I am working with a customer who has a DIR-825. I have encountered nothing but headaches with its configuration (or lack thereof), but let me focus on the showstopper.
I found another thread on this topic, but no working solution was ever posted. Specifically, the issue with DNS is that the local DNS suffix is being appended to all FQDNs (or sometimes the ISP's, its quite random).
I have tried resetting the router's configuration back to stock, no dice. Per the other thread, tested with Advanced DNS Services and DNS Relay both on and off, no change. I have manually set a local domain, with no change.
This appears to be a SERIOUS issue with the software. It has completely soured me to D-link products, as decent QA testing should have caught this. D-link is forcing their customers to beta test software for them, which is utterly unethical.
My current solution is simply to configure each PC on the customer's network with a valid DNS server; unfortunately, this is NOT an acceptable solution because any newly connected device must be manually configured, totally negating the purpose of DHCP.
While I'd like to see a solution posted here to help anyone else who may be encountering this problem, I will likely have the customer replace this steaming brick with an Asus WL-500g; a router of superior specifications, performance, and build quality. I have sworn by D-link products for years in the SMB environment, but will seriously reconsider their products in future, due to this glaring quality control issue, along with several others I've recently encountered, not limited to CHEEEEAP internal components and shoddy construction.