my configuration is:
iface tap0 inet static
pre-up tunctl -u stex
pre-up bash -c 'echo 1 > /proc/sys/net/ipv4/ip_forward'
pre-up iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
post-down tunctl -d stex
iface eth0 inet static
up route add -net 0.0.0.0 dev eth0
up route add -net 0.0.0.0 gw 18.104.22.168
dns-nameservers 22.214.171.124 126.96.36.199
Consider that the last couple of line refer to the fact that I need to go
outside my host box, so for your purpose you don't need them.
I can do whatever I would like with this conf.