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Microsoft TCP/IP Host Name Resolution Order

This article was previously published under Q172218
SUMMARY
This article discusses the different methods of host name to IP addressresolutions used by Microsoft Windows clients. The sequence of methodsis different than the sequence used to resolve NetBIOS names to IPaddresses.
MORE INFORMATION
On a network using the TCP/IP protocol, it is necessary to convert namesof resources to IP addresses to connect to these resources. MicrosoftWindows clients will follow a sequence of methods in attempting to resolvea name to an address, stopping the search when it successfully matches aname to an IP address.

There are two main sequences used in almost all cases: NetBIOS resolutionand Host name resolution. Clients connecting to resources on Microsoftservers, typically through Windows File Manager or Network Neighborhood,most often use NetBIOS name resolution.

For additional information, please see the following article in theMicrosoft Knowledge Base:
119493 NetBIOS over TCP/IP Name Resolution and WINS
Host name resolution resolves the names of TCP/IP resources that do notconnect through the NetBIOS interface. The most common example of this isa Web browser such as Microsoft Internet Explorer. Other examples includeInternet applications such as Ping, FTP, and Telnet. Many modern databaseand mail applications that connect using Winsock, the Microsoft Windowsimplementation of TCP/IP sockets, also use host name resolution. Examplesof these types of applications are Outlook and Exchange.

When troubleshooting name resolution issues, it is important to narrowdown whether the application is resolving a NetBIOS name or a host name.

NOTE: In the context of this article, the term "client" does notnecessarily refer to a workstation. A Windows NT server will take the roleof client when it requires access to resources that require host nameresolution.

Host name resolution generally uses the following sequence:

  1. The client checks to see if the name queried is its own.
  2. The client then searches a local Hosts file, a list of IP address and names stored on the local computer.

    NOTE: The Hosts file location depends on the operating system:
       Windows NT                  %Systemroot%\System32\Drivers\Etc   Windows 95                  <drive>\<Windows folder>   Windows for Workgroups      <drive>\<Windows folder>   Windows 3.1                 <drive>\<Windows folder>   MS-Client 3.0               <Boot volume>\Net   Lan Manager 2.2c Client     <Boot volume>\Net						
    Where %Systemroot% is the folder in which Windows NT is installed, <drive> is the drive on which the OS is installed, and <boot volume> refers to a boot floppy disk or drive C.

    A sample hosts file, Hosts.sam, is installed with the TCP/IP protocol showing the proper format.
  3. Domain Name System (DNS) servers are queried.
  4. If the name is still not resolved, NetBIOS name resolution sequence is used as a backup. This order can be changed by configuring the NetBIOS node type of the client.
The Windows client will try each of these methods until it eithersuccessfully resolves the name or exhausts these methods. Windows NT,Windows 95, and Windows for Workgroups clients using Microsoft TCP/IP3.11b follow this sequence. Lan Manager 2.2c or Microsoft Client 3.0clients will not use NetBIOS name resolution as a backup.

For additional information, please see the following articles in theMicrosoft Knowledge Base:
169141 NetBIOS and hostname resolution for MS-Client and LM 2.2c
When resolving names the client will skip methods for which it is notconfigured. For example, if there is no hosts file on the system, then itwill skip step #2 above and try a query to a DNS server. If no DNS serverIP addresses are entered in the client TCP/IP configuration, then theclient will skip to the next step in the sequence after DNS.

The method for changing host name resolution order differs among operatingsystems and versions. These are documented in the Resource Kits for thespecific operating systems, as well as in the Microsoft Knowledge Base.

For additional information, please see the following articles in theMicrosoft Knowledge Base:
171567 Windows NT 4.0 ServiceProvider Priority Values Not Applied
139270 How to Change Name Resolution Order on Windows 95 and Windows NT
119372 Setting the Name Resolution Search Order for TCP/IP-32

Troubleshooting

Problem: Client is unable to resolve a host name.

Troubleshooting steps:

If a client cannot resolve a host name, then it is best to verify the Hostname resolution sequence listed above that the client should be using. Ifthe name does not exist in any of the resources that the client uses, thenyou must decide to which resource to add it. If the name exists in one ofthe resources, such as a DNS server or a Windows Internet Name Service(WINS) server and the client is not resolving the name correctly, focusyour attention on troubleshooting that specific resource.

Also, confirm that the client is trying to resolve a host name and not aNetBIOS name. Many applications have multiple methods that they canutilize to resolve names, this is especially true of mail and databaseapplications. The application may be configured to connect to resourcesusing NetBIOS. Depending on the client configuration the client may bypasshost name resolution. From there it will be necessary to either change theconnection type to TCP/IP sockets or to troubleshoot the problem as aNetBIOS issue.

Problem: Client resolves a name very slowly, or fails to resolve a name and takes a long time to report a failure.

Troubleshooting steps:

Having DNS servers configured in a client's TCP/IP configuration, but theserver is not available to the client usually causes this. Because theTCP/IP protocol assumes an unreliable network, a client will repeatedlyattempt to connect to a DNS server before abandoning the attempted query.The client will then attempt to query a second DNS server if one isconfigured and take the same time to fail. Only then will the client stepthrough to NetBIOS name resolution as described above.

There are three ways to approach this issue.

  • If the host name is correctly entered in a host file, it will be resolved before the client attempts to query DNS. This solution works well if DNS servers are temporarily unreachable and there is a small number of host names that need to be resolved . Manually configuring Hosts files for numerous clients may be prohibitive. -or-

  • If DNS servers are available, but the DNS Server addresses in the clients TCP/IP configuration are incorrect, then correcting these addresses will allow the clients to contact the DNS servers immediately. Even if a DNS server reports that it cannot resolve a name, this will happen much faster than if the client cannot reach a DNS server at all. -or-

  • If DNS servers are configured on the client, but these servers are permanently unavailable, then remove the IP addresses of the DNS servers from the client configuration. The client will then bypass the DNS lookup without delay. -or-

  • If records in the DNS database are missing or incorrect, then there will be a delay as DNS servers query other DNS servers before reporting that they cannot resolve the name. This will usually cause a delay of just a few seconds.
For additional information on TCP/IP and name resolution, please see thefollowing white paper available on the Microsoft anonymous ftp server:
File Name: Tcpipimp2.doc
Location : ftp://ftp.microsoft.com/bussys/winnt/winnt-docs/papers/
Title : "Microsoft Windows NT 3.5/3.51/4.0: TCP/IP Implementation Details TCP/IP Protocol Stack and Services, Version 2.0. "
Properties

Article ID: 172218 - Last Review: 09/23/2011 22:04:00 - Revision: 5.0

Microsoft Windows NT Server 3.51, Microsoft Windows NT Server 4.0 Standard Edition, Microsoft Windows NT Workstation 3.5, Microsoft Windows NT Workstation 3.51, Microsoft Windows NT Workstation 4.0 Developer Edition, Microsoft Windows 95

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