On 2013/11/04 17:13, ToddAndMargo wrote:
On 11/04/2013 05:07 PM, Yasha Karant wrote:
On 11/04/2013 04:53 PM, ToddAndMargo wrote:
On 11/04/2013 04:21 PM, Yasha Karant wrote:
I need to do a media comparison between a data DVD and the .iso file
that purportedly contains the image of the exact DVD (including any
bootable or autoload binary files, not for an Intel instruction set
architecture).
When burning to the DVD, applications such as K3B and Nero (for Linux)
will do a verify of the burned media. My understanding is that these
applications go through the device driver and device controller
hardware/firmware that may be applying error correction to the raw bit
stream; any such detected "hardware media" errors typically are reported
by the driver to a log file, but typically (if corrected) do not cause
the application to fail.
If one mounts the .iso file, by a command similar to that below,
# mount -t iso9660 -o ro,loop=/dev/loop0 /files/dvdimage.iso
/media1/virtualdisc
and likewise has the physical DVD in the DVD drive and mounted from,
say, /dev/sr0
will a diff /dev/loop0 /dev/sr0 suffice?
Is there a utility that will do the same thing that Nero would do as it
verifies after burning, but not requiring the burn -- that is, verify a
DVD against an ISO image file?
If /dev/sr0 were mounted on, say, /media/someDVD, and the ISO image
file on
/media1/virtualdisk , is there a utility or script to do a "bit by bit"
comparison via the mount points (not just the "raw" mount as /dev/sr0 )?
Yasha Karant
Hi Yasha,
Check the DVD as a raw device.
After you burn the ISO, eject the DVD (clears out something,
I don't know what, but had to learn the hard way):
/usr/bin/eject /dev/sr0
Then inject the DVD (close the door). Can be on the same
line.
/usr/bin/eject -t /dev/sr0
Then make an MD5SUM of each
md5sum /files/dvdimage.iso /dev/sr0
Eyeball the sums. One will be on top of the other.
If you like, I have some leftover code I can send you.
-T
From http://en.wikipedia.org/wiki/Md5sum
As with all such hashing algorithms, there is theoretically an unlimited
number of files that will have any given MD5 hash. However, it is very
unlikely that any two non-identical files in the real world will have
the same MD5 hash, unless they have been specifically created to have
the same hash.
End quote.
I explain the above reality to my students, although I do use MD5SUM
myself. I was hoping for a utility that did a true bit-by-bit
comparison of the two files.
Aside: Note that a (very) clever attacker can embed specific issues
into a file such that the corrupted (and perhaps infected) file will
pass a MD5 hash test. Note that USA NSA and other entities often do
employ such clever persons (do recall the cyber attack on the fissile
material enrichment facilities of a Middle Eastern nation state not in
full agreement with USA foreign policy, albeit an attack not
specifically limited to this mechanism). I am not suggesting that the
DVD and ISO image file I am using are subject to this sort of clever
corruption; but, it is important to understand the limitations of
certain techniques.
Yasha Karant
Hi Yasha,
The likely hood is pretty low.
You could always try using the SHA sums. Maybe do
both.
-T
Or use the "cmp" command, "cmp -l infile outfile" if he really wants a
byte by byte comparison of the disks.
{o.o}
