Internet Engineering Task Force (IETF) P. Kurapati
Request for Comments: 6148 Juniper Networks
Updates: 4388 R. Desetti
Category: Standards Track B. Joshi
ISSN: 2070-1721 Infosys Technologies Ltd.
February 2011 DHCPv4 Lease Query by Relay Agent Remote ID
Some relay agents extract lease information from the DHCP messages
exchanged between the client and DHCP server. This lease information
is used by relay agents for various purposes like antispoofing and
prevention of flooding. RFC 4388 defines a mechanism for relay
agents to retrieve the lease information from the DHCP server when
this information is lost. The existing lease query mechanism is
data-driven, which means that a relay agent can initiate the lease
query only when it starts receiving data to and from the clients. In
certain scenarios, this model is not scalable. This document first
looks at issues in the existing mechanism and then proposes a new
query type, query by Remote ID, to address these issues.
Status of This Memo
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(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.
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Table of Contents
1. Introduction ....................................................32. Terminology .....................................................43. Motivation ......................................................64. Protocol Details ................................................74.1. Sending the DHCPLEASEQUERY Message .........................74.2. Responding to the DHCPLEASEQUERY Message ...................84.3. Building a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message .....84.4. Determining the IP Address to Be Used in Response ..........94.5. Sending a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message ......94.6. Receiving a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message ....94.7. Receiving No Response to the DHCPLEASEQUERY Message .......104.8. Lease-Binding Data Storage Requirements ...................104.9. Using the DHCPLEASEQUERY Message with Multiple
DHCP Servers ..............................................105. RFC 4388 Considerations ........................................116. Security Considerations ........................................117. Acknowledgments ................................................118. References .....................................................128.1. Normative References ......................................128.2. Informative References ....................................121. Introduction
DHCP relay agents snoop DHCP messages and append a Relay Agent
Information option before relaying them to the configured DHCP
server. In this process, some relay agents also glean the lease
information sent by the server and maintain this locally. This
information is used to prevent spoofing attempts from clients and
also sometimes to install routing information. When a relay agent
reboots, this information is lost. RFC 4388 [RFC4388] has defined a
mechanism to retrieve this lease information from the DHCP server.
The existing query types defined by RFC 4388 [RFC4388] are data-
driven. When a client sends data upstream, the relay agent can query
the server about the related lease information, based on the source
MAC/IP address. These mechanisms do not scale well when there are
thousands of clients connected to the relay agent. In the data-
driven model, lease query does not provide the full and consolidated
active lease information associated with a given connection/circuit,
which will result in inefficient anti-spoofing. The relay agent also
has to contend with considerable resources for negative caching,
especially under spoofing attacks.
We need a mechanism for a relay agent to retrieve the consolidated
lease information for a given connection/circuit before upstream
traffic is sent by the clients.
| DHCP | +--------------+
| Server |-...-| DSLAM |
| | | Relay Agent |
| | |
+-----+ +-----+ +-----+
|Node1| |Node2| |Node3|
+-----+ +-----+ +-----+
For example, when a DSLAM (Digital Subscriber Line Access
Multiplexer) acting as a relay agent is rebooted, it should query the
server for the lease information for all the connections/circuits.
Also, as shown in the above figure, there could be multiple clients
on one DSL circuit. The relay agent should get the lease information
of all the clients connected to a DSL circuit. This is possible by
introducing a new query type based on the Remote ID sub-option of the
Relay Agent Information option. This document talks about the
motivation for the new query type and the method to perform it.
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 node 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 RFC 951 [RFC951] and RFC 1542
o DHCP server
A DHCP server is an Internet node that returns configuration
parameters to DHCP clients.
o Fast path
Fast path refers to data transfer that happens through a network
processor or an Application Specific Integrated Circuit (ASIC)
programmed to forward the data at very high speeds.
Gleaning is the extraction of location information from DHCP
messages as the messages are forwarded by the DHCP relay agent
o Location information
Location information is information needed by the access
concentrator to forward traffic to a broadband-accessible node.
This information includes knowledge of the node's hardware
address, the port or virtual circuit that leads to the node,
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 Slow path
Slow path refers to data transfer that happens through the control
plane. This has very limited buffers to store data, and the
speeds are very low compared to the fast path data transfer.
Upstream is the direction from the broadband subscriber towards
the access concentrator.
Consider an access concentrator (e.g., DSLAM) working also as a DHCP
relay agent. A "fast path" and a "slow path" generally exist in most
networking boxes. Fast path processing is done in a network
processor or an ASIC. Slow path processing is done in a normal
processor. As much as possible, regular data forwarding should be
done in the fast path. Slow path processing should be reduced, as it
may become a bottleneck.
For an access concentrator having multiple access ports, multiple IP
addresses may be assigned to a single port using DHCP, and the number
of clients on a port may be unknown. The access concentrator may
also not know the network portions of the IP addresses that are
assigned to its DHCP clients.
The access concentrator gleans IP address or other information from
DHCP negotiations for antispoofing and other purposes. The
antispoofing itself is done in the fast path. The access
concentrator keeps track of only one list of IP addresses: the list
of IP addresses that are assigned by the DHCP servers; upstream
traffic from all other IP addresses is dropped. If a client starts
its data transfer after its DHCP negotiations have been gleaned by
the access concentrator, no legitimate packets will be dropped
because of antispoofing. In other words, antispoofing is effective
(no legitimate packets are dropped, and all spoofed packets are
dropped) and efficient (antispoofing is done in the fast path). The
intention is to achieve similar effective and efficient antispoofing
in the lease query scenario also, when an access concentrator loses
its gleaned information (for example, because of a reboot).
After a deep analysis, we found that the three existing query types
supported by RFC 4388 [RFC4388] do not provide effective and
efficient antispoofing for the above scenario, and a new mechanism is
The existing query types necessitate a data-driven approach: the
lease queries can only be performed when the access concentrator
receives data. This results in
o increased outage time for clients
o excessive negative caching, consuming a lot of resources under a
o antispoofing being done in the slow path instead of the fast path
4. Protocol Details
This section talks about the protocol details for query by Remote ID.
Most of the message handling is similar to RFC 4388 [RFC4388], and
this section highlights only the differences. Readers are advised to
go through RFC 4388 [RFC4388] before going through this section for
complete understanding of the protocol.
When used in this document, the unqualified term "DHCPLEASEQUERY"
indicates a lease query by Remote ID, unless otherwise specified.
RFC 3046 [RFC3046] defines two sub-options for the Relay Agent
Information option. Sub-option 1 corresponds to the Circuit ID that
identifies the local circuit of the access concentrator. This
sub-option is unique to the relay agent. Sub-option 2 corresponds to
the Remote ID that identifies the remote node connected to the access
concentrator. The Remote ID is globally unique in the network and is
configured per circuit/connection in the relay agent.
This document defines a new query type based on the Remote ID
sub-option. Suppose that the access concentrator (e.g., DSLAM) lost
the lease information when it was rebooted. When the access
concentrator comes up, it initiates (for each connection/circuit) a
DHCP lease query by Remote ID as defined in this section. For this
query, the requester supplies an option 82 that includes only a
Remote ID sub-option in the DHCPLEASEQUERY message. The Remote ID is
normally pre-provisioned in the access concentrator per circuit/
connection and hence will remain available to the access concentrator
The DHCP server MUST reply with a DHCPLEASEACTIVE message if there is
an active lease corresponding to the Remote ID that is present in the
DHCPLEASEQUERY message. Otherwise, the server MUST reply with a
DHCPLEASEUNKNOWN message. Servers that do not implement DHCP lease
query based on Remote ID SHOULD simply not respond.
4.1. Sending the DHCPLEASEQUERY Message
The lease query defined in this document will mostly be used by
access concentrators, but it may also be used by other authorized
elements in the network. The DHCPLEASEQUERY message uses the DHCP
message format as described in RFC 2131 [RFC2131], and uses message
number 10 in the DHCP Message Type option (option 53). The
DHCPLEASEQUERY message has the following pertinent message contents:
o There MUST be a Relay Agent Information option (option 82) with
only a Remote ID sub-option (sub-option 2) in the DHCPLEASEQUERY
o The Parameter Request List option [RFC2132] MUST be populated by
the access concentrator with the Associated-IP option code. The
giaddr field and other option codes listed in the Parameter
Request List option are set as explained in Section 6.2 of
RFC 4388 [RFC4388].
o The ciaddr field MUST be set to zero.
o The values of htype, hlen, and chaddr MUST be set to zero.
o The Client Identifier option (option 61) MUST NOT appear in the
The DHCPLEASEQUERY message SHOULD be sent to a DHCP server that is
known to possess authoritative information concerning the Remote ID.
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 Remote ID, it SHOULD
be sent to all DHCP servers configured for the associated relay agent
(if any are known).
4.2. Responding to the DHCPLEASEQUERY Message
There are two possible responses to a DHCPLEASEQUERY message:
The DHCPLEASEUNKNOWN message indicates that the client associated
with the Remote ID sub-option of the DHCPLEASEQUERY message is not
allocated any lease or it is not managed by the server.
The DHCPLEASEACTIVE message indicates that the server not only
knows the client specified in the DHCPLEASEQUERY message, but also
knows that there is an active lease for that client.
4.3. Building a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message
A DHCPLEASEACTIVE message is built by populating information
pertaining to the client associated with the IP address specified in
the ciaddr field.
In the case where more than one IP address has been involved in a
DHCP message exchange with the client specified by the Remote ID,
then the list of all those 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. This is intended for
maintaining backwards compatibility with RFC 4388 [RFC4388].
All other options specified in the Parameter Request List [RFC2132]
are processed as mentioned in Section 6.4.2 of RFC 4388 [RFC4388].
In a DHCPLEASEUNKNOWN response message, the DHCP server MUST echo the
option 82 received in the DHCPLEASEQUERY message. No other option is
included in the message.
4.4. Determining the IP Address to Be Used in Response
The IP address placed in the ciaddr field of a DHCPLEASEACTIVE
message MUST be the IP address with the latest client-last-
transaction-time associated with the client described by the Remote
ID 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
In the case where more than one IP address has been accessed by the
client specified by the Remote ID, 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.
4.5. Sending a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message
The server unicasts the DHCPLEASEACTIVE or DHCPLEASEUNKNOWN message
to the address specified in the giaddr field of the DHCPLEASEQUERY
4.6. Receiving a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message
When a DHCPLEASEACTIVE message is received in response to the
DHCPLEASEQUERY message, it means that there is currently an active
lease associated with the Remote ID in the DHCP server. The access
concentrator SHOULD use the information in the htype, hlen, and
chaddr fields of the DHCPLEASEACTIVE message as well as the Relay
Agent Information option included in the packet to refresh its
location information for this IP address. An access concentrator is
likely to query by IP address for all the IP addresses specified in
the Associated-IP option in the response, if any, at this point in
When a DHCPLEASEUNKNOWN message is received by an access concentrator
that had sent out a DHCPLEASEQUERY message, it means that the DHCP
server does not have definitive information concerning the DHCP
client specified in the Remote ID sub-option of the DHCPLEASEQUERY
message. The access concentrator MAY store this information for
future use. However, another DHCPLEASEQUERY message to the same DHCP
server SHOULD NOT be attempted with the same Remote ID sub-option.
For lease query by Remote ID, the impact of negative caching is
greatly reduced, as the response leads to "definitive" information on
all the nodes connected behind the connection. Note that in the case
of the data-driven approach [RFC4388], a node spoofing several IP
addresses can lead to negative caching of greater magnitude. Another
important change that this document brings is the removal of periodic
lease queries generated from negative caching caused by
DHCPLEASEUNKNOWN messages. Since the information obtained through
query by Remote ID is complete, there is no need to attempt lease
query again for the same connection.
4.7. Receiving No Response to the DHCPLEASEQUERY Message
The condition of an access concentrator receiving no response to a
DHCPLEASEQUERY message is handled in the same manner as suggested in
RFC 4388 [RFC4388].
4.8. Lease-Binding Data Storage Requirements
To generate replies for a lease query by Remote ID efficiently, a
DHCP server should index the lease-binding data structures using
4.9. Using the DHCPLEASEQUERY Message with Multiple DHCP Servers
This scenario is handled in the same way it is done in RFC 4388
5. RFC 4388 Considerations
This document is compatible with RFC 4388-based [RFC4388]
implementations, which means that a client that supports this
extension can work with a server not supporting this document,
provided it uses RFC 4388-defined query types. Also, a server
supporting this document can work with a client not supporting this
query type. However, there are some changes that this document
proposes with respect to RFC 4388 [RFC4388]. Implementers extending
RFC 4388 [RFC4388] implementations to support this document should
take note of the following points:
o There may be cases where a query by IP address/MAC address/Client
Identifier has an option 82 containing a Remote ID. In that case,
the query will still be recognized as a query by IP address/MAC
address/Client Identifier as specified by RFC 4388 [RFC4388].
o Section 6.4 of RFC 4388 [RFC4388] suggests that a DHCPLEASEUNKNOWN
message MUST NOT have any other option present. But for a query
by Remote ID, option 82 MUST be present in the reply.
6. Security Considerations
This document inherits the security concerns present in the original
lease query protocol specification (RFC 4388 [RFC4388]).
This specification introduces one additional issue, beyond those
described in RFC 4388 [RFC4388]. A query by Remote ID will result in
the server replying with consolidated lease-binding information.
Such a query, if done from an unauthorized source, may lead to a leak
of lease-binding information. It is critical to deploy
authentication techniques mentioned in RFC 3118 [RFC3118] to prevent
such unauthorized lease queries.
Copious amounts of text in this document are derived from RFC 4388
[RFC4388]. Kim Kinnear, Damien Neil, Stephen Jacob, Ted Lemon, Ralph
Droms, and Alfred Hoenes provided valuable feedback on this document.