First published on TechNet on Jan 23, 2012
[Today's post comes to us courtesy of John Bay from Commercial Technical Support]

The Small Business Server 2011 Essentials server is designed to seamlessly integrate into an existing network infrastructure. The network card on the server is configured to acquire an IP address from a DHCP server. Typically, the DHCP server will be the router. The server is an Active Directory domain controller and as such it functions as a DNS server. In order for the clients to properly locate the Active Directory, the network card on the client is typically configured to point to the IP address of the SBS 2011 Essentials server. Since the Essentials server is a DHCP client itself, the IP address of the server may change. In Essentials, we had to develop a method to ensure that the client is always pointing to the right IP address for DNS. When the client is joined to the Essentials domain using http://server/connect, a service named Windows Server LAN Configuration is installed on the client. This service is responsible for determining the IP address of the SBS Essentials server and properly configuring the DNS settings on the client machine.

The Windows Server LAN Configuration service will monitor the client IP address. If the client IP address has changed, the service will configure the primary DNS address to be automatic assigned from DHCP and it will send a Universal Plug and Play (UPnP) network call to look for the SBS Essentials server. If the SBS Essentials server is found, a quick DNS test is performed. If the server responds to the DNS request, the primary DNS server on the client will be set to static assigned with the IP address of the SBS 2011 Essentials server that was found on the network via the UPnP call. If the client is a laptop, and changes locations so that they receive a new IP from DHCP either via their home network/wi-fi hotspot or other, the service will send out the UPNP request and in this case no server will be found and the primary DNS address will be set to automatic (via DHCP).

The SBS 2011 Essentials server itself is using a dynamic IP, so the client must make sure that it is updated with the latest IP address for the server. The LAN Configuration Service on the client will contact the server every 30 seconds, and the server will respond with its IP address. If the IP address has changed the client will then update its primary DNS address accordingly. If the server does not respond, the client will switch the DNS address back to automatic assigned from DHCP. So if the SBS Essentials server is down for maintenance, the clients will automatically change the DNS address to DHCP assigned and they will still have Internet access.

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