[Samba] Samba4: net ads join fails: Host is not configured as a member server.

[steve]

Hi everyone
I'm trying to join an Ubuntu 12.04 client to a 12.04 Samba4 DC. xp and 
win7 clients can join fine.

Here is my minmal smb.conf
realm = POLOP.SITE
workgroup = POLOP
security = ADS

Kerberos is working:
kinit Administrator
Password for Administrator at POLOP.SITE:

But then it tell me that the DC is _not_ a DC:
net ads join -UAdministrator
Host is not configured as a member server.
Invalid configuration.  Exiting....
Failed to join domain: This operation is only allowed for the PDC of the 
domain.

and:
net ads testjoin
Failed to open /usr/local/samba/private/secrets.tdb
Join to domain is not valid: Access denied

Can anyone help me tell the Ubuntu client that it really _is_ a DC? Or WHY.

Cheers,
Steve