CS 775

1. CS 775 Microsoft Windows Browser Service Presented by: Stan Bush (from material taken from MSCE study guides)

2. A computer must know what resources are available to it before it can access those resources. For Microsoft networks the service that collects, maintains and distributes that list of resources is called the ‘browser service’. This service should not be confused with an Internet browser. In MS Windows the network browser maintains a list of currently available resources on the network.

3. In a MS Windows environment you see available network resources via the ‘Network Neighborhood’ window as seen below:

4. Or, from the ‘Windows Explorer’ window. Again, do not confuse this function which is the follow on to the old Win 3.1 file manager, to MS Internet Explorer.

5. When a computer joins the network at boot time, it announces its presence and provides a list of resources that it will be making available to the network (this list can be modified at anytime by an authorized user). • The purpose of the browser service is to provide a centralized location and manager for listing network resources. This prevents each workstation and server from having to maintain this list for themselves and greatly decreases network message traffic. • Computers can perform the roles of a master browser, back-up browser, potential browser, or a non-browser.

6. Master Browser(by default is the Primary Domain Controller) • Maintains the master list of all available network resources which is called the ‘browse list’. • It listens for the announcements from computers and adds those computers and their list of available resources to the browse list. • It then distributes that list to the back-up browsers. • It also selects potential browsers for promotion to back-up browsers when necessary. • Only one master browser can exist in a workgroup or domain, except in the case of TCP/IP internetworks. This is because TCP/IP does not route broadcasts and therefore the browsing process will not reveal shared resources across routers (certain routers can be configured to pass-on network broadcasts but this is not the preferred method as this can dramatically increase traffic.

7. Back-up Browser • Receives the list of available resources from the master browser • Supplies the browse list to computers requesting it. • A back-up browser periodically announces itself to the master browser and requests an updated browse list. If it cannot find the master browser it forces an election to elect a new master browser. • One backup browser will be selected by the master browser for every 32 computers on the network up to a maximum of 3, providing of course that there are that many computers available that can assume the role of back-up browser.

8. It, like all computers on the network announces itself and supplies a list of resources it is making available to the network and can use any available resources from other computers if it has the proper permissions. Is simply a computer on the network that is able, if needed, to be promoted to back-up or master browser. Potential Browser

9. Non-Browser • It, like all computers on the network announces itself and supplies a list of resources it is making available to the network and can use any available resources from other computers if it has the proper permissions. • But, it is not eligible to be promoted to back-up or master browser.

10. Preferred Master Browser • A computer can be designated as a preferred master browser by an entry in the windows registry. • When this computer joins the network it announces itself as the master browser, and if one already exists, forces an election that re-evaluates the roles of computers as browsers in the network. • So, if you have a PDC that you do not want to assume the extra task of master browser. Set its registry to specify that it is to be a non-browser and designate another computer as the preferred master browser to off-load those duties. • If the PDC is set to a non-browser and the preferred master browser is not on the network than another qualified computer will be elected as the master.

11. How browsing works…... • A computer starts up and connects to the network, it announces itself to the master browser and if it has resources to share provides a list to the master browser. • When a computer attempts to locate network resources for the first time the computer contacts the master browser and receives a list of back-up browsers. • The computer then contacts one of the back-up browsers and requests the list of available resources. • The back-up browser responds with a list of available resources. • The requesting computer can then contact the owner of the resource that it wants to access and establish a session.

12. Browser elections…... • An election occurs when one of the following occurs: • When a computer on the network attempts to contact the master browser and it cannot be found. • When the current master browser forces an election because it is being shut down. • When more that one master browser exists (i.e. a preferred master came on-line).

13. A client computer, back-up browser, or a preferred master browser can cause an election to occur by broadcasting an ‘election packet’ over the network. All the browsers receive the packet and evaluate their precedence in relation to it. If a computer determines that it is of a higher precedence than the computer that transmitted the packet, it will broadcast one of its own, if it is of lower precedence it will do nothing. This continues until a computer is uncontested as the master browser. During this time the network is considered to be in a state of an ‘election in progress’. Browser elections, cont.

14. Browser elections, cont. • Elections are preformed to determine who gets to be the master browser. The factors that determine the outcome of an election, in order of precedence are: • the designated role of the computer in network browsing • the operating system of the computer • the versions of the operating system

15. Browser elections, cont. • The order of selection by operating system is: • Windows NT server (Primary Domain Controller) • Windows NT Server (Backup Domain Controller) • Windows NT Server • Windows NT Workstation • Windows 98/95 • Windows for Workgroups • The order of selection by version is: • 4.0 • 3.51 • 3.5 • 3.1

16. When a computer initially joins a network it announces itself and continues to announce its continued presence once a minute for the first several minutes and then the interval increases until it reaches 12 minutes. If the master browser does not receive an announcement from a computer for more than 3 periods it removes it and its’ resources from the browse list. This should only happen in the case of an abnormal shutdown or communication failure, as a normal shutdown is announced and the master browser immediately removes that computer from the list. So, as much as a 36-minute delay can occur from when a computer goes down before it is removed from the browse list. Browsing Fundamentals

17. Back-up browsers announce themselves on the network like all other computers. They also request an updated browse list from the master browser every 15 minutes. This coupled with the delay from when a computer is removed from the list can cause a total of 51 minutes to pass before a computer is removed from the browse list maintained at the backup browser, which is the list that is passed on to requesting computers. Browsing Fundamentals, cont.

18. To allow browsing over internetworks or within a WAN using TCP/IP to be successful you would need to implement one of the following solutions: Use the WINS service (Windows Internet Name Service) which dynamically maps NetBIOS names to IP addresses. (browsing is conducted using NetBIOS names which is a unique name assigned each network computer) Use LMHOSTS files – static mapping of NetBIOS names to IP addresses via text file lookup. Configure UDP port 137 to forward browser related broadcasts over network routers. Browsing Fundamentals, cont.