Network Working Group B. Aboba INTERNET-DRAFT Microsoft Category: Informational P. Calhoun <draft-aboba-radius-rfc2869bis-09.txt> Black Storm Networks 9 February 2003 Updates: RFC 2869 RADIUS Support For Extensible Authentication Protocol (EAP) This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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 document defines RADIUS support for the Extensible Authentication Protocol (EAP), an authentication framework which supports multiple authentication mechanisms. While EAP was originally developed for use with PPP, it is also now in use with IEEE 802. This document updates RFC 2869. Aboba & Calhoun Informational [Page 1]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 Table of Contents 1. Introduction .......................................... 3 1.1 Specification of Requirements ................... 3 1.2 Terminology ..................................... 4 2. RADIUS support for EAP ................................ 5 2.1 Protocol overview ............................... 5 2.2 Role reversal ................................... 9 2.3 Retransmission .................................. 9 2.4 Fragmentation ................................... 9 2.5 Invalid packet .................................. 10 2.6 Alternative uses ................................ 10 2.7 Usage guidelines ................................ 11 3. Attributes ............................................ 13 3.1 EAP-Message ..................................... 13 3.2 Message-Authenticator ........................... 15 3.3 Table of attributes ............................. 16 4. Security considerations ............................... 17 4.1 Security requirements ........................... 17 4.2 Security protocol ............................... 18 4.3 Security issues ................................ 20 5. Normative references .................................. 27 6. Informative references ................................ 27 Appendix A - Examples ........................................ 29 Appendix B - Change log ...................................... 37 ACKNOWLEDGMENTS .............................................. 38 AUTHORS' ADDRESSES ........................................... 38 Intellectual property statement .............................. 38 Full Copyright Statement ..................................... 39 Aboba & Calhoun Informational [Page 2]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 1. Introduction The Remote Authentication Dial In User Service (RADIUS) is an authentication, authorization and accounting protocol used to control network access. RADIUS authentication and authorization is specified in [RFC2865], and RADIUS accounting is specified in [RFC2866]. The Extensible Authentication Protocol (EAP), defined in [RFC2284], is an authentication framework which supports multiple authentication mechanisms. EAP may be used on dedicated links as well as switched circuits, and wired as well as wireless links. To date, EAP has been implemented with hosts and routers that connect via switched circuits or dial-up lines using PPP [RFC1661]. It has also been implemented with switches supporting [IEEE802]. EAP encapsulation on IEEE 802 wired media is described in [IEEE8021X]. This specification describes RADIUS attributes supporting the Extensible Authentication Protocol (EAP): EAP-Message and Message-Authenticator. These attributes now have extensive field experience, and so the purpose of this document is to provide clarification and resolve interoperability issues. As noted in [RFC2865], a Network Access Server (NAS) that does not implement a given service MUST NOT implement the RADIUS attributes for that service. This implies that a NAS that is unable to offer EAP service MUST NOT implement the RADIUS attributes for EAP. A NAS MUST treat a RADIUS Access-Accept requesting an unavailable service as an Access-Reject instead. All attributes are comprised of variable length Type-Length-Value 3- tuples. New attribute values can be added without disturbing existing implementations of the protocol. 1.1. Specification of Requirements In this document, several words are used to signify the requirements of the specification. These words are often capitalized. 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 [RFC2119]. Aboba & Calhoun Informational [Page 3]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 1.2. Terminology This document frequently uses the following terms: authenticator The end of the link requiring the authentication. peer The other end of the point-to-point link (PPP), point-to-point LAN segment (IEEE 802.1X) or wireless link, which being authenticated by the authenticator. In IEEE 802.1X, this end is known as the Supplicant. Throughout this specification, the term "user" is used synonymously with peer. authentication server An authentication server is an entity that provides an authentication service to an authenticator. This service verifies from the credentials provided by the peer, the claim of identity made by the peer. Port Access Entity (PAE) The protocol entity associated with a physical or virtual Port. A given PAE may support the protocol functionality associated with the authenticator, peer or both. Silently Discard This means the implementation discards the packet without further processing. The implementation SHOULD provide the capability of logging the error, including the contents of the silently discarded packet, and SHOULD record the event in a statistics counter. Displayable Message This is interpreted to be a human readable string of characters, and MUST NOT affect operation of the protocol. The message encoding MUST follow the UTF-8 transformation format [RFC2279]. Network Access Server (NAS) The device providing access to the network. Service The NAS provides a service to the user, such as IEEE 802 or PPP. Session Each service provided by the NAS to a peer constitutes a session, with the beginning of the session defined as the point where service is first provided and the end of the session defined as the point where service is ended. A peer may have multiple sessions in parallel or series if the NAS Aboba & Calhoun Informational [Page 4]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 supports that, with each session generating a separate start and stop accounting record. 2. RADIUS Support for EAP The Extensible Authentication Protocol (EAP), described in [RFC2284], provides a standard mechanism for support of additional authentication methods without requiring additional functionality on the NAS. Through the use of EAP, support for a number of authentication schemes may be added, including smart cards, Kerberos [RFC1510], Public Key [RFC2716], One Time Passwords [RFC2284], and others. One of the advantages of the EAP architecture is its flexibility. EAP is used to select a specific authentication mechanism. Rather than requiring the NAS to be updated to support each new authentication method, EAP permits the use of an authentication server which may implement some or all authentication methods, with the NAS acting as a pass-through for some or all methods and peers. A NAS MAY authenticate local users while at the same time acting as a pass-through for non-local users and authentication methods it does not implement locally. This means that a NAS implementing RADIUS/EAP is not required to use RADIUS to authenticate every peer. However, once the NAS begins acting as a pass-through for a particular session, it can no longer perform local authentication for that session. In order to support EAP within RADIUS, two new attributes, EAP-Message and Message-Authenticator, are introduced in this document. This section describes how these new attributes may be used for providing EAP support within RADIUS. 2.1. Protocol Overview In RADIUS/EAP, RADIUS is used to shuttle RADIUS-encapsulated EAP Packets between the NAS and an authentication server. The authenticating peer and the NAS begin the EAP conversation by negotiating use of EAP. Once EAP has been negotiated, the NAS SHOULD send an initial EAP-Request message to the authenticating peer. This will typically be an EAP-Request/Identity, although an EAP-Request for an authentication method (Types 4 and greater) is possible. For example, a NAS might be configured to initiate with a default authentication method. This could be useful in cases where the identity is determined by another means (such as the Called-Station-Id or Calling-Station-Id), a single authentication method is required (so that the identity is not needed to determine the method), or where identity hiding is desired, so that the identity is not requested until after a protected channel has been set up. Aboba & Calhoun Informational [Page 5]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 The peer replies with an EAP-Response. The NAS MAY determine from the Response that it should proceed with local authentication. Alternatively, it MAY act as a pass-through, encapsulating the EAP- Response within EAP-Message attribute(s) within a RADIUS Access-Request packet, sent to the RADIUS server. For example, if an EAP-Request/Identity message is sent by the NAS as the first packet, the peer responds with an EAP-Response/Identity. The NAS may determine that the user is local and proceed with local authentication. If no match is found against the list of local users, the NAS encapsulates the EAP-Response/Identity message within EAP- Message attribute(s), enclosed within an Access-Request sent to the RADIUS server. On receiving a valid Access-Request packet containing EAP-Message attribute(s), a RADIUS server supporting and wishing to authenticate with EAP MUST respond with an Access-Challenge packet containing EAP- Message attribute(s). If the RADIUS server does not support EAP or does not wish to authenticate with EAP, it MUST respond with an Access- Reject. The EAP-Message attribute(s) encapsulate a single EAP packet which the NAS decapsulates and passes on to the authenticating peer. The conversation continues until either a RADIUS Access-Reject or Access-Accept packet is received from the RADIUS server. Reception of a RADIUS Access-Reject packet MUST result in the NAS denying access to the authenticating peer. A RADIUS Access-Accept packet successfully ends the authentication phase. Using RADIUS, the NAS can act as a pass-through for an EAP conversation between the peer and authentication server, without needing to implement the EAP method used between them. Where the NAS initiates the conversation by sending an EAP-Request for an authentication method, it may not be required that the NAS fully implement the EAP method sent in the first packet. Depending on the initial method, it may be sufficient for the NAS to be configured with the initial packet to be sent to the peer, and for the NAS to act as a pass-through for subsequent messages. However, this only works well for methods where the initial Request is largely static; otherwise the NAS will need to fully implement the method so as to be able to authenticate the user locally. Where an initial EAP-Request for an authentication Type (4 or greater) is sent by the NAS, the peer may respond with a Nak indicating that it would prefer another authentication method that is not implemented locally. In this case, Access-Request sent by the NAS MAY include an EAP-message attribute encapsulating the received EAP-Response/Nak. This provides the RADIUS server with a hint about the authentication method(s) preferred by the peer, although it does not provide information on the Type of the original Request. In order to evaluate whether the alternatives preferred by the peer are allowed, the RADIUS Aboba & Calhoun Informational [Page 6]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 server will typically need to determine the peer identity, so as to be able to retrieve the associated authentication policy. In order to permit non-EAP aware RADIUS proxies to forward the Access- Request packet, if the NAS initially sends an EAP-Request/Identity message to the peer, the NAS MUST copy the contents of the Type-Data field of the EAP-Response/Identity received from the peer into the User- Name attribute and MUST include the Type-Data field of the EAP- Response/Identity in the User-Name attribute in every subsequent Access- Request. Since RADIUS proxies are assumed to act as a pass-through, they cannot be expected to parse an EAP-Response/Identity encapsulated within EAP-Message attribute(s). The NAS-Port or NAS-Port-Id attributes SHOULD be included by the NAS in the Access-Request packet, and either NAS-Identifier or NAS-IP-Address attributes MUST be included. In order to permit forwarding of the Access-Reply by EAP-unaware proxies, if a User-Name attribute was included in an Access-Request, the RADIUS server MUST include the User- Name attribute in subsequent Access-Challenge, Access-Accept or Access- Reject packets. Without the User-Name attribute, accounting and billing becomes difficult to manage. If the identity is determined by another means, such as the Calling-Station-Id, the NAS MUST include these identifying attributes in every Access-Request, and the RADIUS server MUST copy these identifying attributes into subsequent Access-Challenge, Access-Accept or Access-Reject packets. Having the NAS send the initial EAP-Request packet has a number of advantages: [1] It saves a round trip between the NAS and RADIUS server. [2] An Access-Request is only sent to the RADIUS server if the authenticating peer sends an EAP-Response, confirming that it supports EAP. In situations where peers may be EAP unaware (such as in the case of a switch implementing [IEEE8021X], where there are IEEE 802.1X-unaware hosts), initiating a RADIUS Access-Request on a "carrier sense" or "media up" indication may initiate many authentication exchanges that cannot complete successfully. [3] It allows some users to be authenticated locally. Although having the NAS send the initial EAP-Request packet has substantial advantages, this technique cannot be universally employed. There are circumstances in which the user's identity is already known (such as when authentication and accounting is handled based on Called- Station-Id or Calling-Station-Id), but where the appropriate EAP method may vary based on that identity. Aboba & Calhoun Informational [Page 7]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 Rather than sending an initial EAP-Request packet to the authenticating peer, on detecting the presence of the peer, the NAS MAY send an Access- Request packet to the RADIUS server containing an EAP-Message attribute signifying EAP-Start. The NAS also MAY send an Access-Request packet containing an EAP-Start if, after sending an initial Request for an authentication Type, the peer responds with a Nak. This allows the RADIUS server to take over the task of negotiating a more suitable method. Typically the RADIUS server will respond with an Access- Challenge containing an EAP-Message attribute with an encapsulated EAP- Request/Identity. On receiving an Access-Request containing an encapsulated EAP-Response/Identity, the RADIUS server can retrieve the authentication policy for the peer, in order to choose a suitable method. However, because the RADIUS server is not provided with the contents of the EAP-Response/Nak, it is possible for the RADIUS server to choose an unacceptable method, and for the peer to respond with another Nak. EAP-Start is indicated by sending an EAP-Message attribute with a length of 2 (no data). The Calling-Station-Id SHOULD be included in the User- Name attribute. This results in a RADIUS Access-Request being sent by the NAS to the RADIUS server without first confirming that the peer supports EAP. Since this technique can result in a a large number of uncompleted RADIUS conversations, in situations where EAP unaware peers are common, it SHOULD NOT be employed by default. EAP-Message attributes containing EAP-Start and an EAP-Response/Nak packets MUST NOT be sent together within the same Access-Request. Where the NAS initiates the RADIUS exchange by sending an Access-Request to the server containing an EAP-Start, the Access-Challenge sent by the RADIUS server will typically contain EAP-Message attribute(s) encapsulating an EAP-Request/Identity packet, requesting the peer to identify itself. However, an Access-Challenge containing an EAP-Message attribute encapsulating an EAP-Request for an authentication method (Type 4 or greater) MAY also be sent. The NAS will decapsulate the EAP packet contained within the EAP-Message attribute, send it to the peer, and receive an EAP-Response packet from the peer in return. The NAS will then send a RADIUS Access-Request packet to the RADIUS server, containing EAP-Message attribute(s) encapsulating the EAP-Response packet. For proxied RADIUS requests, there are two methods of processing. If the domain is determined based on the Calling-Station-Id and Called- Station-Id, the RADIUS Server may proxy the initial RADIUS Access- Request/EAP-Start. If the domain is determined based on the user's identity, the local RADIUS Server MUST respond with a RADIUS Access- Challenge/EAP-Identity packet. The response from the authenticating peer MUST be proxied to the final authentication server. Aboba & Calhoun Informational [Page 8]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 For proxied RADIUS requests, the NAS may receive an Access-Reject packet in response to the initial Access-Request packet. This would occur if the message was proxied to a RADIUS server which does not support the EAP-Message attribute. On receiving an Access-Reject, the NAS MUST deny access to the authenticating peer. 2.2. Role reversal Since EAP is a peer-to-peer protocol, an independent and simultaneous authentication may take place in the reverse direction. Both peers may act as authenticators and authenticatees at the same time. However, role reversal is not supported by this specification. A RADIUS server MUST respond to an Access-Request encapsulating an EAP-Request with an Access-Reject. In order to avoid retransmissions by the peer, the Access-Reject MAY include an EAP-Response/Nak packet indicating no preferred method, encapsulated within EAP-Message attribute(s). 2.3. Retransmission As noted in [RFC2284], the EAP authenticator (NAS) is responsible for retransmission of packets between the authenticating peer and the NAS. If an EAP packet is lost in transit between the authenticating peer and the NAS (or vice versa), the NAS will retransmit. As in RADIUS [RFC2865], the RADIUS client is responsible for retransmission of packets between the RADIUS client and the RADIUS server. It may be necessary to adjust retransmission strategies and authentication timeouts in certain cases. For example, when a token card is used additional time may be required to allow the user to find the card and enter the token. Since the NAS will typically not have knowledge of the required parameters, these need to be provided by the RADIUS server. This can be accomplished by inclusion of Session-Timeout attribute within the Access-Challenge packet. If Session-Timeout is present in an Access-Challenge packet that also contains an EAP-Message, the value of the Session-Timeout is used to set the EAP retransmission timer for that EAP Request, and that Request alone. Once the EAP-Request has been sent, the NAS sets the retransmission timer, and if it expires without having received an EAP- Response corresponding to the Request, then the EAP-Request is retransmitted. 2.4. Fragmentation Using the EAP-Message attribute, it is possible for the RADIUS server to encapsulate an EAP packet that is larger than the MTU on the link between the NAS and the peer. Since it is not possible for the RADIUS Aboba & Calhoun Informational [Page 9]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 server to use MTU discovery to ascertain the link MTU, the Framed-MTU attribute may be included in an Access-Request packet containing an EAP- Message attribute so as to provide the RADIUS server with this information. 2.5. Invalid packets EAP methods may contain a per-packet Message Integrity Check (MIC). Although EAP methods such as EAP-TLS [RFC2716] treat MIC check failures as fatal errors, it may be desirable for an EAP method to silently discard an invalid EAP packet, and subsequently continue the conversation. This provides additional resilience against Denial of Service (DoS) attacks. When a RADIUS server receives an Access-Request containing an EAP- Message attribute encapsulating an invalid EAP packet, it SHOULD NOT silently discard the Access-Request without informing the NAS. This would cause the NAS to retransmit the Access-Request, then eventually time-out and initiate failover. As a result, it is best for the RADIUS server to formulate a response of some kind. If the error is considered fatal, an Access-Reject can be sent. In order for the RADIUS server to communicate that an invalid EAP packet has been received, but that the problem is non-fatal, an Access- Challenge containing an EAP-Start MAY be sent. A NAS receiving an Access-Challenge with an EAP-Start MUST discard the EAP packet that it had previously encapsulated within an Access-Request. It will then check whether it has received additional EAP Response packets with an Identifier matching that of the last Request. If so, a new EAP Response packet will be sent to the RADIUS server within an Access-Request packet. Once the RADIUS server responds with a packet containing a non- null EAP-Message attribute, the NAS updates the Identifier value, and any pending Responses that do not match this value can be discarded. 2.6. Alternative uses Currently the conversation between security servers and the RADIUS server is often proprietary because of lack of standardization. In order to increase standardization and provide interoperability between RADIUS vendors and security vendors, it is recommended that RADIUS- encapsulated EAP be used for this conversation. This has the advantage of allowing the RADIUS server to support EAP without the need for authentication-specific code within the RADIUS server. Authentication-specific code can then reside on a security server instead. Aboba & Calhoun Informational [Page 10]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 In the case where RADIUS-encapsulated EAP is used in a conversation between a RADIUS server and a security server, the security server will typically return an Access-Accept message without inclusion of the expected attributes currently returned in an Access-Accept. This means that the RADIUS server MUST add these attributes prior to sending an Access-Accept message to the NAS. 2.7. Usage guidelines 2.7.1. Conflicting messages Within an EAP conversation, a RADIUS Access-Accept will typically contain an EAP-Message attribute encapsulating an EAP Success packet. Similarly, a RADIUS Access-Reject will typically contain an EAP-Message attribute encapsulating an EAP Failure packet. However, in some cases, the authentication result implied by the encapsulated EAP packet may not match the result communicated in the RADIUS message. For example, an EAP Failure packet may be encapsulated within an Access-Accept message and an EAP Success packet may be encapsulated within an Access-Reject. Alternatively, an EAP-Message attribute may not be included within a RADIUS Access-Accept or Access-Reject. Such combinations are likely to cause confusion, because the NAS and peer will arrive at different conclusions as to the outcome of the authentication. For example, if the NAS receives an Access-Reject with an encapsulated EAP Success, it will not grant access to the peer. However, on receiving the Success, the peer will be lead to believe that it authenticated successfully. Similarly, if the NAS receives an Access-Accept with an encapsulated EAP Failure, it will grant access to the peer. However, on receiving an EAP Failure, the peer will be lead to believe that it failed authentication. If no EAP-Message attribute is included within an Access-Accept or Access-Reject, then the peer may not be informed as to the outcome of the authentication, while the NAS will take action to allow or deny access. As described in [RFC2284], the EAP Success and Failure packets are not acknowledged, and these packets terminate the EAP conversation. As a result, if these packets are encapsulated within an Access-Challenge, no response will be received, and therefore no further Access-Requests will be sent to the RADIUS server. As a result, the NAS will not be given an indication of whether to allow or deny access while the peer will be informed as to the outcome of the authentication. To avoid these conflicts, the RADIUS server SHOULD check for agreement between the EAP-Message attribute and the RADIUS message, and SHOULD resolve conflicts before sending the packet. The following combinations Aboba & Calhoun Informational [Page 11]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 SHOULD NOT be sent by a RADIUS server as part of an EAP conversation: Access-Accept/EAP-Message/EAP Failure Access-Accept/no EAP-Message attribute Access-Reject/EAP-Message/EAP Success Access-Reject/no EAP-Message attribute Access-Challenge/EAP-Message/EAP Success Access-Challenge/EAP-Message/EAP Failure Access-Challenge/no EAP-Message attribute Since the responsibility for avoiding these conflicts lies with the RADIUS server, the NAS MUST NOT "manufacture" EAP packets in order to correct contradictory messages that it receives. This behavior, originally mandated within [IEEE8021X], is likely to be deprecated. 2.7.2. Priority In addition to containing EAP-Message attributes, RADIUS messages may also contain other attributes. In order to ensure the correct processing of RADIUS messages, on receiving an Access-Accept, Access-Reject or Access-Challenge, the NAS SHOULD process other attributes first, then decapsulate EAP-Message attribute(s), reconstitute the EAP packet and send it to the peer. 2.7.3. Displayable messages The Reply-Message attribute, defined in section 5.18 of [RFC2865], indicates text which may be displayed to the user. This is similar in concept to EAP Notification, defined in [RFC2284]. When sending a displayable message to a NAS during an EAP conversation, the RADIUS server MUST encapsulate displayable messages within EAP-Message/EAP- Request/Notification attribute(s). Reply-Message attribute(s) MUST NOT be included in any RADIUS message containing an EAP-Message attribute. An EAP-Message/EAP-Request/Notification SHOULD NOT be included within an Access-Accept or Access-Reject packet. In some existing implementations, a NAS receiving Reply-Message attribute(s) copies the Text field(s) into the Type-Data field of an EAP-Request/Notification packet, fills in the Identifier field, and sends this to the peer. However, several issues arise from this: [1] Unexpected Responses. On receiving an EAP-Request/Notification, the peer will send an EAP-Response/Notification, and the NAS will pass this on to the RADIUS server, encapsulated within EAP-Message attribute(s). However, the RADIUS server may not be expecting an Access-Request containing an EAP-Message/EAP-Response/Notification attribute. Aboba & Calhoun Informational [Page 12]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 For example, consider what happens when a Reply-Message is included within an Access-Accept or Access-Reject packet with no EAP-Message attribute(s) present. If the value of the Reply-Message attribute is copied into the Type-Data of an EAP-Request/Notification and sent to the peer, this will result in an Access-Request containing an EAP-Message/EAP-Response/Notification attribute being sent by the NAS to the RADIUS server. Since an Access-Accept or Access- Reject packet terminates the RADIUS conversation, such an Access- Request would not be expected, and could be interpreted as the start of another conversation. [2] Identifier conflicts. While the EAP-Request/Notification is an EAP packet containing an Identifier field, the Reply-Message attribute does not contain an Identifier field. As a result, a NAS receiving a Reply-Message attribute and wishing to translate this to an EAP- Request/Notification will need to choose an Identifier value. It is possible that the chosen Identifier value will conflict with a value chosen by the RADIUS server for another packet within the EAP conversation, potentially causing confusion between a new packet and a retransmission. In order to avoid these problems, a NAS in pass-through mode receiving a Reply-Message attribute from the RADIUS server SHOULD silently discard the attribute. 2.7.4. Multiple EAP-Message attributes When RADIUS is used to enable EAP authentication, Access-Request, Access-Challenge, Access-Accept, and Access-Reject packets SHOULD contain one or more EAP-Message attributes. Where more than one EAP- Message attribute is included, it is assumed that the attributes are to be concatenated to form a single EAP packet. As a result, multiple EAP packets MUST NOT be encoded within EAP-Message attributes contained within a single Access-Challenge, Access-Accept, Access-Reject or Access-Request packet. 3. Attributes 3.1. EAP-Message Description This attribute encapsulates EAP [RFC2284] packets so as to allow the NAS to authenticate users via EAP without having to understand the EAP method it is passing through. The NAS places any EAP messages received from the user into one or more EAP-Message attributes and forwards them to the RADIUS server as Aboba & Calhoun Informational [Page 13]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 part of the Access-Request, which can return EAP messages in Access- Challenge, Access-Accept and Access-Reject packets. The NAS places EAP messages received from the authenticating peer into one or more EAP-Message attributes and forwards them to the RADIUS server within an Access-Request message. If multiple EAP- Messages are contained within an Access-Request or Access- Challenge packet, they MUST be in order and they MUST be consecutive attributes in the Access-Request or Access-Challenge packet. It is expected that EAP will be used to implement a variety of authentication methods, including methods involving strong cryptography. In order to prevent attackers from subverting EAP by attacking RADIUS/EAP, (for example, by modifying the EAP-Success or EAP-Failure packets) it is necessary that RADIUS/EAP provide integrity protection. Therefore the Message-Authenticator attribute MUST be used to protect all Access-Request, Access-Challenge, Access-Accept, and Access- Reject packets containing an EAP-Message attribute. Access-Request packets including EAP-Message attribute(s) without a Message-Authenticator attribute SHOULD be silently discarded by the RADIUS server. A RADIUS server supporting the EAP-Message attribute MUST calculate the correct value of the Message-Authenticator and silently discard the packet if it does not match the value sent. A RADIUS server not supporting the EAP-Message attribute MUST return an Access-Reject if it receives an Access-Request containing an EAP- Message attribute. A RADIUS server receiving an EAP-Message attribute that it does not understand MUST return an Access-Reject. Access-Challenge, Access-Accept, or Access-Reject packets including EAP-Message attribute(s) without a Message-Authenticator attribute SHOULD be silently discarded by the NAS. A NAS supporting the EAP- Message attribute MUST calculate the correct value of the Message- Authenticator and silently discard the packet if it does not match the value sent. A summary of the EAP-Message attribute format is shown below. The fields are transmitted from left to right. 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | String... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type Aboba & Calhoun Informational [Page 14]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 79 for EAP-Message. Length >= 3 String The String field contains EAP packets, as defined in [RFC2284]. If multiple EAP-Message attributes are present in a packet their values should be concatenated; this allows EAP packets longer than 253 octets to be transported by RADIUS. 3.2. Message-Authenticator Description This attribute MAY be used to authenticate and integrity-protect Access-Requests in order to prevent spoofing. It MAY be used in any Access-Request. It MUST be used in any Access-Request, Access- Accept, Access-Reject or Access-Challenge that includes an EAP- Message attribute. A RADIUS server receiving an Access-Request with a Message- Authenticator Attribute present MUST calculate the correct value of the Message-Authenticator and silently discard the packet if it does not match the value sent. A RADIUS Client receiving an Access-Accept, Access-Reject or Access- Challenge with a Message-Authenticator Attribute present MUST calculate the correct value of the Message-Authenticator and silently discard the packet if it does not match the value sent. A summary of the Message-Authenticator attribute format is shown below. The fields are transmitted from left to right. 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | String... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 80 for Message-Authenticator Length Aboba & Calhoun Informational [Page 15]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 18 String When present in an Access-Request packet, Message-Authenticator is an HMAC-MD5 [RFC2104] hash of the entire Access-Request packet, including Type, ID, Length and authenticator, using the shared secret as the key, as follows. Message-Authenticator = HMAC-MD5 (Type, Identifier, Length, Request Authenticator, Attributes) When the message integrity check is calculated the signature string should be considered to be sixteen octets of zero. For Access-Challenge, Access-Accept, and Access-Reject packets, the Message-Authenticator is calculated as follows, using the Request- authenticator from the Access-Request this packet is in reply to: Message-Authenticator = HMAC-MD5 (Type, Identifier, Length, Request Authenticator, Attributes) When the message integrity check is calculated the signature string should be considered to be sixteen octets of zero. The shared secret is used as the key for the HMAC-MD5 message integrity check. The is calculated and inserted in the packet before the Response authenticator is calculated. This attribute is not needed if the User-Password attribute is present, but is useful for preventing attacks on other types of authentication. This attribute is intended to thwart attempts by an attacker to setup a "rogue" NAS, and perform online dictionary attacks against the RADIUS server. It does not afford protection against "offline" attacks where the attacker intercepts packets containing (for example) CHAP challenge and response, and performs a dictionary attack against those packets offline. 3.3. Table of Attributes The following table provides a guide to which attributes may be found in which kind of packets. The EAP-Message and Message-Authenticator attributes specified in this document MUST NOT be present in an Accounting-Request. The following table provides a guide to which attributes may be found in packets sent as part of an EAP conversation, and in what quantity. If a table entry is omitted, the values found in [RFC2548], [RFC2865]-[RFC2869] and [RFC3162] should be assumed. Aboba & Calhoun Informational [Page 16]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 Request Accept Reject Challenge # Attribute 0-1 0-1 0 0 1 User-Name 0 0 0 0 2 User-Password [Note 1] 0 0 0 0 3 CHAP-Password [Note 1] 0 0 0 0 18 Reply-Message 0-1 0-1 0 0-1 24 State 0 0+ 0 0 25 Class 0 0-1 0 0-1 27 Session-Timeout 0+ 0+ 0+ 0+ 33 Proxy-State 0 0 0 0 60 CHAP-Challenge 0-1 0 0 0 70 ARAP-Password [Note 1] 0 0 0 0 75 Password-Retry 1+ 1+ 1+ 1+ 79 EAP-Message [Note 1] 1 1 1 1 80 Message-Authenticator [Note 1] Request Accept Reject Challenge # Attribute [Note 1] An Access-Request that contains either a User-Password or CHAP- Password or ARAP-Password or one or more EAP-Message attributes MUST NOT contain more than one type of those four attributes. If it does not contain any of those four attributes, it SHOULD contain a Message- Authenticator. If any packet type contains an EAP-Message attribute it MUST also contain a Message-Authenticator. The following table defines the meaning of the above table entries. 0 This attribute MUST NOT be present. 0+ Zero or more instances of this attribute MAY be present. 0-1 Zero or one instance of this attribute MAY be present. 1 Exactly one instance of this attribute MUST be present. 1+ One or more of these attributes MUST be present. 4. Security Considerations 4.1. Security requirements RADIUS/EAP is used in order to provide authentication and authorization for network access. As a result, both the RADIUS and EAP portions of the conversation are open to attack. Threats are discussed in [RFC2607], [RFC2865], and [RFC3162]. Examples include: [1] An adversary may attempt to acquire confidential data and identities by snooping RADIUS packets. [2] An adversary may attempt to modify packets containing RADIUS messages. [3] An adversary may attempt to inject packets into a RADIUS conversation. Aboba & Calhoun Informational [Page 17]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 [4] An adversary may launch a dictionary attack against the RADIUS shared secret. [5] An adversary may launch a known plaintext attack, hoping to recover the key stream corresponding to a Request Authenticator. [6] An adversary may attempt to replay a RADIUS exchange. [7] An adversary may attempt to disrupt the EAP negotiation, in order to weaken the authentication, or gain access to user passwords. [8] An authenticated NAS may attempt to forge attributes, including NAS-IP-Address, NAS-Identifier, Called-Station-Id, or Calling-Station-Id. [9] A rogue (unauthenticated) NAS may attempt to impersonate a legitimate NAS. [10] An attacker may attempt to act as a man-in-the-middle. To address these threats, it is necessary to support confidentiality, data origin authentication, integrity, and replay protection on a per- packet basis. Bi-directional authentication between the RADIUS client and server also needs to be provided. There is no requirement that the identities of RADIUS clients and servers be kept confidential (e.g., from a passive eavesdropper). 4.2. Security protocol To address the security threats for RADIUS/EAP, in addition to supporting RADIUS security as defined in [RFC2865] and [RFC2866], RADIUS/EAP implementations SHOULD support IPsec [RFC2401] along with IKE [RFC2409] for key management. IPsec ESP [RFC2406] with non-null transform, and per-packet authentication, integrity and replay protection SHOULD be used, along with IKE for key management. Within RADIUS [RFC2865], a shared secret is used for hiding of attributes such as User-Password, as well as in computation of the Response Authenticator. In RADIUS accounting [RFC2866], the shared secret is used in computation of both the Request Authenticator and the Response Authenticator. Since in RADIUS a shared secret is used to provide confidentiality as well as integrity protection and authentication, only use of IPsec ESP with a non-null transform can provide security services sufficient to substitute for RADIUS application-layer security. Therefore, where IPSEC AH or ESP null is used, it will typically still be necessary to Aboba & Calhoun Informational [Page 18]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 configure a RADIUS shared secret. Where RADIUS is run over IPsec ESP with a non-null transform, the secret shared between the NAS and the RADIUS server may not be configured. In this case, a shared secret of zero length MUST be assumed. However, a RADIUS server that cannot know whether incoming traffic is IPsec- protected MUST be configured with a non-null RADIUS shared secret. When IPsec ESP is used with RADIUS, DES-CBC SHOULD NOT be used as the encryption transform, and per-packet authentication, integrity and replay protection MUST be used. A typical IPsec policy for an IPsec- capable RADIUS client is "Initiate IPsec, from me to any, destination port UDP 1812". This causes an IPsec SA to be set up by the RADIUS client prior to sending RADIUS traffic to any RADIUS server. If some RADIUS servers contacted by the client do not support IPsec, then a more granular policy will be required. For an IPsec-capable RADIUS server, a typical IPsec policy is "Accept IPsec, from any to me, destination port 1812". This causes the RADIUS server to accept (but not require) use of IPsec. It may not be appropriate to require IPsec for all RADIUS clients connecting to an IPsec-enabled RADIUS server, since some RADIUS clients may not support IPsec. Where IPsec is used for security, and no RADIUS shared secret is configured, it is important that trust be demonstrated between the RADIUS client and RADIUS server by some means. For example, before enabling an IKE-authenticated host to act as a RADIUS client, the RADIUS server should check whether the host is authorized to provide network access. For example, the RADIUS server can be configured with the IP addresses (for IKE Aggressive Mode with pre-shared keys) or FQDNs (for certificate authentication) of RADIUS clients. Alternatively, if a separate CA exists for RADIUS clients, then the RADIUS server can configure this CA as a trusted root for use with IPsec. However, unlike SSL/TLS, IKE does not permit certificate policies to be set on a per-port basis, such a policy would need to apply to all uses of IPsec on RADIUS clients and servers. Assuming that only certificate authentication is supported in the deployment, a management station initiating an IPsec-protected telnet session to the RADIUS server would need to obtain a certificate chaining to the RADIUS client CA. Issuing such a certificate might not be appropriate if the management station was not authorized as a RADIUS client. Where RADIUS clients may obtain their IP address dynamically (such as an Access Point supporting DHCP), Main Mode with pre-shared keys [RFC2409] Aboba & Calhoun Informational [Page 19]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 SHOULD NOT be used, since this requires use of a group pre-shared key; instead, Aggressive Mode SHOULD be used. Where RADIUS client addresses are statically assigned either Aggressive Mode or Main Mode MAY be used. With certificate authentication, Main Mode SHOULD be used. Care needs to be taken with IKE Phase 1 Identity Payload selection in order to enable mapping of identities to pre-shared keys even with Aggressive Mode. Where the ID_IPV4_ADDR or ID_IPV6_ADDR Identity Payloads are used and addresses are dynamically assigned, mapping of identities to keys is not possible, so that group pre-shared keys are still a practical necessity. As a result, the ID_FQDN identity payload SHOULD be employed in situations where Aggressive mode is utilized along with pre-shared keys and IP addresses are dynamically assigned. This approach also has other advantages, since it allows the RADIUS server and client to configure themselves based on the fully qualified domain name of their peers. Note that with IPsec, security services are negotiated at the granularity of an IPsec SA, so that RADIUS exchanges requiring a set of security services different from those negotiated with existing IPsec SAs will need to negotiate a new IPsec SA. Separate IPsec SAs are also advisable where quality of service considerations dictate different handling RADIUS conversations. Attempting to apply different quality of service to connections handled by the same IPsec SA can result in reordering, and falling outside the replay window. For a discussion of the issues, see [RFC2983]. 4.3. Security issues This section provides more detail on the vulnerabilities identified in Section 4.1, and how they may be mitigated. Vulnerabilities include: Privacy issues Message-Authenticator security Connection hijacking Dictionary attacks Known plaintext attacks Replay attacks Negotiation attacks Impersonation Man in the middle attacks Separation of EAP server and authenticator Multiple databases 4.3.1. Privacy issues Since RADIUS messages may contain the User-Name attribute as well as NAS-IP-Address or NAS-Identifier attributes, an attacker snooping on Aboba & Calhoun Informational [Page 20]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 RADIUS traffic may be able to determine the geographic location of users in real time. In wireless networks, it is often assumed that RADIUS traffic is physically secure, since it typically travels over the wired network and that this limits the release of location information. However, it is possible for an authenticated attacker send false ARP messages claiming the address of an Access Point in order to cause diversion of RADIUS traffic onto the wireless network. In this way an attacker may obtain RADIUS packets from which it can glean location information, or which it can subject to an offline dictionary attack. To address this vulnerabilities, IPsec ESP with non-null transform and per-packet authentication, integrity and replay protection SHOULD be used to protect both RADIUS authentication [RFC2865] and accounting [RFC2866] traffic. 4.3.2. Message-Authenticator security Access-Request packets with a User-Password attribute establish the identity of both the user and the NAS sending the Access-Request, because of the way the shared secret between NAS and RADIUS server is used. Access-Request packets with CHAP-Password or EAP-Message attributes do not have a User-Password attribute, so the Message- Authenticator attribute SHOULD be used in Access-Request packets that do not have a User-Password attribute, in order to establish the identity of the NAS sending the request. The Message-Authenticator attribute MUST be present in all RADIUS/EAP packets. 4.3.3. Connection hijacking An attacker may attempt to inject packets into the conversation between the NAS and the RADIUS server, or between the RADIUS server and the security server. RADIUS does not support encryption other than attribute hiding, and as described in [RFC2865], only Access-Reply and Access- Challenge packets are integrity protected. Moreover, the integrity protection mechanism described in [RFC2865] is weaker than that likely to be used by some EAP methods, making it possible to subvert those methods by attacking EAP/RADIUS. In order to provide for authentication of all packets in the EAP exchange, all RADIUS/EAP packets MUST include the Message-Authenticator attribute. 4.3.4. Dictionary attacks The RADIUS shared secret is vulnerable to offline dictionary attack, based on capture of the Response authenticator or Message-Authenticator attribute. In order to decrease the level of vulnerability, [RFC2865] recommends: Aboba & Calhoun Informational [Page 21]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 The secret (password shared between the client and the RADIUS server) SHOULD be at least as large and unguessable as a well- chosen password. It is preferred that the secret be at least 16 octets. The risk of an offline dictionary attack can be further reduced by employing IPsec ESP with non-null transform in order to encrypt the RADIUS conversation, as described in Section 4.2. 4.3.5. Known plaintext attacks Since EAP [RFC2284] does not support PAP, the RADIUS User-Password attribute is not used to carry hidden user passwords within EAP conversations. The User-Password hiding mechanism, defined in [RFC2865] utilizes MD5, defined in [RFC1321], in order to generate a key stream based on the RADIUS shared secret and the Request authenticator. Where PAP is in use, it is possible to collect key streams corresponding to a given Request Authenticator value, by capturing RADIUS conversations corresponding to a PAP authentication attempt using a known password. Since the User-Password is known, the key stream corresponding to a given Request Authenticator can be determined and stored. Since the key stream may have been determined previously from a known plaintext attack, if the Request Authenticator repeats, attributes encrypted using the RADIUS attribute hiding mechanism should be considered compromised. In addition to the User-Password attribute, which is not used with EAP, this includes attributes such as Tunnel- Password [RFC2868, section 3.5] and MS-MPPE-Send-Key and MS-MPPE-Recv- Key attributes [RFC2548, section 2.4]. Even though EAP does not support PAP authentication, a security vulnerability can still exist where the same RADIUS shared secret is used for hiding User-Password as well as other attributes. This can occur, for example, if the same RADIUS proxy handles authentication requests for both EAP and PAP. The threat can be mitigated by protecting RADIUS with IPsec ESP with non-null transform, as described in Section 4.2. Where RADIUS shared secrets are configured, the RADIUS shared secret used by a NAS supporting EAP MUST NOT be reused by a NAS supporting PAP authentication, since improper shared secret hygiene could lead to compromise of hidden attributes. 4.3.6. Replay attacks The RADIUS protocol provides only limited support for replay protection. RADIUS Access-Requests include liveness via the 128-bit Request authenticator. However, the Request Authenticator is not a replay counter. Since RADIUS servers may not maintain a cache of previous Aboba & Calhoun Informational [Page 22]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 Request Authenticators, the Request Authenticator does not provide replay protection. Since RADIUS accounting [RFC2866] does not include a nonce within the Request Authenticator, it does not guarantee liveness of the exchange at the protocol level, although an Event-Timestamp attribute may be included to protect against replay. Strong replay protection for RADIUS authentication and accounting can be provided by enabling IPsec replay protection with RADIUS, as described in Section 4.2. 4.3.7. Negotiation attacks In a negotiation attack, a rogue NAS, tunnel server, RADIUS proxy or RADIUS server causes the authenticating peer to choose a less secure authentication method so as to make it easier to obtain the user's password. For example, a session that would normally be authenticated with EAP would instead authenticated via CHAP or PAP; alternatively, a connection that would normally be authenticated via one EAP type occurs via a less secure EAP type, such as MD5. The threat posed by rogue devices, once thought to be remote, has gained currency given compromises of telephone company switching systems, such as those described in [Masters]. Protection against negotiation attacks requires the elimination of downward negotiations. This can be achieved by protecting the RADIUS exchange using IPsec as described in Section 4.2. Alternatively, where IPsec is not used, the vulnerability can be mitigated via implementation of per-connection policy on the part of the authenticating peer, and per-user policy on the part of the RADIUS server. For the authenticating peer, authentication policy should be set on a per- connection basis. Per-connection policy allows an authenticating peer to negotiate a strong EAP method when connecting to one service, while negotiating a weaker EAP method for another service. With per-connection policy, an authenticating peer will only attempt to negotiate EAP for a session in which EAP support is expected. As a result, there is a presumption that an authenticating peer selecting EAP requires that level of security. If it cannot be provided, it is likely that there is some kind of misconfiguration, or even that the authenticating peer is contacting the wrong server. Should the NAS not be able to negotiate EAP, or should the EAP-Request sent by the NAS be of a different EAP type than what is expected, the authenticating peer MUST disconnect. An authenticating peer expecting EAP to be negotiated for a session MUST NOT negotiate a weaker method, such as CHAP or PAP. In wireless networks, the service advertisement itself may be spoof- able, so that an attacker could fool the peer into negotiating an Aboba & Calhoun Informational [Page 23]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 authentication method suitable for a less secure network. For a NAS, it may not be possible to determine whether a peer is required to authenticate with EAP until the peer's identity is known. For example, for shared-uses NASes it is possible for one reseller to implement EAP while another does not. Alternatively, some peer might be authenticated locally by the NAS while other peers are authenticated via RADIUS. In such cases, if any peers of the NAS MUST do EAP, then the NAS MUST attempt to negotiate EAP for every session. This avoids forcing an EAP-capable client to support more than one authentication type, which could weaken security. If CHAP is negotiated, the NAS will pass the User-Name and CHAP- Password attributes to the RADIUS server in an Access-Request packet. If the user is not required to use EAP, then the RADIUS server will respond with an Access-Accept or Access-Reject packet as appropriate. However, if CHAP has been negotiated but EAP is required, the RADIUS server MUST respond with an Access-Reject, rather than an Access- Challenge/EAP-Message/EAP-Request packet. The authenticating peer MUST refuse to renegotiate authentication, even if the renegotiation is from CHAP to EAP. If EAP is negotiated but is not supported by the RADIUS proxy or server, then the server or proxy MUST respond with an Access-Reject. In these cases, a PPP NAS MUST send an LCP-Terminate and disconnect the user. This is the correct behavior since the authenticating peer is expecting EAP to be negotiated, and that expectation cannot be fulfilled. An EAP- capable authenticating peer MUST refuse to renegotiate the authentication protocol if EAP had initially been negotiated. Note that problems with a non-EAP capable RADIUS proxy could prove difficult to diagnose, since a user connecting from one location (with an EAP-capable proxy) might be able to successfully authenticate via EAP, while the same user connecting at another location (and encountering an EAP- incapable proxy) might be consistently disconnected. 4.3.8. Impersonation When RADIUS requests are forwarded by a proxy, the NAS-IP-Address attribute may not correspond to the source address. Since the NAS- Identifier attribute need not contain an FQDN, it also may not correspond to the source address, even indirectly. [RFC2865] Section 3 states: A RADIUS server MUST use the source IP address of the RADIUS UDP packet to decide which shared secret to use, so that RADIUS requests can be proxied. This implies that it is possible for a rogue NAS to forge NAS-IP-Address Aboba & Calhoun Informational [Page 24]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 or NAS-Identifier attributes within a RADIUS Access-Request in order to impersonate another NAS. Since the rogue NAS is authenticated by the RADIUS proxy or server purely based on the source address, other mechanisms are required to detect the forgery. In addition, it is possible for attributes such as the Called-Station-Id and calling- Station-Id to be forged as well. To address this vulnerability RADIUS proxies used with RADIUS/EAP SHOULD check whether the NAS-IP-Address attribute matches the source address of packets originating from the NAS. If the NAS-Identifier attribute is used instead, such a check may not be possible since the NAS-Identifier may not correspond to an FQDN, and therefore may not be resolvable to an IP address to be matched against the source address. Also, where a NAT exists between the RADIUS client and server, checking the NAS-IP-Address attribute may not be feasible. To allow verification of session parameters such as the Called-Station- Id and Calling-Station-Id, they can be sent by the EAP peer to the EAP server, and covered by an EAP methode-specific message integrity check. The RADIUS server can then check the parameters sent by the EAP client against those claimed by the NAS. If a discrepancy is found, an error can be logged. 4.3.9. Man in the middle attacks Since RADIUS security is based on shared secrets, end-to-end security is not provided in the case where authentication or accounting packets are forwarded along a proxy chain. As a result, attackers gaining control of a RADIUS proxy will be able to modify EAP packets in transit. This is the case even where IPsec is used to protect RADIUS. 4.3.10. Separation of EAP server and authenticator It is possible for the EAP peer and authenticator to mutually authenticate, and derive a Master Session Key (MSK) for a ciphersuite used to protect subsequent data traffic. This does not present an issue on the peer, since the peer and EAP client reside on the same machine; all that is required is for the EAP client module to derive and pass a Transient Session Key (TSK) to the ciphersuite module. The situation is more complex when EAP is used with RADIUS, since the authenticator will typically not reside on the same machine as the EAP server. For example, the EAP server may be a security server, or a module residing on the RADIUS server. In the case where the EAP server and authenticator reside on different machines, there are several implications for security. First, mutual authentication will occur between the peer and the EAP server, not Aboba & Calhoun Informational [Page 25]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 between the peer and the authenticator. This means that it is not possible for the peer to validate the identity of the NAS or tunnel server that it is speaking to, using EAP alone. As described in Section 4, when EAP/RADIUS is used to encapsulate EAP packets, IPsec SHOULD be used to provide per-packet authentication, integrity, replay protection and confidentiality. The Message- Authenticator attribute is also required in EAP/RADIUS Access-Requests sent from the NAS or tunnel server to the RADIUS server. Since the Message-Authenticator attribute involves a HMAC-MD5 message integrity check, it is possible for the RADIUS server to verify the integrity of the Access-Request as well as the NAS or tunnel server's identity, even where IPsec is not used. Similarly, Access-Challenge packets sent from the RADIUS server to the NAS are also authenticated and integrity protected using an HMAC-MD5 message integrity check, enabling the NAS or tunnel server to determine the integrity of the packet and verify the identity of the RADIUS server, even where IPsec is not used. Moreover, EAP packets sent using methods that contain their own integrity protection cannot be successfully modified by a rogue NAS or tunnel server. The second issue that arises in the case of an EAP server and authenticator residing on different machines is that the EAP Master Session Key (MSK) negotiated between the peer and EAP server will need to be transmitted to the authenticator. Therefore a mechanism needs to be provided to transmit the MSK from the EAP server to the authenticator or tunnel server that needs it. The specification of the key transport and wrapping mechanism is outside the scope of this document. 4.3.11. Multiple databases In many cases a security server will be deployed along with a RADIUS server in order to provide EAP services. Unless the security server also functions as a RADIUS server, two separate user databases will exist, each containing information about the security requirements for the user. This represents a weakness, since security may be compromised by a successful attack on either of the servers, or their databases. With multiple user databases, adding a new user may require multiple operations, increasing the chances for error. The problems are further magnified in the case where user information is also being kept in an LDAP server. In this case, three stores of user information may exist. In order to address these threats, consolidation of databases is recommended. This can be achieved by having both the RADIUS server and security server store information in the same database; by having the security server provide a full RADIUS implementation; or by consolidating both the security server and the RADIUS server onto the same machine. Aboba & Calhoun Informational [Page 26]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 5. Normative references [RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997.] [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998. [RFC2284] Blunk, L., and J. Vollbrecht, "Extensible Authentication Protocol (EAP)", RFC 2284, March 1998. [RFC2401] Atkinson, R., Kent, S., "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [RFC2406] Kent, S., Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998 [RFC2409] Harkins, D., Carrel, D., "The Internet Key Exchange (IKE)", RFC 2409, November 1998 [RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000. [IEEE802] IEEE Standards for Local and Metropolitan Area Networks: Overview and Architecture, ANSI/IEEE Std 802, 1990. [IEEE8021X] IEEE Standards for Local and Metropolitan Area Networks: Port based Network Access Control, IEEE Std 802.1X-2001, June 2001. 6. Informative references [RFC1510] Kohl, J., Neuman, C., "The Kerberos Network Authentication Service (V5)", RFC 1510, September 1993. [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC2486] Beadles, M., Aboba, B., "The Network Access Identifier", RFC 2486, January 1999. [RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", RFC 2548, March 1999. Aboba & Calhoun Informational [Page 27]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 [RFC2716] Aboba, B., Simon, D.,"PPP EAP TLS Authentication Protocol", RFC 2716, October 1999. [RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000. [RFC2868] Zorn, G. et. al, "RADIUS Attributes for Tunnel Protocol Support", RFC 2868, June 2000. [RFC2983] Black, D. "Differentiated Services and Tunnels", RFC 2983, October 2000. [RFC3162] Aboba, B., Zorn, G., Mitton, D., "RADIUS and IP6", RFC 3162, August 2001. [Masters] Slatalla, M., and Quittner, J., "Masters of Deception." HarperCollins, New York, 1995. [MD5Attack] Dobbertin, H., "The Status of MD5 After a Recent Attack", CryptoBytes Vol.2 No.2, Summer 1996. Aboba & Calhoun Informational [Page 28]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 Appendix A - Examples The examples below illustrate conversations between an authenticating peer, NAS, and RADIUS server. The OTP and EAP-TLS protocols are used only for illustrative purposes; other authentication protocols could also have been used, although they might show somewhat different behavior. Where the NAS sends an EAP-Request/Identity as the initial packet, the exchange appears as follows: Authenticating peer NAS RADIUS server ------------------- --- ------------- <- EAP-Request/ Identity EAP-Response/ Identity (MyID) -> RADIUS Access-Request/ EAP-Message/EAP-Response/ (MyID) -> <- RADIUS Access-Challenge/ EAP-Message/EAP-Request OTP/OTP Challenge <- EAP-Request/ OTP/OTP Challenge EAP-Response/ OTP, OTPpw -> RADIUS Access-Request/ EAP-Message/EAP-Response/ OTP, OTPpw -> <- RADIUS Access-Accept/ EAP-Message/EAP-Success (other attributes) <- EAP-Success Aboba & Calhoun Informational [Page 29]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 In the case where the NAS initiates with an EAP-Request for EAP TLS [RFC2716], and the identity is determined based on the contents of the client certificate, the exchange will appear as follows: Authenticating peer NAS RADIUS server ------------------- --- ------------- <- EAP-Request/ EAP-Type=EAP-TLS (TLS Start, S bit set) EAP-Response/ EAP-Type=EAP-TLS (TLS client_hello)-> RADIUS Access-Request/ EAP-Message/EAP-Response/ EAP-Type=EAP-TLS-> <-RADIUS Access-Challenge/ EAP-Message/ EAP-Request/ EAP-Type=EAP-TLS <- EAP-Request/ EAP-Type=EAP-TLS (TLS server_hello, TLS certificate, [TLS server_key_exchange,] [TLS certificate_request,] TLS server_hello_done) EAP-Response/ EAP-Type=EAP-TLS (TLS certificate, TLS client_key_exchange, [TLS certificate_verify,] TLS change_cipher_spec, TLS finished)-> RADIUS Access-Request/ EAP-Message/EAP-Response/ EAP-Type=EAP-TLS-> <-RADIUS Access-Challenge/ EAP-Message/ EAP-Request/ EAP-Type=EAP-TLS <- EAP-Request/ EAP-Type=EAP-TLS (TLS change_cipher_spec, TLS finished) EAP-Response/ EAP-Type=EAP-TLS -> RADIUS Access-Request/ EAP-Message/EAP-Response/ Aboba & Calhoun Informational [Page 30]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 EAP-Type=EAP-TLS-> <-RADIUS Access-Accept/ EAP-Message/ EAP-Request/ EAP-Success (other attributes) <- EAP-Success In the case where the NAS first sends an EAP-Start packet to the RADIUS server, the conversation would appear as follows: Authenticating peer NAS RADIUS server ------------------- --- ------------- RADIUS Access-Request/ EAP-Message/Start -> <- RADIUS Access-Challenge/ EAP-Message/Identity <- EAP-Request/ Identity EAP-Response/ Identity (MyID) -> RADIUS Access-Request/ EAP-Message/EAP-Response/ (MyID) -> <- RADIUS Access-Challenge/ EAP-Message/EAP-Request OTP/OTP Challenge <- EAP-Request/ OTP/OTP Challenge EAP-Response/ OTP, OTPpw -> RADIUS Access-Request/ EAP-Message/EAP-Response/ OTP, OTPpw -> <- RADIUS Access-Accept/ EAP-Message/EAP-Success (other attributes) <- EAP-Success Aboba & Calhoun Informational [Page 31]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 In the case where the NAS initiates with an EAP-Request for EAP TLS [RFC2716], but the peer responds with a Nak, indicating that it would prefer another method not implemented locally on the NAS, the exchange will appear as follows: Authenticating peer NAS RADIUS server ------------------- --- ------------- <- EAP-Request/ EAP-Type=EAP-TLS (TLS Start, S bit set) EAP-Response/ EAP-Type=Nak (Alternative(s))-> RADIUS Access-Request/ EAP-Message/Start -> <- RADIUS Access-Challenge/ EAP-Message/Identity <- EAP-Request/ Identity EAP-Response/ Identity (MyID) -> RADIUS Access-Request/ EAP-Message/EAP-Response/ (MyID) -> <- RADIUS Access-Challenge/ EAP-Message/EAP-Request OTP/OTP Challenge <- EAP-Request/ OTP/OTP Challenge EAP-Response/ OTP, OTPpw -> RADIUS Access-Request/ EAP-Message/EAP-Response/ OTP, OTPpw -> <- RADIUS Access-Accept/ EAP-Message/EAP-Success (other attributes) <- EAP-Success Aboba & Calhoun Informational [Page 32]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 In the case where the authenticating peer attempts to authenticate the NAS, the conversation would appear as follows: Authenticating Peer NAS RADIUS Server ------------------- --- ------------- EAP-Request/ Challenge, MD5 -> RADIUS Access-Request/ EAP-Message/EAP-Request/ Challenge, MD5 -> <- RADIUS Access-Reject/ EAP-Message/ EAP-Response/ Nak (no alternative) <- EAP-Response/Nak (no alternative) EAP-Failure -> In the case where an invalid EAP Response is inserted by an attacker, the conversation would appear as follows: Authenticating peer NAS RADIUS server ------------------- --- ------------- <- EAP-Request/ EAP-Type=Foo EAP-Response/ EAP-Type=Foo -> RADIUS Access-Request/ EAP-Message/EAP-Response/ EAP-Type=Foo -> <- RADIUS Access-Challenge/ EAP-Message/EAP-Request/ EAP-Type=Foo <- EAP-Request/ EAP-Type=Foo Attacker spoof: EAP-Response/ EAP-Type=Foo -> RADIUS Access-Request/ EAP-Message/EAP-Response/ EAP-Type=Foo -> <- RADIUS Access-Challenge/ EAP-Message/Start EAP-Response/ Aboba & Calhoun Informational [Page 33]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 EAP-Type=Foo -> RADIUS Access-Request/ EAP-Message/EAP-Response/ EAP-Type=Foo -> Access-Accept/ EAP-Message/Success <- EAP Success In the case where the client fails EAP authentication, and an error message is sent prior to disconnection, the conversation would appear as follows: Authenticating peer NAS RADIUS server ------------------- --- ------------- RADIUS Access-Request/ EAP-Message/Start -> <- RADIUS Access-Challenge/ EAP-Message/Identity <- EAP-Request/ Identity EAP-Response/ Identity (MyID) -> RADIUS Access-Request/ EAP-Message/EAP-Response/ (MyID) -> <- RADIUS Access-Challenge/ EAP-Message/EAP-Request OTP/OTP Challenge <- EAP-Request/ OTP/OTP Challenge EAP-Response/ OTP, OTPpw -> RADIUS Access-Request/ EAP-Message/EAP-Response/ OTP, OTPpw -> <- RADIUS Access-Challenge/ EAP-Message/EAP-Request/ Notification <- EAP-Request/ Notification EAP-Response/ Notification -> RADIUS Access-Request/ EAP-Message/EAP-Response/ Notification -> Aboba & Calhoun Informational [Page 34]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 <- RADIUS Access-Reject/ EAP-Message/EAP-Failure <- EAP-Failure (client disconnected) In the case that the RADIUS server or proxy does not support EAP- Message, but no error message is sent, the conversation would appear as follows: Authenticating peer NAS RADIUS server ------------------- --- ------------- RADIUS Access-Request/ EAP-Message/Start -> <- RADIUS Access-Reject (User Disconnected) In the case where the local RADIUS server does support EAP-Message, but the remote RADIUS server does not, the conversation would appear as follows: Authenticating peer NAS RADIUS server ------------------- --- ------------- RADIUS Access-Request/ EAP-Message/Start -> <- RADIUS Access-Challenge/ EAP-Message/Identity <- EAP-Request/ Identity EAP-Response/ Identity (MyID) -> RADIUS Access-Request/ EAP-Message/EAP-Response/ (MyID) -> <- RADIUS Access-Reject (proxied from remote RADIUS server) (User Disconnected) In the case where PPP is the link and the authenticating peer does not support EAP, but where EAP is required for that user, the conversation would appear as follows: Aboba & Calhoun Informational [Page 35]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 Authenticating peer NAS RADIUS server ------------------- --- ------------- <- PPP LCP Request-EAP auth PPP LCP NAK-EAP auth -> <- PPP LCP Request-CHAP auth PPP LCP ACK-CHAP auth -> <- PPP CHAP Challenge PPP CHAP Response -> RADIUS Access-Request/ User-Name, CHAP-Password -> <- RADIUS Access-Reject <- PPP LCP Terminate (User Disconnected) In the case where PPP is the link, the NAS does not support EAP, but where EAP is required for that user, the conversation would appear as follows: Authenticating peer NAS RADIUS server ------------------- --- ------------- <- PPP LCP Request-CHAP auth PP LCP ACK-CHAP auth -> <- PPP CHAP Challenge PPP CHAP Response -> RADIUS Access-Request/ User-Name, CHAP-Password -> <- RADIUS Access-Reject <- PPP LCP Terminate (User Disconnected) Aboba & Calhoun Informational [Page 36]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 Appendix B - Change log The following changes have been made from RFC 2869: A NAS may simultaneously support both local authentication and pass- through; once the NAS enters pass-through mode within a session, it cannot revert back to local authentication (Section 2). The NAS may initiate with an EAP-Request for an authentication Type. If the Request is NAK'd, the NAS may send an initial Access-Request with an EAP-Message attribute containing an EAP-Response/Nak. In such a packet, an EAP-Start must not also be included (Section 2.1) Role reversal is not supported (Section 2.2). The Password-Retry (Section 2.2) and Reply-Message (2.7.3) attributes are deprecated. The RADIUS server is now permitted to treat an invalid EAP Response as a non-fatal error (Section 2.5) Message combinations (e.g. Access-Accept/EAP-Failure) that conflict are discouraged (Section 2.7.1). EAP-Message attributes are processed last (Section 2.7.2). Only a single EAP packet may be encapsulated within a RADIUS message (Section 2.7.4). IPsec ESP with non-null transform SHOULD be used and the usage model is described in detail (Section 4.2). Additional discussion of security vulnerabilities (Section 4.1) and potential fixes (Section 4.3). Separated normative (Section 5) and informative (Section 6) references. Added additional examples (Appendix A): the NAS initiating with an EAP- Request for an authentication Type; attempted role reversal. Aboba & Calhoun Informational [Page 37]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 Acknowledgments Thanks to Dave Dawson and Karl Fox of Ascend, Glen Zorn of Cisco Systems, Jari Arkko of Ericsson and Ashwin Palekar, Tim Moore and Narendra Gidwani of Microsoft for useful discussions of this problem space. The authors would also like to acknowledge Tony Jeffree, Chair of IEEE 802.1 for his assistance in resolving RADIUS/EAP issues in IEEE 802.1X. Author's Addresses Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: +1 425 706 6605 Fax: +1 425 936 7329 EMail: bernarda@microsoft.com Pat R. Calhoun Black Storm Networks 250 Cambridge Avenue, Suite 200 Palo Alto, California, 94306 USA Phone: +1 650-617-2932 Fax: +1 650-786-6445 E-mail: pcalhoun@bstormnetworks.com Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards- related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights Aboba & Calhoun Informational [Page 38]
INTERNET-DRAFT RADIUS & EAP 9 February 2003 which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS 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." Open issues Open issues relating to this specification are tracked on the following web site: http://www.drizzle.com/~aboba/EAP/eapissues.html Expiration Date This memo is filed as <draft-aboba-radius-rfc2869bis-09.txt>, and expires September 24, 2003. Aboba & Calhoun Informational [Page 39]
