You have to create (as root) the file "/etc/dhclient-enter-hooks" with this in it:
echo "nameserver 192.168.1.1" > /etc/resolv.conf
With 192.168.1.1 the actual address of your DNS server.
Next you must make this script executable: use the command "chmod 755 /etc/dhclient-enter-hooks"
Then restart dhclient and see if it worked!
What this thingy does is, it overrides the default make_resolv_conf() function (in /etc/dhclient-script)
with the new behaviour, i.e. just filling in /etc/resolv.conf with whatever we want, regardless of what the DHCP server says.
Let me know if it works for you!