dhclient UDP port 68
As long as you need to use DHCP the DHCP client needs to be able to receive DHCP messages from the network (unless you are running a DHCP server on the same machine).
You can use firewall to filter DHCP traffic if you want but it is a little more complicated.
As long as you've configured the network interface to use DHCP - yes:
A DHCP-request is sendt with 0.0.0.0 as the source address, and the reply is a broadcast-packet as well. It is necessary to expose udp/68 when this occurs.