RADIUS

Remote Authentication Dial-In User Service.

The network access device operates as a client of RADIUS.

RADIUS servers are responsible for:

• Receiving user connection requests.

• Authenticating the user.

• Return all configuration information necessary for the client to deliver service to the user.

Transactions between the client and the RADIUS server are authenticated using a shared secret.

Supports a variety of methods to authenticate a user.

• PAP, CHAP, or MS-CHAP, UNIX login, and other authentication mechanisms.

Combines Authentication and Authorization. Separates Accounting (less flexible than TACACS+).

Uses UDP (less robust).

Unidirectional authentication.

Only encrypts the password (less secure).

RADIUS accounting can hold more information.

Packet

Code - Identifies the type of RADIUS packet.

• (1) Access-Request,

• (2) Access-Accept,

• (3) Access-Reject,

• (4) Accounting-Request,

• (5) Accounting-Response,

• (11) Access-Challenge.

Identifier - Allows the RADIUS client to match a RADIUS response with the correct pending request (usually implemented as a counter).

Authenticator.

• In client Requests – Random value.

• In server Responses - MD5 Hash function of (Code, ID, Length, Request Auth, Attributes, Shared Secret).

Attributes - Section where an arbitrary number of attribute fields can be sent.

• e.g: User-Name and User-Password attributes.

Protocol

RADIUS exchange involving just a username and user password

Only the password is encrypted.

• The shared secret followed by the Request Authenticator is put through an MD5 hash to create a 16 octet value which is XORed with the password entered by the user.

• If the user password is greater than 16 octets, the password is broken into 16-octet blocks and additional MD5 calculations are performed.

The RADIUS protocol has a set of vulnerabilities.

• The Access-Request packet is not authenticated at all.

• Many client implementations do not create Request Authenticators that are sufficiently random.

• Many administrators choose RADIUS shared secrets with insufficient information entropy and many implementations limit the shared secret key space.