I'm having trouble with bind on Ubuntu 14.04. Bind seems to be up and running and I have checked that port 53 is opened. However, when I run a nameserver check on http://reports.internic.net/cgi/whois, it does't work. Therefore on Godaddy, I can't add the nameservers and get the error: "You must enter a registered nameserver."
I figured out the issue. It was due to my lack of knowledge. I'll explain how it was resolved in case others have similar issues. Although I had setup my dns to point my subdomain ns1.example.com to my ip address, there is a further thing that needs to be done. Through your domain registrar, such as GoDaddy, for example.com, you have to register your nameserver. In godaddy, under the Domain Management, it is called Host Names. Once you add the subdomain and ip address, GoDaddy forwards to the authoritative registry, such as Verisign for .com/.net/etc. When that is done you will be able to add the nameserver to the other domain. You can verify that it has been added through the InterNIC Whois Nameserver Search. I still don't fully understand this, but I hope it helps someone!