to do the forwarding: echo "1" > /proc/sys/net/ipv4/ip_forward; iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
iptables --table nat --insert PREROUTING --in-interface eth0 --protocol 
               tcp --destination-port 6881 --jump DNAT --to-destination 192.168.1.2
this is an example only; dont know if it will work for you