3.22 Changing the Preference for a Domain Controller

3.22 Changing the Preference for a Domain Controller

3.22.1 Problem

You want a particular domain controller to be used less frequently for client requests or not at all. This may be necessary if a particular domain controller is overloaded, perhaps due to application requests.

3.22.2 Solution

You can modify the Priority or Weight fields in SRV resource records by modifying the registry on the domain controller. Open regedit or regedt32 on the domain controller and browse to the following key: HKLM\SYSTEM\CurrentControlSet\Services\Netlogon\Parameters. To configure the Priority, add a REG_DWORD with the name LdapSrvPriority. To configure the weight, add a REG_DWORD with the name LdapSrvWeight.

After you make the change, the %SystemRoot%\System32\Config\netlogon.dns file should be updated and the DDNS updates sent to the DNS server within an hour. You can also restart the NetLogon service to expedite the process.

3.22.3 Discussion

Each domain controller registers several SRV records that clients use as part of the DC locator process to find the closest domain controller. Two fields of the SRV record let clients determine which server to use when multiple possibilities are returned. The Priority field is used to dictate if a specific server or set of servers should always be contacted over others unless otherwise unavailable. A server with a higher priority (i.e., lower priority field value) will always be contacted before a server with a lower priority. For example, if DC1 has a SRV priority of 5 and DC2 has a SRV priority of 10, DC1 will always be used unless it is unavailable.

The Weight field, on the other hand, determines the percentage of time clients should use a particular server. You can easily calculate the percentage by dividing the weight by the sum of all Weights for servers with the same Priority. If server's DC1, DC2, and DC3 have Weights of 1, 2, and 3, respectively, then DC1 will be contacted one out of six times (1 / (3 + 2 + 1) ), DC2 will be contacted two out of every six times or 1/3 (2 / (3 + 2 + 1) ), and DC3 will be contacted three out of every six times or 1/2( 3 / (3 + 2 + 1 ) ). Here is an example of how the SRV records look with these weights:

C:\>nslookup -type=SRV _ldap._tcp.dc._msdcs.rallencorp.com Server: dns01.rallencorp.com Address: _ldap._tcp.dc._msdcs.rallencorp.com SRV service location: priority = 0 weight = 1 port = 389 svr hostname = dc1.rallencorp.com _ldap._tcp.dc._msdcs.rallencorp.com SRV service location: priority = 0 weight = 2 port = 389 svr hostname = dc2.rallencorp.com _ldap._tcp.dc._msdcs.rallencorp.com SRV service location: priority = 0 weight = 3 port = 389 svr hostname = dc3.rallencorp.com

In certain situations, having this capability can come in handy. For example, the server acting as the PDC FSMO role owner typically receives more traffic from clients simply because of the nature of tasks that the PDC FSMO has to handle. If you find a certain server like the PDC FSMO has considerably higher load than the rest of the servers, you could change the priority or weight of the SRV records so that it is used less often during the DC locator process. You can increase the Priority to eliminate its use unless all other domain controllers fail. Modify the Weight to reduce how often it will be used.