Determining the Number of Required DNS Servers

  • Article

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

To reduce administrative overhead, use the minimum number of DNS servers required. Be sure to make at least two DNS servers authoritative for each zone to enable fault tolerance and load sharing.

Add additional DNS servers in order to:

  • Provide redundancy when your namespace design requires greater DNS availability.

  • Improve query response time when better DNS performance is required.

  • Reduce WAN traffic for remote locations.

Use the following guidelines to determine the number of DNS servers that you need to deploy:

  • If the ratio of DNS servers to clients is very low and you are experiencing significant name resolution delays, add additional DNS servers to host secondary or Active Directory–integrated zones. Use your anticipated number of queries and dynamic updates per second to determine the number of DNS servers that you need. The Windows Server 2003 DNS Server service is capable of responding to more than 10,000 queries per second on a Pentium III microprocessor running at 700 MHz.

    For information about capacity planning, see "Allocating Hardware Resources" earlier in this chapter.

  • If you delegate zones, add additional DNS servers to handle the delegated zones. Note that you do not need to delegate zones when you have multiple zones. You can host all zones on the same server or servers. One DNS server running Windows Server 2003 can host 20,000 small zones.

  • If you plan to host Active Directory–integrated zones, you must place these zones on Windows 2000–based or Windows Server 2003–based domain controller.

  • If high-volume traffic is a consideration in your environment, add additional DNS servers to balance the workload. Although DNS helps reduce broadcast traffic between local subnets, it does create some traffic between servers and clients, particularly in complex routed environments. In addition, although the DNS service supports incremental zone transfers (IXFRs) and clients and servers can cache recently used names, traffic considerations can still remain an issue, depending on available bandwidth. This is especially true when using short Dynamic Host Configuration Protocol (DHCP) leases, which require more frequent dynamic updates.

  • If you have a high number of client nodes on a single subnet, placing more than one DNS server on the subnet allows for backup and failover in the event that the primary DNS server stops responding.

If your DNS design includes primary and secondary zones and you run a large number of secondary servers for a zone, the primary DNS server can become overloaded when the secondary servers poll to ensure that their zone data is current. You can solve this problem in one of three ways:

  • Use some of the secondary DNS servers as primary servers for the zone. Other secondary servers can poll and request zone updates from these primary servers.

  • Increase the refresh interval so that the secondary servers poll less frequently. Note, however, that a longer refresh interval might cause your secondary zones to be outdated more often.