Backup DNS servers for a domain name - optimal configuration and troublehooting

Question

Let's say we have these name servers for 1 domain.

ns.1.com
ns.2.com
ns.3.com
ns.4.com
backup-ns.1.com
backup-ns.2.com

1. How can I know which NS responded to this specific query, for example using dig? Taking into account DNS cache.
2. In a normal NS query how will they be resolved? Is it round robin?

3. What is the optimal way to list the NS servers, for example:

   ns.1.com
   backup-ns.1.com
   ns.2.com
   backup-ns.2.com
   ns.3.com
   ns.4.com
   

Answer 1

1. dig output includes which server was queried. In the case where the query was made to a caching resolver server the client has no means of knowing which authoritative server the data originated from.

2. If the question is which authoritative server that a resolver server will pick, there is no specified priority - it's at the resolver server's discretion how this selection is done. Also worth noting, there's really no such thing as a "backup" authoritative nameserver, all the authoritative servers must have the current data and can expect to receive queries at all times.

3. This is not really relevant (see 2.)