Some surprise while "dd-ing" a video DVD...

[Phillip Susi]

Noah Dain wrote:
> Also note that this process is NOT good for attempting to get an image
> of a damaged cd/dvd.  As soon as dd hits an error, it still exits. 
> But, there are arguments to use to alter this behaviour:
> 
> basic recovery syntax:
>   dd if=/dev/hdc of=file.txt conv=noerror,sync
> 
>   conv=noerror,sync - do not fail on errors and write zeros to
> destination sectors with cannot be read
> 


Of course, this will result in a partially broken image because the 
parts that failed to read will be filled with zeros not the correct data.


> I don't know what this has to do with udf as all the dvd's i've seen
> are iso9660 filesystem, or at least compatible with it.  The linux
> kernel hasn't even had udf support for all that long now, and write
> support is still experimental.  I know udf is used for packet writing,
> where an optical disk is used read and write like one would a normal
> harddisk.  I don't too much about the specifics, but this guy seems to
> have done his homework:
> http://www.mrichter.com/cdr/primer/udf.htm
> 


All commercial video DVDs use UDF, not iso9660.  The linux kernel has 
supported UDF for several years, which is as long as it has supported 
DVDs.

As for packet writing, I'm actually in the process right now of 
improving the udftools package and fixing some bugs in the kernel to get 
support for this fully integrated into ubuntu in a plug and play way. 
See https://wiki.ubuntu.com/PacketCD


> As far as encryption on dvds go, the dvd itself isn't encrypted, just
> the .vob files on it.  So, if you can't see the VIDEO_TS directory
> with .vob (mpeg) files in it, then the problem is not with normal dvd
> encryption.
> 

Actually, the DVD itelf _is_ encrypted, just not the whole thing.  The 
sectors that contain the vob files are encrypted at the hardware level, 
which is why dd can not read them.  The drive itself performs the 
decryption of those sectors once it has been unlocked.  If the data file 
itself were encrypted, then you would still be able to read and copy the 
file even though you could not understand it's contents.