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rfc2138.txt
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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
Rigney, et. al. Standards Track [Page 1]
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
Rigney, et. al. Standards Track [Page 2]
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.
Rigney, et. al. Standards Track [Page 3]
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.
Rigney, et. al. Standards Track [Page 4]
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.
Rigney, et. al. Standards Track [Page 5]
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.
Rigney, et. al. Standards Track [Page 6]
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
Rigney, et. al. Standards Track [Page 7]
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.
Rigney, et. al. Standards Track [Page 8]
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.
Rigney, et. al. Standards Track [Page 9]
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
Rigney, et. al. Standards Track [Page 10]
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.
Rigney, et. al. Standards Track [Page 11]
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.
Rigney, et. al. Standards Track [Page 12]
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.
Rigney, et. al. Standards Track [Page 13]
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).
Rigney, et. al. Standards Track [Page 14]
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.
Rigney, et. al. Standards Track [Page 15]
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.
Rigney, et. al. Standards Track [Page 16]
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