    Network Working Group                                         K. Kinnear
    Request for Comments: 3527                                      M. Stapp
    Category: Standards Track                                     R. Johnson
                                                               J. Kumarasamy
                                                               Cisco Systems
                                                                  April 2003

                           Link Selection sub-option
               for the Relay Agent Information Option for DHCPv4

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2003).  All Rights Reserved.

    Abstract

       This document describes the link selection sub-option of the relay-
       agent-information option for the Dynamic Host Configuration Protocol
       (DHCPv4).  The giaddr specifies an IP address which determines both a
       subnet, and thereby a link on which a Dynamic Host Configuration
       Protocol (DHCP) client resides as well as an IP address that can be
       used to communicate with the relay agent.  The subnet-selection
       option allows the functions of the giaddr to be split so that when
       one entity is performing as a DHCP proxy, it can specify the
       subnet/link from which to allocate an IP address, which is different
       from the IP address with which it desires to communicate with the
       DHCP server.  Analogous situations exist where the relay agent needs
       to specify the subnet/link on which a DHCP client resides, which is
       different from an IP address that can be used to communicate with the
       relay agent.

    1.  Introduction

       In RFC 2131, the giaddr specifies an IP address which determines a
       subnet (and from there a link) 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

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    RFC 3527               Link Selection sub-option              April 2003

       DHCP proxy, it can specify the subnet/link from which to allocate an
       IP address that is different from the IP address with which it
       desires to communicate with the DHCP server.

       Analogous situations exist where the relay agent needs to specify the
       subnet/link on which a DHCP client resides, which is different from
       an IP address that can be used to communicate with the relay agent.
       Consider 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.
       However, if for any reason, IP address Y is not accessible from the
       DHCP server, this approach will fail.  There are several reasons why
       IP y might be inaccessible from the DHCP server:

          o  There might be some firewall capability in the network element
             in which the relay agent resides that does not allow the DHCP
             server to access the relay agent via IP y.

          o  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/link from which to allocate
       an IP address.  The IP address, which will communicate to the DHCP
       server the subnet/link 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 a
       need to extend the relay-agent-info option with a new sub-option, the
       link-selection sub-option, to allow separation of the specification
       of the subnet/link from the IP address to use when communicating with
       the relay agent.

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    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 BCP 14, RFC 2119 [RFC
       2119].

       This document uses the following terms:

          o  "DHCP client"

             A DHCP client is an Internet host using DHCP to obtain
             configuration 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
             different subnets, per [RFC 951] and [RFC 1542].

          o  "DHCP server"

             A DHCP server is an Internet host that returns configuration
             parameters to DHCP clients.

          o  "link"

             A link is a communications facility or medium over which nodes
             can communicate at the link layer, i.e., the layer immediately
             below IPv4.  Examples are Ethernets (simple or bridged); PPP
             links; X.25, Frame Relay, or ATM networks; and internet (or
             higher) layer "tunnels", such as tunnels over IPv4 or IPv6
             itself.

          o  "subnet"

             A subnet (for the purposes of this document) consists of a
             routable address range.  It may be one of several that exist on
             a link at the same time.

    3.  Link selection sub-option definition

       The link-selection sub-option is used by any DHCP relay agent that
       desires to specify a subnet/link for a DHCP client request that it is
       relaying but needs the subnet/link specification to be different from
       the IP address the DHCP server should use when communicating with the
       relay agent.

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    RFC 3527               Link Selection sub-option              April 2003

       The sub-option contains a single IP address that is an address
       contained in 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).  This determines a
       single subnet, and when allocating an IP address, all of the other
       related subnets on the same link will also be considered in the same
       way as currently specified for the processing of the giaddr in [RFC
       2131, Section 4.3.1, first group of bullets, bullet 4].

       In scenarios where this sub-option is needed, the relay agent adds it
       whenever it sets the giaddr value (i.e., on all messages relayed to
       the DHCP server).

       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 link-selection sub-option, or;

          o  a subnet on the same link (also known as a network segment) as
             the subnet specified by the link-selection sub-option.

       The format of the sub-option is:

                 SubOpt   Len     subnet IP address
                +------+------+------+------+------+------+
                |  5   |   4  |  a1  |  a2  |  a3  |  a4  |
                +------+------+------+------+------+------+

       A relay agent which uses this sub-option MUST assume that the server
       receiving the sub-option supports the sub-option and uses the
       information available in the sub-option to correctly allocate an IP
       address.  A relay agent which uses this sub-option MUST NOT take
       different actions based on whether this sub-option appears or does
       not appear in the response packet from the server.

       It is important to ensure, using administrative techniques, that any
       relay agent employing this sub-option is directed to only send
       packets to a server that 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 (and
       link) 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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    RFC 3527               Link Selection sub-option              April 2003

       In the event that a DHCP server receives a packet that contains both
       a subnet-selection option [RFC 3011], as well as a link-selection
       sub-option, the information contained in the link-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 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 the link (network segment) with which that subnet
       is associated.

       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
       protocol specification [RFC 2131], as well as in the DHCP
       authentication specification [RFC 3118].

       The link-selection sub-option allows a relay agent to specify the
       subnet/link on which to allocate an address for a DHCP client.  Given
       that the subnet-selection option already exists [RFC 3011], no
       fundamental new security issues are raised by the existence of the
       link-selection sub-option specified in this document beyond those
       implied by the subnet-selection option [RFC 3011].

       The existence of either the subnet-selection option or link-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.

       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 that 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 5 from the DHCP Relay Agent sub-options
       space [RFC 3046] for the link-selection sub-option defined in Section
       3.

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    6.  Acknowledgments

       Eric Rosen helped the authors to understand the need for this sub-
       option.  Much of this document was borrowed, with only minimal
       modifications, from the document describing the subnet-selection
       option [RFC 3011].

    7.  References

    7.1.  Normative References

       [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
                  2131, March 1997.

       [RFC 3011] Waters, G. "The IPv4 Subnet Selection Option for DHCP",
                  RFC 3011, November 2000.

       [RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
                  3046, January 2001.

    7.2.  Informative References

       [RFC 951]  Croft, W. and J. Gilmore, "Bootstrap Protocol", RFC 951,
                  September 1985.

       [RFC 1542] Wimer, W., "Clarifications and Extensions for the
                  Bootstrap Protocol", RFC 1542, October 1993.

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    8.  Intellectual Property Statement

       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 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 standard.  Please address the information to the IETF Executive
       Director.

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    RFC 3527               Link Selection sub-option              April 2003

    9.  Authors' Addresses

       Kim Kinnear
       Cisco Systems
       1414 Massachusetts Ave
       Boxborough, Ma. 01719

       Phone: (978) 936-0000
       EMail: kkinnear@cisco.com

       Mark Stapp
       Cisco Systems
       1414 Massachusetts Ave
       Boxborough, Ma. 01719

       Phone: (978) 936-0000
       EMail: mjs@cisco.com

       Jay Kumarasamy
       Cisco Systems
       170 W. Tasman Dr.
       San Jose, CA 95134

       Phone: (408) 526-4000
       EMail: jayk@cisco.com

       Richard Johnson
       Cisco Systems
       170 W. Tasman Dr.
       San Jose, CA 95134

       Phone: (408) 526-4000
       EMail: raj@cisco.com

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    10.  Full Copyright Statement

       Copyright (C) The Internet Society (2003).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
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       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
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       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
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       This document and the information contained herein is provided on an
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    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

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