When a zone that this DNS server hosts is a primary zone, the DNS server is the primary source for information about this zone, and it stores the master copy of zone data in a local file or in AD DS. When the zone is stored in a file, by default the primary zone file is named zone_name.dns and it is located in the %windir%\System32\Dns folder on the server.

When a zone that this DNS server hosts is a secondary zone, this DNS server is a secondary source for information about this zone. The zone at this server must be obtained from another remote DNS server computer that also hosts the zone. This DNS server must have network access to the remote DNS server that supplies this server with updated information about the zone. Because a secondary zone is merely a copy of a primary zone that is hosted on another server, it cannot be stored in AD DS.

Stub zone

When a zone that this DNS server hosts is a stub zone, this DNS server is a source only for information about the authoritative name servers for this zone. The zone at this server must be obtained from another DNS server that hosts the zone. This DNS server must have network access to the remote DNS server to copy the authoritative name server information about the zone.

You can use stub zones to:

Keep delegated zone information current. By updating a stub zone for one of its child zones regularly, the DNS server that hosts both the parent zone and the stub zone will maintain a current list of authoritative DNS servers for the child zone.
Improve name resolution. Stub zones enable a DNS server to perform recursion using the stub zone's list of name servers, without having to query the Internet or an internal root server for the DNS namespace.
Simplify DNS administration. By using stub zones throughout your DNS infrastructure, you can distribute a list of the authoritative DNS servers for a zone without using secondary zones. However, stub zones do not serve the same purpose as secondary zones, and they are not an alternative for enhancing redundancy and load sharing.

There are two lists of DNS servers involved in the loading and maintenance of a stub zone:

The list of master servers from which the DNS server loads and updates a stub zone. A master server may be a primary or secondary DNS server for the zone. In both cases, it will have a complete list of the DNS servers for the zone.
The list of the authoritative DNS servers for a zone. This list is contained in the stub zone using name server (NS) resource records.

When a DNS server loads a stub zone, such as abc.xyz.com, it queries the master servers, which can be in different locations, for the necessary resource records of the authoritative servers for the zone abc.xyz.com. The list of master servers may contain a single server or multiple servers, and it can be changed anytime.

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