Opening port 20 in Win 2003 server

[StorMan]

Hi can someone help us we are a small software company with offices in Oz and NZ. We are tryuing to let the NZ guys copy data to our FTP site on our 2003 server in Brisbane but they cannot. We have a Linux Smoothwall hardware firewall and I have opened port 20 in the Nat and the DMZ. When i port scan our machine from outside the network i cannot see port 20, as if i hadnt opened it.
I do not believe that i can mask it in any way and have not been able to find anything on the net to help us. They use Macs in NZ and i am not sure if this is afffecting this. Any ideas would be great.

I have already opened port 21 but they need port 20 open as well
the strange bit is that they can see port 21 ok.

Thank you

[Chireru]

Port 21 will be held open by your FTP program. Port 20 is used for the actual transfer of data, and shouldn't be openned when nobody's connected.

Are you using PORT mode or PASV mode? If you are using PASV, then you need to open more ports. PASV ports vary depending on FTP server software.

PORT mode needs: 21, 20
PASV mode needs: 21, [PASV port range]