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SIZE is the number of gigabytes and the file name is at the end. Random data is generated by encrypting /dev/zero, similar to other techniques posted on here.
I sometimes have use an usb stick to distribute files to several standalone "internet" pc's. I don't trust these machines period. The sticks I have do not have a write protection. So as a added security measure I fill the unused space on the (small) usb stick with a file with randomly generated bits. Any malware that tries to write to this stick will find no space on it.
Tested on slackware 14
Note: you may need root access to write to the device. This depends on your mount options.
This is to overcome the issue of slow I/O by reading once and forwarding the output to several processes (e. g. 3 in the given command). One could also invoke grep or other programs to work on read data.
This example is taken from Cygwin running on Win7Ent-64. Device names will vary by platform.
Both commands resulted in identical files per the output of md5sum, and ran in the same time down to the second (2m45s), less than 100ms apart. I timed the commands with 'time', which added before 'dd' or 'readom' gives execution times after the command completes. See 'man time' for more info...it can be found on any Unix or Linux newer than 1973. Yeah, that means everywhere.
readom is supposed to guarantee good reads, and does support flags for bypassing bad blocks where dd will either fail or hang.
readom's verbosity gave more interesting output than dd.
On Cygwin, my attempt with 'readom' from the first answer actually ended up reading my hard drive. Both attempts got to 5GB before I killed them, seeing as that is past any CD or standard DVD.
'bs=1M' says "read 1MB into RAM from source, then write that 1MB to output. I also tested 10MB, which shaved the time down to 2m42s.
'if=/dev/scd0' selects Cygwin's representation of the first CD-ROM drive.
'of=./filename.iso' simply means "create filename.iso in the current directory."
'-v' says "be a little noisy (verbose)." The man page implies more verbosity with more 'v's, e.g. -vvv.
dev='D:' in Cygwin explicitly specifies the D-drive. I tried other entries, like '/dev/scd0' and '2,0', but both read from my hard drive instead of the CD-ROM. I imagine my LUN-foo (2,0) was off for my system, but on Cygwin 'D:' sort of "cut to the chase" and did the job.
f='./filename.iso' specifies the output file.
speed=2 simply sets the speed at which the CD is read. I also tried 4, which ran the exact same 2m45s.
retries=8 simply means try reading a block up to 8 times before giving up. This is useful for damaged media (scratches, glue lines, etc.), allowing you to automatically "get everything that can be copied" so you at least have most of the data.
removes all files/filesystems of a harddisk. It removes EVERYTHING of your hard disk. Be carefull when to select a device. It does not prompt for and second check.
Your platform may not have pv by default. If you are using Homebew on OSX, simply 'brew install pv'.
For DVD: dd if=/dev/cdrom of=cd.iso
MBR is first 512B in partions.
The comp.unix.shell posting by St?phane Chazelas also lists the following offsets:
type 32768 (1 byte)
id 32769 (5 bytes)
version 32774 (1 byte)
system_id 32776 (32 bytes)
volume_id 32808 (32 bytes)
volume_space_size 32848 (8 bytes)
escape_sequences 32856 (32 bytes)
volume_set_size 32888 (4 bytes)
volume_sequence_number 32892 (4 bytes)
logical_block_size 32896 (4 bytes)
path_table_size 32900 (8 bytes)
type_l_path_table 32908 (4 bytes)
opt_type_l_path_table 32912 (4 bytes)
type_m_path_table 32916 (4 bytes)
opt_type_m_path_table 32920 (4 bytes)
root_directory_record 32924 (34 bytes)
volume_set_id 32958 (128 bytes)
publisher_id 33086 (128 bytes)
preparer_id 33214 (128 bytes)
application_id 33342 (128 bytes)
copyright_file_id 33470 (37 bytes)
abstract_file_id 33507 (37 bytes)
bibliographic_file_id 33544 (37 bytes)
creation_date 33581 (17 bytes)
modification_date 33598 (17 bytes)
expiration_date 33615 (17 bytes)
effective_date 33632 (17 bytes)
file_structure_version 33649 (1 byte)
application_data 33651 (512 bytes)
It will produce passwords with length of 20 printable characters within a reasonable time.
For shorter or longer passwords just change the 20 in bs=20 to something more convenient.
To create only alpha numeric passwords change [:print:] to [:alnum:]
This is a useful command to backup an sd card with relative total size for piping to pv with a progressbar
This is just a proof of concept: A FILE WHICH CAN AUTOMOUNT ITSELF through a SIMPLY ENCODED script. It takes advantage of the OFFSET option of mount, and uses it as a password (see that 9191? just change it to something similar, around 9k). It works fine, mounts, gets modified, updated, and can be moved by just copying it.
USAGE: SEE SAMPLE OUTPUT
The file is composed of three parts:
a) The legible script (about 242 bytes)
b) A random text fill to reach the OFFSET size (equals PASSWORD minus 242)
c) The actual filesystem
Logically, (a)+(b) = PASSWORD, that means OFFSET, and mount uses that option.
PLEASE NOTE: THIS IS NOT AN ENCRYPTED FILESYSTEM. To improve it, it can be mounted with a better encryption script and used with encfs or cryptfs. The idea was just to test the concept... with one line :)
It applies the original idea of http://www.commandlinefu.com/commands/view/7382/command-for-john-cons for encrypting the file.
The embedded bash script can be grown, of course, and the offset recalculation goes fine. I have my own version with bash --init-file to startup a bashrc with a well-defined environment, aliases, variables.
Blocksize (bs) is not mandatory. It's only needed when the count option is specified.
This example will close the pipe after transferring 100MB at a speed of 3MB per second.
An easy method to generate ISOs from CD/DVD media.
Write 200 blocks of 512k to a dummy file with dd, timing the result. The is useful as a quick test to compare the performance of different file systems.
dd can be used with /dev/zero to easily create a file of all zero-bytes. Pipe that through tr and use octal conversions to change the byte values from zero to 0xff (octal 0377). You can replace 0377 with the byte of your choice. You can also use \\0 and \\377 instead of the quoted version.
This version was mentioned in the comments. Credits go to flatcap.