Configuring a Router to Forward NetBIOS Name Server Broadcasts in Software

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Configuring a Router to Forward NetBIOS Name Server Broadcasts
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If, for some reason, you don't want to use WINS to enable browsing and name services, or if you're not fully converted to a Windows 2000 environment, you can allow the router to forward the broadcasts However, you'll want to carefully evaluate the impact (overhead, administration, etc) of doing so NBT accomplishes broadcast name resolution using UDP port 137, and Server Message Block (SMB) datagram service on UDP port 138 Allowing these two ports to be open through the router will enable one subnet to browse another and resolve addresses However, UDP broadcasts are not forwarded by the routers, thereby limiting you to the local subnet To allow one subnet to broadcast to another, you'll need to take the following steps First, at the router CLI, enter these commands: Router(config)#ip forward-protocol udp 137 Router(config)#ip forward-protocol udp 138 Then enter the configuration mode of the interface from which you want the router to forward broadcasts (in this example, it is ethernet 0): Router(config)#interface ethernet0 Then enter the ip helper statement along with the subnet to which you want the broadcasts forwarded: Router(config-if)#ip helper 10110 If you enter ip forward-protocol without specifying UDP and the port number, the forwarding of the following additional ports will be activated by default: Time Service port 37 TACACS port 49 Domain Name System port 53 BootP port 67 Trivial File Transfer port 69 NoteThis configuration is uni-directional If you wish both subnetworks to be able to browse each other, you'll need to configure both the local and remote routers MULTIPLE SUBNETS If you want to give several subnets the capability to send and receive broadcasts, the ip forwardprotocol should be implemented along with spanning tree on all routers with subnets requiring broadcasts, as shown here: Router(config)#ip forward-protocol spanning tree WarningThe decision to implement spanning tree (or allowing broadcasts in the first place, for that matter) should not be a casual one Before actually using spanning tree, be sure you analyze the effect it will have on the routers, network overhead, general network traffic patterns, and network administration Using spanning tree as a quick fix or as a replacement for WINS or DNS is strongly discouraged If you decide against allowing broadcasts through your routers and don't plan on using the LMHOSTS file, you have other practical solutions: WINS, DNS, or Dynamic DNS (DDNS)
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Resolving NetBIOS Names with DNS
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When using Win 9x or NT, an alternative to using broadcasts or the LMHOSTS file is to use the option to Enable DNS for Windows Resolution available in recent implementations of Microsoft's
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TCP/IP client In a Windows 2000 only environment, WINS is not necessary In mixed Win NT/9x, Windows 2000 environments, however, WINS will be required When Windows 2000 is operating in this environment, it is said to be operating in mixed mode If all systems are Windows 2000, the network is in native mode The most common situation requiring WINS in a Windows 2000 environment will be supporting WINS name resolution for Windows 2000 clients accessing Windows NT 40 workstations and servers The recommended arrangement for this situation is to implement Windows 2000 DNS, create a WINS referral zone, and configure the DNS server to refer to the WINS database Windows 2000 DNS server can then handle name requests from Windows 2000 clients as standard DNS requests In Windows 2000, to reach the WINS configuration dialog box, go to Start | Settings | Network and Dial-up Connections | Local Area Connection | Internet Protocol(TCP/IP) | Properties | Advanced | DNS Figure 11-2 illustrates the Windows 2000 client configuration when the WINS referral zone is named nt4domain
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Figure 11-2: Enabling WINS name resolution in Windows 2000 Since clients configured to use the WINS referral zone feature don't automatically perform dynamic registration of their NetBIOS name when they come up on the network, the naming convention and process of assigning names to the nodes must be managed manually To implement this, the NetBIOS name must match the name entered into DNS as the host name To do so, assign participating nodes a unique machine name, and then enter a DNS host record using the machine name exactly as entered into the TCP/IP configuration The NetBIOS name space is flat, so "matching" a WINS referral zone is limited to a single DNS domain This is because name resolution requests are sent to the DNS under the assumption that the request is for the DNS domain "matched" with the WINS referral zone The reason for limitations has to do with the way NetBIOS name resolution works Let's look at what happens when an application using NetBIOS attempts to locate a NetBIOS-based server In this example, the application is looking for the server named database1 Since the TCP/IP client is configured to use DNS for Windows Resolution, the client constructs a DNS query using the NetBIOS name database1, and then adds the TCP/IP domain name before sending it to the DNS server The query sent to the DNS server becomes a standard query, to which the DNS server can reply without knowing that the name originated from a NetBIOS-based request The request would include the NetBIOS name database1,along with the DNS domain name domaincom as it was earlier entered into the TCP/IP client configuration The standard DNS query sent to the DNS server would request the IP address of database1domaincom The DNS server would reply with an IP address, as it would for any other valid query When the client receives the response from the DNS server, it can pass the IP address to the requesting NetBIOS-based application The application then uses the address as though its request were processed by an LMHOSTS file, a broadcast, or a WINS server
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NoteBe aware that IP address assignments will have to be statically defined in DNS, and using DHCP will allow client addresses to change WINS is not without its limitations; but when properly implemented, it can help ease the administrative workload associated with NetBIOS name services In the next section, we'll take a closer look at WINS
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