DHC Working Grop Michael Patrick Motorola ISG December 11, 1996 DHCP Agent-Supplied Options Status of this Memo This document is an Internet Draft. 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. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress." Please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract Newer high-speed public Internet access technologies call for a high-speed modem to have a LAN attachment to one or more user hosts. It is advantageous to use DHCP to assign user host IP addresses in this environment, but a number of security and scaling problems arise with such "public" DHCP use. This draft calls for the definition of three options which are added by the DHCP relay agent when forwarding public DHCP requests: -- Agent Circuit ID -- Agent Remote ID -- Agent Subnet Mask These options solve the identified problems. Expires June 1997 [Page 1] December 11, 1996 Table of Contents 1 Introduction...........................................2 1.1 High-Speed Circuit Switched Data Networks..............2 1.2 Bridge vs. Router Models...............................3 1.3 DHCP Relay Agent in the Circuit Access Equipment.......4 2.0 Agent-Supplied Options.................................6 2.1 Agent Circuit ID.......................................6 2.2 Agent Remote ID........................................7 2.3 Agent Subnet Mask......................................7 3.0 Issues Resolved........................................8 4.0 References.............................................10 5.0 Author's Address.......................................10 1 Introduction 1.1 High-Speed Circuit Switched Data Networks Public Access to the Internet is usually via a circuit switched data network. Today, this is primarily implemented with dial-up modems connecting to a Remote Access Server. But higher speed circuit access networks also include ISDN, ATM, Frame Relay, and Cable Data Networks. All of these networks can be characterized as a "star" topology where multiple users connect to a central access point. With dial-up modems, only a single host PC attempts to connect to the central point. The PPP protocol is widely used to assign IP addresses to be used by the single host PC. The newer high-speed circuit technologies, however, frequently provide a LAN interface (especially Ethernet) to one or more host PCs. It is desirable to support centralized assignment of the IP addresses of host computers connecting on such circuits via DHCP. The DHCP server can be, but usually is not, co-implemented with the centralized circuit concentration access device. The DHCP server is often connected as a separate server on the "Central LAN" to which the central access device (or devices) attach. A common physical model for high-speed Internet circuit access is shown in Figure 1, below. Expires June 1997 [Page 2] December 11, 1996 +---------------+ Central | DHCP |-- ckt 1--- Modem1-- Host--- Host A LAN | | Relay | Lan +- Host B | | Agent | +- Host C |-----| 1 |-- | | |... +---------+ | +---------------+ | DHCP |--| | Server | | +---------+ | | | +---------------+ +---------+ | | DHCP |-- ckt 1--- Modem2-- Host--- Host D | Other | | | Relay | Lan | Servers |--|-----| Agent | | (Web, | | | 2 |-- ckt 2--- Modem3-- Host--- Host E | DNS) | | | |... Lan | | +---------------+ +---------+ Figure 1: DHCP High Speed Circuit Access Model Note that in this model, the "modem" connects to a LAN at the user site, rather than to a single host. Multiple hosts are implemented at this site. Although it is certainly possible to implement a full IP router at the user site, this requires a relatively expensive piece of equipment (compared to typical modem costs). Furthermore, a router requires an IP address not only for every host, but for the router itself. Finally, a user-side router requires a dedicated Logical IP Subnet (LIS) for each user. While this model is appropriate for relatively small corporate networking environments, it is not appropriate for large, public accessed networks. In this scenario, it is advantageous to implement an IP networking model that does not allocate an IP address for the modem (or other networking equipment device at the user site), and especially not an entire LIS for the user side LAN. 1.2 Bridge vs. Router Models For relay agents implementing a "bridge" model, i.e. where the service offered is to bridge the Host LAN with the Central LAN, the DHCP broadcast from the host is forwarded at the data link layer to the DHCP server. The central access device performs proxy ARP for Expires June 1997 [Page 3] December 11, 1996 hosts, and DHCP proceeds as if the host and the DHCP server are on the same IP subnetwork. It is envisioned, however, that public access devices must support tens of thousands of hosts connecting into the service, e.g. through a cable data service. In this environment, it is unreasonable to require all hosts and servers on the central LAN to maintain ARP tables with tens of thousands of entries. A this scale, then, it is necessary to provides a "Routing" model of public Internet access, where one or more Logical IP Subnets (LISs) are implemented for connecting hosts. The connecting hosts must be assigned, via DHCP, host addresses from a set of LISs. The Routing access model requires at some point between the Host and the Central LAN a Logical IP Subnet (LIS) interface be identified, and that the Host be assigned an IP address on that LIS. The choices in use today by LAN-attached modems include: 1. Modem LIS: A LIS is defined for all hosts connected to the Modem. This describes "dial-up" routers, ISDN routers, IP routing Frame Relay Access Devices, and some cable data modems. 2. Router LIS: The circuit attachment equipment provides only bridging between an intermediate LAN and the Host LAN. A traditional router implements a LIS interface to the intermediate LAN, routing it to the central LAN. This characterizes traditional dial-up Remote Access Servers, and some cable data head-end equipment. 3. Virtual LIS: The circuit attachment equipment provides "virtual" LISs, and permits hosts connecting on its circuits to belong to any of them. This characterizes switching hubs, and some cable data network routers. 1.3 DHCP Relay Agent in the Circuit Access Equipment This document advocates the co-located implementation of a DHCP relay agent with the circuit access equipment. In the Modem LIS and Virtual LIS model, the circuit access equipment acts as both a router to the circuits and as the DHCP relay agent. It is appropriate for any of the circuit access routing models discussed above, although is most appropriate for the Modem LIS and Virtual LIS models. The advantages of co-locating the DHCP relay agent with the circuit access equipment are: Expires June 1997 [Page 4] December 11, 1996 1. DHCP broadcast replies can be routed to only the proper circuit, avoiding, say, the replication of the DCHP reply broadcast onto thousands of access circuits; 2. The same mechanism use to identify the remote connection of the circuit (e.g. a user ID requested by a Remote Access Server acting as teh circuit access equiment) may be used as a host indentifier by DHCP, and used for parameter assignment. This includes centralized static assignment of IP addresses to hosts. This provides a secure remote ID from a trusted source -- the relay agent. A number of issues arise when forwarding DHCP requests from hosts connecting publicly accessed high-speed circuits with LAN connections at the host. Many of these are security issues arising from DHCP client requests from untrusted sources. - How does the relay agent know to which circuit to forward replies? - How does the DHCP server know from which LIS to assign an IP address? - How does the system prevent DHCP IP exhaustion attacks? This is when an attacker requests all available IP addresses from a DHCP server by sending requests with fabricated client MAC addresses. - How can an IP address or LIS be permanently assigned to a particular user or modem? - How does one prevent "spoofing" of client identifer fields used to assign IP addresses? - How does one prevent denial of service by "spoofing" other client's MAC addresses? All of these issues may be addressed by having the circuit access equipment, which is a trusted component, add information to DHCP client requests that it forwards to the DHCP server. 2.0 Agent-Supplied Options This document describes options which are added by the DHCP relay agent. It also specifies handling of those options by the DHCP Expires June 1997 [Page 5] December 11, 1996 server. The option IDs shown have been assigned by IANA, and are in decimal format. 2.1 Agent Circuit ID This option MAY be added by DHCP relay agents which terminate switched or permanent circuits. It encodes a agent-local identifier of the circuit from which a DHCP discover/request packet was received. It is intended for use by agents in relaying DHCP responses back to the proper circuit. Possible uses of this field include - Router interface number - Switching Hub port number - Remote Access Server port number - Frame Relay DLCI - ATM virtual circuit number - Cable Data virtual circuit number The format of the Agent Circuit ID may be further standardized by IETF working groups responsible for IP communication on that type of circuit. In the absence of such standardization, the format may proprietary to the relay agent vendor. DHCP relay agents SHALL NOT modify any existing Agent Circuit ID field which may be in a received DHCP Discover/Request; such an option may have been added by a circuit bridge. DHCP servers supporting this option MUST return the option value unchanged in all Offer and Ack responses. Servers MAY use the information for IP and other parameter assignment policies, but care should be taken due to the potential proprietary format. The DHCP server MAY report the Agent Circuit ID value of current leases in statistical reports (including its MIB) and in logs. Code Len Circuit ID +------+------+------+------+------+------+------+------+-- | 82 | n | c1 | c2 | c3 | c4 | c5 | c6 | ... +------+------+------+------+------+------+------+------+-- 2.2 Agent Remote ID This option MAY be added by DHCP relay agents which terminate switched or permanent circuits and have mechanisms to identify the remote host end of the circuit. The Remote ID field may be used to Expires June 1997 [Page 6] December 11, 1996 encode, for instance: -- a "caller ID" telephone number for dial-up connection -- a "user name" prompted for by a Remote Access Server -- a remote caller ATM address -- a "modem ID" of a cable data modem -- the remote IP address of a point-to-point link -- a remote X.25 address for X.25 connections The format of the Agent Remote ID will depend on the type of circuit connected to the relay agent, and further specification of this field may be standardized by the IETF working groups responsible for IP communications on those circuit types. The only requirement is that the remote ID be administered as globally unique. DHCP servers supporting this option MUST return the option value unchanged in all Offer and Ack responses. DHCP servers MAY use this option to select parameters specific to particular users, hosts, or subscriber modems. The relay agent MAY use this field in addition to or instead of the Agent Circuit ID field to select the circuit on which to forward the DHCP Offer/Ack reply. DHCP relay agents SHALL NOT modify any existing Agent Remote ID field in received broadcasted DHCP Discover/Ack packets; such a field may have been added by a circuit bridge. Code Len Agent Remote ID +------+------+------+------+------+------+------+------+-- | 83 | n | r1 | r2 | r3 | r4 | r5 | r6 | ... +------+------+------+------+------+------+------+------+-- 2.3 Agent Subnet Mask This option MAY be added by DHCP relay agents which terminate multiple Logical IP Subnets. It provides the server with the subnet mask for the LIS on which the relay agent received the request. (The giaddr field provides the agent's IP host address on that LIS.) DHCP servers supporting this option MAY copy the Agent Subnet mask value into the Client Subnet Mask (option 1) parameter returned to the host, and SHOULD have a configurable option to do so. DHCP Servers SHOULD NOT return the Agent Subnet Mask option in the response. This option is intended to avoid the duplicate configuration in both Expires June 1997 [Page 7] December 11, 1996 the relay agent and the server of the agent's circuit subnet masks. A DHCP relay agent terminating a public data switched network may have thousands of such configured circuits and masks. DHCP relay agents SHALL remove any incoming Agent Subnet Mask options on received broadcasted DHCP Discover/Request packets from clients. This option is only appropriately added by the relay agent implementing a LIS interface. Code Len Agent Subnet Mask +------+------+------+------+------+------+ | 84 | 4 | m1 | m2 | m3 | m4 | +------+------+------+------+------+------+ 3.0 Issues Resolved Broadcast Forwarding The circuit access equipment forwards the normally broadcasted DHCP response only on the circuit indicated in the Agent Circuit ID. DHCP LIS selection The DHCP server MAY use the "Gateway IP Address" to select the Logical IP subnet from which to assign IP addresses. It may select either the Subnet Mask provided by the forwarding relay agent, or use local configuration information to select the Subnet Mask based on giaddr (or other info in the DHCP request). Note that the DHCP server SHOULD use the "giaddr" field of the relayed DHCP request for the Router Option reported to the host. DHCP Address Exhaustion In general, the DHCP server may be extended to maintain a database with the "triplet" of (client IP address, client MAC address, client remote ID) Expires June 1997 [Page 8] December 11, 1996 The DHCP server SHOULD implement policies that restrict the number of IP addresses to be assigned to a single remote ID. Static Assignment The DHCP server may use the remote ID to select the IP address to be assigned. It may permit static assignment of IP addresses to particular remote IDs, and disallow an address request from an unauthorized remote ID. IP Spoofing The circuit access device may associate the IP address assigned by a DHCP server in a forwarded DHCP Ack packet with the circuit to which it was forwarded. The circuit access device MAY prevent forwarding of IP packets with source IP addresses -other than- those it has associated with the receiving circuit. This prevents simple IP spoofing attacks on the Central Lan, and IP spoofing of other hosts. Client Identifer Spoofing By using the agent-supplied Agent Remote ID option, the untrusted and as-yet unstandardized client identifer field need not be used by the DHCP server. MAC Address Spoofing By associating a MAC address with an Agent Remote ID, the DHCP server can prevent offering an IP address to an attacker on a different remote ID. 4.0 References [1] Droms, R. "Dynamic Host Configuration Protocol", RFC 1531 [2] Alexander,S. and Droms, R., "DHCP Options and BOOTP Vendor Extension" RFC 1533. 5.0 Author's Address Expires June 1997 [Page 9] December 11, 1996 Michael Patrick Motorola Information Systems Group 20 Cabot Blvd., MS M4-30 Mansfield, MA 02048 Phone: (508) 261-5707 Email: mpatrick@dma.isg.mot.com Expires June 1997 [Page 10]