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 Forum index » Off-Topic Area » Security
How to check Windows MBR code from Linux?
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B.K. Johnson

Joined: 12 Oct 2009
Posts: 78
PostPosted: Wed 29 Jun 2011, 01:51    Post subject:  How to check Windows MBR code from Linux?
Subject description: Checking for boot sector viruses		  
My computer's MBR may have been changed by a virus. I don't want to reboot under Windows until I verify that the MBR is OK. I know some Windows tools I could use to read the MBR and save the output to a file for review, but I want something I can run from puppy (Lucid-5.2.5). Can anyone recommend a Linux tool and create a PET that allows me to do the same?

I could just invoke ms-sys to write a new MBR but I need to know what is there.
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DPUP5520

Joined: 16 Feb 2011
Posts: 798
PostPosted: Wed 29 Jun 2011, 02:18    Post subject:  
use the file command
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nooby

Joined: 29 Jun 2008
Posts: 10522
Location: SwedenEurope
PostPosted: Wed 29 Jun 2011, 02:31    Post subject:  
DPUP5520 wrote:
use the file command


B.K. Johnson maybe know such but all of us that don't get what a Manual file says about the file command. Could you give a suggestion what gives a reasonable result?

file MBR or file mbr or what to write in terminal?
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rcrsn51


Joined: 05 Sep 2006
Posts: 8563
Location: Stratford, Ontario
PostPosted: Wed 29 Jun 2011, 04:04    Post subject:  
Code:
dd if=/dev/sda of=mbr.bin bs=512 count=1
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Bruce B


Joined: 18 May 2005
Posts: 11052
Location: The Peoples Republic of California
PostPosted: Wed 29 Jun 2011, 06:42    Post subject:  
nooby wrote:
DPUP5520 wrote:
use the file command


B.K. Johnson maybe know such but all of us that don't get what a Manual file says about the file command. Could you give a suggestion what gives a reasonable result?

file MBR or file mbr or what to write in terminal?


The first think with any external utility is to use the --help to get a brief summary.

As you need consult the man and info pages for more detail.

http://linux.die.net/man/1/file

Normally typing file foobar is all you need.

For a boot sector you can file /dev/sda for mbr on /dev/sda

I think you will get more details by making a file as shown above by rcrsn51 and then running file mbr.bin

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nooby

Joined: 29 Jun 2008
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Location: SwedenEurope
PostPosted: Wed 29 Jun 2011, 08:43    Post subject:  
Thanks. I guess me too chicken to try. DD is a dangerous thing to use?
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Sylvander

Joined: 15 Dec 2008
Posts: 3263
Location: West Lothian, Scotland, UK
PostPosted: Wed 29 Jun 2011, 09:04    Post subject:  
1. I used the code given by rcrsn51 to make the mbr.bin file in my /00 folder.

2. Here's what I did:
(a) Ran Xfe and navigated to the /00 folder.
(b) Ran a terminal window whilst working in the /00 folder.
(c) Copied and pasted the code...
Code:
dd if=/dev/sda of=mbr.bin bs=512 count=1

From the thread to the terminal command line, and hit <Enter>.
The file mbr.bin was created in the /00 folder.

3. Then in the same terminal window...
I ran the command...
Code:
file mbr.bin

And the following info was displayed:
mbr.bin: x86 boot sector, mbr; partition 2: ID=0xf, starthead 0, startsector 12289725, 7630875 sectors
Not really what I want. Sad

QUESTION:
4. How do I read the makeup of my MBR?
Using the mbr.bin file?
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rcrsn51


Joined: 05 Sep 2006
Posts: 8563
Location: Stratford, Ontario
PostPosted: Wed 29 Jun 2011, 09:26    Post subject:  
Sylvander wrote:
Not really what I want.

QUESTION:
4. How do I read the makeup of my MBR?
Using the mbr.bin file?

What do you want to know about it? The MBR is just a small block of binary code. There are web sites that will show you the standard code for various versions of Windows.
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Aitch


Joined: 04 Apr 2007
Posts: 6825
Location: Chatham, Kent, UK
PostPosted: Wed 29 Jun 2011, 09:42    Post subject:  
rcrsn51 wrote:
Code:
dd if=/dev/sda of=mbr.bin bs=512 count=1


For an explanation of rcrsn51's post see

http://www.miljan.org/main/2007/09/05/easy-way-to-read-mbr/

For an explanation of the mbr/bootloader process, see

http://oldfield.wattle.id.au/luv/boot.html

In my experience, merely repairing the mbr will restore a drive's functionality, provided the fat table is not corrupted....then you'll need more help

Also see M$

http://support.microsoft.com/default.aspx?scid=http://support.microsoft.com:80/support/kb/articles/Q69/0/13.ASP&NoWebContent=1

Aitch Smile
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Flash
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Joined: 04 May 2005
Posts: 10691
Location: Arizona USA
PostPosted: Wed 29 Jun 2011, 10:24    Post subject:  
B.K. Johnson, if all you really want to know is whether the MBR was compromised, maybe this would work: make a file of the existing MBR using rcrsn51's program, reinstall the MBR using Windows, then run rcrsn51's program again and compare the resulting file with the first file. That would show the difference between the two MBRs. If there is no difference, or only a few bits here and there, it seems unlikely that your MBR was compromised. Although I've read that malware written in machine language can be as small as only a few bytes. Twisted Evil
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Aitch


Joined: 04 Apr 2007
Posts: 6825
Location: Chatham, Kent, UK
PostPosted: Wed 29 Jun 2011, 10:50    Post subject:  
Quote:
Although I've read that malware written in machine language can be as small as only a few bytes. Twisted Evil


Just enough to point the boot process to a different address, eh?

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DPUP5520

Joined: 16 Feb 2011
Posts: 798
PostPosted: Wed 29 Jun 2011, 11:23    Post subject:  
Sorry I wasn't around earlier to explain myself but as Flash pointed out if it was a piece of malware written in machine code/assembly language it would be nearly impossible to detect without having an original mbr to compare it to.

Quote:
It has been said that machine code is so unreadable that the Copyright Office cannot even identify whether a particular encoded program is an original work of authorship.

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Sylvander

Joined: 15 Dec 2008
Posts: 3263
Location: West Lothian, Scotland, UK
PostPosted: Wed 29 Jun 2011, 11:25    Post subject:  
1. "What do you want to know about it?"
See the mbr code/contents displayed in this post by Paul Komski.
If someone were to be able to copy his mbr at 2 points in time...
[Before and after possible infection?]
And then display them both...
They might be able to detect changes in the code.
I'd like to explore a method of doing that, if it isn't too difficult.
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rcrsn51


Joined: 05 Sep 2006
Posts: 8563
Location: Stratford, Ontario
PostPosted: Wed 29 Jun 2011, 11:30    Post subject:  
Puppy has the "cmp" command for comparing two files.
Code:
cmp mbr.bin.old mbr.bin.new
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Sylvander

Joined: 15 Dec 2008
Posts: 3263
Location: West Lothian, Scotland, UK
PostPosted: Wed 29 Jun 2011, 11:34    Post subject:  
"Puppy has the "cmp" command for comparing two files."
Good, but it would be nice to be able to also view both...
"A picture is worth a thousand words"...
"Seeing is believing".
How to display them?
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