Strange problems with FTP - please, help!

Hello!

At one of my customers, a FTP server had to be installed. It is the one which
is included in SuSE 8.0 / Kernel 2.4.18-4GB:
220 rohrpostfix FTP server (Version 6.5/OpenBSD, linux port 0.3.2)

What has to be mentioned, is:
The FTP server is behind a NETOPIA router (with ISDN fallback). The router
does IP-masquerading between its internal IP 192.168.1.2 and its official IP,
the server has a WAN-NIC (192.168.1.1).


Here a session transscript from my workstation (SuSE 8.0, also behind a
router with IP-masquerading):
---<snip>------------------------------------------------
Connected to XXXXXXXXXXXXXXXXX
Name (XXXXXXXXXXXXX:rainer): anonymous
331 Guest login ok, type your name as password.
Password: ***********
230 Guest login ok, access restrictions apply.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls
229 Entering Extended Passive Mode (|||17742|)
200 EPRT command successful.
435 Can't build data connection: Illegal seek.
ftp> quit
221 Goodbye.
rainer@miraculix:~>
---<snap>------------------------------------------------


As you can see, the login is possible, but when I enter a command, no data
connection can be established. (Happens with all commands!).
The same happens when several people (not only me!) try to transfer data from
their Windows-PCs (also behind routers), so this cannot be dependend on my
local configuration.

When I use xFTP instead (the one which is included in SuSE Linux), it is no
problem to connect and transfer data. It works fine!!!


Before this netopia-router was installed, a ZyXEL 310 router was there. But it
had to be replaced because it allowed no ISDN fallback and got extreme
problems with ADSL stability (because Telecom changed something in their
system). The ZyXEL had the same problem at the beginning, but then I
configured it to forward *everything* to the server's IP from the outside.
Then it suddenly worked.

What I *guess* is: There is a problem establishing the data connection
from most clients, because they use upper ports, while xFTP seems to work
different. But I could not find out the difference at all.


Does somebody have fixed such a problem? It is really urgent and I must get it
working ASAP.

mfg

  Ing. Rainer Hantsch