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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 February 2002 9 Expires August 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 (2002). All Rights Reserved. 40 Abstract 42 In RFC 2131, 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 RFC 2131, 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. Terminology 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 "DHCP client" 122 A DHCP client is an Internet host using DHCP to obtain confi- 123 guration parameters such as a network address. 125 o "DHCP relay agent" 127 A DHCP relay agent is a third-party agent that transfers BOOTP 128 and DHCP messages between clients and servers residing on dif- 129 ferent subnets, per [RFC 951] and [RFC 1542]. 131 o "DHCP server" 133 A DHCP server is an Internet host that returns configuration 134 parameters to DHCP clients. 136 3. Subnet selection sub-option definition 138 The subnet-selection sub-option is used by any DHCP relay agent which 139 desires to specify a subnet for a DHCP client request that it is 140 relaying but needs the subnet specification to be different from the 141 IP address the DHCP server should use when communicating with the 142 relay agent. 144 The sub-option contains a single IP address that is the address of a 145 subnet. The value for the subnet address is determined by taking any 146 IP address on the subnet and ANDing that address with the subnet mask 147 (i.e.: the network and subnet bits are left alone and the remaining 148 (address) bits are set to zero). When the DHCP server is allocating 149 an address and this sub-option is present then the DHCP server MUST 150 allocate the address on either: 152 o the subnet specified in the subnet selection sub-option, or; 154 o a subnet on the same network segment as the subnet specified in 155 the subnet selection sub-option. 157 The format of the sub-option is: 159 SubOpt Len subnet IP address 160 +------+------+------+------+------+------+ 161 | TBD | 4 | a1 | a2 | a3 | a4 | 162 +------+------+------+------+------+------+ 164 Because the IP address offered to a client is likely to be different 165 if this sub-option is included in a relay-agent-info option than it 166 would be if this sub-option did not appear or was not interpreted, 167 and because some DHCP servers will not support the relay-agent-info 168 option, then relay agents using this sub-option MUST discard 169 DHCPOFFER or DHCPACK packets that do not contain this sub-option in 170 their associated relay-agent-info options. In the case, there will 171 typically not be any relay-agent-info option in the DHCPOFFER or 172 DHCPACK. 174 This will protect against servers who do not implement any support 175 for the relay-agent-info option [RFC 3046]. 177 However, [RFC 3046] states that every server supporting the relay- 178 agent-info option MUST echo the entire contents of the relay-agent- 179 info option in all replies. Thus, a relay agent doesn't have any 180 effective way to tell whether or not a server has actually used the 181 subnet-selection sub-option to drive its choice of subnets on which 182 the IP address is allocated. 184 Because of this situation, it is important to ensure using adminis- 185 trative techniques that any relay agent employing this sub-option is 186 directed to only send packets to a server which supports this sub- 187 option. 189 Support for this sub-option does not require changes to operations or 190 features of the DHCP server other than to select the subnet on which 191 to allocate an address. For example, the handling of DHCPDISCOVER for 192 an unknown subnet should continue to operate unchanged. 194 In the event that a DHCP server receives a packet which contains both 195 a subnet selection option [RFC 3011] as well as a subnet selection 196 sub-option, the information contained in the subnet selection sub- 197 option MUST be used to control the allocation of an IP address in 198 preference to the information contained in the subnet selection 199 option. When this situation occurs, the subnet selection option MUST 200 NOT be echoed in the reply packet to the client, signalling that the 201 subnet selection option was not used to drive the selection of the 202 subnet for IP address allocation. 204 When this sub-option is present and the server supports this sub- 205 option, the server MUST NOT offer an address that is not on the 206 requested subnet or network segment. 208 The IP address to which a DHCP server sends a reply MUST be the same 209 as it would choose when this sub-option is not present. 211 4. Security Considerations 213 Potential attacks on DHCP are discussed in section 7 of the DHCP pro- 214 tocol specification [RFC 2131], as well as in the DHCP authentication 215 specification [RFC 3118]. 217 The subnet selection sub-option allows a relay agent to specify the 218 subnet on which to allocate an address for a DHCP client. Given that 219 the subnet selection option already exists [RFC 3011], no new secu- 220 rity issues are raised by the existance of the subnet selection sub- 221 option specified in this document beyond those implied by the 222 subnet-selection option [RFC 3011]. 224 The existance of either the subnet selection option or subnet selec- 225 tion sub-option documented here would allow a malicious DHCP client 226 to perform a more complete address-pool exhaustion attack than could 227 be performed without the use of these options, since the client would 228 no longer be restricted to attacking address-pools on just its local 229 subnet. 231 There is some minor protection against this form of attack using this 232 sub-option that is not present for the subnet selection option, in 233 that a trusted relay agent which supports the relay-agent-info option 234 MUST discard a packet it receives with a zero giaddr and a relay- 235 agent-info option when that packet arrives on an "untrusted" circuit 236 [RFC 3046, section 2.1]. 238 5. IANA Considerations 240 IANA has assigned a value of TBD from the DHCP Relay Agent Sub- 241 options space [RFC 3046] for the subnet selection sub-option defined 242 in Section 3. 244 6. Acknowledgments 246 Eric Rosen contributed to helping the authors to understand the need 247 for this sub-option. Much of the text of this document was borrowed 248 with only minimal modifications from the document describing the sub- 249 net selection option [RFC 3011]. 251 7. References 253 [RFC 951] Croft, B., Gilmore, J., "Bootstrap Protocol (BOOTP)", RFC 254 951, September 1985. 256 [RFC 1542] Wimer, W., "Clarifications and Extensions for the 257 Bootstrap Protocol", RFC 1542, October 1993. 259 [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate 260 Requirement Levels", RFC 2119, March 1997. 262 [RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 263 2131, March 1997. 265 [RFC 2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor 266 Extensions", Internet RFC 2132, March 1997. 268 [RFC 3011] Waters, G. "The IPv4 Subnet Selection Option for DHCP", 269 Internet RFC 3011, November 2000. 271 [RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 272 3046, January 2001. 274 8. Author's information 275 Kim Kinnear 276 Mark Stapp 277 Cisco Systems 278 250 Apollo Drive 279 Chelmsford, MA 01824 281 Phone: (978) 244-8000 283 EMail: kkinnear@cisco.com 284 mjs@cisco.com 286 Jay Kumarasamy 287 Richard Johnson 288 Cisco Systems 289 170 W. Tasman Dr. 290 San Jose, CA 95134 292 Phone: (408) 526-4000 294 EMail: jayk@cisco.com 295 raj@cisco.com 297 9. Intellectual Property Statement 299 The IETF takes no position regarding the validity or scope of any intel- 300 lectual property or other rights that might be claimed to pertain to 301 the implementation or use of the technology described in this document 302 or the extent to which any license under such rights might or might not 303 be available; neither does it represent that it has made any effort to 304 identify any such rights. Information on the IETF's procedures with 305 respect to rights in standards-track and standards-related documentation 306 can be found in BCP-11. 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