Network Working Group Kim Kinnear INTERNET DRAFT Mark Stapp Richard Johnson Jay Kumarasamy Cisco Systems
JulyNovember 2001 Expires JanuaryMay 2002 Subnet Selection sub-option for the Relay Agent Information Option <draft-ietf-dhc-agent-subnet-selection-00.txt><draft-ietf-dhc-agent-subnet-selection-01.txt> Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract In RFC2131, the giaddr specifies both the subnet on which a DHCP client resides as well as an IP address which can be used to communicate with the relay agent. The subnet selection option [RFC 3011] allows these functions of the giaddr to be split so that when one entity is performing as a DHCP proxy, it can specify the subnet from which to allocate an IP address which is different from the IP address with which it desires to communicate with the DHCP server. Analgous situations exist where the relay agent needs to specify the subnet on which a DHCP client resides which is different from an IP address which can be used to communicate with the relay agent. The subnet-selection sub-option (specified here) of the relay-agent- information option allows a relay agent to do this. 1. Introduction In RFC2131, the giaddr specifies both the subnet on which a DHCP client resides as well as an IP address which can be used to communi- cate with the relay agent. The subnet selection option [RFC 3011] allows these functions of the giaddr to be split so that when one entity is performing as a DHCP proxy, it can specify the subnet from which to allocate an IP address which is different from the IP address with which it desires to communicate with the DHCP server. Analgous situations exist where the relay agent needs to specify the subnet on which a DHCP client resides which is different from an IP address which can be used to communicate with the relay agent. Con- sider the following architecture: +--------+ +---------------+ | DHCP | IP x| |IP y | Server |-.......-| Relay Agent |----+------------+ +--------+ | | | | +---------------+ | +------+ | |Modem | | +------+ | | | +-----+ +-----+ +-----+ |Host1| |Host2| |Host3| +-----+ +-----+ +-----+ In the usual approach, the relay agent would put IP address Y into the giaddr of any packets that it forwarded to the DHCP server. How- ever, if for any reason IP address Y is not accessible from the DHCP server, then this usual approach will fail. There are several rea- sons why IP y might be inaccessible from the DHCP server: 1. IP y might not be unique for this subnet, but might instead be shared as a gateway address by multiple subnets. 2. There might be some firewall capability in the network element in which the relay agent resides that does not allow the DHCP server to access the relay agent via IP y. 3. There might not be an IP y. An example would be the case where there was only one host and this was a point to point link. In any of these or other cases, the relay agent needs to be able to communicate to the DHCP server the subnet from which to allocate an IP address. The IP address which will communicate to the DHCP server the subnet information cannot be used as a way to communicate between the DHCP server and the relay agent. Since the relay agent can modify the client's DHCP DHCPREQUEST in only two ways: the giaddr and the relay-agent-info option, there is thus a need to extend the relay-agent-info option with a new sub- option, the subnet-selection sub-option, to allow separation of the specification of the subnet from the IP address to use when communi- cating with the relay agent. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC 2119]. This document uses the following terms: o "access concentrator" An access concentrator is a router or switch at the service provider's edge of a public access network. This document assumes that the access concentrator includes the DHCP relay agent functionality. o"DHCP client" A DHCP client is an Internet host using DHCP to obtain confi- guration parameters such as a network address. o "DHCP relay agent" A DHCP relay agent is a third-party agent that transfers BOOTP and DHCP messages between clients and servers residing on dif- ferent subnets, per [RFC 951] and [RFC 1542]. o "DHCP server" A DHCP server is an Internet host that returns configuration parameters to DHCP clients. o "downstream" Downstream is the direction from the access concentrator towards the subscriber. o "upstream" Upstream is the direction from the subscriber towards the access concentrator.3. Subnet selection sub-option definition The subnet-selection sub-option MAY be used by any DHCP relay agent which desires to specify a subnet for a DHCP client request that it is relaying but needs the subnet specification to be different from the IP address the DHCP server should use when communicating with the relay agent. The sub-option contains a single IP address that is the address of a subnet. The value for the subnet address is determined by taking any IP address on the subnet and ANDing that address with the subnet mask (i.e.: the network and subnet bits are left alone and the remaining (address) bits are set to zero). When the DHCP server is allocating an address and this option is present then the DHCP server MUST allo- cate the address on either: o the subnet specified in the subnet selection option, or; o a subnet on the same network segment as the subnet specified in the subnet selection option. The format of the option is: SubOpt Len subnet IP address +------+------+------+------+------+------+ | TBD | 4 | a1 | a2 | a3 | a4 | +------+------+------+------+------+------+ Servers supporting this sub-option MUST return an identical copy of the sub-option in the relay-agent-info option to any relay-agent that sends it. Relay agents using this sub-option MUST discard DHCPOFFER or DHCPACK packets that do not contain this option in their associ- ated relay-agent-info options. This option does not require changes to operations or features of the DHCP server other than to select the subnet on which to allocate an address. For example, the handling of DHCPDISCOVER for an unknown subnet should continue to operate unchanged. In the event that a DHCP server receives a packet which contains both a subnet selection option [RFC 3011] as well as a subnet selection sub-option, the information contained in the subnet selection sub- option MUST be used to control the allocation of an IP address in preference to the information contained in the subnet selection option. When this option is present and the server supports this option, the server MUST NOT offer an address that is not on the requested subnet or network segment. The IP address to which a DHCP server sends a reply toMUST be the same as it would chose when this option is not present. 4. Security DHCP currently provides no authentication or security mechanisms. Potential exposures to attack are discussed is section 7 of the pro- tocol specification [RFC 2131]. The subnet selection optionsub-option allows a relay agent to specify the subnet on which to allocate an address for a DHCP client. ThisGiven that the subnet selection option already exists [RFC 3011], no new security issues are raised by the existance of the subnet selection sub-option specified in this docu- ment. The existance of either the subnet selection option or subnet selection sub-option documented here would allow a malicious DHCP client to perform a more complete address-pool exhaustion attack than could be performed without the use of these options, since the client would no longer be restricted to attacking address-pools on just its local subnet. Under the current DHCP security model there are no methods available to circumvent this type of attack. 5. IANA Considerations IANA has assigned the value of TBD for the relay-agent-info option sub-option code for this sub-option. 6. Acknowledgments Eric Rosen contributed to helping the authors to understand the need for this sub-option. 6. ReferencesMuch of the text of this document was borrowed with only minimal modifications from the document describing the sub- net selection option [RFC 826] Plummer, D., "Ethernet Address Resolution Protocol: Or con- verting network protocol addresses to 48.bit Ethernet address for transmission on Ethernet hardware", RFC 826, November 1982.3011]. 7. References [RFC 951] Croft, B., Gilmore, J., "Bootstrap Protocol (BOOTP)", RFC 951, September 1985. [RFC 1542] Wimer, W., "Clarifications and Extensions for the Bootstrap Protocol", RFC 1542, October 1993. [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [RFC 2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [RFC 2132] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor Extensions", Internet RFC 2132, March 1997. [RFC 3011] Waters, G. "The IPv4 Subnet Selection Option for DHCP", Internet RFC 3011, November 2000. [RFC 3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, January 2001. 7.8. Author's information Kim Kinnear Mark Stapp Cisco Systems 250 Apollo Drive Chelmsford, MA 01824 Phone: (978) 244-8000 EMail: email@example.com firstname.lastname@example.org Jay Kumarasamy Richard Johnson Cisco Systems 170 W. Tasman Dr. San Jose, CA 95134 Phone: (408) 526-4000 EMail: email@example.com firstname.lastname@example.org 8.9. Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. This document and translations of it may be copied and furnished to oth- ers, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and dis- tributed,distributed, in whole or in part, without restriction of any kind, providedpro- vided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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