Sorry I'm slow in understanding this.
Ok, so you're saying that if I want the private name servers, the domain registrar needs:
A:
1. the name servers (ns1.mydomain.com) _AND_:
2. IP addresses of the name servers (glue)

Correct?

I just want to make sure the AND condition is there.

Just ignore everything and tell me which answer is correct.

When doing a private name server:
A. The domain registrar only needs the name servers. So they only need to know ns1.mydomain.com
B. The domain registrar needs the name servers AND the IP addresses. They need ns1.mydomain.com, AND the IP address of the name server.

A or B?

If I get the answer to this I'll understand the issue better. This was my original query as well (what do they need?)

B if you are configuring domain mydomain.com (a setup often termed a vanity nameserver, where your nameserver names are beneath your own domain), A if some other domain.

That is, at your registrar, if you are configuring nameservers for a domain and you give it names for those nameservers within that domain, you have to have the IP address (the glue) at the registrar since otherwise it's a catch-22. How does it look up the name in that domain in order to look up the domain? So an entry for domain mydomain.com of ns1.mydomain.com needs to have the IP address.

If, however, those nameservers are in another domain (say, ns1.linode.com) then you're fine, and that nameserver address will be recursively looked up (since it can query an address from linode.com while attempting to identify the nameserver for mydomain.com).

You can have a mixture in the same domain too. So for example, I have a domain where ns1.<mydomain> has an explicit IP address, but then I also have ns1-ns5.linode.com as additional nameservers which don't need an IP address. In my case I then operate the master nameserver on my Linode with the Linode nameservers as slaves, so I get more flexible control over my DNS management but redundancy through the Linode servers without having to operate multiple servers of my own. But my domain registrar information doesn't try to hide the use of the Linode servers.

All of this is largely independent of what address you use for those nameservers, and if they are even your own machines. As pointed out in this thread, there's no problem with you assigning the IP address of Linode's servers as the glue for your vanity nameservers, as long as you accept the responsibility for manually keeping them up to date in the event that they change. Although, in that case, reverse lookups will still likely expose the use of the third party machines so if you truly want your nameservers to completely be part of your domain they will need to be machines you control (or at least fully control the DNS records for).

-- David

B