Domain Master Browser Role

Because the browser service is bound by broadcast segments and each master browser maintains its own separate list, there must be a way to merge these lists into a single domain-wide list. This functionality is provided by the domain master browser that is the PDC for the domain. This functionality is not required for network protocols other than Transmission Control Protocol/Internet Protocol (TCP/IP).

The PDC has is also responsible for connecting to its primary Windows Internet Name Service (WINS) server every 12 minutes to obtain a list of all the DomainName type <1b> entries that are registered by the PDCs throughout the enterprise. This is done by issuing an MSRPC R_WinsGetBrowserNames request. These names, along with the workgroup announcement datagrams collected by the master browsers throughout the WAN, build the full list of domain and workgroup names. The names discovered by workgroup announcements take precedence over those obtained from WINS. These domain and workgroup names also contain the name of the server registering any given computer in the browse list. In the event that a WINS server is not available, or it is not registered, the client's browser requests the list of servers from the computer that registered the name. This operation is done on behalf of the client by its browser and is called a double-hop.

The PDC merges all the lists gathered by the master browsers on each segment across the WAN. Every 12 minutes, the master browser connects to the PDC to obtain the domain-wide list. The list is obtained by first issuing a NetServerEnum request with a flag of 0xFFFFFFFF. This request retrieves the complete list of servers within the domain. The master browser then issues the same request with a flag of 0x8000000, which requests all of the domain and workgroup names.

To signal the PDC to retrieve the list collected by this master browser, the master browser sends the PDC a directed master announcement frame over User Datagram Protocol (UDP) port 138. This signals the PDC to immediately connect to the master browser and retrieve its list. This communication is also performed with two NetServerEnum requests. First, a NetServerEnum request with flag 0x40000000 is issued to request the local list of servers collected by the master browser. Then, a NetServerEnum request with flag 0xC0000000 is sent to retrieve the local workgroup announcement frames sent by the master browser of other domains or workgroups on its segment. Each backup browser on the segment issues a NetServerEnum request with flags of 0xFFFFFFFF and x80000000 at 12-minute intervals to obtain the complete list of servers, domains, and workgroup names.