MIT Kerberos Documentation

OTP Preauthentication

OTP is a preauthentication mechanism for Kerberos 5 which uses One Time Passwords (OTP) to authenticate the client to the KDC. The OTP is passed to the KDC over an encrypted FAST channel in clear-text. The KDC uses the password along with per-user configuration to proxy the request to a third-party RADIUS system. This enables out-of-the-box compatibility with a large number of already widely deployed proprietary systems.

Additionally, our implementation of the OTP system allows for the passing of RADIUS requests over a UNIX domain stream socket. This permits the use of a local companion daemon which can handle the details of authentication.

Defining token types

Token types are defined in either krb5.conf or kdc.conf according to the following format:

    <name> = {
        server = <host:port or filename> (default: see below)
        secret = <filename>
        timeout = <integer> (default: 5 [seconds])
        retries = <integer> (default: 3)
        strip_realm = <boolean> (default: true)

If the server field begins with ‘/’, it will be interpreted as a UNIX socket. Otherwise, it is assumed to be in the format host:port. When a UNIX domain socket is specified, the secret field is optional and an empty secret is used by default. If the server field is not specified, it defaults to RUNSTATEDIR/krb5kdc/<name>.socket.

When forwarding the request over RADIUS, by default the principal is used in the User-Name attribute of the RADIUS packet. The strip_realm parameter controls whether the principal is forwarded with or without the realm portion.

The default token type

A default token type is used internally when no token type is specified for a given user. It is defined as follows:

    DEFAULT = {
        strip_realm = false

The administrator may override the internal DEFAULT token type simply by defining a configuration with the same name.

Token instance configuration

To enable OTP for a client principal, the administrator must define the otp string attribute for that principal. (See set_string.) The otp user string is a JSON string of the format:

    "type": <string>,
    "username": <string>
 }, ...]

This is an array of token objects. Both fields of token objects are optional. The type field names the token type of this token; if not specified, it defaults to DEFAULT. The username field specifies the value to be sent in the User-Name RADIUS attribute. If not specified, the principal name is sent, with or without realm as defined in the token type.

For ease of configuration, an empty array ([]) is treated as equivalent to one DEFAULT token ([{}]).

Other considerations

  1. FAST is required for OTP to work.