scrub(1): patterns on disk/file

NAME

SYNOPSIS

scrub [-f] [-p nnsa|dod|bsi] [-X] [-D newname] [-s size] file

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) file is created, expanded until the file system is full, then scrubbed as in 2). This mode is selected with the -X option. See CAVEATS below.

Scrub accepts the following options:

-p nnsa|dod|bsi|old|fastold: Select the patterns to write. nnsa selects patterns compliant with NNSA Policy Letter NAP-14.x; dod selects patterns compliant with DoD 5220.22-M; bsi selects patterns recommended by the German Center of Security in Information Technologies ( selects pre-version 1.7 scrub patterns; and fastold is old without the random pass. See STANDARDS below for more detail. Default: nnsa.
-b blocksize: Perform read(2) and write(2) calls using the specified blocksize (in bytes). Default: 8192.
-f: Scrub even if target contains signature indicating it has already been scrubbed.
-S: Do not write scrub signature. Scrub will not be able to ascertain if the disk has already been scrubbed.
-X: Create specified regular file and keep appending to it until write returns ENOSPC (file system full), then scrub it as usual.
-D 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 size: Override the device size (in bytes). Without this option, scrub determines media capacity using OS-specific ioctl(2) calls, which may not be accurate. K, M, or G may be appended to the number to change the units to KiBytes, MiBytes, or GiBytes, respectively.

CAVEATS

Scrub may be insufficient to thwart heroic efforts to recover data in an appropriately equipped lab.

Scrub nnsa patterns are reasonable for sanitizing modern PRML/EPRML encoded disk devices. For older MFM/RLL encoded drives, the shred(1) program from GNU binutils does a more thorough and scientific (but more I/O intensive) job of secure deletion, as described in Guttman below.

Scrub makes no attempt to disable write caching on the disk device or deal with spare blocks. RAID and other more sophisticated storage devices may require special handling.

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.

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

STANDARDS

The dod scrub sequence is compliant with the DoD 5220.22-M procedure for sanitizing removeable and non-removeable rigid disks which requires overwriting all addressable locations with a character, its complement, then a random character, and verify. Please refer to the DoD document for additional constraints.

The nnsa (default) scrub sequence is compliant with a Dec. 2005 draft of NNSA Policy Letter NAP-14.x (see reference below) for sanitizing removable and non-removable hard disks, which requires overwriting all locations with a pseudorandom pattern twice and then with a known pattern. Please refer to the NNSA document for additional constraints.

Please consult local authorities regarding your site policy for disk sanitization.

AUTHOR

Jim Garlick <garlick@llnl.gov>