ufw should be installed by default on newer Ubuntu servers. (Although it is most likely disabled)

sudo ufw default allow  # Careful, uwf by default DENIES all incoming traffic. So I first set it to "allow" here to avoid being locked out of the server
sudo ufw enable  

sudo ufw status verbose

sudo ufw deny from 10.101.21.228

sudo ufw deny 53

sudo ufw delete deny from 10.101.21.228

• https://help.ubuntu.com/community/UFW
• http://1000umbrellas.com/2010/04/29/how-to-set-up-the-firewall-using-ufw-on-ubuntu-lucid-lynx-server
• https://help.ubuntu.com/8.04/serverguide/C/firewall.html