Hi @mrpackethead_,
Unfortunately, it's not possible to configure the MX WAN interface to use DNS server which is reachable over the Site to Site VPN from the MX. This is because the MX WAN interface doesn't participate in VPN, and all traffic from the MX management interface will be sent directly to WAN link, so you need to make sure that the server is reachable from WAN interface. It doesn't necessarily mean that it should be a public IP though. If your MX is behind NAT and using an IP in a private range, you can use the DNS server that will be in the private subnet as well.
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