This appears to be somewhat outdated. There's more recent information here: http://trac.secdev.org/scapy/wiki

Includes a FAQ and a great guide for capturing rogue DHCP servers on your network: http://trac.secdev.org/scapy/wiki/IdentifyingRogueDHCPServer...

I've had to use it a few times. It is also (or used to?) be in MacPorts.