Network Working Group R. Woundy
Request for Comments: 4388 Comcast Cable
Category: Standards Track K. Kinnear
Cisco Systems
February 2006
Dynamic Host Configuration Protocol (DHCP) Leasequery
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
A Dynamic Host Configuration Protocol version 4 (DHCPv4) server is
the authoritative source of IP addresses that it has provided to
DHCPv4 clients. Other processes and devices that already make use of
DHCPv4 may need to access this information. The leasequery protocol
provides these processes and devices a lightweight way to access IP
address information.
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RFC 4388 DHCP Leasequery February 2006
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................5
3. Background ......................................................7
4. Design Goals ....................................................7
4.1. Broadcast ARP Is Undesirable ...............................7
4.2. SNMP and LDAP Are Not Appropriate ..........................8
4.3. DHCP Relay Agent Functionality Is Common ...................8
4.4. DHCP Servers Are a Reliable Source of Location
Information ................................................9
4.5. Minimal Additional Configuration Is Required ...............9
5. Protocol Overview ...............................................9
6. Protocol Details ...............................................12
6.1. Definitions Required for DHCPLEASEQUERY Processing ........12
6.2. Sending the DHCPLEASEQUERY Message ........................14
6.3. Receiving the DHCPLEASEQUERY Message ......................15
6.4. Responding to the DHCPLEASEQUERY Message ..................16
6.5. Receiving a DHCPLEASEUNASSIGNED, DHCPLEASEACTIVE, or
DHCPLEASEUNKNOWN Message ..................................20
6.6. Receiving No Response to the DHCPLEASEQUERY Message .......21
6.7. Lease Binding Data Storage Requirements ...................22
6.8. Using the DHCPLEASEQUERY Message with Multiple
DHCP Servers ..............................................23
7. Security Considerations ........................................23
8. IANA Considerations ............................................24
9. Acknowledgements ...............................................24
10. References ....................................................25
10.1. Normative References .....................................25
10.2. Informative References ...................................25
1. Introduction
A DHCPv4 server contains considerable authoritative information
concerning the IP addresses it has leased to DHCP clients. Sometimes
devices or other processes may need access to this information. In
some cases, these devices or processes already have the capability to
send and receive DHCP packets, and so the leasequery protocol is
designed to give these processes and devices a low-overhead way to
access such information.
For example, access concentrators that act as DHCP relay agents
sometimes derive information important to their operation by
extracting data out of the DHCP packets they forward, a process known
as "gleaning". Unfortunately, the typical access concentrator loses
its gleaned information when the access concentrator is rebooted or
is replaced. This memo proposes that when gleaned DHCP information
is not available, the access concentrator/relay agent can obtain the
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location information directly from the DHCP server(s) using the
DHCPLEASEQUERY message.
To continue this example in more depth, in many broadband access
networks, the access concentrator needs to associate an IP address
lease to the correct endpoint location, which includes knowledge of
the host hardware address, the port or virtual circuit that leads to
the host, and/or the hardware address of the intervening subscriber
modem. This is particularly important when one or more IP subnets
are shared among many ports, circuits, and modems. Representative
cable and DSL environments are depicted in Figures 1 and 2 below.
+--------+ +---------------+
| DHCP | | DOCSIS CMTS |
| Server |-...-| or DVB INA |-------------------
+--------+ | (Relay Agent) | | |
+---------------+ +------+ +------+
|Modem1| |Modem2|
+------+ +------+
| | |
+-----+ +-----+ +-----+
|Host1| |Host2| |Host3|
+-----+ +-----+ +-----+
Figure 1: Cable Environment for DHCPLEASEQUERY
+--------+ +---------------+
| DHCP | | DSL Access | +-------+
| Server |-...-| Concentrator |-...-| DSLAM |
+--------+ | (Relay Agent) | +-------+
+---------------+ | |
+------+ +------+
|Modem1| |Modem2|
+------+ +------+
| | |
+-----+ +-----+ +-----+
|Host1| |Host2| |Host3|
+-----+ +-----+ +-----+
Figure 2: DSL Environment for DHCPLEASEQUERY
Knowledge of this location information can benefit the access
concentrator in several ways:
1. The access concentrator can forward traffic to the access
network using the correct access network port, down the
correct virtual circuit, through the correct modem, to the
correct hardware address.
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2. The access concentrator can perform IP source address
verification of datagrams received from the access network.
The verification may be based on the datagram source hardware
address, the incoming access network port, the incoming
virtual circuit, and/or the transmitting modem.
3. The access concentrator can encrypt datagrams that can only be
decrypted by the correct modem, using mechanisms such as [BPI]
or [BPI+].
The access concentrator in this example obtains the location
information primarily from "gleaning" information from DHCP server
responses sent through the relay agent. When location information is
not available from "gleaning", e.g., because the access concentrator
has rebooted, the access concentrator can query the DHCP server(s)
for location information using the DHCPLEASEQUERY message defined in
this document.
The DHCPLEASEQUERY message is a new DHCP message type transmitted
from a DHCP relay agent to a DHCP server. A DHCPLEASEQUERY-aware
relay agent sends the DHCPLEASEQUERY message when it needs to know
the location of an IP endpoint. The DHCPLEASEQUERY-aware DHCP server
replies with a DHCPLEASEUNASSIGNED, DHCPLEASEACTIVE, or
DHCPLEASEUNKNOWN message. The DHCPLEASEACTIVE response to a
DHCPLEASEQUERY message allows the relay agent to determine the IP
endpoint location and the remaining duration of the IP address lease.
The DHCPLEASEUNASSIGNED is similar to a DHCPLEASEACTIVE message, but
indicates that there is no currently active lease on the resultant IP
address but that this DHCP server is authoritative for this IP
address. The DHCPLEASEUNKNOWN message indicates that the DHCP server
has no knowledge of the information specified in the query (e.g., IP
address, MAC address, or Client-identifier option).
The DHCPLEASEQUERY message does not presuppose a particular use for
the information it returns -- it is simply designed to return
information for which the DHCP server is an authoritative source to a
client that requests that information. It is designed to make it
straightforward for processes and devices that already interpret DHCP
packets to access information from the DHCP server.
This document specifies an extension specifically to the DHCPv4
protocol [RFC2131]. Given the nature of the DHCPv6 protocol
[RFC3315], there is no effective way to make the DHCPLEASEQUERY
message interaction common between DHCPv4 and DHCPv6 even should the
desire to do so exist.
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The DHCPLEASEQUERY message was the result of a set of specific real-
world implementation needs that appeared many years after the DHCPv4
protocol was in wide use. Furthermore, at the time of this writing,
the DHCPv6 protocol has yet to be widely deployed. The needs of
access concentrators in yet to be determined DHCPv6 deployment
scenarios are difficult to estimate. If a DHCPLEASEQUERY-like
function is necessary in DHCPv6, many of the ideas of this document
will probably be applicable, while others may not. We have been
cautioned against designing protocol capabilities for which there is
only an imagined consumer, and that is all that exists today in the
realm of DHCPLEASEQUERY for DHCPv6.
Thus, this document applies only to DHCPv4, and for clarity we have
not appended DHCPv4 to every appearance of several common terms. In
this document, all references to IP addresses should be taken to mean
IPv4 addresses, and all references to DHCP servers and DHCP clients
should be taken to mean DHCPv4 servers and DHCPv4 clients.
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 RFC 2119 [RFC2119].
This document uses the following terms:
o "access concentrator"
An access concentrator is a router or switch at the broadband
access provider's edge of a public broadband access network.
This document assumes that the access concentrator includes
the DHCP relay agent functionality.
o "DHCP client"
A DHCP client is an Internet host using DHCP to obtain
configuration parameters such as a network address.
o "DHCP relay agent"
A DHCP relay agent is a third-party agent that transfers
Bootstrap Protocol (BOOTP) and DHCP messages between clients
and servers residing on different subnets, per [RFC951] and
[RFC1542].
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o "DHCP server"
A DHCP server is an Internet host that returns configuration
parameters to DHCP clients.
o "downstream"
Downstream is the direction from the access concentrator
towards the broadband subscriber.
o "gleaning"
Gleaning is the extraction of location information from DHCP
messages, as the messages are forwarded by the DHCP relay
agent function.
o "location information"
Location information is information needed by the access
concentrator to forward traffic to a broadband-accessible
host. This information includes knowledge of the host
hardware address, the port or virtual circuit that leads to
the host, and/or the hardware address of the intervening
subscriber modem.
o "MAC address"
In the context of a DHCP packet, a MAC address consists of the
following fields: hardware type "htype", hardware length
"hlen", and client hardware address "chaddr".
o "stable storage"
Every DHCP server is assumed to have some form of what is
called "stable storage". Stable storage is used to hold
information concerning IP address bindings (among other
things) so that this information is not lost in the event of a
server failure that requires restart of the server.
o "upstream"
Upstream is the direction from the broadband subscriber
towards the access concentrator.
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3. Background
The focus of this document is to enable processes and devices that
wish to access information from the DHCP server in a lightweight and
convenient manner. It is especially appropriate for processes and
devices that already interpret DHCP packets.
One important motivating example is that the DHCPLEASEQUERY message
allows access concentrators to send DHCPLEASEQUERY messages to DHCP
servers to obtain location information of broadband access network
devices.
This document assumes that many access concentrators have an embedded
DHCP relay agent functionality. Typical access concentrators include
DOCSIS Cable Modem Termination Systems (CMTSs) [DOCSIS], DVB
Interactive Network Adapters (INAs) [EUROMODEM], and DSL Access
Concentrators.
The DHCPLEASEQUERY message is an extension to the DHCP protocol
[RFC2131].
The DHCPLEASEQUERY message is a query message only and does not
affect the state of the IP address or the binding information
associated with it.
4. Design Goals
The goal of this document is to provide a lightweight mechanism for
processes or devices to access information contained in the DHCP
server. It is designed to allow processes and devices that already
process and interpret DHCP messages to access this information in a
rapid and lightweight manner.
Some of this information might be acquired in a different way, and
the following sections discuss some of these alternative approaches.
4.1. Broadcast ARP Is Undesirable
The access concentrator can transmit a broadcast Address Resolution
Protocol (ARP) Request [RFC826], and observe the origin and contents
of the ARP Reply, to reconstruct the location information.
The ARP mechanism is undesirable for three reasons:
1. the burden on the access concentrator to transmit over
multiple access ports and virtual circuits (assuming that IP
subnets span multiple ports or virtual circuits),
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2. the burden on the numerous subscriber hosts to receive and
process the broadcast, and
3. the ease by which a malicious host can misrepresent itself as
the IP endpoint.
4.2. SNMP and LDAP Are Not Appropriate
Access concentrator implementations typically do not have Simple
Network Management Protocol (SNMP) management client interfaces nor
Lightweight Directory Access Protocol (LDAP) client interfaces
(although they typically do include SNMP management agents). This is
one reason why this document does not leverage the proposed DHCP
Server MIB [DHCPMIB].
The DHCP Server MIB effort [DHCPMIB] grew out of traffic engineering
and troubleshooting activities at large DHCP installations, and is
primarily intended as a method of gathering performance statistics
about servers the load presented to them.
Despite the presence in the proposed DHCPv4 server MIB of objects
that report configuration and status information, the MIB is intended
to provide more generic, server-wide aggregated or summarized data.
DHCPLEASEQUERY is intended to provide detailed, specific information
about individual leases at a level that would be difficult or
impossible to shoehorn into a MIB.
From an implementation standpoint, the DHCPLEASEQUERY message is not
required to be supported by all DHCPv4 servers. Since it appears
that defining optional MIB objects and objects for optional features
in a MIB is discouraged, trying to support DHCPLEASEQUERY
functionality optionally through a MIB would be similarly discouraged
from an SNMP MIB standpoint.
4.3. DHCP Relay Agent Functionality Is Common
Access concentrators commonly act as DHCP relay agents. Furthermore,
many access concentrators already glean location information from
DHCP server responses, as part of the relay agent function.
The gleaning mechanism as a technique to determine the IP addresses
valid for a particular downstream link is preferred over other
mechanisms (ARP, SNMP, LDAP) because of the lack of additional
network traffic, but sometimes gleaning information can be
incomplete. The access concentrator usually cannot glean information
from any DHCP unicast (i.e., non-relayed) messages due to performance
reasons. Furthermore, the DHCP-gleaned location information often
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does not persist across access concentrator reboots (due to lack of
stable storage), and almost never persists across concentrator
replacements.
4.4. DHCP Servers Are a Reliable Source of Location Information
DHCP servers are the most reliable source of location information for
access concentrators, particularly when the location information is
dynamic and not reproducible by algorithmic means (e.g., when a
single IP subnet extends behind many broadband modems). DHCP servers
participate in all IP lease transactions (and therefore in all
location information updates) with DHCP clients, whereas access
concentrators sometimes miss some important lease transactions.
An access concentrator can be configured with the IP addresses of
multiple different DHCP servers, so that no one DHCP server is a
single point of failure.
4.5. Minimal Additional Configuration Is Required
Access concentrators can usually query the same set of DHCP servers
used for forwarding by the relay agent, thus minimizing configuration
requirements.
5. Protocol Overview
In the following discussion of the DHCPLEASEQUERY message, the client
of the message is assumed to be an access concentrator. Note that
access concentrators are not the only allowed (or required) consumers
of the information provided by the DHCPLEASEQUERY message, but they
do give readers a concrete feel for how the message might be used.
The access concentrator initiates all DHCPLEASEQUERY message
conversations. This document assumes that the access concentrator
gleans location information in its DHCP relay agent function.
However, the location information is usually unavailable after the
reboot or replacement of the access concentrator.
Suppose the access concentrator is a router, and further suppose that
the router receives an IP datagram to forward downstream to the
public broadband access network. If the location information for the
downstream next hop is missing, the access concentrator sends one or
more DHCPLEASEQUERY message(s), each containing the IP address of the
downstream next hop in the "ciaddr" field.
This query will then be answered by returning the information current
when this client's lease was last granted or renewed, allowing the
access concentrator to forward the IP datagram.
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An alternative approach is to send in a DHCPLEASEQUERY message with
the "ciaddr" field empty and the MAC address (i.e., "htype", "hlen",
and "chaddr" fields) with a valid MAC address or a Client-identifier
option (option 61) appearing in the options area. In this case, the
DHCP server must return an IP address in the ciaddr if it has any
record of the client described by the Client-identifier or MAC
address. In the absence of specific configuration information to the
contrary (see Section 6.4), it SHOULD be the IP address with the
latest client-last-transaction-time associated with the client
described by the MAC address or Client-identifier option.
The DHCP servers that implement this protocol always send a response
to the DHCPLEASEQUERY message: either a DHCPLEASEUNASSIGNED,
DHCPLEASEACTIVE, or DHCPLEASEUNKNOWN. The reasons why a
DHCPLEASEUNASSIGNED, DHCPLEASEACTIVE, or DHCPLEASEUNKNOWN message
might be generated are explained in the specific query regimes,
below.
Servers that do not implement the DHCPLEASEQUERY message SHOULD
simply not respond.
The DHCPLEASEQUERY message can support three query regimes: A server
that implements the DHCPLEASEQUERY message must implement all three
query regimes.
o Query by IP address:
For this query, the requester supplies only an IP address in the
DHCPLEASEQUERY message. The DHCP server will return any
information that it has on the most recent client to have been
assigned that IP address.
The DHCP server replies with a DHCPLEASEUNASSIGNED or
DHCPLEASEACTIVE message if the IP address in the DHCPLEASEQUERY
message corresponds to an IP address about which the server has
definitive information (i.e., it is authorized to lease this IP
address). The server replies with a DHCPLEASEUNKNOWN message if
the server does not have definitive information concerning the
address in the DHCPLEASEQUERY message.
o Query by MAC address:
For this query, the requester supplies only a MAC address in the
DHCPLEASEQUERY message. The DHCP server will return any
information that it has on the IP address most recently accessed
by a client with that MAC address. In addition, it may supply
additional IP addresses that have been associated with that MAC
address in different subnets. Information about these bindings
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RFC 4388 DHCP Leasequery February 2006
can then be found using the Query by IP Address, described
above.
The DHCP server replies with a DHCPLEASEACTIVE message if the
MAC address in the DHCPLEASEQUERY message corresponds to a MAC
address with an active lease on an IP address in this server.
The server replies with a DHCPLEASEUNKNOWN message if the server
does not presently have an active lease by a client with this
MAC address in this DHCP server.
o Query by Client-identifier option:
For this query, the requester supplies only a Client-identifier
option in the DHCPLEASEQUERY message. The DHCP server will
return any information that it has on the IP address most
recently accessed by a client with that Client-identifier. In
addition, it may supply additional IP addresses that have been
associated with Client-identifier in different subnets.
Information about these bindings can then be found using the
Query by IP Address, described above.
The DHCP server replies with a DHCPLEASEACTIVE message if the
Client-identifier in the DHCPLEASEQUERY message currently has an
active lease on an IP address in this DHCP server. The server
replies with a DHCPLEASEUNKNOWN message if the server does not
have an active lease by a client with this Client-identifier.
For many DHCP servers, the query by IP address is likely to be the
most efficient form of leasequery. This is the form of
DHCPLEASEQUERY that SHOULD be used if possible.
The DHCPLEASEUNASSIGNED or DHCPLEASEACTIVE message reply must always
contain the IP address in the "ciaddr" field. The DHCPLEASEACTIVE
message SHOULD contain the physical address of the IP address lease
owner in the "htype", "hlen", and "chaddr" fields. The Parameter
Request List (option 55) can be used to request specific options to
be returned about the IP address in the ciaddr. The reply often
contains the time until expiration of the lease, and the original
contents of the Relay Agent Information option [RFC3046]. The access
concentrator uses the "chaddr" field and Relay Agent Information
option to construct location information, which can be cached on the
access concentrator until lease expiration.
Any DHCP server that supports the DHCPLEASEQUERY message SHOULD save
the information from the most recent Relay Agent Information option
(option 82) [RFC3046] associated with every IP address that it
serves. It is assumed that most clients that generate the
DHCPLEASEQUERY message will ask for the Relay Agent Information
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RFC 4388 DHCP Leasequery February 2006
option (option 82) in the Parameter Request List (option 55), and so
supporting the DHCPLEASEQUERY message without having the Relay Agent
Information option around to return to the client is likely to be
less than helpful.
A server that implements DHCPLEASEQUERY SHOULD also save the
information on the most recent Vendor class identifier, option 60,
associated with each IP address, since this option is also likely to
be requested by clients sending the DHCPLEASEQUERY message.
6. Protocol Details
6.1. Definitions Required for DHCPLEASEQUERY Processing
The operation of the DHCPLEASEQUERY message requires the definition
of the following new and extended values for the DHCP packet beyond
those defined by [RFC2131] and [RFC2132]. See also Section 8, IANA
Considerations.
1. The message type option (option 53) from [RFC2132] requires
four new values: one for the DHCPLEASEQUERY message itself and
one for each of its three possible responses
DHCPLEASEUNASSIGNED, DHCPLEASEACTIVE, DHCPLEASEUNKNOWN. The
values of these message types are shown below in an extension
of the table from section 9.6 of [RFC2132]:
Value Message Type
----- ------------
10 DHCPLEASEQUERY
11 DHCPLEASEUNASSIGNED
12 DHCPLEASEUNKNOWN
13 DHCPLEASEACTIVE
2. There is a new option, the client-last-transaction-time:
client-last-transaction-time
This option allows the receiver to determine the time of the
most recent access of the client. It is particularly useful
when DHCPLEASEACTIVE messages from two different DHCP servers
need to be compared, although it can be useful in other
situations. The value is a duration in seconds from the
current time into the past when this IP address was most
recently the subject of communication between the client and
the DHCP server.
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This MUST NOT be an absolute time. This MUST NOT be an
absolute number of seconds since Jan. 1, 1970. Instead, this
MUST be an integer number of seconds in the past from the time
the DHCPLEASEACTIVE message is sent that the client last dealt
with this server about this IP address. In the same way that
the IP Address Lease Time option (option 51) encodes a lease
time that is a number of seconds into the future from the time
the message was sent, this option encodes a value that is a
number of seconds into the past from when the message was
sent.
The code for the this option is 91. The length of the this
option is 4 octets.
Code Len Seconds in the past
+-----+-----+-----+-----+-----+-----+
| 91 | 4 | t1 | t2 | t3 | t4 |
+-----+-----+-----+-----+-----+-----+
3. There in a second new option, the associated-ip option:
associated-ip
This option is used to return all of the IP addresses
associated with the DHCP client specified in a particular
DHCPLEASEQUERY message.
The code for this option is 92. The minimum length for this
option is 4 octets, and the length MUST always be a multiple
of 4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
| 92 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
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6.2. Sending the DHCPLEASEQUERY Message
The DHCPLEASEQUERY message is typically sent by an access
concentrator. The DHCPLEASEQUERY message uses the DHCP message
format as described in [RFC2131], and uses message number 10 in the
DHCP Message Type option (option 53). The DHCPLEASEQUERY message has
the following pertinent message contents:
o The giaddr MUST be set to the IP address of the requester (i.e.,
the access concentrator). The giaddr is independent of the
"ciaddr" field to be searched -- it is simply the return address
of the DHCPLEASEUNASSIGNED, DHCPLEASEACTIVE, or DHCPLEASEUNKNOWN
message from the DHCP server.
Note that this use of the giaddr is consistent with the
definition of giaddr in [RFC2131], where the giaddr is always
used as the return address of the DHCP response message. In some
(but not all) contexts in RFC 2131, the giaddr is used as the
"key" to access the appropriate address pool. The DHCPLEASEQUERY
message is one of those cases where the giaddr MUST NOT be used
as such a "key".
o The Parameter Request List option (option 55) SHOULD be set to
the options of interest to the requester. The interesting
options are likely to include the IP Address Lease Time option
(option 51), the Relay Agent Information option (option 82), and
possibly the Vendor class identifier option (option 60). In the
absence of a Parameter Request List option, the server SHOULD
return the same options it would return for a DHCPREQUEST message
that didn't contain a DHCPLEASEQUERY message, which includes
those mandated by Section 4.3.1 of [RFC2131] as well as any
options that the server was configured to always return to a
client.
Additional details concerning different query types are:
o Query by IP address:
The values of htype, hlen, and chaddr MUST be set to zero.
The "ciaddr" field MUST be set to the IP address of the lease to
be queried.
The Client-identifier option (option 61) MUST NOT appear in the
packet.
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o Query by MAC address:
The values of htype, hlen, and chaddr MUST be set to the value
of the MAC address to search for.
The "ciaddr" field MUST be set to zero.
The Client-identifier option (option 61) MUST NOT appear in the
packet.
o Query by Client-identifier option:
There MUST be a Client-identifier option (option 61) in the
DHCPLEASEQUERY message.
The "ciaddr" field MUST be set to zero.
The values of htype, hlen, and chaddr MUST be set to zero.
The DHCPLEASEQUERY message SHOULD be sent to a DHCP server which is
known to possess authoritative information concerning the IP address.
The DHCPLEASEQUERY message MAY be sent to more than one DHCP server,
and in the absence of information concerning which DHCP server might
possess authoritative information concerning the IP address, it
SHOULD be sent to all DHCP servers configured for the associated
relay agent (if any are known).
Any device expecting to use a DHCPLEASEQUERY message SHOULD ensure
that the Relay Agent Info option that it uses contains information
that unambiguously identifies the device.
6.3. Receiving the DHCPLEASEQUERY Message
A server that implements the DHCPLEASEQUERY message MUST implement
all three query regimes: query by IP address, query by MAC address,
and query by Client-identifier.
A DHCPLEASEQUERY message MUST have a non-zero giaddr. The
DHCPLEASEQUERY message MUST have exactly one of the following: a
non-zero ciaddr, a non-zero htype/hlen/chaddr, or a Client-identifier
option.
The DHCP server that receives a DHCPLEASEQUERY message MUST base its
response on the particular data item used in the query.
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The giaddr is used only for the destination address of any generated
response and, while required, is not otherwise used in generating the
response to the DHCPLEASEQUERY message. It MUST NOT be used to
restrict the processing of the query in any way, and MUST NOT be used
locate a subnet to which the ciaddr (if any) must belong.
Note that this use of the giaddr is consistent with the definition of
giaddr in [RFC2131], where the giaddr is always used as the return
address of the DHCP response message. In some (but not all) contexts
in RFC 2131, the giaddr is used as the "key" to access the
appropriate address pool. The DHCPLEASEQUERY message is one of those
cases where the giaddr MUST NOT be used as such a "key".
6.4. Responding to the DHCPLEASEQUERY Message
There are three possible responses to a DHCPLEASEQUERY message:
o DHCPLEASEUNASSIGNED
The server MUST respond with a DHCPLEASEUNASSIGNED message if
this server has information about the IP address, but there is
no active lease for the IP address. The DHCPLEASEUNASSIGNED
message is only returned for a query by IP address, and
indicates that the server manages this IP address, but there is
no currently active lease on this IP address.
o DHCPLEASEUNKNOWN
The DHCPLEASEUNKNOWN message indicates that the server does not
manage the IP address or the client specified in the
DHCPLEASEQUERY message does not currently have a lease on an IP
address.
When responding with a DHCPLEASEUNKNOWN, the DHCP server MUST
NOT include other DHCP options in the response.
o DHCPLEASEACTIVE
The DHCPLEASEACTIVE message indicates that the server not only
knows about the IP address and client specified in the
DHCPLEASEACTIVE message, but also knows that there is an active
lease by that client for that IP address.
The server MUST respond with a DHCPLEASEACTIVE message when the
IP address returned in the "ciaddr" field is currently leased.
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6.4.1. Determining the IP address about Which to Respond
Since the response to a DHCPLEASEQUERY request can only contain full
information about one IP address -- the one that appears in the
"ciaddr" field -- determination of which IP address about which to
respond is a key issue. Of course, the values of additional IP
addresses for which a client has a lease must also be returned in the
associated-ip option (Section 6.1, #3). This is the only information
returned not directly associated with the IP address in the "ciaddr"
field.
In the event that an IP address appears in the "ciaddr" field of a
DHCPLEASEQUERY message, if that IP address is one managed by the DHCP
server, then that IP address MUST be set in the "ciaddr" field of a
DHCPLEASEUNASSIGNED message.
If the IP address is not managed by the DHCP server, then a
DHCPLEASEUNKNOWN message must be returned.
If the "ciaddr" field of the DHCPLEASEQUERY is zero, then the
DHCPLEASEQUERY message is a query by Client-identifier or MAC
address. In this case, the client's identity is any client that has
proffered an identical Client-identifier option (if the Client-
identifier option appears in the DHCPLEASEQUERY message), or an
identical MAC address (if the MAC address fields in the
DHCPLEASEQUERY message are non-zero). This client matching approach
will, for the purposes of this section, be described as "Client-
identifier or MAC address".
If the "ciaddr" field is zero in a DHCPLEASEQUERY message, then the
IP address placed in the "ciaddr" field of a DHCPLEASEACTIVE message
MUST be that of an IP address for which the client that most recently
used the IP address matches the Client-identifier or MAC address
specified in the DHCPLEASEQUERY message.
If there is only a single IP address that fulfills this criteria,
then it MUST be placed in the "ciaddr" field of the DHCPLEASEACTIVE
message.
In the case where more than one IP address has been accessed by the
client specified by the MAC address or Client-identifier option, then
the DHCP server MUST return the IP address returned to the client in
the most recent transaction with the client unless the DHCP server
has been configured by the server administrator to use some other
preference mechanism.
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RFC 4388 DHCP Leasequery February 2006
If, after all of the above processing, no value is set in the
"ciaddr" field of the DHCPLEASEUNASSIGNED or DHCPLEASEACTIVE message,
then a DHCPLEASEUNKNOWN message MUST be returned instead.
6.4.2. Building a DHCPLEASEUNASSIGNED or DHCPLEASEACTIVE Message Once
the "ciaddr" Field Is Set
Once the "ciaddr" field of the DHCPLEASEUNASSIGNED is set, the
processing for a DHCPLEASEUNASSIGNED message is complete. No other
options are returned for the DHCPLEASEUNASSIGNED message.
For the DHCPLEASEACTIVE message, the rest of the processing largely
involves returning information about the IP address specified in the
"ciaddr" field.
The IP address in the "ciaddr" field of the DHCPLEASEUNASSIGNED or
DHCPLEASEACTIVE message MUST be one for which this server is
responsible (or a DHCPLEASEUNKNOWN message would be have already been
returned early in the processing described in the previous section).
The MAC address of the DHCPLEASEACTIVE message MUST be set to the
values that identify the client associated with the IP address in the
"ciaddr" field of the DHCPLEASEUNASSIGNED message.
If the Client-identifier option (option 61) is specified in the
Parameter Request List option (option 55), then the Client-identifier
(if any) of the client associated with the IP address in the "ciaddr"
field SHOULD be returned in the DHCPLEASEACTIVE message.
In the case where more than one IP address has been involved in a
DHCP message exchange with the client specified by the MAC address
and/or Client-identifier option, then the list of all of the IP
addresses MUST be returned in the associated-ip option, whether or
not that option was requested as part of the Parameter Request List
option.
If the IP Address Lease Time option (option 51) is specified in the
Parameter Request List and if there is a currently valid lease for
the IP address specified in the ciaddr, then the DHCP server MUST
return this option in the DHCPLEASEACTIVE message with its value
equal to the time remaining until lease expiration. If there is no
valid lease for the IP address, then the server MUST NOT return the
IP Address Lease Time option (option 51).
A request for the Renewal (T1) Time Value option or the Rebinding
(T2) Time Value option in the Parameter Request List of the
DHCPLEASEQUERY message MUST be handled like the IP Address Lease Time
option is handled. If there is a valid lease and these times are not
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RFC 4388 DHCP Leasequery February 2006
yet in the past, then the DHCP server SHOULD return these options
(when requested) with the remaining time until renewal or rebinding,
respectively. If these times are already in the past, or if there is
not currently a valid lease for this IP address, the DHCP server MUST
NOT return these options.
If the Relay Agent Information (option 82) is specified in the
Parameter Request List, then the information contained in the most
recent Relay Agent Information option received from the relay agent
associated with this IP address MUST be included in the
DHCPLEASEACTIVE message.
The DHCPLEASEACTIVE message SHOULD include the values of all other
options not specifically discussed above that were requested in the
Parameter Request List of the DHCPLEASEQUERY message and that are
acceptable to return based on the list of "non-sensitive options",
discussed below.
DHCP servers SHOULD be configurable with a list of "non-sensitive
options" that can be returned to the client when specified in the
Parameter Request List of the DHCPLEASEQUERY message. Any option not
on this list SHOULD NOT be returned to a client, even if requested by
that client.
The DHCP server uses information from its lease binding database to
supply the DHCPLEASEACTIVE option values. The values of the options
that were returned to the DHCP client would generally be preferred,
but in the absence of those, options that were sent in DHCP client
requests would be acceptable.
In some cases, the Relay Agent Information option in an incoming
DHCPREQUEST packet is used to help determine the options returned to
the DHCP client that sent the DHCPREQUEST. When responding to a
DHCPLEASEQUERY message, the DHCP server MUST use the saved Relay
Agent Information option just like it did when responding to the DHCP
client in order to determine the values of any options requested by
the DHCPLEASEQUERY message. The goal is to return the same option
values to the DHCPLEASEQUERY as those that were returned to the
DHCPDISCOVER or DHCPREQUEST from the DHCP client (unless otherwise
specified, above).
In the event that two servers are cooperating to provide a high-
availability DHCP server, as supported by [RFC2131], they would have
to communicate some information about IP address bindings to each
other. In order to properly support the DHCPLEASEQUERY message,
these servers MUST ensure that they communicate the Relay Agent
Information option information to each other in addition to any other
IP address binding information.
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RFC 4388 DHCP Leasequery February 2006
6.4.3. Sending a DHCPLEASEUNASSIGNED, DHCPLEASEACTIVE, or
DHCPLEASEUNKNOWN Message
The server expects a giaddr in the DHCPLEASEQUERY message, and
unicasts the DHCPLEASEUNASSIGNED, DHCPLEASEACTIVE, or
DHCPLEASEUNKNOWN message to the giaddr. If the "giaddr" field is
zero, then the DHCP server MUST NOT reply to the DHCPLEASEQUERY
message.
6.5. Receiving a DHCPLEASEUNASSIGNED, DHCPLEASEACTIVE, or
DHCPLEASEUNKNOWN Message
When a DHCPLEASEACTIVE message is received in response to the
DHCPLEASEQUERY message, it means that there is a currently active
lease for this IP address in this DHCP server. The access
concentrator SHOULD use the information in the "htype", "hlen", and
"chaddr" fields of the DHCPLEASEACTIVE as well as any Relay Agent
Information option information included in the packet to refresh its
location information for this IP address.
When a DHCPLEASEUNASSIGNED message is received in response to the
DHCPLEASEQUERY message, that means that there is no currently active
lease for the IP address present in the DHCP server, but that this
server does in fact manage that IP address. In this case, the access
concentrator SHOULD cache this information in order to prevent
unacceptable loads on the access concentrator and the DHCP server in
the face of a malicious or seriously compromised device downstream of
the access concentrator. This caching could be as simple as simply
setting a bit saying that a response was received from a server that
knew about this IP address but that there was no current lease. This
would, of course, need to be cleared when the access concentrator
next "gleaned" that a lease for this IP address came into existence.
In either case, when a DHCPLEASEUNASSIGNED or DHCPLEASEACTIVE message
is received in response to a DHCPLEASEQUERY message, it means that
the DHCP server that responded is a DHCP server that manages the IP
address present in the ciaddr, and the Relay Agent SHOULD cache this
information for later use.
When a DHCPLEASEUNKNOWN message is received by an access concentrator
that has sent out a DHCPLEASEQUERY message, it means that the DHCP
server contacted supports the DHCPLEASEQUERY message but that the
DHCP server does not have definitive information concerning the IP
address contained in the "ciaddr" field of the DHCPLEASEQUERY
message. If there is no IP address in the "ciaddr" field of the
DHCPLEASEQUERY message, then a DHCPLEASEUNKNOWN message means that
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RFC 4388 DHCP Leasequery February 2006
the DHCP server does not have definitive information concerning the
DHCP client specified in the "hlen", "htype", and "chaddr" fields or
the Client-identifier option of the DHCPLEASEQUERY message.
The access concentrator SHOULD cache this information, but only for a
relatively short lifetime, approximately 5 minutes.
Having cached this information, the access concentrator SHOULD only
infrequently direct a DHCPLEASEQUERY message to a DHCP server that
responded to a DHCPLEASEQUERY message for a particular "ciaddr" field
with a DHCPLEASEUNKNOWN.
6.6. Receiving No Response to the DHCPLEASEQUERY Message
When an access concentrator receives no response to a DHCPLEASEQUERY
message, there are several possible reasons:
o The DHCPLEASEQUERY or a corresponding DHCPLEASEUNASSIGNED,
DHCPLEASEACTIVE, or DHCPLEASEUNKNOWN was lost during transmission
or the DHCPLEASEQUERY arrived at the DHCP server but it was
dropped because the server was too busy.
o The DHCP server doesn't support DHCPLEASEQUERY.
In the first of the cases above, a retransmission of the
DHCPLEASEQUERY would be appropriate, but in the second of the two
cases, a retransmission would not be appropriate. There is no way to
tell these two cases apart (other than, perhaps, because of a DHCP
server's response to other DHCPLEASEQUERY messages indicating that it
does or does not support the DHCPLEASEQUERY message).
An access concentrator that utilizes the DHCPLEASEQUERY message
SHOULD attempt to resend DHCPLEASEQUERY messages to servers that do
not respond to them using a backoff algorithm for the retry time that
approximates an exponential backoff. The access concentrator SHOULD
adjust the backoff approach such that DHCPLEASEQUERY messages do not
arrive at a server that is not otherwise known to support the
DHCPLEASEQUERY message at a rate of more than approximately one
packet every 10 seconds, and yet (if the access concentrator needs to
send DHCPLEASEQUERY messages) not less than one DHCPLEASEQUERY per 70
seconds.
In practice, this approach would probably best be handled by a per-
server timer that is restarted whenever a response to a
DHCPLEASEQUERY message is received, and expires after one minute.
The per-server timer would start off expired, and in the expired
state only one DHCPLEASEQUERY message would be queued for the
associated server.
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RFC 4388 DHCP Leasequery February 2006
All DHCPLEASEQUERY messages SHOULD use the exponential backoff
algorithm specified in Section 4.1 of [RFC2131].
Thus, in the initial state, the per-server timer is expired, and a
single DHCPLEASEQUERY message is queued for each server. After the
first response to a DHCPLEASEQUERY message, the per-server timer is
started. At that time, multiple DHCPLEASEQUERY messages can be sent
in parallel to the DHCP server, though the total number SHOULD be
limited to 100 or 200, to avoid swamping the DHCP server. Each of
these messages uses the [RFC2131] exponential backoff algorithm.
Every time a response to any of these messages is received, the per-
server timer is reset and starts counting again up to one minute. In
the event the per-server timer goes off, then all outstanding
messages SHOULD be dropped except for a single DHCPLEASEQUERY message
that is used to poll the server at approximately 64-second intervals
until such time as another (or the first) response to the
DHCPLEASEQUERY is received.
In the event that there is no DHCPLEASEQUERY traffic for one minute,
then the per-server timer will expire. After that time, there will
only be one DHCPLEASEQUERY message allowed to be outstanding to that
server until a response to that message is received.
6.7. Lease Binding Data Storage Requirements
DHCP server implementations that implement the DHCPLEASEQUERY
capability MUST save the most recent Relay Agent Information option
from the most recent DHCPREQUEST packet for two reasons. First, it
is almost certain to be requested by in the dhcp-parameter-request-
list option in any DHCPLEASEQUERY request. Second, the saved Relay
Agent Information option may be necessary to determine the value of
other options given to the DHCP client, if these are requested by the
dhcp-parameter-request list in the DHCPLEASEQUERY request.
This is a list of the information that is required to successfully
implement
o relay-agent-info option from client packet: MUST store with
binding.
o client-last-transaction-time of last client interaction: MUST
store with binding.
o vendor-class-id: SHOULD store with binding.
These data storage requirements are minimally larger than those
required for normal operation of the DHCP protocol, as required to
properly implement [RFC2131].
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RFC 4388 DHCP Leasequery February 2006
6.8. Using the DHCPLEASEQUERY Message with Multiple DHCP Servers
When using the DHCPLEASEQUERY message in an environment where
multiple DHCP servers may contain authoritative information about the
same IP address (such as when two DHCP servers are cooperating to
provide a high-availability DHCP service), multiple, possibly
conflicting, responses might be received.
In this case, some information in the response packet SHOULD be used
to decide among the various responses. The client-last-transaction-
time (if it is available) can be used to decide which server has more
recent information concerning the IP address returned in the "ciaddr"
field.
7. Security Considerations
Access concentrators that use DHCP gleaning, refreshed with
DHCPLEASEQUERY messages, will maintain accurate location information.
Location information accuracy ensures that the access concentrator
can forward data traffic to the intended location in the broadband
access network, can perform IP source address verification of
datagrams from the access network, and can encrypt traffic that can
only be decrypted by the intended access modem (e.g., [BPI] and
[BPI+]). As a result, the access concentrator does not need to
depend on ARP broadcasts across the access network, which is
susceptible to malicious hosts that masquerade as the intended IP
endpoints. Thus, the DHCPLEASEQUERY message allows an access
concentrator to provide considerably enhanced security.
DHCP servers SHOULD prevent exposure of location information
(particularly the mapping of hardware address to IP address lease,
which can be an invasion of broadband subscriber privacy) by
employing the techniques detailed in [RFC3118], "Authentication for
DHCP Messages".
This RFC describes how a DHCP client interacts with a DHCP server.
Access concentrators that send the DHCPLEASEQUERY message are
essentially DHCP clients for the purposes of the DHCPLEASEQUERY
message, even though they perform the functions of a DHCP relay agent
as well. Thus, [RFC3118] is an appropriate mechanism for
DHCPLEASEQUERY messages.
Since [RFC3118] discusses the normal DHCP client interaction,
consisting of a DHCPDISCOVER, DHCPOFFER, DHCPREQUEST, and DHCPACK, it
is necessary to transpose the operations described in [RFC3118] to
the DHCPLEASEQUERY domain. The operations described in [RFC3118] for
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RFC 4388 DHCP Leasequery February 2006
DHCPDISCOVER are performed for DHCPLEASEQUERY, and the operations
described for DHCPOFFER are performed for DHCPLEASEUNASSIGNED,
DHCPLEASEACTIVE, and DHCPLEASEUNKNOWN messages.
Access concentrators SHOULD minimize potential denial of service
attacks on the DHCP servers by minimizing the generation of
DHCPLEASEQUERY messages. In particular, the access concentrator
SHOULD employ negative caching (i.e., cache DHCPLEASEUNASSIGNED,
DHCPLEASEACTIVE, and DHCPLEASEUNKNOWN responses to DHCPLEASEQUERY
messages) and ciaddr restriction (i.e., don't send a DHCPLEASEQUERY
message with a ciaddr outside of the range of the attached broadband
access networks). Together, these mechanisms limit the access
concentrator to transmitting one DHCPLEASEQUERY message (excluding
message retries) per legitimate broadband access network IP address
after a reboot event.
DHCP servers supporting the DHCPLEASEQUERY message SHOULD ensure that
they cannot be successfully attacked by being flooded with large
quantities of DHCPLEASEQUERY messages in a short time.
In some environments, it may be appropriate to configure a DHCP
server with the IP addresses of the relay agents for which it may
respond to DHCPLEASEQUERY messages, thereby allowing it to respond
only to requests from only a handful of relay agents. This does not
provide any true security, but may be useful to thwart
unsophisticated attacks of various sorts.
8. IANA Considerations
IANA has assigned six values for this document. See Section 6.1 for
details. There are four new messages types, which are the value of
the message type option (option 53) from [RFC2132]. The value for
DHCPLEASEQUERY is 10, the value for DHCPLEASEUNASSIGNED is 11, the
value for DHCPLEASEUNKNOWN is 12, and the value for DHCPLEASEACTIVE
is 13. Finally, there are two new DHCP option defined; the client-
last-transaction-time option -- option code 91, and the associated-ip
option -- option code 92.
9. Acknowledgements
Jim Forster, Joe Ng, Guenter Roeck, and Mark Stapp contributed
greatly to the initial creation of the DHCPLEASEQUERY message.
Patrick Guelat suggested several improvements to support static IP
addressing. Thomas Narten made many suggestions for improvements.
Russ Housley pressed effectively for increased security capabilities,
and Ted Hardie suggested ways to minimize undesired information
leakage. Bert Wijnen suggested we clarify our focus to DHCPv4 and
Woundy & Kinnear Standards Track [Page 24]
RFC 4388 DHCP Leasequery February 2006
distinguish our approach from that of the DHCP MIB. R. Barr Hibbs,
one of the authors of the DHCP MIB, supplied information to
effectively distinguish that effort from DHCPLEASEQUERY.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 2001.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages", RFC 3118, June 2001.
10.2. Informative References
[RFC826] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37,
RFC 826, November 1982.
[RFC951] Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
September 1985.
[RFC1542] Wimer, W., "Clarifications and Extensions for the
Bootstrap Protocol", RFC 1542, October 1993.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP
Vendor Extensions", RFC 2132, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[BPI] SCTE Data Standards Subcommittee, "Data-Over-Cable
Service Interface Specifications: DOCSIS 1.0 Baseline
Privacy Interface Specification SCTE 22-2 2002", 2002,
available at http://www.scte.org/standards/.
Woundy & Kinnear Standards Track [Page 25]
RFC 4388 DHCP Leasequery February 2006
[BPI+] CableLabs, "Data-Over-Cable Service Interface
Specifications: Baseline Privacy Plus Interface
Specification CM-SP-BPI+_I12-050812", August 2005,
available at http://www.cablemodem.com/.
[DHCPMIB] Hibbs, R., Waters, G., "Dynamic Host Configuration
Protocol (DHCP) Server MIB", Work in Progress, February
2004.
[DOCSIS] SCTE Data Standards Subcommittee, "Data-Over-Cable
Service Interface Specifications: DOCSIS 1.0 Radio
Frequency Interface Specification SCTE 22-1 2002", 2002,
available at http://www.scte.org/standards/.
[EUROMODEM] ECCA, "Technical Specification of a European Cable Modem
for digital bi-directional communications via cable
networks", Version 1.0, May 1999.
Authors' Addresses
Rich Woundy
Comcast Cable
27 Industrial Ave.
Chelmsford, MA 01824
Phone: (978) 244-4010
EMail: richard_woundy@cable.comcast.com
Kim Kinnear
Cisco Systems
1414 Massachusetts Ave
Boxborough, MA 01719
Phone: (978) 936-0000
EMail: kkinnear@cisco.com
Woundy & Kinnear Standards Track [Page 26]
RFC 4388 DHCP Leasequery February 2006
Full Copyright Statement
Copyright (C) The Internet Society (2006).
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contained in BCP 78, and except as set forth therein, the authors
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Woundy & Kinnear Standards Track [Page 27]
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