From K5Wiki
Jump to: navigation, search
This project was completed in release 1.12.


In 1.11 KRB5 gained client side support for OTP Preauth (RFC 6560), but up until now there is no server side support in tree. Red Hat has created an out-of-tree solution (AuthHub), based upon a plugin model. But our experimenting with this approach has demonstrated:

  1. Access to vendor SDKs is non-trivial
  2. All vendors provide a RADIUS server for OTP validation


The KDC-side support for OTP should read configuration and forward token validation to an appropriate RADIUS server. This plugin will be called otp. In addition to standard RADIUS, the otp plugin will support a non-standard RADIUS over Unix Socket (RoUS; inconceivable!) mode for handling local companion daemons.

Token Type Configuration

krb5.conf or kdc.conf

The configuration is defined as follows:

 <name> = {
  server = <string>
  secret = <string>
  timeout = <integer>
  retries = <integer>
  strip_realm = <boolean>

Name Default Value Format
otp.<name>.server $KDCDIR/<name>.socket host, host:port or /path/to/unix.socket
otp.<name>.secret N/A (RoUS mode) or secret MUST be specified! /path/to/secret.file
otp.<name>.timeout 5 Integer (seconds)
otp.<name>.retries 3 Integer
otp.<name>.strip_realm true Boolean

All values are optional except secret when a non-RoUS server is specified.

NOTE: We only permit a single server to be defined because we are assuming that redundancy will be handled via DNS round-robin.

Default Token Type

There is a reserved token type name: DEFAULT. If no DEFAULT token type configuration is defined, the following configuration will be used internally:

  strip_realm = false

You can override this default token type configuration by defining your own token type configuration with the name of DEFAULT.

Token Instance Configuration

Some portion of the otp plugin configuration is user specific. This value will be stored as the user string otp with the following JSON formatted array of token objects:

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

If type is not specified then it refers to the DEFAULT token type as defined above. The username field overrides the default User-Name attribute sent in the RADIUS packet.

All values above are optional. If the user string is an empty string or is an empty list, then a user string of "[{}]" will be assumed.

OTP Enablement

The otp plugin will be enabled for all principals which have the otp user string set, regardless of its value. Conversely, if the otp user string is unset, the otp plugin will be disabled for the principal.


In the first pass (no PA-OTP-REQUEST present), the otp plugin will look up the otp user string on the given principal. If the string is set (i.e. non-NULL), a generic PA-OTP-CHALLENGE will be sent to the client (no optional fields will be filled in).

Upon receipt of a PA-OTP-REQUEST, the KDC will look up the RADIUS servers using the otp user string and kdc.conf configuration. All RADIUS servers will be used for validation, in the order they were specified in the otp user string, stopping after the first Access-Accept response is received.


The packet sent to the configured RADIUS server will contain:

  • User-Name (default: user principal, realm stripped per config; overridden by username)
  • User-Password (otp-value from PA-OTP-REQUEST)
  • NAS-Identifier (gethostname())
  • Service-Type (Authenticate-Only)

Remaining Issues

FIPS compliance

We are not targeting FIPS compliance, but for those who are interested here is the related information:

  • RADIUS is not FIPS compliant due to the use of MD5 in the protocol
  • EAP might make RADIUS FIPS compliant and Fedora ships a libeap
  • Integration of EAP is not planned at this time

OTP Preauth Challenge Optional Fields

According to RFC 6560, there are many optional fields. We currently do not have any plan to fill these in.


libkrad will be tested with C test programs located in lib/krad. Some of the programs will spawn a Python daemon which uses pyrad to provide a simple RADIUS server to test against.

The OTP module will be tested by a new script tests/t_otp.py, which also uses pyrad to provide a RADIUS server to test against.


A new page admin/otp.rst will describe KDC configuration for OTP.

Mailing list discussions


   8b8f031c6e64360a26c484b548d2158944e09087 Add libkrad
   4b5dd8bcfb10af254fb9efbe4cf39befe5b1e6ac Add server-side otp preauth plugin
   f6cb089daed2615d0b1594d7ccc20d617eb374eb Fix skip logic in t_otp.py
   acb490bd01235511294ecb6b23750e648e48f7dc Fix OTP KDC module get_string error handling

Completed in [krbdev.mit.edu #7678].

Release notes

Administrator experience:

  • Add a FAST OTP preauthentication module for the KDC which uses RADIUS to validate OTP token values.