----[ 1. Scapy crashing on my Mac. ]----

The reason appears to be that Scapy tries t aggressively
enumerate all available network interfaces, and something
about vboxnet0 spooks it. 

Fix: Take vboxnet0 temporarily down ("ifconfig vboxnet0 down") as
root, then start Scapy. After this you can bring vboxnet0 up again
("ifconfig vboxnet0 up" or "ifconfig vboxnet0 192.168.56.1 up") and
both your VM and your scapy will work.

Extra credit: work out a fix for Scapy!

----[ 2. All VMs come up with the same MAC and get the same IP. ]----

To change the MAC address of your VM's eth0, you will need to halt
("power down") your VM. Then, in the "Network" part of the VM
settings, click "Advanced", and change the MAC to whatever you
like. Then boot the VM afresh, and you will see the new MAC for eth0.

The Linux native command for changing the MAC address of an interface
is very simple: "ifconfig eth0 hw ether <new MAC>".  The interface
needs to be brought down first ("ifconfig eth0 down").  After that,
eth0 should come up with a new MAC, and should therefore get a new
dynamic IP via DHCP---but, unfortunately, VirtualBox does not emulate
this driver functionality, and requires changing its own idea of the
MAC while the VM is powered down. If the machine is running or
suspended, you will see the dialog box with the MAC under "Advanced",
but it will be grayed out and impossible to change.