MIT Kerberos Documentation

Authentication indicatorsΒΆ

As of release 1.14, the KDC can be configured to annotate tickets if the client authenticated using a stronger preauthentication mechanism such as PKINIT or OTP. These annotations are called “authentication indicators.” Service principals can be configured to require particular authentication indicators in order to authenticate to that service. An authentication indicator value can be any string chosen by the KDC administrator; there are no pre-set values.

To use authentication indicators with PKINIT or OTP, first configure the KDC to include an indicator when that preauthentication mechanism is used. For PKINIT, use the pkinit_indicator variable in kdc.conf. For OTP, use the indicator variable in the token type definition, or specify the indicators in the otp user string as described in OTP Preauthentication.

To require an indicator to be present in order to authenticate to a service principal, set the require_auth string attribute on the principal to the indicator value to be required. If you wish to allow one of several indicators to be accepted, you can specify multiple indicator values separated by spaces.

For example, a realm could be configured to set the authentication indicator value “strong” when PKINIT is used to authenticate, using a setting in the [realms] subsection:

pkinit_indicator = strong

A service principal could be configured to require the “strong” authentication indicator value:

$ kadmin setstr host/high.value.server require_auth strong
Password for user/admin@KRBTEST.COM:

A user who authenticates with PKINIT would be able to obtain a ticket for the service principal:

$ kinit -X X509_user_identity=FILE:/my/cert.pem,/my/key.pem user
$ kvno host/high.value.server
host/high.value.server@KRBTEST.COM: kvno = 1

but a user who authenticates with a password would not:

$ kinit user
Password for user@KRBTEST.COM:
$ kvno host/high.value.server
kvno: KDC policy rejects request while getting credentials for

GSSAPI server applications can inspect authentication indicators through the auth-indicators name attribute.