Dynamic Host Configuration WG R. Pruss Internet-Draft Cisco Systems Intended status: Experimental February 26, 2007 Expires: August 30, 2007 Authentication Extensions for the Dynamic Host Configuration Protocol draft-pruss-dhcp-auth-dsl-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. This Internet-Draft will expire on August 30, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract This document defines Dynamic Host Configuration Protocol (DHCP) extensions that provide for a Challenge Handshake Authentication Protocol (CHAP) exchange in DHCP prior to configuration of the host. The primary applicability is within a Digital Subscriber Line (DSL) Broadband network environment in order to enable a smooth migration from the Point to Point Protocol (PPP). Pruss Expires August 30, 2007 [Page 1] Internet-Draft DHCP Authentication February 2007 Requirements Language 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 [RFC2119]. Table of Contents 1. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Network Architecture and Terminology . . . . . . . . . . . . . 5 3.1. Where the DHCP Client and DHCP Server reside . . . . . . . 6 4. Applicability Statement . . . . . . . . . . . . . . . . . . . 6 5. Protocol Operation Alternatives . . . . . . . . . . . . . . . 6 5.1. Protocol Operation with Existing Messages . . . . . . . . 7 5.2. Protocol Operation with New Messages . . . . . . . . . . . 8 6. DHCP Options . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.1. DHCP Authentication Protocol Option . . . . . . . . . . . 11 6.2. DHCP Authentication Data Option . . . . . . . . . . . . . 12 6.2.1. Challenge and Response . . . . . . . . . . . . . . . . 13 6.2.2. Success and Failure . . . . . . . . . . . . . . . . . 14 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 9. Messages for operation choice with new messages . . . . . . . 15 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 11.1. Normative References . . . . . . . . . . . . . . . . . . . 17 11.2. Informative References . . . . . . . . . . . . . . . . . . 18 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . . . . 20 Pruss Expires August 30, 2007 [Page 2] Internet-Draft DHCP Authentication February 2007 1. Problem Statement Digital Subscriber Line (DSL) broadband service providers are witnessing a shift in the "last-mile" aggregation technologies and protocols which have traditionally been relied upon. Two primary transitions are from ATM to Ethernet in the access network, and from the Point to Point Protocol (PPP) [RFC1661] for multi-protocol framing and dynamic endpoint configuration to direct encapsulation of IP and DHCP for dynamic endpoint configuration for some devices. The term used by the DSL Forum for the network state associated with an authorized subscriber (that is using DHCP and IP rather than PPP) is "IP session" [WT-146]. While these trends can be readily witnessed, neither are occuring overnight. In addition, they are not necessarily implimented in lock-step. Thus, one may find ATM-based and Ethernet-based access networks running a combination of PPP sessions and IP sessions at any given time, particularly during transition periods. This coexitances will even occur for the same service subscriber. Removing PPP, PPPoA [RFC2364], and PPPoE [RFC2516] from the subscriber access network is relatively straightforward in that most of the properties that DSL providers are interested in going forward are already present in DHCP and IP sessions. Luckily, there are some capabilties of PPP which the markert does not continue to demand. For example, the Dynamic configuration in PPP for IPX or NETBEUI, for example, is no longer of concern. Neither are the multi-link bonding capabilities of PPP [RFC1990] commonly used on separate ISDN B-channels, ar the myriad of other features that PPP developed as the "dial-based" access protocol of choice for framing, authentication, and dynamic configuration for IP and other network layer protocols. Missing from IP sessions and DHCP [RFC2131], however, are isomorphic methods for user authentication and session liveness probing (sometimes referred to as a session "keepalive"). For the latter, existence of a client using a given IP address can be detected by a number of means, including ARP [RFC1433], ICMP Ping [RFC0792], or BFD [I-D.ietf-bfd-base]. This leaves authentication as an open issue needing resolution. Specifically, authentication based on a username and secret password must be covered. This is something that in PPP always occurs before dynamic configuration of an IP address and associated parameters. While most DSL deployments utilize a username and password to authenticate a subscriber and authorize access today, this is not the only method for authentication that has been adopted when moving to DHCP and IP sessions. "Option 82" [RFC3046] is commonly used with DHCP as a credential to authenticate a given subscriber line and authorize service. In this model, the DSL Access Node, which always sits between the DHCP Client and Server, snoops DHCP messages as they Pruss Expires August 30, 2007 [Page 3] Internet-Draft DHCP Authentication February 2007 pass, and inserts pre-configured information for a given line (e.g., an ATM VPI/VCI, Ethernet VLAN, or other tag). That information, while provided in clear text, traverses what is considered a physically secured portion of the access network and is used to determine (typically via a request to an AAA server) whether the DHCP exchange can continue. This fits quite well with current DSL network architecture, as long as the subscriber line itself is all that needs be authorized. However, in some service models it is still necessary for the subscriber to provide credentials directly. From the perspective the Network Access Server (NAS) where the DHCP Server resides, the extensions defined in this document are analogous to the commonly available "Option 82" method. The primary difference between using Option 82 line configuration and a username and password is that the authentication credentials are provided by the subscriber rather than inserted by intervening network equipment. Providing credentials from the subscriber rather than intervening network equipment is particularly important for cases where subscriber line information is unavailable, untrusted, or due to the terms of the service may change at any time. Further, different devices in the home may have different policies and require different credentials. Migration scenarios where PPPoE and DHCP operate on the same network for a period of time lend well to models which utilize identical authentication and authorization credentials across the different data plane encapsulations. 2. Introduction This document defines DHCP Options and procedures that allow for a CHAP-based authentication exchange to occur in DHCP in order to enable smooth migration from PPP sessions to IP sessions in a DSL Broadband network environment. Primary goals are integration of authentication in such a way that it will operate seamlessly with existing RADIUS-based AAA infrastructure and ATM or Ethernet based DSL Networks. As such, only the termination points of PPP in the DSL network are affected, both of which are devices that would logically need to be updated in any transition from PPP to IP sessions. It should be noted that [RFC3118] defines a mechanism that provides authentication of individual DHCP messages. While this mechanism does provide a method of authentication for a DHCP Client based on a shared secret, it does not do so in a manner that can be seamlessly integrated with existing RADIUS-based AAA infrastructure built around PPP CHAP authentication models. Pruss Expires August 30, 2007 [Page 4] Internet-Draft DHCP Authentication February 2007 3. Network Architecture and Terminology The DSL Forum defines its ATM-based network architecture in [TR-059] and Ethernet-based network architecture in [TR-101]. The extensions for DHCP defined in this document are designed to work identically on Ethernet or ATM architectures. The diagram in Figure 1 and following terminology will be used throughout: +------------+ | AAA/RADIUS | | Server | +------------+ | | Sub. +-----+ +--------+ +-----+ +----------+ Home ---| HGW |---| | | | | | Network +-----+ | Access | | | | | | Node |--/Aggregation\--| NAS |---| Internet | Sub. +-----+ | |--\ Network /--| | | | Home ---| HGW |---| | | | | | Network +-----+ +--------+ +-----+ +----------+ | | |----------DSL Access Network --------| Figure 1: DSL Network Architecture o Access Node (AN): Network device, usually located at a service provider central office or street cabinet, that terminates Access Loop connections from Subscribers. In case the Access Loop is a Digital Subscriber Line (DSL), this is often referred to as a DSL Access Multiplexer (DSLAM). The AN may support one or more Access Loop technologies and allow them to inter-work with a common aggregation network technology. o Network Access Server (NAS): Network device that aggregates multiplexed Subscriber traffic from a number of Access Nodes. The NAS plays a central role in per-subscriber policy enforcement and QoS. Often referred to as a Broadband Network Gateway (BNG) or Broadband Remote Access Server (BRAS). A detailed definition of the NAS is given in [RFC2881]. o The Home Gateway (HGW) connects the different Customer Premises Equipment (CPE) to the Access Node and the access network. In case of DSL, the HGW is a DSL Network Termination (NT) that could either operate as a layer 2 bridge or as a layer 3 router. In the latter case, such a device is also referred to as a Routing Gateway (RG). Pruss Expires August 30, 2007 [Page 5] Internet-Draft DHCP Authentication February 2007 3.1. Where the DHCP Client and DHCP Server reside Referring to the DSL network architecture depicted in Figure 1, PPP (via PPPoA [RFC2364] or PPPoE [RFC2516]) operates over the DSL Access Network between the NAS and a device behind the HGW, or between the NAS and the HGW itself. The DHCP Client resides either on a home network device or the HGW, and the DHCP Server protocol state machine resides fully on the NAS. The NAS obtains per-subscriber client configuration information either locally or from the AAA infrastructure (which itself may consult external DHCP servers if necessary) after authentication is successfully completed. The details of how the NAS obtains dynamic configuration to be offered to the DHCP Client are outside the scope of this specification. 4. Applicability Statement The primary target for this extension is for DSL service provider networks where PPP is being phased out to be replaced by native IP and DHCP, or where new devices are being added which will not utilize PPP. Very specific assumptions have been made with respect to the security model, operational methods, and integration requirements for existing AAA mechanisms during the design. It is understood that this mechanism may not be generally applicable in this form for all network environments where DHCP is deployed, though perhaps elements of it may be used to develop a more generic approach while still meeting the specific requirements set out by the DSL network architecture. For example, it is conceivable that EAP [RFC3748] could be carried in order to provide a more extensible and secure set of authentication methods. 5. Protocol Operation Alternatives In this first draft two alternatives for the protocol operation are offered for consideration. The first in the section Protocol Operation with Existing Messages (Section 5.1), uses the existing message set, with small liberties taken on behavior. It has the downside that reverse authentication and other authentication protcols like EAP are not possible in the constraints. The second in the section Protocol Operation with New Messages (Section 5.2), extends the existing message set and should be read coupled with the section on Messages for operation choice with new messages (Section 9) This alternative has the downside of requiring four new messages. Both of these alternatives use common DHCP options. Pruss Expires August 30, 2007 [Page 6] Internet-Draft DHCP Authentication February 2007 5.1. Protocol Operation with Existing Messages In order to integrate with RADIUS CHAP, it is necessary to send a CHAP Challenge to the DHCP Client, hash that Challenge along with an Identifier (ID), Name, and Secret, and return the result in a CHAP Response to the NAS. The NAS, in turn, must send the CHAP Name, ID, Challenge and Response to the RADIUS server to verify the credentials. The Secret is never sent in the clear, and is known only to the AAA server and DHCP Client. The Client's configuration information (IP address, etc) is typically returned in a successful response from the AAA server, which is used to construct the DHCP OFFER. A typical message flow proceeds as shown in Figure 2: Pruss Expires August 30, 2007 [Page 7] Internet-Draft DHCP Authentication February 2007 (HGW) (NAS) (AAA) DHCP Client DHCP Server/ RADIUS Server AAA Client DHCPDISCOVER -------> (w/DHCP-auth-proto chap) <------- DHCPOFFER (w/CHAP Challenge, Name) DHCPDISCOVER -------> (w/CHAP Name, Response, ID) RADIUS Access-Request -------> (w/CHAP Name, ID, Challenge, Response) <-------- RADIUS Access-Accept (Access-Reject if unsuccessful) (DHCP continues normally from this point forward if successful) <------- DHCPOFFER (w/yiaddr) (w/CHAP Failure if unsuccessful) DHCPREQUEST -------> <------- DHCPACK Figure 2: Typical Message Flow It is necessary to "restart" the state machine via the DHCPOFFER with CHAP Failure option if the Access-Reject is received. A CHAP Challenge could also be added to this DHCP offer. Then a DHCPDISCOVER is always answered with a DHCPOFFER with a CHAP Challenge until Access-Accept or the client stops trying. 5.2. Protocol Operation with New Messages Four new DHCP messages are defined: o DHCPAUTH-Challenge Pruss Expires August 30, 2007 [Page 8] Internet-Draft DHCP Authentication February 2007 o DHCPAUTH-Response o DHCPAUTH-Success o DHCPAUTH-Failure to support the new authentication phase which occurs before a DHCPOFFER is sent by the Server. These messages may also be used to integrate other authentication methods, including "in the clear"; password mechanisms (for example, to support One-Time-Password mechanisms), or to carry other types of authentication, including EAP which, unlike DSL Broadband, is widely used in other environments, including 802.11 "Wi-Fi" access networks. In order to facilitate simple deployment on millions of operational Clients, this protocol extension is designed, from the perspective of the client, to be a simple, event driven, "request received, response sent" type exchange with no persistent state necessary. Put simply, any DHCPAUTH-Challenge that is received at any time (as long as conditions are met to thwart malicious attackers, see Security Consideration (Section 7) section for more information) by the DHCP Client Software is responded to with a DHCPAUTH-Response. Any DHCPAUTH-Failure that is received is responded to by relaying to the user (via a GUI or other human-computer interaction) that a failure occurred (e.g., an incorrect password was entered). Reinitializing or halting of the DHCP state machine should follow a DHCPAUTH-Result indicating a failure. A DHCPAUTH-Success message is optional, but SHOULD be sent. The message following this exchange is a DHCP Offer, sent unchanged by the Server. A typical message flow proceeds as shown in Figure 3: Pruss Expires August 30, 2007 [Page 9] Internet-Draft DHCP Authentication February 2007 (HGW) (NAS) (AAA) DHCP Client DHCP Server/ RADIUS Server AAA Client DHCPDISCOVER -------> (w/DHCP-auth-proto chap) <------- DHCPAUTH-Challenge (w/CHAP Challenge, Name) DHCPAUTH-Response -------> (w/CHAP Name, Response, ID) RADIUS Access-Request -------> (w/CHAP Name, ID, Challenge, Response) <-------- RADIUS Access-Accept (Access-Reject if unsuccessful) <------- DHCPAUTH-Success/Failure (DHCP continues normally from this point forward if successful) <------- DHCPOFFER (w/yiaddr) DHCPREQUEST -------> <------- DHCPACK Figure 3: Typical Message Flow with new messages The Client always responds to DHCPAUTH-Challenge messages sent by the BRAS with a DHCPAUTH-Response message. If no DHCPAUTH-Success or DHCPAUTH-Failure message is received after a short period of time the DHCPAUTH-Response is retransmitted (3 seconds is a recommended initial default, exponentially increasing for each retransmit to a maximum of 12 seconds, and timing out after 8 retries, though all values SHOULD be configurable). Each DHCPAUTH-Response messages received by the Server MUST be responded to. The same retransmit procedure is used by the Server for the DHCPAUTH-Challenge message. This message exchange in the figure offers simply one-way user authentication, e.g. the Server verifies the identity of the User at Pruss Expires August 30, 2007 [Page 10] Internet-Draft DHCP Authentication February 2007 the PC Client. It is assumed that the Ethernet access network restricts packets sent to/from the BRAS at the MAC level, providing some trust that DHCPAUTH messages received at the Client are not from a rogue BRAS. In situations where this is not the case, the same set of messages may be sent in reverse for mutual authentication. 6. DHCP Options Two new DHCP Options are defined in this section. The first is used to advertise the support for DHCP Authentication in general and the type of authentication supported. The second is the format used for carrying necessary elements for the CHAP authentication protocol. This is modeled strictly on PPP CHAP [RFC1994] with text lifted liberally from its specification. 6.1. DHCP Authentication Protocol Option The following diagram defines the format of the DHCPAUTH-Protocol option, which is sent from the DHCP Client to the DHCP Server to indicate the authentication algorithm the client prefers. This option MUST be sent in the DHCPDISCOVER if the Client supports DHCP Authentication. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DHCP Code | Length | Authentication-Protocol | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Algorithm | +-+-+-+-+-+-+-+-+ Figure 4: DHCP Authentication Protocol Option DHCP Code: TBA-1 (DHCPAUTH-Protocol) Length: 3 Authentication-Protocol c223 (hex) for Challenge-Handshake Authentication Protocol. Algorithm The Algorithm field is one octet and indicates the authentication method to be used. 5 CHAP with MD5 Pruss Expires August 30, 2007 [Page 11] Internet-Draft DHCP Authentication February 2007 6 CHAP with SHA-1 6.2. DHCP Authentication Data Option +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DHCP Code | Length | CHAP Code | Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data .... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: DHCP Authentication Data Option DHCP Code: TBA-2 (DHCPAUTH-Data) CHAP Code The Code field is one octet and identifies the type of CHAP option and format of data that follow. CHAP Codes are assigned as follows: 1 Challenge 2 Response 3 Success 4 Failure Identifier The Identifier field is one octet and aids in matching challenges, responses and replies. Length The Length field is one octet and indicates the length of the Data plus 2 (the length of CHAP Code plus Identifier). Data The Data field is zero or more octets. The format of the Data field is determined by the CHAP Code field. Pruss Expires August 30, 2007 [Page 12] Internet-Draft DHCP Authentication February 2007 6.2.1. Challenge and Response A summary of the Challenge and Response packet format is shown below. The DHCP Code and Length are described above and repeated here only for reference. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DHCP Code | Length | CHAP Code | Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value-Size | Value ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Name ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: Challenge and Response CHAP Code 1 for Challenge; 2 for Response. Identifier The Identifier field is one octet. The Identifier field MUST be changed each time a Challenge is sent. The Response Identifier MUST be copied from the Identifier field of the Challenge which caused the Response. Value-Size This field is one octet and indicates the length of the Value field. Value The Value field is one or more octets. The Challenge Value is a variable stream of octets. The importance of the uniqueness of the Challenge Value and its relationship to the secret is described above. The Challenge Value MUST be changed each time a Challenge is sent. The length of the Challenge Value depends upon the method used to generate the octets, and is independent of the hash algorithm used. Pruss Expires August 30, 2007 [Page 13] Internet-Draft DHCP Authentication February 2007 The Response Value is the one-way hash calculated over a stream of octets consisting of the Identifier, followed by (concatenated with) the "secret", followed by (concatenated with) the Challenge Value. The length of the Response Value depends upon the hash algorithm used. Name The Name field is one or more octets representing the identification of the system transmitting the packet. There are no limitations on the content of this field. For example, it MAY contain ASCII character strings or globally unique identifiers in ASN.1 syntax. The Name should not be NUL or CR/LF terminated. The size is determined from the Length and Value-Size fields. 6.2.2. Success and Failure If Authentication is successful, the DHCP Server SHOULD transmit the subsequent DHCPOFFER including a DHCPAUTH-Data Option with the CHAP Code set to 3 (Success). If Authentication is unsuccessful, the DHCP Server SHOULD transmit a DHCPOFFER including a DHCPAUTH-Data Option with the CHAP Code set to 4 (Failure). A summary of the Success and Failure packet format is shown below. The DHCP Code and Length are described above and repeated here only for reference. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DHCP Code | Length | CHAP Code | Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Message.... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: Success and Failure Code 3 for Success; 4 for Failure. Identifier Pruss Expires August 30, 2007 [Page 14] Internet-Draft DHCP Authentication February 2007 The Identifier field is one octet and aids in matching requests and replies. The Identifier field MUST be copied from the Identifier field of the Response which caused this reply. 7. Security Considerations The DHCP authentication mechanism described is optimal in catering to a uni-lateral authentication of the client to the NAS, with the authentication based on a one-way hash algorithm used in CHAP. In general, it claims to be no better than the existing security offered by PPPoE [RFC2516]. Given the assumption that the DSL Aggregation Network is trusted, and that direct Layer 2 subscriber-subscriber communication restricted to only occur via the NAS, this mechanism makes no attempt at protecting from man in the middle attacks. However, unlike with PPP, IP DHCP packets including DHCPAUTH messages may be freely routed across multiple IP hops. It is thus conceivable that a remote rogue user may try to exploit known hash weaknesses to derive the secret key of another user through the use of a brute force attack. To keep the relative security equivalent in threat to today's PPPoE environment, the DHCP Auth traffic must be constrained by the access network. The simplest mechanism for doing this is by blocking any DHCP traffic from traversing the NAS. The vast majority of PPPoE and PPPoA wire-line deployments utilize one-way authentication. The mechanism defined in this document is strictly one-way. Mutual authentication is possible, but brings complexity and may ultimately require a new DHCP message type and phase. 8. IANA Considerations This specification requires two values to be assigned by IANA. Both are "BOOTP Vendor Extensions and DHCP Options" TBA-1: (DHCPAUTH-Protocol) TBA-2: (DHCPAUTH-Data) 9. Messages for operation choice with new messages The DHCPAUTH messages follow the format for DHCP messages defined in RFC 2131 [RFC2131]. These new messages are identified by the presence of a DHCP Message Type option, which encodes DHCPAUTH Pruss Expires August 30, 2007 [Page 15] Internet-Draft DHCP Authentication February 2007 message type. DHCPAUTH message use the xid field to match DHCPAUTH- Challenge, DHCPAUTH-Response and DHCPAUTH-Success/DHCPAUTH-Failure messages. Other fields in the DHCP message header, such as siaddr and fname, are left unused. The data in a DHCPAUTH message, such as the challenge or response, is carried in a DHCPAUTH-Data option. The format for this option is: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Data... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: DHCPAUTH Messages Type DHCPAUTH-Data Length number of octets of data carried in the option Data The Data field is zero or more octets carrying the data specific to the m essage The following diagrams defines the data carried in a DHCPAUTH- Challenge or DHCPAUTH-Response message: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value-Size | Value ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Name ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: DHCPAUTH Messages Data Value-Size This field is one octet and indicates the length of the Value field. Value The Value field is one or more octets. The most significant octet is transmitted first. The Challenge Value is a variable stream of octets. The importance of the uniqueness of the Challenge Value and its relationship to the secret is described above. The Challenge Value MUST be changed each Pruss Expires August 30, 2007 [Page 16] Internet-Draft DHCP Authentication February 2007 time a Challenge is sent. The length of the Challenge Value depends upon the method used to generate the octets, and is independent of the hash algorithm used. The Response Value is the one-way hash calculated over a stream of octets consisting of the Identifier, followed by (concatenated with) the "secret", followed by (concatenated with) the Challenge Value. The length of the Response Value depends upon the hash algorithm used (16 octets for MD5). CHAP Name The Name field is one or more octets representing the identification of the system transmitting the packet. There are no limitations on the content of this field. For example, it MAY contain ASCII character strings or globally unique identifiers in ASN.1 syntax. The Name should not be NUL or CR/LF terminated. The size is determined from the Length field. If the existing DHCP client software may be modified, the CHAP Name may also be sent before the authentication phase, either as an additional DHCP Option in the DHCPDISCOVER message, or in the Client ID field that is sent in all DHCP messages. This is an option that may be available even if the DHCP Client software may not be modified directly, as an API to set this value should be readily available even for existing DHCP Client software. 10. Acknowledgements Many thanks to Carlos Pignataro for help editing this document. Thanks Eric Voit, Mark Townsley and Ralph Droms for help with this document. 11. References 11.1. Normative References [RFC1994] Simpson, W., "PPP Challenge Handshake Authentication Protocol (CHAP)", RFC 1994, August 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Pruss Expires August 30, 2007 [Page 17] Internet-Draft DHCP Authentication February 2007 11.2. Informative References [I-D.ietf-bfd-base] Katz, D. and D. Ward, "Bidirectional Forwarding Detection", draft-ietf-bfd-base-05 (work in progress), June 2006. [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [RFC1433] Garrett, J., Hagan, J., and J. Wong, "Directed ARP", RFC 1433, March 1993. [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC1990] Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, August 1996. [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [RFC2364] Gross, G., Kaycee, M., Lin, A., Malis, A., and J. Stephens, "PPP Over AAL5", RFC 2364, July 1998. [RFC2516] Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D., and R. Wheeler, "A Method for Transmitting PPP Over Ethernet (PPPoE)", RFC 2516, February 1999. [RFC2881] Mitton, D. and M. Beadles, "Network Access Server Requirements Next Generation (NASREQNG) NAS Model", RFC 2881, July 2000. [RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046, January 2001. [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001. [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H. Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004. [TR-059] DSL Forum, "DSL Evolution - Architecture Requirements for the Support of QoS-Enabled IP Services", TR 059, September 2003. Pruss Expires August 30, 2007 [Page 18] Internet-Draft DHCP Authentication February 2007 [TR-101] DSL Forum, "Migration to Ethernet Based DSL Aggregation", TR 101, April 2006. [WT-146] DSL Forum, "Internet Protocol (IP) Sessions", WT 146 (work in progress), April 2007. Author's Address Richard Pruss Cisco Systems 80 Albert Street Brisbane, Queensland 4000 Australia Phone: +61 7 3238 8228 Fax: +61 7 3211 3889 Email: ric@cisco.com Pruss Expires August 30, 2007 [Page 19] Internet-Draft DHCP Authentication February 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Pruss Expires August 30, 2007 [Page 20]