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RFC 2865

Remote Authentication Dial In User Service (RADIUS)

Pages: 76
Draft Standard
Errata
Obsoletes:  2138
Updated by:  28683575508069298044
Part 1 of 3 – Pages 1 to 22
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Top   ToC   RFC2865 - Page 1
Network Working Group                                          C. Rigney
Request for Comments: 2865                                    S. Willens
Obsoletes: 2138                                               Livingston
Category: Standards Track                                      A. Rubens
                                                                   Merit
                                                              W. Simpson
                                                              Daydreamer
                                                               June 2000


          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.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

IESG Note:

   This protocol is widely implemented and used.  Experience has shown
   that it can suffer degraded performance and lost data when used in
   large scale systems, in part because it does not include provisions
   for congestion control.  Readers of this document may find it
   beneficial to track the progress of the IETF's AAA working group,
   which may develop a successor protocol that better addresses the
   scaling and congestion control issues.

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. Implementation Note This memo documents the RADIUS protocol. The early deployment of RADIUS was done using UDP port number 1645, which conflicts with the "datametrics" service. The officially assigned port number for RADIUS is 1812.
Top   ToC   RFC2865 - Page 2

Table of Contents

1. Introduction .......................................... 3 1.1 Specification of Requirements ................... 4 1.2 Terminology ..................................... 5 2. Operation ............................................. 5 2.1 Challenge/Response .............................. 7 2.2 Interoperation with PAP and CHAP ................ 8 2.3 Proxy ........................................... 8 2.4 Why UDP? ........................................ 11 2.5 Retransmission Hints ............................ 12 2.6 Keep-Alives Considered Harmful .................. 13 3. Packet Format ......................................... 13 4. Packet Types .......................................... 17 4.1 Access-Request .................................. 17 4.2 Access-Accept ................................... 18 4.3 Access-Reject ................................... 20 4.4 Access-Challenge ................................ 21 5. Attributes ............................................ 22 5.1 User-Name ....................................... 26 5.2 User-Password ................................... 27 5.3 CHAP-Password ................................... 28 5.4 NAS-IP-Address .................................. 29 5.5 NAS-Port ........................................ 30 5.6 Service-Type .................................... 31 5.7 Framed-Protocol ................................. 33 5.8 Framed-IP-Address ............................... 34 5.9 Framed-IP-Netmask ............................... 34 5.10 Framed-Routing .................................. 35 5.11 Filter-Id ....................................... 36 5.12 Framed-MTU ...................................... 37 5.13 Framed-Compression .............................. 37 5.14 Login-IP-Host ................................... 38 5.15 Login-Service ................................... 39 5.16 Login-TCP-Port .................................. 40 5.17 (unassigned) .................................... 41 5.18 Reply-Message ................................... 41 5.19 Callback-Number ................................. 42 5.20 Callback-Id ..................................... 42 5.21 (unassigned) .................................... 43 5.22 Framed-Route .................................... 43 5.23 Framed-IPX-Network .............................. 44 5.24 State ........................................... 45 5.25 Class ........................................... 46 5.26 Vendor-Specific ................................. 47 5.27 Session-Timeout ................................. 48 5.28 Idle-Timeout .................................... 49 5.29 Termination-Action .............................. 49
Top   ToC   RFC2865 - Page 3
      5.30      Called-Station-Id ...............................   50
      5.31      Calling-Station-Id ..............................   51
      5.32      NAS-Identifier ..................................   52
      5.33      Proxy-State .....................................   53
      5.34      Login-LAT-Service ...............................   54
      5.35      Login-LAT-Node ..................................   55
      5.36      Login-LAT-Group .................................   56
      5.37      Framed-AppleTalk-Link ...........................   57
      5.38      Framed-AppleTalk-Network ........................   58
      5.39      Framed-AppleTalk-Zone ...........................   58
      5.40      CHAP-Challenge ..................................   59
      5.41      NAS-Port-Type ...................................   60
      5.42      Port-Limit ......................................   61
      5.43      Login-LAT-Port ..................................   62
      5.44      Table of Attributes .............................   63
   6.     IANA Considerations ...................................   64
      6.1       Definition of Terms .............................   64
      6.2       Recommended Registration Policies ...............   65
   7.     Examples ..............................................   66
      7.1       User Telnet to Specified Host ...................   66
      7.2       Framed User Authenticating with CHAP ............   67
      7.3       User with Challenge-Response card ...............   68
   8.     Security Considerations ...............................   71
   9.     Change Log ............................................   71
   10.    References ............................................   73
   11.    Acknowledgements ......................................   74
   12.    Chair's Address .......................................   74
   13.    Authors' Addresses ....................................   75
   14.    Full Copyright Statement ..............................   76

1. Introduction

This document obsoletes RFC 2138 [1]. A summary of the changes between this document and RFC 2138 is available in the "Change Log" appendix. 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).
Top   ToC   RFC2865 - Page 4
   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.

   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

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [2]. These key words mean the same thing whether capitalized or not. An implementation is not compliant if it fails to satisfy one or more of the must or must not requirements for the protocols it implements. An implementation that satisfies all the must, must not, should and
Top   ToC   RFC2865 - Page 5
   should not requirements for its protocols is said to be
   "unconditionally compliant"; one that satisfies all the must and must
   not requirements but not all the should or should not requirements
   for its protocols is said to be "conditionally compliant".

   A NAS that does not implement a given service MUST NOT implement the
   RADIUS attributes for that service.  For example, a NAS that is
   unable to offer ARAP service MUST NOT implement the RADIUS attributes
   for ARAP.  A NAS MUST treat a RADIUS access-accept authorizing an
   unavailable service as an access-reject instead.

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 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 [3].
Top   ToC   RFC2865 - Page 6
   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 MUST 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 Proxy-State attributes were present in the Access-Request,
   they MUST be copied unmodified and in order into the response packet.
   Other Attributes can be placed before, after, or even between the
   Proxy-State attributes.

   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 (except Proxy-State) 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
   "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 Attribute SHOULD be
Top   ToC   RFC2865 - Page 7
   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 is 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 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
Top   ToC   RFC2865 - Page 8
   came with the Access-Challenge.  The server then sends back either an
   Access-Accept or Access-Reject based on whether the response matches
   the required value, 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 MUST 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.

2.3. Proxy

With proxy RADIUS, one RADIUS server receives an authentication (or accounting) request from a RADIUS client (such as a NAS), forwards the request to a remote RADIUS server, receives the reply from the remote server, and sends that reply to the client, possibly with changes to reflect local administrative policy. A common use for proxy RADIUS is roaming. Roaming permits two or more administrative entities to allow each other's users to dial in to either entity's network for service.
Top   ToC   RFC2865 - Page 9
   The NAS sends its RADIUS access-request to the "forwarding server"
   which forwards it to the "remote server".  The remote server sends a
   response (Access-Accept, Access-Reject, or Access-Challenge) back to
   the forwarding server, which sends it back to the NAS.  The User-Name
   attribute MAY contain a Network Access Identifier [8] for RADIUS
   Proxy operations.  The choice of which server receives the forwarded
   request SHOULD be based on the authentication "realm". The
   authentication realm MAY be the realm part of a Network Access
   Identifier (a "named realm").  Alternatively, the choice of which
   server receives the forwarded request MAY be based on whatever other
   criteria the forwarding server is configured to use, such as Called-
   Station-Id (a "numbered realm").

   A RADIUS server can function as both a forwarding server and a remote
   server, serving as a forwarding server for some realms and a remote
   server for other realms.  One forwarding server can act as a
   forwarder for any number of remote servers.  A remote server can have
   any number of servers forwarding to it and can provide authentication
   for any number of realms.  One forwarding server can forward to
   another forwarding server to create a chain of proxies, although care
   must be taken to avoid introducing loops.

   The following scenario illustrates a proxy RADIUS communication
   between a NAS and the forwarding and remote RADIUS servers:

   1. A NAS sends its access-request to the forwarding server.

   2. The forwarding server forwards the access-request to the remote
      server.

   3. The remote server sends an access-accept, access-reject or
      access-challenge back to the forwarding server.  For this example,
      an access-accept is sent.

   4. The forwarding server sends the access-accept to the NAS.

   The forwarding server MUST treat any Proxy-State attributes already
   in the packet as opaque data.  Its operation MUST NOT depend on the
   content of Proxy-State attributes added by previous servers.

   If there are any Proxy-State attributes in the request received from
   the client, the forwarding server MUST include those Proxy-State
   attributes in its reply to the client.  The forwarding server MAY
   include the Proxy-State attributes in the access-request when it
   forwards the request, or MAY omit them in the forwarded request.  If
   the forwarding server omits the Proxy-State attributes in the
   forwarded access-request, it MUST attach them to the response before
   sending it to the client.
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   We now examine each step in more detail.

   1. A NAS sends its access-request to the forwarding server.  The
      forwarding server decrypts the User-Password, if present, using
      the shared secret it knows for the NAS.  If a CHAP-Password
      attribute is present in the packet and no CHAP-Challenge attribute
      is present, the forwarding server MUST leave the Request-
      Authenticator untouched or copy it to a CHAP-Challenge attribute.

   '' The forwarding server MAY add one Proxy-State attribute to the
      packet.  (It MUST NOT add more than one.)  If it adds a Proxy-
      State, the Proxy-State MUST appear after any other Proxy-States in
      the packet.  The forwarding server MUST NOT modify any other
      Proxy-States that were in the packet (it may choose not to forward
      them, but it MUST NOT change their contents).  The forwarding
      server MUST NOT change the order of any attributes of the same
      type, including Proxy-State.

   2. The forwarding server encrypts the User-Password, if present,
      using the secret it shares with the remote server, sets the
      Identifier as needed, and forwards the access-request to the
      remote server.

   3. The remote server (if the final destination) verifies the user
      using User-Password, CHAP-Password, or such method as future
      extensions may dictate, and returns an access-accept, access-
      reject or access-challenge back to the forwarding server.  For
      this example, an access-accept is sent.  The remote server MUST
      copy all Proxy-State attributes (and only the Proxy-State
      attributes) in order from the access-request to the response
      packet, without modifying them.

   4. The forwarding server verifies the Response Authenticator using
      the secret it shares with the remote server, and silently discards
      the packet if it fails verification.  If the packet passes
      verification, the forwarding server removes the last Proxy-State
      (if it attached one), signs the Response Authenticator using the
      secret it shares with the NAS, restores the Identifier to match
      the one in the original request by the NAS, and sends the access-
      accept to the NAS.

   A forwarding server MAY need to modify attributes to enforce local
   policy.  Such policy is outside the scope of this document, with the
   following restrictions.  A forwarding server MUST not modify existing
   Proxy-State, State, or Class attributes present in the packet.
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   Implementers of forwarding servers should consider carefully which
   values it is willing to accept for Service-Type.  Careful
   consideration must be given to the effects of passing along Service-
   Types of NAS-Prompt or Administrative in a proxied Access-Accept, and
   implementers may wish to provide mechanisms to block those or other
   service types, or other attributes.  Such mechanisms are outside the
   scope of this document.

2.4. 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.
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   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 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
      than 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.

2.5. Retransmission Hints

If the RADIUS server and alternate RADIUS server share the same shared secret, it is OK to retransmit the packet to the alternate RADIUS server with the same ID and Request Authenticator, because the content of the attributes haven't changed. If you want to use a new Request Authenticator when sending to the alternate server, you may. If you change the contents of the User-Password attribute (or any other attribute), you need a new Request Authenticator and therefore a new ID. If the NAS is retransmitting a RADIUS request to the same server as before, and the attributes haven't changed, you MUST use the same Request Authenticator, ID, and source port. If any attributes have changed, you MUST use a new Request Authenticator and ID. A NAS MAY use the same ID across all servers, or MAY keep track of IDs separately for each server, it is up to the implementer. If a NAS needs more than 256 IDs for outstanding requests, it MAY use
Top   ToC   RFC2865 - Page 13
   additional source ports to send requests from, and keep track of IDs
   for each source port.  This allows up to 16 million or so outstanding
   requests at one time to a single server.

2.6. Keep-Alives Considered Harmful

Some implementers have adopted the practice of sending test RADIUS requests to see if a server is alive. This practice is strongly discouraged, since it adds to load and harms scalability without providing any additional useful information. Since a RADIUS request is contained in a single datagram, in the time it would take you to send a ping you could just send the RADIUS request, and getting a reply tells you that the RADIUS server is up. If you do not have a RADIUS request to send, it does not matter if the server is up or not, because you are not using it. If you want to monitor your RADIUS server, use SNMP. That's what SNMP is for.

3. Packet Format

Exactly one RADIUS packet is encapsulated in the UDP Data field [4], where the UDP Destination Port field indicates 1812 (decimal). When a reply is generated, the source and destination ports are reversed. This memo documents the RADIUS protocol. The early deployment of RADIUS was done using UDP port number 1645, which conflicts with the "datametrics" service. The officially assigned port number for RADIUS is 1812.
Top   ToC   RFC2865 - Page 14
   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 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-

   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 are covered in the RADIUS Accounting document [5].
   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.  The RADIUS server can detect a duplicate request if
      it has the same client source IP address and source UDP port and
      Identifier within a short span of time.
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   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
      MUST be treated as padding and ignored on reception.  If the
      packet is shorter than the Length field indicates, it MUST 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.

      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
Top   ToC   RFC2865 - Page 16
         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 provide protection against exhaustive search attacks.
      The secret MUST NOT be empty (length 0) since this would allow
      packets to be trivially forged.

      A RADIUS server MUST use the source IP address of the RADIUS UDP
      packet to decide which shared secret to use, so that RADIUS
      requests can be proxied.

      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 MAY add a Proxy-State Attribute,
      and when the proxy forwards a response, it MUST remove its Proxy-
      State Attribute if it added one.  Proxy-State is always added or
      removed after any other Proxy-States, but no other assumptions
      regarding its location within the list of attributes can be made.
      Since Access-Accept and Access-Reject replies are authenticated on
      the entire packet contents, the stripping of the Proxy-State
      attribute invalidates 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.
Top   ToC   RFC2865 - Page 17

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. An Access-Request SHOULD contain a User-Name attribute. It MUST contain either a NAS-IP-Address attribute or a NAS-Identifier attribute (or both). An Access-Request MUST contain either a User-Password or a CHAP- Password or a State. An Access-Request MUST NOT contain both a User-Password and a CHAP-Password. If future extensions allow other kinds of authentication information to be conveyed, the attribute for that can be used in an Access-Request instead of User-Password or CHAP-Password. An Access-Request 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 [3].
Top   ToC   RFC2865 - Page 18
   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.

   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).
Top   ToC   RFC2865 - Page 19
      On reception of an Access-Accept, the Identifier field is matched
      with a pending Access-Request.  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.

   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.
Top   ToC   RFC2865 - Page 20

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. 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.
Top   ToC   RFC2865 - Page 21

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. Vendor-Specific, Idle-Timeout, Session-Timeout and Proxy-State attributes MAY also be included. No other Attributes defined in this document 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 dialing 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 MUST treat the Access-Challenge as though it had received an Access-Reject instead.
Top   ToC   RFC2865 - Page 22
   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.



(page 22 continued on part 2)

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