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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Kim Kinnear 3 INTERNET DRAFT Mark Stapp 4 Richard Johnson 5 Jay Kumarasamy 6 Cisco Systems 8 July 2001 9 Expires January 2002 11 Subnet Selection sub-option 12 for the Relay Agent Information Option 13 15 Status of this Memo 17 This document is an Internet-Draft and is in full conformance with 18 all provisions of Section 10 of RFC2026. 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF), its areas, and its working groups. Note that 22 other groups may also distribute working documents as Internet- 23 Drafts. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 The list of current Internet-Drafts can be accessed at 31 http://www.ietf.org/ietf/1id-abstracts.txt 33 The list of Internet-Draft Shadow Directories can be accessed at 34 http://www.ietf.org/shadow.html. 36 Copyright Notice 38 Copyright (C) The Internet Society (2001). All Rights Reserved. 40 Abstract 42 In RFC2131, the giaddr specifies both the subnet on which a DHCP 43 client resides as well as an IP address which can be used to 44 communicate with the relay agent. The subnet selection option [RFC 45 3011] allows these functions of the giaddr to be split so that when 46 one entity is performing as a DHCP proxy, it can specify the subnet 47 from which to allocate an IP address which is different from the IP 48 address with which it desires to communicate with the DHCP server. 50 Analgous situations exist where the relay agent needs to specify the 51 subnet on which a DHCP client resides which is different from an IP 52 address which can be used to communicate with the relay agent. The 53 subnet-selection sub-option (specified here) of the relay-agent- 54 information option allows a relay agent to do this. 56 1. Introduction 58 In RFC2131, the giaddr specifies both the subnet on which a DHCP 59 client resides as well as an IP address which can be used to communi- 60 cate with the relay agent. The subnet selection option [RFC 3011] 61 allows these functions of the giaddr to be split so that when one 62 entity is performing as a DHCP proxy, it can specify the subnet from 63 which to allocate an IP address which is different from the IP 64 address with which it desires to communicate with the DHCP server. 66 Analgous situations exist where the relay agent needs to specify the 67 subnet on which a DHCP client resides which is different from an IP 68 address which can be used to communicate with the relay agent. Con- 69 sider the following architecture: 71 +--------+ +---------------+ 72 | DHCP | IP x| |IP y 73 | Server |-.......-| Relay Agent |----+------------+ 74 +--------+ | | | | 75 +---------------+ | +------+ 76 | |Modem | 77 | +------+ 78 | | | 79 +-----+ +-----+ +-----+ 80 |Host1| |Host2| |Host3| 81 +-----+ +-----+ +-----+ 83 In the usual approach, the relay agent would put IP address Y into 84 the giaddr of any packets that it forwarded to the DHCP server. How- 85 ever, if for any reason IP address Y is not accessible from the DHCP 86 server, then this usual approach will fail. There are several rea- 87 sons why IP y might be inaccessible from the DHCP server: 89 1. IP y might not be unique for this subnet, but might instead be 90 shared as a gateway address by multiple subnets. 92 2. There might be some firewall capability in the network element 93 in which the relay agent resides that does not allow the DHCP 94 server to access the relay agent via IP y. 96 3. There might not be an IP y. An example would be the case where 97 there was only one host and this was a point to point link. 99 In any of these or other cases, the relay agent needs to be able to 100 communicate to the DHCP server the subnet from which to allocate an 101 IP address. The IP address which will communicate to the DHCP server 102 the subnet information cannot be used as a way to communicate between 103 the DHCP server and the relay agent. 105 Since the relay agent can modify the client's DHCP DHCPREQUEST in 106 only two ways: the giaddr and the relay-agent-info option, there is 107 thus a need to extend the relay-agent-info option with a new sub- 108 option, the subnet-selection sub-option, to allow separation of the 109 specification of the subnet from the IP address to use when communi- 110 cating with the relay agent. 112 2. 114 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 115 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 116 document are to be interpreted as described in RFC 2119 [RFC 2119]. 118 This document uses the following terms: 120 o "access concentrator" 122 An access concentrator is a router or switch at the service 123 provider's edge of a public access network. This document 124 assumes that the access concentrator includes the DHCP relay 125 agent functionality. 127 o "DHCP client" 129 A DHCP client is an Internet host using DHCP to obtain confi- 130 guration parameters such as a network address. 132 o "DHCP relay agent" 134 A DHCP relay agent is a third-party agent that transfers BOOTP 135 and DHCP messages between clients and servers residing on dif- 136 ferent subnets, per [RFC 951] and [RFC 1542]. 138 o "DHCP server" 139 A DHCP server is an Internet host that returns configuration 140 parameters to DHCP clients. 142 o "downstream" 144 Downstream is the direction from the access concentrator towards 145 the subscriber. 147 o "upstream" 149 Upstream is the direction from the subscriber towards the access 150 concentrator. 152 3. Subnet selection sub-option definition 154 The subnet-selection sub-option MAY be used by any DHCP relay agent 155 which desires to specify a subnet for a DHCP client request that it 156 is relaying but needs the subnet specification to be different from 157 the IP address the DHCP server should use when communicating with the 158 relay agent. 160 The sub-option contains a single IP address that is the address of a 161 subnet. The value for the subnet address is determined by taking any 162 IP address on the subnet and ANDing that address with the subnet mask 163 (i.e.: the network and subnet bits are left alone and the remaining 164 (address) bits are set to zero). When the DHCP server is allocating 165 an address and this option is present then the DHCP server MUST allo- 166 cate the address on either: 168 o the subnet specified in the subnet selection option, or; 170 o a subnet on the same network segment as the subnet specified in 171 the subnet selection option. 173 The format of the option is: 175 SubOpt Len subnet IP address 176 +------+------+------+------+------+------+ 177 | TBD | 4 | a1 | a2 | a3 | a4 | 178 +------+------+------+------+------+------+ 180 Servers supporting this sub-option MUST return an identical copy of 181 the sub-option in the relay-agent-info option to any relay-agent that 182 sends it. Relay agents using this sub-option MUST discard DHCPOFFER 183 or DHCPACK packets that do not contain this option in their associ- 184 ated relay-agent-info options. 186 This option does not require changes to operations or features of the 187 DHCP server other than to select the subnet on which to allocate an 188 address. For example, the handling of DHCPDISCOVER for an unknown 189 subnet should continue to operate unchanged. 191 In the event that a DHCP server receives a packet which contains both 192 a subnet selection option [RFC 3011] as well as a subnet selection 193 sub-option, the information contained in the subnet selection sub- 194 option MUST be used to control the allocation of an IP address in 195 preference to the information contained in the subnet selection 196 option. 198 When this option is present and the server supports this option, the 199 server MUST NOT offer an address that is not on the requested subnet 200 or network segment. 202 The IP address to which a DHCP server sends a reply to MUST be the 203 same as it would chose when this option is not present. 205 4. Security 207 DHCP currently provides no authentication or security mechanisms. 208 Potential exposures to attack are discussed is section 7 of the pro- 209 tocol specification [RFC 2131]. The subnet selection option allows a 210 relay agent to specify the subnet on which to allocate an address for 211 a DHCP client. This would allow a malicious DHCP client to perform a 212 more complete address-pool exhaustion attack since the client would 213 no longer be restricted to attacking address-pools on just its local 214 subnet. Under the current DHCP security model there are no methods 215 available to circumvent this type of attack. 217 5. Acknowledgments 219 Eric Rosen contributed to helping the authors to understand the need 220 for this sub-option. 222 6. References 224 [RFC 826] Plummer, D., "Ethernet Address Resolution Protocol: Or con- 225 verting network protocol addresses to 48.bit Ethernet address for 226 transmission on Ethernet hardware", RFC 826, November 1982. 228 [RFC 951] Croft, B., Gilmore, J., "Bootstrap Protocol (BOOTP)", RFC 229 951, September 1985. 231 [RFC 1542] Wimer, W., "Clarifications and Extensions for the 232 Bootstrap Protocol", RFC 1542, October 1993. 234 [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate 235 Requirement Levels", RFC 2119, March 1997. 237 [RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 238 2131, March 1997. 240 [RFC 2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor 241 Extensions", Internet RFC 2132, March 1997. 243 [RFC 3011] Waters, G. "The IPv4 Subnet Selection Option for DHCP", 244 Internet RFC 3011, November 2000. 246 [RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 247 3046, January 2001. 249 7. Author's information 251 Kim Kinnear 252 Mark Stapp 253 Cisco Systems 254 250 Apollo Drive 255 Chelmsford, MA 01824 257 Phone: (978) 244-8000 259 EMail: kkinnear@cisco.com 260 mjs@cisco.com 262 Jay Kumarasamy 263 Richard Johnson 264 Cisco Systems 265 170 W. Tasman Dr. 266 San Jose, CA 95134 268 Phone: (408) 526-4000 270 EMail: jayk@cisco.com 271 raj@cisco.com 273 8. Full Copyright Statement 275 Copyright (C) The Internet Society (2001). All Rights Reserved. 277 This document and translations of it may be copied and furnished to oth- 278 ers, and derivative works that comment on or otherwise explain it or 279 assist in its implementation may be prepared, copied, published and dis- 280 tributed, in whole or in part, without restriction of any kind, provided 281 that the above copyright notice and this paragraph are included on all 282 such copies and derivative works. However, this document itself may not 283 be modified in any way, such as by removing the copyright notice or 284 references to the Internet Society or other Internet organizations, 285 except as needed for the purpose of developing Internet standards in 286 which case the procedures for copyrights defined in the Internet Stan- 287 dards process must be followed, or as required to translate it into 288 languages other than English. 290 The limited permissions granted above are perpetual and will not be 291 revoked by the Internet Society or its successors or assigns. 293 This document and the information contained herein is provided on an "AS 294 IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK 295 FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT 296 LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT 297 INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FIT- 298 NESS FOR A PARTICULAR PURPOSE.