DHC Working Group Ted Lemon INTERNET DRAFT Nominum Expires:
JulyAugust 2004 Bill Sommerfeld Internet Draft Sun Microsystems Document: <draft-ietf-dhc-3315id-for-v4-01.txt><draft-ietf-dhc-3315id-for-v4-02.txt> Category: Standards Track January,February, 2004 Node-Specific Client Identifiers for DHCPv4 Status of this Memo This document is a submission by the Dynamic Host Configuration Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the email@example.com mailing list. Distribution of this memo is unlimited. 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. Abstract This document specifies the format that is to be used for encoding DHCPv4 [RFC2131 and RFC2132] client identifiers, so that those identifiers will be interchangeable with identifiers used in the DHCPv6 protocol [RFC3315]. Introduction This document specifies the way in which DHCPv4 clients should identify themselves. DHCPv4 client implementations that conform to this specification use a DHCPv6-style DHCP Unique Identifier (DUID) encapsulated in a DHCPv4 client identifier option. This supersedes the behaviour specified in RFC2131 and RFC2132. The reason for making this change is that as we make the transition from IPv4 to IPv6, there will be network devices that must use both DHCPv4 and DHCPv6. Users of these devices will have a smoother network experience if the devices identify themselves consistently, regardless of the version of DHCP they are using at any given moment. Most obviously, DNS updates made by the DHCP server on behalf of the client will not be handled correctly. This change also addresses certain limitations in the functioning of RFC2131/2132-style DHCP client identifiers. This document first describes the problem to be solved. It then states the new technique that is to be used to solve the problem. Finally, it describes the specific changes that one would have to make to RFC2131 and RFC2132 in order for those documents not to contradict what is described in this document. 1.0 Applicability This document updates RFC2131 and RFC2132. DHCPv4 servers implementations SHOULD conform to this document. DHCPv4 clients on network devices that are expected to support DHCPv6 in the future SHOULD conform to this document. This document makes no changes to the behavior of DHCPv6 clients or servers. DHCPv4 clients and servers that are implemented according to this document should be implemented as if the changes specified in section 4.3 and 4.4 have been made to RFC2131 and RFC2132. 2.0 Problem Statement 2.1. Client identities are ephemeral RFC2132 recommends that client identifiers be generated by using the permanent link-layer address of the network interface that the client is trying to configure. In cases where a network interface is removed from the client computer and replaced with a different network interface with a different permanent link-layer address, the identity of the client changes. The client loses its IP address and any other resources associated with its old identifier - for example, its domain name as published through the DHCP server. 2.2. Clients can accidentally present multiple identities Consider a DHCP client that has two network interfaces, one of which is wired and one of which is wireless. There are three interesting cases here: (a) Each network interface is attached to a different link. (b) Both network interface are connected to the same link. (c) Only one network interface is attached to a link. Case (a) is problematic, and is beyond the scope of this document. EvenBriefly, in case (a), there is no obvious way to choose which of the full implicationstwo network interfaces represents the published identity of cases (b)the client, and (c)since the two network interfaces are beyondconnected to different network links, this could make a significant difference. Also, if, as is likely, the scopetwo devices use two different identifiers, but wish to be identified as the same client in the sense of this document. However, it seems safethe domain name on which their A record is published, they will compete for which interface identity gets the single available published identity, and there is no obvious way to point outwrite a DHCP client that casesproduces the right result. Cases (b) and (c) are very common in practice, because many devices such as laptop computers that are popular at the time of this writing have both wireless and wired network interfaces that are installed simultaneously. Both wired and wireless have advantages - wired has the advantage of speed, and wireless the advantage of mobility. So it seems likely that there will be devices that are in states (b) and (c) frequently, and indeed frequently make transitions between these states. If the DHCP client that configures these devices uses the link-layer address of each device as an identifier, the two devices will appear to the DHCP server to be different nodes, and thus will be assigned different IP addresses, and,addresses. As in state (a), in state (b),(b) only one of the two devices will be reachable throughbe able to acquire the domain name registered bypublic identity of the DHCP server.client, although this is less of a problem in case (b) because both interfaces are at least connected to the same network link. Furthermore, if a device in state (b) makes the transition to state (c), it is quite possible that the lease for the device that has lost connectivity will remain valid for some time, and will betime. If the one that gotpublic identity of the registered domain name. Inclient is associated with this case,now-defunct interface the client will not be reachable through its registeredpublished domain name. 2.3. RFC2131/2132 and RFC3315 identifiers are incompatible The 'client identifier' option is used by DHCP clients and servers to identify clients. In some cases, the value of the 'client identifier' option is used to mediate access to resources (for example, the client's domain name, as published through the DHCP server). RFC2132 and RFC3315 specify different methods for deriving client identifiers. These methods guarantee that the DHCPv4 and DHCPv6 identifier will be different. This means that mediation of access to resources using these identifiers will not work correctly in cases where a node may be configured using DHCPv4 in some cases and DHCPv6 in other cases. 2.4. RFC2131 does not require the use of a client identifier RFC2131 allows the DHCP server to identify clients either by using the client identifier option sent by the client, or, if the client did not send one, the client's link-layer address. Like the client identifier format recommended by RFC2131, this suffers from the problems previously described in (2) and (3). 3. Solutions The solution to problem (2.1)2.1 is to use a DHCP client identifier that is persistent - not tied to a particular piece of removable network hardware. Then, when network hardware is swapped in and out, the client identifier does not change, and thus the client has a consistent IP address and consistent use of whatever resources have been associated with its identifier. It is worth noting thatThis creates a new problem in case (2.1), it is harmless for2.2, however: if the devicetwo network interfaces are connected to the same link and use the same client identifier onidentifier, then the server's IP address assignment algorithm will assign the same IP address to both interfaces. But if the DHCP client state machines configuring the two interfaces - inare sufficiently out of sync, the DHCPDISCOVER from the slower interface may be sent after the DHCPACK for the faster interface. In this case, the DHCP server or servers serving these two linkswill seedetect a conflict and abandon the two network interfaces as distinctIP address, because they are connectedthe faster interface is responding to different links, even though theyICMP echo requests. So we can't just use the same identifier.identifier on every interface. The solution to problem (2.2) is the same. Although it2.3 is beyond the scope of this documentto say how ause the DHCP client supporting two network interfacesUnique Identifier as defined in this way would provide a smooth user experience, it does seem safe to say that it will need to use a persistent DHCP client identifier that is the same across the interfaces that it configures. In case (2.2), if both interfaces are connected to the same link, the DHCP server will see requests sent by the client on each interface as being from the same client, and will only allocate one lease to that client. A DHCP client that sends the same client identifier on two interfaces will need to be prepared for the possibility that both interfaces will be assigned the same IP address. It could do this either by shutting down one interface in this case, or it could use some more complicated strategy. It is beyond the scope of this document to describe the details of how this should be done. Obviously, to get the benefit of this strategy, transitions from one device to the other must go unnoticed by the user. The solution to problem (2.3) is to use the DHCP Unique Identifier as defined in RFC3315 asRFC3315 as a client identifier. The DUID provides several different ways of producing persistent DHCP client identifiers, at least one of which is likely to be appropriate for any particular sort of network device. So it turns out that thisThis is also solves problems (1) and (2).a valid way of addressing problem 2.1. To finish addressing problem 2.2, we modify the solution slightly. In addition to the DUID, RFC3315 defines an Identity Association ID (IAID). The IAID, in combination with the DUID, identifies a particular identity with which to associate an IP address. So a DHCP client has a single DUID, but has one IAID for each interface. The DHCP server associates IP addresses with the combination of (DUID, IAID), but uses the DUID to identify the client as a whole. The solution to problem (2.4) is to deprecate the use of the contents of the chaddr field in the DHCP packet as a means of identifying the client. 4. Implementation Requirements Here we specify changes to the behavior of DHCP clients and servers. We also specify changes to the wording in RFC2131 and RFC2132. DHCP clients, servers and relay agents that conform to this specification must implement RFC2131 and RFC2132 with the wording changes specified in sections 4.3 and 4.4. 4.1. DHCP Client behavior DHCP clients conforming to this specification MUST use stable DHCP node identifiers in the dhcp-client-identifier option. DHCP clients MUST NOT use client identifiers based solely on layer two addresses that are hard-wired to the layer two device (e.g., the ethernet MAC address) as suggested in RFC2131, except as allowed in section 9.2 of RFC3315. DHCP clients MUST send a 'client identifier' option containing an Identity Association Unique Identifier, as defined in section 10 of RFC3315, and a DHCP Unique Identifier, as defined in section 9 of RFC3315. These together constitute an RFC3315-style binding identifier. The general format of the DHCPv4 'client identifier' option is defined in section 9.14 of RFC2132. To send a To send a DUIDan RFC3315-style binding identifiier in a DHCPv4 'client identifier' option, the type of the 'client identifier' option should beis set to 255. The type field is immediately followed by the DUID.IAID, which is an opaque 32-bit quantity. The IAID is immediately followed by the DUID, which consumes the remaining contents of the 'client identifier' option. The format of the 'client identifier' option is as follows: Code Len Type DHCP Unique Identifier +-----+-----+-----+-----+-----+-----+-----+---IAID DUID +----+----+-----+----+----+----+----+----+----+--- | 61 | n | 255 | d1i1 | d2i2 | d3i3 | d4i4 | ... +-----+-----+-----+-----+-----+-----+-----+---d1 | d2 |... +----+----+-----+----+----+----+----+----+----+--- Any DHCPv4 or DHCPv6 client that conforms to this specification SHOULD provide a means by which an operator can learn what DUID the client has chosen. Such clients SHOULD also provide a means by which the operator can configure the DUID. A device that is normally configured with both a DHCPv4 and DHCPv6 client SHOULD automatically use the same DUID for DHCPv4 and DHCPv6 without any operator intervention. DHCP clients that support more than one network interface SHOULD use the same client identifierDUID on eachevery interface. SuchDHCP clients SHOULD be prepared for the possibilitythat the DHCP server will allocate the same IP address to both interfaces.support more than one network interface SHOULD use a different IAID on each interface. 4.2. DHCPv4 Server behavior This document does not require any change to DHCPv4 or DHCPv6 servers that follow RFC2131 and RFC2132. However, some DHCPv4 servers can be configured not to conform to RFC2131 and RFC2131, in the sense that they ignore the 'client identifier' option and use the client's hardware address instead. Some DHCP servers do not take into account the possibility that the same client identifier may be used on separate links, and thus will behave incorrectly when a DHCP client acquires leases on two different IP addresses on two different links at the same time.DHCP servers that conform to this specification MUST use the 'client identifier' option to identify the client if the client sends it. DHCP servers MUST assume that when a lease on an IP address is bound to a particular DHCP client identifier, all other still-valid leases on IP addresses bound to that client identifier are still in use. DHCP serversMAY use administrator-supplied values for chaddr and htype to identify the client in the case where the administrator is assigning a fixed IP address to the client, even if the client sends an client identifier option. This is ONLY permitted in the case where the DHCP server administrator has provided the values for chaddr and htype, because in this case if it causes a problem, the administrator can correct the problem by removing the offending configuration information. 4.3. Changes from RFC2131 In section 2 of RFC2131, on page 9, the text that reads "; for example, the 'client identifier' may contain a hardware address, identical to the contents of the 'chaddr' field, or it may contain another type of identifier, such as a DNS name" is deleted. In section 4.2 of RFC2131, the text "The client MAY choose to explicitly provide the identifier through the 'client identifier' option. If the client supplies a 'client identifier', the client MUST use the same 'client identifier' in all subsequent messages, and the server MUST use that identifier to identify the client. If the client does not provide a 'client identifier' option, the server MUST use the contents of the 'chaddr' field to identify the client." is replaced by the text "The client MUST explicitly provide a client identifier through the 'client identifier' option." In the same section, the text "Use of 'chaddr' as the client's unique identifier may cause unexpected results, as that identifier may be associated with a hardware interface that could be moved to a new client. Some sites may choose to use a manufacturer's serial number as the 'client identifier', to avoid unexpected changes in a clients network address due to transfer of hardware interfaces among computers. Sites may also choose to use a DNS name as the 'client identifier', causing address leases to be associated with the DNS name rather than a specific hardware box." is replaced by the text "The DHCP client MUST NOT rely on the 'chaddr' field to identify it." In section 4.4.1 of RFC2131, the text "The client MAY include a different unique identifier" is replaced with "The client MUST include a unique identifier". In sections 3.1, item 4 and 6, 3.2 item 3 and 4, and 4.3.1, where RFC2131 says that 'chaddr' may be used instead of the 'client identifier' option, the text that says "or 'chaddr'" and "'chaddr' or" is deleted. Note that these changes do not relieve the DHCP server of the obligation to use 'chaddr' as an identifier if the client does not send a 'client identifier' option. Rather, they oblige clients that conform with this document to send a 'client identifier' option, and not rely on 'chaddr' for identification. DHCP servers MUST use 'chaddr' as an identifier in cases where 'client identifier' is not sent, in order to support old clients that do not conform with this document. 4.4. Changes from RFC2132 The text in section 9.14, beginning with "The client identifier MAY consist of" through "that meet this requirement for uniqueness." is replaced with "the client identifier consists of a type field whose value is normally 255, followed by a two-byte type field, followed by the contents of the identifier. The two-byte type field and the format of the contents of the identifier are defined in RF3315, section 9." The text "its minimum length is 2" in the following paragraph is deleted. 5. Security Considerations This document raises no new security issues. Potential exposure to attack in the DHCPv4 protocol are discussed in section 7 of the DHCP protocol specification [RFC2131] and in Authentication for DHCP messages [RFC3118]. Potential exposure to attack in the DHCPv6 protocol is discussed in section 23 of RFC3315. 6. IANA Considerations This document defines no new name spaces that need to be administered by the IANA. This document deprecates all 'client identifier' type codes other than 255, and thus there is no need for the IANA to track possible values for the type field of the 'client identifier' option. 7. Normative References [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [RFC2132] S. Alexander, R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC2132, March, 1997 [RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., Carney, M., "Dynamic Host Configuration Protocol for IPv6 (DHCPV6)", July, 2003 8. Informative References [RFC3118] Droms, R., Arbaugh, W., "Authentication for DHCP Messages", RFC3118, June, 2001 Author's Addresses Ted Lemon Nominum 2385 Bay Road Redwood City, CA 94063 USA +1 650 381 6000 firstname.lastname@example.org Bill Sommerfeld Sun Microsystems 1 Network Drive Burlington, MA 01824 +1 781 442 3458 email@example.com Full Copyright Statement "Copyright (C) 2003, 2004 The Internet Society. All Rights Reserved. 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