auth_krb5 - Man Page

auth_krb5 [-i instance]

This program does authentication for nnrpd against a Kerberos v5 KDC. This is NOT real Kerberos authentication using service tickets; instead, a username and password is used to attempt to obtain a Kerberos v5 TGT to confirm that they are valid.  As such, this authenticator assumes that nnrpd has been given the user's username and password, and therefore is not as secure as real Kerberos authentication.  It generally should only be used with NNTP over TLS to protect the password from sniffing.

Normally, you do not want to use this authenticator.  Instead, use ckpasswd with PAM support and configure the nnrpd PAM stack to use a Kerberos PAM module.  A full Kerberos PAM module is more sophisticated about how it validates passwords and has a much broader array of options than this authenticator.

-i instance

If this option is given, instance will be used as the instance of the principal received from nnrpd and authentication will be done against that principal instead of the base principal.  In other words, a principal like user, when passed to auth_krb5 invoked with -i nntp, will be transformed into user/nntp before attempting Kerberos authentication.

Since giving one's password to nnrpd is not as secure as normal Kerberos authentication, this option supports a configuration where all users are given a separate instance just for news authentication with its own password, so their regular account password isn't exposed via NNTP.

The following readers.conf(5) fragment tells nnrpd to authenticate users by attempting to obtain Kerberos v5 TGTs for them, appending an instance of nntp to usernames before doing so:

    auth kerberos {
        auth: "auth_krb5 -i nntp"
    }

    access kerberos {
        users: "*/nntp"
        newsgroups: example.*
    }

Access is granted to the example.* groups for all users who successfully authenticate.

Currently, any username containing realm information (containing @) is rejected.  This is to prevent someone from passing in a username corresponding to a principal in another realm that they have access to and gaining access to the news server via it.  However, this is also something that people may wish to do under some circumstances, so there should be a better way of handling it (such as, perhaps, a list of acceptable realms or a -r flag specifying the realm in which to attempt authentication).

It's not clear the right thing to do when the username passed in contains a / and -i was also given.  Right now, auth_krb5 will create a malformed Kerberos principal with multiple instances and attempt to authenticate against it, which will fail but perhaps not with the best error message.

Originally written by Christopher P. Lindsey.  This documentation was written by Russ Allbery <eagle@eyrie.org> based on Christopher's original README file.

ckpasswd(8), nnrpd(8), readers.conf(5).

readers.conf(5).