    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].

    Lemon, et. al.              Standards Track                     [Page 1]

    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].

    Lemon, et. al.              Standards Track                     [Page 2]

    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.

    Lemon, et. al.              Standards Track                     [Page 3]

    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.

    Lemon, et. al.              Standards Track                     [Page 4]

    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

    Lemon, et. al.              Standards Track                     [Page 5]

    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

    Lemon, et. al.              Standards Track                     [Page 6]

    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.

    Lemon, et. al.              Standards Track                     [Page 7]

    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
       rights which may cover technology that may be required to practice
       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: Ted.Lemon@nominum.com

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

       Phone: +1 408 974 3207
       EMail: rfc@stuartcheshire.org

       Bernie Volz
       Ericsson
       959 Concord Street
       Framingham, MA, 01701

       Phone: +1 508 875 3162
       EMail: bernie.volz@ericsson.com

    Lemon, et. al.              Standards Track                     [Page 8]

    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
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       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
       copyrights defined in the Internet Standards process must be
       followed, or as required to 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.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Acknowledgement

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

    Lemon, et. al.              Standards Track                     [Page 9]