about ns record in child domain

[Tech W.]

Greetings,

I have a domain said example.com.
in example.com's main DNS I added a NS record:

cdn.example.com.  IN  NS  otherdns.example.com.
otherdns.example.com.  IN  A  12.34.56.78


Then in DNS of otherdns.example.com, I set something in named.conf like:

      zone "cdn.example.com" {
           type master;
           file "/var/cdn.example.com.db";
      };

in cdn.example.com.db:

www.cdn.example.com.  IN  A  11.22.33.44
mail.cdn.example.com   IN  A  xx.xx.xx.xx
...


But, my question is, how can I add NS records for zone itself of cdn.example.com? I mean, I can't add something like this in cdn.example.com.db:

otherdns.example.com.  IN  A  12.34.56.78
(this NS item was defined in example.com's main dns.)


Please help, thanks!

Regards.
Ken.




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