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rfc:rfc2058

Network Working Group C. Rigney Request for Comments: 2058 Livingston Category: Standards Track A. Rubens

                                                                 Merit
                                                            W. Simpson
                                                            Daydreamer
                                                            S. Willens
                                                            Livingston
                                                          January 1997
        Remote Authentication Dial In User Service (RADIUS)

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document describes a protocol for carrying authentication,
 authorization, and configuration information between a Network Access
 Server which desires to authenticate its links and a shared
 Authentication Server.

Table of Contents

 1.     Introduction ..........................................    3
    1.1       Specification of Requirements ...................    4
    1.2       Terminology .....................................    4
 2.     Operation .............................................    5
    2.1       Challenge/Response ..............................    6
    2.2       Interoperation with PAP and CHAP ................    7
    2.3       Why UDP? ........................................    8
 3.     Packet Format .........................................    9
 4.     Packet Types ..........................................   12
    4.1       Access-Request ..................................   12
    4.2       Access-Accept ...................................   14
    4.3       Access-Reject ...................................   15
    4.4       Access-Challenge ................................   16
 5.     Attributes ............................................   17
    5.1       User-Name .......................................   20
    5.2       User-Password ...................................   21
    5.3       CHAP-Password ...................................   22
    5.4       NAS-IP-Address ..................................   23

Rigney, et. al. Informational [Page 1] RFC 2058 RADIUS January 1997

    5.5       NAS-Port ........................................   24
    5.6       Service-Type ....................................   25
    5.7       Framed-Protocol .................................   27
    5.8       Framed-IP-Address ...............................   28
    5.9       Framed-IP-Netmask ...............................   29
    5.10      Framed-Routing ..................................   29
    5.11      Filter-Id .......................................   30
    5.12      Framed-MTU ......................................   31
    5.13      Framed-Compression ..............................   32
    5.14      Login-IP-Host ...................................   33
    5.15      Login-Service ...................................   33
    5.16      Login-TCP-Port ..................................   34
    5.17      (unassigned) ....................................   35
    5.18      Reply-Message ...................................   35
    5.19      Callback-Number .................................   36
    5.20      Callback-Id .....................................   37
    5.21      (unassigned) ....................................   37
    5.22      Framed-Route ....................................   38
    5.23      Framed-IPX-Network ..............................   39
    5.24      State ...........................................   39
    5.25      Class ...........................................   40
    5.26      Vendor-Specific .................................   41
    5.27      Session-Timeout .................................   43
    5.28      Idle-Timeout ....................................   44
    5.29      Termination-Action ..............................   44
    5.30      Called-Station-Id ...............................   45
    5.31      Calling-Station-Id ..............................   46
    5.32      NAS-Identifier ..................................   47
    5.33      Proxy-State .....................................   48
    5.34      Login-LAT-Service ...............................   49
    5.35      Login-LAT-Node ..................................   50
    5.36      Login-LAT-Group .................................   51
    5.37      Framed-AppleTalk-Link ...........................   52
    5.38      Framed-AppleTalk-Network ........................   53
    5.39      Framed-AppleTalk-Zone ...........................   53
    5.40      CHAP-Challenge ..................................   54
    5.41      NAS-Port-Type ...................................   55
    5.42      Port-Limit ......................................   56
    5.43      Login-LAT-Port ..................................   57
    5.44      Table of Attributes .............................   58
 6.     Examples ..............................................   59
    6.1       User Telnet to Specified Host ...................   59
    6.2       Framed User Authenticating with CHAP ............   60
    6.3       User with Challenge-Response card ...............   61
 SECURITY CONSIDERATIONS ......................................   62
 REFERENCES ...................................................   63
 ACKNOWLEDGEMENTS .............................................   63
 CHAIR'S ADDRESS ..............................................   64

Rigney, et. al. Informational [Page 2] RFC 2058 RADIUS January 1997

 AUTHORS' ADDRESSES ...........................................   64

1. Introduction

 Managing dispersed serial line and modem pools for large numbers of
 users can create the need for significant administrative support.
 Since modem pools are by definition a link to the outside world, they
 require careful attention to security, authorization and accounting.
 This can be best achieved by managing a single "database" of users,
 which allows for authentication (verifying user name and password) as
 well as configuration information detailing the type of service to
 deliver to the user (for example, SLIP, PPP, telnet, rlogin).
 Key features of RADIUS are:
 Client/Server Model
    A Network Access Server (NAS) operates as a client of RADIUS.  The
    client is responsible for passing user information to designated
    RADIUS servers, and then acting on the response which is returned.
    RADIUS servers are responsible for receiving user connection
    requests, authenticating the user, and then returning all
    configuration information necessary for the client to deliver
    service to the user.
    A RADIUS server can act as a proxy client to other RADIUS servers
    or other kinds of authentication servers.
 Network Security
    Transactions between the client and RADIUS server are
    authenticated through the use of a shared secret, which is never
    sent over the network.  In addition, any user passwords are sent
    encrypted between the client and RADIUS server, to eliminate the
    possibility that someone snooping on an unsecure network could
    determine a user's password.
 Flexible Authentication Mechanisms
    The RADIUS server can support a variety of methods to authenticate
    a user.  When it is provided with the user name and original
    password given by the user, it can support PPP PAP or CHAP, UNIX
    login, and other authentication mechanisms.

Rigney, et. al. Informational [Page 3] RFC 2058 RADIUS January 1997

 Extensible Protocol
    All transactions are comprised of variable length Attribute-
    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.
 MUST      This word, or the adjective "required", means that the
           definition is an absolute requirement of the specification.
 MUST NOT  This phrase means that the definition is an absolute
           prohibition of the specification.
 SHOULD    This word, or the adjective "recommended", means that there
           may exist valid reasons in particular circumstances to
           ignore this item, but the full implications must be
           understood and carefully weighed before choosing a
           different course.
 MAY       This word, or the adjective "optional", means that this
           item is one of an allowed set of alternatives.  An
           implementation which does not include this option MUST be
           prepared to interoperate with another implementation which
           does include the option.

1.2. Terminology

 This document frequently uses the following terms:
 service   The NAS provides a service to the dial-in user, such as PPP
           or Telnet.
 session   Each service provided by the NAS to a dial-in user
           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 user may have multiple sessions in parallel or
           series if the NAS supports that.
 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

Rigney, et. al. Informational [Page 4] RFC 2058 RADIUS January 1997

           in a statistics counter.

2. Operation

 When a client is configured to use RADIUS, any user of the client
 presents authentication information to the client.  This might be
 with a customizable login prompt, where the user is expected to enter
 their username and password.  Alternatively, the user might use a
 link framing protocol such as the Point-to-Point Protocol (PPP),
 which has authentication packets which carry this information.
 Once the client has obtained such information, it may choose to
 authenticate using RADIUS.  To do so, the client creates an "Access-
 Request" containing such Attributes as the user's name, the user's
 password, the ID of the client and the Port ID which the user is
 accessing.  When a password is present, it is hidden using a method
 based on the RSA Message Digest Algorithm MD5 [1].
 The Access-Request is submitted to the RADIUS server via the network.
 If no response is returned within a length of time, the request is
 re-sent a number of times.  The client can also forward requests to
 an alternate server or servers in the event that the primary server
 is down or unreachable.  An alternate server can be used either after
 a number of tries to the primary server fail, or in a round-robin
 fashion.  Retry and fallback algorithms are the topic of current
 research and are not specified in detail in this document.
 Once the RADIUS server receives the request, it validates the sending
 client.  A request from a client for which the RADIUS server does not
 have a shared secret should be silently discarded.  If the client is
 valid, the RADIUS server consults a database of users to find the
 user whose name matches the request.  The user entry in the database
 contains a list of requirements which must be met to allow access for
 the user.  This always includes verification of the password, but can
 also specify the client(s) or port(s) to which the user is allowed
 access.
 The RADIUS server MAY make requests of other servers in order to
 satisfy the request, in which case it acts as a client.
 If any condition is not met, the RADIUS server sends an "Access-
 Reject" response indicating that this user request is invalid.  If
 desired, the server MAY include a text message in the Access-Reject
 which MAY be displayed by the client to the user.  No other
 Attributes are permitted in an Access-Reject.
 If all conditions are met and the RADIUS server wishes to issue a
 challenge to which the user must respond, the RADIUS server sends an

Rigney, et. al. Informational [Page 5] RFC 2058 RADIUS January 1997

 "Access-Challenge" response.  It MAY include a text message to be
 displayed by the client to the user prompting for a response to the
 challenge, and MAY include a State attribute.  If the client receives
 an Access-Challenge and supports challenge/response it MAY display
 the text message, if any, to the user, and then prompt the user for a
 response.  The client then re-submits its original Access-Request
 with a new request ID, with the User-Password Attribute replaced by
 the response (encrypted), and including the State Attribute from the
 Access-Challenge, if any.  Only 0 or 1 instances of the State
 Attributes should be present in a request.  The server can respond to
 this new Access-Request with either an Access-Accept, an Access-
 Reject, or another Access-Challenge.
 If all conditions are met, the list of configuration values for the
 user are placed into an "Access-Accept" response.  These values
 include the type of service (for example: SLIP, PPP, Login User) and
 all necessary values to deliver the desired service.  For SLIP and
 PPP, this may include values such as IP address, subnet mask, MTU,
 desired compression, and desired packet filter identifiers.  For
 character mode users, this may include values such as desired
 protocol and host.

2.1. Challenge/Response

 In challenge/response authentication, the user is given an
 unpredictable number and challenged to encrypt it and give back the
 result. Authorized users are equipped with special devices such as
 smart cards or software that facilitate calculation of the correct
 response with ease. Unauthorized users, lacking the appropriate
 device or software and lacking knowledge of the secret key necessary
 to emulate such a device or software, can only guess at the response.
 The Access-Challenge packet typically contains a Reply-Message
 including a challenge to be displayed to the user, such as a numeric
 value unlikely ever to be repeated. Typically this is obtained from
 an external server that knows what type of authenticator should be in
 the possession of the authorized user and can therefore choose a
 random or non-repeating pseudorandom number of an appropriate radix
 and length.
 The user then enters the challenge into his device (or software) and
 it calculates a response, which the user enters into the client which
 forwards it to the RADIUS server via a second Access-Request.  If the
 response matches the expected response the RADIUS server replies with
 an Access-Accept, otherwise an Access-Reject.
 Example: The NAS sends an Access-Request packet to the RADIUS Server
 with NAS-Identifier, NAS-Port, User-Name, User-Password (which may

Rigney, et. al. Informational [Page 6] RFC 2058 RADIUS January 1997

 just be a fixed string like "challenge" or ignored).  The server
 sends back an Access-Challenge packet with State and a Reply-Message
 along the lines of "Challenge 12345678, enter your response at the
 prompt" which the NAS displays.  The NAS prompts for the response and
 sends a NEW Access-Request to the server (with a new ID) with NAS-
 Identifier, NAS-Port, User-Name, User-Password (the response just
 entered by the user, encrypted), and the same State Attribute that
 came with the Access-Challenge.  The server then sends back either an
 Access-Accept or Access-Reject based on whether the response matches
 what it should be, or it can even send another Access-Challenge.

2.2. Interoperation with PAP and CHAP

 For PAP, the NAS takes the PAP ID and password and sends them in an
 Access-Request packet as the User-Name and User-Password. The NAS MAY
 include the Attributes Service-Type = Framed-User and Framed-Protocol
 = PPP as a hint to the RADIUS server that PPP service is expected.
 For CHAP, the NAS generates a random challenge (preferably 16 octets)
 and sends it to the user, who returns a CHAP response along with a
 CHAP ID and CHAP username.  The NAS then sends an Access-Request
 packet to the RADIUS server with the CHAP username as the User-Name
 and with the CHAP ID and CHAP response as the CHAP-Password
 (Attribute 3).  The random challenge can either be included in the
 CHAP-Challenge attribute or, if it is 16 octets long, it can be
 placed in the Request Authenticator field of the Access-Request
 packet.  The NAS MAY include the Attributes Service-Type = Framed-
 User and Framed-Protocol = PPP as a hint to the RADIUS server that
 PPP service is expected.
 The RADIUS server looks up a password based on the User-Name,
 encrypts the challenge using MD5 on the CHAP ID octet, that password,
 and the CHAP challenge (from the CHAP-Challenge attribute if present,
 otherwise from the Request Authenticator), and compares that result
 to the CHAP-Password.  If they match, the server sends back an
 Access-Accept, otherwise it sends back an Access-Reject.
 If the RADIUS server is unable to perform the requested
 authentication it should return an Access-Reject.  For example, CHAP
 requires that the user's password be available in cleartext to the
 server so that it can encrypt the CHAP challenge and compare that to
 the CHAP response.  If the password is not available in cleartext to
 the RADIUS server then the server MUST send an Access-Reject to the
 client.

Rigney, et. al. Informational [Page 7] RFC 2058 RADIUS January 1997

2.3. Why UDP?

 A frequently asked question is why RADIUS uses UDP instead of TCP as
 a transport protocol.  UDP was chosen for strictly technical reasons.
 There are a number of issues which must be understood.  RADIUS is a
 transaction based protocol which has several interesting
 characteristics:
 1.   If the request to a primary Authentication server fails, a
      secondary server must be queried.
      To meet this requirement, a copy of the request must be kept
      above the transport layer to allow for alternate transmission.
      This means that retransmission timers are still required.
 2.   The timing requirements of this particular protocol are
      significantly different than TCP provides.
      At one extreme, RADIUS does not require a "responsive" detection
      of lost data.  The user is willing to wait several seconds for
      the authentication to complete.  The generally aggressive TCP
      retransmission (based on average round trip time) is not
      required, nor is the acknowledgement overhead of TCP.
      At the other extreme, the user is not willing to wait several
      minutes for authentication.  Therefore the reliable delivery of
      TCP data two minutes later is not useful.  The faster use of an
      alternate server allows the user to gain access before giving
      up.
 3.   The stateless nature of this protocol simplifies the use of UDP.
      Clients and servers come and go.  Systems are rebooted, or are
      power cycled independently.  Generally this does not cause a
      problem and with creative timeouts and detection of lost TCP
      connections, code can be written to handle anomalous events.
      UDP however completely eliminates any of this special handling.
      Each client and server can open their UDP transport just once
      and leave it open through all types of failure events on the
      network.
 4.   UDP simplifies the server implementation.
      In the earliest implementations of RADIUS, the server was single
      threaded.  This means that a single request was received,
      processed, and returned.  This was found to be unmanageable in
      environments where the back-end security mechanism took real

Rigney, et. al. Informational [Page 8] RFC 2058 RADIUS January 1997

      time (1 or more seconds).  The server request queue would fill
      and in environments where hundreds of people were being
      authenticated every minute, the request turn-around time
      increased to longer that users were willing to wait (this was
      especially severe when a specific lookup in a database or over
      DNS took 30 or more seconds).  The obvious solution was to make
      the server multi-threaded.  Achieving this was simple with UDP.
      Separate processes were spawned to serve each request and these
      processes could respond directly to the client NAS with a simple
      UDP packet to the original transport of the client.
 It's not all a panacea.  As noted, using UDP requires one thing which
 is built into TCP: with UDP we must artificially manage
 retransmission timers to the same server, although they don't require
 the same attention to timing provided by TCP.  This one penalty is a
 small price to pay for the advantages of UDP in this protocol.
 Without TCP we would still probably be using tin cans connected by
 string.  But for this particular protocol, UDP is a better choice.

3. Packet Format

 Exactly one RADIUS packet is encapsulated in the UDP Data field [2],
 where the UDP Destination Port field indicates 1812 (decimal).
 When a reply is generated, the source and destination ports are
 reversed.
 A summary of the RADIUS data 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                         Authenticator                         |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Attributes ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-

Rigney, et. al. Informational [Page 9] RFC 2058 RADIUS January 1997

Code

 The Code field is one octet, and identifies the type of RADIUS
 packet.  When a packet is received with an invalid Code field, it is
 silently discarded.
    RADIUS Codes (decimal) are assigned as follows:
         1       Access-Request
         2       Access-Accept
         3       Access-Reject
         4       Accounting-Request
         5       Accounting-Response
        11       Access-Challenge
        12       Status-Server (experimental)
        13       Status-Client (experimental)
       255       Reserved
 Codes 4 and 5 will be covered in the RADIUS Accounting document [9],
 and are not further mentioned here.  Codes 12 and 13 are reserved for
 possible use, but are not further mentioned here.

Identifier

 The Identifier field is one octet, and aids in matching requests and
 replies.

Length

 The Length field is two octets.  It indicates the length of the
 packet including the Code, Identifier, Length, Authenticator and
 Attribute fields.  Octets outside the range of the Length field
 should be treated as padding and should be ignored on reception.  If
 the packet is shorter than the Length field indicates, it should be
 silently discarded.  The minimum length is 20 and maximum length is
 4096.

Authenticator

 The Authenticator field is sixteen (16) octets.  The most significant
 octet is transmitted first.  This value is used to authenticate the
 reply from the RADIUS server, and is used in the password hiding
 algorithm.

Rigney, et. al. Informational [Page 10] RFC 2058 RADIUS January 1997

Request Authenticator

 In Access-Request Packets, the Authenticator value is a 16 octet
 random number, called the Request Authenticator.  The value SHOULD be
 unpredictable and unique over the lifetime of a secret (the password
 shared between the client and the RADIUS server), since repetition of
 a request value in conjunction with the same secret would permit an
 attacker to reply with a previously intercepted response.  Since it
 is expected that the same secret MAY be used to authenticate with
 servers in disparate geographic regions, the Request Authenticator
 field SHOULD exhibit global and temporal uniqueness.
 The Request Authenticator value in an Access-Request packet SHOULD
 also be unpredictable, lest an attacker trick a server into
 responding to a predicted future request, and then use the response
 to masquerade as that server to a future Access-Request.
 Although protocols such as RADIUS are incapable of protecting against
 theft of an authenticated session via realtime active wiretapping
 attacks, generation of unique unpredictable requests can protect
 against a wide range of active attacks against authentication.
 The NAS and RADIUS server share a secret.  That shared secret
 followed by the Request Authenticator is put through a one-way MD5
 hash to create a 16 octet digest value which is xored with the
 password entered by the user, and the xored result placed in the
 User-Password attribute in the Access-Request packet.  See the entry
 for User-Password in the section on Attributes for a more detailed
 description.

Response Authenticator

   The value of the Authenticator field in Access-Accept, Access-
   Reject, and Access-Challenge packets is called the Response
   Authenticator, and contains a one-way MD5 hash calculated over a
   stream of octets consisting of: the RADIUS packet, beginning with
   the Code field, including the Identifier, the Length, the Request
   Authenticator field from the Access-Request packet, and the
   response Attributes, followed by the shared secret.  That is,
   ResponseAuth = MD5(Code+ID+Length+RequestAuth+Attributes+Secret)
   where + denotes concatenation.

Administrative Note

 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.
 This is to ensure a sufficiently large range for the secret to

Rigney, et. al. Informational [Page 11] RFC 2058 RADIUS January 1997

 provide protection against exhaustive search attacks.  A RADIUS
 server SHOULD use the source IP address of the RADIUS UDP packet to
 decide which shared secret to use, so that RADIUS requests can be
 proxied.
 When using a forwarding proxy, the proxy must be able to alter the
 packet as it passes through in each direction - when the proxy
 forwards the request, the proxy can add a Proxy-State Attribute, and
 when the proxy forwards a response, it removes the Proxy-State
 Attribute. Since Access-Accept and Access-Reject replies are
 authenticated on the entire packet contents, the stripping of the
 Proxy-State attribute would invalidate the signature in the packet -
 so the proxy has to re-sign it.
 Further details of RADIUS proxy implementation are outside the scope
 of this document.

Attributes

 Many Attributes may have multiple instances, in such a case the order
 of Attributes of the same Type SHOULD be preserved.  The order of
 Attributes of different Types is not required to be preserved.
 In the section below on "Attributes" where the text refers to which
 packets an attribute is allowed in, only packets with Codes 1, 2, 3
 and 11 and attributes defined in this document are covered in this
 document.  A summary table is provided at the end of the "Attributes"
 section.  To determine which Attributes are allowed in packets with
 codes 4 and 5 refer to the RADIUS Accounting document [9].

4. Packet Types

 The RADIUS Packet type is determined by the Code field in the first
 octet of the Packet.

4.1. Access-Request

 Description
   Access-Request packets are sent to a RADIUS server, and convey
   information used to determine whether a user is allowed access to a
   specific NAS, and any special services requested for that user.  An
   implementation wishing to authenticate a user MUST transmit a
   RADIUS packet with the Code field set to 1 (Access-Request).
   Upon receipt of an Access-Request from a valid client, an
   appropriate reply MUST be transmitted.

Rigney, et. al. Informational [Page 12] RFC 2058 RADIUS January 1997

   An Access-Request MUST contain a User-Name attribute.  It SHOULD
   contain either a NAS-IP-Address attribute or NAS-Identifier
   attribute (or both, although that is not recommended).  It MUST
   contain either a User-Password attribute or CHAP-Password
   attribute.  It SHOULD contain a NAS-Port or NAS-Port-Type attribute
   or both unless the type of access being requested does not involve
   a port or the NAS does not distinguish among its ports.
   An Access-Request MAY contain additional attributes as a hint to
   the server, but the server is not required to honor the hint.
   When a User-Password is present, it is hidden using a method based
   on the RSA Message Digest Algorithm MD5 [1].
 A summary of the Access-Request packet 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                     Request Authenticator                     |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Attributes ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-
 Code
    1 for Access-Request.
 Identifier
    The Identifier field MUST be changed whenever the content of the
    Attributes field changes, and whenever a valid reply has been
    received for a previous request.  For retransmissions, the
    Identifier MUST remain unchanged.
 Request Authenticator
    The Request Authenticator value MUST be changed each time a new
    Identifier is used.

Rigney, et. al. Informational [Page 13] RFC 2058 RADIUS January 1997

 Attributes
    The Attribute field is variable in length, and contains the list
    of Attributes that are required for the type of service, as well
    as any desired optional Attributes.

4.2. Access-Accept

 Description
   Access-Accept packets are sent by the RADIUS server, and provide
   specific configuration information necessary to begin delivery of
   service to the user.  If all Attribute values received in an
   Access-Request are acceptable then the RADIUS implementation MUST
   transmit a packet with the Code field set to 2 (Access-Accept).  On
   reception of an Access-Accept, the Identifier field is matched with
   a pending Access-Request.  Additionally, the Response Authenticator
   field MUST contain the correct response for the pending Access-
   Request.  Invalid packets are silently discarded.
 A summary of the Access-Accept packet 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                     Response Authenticator                    |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Attributes ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-
 Code
    2 for Access-Accept.
 Identifier
    The Identifier field is a copy of the Identifier field of the
    Access-Request which caused this Access-Accept.

Rigney, et. al. Informational [Page 14] RFC 2058 RADIUS January 1997

 Response Authenticator
    The Response Authenticator value is calculated from the Access-
    Request value, as described earlier.
 Attributes
    The Attribute field is variable in length, and contains a list of
    zero or more Attributes.

4.3. Access-Reject

 Description
   If any value of the received Attributes is not acceptable, then the
   RADIUS server MUST transmit a packet with the Code field set to 3
   (Access-Reject).  It MAY include one or more Reply-Message
   Attributes with a text message which the NAS MAY display to the
   user.
 A summary of the Access-Reject packet 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                     Response Authenticator                    |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Attributes ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-
 Code
    3 for Access-Reject.
 Identifier
    The Identifier field is a copy of the Identifier field of the
    Access-Request which caused this Access-Reject.

Rigney, et. al. Informational [Page 15] RFC 2058 RADIUS January 1997

 Response Authenticator
    The Response Authenticator value is calculated from the Access-
    Request value, as described earlier.
 Attributes
    The Attribute field is variable in length, and contains a list of
    zero or more Attributes.

4.4. Access-Challenge

    Description
   If the RADIUS server desires to send the user a challenge requiring
   a response, then the RADIUS server MUST respond to the Access-
   Request by transmitting a packet with the Code field set to 11
   (Access-Challenge).
   The Attributes field MAY have one or more Reply-Message Attributes,
   and MAY have a single State Attribute, or none.  No other
   Attributes are permitted in an Access-Challenge.
   On receipt of an Access-Challenge, the Identifier field is matched
   with a pending Access-Request.  Additionally, the Response
   Authenticator field MUST contain the correct response for the
   pending Access-Request.  Invalid packets are silently discarded.
   If the NAS does not support challenge/response, it MUST treat an
   Access-Challenge as though it had received an Access-Reject
   instead.
   If the NAS supports challenge/response, receipt of a valid Access-
   Challenge indicates that a new Access-Request SHOULD be sent.  The
   NAS MAY display the text message, if any, to the user, and then
   prompt the user for a response.  It then sends its original
   Access-Request with a new request ID and Request Authenticator,
   with the User-Password Attribute replaced by the user's response
   (encrypted), and including the State Attribute from the Access-
   Challenge, if any.  Only 0 or 1 instances of the State Attribute
   can be present in an Access-Request.
   A NAS which supports PAP MAY forward the Reply-Message to the
   dialin client and accept a PAP response which it can use as though
   the user had entered the response.  If the NAS cannot do so, it
   should treat the Access-Challenge as though it had received an
   Access-Reject instead.

Rigney, et. al. Informational [Page 16] RFC 2058 RADIUS January 1997

 A summary of the Access-Challenge packet 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Code      |  Identifier   |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 |                     Response Authenticator                    |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  Attributes ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-
 Code
    11 for Access-Challenge.
 Identifier
    The Identifier field is a copy of the Identifier field of the
    Access-Request which caused this Access-Challenge.
 Response Authenticator
    The Response Authenticator value is calculated from the Access-
    Request value, as described earlier.
 Attributes
    The Attributes field is variable in length, and contains a list of
    zero or more Attributes.

5. Attributes

 RADIUS Attributes carry the specific authentication, authorization,
 information and configuration details for the request and reply.
 Some Attributes MAY be included more than once.  The effect of this
 is Attribute specific, and is specified in each Attribute
 description.
 The end of the list of Attributes is indicated by the Length of the
 RADIUS packet.

Rigney, et. al. Informational [Page 17] RFC 2058 RADIUS January 1997

 A summary of the 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     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 [3].
    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:
    A RADIUS server MAY ignore Attributes with an unknown Type.
    A RADIUS client MAY ignore Attributes with an unknown Type.
        1      User-Name
        2      User-Password
        3      CHAP-Password
        4      NAS-IP-Address
        5      NAS-Port
        6      Service-Type
        7      Framed-Protocol
        8      Framed-IP-Address
        9      Framed-IP-Netmask
       10      Framed-Routing
       11      Filter-Id
       12      Framed-MTU
       13      Framed-Compression
       14      Login-IP-Host
       15      Login-Service
       16      Login-TCP-Port
       17      (unassigned)
       18      Reply-Message
       19      Callback-Number
       20      Callback-Id
       21      (unassigned)
       22      Framed-Route
       23      Framed-IPX-Network
       24      State
       25      Class
       26      Vendor-Specific

Rigney, et. al. Informational [Page 18] RFC 2058 RADIUS January 1997

       27      Session-Timeout
       28      Idle-Timeout
       29      Termination-Action
       30      Called-Station-Id
       31      Calling-Station-Id
       32      NAS-Identifier
       33      Proxy-State
       34      Login-LAT-Service
       35      Login-LAT-Node
       36      Login-LAT-Group
       37      Framed-AppleTalk-Link
       38      Framed-AppleTalk-Network
       39      Framed-AppleTalk-Zone
       40-59   (reserved for accounting)
       60      CHAP-Challenge
       61      NAS-Port-Type
       62      Port-Limit
       63      Login-LAT-Port
 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 an Access-Request but with an invalid
   Length, an Access-Reject SHOULD be transmitted.  If an Attribute is
   received in an Access-Accept, Access-Reject or Access-Challenge
   packet with an invalid length, the packet MUST either be treated as
   an Access-Reject or else 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 a "string" in RADIUS does not require termination by an
   ASCII NUL because the Attribute already has a length field.
   The format of the value field is one of four data types.
    string    0-253 octets
    address   32 bit value, most significant octet first.
    integer   32 bit value, most significant octet first.

Rigney, et. al. Informational [Page 19] RFC 2058 RADIUS January 1997

    time      32 bit value, most significant octet first -- seconds
              since 00:00:00 GMT, January 1, 1970.  The standard
              Attributes do not use this data type but it is presented
              here for possible use within Vendor-Specific attributes.

5.1. User-Name

 Description
   This Attribute indicates the name of the user to be authenticated.
   It is only used in Access-Request packets.
 A summary of the User-Name 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
   1 for User-Name.
 Length
   >= 3
 String
   The String field is one or more octets.  The NAS may limit the
   maximum length of the User-Name but the ability to handle at least
   63 octets is recommended.

Rigney, et. al. Informational [Page 20] RFC 2058 RADIUS January 1997

   The format of the username MAY be one of several forms:
   monolithic Consisting only of alphanumeric characters.  This
              simple form might be used to locally manage a NAS.
   simple     Consisting only of printable ASCII characters.
   name@fqdn SMTP address.  The Fully Qualified Domain Name (with or
             without trailing dot) indicates the realm in which the
             name part applies.
   distinguished name
             A name in ASN.1 form used in Public Key authentication
             systems.

5.2. User-Password

 Description
   This Attribute indicates the password of the user to be
   authenticated, or the user's input following an Access-Challenge.
   It is only used in Access-Request packets.
   On transmission, the password is hidden.  The password is first
   padded at the end with nulls to a multiple of 16 octets.  A one-way
   MD5 hash is calculated over a stream of octets consisting of the
   shared secret followed by the Request Authenticator.  This value is
   XORed with the first 16 octet segment of the password and placed in
   the first 16 octets of the String field of the User-Password
   Attribute.
   If the password is longer than 16 characters, a second one-way MD5
   hash is calculated over a stream of octets consisting of the shared
   secret followed by the result of the first xor.  That hash is XORed
   with the second 16 octet segment of the password and placed in the
   second 16 octets of the String field of the User-Password
   Attribute.
   If necessary, this operation is repeated, with each xor result
   being used along with the shared secret to generate the next hash
   to xor the next segment of the password, to no more than 128
   characters.
   The method is taken from the book "Network Security" by Kaufman,
   Perlman and Speciner [4] pages 109-110.  A more precise explanation
   of the method follows:

Rigney, et. al. Informational [Page 21] RFC 2058 RADIUS January 1997

   Call the shared secret S and the pseudo-random 128-bit Request
   Authenticator RA.  Break the password into 16-octet chunks p1, p2,
   etc.  with the last one padded at the end with nulls to a 16-octet
   boundary.  Call the ciphertext blocks c(1), c(2), etc.  We'll need
   intermediate values b1, b2, etc.
       b1 = MD5(S + RA)       c(1) = p1 xor b1
       b2 = MD5(S + c(1))     c(2) = p2 xor b2
              .                       .
              .                       .
              .                       .
       bi = MD5(S + c(i-1))   c(i) = pi xor bi
   The String will contain c(1)+c(2)+...+c(i) where + denotes
   concatenation.
   On receipt, the process is reversed to yield the original password.
 A summary of the User-Password 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
    |     Type      |    Length     |  String ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
    Type
       2 for User-Password.
    Length
       At least 18 and no larger than 130.
    String
       The String field is between 16 and 128 octets long, inclusive.

5.3. CHAP-Password

 Description
   This Attribute indicates the response value provided by a PPP
   Challenge-Handshake Authentication Protocol (CHAP) user in response
   to the challenge.  It is only used in Access-Request packets.

Rigney, et. al. Informational [Page 22] RFC 2058 RADIUS January 1997

   The CHAP challenge value is found in the CHAP-Challenge Attribute
   (60) if present in the packet, otherwise in the Request
   Authenticator field.
 A summary of the CHAP-Password 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 4 5 6 7 8 9
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  CHAP Ident   |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    3 for CHAP-Password.
 Length
    19
 CHAP Ident
    This field is one octet, and contains the CHAP Identifier from the
    user's CHAP Response.
 String
    The String field is 16 octets, and contains the CHAP Response from
    the user.

5.4. NAS-IP-Address

 Description
   This Attribute indicates the identifying IP Address of the NAS
   which is requesting authentication of the user.  It is only used in
   Access-Request packets.  Either NAS-IP-Address or NAS-Identifier
   SHOULD be present in an Access-Request packet.

Rigney, et. al. Informational [Page 23] RFC 2058 RADIUS January 1997

 A summary of the NAS-IP-Address 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     |            Address
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Address (cont)         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
    4 for NAS-IP-Address.
 Length
    6
 Address
    The Address field is four octets.

5.5. NAS-Port

 Description
   This Attribute indicates the physical port number of the NAS which
   is authenticating the user.  It is only used in Access-Request
   packets.  Note that this is using "port" in its sense of a physical
   connection on the NAS, not in the sense of a TCP or UDP port
   number.  Either NAS-Port or NAS-Port-Type (61) or both SHOULD be
   present in an Access-Request packet, if the NAS differentiates
   among its ports.

Rigney, et. al. Informational [Page 24] RFC 2058 RADIUS January 1997

 A summary of the NAS-Port 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
    5 for NAS-Port.
 Length
    6
 Value
    The Value field is four octets.  Despite the size of the field,
    values range from 0 to 65535.

5.6. Service-Type

 Description
   This Attribute indicates the type of service the user has
   requested, or the type of service to be provided.  It MAY be used
   in both Access-Request and Access-Accept packets.  A NAS is not
   required to implement all of these service types, and MUST treat
   unknown or unsupported Service-Types as though an Access-Reject had
   been received instead.
 A summary of the Service-Type 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)         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Rigney, et. al. Informational [Page 25] RFC 2058 RADIUS January 1997

 Type
    6 for Service-Type.
 Length
    6
 Value
    The Value field is four octets.
     1      Login
     2      Framed
     3      Callback Login
     4      Callback Framed
     5      Outbound
     6      Administrative
     7      NAS Prompt
     8      Authenticate Only
     9      Callback NAS Prompt
    The service types are defined as follows when used in an Access-
    Accept.  When used in an Access-Request, they should be considered
    to be a hint to the RADIUS server that the NAS has reason to
    believe the user would prefer the kind of service indicated, but
    the server is not required to honor the hint.
    Login               The user should be connected to a host.
    Framed              A Framed Protocol should be started for the
                        User, such as PPP or SLIP.
    Callback Login      The user should be disconnected and called
                        back, then connected to a host.
    Callback Framed     The user should be disconnected and called
                        back, then a Framed Protocol should be started
                        for the User, such as PPP or SLIP.
    Outbound            The user should be granted access to outgoing
                        devices.
    Administrative      The user should be granted access to the
                        administrative interface to the NAS from which
                        privileged commands can be executed.

Rigney, et. al. Informational [Page 26] RFC 2058 RADIUS January 1997

    NAS Prompt          The user should be provided a command prompt
                        on the NAS from which non-privileged commands
                        can be executed.
    Authenticate Only   Only Authentication is requested, and no
                        authorization information needs to be returned
                        in the Access-Accept (typically used by proxy
                        servers rather than the NAS itself).
    Callback NAS Prompt The user should be disconnected and called
                        back, then provided a command prompt on the
                        NAS from which non-privileged commands can be
                        executed.

5.7. Framed-Protocol

 Description
    This Attribute indicates the framing to be used for framed access.
    It MAY be used in both Access-Request and Access-Accept packets.
 A summary of the Framed-Protocol 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
    7 for Framed-Protocol.
 Length
    6

Rigney, et. al. Informational [Page 27] RFC 2058 RADIUS January 1997

 Value
    The Value field is four octets.
     1      PPP
     2      SLIP
     3      AppleTalk Remote Access Protocol (ARAP)
     4      Gandalf proprietary SingleLink/MultiLink protocol
     5      Xylogics proprietary IPX/SLIP

5.8. Framed-IP-Address

 Description
    This Attribute indicates the address to be configured for the
    user.  It MAY be used in Access-Accept packets.  It MAY be used in
    an Access-Request packet as a hint by the NAS to the server that
    it would prefer that address, but the server is not required to
    honor the hint.
 A summary of the Framed-IP-Address 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     |            Address
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Address (cont)         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
    8 for Framed-IP-Address.
 Length
    6
 Address
    The Address field is four octets.  The value 0xFFFFFFFF indicates
    that the NAS should allow the user to select an address (e.g.
    Negotiated).  The value 0xFFFFFFFE indicates that the NAS should
    select an address for the user (e.g. Assigned from a pool of
    addresses kept by the NAS).  Other valid values indicate that the
    NAS should use that value as the user's IP address.

Rigney, et. al. Informational [Page 28] RFC 2058 RADIUS January 1997

5.9. Framed-IP-Netmask

 Description
    This Attribute indicates the IP netmask to be configured for the
    user when the user is a router to a network.  It MAY be used in
    Access-Accept packets.  It MAY be used in an Access-Request packet
    as a hint by the NAS to the server that it would prefer that
    netmask, but the server is not required to honor the hint.
 A summary of the Framed-IP-Netmask 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     |            Address
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Address (cont)         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
    9 for Framed-IP-Netmask.
 Length
    6
 Address
    The Address field is four octets specifying the IP netmask of the
    user.

5.10. Framed-Routing

 Description
    This Attribute indicates the routing method for the user, when the
    user is a router to a network.  It is only used in Access-Accept
    packets.

Rigney, et. al. Informational [Page 29] RFC 2058 RADIUS January 1997

 A summary of the Framed-Routing 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
    10 for Framed-Routing.
 Length
    6
 Value
    The Value field is four octets.
     0      None
     1      Send routing packets
     2      Listen for routing packets
     3      Send and Listen

5.11. Filter-Id

 Description
    This Attribute indicates the name of the filter list for this
    user.  Zero or more Filter-Id attributes MAY be sent in an
    Access-Accept packet.
    Identifying a filter list by name allows the filter to be used on
    different NASes without regard to filter-list implementation
    details.
 A summary of the Filter-Id 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Rigney, et. al. Informational [Page 30] RFC 2058 RADIUS January 1997

 Type
    11 for Filter-Id.
 Length
    >= 3
 String
    The String field is one or more octets, and its contents are
    implementation dependent.  It is intended to be human readable and
    MUST NOT affect operation of the protocol.  It is recommended that
    the message contain displayable ASCII characters from the range 32
    through 126 decimal.

5.12. Framed-MTU

 Description
    This Attribute indicates the Maximum Transmission Unit to be
    configured for the user, when it is not negotiated by some other
    means (such as PPP).  It is only used in Access-Accept packets.
 A summary of the Framed-MTU 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
    12 for Framed-MTU.
 Length
    6
 Value
    The Value field is four octets.  Despite the size of the field,
    values range from 64 to 65535.

Rigney, et. al. Informational [Page 31] RFC 2058 RADIUS January 1997

5.13. Framed-Compression

 Description
   This Attribute indicates a compression protocol to be used for the
   link.  It MAY be used in Access-Accept packets.  It MAY be used in
   an Access-Request packet as a hint to the server that the NAS would
   prefer to use that compression, but the server is not required to
   honor the hint.
   More than one compression protocol Attribute MAY be sent.  It is
   the responsibility of the NAS to apply the proper compression
   protocol to appropriate link traffic.
 A summary of the Framed-Compression 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
       13 for Framed-Compression.
    Length
       6
    Value
       The Value field is four octets.
        0      None
        1      VJ TCP/IP header compression [5]
        2      IPX header compression

Rigney, et. al. Informational [Page 32] RFC 2058 RADIUS January 1997

5.14. Login-IP-Host

 Description
    This Attribute indicates the system with which to connect the
    user, when the Login-Service Attribute is included.  It MAY be
    used in Access-Accept packets.  It MAY be used in an Access-
    Request packet as a hint to the server that the NAS would prefer
    to use that host, but the server is not required to honor the
    hint.
 A summary of the Login-IP-Host 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     |            Address
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Address (cont)         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
    14 for Login-IP-Host.
 Length
    6
 Address
    The Address field is four octets.  The value 0xFFFFFFFF indicates
    that the NAS SHOULD allow the user to select an address.  The
    value 0 indicates that the NAS SHOULD select a host to connect the
    user to.  Other values indicate the address the NAS SHOULD connect
    the user to.

5.15. Login-Service

 Description
    This Attribute indicates the service which should be used to
    connect the user to the login host.  It is only used in Access-
    Accept packets.

Rigney, et. al. Informational [Page 33] RFC 2058 RADIUS January 1997

 A summary of the Login-Service 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
    15 for Login-Service.
 Length
    6
 Value
    The Value field is four octets.
     0      Telnet
     1      Rlogin
     2      TCP Clear
     3      PortMaster (proprietary)
     4      LAT

5.16. Login-TCP-Port

 Description
    This Attribute indicates the TCP port with which the user is to be
    connected, when the Login-Service Attribute is also present.  It
    is only used in Access-Accept packets.
 A summary of the Login-TCP-Port 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)         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Rigney, et. al. Informational [Page 34] RFC 2058 RADIUS January 1997

 Type
    16 for Login-TCP-Port.
 Length
    6
 Value
    The Value field is four octets.  Despite the size of the field,
    values range from 0 to 65535.

5.17. (unassigned)

 Description
    ATTRIBUTE TYPE 17 HAS NOT BEEN ASSIGNED.

5.18. Reply-Message

 Description
    This Attribute indicates text which MAY be displayed to the user.
    When used in an Access-Accept, it is the success message.
    When used in an Access-Reject, it is the failure message.  It MAY
    indicate a dialog message to prompt the user before another
    Access-Request attempt.
    When used in an Access-Challenge, it MAY indicate a dialog message
    to prompt the user for a response.
    Multiple Reply-Message's MAY be included and if any are displayed,
    they MUST be displayed in the same order as they appear in the
    packet.
 A summary of the Reply-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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Rigney, et. al. Informational [Page 35] RFC 2058 RADIUS January 1997

 Type
    18 for Reply-Message.
 Length
    >= 3
 String
    The String field is one or more octets, and its contents are
    implementation dependent.  It is intended to be human readable,
    and MUST NOT affect operation of the protocol.  It is recommended
    that the message contain displayable ASCII characters from the
    range 10, 13, and 32 through 126 decimal.  Mechanisms for
    extension to other character sets are beyond the scope of this
    specification.

5.19. Callback-Number

 Description
    This Attribute indicates a dialing string to be used for callback.
    It MAY be used in Access-Accept packets.  It MAY be used in an
    Access-Request packet as a hint to the server that a Callback
    service is desired, but the server is not required to honor the
    hint.
 A summary of the Callback-Number 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    19 for Callback-Number.
 Length
    >= 3

Rigney, et. al. Informational [Page 36] RFC 2058 RADIUS January 1997

 String
    The String field is one or more octets.  The actual format of the
    information is site or application specific, and a robust
    implementation SHOULD support the field as undistinguished octets.
    The codification of the range of allowed usage of this field is
    outside the scope of this specification.

5.20. Callback-Id

 Description
    This Attribute indicates the name of a place to be called, to be
    interpreted by the NAS.  It MAY be used in Access-Accept packets.
 A summary of the Callback-Id 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    20 for Callback-Id.
 Length
    >= 3
 String
    The String field is one or more octets.  The actual format of the
    information is site or application specific, and a robust
    implementation SHOULD support the field as undistinguished octets.
    The codification of the range of allowed usage of this field is
    outside the scope of this specification.

5.21. (unassigned)

 Description
    ATTRIBUTE TYPE 21 HAS NOT BEEN ASSIGNED.

Rigney, et. al. Informational [Page 37] RFC 2058 RADIUS January 1997

5.22. Framed-Route

 Description
    This Attribute provides routing information to be configured for
    the user on the NAS.  It is used in the Access-Accept packet and
    can appear multiple times.
 A summary of the Framed-Route 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
    22 for Framed-Route.
 Length
    >= 3
 String
    The String field is one or more octets, and its contents are
    implementation dependent.  It is intended to be human readable and
    MUST NOT affect operation of the protocol.  It is recommended that
    the message contain displayable ASCII characters from the range 32
    through 126 decimal.
    For IP routes, it SHOULD contain a destination prefix in dotted
    quad form optionally followed by a slash and a decimal length
    specifier stating how many high order bits of the prefix should
    be used.  That is followed by a space, a gateway address in
    dotted quad form, a space, and one or more metrics separated by
    spaces.  For example, "192.168.1.0/24 192.168.1.1 1 2 -1 3 400".
    The length specifier may be omitted in which case it should
    default to 8 bits for class A prefixes, 16 bits for class B
    prefixes, and 24 bits for class C prefixes.  For example,
    "192.168.1.0 192.168.1.1 1".
    Whenever the gateway address is specified as "0.0.0.0" the IP
    address of the user SHOULD be used as the gateway address.

Rigney, et. al. Informational [Page 38] RFC 2058 RADIUS January 1997

5.23. Framed-IPX-Network

 Description
    This Attribute indicates the IPX Network number to be configured
    for the user.  It is used in Access-Accept packets.
 A summary of the Framed-IPX-Network 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
    23 for Framed-IPX-Network.
 Length
    6
 Value
    The Value field is four octets.  The value 0xFFFFFFFE indicates
    that the NAS should select an IPX network for the user (e.g.
    assigned from a pool of one or more IPX networks kept by the NAS).
    Other values should be used as the IPX network for the link to the
    user.

5.24. State

 Description
    This Attribute is available to be sent by the server to the client
    in an Access-Challenge and MUST be sent unmodified from the client
    to the server in the new Access-Request reply to that challenge,
    if any.
    This Attribute is available to be sent by the server to the client
    in an Access-Accept that also includes a Termination-Action
    Attribute with the value of RADIUS-Request.  If the NAS performs
    the Termination-Action by sending a new Access-Request upon

Rigney, et. al. Informational [Page 39] RFC 2058 RADIUS January 1997

    termination of the current session, it MUST include the State
    attribute unchanged in that Access-Request.
    In either usage, no interpretation by the client should be made.
    A packet may have only one State Attribute.  Usage of the State
    Attribute is implementation dependent.
 A summary of the State 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    24 for State.
 Length
    >= 3
 String
    The String field is one or more octets.  The actual format of the
    information is site or application specific, and a robust
    implementation SHOULD support the field as undistinguished octets.
    The codification of the range of allowed usage of this field is
    outside the scope of this specification.

5.25. Class

 Description
    This Attribute is available to be sent by the server to the client
    in an Access-Accept and should be sent unmodified by the client to
    the accounting server as part of the Accounting-Request packet if
    accounting is supported.  No interpretation by the client should
    be made.

Rigney, et. al. Informational [Page 40] RFC 2058 RADIUS January 1997

 A summary of the Class 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    25 for Class.
 Length
    >= 3
 String
    The String field is one or more octets.  The actual format of the
    information is site or application specific, and a robust
    implementation SHOULD support the field as undistinguished octets.
    The codification of the range of allowed usage of this field is
    outside the scope of this specification.

5.26. Vendor-Specific

 Description
    This Attribute is available to allow vendors to support their own
    extended Attributes not suitable for general usage.  It MUST not
    affect the operation of the RADIUS protocol.
    Servers not equipped to interpret the vendor-specific information
    sent by a client MUST ignore it (although it may be reported).
    Clients which do not receive desired vendor-specific information
    SHOULD make an attempt to operate without it, although they may do
    so (and report they are doing so) in a degraded mode.

Rigney, et. al. Informational [Page 41] RFC 2058 RADIUS January 1997

 A summary of the Vendor-Specific 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       |            Vendor-Id
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      Vendor-Id (cont)           |  String...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    26 for Vendor-Specific.
 Length
    >= 7
 Vendor-Id
    The high-order octet is 0 and the low-order 3 octets are the SMI
    Network Management Private Enterprise Code of the Vendor in
    network byte order, as defined in the Assigned Numbers RFC [2].
 String
    The String field is one or more octets.  The actual format of the
    information is site or application specific, and a robust
    implementation SHOULD support the field as undistinguished octets.
    The codification of the range of allowed usage of this field is
    outside the scope of this specification.

Rigney, et. al. Informational [Page 42] RFC 2058 RADIUS January 1997

    It SHOULD be encoded as a sequence of vendor type / vendor length
    / value fields, as follows.  The Attribute-Specific field is
    dependent on the vendor's definition of that attribute.  An
    example encoding of the Vendor-Specific attribute using this
    method follows:
     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       |            Vendor-Id
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         Vendor-Id (cont)           | Vendor type   | Vendor length |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Attribute-Specific...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

5.27. Session-Timeout

 Description
    This Attribute sets the maximum number of seconds of service to be
    provided to the user before termination of the session or prompt.
    This Attribute is available to be sent by the server to the client
    in an Access-Accept or Access-Challenge.
 A summary of the Session-Timeout 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
    27 for Session-Timeout.
 Length
    6

Rigney, et. al. Informational [Page 43] RFC 2058 RADIUS January 1997

 Value
    The field is 4 octets, containing a 32-bit unsigned integer with
    the maximum number of seconds this user should be allowed to
    remain connected by the NAS.

5.28. Idle-Timeout

 Description
    This Attribute sets the maximum number of consecutive seconds of
    idle connection allowed to the user before termination of the
    session or prompt.  This Attribute is available to be sent by the
    server to the client in an Access-Accept or Access-Challenge.
 A summary of the Idle-Timeout 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
    28 for Idle-Timeout.
 Length
    6
 Value
    The field is 4 octets, containing a 32-bit unsigned integer with
    the maximum number of consecutive seconds of idle time this user
    should be permitted before being disconnected by the NAS.

5.29. Termination-Action

 Description
    This Attribute indicates what action the NAS should take when the
    specified service is completed.  It is only used in Access-Accept
    packets.

Rigney, et. al. Informational [Page 44] RFC 2058 RADIUS January 1997

 A summary of the Termination-Action 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
    29 for Termination-Action.
 Length
    6
 Value
    The Value field is four octets.
     0      Default
     1      RADIUS-Request
    If the Value is set to RADIUS-Request, upon termination of the
    specified service the NAS MAY send a new Access-Request to the
    RADIUS server, including the State attribute if any.

5.30. Called-Station-Id

 Description
    This Attribute allows the NAS to send in the Access-Request
    packet the phone number that the user called, using  Dialed
    Number Identification (DNIS) or similar technology.  Note that
    this may be different from the phone number the call comes in
    on.  It is only used in Access-Request packets.

Rigney, et. al. Informational [Page 45] RFC 2058 RADIUS January 1997

 A summary of the Called-Station-Id 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    30 for Called-Station-Id.
 Length
    >= 3
 String
    The String field is one or more octets, containing the phone
    number that the user's call came in on.
    The actual format of the information is site or application
    specific.  Printable ASCII is recommended, but a robust
    implementation SHOULD support the field as undistinguished octets.
    The codification of the range of allowed usage of this field is
    outside the scope of this specification.

5.31. Calling-Station-Id

 Description
    This Attribute allows the NAS to send in the Access-Request
    packet the phone number that the call came from, using Automatic
    Number Identification (ANI) or similar technology.  It is only
    used in Access-Request packets.
 A summary of the Calling-Station-Id 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Rigney, et. al. Informational [Page 46] RFC 2058 RADIUS January 1997

 Type
    31 for Calling-Station-Id.
 Length
    >= 3
 String
    The String field is one or more octets, containing the phone
    number that the user placed the call from.
    The actual format of the information is site or application
    specific.  Printable ASCII is recommended, but a robust
    implementation SHOULD support the field as undistinguished octets.
    The codification of the range of allowed usage of this field is
    outside the scope of this specification.

5.32. NAS-Identifier

 Description
    This Attribute contains a string identifying the NAS originating
    the Access-Request.  It is only used in Access-Request packets.
    Either NAS-IP-Address or NAS-Identifier SHOULD be present in an
    Access-Request packet.
 A summary of the NAS-Identifier 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    32 for NAS-Identifier.
 Length
    >= 3

Rigney, et. al. Informational [Page 47] RFC 2058 RADIUS January 1997

 String
    The String field is one or more octets, and should be unique to
    the NAS within the scope of the RADIUS server.  For example, a
    fully qualified domain name would be suitable as a NAS-Identifier.
    The actual format of the information is site or application
    specific, and a robust implementation SHOULD support the field as
    undistinguished octets.
    The codification of the range of allowed usage of this field is
    outside the scope of this specification.

5.33. Proxy-State

 Description
    This Attribute is available to be sent by a proxy server to
    another server when forwarding an Access-Request and MUST be
    returned unmodified in the Access-Accept, Access-Reject or
    Access-Challenge.  This attribute should be removed by the proxy
    server before the response is forwarded to the NAS.
    Usage of the Proxy-State Attribute is implementation dependent.  A
    description of its function is outside the scope of this
    specification.
 A summary of the Proxy-State 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    33 for Proxy-State.
 Length
    >= 3

Rigney, et. al. Informational [Page 48] RFC 2058 RADIUS January 1997

 String
    The String field is one or more octets.  The actual format of the
    information is site or application specific, and a robust
    implementation SHOULD support the field as undistinguished octets.
    The codification of the range of allowed usage of this field is
    outside the scope of this specification.

5.34. Login-LAT-Service

 Description
    This Attribute indicates the system with which the user is to be
    connected by LAT.  It MAY be used in Access-Accept packets, but
    only when LAT is specified as the Login-Service.  It MAY be used
    in an Access-Request packet as a hint to the server, but the
    server is not required to honor the hint.
    Administrators use the service attribute when dealing with
    clustered systems, such as a VAX or Alpha cluster. In such an
    environment several different time sharing hosts share the same
    resources (disks, printers, etc.), and administrators often
    configure each to offer access (service) to each of the shared
    resources. In this case, each host in the cluster advertises its
    services through LAT broadcasts.
    Sophisticated users often know which service providers (machines)
    are faster and tend to use a node name when initiating a LAT
    connection.  Alternately, some administrators want particular
    users to use certain machines as a primitive form of load
    balancing (although LAT knows how to do load balancing itself).
 A summary of the Login-LAT-Service 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    34 for Login-LAT-Service.

Rigney, et. al. Informational [Page 49] RFC 2058 RADIUS January 1997

 Length
    >= 3
 String
    The String field is one or more octets, and contains the identity
    of the LAT service to use.  The LAT Architecture allows this
    string to contain $ (dollar), - (hyphen), . (period), _
    (underscore), numerics, upper and lower case alphabetics, and the
    ISO Latin-1 character set extension [6].  All LAT string
    comparisons are case insensitive.

5.35. Login-LAT-Node

 Description
    This Attribute indicates the Node with which the user is to be
    automatically connected by LAT.  It MAY be used in Access-Accept
    packets, but only when LAT is specified as the Login-Service.  It
    MAY be used in an Access-Request packet as a hint to the server,
    but the server is not required to honor the hint.
 A summary of the Login-LAT-Node 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    35 for Login-LAT-Node.
 Length
    >= 3

Rigney, et. al. Informational [Page 50] RFC 2058 RADIUS January 1997

 String
    The String field is one or more octets, and contains the identity
    of the LAT Node to connect the user to.  The LAT Architecture
    allows this string to contain $ (dollar), - (hyphen), . (period),
    _ (underscore), numerics, upper and lower case alphabetics, and
    the ISO Latin-1 character set extension.  All LAT string
    comparisons are case insensitive.

5.36. Login-LAT-Group

 Description
    This Attribute contains a string identifying the LAT group codes
    which this user is authorized to use.  It MAY be used in Access-
    Accept packets, but only when LAT is specified as the Login-
    Service.  It MAY be used in an Access-Request packet as a hint to
    the server, but the server is not required to honor the hint.
    LAT supports 256 different group codes, which LAT uses as a form
    of access rights.  LAT encodes the group codes as a 256 bit
    bitmap.
    Administrators can assign one or more of the group code bits at
    the LAT service provider; it will only accept LAT connections that
    have these group codes set in the bit map. The administrators
    assign a bitmap of authorized group codes to each user; LAT gets
    these from the operating system, and uses these in its requests to
    the service providers.
 A summary of the Login-LAT-Group 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    36 for Login-LAT-Group.
 Length
    34

Rigney, et. al. Informational [Page 51] RFC 2058 RADIUS January 1997

 String
    The String field is a 32 octet bit map, most significant octet
    first.  A robust implementation SHOULD support the field as
    undistinguished octets.
    The codification of the range of allowed usage of this field is
    outside the scope of this specification.

5.37. Framed-AppleTalk-Link

 Description
    This Attribute indicates the AppleTalk network number which should
    be used for the serial link to the user, which is another
    AppleTalk router.  It is only used in Access-Accept packets.  It
    is never used when the user is not another router.
 A summary of the Framed-AppleTalk-Link 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
    37 for Framed-AppleTalk-Link.
 Length
    6
 Value
    The Value field is four octets.  Despite the size of the field,
    values range from 0 to 65535.  The special value of 0 indicates
    that this is an unnumbered serial link.  A value of 1-65535 means
    that the serial line between the NAS and the user should be
    assigned that value as an AppleTalk network number.

Rigney, et. al. Informational [Page 52] RFC 2058 RADIUS January 1997

5.38. Framed-AppleTalk-Network

 Description
    This Attribute indicates the AppleTalk Network number which the
    NAS should probe to allocate an AppleTalk node for the user.  It
    is only used in Access-Accept packets.  It is never used when the
    user is another router.  Multiple instances of this Attribute
    indicate that the NAS may probe using any of the network numbers
    specified.
 A summary of the Framed-AppleTalk-Network 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
    38 for Framed-AppleTalk-Network.
 Length
    6
 Value
    The Value field is four octets.  Despite the size of the field,
    values range from 0 to 65535.  The special value 0 indicates that
    the NAS should assign a network for the user, using its default
    cable range.  A value between 1 and 65535 (inclusive) indicates
    the AppleTalk Network the NAS should probe to find an address for
    the user.

5.39. Framed-AppleTalk-Zone

 Description
    This Attribute indicates the AppleTalk Default Zone to be used for
    this user.  It is only used in Access-Accept packets.  Multiple
    instances of this attribute in the same packet are not allowed.

Rigney, et. al. Informational [Page 53] RFC 2058 RADIUS January 1997

 A summary of the Framed-AppleTalk-Zone 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 4
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    39 for Framed-AppleTalk-Zone.
 Length
    >= 3
 String
    The name of the Default AppleTalk Zone to be used for this user.
    A robust implementation SHOULD support the field as
    undistinguished octets.
    The codification of the range of allowed usage of this field is
    outside the scope of this specification.

5.40. CHAP-Challenge

 Description
    This Attribute contains the CHAP Challenge sent by the NAS to a
    PPP Challenge-Handshake Authentication Protocol (CHAP) user.  It
    is only used in Access-Request packets.
    If the CHAP challenge value is 16 octets long it MAY be placed in
    the Request Authenticator field instead of using this attribute.
 A summary of the CHAP-Challenge 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...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Rigney, et. al. Informational [Page 54] RFC 2058 RADIUS January 1997

 Type
    60 for CHAP-Challenge.
 Length
    >= 7
 String
    The String field contains the CHAP Challenge.

5.41. NAS-Port-Type

 Description
    This Attribute indicates the type of the physical port of the NAS
    which is authenticating the user.  It can be used instead of or in
    addition to the NAS-Port (5) attribute.  It is only used in
    Access-Request packets.  Either NAS-Port (5) or NAS-Port-Type or
    both SHOULD be present in an Access-Request packet, if the NAS
    differentiates among its ports.
 A summary of the NAS-Port-Type 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
    61 for NAS-Port-Type.
 Length
    6
 Value
    The Value field is four octets.  "Virtual" refers to a connection
    to the NAS via some transport protocol, instead of through a
    physical port.  For example, if a user telnetted into a NAS to

Rigney, et. al. Informational [Page 55] RFC 2058 RADIUS January 1997

    authenticate himself as an Outbound-User, the Access-Request might
    include NAS-Port-Type = Virtual as a hint to the RADIUS server
    that the user was not on a physical port.
    0       Async
    1       Sync
    2       ISDN Sync
    3       ISDN Async V.120
    4       ISDN Async V.110
    5       Virtual

5.42. Port-Limit

 Description
    This Attribute sets the maximum number of ports to be provided to
    the user by the NAS.  This Attribute MAY be sent by the server to
    the client in an Access-Accept packet.  It is intended for use in
    conjunction with Multilink PPP [7] or similar uses.  It MAY also
    be sent by the NAS to the server as a hint that that many ports
    are desired for use, but the server is not required to honor the
    hint.
 A summary of the Port-Limit 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
    62 for Port-Limit.
 Length
    6
 Value
    The field is 4 octets, containing a 32-bit unsigned integer with
    the maximum number of ports this user should be allowed to connect
    to on the NAS.

Rigney, et. al. Informational [Page 56] RFC 2058 RADIUS January 1997

5.43. Login-LAT-Port

 Description
    This Attribute indicates the Port with which the user is to be
    connected by LAT.  It MAY be used in Access-Accept packets, but
    only when LAT is specified as the Login-Service.  It MAY be used
    in an Access-Request packet as a hint to the server, but the
    server is not required to honor the hint.
 A summary of the Login-LAT-Port 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 |     Type      |    Length     |  String ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
    63 for Login-LAT-Port.
 Length
    >= 3
 String
    The String field is one or more octets, and contains the identity
    of the LAT port to use.  The LAT Architecture allows this string
    to contain $ (dollar), - (hyphen), . (period), _ (underscore),
    numerics, upper and lower case alphabetics, and the ISO Latin-1
    character set extension.  All LAT string comparisons are case
    insensitive.

Rigney, et. al. Informational [Page 57] RFC 2058 RADIUS January 1997

5.44. Table of Attributes

 The following table provides a guide to which attributes may be found
 in which kinds of packets, and in what quantity.
 Request   Accept   Reject   Challenge   #    Attribute
 1         0        0        0            1   User-Name
 0-1       0        0        0            2   User-Password [Note 1]
 0-1       0        0        0            3   CHAP-Password [Note 1]
 0-1       0        0        0            4   NAS-IP-Address
 0-1       0        0        0            5   NAS-Port
 0-1       0-1      0        0            6   Service-Type
 0-1       0-1      0        0            7   Framed-Protocol
 0-1       0-1      0        0            8   Framed-IP-Address
 0-1       0-1      0        0            9   Framed-IP-Netmask
 0         0-1      0        0           10   Framed-Routing
 0         0+       0        0           11   Filter-Id
 0         0-1      0        0           12   Framed-MTU
 0+        0+       0        0           13   Framed-Compression
 0+        0+       0        0           14   Login-IP-Host
 0         0-1      0        0           15   Login-Service
 0         0-1      0        0           16   Login-TCP-Port
 0         0+       0+       0+          18   Reply-Message
 0-1       0-1      0        0           19   Callback-Number
 0         0-1      0        0           20   Callback-Id
 0         0+       0        0           22   Framed-Route
 0         0-1      0        0           23   Framed-IPX-Network
 0-1       0-1      0        0-1         24   State
 0         0+       0        0           25   Class
 0+        0+       0        0+          26   Vendor-Specific
 0         0-1      0        0-1         27   Session-Timeout
 0         0-1      0        0-1         28   Idle-Timeout
 0         0-1      0        0           29   Termination-Action
 0-1       0        0        0           30   Called-Station-Id
 0-1       0        0        0           31   Calling-Station-Id
 0-1       0        0        0           32   NAS-Identifier
 0+        0+       0+       0+          33   Proxy-State
 0-1       0-1      0        0           34   Login-LAT-Service
 0-1       0-1      0        0           35   Login-LAT-Node
 0-1       0-1      0        0           36   Login-LAT-Group
 0         0-1      0        0           37   Framed-AppleTalk-Link
 0         0+       0        0           38   Framed-AppleTalk-Network
 0         0-1      0        0           39   Framed-AppleTalk-Zone
 0-1       0        0        0           60   CHAP-Challenge
 0-1       0        0        0           61   NAS-Port-Type
 0-1       0-1      0        0           62   Port-Limit
 0-1       0-1      0        0           63   Login-LAT-Port
 Request   Accept   Reject   Challenge   #    Attribute

Rigney, et. al. Informational [Page 58] RFC 2058 RADIUS January 1997

 [Note 1] An Access-Request MUST contain either a User-Password or a
 CHAP-Password, and MUST NOT contain both.
 The following table defines the meaning of the above table entries.

0 This attribute MUST NOT be present in packet. 0+ Zero or more instances of this attribute MAY be present in packet. 0-1 Zero or one instance of this attribute MAY be present in packet. 1 Exactly one instance of this attribute MUST be present in packet.

6. Examples

 A few examples are presented to illustrate the flow of packets and
 use of typical attributes.  These examples are not intended to be
 exhaustive, many others are possible.

6.1. User Telnet to Specified Host

 The NAS at 192.168.1.16 sends an Access-Request UDP packet to the
 RADIUS Server for a user named nemo logging in on port 3.
    Code = 1        (Access-Request)
    ID = 0
    Request Authenticator = {16 octet random number}
    Attributes:
        User-Name = "nemo"
        User-Password = {16 octets of Password padded at end with nulls,
                    XORed with MD5(shared secret|Request Authenticator)}
        NAS-IP-Address = 192.168.1.16
        NAS-Port = 3
 The RADIUS server authenticates nemo, and sends an Access-Accept UDP
 packet to the NAS telling it to telnet nemo to host 192.168.1.3.
    Code = 2        (Access-Accept)
    ID = 0          (same as in Access-Request)
    Response Authenticator = {16-octet MD-5 checksum of the code (2),
                    id (0), the Request Authenticator from above, the
                    attributes in this reply, and the shared secret}
    Attributes:
        Service-Type = Login-User
        Login-Service = Telnet
        Login-Host = 192.168.1.3

Rigney, et. al. Informational [Page 59] RFC 2058 RADIUS January 1997

6.2. Framed User Authenticating with CHAP

 The NAS at 192.168.1.16 sends an Access-Request UDP packet to the
 RADIUS Server for a user named flopsy logging in on port 20 with PPP,
 authenticating using CHAP.  The NAS sends along the Service-Type and
 Framed-Protocol attributes as a hint to the RADIUS server that this
 user is looking for PPP, although the NAS is not required to do so.
    Code = 1        (Access-Request)
    ID = 1
    Request Authenticator = {16 octet random number also used as
                             CHAP challenge}
    Attributes:
        User-Name = "flopsy"
        CHAP-Password = {1 octet CHAP ID followed by 16 octet
                         CHAP response}
        NAS-IP-Address = 192.168.1.16
        NAS-Port = 20
        Service-Type = Framed-User
        Framed-Protocol = PPP
 The RADIUS server authenticates flopsy, and sends an Access-Accept
 UDP packet to the NAS telling it to start PPP service and assign an
 address for the user out of its dynamic address pool.
    Code = 2        (Access-Accept)
    ID = 1          (same as in Access-Request)
    Response Authenticator = {16-octet MD-5 checksum of the code (2),
                    id (1), the Request Authenticator from above, the
                    attributes in this reply, and the shared secret}
    Attributes:
        Service-Type = Framed-User
        Framed-Protocol = PPP
        Framed-IP-Address = 255.255.255.254
        Framed-Routing = None
        Framed-Compression = 1      (VJ TCP/IP Header Compression)
        Framed-MTU = 1500

Rigney, et. al. Informational [Page 60] RFC 2058 RADIUS January 1997

6.3. User with Challenge-Response card

 The NAS at 192.168.1.16 sends an Access-Request UDP packet to the
 RADIUS Server for a user named mopsy logging in on port 7.

Code = 1 (Access-Request) ID = 2 Request Authenticator = {16 octet random number} Attributes:

  User-Name = "mopsy"
  User-Password = {16 octets of Password padded at end with nulls,
              XORed with MD5(shared secret|Request Authenticator)}
  NAS-IP-Address = 192.168.1.16
  NAS-Port = 7
 The RADIUS server decides to challenge mopsy, sending back a
 challenge string and looking for a response.  The RADIUS server
 therefore and sends an Access-Challenge UDP packet to the NAS.

Code = 11 (Access-Challenge} ID = 2 (same as in Access-Request) Response Authenticator = {16-octet MD-5 checksum of the code (11),

              id (2), the Request Authenticator from above, the
              attributes in this reply, and the shared secret}

Attributes:

  Reply-Message = "Challenge 32769430.  Enter response at prompt."
  State =     {Magic Cookie to be returned along with user's response}
 The user enters his response, and the NAS send a new Access-Request
 with that response, and includes the State Attribute.

Code = 1 (Access-Request) ID = 3 (Note that this changes) Request Authenticator = {NEW 16 octet random number} Attributes:

  User-Name = "mopsy"
  User-Password = {16 octets of Response padded at end with
              nulls, XORed with MD5 checksum of shared secret
              plus above Request Authenticator}
  NAS-IP-Address = 192.168.1.16
  NAS-Port = 7
  State =     {Magic Cookie from Access-Challenge packet, unchanged}

Rigney, et. al. Informational [Page 61] RFC 2058 RADIUS January 1997

 The Response was incorrect, so the RADIUS server tells the NAS to
 reject the login attempt.
    Code = 3        (Access-Reject)
    ID = 3          (same as in Access-Request)
    Response Authenticator = {16-octet MD-5 checksum of the code (3),
                    id (3), the Request Authenticator from above, the
                    attributes in this reply if any, and the shared
                     secret}
    Attributes:
            (none, although a Reply-Message could be sent)

Security Considerations

 Security issues are the primary topic of this document.
 In practice, within or associated with each RADIUS server, there is a
 database which associates "user" names with authentication
 information ("secrets").  It is not anticipated that a particular
 named user would be authenticated by multiple methods.  This would
 make the user vulnerable to attacks which negotiate the least secure
 method from among a set.  Instead, for each named user there should
 be an indication of exactly one method used to authenticate that user
 name.  If a user needs to make use of different authentication
 methods under different circumstances, then distinct user names
 SHOULD be employed, each of which identifies exactly one
 authentication method.
 Passwords and other secrets should be stored at the respective ends
 such that access to them is as limited as possible.  Ideally, the
 secrets should only be accessible to the process requiring access in
 order to perform the authentication.
 The secrets should be distributed with a mechanism that limits the
 number of entities that handle (and thus gain knowledge of) the
 secret.  Ideally, no unauthorized person should ever gain knowledge
 of the secrets.  It is possible to achieve this with SNMP Security
 Protocols [8], but such a mechanism is outside the scope of this
 specification.
 Other distribution methods are currently undergoing research and
 experimentation.  The SNMP Security document [8] also has an
 excellent overview of threats to network protocols.

Rigney, et. al. Informational [Page 62] RFC 2058 RADIUS January 1997

References

 [1]   Rivest, R., and S. Dusse, "The MD5 Message-Digest Algorithm",
       RFC 1321, MIT Laboratory for Computer Science, RSA Data
       Security Inc., April 1992.
 [2]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
       USC/Information Sciences Institute, August 1980.
 [3]   Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
       1700, USC/Information Sciences Institute, October 1994.
 [4]   Kaufman, C., Perlman, R., and Speciner, M., "Network Security:
       Private Communications in a Public World", Prentice Hall, March
       1995, ISBN 0-13-061466-1.
 [5]   Jacobson, V., "Compressing TCP/IP headers for low-speed serial
       links", RFC 1144, Lawrence Berkeley Laboratory, February 1990.
 [6]   ISO 8859. International Standard -- Information Processing --
       8-bit Single-Byte Coded Graphic Character Sets -- Part 1: Latin
       Alphabet No. 1, ISO 8859-1:1987.
       <URL:http://www.iso.ch/cate/d16338.html>
 [7]   Sklower, K., Lloyd, B., McGregor, G., and Carr, D., "The PPP
       Multilink Protocol (MP)", RFC 1717, University of California
       Berkeley, Lloyd Internetworking, Newbridge Networks
       Corporation, November 1994.
 [8]   Galvin, J., McCloghrie, K., and J. Davin, "SNMP Security
       Protocols", RFC 1352, Trusted Information Systems, Inc., Hughes
       LAN Systems, Inc., MIT Laboratory for Computer Science, July
       1992.
 [9]   Rigney, C., "RADIUS Accounting", RFC 2059, January 1997.

Acknowledgments

 RADIUS was originally developed by Livingston Enterprises for their
 PortMaster series of Network Access Servers.

Rigney, et. al. Informational [Page 63] RFC 2058 RADIUS January 1997

Chair's Address

 The working group can be contacted via the current chair:
 Carl Rigney
 Livingston Enterprises
 6920 Koll Center Parkway, Suite 220
 Pleasanton, California  94566
 Phone: +1 510 426 0770
 EMail: cdr@livingston.com

Authors' Addresses

 Questions about this memo can also be directed to:
 Carl Rigney
 Livingston Enterprises
 6920 Koll Center Parkway, Suite 220
 Pleasanton, California  94566
 Phone: +1 510 426 0770
 EMail: cdr@livingston.com
 Allan C. Rubens
 Merit Network, Inc.
 4251 Plymouth Road
 Ann Arbor, Michigan  48105-2785
 EMail: acr@merit.edu
 William Allen Simpson
 Daydreamer
 Computer Systems Consulting Services
 1384 Fontaine
 Madison Heights, Michigan  48071
 EMail: wsimpson@greendragon.com
 Steve Willens
 Livingston Enterprises
 6920 Koll Center Parkway, Suite 220
 Pleasanton, California  94566
 EMail: steve@livingston.com

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