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.
Primary Servers
A primary server is
created when a primary zone is added, either through the New Zone
Wizard, the Configure A DNS Server Wizard, or command-line tools.
The primary server for a
zone acts as the zone’s central point of update. Newly created zones are
always this type. With Windows Server 2003, you can deploy primary
zones in one of two ways: as standard primary zones or primary zones
integrated with Active Directory.
Standard Primary Zones
For
standard primary zones, only a single server can host and load the
master copy of the zone. If you create a zone and keep it as a standard
primary zone, no additional primary servers for the zone are permitted.
The standard primary
model implies a single point of failure. For example, if the primary
server for a zone is unavailable to the network, no changes to the zone
can be made. Note that queries for names in the zone are not affected
and can continue uninterrupted, as long as secondary servers for the
zone are available to answer them.
Active Directory-Integrated Zones
When you deploy
an Active Directory–integrated zone, zone data is stored and replicated
in Active Directory. Using an Active Directory–integrated zone
increases fault tolerance and (by default) turns every domain controller
in the domain running a DNS server into a primary server. To configure a
primary zone as an Active Directory–integrated zone, the original DNS
server on which the zone is created must be an Active Directory domain
controller.
Secondary Servers
DNS design
specifications recommend that at least two DNS servers be used to host
each zone. For standard primary zones, a secondary server is required to
allow the zone to appear to other DNS servers in the network.
Secondary servers
provide a means to offload DNS query traffic in areas of the network
where a zone is heavily queried and used. Additionally, if a primary
server is down, a secondary server provides name resolution in the zone
until the primary server is available.
The servers from which secondary servers acquire zone information are called masters.
A master can be the primary server or another secondary server. You
specify the secondary server’s master servers when the server’s
secondary zone is created, through either the New Zone Wizard, the
Configure A DNS Server Wizard, or command-line tools.
Tip
Secondary
servers are best placed as close as possible to clients that have a
high demand for names used in the zone. Also, you should consider
placing secondary servers across a router, either on other subnets or
across wide area network (WAN) links. This setup provides efficient use
of a secondary server as a backup in cases where an intermediate network
link becomes the point of failure between DNS servers and clients that
use the zone. |
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.
Caching-Only Servers
Caching-only servers
do not host any zones and are not authoritative for any particular
domain. The information they contain is limited to what has been cached
while resolving queries.
In determining when to
use this kind of server, note that when it is initially started, it has
no cached information. The information is obtained over time as client
requests are serviced. However, if you are dealing with a slow WAN link
between sites, this option might be ideal because once the cache is
built, traffic across the WAN link decreases. DNS queries are also
resolved faster, improving the performance of network applications. In
addition, the caching-only server does not perform zone transfers, which
can also be network-intensive in WAN environments. Finally, a
caching-only DNS server can be valuable at a site where DNS
functionality is needed locally, but administering domains or zones is
not desirable for that location.
Tip
When
you need to minimize name resolution traffic across WAN links without
increasing zone transfer traffic, install a caching-only server. |
By default, the DNS Server service acts as a caching-only server. Caching-only servers thus require little or no configuration.
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. |
