RADIUS

Remote Authentication Dial-In User Service (RADIUS) is a distributed information interaction protocol that uses a client/server model. The protocol can protect networks against unauthorized access and is often used in network environments that require both high security and remote user access.

The RADIUS authorization process is combined with the RADIUS authentication process, and user authorization information is piggybacked in authentication responses. RADIUS uses UDP port 1812 for authentication and UDP port 1813 for accounting.

RADIUS was originally designed for dial-in user access, and has been extended to support additional access methods, such as Ethernet and ADSL.

Client/server model

The RADIUS client runs on the NASs located throughout the network. It passes user information to RADIUS servers and acts on the responses to, for example, reject or accept user access requests.

The RADIUS server runs on the computer or workstation at the network center and maintains information related to user authentication and network service access.

The RADIUS server operates using the following process:

  1. Receives authentication, authorization, and accounting requests from RADIUS clients.

  2. Performs user authentication, authorization, or accounting.

  3. Returns user access control information (for example, rejecting or accepting the user access request) to the clients.

The RADIUS server can also act as the client of another RADIUS server to provide authentication proxy services.

The RADIUS server maintains the following databases:

Figure 2: RADIUS server databases

Information exchange security mechanism

The RADIUS client and server exchange information between them with the help of shared keys, which are preconfigured on the client and server. A RADIUS packet has a 16-byte field called Authenticator. This field includes a signature generated by using the MD5 algorithm, the shared key, and some other information. The receiver of the packet verifies the signature and accepts the packet only when the signature is correct. This mechanism ensures the security of information exchanged between the RADIUS client and server.

The shared keys are also used to encrypt user passwords that are included in RADIUS packets.

User authentication methods

The RADIUS server supports multiple user authentication methods, such as PAP, CHAP, and EAP.

Basic RADIUS packet exchange process

Figure 3 illustrates the interactions between a user host, the RADIUS client, and the RADIUS server.

Figure 3: Basic RADIUS packet exchange process

RADIUS uses in the following workflow:

  1. The host sends a connection request that includes the user's username and password to the RADIUS client.

  2. The RADIUS client sends an authentication request (Access-Request) to the RADIUS server. The request includes the user's password, which has been processed by the MD5 algorithm and shared key.

  3. The RADIUS server authenticates the username and password. If the authentication succeeds, the server sends back an Access-Accept packet that contains the user's authorization information. If the authentication fails, the server returns an Access-Reject packet.

  4. The RADIUS client permits or denies the user according to the authentication result. If the result permits the user, the RADIUS client sends a start-accounting request (Accounting-Request) packet to the RADIUS server.

  5. The RADIUS server returns an acknowledgment (Accounting-Response) packet and starts accounting.

  6. The user accesses the network resources.

  7. The host requests the RADIUS client to tear down the connection.

  8. The RADIUS client sends a stop-accounting request (Accounting-Request) packet to the RADIUS server.

  9. The RADIUS server returns an acknowledgment (Accounting-Response) and stops accounting for the user.

  10. The RADIUS client notifies the user of the termination.

RADIUS packet format

RADIUS uses UDP to transmit packets. The protocol also uses a series of mechanisms to ensure smooth packet exchange between the RADIUS server and the client. These mechanisms include the timer mechanism, the retransmission mechanism, and the backup server mechanism.

Figure 4: RADIUS packet format

Descriptions of the fields are as follows:

Code

Packet type

Description

1

Access-Request

From the client to the server. A packet of this type includes user information for the server to authenticate the user. It must contain the User-Name attribute and can optionally contain the attributes of NAS-IP-Address, User-Password, and NAS-Port.

2

Access-Accept

From the server to the client. If all attribute values included in the Access-Request are acceptable, the authentication succeeds, and the server sends an Access-Accept response.

3

Access-Reject

From the server to the client. If any attribute value included in the Access-Request is unacceptable, the authentication fails, and the server sends an Access-Reject response.

4

Accounting-Request

From the client to the server. A packet of this type includes user information for the server to start or stop accounting for the user. The Acct-Status-Type attribute in the packet indicates whether to start or stop accounting.

5

Accounting-Response

From the server to the client. The server sends a packet of this type to notify the client that it has received the Accounting-Request and has successfully recorded the accounting information.

No.

Attribute

No.

Attribute

1

User-Name

45

Acct-Authentic

2

User-Password

46

Acct-Session-Time

3

CHAP-Password

47

Acct-Input-Packets

4

NAS-IP-Address

48

Acct-Output-Packets

5

NAS-Port

49

Acct-Terminate-Cause

6

Service-Type

50

Acct-Multi-Session-Id

7

Framed-Protocol

51

Acct-Link-Count

8

Framed-IP-Address

52

Acct-Input-Gigawords

9

Framed-IP-Netmask

53

Acct-Output-Gigawords

10

Framed-Routing

54

(unassigned)

11

Filter-ID

55

Event-Timestamp

12

Framed-MTU

56-59

(unassigned)

13

Framed-Compression

60

CHAP-Challenge

14

Login-IP-Host

61

NAS-Port-Type

15

Login-Service

62

Port-Limit

16

Login-TCP-Port

63

Login-LAT-Port

17

(unassigned)

64

Tunnel-Type

18

Reply-Message

65

Tunnel-Medium-Type

19

Callback-Number

66

Tunnel-Client-Endpoint

20

Callback-ID

67

Tunnel-Server-Endpoint

21

(unassigned)

68

Acct-Tunnel-Connection

22

Framed-Route

69

Tunnel-Password

23

Framed-IPX-Network

70

ARAP-Password

24

State

71

ARAP-Features

25

Class

72

ARAP-Zone-Access

26

Vendor-Specific

73

ARAP-Security

27

Session-Timeout

74

ARAP-Security-Data

28

Idle-Timeout

75

Password-Retry

29

Termination-Action

76

Prompt

30

Called-Station-Id

77

Connect-Info

31

Calling-Station-Id

78

Configuration-Token

32

NAS-Identifier

79

EAP-Message

33

Proxy-State

80

Message-Authenticator

34

Login-LAT-Service

81

Tunnel-Private-Group-ID

35

Login-LAT-Node

82

Tunnel-Assignment-id

36

Login-LAT-Group

83

Tunnel-Preference

37

Framed-AppleTalk-Link

84

ARAP-Challenge-Response

38

Framed-AppleTalk-Network

85

Acct-Interim-Interval

39

Framed-AppleTalk-Zone

86

Acct-Tunnel-Packets-Lost

40

Acct-Status-Type

87

NAS-Port-Id

41

Acct-Delay-Time

88

Framed-Pool

42

Acct-Input-Octets

89

(unassigned)

43

Acct-Output-Octets

90

Tunnel-Client-Auth-id

44

Acct-Session-Id

91

Tunnel-Server-Auth-id

Extended RADIUS attributes

The RADIUS protocol features excellent extensibility. The Vendor-Specific attribute (attribute 26) allows a vendor to define extended attributes. The extended attributes can implement functions that the standard RADIUS protocol does not provide.

A vendor can encapsulate multiple subattributes in the TLV format in attribute 26 to provide extended functions. As shown in Figure 5, a subattribute encapsulated in attribute 26 consists of the following parts:

The device supports RADIUS subattributes with a vendor ID of 25506. For more information, see "Proprietary RADIUS subattributes (vendor ID 25506)."

Figure 5: Format of attribute 26