Here is the setup...
One main office that has two physical DNS servers. 184.108.40.206 and 220.127.116.11
6 satellite offices (in same city) that are very small and connect to the main site by MPLS or VPN.
Each satellite has a Sonicwall TZ100 firewall with DNS configured as the following
DNS SErver 1: 18.104.22.168
DNS Server 2: 22.214.171.124
DNS Server 3: null
When they lose connectivity to the main site they cannot surf the internet. We attempted to add a public DNS (ex:126.96.36.199) for the third DNS server. When we do this it causes problems. The computers will randomly not be able to ping main site hostnames and/or connect to the terminal server farm. We have to manually log into the machine and flushdns registerdns to get them back up.
Having a DNS server at each satellite is out of the question. What is the best way to configure this? Also how can I prevent the problem i am having? Any assistance is appreciated.