No DNS resolution if custom configured DNS server is not reachable - how to fallback to default DNS How can I configure the DNS server in the network preferences with 8.8.8.8 (Google DNS) for my Mac but have it default to local network DNS servers in case Google's DNS server is not reachable from a LAN?
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.