Network Working Group                                           T. Lemon
Request for Comments: 3442                                 Nominum, Inc.
Updates: 2132                                                S. Cheshire
Category: Standards Track                           Apple Computer, Inc.
                                                                B. Volz
                                                               Ericsson
                                                          December 2002


                The Classless Static Route Option for
         Dynamic Host Configuration Protocol (DHCP) version 4

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 (2002).  All Rights Reserved.

Abstract

  This document defines a new Dynamic Host Configuration Protocol
  (DHCP) option which is passed from the DHCP Server to the DHCP Client
  to configure a list of static routes in the client.  The network
  destinations in these routes are classless - each routing table entry
  includes a subnet mask.

Introduction

  This option obsoletes the Static Route option (option 33) defined in
  RFC 2132 [4].

  The IP protocol [1] uses routers to transmit packets from hosts
  connected to one IP subnet to hosts connected to a different IP
  subnet.  When an IP host (the source host) wishes to transmit a
  packet to another IP host (the destination), it consults its routing
  table to determine the IP address of the router that should be used
  to forward the packet to the destination host.

  The routing table on an IP host can be maintained in a variety of
  ways - using a routing information protocol such as RIP [8], ICMP
  router discovery [6,9] or using the DHCP Router option, defined in
  RFC 2132 [4].



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RFC 3442        Classless Static Route Option for DHCPv4   December 2002


  In a network that already provides DHCP service, using DHCP to update
  the routing table on a DHCP client has several virtues.  It is
  efficient, since it makes use of messages that would have been sent
  anyway.  It is convenient - the DHCP server configuration is already
  being maintained, so maintaining routing information, at least on a
  relatively stable network, requires little extra work.  If DHCP
  service is already in use, no additional infrastructure need be
  deployed.

  The DHCP protocol as defined in RFC 2131 [3] and the options defined
  in RFC 2132 [4] only provide a mechanism for installing a default
  route or installing a table of classful routes.  Classful routes are
  routes whose subnet mask is implicit in the subnet number - see
  section 3.2 of STD 5, RFC 791 [1] for details on classful routing.

  Classful routing is no longer in common use, so the DHCP Static Route
  option is no longer useful.  Currently, classless routing [7, 10] is
  the most commonly-deployed form of routing on the Internet.  In
  classless routing, IP addresses consist of a network number (the
  combination of the network number and subnet number described in RFC
  950 [7]) and a host number.

  In classful IP, the network number and host number are derived from
  the IP address using a bitmask whose value is determined by the first
  few bits of the IP address.  In classless IP, the network number and
  host number are derived from the IP address using a separate
  quantity, the subnet mask.  In order to determine the network to
  which a given route applies, an IP host must know both the network
  number AND the subnet mask for that network.

  The Static Routes option (option 33) does not provide a subnet mask
  for each route - it is assumed that the subnet mask is implicit in
  whatever network number is specified in each route entry.  The
  Classless Static Routes option does provide a subnet mask for each
  entry, so that the subnet mask can be other than what would be
  determined using the algorithm specified in STD 5, RFC 791 [1] and
  STD 5, RFC 950 [7].

Definitions

  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 [2].








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RFC 3442        Classless Static Route Option for DHCPv4   December 2002


  This document also uses the following terms:

     "DHCP client"

        DHCP client or "client" is an Internet host using DHCP to
        obtain configuration parameters such as a network address.

     "DHCP server"

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

     "link"

        Any set of network attachment points that will all receive a
        link-layer broadcast sent on any one of the attachment points.
        This term is used in DHCP because in some cases more than one
        IP subnet may be configured on a link.  DHCP uses a local-
        network (all-ones) broadcast, which is not subnet-specific, and
        will therefore reach all nodes connected to the link,
        regardless of the IP subnet or subnets on which they are
        configured.

        A "link" is sometimes referred to as a broadcast domain or
        physical network segment.

Classless Route Option Format

  The code for this option is 121, and its minimum length is 5 bytes.
  This option can contain one or more static routes, each of which
  consists of a destination descriptor and the IP address of the router
  that should be used to reach that destination.

   Code Len Destination 1    Router 1
  +-----+---+----+-----+----+----+----+----+----+
  | 121 | n | d1 | ... | dN | r1 | r2 | r3 | r4 |
  +-----+---+----+-----+----+----+----+----+----+

   Destination 2       Router 2
  +----+-----+----+----+----+----+----+
  | d1 | ... | dN | r1 | r2 | r3 | r4 |
  +----+-----+----+----+----+----+----+

  In the above example, two static routes are specified.







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RFC 3442        Classless Static Route Option for DHCPv4   December 2002


  Destination descriptors describe the IP subnet number and subnet mask
  of a particular destination using a compact encoding.  This encoding
  consists of one octet describing the width of the subnet mask,
  followed by all the significant octets of the subnet number.

  The width of the subnet mask describes the number of one bits in the
  mask, so for example a subnet with a subnet number of 10.0.127.0 and
  a netmask of 255.255.255.0 would have a subnet mask width of 24.

  The significant portion of the subnet number is simply all of the
  octets of the subnet number where the corresponding octet in the
  subnet mask is non-zero.  The number of significant octets is the
  width of the subnet mask divided by eight, rounding up, as shown in
  the following table:

       Width of subnet mask     Number of significant octets
                    0                     0
                 1- 8                     1
                 9-16                     2
                17-24                     3
                25-32                     4

  The following table contains some examples of how various subnet
  number/mask combinations can be encoded:

  Subnet number   Subnet mask      Destination descriptor
  0               0                0
  10.0.0.0        255.0.0.0        8.10
  10.0.0.0        255.255.255.0    24.10.0.0
  10.17.0.0       255.255.0.0      16.10.17
  10.27.129.0     255.255.255.0    24.10.27.129
  10.229.0.128    255.255.255.128  25.10.229.0.128
  10.198.122.47   255.255.255.255  32.10.198.122.47

Local Subnet Routes

  In some cases more than one IP subnet may be configured on a link.
  In such cases, a host whose IP address is in one IP subnet in the
  link could communicate directly with a host whose IP address is in a
  different IP subnet on the same link.  In cases where a client is
  being assigned an IP address on an IP subnet on such a link, for each
  IP subnet in the link other than the IP subnet on which the client
  has been assigned the DHCP server MAY be configured to specify a
  router IP address of 0.0.0.0.







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RFC 3442        Classless Static Route Option for DHCPv4   December 2002


  For example, consider the case where there are three IP subnets
  configured on a link: 10.0.0/24, 192.168.0/24, 10.0.21/24.  If the
  client is assigned an IP address of 10.0.21.17, then the server could
  include a route with a destination of 10.0.0/24 and a router address
  of 0.0.0.0, and also a route with a destination of 192.168.0/24 and a
  router address of 0.0.0.0.

  A DHCP client whose underlying TCP/IP stack does not provide this
  capability MUST ignore routes in the Classless Static Routes option
  whose router IP address is 0.0.0.0.  Please note that the behavior
  described here only applies to the Classless Static Routes option,
  not to the Static Routes option nor the Router option.

DHCP Client Behavior

  DHCP clients that do not support this option MUST ignore it if it is
  received from a DHCP server.  DHCP clients that support this option
  MUST install the routes specified in the option, except as specified
  in the Local Subnet Routes section.  DHCP clients that support this
  option MUST NOT install the routes specified in the Static Routes
  option (option code 33) if both a Static Routes option and the
  Classless Static Routes option are provided.

  DHCP clients that support this option and that send a DHCP Parameter
  Request List option MUST request both this option and the Router
  option [4] in the DHCP Parameter Request List.

  DHCP clients that support this option and send a parameter request
  list MAY also request the Static Routes option, for compatibility
  with older servers that don't support Classless Static Routes.  The
  Classless Static Routes option code MUST appear in the parameter
  request list prior to both the Router option code and the Static
  Routes option code, if present.

  If the DHCP server returns both a Classless Static Routes option and
  a Router option, the DHCP client MUST ignore the Router option.

  Similarly, if the DHCP server returns both a Classless Static Routes
  option and a Static Routes option, the DHCP client MUST ignore the
  Static Routes option.

  After deriving a subnet number and subnet mask from each destination
  descriptor, the DHCP client MUST zero any bits in the subnet number
  where the corresponding bit in the mask is zero. In other words, the
  subnet number installed in the routing table is the logical AND of
  the subnet number and subnet mask given in the Classless Static
  Routes option. For example, if the server sends a route with a
  destination of 129.210.177.132 (hexadecimal 81D4B184) and a subnet



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RFC 3442        Classless Static Route Option for DHCPv4   December 2002


  mask of 255.255.255.128 (hexadecimal FFFFFF80), the client will
  install a route with a destination of 129.210.177.128 (hexadecimal
  81D4B180).

Requirements to Avoid Sizing Constraints

  Because a full routing table can be quite large, the standard 576
  octet maximum size for a DHCP message may be too short to contain
  some legitimate Classless Static Route options.  Because of this,
  clients implementing the Classless Static Route option SHOULD send a
  Maximum DHCP Message Size [4] option if the DHCP client's TCP/IP
  stack is capable of receiving larger IP datagrams.  In this case, the
  client SHOULD set the value of this option to at least the MTU of the
  interface that the client is configuring.  The client MAY set the
  value of this option higher, up to the size of the largest UDP packet
  it is prepared to accept.  (Note that the value specified in the
  Maximum DHCP Message Size option is the total maximum packet size,
  including IP and UDP headers.)

  DHCP clients requesting this option, and DHCP servers sending this
  option, MUST implement DHCP option concatenation [5].  In the
  terminology of RFC 3396 [5], the Classless Static Route Option is a
  concatenation-requiring option.

DHCP Server Administrator Responsibilities

  Many clients may not implement the Classless Static Routes option.
  DHCP server administrators should therefore configure their DHCP
  servers to send both a Router option and a Classless Static Routes
  option, and should specify the default router(s) both in the Router
  option and in the Classless Static Routes option.

  When a DHCP client requests the Classless Static Routes option and
  also requests either or both of the Router option and the Static
  Routes option, and the DHCP server is sending Classless Static Routes
  options to that client, the server SHOULD NOT include the Router or
  Static Routes options.

Security Considerations

  Potential exposures to attack in the DHCP protocol are discussed in
  section 7 of the DHCP protocol specification [3] and in
  Authentication for DHCP Messages [11].

  The Classless Static Routes option can be used to misdirect network
  traffic by providing incorrect IP addresses for routers.  This can be
  either a Denial of Service attack, where the router IP address given
  is simply invalid, or can be used to set up a man-in-the-middle



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RFC 3442        Classless Static Route Option for DHCPv4   December 2002


  attack by providing the IP address of a potential snooper.  This is
  not a new problem - the existing Router and Static Routes options
  defined in RFC 2132 [4] exhibit the same vulnerability.

IANA Considerations

  This DHCP option has been allocated the option code 121 in the list
  of DHCP option codes that the IANA maintains.

Normative References

  [1]  Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.

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

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

  [4]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
       Extensions", RFC 2132, March 1997.

  [5]  Lemon, T. and S. Cheshire, "Encoding Long Options in the Dynamic
       Host Configuration Protocol (DHCPv4)", RFC 3396, November 2002.

Informative References

  [6]  Postel, J., "Internet Control Message Protocol", STD 5, RFC 792,
       September 1981.

  [7]  Mogul, J. and J. Postel, "Internet Standard Subnetting
       Procedure", STD 5, RFC 950, August 1985.

  [8]  Hedrick, C., "Routing Information Protocol", RFC 1058, June
       1988.

  [9]  Deering, S., "ICMP Router Discovery Messages", RFC 1256,
       September 1991.

  [10] Pummill, T. and B. Manning, "Variable Length Subnet Table For
       IPv4", RFC 1878, December 1995.

  [11] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
       RFC 3118, June 2001.







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RFC 3442        Classless Static Route Option for DHCPv4   December 2002


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

Authors' Addresses

  Ted Lemon
  Nominum, Inc.
  2385 Bay Road
  Redwood City, CA 94063

  EMail: [email protected]

  Stuart Cheshire
  Apple Computer, Inc.
  1 Infinite Loop
  Cupertino
  California 95014
  USA

  Phone: +1 408 974 3207
  EMail: [email protected]

  Bernie Volz
  Ericsson
  959 Concord Street
  Framingham, MA, 01701

  Phone: +1 508 875 3162
  EMail: [email protected]



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RFC 3442        Classless Static Route Option for DHCPv4   December 2002


Full Copyright Statement

  Copyright (C) The Internet Society (2002).  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
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  The limited permissions granted above are perpetual and will not be
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Acknowledgement

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



















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