Sending DNS Resolver traffic through OpenVPN Client (NAT problem)
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You cannot policy route traffic originating on the firewall. Period. It happens when traffic enters an interface. Traffic originating on the firewall never does that.
OPNsense can do it. I have my VPN clients on a VLAN and my non-VPN clients on the LAN. I added a rule to the VLAN that routes any TCP/UDP traffic with destination This Firewall and ports DNS-DNS to the VPN gateway, and a rule to the LAN that routes any TCP/UDP traffic with destination This Firewall and ports DNS-DNS to the WAN gateway. Works great. No DNS leaks for VPN clients and the non-VPN clients get faster DNS lookups.
Tried it in pfSense and it doesn't work. Sure would be nice if it did.
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No idea what OPNsense does for DNS. But it sounds like you have that and pfSense configured completely differently.