Network Working Group C. Rigney Request for Comments: 2869 Livingston Category: Informational W. Willats Cyno Technologies P. Calhoun Sun Microsystems June 2000 RADIUS Extensions Status of this Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved.
AbstractThis document describes additional attributes for carrying authentication, authorization and accounting information between a Network Access Server (NAS) and a shared Accounting Server using the Remote Authentication Dial In User Service (RADIUS) protocol described in RFC 2865  and RFC 2866 . 1. Introduction .......................................... 2 1.1 Specification of Requirements ................... 3 1.2 Terminology ..................................... 3 2. Operation ............................................. 4 2.1 RADIUS support for Interim Accounting Updates.... 4 2.2 RADIUS support for Apple Remote Access Protocol ........................................ 5 2.3 RADIUS Support for Extensible Authentication Protocol (EAP) .................................. 11 2.3.1 Protocol Overview ............................... 11 2.3.2 Retransmission .................................. 13 2.3.3 Fragmentation ................................... 14 2.3.4 Examples ........................................ 14 2.3.5 Alternative uses ................................ 19 3. Packet Format ......................................... 19 4. Packet Types .......................................... 19 5. Attributes ............................................ 20
5.1 Acct-Input-Gigawords ............................ 22 5.2 Acct-Output-Gigawords ........................... 23 5.3 Event-Timestamp ................................. 23 5.4 ARAP-Password ................................... 24 5.5 ARAP-Features ................................... 25 5.6 ARAP-Zone-Access ................................ 26 5.7 ARAP-Security ................................... 27 5.8 ARAP-Security-Data .............................. 28 5.9 Password-Retry .................................. 28 5.10 Prompt .......................................... 29 5.11 Connect-Info .................................... 30 5.12 Configuration-Token ............................. 31 5.13 EAP-Message ..................................... 32 5.14 Message-Authenticator ........................... 33 5.15 ARAP-Challenge-Response ......................... 35 5.16 Acct-Interim-Interval ........................... 36 5.17 NAS-Port-Id ..................................... 37 5.18 Framed-Pool ..................................... 37 5.19 Table of Attributes ............................. 38 6. IANA Considerations ................................... 39 7. Security Considerations ............................... 39 7.1 Message-Authenticator Security .................. 39 7.2 EAP Security .................................... 39 7.2.1 Separation of EAP server and PPP authenticator .. 40 7.2.2 Connection hijacking ............................ 41 7.2.3 Man in the middle attacks ....................... 41 7.2.4 Multiple databases .............................. 41 7.2.5 Negotiation attacks ............................. 42 8. References ............................................ 43 9. Acknowledgements ...................................... 44 10. Chair's Address ....................................... 44 11. Authors' Addresses .................................... 45 12. Full Copyright Statement .............................. 47 RFC 2865  describes the RADIUS Protocol as it is implemented and deployed today, and RFC 2866  describes how Accounting can be performed with RADIUS.
This memo suggests several additional Attributes that can be added to RADIUS to perform various useful functions. These Attributes do not have extensive field experience yet and should therefore be considered experimental. The Extensible Authentication Protocol (EAP)  is a PPP extension that provides support for additional authentication methods within PPP. This memo describes how the EAP-Message and Message- Authenticator attributes may be used for providing EAP support within RADIUS. 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. RFC 2119 . An implementation is not compliant if it fails to satisfy one or more of the must or must not requirements for the protocols it implements. An implementation that satisfies all the must, must not, should and should not requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the must and must not requirements but not all the should or should not requirements for its protocols is said to be "conditionally compliant." A NAS that does not implement a given service MUST NOT implement the RADIUS attributes for that service. For example, a NAS that is unable to offer ARAP service MUST NOT implement the RADIUS attributes for ARAP. A NAS MUST treat a RADIUS access-request requesting an unavailable service as an access-reject instead.
is ended. A user may have multiple sessions in parallel or series if the NAS supports that, with each session generating a separate start and stop accounting record. 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. RFC 2865  and RFC 2866 .
A NAS MAY use a fudge factor to add a random delay between Interim Accounting messages for separate sessions. This will ensure that a cycle where all messages are sent at once is prevented, such as might otherwise occur if a primary link was recently restored and many dial-up users were directed to the same NAS at once. The Network and NAS CPU load of using Interim Updates should be carefully considered, and appropriate values of Acct-Interim-Interval chosen.
an Access-Accept may be sent down to the user as a sign-on message of the day string using the out-of-band channel. We have tried to respect the spirit of the existing RADIUS protocol as much as possible, making design decisions compatible with prior art. Further, we have tried to strike a balance between flooding the RADIUS world with new attributes, and hiding all of ARAP operation within a single multiplexed ARAP attribute string or within Extended Authentication Protocol (EAP)  machinery. However, we feel ARAP is enough of a departure from PPP to warrant a small set of similarly named attributes of its own. We have assumed that an ARAP-aware RADIUS server will be able to do DES encryption and generate security module challenges. This is in keeping with the general RADIUS goal of smart server / simple NAS. ARAP authenticates a connection in two phases. The first is a "Two- Way DES" random number exchange, using the user's password as a key. We say "Two-Way" because the ARAP NAS challenges the dial-in client to authenticate itself, and the dial-in client challenges the ARAP NAS to authenticate itself. Specifically, ARAP does the following: 1. The NAS sends two 32-bit random numbers to the dial-in client in an ARAP msg_auth_challenge packet. 2. The dial-in client uses the user's password to DES encrypt the two random numbers sent to it by the NAS. The dial-in client then sends this result, the user's name and two 32-bit random numbers of its own back to the NAS in an ARAP msg_auth_request packet. 3. The NAS verifies the encrypted random numbers sent by the dial-in client are what it expected. If so, it encrypts the dial-in client's challenge using the password and sends it back to the dial-in client in an ARAP msg_auth_response packet. Note that if the dial-in client's response was wrong, meaning the user has the wrong password, the server can initiate a retry sequence up to the maximum amount of retries allowed by the NAS. In this case, when the dial-in client receives the ARAP msg_auth_response packet it will acknowledge it with an ARAP msg_auth_again packet. After this first "DES Phase" the ARAP NAS MAY initiate a secondary authentication phase using what Apple calls "Add-In Security Modules." Security Modules are small pieces of code which run on
both the client and server and are allowed to read and write arbitrary data across the communications link to perform additional authentication functions. Various security token vendors use this mechanism to authenticate ARA callers. Although ARAP allows security modules to read and write anything they like, all existing security modules use simple challenge and response cycles, with perhaps some overall control information. This document assumes all existing security modules can be supported with one or more challenge/response cycles. To complicate RADIUS and ARAP integration, ARAP sends down some profile information after the DES Phase and before the Security Module phase. This means that besides the responses to challenges, this profile information must also be present, at somewhat unusual times. Fortunately the information is only a few pieces of numeric data related to passwords, which this document packs into a single new attribute. Presenting an Access-Request to RADIUS on behalf of an ARAP connection is straightforward. The ARAP NAS generates the random number challenge, and then receives the dial-in client's response, the dial-in client's challenge, and the user's name. Assuming the user is not a guest, the following information is forwarded in an Access-Request packet: User-Name (up to 31 characters long), Framed-Protocol (set to 3, ARAP), ARAP-Password, and any additional attributes desired, such as Service-Type, NAS-IP-Address, NAS-Id, NAS-Port-Type, NAS-Port, NAS-Port-Id, Connect-Info, etc. The Request Authenticator is a NAS-generated 16 octet random number. The low-order 8 octets of this number are sent to the dial-in user as the two 4 octet random numbers required in the ARAP msg_auth_challenge packet. Octets 0-3 are the first random number and Octets 4-7 are the second random number. The ARAP-Password in the Access-Request contains a 16 octet random number field, and is used to carry the dial-in user's response to the NAS challenge and the client's own challenge to the NAS. The high- order octets contain the dial-in user's challenge to the NAS (2 32- bit numbers, 8 octets) and the low-order octets contain the dial-in user's response to the NAS challenge (2 32-bit numbers, 8 octets). Only one of User-Password, CHAP-Password, or ARAP-Password needs to be present in an Access-Request, or one or more EAP-Messages. If the RADIUS server does not support ARAP it SHOULD return an Access-Reject to the NAS.
If the RADIUS server does support ARAP, it should verify the user's response using the Challenge (from the lower order 8 octets of the Request Authenticator) and the user's response (from the low order 8 octets of the ARAP-Password). If that authentication fails, the RADIUS server should return an Access-Reject packet to the NAS, with optional Password-Retry and Reply-Messages attributes. The presence of Password-Retry indicates the ARAP NAS MAY choose to initiate another challenge-response cycle, up to a total number of times equal to the integer value of the Password-Retry attribute. If the user is authenticated, the RADIUS server should return an Access-Accept packet (Code 2) to the NAS, with ID and Response Authenticator as usual, and attributes as follows: Service-Type of Framed-Protocol. Framed-Protocol of ARAP (3). Session-Timeout with the maximum connect time for the user in seconds. If the user is to be given unlimited time, Session-Timeout should not be included in the Access-Accept packet, and ARAP will treat that as an unlimited timeout (-1). ARAP-Challenge-Response, containing 8 octets with the response to the dial-in client's challenge. The RADIUS server calculates this value by taking the dial-in client's challenge from the high order 8 octets of the ARAP-Password attribute and performing DES encryption on this value with the authenticating user's password as the key. If the user's password is less than 8 octets in length, the password is padded at the end with NULL octets to a length of 8 before using it as a key. If the user's password is greater than 8 octets in length, an Access-Reject MUST be sent instead. ARAP-Features, containing information that the NAS should send to the user in an ARAP "feature flags" packet. Octet 0: If zero, user cannot change their password. If non- zero user can. (RADIUS does not handle the password changing, just the attribute which indicates whether ARAP indicates they can.) Octet 1: Minimum acceptable password length (0-8).
Octet 2-5: Password creation date in Macintosh format, defined as 32 bits unsigned representing seconds since Midnight GMT January 1, 1904. Octet 6-9 Password Expiration Delta from create date in seconds. Octet 10-13: Current RADIUS time in Macintosh format Optionally, a single Reply-Message with a text string up to 253 characters long which MAY be sent down to the user to be displayed in a sign-on/message of the day dialog. Framed-AppleTalk-Network may be included. Framed-AppleTalk-Zone, up to 32 characters in length, may be included. ARAP defines the notion of a list of zones for a user. Along with a list of zone names, a Zone Access Flag is defined (and used by the NAS) which says how to use the list of zone names. That is, the dial-in user may only be allowed to see the Default Zone, or only the zones in the zone list (inclusive) or any zone except those in the zone list (exclusive). The ARAP NAS handles this by having a named filter which contains (at least) zone names. This solves the problem where a single RADIUS server is managing disparate NAS clients who may not be able to "see" all of the zone names in a user zone list. Zone names only have meaning "at the NAS." The disadvantage of this approach is that zone filters must be set up on the NAS somehow, then referenced by the RADIUS Filter-Id. ARAP-Zone-Access contains an integer which specifies how the "zone list" for this user should be used. If this attribute is present and the value is 2 or 4 then a Filter-Id must also be present to name a zone list filter to apply the access flag to. The inclusion of a Callback-Number or Callback-Id attribute in the Access-Accept MAY cause the ARAP NAS to disconnect after sending the Feature Flags to begin callback processing in an ARAP specific way.
Other attributes may be present in the Access-Accept packet as well. An ARAP NAS will need other information to finish bringing up the connection to the dial in client, but this information can be provided by the ARAP NAS without any help from RADIUS, either through configuration by SNMP, a NAS administration program, or deduced by the AppleTalk stack in the NAS. Specifically: 1. AppearAsNet and AppearAsNode values, sent to the client to tell it what network and node numbers it should use in its datagram packets. AppearAsNet can be taken from the Framed-AppleTalk- Network attribute or from the configuration or AppleTalk stack onthe NAS. 2. The "default" zone - that is the name of the AppleTalk zone in which the dial-in client will appear. (Or can be specified with the Framed-AppleTalk-Zone attribute.) 3. Other very NAS specific stuff such as the name of the NAS, and smartbuffering information. (Smartbuffering is an ARAP mechanism for replacing common AppleTalk datagrams with small tokens, to improve slow link performance in a few common traffic situations.) 4. "Zone List" information for this user. The ARAP specification defines a "zone count" field which is actually unused. RADIUS supports ARAP Security Modules in the following manner. After DES authentication has been completed, the RADIUS server may instruct the ARAP NAS to run one or more security modules for the dial-in user. Although the underlying protocol supports executing multiple security modules in series, in practice all current implementations only allow executing one. Through the use of multiple Access-Challenge requests, multiple modules can be supported, but this facility will probably never be used. We also assume that, even though ARAP allows a free-form dialog between security modules on each end of the point-to-point link, in actual practice all security modules can be reduced to a simple challenge/response cycle. If the RADIUS server wishes to instruct the ARAP NAS to run a security module, it should send an Access-Challenge packet to the NAS with (optionally) the State attribute, plus the ARAP-Challenge- Response, ARAP-Features, and two more attributes:
ARAP-Security: a four octet security module signature, containing a Macintosh OSType. ARAP-Security-Data, a string to carry the actual security module challenge and response. When the security module finishes executing, the security module response is passed in an ARAP-Security-Data attribute from the NAS to the RADIUS server in a second Access-Request, also including the State from the Access-Challenge. The authenticator field contains no special information in this case, and this can be discerned by the presence of the State attribute. 3], provides a standard mechanism for support of additional authentication methods within PPP. Through the use of EAP, support for a number of authentication schemes may be added, including smart cards, Kerberos, Public Key, One Time Passwords, and others. In order to provide for support of 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. In the proposed scheme, the RADIUS server is used to shuttle RADIUS- encapsulated EAP Packets between the NAS and a backend security server. While the conversation between the RADIUS server and the backend security server will typically occur using a proprietary protocol developed by the backend security server vendor, it is also possible to use RADIUS-encapsulated EAP via the EAP-Message attribute. This has the advantage of allowing the RADIUS server to support EAP without the need for authentication-specific code, which can instead reside on the backend security server.
In order to permit non-EAP aware RADIUS proxies to forward the Access-Request packet, if the NAS sends the EAP-Request/Identity, the NAS MUST copy the contents of the EAP-Response/Identity into the User-Name attribute and MUST include the EAP-Response/Identity in the User-Name attribute in every subsequent Access-Request. NAS-Port or NAS-Port-Id SHOULD be included in the attributes issued by the NAS in the Access-Request packet, and either NAS-Identifier or NAS-IP- Address 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-Accept packets. Without the User-Name attribute, accounting and billing becomes very difficult to manage. If identity is determined via another means such as Called-Station-Id or Calling-Station-Id, the NAS MUST include these identifying attributes in every Access-Request. While this approach will save a round-trip, it cannot be universally employed. There are circumstances in which the user's identity may not be needed (such as when authentication and accounting is handled based on Called-Station-Id or Calling-Station-Id), and therefore an EAP-Request/Identity packet may not necessarily be issued by the NAS to the authenticating peer. In cases where an EAP-Request/Identity packet will not be sent, the NAS will send to the RADIUS server a RADIUS Access-Request packet containing an EAP-Message attribute signifying EAP-Start. EAP-Start is indicated by sending an EAP- Message attribute with a length of 2 (no data). However, it should be noted that since no User-Name attribute is included in the Access- Request, this approach is not compatible with RADIUS as specified in , nor can it easily be applied in situations where proxies are deployed, such as roaming or shared use networks. If the RADIUS server supports EAP, it MUST respond with an Access- Challenge packet containing an EAP-Message attribute. If the RADIUS server does not support EAP, it MUST respond with an Access-Reject. The EAP-Message attribute includes an encapsulated EAP packet which is then passed on to the authenticating peer. In the case where the NAS does not initially send an EAP-Request/Identity message to the peer, the Access-Challenge typically will contain an EAP-Message attribute encapsulating an EAP-Request/Identity message, requesting the dial-in user to identify themself. The NAS will then respond with a RADIUS Access-Request packet containing an EAP-Message attribute encapsulating an EAP-Response. The conversation continues until either a RADIUS Access-Reject or Access-Accept packet is received.
Reception of a RADIUS Access-Reject packet, with or without an EAP- Message attribute encapsulating EAP-Failure, MUST result in the NAS issuing an LCP Terminate Request to the authenticating peer. A RADIUS Access-Accept packet with an EAP-Message attribute encapsulating EAP-Success successfully ends the authentication phase. The RADIUS Access-Accept/EAP-Message/EAP-Success packet MUST contain all of the expected attributes which are currently returned in an Access-Accept packet. The above scenario creates a situation in which the NAS never needs to manipulate an EAP packet. An alternative may be used in situations where an EAP-Request/Identity message will always be sent by the NAS to the authenticating peer. For proxied RADIUS requests there are two methods of processing. If the domain is determined based on the 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. For proxied RADIUS requests, the NAS may receive an Access-Reject packet in response to its Access-Request/EAP-Identity packet. This would occur if the message was proxied to a RADIUS Server which does not support the EAP-Message extension. On receiving an Access-Reject, the NAS MUST send an LCP Terminate Request to the authenticating peer, and disconnect. 3], the EAP authenticator (NAS) is responsible for retransmission of packets between the authenticating peer and the NAS. Thus 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 , the RADIUS client is responsible for retransmission of packets between the RADIUS client and the RADIUS server. Note that 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 and Password-Retry attributes 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 provides the NAS with the maximum number of seconds the NAS should wait for an EAP-Response before retransmitting the EAP-Message to the dial-in user.
RADIUS Access-Request/ EAP-Message/ EAP-Response/ OTP, OTPpw -> <- RADIUS Access-Accept/ EAP-Message/EAP-Success (other attributes) <- PPP EAP-Success PPP Authentication Phase complete, NCP Phase starts 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 ------------------- --- ------------- <- PPP LCP Request-EAP auth PPP LCP ACK-EAP auth -> RADIUS Access-Request/ EAP-Message/Start -> <- RADIUS Access-Challenge/ EAP-Message/Identity <- PPP EA-Request/ Identity PPP EAP-Response/ Identity (MyID) -> RADIUS Access-Request/ EAP-Message/ EAP-Response/ (MyID) -> <- RADIUS Access-Challenge/ EAP-Message/EAP-Request OTP/OTP Challenge <- PPP EAP-Request/ OTP/OTP Challenge PPP EAP-Response/ OTP, OTPpw ->
RADIUS Access-Request/ EAP-Message/ EAP-Response/ OTP, OTPpw -> <- RADIUS Access-Accept/ EAP-Message/EAP-Success (other attributes) <- PPP EAP-Success PPP Authentication Phase complete, NCP Phase starts In the case where the client fails EAP authentication, the conversation would appear as follows: Authenticating Peer NAS RADIUS Server ------------------- --- ------------- <- PPP LCP Request-EAP auth PPP LCP ACK-EAP auth -> Access-Request/ EAP-Message/Start -> <- RADIUS Access-Challenge/ EAP-Message/Identity <- PPP EAP-Request/ Identity PPP EAP-Response/ Identity (MyID) -> RADIUS Access-Request/ EAP-Message/ EAP-Response/ (MyID) -> <- RADIUS Access-Challenge/ EAP-Message/EAP-Request OTP/OTP Challenge <- PPP EAP-Request/ OTP/OTP Challenge PPP EAP-Response/ OTP, OTPpw -> RADIUS Access-Request/
EAP-Message/ EAP-Response/ OTP, OTPpw -> <- RADIUS Access-Reject/ EAP-Message/EAP-Failure <- PPP EAP-Failure (client disconnected) In the case that the RADIUS server or proxy does not support EAP-Message, the conversation would appear as follows: Authenticating Peer NAS RADIUS Server ------------------- --- ------------- <- PPP LCP Request-EAP auth PPP LCP ACK-EAP auth -> RADIUS Access-Request/ EAP-Message/Start -> <- RADIUS Access-Reject <- PPP LCP Terminate (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 ------------------- --- ------------- <- PPP LCP Request-EAP auth PPP LCP ACK-EAP auth -> RADIUS Access-Request/ EAP-Message/Start -> <- RADIUS Access-Challenge/ EAP-Message/Identity <- PPP EAP-Request/ Identity
PPP EAP-Response/ Identity (MyID) -> RADIUS Access-Request/ EAP-Message/EAP-Response/ (MyID) -> <- RADIUS Access-Reject (proxied from remote RADIUS Server) <- PPP LCP Terminate (User Disconnected) In the case where the authenticating peer 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-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 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) RFC 2865  and 2866 . RFC 2865  and 2866 . See "Table of Attributes" below to determine which types of packets can contain which attributes defined here.
RFC 2865  but is included here for ease of reference. 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 | Value ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type The Type field is one octet. Up-to-date values of the RADIUS Type field are specified in the most recent "Assigned Numbers" RFC . Values 192-223 are reserved for experimental use, values 224-240 are reserved for implementation-specific use, and values 241-255 are reserved and should not be used. This specification concerns the following values: 1-39 (refer to RFC 2865 , "RADIUS") 40-51 (refer to RFC 2866 , "RADIUS Accounting") 52 Acct-Input-Gigawords 53 Acct-Output-Gigawords 54 Unused 55 Event-Timestamp 56-59 Unused 60-63 (refer to RFC 2865 , "RADIUS") 64-67 (refer to ) 68 (refer to ) 69 (refer to ) 70 ARAP-Password 71 ARAP-Features 72 ARAP-Zone-Access
73 ARAP-Security 74 ARAP-Security-Data 75 Password-Retry 76 Prompt 77 Connect-Info 78 Configuration-Token 79 EAP-Message 80 Message-Authenticator 81-83 (refer to ) 84 ARAP-Challenge-Response 85 Acct-Interim-Interval 86 (refer to ) 87 NAS-Port-Id 88 Framed-Pool 89 Unused 90-91 (refer to ) 92-191 Unused Length The Length field is one octet, and indicates the length of this attribute including the Type, Length and Value fields. If an attribute is received in a packet with an invalid Length, the entire request should be silently discarded. Value The Value field is zero or more octets and contains information specific to the attribute. The format and length of the Value field is determined by the Type and Length fields. Note that none of the types in RADIUS terminate with a NUL (hex 00). In particular, types "text" and "string" in RADIUS do not terminate with a NUL (hex 00). The Attribute has a length field and does not use a terminator. Text contains UTF-8 encoded 10646  characters and String contains 8-bit binary data. Servers and servers and clients MUST be able to deal with embedded nulls. RADIUS implementers using C are cautioned not to use strcpy() when handling strings. The format of the value field is one of five data types. Note that type "text" is a subset of type "string." text 1-253 octets containing UTF-8 encoded 10646  characters. Text of length zero (0) MUST NOT be sent; omit the entire attribute instead.
string 1-253 octets containing binary data (values 0 through 255 decimal, inclusive). Strings of length zero (0) MUST NOT be sent; omit the entire attribute instead. address 32 bit unsigned value, most significant octet first. integer 32 bit unsigned value, most significant octet first. time 32 bit unsigned value, most significant octet first -- seconds since 00:00:00 UTC, January 1, 1970.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Value +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Value (cont) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 55 for Event-Timestamp Length 6 Value The Value field is four octets encoding an unsigned integer with the number of seconds since January 1, 1970 00:00 UTC.
Type 70 for ARAP-Password. Length 18 Value This attribute contains a 16 octet string, used to carry the dial-in user's response to the NAS challenge and the client's own challenge to the NAS. The high-order octets (Value1 and Value2) contain the dial-in user's challenge to the NAS (2 32-bit numbers, 8 octets) and the low-order octets (Value3 and Value4) contain the dial-in user's response to the NAS challenge (2 32-bit numbers, 8 octets).
Value The Value field is a compound string containing information the NAS should send to the user in the ARAP "feature flags" packet. Value1: If zero, user cannot change their password. If non-zero user can. (RADIUS does not handle the password changing, just the attribute which indicates whether ARAP indicates they can.) Value2: Minimum acceptable password length, from 0 to 8. Value3: Password creation date in Macintosh format, defined as 32 unsigned bits representing seconds since Midnight GMT January 1, 1904. Value4: Password Expiration Delta from create date in seconds. Value5: Current RADIUS time in Macintosh format.
Value The Value field is four octets encoding an integer with one of the following values: 1 Only allow access to default zone 2 Use zone filter inclusively 4 Use zone filter exclusively The value 3 is skipped, not because these are bit flags, but because 3 in some ARAP implementations means "all zones" which is the same as not specifying a list at all under RADIUS. If this attribute is present and the value is 2 or 4 then a Filter-Id must also be present to name a zone list filter to apply the access flag to.
Value The Value field is four octets, containing an integer specifying the security module signature, which is a Macintosh OSType. (Macintosh OSTypes are 4 ascii characters cast as a 32-bit integer)
A summary of the Password-Retry attribute format is shown below. The fields are transmitted from left to right. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Value +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Value (cont) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 75 for Password-Retry. Length 6 Value The Value field is four octets, containing an integer specifying the number of password retry attempts to permit the user.
Length 6 Value The Value field is four octets. 0 No Echo 1 Echo 8] characters. The connection speed SHOULD be included at the beginning of the first Connect-Info attribute in the packet. If the transmit and receive connection speeds differ, they may both be included in the first attribute with the transmit speed first (the speed the NAS modem transmits at), a slash (/), the receive speed, then optionally other information.
For example, "28800 V42BIS/LAPM" or "52000/31200 V90" More than one Connect-Info attribute may be present in an Accounting-Request packet to accommodate expected efforts by ITU to have modems report more connection information in a standard format that might exceed 252 octets.
3] packets so as to allow the NAS to authenticate dial-in users via EAP without having to understand the EAP protocol. The NAS places any EAP messages received from the user into one or more EAP attributes and forwards them to the RADIUS Server as part of the Access-Request, which can return EAP messages in Access- Challenge, Access-Accept and Access-Reject packets. A RADIUS Server receiving EAP messages that it does not understand SHOULD return an Access-Reject. 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. Access-Accept and Access-Reject packets SHOULD only have ONE EAP-Message attribute in them, containing EAP-Success or EAP- Failure. 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 at least as strong as those used in the EAP methods themselves. 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 an EAP-Message attribute without a Message-Authenticator attribute SHOULD be silently discarded by the RADIUS server. A RADIUS Server supporting EAP-Message 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 EAP-Message 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 an EAP-Message attribute without a Message-Authenticator attribute SHOULD be silently discarded by the NAS. A NAS supporting EAP-Message 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 79 for EAP-Message. Length >= 3 String The String field contains EAP packets, as defined in . 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 passed by RADIUS.
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. Earlier drafts of this memo used "Signature" as the name of this attribute, but Message-Authenticator is more precise. Its operation has not changed, just the name. 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 18 String When present in an Access-Request packet, Message-Authenticator is an HMAC-MD5  checksum 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 checksum 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 checksum 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 hash. 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. IP Security will eventually make this attribute unnecessary, so it should be considered an interim measure.
Value The Value field contains an 8 octet response to the dial-in client's challenge. The RADIUS server calculates this value by taking the dial-in client's challenge from the high order 8 octets of the ARAP-Password attribute and performing DES encryption on this value with the authenticating user's password as the key. If the user's password is less than 8 octets in length, the password is padded at the end with NULL octets to a length of 8 before using it as a key.
A summary of the Framed-Pool 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 88 for Framed-Pool Length >= 3 String The string field contains the name of an assigned address pool configured on the NAS.
[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 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], in accordance with BCP 26 . 1].
1], only Access-Reply and Access-Challenge packets are integrity protected. Moreover, the integrity protection mechanism described in  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 EAP/RADIUS packets MUST be authenticated using the Message-Authenticator attribute, as described previously.
11]. Protection against negotiation attacks requires the elimination of downward negotiations. This can be achieved 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 EAP when calling one service, while negotiating CHAP for another service, even if both services are accessible via the same phone number. 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 CHAP or PAP. For a NAS, it may not be possible to determine whether a user is required to authenticate with EAP until the user's identity is known. For example, for shared-uses NASes it is possible for one reseller to implement EAP while another does not. In such cases, if any users of the NAS MUST do EAP, then the NAS MUST attempt to negotiate EAP for every call. This avoids forcing an EAP-capable client to do more than one authentication, which weakens 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, the 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 dialing in from one location (with an EAP-capable proxy) might be able to successfully authenticate via EAP, while the same user dialing into another location (and encountering an EAP-incapable proxy) might be consistently disconnected.  Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000.  Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.  Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication Protocol (EAP)", RFC 2284, March 1998.  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March, 1997.  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700, October 1994.  Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M. and I. Goyret, "RADIUS Attributes for Tunnel Protocol Support", RFC 2868, June 2000.  Zorn, G., Aboba, B. and D. Mitton, "RADIUS Accounting Modifications for Tunnel Protocol Support", RFC 2867, June 2000.  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998.
 Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997.  Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.  Slatalla, M., and Quittner, J., "Masters of Deception." HarperCollins, New York, 1995.
Questions on EAP and RADIUS may be directed to any of the following: Pat R. Calhoun Network and Security Research Center Sun Microsystems, Inc. 15 Network Circle Menlo Park, CA 94025 Phone: +1 650 786 7733 EMail: firstname.lastname@example.org Allan C. Rubens Tut Systems, Inc. 220 E. Huron, Suite 260 Ann Arbor, MI 48104 Phone: +1 734 995 1697 EMail: email@example.com Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: +1 425 936 6605 EMail: firstname.lastname@example.org
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