Dd -
dd, sometimes called GNU dd, is the oldest disk imaging tool still used. Although it is functional and requires only minimal resources to run, it lacks some of the useful features found in more modern imagers such as metadata gathering, error correction, piecewise hashing, and a user-friendly interface. dd is a command line program that uses several obscure command line arguments to control the imaging process. Because some of these flags are similar and, if confused, can destroy the source media the examiner is trying to duplicate, users should be careful when running this program. The program generates raw image files which can be read by many other programs.
dd is part of the GNU Coreutils package which in turn has been ported to many operating systems.
There are a few forks of dd for forensic purposes including:
dcfldd
sdd
dd_rescue
ddrescue
dccidd, including a Microsoft Windows version that supports reading physical memory
Example
Here are two common dd command lines:
UNIX/Linux
dd if=/dev/hda of=mybigfile.img bs=65536 conv=noerror,sync
Windows
dd.exe if=\\.\PhysicalDrive0 of=d:\images\PhysicalDrive0.img --md5sum --verifymd5 --md5out=d:\images\PhysicalDrive0.img.md5
Tips
With linux in addition to
dd if=/dev/hda of=mybigfile.img bs=65536 conv=noerror,sync
You can wipe a drive with:
dd if=/dev/zero of=/dev/hda bs=4K conv=noerror,sync
For imaging a useful alternate invocation in Linux or UNIX is:
dd if=/dev/hda bs=4K conv=sync,noerror | tee mybigfile.img | md5sum > mybigfile.md5
The above alternate imaging command uses dd to read the harddrive being imaged and outputs the data to tee. tee saves a copy of the data as your image file and also outputs a copy of the data to md5sum. md5sum calculates the hash which gets saved in mybgifile.md5
For all of the above
if => input file /dev/hda => the linux name of a physical disk. Mac has their own names. /dev/zero => in linux, this is an infinite source of nulls of => output file mybigfile.img => The name of the image file you are creating bs => blocksize 65536 => 64K (I normally use 4K in linux. That is what the linux kernel uses as a page size.) noerror => don't die if you have a read error from the source drive sync => if there is an error, null fill the rest of the block.
In linux, the blocksize value can have a multiplicative suffix:
c =1 w =2 b =512 kB =1000, K =1024 MB =1000*1000, M =1024*1024 GB =1000*1000*1000, G =1024*1024*1024 and so on for T, P, E, Z, Y.
Things to know:
Having a bigger blocksize is more efficient, but if you use a 1MB block as an example and have a read error in the first sector, then dd will null fill the entire MB. Thus you should use as small a blocksize as feasible.
But with linux if you go below 4KB blocksize, you can hit really bad performance issues. It can be as much as 10x slower to use the default 512 byte block as it is to use a 4KB block.
Without noerror and sync, you basically don't have a forensic image. For forensic images they are mandatory.
dd by itself does not hash, that is why the alternate command is provided.
Cautions
Reversing Args can cause evidence erasure
Use extreme care when typing the command line for this program. Reversing the if and of flags will cause the computer to erase your evidence!
Use extreme caution if reading from a tape drive
At least with Linux/UNIX, tape drives have functional differences from disk that make them more complex to "image". Specifically they have EOF and EOT markings on the tape media that do not have a corresponding functionality with disks.
Most commercial backup software use EOF separators to allow a single tape to hold multiple backup sessions.
backup1-- EOF -- backup2 -- EOF -- backup3 -- EOT
A simple dd if=/dev/st0 of=image.dd will only preserve the first backup session.
For testing, from Linux you can create a multi-session backup tape via:
mt rewind -f /dev/st0 tar -cf /dev/nst0 /home tar -cf /dev/nst0 /srv
The nst device driver considers the closing of /dev/nst0 to signal the end of a tape file, so it appends a EOF mark after each invocation of tar.
So the tape would have:
home_tar_archive -- EOF -- srv_tar_archive -- EOF -- EOT
If you start reading from the start of the tape with either dd or tar, they will stop when the first EOF is hit and thus will only extract the home archive and will miss the srv archive.
See also
aimage
Blackbag
dc3dd
dcfldd
dd_rescue
ddrescue
mdd
Raw Image Format
External Links
LinuxJournal article about dd