RFC 2865
Remote Authentication Dial In User Service (RADIUS)
Pages: 76
Draft Standard
→ Errata
Obsoletes: 2138
Updated by: 2868 3575 5080 6929 8044
Draft Standard
→ Errata
Obsoletes: 2138
Updated by: 2868 3575 5080 6929 8044
Part 1 of 3 – Pages 1 to 22
Network Working Group C. Rigney Request for Comments: 2865 S. Willens Obsoletes: 2138 Livingston Category: Standards Track A. Rubens Merit W. Simpson Daydreamer June 2000 Remote Authentication Dial In User Service (RADIUS) Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved. IESG Note: This protocol is widely implemented and used. Experience has shown that it can suffer degraded performance and lost data when used in large scale systems, in part because it does not include provisions for congestion control. Readers of this document may find it beneficial to track the progress of the IETF's AAA working group, which may develop a successor protocol that better addresses the scaling and congestion control issues.Abstract
This document describes a protocol for carrying authentication, authorization, and configuration information between a Network Access Server which desires to authenticate its links and a shared Authentication Server. Implementation Note This memo documents the RADIUS protocol. The early deployment of RADIUS was done using UDP port number 1645, which conflicts with the "datametrics" service. The officially assigned port number for RADIUS is 1812.
Table of Contents
1. Introduction .......................................... 3 1.1 Specification of Requirements ................... 4 1.2 Terminology ..................................... 5 2. Operation ............................................. 5 2.1 Challenge/Response .............................. 7 2.2 Interoperation with PAP and CHAP ................ 8 2.3 Proxy ........................................... 8 2.4 Why UDP? ........................................ 11 2.5 Retransmission Hints ............................ 12 2.6 Keep-Alives Considered Harmful .................. 13 3. Packet Format ......................................... 13 4. Packet Types .......................................... 17 4.1 Access-Request .................................. 17 4.2 Access-Accept ................................... 18 4.3 Access-Reject ................................... 20 4.4 Access-Challenge ................................ 21 5. Attributes ............................................ 22 5.1 User-Name ....................................... 26 5.2 User-Password ................................... 27 5.3 CHAP-Password ................................... 28 5.4 NAS-IP-Address .................................. 29 5.5 NAS-Port ........................................ 30 5.6 Service-Type .................................... 31 5.7 Framed-Protocol ................................. 33 5.8 Framed-IP-Address ............................... 34 5.9 Framed-IP-Netmask ............................... 34 5.10 Framed-Routing .................................. 35 5.11 Filter-Id ....................................... 36 5.12 Framed-MTU ...................................... 37 5.13 Framed-Compression .............................. 37 5.14 Login-IP-Host ................................... 38 5.15 Login-Service ................................... 39 5.16 Login-TCP-Port .................................. 40 5.17 (unassigned) .................................... 41 5.18 Reply-Message ................................... 41 5.19 Callback-Number ................................. 42 5.20 Callback-Id ..................................... 42 5.21 (unassigned) .................................... 43 5.22 Framed-Route .................................... 43 5.23 Framed-IPX-Network .............................. 44 5.24 State ........................................... 45 5.25 Class ........................................... 46 5.26 Vendor-Specific ................................. 47 5.27 Session-Timeout ................................. 48 5.28 Idle-Timeout .................................... 49 5.29 Termination-Action .............................. 49
5.30 Called-Station-Id ............................... 50
5.31 Calling-Station-Id .............................. 51
5.32 NAS-Identifier .................................. 52
5.33 Proxy-State ..................................... 53
5.34 Login-LAT-Service ............................... 54
5.35 Login-LAT-Node .................................. 55
5.36 Login-LAT-Group ................................. 56
5.37 Framed-AppleTalk-Link ........................... 57
5.38 Framed-AppleTalk-Network ........................ 58
5.39 Framed-AppleTalk-Zone ........................... 58
5.40 CHAP-Challenge .................................. 59
5.41 NAS-Port-Type ................................... 60
5.42 Port-Limit ...................................... 61
5.43 Login-LAT-Port .................................. 62
5.44 Table of Attributes ............................. 63
6. IANA Considerations ................................... 64
6.1 Definition of Terms ............................. 64
6.2 Recommended Registration Policies ............... 65
7. Examples .............................................. 66
7.1 User Telnet to Specified Host ................... 66
7.2 Framed User Authenticating with CHAP ............ 67
7.3 User with Challenge-Response card ............... 68
8. Security Considerations ............................... 71
9. Change Log ............................................ 71
10. References ............................................ 73
11. Acknowledgements ...................................... 74
12. Chair's Address ....................................... 74
13. Authors' Addresses .................................... 75
14. Full Copyright Statement .............................. 76
1. Introduction
This document obsoletes RFC 2138 [1]. A summary of the changes
between this document and RFC 2138 is available in the "Change Log"
appendix.
Managing dispersed serial line and modem pools for large numbers of
users can create the need for significant administrative support.
Since modem pools are by definition a link to the outside world, they
require careful attention to security, authorization and accounting.
This can be best achieved by managing a single "database" of users,
which allows for authentication (verifying user name and password) as
well as configuration information detailing the type of service to
deliver to the user (for example, SLIP, PPP, telnet, rlogin).
Key features of RADIUS are:
Client/Server Model
A Network Access Server (NAS) operates as a client of RADIUS. The
client is responsible for passing user information to designated
RADIUS servers, and then acting on the response which is returned.
RADIUS servers are responsible for receiving user connection
requests, authenticating the user, and then returning all
configuration information necessary for the client to deliver
service to the user.
A RADIUS server can act as a proxy client to other RADIUS servers
or other kinds of authentication servers.
Network Security
Transactions between the client and RADIUS server are
authenticated through the use of a shared secret, which is never
sent over the network. In addition, any user passwords are sent
encrypted between the client and RADIUS server, to eliminate the
possibility that someone snooping on an unsecure network could
determine a user's password.
Flexible Authentication Mechanisms
The RADIUS server can support a variety of methods to authenticate
a user. When it is provided with the user name and original
password given by the user, it can support PPP PAP or CHAP, UNIX
login, and other authentication mechanisms.
Extensible Protocol
All transactions are comprised of variable length Attribute-
Length-Value 3-tuples. New attribute values can be added without
disturbing existing implementations of the protocol.
1.1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14 [2]. These key
words mean the same thing whether capitalized or not.
An implementation is not compliant if it fails to satisfy one or more
of the must or must not requirements for the protocols it implements.
An implementation that satisfies all the must, must not, should and
should not requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the must and must not requirements but not all the should or should not requirements for its protocols is said to be "conditionally compliant". A NAS that does not implement a given service MUST NOT implement the RADIUS attributes for that service. For example, a NAS that is unable to offer ARAP service MUST NOT implement the RADIUS attributes for ARAP. A NAS MUST treat a RADIUS access-accept authorizing an unavailable service as an access-reject instead.1.2. Terminology
This document frequently uses the following terms: service The NAS provides a service to the dial-in user, such as PPP or Telnet. session Each service provided by the NAS to a dial-in user constitutes a session, with the beginning of the session defined as the point where service is first provided and the end of the session defined as the point where service is ended. A user may have multiple sessions in parallel or series if the NAS supports that. silently discard This means the implementation discards the packet without further processing. The implementation SHOULD provide the capability of logging the error, including the contents of the silently discarded packet, and SHOULD record the event in a statistics counter.2. Operation
When a client is configured to use RADIUS, any user of the client presents authentication information to the client. This might be with a customizable login prompt, where the user is expected to enter their username and password. Alternatively, the user might use a link framing protocol such as the Point-to-Point Protocol (PPP), which has authentication packets which carry this information. Once the client has obtained such information, it may choose to authenticate using RADIUS. To do so, the client creates an "Access- Request" containing such Attributes as the user's name, the user's password, the ID of the client and the Port ID which the user is accessing. When a password is present, it is hidden using a method based on the RSA Message Digest Algorithm MD5 [3].
The Access-Request is submitted to the RADIUS server via the network. If no response is returned within a length of time, the request is re-sent a number of times. The client can also forward requests to an alternate server or servers in the event that the primary server is down or unreachable. An alternate server can be used either after a number of tries to the primary server fail, or in a round-robin fashion. Retry and fallback algorithms are the topic of current research and are not specified in detail in this document. Once the RADIUS server receives the request, it validates the sending client. A request from a client for which the RADIUS server does not have a shared secret MUST be silently discarded. If the client is valid, the RADIUS server consults a database of users to find the user whose name matches the request. The user entry in the database contains a list of requirements which must be met to allow access for the user. This always includes verification of the password, but can also specify the client(s) or port(s) to which the user is allowed access. The RADIUS server MAY make requests of other servers in order to satisfy the request, in which case it acts as a client. If any Proxy-State attributes were present in the Access-Request, they MUST be copied unmodified and in order into the response packet. Other Attributes can be placed before, after, or even between the Proxy-State attributes. If any condition is not met, the RADIUS server sends an "Access- Reject" response indicating that this user request is invalid. If desired, the server MAY include a text message in the Access-Reject which MAY be displayed by the client to the user. No other Attributes (except Proxy-State) are permitted in an Access-Reject. If all conditions are met and the RADIUS server wishes to issue a challenge to which the user must respond, the RADIUS server sends an "Access-Challenge" response. It MAY include a text message to be displayed by the client to the user prompting for a response to the challenge, and MAY include a State attribute. If the client receives an Access-Challenge and supports challenge/response it MAY display the text message, if any, to the user, and then prompt the user for a response. The client then re- submits its original Access-Request with a new request ID, with the User-Password Attribute replaced by the response (encrypted), and including the State Attribute from the Access-Challenge, if any. Only 0 or 1 instances of the State Attribute SHOULD be
present in a request. The server can respond to this new Access- Request with either an Access-Accept, an Access-Reject, or another Access-Challenge. If all conditions are met, the list of configuration values for the user are placed into an "Access-Accept" response. These values include the type of service (for example: SLIP, PPP, Login User) and all necessary values to deliver the desired service. For SLIP and PPP, this may include values such as IP address, subnet mask, MTU, desired compression, and desired packet filter identifiers. For character mode users, this may include values such as desired protocol and host.2.1. Challenge/Response
In challenge/response authentication, the user is given an unpredictable number and challenged to encrypt it and give back the result. Authorized users are equipped with special devices such as smart cards or software that facilitate calculation of the correct response with ease. Unauthorized users, lacking the appropriate device or software and lacking knowledge of the secret key necessary to emulate such a device or software, can only guess at the response. The Access-Challenge packet typically contains a Reply-Message including a challenge to be displayed to the user, such as a numeric value unlikely ever to be repeated. Typically this is obtained from an external server that knows what type of authenticator is in the possession of the authorized user and can therefore choose a random or non-repeating pseudorandom number of an appropriate radix and length. The user then enters the challenge into his device (or software) and it calculates a response, which the user enters into the client which forwards it to the RADIUS server via a second Access-Request. If the response matches the expected response the RADIUS server replies with an Access-Accept, otherwise an Access-Reject. Example: The NAS sends an Access-Request packet to the RADIUS Server with NAS-Identifier, NAS-Port, User-Name, User-Password (which may just be a fixed string like "challenge" or ignored). The server sends back an Access-Challenge packet with State and a Reply-Message along the lines of "Challenge 12345678, enter your response at the prompt" which the NAS displays. The NAS prompts for the response and sends a NEW Access-Request to the server (with a new ID) with NAS- Identifier, NAS-Port, User-Name, User-Password (the response just entered by the user, encrypted), and the same State Attribute that
came with the Access-Challenge. The server then sends back either an Access-Accept or Access-Reject based on whether the response matches the required value, or it can even send another Access-Challenge.2.2. Interoperation with PAP and CHAP
For PAP, the NAS takes the PAP ID and password and sends them in an Access-Request packet as the User-Name and User-Password. The NAS MAY include the Attributes Service-Type = Framed-User and Framed-Protocol = PPP as a hint to the RADIUS server that PPP service is expected. For CHAP, the NAS generates a random challenge (preferably 16 octets) and sends it to the user, who returns a CHAP response along with a CHAP ID and CHAP username. The NAS then sends an Access-Request packet to the RADIUS server with the CHAP username as the User-Name and with the CHAP ID and CHAP response as the CHAP-Password (Attribute 3). The random challenge can either be included in the CHAP-Challenge attribute or, if it is 16 octets long, it can be placed in the Request Authenticator field of the Access-Request packet. The NAS MAY include the Attributes Service-Type = Framed- User and Framed-Protocol = PPP as a hint to the RADIUS server that PPP service is expected. The RADIUS server looks up a password based on the User-Name, encrypts the challenge using MD5 on the CHAP ID octet, that password, and the CHAP challenge (from the CHAP-Challenge attribute if present, otherwise from the Request Authenticator), and compares that result to the CHAP-Password. If they match, the server sends back an Access-Accept, otherwise it sends back an Access-Reject. If the RADIUS server is unable to perform the requested authentication it MUST return an Access-Reject. For example, CHAP requires that the user's password be available in cleartext to the server so that it can encrypt the CHAP challenge and compare that to the CHAP response. If the password is not available in cleartext to the RADIUS server then the server MUST send an Access-Reject to the client.2.3. Proxy
With proxy RADIUS, one RADIUS server receives an authentication (or accounting) request from a RADIUS client (such as a NAS), forwards the request to a remote RADIUS server, receives the reply from the remote server, and sends that reply to the client, possibly with changes to reflect local administrative policy. A common use for proxy RADIUS is roaming. Roaming permits two or more administrative entities to allow each other's users to dial in to either entity's network for service.
The NAS sends its RADIUS access-request to the "forwarding server" which forwards it to the "remote server". The remote server sends a response (Access-Accept, Access-Reject, or Access-Challenge) back to the forwarding server, which sends it back to the NAS. The User-Name attribute MAY contain a Network Access Identifier [8] for RADIUS Proxy operations. The choice of which server receives the forwarded request SHOULD be based on the authentication "realm". The authentication realm MAY be the realm part of a Network Access Identifier (a "named realm"). Alternatively, the choice of which server receives the forwarded request MAY be based on whatever other criteria the forwarding server is configured to use, such as Called- Station-Id (a "numbered realm"). A RADIUS server can function as both a forwarding server and a remote server, serving as a forwarding server for some realms and a remote server for other realms. One forwarding server can act as a forwarder for any number of remote servers. A remote server can have any number of servers forwarding to it and can provide authentication for any number of realms. One forwarding server can forward to another forwarding server to create a chain of proxies, although care must be taken to avoid introducing loops. The following scenario illustrates a proxy RADIUS communication between a NAS and the forwarding and remote RADIUS servers: 1. A NAS sends its access-request to the forwarding server. 2. The forwarding server forwards the access-request to the remote server. 3. The remote server sends an access-accept, access-reject or access-challenge back to the forwarding server. For this example, an access-accept is sent. 4. The forwarding server sends the access-accept to the NAS. The forwarding server MUST treat any Proxy-State attributes already in the packet as opaque data. Its operation MUST NOT depend on the content of Proxy-State attributes added by previous servers. If there are any Proxy-State attributes in the request received from the client, the forwarding server MUST include those Proxy-State attributes in its reply to the client. The forwarding server MAY include the Proxy-State attributes in the access-request when it forwards the request, or MAY omit them in the forwarded request. If the forwarding server omits the Proxy-State attributes in the forwarded access-request, it MUST attach them to the response before sending it to the client.
We now examine each step in more detail.
1. A NAS sends its access-request to the forwarding server. The
forwarding server decrypts the User-Password, if present, using
the shared secret it knows for the NAS. If a CHAP-Password
attribute is present in the packet and no CHAP-Challenge attribute
is present, the forwarding server MUST leave the Request-
Authenticator untouched or copy it to a CHAP-Challenge attribute.
'' The forwarding server MAY add one Proxy-State attribute to the
packet. (It MUST NOT add more than one.) If it adds a Proxy-
State, the Proxy-State MUST appear after any other Proxy-States in
the packet. The forwarding server MUST NOT modify any other
Proxy-States that were in the packet (it may choose not to forward
them, but it MUST NOT change their contents). The forwarding
server MUST NOT change the order of any attributes of the same
type, including Proxy-State.
2. The forwarding server encrypts the User-Password, if present,
using the secret it shares with the remote server, sets the
Identifier as needed, and forwards the access-request to the
remote server.
3. The remote server (if the final destination) verifies the user
using User-Password, CHAP-Password, or such method as future
extensions may dictate, and returns an access-accept, access-
reject or access-challenge back to the forwarding server. For
this example, an access-accept is sent. The remote server MUST
copy all Proxy-State attributes (and only the Proxy-State
attributes) in order from the access-request to the response
packet, without modifying them.
4. The forwarding server verifies the Response Authenticator using
the secret it shares with the remote server, and silently discards
the packet if it fails verification. If the packet passes
verification, the forwarding server removes the last Proxy-State
(if it attached one), signs the Response Authenticator using the
secret it shares with the NAS, restores the Identifier to match
the one in the original request by the NAS, and sends the access-
accept to the NAS.
A forwarding server MAY need to modify attributes to enforce local
policy. Such policy is outside the scope of this document, with the
following restrictions. A forwarding server MUST not modify existing
Proxy-State, State, or Class attributes present in the packet.
Implementers of forwarding servers should consider carefully which values it is willing to accept for Service-Type. Careful consideration must be given to the effects of passing along Service- Types of NAS-Prompt or Administrative in a proxied Access-Accept, and implementers may wish to provide mechanisms to block those or other service types, or other attributes. Such mechanisms are outside the scope of this document.2.4. Why UDP?
A frequently asked question is why RADIUS uses UDP instead of TCP as a transport protocol. UDP was chosen for strictly technical reasons. There are a number of issues which must be understood. RADIUS is a transaction based protocol which has several interesting characteristics: 1. If the request to a primary Authentication server fails, a secondary server must be queried. To meet this requirement, a copy of the request must be kept above the transport layer to allow for alternate transmission. This means that retransmission timers are still required. 2. The timing requirements of this particular protocol are significantly different than TCP provides. At one extreme, RADIUS does not require a "responsive" detection of lost data. The user is willing to wait several seconds for the authentication to complete. The generally aggressive TCP retransmission (based on average round trip time) is not required, nor is the acknowledgement overhead of TCP. At the other extreme, the user is not willing to wait several minutes for authentication. Therefore the reliable delivery of TCP data two minutes later is not useful. The faster use of an alternate server allows the user to gain access before giving up. 3. The stateless nature of this protocol simplifies the use of UDP. Clients and servers come and go. Systems are rebooted, or are power cycled independently. Generally this does not cause a problem and with creative timeouts and detection of lost TCP connections, code can be written to handle anomalous events. UDP however completely eliminates any of this special handling. Each client and server can open their UDP transport just once and leave it open through all types of failure events on the network.
4. UDP simplifies the server implementation.
In the earliest implementations of RADIUS, the server was single
threaded. This means that a single request was received,
processed, and returned. This was found to be unmanageable in
environments where the back-end security mechanism took real time
(1 or more seconds). The server request queue would fill and in
environments where hundreds of people were being authenticated
every minute, the request turn-around time increased to longer
than users were willing to wait (this was especially severe when a
specific lookup in a database or over DNS took 30 or more
seconds). The obvious solution was to make the server multi-
threaded. Achieving this was simple with UDP. Separate processes
were spawned to serve each request and these processes could
respond directly to the client NAS with a simple UDP packet to the
original transport of the client.
It's not all a panacea. As noted, using UDP requires one thing which
is built into TCP: with UDP we must artificially manage
retransmission timers to the same server, although they don't require
the same attention to timing provided by TCP. This one penalty is a
small price to pay for the advantages of UDP in this protocol.
Without TCP we would still probably be using tin cans connected by
string. But for this particular protocol, UDP is a better choice.
2.5. Retransmission Hints
If the RADIUS server and alternate RADIUS server share the same
shared secret, it is OK to retransmit the packet to the alternate
RADIUS server with the same ID and Request Authenticator, because the
content of the attributes haven't changed. If you want to use a new
Request Authenticator when sending to the alternate server, you may.
If you change the contents of the User-Password attribute (or any
other attribute), you need a new Request Authenticator and therefore
a new ID.
If the NAS is retransmitting a RADIUS request to the same server as
before, and the attributes haven't changed, you MUST use the same
Request Authenticator, ID, and source port. If any attributes have
changed, you MUST use a new Request Authenticator and ID.
A NAS MAY use the same ID across all servers, or MAY keep track of
IDs separately for each server, it is up to the implementer. If a
NAS needs more than 256 IDs for outstanding requests, it MAY use
additional source ports to send requests from, and keep track of IDs for each source port. This allows up to 16 million or so outstanding requests at one time to a single server.2.6. Keep-Alives Considered Harmful
Some implementers have adopted the practice of sending test RADIUS requests to see if a server is alive. This practice is strongly discouraged, since it adds to load and harms scalability without providing any additional useful information. Since a RADIUS request is contained in a single datagram, in the time it would take you to send a ping you could just send the RADIUS request, and getting a reply tells you that the RADIUS server is up. If you do not have a RADIUS request to send, it does not matter if the server is up or not, because you are not using it. If you want to monitor your RADIUS server, use SNMP. That's what SNMP is for.3. Packet Format
Exactly one RADIUS packet is encapsulated in the UDP Data field [4], where the UDP Destination Port field indicates 1812 (decimal). When a reply is generated, the source and destination ports are reversed. This memo documents the RADIUS protocol. The early deployment of RADIUS was done using UDP port number 1645, which conflicts with the "datametrics" service. The officially assigned port number for RADIUS is 1812.
A summary of the RADIUS data format is shown below. The fields are
transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authenticator |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
The Code field is one octet, and identifies the type of RADIUS
packet. When a packet is received with an invalid Code field, it
is silently discarded.
RADIUS Codes (decimal) are assigned as follows:
1 Access-Request
2 Access-Accept
3 Access-Reject
4 Accounting-Request
5 Accounting-Response
11 Access-Challenge
12 Status-Server (experimental)
13 Status-Client (experimental)
255 Reserved
Codes 4 and 5 are covered in the RADIUS Accounting document [5].
Codes 12 and 13 are reserved for possible use, but are not further
mentioned here.
Identifier
The Identifier field is one octet, and aids in matching requests
and replies. The RADIUS server can detect a duplicate request if
it has the same client source IP address and source UDP port and
Identifier within a short span of time.
Length
The Length field is two octets. It indicates the length of the
packet including the Code, Identifier, Length, Authenticator and
Attribute fields. Octets outside the range of the Length field
MUST be treated as padding and ignored on reception. If the
packet is shorter than the Length field indicates, it MUST be
silently discarded. The minimum length is 20 and maximum length
is 4096.
Authenticator
The Authenticator field is sixteen (16) octets. The most
significant octet is transmitted first. This value is used to
authenticate the reply from the RADIUS server, and is used in the
password hiding algorithm.
Request Authenticator
In Access-Request Packets, the Authenticator value is a 16
octet random number, called the Request Authenticator. The
value SHOULD be unpredictable and unique over the lifetime of a
secret (the password shared between the client and the RADIUS
server), since repetition of a request value in conjunction
with the same secret would permit an attacker to reply with a
previously intercepted response. Since it is expected that the
same secret MAY be used to authenticate with servers in
disparate geographic regions, the Request Authenticator field
SHOULD exhibit global and temporal uniqueness.
The Request Authenticator value in an Access-Request packet
SHOULD also be unpredictable, lest an attacker trick a server
into responding to a predicted future request, and then use the
response to masquerade as that server to a future Access-
Request.
Although protocols such as RADIUS are incapable of protecting
against theft of an authenticated session via realtime active
wiretapping attacks, generation of unique unpredictable
requests can protect against a wide range of active attacks
against authentication.
The NAS and RADIUS server share a secret. That shared secret
followed by the Request Authenticator is put through a one-way
MD5 hash to create a 16 octet digest value which is xored with
the password entered by the user, and the xored result placed
in the User-Password attribute in the Access-Request packet.
See the entry for User-Password in the section on Attributes
for a more detailed description.
Response Authenticator
The value of the Authenticator field in Access-Accept, Access-
Reject, and Access-Challenge packets is called the Response
Authenticator, and contains a one-way MD5 hash calculated over
a stream of octets consisting of: the RADIUS packet, beginning
with the Code field, including the Identifier, the Length, the
Request Authenticator field from the Access-Request packet, and
the response Attributes, followed by the shared secret. That
is, ResponseAuth =
MD5(Code+ID+Length+RequestAuth+Attributes+Secret) where +
denotes concatenation.
Administrative Note
The secret (password shared between the client and the RADIUS
server) SHOULD be at least as large and unguessable as a well-
chosen password. It is preferred that the secret be at least 16
octets. This is to ensure a sufficiently large range for the
secret to provide protection against exhaustive search attacks.
The secret MUST NOT be empty (length 0) since this would allow
packets to be trivially forged.
A RADIUS server MUST use the source IP address of the RADIUS UDP
packet to decide which shared secret to use, so that RADIUS
requests can be proxied.
When using a forwarding proxy, the proxy must be able to alter the
packet as it passes through in each direction - when the proxy
forwards the request, the proxy MAY add a Proxy-State Attribute,
and when the proxy forwards a response, it MUST remove its Proxy-
State Attribute if it added one. Proxy-State is always added or
removed after any other Proxy-States, but no other assumptions
regarding its location within the list of attributes can be made.
Since Access-Accept and Access-Reject replies are authenticated on
the entire packet contents, the stripping of the Proxy-State
attribute invalidates the signature in the packet - so the proxy
has to re-sign it.
Further details of RADIUS proxy implementation are outside the
scope of this document.
4. Packet Types
The RADIUS Packet type is determined by the Code field in the first octet of the Packet.4.1. Access-Request
Description Access-Request packets are sent to a RADIUS server, and convey information used to determine whether a user is allowed access to a specific NAS, and any special services requested for that user. An implementation wishing to authenticate a user MUST transmit a RADIUS packet with the Code field set to 1 (Access-Request). Upon receipt of an Access-Request from a valid client, an appropriate reply MUST be transmitted. An Access-Request SHOULD contain a User-Name attribute. It MUST contain either a NAS-IP-Address attribute or a NAS-Identifier attribute (or both). An Access-Request MUST contain either a User-Password or a CHAP- Password or a State. An Access-Request MUST NOT contain both a User-Password and a CHAP-Password. If future extensions allow other kinds of authentication information to be conveyed, the attribute for that can be used in an Access-Request instead of User-Password or CHAP-Password. An Access-Request SHOULD contain a NAS-Port or NAS-Port-Type attribute or both unless the type of access being requested does not involve a port or the NAS does not distinguish among its ports. An Access-Request MAY contain additional attributes as a hint to the server, but the server is not required to honor the hint. When a User-Password is present, it is hidden using a method based on the RSA Message Digest Algorithm MD5 [3].
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. 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. Vendor-Specific, Idle-Timeout, Session-Timeout and Proxy-State attributes MAY also be included. No other Attributes defined in this document are permitted in an Access-Challenge. On receipt of an Access-Challenge, the Identifier field is matched with a pending Access-Request. Additionally, the Response Authenticator field MUST contain the correct response for the pending Access-Request. Invalid packets are silently discarded. If the NAS does not support challenge/response, it MUST treat an Access-Challenge as though it had received an Access-Reject instead. If the NAS supports challenge/response, receipt of a valid Access-Challenge indicates that a new Access-Request SHOULD be sent. The NAS MAY display the text message, if any, to the user, and then prompt the user for a response. It then sends its original Access-Request with a new request ID and Request Authenticator, with the User-Password Attribute replaced by the user's response (encrypted), and including the State Attribute from the Access-Challenge, if any. Only 0 or 1 instances of the State Attribute can be present in an Access-Request. A NAS which supports PAP MAY forward the Reply-Message to the dialing client and accept a PAP response which it can use as though the user had entered the response. If the NAS cannot do so, it MUST treat the Access-Challenge as though it had received an Access-Reject instead.
A summary of the Access-Challenge packet format is shown below. The
fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Response Authenticator |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
11 for Access-Challenge.
Identifier
The Identifier field is a copy of the Identifier field of the
Access-Request which caused this Access-Challenge.
Response Authenticator
The Response Authenticator value is calculated from the Access-
Request value, as described earlier.
Attributes
The Attributes field is variable in length, and contains a list of
zero or more Attributes.
(page 22 continued on part 2)