This article was previously published under Q217136
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IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base:
256986 Description of the Microsoft Windows Registry
After you apply Windows NT Service Pack 4 to the Windows NT DNS server, clients that try to access a UNIX computer receive the following error message:
To resolve this problem, obtain the latest service pack for Windows NT 4.0 or the individual software update. For information on obtaining the latest service pack, please go to:
152734 how to obtain the latest windows nt 4.0 service pack
For information on obtaining the individual software update, contact Microsoft Product Support Services. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web:
Microsoft has confirmed that this is a problem in Windows NT 4.0. This problem was first corrected in Windows NT version 4.0 Service Pack 5.
The UNIX computer can use Reverse Lookup against the Windows NT DNS server to identify and validate its clients. For additional information about Reverse Lookup, please see the following article(s) in the Microsoft Knowledge Base:
Windows NT SP3 Dns.exe maintained mixed case host names. The list of names on the UNIX machine that is used to validate clients against was held using mixed case names also. Doing a Reverse Lookup, the names returned from the DNS server matched the names on the UNIX side and the clients were granted access.
Windows NT SP4 Dns.exe, however, converts all names to lowercase. As lowercase names are returned upon Reverse Lookup requests, the names returned do not match the names on the UNIX side and, therefore, "access denied" is returned.
To resolve this problem, a new Dns.exe now has a registry entry that makes uppercase and lowercase handling configurable.
WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.
This has been resolved because of compatibility reasons. Please note, however, that RFC 952 describes that there should be no code and functionality depending on uppercase or lowercase of a DNS response. Therefore, maintaining a mixed case host list is not according RFC.