IPFW firewall NAT, port address translation, and "active" FTP

[Brett Glass]

Everyone:

I've just attempted to build a router using FreeBSD 8.0 with IPFW's 
firewall NAT. I've included the following NAT parameters:

ipfw nat 123 config if xl0 log redirect_port tcp 10.0.1.99:21 21 
redirect_port tcp 10.0.1.99:20 20

Note that, among other things, incoming FTP is redirected to the 
host at 10.0.1.99 inside the firewall.

The problem we're having is that users are having trouble reaching 
the FTP server with some clients -- in particular, Microsoft 
Internet Exploder. (I don't WANT them to be using IE, but I do not 
have control over this.) Does anyone know if I need to set anything 
special to make the firewall track FTP data ports?

--Brett Glass