NetBIOS uses broadcast traffic for many of the functions that it performs. Remember, broadcast packets do not get forwarded across a router. LMHOSTS and WINS help NetBIOS overcome this short coming.
Basics
A server boots and as it initializes, the server service will be announced. It announces itself every minute for the first five minutes, then once every 12 minutes. These announcements are made over NetBIOS Datagram service port 138.
The master browser listens for computers as they announce themselves on the network. It adds any servers that announce themselves to a list of servers that it keeps.
The master browser now has a copy of all the servers on the network. So that clients know which computer is the master browser, it registers an additional NetBIOS name on the network called MBROWSE. The client only needs to request this list from this name. Once the client receives the list from the master browser, it doesn't need to talk to it again, it can receive the list from a backup browser. (NOTE:_ If the client restarts, or the backup browser is unavailable, it will need to contact the master browser.)
Now the client has a list of all the computers on the local subnet.
The backup browser retrieves a copy of the list from the master browser every 15 minutes. This is the default time interval and cannot be changed.
One short-coming of this process is the Master Browser keeps an entry in its list until that entry has missed three announcement periods. This could be up to 36 minutes (every 12 minutes) and because the Backup Browser retrieves a new copy of the list every 15 minutes only, the delay could be up to 36 even up to 51 minutes. Meaning, a crash system could remain on the list up to 51 minutes. (NOTE:_ if a system shuts down properly, it is removed from all lists (NetBIOS name release).)
Types of Browsers
Domain Master Browser- Master Browser responsible for an entire domain (PDC of a domain is the DMB) The Domain Master Browser makes a domain announcement every 15 minutes.
Master Browser- is the system that builds the browser list and distributes it to the Backup Browser when requested, in can be on any version of windows when no BDC is in domain. (announces every 12 minutes)
Backup Browser- nearly always the BDC in the domain model. It can be on any version of windows. (announces every 15 minutes)
Potential Browser- this can act as a browser, but is not currently a browser.
Non Browser- a system configured not to participate in browsing.
Configuring the Browser Type
In NT and 95 browser configuration takes place in the registry. (see page 305) There are two settings within the registry
ISDomainMaster is either true or false. The default is false.
MaintainServerList is a setting that tells your browser service whether to keep the browse list. There are three settings: Yes, No, Auto. The default is Auto. If you choose Yes, the system usually becomes a backup browser. If you set to No, this ensures your computer does not participate in browsing.
Browser Elections
The process of choosing the Master Browser is handled through elections (broadcast) by the computers on the subnet. Each computer votes for itself, and the first to "cast the ballot" wins the election and is the Master Browser. There are two main criteria determine how long a system waits before casting its ballot:
Current browser role: backup browser wins over a Potential browser.
Operating system and version NT4 wins over 3.51. And both win over 95. 95 wins over W4WG.
Methods for tie-breaking:
Time up- longest system online
Computer name alphabetic order if systems have been online the same amount of time.
Timing for an election are determined by three reasons. To call for an election the computer calling sends a broadcast to the network. All systems receive this broadcast and the system with the highest criteria is the first to broadcast a reply, hence becoming the master browser.
Master Browser shuts down the last thing it does is call for an election to determine its replacement.
Backup Browser fails to update its copy of the browser list it will call for an election.
A client fails to contact the Master Browser it will call for an election.
Browsing Workgroups and Domains
The browsers and clients that talk with each other all have to be in the same domain or the same workgroup. If there are more than one domain or more than one workgroup, each must have its own Master Browser and Backup Browsers.
There is always only one master browser. And in a domain model, the Backup Browsers are the BDCs. You can have up to three BDCs acting as backup browsers on each subnet, but not more than.
In the workgroup model, the election process is very important and very common. By default Microsoft designed the system to choose one Backup Browser for every 32 systems. Thus no one system is overloaded with requests.
The problem of browser service and the problem of domain activity can be resolved by either using a LMHOSTS file or a WINS (NetBIOS Name Server).
Using LMHOSTS
For the Domain controllers to synchronize with each other, they need to be added to an LMHOSTS file which is placed on each controller. The file should give the address of the controllers, and using the #DOM tag identify the system as a domain controller. Each workstation should also contain a copy of this file just in case the local domain controller should fail, they can still get authenticate and have browsing capability.
This doesn't quite resolve the problem. Even the LMHOSTS file doesn't cross a router. What happens is the PDC takes on the role of Domain Master Browser. Then another system will act as Master Browser in that particular subnet. Each subnet has a master browser, which receives it update from the Domain Master Browser. The Domain Master Browser creates a browser list for the entire domain and then passes this information on to the master browsers on each subnet, who in turn give the list to the backup browsers. This allows a host on any subnet to see all the resources of the network that is available.
Because the BDCs already know how to find the PDCs so it would make sense to make them the Master Browser per each subnet.
Using WINS to enable NBT
Wins is a NetBIOS Name Server (NBNS). This means it handles the resolution of a NetBIOS computer name over TCP/IP address. It also helps you get around the domain activity problem and enables you to facilitate the browsing services better.
When a WINS client initiates it registers its name with the WINS server's database. This database contains a special group called Domain Name Group. This group is used to register domain controllers. This group can have up to 25 IP addresses, including the PDC and 24 BDCs. This way, when a WINS client needs to be authenticated, the client asks the WINS server. The WINS server then provides the list of IP addresses available for logon. Hence, if you have more than 25 domain controllers, you will need more than one WINS server.
WINS server can also facilitate the browsing process because the 25 IP addresses, the PDC (Domain Master Browser) and the 24 BDCs (usually the Master Browsers) are aware of each other.