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Versions: 00 01 02 03 04 05 06 07 rfc3442                               
Network Working Group                                            Ted Lemon
Internet Draft                                                Nominum, Inc.

Obsoletes: draft-ietf-dhc-csr-00.txt                           March, 2000
                                                    Expires September 2000

              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,
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   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 [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 first checks to see
   if the IP address of the destination host to see if it is on a
   subnet to which the source host is connected.    If the destination
   host's IP address is not on a subnet to which the source host is
   connected, then the source host 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

   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 [1] and the options defined in [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 [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 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 [4] and [8].


   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.

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 non-zero 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 non-zero portion of the subnet number is simply all of the
   octets of the subnet number, with the least significant octets that
   are zero omitted.   For a subnet mask width of between 25 and 32,
   the subnet number will be four octets.   Mask widths of between 17
   and 24 indicate a three-octet subnet number; between 9 and 16
   indicate a two-octet subnet number, between 1 and 8 indicate a
   one-octet number.   As a special case, the default route may be
   represented by a zero width, with no following subnet number.
   Host routes are represented by a mask width of 32, followed by four
   octets containing the IP address of the host.

   The following table contains some examples:

   Subnet number   Subnet mask      Destination descriptor
   0               0                0        8.10      16.10.17

DHCP Client Behavior

   The DHCP client MAY use this option to install a set of static
   routes in its routing table.   A DHCP client that implements this
   option SHOULD use this option in preference to the Static routes
   option if both are present in a reply from the DHCP server.   The
   client MAY request both options.

   After deriving a subnet number and subnet mask from each
   destination descriptor, the DHCP client SHOULD check each route to
   determine if are any bits in the destination network number whose
   value is one whose corresponding value in the subnet mask is zero,
   and SHOULD NOT install any routes for which this is the case.  For
   example, the client should not install a route with a destination
   of 129.210.377.4 and a subnet mask of

   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 the MTU of
   the interface that the client is configuring.

DHCP Server administrator responsibilities

   The client's behaviour if both a Routers option and a Classless
   Static Routes option default route (network number, network
   mask are specified is not defined in this document, so as
   to avoid placing onerous requirements on the client and server
   implementations.   Therefore, the DHCP server administrator SHOULD
   NOT configure the DHCP server so that it sends both a Routers
   option and a Classless Static Routes option containing a default
   route.   Either no Routers option should be configured (this is
   probably preferable in the near term, since only newer DHCP clients
   will implement this option), or the Classless Static Routes option
   should not contain a default route.

   The client's behaviour is also not defined in the case where the
   server sends a classless static route in which some bits in the
   network number are 1, and corresponding bits in the subnet mask are
   zero.   Therefore, DHCP server administrators SHOULD NOT configure
   the DHCP server to send such a route.

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.


   [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

Author Information

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


   This document will expire on July 31, 2000.

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