[gentoo-user] Frox iptables ftp proxy

[D.H.]

I'd like to set up an ftp proxy on my home firewall so I can scan for
viruses using clamd.  I found frox.  Which looks like it will do what I
want.  I've pretty much used the default install which makes frox listen
on 127.0.0.1:2121.  But, I'm not sure the firewall rules are working
right.

eth1 is the internal interface

iptables -A FORWARD -p tcp -i eth1 --destination-port 2121 \
     --destination 127.0.0.1 -j ACCEPT

iptables -t nat -A PREROUTING -p tcp -i eth1 --destination-port 21 \
     -j DNAT --to-destination 127.0.0.1:2121

Either that, or frox itself is having issues.  Any ideas?  While I'm at it, 
is there an alternative to frox?