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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 April 2002 9 Expires October 2002 11 Link 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 (2002). All Rights Reserved. 40 Abstract 42 In RFC 2131, the giaddr specifies an IP address which determines both 43 a subnet and thereby a link on which a DHCP client resides as well as 44 an IP address which can be used to communicate with the relay agent. 45 The subnet-selection option [RFC 3011] allows these functions of the 46 giaddr to be split so that when one entity is performing as a DHCP 47 proxy, it can specify the subnet/link from which to allocate an IP 48 address which is different from the IP address with which it desires 49 to communicate with the DHCP server. Analgous situations exist where 50 the relay agent needs to specify the subnet/link on which a DHCP 51 client resides which is different from an IP address which can be 52 used to communicate with the relay agent. The link-selection sub- 53 option (specified here) of the relay-agent-information option allows 54 a relay agent to do this. 56 1. Introduction 58 In RFC 2131, the giaddr specifies and IP address which determines a 59 subnet (and from there a link) on which a DHCP client resides as well 60 as an IP address which can be used to communicate with the relay 61 agent. The subnet-selection option [RFC 3011] allows these functions 62 of the giaddr to be split so that when one entity is performing as a 63 DHCP proxy, it can specify the subnet/link from which to allocate an 64 IP address which is different from the IP address with which it 65 desires to communicate with the DHCP server. 67 Analgous situations exist where the relay agent needs to specify the 68 subnet/link on which a DHCP client resides which is different from an 69 IP address which can be used to communicate with the relay agent. 70 Consider the following architecture: 72 +--------+ +---------------+ 73 | DHCP | IP x| |IP y 74 | Server |-.......-| Relay Agent |----+------------+ 75 +--------+ | | | | 76 +---------------+ | +------+ 77 | |Modem | 78 | +------+ 79 | | | 80 +-----+ +-----+ +-----+ 81 |Host1| |Host2| |Host3| 82 +-----+ +-----+ +-----+ 84 In the usual approach, the relay agent would put IP address Y into 85 the giaddr of any packets that it forwarded to the DHCP server. How- 86 ever, if for any reason IP address Y is not accessible from the DHCP 87 server, then this usual approach will fail. There are several rea- 88 sons why IP y might be inaccessible from the DHCP server: 90 1. IP y might not be unique for this subnet, but might instead be 91 shared as a gateway address by multiple subnets. 93 2. There might be some firewall capability in the network element 94 in which the relay agent resides that does not allow the DHCP 95 server to access the relay agent via IP y. 97 3. There might not be an IP y. An example would be the case where 98 there was only one host and this was a point to point link. 100 In any of these or other cases, the relay agent needs to be able to 101 communicate to the DHCP server the subnet/link from which to allocate 102 an IP address. The IP address which will communicate to the DHCP 103 server the subnet/link information cannot be used as a way to commun- 104 icate between the DHCP server and the relay agent. 106 Since the relay agent can modify the client's DHCP DHCPREQUEST in 107 only two ways: the giaddr and the relay-agent-info option, there is 108 thus a need to extend the relay-agent-info option with a new sub- 109 option, the link-selection sub-option, to allow separation of the 110 specification of the subnet/link from the IP address to use when com- 111 municating with the relay agent. 113 2. Terminology 115 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 116 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 117 document are to be interpreted as described in RFC 2119 [RFC 2119]. 119 This document uses the following terms: 121 o "DHCP client" 123 A DHCP client is an Internet host using DHCP to obtain confi- 124 guration parameters such as a network address. 126 o "DHCP relay agent" 128 A DHCP relay agent is a third-party agent that transfers BOOTP 129 and DHCP messages between clients and servers residing on dif- 130 ferent subnets, per [RFC 951] and [RFC 1542]. 132 o "DHCP server" 134 A DHCP server is an Internet host that returns configuration 135 parameters to DHCP clients. 137 o "link" 139 A link is a collection of subnets that all coexist on the same 140 physical medium. Sometimes called a lan segment or network seg- 141 ment in other contexts. 143 o "subnet" 145 A subnet (for the purposes of this document) consists on a rout- 146 able address range. It may be one of several that exist on a 147 link at the same time. 149 3. Link selection sub-option definition 151 The link-selection sub-option is used by any DHCP relay agent which 152 desires to specify a subnet/link for a DHCP client request that it is 153 relaying but needs the subnet/link specification to be different from 154 the IP address the DHCP server should use when communicating with the 155 relay agent. 157 The sub-option contains a single IP address that is an address con- 158 tained in a subnet. The value for the subnet address is determined by 159 taking any IP address on the subnet and ANDing that address with the 160 subnet mask (i.e.: the network and subnet bits are left alone and the 161 remaining (address) bits are set to zero). This determines a single 162 subnet, and when allocating an IP address, all of the other related 163 subnets on the same link will also be considered in the same way as 164 currently specified for the processing of the giaddr in [RFC 2131]. 166 When the DHCP server is allocating an address and this sub-option is 167 present then the DHCP server MUST allocate the address on either: 169 o the subnet specified in the link-selection sub-option, or; 171 o a subnet on the same link (also known as a network segment) as 172 the subnet specified by the link-selection sub-option. 174 The format of the sub-option is: 176 SubOpt Len subnet IP address 177 +------+------+------+------+------+------+ 178 | TBD | 4 | a1 | a2 | a3 | a4 | 179 +------+------+------+------+------+------+ 181 A relay agent which uses this sub-option MUST assume that the server 182 receiving the sub-option supports the sub-option and used the 183 information available in the sub-option to correctly allocate an IP 184 address. A relay agent which uses this sub-option MUST NOT take dif- 185 ferent actions based on whether this sub-option appears or does not 186 appear in the response packet from the server. 188 It is important to ensure using administrative techniques that any 189 relay agent employing this sub-option is directed to only send pack- 190 ets to a server which supports this sub-option. 192 Support for this sub-option does not require changes to operations or 193 features of the DHCP server other than to select the subnet (and 194 link) on which to allocate an address. For example, the handling of 195 DHCPDISCOVER for an unknown subnet should continue to operate 196 unchanged. 198 In the event that a DHCP server receives a packet which contains both 199 a subnet-selection option [RFC 3011] as well as a link-selection 200 sub-option, the information contained in the link-selection sub- 201 option MUST be used to control the allocation of an IP address in 202 preference to the information contained in the subnet-selection 203 option. 205 When this sub-option is present and the server supports this sub- 206 option, the server MUST NOT offer an address that is not on the 207 requested subnet or the link (network segment) with which that subnet 208 is associated. 210 The IP address to which a DHCP server sends a reply MUST be the same 211 as it would choose when this sub-option is not present. 213 4. Security Considerations 215 Potential attacks on DHCP are discussed in section 7 of the DHCP pro- 216 tocol specification [RFC 2131], as well as in the DHCP authentication 217 specification [RFC 3118]. 219 The link-selection sub-option allows a relay agent to specify the 220 subnet/link on which to allocate an address for a DHCP client. Given 221 that the subnet-selection option already exists [RFC 3011], no funda- 222 mental new security issues are raised by the existance of the link- 223 selection sub-option specified in this document beyond those implied 224 by the subnet-selection option [RFC 3011]. 226 The existance of either the subnet-selection option or link-selection 227 sub-option documented here would allow a malicious DHCP client to 228 perform a more complete address-pool exhaustion attack than could be 229 performed without the use of these options, since the client would no 230 longer be restricted to attacking address-pools on just its local 231 subnet. 233 There is some minor protection against this form of attack using this 234 sub-option that is not present for the subnet-selection option, in 235 that a trusted relay agent which supports the relay-agent-info option 236 MUST discard a packet it receives with a zero giaddr and a relay- 237 agent-info option when that packet arrives on an "untrusted" circuit 238 [RFC 3046, section 2.1]. 240 5. IANA Considerations 242 IANA has assigned a value of TBD from the DHCP Relay Agent Sub- 243 options space [RFC 3046] for the link-selection sub-option defined in 244 Section 3. 246 6. Acknowledgments 248 Eric Rosen contributed to helping the authors to understand the need 249 for this sub-option. Much of the text of this document was borrowed 250 with only minimal modifications from the document describing the 251 subnet-selection option [RFC 3011]. 253 7. References 255 [RFC 951] Croft, B., Gilmore, J., "Bootstrap Protocol (BOOTP)", RFC 256 951, September 1985. 258 [RFC 1542] Wimer, W., "Clarifications and Extensions for the 259 Bootstrap Protocol", RFC 1542, October 1993. 261 [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate 262 Requirement Levels", RFC 2119, March 1997. 264 [RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 265 2131, March 1997. 267 [RFC 2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor 268 Extensions", Internet RFC 2132, March 1997. 270 [RFC 3011] Waters, G. "The IPv4 Subnet Selection Option for DHCP", 271 Internet RFC 3011, November 2000. 273 [RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 274 3046, January 2001. 276 8. Author's information 278 Kim Kinnear 279 Mark Stapp 280 Cisco Systems 281 250 Apollo Drive 282 Chelmsford, MA 01824 284 Phone: (978) 244-8000 286 EMail: kkinnear@cisco.com 287 mjs@cisco.com 289 Jay Kumarasamy 290 Richard Johnson 291 Cisco Systems 292 170 W. Tasman Dr. 293 San Jose, CA 95134 295 Phone: (408) 526-4000 297 EMail: jayk@cisco.com 298 raj@cisco.com 300 9. Intellectual Property Statement 302 The IETF takes no position regarding the validity or scope of any intel- 303 lectual property or other rights that might be claimed to pertain to 304 the implementation or use of the technology described in this document 305 or the extent to which any license under such rights might or might not 306 be available; neither does it represent that it has made any effort to 307 identify any such rights. Information on the IETF's procedures with 308 respect to rights in standards-track and standards-related documentation 309 can be found in BCP-11. 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