Internet Engineering Task Force Richard Johnson Internet Draft Kim Kinnear Expiration: August 2003 Mark Stapp File: draft-ietf-dhc-server-override-00.txt Jay Kumarasamy Cisco Systems, Inc. DHCP Server-ID Override Suboption February 5, 2003 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 (2003). All Rights Reserved. Abstract This memo defines a new suboption of the DHCP relay information option [6] which allows the DHCP relay to specify a new value for the Server-ID option, which is inserted by the DHCP Server. In some cases it is convenient for the DHCP relay to act as the actual DHCP server such that DHCP RENEWAL requests will come to the relay instead of going to the server directly. This gives the relay the opportunity to include the Relay Agent option with appropriate Johnson, et. al. [Page 1] Internet Draft DHCP Server-ID Override Suboption February 2003 suboptions even on RENEWAL messages. This new relay agent suboption allows the relay to tell the DHCP server what value to use in the Server-ID option [3]. If this suboption is not present, the server should build the Server-ID option in the normal fashion. 1.0 Introduction There are many situations where the DHCP relay is involved and can insert a relay agent option with appropriate suboptions easily into DHCP DISCOVER messages. Once the lease has been granted, however, future DHCP RENEWAL messages are sent directly to the DHCP Server as specified in the Server-ID option. This means that the relay may not see the DHCP RENEWAL messages (depending upon network topology) and thus can not provide the same relay agent option information in the RENEWAL messages. This new DHCP relay agent suboption, Server-ID override, allows the relay to tell the DHCP server what value to place into the Server-ID option. Using this, the relay agent can force RENEWAL messages to come to it instead of the server. The relay may then insert the relay agent option with appropriate suboptions and relay the request to the actual server. In this fashion the DHCP server will be provided with the same relay agent information upon renewals (such as Circuit-ID, Remote-ID, Device Class, etc.) as was provided in the initial DISCOVER message. In effect, this makes a RENEWAL into a REBINDING. This new suboption could also be used by the DHCP relay in order to allow the relay to appear as the actual DHCP server to the client. This has the advantage that the relay can more easily keep up-to-date information about leases granted, etc. In short, this new suboption allows the DHCPv4 relay to function in the same fashion as the DHCPv6 relay currently does. 1.1 Conventions 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 [1]. 2.0 Server-ID Override Suboption Definition The format of the suboption is: Code Len Overridden Server-ID address Johnson, et. al. [Page 2] Internet Draft DHCP Server-ID Override Suboption February 2003 +-----+-----+-----+-----+-----+-----+ | TBD | n | a1 | a2 | a3 | a4 | +-----+-----+-----+-----+-----+-----+ The option length (n) is 4. The octets "a1" through "a4" specify the value which SHOULD be inserted into the Server-ID option by the DHCP Server upon reply. DHCP Servers SHOULD use this value as the value to insert into the Server-ID option ONLY when the protocol is in the SELECTING and REQUESTING and REBINDING states. If this suboption appears in a DHCP request which is part of a lease RENEWAL, it SHOULD be ignored. 3.0 IANA Considerations None. 4.0 Acknowledgements This document is the result of work done within Cisco Systems. Thanks to Jay Kumarasamy, Kim Kinnear, and Mark Stapp for their work on this suboption definition and the other related work for which this is necessary. 5.0 Security Considerations Message authentication in DHCP for intradomain use where the out-of- band exchange of a shared secret is feasible is defined in RFC 3118 [5]. Potential exposures to attack are discussed in section 7 of the DHCP protocol specification in RFC 2131 [2]. The DHCP Relay Agent option depends on a trusted relationship between the DHCP relay agent and the server, as described in section 5 of RFC 3046. While the introduction of fraudulent relay-agent options can be prevented by a perimeter defense that blocks these options unless the relay agent is trusted, a deeper defense using the authentication option for relay agent options [4] SHOULD be deployed as well. If a rouge DHCP relay were inserted between the client and the server, it could redirect clients to it using this suboption. This would allow such a system to later deny renew requests and thus force clients to discontinue use of their allocated address. This interception, however, would need to be done during the initial DISCOVER and OFFER phase, since the suboption value SHOULD be ignored by the server during RENEWAL state. Either DHCP Authentication [5] or DHCP Relay Agent option authentication [4] would address this case. Johnson, et. al. [Page 3] Internet Draft DHCP Server-ID Override Suboption February 2003 References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, BCP 14, March 1997. [2] Droms, R. "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [3] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997. [4] Stapp, M. "The Authentication Suboption for the DHCP Relay Agent Option", draft-ietf-dhc-auth-suboption-00.txt, June 23, 2002 [5] Droms, R. "Authentication for DHCP Messages", RFC 3118, June 2001 [6] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, January 2001 Author Information: Richard Johnson Jay Kumarasamy Cisco Systems 170 W. Tasman Dr. San Jose, CA 95134 Phone: (408) 526-4000 EMail: jayk@cisco.com raj@cisco.com Kim Kinnear Mark Stapp Cisco Systems 250 Apollo Drive Chelmsford, MA 01824 Phone: (978) 244-8000 EMail: kkinnear@cisco.com mjs@cisco.com Johnson, et. al. [Page 4]