scrub(1)scrub(1)NAMEscrub - write patterns on disk/file
SYNOPSISscrub [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 overwrit‐
ing 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 overwrit‐
ing all addressable locations with a character, its complement,
a random character, then verify. NOTE: scrub performs the ran‐
dom 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 Cryptogra‐
phy" (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 mag‐
netic 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 tar‐
get 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 tradi‐
tional 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 Liver‐
more 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.
SEE ALSO
DoD 5220.22-M, "National Industrial Security Program Operating Manual",
Chapter 8, 01/1995.
NNSA Policy Letter: NAP-14.1-C, "Clearing, Sanitizing, and Destroying
Information System Storage Media, Memory Devices, and other Related
Hardware", 05-02-08, page XVI-8.
"Secure Deletion of Data from Magnetic and Solid-State Memory", by
Peter Gutmann, Sixth USENIX Security Symposium, San Jose, CA, July
22-25, 1996.
"Gutmann Method", Wikipedia, http://en.wikipedia.org/wiki/Gut‐
mann_method.
Darik's boot and Nuke FAQ: http://dban.sourceforge.net/faq/index.html
"Tutorial on Disk Drive Data Sanitization", by Gordon Hugues and Tom
Coughlin,
http://cmrr.ucsd.edu/people/Hughes/DataSanitizationTutorial.pdf.
"Guidelines for Media Sanitization", NIST special publication 800-88,
Kissel et al, September, 2006.
shred(1), hdparm(8)scrub(1)
