If you only want specific interfaces to get ip address via the DHCP (in this example, let’s say eth0 and wlan0 only) then remove every thing else for the file with the exception of the lo interface.
See my sample /etc/network/interfaces file.
Note: Backup first your /etc/network/interfaces file if you’re having some doubt with what you are doing, in this way you can revert back to the original file. To do this enter cp /etc/network/interfaces /etc/network/interfaces.bak (this create a backup of the /etc/network/interfaces file)
root@wiztechie:~# cat /etc/network/interfaces
iface lo inet loopback
iface eth0 inet dhcp
iface wlan0 inet dhcp
If you want to autostart or run this at startup, enter the command.
update-rc.d networking defaults
Hope this guide help you with your networking problems with BackTrack.