Na página man do dhclient (8):
If the DHCP client should listen and transmit on a port other than the standard (port 68), the -p flag may used. It should be followed by the udp port number that dhclient should use. This is mostly useful for debugging purposes. If a different port is specified for the client to lis- ten on and transmit on, the client will also use a different destination port - one less than the specified port.
(Note que isto é do pacote isc-dhcp-client no Debian squeeze. Eu não sei se a sua versão do Ubuntu vem com um pacote dhclient diferente)