Chance of equal checksum and changing blocks

David de Lama david.delama at
Tue Jan 27 10:03:30 GMT 2009

>Here's an example.  Suppose the block size is 900 bytes (i.e., you
>either specified --block-size=900 or saw blength=900 in the output).
>Then you could use the following command to overwrite blocks 40 through
>45 of the file (counting from 0) with zeros:
>dd bs=900 if=/dev/zero of=/home/ seek=40 count=6
>To overwrite with random data, change the if=/dev/zero to
>if=/dev/urandom .

Hi Matt and thanks for the fast answer!

I testet the command to overwrite the desired amount of blocks but it didn't work!
I created a 100MB file with:
dd if=/dev/zero of=/home/test/ bs=1M count=100

Then I wanted to overwrite 25 blocks:
dd bs=1M if=/dev/urandom of=/home/ seek=50 count=25

But when I look at the file, its size is now 76.8MB! So all the Blocks after block 75 are deleted! :(
Need help, please!


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