Block ad and other undesirable sites in linux

As root, edit the /etc/hosts file, add a line for each site to block, including subdomains, like this:

127.0.0.1 hosttoblock.com
127.0.0.1 www.hosttoblock.com
127.0.0.1 mail.hosttoblock.com

Then flush DNS cache:

# service nscd restart

# /etc/init.d/nscd restart

# /etc/init.d/dnsmasq restart