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Redundant DNS over VPN connection

I have a hub and spoke design where the branch locations use  a VPN tunnel to get back to HQ for their primary DNS servers.

To allow the branches to still browse the Internet if HQ has an outage, the branch routers are configured with 4 DNS server.  The first two are Private.  The second two are public DNS servers.

When the branch has any type of outage which causes the VPN tunnel to drop, their DNS reverts to the Public DNS servers.  Once the VPN tunnel comes back up, they cannot resolve internal websites since they are still using the Public DNS.

We can perform a reboot of the computers once the tunnel is up or perform an ipconfig /flushdns.

Does anyone know if the secondary DNS will time out or can someone explain how they are accomplishing redundant DNS?                  

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