i want to assign manually configure my eth0 for my linux laptop. I know the ifconfig command to add the ip address, default gateway and broadast address, but i don't know how to add the dns and change the mac address
Whoops sorry, I read the post in a hurry and it seems I missed the whole question. Like taqqi44 said, although the MAC address can be changed on runtime through ifconfig, to make the change take place in each boot automatically you need to pass it through you distribution's appropriate script. You could also use any script that is executed on boot after the networking script, like rc.local, however that can cause problems if some service that loads before it depend on the network.