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LMHOSTS File Information and Predefined Keywords

This article was previously published under Q102725
The LMHOSTS file is a local text file that maps IP addresses to NetBIOSnames of remote servers with which you want to communicate over the TCP/IPprotocol. Windows recognizes names instead of IP addresses for networkrequests and a name discovery process is used to correctly route networkrequests with TCP/IP. Because the name discovery process is generally notrouted by an IP router, the LMHOSTS file allows Windows machines tocommunicate using TCP/IP across a subnet.

This article is divided into the following sections:
  • Comprehensive Information
  • LMHOSTS Entry Cache
  • Predefined Keywords
  • More Information on Block Inclusion

Comprehensive Information

  • LMHOSTS contains IP address to "NetBIOS over TCP/IP" name translations.
  • LMHOSTS is only used by the NBT (NetBIOS over TCP/IP) interface.
  • LMHOSTS file contains some valuable additions to the LAN Manager and Windows for Workgroups LMHOSTS file, such as the ability to support routed domain logon validation.
  • LMHOSTS contains static information about TCP/IP addresses, but using logon scripts and/or the replicator service, the "master" file can be distributed transparently across all stations.
  • By default, the LMHOSTS file should be located in the directory %SYSTEMROOT%\SYSTEM32\DRIVERS\ETC (usually C:\WINNT\SYSTEM32\DRIVERS\ETC).

LMHOSTS Entry Cache

By default, entries specified with the #PRE keyword are cached atinitialization up to a maximum of 100 entries. More initially cachedentries can be obtained modifying the following Registry value:
\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services \NetBT\Parameters\MaxPreload
The sequence used to determine NetBIOS to IP address resolution is thefollowing: the cache is verified, a broadcast to the subnet is performed,and then additional entries from LMHOSTS are checked. If a connection isnot established within 15 seconds, the TDI times-out and an error messageis displayed.

Predefined Keywords

All predefined keywords that are valid for Windows and Windows Advanced Server computers are not valid for LMHOSTS files on Microsoft LANManager or Microsoft Windows for Workgroups computers. These computerscan't be validated by routed domain controllers the same way Windows NTcomputers can. To be able to validate them, use the LAN Manager TCP/IPExtensions Service available on all OS/2 LAN Manager (version 2.1a orlater) servers.
# <remark line>
Used to remark IP address to NetBIOS names.
#PRE
Defines which entries should be initially preloaded into the name cache.
#DOM:<domain_name>
Distinguishes Windows Advanced Server servers from Windows workstations, a LAN Manager server, or a Windows for Workgroups computer. This keyword is needed for all servers that validate logon requests.
#NOFNR
Avoid Directed Name Queries (DNS). Some earlier versions of LAN Manager cannot handle NetBIOS DNS requests and return error 53. Specifying this flag on the same line will avoid using NetBIOS directed name queries.
#BEGIN_ALTERNATE
      #INCLUDE <\\server_name01\sharename\LMHOSTS>      #INCLUDE <\\server_name02\sharename\LMHOSTS>      ...   #END_ALTERNATE      Defines a redundant list of alternate locations for LMHOSTS files.      The recommended way to #INCLUDE remote files is using a UNC path,      to ensure access to the file. Of course, the UNC names must exist      in the LMHOSTS file with a proper IP address to NetBIOS name      translation.   #MH      Associates a single, unique NetBios computer name to an IP address.      You can create multiple entries for the same NetBios computer name      for each NIC in the multihomed device, up to a maximum of 25 different       IP addresses for the same name.		
#SG
Used to define a special group, such as printers or computers that belong to groups on the intranet for easy reference, browsing or broadcasting. Special groups are limited to a total of 25 members.
The first 3 keywords can be used together to specify 1 NetBIOS over TCP/IPname to IP address translation. For example:
   102.54.94.97 rhinodc #PRE  #DOM:rhino  #PDC for rhino domain    102.54.94.99 zoobdc  #PRE  #DOM:rhino  #BDC for rhino domain 
NOTE: PDC is primary domain controller and BDC is backup domain controller.

To start with the IP Address followed by the NetBIOS name, the predefinedentries can be defined in diverse order. It is recommended to insertimportant entries first, because the the file is accessed in sequentialorder.

More Information on Block Inclusion Keywords

Windows provides new block commands that permit centralized locationandmanagement of the LMHOSTS file. In addition, provisions are included topermit redundant backup copies on multiple servers. The following Windows LMHOSTS file block commands are discussed in detail below:
  • #INCLUDE
  • #BEGIN_INCLUDE
  • #END_INCLUDE
  • #BEGIN_ALTERNATE
  • #END_ALTERNATE
Block Inclusion Keywords (#BEGIN_INCLUDE, #END_INCLUDE):
   130.20.26.169  Trustedserver  #PRE   #DOM:Testing  # Test group domain		
   #BEGIN_INCLUDE   #INCLUDE \\localsrv\public\lmhosts       # Local domain   #INCLUDE \\Trustedserver\public\lmhosts  # Server in Testing domain   #END_INCLUDE		
In the above block inclusion, the specified LMHOSTS file is scanned underthe following conditions:
  • During TCP/IP initialization, the local LMHOSTS file is scanned for #PRE commands. It places all these entries in its name cache. In addition, all LMHOSTS files specified by the #INCLUDE command are scanned.
  • When resolving a computer name (NetBIOS name) to an IP address, the #INCLUDE file is also searched if the name cannot be found in the preloaded entries of the cache.
  • The LMHOSTS files are scanned to resolve the address of a domain controller of a domain that is not found in the preloaded cache name list.
The benefit of this command is that it allows system administrators toeasily maintain a centrally-located master LMHOSTS file that can beaccessed over the network by each node. This method also works acrosssubnets because of the use of UNC (universal naming convention) names andLMHOSTS mappings for these entries.

If the domain spans multiple subnets or if the domain controller for adomain is across a router, it is necessary to have an entry in the LMHOSTSfile for locating the domain controller for that server, with a #DOMprefix added (for our example, trustedserver is the domain controller for domainTESTING). See the IP line above the block inclusion. This line must appearbefore the #INCLUDE.

In the above case, the TESTING domain has a trust relationship with thelocal domain. This is important for servers included in the block that arenot in the local domain.

NOTE: The LMHOSTS files located on servers that are in a trusted domainshould have read privileges for the Everyone group, which includes all ofthe local users and the trusted domain users.

Alternate Block Inclusion (#BEGIN_ALTERNATE, #END_ALTERNATE):
   #BEGIN_ALTERNATE   #INCLUDE \\localsrv\public\lmhosts      # Local domain   #INCLUDE \\Trustedserver\public\lmhosts # Server in Testing Domain   #END_ALTERNATE		
In this case, the "Alternate Include" condition is satisfied if one of the#INCLUDE's is successful. This means that only one LMHOSTS file will bescanned. The first successful #INCLUDE is the only LMHOSTS file that willbe read.

This is helpful in keeping multiple copies of the same file on differentservers. If one of the servers is down, it will attempt to read theLMHOSTSfile from the alternate server identified in the block inclusion.

Additional information, including a full description of all thesefeatures, is available in the Windows Resource Kit.
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Article ID: 102725 - Last Review: 02/20/2007 01:53:00 - Revision: 3.2

  • Microsoft Windows 2000 Server
  • Microsoft Windows 2000 Advanced Server
  • Microsoft Windows 2000 Professional Edition
  • Microsoft Windows 2000 Datacenter Server
  • Microsoft Windows NT Advanced Server 3.1
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  • Microsoft Windows NT Server 3.51
  • Microsoft Windows NT Server 4.0 Standard Edition
  • Microsoft Windows NT Workstation 3.1
  • Microsoft Windows NT Workstation 3.5
  • Microsoft Windows NT Workstation 3.51
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Advanced Server 3.1
  • kbnetwork KB102725
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