My problem:
I have a main office with 20 PC in WinNT 4.0 Domain and 2 remote offices with 6-8 PC on each. IN main office I use Slackware 10.2 as a VPN server ( with masquerade internal network) on each remote office I have 1 Slackware as a VPN client connected to main office (each remote office is in different network like 10.20.0.0/24, 10.20.1.0/24)They all have DNS from main office and no problem to open websites from webserver in main office.
All work perfect BUT now I need to join all PC's from remote offices to Domain in main office. When I try to join a PC to Domain I receive "A domain controller for the domain "DOMAIN_NAME" could not be contacted"
Any advice pls!!

I don't think it will work easily.

I tried to get some Windows pcs to join an NT4.0 domain through a masquerading firewall (running Devil Linux).

You might look at "How to write an Lmhosts file for domain validation and other name resolution issues":

http://support.microsoft.com/kb/180094/EN-US/ for Win2000
and http://support.microsoft.com/kb/314108/EN-US/ for WinXP

...though I couldn't get this to work.

I think it had something to do with (NETBIOS over TCP/IP??) broadcast packets being unable to be routed through the firewall to the machine. Bah! I just gave up, and cached the domain account on the workstation - luckily only needing one user per machine.