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Network Working Group                                           T. LemonRequest for Comments: 3442                                 Nominum, Inc.Updates:2132                                                S. CheshireCategory: Standards Track                           Apple Computer, Inc.                                                                 B. Volz                                                                Ericsson                                                           December 2002The Classless Static Route Option forDynamic Host Configuration Protocol (DHCP) version 4Status 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 inRFC 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 inRFC 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 inRFC 2131 [3] and the options defined   inRFC 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 - seesection 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 inRFC950 [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 inBCP 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  32.10.198.122.47Local 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 subnetLemon, 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 ofRFC 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 insection 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-middleLemon, 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 inRFC 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 2002Intellectual 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 inBCP-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.comLemon, et. al.              Standards Track                     [Page 8]

RFC 3442        Classless Static Route Option for DHCPv4   December 2002Full 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]