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rfc:rfc3442

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

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 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
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 or assist in its implementation may be prepared, copied, published
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 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
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Acknowledgement

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

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

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