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
December 2015 Active DHCPv4 Lease Query
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
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.
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.
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
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
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].
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
o "catch-up phase"
The period while the catch-up information is being sent is the
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
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].
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
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".
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
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
reachability to the DHCPv4 server. The message framing for TCP is
discussed in Section 5.1. No relaying for Active Leasequery is
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
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
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.
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
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
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
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
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 |
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. |
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
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
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.
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.
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
When operating in insecure mode, the requestor sends a
DHCPACTIVELEASEQUERY request after the establishment of a TCP
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
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
1. No response from the DHCPv4 server.
2. The DHCPv4 server closes the TCP connection after it receives the
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
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].
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
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
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
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
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.
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
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
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
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
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
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
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.
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
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
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
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
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
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
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
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
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.
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
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
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
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
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
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.
Cisco Systems, Inc.
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Cisco Systems, Inc.
1414 Massachusetts Ave
Boxborough, MA 01719
Cisco Systems, Inc.
1414 Massachusetts Ave
Boxborough, MA 01719
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