Network Working Group Kim Kinnear INTERNET DRAFT Mark Stapp Richard Johnson Jay Kumarasamy Cisco Systems
FebruaryApril 2002 Expires AugustOctober 2002 SubnetLink Selection sub-option for the Relay Agent Information Option <draft-ietf-dhc-agent-subnet-selection-02.txt><draft-ietf-dhc-agent-subnet-selection-03.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 (2002). All Rights Reserved. Abstract In RFC 2131, the giaddr specifies an IP address which determines both thea subnet and thereby a link on which a DHCP client resides as well as an IP address which can be used to communicate with the relay agent. The subnet selectionsubnet-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 subnetsubnet/link 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 subnetsubnet/link 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-optionlink-selection sub- option (specified here) of the relay-agent- informationrelay-agent-information option allows a relay agent to do this. 1. Introduction In RFC 2131, the giaddr specifies both theand IP address which determines a subnet (and from there a link) on which a DHCP client resides as well as an IP address which can be used to communi- catecommunicate with the relay agent. The subnet selectionsubnet-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 subnetsubnet/link 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 subnetsubnet/link on which a DHCP client resides which is different from an IP address which can be used to communicate with the relay agent. Con- siderConsider 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 subnetsubnet/link from which to allocate an IP address. The IP address which will communicate to the DHCP server the subnetsubnet/link information cannot be used as a way to communicatecommun- icate 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-selectionlink-selection sub-option, to allow separation of the specification of the subnetsubnet/link from the IP address to use when communi- catingcom- municating 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 "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 "link" A link is a collection of subnets that all coexist on the same physical medium. Sometimes called a lan segment or network seg- ment in other contexts. o "subnet" A subnet (for the purposes of this document) consists on a rout- able address range. It may be one of several that exist on a link at the same time. 3. SubnetLink selection sub-option definition The subnet-selectionlink-selection sub-option is used by any DHCP relay agent which desires to specify a subnetsubnet/link for a DHCP client request that it is relaying but needs the subnetsubnet/link 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 thean address ofcon- tained in 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). This determines a single subnet, and when allocating an IP address, all of the other related subnets on the same link will also be considered in the same way as currently specified for the processing of the giaddr in [RFC 2131]. When the DHCP server is allocating an address and this sub-option is present then the DHCP server MUST allocate the address on either: o the subnet specified in the subnet selectionlink-selection sub-option, or; o a subnet on the same link (also known as a network segmentsegment) as the subnet specified inby the subnet selectionlink-selection sub-option. The format of the sub-option is: SubOpt Len subnet IP address +------+------+------+------+------+------+ | TBD | 4 | a1 | a2 | a3 | a4 | +------+------+------+------+------+------+ Because the IP address offered to a client is likely to be different if this sub-option is included in a relay-agent-info option than it would be if this sub-option did not appear or was not interpreted, and because some DHCP servers will not support the relay-agent-info option, thenA relay agents usingagent which uses this sub-option MUST discard DHCPOFFER or DHCPACK packetsassume that do not contain this sub-option in their associated relay-agent-info options. In the case, there will typically not be any relay-agent-info option in the DHCPOFFER or DHCPACK. This will protect against servers who do not implement any support forthe relay-agent-info option [RFC 3046]. However, [RFC 3046] states that everyserver supportingreceiving the relay- agent-info option MUST echosub-option supports the entire contents ofsub-option and used the relay-agent- info optioninformation available in all replies. Thus, athe sub-option to correctly allocate an IP address. A relay agent doesn't have any effective way to tellwhich uses this sub-option MUST NOT take dif- ferent actions based on whether this sub-option appears or does not a server has actually usedappear in the subnet-selection sub-option to drive its choice of subnets on whichresponse packet from the IP address is allocated. Because of this situation, itserver. It is important to ensure using adminis- trativeadministrative techniques that any relay agent employing this sub-option is directed to only send packetspack- ets to a server which supports this sub- option.sub-option. Support for this sub-option does not require changes to operations or features of the DHCP server other than to select the subnet (and link) 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 selectionsubnet-selection option [RFC 3011] as well as a subnet selectionlink-selection sub-option, the information contained in the subnet selectionlink-selection sub- option MUST be used to control the allocation of an IP address in preference to the information contained in the subnet selectionsubnet-selection option. When this situation occurs, the subnet selection option MUST NOT be echoed in the reply packet to the client, signalling that the subnet selection option was not used to drive the selection of the subnet for IP address allocation. When thissub-option is present and the server supports this sub- option, the server MUST NOT offer an address that is not on the requested subnet or network segment.the link (network segment) with which that subnet is associated. The IP address to which a DHCP server sends a reply MUST be the same as it would choose when this sub-option is not present. 4. Security Considerations Potential attacks on DHCP are discussed in section 7 of the DHCP pro- tocol specification [RFC 2131], as well as in the DHCP authentication specification [RFC 3118]. The subnet selectionlink-selection sub-option allows a relay agent to specify the subnetsubnet/link on which to allocate an address for a DHCP client. Given that the subnet selectionsubnet-selection option already exists [RFC 3011], no funda- mental new secu- ritysecurity issues are raised by the existance of the subnetlink- selection sub- optionsub-option specified in this document beyond those implied by the subnet-selection option [RFC 3011]. The existance of either the subnet selectionsubnet-selection option or subnet selec- tionlink-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. There is some minor protection against this form of attack using this sub-option that is not present for the subnet selectionsubnet-selection option, in that a trusted relay agent which supports the relay-agent-info option MUST discard a packet it receives with a zero giaddr and a relay- agent-info option when that packet arrives on an "untrusted" circuit [RFC 3046, section 2.1]. 5. IANA Considerations IANA has assigned a value of TBD from the DHCP Relay Agent Sub- options space [RFC 3046] for the subnet selectionlink-selection sub-option defined in Section 3. 6. Acknowledgments Eric Rosen contributed to helping the authors to understand the need for this sub-option. Much of the text of this document was borrowed with only minimal modifications from the document describing the sub- net selectionsubnet-selection option [RFC 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. 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 9. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intel- lectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this stan- dard. Please address the information to the IETF Executive Director. 10. Full Copyright Statement Copyright (C) The Internet Society (2002). 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, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Stan- dards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FIT- NESS FOR A PARTICULAR PURPOSE.