RFC 2138: Remote Authentication Dial In User Service (RADIUS)

Network Working Group C. Rigney
Request for Comments: 2138 Livingston
Obsoletes: 2058 A. Rubens
Category: Standards Track Merit
 W. Simpson
 Daydreamer
 S. Willens
 Livingston
 April 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.
Implementation Note
 This memo documents the RADIUS protocol. There has been some
 confusion in the assignment of port numbers for this protocol. The
 early deployment of RADIUS was done using the erroneously chosen port
 number 1645, which conflicts with the "datametrics" service. The
 officially assigned port number for RADIUS is 1812.
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 ................ 7
 2.3 Why UDP? ........................................ 8
 3. Packet Format ......................................... 10
 4. Packet Types .......................................... 13
 4.1 Access-Request .................................. 13
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RFC 2138 RADIUS April 1997
 4.2 Access-Accept ................................... 14
 4.3 Access-Reject ................................... 15
 4.4 Access-Challenge ................................ 17
 5. Attributes ............................................ 18
 5.1 User-Name ....................................... 21
 5.2 User-Password ................................... 22
 5.3 CHAP-Password ................................... 23
 5.4 NAS-IP-Address .................................. 24
 5.5 NAS-Port ........................................ 25
 5.6 Service-Type .................................... 26
 5.7 Framed-Protocol ................................. 28
 5.8 Framed-IP-Address ............................... 29
 5.9 Framed-IP-Netmask ............................... 29
 5.10 Framed-Routing .................................. 30
 5.11 Filter-Id ....................................... 31
 5.12 Framed-MTU ...................................... 32
 5.13 Framed-Compression .............................. 33
 5.14 Login-IP-Host ................................... 33
 5.15 Login-Service ................................... 34
 5.16 Login-TCP-Port .................................. 35
 5.17 (unassigned) .................................... 36
 5.18 Reply-Message ................................... 36
 5.19 Callback-Number ................................. 37
 5.20 Callback-Id ..................................... 38
 5.21 (unassigned) .................................... 38
 5.22 Framed-Route .................................... 39
 5.23 Framed-IPX-Network .............................. 40
 5.24 State ........................................... 40
 5.25 Class ........................................... 41
 5.26 Vendor-Specific ................................. 42
 5.27 Session-Timeout ................................. 44
 5.28 Idle-Timeout .................................... 44
 5.29 Termination-Action .............................. 45
 5.30 Called-Station-Id ............................... 46
 5.31 Calling-Station-Id .............................. 47
 5.32 NAS-Identifier .................................. 48
 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 ........................... 54
 5.40 CHAP-Challenge .................................. 55
 5.41 NAS-Port-Type ................................... 55
 5.42 Port-Limit ...................................... 56
 5.43 Login-LAT-Port .................................. 57
 5.44 Table of Attributes ............................. 58
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RFC 2138 RADIUS April 1997
 6. Examples .............................................. 59
 6.1 User Telnet to Specified Host ................... 60
 6.2 Framed User Authenticating with CHAP ............ 60
 6.3 User with Challenge-Response card ............... 61
 Security Considerations ...................................... 63
 References ................................................... 64
 Acknowledgements ............................................. 64
 Chair's Address .............................................. 65
 Author's Addresses ........................................... 65
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.
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RFC 2138 RADIUS April 1997
 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
 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.
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RFC 2138 RADIUS April 1997
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 [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.
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RFC 2138 RADIUS April 1997
 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
 "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.
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RFC 2138 RADIUS April 1997
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
 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
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RFC 2138 RADIUS April 1997
 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.
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.
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RFC 2138 RADIUS April 1997
 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 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.
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RFC 2138 RADIUS April 1997
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.
 This memo documents the RADIUS protocol. There has been some
 confusion in the assignment of port numbers for this protocol. The
 early deployment of RADIUS was done using the erroneously chosen port
 number 1645, which conflicts with the "datametrics" service. The
 officially assigned port number for RADIUS is 1812.
 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
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RFC 2138 RADIUS April 1997
 Codes 4 and 5 are 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.
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.
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RFC 2138 RADIUS April 1997
 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
 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.
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RFC 2138 RADIUS April 1997
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.
 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.
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RFC 2138 RADIUS April 1997
 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).
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RFC 2138 RADIUS April 1997
 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.
 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.
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RFC 2138 RADIUS April 1997
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.
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RFC 2138 RADIUS April 1997
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. Standards Track [Page 17]

RFC 2138 RADIUS April 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. Standards Track [Page 18]

RFC 2138 RADIUS April 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
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RFC 2138 RADIUS April 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
 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.
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RFC 2138 RADIUS April 1997
 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.
 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.
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RFC 2138 RADIUS April 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:
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RFC 2138 RADIUS April 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. Standards Track [Page 23]

RFC 2138 RADIUS April 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. Standards Track [Page 24]

RFC 2138 RADIUS April 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.
 A summary of the NAS-Port Attribute format is shown below. The
 fields are transmitted from left to right.
Rigney, et. al. Standards Track [Page 25]

RFC 2138 RADIUS April 1997
 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) |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
 6 for Service-Type.
Rigney, et. al. Standards Track [Page 26]

RFC 2138 RADIUS April 1997
 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.
 NAS Prompt The user should be provided a command prompt
 on the NAS from which non-privileged commands
 can be executed.
Rigney, et. al. Standards Track [Page 27]

RFC 2138 RADIUS April 1997
 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
 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
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RFC 2138 RADIUS April 1997
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.
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.
Rigney, et. al. Standards Track [Page 29]

RFC 2138 RADIUS April 1997
 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.
 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.
Rigney, et. al. Standards Track [Page 30]

RFC 2138 RADIUS April 1997
 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 ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
 11 for Filter-Id.
 Length
 >= 3
Rigney, et. al. Standards Track [Page 31]

RFC 2138 RADIUS April 1997
 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. Standards Track [Page 32]

RFC 2138 RADIUS April 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
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-
Rigney, et. al. Standards Track [Page 33]

RFC 2138 RADIUS April 1997
 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.
 A summary of the Login-Service Attribute format is shown below. The
 fields are transmitted from left to right.
Rigney, et. al. Standards Track [Page 34]

RFC 2138 RADIUS April 1997
 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) |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
 16 for Login-TCP-Port.
Rigney, et. al. Standards Track [Page 35]

RFC 2138 RADIUS April 1997
 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 ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 Type
 18 for Reply-Message.
Rigney, et. al. Standards Track [Page 36]

RFC 2138 RADIUS April 1997
 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. Standards Track [Page 37]

RFC 2138 RADIUS April 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.
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RFC 2138 RADIUS April 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.
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RFC 2138 RADIUS April 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.
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RFC 2138 RADIUS April 1997
 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
 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.
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RFC 2138 RADIUS April 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.
 A summary of the Vendor-Specific Attribute format is shown below.
 The fields are transmitted from left to right.
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RFC 2138 RADIUS April 1997
 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 [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.
 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...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
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RFC 2138 RADIUS April 1997
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
 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.
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RFC 2138 RADIUS April 1997
 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.
 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.
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RFC 2138 RADIUS April 1997
 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.
 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.
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RFC 2138 RADIUS April 1997
 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 ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 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.
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RFC 2138 RADIUS April 1997
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
 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.
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RFC 2138 RADIUS April 1997
 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
 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.
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RFC 2138 RADIUS April 1997
 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.
 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.
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RFC 2138 RADIUS April 1997
 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
 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.
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RFC 2138 RADIUS April 1997
 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
 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) |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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RFC 2138 RADIUS April 1997
 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.
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
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RFC 2138 RADIUS April 1997
 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.
 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.
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RFC 2138 RADIUS April 1997
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...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
 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.
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RFC 2138 RADIUS April 1997
 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
 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.
Rigney, et. al. Standards Track [Page 56]

RFC 2138 RADIUS April 1997
 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.
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.
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RFC 2138 RADIUS April 1997
 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.
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
Rigney, et. al. Standards Track [Page 58]

RFC 2138 RADIUS April 1997
 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
 [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.
Rigney, et. al. Standards Track [Page 59]

RFC 2138 RADIUS April 1997
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
 Length = 56
 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)
 Length = 38
 Response Authenticator = {16-octet MD-5 checksum of the code (2),
 id (0), Length (38), 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
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.
Rigney, et. al. Standards Track [Page 60]

RFC 2138 RADIUS April 1997
 Code = 1 (Access-Request)
 ID = 1
 Length = 71
 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)
 Length = 56
 Response Authenticator = {16-octet MD-5 checksum of the code (2),
 id (1), Length (56), 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
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
 Length = 57
 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
Rigney, et. al. Standards Track [Page 61]

RFC 2138 RADIUS April 1997
 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)
 Length = 78
 Response Authenticator = {16-octet MD-5 checksum of the code (11),
 id (2), length (78), 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;
 in this example 8 octets of data}
 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)
 Length = 67
 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}
 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)
 Length = 20
 Response Authenticator = {16-octet MD-5 checksum of the code (3),
 id (3), length(20), 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)
Rigney, et. al. Standards Track [Page 62]

RFC 2138 RADIUS April 1997
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. Standards Track [Page 63]

RFC 2138 RADIUS April 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 Davin, J., "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 2139, April 1997.
Acknowledgments
 RADIUS was originally developed by Livingston Enterprises for their
 PortMaster series of Network Access Servers.
Rigney, et. al. Standards Track [Page 64]

RFC 2138 RADIUS April 1997
Chair's Address
 The working group can be contacted via the current chair:
 Carl Rigney
 Livingston Enterprises
 4464 Willow Road
 Pleasanton, California 94588
 Phone: +1 510 426 0770
 EMail: cdr@livingston.com
Authors' Addresses
 Questions about this memo can also be directed to:
 Carl Rigney
 Livingston Enterprises
 4464 Willow Road
 Pleasanton, California 94588
 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
 4464 Willow Road
 Pleasanton, California 94588
 EMail: steve@livingston.com
Rigney, et. al. Standards Track [Page 65]

Document	Document type	RFC - Proposed Standard April 1997 Report errata Obsoleted by RFC 2865 Obsoletes RFC 2058 Was draft-ietf-radius-radius (radius WG)
Select version	04 RFC 2138
Compare versions
Authors	Allan Rubens , Carl Rigney , Steve Willens , William A. Simpson Email authors
RFC stream	IETF Logo IETF Logo
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Additional resources	Mailing list discussion

RFC 2138 - Remote Authentication Dial In User Service (RADIUS)