Identifying and Troubleshooting Name Resolution Problems

     

Identifying and Troubleshooting Name Resolution Problems

Each machine on a computer network is assigned a unique network address. Computers communicate with each other across networks by connecting to these network addresses. These numbers , also known as IP addresses , are 12 digits long and difficult for people to remember. To solve this dilemma, a system was developed whereby people can use "friendly" names that are then translated into the network addresses the computers use to locate each other and to communicate. These names are called hostnames , and each machine is assigned one. Groups of hosts form a domain. The software that translates these names to network addresses is called the Domain Name System ( DNS ).

Domain Name Servers have been in use on the Internet for many years . Prior to DNS, HOSTS files were used for name resolution, but as the Internet quickly grew in size and popularity, maintaining HOSTS files and keeping them up to date became impossible . When the Internet community realized a need for a more manageable, scalable, and efficient name resolution system, DNS was created.

Prior to the introduction of Windows 2000, Network Basic Input/Output System (NetBIOS) names were used to identify computers, services, and other resources on Windows-based machines. In the early days of Windows networks, LMHOSTS files were used for NetBIOS name resolution. Later, these names were often resolved to Internet Protocol (IP) addresses using a NetBIOS Name Server (NBNS).

The following sections identify some of the common name resolution problems you may encounter and the tools you can use to troubleshoot name resolution problems.

Troubleshooting DNS

As already mentioned, DNS is required to resolve hostnames or fully qualified domain names (FQDNs) to IP addresses. DNS name resolution problems usually occur in the following two areas:

  • A user receives no response when attempting to resolve a hostname.

  • A successful response is received from a DNS server, but the information returned is incorrect.

One of the more common errors that you will encounter in regards to name resolution is a server not found error. This error indicates a problem with name resolution that can be related to incorrect parameters on the client or the DNS server not being online. If an error message of this type is reported (and you have eliminated the hardware and network connection as the source of the problem), verify that the user is configured with the correct IP address of a DNS server. You can do so through the properties of the Internet or LAN connection if static IP addressing is used. You can use the ipconfig command to check DNS settings when dynamic IP addressing is being used.

DNS can support both static and dynamic entries. An administrator can manually add resource records to the database, thereby creating static entries. Conversely, through Dynamic DNS, DNS clients can create and update their own resource records. This capability is particularly useful if dynamic IP addressing is used.

When dynamic IP addresses are assigned to computers, problems may occur if the DNS server stores static entries in the DNS database. The static entries contain the IP address and hostname of the computer. With this information, the DNS server answers name resolution requests for computers located on the network. Problems that can occur include the IP address of a computer being changed but the record in the DNS database not being updated or the DNS server having incorrect information within the DNS cache. If the DNS server has incorrect information, it causes name resolution errors to occur.

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You can update the DNS database with new and correct IP information by using the ipconfig /registerdns command.


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As simple as it sounds, you should verify that the user is referencing the correct domain name. A simple typing mistake can return a server not found error.


NSLOOKUP

One of the commands you can use to diagnose DNS problems on the network is nslookup . For example, you can use the command to query a specific DNS server to resolve the computer name registered for a given IP address. The following parameters are available with the command:

  • subcommands ” Allows you to specify additional subcommands to be used with nslookup .

  • Computertofind ” Queries the default DNS server for information about the computer specified. This can be an IP address or a computer name.

  • Server ” Specifies which DNS server to query.

If you issue the command and receive a No response from server error message, the DNS server is more than likely not online. If you receive a No records error message, the DNS server does not have a record for the computer specified in the query.

Netsh Diagnostics Commands

The Netsh Network Diagnostics command can be used to troubleshoot network services including DNS from the command line.

You can use the following commands to troubleshoot DNS:

  • ping dns ” Verifies connectivity with DNS servers configured through the TCP/IP Properties dialog box.

  • show dns ” Lists the DNS servers configured for a given interface. If no parameters are used, DNS servers configured for all adapters are displayed.

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The Netsh diag context is a new feature of Windows XP. It is not supported by previous versions of Windows.


Troubleshooting NetBIOS Name Resolution

Some clients and/or applications require NetBIOS names to communicate with other hosts on a network. A NetBIOS name is a 16-character name in which the first 15 characters identify a unique host, and the 16th character identifies a service or application running on the host such as the Workstation or Server service.

As with domain names, NetBIOS names must be resolved to an IP address before two hosts can communicate. Numerous methods are available for name resolution, and the method employed depends on the environment. For example, some networks may use Windows Internet Naming Service (WINS), whereas others may opt to rely on broadcasts for name resolution.

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If your network uses both NetBIOS and DNS, you may need to identify where name resolution is failing. If an application is generating name resolution errors, you should determine whether the application uses NetBIOS or DNS. If the application uses Window Sockets (for example, Internet Explorer and Telnet), the error is related to hostname resolution. Name resolution errors generated by applications using NetBIOS are obviously related to NetBIOS name resolution.


The most common error message you will probably encounter with NetBIOS name resolution is a computer not found message. Often this error may be the result of a user typing in the incorrect NetBIOS name. You can easily determine whether name resolution is failing by using the ping command. A successful ping to the computer's IP address will indicate that the computer is online and eliminate network issues as the source of the problem. How you troubleshoot the problem depends on the method of name resolution used on the network. For example, if a WINS server is being used, verify that an entry for the computer appears in the WINS database, or if an LMHOSTS file is used, verify that a mapping exists for the computer.

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You can use numerous methods to resolve NetBIOS names. Windows Internet Naming Service (WINS) is one of the most common. A WINS server is similar to a DNS server. It maintains dynamic database mappings for NetBIOS names and their corresponding IP addresses. An LMHOSTS file may be used; it is a static text file containing mappings for NetBIOS names and IP addresses. Alternatively, some networks may rely on broadcast messages for name resolution.


Nbtstat Command

The Nbtstat command can be used to troubleshoot NetBIOS-related problems. The output of the command can display NetBIOS over TCP/IP protocol statistics, the NetBIOS name tables for a local or remote computer, as well as the contents of the NetBIOS name cache. Table 7.2 summarizes the parameters available for use with the command.

Table 7.2. Nbtstat Command-Line Parameters

Parameter

Description

-a remotename

Displays the NetBIOS name table of the computer specified. The remotename portion is the NetBIOS name of the computer.

-A ipaddress

Displays the NetBIOS name table of the computer specified. The ipaddress portion is the IP address assigned to the computer.

-c

Displays the contents of the NetBIOS name cache.

-r

Displays NetBIOS name resolution statistics.

-R

Purges the contents of the NetBIOS name cache and reloads the #pre-tagged entries from the LMHOSTS file.

-RR

Releases and refreshes NetBIOS names for a local computer that are registered with a WINS server.


Troubleshooting Name Resolution and Applications

When troubleshooting name resolution within applications, you must watch out for certain issues. For example, when a hyperlink identifies a file on a mapped drive S, the user who is trying to access the link must have the same network drive mapped to the same drive letter. If the user has the wrong drive mapped to drive S, the link produces an error that the file is not found. All links within files should use the UNC path , so regardless of which drive mappings the user has on his system, he can locate the destination file.

Figure 7.4 displays the common error message that appears when a hyperlink uses an unavailable drive mapping.

Figure 7.4. Cannot open specified file error message.

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Hyperlinks are not the only problem. Shortcuts located on desktops or within folders should also use the UNC path to files. The same reasoning applies. If a drive is mapped to the wrong network drive or has been remapped to a different letter, it gen erates the same error shown in Figure 7.4.



MCDST 70-272 Exam Cram 2. Supporting Users & Troubleshooting Desktop Applications on a Windows XP Operating System (Exam Cram 2)
MCSA/MCSE 70-291 Exam Cram: Implementing, Managing, and Maintaining a Microsoft Windows Server 2003 Network Infrastructure (2nd Edition)
ISBN: 0789736187
EAN: 2147483647
Year: 2003
Pages: 119
Authors: Diana Huggins

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