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Re: How to validate Debian woody CDs?



On Thu, Nov 28, 2002 at 01:51:02PM +0100, Jochen Voss wrote:
> Is this a good way to check wheter the CDs are official Debian CDs?

Yes! Except md5sum should be called differently, see below.

> The result was that the checksums did match for CD #1, but did NOT
> match for the remaining CDs. When she asked the dealer he told her,
> that this does not indicate a problem, but could be caused by
> "additional bits at the beginning or end of the CD". Does this make
> sense?

It does, unfortunately ("at the end", at least).

According to the mkisofs docs, many OSs break if the length of a CD is
not a multiple of the OS page size, or something like that. To avoid
this problem, mkisofs adds a few sectors of zeroes to the end of the
CD image. These zeroes are included in the md5sum.

However, when writing the image to CD, I think some programs add yet
more padding to the end of the written data, "just to be safe". 
cdrecord needs an explicit "-pad" to do this, but other programs may
add the padding by default.

But it is possible to pass the correct data to md5sum:

First, mount the CD. Just working on the unmounted device doesn't
always seem to work. Then, try one of two things:

  cat /dev/cdrom | md5sum

Apparently, this is preferable to "md5sum /dev/cdrom" because cat does
a better job of really reading all the available data - YMMV. I'm not
sure whether the above will work if more padding was added e.g. by
cdrecord, so better try the alternative:

  dd if=/dev/cdrom bs=1k count=##### | md5sum

where the ##### is the "1k-blocks" value from the output of
"df /cdrom".

HTH,

  Richard

-- 
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  |_) /|  Richard Atterer     |  CS student at the Technische  |  GnuPG key:
  | \/¯|  http://atterer.net  |  Universität München, Germany  |  0x888354F7
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