Chapter 7. Name Resolution and Browsing

WINS Clients and Server Interaction

There are two types of interaction between a WINS client and a server: the client keeps its own NetBIOS name[1] registered with the server and queries the server to get the IP address corresponding to the NetBIOS name of another system.

When a WINS client joins the network, it registers its NetBIOS name with the WINS server, which stores it along with the client's IP address in the WINS database. This entry is marked active. The client is then expected to renew the registration of its name periodically (typically, every four days) to inform the server that it is still using the name. This period is called the time to live, or TTL. When the client leaves the network by being shut down gracefully, it informs the server, and the server marks the client's entry in its database as released.

When a client leaves the network without telling the WINS server to release its name, the server waits until after it fails to receive the expected registration renewal from the client and then marks the entry as released.

In either case, the released name is available for use by other clients joining the network. It might persist in the released state in the WINS database, and if it is not reregistered, the entry will eventually be deleted.

More information on WINS can be found in the Microsoft white paper Windows Internet Naming Service (WINS) Architecture and Capacity Planning. It can be downloaded from the Microsoft web site at http://www.microsoft.com.

The lmhosts File

In Chapter 3 we showed you how to configure Windows systems to use the LMHOSTS file as an alternative to the WINS server for name resolution. Samba also can use an LMHOSTS file, which by default is /usr/local/samba/lib/lmhosts. Samba's lmhosts is the same format as the Windows version. A simple lmhosts file might look like this:

172.16.1.1    toltec
172.16.1.6    maya

The names on the right side of the entries are NetBIOS names, so you can assign resource types to them and add additional entries for computers:

172.16.1.1    toltec#20
172.16.1.1    metran#1b
172.16.1.6    maya#20

Here, we've made toltec the primary domain controller of the METRAN domain on the second line. This line starts with toltec's IP address, followed by the name metran and the resource type <1B>. The other lines are entries for toltec and maya as standard workstations.

If you wish to place an lmhosts file somewhere other than the default location, you will need to notify the nmbd process upon startup using the -H option, followed by the name of your lmhosts file, as follows:

# nmbd -H /etc/samba/lmhosts -D

Configuring Name Resolution for the Samba Suite

Various daemons and tools in the Samba suite need to perform name resolution. You can define the order in which the programs try each name-resolution method through the name resolve order parameter, like this:

[global]
    name resolve order = wins lmhosts hosts bcast

The string used to define the parameter can take up to four values:

lmhosts

Uses the Samba server's local lmhosts file

hosts

Uses the standard Unix name-resolution methods, which can be /etc/hosts, DNS, NIS, or a combination, depending on how the local system is configured

wins

Uses the WINS server

bcast

Uses the broadcast method

The order in which they are specified is the order in which name resolution will be attempted. In our example, Samba will attempt to use its WINS server first for name resolution, followed by the lmhosts file on the local system. Next, the hosts value tells it to use Unix name-resolution methods. The word hosts can be misleading; it covers not only the /etc/hosts file, but also the use of DNS or NIS (as configured on the Unix host). Finally, if those three do not work, it will perform a broadcast name resolution.

Setting Up Samba as a WINS Server

You can set up Samba as a WINS server by setting the wins support parameter in the configuration file, like this:

[global]
    wins support = yes

Believe it or not, that's all you need to do! The wins support option turns Samba into a WINS server. For most installations, Samba's default configuration is sufficient.

WARNING

Remember, Samba cannot communicate with Windows WINS servers. If you are using Samba as your WINS server, you must make sure not to allow any Windows systems or other Samba servers on your network to be configured as WINS servers. If you do, their WINS databases will not synchronize, resulting in inconsistent name resolution.

Setting Up Samba to Use Another WINS Server

You can configure Samba to use a WINS server somewhere else on the network by simply providing it with the IP address of the WINS server. This is done with the global wins server configuration option, as shown here:

[global]
    wins server = 172.16.1.1

With this option enabled, Samba will direct all WINS requests to the server located at 172.16.1.1. Note that because the request is directed at a single machine, we don't have to worry about any of the problems inherent in broadcasting. However, Samba will not necessarily use the WINS server before other forms of name resolution. The order in which Samba attempts various name-resolution techniques is given with the name resolve order configuration option, which we discussed earlier.

The wins support and the wins server parameters are mutually exclusive; you cannot simultaneously offer Samba as the WINS server and use another system as the server! Typically, one Samba server is set up as the WINS server using wins support, and all other Samba servers are configured with the wins server parameter pointing to the Samba WINS server.

Name-Resolution Configuration Options

Samba's name-resolution options are shown in Table 7-1.

Table 7-1. Name-resolution options

Browsing in a Windows Network

To keep things simple, we will first describe network browsing in a network that contains only Windows systems and then show you how to add a Samba server.

The basic way browsing works is that one computer in the network takes on the role of the master browser (also called local master browser, browse master, or browse server) and keeps a list of all the computers on the local subnet that are acting as SMB servers. The list of computers is called the browse list and includes all Samba servers, Windows NT/2000/XP systems, and any Windows 95/98/Me systems that have the "File and printer sharing for Microsoft Networks" networking component installed. The browse list also contains the names of all workgroups and domains. At this level, browsing is limited to the local subnet because the browsing protocol depends on broadcast packets, which are typically not forwarded to other subnets by routers.

A user at any Windows system can view the browse list by opening up the Network Neighborhood (or My Network Places), as we showed you in Chapter 1. Or, the net view command can be used from a Windows command prompt:

C:\>net view
Server Name            Remark

-------------------------------------------------------------------------------
\\MAYA                 Windows 98
\\MIXTEC               Samba 2.2.5
\\OLMEC                Windows XP Pro on Pentium/ASUS
\\TOLTEC               Samba 2.2.5
\\YAQUI                Windows 95 on mixtec/VMware
\\ZAPOTEC
The command completed successfully.

Then, net view can be used with a computer name as an argument to contact a server directly and list the resources it is sharing:

C:\>net view \\maya
Shared resources at \\maya

Windows 98

Share name   Type         Used as  Comment

-------------------------------------------------------------------------------
D            Disk
E            Disk
HP           Print
The command completed successfully.

The computers on the network involved in browsing are more than just the master browser and its clients. There are also backup browsers, which maintain copies of the browse list and respond to client requests for it. Backup browsers are therefore able to take over the role of master browser seamlessly in case it fails. The master browser usually doesn't serve the browse list directly to clients. Instead, its job is mainly to keep the master copy of the browse list up-to-date, and also periodically update the backup browsers. Clients are expected to get their copies of the browse list from backup browsers, selecting among them randomly to help to distribute the load on the backup browsers more evenly. Ideally, the interaction between any client and the master browser is limited to the client announcing when it joins or leaves the network (if it is a server) and requesting a list of backup browsers.

There can be more than one backup browser. A workgroup will have a backup browser if two or more computers are running Windows 95/98/Me or Windows NT Workstation (or another nonserver version of Windows NT/2000/XP) on the subnet. For every 32 additional computers, another backup browser is added.

In a Windows NT domain, the primary domain controller is always the local master browser, and if it fails, another Windows NT/2000 server (if one exists) will take over the role of local master browser. Other versions of Windows can function as backup browsers, but will never become a master browser if a Windows NT/2000 server is available.

In addition to acting as the local master browser, the primary domain controller also acts as the domain master browser, which ties subnets together and allows browse lists to be shared between master and backup browsers on separate subnets. This is how browsing is extended to function beyond the local subnet. Each subnet functions as a separate browsing entity, and the domain master browser synchronizes the master browsers of each subnet. In a Windows-only network, browsing cannot function across subnets unless a Windows NT/2000 PDC exists on the network. Samba can act as a domain master browser and can perform that task even in a workgroup network, which means that the Windows PDC is not required for this task. (It is also possible to use the remote browse sync parameter to configure a Samba server to synchronize its browse list with a Samba server on another subnet. In this case, each server must be acting as the local master browser of its subnet.)

Unless it is configured never to act as a browser, each computer on the subnet is considered a potential browser and can be ordered by the browse master to become a backup browser, or it can identify itself as a backup browser and accept the role on its own.

Browser Elections

When no master browser is running on the subnet, potential browsers choose a new master browser among themselves in a process called an election. An election is started by a computer in the subnet when it discovers that no master browser is currently running. If a master browser is shut down gracefully, it will broadcast an election request datagram, initiating an election by the remaining computers. If the master browser fails, the election can be started by a client computer that requests a list of backup browsers from the master browser or by a backup browser that requests to have its browse list updated from the master browser. In each case, the system fails to receive a reply from the master browser and initiates the election.

Browser elections are decided in multiple rounds of self-elimination. During each round, potential browsers broadcast election request datagrams containing their qualifications to notify other potential browsers that an election is happening and that if the recipient is more qualified, it should also broadcast a bid. When a potential browser receives an election request datagram from a more qualified opponent, it drops out, disqualifying itself from becoming the master browser. Otherwise, it responds with its own election request datagram. After a few rounds, only one potential browser is left in the election. After an additional four rounds of sending out an election request datagram and receiving no response, it becomes the master browser and sends a broadcast datagram announcing itself as the local master browser for the subnet. It then assigns runners-up in the election as backup browsers, as needed.

A potential browser's qualifications include the following:

• Whether it has recently lost an election

• The version of the election protocol it is running

• Its election criteria

• The amount of time the system has been up

• The computer's NetBIOS name

If the potential browser has lost an election recently, it immediately disqualifies itself. The version of the election protocol it is running is checked, but so far, all Windows systems (and Samba) use the same election protocol, so the check is not very meaningful. The election criteria are usually what determine which computer becomes the local master browser. There are two parts to the election criteria, shown in Tables Table 7-2 and Table 7-3.

Table 7-2. Operating-system values in an election

Table 7-3. Computer-role settings in an election

The operating-system type is compared first, and the system with the highest value wins. The values have been chosen to cause the primary domain controller, if there is one, to become the local master browser. Otherwise, a Windows NT/2000/XP system will win over a Windows for Workgroups or Windows 95/98/Me system.

When an operating-system type comparison results in a tie, the role of the computer is compared. A computer can have more than one of the values in Table 7-3, in which case the values are added.

A domain master browser has a role value of 128 to weight the election so heavily in its favor that it will also become the local master browser on its own subnet. Although the primary domain controller (which is always the domain master browser) will win the election based solely on its operating system value, sometimes there is no primary domain controller on the network, and the domain master browser would not otherwise be distinguished from other potential browsers.

Systems that are using a WINS server for name resolution are weighted heavily over ones that use broadcast name resolution with a role value of 32.

A preferred master is a computer that has been selected and configured manually by a system administrator to be favored as the choice master browser. When a preferred master starts up, it forces a browser election, even if an existing master browser is still active. A preferred master has a role value of 8, and the existing master browser gets a value of 4.

A backup browser that has recently been a master browser and still has an up-to-date browse list is given a role value of 2, and a potential browser that has been running as a backup browser gets a value of 1.

If comparing the operating-system type and role results in a tie, the computer that has been running the longest wins. In the unlikely event that the two have been up for the same amount of time, the computer that wins is the one with the NetBIOS name that sorts first alphabetically.

You can tell if a machine is a local master browser by using the Windows nbtstat command. Place the NetBIOS name of the machine you wish to check after the -a option:

C:\>nbtstat -a toltec

Local Area Connection:
Node IpAddress: [172.16.1.4] Scope Id: []

           NetBIOS Remote Machine Name Table

       Name               Type         Status
    ---------------------------------------------
    TOLTEC         <00>  UNIQUE      Registered
    TOLTEC         <03>  UNIQUE      Registered
    TOLTEC         <20>  UNIQUE      Registered
    ..__MSBROWSE__.<01>  GROUP       Registered
    METRAN         <00>  GROUP       Registered
    METRAN         <1B>  UNIQUE      Registered
    METRAN         <1C>  GROUP       Registered
    METRAN         <1D>  UNIQUE      Registered
    METRAN         <1E>  GROUP       Registered

    MAC Address = 00-00-00-00-00-00

The resource entry that you're looking for is .._ _MSBROWSE_ _.<01>. This indicates that the server is currently acting as the local master browser for the current subnet. If the machine is a Samba server, you can check the Samba nmbd log file for an entry such as:

nmbd/nmbd_become_lmb.c:become_local_master_stage2(406)
*****
Samba name server TOLTEC is now a local master browser for
workgroup METRAN on subnet 172.16.1.0

Or, you can use the nmblookup command with the -M option and the workgroup or domain name on any Samba server to find the IP address of the local master:

$ nmblookup -M metran
querying metran on 172.16.1.255
172.16.1.1 metran<1d>

Server Announcements

After the master browser election is decided, each server on the network announces itself to the network to allow the master and backup browsers to build their browse lists. At first, the server announcements happen every minute, but the interval is gradually stretched out to every 12 minutes. When a server is shut down gracefully, it sends an announcement that it is going offline to allow the master and backup browsers to remove it from the browse list. However, when a server goes offline by crashing or by some other failure, the master browser notices its disappearance only because it stops receiving server announcements. The master browser waits for three of the server's announcement periods before deciding that it is offline, which can take up to 36 minutes. Because backup browsers have their browse lists updated from the master browser once every 15 minutes, it can take up to 51 minutes for clients to be informed of a failed server.

For more detailed information on Microsoft's browsing protocols, consult the Microsoft documents Browsing and Windows 95 Networking and CIFS/E Browser Protocol. You can find these by searching for the titles on the Microsoft web site at http://www.microsoft.com.

More information on configuring Samba for browsing can be found in BROWSING.txt and BROWSING-Config.txt in the Samba distribution's docs/textdocs directory.

Configuring Samba for Browsing

Samba has full support for browsing and can participate as a master browser, a backup browser, a domain master browser, a potential browser, or just a server that doesn't participate in browsing elections. If you want to make sure your Samba server never becomes a master or backup browser, simply set:

[global]
    local master = no

Usually, you will want Samba to be available as a local master or at least a backup browser. In the simplest case, you don't need to do anything because Samba's default is to participate in browsing elections with its operating system value set to 20, which will beat any Windows system less than a Windows NT/2000 primary domain controller (see Table 7-2). The operating-system value Samba reports for itself in browser elections can be set using the os level parameter:

[global]
    os level = 33

The preceding value will allow Samba to beat even a Windows 2000 Advanced Server acting as a primary domain controller. As we show in the following section, though, forcing Samba to win this way is not recommended.

If you want to allow a Windows XP Professional system to be the master browser, you would need to set Samba lower:

[global]
    os level = 8

The maximum value for os level is 255 because it is handled as an 8-bit unsigned integer. Supposing we wanted to make absolutely sure our Samba server will be the local master browser at all times, we might say:

[global]
    local master = yes
    os level = 255
    preferred master = yes

The addition of the preferred master parameter causes Samba to start a browser election as soon as it starts up, and the os level of 255 allows it to beat any other system on the network. This includes other Samba servers, assuming they are configured properly! If another server is using a similar configuration file (with os level = 255 and preferred master = yes), the two will fight each other for the master browser role, winning elections based on minor criteria, such as uptime or their current role. To avoid this, other Samba servers should be set with a lower os level and not configured to be the preferred master.

Samba as the Domain Master Browser

Previously we mentioned that for a Windows workgroup or domain to extend into multiple subnets, one system would have to take the role of the domain master browser. The domain master browser propagates browse lists across each subnet in the workgroup. This works because each local master browser periodically synchronizes its browse list with the domain master browser. During this synchronization, the local master browser passes on the name of any server that the domain master browser does not have in its browse list, and vice versa. Each local master browser eventually holds the browse list for the entire domain.

There is no election to determine which machine assumes the role of the domain master browser. Instead, the administrator has to set it manually. By Microsoft design, however, the domain master browser and the PDC both register a resource type of <1B>, so the roles—and the machines—are inseparable.

If you have a Windows NT server on the network acting as a PDC, we recommend that you do not try to use Samba to become the domain master browser. The reverse is true as well: if Samba is taking on the responsibilities of a PDC, we recommend making it the domain master browser as well. Although it is possible to split the roles with Samba, this is not a good idea. Using two different machines to serve as the PDC and the domain master browser can cause random errors to occur in a Windows workgroup.

Samba can assume the role of a domain master browser for all subnets in the workgroup with the following options:

[global]
    domain master = yes
    preferred master = yes
    local master = yes
    os level = 255

The final three parameters ensure that the server is also the local master browser, which is vital for it to work properly as the domain master browser. You can verify that a Samba machine is in fact the domain master browser by checking the nmbd log file:

nmbd/nmbd_become_dmb.c:become_domain_master_stage2(118)
*****
Samba name server TOLTEC is now a domain master browser for
workgroup METRAN on subnet 172.16.1.0

Or you can use the nmblookup command that comes with the Samba distribution to query for a unique <1B> resource type in the workgroup:

# nmblookup METRAN#1B
Sending queries to 172.16.1.255
172.16.1.1 METRAN<1b>