Windows NT Contains File System Tunneling Capabilities

• Microsoft Windows NT Workstation version 4.0
• Microsoft Windows NT Server version 4.0

SUMMARY

Windows NT 4.0 contains file system tunneling capabilities to allow for compatibility with applications that rely on file systems being able to hold onto file meta-info for a short period of time. This occurs after deletion or renaming and re-introducing a new directory entry with that meta-info (if a create or rename occurs to cause a file of that name to appear again in a short period of time).

When a name is removed from a directory (rename or delete), its short/long name pair and creation time are saved in a cache, keyed by the name that was removed. When a name is added to a directory (rename or create), the cache is searched to see if there is information to restore. The cache is effective per instance of a directory. If a directory is deleted, the cache for it is removed.

These paired operations will cause tunneling on "name."

• delete(name)/create(name)
• delete(name)/rename(source, name)
• rename(name, newname)/create(name)
• rename(name, newname)/rename(source, name)

How to See the Effects of Tunneling

1. Create a file called file1.

2. Wait for one to two minutes.

3. Create a file called file2.

4. Perform a DIR /TC. Please note the creation times.

5. Rename file1 to file.

6. Rename file2 to file1.

7. Perform a DIR /TC. Please note the creation times are identical.

MORE INFORMATION

Tunneling cache time can be adjusted from the default time of 15 seconds or if tunneling capabilities are undesirable can be disabled by adding a value in the Windows NT Registry.

If tunneling is disabled, Apps that use this safe save method will lose the name they are unaware of, usually the LFN, and the rediscovery of shortcut targets could be impaired since the creation timestamps will not remain constant for files manipulated by such apps. Note that creation timestamp maintenance is possible in the absence of tunneling if an application is smart enough. The same is not true for the long filenames.

WARNING: Using Registry Editor incorrectly can cause serious, system-wide problems that may require you to reinstall Windows NT to correct them. Microsoft cannot guarantee that any problems resulting from the use of Registry Editor can be solved. Use this tool at your own risk.

1. Start Registry Editor (Regedt32.exe), and select the following subkey:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem

2. From the Edit menu, click Add Value.

3. Type the following value to reduce the default cache time:

         Value Name: MaximumTunnelEntryAgeInSeconds
         Data Type: REG_DWORD
         Value: 15  <-- change to fit your needs in seconds.
   

   Type the following value to disable tunneling:

      Value Name: MaximumTunnelEntries
      Data Type: REG_DWORD
      Value: 0