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

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
   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.

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.

   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.

   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.

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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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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