DHC Working Group P. Kurapati
Internet-Draft R. Desetti
Expires: April 29, 2010 B. Joshi
Infosys Technologies Ltd.
October 26, 2009
DHCPv4 Leasequery by relay agent remote ID
draft-ietf-dhc-leasequery-by-remote-id-03.txt
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Abstract
Some Relay Agents extract lease information from the DHCP message
exchanged between the client and DHCP server. This lease information
is used by relay agents for various purposes like antispoofing,
prevention of flooding. RFC 4388 defines a mechanism for relay
agents to retrieve the lease information from the DHCP server as and
when this information is lost. Existing leasequery mechanism is data
driven which means that a relay agent can initiate the leasequery
only when it starts receiving data from/to the clients. In certain
scenarios, this model is not scalable. This document first looks at
issues in existing mechanism and then proposes a new query type,
query by remote ID, to address these issues.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Information Acquisition before Data Starts . . . . . . . . 9
4.2. Lessen Negative Caching . . . . . . . . . . . . . . . . . 9
4.3. Antispoofing in 'Fast Path' . . . . . . . . . . . . . . . 9
5. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 10
6. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Sending the DHCPLEASEQUERY Message . . . . . . . . . . . . 11
6.2. Receiving the DHCPLEASEQUERY Message . . . . . . . . . . . 12
6.3. Responding to the DHCPLEASEQUERY Message . . . . . . . . . 12
6.4. Determining the IP address to be used in response . . . . 12
6.5. Building a DHCPLEASEUNKNOWN or DHCPLEASEACTIVE Messages . 13
6.6. Sending a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message . . 15
6.7. Receiving a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message . 15
6.8. Receiving No Response to the DHCPLEASEQUERY Message . . . 15
6.9. Lease Binding Data Storage Requirements . . . . . . . . . 15
6.10. Using the DHCPLEASEQUERY Message with Multiple DHCP
Servers . . . . . . . . . . . . . . . . . . . . . . . . . 16
7. RFC 4388 Considerations . . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative Reference . . . . . . . . . . . . . . . . . . . 21
11.2. Informative Reference . . . . . . . . . . . . . . . . . . 21
Appendix A. Why a New Leasequery is Required? . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
DHCP relay agents snoop DHCP messages and append relay agent
information option before relaying it to the configured DHCP Servers.
In this process, some relay agents also glean the lease information
sent by the server and maintain this locally. This information is
used for prevention of spoofing attempts from the clients and also
sometimes used to install routing information. When 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 client initiates data, based on the source MAC/IP
address, relay agent can query the server about the lease
information. These mechanisms do not scale well when there are
thousands of clients connected to the relay agent. In data driven
model, DHCP Leasequery does not provide all the active Lease
informations associated with a given connection/circuit [consolidated
information] which will result into an inefficient anti-spoofing. It
also has to contend with considerable resources for negative caching
specially under spoof attacks.
We need a mechanism for relay agent to retrieve the consolidated
lease information for a given connection/circuit before traffic is
initiated by the clients.
+--------+
| DHCP | +--------------+
| Server |-...-| DSLAM |
| | | Relay Agent |
+--------+ +--------------+
| |
+------+ +------+
|Modem1| |Modem2|
+------+ +------+
| | |
+-----+ +-----+ +-----+
|Host1| |Host2| |Host3|
+-----+ +-----+ +-----+
Figure 1
For example, when a DSLAM 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. 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 Relay Agent Information option. This document talks
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about the motivation for the new query type and the method to do the
same.
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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[RFC951] and
RFC1542[RFC1542].
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 "fast path"
Data transfer which happens through Network Processor or an ASIC
which are programmed to forward the data at very high speeds.
o "gleaning"
Gleaning is the extraction of location information from DHCP
messages, as the messages are forwarded by the DHCP relay agent
function.
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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 "slow path"
Data transfer which happens through the control plane. Typically
this has very limited buffers to store data and the speeds are very
low compared to fast path data transfer.
o "upstream"
Upstream is the direction from the broadband subscriber towards the
access concentrator.
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3. Motivation
Consider a typical access concentrator (e.g., DSLAM) working also as
a DHCP relay agent. "Fast path" and "slow path" generally exist in
most networking boxes. Fast path processing is done in network
processor or an ASIC (Application Specific Integrated Circuit). Slow
path processing is done in a normal processor. As much as possible,
regular data handling code should be in 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 using DHCP to a single port 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 for
antispoofing and for other purposes from DHCP negotiations. The
antispoofing itself is done in fast path. Access concentrator keeps
track of only one list of IP addresses: list of IP addresses that are
assigned by DHCP server. Traffic for all other IP addresses is
dropped. If client starts its data transfer after its DHCP
negotiations are gleaned by 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
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 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 required.
The existing query types
o necessitate a data driven approach: the lease queries can only be
done when access concentrator receives data. That results in
increased outage time for clients.
o result in excessive negative caching consuming lot of resources
under a spoofing attack.
o result in antispoofing being done in slow path instead of fast
path.
The deeper analysis, which led to the above conclusions, itself
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appears as an Appendix to this document.
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4. Design Goals
The goal of this document is to provide a lightweight mechanism for
access concentrator to retrieve lease information available in the
DHCP server. The mechanism SHOULD also support an access
concentrator to retrieve consolidated lease information for a
connection/circuit.
4.1. Information Acquisition before Data Starts
Existing data driven approach by RFC 4388 [RFC4388] means that the
lease queries can only be done when access concentrator receives
data. If an approach exists to initiate lease queries even before
the calls come up, then it will be more effective. For antispoofing,
packets need to be dropped until it gets the lease information from
DHCP server. If access concentrator finishes the lease queries
before it start receiving data, then there is no need to drop
legitimate packets. So, effectively outage time may be reduced. The
lease queries should help in retrieving lease information even before
the data starts flowing and should be independent of data traffic.
4.2. Lessen Negative Caching
If lease queries result in negative caches, then that puts additional
overhead on access concentrator. The negative caches not only
consume precious resources they also need to be managed. Hence they
should be avoided as much as possible. The lease queries should
reduce the need for negative caching as far as possible.
4.3. Antispoofing in 'Fast Path'
If Antispoofing is not done in fast path, it will become a bottleneck
and may lead to denial of service of access concentrator. The lease
queries should make it possible to do antispoofing in fast path.
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5. Protocol Overview
RFC 3046 [RFC3046] defines two sub-options for Relay Agent
Information option. Sub-option 1 corresponds to circuit ID which
identifies the local circuit of the access concentrator. This sub-
option is unique to the relay agent. Sub-option 2 corresponds to
remote ID which identifies the remote host end of the circuit. This
is globally unique in the network.
This document defines a new query type based on 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 would initiate a DHCPLEASEQUERY message for each connection/
circuit containing the Relay Agent Information option [RFC3046] with
sub-option remote ID. DHCP server must return an IP address in the
ciaddr if it has any record of the client described by the remote ID.
In the absence of specific configuration information to the contrary,
it SHOULD be the IP address with the latest client-last-transaction-
time associated with the client described by the remote ID. The DHCP
servers that implement this document always send a response
(DHCPLEASEACTIVE or DHCPLEASEUNKNOWN) to the DHCPLEASEQUERY message.
The reasons why a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN message might
be generated are explained in the specific query regimes below.
Servers that do not implement the DHCPLEASEQUERY based on remote ID
message SHOULD simply not respond.
The query regime is described below:
o Query by Agent Remote ID sub-option:
For this query, the requester supplies only a option 82 which will
include only an Agent Remote ID sub-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 Agent
Remote ID. In addition, it SHOULD supply any additional IP addresses
that have been associated with Agent Remote ID in different subnets.
Information about these bindings can then be found using the Query by
IP Address, as described in RFC 4388[RFC4388].
The DHCP server MUST reply with a DHCPLEASEACTIVE message if the
Agent Remote ID in the DHCPLEASEQUERY message currently has an active
lease on an IP address in this DHCP server. Otherwise, the server
MUST reply with a DHCPLEASEUNKNOWN message.
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6. Protocol Details
In this section, DHCPLEASEQUERY message refers to DHCPLEASEQUERY
message with query by remote ID.
6.1. 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[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 the return address of the
DHCPLEASEACTIVE or DHCPLEASEUNKNOWN message from the DHCP server.
Note that this use of the giaddr is consistent with the definition
of giaddr in RFC2131[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 address to allocate to a client is
selected based on 'giaddr'.
o The Parameter Request List option (option 55) SHOULD be set to the
options of interest to the requester. It MUST include the Relay
Agent Information option (option 82). The other interesting
options are likely to include the IP Address Lease Time option
(option 51), 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 Parameter Request List
option (option 55), 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 Agent Remote ID sub-option:
* There MUST be a Relay Agent Information option (option 82) with
only Agent Remote ID sub-option (sub-option 2) 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 Client-identifier option (option 61) MUST NOT appear in the
packet.
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The DHCPLEASEQUERY message SHOULD be sent to a DHCP server which 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).
6.2. Receiving the DHCPLEASEQUERY Message
A DHCPLEASEQUERY message MUST have a non-zero giaddr. The
DHCPLEASEQUERY message MUST have a zero ciaddr, a zero htype/hlen/
chaddr, and no Client-identifier option. The DHCPLEASEQUERY message
MUST have a relay agent option 82 with only remote ID sub-option.
6.3. Responding to the DHCPLEASEQUERY Message
There are two possible responses to a DHCPLEASEQUERY message:
o DHCPLEASEUNKNOWN
The DHCPLEASEUNKNOWN message indicates that the client specified in
the DHCPLEASEQUERY message is not allocated any lease or it is not
managed by the server.
o DHCPLEASEACTIVE
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.
6.4. Determining the IP address to be used in response
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 (RFC 4388[RFC4388], Section 6.1, #3). This is
the only information returned not directly associated with the IP
address in the "ciaddr" field.
The client's identity is any client that has proffered an identical
Agent Remote ID (if the option 82 with Agent Remote ID sub-option
appears in DHCPLEASEQUERY message). This client matching approach
will, for the purposes of this section, be described as "remote ID".
The IP address placed in the "ciaddr" field of a DHCPLEASEACTIVE
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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
message.
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.
6.5. Building a DHCPLEASEUNKNOWN or DHCPLEASEACTIVE Messages
For a DHCPLEASEUNKNOWN message, DHCP server MUST echo the received
Option 82 available in DHCPLEASEQUERY in the response. No other
options are returned for this message. With that the processing for
a DHCPLEASEUNKNOWN message is complete.
For the DHCPLEASEACTIVE message, the rest of the processing largely
involves returning information about the IP address specified in the
"ciaddr" field.
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 DHCPLEASEACTIVE 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 Agent 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.
If the IP Address Lease Time option (option 51) is specified in the
Parameter Request List then the DHCP server MUST return this option
in the DHCPLEASEACTIVE message with its value equal to the time
remaining until lease expiration.
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
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option is handled. DHCP server SHOULD return these options (when
requested) with the remaining time until renewal or rebinding,
respectively.
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 access concentrator when
specified in the Parameter Request List of the DHCPLEASEQUERY
message. Any option not on this list SHOULD NOT be returned to an
access concentrator, even if requested by that access concentrator.
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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6.6. Sending a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message
The server expects a giaddr in the DHCPLEASEQUERY message, and
unicasts the DHCPLEASEACTIVE or DHCPLEASEUNKNOWN message to the
giaddr.
6.7. Receiving a 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 Relay Agent
Information option information 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
time.
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 Agent Remote ID sub-option of the
DHCPLEASEQUERY message. The access concentrator MAY store this
information for future use. However, a DHCPLEASEQUERY SHOULD NOT be
attempted with the same Remote ID su-option.
For leasequry by remote-id, the impact of negative caching is greatly
reduced as the the response leads to "definitive" information on all
the hosts connected behind the connection. Note that in the case of
RFC 4388 [RFC4388], a host spoofing several IP addresses can lead to
negative caching of greater magnitude. Another important change this
draft brings is the removal of "periodic" leasequeries generated from
negative caching caused by DHCPLEASEUNKNOWN. Since the information
obtained through query by remote-id is complete, there is no need of
attempting leasequery again for the same remote-id.
6.8. Receiving No Response to the DHCPLEASEQUERY Message
When an access concentrator receives no response to a DHCPLEASEQUERY
message, it should be handled in the same manner as suggested in RFC
4388 [RFC4388].
6.9. Lease Binding Data Storage Requirements
IMPLEMENTATION:
To generate replies for a lease query by remote-id effeciently, a
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DHCP server should index the lease binding data structures using
remote-id.
6.10. Using the DHCPLEASEQUERY Message with Multiple DHCP Servers
This scenario should be handled in the same way it is done in RFC
4388 [RFC4388].
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7. RFC 4388 Considerations
This document is compatible with RFC 4388 [RFC4388] based
implementations which means that a client which supports this
extension can work with a server not supporting this document
provided it uses RFC 4388 [RFC4388] 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]. Implementors extending
RFC 4388 [RFC4388] implementation to support this document, should
take note of the following points:
o RFC 4388 [RFC4388] suggests that a DHCPLEASEUNASSIGNED is returned
only in the case of 'query by IP address'. All other query types
will have a return message of either DHCPLEASEACTIVE or
DHCPLEASEUNKNOWN'. Although it is possible to return
DHCPLEASEUNASSIGNED in case of a query by remote-id, in order to
maintain compatibility with other similar query types (MAC and
Client-id) a query by remote-id does not support a
DHCPLEASEUNASSIGNED response.
o There may be cases where a query by IP address/MAC address/Client
Identifier has an option 82 containing remote ID. In that case,
the query will still be recognized as 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
MUST NOT have any other option present. But for a query by remote
ID, option 82 MUST be present in the reply.
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8. Security Considerations
This document does not introduce any new security concerns beyond
those specified in the original leasequery protocol RFC 4388
[RFC4388] specifications.
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9. IANA Considerations
This document does not introduce any new namespaces for the IANA to
manage.
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10. Acknowledgments
Copious amounts of text in this document are derived from RFC 4388
[RFC4388]. Kim kinnear provided valuable feedback on this document.
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11. References
11.1. Normative Reference
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration
Protocol (DHCP) Leasequery", RFC 4388, February 2006.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, March 1997.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option",
RFC 3046, January 2001.
11.2. Informative Reference
[RFC951] Croft, B. and J. Gilmore, "Bootstrap Protocol (BOOTP)",
RFC 951, September 1985.
[RFC1542] Wimer, W., "Clarifications and Extensions for the
Bootstrap Protocol", RFC 1542, October 1993.
[RFC2132] Droms, R. and S. Alexander, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
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Appendix A. Why a New Leasequery is Required?
The three existing query types supported by RFC 4388 do not provide
effective and efficient antispoofing for the above scenario.
o Query by Client Identifier
Query by Client Identifier is not possible because to use that access
concentrator need to glean client identifier also but the whole issue
is that we need leasequeries because the gleaned information was
lost. On the other hand, we can query by client identifier when
client sends a DHCP request, but then there may not be any need for
lease query as such -- regular gleaning may be enough.
o Query by IP Address
RFC 4388 suggests that it is preferable to use Query by IP Address
when getting downstream traffic.
Query by IP address is not very useful in downstream traffic because
downstream traffic may not exist for the clients on a access port.
(In most Internet applications, downstream traffic exists only when a
client sends upstream traffic). In other words, the client will be
denied service until it gets downstream traffic, which may never
come.
Query by IP address may be used for upstream traffic. Then whenever
an upstream packet comes whose IP address is unknown to the access
concentrator, a lease query may be initiated. A related question is
what to do with that upstream traffic itself until lease query
response comes? If the traffic is dropped, we may be dropping
legitimate traffic. If the traffic is forwarded, we may be
forwarding spoofed packets. Once the lease response comes,
subsequent traffic is handled depending on the response. If a
DHCPLEASEACTIVE response comes, access concentrator will accept the
traffic. If a DHCPLEASEUNASSIGNED response comes, access
concentrator will drop the traffic corresponding to the IP address.
If a DHCPLEASEUNKNOWN response comes, access concentrator may drop
the traffic corresponding to the IP address but will have to
periodically send the lease query for that IP address again
(additional overhead). The process is triggered whenever an unknown
IP address comes.
Note that access concentrator needs to keep track of 4 lists of IP
addresses: (1) List of IP addresses for which it got DHCPLEASEACTIVE
responses; (2) List of IP addresses for which it got
DHCPLEASEUNASSIGNED responses; (3) List of IP addresses for which it
got DHCPLEASEUNKNOWN responses; (4) All other IP addresses.
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This approach may be acceptable if only legitimate traffic is
received. Consider the case when someone sends packets that uses
spoofed IP addresses. In that case, lease response will be
DHCPLEASEUNASSIGNED or DHCPLEASEUNKNOWN. RFC 4388 suggests usage of
negative caching in this regard (which involves additional
resources).
In a spoofing type of attack, negative caching information may grow
considerably if attacker varies the source IP address. For each such
new source IP address, traffic will come to slow path, a new lease
query needs to be initiated, response will be processed, and negative
caching to be done. That will mean using many resources for negative
caching.
RFC 4388 suggests that if the access concentrator knows the network
portion of the IP addresses that are assigned to its clients, then
some amount of antispoofing can be done in fast path and some lease
queries may be avoided. But as indicated before, that information
may not always be available to access concentrators.
Effectively, antispoofing support involves considerable slow path
processing and considerable resources tied for negative caching.
RFC 4388 says that DHCP server should be protected from being flooded
with too many leasequery requests and access concentrator also should
not send too many lease query messages at a time. This would mean
that legitimate clients may be excessively delayed getting their
information in the face of antispoofing attacks.
It is concluded that antispoofing is neither effective nor efficient
with this query type.
o Query by MAC Address
Query by MAC address can also be used similar to query by IP address
described above. Indeed, query by MAC address may be better than
query by IP address in one sense because of the possible presence of
associated-ip option in lease responses (Note that associated-ip
option does not appear in responses for query by IP address). With
associated-ip option, access concentrator can get information not
only about the IP address/MAC address that triggered the lease query
but also about other IP addresses that are associated with the
original MAC address. That way, when traffic that uses the other IP
addresses comes along, access concentrator is already prepared to
deal with them.
Although, query by MAC address is better than query by IP address in
the above respect, it has a specific problem which is not shared by
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query by IP address. For a query by MAC address, only two types of
responses are possible: DHCPLEASEUNKNOWN and DHCPLEASEACTIVE;
DHCPLEASEUNASSIGNED is not supported. This is particularly
troublesome when a DHCP server indeed has definitive information that
no IP addresses are associated with the specified MAC address in the
leasequery, but it is forced to respond with DHCPLEASEUNKNOWN instead
of DHCPLEASEUNASSIGNED. As we have seen above, unlike
DHCPLEASEUNASSIGNED, DHCPLEASEUNKNOWN requires periodic querying with
DHCP server, an additional overhead.
Moreover, query by MAC address also shares all other issues we
discussed above for query by IP address.
We conclude that existing lease query types are not appropriate to
achieve effective and efficient antispoofing.
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Authors' Addresses
Pavan Kurapati
Infosys Technologies Ltd.
44 Electronics City, Hosur Road
Bangalore 560 100
India
Email: pavan_kurapati@infosys.com
URI: http://www.infosys.com/
D.T.V Ramakrishna Rao
Infosys Technologies Ltd.
44 Electronics City, Hosur Road
Bangalore 560 100
India
Email: ramakrishnadtv@infosys.com
URI: http://www.infosys.com/
Bharat Joshi
Infosys Technologies Ltd.
44 Electronics City, Hosur Road
Bangalore 560 100
India
Email: bharat_joshi@infosys.com
URI: http://www.infosys.com/
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