I have been searching for this answer for awhile and came up with nothing. I recently upgraded from ubuntu 10.04 to 12.04 and I understand the auto config of the resolv.conf file and I have edited the /network/interface to include the changes but it keeps old configurations from when it was 10.04.

EXAMPLE:
network/interface=

auto eth0
iface eth0 inet static
address <IP_ADDRESS>
netmask 255.255.255.0
gateway <IP_ADDRESS>
network <IP_ADDRESS>
broadcast <IP_ADDRESS>

dns-search <NEW_DOMAIN>
dns-domain <NEW_DOMAIN>
dns-nameservers <NEW_IP_ADDRESS> 4.2.2.1 8.8.8.8

I look into the /resolv.conf and it shows this:

nameserver <NEW_IP_ADDRESS>
nameserver 4.2.2.1
nameserver 8.8.8.8
search <NEW_DOMAIN>
domain <OLD_DOMAIN>
search <OLD_DOMAIN>
nameserver <OLD_IP_ADDRESS>
nameserver 4.2.2.1
nameserver 4.2.2.2

Any suggestions would be great