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Note that other 17 groups may also distribute working documents as Internet-Drafts. 19 Internet-Drafts are draft documents valid for a maximum of six months 20 and may be updated, replaced, or obsoleted by other documents at any 21 time. It is inappropriate to use Internet-Drafts as reference 22 material or to cite them other than as "work in progress." 24 The list of current Internet-Drafts can be accessed at 25 http://www.ietf.org/ietf/1id-abstracts.txt 27 The list of Internet-Draft Shadow Directories can be accessed at 28 http://www.ietf.org/shadow.html. 30 To learn the current status of any Internet-Draft, please check the 31 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow 32 Directories on ds.internic.net (US East Coast), nic.nordu.net 33 (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). 35 Copyright Notice 37 Copyright (C) The Internet Society (2000). All Rights Reserved. 39 Abstract 41 This memo defines a new DHCP option for selecting the subnet on which 42 to allocate an address. This option would override a DHCP server's 43 normal methods of selecting the subnet on which to allocate an address 44 for a client. 46 Table of Contents 48 1. Introduction......................................................2 49 1.1. Motivational Example.........................................2 50 2. Subnet Selection Option Definition................................3 51 3. Intellectual Property.............................................4 52 4. IANA Considerations...............................................5 53 5. Acknowledgements..................................................5 54 6. Security Considerations...........................................5 55 7. References........................................................5 56 8. Editor's Addresses................................................6 57 9. Full Copyright Statement..........................................6 59 1. Introduction 61 The Dynamic Host Configuration Protocol (DHCP) [RFC2131] provides a 62 framework for passing configuration information to hosts on a TCP/IP 63 network. RFC 2132 [RFC2132] specifies DHCP option configuration 64 information that may be carried in DHCP packets to/from the DHCP 65 server and the DHCP client. This document specifies a new DHCP option. 67 To select the subnet on which to allocate an address, the DHCP server 68 determines the subnet from which the request originated, and then 69 selects an address on the originating subnet or on a subnet that is on 70 the same network segment as the originating subnet. The subnet from 71 which the request originates can be determined by: 73 o Using the subnet address of the giaddr field in the DHCP packet 74 header, or if the giaddr field is zero; 76 o Using the subnet address of the local interface on which the DHCP 77 server received the packet. 79 This memo defines a new DHCP option, the subnet selection option, 80 which allows the DHCP client to specify the subnet on which to 81 allocate an address. This option takes precedence over the methods 82 that the DHCP server uses to determine the subnet on which to select 83 an address. 85 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 86 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 87 document are to be interpreted as described in [RFC2119]. 89 1.1. Motivational Example 91 An example of where this option could be useful is in a device (e.g.: 92 a RAS device) that is allocating addresses on behalf of its clients. 93 In this case the device would be allocating addresses through DHCP and 94 then managing those addresses among its clients. 96 In this scenario, the device is connected to a private "internal" 97 network on which the DHCP server would be located. The device is also 98 connected to one or more service providing "external" networks (i.e.: 99 the networks that the device's clients are connected to). Furthermore, 100 the internal network is not IP connected to the external networks, 101 although inside the device there is connectivity between the internal 102 and external networks (e.g.: though the backplane). 104 Recall that the device is allocating addresses for its clients on the 105 external networks and that there is no IP connectivity between the 106 internal network and the external networks. The DHCP requests cannot 107 originate from the external networks since packets cannot be routed 108 between the external network and the internal network. Thus, the DHCP 109 requests must originate from the internal network. The problem with 110 originating the DHCP requests from the internal network is that the 111 DHCP server will allocate addresses on the internal network's subnet, 112 when what is required are addresses on the external subnets. The 113 subnet selection option provides a solution to this problem. 115 The device would send its DHCP request on the internal subnet, but 116 would include the subnet selection option containing the address of 117 the external subnet on which it requires the address. The subnet 118 selection option instructs the DHCP server to allocate the address on 119 the requested subnet as opposed to the normal operation of allocating 120 the address on the subnet from which the DHCP request originated. 122 2. Subnet Selection Option Definition 124 The subnet selection option is a DHCP option. The option contains a 125 single IPv4 address that is the address of a subnet. The value for the 126 subnet address is determined by taking any IPv4 address on the subnet 127 and ANDing that address with the subnet mask (i.e.: the network and 128 subnet bits are left alone and the remaining (address) bits are set to 129 zero). When the DHCP server is configured to respond to this option, 130 is allocating an address, and this option is present then the DHCP 131 server MUST allocate the address on either: 133 o the subnet specified in the subnet selection option, or; 135 o a subnet on the same network segment as the subnet specified in the 136 subnet selection option. 138 The format of the option is: 140 Code Len IPv4 Address 141 +-----+-----+-----+-----+-----+-----+ 142 | TBD | 4 | A1 | A2 | A3 | A4 | 143 +-----+-----+-----+-----+-----+-----+ 144 Servers configured to support this option MUST return an identical 145 copy of the option to any client that sends it, regardless of whether 146 or not the client requests the option in a parameter request list. 147 Clients using this option MUST discard DHCPOFFER or DHCPACK packets 148 that do not contain this option. 150 This option does not require changes to operations or features of the 151 DHCP server other than to select the subnet on which to allocate an 152 address. For example, the handling of DHCPDISCOVER for an unknown 153 subnet should continue to operate unchanged. 155 When this option is present and the server is configured to support 156 this option, the server MUST NOT offer an address that is not on the 157 requested subnet or network segment. Servers that do not understand 158 this option will allocate an address using their normal algorithms and 159 will not return this option in the DHCPOFFER or DHCPACK. In this case 160 the client will discard the DHCPOFFER or DHCPACK. Servers that 161 understand this option but are administratively configured to ignore 162 the option MUST ignore the option, use their normal algorithms to 163 allocate an address, and MUST NOT return this option in the DHCPOFFER 164 or DHCPACK. In this case the client will discard the DHCPOFFER or 165 DHCPACK. 167 During an address renew, the DHCP server may send a DHCPACK directly 168 to the allocated address, however packets from the DHCP server may not 169 be routable to the address. Thus, in all packets that the DHCP client 170 sends that contain the subnet selection option, the giaddr field in 171 the BOOTP header MUST be set to an IPv4 address on which the DHCP 172 client will accept DHCP packets (e.g.: the address on the subnet 173 connected to the internal network). 175 The IPv4 address to which a DHCP server sends a reply to MUST be the 176 same as it would chose when this option is not present. 178 3. Intellectual Property 180 The IETF takes no position regarding the validity or scope of any 181 intellectual property or other rights that might be claimed to pertain 182 to the implementation or use of the technology described in this 183 document or the extent to which any license under such rights might or 184 might not be available; neither does it represent that it has made any 185 effort to identify any such rights. Information on the IETF's 186 procedures with respect to rights in standards-track and standards- 187 related documentation can be found in BCP-11. 189 Copies of claims of rights made available for publication and any 190 assurances of licenses to be made available, or the result of an 191 attempt made to obtain a general license or permission for the use of 192 such proprietary rights by implementers or users of this specification 193 can be obtained from the IETF Secretariat. 195 The IETF invites any interested party to bring to its attention any 196 copyrights, patents or patent applications, or other proprietary 197 rights which may cover technology that may be required to practice 198 this standard. Please address the information to the IETF Executive 199 Director. 201 4. IANA Considerations 203 IANA has assigned a value of TBD for the DHCP option code described in 204 this document. 206 5. Acknowledgements 208 This document is the result of work undertaken the by DHCP working 209 group. Thanks to Ted Lemon, Tim Aston and Ralph Droms for their 210 helpful comments in this work. 212 6. Security Considerations 214 DHCP currently provides no authentication or security mechanisms. 215 Potential exposures to attack are discussed is section 7 of the 216 protocol specification [RFC2131]. 218 The subnet selection option allows for the DHCP client to specify the 219 subnet on which to allocate an address. This would allow a client to 220 perform a more complete address-pool exhaustion attack since the 221 client would no longer be restricted to attacking address-pools on 222 just its local subnet. 224 Servers that implement the subnet selection option MUST by default 225 disable use of the feature; it must specifically be enabled through 226 configuration. Moreover, a server SHOULD provide the ability to 227 selectively enable use of the feature under restricted conditions, 228 e.g., by enabling use of the option only from explicitly configured 229 client-ids, enabling its use only by clients on a particular subnet, 230 or restricting the subnets (as indicated in the subnet selection 231 option) from which addresses may be requested. 233 7. References 235 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 236 Requirement Levels", RFC 2119, BCP 14, March 1997. 238 [RFC2131] Droms, R. "Dynamic Host Configuration Protocol", RFC 2131, 239 March 1997. 241 [RFC2132] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor 242 Extensions", RFC 2132, March 1997. 244 8. Editor's Addresses 246 Glenn Waters 247 Nortel Networks 248 310-875 Carling Avenue, 249 Ottawa, Ontario K1S 5P1 250 Canada 252 Phone: +1 613-795-0249 253 Email: gww@nortelnetworks.com 255 9. Full Copyright Statement 257 Copyright (C) The Internet Society (2000). All Rights Reserved. 259 This document and translations of it may be copied and furnished to 260 others, and derivative works that comment on or otherwise explain it 261 or assist in its implementation may be prepared, copied, published and 262 distributed, in whole or in part, without restriction of any kind, 263 provided that the above copyright notice and this paragraph are 264 included on all such copies and derivative works. However, this 265 document itself may not be modified in any way, such as by removing 266 the copyright notice or references to the Internet Society or other 267 Internet organizations, except as needed for the purpose of developing 268 Internet standards in which case the procedures for copyrights defined 269 in the Internet Standards process must be followed, or as required to 270 translate it into languages other than English. 272 The limited permissions granted above are perpetual and will not be 273 revoked by the Internet Society or its successors or assigns. 275 This document and the information contained herein is provided on an 276 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 277 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT 278 NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN 279 WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 280 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.