But my first guess would be that you are leaving the nameserver field blank, have entered an IP that is not a nameserver, entered an invalid IP or entered a FQDN rather than an IP for your nameserver.

You could try setting google's public DNS (8.8.8.8) and see what happens...

Beyond that, without more info of your config, environment, etc it's hard to offer many more suggestions...

In other words the IP(s) of where your TKL appliance will get it's DNS from. You need to specify at least one. If you are running a local DNS server then use the IP of that, if not, then often consumer grade modems do a DNS passthrough (i.e. you set the DNS provider in your router then all other connected computers use the IP of your router as the DNS/nameserver IP), or you can use an external DNS provider (assuming that your appliance has internet connection). Generally your ISP will provide a DNS service although in my experience they are usually sub-optimal. Like I said above, I'd recommend using google's public DNS: 8.8.8.8