Resolvi meu problema. Script Iptables segue:
/sbin/iptables -t nat -A PREROUTING -i eth0 -p tcp -j DNAT --to --dport 3389 192.168.100.21:3389
/sbin/iptables -A FORWARD -p tcp --dport 3389 192.168.100.21 d j ACCEPT
Eth0 -> Board receiving the internret
192.168.100.21 -> Terminal Server ip in ETH1
3389 -> default port remote access rdp