Re: [dm-crypt] yet another header reconstruction question

[Arno Wagner <arno@wagner.name>]

On Tue, Feb 16, 2016 at 12:32:06 CET, Florian Dotzer wrote:
> sent again as text. sorry for HTML input. hopefully it is more readable now . 

fine now.
 
> Dear Readers of the List .
>  
> I had a encrypted RAID 5 on my QNAP Device  in /dev/md0.
>  
> It worked without any problems for about 6 years . 
> But space went low and desaster began.
> After adding a disk , mdadm had overwritten the 
> header (block device /dev/md0 ) like this :
>  
> 6d 64 61 64 6d 3a 20 61 64 64 65 64 20 2f 64 65  mdadm: added /de
> 76 2f 73 64 63 33 0a 00 00 00 00 00 00 00 00 00  v/sdc3..........
> 00 00 00 00 00 00 00 00 63 62 63 2d 70 6c 61 69  ........cbc-plai
> 6e 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  n...............
> 00 00 00 00 00 00 00 00 73 68 61 31 00 00 00 00  ........sha1....
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 00 00 00 00 00 00 00 00 00 00 08 08 00 00 00 20  ...............
> bc 31 80 6d da a4 f0 5c ed 9f 24 96 fc 1b 72 6a

What did you do? 
Did you pipe the output from mdadm into /dev/md0?
If so, there might be a chance to recover this.

> According to the documentation on-disk-format ,
> magic (LUKS 0xba 0xbe )  , version and cipher name 
> seem to be overwritten.
>  
> I'd like to know the possible versions ( 00 01 ? ) and cipher names 
> (aes ?) in order to be able to reconstruct the header (fingers crossed )

First, make that header backup now, if you have not already.

The data that is missing depends on the QNAP device. 
Defaults can be changed on compile. Easiest option is 
likely to make a new LUKS container on it and try what 
is in there. You should be able to use the loop-device
procedure from FAQ Item 2.6 from the commandline for 
that. If those values do not work, next option is to
have the QNAP create a LUKS container on a new disk
and see what it does.
  
Regards,
Arno

-- 
Arno Wagner,     Dr. sc. techn., Dipl. Inform.,    Email: arno@wagner.name
GnuPG: ID: CB5D9718  FP: 12D6 C03B 1B30 33BB 13CF  B774 E35C 5FA1 CB5D 9718
----
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