Re: How to check a set of Woody CD's for integrity

["D. Joe Anderson" <deejoe@raccoon.com>]

On Wed, Sep 18, 2002 at 12:53:39AM +0530, Raghavendra Bhat wrote:
> Vineet Kumar:
> 
> >> If it doesn't, then think about digging deeper to find out what differs
> 
> I have  had downloaded the  ISO images and  the checksums match.  A  small doubt
> though, I can burn CD-Rs using these  images without any hitch.  But when I do a
> `dd' of the resultant CD/s, the generated image md5sum differs from the original
> md5sum  of  the  downloaded  ISO.   Why  is  it so  ?   Is  it  due  to  control
> data/something else being added to the CD during the `cdrecord' burn process ?

The ISO images are rarely the exact size of the media on which you
burn them.  In order to use dd with md5sum on a CD burned from the
images in such a way to reproduce the md5sum of the image file, you
have to direct the dd program to read exactly the same number of
bytes from the CD as is in the image file.

It tends to work better for me if I read the bytes from the CD in
large chunks, I've found.  So, I'll use the bs= and count= arguments
to dd to do this.

bs x count = total number of bytes in image file

Using as an example this old ISO image I have sitting around:

$ ls -ld /opt/iso/Debian/debiani386-2.2-1.iso
-rw-r--r--    1 root     root     658933760 Oct  8  2000
/opt/iso/Debian/debiani386-2.2-1.iso

I calculate how many 1024byte blocks it is using 'dc':

$ echo " 4 k 65893376 1024 / p" | dc
64349.0000

So, having burned that image to a disk in my CD-RW drive, I can check
its md5sum this way:

$ dd if=/dev/scd0 count=64349 bs=1024 | md5sum

The result of this should be the same as running:

$ md5sum debiani386-2.2-1.iso

I tend to write the size of the iso image on my CD-R labels, along
with several digits of the md5 checksum, so that I can check their
integrity this way.

--Joe