Network Working Group Ted Lemon Internet Draft Nominum, Inc. Obsoletes: draft-ietf-dhc-csr-06.txt Stuart Cheshire Apple Computer, Inc. Bernie Volz Ericsson
Obsoletes: draft-ietf-dhc-csr-05.txt October, 2001 Expires April,July, 2002 Expires January, 2003 The Classless Static Route Option for DHCP <draft-ietf-dhc-csr-06.txt>DHCPv4 <draft-ietf-dhc-csr-07.txt> Status of this Memo This document is an Internet-Draft and is in full conformance withsubject to 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-DraftsInternet- Drafts as reference material or to cite them other than as "work in progress".progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txthttp://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.http://www.ietf.org/shadow.html Abstract 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. The network destinations in these routes are classless - each routing table entry includes a subnet mask. Introduction This option supersedes the Static Route option (option 33) defined in RFC2132 . IntroductionThe IP protocol  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 ^L ways - using a routing information protocol such as RIP , ICMP router discovery [6,7] or using the DHCP Router option, defined in RFC2132 . 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  and the options defined in RFC2132  only provide a mechanism for installing a default route or installing a table of classedclassful routes. ClassedClassful routes are routes whose subnet mask is implicit in the subnet number - see section 3.2 of RFC791  for details on classedclassful routing. ClassedClassful routing is no longer in common use, so the DHCP Static Route option is no longer useful. Currently, classless routing, described in  and , 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 ) and a host number. In classedclassful 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  and RFC950 . 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 RFC 2119 . 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. ^L "DHCP server" A DHCP server or "server" is an Internet host that returns configuration parameters to DHCP clients. "link" 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 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 ^L 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 18.104.22.168 10.17.0.0 255.255.0.0 16.10.17 10.27.129.0 255.255.255.0 22.214.171.124 10.229.0.128 255.255.255.128 126.96.36.199.128 10.198.122.47 255.255.255.255 188.8.131.52.47 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. 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. ^L DHCP clients that support this option and that send a DHCP Parameter Request List option MUST request both this option and the Router option  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 option. 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 184.108.40.206 (hexadecimal 81D4B184) and a subnet mask of 255.255.255.128 (hexadecimal FFFFFF80), the client will install a route with a destination of 220.127.116.11 (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  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  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  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 ^L 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 in the DHCP protocol are discussed in section 7 of the DHCP protocol specification . and in Authentication for DHCP Messages . 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 valid, or can be used to set up a man-in-the-middle 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 RFC2132  exhibit the same vulnerability. IANA Considerations This DHCP option will require the allocation of an option code in the list of DHCP option codes that the IANA maintains. References  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, Bucknell University, March 1997.  Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell University, March 1997.  Bradner, S., "Key words for use in RFCs to indicate requirement levels", RFC 2119, Harvard University, March 1997.  Postel, J., "Internet Protocol", RFC 791, USC/Information Sciences Institute, September 1981.  Hedrick, C.L., "Routing Information Protocol", RFC 1058, Rutgers University, June 1, 1988.  Deering, S., "ICMP Router Discovery Messages", RFC 1256, Xerox PARC, September 1991.  Postel, J., "Internet Control Message Protocol", RFC 792, USC/Information Sciences Institute, September 1981.  Mogul, J., Postel, J., "Internet Standard Subnetting Procedure", RFC950, Stanford University, USC/Information Sciences Institute, August 1985.  Pummill, T., Manning, B., "Variable Length Subnet Table For IPv4", RFC1878, Alantec, USC/Information Sciences Institute, December, 1995.  Lemon, T., Cheshire, S., "Encoding Long DHCP Options", draft-ietf-dhc-concat-02.txt,draft-ietf-dhc-concat-05.txt, Nominum, Inc., October, 2001.Apple Computer, Inc., July, 2002. ^L Author Information Ted Lemon Nominum, Inc. 950 Charter Street2385 Bay Road Redwood City, CA 9404394063 email: Ted.Lemon@nominum.com Stuart Cheshire Apple Computer, Inc. 1 Infinite Loop Cupertino California 95014 USA Phone: +1 408 974 3207 EMail: email@example.com Bernie Volz Ericsson 959 Concord Street Framingham, MA, 01701 Phone: +1 508 875 3162 EMail: firstname.lastname@example.org Expiration This document will expire on AprilDecember 31, 2002. Full Copyright Statement Copyright (C) The Internet Society (2000-2001).(2000-2002). All Rights Reserved. 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