[Masq] FTP DNAT

[Willliam Klein]

Title: FTP DNAT

Hi all;

I'm trying to setup an ftp server inside my firewall.  I know I need to DNAT port 21. so my rules are:

        $IPTABLES -t nat -A PREROUTING -i $EXTERNAL  -p tcp  \
                  -d $EXT_IP --dport 21 \
                  -j DNAT --to $REMOTEBACKUP:21

        $IPTABLES -A FORWARD  -p tcp  -s any/0 \
                  -d $EXT_IP --dport 21 \
                -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT

Now how do I get the random unprivliged port that the client and ftp server agree upon to DNAT?

Thanks

willie