Server planning for DNS

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Server planning for DNS

When planning for your DNS servers, it is important to consider the following:

  • Perform capacity planning and review server hardware requirements.

  • Determine how many DNS servers you need and their role in your network.

    When deciding the number of DNS servers to use, decide which servers will host primary and secondary copies of zones. Also, if you are using Active Directory, determine whether the server computer will perform as a domain controller or a member server for the domain.

  • Decide where you are going to place DNS servers on your network for traffic loads, replication, and fault tolerance.

  • Decide whether to use only DNS servers running Windows Server 2003 for all your DNS servers or if you are operating a mixture of Windows and other DNS server implementations.

    For more information about migration and interoperability of the DNS Server service with other DNS server implementations, see Migrating servers or Interoperability issues.

Server capacity planning

Planning and deploying DNS servers on your network involves examining several aspects of your network and the capacity requirements for any DNS servers you intend to use in it. Some questions to consider when planning include the following:

  • How many zones is the DNS server expected to load and host?

  • For each zone the server is loading for service, how large is the zone (based on the size of the zone file or the number of resource records used in the zone)?

  • For a multihomed DNS server, how many interfaces are to be enabled for listening to and servicing DNS clients on each of the server's connected subnets?

  • How many total or overall DNS query requests from all of its clients is a DNS server expected to receive and service?

In many cases, adding more RAM to a DNS server can provide the most noticeable improvements in performance. This is because the DNS Server service fully loads all of its configured zones into memory at startup. If your server is operating and loading a large number of zones, and dynamic updates occur frequently for zone clients, additional memory can be helpful.

Keep in mind that for typical usage, the DNS server consumes system memory as follows:

  • Approximately 4 MB of RAM is used when the DNS server is started without any zones.

  • For each addition of zones or resource records to the server, the DNS server consumes additional server memory.

  • It is estimated that for the addition of every resource record to a server zone, an average of approximately 100 bytes of server memory is used.

    For example, if a zone containing 1000 resource records is added to a server, it would require approximately 100 KB (kilobytes) of server memory.

In determining your DNS server plans, you can start by reviewing sample DNS server performance test results collected by the DNS development and testing teams. In addition, you can use DNS server-related counters provided for use with monitoring tools to obtain your own performance measurements. For more information, see Monitoring DNS server performance.


  • The previous recommendations are not intended to indicate maximum performance or limitations for DNS servers.

  • These numbers are approximate and can be influenced by the type of resource records entered in zones, the number of resource records with the same owner name, and the number of zones in use at a specific DNS server.

Where to place DNS servers

In general, place your DNS servers at a location on your network that is centrally accessible to your clients. It is often most practical to use a DNS server on each subnet. There are several factors to consider when deciding where a DNS server is needed:

  • If you are deploying DNS to support Active Directory, is the DNS server computer also a domain controller or likely to be promoted to one in the future?

  • If the DNS server stops responding, are its local clients able to gain access to an alternate DNS server?

  • If the DNS server is located on a subnet that is remote to some of its clients, what other DNS servers or name resolution options are available if the routed connection stops responding?

For DNS server installations where the use of Active Directory is an issue, review special interoperability issues and installation details. For more information, see Interoperability issues.

For all DNS server installations including those in which the use of Active Directory is not an issue, the following server placement and planning guidelines can be usefully applied.

For example, if you have a routed local area network and high-speed links that are fairly reliable, you might be able to use one DNS server for a larger, multiple subnetted network area. If you have a high number of client nodes on a single subnet design, you might want to add more than one DNS server to the subnet to provide backup and failover if the preferred DNS server stops responding.

When determining the number of DNS servers you need to use, assess the effect of zone transfers and DNS query traffic on slower links in your network. Although DNS is designed to help reduce broadcast traffic between local subnets, it does create some traffic between servers and clients that should be reviewed, particularly when used in complexly routed LAN or WAN environments.

Consider the effects of zone transfer over slower speed links, like those typically used for a wide area network (WAN) connection. Although the DNS Server service supports incremental zone transfers and DNS clients and servers can cache recently used names, traffic considerations are sometimes still an issue, particularly when DHCP leases are shortened and, as a result, dynamic updates in DNS are performed more frequently. One option for dealing with remote locations on WAN links is to set up a DNS server at these locations to provide caching-only DNS service.

With most installations, you should have at least two server computers hosting each of your DNS zones for fault tolerance. DNS was designed to have two servers for each zone, one as a primary server and the other as a backup or secondary server. When making any final determinations about the number of servers to use, first assess the level of fault tolerance you need for your network.


  • When only a single DNS server is used on a small LAN in a single-subnet environment, you can configure the single server to simulate both the primary and secondary servers for a zone.

  • For best results and simplified DNS administration, consider using the Windows Server 2003 DNS Server service for all of your DNS servers.

  • When administering zone files created by the DNS Server service, it is recommended that you use the DNS console tool to make changes. As an alternate option, it is possible to administer all of your DNS zone files manually using any application that supports file saving in text mode. Whether you decide to edit zone files manually in a text editor or use the DNS console to modify them, select one method for updating zones and use it consistently. This helps prevent having zone edits overwritten or rejected, which might happen if you change from one method to another.