ipv4

1. First thing to do is to enable IP-forwarding (obviously)
2. Then we change the DESTINATION of packets coming in on ppp0
3. Still the SOURCE of those packets points to some computer on the internet

#!/bin/sh
IF=ppp0
IPT=iptables
IPTNAT="$IPT -t nat -A"
PORT=63392
SERVER=insert_ip_here
GW=insert_ip_here

echo "Activating forwarding... beware!"
echo "1" > /proc/sys/net/ipv4/ip_forward

#$IPT --flush -t nat

# packets coming in from $IF are routed to $SERVER
$IPTNAT PREROUTING -p tcp --dport $PORT -i $IF -j DNAT --to $SERVER:$PORT
$IPTNAT PREROUTING -p udp --dport $PORT -i $IF -j DNAT --to $SERVER:$PORT

# before leaving those packets are modified to look as if they came
# from $GW, so the return packets from $SERVER find their way back to $GW
$IPTNAT POSTROUTING -p tcp --dport $PORT -d $SERVER -j SNAT --to $GW
$IPTNAT POSTROUTING -p udp --dport $PORT -d $SERVER -j SNAT --to $GW

$IPT -L -t nat -vn

// posted by Jo-Chen @ Wednesday, August 24, 2005