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IETF RFC 3527
Link Selection sub-option for the Relay Agent Information Option for DHCPv4
Last modified on Wednesday, April 23rd, 2003
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Network Working Group K. Kinnear
Request for Comments: 3527 M. Stapp
Category: Standards Track R. Johnson
J. Kumarasamy
Cisco Systems
April 2003
Link Selection sub-option
for the Relay Agent Information Option for DHCPv4
 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 © The Internet Society (2003). All Rights Reserved.
Abstract
This document describes the link selection sub-option of the relay-
agent-information option for the Dynamic Host Configuration Protocol
(DHCPv4). The giaddr specifies an IP address which determines both a
subnet, and thereby a link on which a Dynamic Host Configuration
Protocol (DHCP) client resides as well as an IP address that can be
used to communicate with the relay agent. The subnet-selection
option allows the functions of the giaddr to be split so that when
one entity is performing as a DHCP proxy, it can specify the
subnet/link from which to allocate an IP address, which is different
from the IP address with which it desires to communicate with the
DHCP server. Analogous situations exist where the relay agent needs
to specify the subnet/link on which a DHCP client resides, which is
different from an IP address that can be used to communicate with the
relay agent.
1. Introduction
In RFC 2131, the giaddr specifies an IP address which determines a
subnet (and from there a link) on which a DHCP client resides as well
as an IP address which can be used to communicate with the relay
agent. The subnet-selection option [RFC 3011] allows these functions
of the giaddr to be split, so that when one entity is performing as a
Kinnear, et al. Standards Track PAGE 1 
RFC 3527 Link Selection sub-option April 2003
DHCP proxy, it can specify the subnet/link from which to allocate an
IP address that is different from the IP address with which it
desires to communicate with the DHCP server.
Analogous situations exist where the relay agent needs to specify the
subnet/link on which a DHCP client resides, which is different from
an IP address that can be used to communicate with the relay agent.
Consider the following architecture:
+--------+ +---------------+
| DHCP | IP x| |IP y
| Server |-.......-| Relay Agent |----+------------+
+--------+ | | | |
+---------------+ | +------+
| |Modem |
| +------+
| | |
+-----+ +-----+ +-----+
|Host1| |Host2| |Host3|
+-----+ +-----+ +-----+
In the usual approach, the relay agent would put IP address Y into
the giaddr of any packets that it forwarded to the DHCP server.
However, if for any reason, IP address Y is not accessible from the
DHCP server, this approach will fail. There are several reasons why
IP y might be inaccessible from the DHCP server:
o There might be some firewall capability in the network element
in which the relay agent resides that does not allow the DHCP
server to access the relay agent via IP y.
o There might not be an IP y. An example would be the case where
there was only one host and this was a point to point link.
In any of these or other cases, the relay agent needs to be able to
communicate to the DHCP server the subnet/link from which to allocate
an IP address. The IP address, which will communicate to the DHCP
server the subnet/link information, cannot be used as a way to
communicate between the DHCP server and the relay agent.
Since the relay agent can modify the client's DHCP DHCPREQUEST in
only two ways, the giaddr and the relay-agent-info option, there is a
need to extend the relay-agent-info option with a new sub-option, the
link-selection sub-option, to allow separation of the specification
of the subnet/link from the IP address to use when communicating with
the relay agent.
Kinnear, et al. Standards Track PAGE 2
RFC 3527 Link Selection sub-option April 2003
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 BCP 14, RFC 2119 [RFC
2119].
This document uses the following terms:
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 BOOTP
and DHCP messages between clients and servers residing on
different subnets, per [RFC 951] and [RFC 1542].
o "DHCP server"
A DHCP server is an Internet host that returns configuration
parameters to DHCP clients.
o "link"
A link is a communications facility or medium over which nodes
can communicate at the link layer, i.e., the layer immediately
below IPv4. Examples are Ethernets (simple or bridged); PPP
links; X.25, Frame Relay, or ATM networks; and internet (or
higher) layer "tunnels", such as tunnels over IPv4 or IPv6
itself.
o "subnet"
A subnet (for the purposes of this document) consists of a
routable address range. It may be one of several that exist on
a link at the same time.
3. Link selection sub-option definition
The link-selection sub-option is used by any DHCP relay agent that
desires to specify a subnet/link for a DHCP client request that it is
relaying but needs the subnet/link specification to be different from
the IP address the DHCP server should use when communicating with the
relay agent.
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RFC 3527 Link Selection sub-option April 2003
The sub-option contains a single IP address that is an address
contained in a subnet. The value for the subnet address is determined
by taking any IP address on the subnet and ANDing that address with
the subnet mask (i.e., the network and subnet bits are left alone and
the remaining (address) bits are set to zero). This determines a
single subnet, and when allocating an IP address, all of the other
related subnets on the same link will also be considered in the same
way as currently specified for the processing of the giaddr in [RFC
2131, Section 4.3.1, first group of bullets, bullet 4].
In scenarios where this sub-option is needed, the relay agent adds it
whenever it sets the giaddr value (i.e., on all messages relayed to
the DHCP server).
When the DHCP server is allocating an address and this sub-option is
present, then the DHCP server MUST allocate the address on either:
o the subnet specified in the link-selection sub-option, or;
o a subnet on the same link (also known as a network segment) as
the subnet specified by the link-selection sub-option.
The format of the sub-option is:
SubOpt Len subnet IP address
+------+------+------+------+------+------+
| 5 | 4 | a1 | a2 | a3 | a4 |
+------+------+------+------+------+------+
A relay agent which uses this sub-option MUST assume that the server
receiving the sub-option supports the sub-option and uses the
information available in the sub-option to correctly allocate an IP
address. A relay agent which uses this sub-option MUST NOT take
different actions based on whether this sub-option appears or does
not appear in the response packet from the server.
It is important to ensure, using administrative techniques, that any
relay agent employing this sub-option is directed to only send
packets to a server that supports this sub-option.
Support for this sub-option does not require changes to operations or
features of the DHCP server other than to select the subnet (and
link) on which to allocate an address. For example, the handling of
DHCPDISCOVER for an unknown subnet should continue to operate
unchanged.
Kinnear, et al. Standards Track PAGE 4
RFC 3527 Link Selection sub-option April 2003
In the event that a DHCP server receives a packet that contains both
a subnet-selection option [RFC 3011], as well as a link-selection
sub-option, the information contained in the link-selection sub-
option MUST be used to control the allocation of an IP address in
preference to the information contained in the subnet-selection
option.
When this sub-option is present and the server supports this sub-
option, the server MUST NOT offer an address that is not on the
requested subnet or the link (network segment) with which that subnet
is associated.
The IP address to which a DHCP server sends a reply MUST be the same
as it would choose when this sub-option is not present.
4. Security Considerations
Potential attacks on DHCP are discussed in section 7 of the DHCP
protocol specification [RFC 2131], as well as in the DHCP
authentication specification [RFC 3118].
The link-selection sub-option allows a relay agent to specify the
subnet/link on which to allocate an address for a DHCP client. Given
that the subnet-selection option already exists [RFC 3011], no
fundamental new security issues are raised by the existence of the
link-selection sub-option specified in this document beyond those
implied by the subnet-selection option [RFC 3011].
The existence of either the subnet-selection option or link-selection
sub-option documented here would allow a malicious DHCP client to
perform a more complete address-pool exhaustion attack than could be
performed without the use of these options, since the client would no
longer be restricted to attacking address-pools on just its local
subnet.
There is some minor protection against this form of attack using this
sub-option that is not present for the subnet-selection option, in
that a trusted relay agent that supports the relay-agent-info option
MUST discard a packet it receives with a zero giaddr and a relay-
agent-info option when that packet arrives on an "untrusted" circuit
[RFC 3046, section 2.1].
5. IANA Considerations
IANA has assigned a value of 5 from the DHCP Relay Agent sub-options
space [RFC 3046] for the link-selection sub-option defined in Section
3.
Kinnear, et al. Standards Track PAGE 5
RFC 3527 Link Selection sub-option April 2003
6. Acknowledgments
Eric Rosen helped the authors to understand the need for this sub-
option. Much of this document was borrowed, with only minimal
modifications, from the document describing the subnet-selection
option [RFC 3011].
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC 3011] Waters, G. "The IPv4 Subnet Selection Option for DHCP",
RFC 3011, November 2000.
[RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 2001.
7.2. Informative References
[RFC 951] Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
September 1985.
[RFC 1542] Wimer, W., "Clarifications and Extensions for the
Bootstrap Protocol", RFC 1542, October 1993.
Kinnear, et al. Standards Track PAGE 6
RFC 3527 Link Selection sub-option April 2003
8. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Kinnear, et al. Standards Track PAGE 7
RFC 3527 Link Selection sub-option April 2003
9. Authors' Addresses
Kim Kinnear
Cisco Systems
1414 Massachusetts Ave
Boxborough, Ma. 01719
Phone: (978) 936-0000
EMail: kkinnear@cisco.com
Mark Stapp
Cisco Systems
1414 Massachusetts Ave
Boxborough, Ma. 01719
Phone: (978) 936-0000
EMail: mjs@cisco.com
Jay Kumarasamy
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
Phone: (408) 526-4000
EMail: jayk@cisco.com
Richard Johnson
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
Phone: (408) 526-4000
EMail: raj@cisco.com
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RFC 3527 Link Selection sub-option April 2003
10. Full Copyright Statement
Copyright © The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Kinnear, et al. Standards Track PAGE 9
Link Selection sub-option for the Relay Agent Information Option for DHCPv4
RFC TOTAL SIZE: 16831 bytes
PUBLICATION DATE: Wednesday, April 23rd, 2003
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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