Faking a MAC address
From NetBSD Wiki
This should give some idea:
ifconfig bridge0 create ifconfig bridge0 up ifconfig vr0 up brconfig bridge0 add vr0 ifconfig tap0 create sysctl -w net.link.tap.tap0=00:00:de:ad:be:ef ifconfig tap0 up brconfig bridge0 add tap0
Now you can get an ip if from dhcp with the fake MAC:
dhclient tap0
