reply with transaction-ids matching a single
 DHCPACTIVELEASEQUERY message from a single DHCPv4 server.
 A DHCPACTIVELEASEQUERY has two regimes -- during the catch-up phase,
 if any, and after any catch-up phase.  If the DHCPACTIVELASEQUERY
 request had a query-start-time, then the DHCPACTIVELEASEQUERY starts
 out in the catch-up phase.  See Section 7.4.1 for information on
 processing during the catch-up phase, as well as how to determine
 when the catch-up phase is complete.
 After the catch-up phase, or during the entire series of messages
 received as the response to a DHCPACTIVELEASEQUERY request with no
 query-start-time (and therefore no catch-up phase), the base-time
 option of the most recent message SHOULD be saved as a record of the
 most recent time that data was received.  This base-time (in the
 context of the DHCPv4 server) can be used in a subsequent
 DHCPACTIVELEASEQUERY message's query-start-time or in a
 DHCPBULKLEASEQUERY message's query-start-time, if one is required,
 after a loss of the Active Leasequery connection.
 The DHCPLEASEQUERYSTATUS message MAY unilaterally terminate a
 successful DHCPACTIVELEASEQUERY request that is currently in progress
 in the event that the DHCPv4 server determines that it cannot
 continue processing a DHCPACTIVELEASEQUERY request.  For example,
 when a server is requested to shut down, it SHOULD send a
 DHCPLEASEQUERYSTATUS message with a dhcp-status-code of
 QueryTerminated and include in the message a base-time.  This MUST be
 the last message on that connection, and once the message has been
 transmitted, the server MUST close the connection.
 After receiving DHCPLEASEQUERYSTATUS with a QueryTerminated status
 from a server, the Requestor MAY close the TCP connection to that
 server.

Kinnear, et al. Standards Track [Page 16] RFC 7724 Active DHCPv4 Lease Query December 2015

 The DHCPv4 Leasequery protocol uses the associated-ip option as an
 indicator that multiple bindings were present in response to a single
 client-based query.  For Active Leasequery, client-based queries are
 not supported, and so the associated-ip option is not used and MUST
 NOT be present in replies.

7.4.1. Processing Replies from a Request Containing a query-start-time

 If the DHCPACTIVELEASEQUERY was requested with a query-start-time,
 the DHCPv4 server will attempt to send information about all bindings
 that changed since the time specified in the query-start-time.  This
 is the catch-up phase of the DHCPACTIVELEASEQUERY processing.  The
 DHCPv4 server MAY also begin immediate updates over the same
 connection of real-time binding information changes.  Thus, the
 catch-up phase can run in parallel with the normal updates generated
 by the DHCPACTIVELEASEQUERY request.
 A DHCPv4 server MAY keep only a limited amount of time-ordered
 information available to respond to a DHCPACTIVELEASEQUERY request
 containing a query-start-time.  Thus, it is possible that the time
 specified in the query-start-time represents a time not covered by
 the time-ordered information kept by the DHCPv4 server.  In such
 case, when there is not enough data saved in the DHCPv4 server to
 satisfy the request specified by the query-start-time option, the
 DHCPv4 server will reply immediately with a DHCPLEASEQUERYSTATUS
 message with a dhcp-status-code of DataMissing with a base-time
 option equal to the server's current time.  This will signal the end
 of the catch-up phase, and the only updates that will subsequently be
 received on this connection are the real-time updates from the
 DHCPACTIVELEASEQUERY request.
 If there is enough data saved to satisfy the request, then
 DHCPLEASEACTIVE and DHCPLEASEUNASSIGNED messages will begin arrive
 from the DHCPv4 server.  Some of these messages will be related to
 the query-start-time request and be part of the catch-up phase.  Some
 of these messages will be real-time updates of binding changes taking
 place in the DHCPv4 server.  In general, there is no way to determine
 the source of each message.
 The updates sent by the DHCPv4 server during the catch-up phase are
 not in the order that the binding data was updated.  Therefore, until
 the catch-up phase is complete, the latest base-time value received
 from a DHCPv4 server processing an Active Leasequery request cannot
 be reset from the incoming messages (and used in a subsequent Active
 Leasequery's query-start-time option), because to do so would
 compromise the ability to recover lost information if the
 DHCPACTIVELEASEQUERY were to terminate prior to the completion of the
 catch-up phase.

Kinnear, et al. Standards Track [Page 17] RFC 7724 Active DHCPv4 Lease Query December 2015

 The requestor will know that the catch-up phase is complete because
 the DHCPv4 server will transmit a DHCPLEASEQUERYSTATUS message with
 the dhcp-status-code of CatchUpComplete (or, as discussed above,
 DataMissing).  Once this message is transmitted, all additional
 DHCPLEASEACTIVE and DHCPLEASEUNASSIGNED messages will relate to real-
 time ("new") binding changes in the DHCPv4 server.
 As discussed in Section 6.3, the requestor SHOULD keep track of the
 latest base-time option value received over a particular connection,
 to be used in a subsequent DHCPACTIVELEASEQUERY request -- but only
 if the catch-up phase is complete.  Prior to the completion of the
 catch-up phase, if the connection should go away or if the requestor
 receives a DHCPLEASEQUERYDONE message, then when it reconnects it
 MUST use the base-time value from the previous connection and not any
 base-time value received from the recently closed connection.
 In the event that there was enough data available to the DHCPv4
 server to begin to satisfy the request implied by the query-start-
 time option, but during the processing of that data the server found
 that it was unable to continue (perhaps there was barely enough, the
 connection was very slow, and the aging algorithm caused the saved
 data to become unavailable), the DHCPv4 server will terminate the
 catch-up phase of processing immediately by sending a
 DHCPLEASEQUERYSTATUS message with a dhcp-status-code of DataMissing
 and with a base-time option of the current time.
 The requestor must not assume that every individual state change of
 every binding during the period from the time specified in the query-
 start-time and the present is replicated in an Active Leasequery
 reply message.  See Section 6.  The requestor MAY assume that at
 least one Active Leasequery reply message will exist for every
 binding that had one or more changes of state during the period
 specified by the query-start-time and the current time.  The last
 message for each binding will contain the state at the current time,
 and there can be one or more messages concerning a single binding
 during the catch-up phase of processing.
 Bindings can change multiple times while the requestor is not
 connected.  The requestor will only receive information about the
 current state of the binding, not information about each state change
 that occurred during the period from the query-start-time to the
 present.
 If the DHCPLEASEQUERYSTATUS message containing a dhcp-status-code of
 DataMissing is received and the requestor is interested in keeping
 its database up to date with respect to the current state of the
 bindings in the DHCPv4 server, then the requestor SHOULD issue a
 DHCPBULKLEASEQUERY request to recover the information missing from

Kinnear, et al. Standards Track [Page 18] RFC 7724 Active DHCPv4 Lease Query December 2015

 its database.  This DHCPBULKLEASEQUERY should include a query-start-
 time option, set to the same value as the query-start-time option
 previously included in the DHCPACTIVELEASEQUERY responses from the
 DHCPv4 server, and a query-end-time option equal to the base-time
 option returned by the DHCPv4 server in the DHCPLEASEQUERYSTATUS
 message with the dhcp-status-code of DataMissing.
 Typically, the requestor would have one connection open to a DHCPv4
 server for a DHCPACTIVELEASEQUERY request and possibly one additional
 connection open for a DHCPBULKLEASEQUERY request to the same DHCPv4
 server to fill in the data that might have been missed prior to the
 initiation of the DHCPACTIVELEASEQUERY.  The Bulk Leasequery
 connection would typically run to completion and be closed, leaving
 one Active Leasequery connection open to a single DHCPv4 server.

7.5. Closing Connections

 The Requestor or DHCPv4 leasequery server MAY close its end of the
 TCP connection at any time.  The Requestor MAY choose to retain the
 connection if it intends to issue additional queries.  Note that this
 requestor behavior does not guarantee that the connection will be
 available for additional queries: the server might decide to close
 the connection based on its own configuration.

8. Server Behavior

 A DHCPv4 server that supports Active Leasequery MUST support Bulk
 Leasequery [RFC6926] as well.

8.1. Accepting Connections

 DHCPv4 servers that implement DHCPv4 Active Leasequery listen for
 incoming TCP connections.  The approach used in accepting the
 requestor's connection is the same as specified in DHCPv4 Bulk
 Leasequery [RFC6926], with the exception that support for Active
 Leasequery MUST NOT be enabled by default, and MUST require an
 explicit configuration step to be performed before it will operate.
 DHCPv4 servers SHOULD be able to operate in either insecure or secure
 mode.  See Section 9.  This MAY be a mode that is administratively
 controlled, where the server will require a TLS connection to operate
 or will only operate without a TLS connection.  In either case,
 operation in insecure mode MUST NOT be the default, even if operation
 in secure mode is not supported.  Operation in insecure mode MUST
 always require an explicit configuration step, separate from the
 configuration step required to enable support for Active Leasequery.

Kinnear, et al. Standards Track [Page 19] RFC 7724 Active DHCPv4 Lease Query December 2015

 When operating in insecure mode, the DHCPv4 server simply waits for
 the requestor to send the Active Leasequery after the establishment
 of TCP connection.  If it receives a DHCPTLS message, it will respond
 with TLSConnectionRefused in a DHCPTLS message.
 When operating in secure mode, DHCPv4 servers MUST support TLS
 [RFC5246] to protect the integrity and privacy of the data
 transmitted over the TCP connection.  When operating in secure mode,
 DHCPv4 servers MUST be configurable with regard to which requestors
 they will communicate.  The certificate presented by a requestor when
 initiating the TLS connection is used to distinguish between
 acceptable and unacceptable requestors.
 When operating in secure mode, a DHCPv4 server MUST begin to
 negotiate a TLS connection with a requestor who asks for one, and
 MUST close TCP connections that are not secured with TLS or for which
 the requestor's certificate is deemed unacceptable.  The
 recommendations in [RFC7525] apply when negotiating a TLS connection.
 A requestor will request a TLS connection by sending a DHCPTLS as the
 first message over a newly created TCP connection.  If the DHCPv4
 server supports TLS connections and has not been configured to not
 allow them on this link, the DHCPv4 server MUST respond to this
 DHCPTLS message by sending a DHCPTLS message with no dhcp-status-code
 back to the requestor.  This indicates to the requestor that the
 DHCPv4 server will support the negotiation of a TLS connection over
 this existing TCP connection.
 If a connection is to be rejected because of a limitation of the
 number of open connections, the TCP connection itself should be
 rejected, or the subsequent ACTIVELEASEQUERY message should be
 rejected.  Capacity-related rejections SHOULD NOT affect the response
 to the DHCPTLS message.
 Any options appearing in a DHCPTLS message received by a DHCPv4
 server SHOULD be ignored.  This is a "SHOULD" instead of a "MUST" in
 order to allow use of the DHCPTLS message in later documents,
 possibly with the use of options, without requiring those documents
 to update this document.
 If for some reason the DHCPv4 server cannot support or has been
 configured to not support a TLS connection, then it sends a DHCPTLS
 message with a dhcp-status-code of TLSConnectionRefused back to the
 requestor.
 In the event that the DHCPv4 server sends a DHCPTLS message with no
 dhcp-status-code option included (which indicates success), the
 requestor is supposed to initiate a TLS handshake [RFC5246] (see

Kinnear, et al. Standards Track [Page 20] RFC 7724 Active DHCPv4 Lease Query December 2015

 Section 7.2).  During the TLS handshake, the DHCPv4 server MUST
 validate the requestor's digital certificate.  In addition, the
 digital certificate presented by the requestor is used to decide if
 this requestor is allowed to perform an Active Leasequery.  If this
 requestor's certificate is deemed unacceptable, the server MUST abort
 the creation of the TLS connection.
 All TLS connections established between a requestor and a DHCPv4
 server for the purposes of supporting Active Leasequery MUST be
 mutually authenticated.
 If the TLS handshake is not successful in creating a TLS connection,
 the server MUST close the TCP connection.
 If the TCP connection becomes blocked while the server is accepting a
 connection or reading a query, it SHOULD terminate the connection
 after a BULK_LQ_DATA_TIMEOUT.  We make this recommendation to allow
 servers to control the period of time they are willing to wait before
 abandoning an inactive connection, independent of the TCP
 implementations they may be using.

8.1.1. Update to RFC 6926

 In an update to the DHCPv4 Bulk Leasequery protocol [RFC6926] (which
 didn't discuss this situation explicitly), if the DHCPv4 server
 receives a DHCPv4 message containing a dhcp-message-type option with
 a value that is not supported over a TCP connection, it MUST close
 the TCP connection.

8.2. Replying to an Active Leasequery

 If the connection becomes blocked while the server is attempting to
 send reply messages, the server SHOULD terminate the TCP connection
 after ACTIVE_LQ_SEND_TIMEOUT.  This timeout governs how long the
 DHCPv4 server is prepared to wait for the requestor to read and
 process enough information to unblock the TCP connection.  The
 default is two minutes, which means that if more than two minutes
 goes by without the requestor reading enough information to unblock
 the TCP connection, the DHCPv4 server SHOULD close the TCP
 connection.
 If the DHCPv4 server encounters an error during processing of the
 DHCPACTIVELEASEQUERY message, either during initial processing or
 later during the message processing, it SHOULD send a
 DHCPLEASEQUERYSTATUS containing an error code of some kind in a dhcp-
 status-code option.  It SHOULD close the connection after this error
 is signaled.

Kinnear, et al. Standards Track [Page 21] RFC 7724 Active DHCPv4 Lease Query December 2015

 Every reply to a DHCPACTIVELEASEQUERY request MUST contain the
 information specified in replies to a DHCPBULKLEASEQUERY request
 [RFC6926], with the exception that a server implementing Active
 Leasequery SHOULD be able to be configured to prevent specific data
 items from being sent to the requestor even if these data items were
 requested in the dhcp-parameter-request-list option.
 Some servers can be configured to respond to a DHCPv4 Leasequery
 [RFC4388] or a DHCPBULKLEASEQUERY [RFC6926] for an IPv4 binding that
 is reserved in such a way that it appears that the IPv4 binding is
 leased to the DHCP client for which it is reserved.  These servers
 SHOULD also respond to a DHCPACTIVELEASEQUERY request with the same
 information as they would to a DHCPBULKLEASEQUERY request when they
 first determine that the IPv4 binding is reserved to a DHCP client.
 If a DHCPACTIVELEASEQUERY request contains a query-start-time option,
 it indicates that the requestor would like the DHCPv4 server to send
 it not only messages that correspond to DHCPv4 binding activity that
 occurs subsequent to the receipt of the DHCPLEASEACTIVE request, but
 also messages that correspond to DHCPv4 binding activity that
 occurred prior to the DHCPACTIVELEASEQUERY request.
 If a query-end-time option appears in a DHCPACTIVELEASEQUERY the
 DHCPv4 server should send a DHCPLEASEQUERYSTATUS message with a dhcp-
 status-code of MalformedQuery and terminate the connection.
 In order to implement a meaningful response to this query, the DHCPv4
 server MAY keep track of the binding activity and associate changes
 with particular base-time values from the messages.  Then, when
 requested to do so by a DHCPACTIVELEASEQUERY request containing a
 query-start-time option, the DHCPv4 server can respond with replies
 for all binding activity occurring on that query-start-time or later
 times.
 These replies based on the query-start-time MAY be interleaved with
 the messages generated due to current binding activity.
 Once the transmission of the DHCPv4 Leasequery messages associated
 with the query-start-time option are complete, a DHCPLEASEQUERYSTATUS
 message MUST be sent with a dhcp-status-code value of
 CatchUpComplete.
 The DHCPv4 server SHOULD keep track of previous binding activity.  It
 SHOULD limit the amount of previous binding activity it keeps track
 of.  The DHCPv4 server MAY choose to only do this in the event that
 it has received at least one DHCPACTIVELEASEQUERY request in the
 past, as to do so will almost certainly entail some utilization of
 resources that would be wasted if there are no DHCPACTIVELEASEQUERY

Kinnear, et al. Standards Track [Page 22] RFC 7724 Active DHCPv4 Lease Query December 2015

 requestors for this DHCPv4 server.  The DHCPv4 server SHOULD make the
 amount of previous binding activity it retains configurable.  There
 is no requirement on the DHCPv4 server to retain this information
 over a server restart (or even to retain such information at all).
 Unless there is an error or some requirement to cease processing a
 DHCPACTIVELEASEQUERY request yielding a DHCPLEASEQUERYSTATUS message,
 such as a server shutdown, there will be no DHCPLEASEQUERYSTATUS
 message at the conclusion of the DHCPACTIVELEASEQUERY processing
 because that processing will not conclude but will continue until
 either the requestor or the server closes the connection.
 While the form of the data being sent by a DHCPACTIVELEASEQUERY is
 essentially the same as that being sent by a DHCPBULKLEASEQUERY, the
 reasons for sending information differs considerably between these
 two capabilities.  In the DHCPBULKLEASEQUERY context, the entire
 contents of the lease state database (subject to the constraints of
 the various query options) are returned to the requestor.  In the
 DHCPACTIVELEASEQUERY context, changes to the lease state database are
 returned to the requestor essentially as they happen.  For instance,
 when an IPv4 binding transitions from the leased state to some other
 state, the DHCPACTIVELEASEQUERY will send a DHCPLEASEUNASSIGNED
 packet with information regarding that binding.  The server may then
 entirely forget about that IPv4 binding (or not), but it is important
 to tell the DHCPACTIVELEASEQUERY requestor that a binding has
 transitioned away from the leased state.
 The relationship between the time that the server replies to a DHCP
 client request and the time that the DHCP server sends a reply to a
 DHCPACTIVELEASEQUERY message is a matter of implementation (and thus
 not defined by this document).  However, the server SHOULD NOT delay
 responding to the DHCP client in order to transmit a reply to a
 DHCPACTIVELEASEQUERY message, and the server SHOULD send the reply to
 the DHCPACTIVELASEQUERY message as soon as possible after responding
 to the client.

8.3. Multiple or Parallel Queries

 Every Active Leasequery request MUST be made on a single TCP
 connection where there is no other request active at the time the
 request is made.  Note that this is different than what was allowed
 in Section 7.7 of [RFC6926] for Bulk Leasequery requests.
 Typically, a requestor of an Active Leasequery would not need to send
 a second Active Leasequery while the first is still active.  However,
 sending an Active Leasequery and a Bulk Leasequery in parallel would
 be possible and reasonable.  In case of parallel Active and Bulk
 Leasequery requests, the requestor MUST use different connections.

Kinnear, et al. Standards Track [Page 23] RFC 7724 Active DHCPv4 Lease Query December 2015

 This MAY be a feature that is administratively controlled.  Servers
 that are able to process queries in parallel SHOULD offer
 configuration that limits the number of simultaneous queries
 permitted from any one requestor, in order to control resource use if
 there are multiple requestors seeking service.

8.4. Closing Connections

 The server MAY end communication by sending a DHCPLEASEQUERYSTATUS
 message and then immediately closing the TCP connection.
 Alternatively, the server MAY retain the connection and wait for
 additional queries from the requestor.  The server SHOULD limit the
 number of connections it maintains and SHOULD close idle connections
 to enforce the limit.
 The server MUST close its end of the TCP connection if it encounters
 an error sending data on the connection.  The server MUST close its
 end of the TCP connection if it finds that it has to abort an in-
 process request.  A server aborting an in-process request SHOULD
 attempt to signal that to its requestors by using the QueryTerminated
 status code in the dhcp-status-code option in a DHCPLEASEQUERYSTATUS
 message.  If the server detects that the requestor end has been
 closed, the server MUST close its end of the connection.

9. Security Considerations

 The Security Considerations section of [RFC2131] details the general
 threats to DHCPv4.  The DHCPv4 Leasequery specification [RFC4388]
 describes recommendations for the Leasequery protocol, especially
 with regard to relayed LEASEQUERY messages, mitigation of packet-
 flooding DoS attacks, restriction to trusted requestors, and use of
 IPsec [RFC4301].
 The use of TCP introduces some additional concerns.  Attacks that
 attempt to exhaust the DHCPv4 server's available TCP connection
 resources can compromise the ability of legitimate clients to receive
 service.  Malicious requestors who succeed in establishing
 connections, but who then send invalid queries, partial queries, or
 no queries at all also can exhaust a server's pool of available
 connections.
 Two modes of operation exist for this protocol, insecure mode and
 secure mode.  These two modes exist because there are essentially two
 models of use for this protocol.  In one model, the requestor of an
 Active Leasequery is connected to the Internet in an arbitrary
 location, and the information transmitted needs to be protected

Kinnear, et al. Standards Track [Page 24] RFC 7724 Active DHCPv4 Lease Query December 2015

 during transmission.  In addition, the identities of both requestor
 and server need to be verified.  For this model of use, the secure
 mode is appropriate.
 The other model of use is where the requestor of the Active
 Leasequery resides in a network element that is essentially "next to"
 the element containing the DHCP server, and both of these elements
 are inside a protected environment.  For this model, the insecure
 mode is sufficient since there are other, more global, protections in
 place to protect this information.
 When operating in secure mode, TLS [RFC5246] is used to secure the
 connection.  The recommendations in [RFC7525] apply when negotiating
 a TLS connection.
 Operating in insecure mode (see Section 8.1) does not provide any way
 to validate the authorization of requestors of a DHCPV4 Active
 Leasequery request.
 Servers SHOULD offer configuration parameters to limit the sources of
 incoming connections through validation and use of the digital
 certificates presented to create a TLS connection.  They SHOULD also
 limit the number of accepted connections and limit the period of time
 during which an idle connection will be left open.
 The data acquired by using an Active Leasequery is subject to the
 same potential abuse as the data held by the DHCPv4 server from which
 it was acquired and SHOULD be secured by mechanisms as strong as
 those used for the data held by that DHCPv4 server.  The data
 acquired by using an Active Leasequery SHOULD be deleted as soon as
 possible after the use for which it was acquired has passed.
 Servers that implement the Bulk Leasequery protocol [RFC6926] but do
 not implement the Active Leasequery protocol SHOULD implement the
 update to [RFC6926] discussed in Section 8.1.1.

10. IANA Considerations

 IANA has assigned the following new DHCP message types from the
 registry "DHCP Message Type 53 Values" maintained at
 <http://www.iana.org/assignments/bootp-dhcp-parameters>:
 1.  A dhcp-message-type of 16 for DHCPACTIVELEASEQUERY.
 2.  A dhcp-message-type of 17 for DHCPLEASEQUERYSTATUS.
 3.  A dhcp-message-type of 18 for DHCPTLS.

Kinnear, et al. Standards Track [Page 25] RFC 7724 Active DHCPv4 Lease Query December 2015

 IANA has assigned the following new DHCP status codes from the
 registry "DHCP Status Code Type 151 Values" maintained at
 <http://www.iana.org/assignments/bootp-dhcp-parameters>:
                +----------------------+-------------+
                | Name                 | Status-Code |
                +----------------------+-------------+
                | DataMissing          | 5           |
                | ConnectionActive     | 6           |
                | CatchUpComplete      | 7           |
                | TLSConnectionRefused | 8           |
                +----------------------+-------------+

11. References

11.1. 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,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
            RFC 2131, DOI 10.17487/RFC2131, March 1997,
            <http://www.rfc-editor.org/info/rfc2131>.
 [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>.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246,
            DOI 10.17487/RFC5246, August 2008,
            <http://www.rfc-editor.org/info/rfc5246>.
 [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>.
 [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
            "Recommendations for Secure Use of Transport Layer
            Security (TLS) and Datagram Transport Layer Security
            (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
            2015, <http://www.rfc-editor.org/info/rfc7525>.

Kinnear, et al. Standards Track [Page 26] RFC 7724 Active DHCPv4 Lease Query December 2015

11.2. Informative References

 [RFC951]   Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
            DOI 10.17487/RFC0951, September 1985,
            <http://www.rfc-editor.org/info/rfc951>.
 [RFC1542]  Wimer, W., "Clarifications and Extensions for the
            Bootstrap Protocol", RFC 1542, DOI 10.17487/RFC1542,
            October 1993, <http://www.rfc-editor.org/info/rfc1542>.
 [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
            Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
            <http://www.rfc-editor.org/info/rfc2132>.
 [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
            Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
            December 2005, <http://www.rfc-editor.org/info/rfc4301>.
 [RFC7414]  Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
            Zimmermann, "A Roadmap for Transmission Control Protocol
            (TCP) Specification Documents", RFC 7414,
            DOI 10.17487/RFC7414, February 2015,
            <http://www.rfc-editor.org/info/rfc7414>.

Acknowledgments

 The ideas in this document came in part from work in DHCPv6 and
 DHCPv4 Bulk Leasequery as well as from in depth discussions between
 the authors.  Useful review comments by Ted Lemon, Scott Bradner,
 Francis Dupont, and Stephen Farrell on drafts for DHCPv6 Active
 Leasequery were also included in this draft.  Brian Haberman's review
 brought this document into much closer alignment with DHCPv6 Active
 Leasequery.  Additional reviews by Alissa Cooper, Spencer Dawkins,
 Christer Holmberg, and Ben Campbell added clarity to this document.

Kinnear, et al. Standards Track [Page 27] RFC 7724 Active DHCPv4 Lease Query December 2015

Authors' Addresses

 Kim Kinnear
 Cisco Systems, Inc.
 1414 Massachusetts Ave
 Boxborough, MA  01719
 United States
 Email: kkinnear@cisco.com
 Mark Stapp
 Cisco Systems, Inc.
 1414 Massachusetts Ave
 Boxborough, MA  01719
 United States
 Email: mjs@cisco.com
 Bernie Volz
 Cisco Systems, Inc.
 1414 Massachusetts Ave
 Boxborough, MA  01719
 United States
 Email: volz@cisco.com
 Neil Russell
 Staples
 500 Staples Drive
 Framingham, MA  01702
 United States
 Email: neil.e.russell@gmail.com

Kinnear, et al. Standards Track [Page 28]

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