Internet Engineering Task Force (IETF) P. Kurapati Request for Comments: 6148 Juniper Networks Updates: 4388 R. Desetti Category: Standards Track B. Joshi ISSN: 2070-1721 Infosys Technologies Ltd. February 2011 DHCPv4 Lease Query by Relay Agent Remote ID
AbstractSome relay agents extract lease information from the DHCP messages exchanged between the client and DHCP server. This lease information is used by relay agents for various purposes like antispoofing and prevention of flooding. RFC 4388 defines a mechanism for relay agents to retrieve the lease information from the DHCP server when this information is lost. The existing lease query mechanism is data-driven, which means that a relay agent can initiate the lease query only when it starts receiving data to and from the clients. In certain scenarios, this model is not scalable. This document first looks at issues in the existing mechanism and then proposes a new query type, query by Remote ID, to address these issues. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6148.
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1. Introduction ....................................................3 2. Terminology .....................................................4 3. Motivation ......................................................6 4. Protocol Details ................................................7 4.1. Sending the DHCPLEASEQUERY Message .........................7 4.2. Responding to the DHCPLEASEQUERY Message ...................8 4.3. Building a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message .....8 4.4. Determining the IP Address to Be Used in Response ..........9 4.5. Sending a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message ......9 4.6. Receiving a DHCPLEASEACTIVE or DHCPLEASEUNKNOWN Message ....9 4.7. Receiving No Response to the DHCPLEASEQUERY Message .......10 4.8. Lease-Binding Data Storage Requirements ...................10 4.9. Using the DHCPLEASEQUERY Message with Multiple DHCP Servers ..............................................10 5. RFC 4388 Considerations ........................................11 6. Security Considerations ........................................11 7. Acknowledgments ................................................11 8. References .....................................................12 8.1. Normative References ......................................12 8.2. Informative References ....................................12 RFC4388] has defined a mechanism to retrieve this lease information from the DHCP server. The existing query types defined by RFC 4388 [RFC4388] are data- driven. When a client sends data upstream, the relay agent can query the server about the related lease information, based on the source MAC/IP address. These mechanisms do not scale well when there are thousands of clients connected to the relay agent. In the data- driven model, lease query does not provide the full and consolidated active lease information associated with a given connection/circuit, which will result in inefficient anti-spoofing. The relay agent also has to contend with considerable resources for negative caching, especially under spoofing attacks. We need a mechanism for a relay agent to retrieve the consolidated lease information for a given connection/circuit before upstream traffic is sent by the clients.
+--------+ | DHCP | +--------------+ | Server |-...-| DSLAM | | | | Relay Agent | +--------+ +--------------+ | | +------+ +------+ |Modem1| |Modem2| +------+ +------+ | | | +-----+ +-----+ +-----+ |Node1| |Node2| |Node3| +-----+ +-----+ +-----+ Figure 1 For example, when a DSLAM (Digital Subscriber Line Access Multiplexer) acting as a relay agent is rebooted, it should query the server for the lease information for all the connections/circuits. Also, as shown in the above figure, there could be multiple clients on one DSL circuit. The relay agent should get the lease information of all the clients connected to a DSL circuit. This is possible by introducing a new query type based on the Remote ID sub-option of the Relay Agent Information option. This document talks about the motivation for the new query type and the method to perform it. RFC2119]. This document uses the following terms: o Access Concentrator An access concentrator is a router or switch at the broadband access provider's edge of a public broadband access network. This document assumes that the access concentrator includes the DHCP relay agent functionality. o DHCP client A DHCP client is an Internet node using DHCP to obtain configuration parameters such as a network address.
o DHCP relay agent A DHCP relay agent is a third-party agent that transfers Bootstrap Protocol (BOOTP) and DHCP messages between clients and servers residing on different subnets, per RFC 951 [RFC951] and RFC 1542 [RFC1542]. o DHCP server A DHCP server is an Internet node that returns configuration parameters to DHCP clients. o Fast path Fast path refers to data transfer that happens through a network processor or an Application Specific Integrated Circuit (ASIC) programmed to forward the data at very high speeds. o Gleaning Gleaning is the extraction of location information from DHCP messages as the messages are forwarded by the DHCP relay agent function. o Location information Location information is information needed by the access concentrator to forward traffic to a broadband-accessible node. This information includes knowledge of the node's hardware address, the port or virtual circuit that leads to the node, and/or the hardware address of the intervening subscriber modem. o MAC address In the context of a DHCP packet, a MAC address consists of the following fields: hardware type ("htype"), hardware length ("hlen"), and client hardware address ("chaddr"). o Slow path Slow path refers to data transfer that happens through the control plane. This has very limited buffers to store data, and the speeds are very low compared to the fast path data transfer. o Upstream Upstream is the direction from the broadband subscriber towards the access concentrator.
RFC4388] do not provide effective and efficient antispoofing for the above scenario, and a new mechanism is required. The existing query types necessitate a data-driven approach: the lease queries can only be performed when the access concentrator receives data. This results in o increased outage time for clients o excessive negative caching, consuming a lot of resources under a spoofing attack o antispoofing being done in the slow path instead of the fast path
RFC4388], and this section highlights only the differences. Readers are advised to go through RFC 4388 [RFC4388] before going through this section for complete understanding of the protocol. When used in this document, the unqualified term "DHCPLEASEQUERY" indicates a lease query by Remote ID, unless otherwise specified. RFC 3046 [RFC3046] defines two sub-options for the Relay Agent Information option. Sub-option 1 corresponds to the Circuit ID that identifies the local circuit of the access concentrator. This sub-option is unique to the relay agent. Sub-option 2 corresponds to the Remote ID that identifies the remote node connected to the access concentrator. The Remote ID is globally unique in the network and is configured per circuit/connection in the relay agent. This document defines a new query type based on the Remote ID sub-option. Suppose that the access concentrator (e.g., DSLAM) lost the lease information when it was rebooted. When the access concentrator comes up, it initiates (for each connection/circuit) a DHCP lease query by Remote ID as defined in this section. For this query, the requester supplies an option 82 that includes only a Remote ID sub-option in the DHCPLEASEQUERY message. The Remote ID is normally pre-provisioned in the access concentrator per circuit/ connection and hence will remain available to the access concentrator after reboot. The DHCP server MUST reply with a DHCPLEASEACTIVE message if there is an active lease corresponding to the Remote ID that is present in the DHCPLEASEQUERY message. Otherwise, the server MUST reply with a DHCPLEASEUNKNOWN message. Servers that do not implement DHCP lease query based on Remote ID SHOULD simply not respond. RFC2131], and uses message number 10 in the DHCP Message Type option (option 53). The DHCPLEASEQUERY message has the following pertinent message contents:
o There MUST be a Relay Agent Information option (option 82) with only a Remote ID sub-option (sub-option 2) in the DHCPLEASEQUERY message. o The Parameter Request List option [RFC2132] MUST be populated by the access concentrator with the Associated-IP option code. The giaddr field and other option codes listed in the Parameter Request List option are set as explained in Section 6.2 of RFC 4388 [RFC4388]. o The ciaddr field MUST be set to zero. o The values of htype, hlen, and chaddr MUST be set to zero. o The Client Identifier option (option 61) MUST NOT appear in the packet. The DHCPLEASEQUERY message SHOULD be sent to a DHCP server that is known to possess authoritative information concerning the Remote ID. The DHCPLEASEQUERY message MAY be sent to more than one DHCP server, and in the absence of information concerning which DHCP server might possess authoritative information concerning the Remote ID, it SHOULD be sent to all DHCP servers configured for the associated relay agent (if any are known).
In the case where more than one IP address has been involved in a DHCP message exchange with the client specified by the Remote ID, then the list of all those IP addresses MUST be returned in the Associated-IP option, whether or not that option was requested as part of the Parameter Request List option. This is intended for maintaining backwards compatibility with RFC 4388 [RFC4388]. All other options specified in the Parameter Request List [RFC2132] are processed as mentioned in Section 6.4.2 of RFC 4388 [RFC4388]. In a DHCPLEASEUNKNOWN response message, the DHCP server MUST echo the option 82 received in the DHCPLEASEQUERY message. No other option is included in the message.
Agent Information option included in the packet to refresh its location information for this IP address. An access concentrator is likely to query by IP address for all the IP addresses specified in the Associated-IP option in the response, if any, at this point in time. When a DHCPLEASEUNKNOWN message is received by an access concentrator that had sent out a DHCPLEASEQUERY message, it means that the DHCP server does not have definitive information concerning the DHCP client specified in the Remote ID sub-option of the DHCPLEASEQUERY message. The access concentrator MAY store this information for future use. However, another DHCPLEASEQUERY message to the same DHCP server SHOULD NOT be attempted with the same Remote ID sub-option. For lease query by Remote ID, the impact of negative caching is greatly reduced, as the response leads to "definitive" information on all the nodes connected behind the connection. Note that in the case of the data-driven approach [RFC4388], a node spoofing several IP addresses can lead to negative caching of greater magnitude. Another important change that this document brings is the removal of periodic lease queries generated from negative caching caused by DHCPLEASEUNKNOWN messages. Since the information obtained through query by Remote ID is complete, there is no need to attempt lease query again for the same connection. RFC4388]. RFC4388].
RFC 4388-based [RFC4388] implementations, which means that a client that supports this extension can work with a server not supporting this document, provided it uses RFC 4388-defined query types. Also, a server supporting this document can work with a client not supporting this query type. However, there are some changes that this document proposes with respect to RFC 4388 [RFC4388]. Implementers extending RFC 4388 [RFC4388] implementations to support this document should take note of the following points: o There may be cases where a query by IP address/MAC address/Client Identifier has an option 82 containing a Remote ID. In that case, the query will still be recognized as a query by IP address/MAC address/Client Identifier as specified by RFC 4388 [RFC4388]. o Section 6.4 of RFC 4388 [RFC4388] suggests that a DHCPLEASEUNKNOWN message MUST NOT have any other option present. But for a query by Remote ID, option 82 MUST be present in the reply. RFC4388]). This specification introduces one additional issue, beyond those described in RFC 4388 [RFC4388]. A query by Remote ID will result in the server replying with consolidated lease-binding information. Such a query, if done from an unauthorized source, may lead to a leak of lease-binding information. It is critical to deploy authentication techniques mentioned in RFC 3118 [RFC3118] to prevent such unauthorized lease queries. RFC4388]. Kim Kinnear, Damien Neil, Stephen Jacob, Ted Lemon, Ralph Droms, and Alfred Hoenes provided valuable feedback on this document.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4388] Woundy, R. and K. Kinnear, "Dynamic Host Configuration Protocol (DHCP) Leasequery", RFC 4388, February 2006. [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997. [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, January 2001. [RFC3118] Droms, R., Ed. and W. Arbaugh, Ed., "Authentication for DHCP Messages", RFC 3118, June 2001. [RFC951] Croft, B. and J. Gilmore, "Bootstrap Protocol (BOOTP)", RFC 951, September 1985. [RFC1542] Wimer, W., "Clarifications and Extensions for the Bootstrap Protocol", RFC 1542, October 1993.
http://www.juniper.net/ D.T.V Ramakrishna Rao Infosys Technologies Ltd. 44 Electronics City, Hosur Road Bangalore 560 100 India EMail: firstname.lastname@example.org URI: http://www.infosys.com/ Bharat Joshi Infosys Technologies Ltd. 44 Electronics City, Hosur Road Bangalore 560 100 India EMail: email@example.com URI: http://www.infosys.com/