-> Understanding Server Types

The DNS server type refers to the type of zone the server is hosting—or, in the case of caching-only servers, whether it is hosting a zone at all. The following sections describe some of the essential features of the various server types.
(i)Primary DNS Servers
A standard primary DNS server is a name server that obtains zone data from the local DNS database. This makes the primary DNS server authoritative for the zone data that it contains. When a change needs to be made to the resource records of the zone, it has to be done on the primary DNS server so that is can be included in the local zone database.

A DNS primary server is created when a new primary zone is added. The primary server that is created becomes the mechanism for updating the specific primary zone.

When a query is sent to the standard primary DNS server for name resolution, the following events take place:
i. The request for name resolution is sent to the primary DNS server.
ii. The primary DNS server compares the requested name to the information it contains in its local zone database.
iii. If the primary DNS server locates a match for the queried name, the requested information is returned to the client.
iv. If the DNS server cannot find a matching record in its local zone database file, the DNS server then attempts a number of name resolution methods to resolve the request on behalf of the client.
v. If all attempts for name resolution in unsuccessful, the DNS server returns an error message to the client.

(ii)Secondary Servers
This DNS server type obtains a read-only copy of zone information through DNS zone transfers. A secondary DNS server cannot make any changes to the information contained in its read-only zone copy. A secondary DNS server can however resolve queries for name resolution.

Secondary DNS servers are usually implemented to provide fault tolerance, provide fast access for clients in remote locations, and to distribute the DNS server processing load evenly. If a secondary DNS server is implemented, that DNS server can continue to handle queries when the primary DNS becomes unavailable. Secondary DNS servers also assist in reducing the processing load of the primary DNS server. It is recommended to install at least one primary DNS server, and one secondary DNS server for each DNS zone.

A secondary DNS server obtains its data from the primary DNS server's zone database, as a copy of that database. During zone transfer, the primary DNS server's zone database is replicated to the secondary DNS server. A secondary DNS server cannot make changes to its zone information. All changes have to be made on the primary zone, and then have to be replicated to the secondary DNS server through DNS zone transfer.

DNS Notify is a mechanism that enables a primary DNS server to inform secondary DNS servers when its database has been updated. The mechanism informs the secondary DNS servers when they need to initiate a zone transfer so that the updates of the primary DNS server can be replicated to them. When a secondary DNS server receives the notification from the primary DNS server, it can start an incremental zone transfer or a full zone transfer to pull zone changes from the primary DNS server.

(iii)Caching-Only Servers
The main characteristics of caching-only DNS servers are:
* Caching-only DNS servers do not host zones.
*They are not authoritative for any DNS domain.
*The information stored by caching-only DNS servers is the name resolution data that it has collected through name resolution queries.

A caching-only DNS server just performs queries and then stores the results of these queries. All information stored on the caching-only DNS server is therefore only that data which has been cached while the server performed queries. Caching-only DNS servers only cache information when the queries have been resolved.

When a caching-only DNS servers starts for the first time, it has no cached information. The caching-only DNS server collects information as it sends and resolves queries. One of the main advantages of implementing caching-only DNS servers is that they are excluded from the zone transfer process, and therefore do not generate network traffic from zone transfers.
To install a caching-only DNS server, complete the following steps:
1. Install the DNS server role on the server computer.
2. Do not configure the DNS server (as you might normally) to load any zones.
3. Verify that server root hints are configured or updated correctly.

(iv)Master DNS Servers
The servers from which secondary DNS servers obtain zone information in the DNS hierarchy are called master servers. When a secondary DNS server is configured, you have to specify the master server from whom it will obtain zone information. Zone transfer enables a secondary DNS server to obtain zone information from its configured primary DNS server, and enables these servers to continue handling queries if the primary DNS server fails. In this case, the primary DNS server is the master server of the secondary DNS server. A secondary DNS server can also transfer its zone data to other secondary DNS servers, who are beneath it in the DNS hierarchy. In this case, the secondary DNS server is regarded as the master server to the other subordinate secondary DNS servers. A secondary DNS server initiates the zone transfer process from its particular master server when it is brought online.

(v) Stub Servers
Stub DNS servers host stub zones: abbreviated copies of a zone that contain only a list of the authoritative name servers for its master zone. A DNS server hosting a stub zone attempts to resolve queries for computer names in the master zone by querying these name servers listed. Stub zones are most frequently used to enable a parent zone to keep an updated list of the name servers available in a child zone.

(vi)Dynamic DNS Servers
Windows 2000, Windows XP and Windows Server 2003 computers can dynamically update the resource records of a DNS server when a client's IP addressing information is added, or renewed via Dynamic Host Configuration Protocol (DHCP). Both DHCP and Dynamic DNS (DDNS) updates make this possible. When dynamic DNS updates are enabled, a client sends a message to the DNS server when changes are made to its IP addressing data. This indicates to the DNS server that the A type resource record of the client needs to be updated.