Microsoft KB Archive/246322

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System Administration of Backups, Tar, and Pax in Interix

Article ID: 246322

Article Last Modified on 11/1/2006


  • Microsoft Interix 2.2 Standard Edition
  • Microsoft Interix 2.2 Standard Edition
  • Microsoft Interix 2.2 Standard Edition
  • Microsoft Interix 2.2 Standard Edition

This article was previously published under Q246322


This article describes the system administration of backups, tar, and pax in Interix.


Traditionally, tape backups are made by using tape archive (tar) and cpio. Although it may appear to be beneficial to continue using tar or portable archive exchange (pax) on an Interix system, it is not recommended. Instead, use a 32-bit Windows backup tool. A reason for this is that the Microsoft Windows NT security system uses 32-bit user IDs. The tar file format as specified by Portable Operating System Interface for UNIX (POSIX) uses a 16-bit user ID. The Interix user ID is generated from the Windows NT Security ID (SID in Windows NT documentation).

As a side effect, the Interix user ID is not identical to the Windows NT security ID. The Windows NT security ID for a user is unique everywhere, but a POSIX user ID is unique only within the network of a trusted domain.

From the tar man page:

Sometimes when creating or writing to an archive, the following message is printed to standard error:

uid nnnnnn too large to fit in header: substituting 32666

On traditional computers, the user ID and group ID numbers fit into 16 bits, but on Interix, they can be 32 bits long. The Interix user IDs and group IDs don't fit into the tar or USTAR archive formats. This implementation substitutes the value 32666 (decimal) for the original user ID or group ID.

As a consequence, you can receive error messages when you unpack tar archives you created because they do not contain your user ID.

You may consider using the pax program as a partial solution, because the USTAR format created by pax permits the archive to store the string that represents the ID numbers. But neither tar or pax back up the access control lists (ACLs) associated with 32-bit Windows files. Because they have not been backed up, they cannot be restored. 32-bit Windows files will not be properly backed up or restored. Using pax is only a solution for files restricted to the Interix environment.

A better solution is to use a 32-bit Windows backup program. It will store all of the ACLs associated with all of the files, and do so from a single point of control.

For additional information, see the following Interix Web site:

Additional query words: 2.2.10 2.2.20 2.2.30 2.2.40

Keywords: kbinfo KB246322