HI all,

It transpires from another thread that there's more to the space before the the start of the first partition than just the MBR. In order to be sure of wiping out any viruses lurking in the pre-first partition space it turns out one would have to wipe not just the first 512 bytes, but the first 62 sectors:

One could then still boot the system from a rescue CD. I just thought I'd run this idea past the panel to see if there's any problem with it I haven't envisaged?

I think there's hiding room elsewhere too. Why not nuke the whole disk:

sudo dd if=/dev/random of=/dev/sda

if I have the syntax right...

*harmless;
He said he still wanted to boot from a rescue CD, so wiping the whole disk is not the right answer.

*Clueless;
63 sectors, not 62-----#0 is the MBR, 1st partition typically starts on #63 (Doesn't everyone count from zero?...)
So:
sudo dd if=/dev/zero of=/dev/sda bs=32256 count=1
OR
sudo dd if=/dev/zero of=/dev/sda bs=512 count=63

GRUB typically puts stage 1.5 and stage 2 after the MBR, so your boot method has to be compatible with all that being gone.

Why not wipe the area and then just re-install GRUB???

There's 63 empty sectors at the start of an extended, and all logical, partitions. Otherwise, where is there more hiding space?

Whoops, so he did, my mistake. Isn't there also some space at the end of the disk where metadata for software RAID sits?

I suppose so, but keep in mind that a RAID setup creates a "virtual device" which looks to the OS like a single physical drive. At that point, what is actually written to the physical drives is not relevant to the typical user.

That's the understatement of the decade!
I do like his sig, though. :-)

These aren't equivalent, are they? The first command nukes the first 63 sectors once; the second over-writes the MBR 63 times, n'est pas?

The first command writes 32256 bytes in one large chunk. The second command writes 32256 bytes in 512 byte chunks (63 of them).

EDIT:
That command would never finish, you would have to use '/dev/urandom', but I don't see why not just use /dev/zero ... unless you wanted to cover up the data, but then you'd still have to do it more than once.

Sigh... Oh well, I obviously need to look into "dd" a bit more.

So in that case, say one wanted to over-write the MBR 35 times with random data (the so-called Guttmann Method for secure wiping) what would the command be?

dd inside a loop?

Well clearly, yes. But ISTR there is a concise way of doing it just by appending an option to 'dd'. Dashed if I can remember what it is, though. :-/

No, there's no such option AFAIK, but why 35 times ? Whatever virus might have been there is gone after the first wipe. Multiple wipes are only done to make sure forensic tools used by agencies cannot recover any data even with the most sophisticated of technology.

It was only by way of an example, TM. Replace 35X with whatever factor >1 you please. I'm just curious as to what the syntax of the command would look like. However, if you say no such option for multiple overwrites exists with 'dd' then unless someone else knows better, I'm quite happy to accept your assertion. ;-)

man dd

dd is not very complicated, and I don't think you will find any ambiguity in the answer to your question.

dd may or may not be complicated, but the man pages for it (and for so many other commands) are an atrocity against the English language. Whoever writes such gibberish should be strung up.

BTW, you're up awfully early aren't you?

If you think you can write a better man page then do it and submit it, I'm sure they'll be happy to get a newer, better man page.