losing internet connectivity when using arpspoof under BT R3 in Virtual machine

[eeran]

Im trying to experience with Backtrack in a virtual machine on my macbook air.
I setuo the network device to be bridged so that the vm will have my true internal ip in the network
i also setup the vm to have my true mac address.
then, i started arpspoof but once i do it (or ettercap, tried that too) i lose internet connectivity.
the idea works, the victim target sees my mac address as the router's mac address.
but why do i lose internet connection?

[rastamouse]

Enabled IP forwarding?

echo 1 > /proc/sys/net/ipv4/ip_forward

[eeran]

Yes, sorry i didnt mention that.
i did the following:
echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A PREROUTING -p tcp --destination-port 443 -j REDIRECT --to-ports 10000
iptables -t nat -A PREROUTING -p tcp --destination-port 80 -j REDIRECT --to-ports
arpspoof .....
then i can see with arp -na that the arp is updated, but internet connectivity is gone