Dynamic Host Configuration Working Group Ted Lemon Internet Draft Nominum, Inc. Obsoletes: draft-ietf-dhc-csr-03.txt February, 2001 Expires August, 2001 The Classless Static Route Option for DHCP <draft-ietf-dhc-csr-04.txt> 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 http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at 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. This option supersedes the Static Route option (option 33) defined in . Introduction The 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 ways - using a routing information protocol such as RIP , 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  and the options defined in  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  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  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 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  and . 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. "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 10.0.127.0 and a netmask of 255.255.255.0 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 10.0.0.0 255.0.0.0 8.10 10.17.0.0 255.255.0.0 16.10.17 10.27.129.0 255.255.255.0 188.8.131.52 10.229.0.128 255.255.255.128 184.108.40.206.128 10.198.122.47 255.255.255.255 220.127.116.11.47 Local Subnet Routes In the case where there is more than one IP subnet connected to the local network, the DHCP server MAY send routes for those subnets specifying an IP destination address of 0.0.0.0. This statement applies strictly to the Classless Static Routes option. The behaviour of the DHCP client in the case that a Routers option contains a destination of 0.0.0.0 is not specified here. 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. 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  in the DHCP Parameter Request List. DHCP clients that support this option and send a parameter request list MUST NOT request the Static Routes option. The Classless Static Routes option code SHOULD appear in the parameter request list prior to the Routers option code. If the DHCP server returns both a Router option and a Classless Static Routes option, the DHCP client MUST ignore the Routers option. Some TCP/IP stacks can be configured to send ARP request messages on an interface for IP addresses that are on subnets not configured for that interface. Consequently, DHCP clients that implement the Classless Static Routes option MUST check each route to see if the IP destination is 0.0.0.0, and MUST EITHER configure their IP stack to ARP for IP addresses whose routing destination is 0.0.0.0, OR ignore routes found in the Classless Static Routes option that have a destination of 0.0.0.0. After deriving a subnet number and subnet mask from each destination descriptor, the DHCP client SHOULD check each route to determine if there 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 255.255.255.128. 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 the MTU of the interface that the client is configuring. 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 Routers option and a Classless Static Routes option, and should specify the default router(s) both in the Routers option and in the Classless Static Routes option. DHCP Server Considerations When a DHCP client requests both the Routers option and the Classless Static Routes option, and the DHCP server is configured with both a Classless Static Routes option and a Routers option that applies to the client, the DHCP server MAY exclude the Routers option from its response. Security Considerations DHCP currently provides no authentication or security mechanisms. Potential exposures to attack are discussed in section 7 of the DHCP protocol specification . The Classless Static Routes option can be used to misdirect network traffic by providing incorrect IP addresses for routers. 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., "Encoding Long DHCP Options", draft-ietf-dhc-concat-00.txt, Nominum, Inc., February, 2001. Author Information Ted Lemon Nominum, Inc. 950 Charter Street Redwood City, CA 94043 email: Ted.Lemon@nominum.com Expiration This document will expire on August 31, 2001. Full Copyright Statement Copyright (C) The Internet Society (2000-2001). All Rights Reserved. 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