Windows Vista and Windows Server 2008 DNS clients do not honor DNS round robin by default

Source: Microsoft Support
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By default, Windows Vista and Windows Server 2008 follow RFC 3484 for destination IP address selection, which may not honor DNS round robin. 
Cause
Windows Vista and Windows Server 2008 have a new TCPIP stack that supports IPV4 and IPv6 natively.  This new stack follows RFC 3484 “Default Address Selection for Internet Protocol version 6” for IPV6 and for IPV4 when possible.  This RFC attempts to select the closest IP address rather than using DNS round robin’s random selection. 
Resolution


To resolve this issue, add a registry key that disables subnet prioritization. Add a new registry key with the following settings:

 

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

DWORD = OverrideDefaultAddressSelection
Value data: = 1

 

Note: A value of 1 disables subnet prioritization. This is by design for IPv4 and IPv6 in order to comply with RFC 3484
More Information


RFC 3484 works well for IPV6 but has the following problem for IPV4 where NATS are popular.

For example, when clients behind NATs use 192.168.X.Y source addresses and follow RFC 3484, servers with the best IP match to this public IP range will get used more than other servers.







For more information on RFC 3484 visit the following link:

http://www.ietf.org/rfc/rfc3484.txt





Note: This registry option does not exist on Windows Vista pre-SP1 systems.

For Windows Vista and Windows Server 2008, the default value for this registry key is 0.

For Windows 2008 R2 and Windows 7, the default value for this registry key is 1.
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Article ID: 968920 - Last Review: 03/31/2009 21:42:01 - Revision: 2.0

Windows Vista Business, Windows Vista Enterprise, Windows Vista Home Premium, Windows Vista Home Basic, Windows Vista Ultimate, Windows Server 2008 Standard, Windows Server 2008 Enterprise, Windows Server 2008 Datacenter

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