Network Working Group Kim Kinnear
INTERNET DRAFT Mark Stapp
Richard Johnson
Jay Kumarasamy
Cisco Systems
February 2002
Expires August 2002
Subnet Selection sub-option
for the Relay Agent Information Option
<draft-ietf-dhc-agent-subnet-selection-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
In RFC 2131, 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.
Kinnear, et. al. Expires August 2002 [Page 1]
Internet Draft Subnet Selection sub-option February 2002
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 RFC 2131, 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
Kinnear, et. al. Expires August 2002 [Page 2]
Internet Draft Subnet Selection sub-option February 2002
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 is used by any DHCP relay agent which
Kinnear, et. al. Expires August 2002 [Page 3]
Internet Draft Subnet Selection sub-option February 2002
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 sub-option is present then the DHCP server MUST
allocate the address on either:
o the subnet specified in the subnet selection sub-option, or;
o a subnet on the same network segment as the subnet specified in
the subnet selection sub-option.
The format of the sub-option is:
SubOpt Len subnet IP address
+------+------+------+------+------+------+
| TBD | 4 | a1 | a2 | a3 | a4 |
+------+------+------+------+------+------+
Because the IP address offered to a client is likely to be different
if this sub-option is included in a relay-agent-info option than it
would be if this sub-option did not appear or was not interpreted,
and because some DHCP servers will not support the relay-agent-info
option, then relay agents using this sub-option MUST discard
DHCPOFFER or DHCPACK packets that do not contain this sub-option in
their associated relay-agent-info options. In the case, there will
typically not be any relay-agent-info option in the DHCPOFFER or
DHCPACK.
This will protect against servers who do not implement any support
for the relay-agent-info option [RFC 3046].
However, [RFC 3046] states that every server supporting the relay-
agent-info option MUST echo the entire contents of the relay-agent-
info option in all replies. Thus, a relay agent doesn't have any
effective way to tell whether or not a server has actually used the
subnet-selection sub-option to drive its choice of subnets on which
the IP address is allocated.
Kinnear, et. al. Expires August 2002 [Page 4]
Internet Draft Subnet Selection sub-option February 2002
Because of this situation, it is important to ensure using adminis-
trative techniques that any relay agent employing this sub-option is
directed to only send packets 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 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 situation occurs, the subnet selection option MUST
NOT be echoed in the reply packet to the client, signalling that the
subnet selection option was not used to drive the selection of the
subnet for IP address allocation.
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 network segment.
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 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 secu-
rity issues are raised by the existance of the subnet 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 subnet selec-
tion 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
Kinnear, et. al. Expires August 2002 [Page 5]
Internet Draft Subnet Selection sub-option February 2002
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 subnet 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 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.
[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
Kinnear, et. al. Expires August 2002 [Page 6]
Internet Draft Subnet Selection sub-option February 2002
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. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any intel-
lectual 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 stan-
dard. Please address the information to the IETF Executive Director.
Kinnear, et. al. Expires August 2002 [Page 7]
Internet Draft Subnet Selection sub-option February 2002
10. Full Copyright Statement
Copyright (C) The Internet Society (2002). 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 dis-
tributed, 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 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
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 FIT-
NESS FOR A PARTICULAR PURPOSE.
Kinnear, et. al. Expires August 2002 [Page 8]
