We have 2 offices and both have different ISPs. We set up a VPN between both offices but when the VPN Tunnel is enabled DNS will not work in the remote office unless we use the main office's router IP as the DNS (therefore forcing resolution over the VPN). But, if we turn the tunnel off, local DNS works fine. We only have this issue in the remote office.
We tested the same router that was in the trouble location on a connection from the same ISP that we have in our main office and it works fine, no DNS issues.
What are we doing wrong?
The routers are Linksys RVS4000's running 1.2.10 firmware.