To flush your system DNS Cache Resolver in Ubuntu 12.04, press Ctrl – Alt – T on your keyboard to open Terminal. When terminal opens, run the commands below to clear it.
sudo /etc/init.d/dns-clean
http://www.liberiangeek.net/2012/05/windows-7-vs-ubuntu-12-04-how-to-flush-your-system-dns-cache/
Add custom DNS
To avoid having
your settings get revoked after reboots, or after periods of inactivity,
you may need to make the following changes via the command line:
$ sudo cp /etc/resolv.conf /etc/resolv.conf.auto
NOTE: The dhclient.conf location may vary depending on you linux distribution.
To locate the correct file you can use:
$ sudo find /etc -name dhclient.conf
$ gksudo gedit /etc/dhcp3/dhclient.conf
# add the following line to the document before the 'return subnet-mask' command
supersede domain-name-servers 208.67.222.222,208.67.220.220;
# save and exitYou may be required to change eth0 to your own network device's name if it uses a non-standard name.
$ sudo ifdown eth0 && sudo ifup eth0
https://store.opendns.com/setup/device/ubuntu/
Check domain with custom DNS
DNS records are likely cached by your provider's DNS servers so if you want to check if the DNS changes you made were successful you can interrogate a DNS server from your domain hosting service with dig:
dig -t a ns1.myhostingcompany.com @domain_registrar_dns_server
It you want Ubuntu to start caching dns I recommend installing pdnsd together with resolvconf. nscd is buggy and not advisable.
Example: dig -t a @8.8.8.8 example.com
http://askubuntu.com/questions/2219/how-do-i-clear-the-dns-cache
Check your PC DNS in command-line
nm-tool
http://askubuntu.com/questions/152593/command-line-to-list-dns-servers
Check DNS related stuff online
http://www.iptools.com/
