[Samba] Failed to join domain: failed to find DC for administrator at XYZ

[Yanni]

Hello everyone

I had samba 4.1 working as a member server to a windows 2012 AD in a
test environment for a while now until I had to re-install the windows
server from scratch. I have the new AD setup with the same NETBIOS name
but now I have problems re-joining the samba box to the new AD. It looks
like a DNS issue but I am unable to understand why. After the
ire-install, I got a new Kerberos ticket with no problem:

[root at gouda ~]# klist
Ticket cache: FILE:/tmp/krb5cc_0
Default principal: administrator at TESTAD.BIO.UCL.AC.UK

Valid starting       Expires              Service principal
03/09/2015 10:12:56  03/09/2015 10:22:53
krbtgt/TESTAD.BIO.UCL.AC.UK at TESTAD.BIO.UCL.AC.UK
-------------------------------------------

Then I tried to join samba to the new AD:

[root at gouda ~]# net ads join administrator at TESTAD.BIO.UCL.AC.UK
Enter root's password:
Failed to join domain: failed to find DC for domain
administrator at TESTAD.BIO.UCL.AC.UK
--------------------------------------------
some info:
AD IP: 192.21.35.5
Samba: 192.21.35.136
DNS: is running on the windows server.
--------------------------------------------

On the samba box resolv.conf looks like this:

nameserver 192.21.35.5
search testad.bio.ucl.ac.uk
domain testad.bio.ucl.ac.uk

etc/hosts looks like this:

127.0.0.1   localhost localhost.localdomain localhost4
localhost4.localdomain4
::1         localhost localhost.localdomain localhost6
localhost6.localdomain6

192.21.35.5 testserver1 testserver1.testad.bio.ucl.ac.uk
-------------------------------------------
On the windows server:
Primary DNS: 192.21.35.5
Secondary DNS: 127.0.0.1

The hosts file in C:\Windows\System32\drivers\etc is empty.

Do you know why I can't join the samba box to the new AD domain?
If you have any ideas/suggestions please let me know.

Many thanks
Yanni