Virtualizing complex Linux systems

No Partition Table?

The reason fdisk reports a missing partition table is that the RAID controller shifts the Master Boot Record (MBR) to a currently unknown sector somewhere at the back of the disk. The good news is that you can find the MBR with forensic tools and some basic skills.

You can find an explanation of the MBR structure online [5]; in this case, all you need to know is that all popular systems have one thing in common by definition: Each MBR ends with a signature of 0x55 and 0xAA, or, in a null-indexed view, bytes 510 and 511 (Figure  4).

© © WikipediaFigure 4: Each MBR ends with the signature 0x55 and 0xAA, which makes it easy to find.

Luckily, The Sleuth Kit has exactly the right command for this: sigfind (Listing 1).

Listing 1

sigfind

 

In most cases, sigfind gives you enough information to load the partitions and fire up the RAID array. One thing is already clear; this is not a mirrored RAID, or it would have stored the same offsets on both disks. The partition table is 1024 bytes farther back than normal, as the mmls command shows (Listing 2). Remember, then, that the actual partition offset is 1024 plus 2048 bytes.

Listing 2

mmls -o

 

No MBR on the Second Disk

Unfortunately, the second disk has no MBR at all – in other words, this really is a RAID 0, which will not be easy to reconstruct. Without any information on the second disk, the only option an admin has is to use a system with an internal controller (as shown in Figure 1) to create an image from the 160GB disk. Using ewfacquire, you can create, for example, a help.E01 file:

-rw-r--r--  1 root root  443976036 Jun 21 15:09 image_source1.E01

EWF uses very efficient compression methods, but you can only squeeze a 160GB disk into 444MB if it is more or less empty.

Analyzing the System Disk

To retrieve more information about the RAID array and access the EWF image, the next step is to investigate the disk on the Linux system. Figure 1 shows the sda1 partition on the 40GB disk sda, which you will now look at more closely. On the evaluation system, the disk will then become sdi. The command

fls -o 2048 /dev/sdi

tells you more (Figure 5): Yes, this disk really does contain a Linux system.

Figure 5: The directory tree. The fls command output leaves no doubt that this is a Linux disk.

To determine the mountpoint of the RAID disks, you then need to read /etc/fstab (Listing 3). The plus signs show the directory depth – that is, the number of directories in the path to the result in question. Line  1 only has one plus sign at the start, and thus a directory depth of 1, so you can assume /etc/fstab resides here. The command

Listing 3

fls -r -o

 

icat -o 2048 /dev/sdi 130830

shows the content (see Figure 6): /dev/sdb1 with the XFS filesystem is mounted in /var on the original system, at least if you believe the comments.

Figure 6: Content of node 130830 output by icat: Behold, this is the /etc/fstab file, which will help in understanding the RAID array.

If you don't want to take this at face value, you can check the symlink targets on the system below /dev/disk/by-uuid/ and check which UUID is assigned to which partition.

For virtualization purposes, you still need to merge the 40GB disk and the two 80GB disks from the RAID 0 array. In order to do this, you need to sync the content of the two images to create one .vdi disk using Rsync. Because the RAID array was almost empty, you only need to assign an empty disk with 20GB capacity to the new VirtualBox instance. Now you have five steps to complete:

  • Partitioning and formatting
  • Transferring the files from the 40GB hard disk to / with Rsync
  • Transferring the files from the 160GB RAID disk to /var
  • Making the system bootable; that is, creating an MBR
  • Modifying /etc/fstab and /boot/grub/grub.cfg and making them bootable

After working your way through this list, you should have a new system with the data from the old machine, and you should be able to look forward to a successful boot.

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