Advanced Linux Networking

NetBEUI is similar in many ways to AppleTalk and IPX, but it has historically been used primarily by IBM and Microsoft as the basis for networking in DOS, Windows, and OS/2. The Linux kernel does not, as of the 2.4. x series, include a standard NetBEUI stack. This omission is offset by two facts, though. First, the common uses of NetBEUI can be served by NetBIOS over TCP/IP (sometimes called NBT ), which Linux does support. Second, a third-party NetBEUI stack is available, although it's of limited utility.

NetBEUI Features and Capabilities

Like AppleTalk and IPX, NetBEUI was designed with small networks in mind. In fact, NetBEUI is even more limited than its competing small-network protocols, because it's restricted to networks of just 256 computers. NetBEUI uses computer names similar to TCP/IP hostnames, but there is no underlying numeric addressing system, as is true of TCP/IP, AppleTalk, and IPX; NetBEUI uses the computer's name directly. These names are two-tiered in nature, much like AppleTalk's names (which include a computer name and a network zone name). NetBEUI calls its higher-order groupings workgroups or domains, depending upon whether a centralized computer exists to control logins (domains support this feature, but workgroups leave authentication to individual servers). When it starts up and periodically thereafter, a computer configured to use NetBEUI makes a broadcast to announce its presence.

NetBEUI can theoretically be used over just about any network medium, but it's most commonly used over Ethernet. Like AppleTalk and IPX, NetBEUI can coexist on Ethernet with TCP/IP or other network stacks.

NetBEUI is most frequently used in conjunction with the SMB/CIFS file- and printer-sharing protocols. These are comparable in scope to NFS/ lpd for Unix and Linux, AppleTalk's equivalent protocols, or NCP. They can also be used over TCP/IP, and in fact this configuration is very common, even on networks that contain Windows computers exclusively. Because it's not easily routed, however, NetBEUI offers some security benefits ”a distant attacker is unlikely to be able to launch a successful attack against a NetBEUI server.

Obtaining a NetBEUI Stack for Linux

Few Linux computers participate in NetBEUI communications because the standard kernel lacks a NetBEUI stack. In 2000, Procom Technologies (http://www.procom.com) released an open source NetBEUI stack for Linux, as well as patches to Samba (described in Chapter 7, File and Printer Sharing via Samba), to allow Samba to operate over NetBEUI rather than TCP/IP. These patches have not become commonplace, and in fact they aren't posted directly on Procom's Web site, although you can request the stack from their technical support department. The NetBEUI stack may not work properly with kernels beyond the 2.0. x series (I was unable to get the patches to compile with a 2.2.18 kernel, for instance). The NetBEUI stack was also designed for versions of Samba before 2.0.7, although there's been some talk of adding the NetBEUI support to Samba sometime before Samba 3.0 is released. The support also requires recompiling both your kernel and Samba. For these reasons, you're probably better off foregoing the use of NetBEUI unless you have a very compelling reason to use it, such as a network on which TCP/IP is forbidden. If you really must use NetBEUI, you may need to use it with an older 2.0. x kernel and Samba 2.0.6 or earlier.

In addition to patching the Linux kernel and Samba, the NetBEUI stack comes with a number of tools that let you configure and manipulate it. This configuration tends to be fairly simple, and in most cases you'll use new Samba options to control the computer's NetBEUI behavior, but the separate utilities can be useful for troubleshooting and for learning more about NetBEUI. One utility in particular, netb , is also required to start up the NetBIOS stack, as described shortly.

Using Linux NetBEUI Software

The NetBEUI stack includes a README file with complete installation and use instructions. This file outlines two methods of installation. One requires you to edit the Makefile to point to your Linux kernel and Samba source code trees and set a few other system-specific options. You can then recompile both Linux and Samba with a single command, install your new kernel, and reboot the system. The second procedure also requires you to edit the Makefile , but proceeds to give instructions on performing individual steps in a piecemeal fashion. This second approach is likely to be superior if you run into any difficulties, because you'll be better able to isolate and correct the problem.

Whichever way you do it, you'll need the source code to both the Linux kernel and Samba. You can obtain these from http://www.kernel.org and http://www.samba.org, respectively, or from many common Linux download sites, like ftp://sunsite.unc.edu. Both packages can take several minutes to compile and install, so even if you don't run into problems, installing NetBEUI support is likely to take several minutes.

Once you've installed NetBEUI support, you can use several commands to enable or manipulate this support. These commands, included with the NetBEUI stack, are as follows :

• netb ” Type this command followed by start to start NetBEUI on Linux. To stop NetBEUI, type netb stop . You must use this utility before you can use NetBEUI on Linux.

• nbview ” Use this command if you want to check on the status of the local NetBEUI stack. This command reads the /proc/sys/netbeui file, which contains this information, and formats and parses the information for easier human consumption.

• nbstatus ” This command displays information on the specified machine or workgroup; for instance, nbstatus SERVER displays information on the computer called SERVER .

• nbadmin ” This command allows you to bind NetBEUI to a specific network interface, unbind NetBEUI from a specific interface, or drop a specific NetBEUI session. It does this using the bind , unbind , and drop commands, respectively, as in nbadmin bind eth0 or nbadmin drop 102 . (You can obtain NetBEUI session numbers from nbview .)

For the most part, you'll only need to issue a netb start command, then start Samba. The NetBEUI stack adds a parameter to nmbd (the NetBIOS name daemon), smbd (the SMB daemon), and smbclient (the text-mode Samba client) to specify whether to use TCP/IP or NetBEUI. This parameter is -Z <NETBEUI TCPIP> . For instance, to launch smbd to use NetBEUI, you'd type smbd -Z NETBEUI . You can also use the new -S NAME parameter to smbd to set the system's NetBEUI name to NAME .

To sum up, you can turn Linux into a NetBEUI SMB/CIFS server called NAME by recompiling the kernel and Samba with the Procom NetBEUI stack, rebooting, shutting down Samba (if necessary), and typing the following commands:

# netb start # nmbd -Z NETBEUI # smbd -Z NETBEUI -S NAME

You can place these commands in a startup script, or modify your regular Samba startup script to incorporate these changes. Other Samba features, such as the definitions of shares, all function as described in Chapter 7. The advantage of this procedure boils down to two factors. First, it can be used with some older clients that support NetBEUI but not TCP/IP, and it reduces the chance of malicious outside access to the computer via Samba, because NetBEUI isn't normally routed over the Internet. On the downside, NetBEUI support is duplicated in NetBIOS over TCP/IP, which works with all recent Linux kernels and versions of Samba, and doesn't require patching or recompiling the kernel or Samba.