Postado originalmente por wrochal
Caro,
Vamos lá:
IPT=caminho_do_iptables
IF_WAN=interface_da_internet
IP_WAN=IP_da_internet
$IPT -t nat -A PREROUTING -i $IF_WAN -p tcp --dport 1723 -j DNAT --to 192.168.0.150:1723
$IPT -t nat -A PREROUTING -i $IF_WAN -p tcp --dport 1723 -j DNAT --to 192.168.0.150
$IPT -t nat -A PREROUTING -i $IF_WAN -p tcp --dport 1723 -j DNAT --to 192.168.0.150
$IPT -t nat -A PREROUTING -i $IF_WAN -p 47 -j DNAT --to 192.168.0.150
$IPT -A INPUT -i eth0 -p tcp -d $IP_WAN --dport 1723 -j ACCEPT
$IPT -A INPUT -i eth0 -p 47 -j ACCEPT
$IPT -A FORWARD -p TCP --dport 1723 -j ACCEPT
$IPT -A FORWARD -p 47 -j ACCEPT
Vale lembrar que 192.168.0.150 seria IP do Servidor Interno, o windows.