I have a RB411UAHR with 2 WAN interfaces:
* ADSL (pppoe-out1)
* 3G (ppp-out1)

Both from different ISP's, both use different DNS servers.

"Use Peer DNS" is enabled on both interfaces.
"IP/DNS/allow-remote-requests" is enabled.

This is a failover solution with both ADSL & 3G permanently connected.
The default route for the 3G has a greater distance, so that failover occurs when the ADSL goes down.

BUT

My IP/DNS settings are also affected. I often find routers running on ADSL, having the DNS addresses of the 3G ISP, this causes DNS lookup failures.

Is there a better way to ensure that I'm using the peer DNS of the shortest active default route's interface?

Remove the "use peer dns" from the pppoe settings, and set manually some public DNS servers, like the
google DNS server for example.

Remove the "use peer dns" from the pppoe settings, and set manually some public DNS servers, like the
google DNS server for example.

I'll try that thanks, the only reason I've been avoiding this option is because our ADSL lines may become capped and then only local traffic is allowed by the ISP. In this case, I will probably not have access to the USA-hosted DNS servers (8.8.8.8 & 8.8.4.4).
BUT, reconfiguring the DNS when that happens may be less troublesome than struggling with DNS related issues throughout the month.