Microsoft TCP/IP Host Name Resolution Order

Host name resolution resolves the names of TCP/IP resources that do not connect through the NetBIOS interface. The most common example of this is a Web browser such as Microsoft Internet Explorer. Other examples include Internet applications such as Ping, FTP, and Telnet. Many modern database and mail applications that connect using Winsock, the Microsoft Windows implementation of TCP/IP sockets, also use host name resolution. Examples of these types of applications are Outlook and Exchange.

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

NOTE: In the context of this article, the term "client" does not necessarily refer to a workstation. A Windows NT server will take the role of client when it requires access to resources that require host name resolution.

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 either successfully resolves the name or exhausts these methods. Windows NT, Windows 95, and Windows for Workgroups clients using Microsoft TCP/IP 3.11b follow this sequence. Lan Manager 2.2c or Microsoft Client 3.0 clients will not use NetBIOS name resolution as a backup.

For additional information, please see the following articles in the Microsoft Knowledge Base:
When resolving names the client will skip methods for which it is not configured. For example, if there is no hosts file on the system, then it will skip step #2 above and try a query to a DNS server. If no DNS server IP addresses are entered in the client TCP/IP configuration, then the client will skip to the next step in the sequence after DNS.

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

For additional information, please see the following articles in the Microsoft Knowledge Base:

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 Host name resolution sequence listed above that the client should be using. If the name does not exist in any of the resources that the client uses, then you must decide to which resource to add it. If the name exists in one of the resources, such as a DNS server or a Windows Internet Name Service (WINS) server and the client is not resolving the name correctly, focus your attention on troubleshooting that specific resource.

Also, confirm that the client is trying to resolve a host name and not a NetBIOS name. Many applications have multiple methods that they can utilize to resolve names, this is especially true of mail and database applications. The application may be configured to connect to resources using NetBIOS. Depending on the client configuration the client may bypass host name resolution. From there it will be necessary to either change the connection type to TCP/IP sockets or to troubleshoot the problem as a NetBIOS 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 the server is not available to the client usually causes this. Because the TCP/IP protocol assumes an unreliable network, a client will repeatedly attempt to connect to a DNS server before abandoning the attempted query. The client will then attempt to query a second DNS server if one is configured and take the same time to fail. Only then will the client step through 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 the following white paper available on the Microsoft anonymous ftp server: