Network Working Group Kim Kinnear
INTERNET DRAFT Mark Stapp
Richard Johnson
Jay Kumarasamy
Cisco Systems
November 2001
Expires May 2002
Subnet Selection sub-option
for the Relay Agent Information Option
<draft-ietf-dhc-agent-subnet-selection-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
In RFC2131, the giaddr specifies both the subnet 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
from which to allocate an IP address which is different from the IP
address with which it desires to communicate with the DHCP server.
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Analgous situations exist where the relay agent needs to specify the
subnet on which a DHCP client resides which is different from an IP
address which can be used to communicate with the relay agent. The
subnet-selection sub-option (specified here) of the relay-agent-
information option allows a relay agent to do this.
1. Introduction
In RFC2131, the giaddr specifies both the subnet on which a DHCP
client resides as well as an IP address which can be used to communi-
cate 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 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 on which a DHCP client resides which is different from an IP
address which can be used to communicate with the relay agent. Con-
sider 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.
2. There might be some firewall capability in the network element
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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 from which to allocate an
IP address. The IP address which will communicate to the DHCP server
the subnet 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
thus a need to extend the relay-agent-info option with a new sub-
option, the subnet-selection sub-option, to allow separation of the
specification of the subnet from the IP address to use when communi-
cating 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.
3. Subnet selection sub-option definition
The subnet-selection sub-option MAY be used by any DHCP relay agent
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Internet Draft Subnet Selection sub-option November 2001
which desires to specify a subnet for a DHCP client request that it
is relaying but needs the subnet 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 the address of 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). When the DHCP server is allocating
an address and this option is present then the DHCP server MUST allo-
cate the address on either:
o the subnet specified in the subnet selection option, or;
o a subnet on the same network segment as the subnet specified in
the subnet selection option.
The format of the option is:
SubOpt Len subnet IP address
+------+------+------+------+------+------+
| TBD | 4 | a1 | a2 | a3 | a4 |
+------+------+------+------+------+------+
Servers supporting this sub-option MUST return an identical copy of
the sub-option in the relay-agent-info option to any relay-agent that
sends it. Relay agents using this sub-option MUST discard DHCPOFFER
or DHCPACK packets that do not contain this option in their associ-
ated relay-agent-info options.
This option does not require changes to operations or features of the
DHCP server other than to select the subnet 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 subnet selection
sub-option, the information contained in the subnet 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 option is present and the server supports this option, the
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Internet Draft Subnet Selection sub-option November 2001
server MUST NOT offer an address that is not on the requested subnet
or network segment.
The IP address to which a DHCP server sends a reply MUST be the same
as it would chose when this option is not present.
4. Security
DHCP currently provides no authentication or security mechanisms.
Potential exposures to attack are discussed is section 7 of the pro-
tocol specification [RFC 2131]. The subnet selection sub-option
allows a relay agent to specify the subnet on which to allocate an
address for a DHCP client. Given that the subnet selection option
already exists [RFC 3011], no new security issues are raised by the
existance of the subnet selection sub-option specified in this docu-
ment. The existance of either the subnet selection option or subnet
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. Under the current DHCP security model there are no
methods available to circumvent this type of attack.
5. IANA Considerations
IANA has assigned the value of TBD for the relay-agent-info option
sub-option code for this sub-option.
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 sub-
net 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.
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Internet Draft Subnet Selection sub-option November 2001
[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.
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
9. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to oth-
ers, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and
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Internet Draft Subnet Selection sub-option November 2001
distributed, in whole or in part, without restriction of any kind, pro-
vided 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 Stan-
dards 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
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LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT
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