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Configuring round robin

Updated: January 21, 2005

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

Configuring round robin

Round robin is a local balancing mechanism used by DNS servers to share and distribute network resource loads. You can use it to rotate all resource record (RR) types contained in a query answer if multiple RRs are found.

By default, DNS uses round robin to rotate the order of RR data returned in query answers where multiple RRs of the same type exist for a queried DNS domain name. This feature provides a simple method for load balancing client use of Web servers and other frequently queried multihomed computers.

If round robin is disabled for a DNS server, the order of the response for these queries is based on a static ordering of RRs in the answer list as they are stored in the zone (either its zone file or Active Directory).

Example: Round-robin rotation

A forward lookup-type query (for all A RRs that match a DNS domain name) is made for a multihomed computer (multihomed.example.microsoft.com) that has three IP addresses. Separate A RRs are used to map the host's name to each of these IP addresses in the zone. In the stored example.microsoft.com zone, the RRs appear in this fixed order:

multihomed   IN  A  10.0.0.1

multihomed   IN  A  10.0.0.2

multihomed   IN  A  10.0.0.3

The first DNS client that queries the server to resolve this host's name receives the list in default order. When a second client sends a subsequent query to resolve this name, the list is rotated as follows:

multihomed   IN  A  10.0.0.2

multihomed   IN  A  10.0.0.3

multihomed   IN  A  10.0.0.1

Restricting round-robin rotation for selected RR types

By default, DNS will perform round-robin rotation for all RR types. You can specify that certain RR types are not to be round-robin rotated in the registry. There is a registry entry called DoNotRoundRobinTypes (REG_SZ) with a string value containing a list of RR types. By modifying this entry, you turn off round-robin rotation for specific RR types. For example, to prevent round-robin rotation for A, PTR, SRV, and NS record types, you would enter the following value for the registry entry:

a ptr srv ns

Restricting round-robin rotation for all RR types

The default setting for round-robin rotation is contained in the registry entry RoundRobin (REG_DWORD). By default, this entry's value is 1, rotating all RR types except those listed in the DoNotRoundRobinTypes registry entry. If the value of RoundRobin is set to 0, then no RR types will be round-robin rotated.

Caution

  • Incorrectly editing the registry may severely damage your system. Before making changes to the registry, you should back up any valued data on the computer. You can also use the Last Known Good Configuration startup option if you encounter problems after manual changes have been applied.

Notes

  • The string value for the DoNotRoundRobinTypes registry entry may contain types in numeric (as shown above) of mnemonic formats.

  • Both of the round robin registry entries must be created and stored in the following registry subkey:

    HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\DNS\Parameters

  • Local subnet priority supersedes the use of round-robin rotation for multihomed names. When enabled, round-robin continues to be a secondary method used to sort multiple RRs returned in a listed answer. For more information on local subnet priority, see Prioritizing local subnets.

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