Network Working Group Kim Kinnear
INTERNET DRAFT Mark Stapp
Richard Johnson
Jay Kumarasamy
Cisco Systems
April 2002
Expires October 2002
Link Selection sub-option
for the Relay Agent Information Option
<draft-ietf-dhc-agent-subnet-selection-03.txt>
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
In RFC 2131, the giaddr specifies an IP address which determines both
a subnet and thereby 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 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
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to communicate with the DHCP server. Analgous 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 which can be
used to communicate with the relay agent. The link-selection sub-
option (specified here) of the relay-agent-information option allows
a relay agent to do this.
1. Introduction
In RFC 2131, the giaddr specifies and 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
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.
Analgous 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 which 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. How-
ever, if for any reason IP address Y is not accessible from the DHCP
server, then this usual approach will fail. There are several rea-
sons why IP y might be inaccessible from the DHCP server:
1. IP y might not be unique for this subnet, but might instead be
shared as a gateway address by multiple subnets.
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2. 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.
3. 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 commun-
icate 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
thus 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 com-
municating with the relay agent.
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 [RFC 2119].
This document uses the following terms:
o "DHCP client"
A DHCP client is an Internet host using DHCP to obtain confi-
guration 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 dif-
ferent 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 collection of subnets that all coexist on the same
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physical medium. Sometimes called a lan segment or network seg-
ment in other contexts.
o "subnet"
A subnet (for the purposes of this document) consists on a rout-
able 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 which
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.
The sub-option contains a single IP address that is an address con-
tained 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].
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
+------+------+------+------+------+------+
| TBD | 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 used the
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information available in the sub-option to correctly allocate an IP
address. A relay agent which uses this sub-option MUST NOT take dif-
ferent 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 pack-
ets to a server which 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.
In the event that a DHCP server receives a packet which 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 pro-
tocol 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 funda-
mental new security issues are raised by the existance of the link-
selection sub-option specified in this document beyond those implied
by the subnet-selection option [RFC 3011].
The existance 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
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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 which 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 TBD from the DHCP Relay Agent Sub-
options space [RFC 3046] for the link-selection sub-option defined in
Section 3.
6. Acknowledgments
Eric Rosen contributed to helping the authors to understand the need
for this sub-option. Much of the text of this document was borrowed
with only minimal modifications from the document describing the
subnet-selection option [RFC 3011].
7. References
[RFC 951] Croft, B., Gilmore, J., "Bootstrap Protocol (BOOTP)", RFC
951, September 1985.
[RFC 1542] Wimer, W., "Clarifications and Extensions for the
Bootstrap Protocol", RFC 1542, October 1993.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC 2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor
Extensions", Internet RFC 2132, March 1997.
[RFC 3011] Waters, G. "The IPv4 Subnet Selection Option for DHCP",
Internet RFC 3011, November 2000.
[RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 2001.
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8. Author's information
Kim Kinnear
Mark Stapp
Cisco Systems
250 Apollo Drive
Chelmsford, MA 01824
Phone: (978) 244-8000
EMail: kkinnear@cisco.com
mjs@cisco.com
Jay Kumarasamy
Richard Johnson
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
Phone: (408) 526-4000
EMail: jayk@cisco.com
raj@cisco.com
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