How to update/flush dns cache on clients?
Solution 1:
You can restart BIND on Ubuntu using sudo rndc reload, on Centos I believe you have to /etc/init.d/bind restart.
You can also execute /etc/init.d/networking restart to flush the DNS/networking for local desktops running Ubuntu.
Solution 2:
I have found that a simple restart does not always clear the cache. The other way of doing it, would be to use the nscd command and invalidate the hosts cache.
# nscd -i hosts
Then you don't even need to restart the caching service.