draft-ietf-dhc-agent-subnet-selection-01.txt   draft-ietf-dhc-agent-subnet-selection-02.txt 
Network Working Group Kim Kinnear Network Working Group Kim Kinnear
INTERNET DRAFT Mark Stapp INTERNET DRAFT Mark Stapp
Richard Johnson Richard Johnson
Jay Kumarasamy Jay Kumarasamy
Cisco Systems Cisco Systems
November 2001 February 2002
Expires May 2002 Expires August 2002
Subnet Selection sub-option Subnet Selection sub-option
for the Relay Agent Information Option for the Relay Agent Information Option
<draft-ietf-dhc-agent-subnet-selection-01.txt> <draft-ietf-dhc-agent-subnet-selection-02.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved. Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract Abstract
In RFC2131, the giaddr specifies both the subnet on which a DHCP 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 client resides as well as an IP address which can be used to
communicate with the relay agent. The subnet selection option [RFC communicate with the relay agent. The subnet selection option [RFC
3011] allows these functions of the giaddr to be split so that when 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 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 from which to allocate an IP address which is different from the IP
address with which it desires to communicate with the DHCP server. address with which it desires to communicate with the DHCP server.
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and DHCP messages between clients and servers residing on dif- and DHCP messages between clients and servers residing on dif-
ferent subnets, per [RFC 951] and [RFC 1542]. ferent subnets, per [RFC 951] and [RFC 1542].
o "DHCP server" o "DHCP server"
A DHCP server is an Internet host that returns configuration A DHCP server is an Internet host that returns configuration
parameters to DHCP clients. parameters to DHCP clients.
3. Subnet selection sub-option definition 3. Subnet selection sub-option definition
The subnet-selection sub-option MAY be used by any DHCP relay agent The subnet-selection sub-option is used by any DHCP relay agent which
which desires to specify a subnet for a DHCP client request that it desires to specify a subnet for a DHCP client request that it is
is relaying but needs the subnet specification to be different from relaying but needs the subnet specification to be different from the
the IP address the DHCP server should use when communicating with the IP address the DHCP server should use when communicating with the
relay agent. relay agent.
The sub-option contains a single IP address that is the address of a 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 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 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 (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 (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- an address and this sub-option is present then the DHCP server MUST
cate the address on either: allocate the address on either:
o the subnet specified in the subnet selection option, or; o the subnet specified in the subnet selection sub-option, or;
o a subnet on the same network segment as the subnet specified in o a subnet on the same network segment as the subnet specified in
the subnet selection option. the subnet selection sub-option.
The format of the option is: The format of the sub-option is:
SubOpt Len subnet IP address SubOpt Len subnet IP address
+------+------+------+------+------+------+ +------+------+------+------+------+------+
| TBD | 4 | a1 | a2 | a3 | a4 | | TBD | 4 | a1 | a2 | a3 | a4 |
+------+------+------+------+------+------+ +------+------+------+------+------+------+
Servers supporting this sub-option MUST return an identical copy of Because the IP address offered to a client is likely to be different
the sub-option in the relay-agent-info option to any relay-agent that if this sub-option is included in a relay-agent-info option than it
sends it. Relay agents using this sub-option MUST discard DHCPOFFER would be if this sub-option did not appear or was not interpreted,
or DHCPACK packets that do not contain this option in their associ- and because some DHCP servers will not support the relay-agent-info
ated relay-agent-info options. 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 option does not require changes to operations or features of the This will protect against servers who do not implement any support
DHCP server other than to select the subnet on which to allocate an for the relay-agent-info option [RFC 3046].
address. For example, the handling of DHCPDISCOVER for an unknown
subnet should continue to operate unchanged. 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.
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 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 a subnet selection option [RFC 3011] as well as a subnet selection
sub-option, the information contained in the subnet selection sub- sub-option, the information contained in the subnet selection sub-
option MUST be used to control the allocation of an IP address in option MUST be used to control the allocation of an IP address in
preference to the information contained in the subnet selection preference to the information contained in the subnet selection
option. 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 option is present and the server supports this option, the When this sub-option is present and the server supports this sub-
server MUST NOT offer an address that is not on the requested subnet option, the server MUST NOT offer an address that is not on the
or network segment. requested subnet or network segment.
The IP address to which a DHCP server sends a reply MUST be the same 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. as it would choose when this sub-option is not present.
4. Security 4. Security Considerations
DHCP currently provides no authentication or security mechanisms. Potential attacks on DHCP are discussed in section 7 of the DHCP pro-
Potential exposures to attack are discussed is section 7 of the pro- tocol specification [RFC 2131], as well as in the DHCP authentication
tocol specification [RFC 2131]. The subnet selection sub-option specification [RFC 3118].
allows a relay agent to specify the subnet on which to allocate an
address for a DHCP client. Given that the subnet selection option The subnet selection sub-option allows a relay agent to specify the
already exists [RFC 3011], no new security issues are raised by the subnet on which to allocate an address for a DHCP client. Given that
existance of the subnet selection sub-option specified in this docu- the subnet selection option already exists [RFC 3011], no new secu-
ment. The existance of either the subnet selection option or subnet rity issues are raised by the existance of the subnet selection sub-
selection sub-option documented here would allow a malicious DHCP option specified in this document beyond those implied by the
client to perform a more complete address-pool exhaustion attack than subnet-selection option [RFC 3011].
could be performed without the use of these options, since the client
would no longer be restricted to attacking address-pools on just its The existance of either the subnet selection option or subnet selec-
local subnet. Under the current DHCP security model there are no tion sub-option documented here would allow a malicious DHCP client
methods available to circumvent this type of attack. 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 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 5. IANA Considerations
IANA has assigned the value of TBD for the relay-agent-info option IANA has assigned a value of TBD from the DHCP Relay Agent Sub-
sub-option code for this sub-option. options space [RFC 3046] for the subnet selection sub-option defined
in Section 3.
6. Acknowledgments 6. Acknowledgments
Eric Rosen contributed to helping the authors to understand the need Eric Rosen contributed to helping the authors to understand the need
for this sub-option. Much of the text of this document was borrowed for this sub-option. Much of the text of this document was borrowed
with only minimal modifications from the document describing the sub- with only minimal modifications from the document describing the sub-
net selection option [RFC 3011]. net selection option [RFC 3011].
7. References 7. References
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Richard Johnson Richard Johnson
Cisco Systems Cisco Systems
170 W. Tasman Dr. 170 W. Tasman Dr.
San Jose, CA 95134 San Jose, CA 95134
Phone: (408) 526-4000 Phone: (408) 526-4000
EMail: jayk@cisco.com EMail: jayk@cisco.com
raj@cisco.com raj@cisco.com
9. Full Copyright Statement 9. Intellectual Property Statement
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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
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