Network Working Group                                        G. Waters
 INTERNET-DRAFT                                         Nortel Networks
                                                             April 2000
                    The Subnet Selection Option for DHCP
                      Friday, April 7, 2000, 12:11 PM
 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 (2000).  All Rights Reserved.
   This memo defines a new DHCP option for selecting the subnet on which
   to allocate an address. This option would override a DHCP server's
   normal methods of selecting the subnet on which to allocate an address
   for a client.
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   Table of Contents
   1. Introduction......................................................2
      1.1. Motivational Example.........................................2
   2. Subnet Selection Option Definition................................3
   3. Intellectual Property.............................................4
   4. Acknowledgements..................................................4
   5. Security Considerations...........................................4
   6. References........................................................5
   7. Editor's Addresses................................................5
   8. Full Copyright Statement..........................................5
 1. Introduction
   To select the subnet on which to allocate an address, the DHCP server
   determines the subnet from which the request originated, and then
   selects an address on the originating subnet or on a subnet that is on
   the same network segment as the originating subnet. The subnet from
   which the request originates can be determined by:
   o Using the subnet address of the giaddr field in the DHCP packet
     header, or if the giaddr field is zero;
   o Using the subnet address of the local interface on which the DHCP
     server received the packet.
   This memo defines a new DHCP option, the subnet selection option,
   which allows the DHCP client to specify the subnet on which to
   allocate an address. This option takes precedence over the methods
   that the DHCP server uses to determine the subnet on which to select
   an address.
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].
 1.1. Motivational Example
   An example of where this option could be useful is in a device (e.g.:
   a RAS device) that is allocating addresses on behalf of its clients.
   In this case the device would be allocating addresses through DHCP and
   then managing those addresses among its clients.
   In this scenario, the device is connected to a private "internal"
   network on which the DHCP server would be located. The device is also
   connected to one or more service providing "external" networks (i.e.:
   the networks that the device's clients are connected to). Furthermore,
   the internal network is not IP connected to the external networks,
   although inside the device there is connectivity between the internal
   and external networks (e.g.: though the backplane).
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   Recall that the device is allocating addresses for its clients on the
   external networks and that there is no IP connectivity between the
   internal network and the external networks. The DHCP requests cannot
   originate from the external networks since packets cannot be routed
   between the external network and the internal network. Thus, the DHCP
   requests must originate from the internal network. The problem with
   originating the DHCP requests from the internal network is that the
   DHCP server will allocate addresses on the internal network's subnet,
   when what is required are addresses on the external subnets. The
   subnet selection option provides a solution to this problem.
   The device would send its DHCP request on the internal subnet, but
   would include the subnet selection option containing the address of
   the external subnet on which it requires the address. The subnet
   selection option instructs the DHCP server to allocate the address on
   the requested subnet as opposed to the normal operation of allocating
   the address on the subnet from which the DHCP request originated.
 2. Subnet Selection Option Definition
   The subnet selection option is a DHCP option. The 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 allocate 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:
        Code   Len        IP Address
       | TBD |  4  | A1  | A2  | A3  | A4  |
   Servers supporting this option MUST return an identical copy of the
   option to any client that sends it, regardless of whether or not the
   client requests the option in a parameter request list. Clients using
   this option MUST discard DHCPOFFER or DHCPACK packets that do not
   contain this option.
   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.
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   When this option is present and the server supports this option, the
   server MUST NOT offer an address that is not on the requested subnet
   or network segment.
   During an address renew, the DHCP server may send a DHCPACK directly
   to the allocated address, however packets from the DHCP server may not
   be routable to the address. Thus, in all packets that the DHCP client
   sends that contain the subnet selection option, the giaddr field in
   the BOOTP header MUST be set to an IP address on which the DHCP client
   will accept DHCP packets (e.g.: the address of the subnet connected to
   the internal network).
   The IP address to which a DHCP server sends a reply to MUST be the
   same as it would chose when this option is not present.
 3. Intellectual Property
   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 implementers 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
 4. Acknowledgements
   This document is the result of work undertaken the by DHCP working
   group. Thanks to Ted Lemon, Tim Aston and Ralph Droms for their
   helpful comments in this work.
 5. Security Considerations
   DHCP currently provides no authentication or security mechanisms.
   Potential exposures to attack are discussed is section 7 of the
   protocol specification [RFC2131].
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   The subnet selection option allows for the DHCP client to specify the
   subnet on which to allocate an address. This would allow a client to
   perform a more complete address-pool exhaustion attack since the
   client would no longer be restricted to attacking address-pools on
   just its local subnet. Under the current DHCP security model there is
   no methods available to circumvent this type of attack.
 6. References
   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
        Requirement Levels", RFC 2119, BCP 14, March 1997.
   [RFC2131] Droms, R. "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.
   [RFC2132] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
        Extensions", RFC 2132, March 1997.
 7. Editor's Addresses
   Glenn Waters
   Nortel Networks
   310-875 Carling Avenue,
   Ottawa, Ontario K1S 5P1
   Phone:  +1 613-798-4925
 8. Full Copyright Statement
   Copyright (C) The Internet Society (2000).  All Rights Reserved.
   This document and translations of it may be copied and furnished to
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   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.
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   This document and the information contained herein is provided on an
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