Use hivex to unpack a Windows Boot Configuration Data (BCD) hive

Thanks to “TJ” for tipping me off about another use of the Registry “hive” format in recent versions of Windows.

There are scant details available, but if you have a version of Windows Vista or later, then the boot loader is no longer configured through a plain text file (“BOOT.INI”) but via a binary blob. Microsoft provides a tool called “BCDEDIT.EXE” that you are supposed to use to edit this, but the blob is a hive so you can use hivex to display or modify it.

We first use guestfish to download the blob:

$ guestfish --ro -a /dev/vg_trick/Windows7x64 -m /dev/sda1

Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.

Type: 'help' for help with commands
      'quit' to quit the shell

><fs> ll /
total 392
drwxrwxrwx  1 root root   4096 Dec 15 04:48 .
dr-xr-xr-x 20 root root      0 Mar 30 13:30 ..
-rwxrwxrwx  1 root root   8192 Dec 15 12:47 BOOTSECT.BAK
drwxrwxrwx  1 root root   4096 Dec 15 12:47 Boot
drwxrwxrwx  1 root root      0 Dec 15 04:48 System Volume Information
-rwxrwxrwx  1 root root 383562 Jul 13  2009 bootmgr
><fs> ll /Boot/
total 596
drwxrwxrwx 1 root root   4096 Dec 15 12:47 .
drwxrwxrwx 1 root root   4096 Dec 15 04:48 ..
-rwxrwxrwx 1 root root  24576 Mar 25 12:25 BCD
-rwxrwxrwx 1 root root  21504 Mar 25 12:25 BCD.LOG
-rwxrwxrwx 2 root root      0 Dec 15 12:47 BCD.LOG1
-rwxrwxrwx 2 root root      0 Dec 15 12:47 BCD.LOG2
-rwxrwxrwx 1 root root  65536 Dec 15 12:47 BOOTSTAT.DAT
[snipped]
><fs> download /Boot/BCD /tmp/BCD
><fs> ^D

Then we can dump the contents out using hivexregedit. (We could also browse the contents with hivexsh).

$ hivexregedit --export /tmp/BCD '\' > /tmp/BCD.reg

In typical Microsoft style, the contents themselves are obscure, consisting of plenty of subkeys that look like this:

[\Objects\{1afa9c49-16ab-4a5c-901b-212802da9460}\Elements\14000006]
"Element"=hex(7):7b,00,37,00,65,00,61,00,32,00,65,00,31,00,61,00,63,\
  00,2d,00,32,00,65,00,36,00,31,00,2d,00,34,00,37,00,32,00,38,00,2d,00,\
  61,00,61,00,61,00,33,00,2d,00,38,00,39,00,36,00,64,00,39,00,64,00,30,\
  00,61,00,39,00,66,00,30,00,65,00,7d,00,00,00,00,00

(Note that “type 7″ is a list of strings, and the whole thing is encoded in UTF-16LE, so this requires some further work to parse).

There’s scope here to extend virt-inspector to understand this stuff, or even to write a BCDEDIT-style tool to modify the way Window VMs boot. Apparently the current BCDEDIT tool is half-arsed, so here’s another opportunity to beat Microsoft’s own tooling.

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2 Comments

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2 responses to “Use hivex to unpack a Windows Boot Configuration Data (BCD) hive

  1. Hi Richard,

    Hivex is great! It was the last thing I needed to get Windows 7 deployment over PXE to work for my Ulimate Deployment Appliance. I wrote a perl script that uses hivexsh to do the edits to a bcd file. Since hivex can not create a BCD file I just use the one I find on the installation DVD, clean it out entirely and then create the appropriate entries for PXE booting. Get the
    script at: ultimatedeployment.org

    The script currently only supports creating a PXE bootable BCD store, but it can quite easily be extended.

    The dude

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