Errormessage "Could not Find Primary DC" appears in Server Manager or when you would join a domain or set up a trust relationship between two domains.

This behavior can occur if the 1b (domain master browser) and 1c (domain controller) NetBIOS names for the PDC are not registered in the Windows Internet Naming Service (WINS). This can occur when the WINS servers in the two domains do not replicate to each other or there aren't any WINS server available at a remote place (on the other side of a firewall or router)

Resolution:

Make these entries in LMHOSTS:

  10.0.0.1   PDCName   #PRE #DOM:DomainName
  10.0.0.1   "Domain         \0x1b"   #PRE  

- Replace 10.0.0.1 in the example with the IP address of the PDC in the remote domain.
- Replace the PDCName with the NetBIOS name of the domain PDC.
- Replace DomainName with the Windows NT 4.0 domain name of the target domain.

So far, this is a common thing, but the next line is also required:

A NetBIOS type, also called NetBIOS suffix, must be indicated for the appropriate domain. Specially important is, that this suffix must be at the end of the domain name, with is 15 characters plus 1, total 16 characters in lenght. When you specify the NetBIOS suffix (\0x1b) after the Domain name (must be the sixteenth character), the spacing between the quotation marks is critical. There must be a total of 20 characters within the quotation marks (the domain name plus the appropriate number of spaces to pad up to 15 characters plus the backslash (\) plus the NetBIOS hex representation of the service type).