You are setting up a LAN with multiple computers running Linux?
How are you connecting the computers? Via a Switch, router or hub?
If you are using a router then why setup Ubuntu as a DNS and DHCP? Your router/modem will serve as a DHCP and DNS via your ISP.
Why setup a local DNS for such a small LAN? You could use your '/etc/hosts' file;
excerpt 'man hosts';
hosts - The static table lookup for host names
This manual page describes the format of the /etc/hosts file. This
file is a simple text file that associates IP addresses with host-
names, one line per IP address. For each host a single line should
be present with the following information:
IP_address canonical_hostname [aliases...]
If your LAN is connect to the Internet then you must be using a ISP. The ISP will supply with a DNS that you could place in your '/etc/resolv.conf' file. Or through your modem/router.
This has been covered a lot on LQ. Do a search!