This is a negative test scenario for samba 3.0.20 and 3.0.21c
Used AD at Windows 2003 SP1 server for this scenario.
Network configuration: 
---- samba client ------ Windows 2003 SP1 server -----
Windows 2003 SP1 server runs a name service

The steps to reproduce the problem:
1. create the valid smb.conf and krb5.conf for ADS authentication below
2. configure DC as a name server; 
  # more /etc/resolv.conf 
  search eng.test.com  
  nameserver 2.9.192.20
3. register (with ADS option) the samba client in AD’s Computers section with successful result 
  # net ads join -U Administrator%password
  Using short domain name -- ENG
  Joined 'CLIENT-11' to realm 'ENG.TEST.COM'
4. samba client leaves/unregister (with ADS option) AD’s Computers section successfully 
  # net ads leave -U Administrator%password
  Removed 'CLIENT-11' from realm 'ENG.TEST.COM'

5. modify the name server configuration, where the primary name server is unreachable (bogus) ip address. The secondary name server is valid ip address of DC where DNS is running 
# more /etc/resolv.conf 
search eng.test.com  
nameserver 1.1.1.1               <----- bogus the primary nameserver   
nameserver 2.9.192.20
6. register (with ADS option) the samba client in AD’s Computers section  
  # net ads join -U Administrator%password
samba client registration hangs forever until Ctrl+C is executed to terminate the command
7. register (with RPC option) the samba client in AD’s Computers section with successful result 
# net rpc join -U Administrator%password
Joined domain ENG.


#testparm
Load smb config files from /etc/samba/smb.conf
WARNING: The "printer admin" option is deprecated
Processing section "[print$]"
Processing section "[printers]"
Loaded services file OK.
Server role: ROLE_DOMAIN_MEMBER
Press enter to see a dump of your service definitions

[global]
        workgroup = ENG
        realm = ENG.TEST.COM
        security = ADS
        client schannel = No
        password server = 2.9.192.20, *
        log level = 5
        log file = /local/local1/errorlog/samba.log
        max log size = 1000
        smb ports = 50139
        lpq cache time = 10
        socket options = TCP_NODELAY SO_RCVBUF=8192 SO_SNDBUF=8192
        printcap cache time = 60
        printcap name = cups
        preferred master = No
        local master = No
        domain master = No
        dns proxy = No
        wins server = 2.9.192.20
        ldap ssl = start tls
        idmap uid = 70000-200000
        idmap gid = 70000-200000
        template homedir = /local/local1/
        template shell = /admin-shell
        winbind cache time = 900
        winbind enum users = No
        winbind enum groups = No
        winbind use default domain = Yes
        comment = test printing
        printer admin = @cupsAdmin
        cups options = "raw"
        default devmode = Yes
        force printername = Yes

[print$]
        path = /state/samba/printers
        write list = @cupsAdmin
        force user = root
        force group = root
        guest ok = Yes

[printers]
        path = /local/local1/spool/samba
        guest ok = Yes
        printable = Yes
        browseable = No
 

# more /etc/krb5.conf
[logging]
 default = FILE:/var/log/krb5libs.log
 kdc = FILE:/var/log/krb5kdc.log
 admin_server = FILE:/var/log/kadmind.log

[libdefaults]
 ticket_lifetime = 24000
 default_realm = ENG.TEST.COM
 dns_lookup_realm = false
 dns_lookup_kdc = false

[realms]
 ENG.TEST.COM = {
  kdc = server-2003-ent1.eng.test.com:88
  admin_server = server-2003-ent1.eng.test.com:749
  default_domain = ENG.TEST.COM
 }

[domain_realm]
 .eng.test.com = ENG.TEST.COM
 eng.test.com = ENG.TEST.COM

[kdc]
 profile = /var/kerberos/krb5kdc/kdc.conf

[pam]
 debug = false
 ticket_lifetime = 36000
 renew_lifetime = 36000
 forwardable = true
 krb4_convert = false