IP Networking

We have set-up a FTP server on a Linux machine, which sits behind a firewall. Most users outside our firewall can access it, but one user in particular cannot. I assume it has something to do with their firewall rules. It appears that they can connect with no problem but the server does not grant full access for long, as there is appears to be a failure in staying connected to the port, e.g. an extract from the log

WINSOCK.DLL: WinSock 2.0
WS_FTP Pro, Version 7.0, 2001.06.07
Connecting to XX.XX.XXX.XXX:XX
Connected to XX.XX.XXX.XXX:XX in 0.430, Waiting for Server Response 220 localhost.localdomain FTP server (Version wu-2.6.2-12) ready.
Host type (1): Automatic detect
USER username
331 Password required for username.
PASS (hidden)
230 User username logged in. Access restrictions apply.
SYST
215 UNIX Type: L8
Host type (2): UNIX (standard)
PWD
257 "/" is current directory.
TYPE A
200 Type set to A.
PORT 192,168,0,41,7,220
connection reset; the server has reset the connection for some unkown reason.
Port failed
PASV
connection reset; the server has reset the connection for some unkown reason.
It appears that the connection is dead. Attempting reconnect...
==========================================================
and tries to connect again with the end result:


connection reset; the server has reset the connection for some unkown reason.

==========================================================

They are using a NAT (Network Address Translation) security method with their firewall, maybe this is overriding the firewall rules? Has anyone seen this before?
Thanks

We resolved this, if anyone else expereices a similar problem the NAT configuration has a passive mode setting which needs to be deactivated before connecting to the Linux hosted FTP server.