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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group Kim Kinnear 2 INTERNET DRAFT Mark Stapp 3 Richard Johnson 4 Jay Kumarasamy 5 Cisco Systems 7 November 2001 8 Expires May 2002 10 Subnet Selection sub-option 11 for the Relay Agent Information Option 12 14 Status of this Memo 16 This document is an Internet-Draft and is in full conformance with 17 all provisions of Section 10 of RFC2026. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 35 Copyright Notice 37 Copyright (C) The Internet Society (2001). All Rights Reserved. 39 Abstract 41 In RFC2131, the giaddr specifies both the subnet on which a DHCP 42 client resides as well as an IP address which can be used to 43 communicate with the relay agent. The subnet selection option [RFC 44 3011] allows these functions of the giaddr to be split so that when 45 one entity is performing as a DHCP proxy, it can specify the subnet 46 from which to allocate an IP address which is different from the IP 47 address with which it desires to communicate with the DHCP server. 49 Analgous situations exist where the relay agent needs to specify the 50 subnet on which a DHCP client resides which is different from an IP 51 address which can be used to communicate with the relay agent. The 52 subnet-selection sub-option (specified here) of the relay-agent- 53 information option allows a relay agent to do this. 55 1. Introduction 57 In RFC2131, the giaddr specifies both the subnet on which a DHCP 58 client resides as well as an IP address which can be used to communi- 59 cate with the relay agent. The subnet selection option [RFC 3011] 60 allows these functions of the giaddr to be split so that when one 61 entity is performing as a DHCP proxy, it can specify the subnet from 62 which to allocate an IP address which is different from the IP 63 address with which it desires to communicate with the DHCP server. 65 Analgous situations exist where the relay agent needs to specify the 66 subnet on which a DHCP client resides which is different from an IP 67 address which can be used to communicate with the relay agent. Con- 68 sider the following architecture: 70 +--------+ +---------------+ 71 | DHCP | IP x| |IP y 72 | Server |-.......-| Relay Agent |----+------------+ 73 +--------+ | | | | 74 +---------------+ | +------+ 75 | |Modem | 76 | +------+ 77 | | | 78 +-----+ +-----+ +-----+ 79 |Host1| |Host2| |Host3| 80 +-----+ +-----+ +-----+ 82 In the usual approach, the relay agent would put IP address Y into 83 the giaddr of any packets that it forwarded to the DHCP server. How- 84 ever, if for any reason IP address Y is not accessible from the DHCP 85 server, then this usual approach will fail. There are several rea- 86 sons why IP y might be inaccessible from the DHCP server: 88 1. IP y might not be unique for this subnet, but might instead be 89 shared as a gateway address by multiple subnets. 91 2. There might be some firewall capability in the network element 92 in which the relay agent resides that does not allow the DHCP 93 server to access the relay agent via IP y. 95 3. There might not be an IP y. An example would be the case where 96 there was only one host and this was a point to point link. 98 In any of these or other cases, the relay agent needs to be able to 99 communicate to the DHCP server the subnet from which to allocate an 100 IP address. The IP address which will communicate to the DHCP server 101 the subnet information cannot be used as a way to communicate between 102 the DHCP server and the relay agent. 104 Since the relay agent can modify the client's DHCP DHCPREQUEST in 105 only two ways: the giaddr and the relay-agent-info option, there is 106 thus a need to extend the relay-agent-info option with a new sub- 107 option, the subnet-selection sub-option, to allow separation of the 108 specification of the subnet from the IP address to use when communi- 109 cating with the relay agent. 111 2. Terminology 113 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 114 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 115 document are to be interpreted as described in RFC 2119 [RFC 2119]. 117 This document uses the following terms: 119 o "DHCP client" 121 A DHCP client is an Internet host using DHCP to obtain confi- 122 guration parameters such as a network address. 124 o "DHCP relay agent" 126 A DHCP relay agent is a third-party agent that transfers BOOTP 127 and DHCP messages between clients and servers residing on dif- 128 ferent subnets, per [RFC 951] and [RFC 1542]. 130 o "DHCP server" 132 A DHCP server is an Internet host that returns configuration 133 parameters to DHCP clients. 135 3. Subnet selection sub-option definition 137 The subnet-selection sub-option MAY be used by any DHCP relay agent 138 which desires to specify a subnet for a DHCP client request that it 139 is relaying but needs the subnet specification to be different from 140 the IP address the DHCP server should use when communicating with the 141 relay agent. 143 The sub-option contains a single IP address that is the address of a 144 subnet. The value for the subnet address is determined by taking any 145 IP address on the subnet and ANDing that address with the subnet mask 146 (i.e.: the network and subnet bits are left alone and the remaining 147 (address) bits are set to zero). When the DHCP server is allocating 148 an address and this option is present then the DHCP server MUST allo- 149 cate the address on either: 151 o the subnet specified in the subnet selection option, or; 153 o a subnet on the same network segment as the subnet specified in 154 the subnet selection option. 156 The format of the option is: 158 SubOpt Len subnet IP address 159 +------+------+------+------+------+------+ 160 | TBD | 4 | a1 | a2 | a3 | a4 | 161 +------+------+------+------+------+------+ 163 Servers supporting this sub-option MUST return an identical copy of 164 the sub-option in the relay-agent-info option to any relay-agent that 165 sends it. Relay agents using this sub-option MUST discard DHCPOFFER 166 or DHCPACK packets that do not contain this option in their associ- 167 ated relay-agent-info options. 169 This option does not require changes to operations or features of the 170 DHCP server other than to select the subnet on which to allocate an 171 address. For example, the handling of DHCPDISCOVER for an unknown 172 subnet should continue to operate unchanged. 174 In the event that a DHCP server receives a packet which contains both 175 a subnet selection option [RFC 3011] as well as a subnet selection 176 sub-option, the information contained in the subnet selection sub- 177 option MUST be used to control the allocation of an IP address in 178 preference to the information contained in the subnet selection 179 option. 181 When this option is present and the server supports this option, the 182 server MUST NOT offer an address that is not on the requested subnet 183 or network segment. 185 The IP address to which a DHCP server sends a reply MUST be the same 186 as it would chose when this option is not present. 188 4. Security 190 DHCP currently provides no authentication or security mechanisms. 191 Potential exposures to attack are discussed is section 7 of the pro- 192 tocol specification [RFC 2131]. The subnet selection sub-option 193 allows a relay agent to specify the subnet on which to allocate an 194 address for a DHCP client. Given that the subnet selection option 195 already exists [RFC 3011], no new security issues are raised by the 196 existance of the subnet selection sub-option specified in this docu- 197 ment. The existance of either the subnet selection option or subnet 198 selection sub-option documented here would allow a malicious DHCP 199 client to perform a more complete address-pool exhaustion attack than 200 could be performed without the use of these options, since the client 201 would no longer be restricted to attacking address-pools on just its 202 local subnet. Under the current DHCP security model there are no 203 methods available to circumvent this type of attack. 205 5. IANA Considerations 207 IANA has assigned the value of TBD for the relay-agent-info option 208 sub-option code for this sub-option. 210 6. Acknowledgments 212 Eric Rosen contributed to helping the authors to understand the need 213 for this sub-option. Much of the text of this document was borrowed 214 with only minimal modifications from the document describing the sub- 215 net selection option [RFC 3011]. 217 7. References 219 [RFC 951] Croft, B., Gilmore, J., "Bootstrap Protocol (BOOTP)", RFC 220 951, September 1985. 222 [RFC 1542] Wimer, W., "Clarifications and Extensions for the 223 Bootstrap Protocol", RFC 1542, October 1993. 225 [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate 226 Requirement Levels", RFC 2119, March 1997. 228 [RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 229 2131, March 1997. 231 [RFC 2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor 232 Extensions", Internet RFC 2132, March 1997. 234 [RFC 3011] Waters, G. "The IPv4 Subnet Selection Option for DHCP", 235 Internet RFC 3011, November 2000. 237 [RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 238 3046, January 2001. 240 8. Author's information 242 Kim Kinnear 243 Mark Stapp 244 Cisco Systems 245 250 Apollo Drive 246 Chelmsford, MA 01824 248 Phone: (978) 244-8000 250 EMail: kkinnear@cisco.com 251 mjs@cisco.com 253 Jay Kumarasamy 254 Richard Johnson 255 Cisco Systems 256 170 W. Tasman Dr. 257 San Jose, CA 95134 259 Phone: (408) 526-4000 261 EMail: jayk@cisco.com 262 raj@cisco.com 264 9. Full Copyright Statement 266 Copyright (C) The Internet Society (2001). All Rights Reserved. 268 This document and translations of it may be copied and furnished to oth- 269 ers, and derivative works that comment on or otherwise explain it or 270 assist in its implementation may be prepared, copied, published and 271 distributed, in whole or in part, without restriction of any kind, pro- 272 vided that the above copyright notice and this paragraph are included on 273 all such copies and derivative works. However, this document itself may 274 not be modified in any way, such as by removing the copyright notice or 275 references to the Internet Society or other Internet organizations, 276 except as needed for the purpose of developing Internet standards in 277 which case the procedures for copyrights defined in the Internet Stan- 278 dards process must be followed, or as required to translate it into 279 languages other than 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 "AS 285 IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK 286 FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT 287 LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT 288 INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FIT- 289 NESS FOR A PARTICULAR PURPOSE.