Remote Authentication Dial-In User Service (RADIUS)

Managing a large number of dispersed users on WLANs, wireless personal area networks (WPANs), and wireless wide area networks (WWANs) in general, and on serial line and modem pools in particular, can create the need for significant administrative support. Since WLANs, WPANs, WWANs, and 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 as well as configuration information for the type of service to deliver to the user (such as the Serial Line Interface Protocol [SLIP], PPP, telnet, or rlogin). What follows is a brief summary of RADIUS functions, based on RFC 2138.^[55] Figure 4-22 applies RADIUS to the hotspot service environment.

Figure 4-22: RADIUS application

Key features of RADIUS are as follows:

• 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 that 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 unsecured network could determine a user’s password.

• Flexible authentication mechanisms The RADIUS server can support a variety of methods for authenticating 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 three-tuples. New attribute values can be added without disturbing the existing implementations of the protocol.

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 his or her username and password. Alternatively, the user might use a link-framing protocol such as PPP, which has authentication packets to 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 client’s ID, and the port ID the user is accessing. When a password is present, it is hidden using a method based on the RSA Message Digest algorithm MD-5.^[53]

The access-request is submitted to the RADIUS server via the network. If no response is returned within a certain length of time, the request is resent 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.^[54] 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 to be met before the user is granted access. It 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 that 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 it may include a state attribute. If the client receives an access-challenge and supports a challenge/response, it may display the text message, if any, to the user and then prompt the user for a response. The client then resubmits 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 zero or one instance of the state attributes should be present in a request. The server can respond to this new access-request with an access-accept, an access-reject, or another access-challenge.

If all conditions are met, the list of configuration values for the user is placed into an access-accept response. These values include the type of service (such as SLIP, PPP, and login user) and all necessary values to deliver the desired service. For SLIP and PPP, this may include values such as the IP address, subnet mask, Maximum Transmission Unit (MTU), desired compression, and desired packet filter identifiers. For character mode users, this may include values such as the desired protocol and host.

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 facilitates calculation of the correct response with ease. Unauthorized users, lacking the appropriate device or software as well as 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 or her device (or software) and it calculates a response, which the user enters into the client that 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, or otherwise with an access-reject.

Let’s look at an example. The NAS sends an access-request packet to the RADIUS server with NAS-identifier, NAS-port, user-name, and 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.

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 MD-5 on the CHAP ID octet, the 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.

Use of User Datagram Protocol (UDP) A frequently asked question is why RADIUS uses the User Datagram Protocol (UDP) instead of TCP as a transport protocol. UDP was chosen for strictly technical reasons.

A number of issues must be understood. RADIUS is a transaction-based protocol that has several interesting characteristics:

• 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.

• 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 the 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.

• 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 the 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.

• 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 backend security mechanism took real time (one or more seconds). The server request queue would fill and in environments where hundreds of people were being authenticated every minute, the request turnaround 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 multithreaded. 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 is not all a panacea. As noted, using UDP requires one thing that 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.

Packet Format

Exactly one RADIUS packet is encapsulated in the UDP Data field,^[55] 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 in Figure 4-23. Fields are transmitted from left to right.

Figure 4-23: RADIUS data format

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

The reader is referred to the RFC for additional protocol details.

^[55]RFC 2977 is Copyright © The Internet Society (2000). This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published, and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works.

^[53]B. Aboba and G. Zorn, “RFC 2477: Criteria for Evaluating Roaming Protocols.” December 1998.

^[54]B. Aboba and J. Vollbrecht, “RFC 2607: Proxy Chaining and Policy Implementation in Roaming.” June 1999.

^[55]RFC 2977 is Copyright © The Internet Society (2000). This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published, and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works.