Dynamic ip with permanent DNS

BSD:

vi /etc/dhclient.conf
add
supersede domain-name-servers x.x.x.x , y.y.y.y ;

x.x.x.x = dns-server-1
y.y.y.y = dns-server-2

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Ubuntu :

root@mail:~# more /etc/dhcp3/dhclient.conf
request subnet-mask, broadcast-address, time-offset, routers, host-name, ntp-servers;
supersede domain-name "ubuntu8.com";
prepend domain-name-servers 8.8.8.8;