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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Downref: Normative reference to an Historic RFC: RFC 1058 (ref. '5') ** Downref: Normative reference to an Historic RFC: RFC 1878 (ref. '9') Summary: 8 errors (**), 0 flaws (~~), 7 warnings (==), 4 comments (--). 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-03.txt December, 2000 5 Expires May, 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 first checks the IP 44 address of the destination host to see if it is on a subnet to 45 which the source host is connected. If the destination host's IP 46 address is not on a subnet to which the source host is connected, 47 then the source host consults its routing table to determine the IP 48 address of the router that should be used to forward the packet to 49 the destination host. 51 The routing table on an IP host can be maintained in a variety of 52 ways - using a routing information protocol such as RIP [5], ICMP 53 router discovery [6,7] or using the DHCP Router option, defined in 54 [2]. 56 In a network that already provides DHCP service, using DHCP to 57 update the routing table on a DHCP client has several virtues. It 58 is efficient, since it makes use of messages that would have been 59 sent anyway. It is convenient - the DHCP server configuration 60 is already being maintained, so maintaining routing information, at 61 least on a relatively stable network, requires little extra work. 62 If DHCP service is already in use, no additional infrastructure 63 need be deployed. 65 The DHCP protocol as defined in [1] and the options defined in [2] 66 only provide a mechanism for installing a default route or 67 installing a table of classed routes. Classed routes are routes 68 whose subnet mask is implicit in the subnet number - see section 69 3.2 of [4] for details on classed routing. 71 Classed routing is no longer in common use, so the DHCP Static 72 Route option is no longer useful. Currently, classless routing, 73 described in [8] and [9], is the most commonly-deployed form of 74 routing on the Internet. In classless routing, IP addresses 75 consist of a network number (the combination of the network number 76 and subnet number described in [8]) and a host number. 78 In classed IP, the network number and host number are derived from 79 the IP address using a bitmask whose value is determined by the first 80 few bits of the IP address. In classless IP, the network number 81 and host number are derived from the IP address using a seperate 82 quantity, the subnet mask. In order to determine the network to 83 which a given route applies, an IP host must know both the network 84 number AND the subnet mask for that network. 86 The Static Routes option does not provide a subnet mask for each 87 route - it is assumed that the subnet mask is implicit in whatever 88 network number is specified in each route entry. The Classless 89 Static Routes option does provide a subnet mask for each entry, so 90 that the subnet mask can be other than what would be determined 91 using the algorithm specified in [4] and [8]. 93 Definitions 95 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 96 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this 97 document are to be interpreted as described in RFC 2119 [3]. 99 This document also uses the following terms: 101 "DHCP client" 103 DHCP client or "client" is an Internet host using DHCP to 104 obtain configuration parameters such as a network address. 106 "DHCP server" 108 A DHCP server or "server" is an Internet host that returns 109 configuration parameters to DHCP clients. 111 Classless Route Option Format 113 The code for this option is TBD, and its minimum length is 5 bytes. 114 This option can contain one or more static routes, each of which 115 consists of a destination descriptor and the IP address of the 116 router that should be used to reach that destination. 118 Code Len Destination 1 Router 1 119 +-----+---+----+-----+----+----+----+----+----+ 120 | TBD | n | d1 | ... | dN | r1 | r2 | r3 | r4 | 121 +-----+---+----+-----+----+----+----+----+----+ 123 Destination 2 Router 2 124 +----+-----+----+----+----+----+----+ 125 | d1 | ... | dN | r1 | r2 | r3 | r4 | 126 +----+-----+----+----+----+----+----+ 128 In the above example, two static routes are specified. 130 Destination descriptors describe the IP subnet number and subnet 131 mask of a particular destination using a compact encoding. This 132 encoding consists of one octet describing the width of the subnet 133 mask, followed by all the non-zero octets of the subnet number. 135 The width of the subnet mask describes the number of one bits in 136 the mask, so for example a subnet with a subnet number of 137 10.0.127.0 and a netmask of 255.255.255.0 would have a subnet mask 138 width of 24. 140 The non-zero portion of the subnet number is simply all of the 141 octets of the subnet number, with the least significant octets that 142 are zero omitted. For a subnet mask width of between 25 and 32, 143 the subnet number will be four octets. Mask widths of between 17 144 and 24 indicate a three-octet subnet number; between 9 and 16 145 indicate a two-octet subnet number, between 1 and 8 indicate a 146 one-octet number. As a special case, the default route may be 147 represented by a zero width, with no following subnet number. 148 Host routes are represented by a mask width of 32, followed by four 149 octets containing the IP address of the host. 151 The following table contains some examples: 153 Subnet number Subnet mask Destination descriptor 154 0 0 0 155 10.0.0.0 255.0.0.0 8.10 156 10.17.0.0 255.255.0.0 16.10.17 157 10.27.129.0 255.255.255.0 24.10.27.129 158 10.229.0.128 255.255.255.128 25.10.229.0.128 159 10.198.122.47 255.255.255.255 32.10.198.122.47 160 Local Subnet Routes 162 In the case where there is more than one IP subnet connected to the 163 local network, the DHCP server MAY send routes for those subnets 164 that specify an IP destination address of 0.0.0.0. DHCP clients 165 that implement this option MUST check for an IP destination address 166 of 0.0.0.0, and MUST EITHER configure their IP stack to ARP for IP 167 addresses whose routing destination is 0.0.0.0, OR ignore routes 168 with a destination of 0.0.0.0. DHCP clients that support ARPing 169 as described here MUST ignore the Router option (option code 3) if 170 the Router option contains the client's own IP address. 172 DHCP Client Behavior 174 DHCP clients that do not support this option MUST ignore it if it 175 is received from a DHCP server. DHCP clients that support this 176 option MUST install the routes specified in the option. DHCP 177 clients that support this option MUST NOT install the routes 178 specified in the Static Routes option (option code 33) if both a 179 Static Routes option and the Classless Static Routes option are 180 provided. 182 DHCP clients that support this option and that send a DHCP 183 Parameter Request List option MUST request both this option and the 184 Router option [2] in the DHCP Parameter Request List. DHCP clients 185 that support this option and send a parameter request list MUST NOT 186 request the Static Routes option. 188 If the DHCP server returns a Router option, clients that support 189 the Classless Static Routes option MUST use the default route(s) 190 listed in the Router option in addition to the routes listed 191 in the Classless Static Routes option. 193 After deriving a subnet number and subnet mask from each 194 destination descriptor, the DHCP client SHOULD check each route to 195 determine if are any bits in the destination network number whose 196 value is one whose corresponding value in the subnet mask is zero, 197 and SHOULD NOT install any routes for which this is the case. For 198 example, the client should not install a route with a destination 199 of 129.210.377.4 and a subnet mask of 255.255.255.128. 201 Because a full routing table can be quite large, the standard 576 202 octet maximum size for a DHCP message may be too short to contain 203 some legitimate Classless Static Route options. Because of this, 204 clients implementing the Classless Static Route option SHOULD send 205 a Maximum DHCP Message Size [2] option if the DHCP client's TCP/IP 206 stack is capable of reassembling fragmented IP datagrams. In this 207 case, the client SHOULD set the value of this option to the MTU of 208 the interface that the client is configuring. 210 DHCP Server administrator responsibilities 212 Many clients may not implement the Classless Static Routes option. 213 DHCP server administrators should therefore configure their DHCP 214 servers to send both a Routers option and a Classless Static 215 Routes option, and should specify all default routes in the Routers 216 option, and not specify any default routes in the Classless 217 Static Routes option. 219 Security Considerations 221 DHCP currently provides no authentication or security mechanisms. 222 Potential exposures to attack are discussed in section 7 of the DHCP 223 protocol specification [1]. The Classless Static Routes option can 224 be used to misdirect network traffic by providing incorrect IP 225 addresses for routers. 227 References 229 [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, 230 Bucknell University, March 1997. 231 [2] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor 232 Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell 233 University, March 1997. 234 [3] Bradner, S., "Key words for use in RFCs to indicate requirement 235 levels", RFC 2119, Harvard University, March 1997. 236 [4] Postel, J., "Internet Protocol", RFC 791, USC/Information 237 Sciences Institute, September 1981. 238 [5] Hedrick, C.L., "Routing Information Protocol", RFC 1058, 239 Rutgers University, June 1, 1988. 240 [6] Deering, S., "ICMP Router Discovery Messages", RFC 1256, 241 Xerox PARC, September 1991. 242 [7] Postel, J., "Internet Control Message Protocol", RFC 792, 243 USC/Information Sciences Institute, September 1981. 244 [8] Mogul, J., Postel, J., "Internet Standard Subnetting 245 Procedure", RFC950, Stanford University, USC/Information 246 Sciences Institute, August 1985. 247 [9] Pummill, T., Manning, B., "Variable Length Subnet Table For 248 IPv4", RFC1878, Alantec, USC/Information Sciences Institute, 249 December, 1995 251 Author Information 253 Ted Lemon 254 Nominum, Inc. 255 950 Charter Street 256 Redwood City, CA 94043 257 email: Ted.Lemon@nominum.com 259 Expiration 261 This document will expire on May 31, 2001. 263 Full Copyright Statement 265 Copyright (C) The Internet Society (2000). All Rights Reserved. 267 This document and translations of it may be copied and furnished to 268 others, and derivative works that comment on or otherwise explain it 269 or assist in its implementation may be prepared, copied, published 270 and distributed, in whole or in part, without restriction of any 271 kind, provided that the above copyright notice and this paragraph are 272 included on all such copies and derivative works. However, this 273 document itself may not be modified in any way, such as by removing 274 the copyright notice or references to the Internet Society or other 275 Internet organizations, except as needed for the purpose of 276 developing Internet standards in which case the procedures for 277 copyrights defined in the Internet Standards process must be 278 followed, or as required to translate it into languages other than 279 English. 281 The limited permissions granted above are perpetual and will not be 282 revoked by the Internet Society or its successors or assigns. 284 This document and the information contained herein is provided on an 285 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 286 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 287 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 288 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 289 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.