cannot ftp files to a host behind firewall

["Joe Hu" <joehu@yahoo.com>]

I'm having a problem to make ftp work for an internal machine. On the
router, I set portfw so that port 20 and 21 (both tcp) are forwarded to to
an internal host. From a machine outside the firewall, I run ftp to connect
to the router. It did connect to the internal host. That means, the portfw
worked. However, I counldn't transfer files to and from the internal host.
Could someone help me out?

Thanks in advance.

joe