Ubuntu Manpage: scrub - write patterns on disk/file

NAME

       scrub - write patterns on disk/file

SYNOPSIS

       scrub [OPTIONS] special-file
       scrub [OPTIONS] file
       scrub -X [OPTIONS] directory

DESCRIPTION

       Scrub  iteratively  writes  patterns  on files or disk devices to make retrieving the data
       more difficult.  Scrub operates in one of three modes:

       1) The special file corresponding to an entire disk is scrubbed and  all  data  on  it  is
       destroyed.   This  mode is selected if file is a character or block special file.  This is
       the most effective method.

       2) A regular file is scrubbed and only the data in the file (and optionally  its  name  in
       the  directory entry) is destroyed.  The file size is rounded up to fill out the last file
       system block.  This mode is selected if file is a regular file.  See CAVEATS below.

       3) directory is created and filled with files until the file  system  is  full,  then  the
       files are scrubbed as in 2). This mode is selected with the -X option.  See CAVEATS below.

OPTIONS

Scrub accepts the following options:

-v, --version
Print scrub version and exit.

-r, --remove
Remove the file after scrubbing.

-p, --pattern PATTERN
Select the patterns to write. See SCRUB METHODS below. The default, nnsa, is
reasonable for sanitizing modern PRML/EPRML encoded disk devices.

-b, --blocksize blocksize
Perform read(2) and write(2) calls using the specified blocksize (in bytes). K, M,
or G may be appended to the number to change the units to KiBytes, MiBytes, or
GiBytes, respectively. Default: 4M.

-f, --force
Scrub even if target contains signature indicating it has already been scrubbed.

-S, --no-signature
Do not write scrub signature. Later, scrub will not be able to ascertain if the
disk has already been scrubbed.

-X, --freespace
Create specified directory and fill it with files until write returns ENOSPC (file
system full), then scrub the files as usual. The size of each file can be set with
-s, otherwise it will be the maximum file size creatable given the user's file size
limit or 1g if unlimited.

-D, --dirent newname
After scrubbing the file, scrub its name in the directory entry, then rename it to
the new name. The scrub patterns used on the directory entry are constrained by
the operating system and thus are not compliant with cited standards.

-s, --device-size size
Override the device size (in bytes). Without this option, scrub determines media
capacity using OS-specific ioctl(2) calls. K, M, or G may be appended to the
number to change the units to KiBytes, MiBytes, or GiBytes, respectively.

-L, --no-link
If file is a symbolic link, do not scrub the link target. Do remove it, however,
if --remove is specified.

-R, --no-hwrand
Don't use a hardware random number generator even if one is available.

-t, --no-threads
Don't generate random data in parallel with I/O.

-h, --help
Print a summary of command line options on stderr.

SCRUB METHODS

       nnsa   4-pass  NNSA  Policy  Letter  NAP-14.1-C  (XVI-8) for sanitizing removable and non-
              removable hard disks, which requires overwriting all locations with a  pseudorandom
              pattern twice and then with a known pattern: random(x2), 0x00, verify.

       dod    4-pass  DoD 5220.22-M section 8-306 procedure (d) for sanitizing removable and non-
              removable rigid disks which requires overwriting all addressable locations  with  a
              character,  its  complement, a random character, then verify.  NOTE: scrub performs
              the random pass first to make verification easier: random, 0x00, 0xff, verify.

       bsi    9-pass  method  recommended  by  the  German  Center  of  Security  in  Information
              Technologies  (http://www.bsi.bund.de):  0xff,  0xfe, 0xfd, 0xfb, 0xf7, 0xef, 0xdf,
              0xbf, 0x7f.

       gutmann
              The canonical 35-pass sequence described in Gutmann's paper cited below.

       schneier
              7-pass method described by Bruce Schneier in "Applied Cryptography"  (1996):  0x00,
              0xff, random(x5)

       pfitzner7
              Roy Pfitzner's 7-random-pass method: random(x7).

       pfitzner33
              Roy Pfitzner's 33-random-pass method: random(x33).

       usarmy US  Army  AR380-19  method:  0x00,  0xff, random.  (Note: identical to DoD 522.22-M
              section 8-306 procedure (e) for sanitizing magnetic core memory).

       fillzero
              1-pass pattern: 0x00.

       fillff 1-pass pattern: 0xff.

       random 1-pass pattern: random(x1).

       random2
              2-pass pattern: random(x2).

       old    6-pass pre-version 1.7 scrub method: 0x00, 0xff, 0xaa, 0x00, 0x55, verify.

       fastold
              5-pass pattern: 0x00, 0xff, 0xaa, 0x55, verify.

       custom=string
              1-pass custom pattern.  String may contain C-style numerical escapes: \nnn  (octal)
              or \xnn (hex).

CAVEATS

Scrub may be insufficient to thwart heroic efforts to recover data in an appropriately
equipped lab. If you need this level of protection, physical destruction is your best
bet.

The effectiveness of scrubbing regular files through a file system will be limited by the
OS and file system. File systems that are known to be problematic are journaled, log
structured, copy-on-write, versioned, and network file systems. If in doubt, scrub the
raw disk device.

Scrubbing free blocks in a file system with the -X method is subject to the same caveats
as scrubbing regular files, and in addition, is only useful to the extent the file system
allows you to reallocate the target blocks as data blocks in a new file. If in doubt,
scrub the raw disk device.

On MacOS X HFS file system, scrub attempts to overwrite a file's resource fork if it
exists. Although MacOS X claims it will support additional named forks in the future,
scrub is only aware of the traditional data and resource forks.

scrub cannot access disk blocks that have been spared out by the disk controller. For
SATA/PATA drives, the ATA "security erase" command built into the drive controller can do
this. Similarly, the ATA "enhanced security erase" can erase data on track edges and
between tracks. The DOS utility HDDERASE from the UCSD Center for Magnetic Recording
Research can issue these commands, as can modern versions of Linux hdparm. Unfortunately,
the analogous SCSI command is optional according to T-10, and not widely implemented.

EXAMPLES

       To scrub a raw device /dev/sdf1 with default NNSA patterns:

              # scrub /dev/sdf1
              scrub: using NNSA NAP-14.1-C patterns
              scrub: please verify that device size below is correct!
              scrub: scrubbing /dev/sdf1 1995650048 bytes (~1GB)
              scrub: random  |................................................|
              scrub: random  |................................................|
              scrub: 0x00    |................................................|
              scrub: verify  |................................................|

       To scrub the file /tmp/scrubme with a sequence of 0xff 0xaa bytes:

              # scrub -p custom="\xff\xaa" /tmp/scrubme
              scrub: using Custom single-pass patterns
              scrub: scrubbing /tmp/scrubme 78319616 bytes (~74MB)
              scrub: 0xffaa  |................................................|

AUTHOR

       Jim Garlick <garlick@llnl.gov>

       This work was produced at  the  University  of  California,  Lawrence  Livermore  National
       Laboratory  under Contract No. W-7405-ENG-48 with the DOE.  Designated UCRL-CODE-2003-006,
       scrub is licensed under terms of the GNU General Public License.