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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group Ted Lemon 2 Internet Draft Nominum, Inc. 4 Obsoletes: draft-ietf-dhc-csr-04.txt July, 2001 5 Expires January, 2002 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. 58 If DHCP service is already in use, no additional infrastructure 59 need be deployed. 61 The DHCP protocol as defined in [1] and the options defined in [2] 62 only provide a mechanism for installing a default route or 63 installing a table of classed routes. Classed routes are routes 64 whose subnet mask is implicit in the subnet number - see section 65 3.2 of [4] for details on classed routing. 67 Classed routing is no longer in common use, so the DHCP Static 68 Route option is no longer useful. Currently, classless routing, 69 described in [8] and [9], is the most commonly-deployed form of 70 routing on the Internet. In classless routing, IP addresses 71 consist of a network number (the combination of the network number 72 and subnet number described in [8]) and a host number. 74 In classed IP, the network number and host number are derived from 75 the IP address using a bitmask whose value is determined by the first 76 few bits of the IP address. In classless IP, the network number 77 and host number are derived from the IP address using a seperate 78 quantity, the subnet mask. In order to determine the network to 79 which a given route applies, an IP host must know both the network 80 number AND the subnet mask for that network. 82 The Static Routes option (option 33) does not provide a subnet mask 83 for each route - it is assumed that the subnet mask is implicit in 84 whatever network number is specified in each route entry. The 85 Classless Static Routes option does provide a subnet mask for each 86 entry, so that the subnet mask can be other than what would be 87 determined using the algorithm specified in [4] and [8]. 89 Definitions 91 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 92 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this 93 document are to be interpreted as described in RFC 2119 [3]. 95 This document also uses the following terms: 97 "DHCP client" 99 DHCP client or "client" is an Internet host using DHCP to 100 obtain configuration parameters such as a network address. 102 "DHCP server" 104 A DHCP server or "server" is an Internet host that returns 105 configuration parameters to DHCP clients. 107 Classless Route Option Format 109 The code for this option is TBD, and its minimum length is 5 bytes. 110 This option can contain one or more static routes, each of which 111 consists of a destination descriptor and the IP address of the 112 router that should be used to reach that destination. 114 Code Len Destination 1 Router 1 115 +-----+---+----+-----+----+----+----+----+----+ 116 | TBD | n | d1 | ... | dN | r1 | r2 | r3 | r4 | 117 +-----+---+----+-----+----+----+----+----+----+ 119 Destination 2 Router 2 120 +----+-----+----+----+----+----+----+ 121 | d1 | ... | dN | r1 | r2 | r3 | r4 | 122 +----+-----+----+----+----+----+----+ 124 In the above example, two static routes are specified. 126 Destination descriptors describe the IP subnet number and subnet 127 mask of a particular destination using a compact encoding. This 128 encoding consists of one octet describing the width of the subnet 129 mask, followed by all the non-zero octets of the subnet number. 131 The width of the subnet mask describes the number of one bits in 132 the mask, so for example a subnet with a subnet number of 133 10.0.127.0 and a netmask of 255.255.255.0 would have a subnet mask 134 width of 24. 136 The non-zero portion of the subnet number is simply all of the 137 octets of the subnet number, with the least significant octets that 138 are zero omitted. For a subnet mask width of between 25 and 32, 139 the subnet number will be four octets. Mask widths of between 17 140 and 24 indicate a three-octet subnet number; between 9 and 16 141 indicate a two-octet subnet number, between 1 and 8 indicate a 142 one-octet number. As a special case, the default route may be 143 represented by a zero width, with no following subnet number. 144 Host routes are represented by a mask width of 32, followed by four 145 octets containing the IP address of the host. 147 The following table contains some examples: 149 Subnet number Subnet mask Destination descriptor 150 0 0 0 151 10.0.0.0 255.0.0.0 8.10 152 10.17.0.0 255.255.0.0 16.10.17 153 10.27.129.0 255.255.255.0 24.10.27.129 154 10.229.0.128 255.255.255.128 25.10.229.0.128 155 10.198.122.47 255.255.255.255 32.10.198.122.47 157 Local Subnet Routes 159 In some cases more than one IP subnet may be configured within a 160 given network broadcast domain. In such cases, a host whose IP 161 address is in one IP subnet in the broadcast domain could communicate 162 directly with a host whose IP address is in a different IP subnet in 163 the same broadcast domain. In cases where a client is being 164 assigned an IP address on an IP subnet in such a broadcast domain, 165 for each IP subnet in the broadcast domain other than the IP subnet 166 on which the client has been assigned the DHCP server MAY be 167 configured to specify a router IP address of 0.0.0.0. 169 For example, consider the case where there are three IP subnets 170 configured on a particular broadcast domain: 10.0.0/24, 171 192.168.0/24, 10.0.21/24. If the client is assigned an IP address 172 of 10.0.21.17, then the server could include a route with a 173 destination of 10.0.0/24 and a router address of 0.0.0.0, and also 174 a route with a destination of 192.168.0/24 and a router address of 175 0.0.0.0. 177 A DHCP client whose underlying TCP/IP stack does not provide this 178 capability MUST ignore routes in the Classless Static Routes option 179 whose router IP address is 0.0.0.0. Please note that the behavior 180 described here only applies to the Classless Static Routes option, 181 not to the Static Routes option nor the Router option. 183 DHCP Client Behavior 185 DHCP clients that do not support this option MUST ignore it if it 186 is received from a DHCP server. DHCP clients that support this 187 option MUST install the routes specified in the option, except as 188 specified in the Local Subnet Routes section. DHCP clients that 189 support this option MUST NOT install the routes specified in the 190 Static Routes option (option code 33) if both a Static Routes 191 option and the Classless Static Routes option are provided. 193 DHCP clients that support this option and that send a DHCP 194 Parameter Request List option MUST request both this option and the 195 Router option [2] in the DHCP Parameter Request List. DHCP clients 196 that support this option and send a parameter request list MUST NOT 197 request the Static Routes option. The Classless Static Routes 198 option code SHOULD appear in the parameter request list prior to 199 the Router option code. 201 If the DHCP server returns both a Router option and a Classless 202 Static Routes option, the DHCP client MUST ignore the Router 203 option. 205 After deriving a subnet number and subnet mask from each 206 destination descriptor, the DHCP client SHOULD check the 207 combination of the network number and the subnet mask for validity. 208 If the network number contains nonzero bits beyond the subnet mask, 209 the client SHOULD discard that route. For example, the client 210 should not install a route with a destination of 129.210.377.4 and 211 a subnet mask of 255.255.255.128. 213 Requirements to avoid sizing constraints 215 Because a full routing table can be quite large, the standard 576 216 octet maximum size for a DHCP message may be too short to contain 217 some legitimate Classless Static Route options. Because of this, 218 clients implementing the Classless Static Route option SHOULD send 219 a Maximum DHCP Message Size [2] option if the DHCP client's TCP/IP 220 stack is capable of reassembling fragmented IP datagrams. In this 221 case, the client SHOULD set the value of this option to the MTU of 222 the interface that the client is configuring. If the client 223 supports UDP fragmentation, it MAY set the value of this option to 224 the size of the largest UDP packet it is prepared to accept. 226 DHCP servers sending this option MUST use the technique described 227 in [10] for sending options larger than 255 bytes when storing this 228 option in outgoing DHCP packets. DHCP clients supporting this 229 option MUST support the technique described in [10] when reading 230 this option from incoming DHCP packets. 232 DHCP Server administrator responsibilities 234 Many clients may not implement the Classless Static Routes option. 235 DHCP server administrators should therefore configure their DHCP 236 servers to send both a Router option and a Classless Static Routes 237 option, and should specify the default router(s) both in the 238 Router option and in the Classless Static Routes option. 240 DHCP Server Considerations 242 When a DHCP client requests both the Router option and the 243 Classless Static Routes option, and the DHCP server is configured 244 with both a Classless Static Routes option and a Router option 245 that applies to the client, the DHCP server MAY exclude the Router 246 option from its response. 248 Security Considerations 250 DHCP currently provides no authentication or security mechanisms. 251 Potential exposures to attack are discussed in section 7 of the DHCP 252 protocol specification [1]. The Classless Static Routes option can 253 be used to misdirect network traffic by providing incorrect IP 254 addresses for routers. 256 References 258 [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, 259 Bucknell University, March 1997. 260 [2] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor 261 Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell 262 University, March 1997. 263 [3] Bradner, S., "Key words for use in RFCs to indicate requirement 264 levels", RFC 2119, Harvard University, March 1997. 265 [4] Postel, J., "Internet Protocol", RFC 791, USC/Information 266 Sciences Institute, September 1981. 267 [5] Hedrick, C.L., "Routing Information Protocol", RFC 1058, 268 Rutgers University, June 1, 1988. 269 [6] Deering, S., "ICMP Router Discovery Messages", RFC 1256, 270 Xerox PARC, September 1991. 271 [7] Postel, J., "Internet Control Message Protocol", RFC 792, 272 USC/Information Sciences Institute, September 1981. 273 [8] Mogul, J., Postel, J., "Internet Standard Subnetting 274 Procedure", RFC950, Stanford University, USC/Information 275 Sciences Institute, August 1985. 276 [9] Pummill, T., Manning, B., "Variable Length Subnet Table For 277 IPv4", RFC1878, Alantec, USC/Information Sciences Institute, 278 December, 1995. 279 [10] Lemon, T., "Encoding Long DHCP Options", 280 draft-ietf-dhc-concat-01.txt, Nominum, Inc., July, 2001. 282 Author Information 284 Ted Lemon 285 Nominum, Inc. 286 950 Charter Street 287 Redwood City, CA 94043 288 email: Ted.Lemon@nominum.com 290 Expiration 292 This document will expire on January 31, 2002. 294 Full Copyright Statement 296 Copyright (C) The Internet Society (2000-2001). 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