problem with tun

Hi All,

I have downloaded lwIP (a opensource TCP stack). This communicates
over the network using the "tun" device.
I used tcpdump to watch the network interface.
The problem is that packet sent on "tun0" never seems to reach the
actual network. Similarly no packet is seen on "tun0".
A normal flow of packets is seen over the "eth0"

Please help.

Regards,
Yadnesh