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Versions: 00 01 02 03 RFC 3527

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
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   Drafts.

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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.



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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 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



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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 is used by any DHCP relay agent which



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   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.



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   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



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   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





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      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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10.  Full Copyright Statement

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