RFC2058 - Remote Authentication Dial In User Service (RADIUS)

Network Working Group C. Rigney
Request for Comments: 2058 Livingston
Category: Standards Track A. Rubens
Merit
W. Simpson
Daydreamer
S. Willens
Livingston
January 1997
Remote Authentication Dial In User Service (RADIUS)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document describes a protocol for carrying authentication,
authorization, and configuration information between a Network Access
Server which desires to authenticate its links and a shared
Authentication Server.
Table of Contents
1. IntrodUCtion .......................................... 3
1.1 Specification of Requirements ................... 4
1.2 Terminology ..................................... 4
2. Operation ............................................. 5
2.1 Challenge/Response .............................. 6
2.2 Interoperation with PAP and CHAP ................ 7
2.3 Why UDP? ........................................ 8
3. Packet Format ......................................... 9
4. Packet Types .......................................... 12
4.1 Access-Request .................................. 12
4.2 Access-Accept ................................... 14
4.3 Access-Reject ................................... 15
4.4 Access-Challenge ................................ 16
5. Attributes ............................................ 17
5.1 User-Name ....................................... 20
5.2 User-PassWord ................................... 21
5.3 CHAP-Password ................................... 22
5.4 NAS-IP-Address .................................. 23
5.5 NAS-Port ........................................ 24
5.6 Service-Type .................................... 25
5.7 Framed-Protocol ................................. 27
5.8 Framed-IP-Address ............................... 28
5.9 Framed-IP-Netmask ............................... 29
5.10 Framed-Routing .................................. 29
5.11 Filter-Id ....................................... 30
5.12 Framed-MTU ...................................... 31
5.13 Framed-Compression .............................. 32
5.14 Login-IP-Host ................................... 33
5.15 Login-Service ................................... 33
5.16 Login-TCP-Port .................................. 34
5.17 (unassigned) .................................... 35
5.18 Reply-Message ................................... 35
5.19 Callback-Number ................................. 36
5.20 Callback-Id ..................................... 37
5.21 (unassigned) .................................... 37
5.22 Framed-Route .................................... 38
5.23 Framed-IPX-Network .............................. 39
5.24 State ........................................... 39
5.25 Class ........................................... 40
5.26 Vendor-Specific ................................. 41
5.27 Session-Timeout ................................. 43
5.28 Idle-Timeout .................................... 44
5.29 Termination-Action .............................. 44
5.30 Called-Station-Id ............................... 45
5.31 Calling-Station-Id .............................. 46
5.32 NAS-Identifier .................................. 47
5.33 Proxy-State ..................................... 48
5.34 Login-LAT-Service ............................... 49
5.35 Login-LAT-Node .................................. 50
5.36 Login-LAT-Group ................................. 51
5.37 Framed-AppleTalk-Link ........................... 52
5.38 Framed-AppleTalk-Network ........................ 53
5.39 Framed-AppleTalk-Zone ........................... 53
5.40 CHAP-Challenge .................................. 54
5.41 NAS-Port-Type ................................... 55
5.42 Port-Limit ...................................... 56
5.43 Login-LAT-Port .................................. 57
5.44 Table of Attributes ............................. 58
6. Examples .............................................. 59
6.1 User Telnet to Specified Host ................... 59
6.2 Framed User Authenticating with CHAP ............ 60
6.3 User with Challenge-Response card ............... 61
SECURITY CONSIDERATIONS ...................................... 62
REFERENCES ................................................... 63
ACKNOWLEDGEMENTS ............................................. 63
CHAIR"S ADDRESS .............................................. 64
AUTHORS" ADDRESSES ........................................... 64
1. Introduction
Managing dispersed serial line and modem pools for large numbers of
users can create the need for significant administrative support.
Since modem pools are by definition a link to the outside world, they
require careful attention to security, authorization and accounting.
This can be best achieved by managing a single "database" of users,
which allows for authentication (verifying user name and password) as
well as configuration information detailing the type of service to
deliver to the user (for example, SLIP, PPP, telnet, rlogin).
Key features of RADIUS are:
Client/Server Model
A Network Access Server (NAS) operates as a client of RADIUS. The
client is responsible for passing user information to designated
RADIUS servers, and then acting on the response which is returned.
RADIUS servers are responsible for receiving user connection
requests, authenticating the user, and then returning all
configuration information necessary for the client to deliver
service to the user.
A RADIUS server can act as a proxy client to other RADIUS servers
or other kinds of authentication servers.
Network Security
Transactions between the client and RADIUS server are
authenticated through the use of a shared secret, which is never
sent over the network. In addition, any user passwords are sent
encrypted between the client and RADIUS server, to eliminate the
possibility that someone snooping on an unsecure network could
determine a user"s password.
Flexible Authentication Mechanisms
The RADIUS server can support a variety of methods to authenticate
a user. When it is provided with the user name and original
password given by the user, it can support PPP PAP or CHAP, UNIX
login, and other authentication mechanisms.
Extensible Protocol
All transactions are comprised of variable length Attribute-
Length-Value 3-tuples. New attribute values can be added without
disturbing existing implementations of the protocol.
1.1. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.
MUST NOT This phrase means that the definition is an absolute
prohibition of the specification.
SHOULD This word, or the adjective "recommended", means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications must be
understood and carefully weighed before choosing a
different course.
MAY This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST be
prepared to interoperate with another implementation which
does include the option.
1.2. Terminology
This document frequently uses the following terms:
service The NAS provides a service to the dial-in user, such as PPP
or Telnet.
session Each service provided by the NAS to a dial-in user
constitutes a session, with the beginning of the session
defined as the point where service is first provided and
the end of the session defined as the point where service
is ended. A user may have multiple sessions in parallel or
series if the NAS supports that.
silently discard
This means the implementation discards the packet without
further processing. The implementation SHOULD provide the
capability of logging the error, including the contents of
the silently discarded packet, and SHOULD record the event
in a statistics counter.
2. Operation
When a client is configured to use RADIUS, any user of the client
presents authentication information to the client. This might be
with a customizable login prompt, where the user is eXPected to enter
their username and password. Alternatively, the user might use a
link framing protocol such as the Point-to-Point Protocol (PPP),
which has authentication packets which carry this information.
Once the client has oBTained such information, it may choose to
authenticate using RADIUS. To do so, the client creates an "Access-
Request" containing such Attributes as the user"s name, the user"s
password, the ID of the client and the Port ID which the user is
accessing. When a password is present, it is hidden using a method
based on the RSA Message Digest Algorithm MD5 [1].
The Access-Request is submitted to the RADIUS server via the network.
If no response is returned within a length of time, the request is
re-sent a number of times. The client can also forward requests to
an alternate server or servers in the event that the primary server
is down or unreachable. An alternate server can be used either after
a number of tries to the primary server fail, or in a round-robin
fashion. Retry and fallback algorithms are the topic of current
research and are not specified in detail in this document.
Once the RADIUS server receives the request, it validates the sending
client. A request from a client for which the RADIUS server does not
have a shared secret should be silently discarded. If the client is
valid, the RADIUS server consults a database of users to find the
user whose name matches the request. The user entry in the database
contains a list of requirements which must be met to allow access for
the user. This always includes verification of the password, but can
also specify the client(s) or port(s) to which the user is allowed
access.
The RADIUS server MAY make requests of other servers in order to
satisfy the request, in which case it acts as a client.
If any condition is not met, the RADIUS server sends an "Access-
Reject" response indicating that this user request is invalid. If
desired, the server MAY include a text message in the Access-Reject
which MAY be displayed by the client to the user. No other
Attributes are permitted in an Access-Reject.
If all conditions are met and the RADIUS server wishes to issue a
challenge to which the user must respond, the RADIUS server sends an
"Access-Challenge" response. It MAY include a text message to be
displayed by the client to the user prompting for a response to the
challenge, and MAY include a State attribute. If the client receives
an Access-Challenge and supports challenge/response it MAY display
the text message, if any, to the user, and then prompt the user for a
response. The client then re-submits its original Access-Request
with a new request ID, with the User-Password Attribute replaced by
the response (encrypted), and including the State Attribute from the
Access-Challenge, if any. Only 0 or 1 instances of the State
Attributes should be present in a request. The server can respond to
this new Access-Request with either an Access-Accept, an Access-
Reject, or another Access-Challenge.
If all conditions are met, the list of configuration values for the
user are placed into an "Access-Accept" response. These values
include the type of service (for example: SLIP, PPP, Login User) and
all necessary values to deliver the desired service. For SLIP and
PPP, this may include values such as IP address, subnet mask, MTU,
desired compression, and desired packet filter identifiers. For
character mode users, this may include values such as desired
protocol and host.
2.1. Challenge/Response
In challenge/response authentication, the user is given an
unpredictable number and challenged to encrypt it and give back the
result. Authorized users are equipped with special devices such as
smart cards or software that facilitate calculation of the correct
response with ease. Unauthorized users, lacking the appropriate
device or software and lacking knowledge of the secret key necessary
to emulate such a device or software, can only guess at the response.
The Access-Challenge packet typically contains a Reply-Message
including a challenge to be displayed to the user, such as a numeric
value unlikely ever to be repeated. Typically this is obtained from
an external server that knows what type of authenticator should be in
the possession of the authorized user and can therefore choose a
random or non-repeating pseudorandom number of an appropriate radix
and length.
The user then enters the challenge into his device (or software) and
it calculates a response, which the user enters into the client which
forwards it to the RADIUS server via a second Access-Request. If the
response matches the expected response the RADIUS server replies with
an Access-Accept, otherwise an Access-Reject.
Example: The NAS sends an Access-Request packet to the RADIUS Server
with NAS-Identifier, NAS-Port, User-Name, User-Password (which may
just be a fixed string like "challenge" or ignored). The server
sends back an Access-Challenge packet with State and a Reply-Message
along the lines of "Challenge 12345678, enter your response at the
prompt" which the NAS displays. The NAS prompts for the response and
sends a NEW Access-Request to the server (with a new ID) with NAS-
Identifier, NAS-Port, User-Name, User-Password (the response just
entered by the user, encrypted), and the same State Attribute that
came with the Access-Challenge. The server then sends back either an
Access-Accept or Access-Reject based on whether the response matches
what it should be, or it can even send another Access-Challenge.
2.2. Interoperation with PAP and CHAP
For PAP, the NAS takes the PAP ID and password and sends them in an
Access-Request packet as the User-Name and User-Password. The NAS MAY
include the Attributes Service-Type = Framed-User and Framed-Protocol
= PPP as a hint to the RADIUS server that PPP service is expected.
For CHAP, the NAS generates a random challenge (preferably 16 octets)
and sends it to the user, who returns a CHAP response along with a
CHAP ID and CHAP username. The NAS then sends an Access-Request
packet to the RADIUS server with the CHAP username as the User-Name
and with the CHAP ID and CHAP response as the CHAP-Password
(Attribute 3). The random challenge can either be included in the
CHAP-Challenge attribute or, if it is 16 octets long, it can be
placed in the Request Authenticator field of the Access-Request
packet. The NAS MAY include the Attributes Service-Type = Framed-
User and Framed-Protocol = PPP as a hint to the RADIUS server that
PPP service is expected.
The RADIUS server looks up a password based on the User-Name,
encrypts the challenge using MD5 on the CHAP ID octet, that password,
and the CHAP challenge (from the CHAP-Challenge attribute if present,
otherwise from the Request Authenticator), and compares that result
to the CHAP-Password. If they match, the server sends back an
Access-Accept, otherwise it sends back an Access-Reject.
If the RADIUS server is unable to perform the requested
authentication it should return an Access-Reject. For example, CHAP
requires that the user"s password be available in cleartext to the
server so that it can encrypt the CHAP challenge and compare that to
the CHAP response. If the password is not available in cleartext to
the RADIUS server then the server MUST send an Access-Reject to the
client.
2.3. Why UDP?
A frequently asked question is why RADIUS uses UDP instead of TCP as
a transport protocol. UDP was chosen for strictly technical reasons.
There are a number of issues which must be understood. RADIUS is a
transaction based protocol which has several interesting
characteristics:
1. If the request to a primary Authentication server fails, a
secondary server must be queried.
To meet this requirement, a copy of the request must be kept
above the transport layer to allow for alternate transmission.
This means that retransmission timers are still required.
2. The timing requirements of this particular protocol are
significantly different than TCP provides.
At one extreme, RADIUS does not require a "responsive" detection
of lost data. The user is willing to wait several seconds for
the authentication to complete. The generally aggressive TCP
retransmission (based on average round trip time) is not
required, nor is the acknowledgement overhead of TCP.
At the other extreme, the user is not willing to wait several
minutes for authentication. Therefore the reliable delivery of
TCP data two minutes later is not useful. The faster use of an
alternate server allows the user to gain access before giving
up.
3. The stateless nature of this protocol simplifies the use of UDP.
Clients and servers come and go. Systems are rebooted, or are
power cycled independently. Generally this does not cause a
problem and with creative timeouts and detection of lost TCP
connections, code can be written to handle anomalous events.
UDP however completely eliminates any of this special handling.
Each client and server can open their UDP transport just once
and leave it open through all types of failure events on the
network.
4. UDP simplifies the server implementation.
In the earliest implementations of RADIUS, the server was single
threaded. This means that a single request was received,
processed, and returned. This was found to be unmanageable in
environments where the back-end security mechanism took real
time (1 or more seconds). The server request queue would fill
and in environments where hundreds of people were being
authenticated every minute, the request turn-around time
increased to longer that users were willing to wait (this was
especially severe when a specific lookup in a database or over
DNS took 30 or more seconds). The obvious solution was to make
the server multi-threaded. Achieving this was simple with UDP.
Separate processes were spawned to serve each request and these
processes could respond directly to the client NAS with a simple
UDP packet to the original transport of the client.
It"s not all a panacea. As noted, using UDP requires one thing which
is built into TCP: with UDP we must artificially manage
retransmission timers to the same server, although they don"t require
the same attention to timing provided by TCP. This one penalty is a
small price to pay for the advantages of UDP in this protocol.
Without TCP we would still probably be using tin cans connected by
string. But for this particular protocol, UDP is a better choice.
3. Packet Format
Exactly one RADIUS packet is encapsulated in the UDP Data field [2],
where the UDP Destination Port field indicates 1812 (decimal).
When a reply is generated, the source and destination ports are
reversed.
A summary of the RADIUS data format is shown below. The fields are
transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authenticator


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
The Code field is one octet, and identifies the type of RADIUS
packet. When a packet is received with an invalid Code field, it is
silently discarded.
RADIUS Codes (decimal) are assigned as follows:
1 Access-Request
2 Access-Accept
3 Access-Reject
4 Accounting-Request
5 Accounting-Response
11 Access-Challenge
12 Status-Server (experimental)
13 Status-Client (experimental)
255 Reserved
Codes 4 and 5 will be covered in the RADIUS Accounting document [9],
and are not further mentioned here. Codes 12 and 13 are reserved for
possible use, but are not further mentioned here.
Identifier
The Identifier field is one octet, and aids in matching requests and
replies.
Length
The Length field is two octets. It indicates the length of the
packet including the Code, Identifier, Length, Authenticator and
Attribute fields. Octets outside the range of the Length field
should be treated as padding and should be ignored on reception. If
the packet is shorter than the Length field indicates, it should be
silently discarded. The minimum length is 20 and maximum length is
4096.
Authenticator
The Authenticator field is sixteen (16) octets. The most significant
octet is transmitted first. This value is used to authenticate the
reply from the RADIUS server, and is used in the password hiding
algorithm.
Request Authenticator
In Access-Request Packets, the Authenticator value is a 16 octet
random number, called the Request Authenticator. The value SHOULD be
unpredictable and unique over the lifetime of a secret (the password
shared between the client and the RADIUS server), since repetition of
a request value in conjunction with the same secret would permit an
attacker to reply with a previously intercepted response. Since it
is expected that the same secret MAY be used to authenticate with
servers in disparate geographic regions, the Request Authenticator
field SHOULD exhibit global and temporal uniqueness.
The Request Authenticator value in an Access-Request packet SHOULD
also be unpredictable, lest an attacker trick a server into
responding to a predicted future request, and then use the response
to masquerade as that server to a future Access-Request.
Although protocols such as RADIUS are incapable of protecting against
theft of an authenticated session via realtime active wiretapping
attacks, generation of unique unpredictable requests can protect
against a wide range of active attacks against authentication.
The NAS and RADIUS server share a secret. That shared secret
followed by the Request Authenticator is put through a one-way MD5
hash to create a 16 octet digest value which is xored with the
password entered by the user, and the xored result placed in the
User-Password attribute in the Access-Request packet. See the entry
for User-Password in the section on Attributes for a more detailed
description.
Response Authenticator
The value of the Authenticator field in Access-Accept, Access-
Reject, and Access-Challenge packets is called the Response
Authenticator, and contains a one-way MD5 hash calculated over a
stream of octets consisting of: the RADIUS packet, beginning with
the Code field, including the Identifier, the Length, the Request
Authenticator field from the Access-Request packet, and the
response Attributes, followed by the shared secret. That is,
ResponseAuth = MD5(Code+ID+Length+RequestAuth+Attributes+Secret)
where + denotes concatenation.
Administrative Note
The secret (password shared between the client and the RADIUS server)
SHOULD be at least as large and unguessable as a well-chosen
password. It is preferred that the secret be at least 16 octets.
This is to ensure a sufficiently large range for the secret to
provide protection against exhaustive search attacks. A RADIUS
server SHOULD use the source IP address of the RADIUS UDP packet to
decide which shared secret to use, so that RADIUS requests can be
proxied.
When using a forwarding proxy, the proxy must be able to alter the
packet as it passes through in each direction - when the proxy
forwards the request, the proxy can add a Proxy-State Attribute, and
when the proxy forwards a response, it removes the Proxy-State
Attribute. Since Access-Accept and Access-Reject replies are
authenticated on the entire packet contents, the stripping of the
Proxy-State attribute would invalidate the signature in the packet -
so the proxy has to re-sign it.
Further details of RADIUS proxy implementation are outside the scope
of this document.
Attributes
Many Attributes may have multiple instances, in such a case the order
of Attributes of the same Type SHOULD be preserved. The order of
Attributes of different Types is not required to be preserved.
In the section below on "Attributes" where the text refers to which
packets an attribute is allowed in, only packets with Codes 1, 2, 3
and 11 and attributes defined in this document are covered in this
document. A summary table is provided at the end of the "Attributes"
section. To determine which Attributes are allowed in packets with
codes 4 and 5 refer to the RADIUS Accounting document [9].
4. Packet Types
The RADIUS Packet type is determined by the Code field in the first
octet of the Packet.
4.1. Access-Request
Description
Access-Request packets are sent to a RADIUS server, and convey
information used to determine whether a user is allowed access to a
specific NAS, and any special services requested for that user. An
implementation wishing to authenticate a user MUST transmit a
RADIUS packet with the Code field set to 1 (Access-Request).
Upon receipt of an Access-Request from a valid client, an
appropriate reply MUST be transmitted.
An Access-Request MUST contain a User-Name attribute. It SHOULD
contain either a NAS-IP-Address attribute or NAS-Identifier
attribute (or both, although that is not recommended). It MUST
contain either a User-Password attribute or CHAP-Password
attribute. It SHOULD contain a NAS-Port or NAS-Port-Type attribute
or both unless the type of access being requested does not involve
a port or the NAS does not distinguish among its ports.
An Access-Request MAY contain additional attributes as a hint to
the server, but the server is not required to honor the hint.
When a User-Password is present, it is hidden using a method based
on the RSA Message Digest Algorithm MD5 [1].
A summary of the Access-Request packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Request Authenticator


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
1 for Access-Request.
Identifier
The Identifier field MUST be changed whenever the content of the
Attributes field changes, and whenever a valid reply has been
received for a previous request. For retransmissions, the
Identifier MUST remain unchanged.
Request Authenticator
The Request Authenticator value MUST be changed each time a new
Identifier is used.
Attributes
The Attribute field is variable in length, and contains the list
of Attributes that are required for the type of service, as well
as any desired optional Attributes.
4.2. Access-Accept
Description
Access-Accept packets are sent by the RADIUS server, and provide
specific configuration information necessary to begin delivery of
service to the user. If all Attribute values received in an
Access-Request are acceptable then the RADIUS implementation MUST
transmit a packet with the Code field set to 2 (Access-Accept). On
reception of an Access-Accept, the Identifier field is matched with
a pending Access-Request. Additionally, the Response Authenticator
field MUST contain the correct response for the pending Access-
Request. Invalid packets are silently discarded.
A summary of the Access-Accept packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Response Authenticator


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
2 for Access-Accept.
Identifier
The Identifier field is a copy of the Identifier field of the
Access-Request which caused this Access-Accept.
Response Authenticator
The Response Authenticator value is calculated from the Access-
Request value, as described earlier.
Attributes
The Attribute field is variable in length, and contains a list of
zero or more Attributes.
4.3. Access-Reject
Description
If any value of the received Attributes is not acceptable, then the
RADIUS server MUST transmit a packet with the Code field set to 3
(Access-Reject). It MAY include one or more Reply-Message
Attributes with a text message which the NAS MAY display to the
user.
A summary of the Access-Reject packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Identifier Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Response Authenticator


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
3 for Access-Reject.
Identifier
The Identifier field is a copy of the Identifier field of the
Access-Request which caused this Access-Reject.
Response Authenticator
The Response Authenticator value is calculated from the Access-
Request value, as described earlier.
Attributes
The Attribute field is variable in length, and contains a list of
zero or more Attributes.
4.4. Access-Challenge
Description
If the RADIUS server desires to send the user a challenge requiring
a response, then the RADIUS server MUST respond to the Access-
Request by transmitting a packet with the Code field set to 11
(Access-Challenge).
The Attributes field MAY have one or more Reply-Message Attributes,
and MAY have a single State Attribute, or none. No other
Attributes are permitted in an Access-Challenge.
On receipt of an Access-Challenge, the Identifier field is matched
with a pending Access-Request. Additionally, the Response
Authenticator field MUST contain the correct response for the
pending Access-Request. Invalid packets are silently discarded.
If the NAS does not support challenge/response, it MUST treat an
Access-Challenge as though it had received an Access-Reject
instead.
If the NAS supports challenge/response, receipt of a valid Access-
Challenge indicates that a new Access-Request SHOULD be sent. The
NAS MAY display the text message, if any, to the user, and then
prompt the user for a response. It then sends its original
Access-Request with a new request ID and Request Authenticator,
with the User-Password Attribute replaced by the user"s response
(encrypted), and including the State Attribute from the Access-
Challenge, if any. Only 0 or 1 instances of the State Attribute
can be present in an Access-Request.
A NAS which supports PAP MAY forward the Reply-Message to the
dialin client and accept a PAP response which it can use as though
the user had entered the response. If the NAS cannot do so, it
should treat the Access-Challenge as though it had received an
Access-Reject instead.
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.
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
27 Session-Timeout
28 Idle-Timeout
29 Termination-Action
30 Called-Station-Id
31 Calling-Station-Id
32 NAS-Identifier
33 Proxy-State
34 Login-LAT-Service
35 Login-LAT-Node
36 Login-LAT-Group
37 Framed-AppleTalk-Link
38 Framed-AppleTalk-Network
39 Framed-AppleTalk-Zone
40-59 (reserved for accounting)
60 CHAP-Challenge
61 NAS-Port-Type
62 Port-Limit
63 Login-LAT-Port
Length
The Length field is one octet, and indicates the length of this
Attribute including the Type, Length and Value fields. If an
Attribute is received in an Access-Request but with an invalid
Length, an Access-Reject SHOULD be transmitted. If an Attribute is
received in an Access-Accept, Access-Reject or Access-Challenge
packet with an invalid length, the packet MUST either be treated as
an Access-Reject or else silently discarded.
Value
The Value field is zero or more octets and contains information
specific to the Attribute. The format and length of the Value
field is determined by the Type and Length fields.
Note that a "string" in RADIUS does not require termination by an
ASCII NUL because the Attribute already has a length field.
The format of the value field is one of four data types.
string 0-253 octets
address 32 bit value, most significant octet first.
integer 32 bit value, most significant octet first.
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.
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:
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.
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.
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.
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.
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.
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
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.
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.
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
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.
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-
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.
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.
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.
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
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.
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.
5.23. Framed-IPX-Network
Description
This Attribute indicates the IPX Network number to be configured
for the user. It is used in Access-Accept packets.
A summary of the Framed-IPX-Network Attribute format is shown below.
The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Length Value
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Value (cont)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
23 for Framed-IPX-Network.
Length
6
Value
The Value field is four octets. The value 0xFFFFFFFE indicates
that the NAS should select an IPX network for the user (e.g.
assigned from a pool of one or more IPX networks kept by the NAS).
Other values should be used as the IPX network for the link to the
user.
5.24. State
Description
This Attribute is available to be sent by the server to the client
in an Access-Challenge and MUST be sent unmodified from the client
to the server in the new Access-Request reply to that challenge,
if any.
This Attribute is available to be sent by the server to the client
in an Access-Accept that also includes a Termination-Action
Attribute with the v