Block Google DNS on Tomato routers
If you have a Tomato based router, then you are in luck, as they have a neat setting to prevent devices from contacting hardcoded DNS servers, like the Chromecast does.
- Go to your router web utility, click Advanced, and then DHCP/DNS.
- Once you're there, check the box that says Intercept DNS port (UDP 53). That forces all traffic to be sent through the DNS servers that are specified on the router. This will prevent the Chromecast and Roku from communicating with Google's DNS servers.