auto lo
iface lo inet loopback

# The primary network interface
auto eth0
#iface eth0 inet dhcp

allow-hotplug usb0

mapping hotplug
        script grep
        map usb0

iface usb0 inet static
       address 192.168.2.14
       netmask 255.255.255.0
       broadcast 192.168.2.255
#       up iptables -I INPUT 1 -s 192.168.2.15 -j ACCEPT
up iptables -t nat -A POSTROUTING -o eth0 -s 192.168.2.15 -j MASQUERADE
up echo 1 > /proc/sys/net/ipv4/ip_forward
down iptables -t nat -D POSTROUTING -o eth0 -s 192.168.2.15 -j MASQUERADE
down echo 0 > /proc/sys/net/ipv4/ip_forward


iface br3 inet dhcp
         hostname ash
         pre-up   echo     "Adding ethernet bridge between LAN and PAN"
         pre-up   ifconfig ath0 0.0.0.0
         pre-up   brctl    addbr br3
         pre-up   brctl    setfd br3 0
         pre-up   brctl    stp br3 off
         pre-up   brctl    addif br3 ath0         
         pre-down echo     "Removing ethernet bridge between LAN and PAN"
         post-down brctl    delif br3 ath0
         post-down brctl    delbr br3
         post-down ifdown ath0
         post-down ifup ath0

$ sudo ifup usb0
SIOCSIFADDR: No such device
usb0: ERROR while getting interface flags: No such device
SIOCSIFNETMASK: No such device
SIOCSIFBRDADDR: No such device
usb0: ERROR while getting interface flags: No such device
usb0: ERROR while getting interface flags: No such device
Failed to bring up usb0.

auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
auto usb0
iface usb0 inet static
        address 192.168.2.15
        netmask 255.255.255.0
        gateway 192.168.2.14

Nokia-N810-43-7:~# ifdown usb0
mc-disconnect[2548]: GLIB DEBUG default - checking MC status
mc-disconnect[2548]: GLIB DEBUG default - mission_control_get_presence: MC not running.
run-parts: /etc/network/if-post-down.d/zz_proxy_unset exited with return code 1
Nokia-N810-43-7:~# ifup usb0
run-parts: /etc/network/if-up.d/00_proxy_set exited with return code 1