&author.jht; July 5, 1998 Updated: April 21, 2003 This document contains detailed informataion as well as a fast track guide to implementing browsing across subnets and / or across workgroups (or domains). WINS is the best tool for resolution of NetBIOS names to IP addesses. WINS is NOT involved in browse list handling except by way of name to address resolution. MS Windows 2000 and later can be configured to operate with NO NetBIOS over TCP/IP. Samba-3 and later also supports this mode of operation. When the use of NetBIOS over TCP/IP has been disabled then the primary means for resolution of MS Windows machine names is via DNS and Active Directory. The following information assumes that your site is running NetBIOS over TCP/IP. To most people browsing means that they can see the MS Windows and Samba servers in the Network Neighborhood, and when the computer icon for a particular server is clicked, it opens up and shows the shares and printers available on the target server. What seems so simple is in fact a very complex interaction of different technologies. The technologies (or methods) employed in making all of this work includes: MS Windows machines register their presence to the network Machines announce themselves to other machines on the network One or more machine on the network collates the local announcements The client machine finds the machine that has the collated list of machines The client machine is able to resolve the machine names to IP addresses The client machine is able to connect to a target machine The samba application that controls/manages browse list management and name resolution is called nmbd. The configuration parameters involved in nmbd's operation are: Browsing options: ----------------- * os level lm announce lm interval * preferred master * local master * domain master browse list enhanced browsing Name Resolution Method: ----------------------- * name resolve order WINS options: ------------- dns proxy wins proxy * wins server * wins support wins hook WINS Server and WINS Support are mutually exclusive options. Those marked with an '*' are the only options that commonly MAY need to be modified. Even if not one of these parameters is set nmbd will still do it's job. Firstly, all MS Windows networking is based on SMB (Server Message Block) based messaging. SMB messaging may be implemented using NetBIOS or without NetBIOS. Samba implements NetBIOS by encapsulating it over TCP/IP. MS Windows products can do likewise. NetBIOS based networking uses broadcast messaging to affect browse list management. When running NetBIOS over TCP/IP this uses UDP based messaging. UDP messages can be broadcast or unicast. Normally, only unicast UDP messaging can be forwarded by routers. The remote announce parameter to smb.conf helps to project browse announcements to remote network segments via unicast UDP. Similarly, the remote browse sync parameter of smb.conf implements browse list collation using unicast UDP. Secondly, in those networks where Samba is the only SMB server technology wherever possible nmbd should be configured on one (1) machine as the WINS server. This makes it easy to manage the browsing environment. If each network segment is configured with it's own Samba WINS server, then the only way to get cross segment browsing to work is by using the remote announce and the remote browse sync parameters to your smb.conf file. If only one WINS server is used for an entire multi-segment network then the use of the remote announce and the remote browse sync parameters should NOT be necessary. As of Samba 3 WINS replication is being worked on. The bulk of the code has been committed, but it still needs maturation. Right now samba WINS does not support MS-WINS replication. This means that when setting up Samba as a WINS server there must only be one nmbd configured as a WINS server on the network. Some sites have used multiple Samba WINS servers for redundancy (one server per subnet) and then used remote browse sync and remote announce to affect browse list collation across all segments. Note that this means clients will only resolve local names, and must be configured to use DNS to resolve names on other subnets in order to resolve the IP addresses of the servers they can see on other subnets. This setup is not recommended, but is mentioned as a practical consideration (ie: an 'if all else fails' scenario). Lastly, take note that browse lists are a collection of unreliable broadcast messages that are repeated at intervals of not more than 15 minutes. This means that it will take time to establish a browse list and it can take up to 45 minutes to stabilise, particularly across network segments. As stated above, MS Windows machines register their NetBIOS names (ie: the machine name for each service type in operation) on start up. Also, as stated above, the exact method by which this name registration takes place is determined by whether or not the MS Windows client/server has been given a WINS server address, whether or not LMHOSTS lookup is enabled, or if DNS for NetBIOS name resolution is enabled, etc. In the case where there is no WINS server all name registrations as well as name lookups are done by UDP broadcast. This isolates name resolution to the local subnet, unless LMHOSTS is used to list all names and IP addresses. In such situations Samba provides a means by which the samba server name may be forcibly injected into the browse list of a remote MS Windows network (using the remote announce parameter). Where a WINS server is used, the MS Windows client will use UDP unicast to register with the WINS server. Such packets can be routed and thus WINS allows name resolution to function across routed networks. During the startup process an election will take place to create a local master browser if one does not already exist. On each NetBIOS network one machine will be elected to function as the domain master browser. This domain browsing has nothing to do with MS security domain control. Instead, the domain master browser serves the role of contacting each local master browser (found by asking WINS or from LMHOSTS) and exchanging browse list contents. This way every master browser will eventually obtain a complete list of all machines that are on the network. Every 11-15 minutes an election is held to determine which machine will be the master browser. By the nature of the election criteria used, the machine with the highest uptime, or the most senior protocol version, or other criteria, will win the election as domain master browser. Clients wishing to browse the network make use of this list, but also depend on the availability of correct name resolution to the respective IP address/addresses. Any configuration that breaks name resolution and/or browsing intrinsics will annoy users because they will have to put up with protracted inability to use the network services. Samba supports a feature that allows forced synchonisation of browse lists across routed networks using the remote browse sync parameter in the smb.conf file. This causes Samba to contact the local master browser on a remote network and to request browse list synchronisation. This effectively bridges two networks that are separated by routers. The two remote networks may use either broadcast based name resolution or WINS based name resolution, but it should be noted that the remote browse sync parameter provides browse list synchronisation - and that is distinct from name to address resolution, in other words, for cross subnet browsing to function correctly it is essential that a name to address resolution mechanism be provided. This mechanism could be via DNS, /etc/hosts, and so on. To set up cross subnet browsing on a network containing machines in up to be in a WORKGROUP, not an NT Domain you need to set up one Samba server to be the Domain Master Browser (note that this is *NOT* the same as a Primary Domain Controller, although in an NT Domain the same machine plays both roles). The role of a Domain master browser is to collate the browse lists from local master browsers on all the subnets that have a machine participating in the workgroup. Without one machine configured as a domain master browser each subnet would be an isolated workgroup, unable to see any machines on any other subnet. It is the presense of a domain master browser that makes cross subnet browsing possible for a workgroup. In an WORKGROUP environment the domain master browser must be a Samba server, and there must only be one domain master browser per workgroup name. To set up a Samba server as a domain master browser, set the following option in the [global] section of the &smb.conf; file : domain master = yes The domain master browser should also preferrably be the local master browser for its own subnet. In order to achieve this set the following options in the [global] section of the &smb.conf; file : domain master = yes local master = yes preferred master = yes os level = 65 The domain master browser may be the same machine as the WINS server, if you require. Next, you should ensure that each of the subnets contains a machine that can act as a local master browser for the workgroup. Any MS Windows NT/2K/XP/2003 machine should be able to do this, as will Windows 9x machines (although these tend to get rebooted more often, so it's not such a good idea to use these). To make a Samba server a local master browser set the following options in the [global] section of the &smb.conf; file : domain master = no local master = yes preferred master = yes os level = 65 Do not do this for more than one Samba server on each subnet, or they will war with each other over which is to be the local master browser. The local master parameter allows Samba to act as a local master browser. The preferred master causes nmbd to force a browser election on startup and the os level parameter sets Samba high enough so that it should win any browser elections. If you have an NT machine on the subnet that you wish to be the local master browser then you can disable Samba from becoming a local master browser by setting the following options in the [global] section of the &smb.conf; file : domain master = no local master = no preferred master = no os level = 0 If you are adding Samba servers to a Windows NT Domain then you must not set up a Samba server as a domain master browser. By default, a Windows NT Primary Domain Controller for a Domain name is also the Domain master browser for that name, and many things will break if a Samba server registers the Domain master browser NetBIOS name (DOMAIN<1B>) with WINS instead of the PDC. For subnets other than the one containing the Windows NT PDC you may set up Samba servers as local master browsers as described. To make a Samba server a local master browser set the following options in the [global] section of the &smb.conf; file : domain master = no local master = yes preferred master = yes os level = 65 If you wish to have a Samba server fight the election with machines on the same subnet you may set the os level parameter to lower levels. By doing this you can tune the order of machines that will become local master browsers if they are running. For more details on this see the section Forcing samba to be the master browser below. If you have Windows NT machines that are members of the domain on all subnets, and you are sure they will always be running then you can disable Samba from taking part in browser elections and ever becoming a local master browser by setting following options in the [global] section of the &smb.conf; file : domain master = no local master = no preferred master = no os level = 0 Who becomes the master browser is determined by an election process using broadcasts. Each election packet contains a number of parameters which determine what precedence (bias) a host should have in the election. By default Samba uses a very low precedence and thus loses elections to just about anyone else. If you want Samba to win elections then just set the os level global option in &smb.conf; to a higher number. It defaults to 0. Using 34 would make it win all elections over every other system (except other samba systems!) A os level of 2 would make it beat WfWg and Win95, but not MS Windows NT/2K Server. A MS Windows NT/2K Server domain controller uses level 32. The maximum os level is 255 If you want samba to force an election on startup, then set the preferred master global option in &smb.conf; to "yes". Samba will then have a slight advantage over other potential master browsers that are not preferred master browsers. Use this parameter with care, as if you have two hosts (whether they are windows 95 or NT or samba) on the same local subnet both set with preferred master to "yes", then periodically and continually they will force an election in order to become the local master browser. If you want samba to be a domain master browser, then it is recommended that you also set preferred master to "yes", because samba will not become a domain master browser for the whole of your LAN or WAN if it is not also a local master browser on its own broadcast isolated subnet. It is possible to configure two samba servers to attempt to become the domain master browser for a domain. The first server that comes up will be the domain master browser. All other samba servers will attempt to become the domain master browser every 5 minutes. They will find that another samba server is already the domain master browser and will fail. This provides automatic redundancy, should the current domain master browser fail. The domain master is responsible for collating the browse lists of multiple subnets so that browsing can occur between subnets. You can make samba act as the domain master by setting domain master = yes in &smb.conf;. By default it will not be a domain master. Note that you should NOT set Samba to be the domain master for a workgroup that has the same name as an NT Domain. When samba is the domain master and the master browser it will listen for master announcements (made roughly every twelve minutes) from local master browsers on other subnets and then contact them to synchronise browse lists. If you want samba to be the domain master then I suggest you also set the os level high enough to make sure it wins elections, and set preferred master to "yes", to get samba to force an election on startup. Note that all your servers (including samba) and clients should be using a WINS server to resolve NetBIOS names. If your clients are only using broadcasting to resolve NetBIOS names, then two things will occur: your local master browsers will be unable to find a domain master browser, as it will only be looking on the local subnet. if a client happens to get hold of a domain-wide browse list, and a user attempts to access a host in that list, it will be unable to resolve the NetBIOS name of that host. If, however, both samba and your clients are using a WINS server, then: your local master browsers will contact the WINS server and, as long as samba has registered that it is a domain master browser with the WINS server, your local master browser will receive samba's ip address as its domain master browser. when a client receives a domain-wide browse list, and a user attempts to access a host in that list, it will contact the WINS server to resolve the NetBIOS name of that host. as long as that host has registered its NetBIOS name with the same WINS server, the user will be able to see that host. If your network uses a "0" based broadcast address (for example if it ends in a 0) then you will strike problems. Windows for Workgroups does not seem to support a 0's broadcast and you will probably find that browsing and name lookups won't work. Samba now supports machines with multiple network interfaces. If you have multiple interfaces then you will need to use the interfaces option in &smb.conf; to configure them. The remote announce parameter of smb.conf can be used to forcibly ensure that all the NetBIOS names on a network get announced to a remote network. The syntax of the remote announce parameter is: remote announce = a.b.c.d [e.f.g.h] ... _or_ remote announce = a.b.c.d/WORKGROUP [e.f.g.h/WORKGROUP] ... where: a.b.c.d and e.f.g.h is either the LMB (Local Master Browser) IP address or the broadcst address of the remote network. ie: the LMB is at 192.168.1.10, or the address could be given as 192.168.1.255 where the netmask is assumed to be 24 bits (255.255.255.0). When the remote announcement is made to the broadcast address of the remote network every host will receive our announcements. This is noisy and therefore undesirable but may be necessary if we do NOT know the IP address of the remote LMB. WORKGROUP is optional and can be either our own workgroup or that of the remote network. If you use the workgroup name of the remote network then our NetBIOS machine names will end up looking like they belong to that workgroup, this may cause name resolution problems and should be avoided. The remote browse sync parameter of smb.conf is used to announce to another LMB that it must synchronise it's NetBIOS name list with our Samba LMB. It works ONLY if the Samba server that has this option is simultaneously the LMB on it's network segment. The syntax of the remote browse sync parameter is: remote browse sync = a.b.c.d where a.b.c.d is either the IP address of the remote LMB or else is the network broadcast address of the remote segment. Use of WINS (either Samba WINS _or_ MS Windows NT Server WINS) is highly recommended. Every NetBIOS machine registers it's name together with a name_type value for each of of several types of service it has available. eg: It registers it's name directly as a unique (the type 0x03) name. It also registers it's name if it is running the lanmanager compatible server service (used to make shares and printers available to other users) by registering the server (the type 0x20) name. All NetBIOS names are up to 15 characters in length. The name_type variable is added to the end of the name - thus creating a 16 character name. Any name that is shorter than 15 characters is padded with spaces to the 15th character. ie: All NetBIOS names are 16 characters long (including the name_type information). WINS can store these 16 character names as they get registered. A client that wants to log onto the network can ask the WINS server for a list of all names that have registered the NetLogon service name_type. This saves broadcast traffic and greatly expedites logon processing. Since broadcast name resolution can not be used across network segments this type of information can only be provided via WINS _or_ via statically configured lmhosts files that must reside on all clients in the absence of WINS. WINS also serves the purpose of forcing browse list synchronisation by all LMB's. LMB's must synchronise their browse list with the DMB (domain master browser) and WINS helps the LMB to identify it's DMB. By definition this will work only within a single workgroup. Note that the domain master browser has NOTHING to do with what is referred to as an MS Windows NT Domain. The later is a reference to a security environment while the DMB refers to the master controller for browse list information only. Use of WINS will work correctly only if EVERY client TCP/IP protocol stack has been configured to use the WINS server/s. Any client that has not been configured to use the WINS server will continue to use only broadcast based name registration so that WINS may NEVER get to know about it. In any case, machines that have not registered with a WINS server will fail name to address lookup attempts by other clients and will therefore cause workstation access errors. To configure Samba as a WINS server just add wins support = yes to the smb.conf file [globals] section. To configure Samba to register with a WINS server just add "wins server = a.b.c.d" to your smb.conf file [globals] section. Never use both wins support = yes together with wins server = a.b.c.d particularly not using it's own IP address. Specifying both will cause &nmbd; to refuse to start! Either a Samba machine or a Windows NT Server machine may be set up as a WINS server. To set a Samba machine to be a WINS server you must add the following option to the &smb.conf; file on the selected machine : in the [globals] section add the line wins support = yes Versions of Samba prior to 1.9.17 had this parameter default to yes. If you have any older versions of Samba on your network it is strongly suggested you upgrade to a recent version, or at the very least set the parameter to 'no' on all these machines. Machines with wins support = yes will keep a list of all NetBIOS names registered with them, acting as a DNS for NetBIOS names. You should set up only ONE wins server. Do NOT set the wins support = yes option on more than one Samba server. To set up a Windows NT Server as a WINS server you need to set up the WINS service - see your NT documentation for details. Note that Windows NT WINS Servers can replicate to each other, allowing more than one to be set up in a complex subnet environment. As Microsoft refuse to document these replication protocols Samba cannot currently participate in these replications. It is possible in the future that a Samba->Samba WINS replication protocol may be defined, in which case more than one Samba machine could be set up as a WINS server but currently only one Samba server should have the wins support = yes parameter set. After the WINS server has been configured you must ensure that all machines participating on the network are configured with the address of this WINS server. If your WINS server is a Samba machine, fill in the Samba machine IP address in the "Primary WINS Server" field of the "Control Panel->Network->Protocols->TCP->WINS Server" dialogs in Windows 95 or Windows NT. To tell a Samba server the IP address of the WINS server add the following line to the [global] section of all &smb.conf; files : wins server = <name or IP address> where <name or IP address> is either the DNS name of the WINS server machine or its IP address. Note that this line MUST NOT BE SET in the &smb.conf; file of the Samba server acting as the WINS server itself. If you set both the wins support = yes option and the wins server = <name> option then nmbd will fail to start. There are two possible scenarios for setting up cross subnet browsing. The first details setting up cross subnet browsing on a network containing Windows 95, Samba and Windows NT machines that are not configured as part of a Windows NT Domain. The second details setting up cross subnet browsing on networks that contain NT Domains. Samba-3 permits WINS replication through the use of the wrepld utility. This tool is not currently capable of being used as it is still in active development. As soon as this tool becomes moderately functional we will prepare man pages and enhance this section of the documentation to provide usage and technical details. New to Samba-3 is a tool called winsedit that may be used to add static WINS entries to the WINS database. This tool can be used also to modify entries existing in the WINS database. The development of the winsedit tool was made necessary due to the migration of the older style wins.dat file into a new tdb binary backend data store. The following hints should be carefully considered as they are stumbling points for many new network administrators. Do NOT use more than one (1) protocol on MS Windows machines A very common cause of browsing problems results from installing more than one protocol on an MS Windows machine. Every NetBIOS machine takes part in a process of electing the LMB (and DMB) every 15 minutes. A set of election criteria is used to determine the order of precidence for winning this election process. A machine running Samba or Windows NT will be biased so that the most suitable machine will predictably win and thus retain it's role. The election process is "fought out" so to speak over every NetBIOS network interface. In the case of a Windows 9x machine that has both TCP/IP and IPX installed and has NetBIOS enabled over both protocols the election will be decided over both protocols. As often happens, if the Windows 9x machine is the only one with both protocols then the LMB may be won on the NetBIOS interface over the IPX protocol. Samba will then lose the LMB role as Windows 9x will insist it knows who the LMB is. Samba will then cease to function as an LMB and thus browse list operation on all TCP/IP only machines will fail. Windows 95, 98, 98se, Me are referred to generically as Windows 9x. The Windows NT4, 2000, XP and 2003 use common protocols. These are roughly referred to as the WinNT family, but it should be recognised that 2000 and XP/2003 introduce new protocol extensions that cause them to behave differently from MS Windows NT4. Generally, where a server does NOT support the newer or extended protocol, these will fall back to the NT4 protocols. The safest rule of all to follow it this - USE ONLY ONE PROTOCOL! Resolution of NetBIOS names to IP addresses can take place using a number of methods. The only ones that can provide NetBIOS name_type information are: WINS: the best tool! LMHOSTS: is static and hard to maintain. Broadcast: uses UDP and can not resolve names across remote segments. Alternative means of name resolution includes: /etc/hosts: is static, hard to maintain, and lacks name_type info DNS: is a good choice but lacks essential name_type info. Many sites want to restrict DNS lookups and want to avoid broadcast name resolution traffic. The "name resolve order" parameter is of great help here. The syntax of the "name resolve order" parameter is: name resolve order = wins lmhosts bcast host _or_ name resolve order = wins lmhosts (eliminates bcast and host) The default is: name resolve order = host lmhost wins bcast where "host" refers the the native methods used by the Unix system to implement the gethostbyname() function call. This is normally controlled by /etc/host.conf, /etc/nsswitch.conf and /etc/resolv.conf. SMB networking provides a mechanism by which clients can access a list of machines in a network, a so-called browse list. This list contains machines that are ready to offer file and/or print services to other machines within the network. Thus it does not include machines which aren't currently able to do server tasks. The browse list is heavily used by all SMB clients. Configuration of SMB browsing has been problematic for some Samba users, hence this document. MS Windows 2000 and later, as with Samba 3 and later, can be configured to not use NetBIOS over TCP/IP. When configured this way it is imperative that name resolution (using DNS/LDAP/ADS) be correctly configured and operative. Browsing will NOT work if name resolution from SMB machine names to IP addresses does not function correctly. Where NetBIOS over TCP/IP is enabled use of a WINS server is highly recommended to aid the resolution of NetBIOS (SMB) names to IP addresses. WINS allows remote segment clients to obtain NetBIOS name_type information that can NOT be provided by any other means of name resolution. Samba facilitates browsing. The browsing is supported by &nmbd; and is also controlled by options in the &smb.conf; file. Samba can act as a local browse master for a workgroup and the ability to support domain logons and scripts is now available. Samba can also act as a domain master browser for a workgroup. This means that it will collate lists from local browse masters into a wide area network server list. In order for browse clients to resolve the names they may find in this list, it is recommended that both samba and your clients use a WINS server. Note that you should NOT set Samba to be the domain master for a workgroup that has the same name as an NT Domain: on each wide area network, you must only ever have one domain master browser per workgroup, regardless of whether it is NT, Samba or any other type of domain master that is providing this service. Nmbd can be configured as a WINS server, but it is not necessary to specifically use samba as your WINS server. MS Windows NT4, Server or Advanced Server 2000 or 2003 can be configured as your WINS server. In a mixed NT/2000/2003 server and samba environment on a Wide Area Network, it is recommended that you use the Microsoft WINS server capabilities. In a samba-only environment, it is recommended that you use one and only one Samba server as your WINS server. To get browsing to work you need to run nmbd as usual, but will need to use the workgroup option in &smb.conf; to control what workgroup Samba becomes a part of. Samba also has a useful option for a Samba server to offer itself for browsing on another subnet. It is recommended that this option is only used for 'unusual' purposes: announcements over the internet, for example. See remote announce in the &smb.conf; man page. If something doesn't work then hopefully the log.nmb file will help you track down the problem. Try a debug level of 2 or 3 for finding problems. Also note that the current browse list usually gets stored in text form in a file called browse.dat. Note that if it doesn't work for you, then you should still be able to type the server name as \\SERVER in filemanager then hit enter and filemanager should display the list of available shares. Some people find browsing fails because they don't have the global guest account set to a valid account. Remember that the IPC$ connection that lists the shares is done as guest, and thus you must have a valid guest account. MS Windows 2000 and upwards (as with Samba) can be configured to disallow anonymous (ie: Guest account) access to the IPC$ share. In that case, the MS Windows 2000/XP/2003 machine acting as an SMB/CIFS client will use the name of the currently logged in user to query the IPC$ share. MS Windows 9X clients are not able to do this and thus will NOT be able to browse server resources. The other big problem people have is that their broadcast address, netmask or IP address is wrong (specified with the "interfaces" option in &smb.conf;) Since the release of Samba 1.9.17(alpha1) Samba has been updated to enable it to support the replication of browse lists across subnet boundaries. New code and options have been added to achieve this. This section describes how to set this feature up in different settings. To see browse lists that span TCP/IP subnets (ie. networks separated by routers that don't pass broadcast traffic) you must set up at least one WINS server. The WINS server acts as a DNS for NetBIOS names, allowing NetBIOS name to IP address translation to be done by doing a direct query of the WINS server. This is done via a directed UDP packet on port 137 to the WINS server machine. The reason for a WINS server is that by default, all NetBIOS name to IP address translation is done by broadcasts from the querying machine. This means that machines on one subnet will not be able to resolve the names of machines on another subnet without using a WINS server. Remember, for browsing across subnets to work correctly, all machines, be they Windows 95, Windows NT, or Samba servers must have the IP address of a WINS server given to them by a DHCP server, or by manual configuration (for Win95 and WinNT, this is in the TCP/IP Properties, under Network settings) for Samba this is in the &smb.conf; file. Cross subnet browsing is a complicated dance, containing multiple moving parts. It has taken Microsoft several years to get the code that achieves this correct, and Samba lags behind in some areas. Samba is capable of cross subnet browsing when configured correctly. Consider a network set up as follows : (DMB) N1_A N1_B N1_C N1_D N1_E | | | | | ------------------------------------------------------- | subnet 1 | +---+ +---+ |R1 | Router 1 Router 2 |R2 | +---+ +---+ | | | subnet 2 subnet 3 | -------------------------- ------------------------------------ | | | | | | | | N2_A N2_B N2_C N2_D N3_A N3_B N3_C N3_D (WINS) Consisting of 3 subnets (1, 2, 3) connected by two routers (R1, R2) - these do not pass broadcasts. Subnet 1 has 5 machines on it, subnet 2 has 4 machines, subnet 3 has 4 machines. Assume for the moment that all these machines are configured to be in the same workgroup (for simplicities sake). Machine N1_C on subnet 1 is configured as Domain Master Browser (ie. it will collate the browse lists for the workgroup). Machine N2_D is configured as WINS server and all the other machines are configured to register their NetBIOS names with it. As all these machines are booted up, elections for master browsers will take place on each of the three subnets. Assume that machine N1_C wins on subnet 1, N2_B wins on subnet 2, and N3_D wins on subnet 3 - these machines are known as local master browsers for their particular subnet. N1_C has an advantage in winning as the local master browser on subnet 1 as it is set up as Domain Master Browser. On each of the three networks, machines that are configured to offer sharing services will broadcast that they are offering these services. The local master browser on each subnet will receive these broadcasts and keep a record of the fact that the machine is offering a service. This list of records is the basis of the browse list. For this case, assume that all the machines are configured to offer services so all machines will be on the browse list. For each network, the local master browser on that network is considered 'authoritative' for all the names it receives via local broadcast. This is because a machine seen by the local master browser via a local broadcast must be on the same network as the local master browser and thus is a 'trusted' and 'verifiable' resource. Machines on other networks that the local master browsers learn about when collating their browse lists have not been directly seen - these records are called 'non-authoritative'. At this point the browse lists look as follows (these are the machines you would see in your network neighborhood if you looked in it on a particular network right now). Subnet Browse Master List ------ ------------- ---- Subnet1 N1_C N1_A, N1_B, N1_C, N1_D, N1_E Subnet2 N2_B N2_A, N2_B, N2_C, N2_D Subnet3 N3_D N3_A, N3_B, N3_C, N3_D Note that at this point all the subnets are separate, no machine is seen across any of the subnets. Now examine subnet 2. As soon as N2_B has become the local master browser it looks for a Domain master browser to synchronize its browse list with. It does this by querying the WINS server (N2_D) for the IP address associated with the NetBIOS name WORKGROUP<1B>. This name was registerd by the Domain master browser (N1_C) with the WINS server as soon as it was booted. Once N2_B knows the address of the Domain master browser it tells it that is the local master browser for subnet 2 by sending a MasterAnnouncement packet as a UDP port 138 packet. It then synchronizes with it by doing a NetServerEnum2 call. This tells the Domain Master Browser to send it all the server names it knows about. Once the domain master browser receives the MasterAnnouncement packet it schedules a synchronization request to the sender of that packet. After both synchronizations are done the browse lists look like : Subnet Browse Master List ------ ------------- ---- Subnet1 N1_C N1_A, N1_B, N1_C, N1_D, N1_E, N2_A(*), N2_B(*), N2_C(*), N2_D(*) Subnet2 N2_B N2_A, N2_B, N2_C, N2_D N1_A(*), N1_B(*), N1_C(*), N1_D(*), N1_E(*) Subnet3 N3_D N3_A, N3_B, N3_C, N3_D Servers with a (*) after them are non-authoritative names. At this point users looking in their network neighborhood on subnets 1 or 2 will see all the servers on both, users on subnet 3 will still only see the servers on their own subnet. The same sequence of events that occured for N2_B now occurs for the local master browser on subnet 3 (N3_D). When it synchronizes browse lists with the domain master browser (N1_A) it gets both the server entries on subnet 1, and those on subnet 2. After N3_D has synchronized with N1_C and vica-versa the browse lists look like. Subnet Browse Master List ------ ------------- ---- Subnet1 N1_C N1_A, N1_B, N1_C, N1_D, N1_E, N2_A(*), N2_B(*), N2_C(*), N2_D(*), N3_A(*), N3_B(*), N3_C(*), N3_D(*) Subnet2 N2_B N2_A, N2_B, N2_C, N2_D N1_A(*), N1_B(*), N1_C(*), N1_D(*), N1_E(*) Subnet3 N3_D N3_A, N3_B, N3_C, N3_D N1_A(*), N1_B(*), N1_C(*), N1_D(*), N1_E(*), N2_A(*), N2_B(*), N2_C(*), N2_D(*) Servers with a (*) after them are non-authoritative names. At this point users looking in their network neighborhood on subnets 1 or 3 will see all the servers on all sunbets, users on subnet 2 will still only see the servers on subnets 1 and 2, but not 3. Finally, the local master browser for subnet 2 (N2_B) will sync again with the domain master browser (N1_C) and will recieve the missing server entries. Finally - and as a steady state (if no machines are removed or shut off) the browse lists will look like : Subnet Browse Master List ------ ------------- ---- Subnet1 N1_C N1_A, N1_B, N1_C, N1_D, N1_E, N2_A(*), N2_B(*), N2_C(*), N2_D(*), N3_A(*), N3_B(*), N3_C(*), N3_D(*) Subnet2 N2_B N2_A, N2_B, N2_C, N2_D N1_A(*), N1_B(*), N1_C(*), N1_D(*), N1_E(*) N3_A(*), N3_B(*), N3_C(*), N3_D(*) Subnet3 N3_D N3_A, N3_B, N3_C, N3_D N1_A(*), N1_B(*), N1_C(*), N1_D(*), N1_E(*), N2_A(*), N2_B(*), N2_C(*), N2_D(*) Servers with a (*) after them are non-authoritative names. Synchronizations between the domain master browser and local master browsers will continue to occur, but this should be a steady state situation. If either router R1 or R2 fails the following will occur: Names of computers on each side of the inaccessible network fragments will be maintained for as long as 36 minutes, in the network neighbourhood lists. Attempts to connect to these inaccessible computers will fail, but the names will not be removed from the network neighbourhood lists. If one of the fragments is cut off from the WINS server, it will only be able to access servers on its local subnet, by using subnet-isolated broadcast NetBIOS name resolution. The effects are similar to that of losing access to a DNS server.