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Versions: 00 01 02 03 04 05 06 07 rfc3442                               
Network Working Group                                          Ted Lemon
Internet Draft                                             Nominum, Inc.
                                                         Stuart Cheshire
                                                    Apple Computer, Inc.
                                                             Bernie Volz

Obsoletes: draft-ietf-dhc-csr-05.txt                       October, 2001
                                                     Expires April, 2002

              The Classless Static Route Option for DHCP

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

     The list of current Internet-Drafts can be accessed at

     The list of Internet-Draft Shadow Directories can be accessed at


   This document defines a new DHCP option which is passed from the
   DHCP Server to the DHCP Client to configure a list of static routes
   in the client.   This option supersedes the Static Route option
   (option 33) defined in RFC2132 [2].


   The IP protocol [4] 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 [5], ICMP
   router discovery [6,7] or using the DHCP Router option, defined in
   RFC2132 [2].

   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 RFC2131 [1] and the options defined
   in RFC2132 [2] only provide a mechanism for installing a default
   route or installing a table of classed routes.  Classed routes are
   routes whose subnet mask is implicit in the subnet number - see
   section 3.2 of RFC791 [4] for details on classed routing.

   Classed routing is no longer in common use, so the DHCP Static
   Route option is no longer useful.  Currently, classless routing,
   described in [8] and [9], 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 [8]) and a host number.

   In classed 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 seperate
   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 RFC791 [4] and RFC950


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [3].

   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.


           Any set of all network attachment points that will recieve
           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 TBD, 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
   | TBD | 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.

   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 and a netmask of 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        8.10      16.10.17

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

   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, then the server
   could include a route with a destination of 10.0.0/24 and a router
   address of, and also a route with a destination of
   192.168.0/24 and a router address of

   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  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 [2] 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 Router option and a Classless
   Static Routes option, the DHCP client MUST ignore the Router

   After deriving a subnet number and subnet mask from each
   destination descriptor, the DHCP client MUST set any bits in the
   subnet number that are zero in the subnet mask to zero.  For
   example, if the server sends a route with a destination of (hexadecimal 81D4B184) and a subnet mask of (hexadecimal FFFFFF80), the client will install a
   route with a destination of (hexadecimal

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 [2] option if the DHCP client's TCP/IP
   stack is capable of reassembling fragmented 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 servers sending this option MUST use the technique described
   in [10] for sending options larger than 255 bytes when storing this
   option in outgoing DHCP packets.  DHCP clients supporting this
   option MUST support the technique described in [10] when reading
   this option from incoming DHCP packets.

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.

DHCP Server Considerations

   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

   DHCP currently provides no authentication or security mechanisms.
   Potential exposures to attack are discussed in section 7 of the DHCP
   protocol specification [1]. The Classless Static Routes option can
   be used to misdirect network traffic by providing incorrect IP
   addresses for routers.

IANA Considerations

   This DHCP option will require the allocation of an option code in
   the list DHCP option codes that the IANA maintains.


   [1]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        Bucknell University, March 1997.
   [2]  Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
        Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell
        University, March 1997.
   [3]  Bradner, S., "Key words for use in RFCs to indicate requirement
        levels", RFC 2119, Harvard University, March 1997.
   [4]  Postel, J., "Internet Protocol", RFC 791, USC/Information
        Sciences Institute, September 1981.
   [5]  Hedrick, C.L., "Routing Information Protocol", RFC 1058,
        Rutgers University, June 1, 1988.
   [6]  Deering, S., "ICMP Router Discovery Messages", RFC 1256,
        Xerox PARC, September 1991.
   [7]  Postel, J., "Internet Control Message Protocol", RFC 792,
        USC/Information Sciences Institute, September 1981.
   [8]  Mogul, J., Postel, J., "Internet Standard Subnetting
        Procedure", RFC950, Stanford University, USC/Information
        Sciences Institute, August 1985.
   [9]  Pummill, T., Manning, B., "Variable Length Subnet Table For
        IPv4", RFC1878, Alantec, USC/Information Sciences Institute,
        December, 1995.
   [10] Lemon, T., Cheshire, S., "Encoding Long DHCP Options",
        draft-ietf-dhc-concat-02.txt, Nominum, Inc., October, 2001.

Author Information

Ted Lemon
Nominum, Inc.
950 Charter Street
Redwood City, CA 94043
email: Ted.Lemon@nominum.com

Stuart Cheshire
Apple Computer, Inc.
1 Infinite Loop
California 95014
Phone: +1 408 974 3207
EMail: rfc@stuartcheshire.org

Bernie Volz
959 Concord Street
Framingham, MA, 01701
Phone: +1 508 875 3162
EMail: bernie.volz@ericsson.com


   This document will expire on April 31, 2002.

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