Since casting JF to a Chromecast requires that it be resolvable by Google's DNS (or at least, it used to be that way I think), here's a fun trick to get it working: point your public DNS record to your private IP. It's apparently not always supported by your DNS provider, but it works great for me (namecheap).

No need to expose your JF instance over the Internet this way, and no need for complicated DNS interception stuff with your router.

You may need to have SSL certs for casting, not sure.