Chapter 1. Definition of NetBIOS Protocol and Name Resolution Modes
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## Chapter 1. Definition of NetBIOS Protocol and Name Resolution Modes

### Luke Leighton

12 June 1997

## NETBIOS

NetBIOS runs over the following transports: TCP/IP; NetBEUI and IPX/SPX. Samba only uses NetBIOS over TCP/IP. For details on the TCP/IP NetBIOS Session Service NetBIOS Datagram Service, and NetBIOS Names, see rfc1001.txt and rfc1002.txt.

NetBEUI is a raw NetBIOS frame protocol implementation that allows NetBIOS datagrams to be sent out over the 'wire' embedded within LLC frames. NetBEUI is not required when using NetBIOS over TCP/IP protocols and it is preferable NOT to install NetBEUI if it can be avoided.

IPX/SPX is also not required when using NetBIOS over TCP/IP, and it is preferable NOT to install the IPX/SPX transport unless you are using Novell servers. At the very least, it is recommended that you do not install 'NetBIOS over IPX/SPX'.

[When installing Windows 95, you will find that NetBEUI and IPX/SPX are installed as the default protocols. This is because they are the simplest to manage: no Windows 95 user-configuration is required].

NetBIOS applications (such as samba) offer their services (for example, SMB file and print sharing) on a NetBIOS name. They must claim this name on the network before doing so. The NetBIOS session service will then accept connections on the application's behalf (on the NetBIOS name claimed by the application). A NetBIOS session between the application and the client can then commence.

NetBIOS names consist of 15 characters plus a 'type' character. This is similar, in concept, to an IP address and a TCP port number, respectively. A NetBIOS-aware application on a host will offer different services under different NetBIOS name types, just as a host will offer different TCP/IP services on different port numbers.

NetBIOS names must be claimed on a network, and must be defended. The use of NetBIOS names is most suitable on a single subnet; a Local Area Network or a Wide Area Network.

NetBIOS names are either UNIQUE or GROUP. Only one application can claim a UNIQUE NetBIOS name on a network.

There are two kinds of NetBIOS Name resolution: Broadcast and Point-to-Point.

## BROADCAST NetBIOS

Clients can claim names, and therefore offer services on successfully claimed names, on their broadcast-isolated subnet. One way to get NetBIOS services (such as browsing: see ftp.microsoft.com/drg/developr/CIFS/browdiff.txt; and SMB file/print sharing: see cifs4.txt) working on a LAN or WAN is to make your routers forward all broadcast packets from TCP/IP ports 137, 138 and 139.

This, however, is not recommended. If you have a large LAN or WAN, you will find that some of your hosts spend 95 percent of their time dealing with broadcast traffic. [If you have IPX/SPX on your LAN or WAN, you will find that this is already happening: a packet analyzer will show, roughly every twelve minutes, great swathes of broadcast traffic!].

## NBNS NetBIOS

rfc1001.txt describes, amongst other things, the implementation and use of, a 'NetBIOS Name Service'. NT/AS offers 'Windows Internet Name Service' which is fully rfc1001/2 compliant, but has had to take specific action with certain NetBIOS names in order to make it useful. (for example, it deals with the registration of <1c> <1d> <1e> names all in different ways. I recommend the reading of the Microsoft WINS Server Help files for full details).

The use of a WINS server cuts down on broadcast network traffic for NetBIOS name resolution. It has the effect of pulling all the broadcast isolated subnets together into a single NetBIOS scope, across your LAN or WAN, while avoiding the use of TCP/IP broadcast packets.

When you have a WINS server on your LAN, WINS clients will be able to contact the WINS server to resolve NetBIOS names. Note that only those WINS clients that have registered with the same WINS server will be visible. The WINS server _can_ have static NetBIOS entries added to its database (usually for security reasons you might want to consider putting your domain controllers or other important servers as static entries, but you should not rely on this as your sole means of security), but for the most part, NetBIOS names are registered dynamically.

This provides some confusion for lots of people, and is worth mentioning here: a Browse Server is NOT a WINS Server, even if these services are implemented in the same application. A Browse Server _needs_ a WINS server because a Browse Server is a WINS client, which is _not_ the same thing].

Clients can claim names, and therefore offer services on successfully claimed names, on their broadcast-isolated subnet. One way to get NetBIOS services (such as browsing: see ftp.microsoft.com/drg/developr/CIFS/browdiff.txt; and SMB file/print sharing: see cifs6.txt) working on a LAN or WAN is to make your routers forward all broadcast packets from TCP/IP ports 137, 138 and 139. You will find, however, if you do this on a large LAN or a WAN, that your network is completely swamped by NetBIOS and browsing packets, which is why WINS was developed to minimise the necessity of broadcast traffic.

WINS Clients therefore claim names from the WINS server. If the WINS server allows them to register a name, the client's NetBIOS session service can then offer services on this name. Other WINS clients will then contact the WINS server to resolve a NetBIOS name.

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