No DNS resolution if custom configured DNS server is not reachable - how to fallback to default DNS
First go to the terminal to find out what's your current DNSs using the command:

cat /etc/resolv.conf | grep nameserver


Then, 

in your network settings, under "Advanced"



enter:

8.8.8.8
8.8.4.4
x.x.x.x //local DNS
x.x.x.x //local DNS


If Google DNS won't work it will fallback to google other DNS server. If that fails too the fallback will be your local DNSs.