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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Dynamic Host Configuration Working Group Ted Lemon 2 Internet Draft Nominum, Inc. 4 Obsoletes: draft-ietf-dhc-csr-03.txt February, 2001 5 Expires August, 2001 7 The Classless Static Route Option for DHCP 8 10 Status of this Memo 12 This document is an Internet-Draft and is in full conformance with 13 all provisions of Section 10 of RFC2026. 15 This document is an Internet-Draft. Internet-Drafts are working 16 documents of the Internet Engineering Task Force (IETF), its areas, 17 and its working groups. Note that other groups may also distribute 18 working documents as Internet-Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as "work in progress". 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/ietf/1id-abstracts.txt 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 Abstract 33 This document defines a new DHCP option which is passed from the 34 DHCP Server to the DHCP Client to configure a list of static routes 35 in the client. This option supersedes the Static Route option 36 (option 33) defined in [2]. 38 Introduction 40 The IP protocol [4] uses routers to transmit packets from hosts 41 connected to one IP subnet to hosts connected to a different IP 42 subnet. When an IP host (the source host) wishes to transmit a 43 packet to another IP host (the destination), it consults its 44 routing table to determine the IP address of the router that should 45 be used to forward the packet to the destination host. 47 The routing table on an IP host can be maintained in a variety of 48 ways - using a routing information protocol such as RIP [5], ICMP 49 router discovery [6,7] or using the DHCP Router option, defined in 50 [2]. 52 In a network that already provides DHCP service, using DHCP to 53 update the routing table on a DHCP client has several virtues. It 54 is efficient, since it makes use of messages that would have been 55 sent anyway. It is convenient - the DHCP server configuration 56 is already being maintained, so maintaining routing information, at 57 least on a relatively stable network, requires little extra work. 59 If DHCP service is already in use, no additional infrastructure 60 need be deployed. 62 The DHCP protocol as defined in [1] and the options defined in [2] 63 only provide a mechanism for installing a default route or 64 installing a table of classed routes. Classed routes are routes 65 whose subnet mask is implicit in the subnet number - see section 66 3.2 of [4] for details on classed routing. 68 Classed routing is no longer in common use, so the DHCP Static 69 Route option is no longer useful. Currently, classless routing, 70 described in [8] and [9], is the most commonly-deployed form of 71 routing on the Internet. In classless routing, IP addresses 72 consist of a network number (the combination of the network number 73 and subnet number described in [8]) and a host number. 75 In classed IP, the network number and host number are derived from 76 the IP address using a bitmask whose value is determined by the first 77 few bits of the IP address. In classless IP, the network number 78 and host number are derived from the IP address using a seperate 79 quantity, the subnet mask. In order to determine the network to 80 which a given route applies, an IP host must know both the network 81 number AND the subnet mask for that network. 83 The Static Routes option (option 33) does not provide a subnet mask 84 for each route - it is assumed that the subnet mask is implicit in 85 whatever network number is specified in each route entry. The 86 Classless Static Routes option does provide a subnet mask for each 87 entry, so that the subnet mask can be other than what would be 88 determined using the algorithm specified in [4] and [8]. 90 Definitions 92 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 93 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this 94 document are to be interpreted as described in RFC 2119 [3]. 96 This document also uses the following terms: 98 "DHCP client" 100 DHCP client or "client" is an Internet host using DHCP to 101 obtain configuration parameters such as a network address. 103 "DHCP server" 105 A DHCP server or "server" is an Internet host that returns 106 configuration parameters to DHCP clients. 108 Classless Route Option Format 110 The code for this option is TBD, and its minimum length is 5 bytes. 111 This option can contain one or more static routes, each of which 112 consists of a destination descriptor and the IP address of the 113 router that should be used to reach that destination. 115 Code Len Destination 1 Router 1 116 +-----+---+----+-----+----+----+----+----+----+ 117 | TBD | n | d1 | ... | dN | r1 | r2 | r3 | r4 | 118 +-----+---+----+-----+----+----+----+----+----+ 120 Destination 2 Router 2 121 +----+-----+----+----+----+----+----+ 122 | d1 | ... | dN | r1 | r2 | r3 | r4 | 123 +----+-----+----+----+----+----+----+ 125 In the above example, two static routes are specified. 127 Destination descriptors describe the IP subnet number and subnet 128 mask of a particular destination using a compact encoding. This 129 encoding consists of one octet describing the width of the subnet 130 mask, followed by all the non-zero octets of the subnet number. 132 The width of the subnet mask describes the number of one bits in 133 the mask, so for example a subnet with a subnet number of 134 10.0.127.0 and a netmask of 255.255.255.0 would have a subnet mask 135 width of 24. 137 The non-zero portion of the subnet number is simply all of the 138 octets of the subnet number, with the least significant octets that 139 are zero omitted. For a subnet mask width of between 25 and 32, 140 the subnet number will be four octets. Mask widths of between 17 141 and 24 indicate a three-octet subnet number; between 9 and 16 142 indicate a two-octet subnet number, between 1 and 8 indicate a 143 one-octet number. As a special case, the default route may be 144 represented by a zero width, with no following subnet number. 145 Host routes are represented by a mask width of 32, followed by four 146 octets containing the IP address of the host. 148 The following table contains some examples: 150 Subnet number Subnet mask Destination descriptor 151 0 0 0 152 10.0.0.0 255.0.0.0 8.10 153 10.17.0.0 255.255.0.0 16.10.17 154 10.27.129.0 255.255.255.0 24.10.27.129 155 10.229.0.128 255.255.255.128 25.10.229.0.128 156 10.198.122.47 255.255.255.255 32.10.198.122.47 157 Local Subnet Routes 159 In the case where there is more than one IP subnet connected to the 160 local network, the DHCP server MAY send routes for those subnets 161 specifying an IP destination address of 0.0.0.0. This statement 162 applies strictly to the Classless Static Routes option. The 163 behaviour of the DHCP client in the case that a Routers option 164 contains a destination of 0.0.0.0 is not specified here. 166 DHCP Client Behavior 168 DHCP clients that do not support this option MUST ignore it if it 169 is received from a DHCP server. DHCP clients that support this 170 option MUST install the routes specified in the option. DHCP 171 clients that support this option MUST NOT install the routes 172 specified in the Static Routes option (option code 33) if both a 173 Static Routes option and the Classless Static Routes option are 174 provided. 176 DHCP clients that support this option and that send a DHCP 177 Parameter Request List option MUST request both this option and the 178 Router option [2] in the DHCP Parameter Request List. DHCP clients 179 that support this option and send a parameter request list MUST NOT 180 request the Static Routes option. The Classless Static Routes 181 option code SHOULD appear in the parameter request list prior to 182 the Routers option code. 184 If the DHCP server returns both a Router option and a Classless 185 Static Routes option, the DHCP client MUST ignore the Routers 186 option. 188 Some TCP/IP stacks can be configured to send ARP request messages 189 on an interface for IP addresses that are on subnets not configured 190 for that interface. Consequently, DHCP clients that implement the 191 Classless Static Routes option MUST check each route to see if the 192 IP destination is 0.0.0.0, and MUST EITHER configure their IP stack 193 to ARP for IP addresses whose routing destination is 0.0.0.0, OR 194 ignore routes found in the Classless Static Routes option that have 195 a destination of 0.0.0.0. 197 After deriving a subnet number and subnet mask from each 198 destination descriptor, the DHCP client SHOULD check each route to 199 determine if there are any bits in the destination network number 200 whose value is one whose corresponding value in the subnet mask is 201 zero, and SHOULD NOT install any routes for which this is the case. 202 For example, the client should not install a route with a 203 destination of 129.210.377.4 and a subnet mask of 255.255.255.128. 205 Requirements to avoid sizing constraints 207 Because a full routing table can be quite large, the standard 576 208 octet maximum size for a DHCP message may be too short to contain 209 some legitimate Classless Static Route options. Because of this, 210 clients implementing the Classless Static Route option SHOULD send 211 a Maximum DHCP Message Size [2] option if the DHCP client's TCP/IP 212 stack is capable of reassembling fragmented IP datagrams. In this 213 case, the client SHOULD set the value of this option to the MTU of 214 the interface that the client is configuring. 216 DHCP servers sending this option MUST use the technique described 217 in [10] for sending options larger than 255 bytes when storing this 218 option in outgoing DHCP packets. DHCP clients supporting this 219 option MUST support the technique described in [10] when reading 220 this option from incoming DHCP packets. 222 DHCP Server administrator responsibilities 224 Many clients may not implement the Classless Static Routes option. 225 DHCP server administrators should therefore configure their DHCP 226 servers to send both a Routers option and a Classless Static Routes 227 option, and should specify the default router(s) both in the 228 Routers option and in the Classless Static Routes option. 230 DHCP Server Considerations 232 When a DHCP client requests both the Routers option and the 233 Classless Static Routes option, and the DHCP server is configured 234 with both a Classless Static Routes option and a Routers option 235 that applies to the client, the DHCP server MAY exclude the Routers 236 option from its response. 238 Security Considerations 240 DHCP currently provides no authentication or security mechanisms. 241 Potential exposures to attack are discussed in section 7 of the DHCP 242 protocol specification [1]. The Classless Static Routes option can 243 be used to misdirect network traffic by providing incorrect IP 244 addresses for routers. 246 References 248 [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, 249 Bucknell University, March 1997. 250 [2] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor 251 Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell 252 University, March 1997. 253 [3] Bradner, S., "Key words for use in RFCs to indicate requirement 254 levels", RFC 2119, Harvard University, March 1997. 255 [4] Postel, J., "Internet Protocol", RFC 791, USC/Information 256 Sciences Institute, September 1981. 257 [5] Hedrick, C.L., "Routing Information Protocol", RFC 1058, 258 Rutgers University, June 1, 1988. 259 [6] Deering, S., "ICMP Router Discovery Messages", RFC 1256, 260 Xerox PARC, September 1991. 261 [7] Postel, J., "Internet Control Message Protocol", RFC 792, 262 USC/Information Sciences Institute, September 1981. 263 [8] Mogul, J., Postel, J., "Internet Standard Subnetting 264 Procedure", RFC950, Stanford University, USC/Information 265 Sciences Institute, August 1985. 267 [9] Pummill, T., Manning, B., "Variable Length Subnet Table For 268 IPv4", RFC1878, Alantec, USC/Information Sciences Institute, 269 December, 1995. 270 [10] Lemon, T., "Encoding Long DHCP Options", 271 draft-ietf-dhc-concat-00.txt, Nominum, Inc., February, 2001. 273 Author Information 275 Ted Lemon 276 Nominum, Inc. 277 950 Charter Street 278 Redwood City, CA 94043 279 email: Ted.Lemon@nominum.com 281 Expiration 283 This document will expire on August 31, 2001. 285 Full Copyright Statement 287 Copyright (C) The Internet Society (2000-2001). 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