21 Tbackup on a multiuser system

Contents of this section

Tbackup can only be run by the root user. This is done to avoid difficulties in accessing restricted resources like devices and home directories of other users.

21.1 User access while running tbackup.

You may want to prevent other users from using the system while tbackup is running, especially when it is backing up the user directories.

When a (user) directory is altered while being backed up, tbackup may get confused and fail to back up some of the files in the directory. Files that are missed in this way will also not get backed up in an incremental backup made some time later.

If preventing user access during a backup is unacceptable, you may want to show a warning message while the backup is being made, e.g.

                      ** WARNING **
        A backup of the user files is in progress.

-  Do not move or rename any of your files or directories,
   this may cause them not to be backed up.
-  Do not edit or change files, this may cause the contents of
   these files to be backed up incorrectly.
-  It is however safe to create new files.

See the wall(1) and motd(5) manual pages for information on showing messages.

21.2 Security and privacy considerations.

If other users can use the system while a backup is being made, they may interfere with the backup process if they can access the backup device. This can lead to a corrupted backup without you getting any error message.

Check the write permissions on your floppy and/or tape drive. While the backup is being made the backup device should only be accessible by the root user, i.e. have mode 600 (rw-------) or 660 (rw-rw---).

Tbackup automatically makes its working directories inaccessible to non-root users to prevent them from reading secret or private information from its temporary files.

You should ensure that index files produced by tbackup are unreadable for non-root users; a backup index can reveal the file names in private (non world readable) directories to other users.

21.3 Restoring backups you didn't make yourself.

In general you should only restore or list archives made by people you trust. Using an archive made by someone else can compromise your system in two ways:

  1. An archive can contain SUID programs or trojan horses that can be used by crackers to get root privileges when restored. Whether this is the case may be hard to detect, even if you have extensive UNIX knowledge.
  2. The `restore parameters' in the DESCR2K files on tbackup floppies are actually shell commands. These are executed by the floppy method when doing a trestore or tlist. Thus, restoring or listing an archive with a sabotaged DESCR2K file can have serious consequences.

Before executing the restore parameters trestore and tlist first print them on the screen. You are asked to

Press enter to use this backup set, ^C to abort.
If the restore parameters on an outside disk set look suspicious (you need to know shell programming to see this), press ctrl-C.

Next Chapter, Previous Chapter

Table of contents of this chapter, General table of contents

Top of the document, Beginning of this Chapter