25.4 Name Resolution as Used within MS Windows Networking
MS Windows networking is predicated about the name each machine is given. This name is known variously (and inconsistently) as the " computer name ," " machine name ," " networking name ," " netbios name ," or " SMB name ." All terms mean the same thing with the exception of " netbios name " that can also apply to the name of the workgroup or the domain name. The terms " workgroup " and " domain " are really just a simple name with which the machine is associated. All NetBIOS names are exactly 16 characters in length. The 16 th character is reserved. It is used to store a one-byte value that indicates service level information for the NetBIOS name that is registered. A NetBIOS machine name is, therefore, registered for each service type that is provided by the client/server.
Table 25.1 and Table 25.2 list typical NetBIOS name/service type registrations.
Table 25.1. Unique NetBIOS Names
Table 25.2. Group Names
It should be noted that all NetBIOS machines register their own names as per the above. This is in vast contrast to TCP/IP installations where traditionally the system administrator will determine in the /etc/ hosts or in the DNS database what names are associated with each IP address.
One further point of clarification should be noted. The /etc/hosts file and the DNS records do not provide the NetBIOS name type information that MS Windows clients depend on to locate the type of service that may be needed. An example of this is what happens when an MS Windows client wants to locate a domain logon server. It finds this service and the IP address of a server that provides it by performing a lookup (via a NetBIOS broadcast) for enumeration of all machines that have registered the name type *<1c>. A logon request is then sent to each IP address that is returned in the enumerated list of IP addresses. Whichever machine first replies, it then ends up providing the logon services.
The name " workgroup " or " domain " really can be confusing since these have the added significance of indicating what is the security architecture of the MS Windows network. The term " workgroup " indicates that the primary nature of the network environment is that of a peer-to-peer design. In a WORKGROUP, all machines are responsible for their own security, and generally such security is limited to the use of just a password (known as Share Level security). In most situations with peer-to-peer networking, the users who control their own machines will simply opt to have no security at all. It is possible to have User Level Security in a WORKGROUP environment, thus requiring the use of a user name and a matching password.
MS Windows networking is thus predetermined to use machine names for all local and remote machine message passing. The protocol used is called Server Message Block (SMB) and this is implemented using the NetBIOS protocol (Network Basic Input Output System). NetBIOS can be encapsulated using LLC (Logical Link Control) protocol ” in which case the resulting protocol is called NetBEUI (Network Basic Extended User Interface). NetBIOS can also be run over IPX (Internetworking Packet Exchange) protocol as used by Novell NetWare, and it can be run over TCP/IP protocols ” in which case the resulting protocol is called NBT or NetBT, the NetBIOS over TCP/IP.
MS Windows machines use a complex array of name resolution mechanisms. Since we are primarily concerned with TCP/IP, this demonstration is limited to this area.
25.4.1 The NetBIOS Name Cache
All MS Windows machines employ an in-memory buffer in which is stored the NetBIOS names and IP addresses for all external machines that machine has communicated with over the past 10-15 minutes. It is more efficient to obtain an IP address for a machine from the local cache than it is to go through all the configured name resolution mechanisms.
If a machine whose name is in the local name cache has been shut down before the name had been expired and flushed from the cache, then an attempt to exchange a message with that machine will be subject to time-out delays. Its name is in the cache, so a name resolution lookup will succeed, but the machine cannot respond. This can be frustrating for users but is a characteristic of the protocol.
The MS Windows utility that allows examination of the NetBIOS name cache is called " nbtstat ". The Samba equivalent of this is called nmblookup .
25.4.2 The LMHOSTS File
This file is usually located in MS Windows NT 4.0 or Windows 200x/XP in the directory C:\WINNT\SYSTEM32\DRIVERS\ETC and contains the IP Address and the machine name in matched pairs. The LMHOSTS file performs NetBIOS name to IP address mapping.
It typically looks like this:
# Copyright (c) 1998 Microsoft Corp. # # This is a sample LMHOSTS file used by the Microsoft Wins Client (NetBIOS # over TCP/IP) stack for Windows98 # # This file contains the mappings of IP addresses to NT computernames # (NetBIOS) names. Each entry should be kept on an individual line. # The IP address should be placed in the first column followed by the # corresponding computername. The address and the computername # should be separated by at least one space or tab. The "#" character # is generally used to denote the start of a comment (see the exceptions # below). # # This file is compatible with Microsoft LAN Manager 2.x TCP/IP lmhosts # files and offers the following extensions: # # #PRE # #DOM:<domain> # #INCLUDE <filename> # #BEGIN_ALTERNATE # #END_ALTERNATE # \0xnn (non-printing character support) # # Following any entry in the file with the characters "#PRE" will cause # the entry to be preloaded into the name cache. By default, entries are # not preloaded, but are parsed only after dynamic name resolution fails. # # Following an entry with the "#DOM:<domain>" tag will associate the # entry with the domain specified by <domain>. This effects how the # browser and logon services behave in TCP/IP environments. To preload # the host name associated with #DOM entry, it is necessary to also add a # #PRE to the line. The <domain> is always preloaded although it will not # be shown when the name cache is viewed . # # Specifying "#INCLUDE <filename>" will force the RFC NetBIOS (NBT) # software to seek the specified <filename> and parse it as if it were # local. <filename> is generally a UNC-based name, allowing a # centralized lmhosts file to be maintained on a server. # It is ALWAYS necessary to provide a mapping for the IP address of the # server prior to the #INCLUDE. This mapping must use the #PRE directive. # In addition the share "public" in the example below must be in the # LanManServer list of "NullSessionShares" in order for client machines to # be able to read the lmhosts file successfully. This key is under # \machine\system\currentcontrolset\services\lanmanserver\ # parameters\nullsessionshares # in the registry. Simply add "public" to the list found there. # # The #BEGIN_ and #END_ALTERNATE keywords allow multiple #INCLUDE # statements to be grouped together. Any single successful include # will cause the group to succeed. # # Finally, non-printing characters can be embedded in mappings by # first surrounding the NetBIOS name in quotations, then using the # \0xnn notation to specify a hex value for a non-printing character. # # The following example illustrates all of these extensions: # # 18.104.22.168 rhino #PRE #DOM:networking #net group's DC # 22.214.171.124 "appname \0x14" #special app server # 126.96.36.199 popular #PRE #source server # 188.8.131.52 localsrv #PRE #needed for the include # # #BEGIN_ALTERNATE # #INCLUDE \\localsrv\public\lmhosts # #INCLUDE \\ rhino \public\lmhosts # #END_ALTERNATE # # In the above example, the " appname " server contains a special # character in its name, the "popular" and "localsrv" server names are # preloaded, and the "rhino" server name is specified so it can be used # to later #INCLUDE a centrally maintained lmhosts file if the "localsrv" # system is unavailable. # # Note that the whole file is parsed including comments on each lookup, # so keeping the number of comments to a minimum will improve performance. # Therefore it is not advisable to simply add lmhosts file entries onto the # end of this file.
25.4.3 HOSTS File
This file is usually located in MS Windows NT 4.0 or Windows 200x/XP in the directory C:\WINNT\SYSTEM32\DRIVERS\ETC and contains the IP Address and the IP hostname in matched pairs. It can be used by the name resolution infrastructure in MS Windows, depending on how the TCP/IP environment is configured. This file is in every way the equivalent of the UNIX/Linux /etc/hosts file.
25.4.4 DNS Lookup
This capability is configured in the TCP/IP setup area in the network configuration facility. If enabled, an elaborate name resolution sequence is followed, the precise nature of which is dependant on how the NetBIOS Node Type parameter is configured. A Node Type of O means that NetBIOS broadcast (over UDP broadcast) is used if the name that is the subject of a name lookup is not found in the NetBIOS name cache. If that fails then DNS, HOSTS and LMHOSTS are checked. If set to Node Type 8, then a NetBIOS Unicast (over UDP Unicast) is sent to the WINS Server to obtain a lookup before DNS, HOSTS, LMHOSTS, or broadcast lookup is used.
25.4.5 WINS Lookup
A WINS (Windows Internet Name Server) service is the equivalent of the rfc1001/1002 specified NBNS (NetBIOS Name Server). A WINS server stores the names and IP addresses that are registered by a Windows client if the TCP/IP setup has been given at least one WINS Server IP Address.
To configure Samba to be a WINS server, the following parameter needs to be added to the smb.conf file:
wins support = Yes
To configure Samba to use a WINS server, the following parameters are needed in the smb.conf file:
wins support = No wins server = xxx.xxx.xxx.xxx
where xxx.xxx.xxx.xxx is the IP address of the WINS server.
For information about setting up Samba as a WINS server, read Chapter 9, Network Browsing .