Network Working Group Kim Kinnear INTERNET DRAFT Mark Stapp Richard Johnson Jay Kumarasamy Cisco Systems
November 2001February 2002 Expires MayAugust 2002 Subnet Selection sub-option for the Relay Agent Information Option <draft-ietf-dhc-agent-subnet-selection-01.txt><draft-ietf-dhc-agent-subnet-selection-02.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).(2002). All Rights Reserved. Abstract In RFC2131,RFC 2131, 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,RFC 2131, 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 "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. 3. Subnet selection sub-option definition The subnet-selection sub-option MAY beis 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 optionsub-option is present then the DHCP server MUST allo- cateallocate the address on either: o the subnet specified in the subnet selection option,sub-option, or; o a subnet on the same network segment as the subnet specified in the subnet selection option.sub-option. The format of the optionsub-option is: SubOpt Len subnet IP address +------+------+------+------+------+------+ | TBD | 4 | a1 | a2 | a3 | a4 | +------+------+------+------+------+------+ Servers supporting this sub-option MUST return an identical copy ofBecause the IP address offered to a client is likely to be different if this sub-option is included in thea relay-agent-info option to any relay-agent that sends it. Relaythan 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, then relay agents using this sub-option MUST discard DHCPOFFER or DHCPACK packets that do not contain this optionsub-option in their associ- atedassociated 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 for the relay-agent-info option [RFC 3046]. However, [RFC 3046] states that every server supporting the relay- agent-info option MUST echo the entire contents of the relay-agent- info option in all replies. Thus, a relay agent doesn't have any effective way to tell whether or not a server has actually used the subnet-selection sub-option to drive its choice of subnets on which the IP address is allocated. Because of this situation, it is important to ensure using adminis- trative techniques that any relay agent employing this sub-option is directed to only send packets to a server which supports this 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 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 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 this sub-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 IP address to which a DHCP server sends a reply MUST be the same as it would chosechoose when this optionsub-option is not present. 4. Security DHCP currently provides no authentication or security mechanisms.Considerations Potential exposures to attackattacks on DHCP are discussed isin section 7 of the DHCP pro- tocol specification [RFC 2131].2131], as well as in the DHCP authentication specification [RFC 3118]. The subnet selection sub-option allows a relay agent to specify the subnet on which to allocate an address for a DHCP client. Given that the subnet selection option already exists [RFC 3011], no new securitysecu- rity issues are raised by the existance of the subnet selection sub-optionsub- option specified in this docu- ment.document beyond those implied by the subnet-selection option [RFC 3011]. The existance of either the subnet selection option or subnet selectionselec- tion 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 circumventThere is some minor protection against this typeform of attack.attack using this sub-option that is not present for the subnet 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 thea value of TBD from the DHCP Relay Agent Sub- options space [RFC 3046] for the relay-agent-info optionsubnet selection sub-option code for this 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 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. 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