Most DNS clients cache the results of name resolution requests to speed up name resolution if multiple lookups are done to the same address, such as is common when browsing the web.

Sometimes a invalid DNS entry will be cached and you will need to either flush the DNS cache to get rid of it, or wait up to 24 hours for it to be dropped from the cache automatically

Most results from google are some to install nscd and restart nscd service to flush DNS cache.

sudo aptitude install nscd
sudo /etc/init.d/nscd restart

Actually, it’s not need to take time installing a nscd service,directly run this following command will do the same job:

sudo /etc/init.d/dns-clean start