Rekall’s advanced memory analysis capabilities are powered by a powerful binary parsing engine - capable of parsing very complex binary formats. Not only can this be applied to memory forensics, but also traditional filesystem forensics can be implemented using this capability.

Recent versions of Rekall have introduced a complete NTFS parsing subsystem using the familiar Rekall interface. This blog post introduces the NTFS filesystem plugins. In the first part of this blog post, I will show how NTFS disk images can be analysed, while in the second part I will talk a bit about the implementation details within Rekall.

Why did we add NTFS support to a memory forensic tool?

When we added support for using the windows pagefile to supplement memory analysis it became apparent that we needed to read the pagefile directly from the NTFS since the file is normally locked - so normal file APIs are not usable. We considered using tricky kernel hacking to bypass the file lock restrictions but this seems fragile and NTFS parsing is not that complicated. For memory acquisition using WinPmem we included the fcat tool from the Sleuthkit to copy the pagefile out (alternatively we could have linked libtsk directly). But one of the more important uses of Rekall is live analysis, and this does not really solve it.
We knew that Rekall’s binary parsing library was up to the task of handling NTFS. It was a good exercise to learn NTFS and document it in the Rekall implementation. I used Brian Carrier’s excellent book File System Forensic Analysis to learn about the NTFS and implement it in Rekall.
Although we also maintain pytsk as a python binding to the TSK library, it is a bit of a pain to use. The bindings are sometimes fragile and can cause crashes under some situations. They also need to be frequently updated when TSK evolves. Performance is not great - TSK needs to parse the entire MFT each time it is loaded. Pytsk has a lot of trouble parsing a live filesystems since TSK caches its analysis of the MFT and might not see new files created after this initial MFT parsing. Rebuilding the caches is quite slow too due to the IO required in reading the entire MFT each time. I felt that a pure python implementation of NTFS parsing can be useful in some situations and use cases and possibly be more efficient (Despite being written in Python :-).
Probably the most important reason for implementing NTFS parsing in Rekall was that it was a hell of a lot of fun! Rekall’s programming APIs are very easy to work with and the final plugins were really fast and powerful.

Rekall’s implementation is not supposed to be a replacement for TSK. The Sleuthkit actually converts much of the information found in NTFS into a common format to fit all filesystems. So for example, it extends the inode abstraction from other filesystems from a simple integer to a string which may contain type and id (e.g. 50-144-8). Some of the timestamps are also omitted from the tool’s output (but may be found using the low level API). This extra layer of abstraction is good in the general case (e.g. autopsy works with all filesystems in the same way) but may actually be hiding some important forensic information in some cases. Its useful to have an implementation of NTFS parsing with no abstractions at all - to allow examiners to corroborate some of the low level information available.

1. Rekall NTFS plugins.

In order to analyze an NTFS disk image, simply load it with the familiar -f switch. In the following example I use the vdfuse tool in order to export my Virtual Box VDI disk partitions as raw devices:

$ vdfuse -r -f ~/VirtualBox\ VMs/win7/win7.vdi /tmp/mnt/
$ rekal -f /tmp/mnt/Partition2
 --------------------------------------------------------------------------
The Rekall Memory Forensic framework 1.2.0 (Col de la Croix).

"We can remember it for you wholesale!"

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License.

See http://www.rekall-forensic.com/docs/Manual/tutorial.html to get started.
 --------------------------------------------------------------------------
[1] Partition2 15:57:14> istat
MFT Entry Header Values:
Entry: 5        Sequence: 5
$LogFile Sequence Number: 13730649171
Links: 1

$STANDARD_INFORMATION Attribute Values:
Flags                          COMPRESSED, HIDDEN, SYSTEM
Owner ID                       0
SID                            265
Created                        2009-07-14 02:38:56+0000
File Modified                  2014-10-31 21:36:56+0000
MFT Modified                   2014-10-31 21:36:56+0000
Accessed                       2014-10-31 21:36:56+0000

Attributes:
     Inode                   Type                 Name     Res     Size    Comment
--------------- ------------------------------ ---------- ----- ---------- -------
         5-16-0 $STANDARD_INFORMATION                     True          72
         5-48-1 $FILE_NAME                                True          68 .
        5-144-6 $INDEX_ROOT                    $I30       True         168
        5-160-8 $INDEX_ALLOCATION              $I30       False       8192
        5-176-7 $BITMAP                        $I30       True           8
        5-256-9 $LOGGED_UTILITY_STREAM         $TXF_DATA  True          56

$I30 Analysis:
   MFT      Seq           Created                  File Mod                   MFT Mod                   Access              Size    Filename
---------- ----- ------------------------- ------------------------- ------------------------- ------------------------- ---------- --------
         4     4 -                         -                         -                         -                                  0 $AttrDef
         8     8 -                         -                         -                         -                                  0 $BadClus
         6     6 -                         -                         -                         -                                  0 $Bitmap
         7     7 -                         -                         -                         -                                  0 $Boot
        11    11 2013-02-20 02:35:15+0000  2013-02-20 02:35:15+0000  2013-02-20 02:35:15+0000  2013-02-20 02:35:15+0000           0 $Extend
         2     2 -                         -                         -                         -                                  0 $LogFile
         0     1 2013-02-20 02:35:15+0000  2013-02-20 02:35:15+0000  2013-02-20 02:35:15+0000  2013-02-20 02:35:15+0000       16384 $MFT
         1     1 -                         -                         -                         -                                  0 $MFTMirr
        57     3 2009-07-14 03:18:56+0000  2013-02-19 17:51:59+0000  2013-02-19 17:51:59+0000  2013-02-19 17:51:59+0000           0 $Recycle.Bin
         9     9 2013-02-20 02:35:15+0000  2013-02-20 02:35:15+0000  2013-02-20 02:35:15+0000  2013-02-20 02:35:15+0000           0 $Secure
        10    10 -                         -                         -                         -                                  0 $UpCase
         3     3 -                         -                         -                         -                                  0 $Volume
         5     5 2009-07-14 02:38:56+0000  2014-10-31 21:36:56+0000  2014-10-31 21:36:56+0000  2014-10-31 21:36:56+0000           0 .
    145393    16 2013-02-24 22:22:28+0000  2014-10-28 09:15:59+0000  2014-10-28 09:15:59+0000  2014-10-28 09:15:59+0000           0 Config.Msi
     63072    11 2013-02-19 18:31:29+0000  2014-10-23 15:24:31+0000  2014-10-23 15:24:31+0000  2014-10-23 15:24:31+0000           0 cygwin
     13692     1 2009-07-14 05:08:56+0000  2009-07-14 05:08:56+0000  2013-02-20 02:46:34+0000  2009-07-14 05:08:56+0000           0 Documents and Settings
    165336    45 2013-12-28 19:17:49+0000  2013-12-28 19:17:55+0000  2013-12-28 19:17:55+0000  2013-12-28 19:17:55+0000           0 MinGW
      1251    15 2014-08-25 13:45:38+0000  2014-10-27 14:08:43+0000  2014-10-27 14:08:43+0000  2014-08-25 13:45:38+0000  1207721984 pagefile.sys
        58     1 2009-07-14 03:20:08+0000  2009-07-14 03:20:08+0000  2013-02-20 02:46:13+0000  2009-07-14 03:20:08+0000           0 PerfLogs
        60     1 2009-07-14 03:20:08+0000  2014-08-27 23:30:42+0000  2014-08-27 23:30:42+0000  2014-08-27 23:30:42+0000           0 Program Files
       247     1 2009-07-14 03:20:08+0000  2014-10-28 09:11:38+0000  2014-10-28 09:11:38+0000  2014-10-28 09:11:38+0000           0 Program Files (x86)
       363     1 2009-07-14 03:20:08+0000  2013-02-19 18:27:21+0000  2013-02-19 18:27:21+0000  2013-02-19 18:27:21+0000           0 ProgramData
        60     1 2009-07-14 03:20:08+0000  2014-08-27 23:30:42+0000  2014-08-27 23:30:42+0000  2014-08-27 23:30:42+0000           0 PROGRA~1
       247     1 2009-07-14 03:20:08+0000  2014-10-28 09:11:38+0000  2014-10-28 09:11:38+0000  2014-10-28 09:11:38+0000           0 PROGRA~2
       363     1 2009-07-14 03:20:08+0000  2013-02-19 18:27:21+0000  2013-02-19 18:27:21+0000  2013-02-19 18:27:21+0000           0 PROGRA~3
     88993     1 2013-02-19 18:58:43+0000  2014-08-27 22:34:33+0000  2014-10-21 16:39:13+0000  2014-08-27 22:34:33+0000           0 Python27
    118195     3 2013-02-19 22:37:36+0000  2013-05-30 13:28:51+0000  2013-05-30 13:28:51+0000  2013-05-30 13:28:51+0000           0 Python27.32
     27376     2 2013-02-19 17:51:27+0000  2013-02-19 17:51:27+0000  2013-02-19 17:51:27+0000  2013-02-19 17:51:27+0000           0 Recovery
    149334    13 2014-08-06 18:56:26+0000  2014-09-11 14:18:27+0000  2014-09-11 14:18:27+0000  2014-09-11 14:18:27+0000           0 rekall-profiles
    149334    13 2014-08-06 18:56:26+0000  2014-09-11 14:18:27+0000  2014-09-11 14:18:27+0000  2014-09-11 14:18:27+0000           0 REKALL~1
     16393     2 2013-02-20 02:47:16+0000  2014-10-31 22:07:56+0000  2014-10-31 22:07:56+0000  2014-10-31 22:07:56+0000           0 System Volume Information
     16393     2 2013-02-20 02:47:16+0000  2014-10-31 22:07:56+0000  2014-10-31 22:07:56+0000  2014-10-31 22:07:56+0000           0 SYSTEM~1
       457     1 2009-07-14 03:20:08+0000  2013-02-19 17:51:39+0000  2014-08-27 22:06:23+0000  2013-02-19 17:51:39+0000           0 Users
    154403     2 2013-02-20 13:12:13+0000  2013-02-20 13:15:48+0000  2013-02-20 13:15:48+0000  2013-02-20 13:15:48+0000           0 websymbols
    154403     2 2013-02-20 13:12:13+0000  2013-02-20 13:15:48+0000  2013-02-20 13:15:48+0000  2013-02-20 13:15:48+0000           0 WEBSYM~1
     58269     7 2013-02-19 18:28:16+0000  2013-02-19 18:28:16+0000  2013-02-19 18:28:16+0000  2013-02-19 18:28:16+0000           0 WinDDK
       619     1 2009-07-14 03:20:08+0000  2014-10-21 23:41:52+0000  2014-10-21 23:41:52+0000  2014-10-21 23:41:52+0000           0 Windows

The istat plugin displays information about a particular MFT entry. By default it shows entry 5 (The root directory). If the entry has an I30 attribute (which represents a directory index) the plugin further parses the entry and displays all files in the directory recovered from the I30 attribute stream. Note that the I30 stream contains 3 timestamps for each entry which are separated from the timestamps actually present in the MFT’s $STANDARD_INFORMATION attribute.

The output of istat also lists the attributes and their types in a similar notation to that found in, e.g. the Sleuthkit. That is as a tuple separated by dashes, MFT-TYPE-ID.

The fls plugin works in a similar way, but lists directories based on a filename, rooted at the root of the filesystem. The filename may use forward or backslash for separators.

[1] Partition2 16:06:52> fls "Python27"
-----------------------> fls("Python27")
   MFT      Seq           Created                  File Mod                   MFT Mod                   Access              Size    Filename
---------- ----- ------------------------- ------------------------- ------------------------- ------------------------- ---------- --------
    213703    19 2013-12-28 19:37:46+0000  2013-12-28 19:37:47+0000  2013-12-28 19:37:47+0000  2013-12-28 19:37:46+0000        1315 distorm3-wininst.log
    213703    19 2013-12-28 19:37:46+0000  2013-12-28 19:37:47+0000  2013-12-28 19:37:47+0000  2013-12-28 19:37:46+0000        1315 DISTOR~1.LOG
     91798     1 2013-02-19 18:59:11+0000  2013-12-28 17:23:18+0000  2013-12-28 17:23:18+0000  2013-12-28 17:23:18+0000           0 DLLs
     94581     1 2013-02-19 18:59:34+0000  2013-02-19 18:59:34+0000  2013-02-19 18:59:34+0000  2013-02-19 18:59:34+0000           0 Doc
     92031     1 2013-02-19 18:59:15+0000  2014-10-03 00:01:56+0000  2014-10-03 00:01:56+0000  2014-10-03 00:01:56+0000           0 include
     89201     1 2013-02-19 18:58:47+0000  2014-10-31 21:17:03+0000  2014-10-31 21:17:03+0000  2014-10-31 21:17:03+0000           0 Lib
     92334     1 2013-02-19 18:59:18+0000  2013-02-19 18:59:18+0000  2013-02-19 18:59:18+0000  2013-02-19 18:59:18+0000           0 libs
     89063     1 2012-04-10 22:31:16+0000  2012-04-10 22:31:16+0000  2013-02-19 18:58:44+0000  2013-02-19 18:58:44+0000       40092 LICENSE.txt
    127389     7 2013-05-30 13:01:27+0000  2013-05-30 13:01:29+0000  2013-05-30 13:01:29+0000  2013-05-30 13:01:27+0000        9973 M2Crypto-wininst.log
    127389     7 2013-05-30 13:01:27+0000  2013-05-30 13:01:29+0000  2013-05-30 13:01:29+0000  2013-05-30 13:01:27+0000        9973 M2CRYP~1.LOG
     89035     1 2012-04-10 22:18:52+0000  2012-04-10 22:18:52+0000  2013-02-19 18:58:44+0000  2013-02-19 18:58:44+0000      310875 NEWS.txt
    129217     7 2013-05-30 13:02:47+0000  2013-05-30 13:02:47+0000  2013-05-30 13:02:47+0000  2013-05-30 13:02:47+0000        2645 psutil-wininst.log
    129217     7 2013-05-30 13:02:47+0000  2013-05-30 13:02:47+0000  2013-05-30 13:02:47+0000  2013-05-30 13:02:47+0000        2645 PSUTIL~1.LOG
      1267     8 2014-08-27 22:34:33+0000  2014-08-27 22:34:36+0000  2014-08-27 22:34:36+0000  2014-08-27 22:34:36+0000           0 PyInstaller-2.1
      1267     8 2014-08-27 22:34:33+0000  2014-08-27 22:34:36+0000  2014-08-27 22:34:36+0000  2014-08-27 22:34:36+0000           0 PYINST~1.1
     89064     1 2012-04-10 22:24:54+0000  2012-04-10 22:24:54+0000  2013-02-19 18:58:44+0000  2013-02-19 18:58:44+0000       27136 python.exe
     89065     1 2012-04-10 22:24:58+0000  2012-04-10 22:24:58+0000  2013-02-19 18:58:44+0000  2013-02-19 18:58:44+0000       27648 pythonw.exe
    116396     2 2013-02-19 19:45:14+0000  2013-02-19 23:42:49+0000  2013-02-19 23:42:49+0000  2013-02-19 19:45:14+0000      238566 pywin32-wininst.log
    116396     2 2013-02-19 19:45:14+0000  2013-02-19 23:42:49+0000  2013-02-19 23:42:49+0000  2013-02-19 19:45:14+0000      238566 PYWIN3~1.LOG
     89009     2 2012-03-18 22:58:32+0000  2013-05-30 13:36:32+0000  2013-05-30 13:36:32+0000  2013-05-30 13:36:32+0000        2797 readme.txt
    213706    13 2013-12-28 19:37:46+0000  2013-12-28 19:37:46+0000  2013-12-28 19:37:46+0000  2013-12-28 19:37:46+0000      223744 Removedistorm3.exe
    127390     5 2013-05-30 13:01:27+0000  2013-05-30 13:01:27+0000  2013-05-30 13:01:27+0000  2013-05-30 13:01:27+0000      223744 RemoveM2Crypto.exe
    129218     5 2013-05-30 13:02:47+0000  2013-05-30 13:02:47+0000  2013-05-30 13:02:47+0000  2013-05-30 13:02:47+0000      223744 Removepsutil.exe
    116397     2 2013-02-19 19:45:14+0000  2013-02-19 23:42:21+0000  2013-02-19 23:42:21+0000  2013-02-19 19:45:14+0000      223744 Removepywin32.exe
    116397     2 2013-02-19 19:45:14+0000  2013-02-19 23:42:21+0000  2013-02-19 23:42:21+0000  2013-02-19 19:45:14+0000      223744 REMOVE~1.EXE
    127390     5 2013-05-30 13:01:27+0000  2013-05-30 13:01:27+0000  2013-05-30 13:01:27+0000  2013-05-30 13:01:27+0000      223744 REMOVE~2.EXE
    129218     5 2013-05-30 13:02:47+0000  2013-05-30 13:02:47+0000  2013-05-30 13:02:47+0000  2013-05-30 13:02:47+0000      223744 REMOVE~3.EXE
    213706    13 2013-12-28 19:37:46+0000  2013-12-28 19:37:46+0000  2013-12-28 19:37:46+0000  2013-12-28 19:37:46+0000      223744 REMOVE~4.EXE
    117061     1 2013-02-19 19:45:15+0000  2014-08-27 23:48:19+0000  2014-08-27 23:48:19+0000  2014-08-27 23:48:19+0000           0 Scripts
    233676    26 2014-02-19 22:43:58+0000  2014-02-19 22:43:58+0000  2014-02-19 22:43:58+0000  2014-02-19 22:43:58+0000           0 share
     92364     1 2013-02-19 18:59:18+0000  2013-02-19 18:59:33+0000  2013-02-19 18:59:33+0000  2013-02-19 18:59:33+0000           0 tcl
     94437     1 2013-02-19 18:59:33+0000  2013-02-19 18:59:34+0000  2013-02-19 18:59:34+0000  2013-02-19 18:59:34+0000           0 Tools

Similarly fstat is analogous to istat except takes a filename as an argument.

Rekall supports NTFS compressed files too. Consider the following file:

[1] Partition2 16:06:54> istat 89063
MFT Entry Header Values:
Entry: 89063        Sequence: 1
$LogFile Sequence Number: 12520239903
Links: 1

$STANDARD_INFORMATION Attribute Values:
Flags                          ARCHIVE, COMPRESSED
Owner ID                       0
SID                            713
Created                        2012-04-10 22:31:16+0000
File Modified                  2012-04-10 22:31:16+0000
MFT Modified                   2013-02-19 18:58:44+0000
Accessed                       2013-02-19 18:58:44+0000

Attributes:
     Inode                   Type                 Name     Res     Size    Comment
--------------- ------------------------------ ---------- ----- ---------- -------
     89063-16-0 $STANDARD_INFORMATION                     True          72
     89063-48-2 $FILE_NAME                                True          88 LICENSE.txt
    89063-128-3 $DATA                                     False      40092 VCN: 0-15

Clusters (128-3):
3456320-3456326(6)        Sparse(10)

NTFS compression works by compressing every 16 clusters together, and inserting a sparse cluster to cover the compressed region. We can see this in the above cluster listing.

Rekall provides the idump plugin which is analogous to the regular dump plugin, and displays a hexdump of the MTF entry.

[1] Partition2 16:06:56> idump 89063
    Offset                           Hex                              Data
-------------- ------------------------------------------------ ----------------
           0x0 41 2e 20 48 49 53 54 4f 52 59 20 4f 46 20 54 48  A..HISTORY.OF.TH -
          0x10 45 20 53 4f 46 54 57 41 52 45 0d 0a 3d 3d 3d 3d  E.SOFTWARE..==== -
          0x20 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d 3d  ================ -
          0x30 3d 3d 3d 3d 3d 3d 0d 0a 0d 0a 50 79 74 68 6f 6e  ======....Python -
          0x40 20 77 61 73 20 63 72 65 61 74 65 64 20 69 6e 20  .was.created.in. -
          0x50 74 68 65 20 65 61 72 6c 79 20 31 39 39 30 73 20  the.early.1990s. -
          0x60 62 79 20 47 75 69 64 6f 20 76 61 6e 20 52 6f 73  by.Guido.van.Ros -
          0x70 73 75 6d 20 61 74 20 53 74 69 63 68 74 69 6e 67  sum.at.Stichting -
          0x80 0d 0a 4d 61 74 68 65 6d 61 74 69 73 63 68 20 43  ..Mathematisch.C -
          0x90 65 6e 74 72 75 6d 20 28 43 57 49 2c 20 73 65 65  entrum.(CWI,.see -

If you want to copy a file out of the NTFS filesystem, use the iexport plugin.

[1] Partition2 16:50:46> iexport 89063, dump_dir="/tmp/"
Writing MFT Entry 89063 as Python27/LICENSE.txt
[1] Partition2 16:51:11> !head /tmp/Python27%2fLICENSE.txt
A. HISTORY OF THE SOFTWARE
==========================

Python was created in the early 1990s by Guido van Rossum at Stichting
Mathematisch Centrum (CWI, see http://www.cwi.nl) in the Netherlands
as a successor of a language called ABC.  Guido remains Python's
principal author, although it includes many contributions from others.

In 1995, Guido continued his work on Python at the Corporation for
National Research Initiatives (CNRI, see http://www.cnri.reston.va.us)

2. Rekall’s NTFS implementation notes.

This section is intended for Rekall developers who want to learn a bit about how Rekall’s NTFS implementation uses some of the common features in the Rekall API.

2.1. Autodetection of NTFS

To make Rekall as easy to use as possible, we use autodetection as much as we can. Ideally a user should simply provide the image file, and Rekall will detect the image format and the profile required. To support this, Rekall has an autodetection plugin system. A detector class simply registers by extending guess_profile.DetectionMethod:

class NTFSDetector(guess_profile.DetectionMethod):
    name = "ntfs"

    def Offsets(self):
        return [0]

    def DetectFromHit(self, hit, _, address_space):
        ntfs_profile = self.session.LoadProfile("ntfs")
        try:
            ntfs = NTFS(address_space=address_space, session=self.session)
            self.session.SetParameter("ntfs", ntfs)

            return ntfs_profile
        except NTFSParseError:
            return

The detector can provide a string on which to fire, or a list of offsets to check in its Offsets() method. The framework will then call it when a hit is found.

2.2. Implementing Fixups

One of the more interesting features of NTFS is the use of Fixups. When the NTFS writes to disk certain data structures, it replaces some bytes in the cluster with a random sequence. It then stores the bytes that used to be there as fixups in a list. When NTFS reads the cluster from disk it applies the fixups to get the original data.

This means that we can not simply read clusters from the disk - we must apply the relevant fixups. In Rekall we have an Address Space abstraction to read data. Address Spaces typically layer on top of other address spaces. Hence we can implement the FixupAddressSpace so it can be layered on top of another address space:

class FixupAddressSpace(addrspace.BaseAddressSpace):
    """An address space to implement record fixup."""

    def __init__(self, fixup_magic, fixup_table, base_offset, length, **kwargs):
        super(FixupAddressSpace, self).__init__(**kwargs)
        self.as_assert(self.base is not None, "Address space must be stacked.")
        self.base_offset = base_offset
        self.fixup_table = fixup_table
        self.fixup_magic = fixup_magic

        # We read the entire region into a mutable buffer then apply the fixups.
        self.buffer = array.array("c", self.base.read(base_offset, length))
        for i, fixup_value in enumerate(fixup_table):
            fixup_offset = (i+1) * 512 - 2
            if (self.buffer[fixup_offset:fixup_offset+2].tostring() !=
                    fixup_magic.v()):
                raise NTFSParseError("Fixup error")

            self.buffer[fixup_offset:fixup_offset+2] = array.array(
                "c", fixup_value.v())

    def read(self, address, length):
        buffer_offset = address - self.base_offset
        return self.buffer[buffer_offset:buffer_offset+length].tostring()

We can then apply the fixup to arbitary structures. The below code will automatically apply the fixup every time we instantiate an MFT_ENTRY struct. Therefore the fixups become completely transparent now:

class MFT_ENTRY(obj.Struct):
    def __init__(self, **kwargs):
        super(MFT_ENTRY, self).__init__(**kwargs)

        # We implement fixup by wrapping the base address space with a fixed
        # one:
        self.obj_vm = FixupAddressSpace(fixup_magic=self.fixup_magic,
                                        fixup_table=self.fixup_table,
                                        base_offset=self.obj_offset,
                                        length=self.mft_entry_allocated,
                                        base=self.obj_vm)

2.3. Runlists

NTFS attributes can be fragmented. The actual blocks they occupy on disk are described using a run list. Rekall already has an address space primitive called a RunBasedAddressSpace. This type of address space is simply initialized with a list of runs specifying tuples of the form (file address, disk address length), and then layered on top of the Physical Address Space (i.e. the disk image).

Supporting compressed files makes the implementation slightly more complex, but in general all one has to do is derive an address space from the RunBasedAddressSpace and in the constructor populate the self.runs collection. The following shows the simplified implementation ignoring compression.

class RunListAddressSpace(addrspace.RunBasedAddressSpace):
    """An address space which is initialized from a runlist."""

    def __init__(self, run_list, cluster_size=None, size=0, **kwargs):
        super(RunListAddressSpace, self).__init__(**kwargs)
        self.PAGE_SIZE = cluster_size or self.session.cluster_size
        self.compression_unit_size = 16 * self.PAGE_SIZE
        self._end = size

        # In clusters.
        file_offset = 0
        for range_start, range_length in run_list:
            self._store_run(
                file_offset, range_start, uncompressed_range_length)

    def _store_run(self, file_offset, range_start, length):
        """Store a new run with all items given in self.PAGE_SIZE."""
        self.runs.insert(
            [file_offset * self.PAGE_SIZE,
             range_start * self.PAGE_SIZE,
             length * self.PAGE_SIZE,
             False])
...

Once the mapping is defined, the address space takes care of efficiently locating and using the correct run for arbitrary read operations.

2.4. Further abstractions

Rekall uses rekall.obj.Struct classes to represent arbitrary structs in memory. There is a mechanism to extend these and provide methods for these structs. The methods can be used to define a kind of API for accessing other data. For example, we can attach convenience methods to an MFT_ENTRY:

class MFT_ENTRY(obj.Struct):

    @property
    def attributes(self):
        """Iterate over all attributes, even ones in $ATTRIBUTE_LIST."""
        seen = set()

        for attribute in self._attributes:
            if attribute.type == 0xFFFFFFFF:
                break

            if attribute in seen:
                continue

            seen.add(attribute)
            yield attribute

            if attribute.type == "$ATTRIBUTE_LIST":
                for sub_attr in attribute.DecodeAttribute():
                    if sub_attr.mftReference == self.mft_entry:
                        continue

                    result = sub_attr.attribute
                    if result in seen:
                        continue

                    yield result

    def open_file(self):
        """Returns an address space which maps the content of the file's data.

        If this MFT does not contain any $DATA streams, returns a NoneObject().

        The returned address space is formed by joining all $DATA streams' run
        lists in this MFT into a contiguous mapping.
        """
        ....

    def list_files(self):
        """List the files contained in this directory.

        Note that any file can contain other files (i.e. be a directory) if it
        has an $I30 stream. Thats is directories may also contain data and
        behave as files!

        Returns:
          An iterator over all INDEX_RECORD_ENTRY.
        """
        ....

The above is a sample of some of the convenience methods attached to the MFT_ENTRY. The first combines the attributes defined within the MFT with those defined inside the $ATTRIBUTE_LISTattribute (Typically an MFT will start with some built in attributes until it runs out of room, then it will move some attributes to an $ATTRIBUTE_LIST attribute which is non resident. But this is an implementation detail of the MFT and should really be abstracted.

Similarly we have the list_files() method which simply finds the $INDEX_ROOT and $INDEX_ALLOCATION attributes and enumerates all entries within.

Similarly file data can be stored in multiple $DATA attributes (with different VCN ranges). Its a bit tedious to combine these $DATA attributes and so we have the open_file() convenience method to return a suitable address space over the file.

3. Using the NTFS API.

Using these method it is easy to use the API to open and read arbitrary MFT entries:

$ rekal -f /tmp/mnt/Partition2
 ----------------------------------------------------------------------------
The Rekall Memory Forensic framework 1.2.0 (Col de la Croix).

"We can remember it for you wholesale!"

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License.

See http://www.rekall-forensic.com/docs/Manual/tutorial.html to get started.
 ----------------------------------------------------------------------------
# This gets a reference to the ntfs object which represents the filesystem.
[1] Partition2 19:28:33> ntfs = session.GetParameter("ntfs")

# The NTFS object contains a reference to the MFT
[1] Partition2 19:28:38> mft = ntfs.mft[89035]

# Which is just an array of MFT_ENTRY structs
[1] Partition2 19:28:40> print mft
[MFT_ENTRY Array[89035] ] @ 0x056F2C00
  0x00 magic                    [String:magic]: 'FILE'
  0x04 fixup_offset             [unsigned short:fixup_offset]: 0x00000030
  0x06 fixup_count              [unsigned short:fixup_count]: 0x00000003
  0x08 logfile_sequence_number  [unsigned long long:logfile_sequence_number]: 0x2C601977B
  0x10 sequence_value           [unsigned short:sequence_value]: 0x00000001
  0x12 link_count               [unsigned short:link_count]: 0x00000001
  0x14 attribute_offset         [unsigned short:attribute_offset]: 0x00000038
  0x16 flags                    [Flags:flags]: 0x00000001 (ALLOCATED)
  0x18 mft_entry_size           [unsigned short:mft_entry_size]: 0x00000178
  0x1C mft_entry_allocated      [unsigned short:mft_entry_allocated]: 0x00000400
  0x20 base_record_reference    [unsigned long long:base_record_reference]: 0x00000000
  0x28 next_attribute_id        [unsigned short:next_attribute_id]: 0x00000004
  0x30 fixup_magic              [String:fixup_magic]: '\x0f\x00'
  0x32 fixup_table             <Array 2 x String @ 0x056F2C32>
  0x38 _attributes             <ListArray 0 x NTFS_ATTRIBUTE @ 0x056F2C38>

# We use the convenience method to open the file, returning a suitable address space.
[1] Partition2 19:28:41> fd = mft.open_file()

# We can just read the address space.
[1] Partition2 19:28:45> fd.read(0, 20)
                  Out  > 'Python News\r\n+++++++'

We can also list files in a directory:

[1] Partition2 19:38:00> for record in ntfs.mft[5].list_files():
                    |..>         print record.file.name
$AttrDef
$BadClus
$Bitmap
$Boot
$Extend
$LogFile
$MFT
$MFTMirr
$Recycle.Bin
$Secure
$UpCase
$Volume
.
Config.Msi
cygwin
Documents and Settings
MinGW
pagefile.sys
PerfLogs
Program Files
Program Files (x86)
ProgramData
PROGRA~1
PROGRA~2
PROGRA~3
Python27
Python27.32
Recovery
rekall-profiles
REKALL~1
System Volume Information
SYSTEM~1
Users
websymbols
WEBSYM~1
WinDDK
Windows
[1] Partition2 19:38:16> print record
[INDEX_RECORD_ENTRY ListArray[20] ] @ 0x00001890
  0x00 mftReference      [BitField(0-48):mftReference]: 0x0000026B
  0x06 seq_num           [short int:seq_num]: 0x00000001
  0x08 sizeOfIndexEntry  [unsigned short:sizeOfIndexEntry]: 0x00000060
  0x0A filenameOffset    [unsigned short:filenameOffset]: 0x00000050
  0x0C flags             [unsigned int:flags]: 0x00000000
  0x10 file             [FILE_NAME file] @ 0x000018A0

[1] Partition2 19:38:18> print record.file
[FILE_NAME file] @ 0x000018A0
  0x00 mftReference     [BitField(0-48):mftReference]: 0x00000005
  0x06 seq_num          [short int:seq_num]: 0x00000005
  0x08 created          [WinFileTime:created]: 0x4A5BF968 (2009-07-14 03:20:08+0000)
  0x10 file_modified    [WinFileTime:file_modified]: 0x5446EF40 (2014-10-21 23:41:52+0000)
  0x18 mft_modified     [WinFileTime:mft_modified]: 0x5446EF40 (2014-10-21 23:41:52+0000)
  0x20 file_accessed    [WinFileTime:file_accessed]: 0x5446EF40 (2014-10-21 23:41:52+0000)
  0x28 allocated_size   [unsigned long long:allocated_size]: 0x00000000
  0x30 size             [unsigned long long:size]: 0x00000000
  0x38 flags            [Flags:flags]: 0x10000800 ()
  0x3C reparse_value    [unsigned int:reparse_value]: 0x00000000
  0x40 _length_of_name  [byte:_length_of_name]: 0x00000007
  0x41 name_type        [Enumeration:name_type]: 0x00000000 (POSIX)
  0x42 name             [UnicodeString:name]: u'Windows' (Windows)

Note that iterating over the index produces a list of INDEX_RECORD_ENTRY structs which also contain FILE_NAME structs within them. The FILE_NAME structs contain the 4 NTFS timestamps quite independently from the timestamps stored in the actual MFT for the file itself (This FILE_NAME struct came from the directory index), this can be forensically significant.

The next example shows how to get the $STANDARD_INFORMATION record for each file:

[1] Partition2 19:56:32> x=ntfs.mft[89035].get_attribute("$STANDARD_INFORMATION")
[1] Partition2 19:57:01> print x.DecodeAttribute()
[STANDARD_INFORMATION STANDARD_INFORMATION] @ 0x00000000
  0x00 create_time         [WinFileTime:create_time]: 0x4F84B1CC (2012-04-10 22:18:52+0000)
  0x08 file_altered_time   [WinFileTime:file_altered_time]: 0x4F84B1CC (2012-04-10 22:18:52+0000)
  0x10 mft_altered_time    [WinFileTime:mft_altered_time]: 0x5123CB64 (2013-02-19 18:58:44+0000)
  0x18 file_accessed_time  [WinFileTime:file_accessed_time]: 0x5123CB64 (2013-02-19 18:58:44+0000)
  0x20 flags               [Flags:flags]: 0x00000820 (ARCHIVE, COMPRESSED)
  0x24 max_versions        [unsigned int:max_versions]: 0x00000000
  0x28 version             [unsigned int:version]: 0x00000000
  0x2C class_id            [unsigned int:class_id]: 0x00000000
  0x30 owner_id            [unsigned int:owner_id]: 0x00000000
  0x34 sid                 [unsigned int:sid]: 0x000002C9
  0x38 quota               [unsigned long long:quota]: 0x00000000
  0x40 usn                 [unsigned int:usn]: 0xC54A40B8

4. Conclusions

Although the NTFS support in Rekall is still pretty immature we want to make it better and more useful. For a relatively complex filesystem, such as NTFS, the Rekall implementation is pretty small, coming in at around 1000 lines of code (not including the implementation for lznt1 - the NTFS compression algorithm. Additional lines are for plugins etc). It should be possible to support additional filesystems as well. We also want to write more interesting plugins, please let us know any ideas for a good NTFS plugin :-)

Performance is pretty good. One thing you should notice is that Rekall starts up pretty fast since it does not scan the MFT like TSK does. Of course this means that Rekall cant find orphaned files like TSK does! Rekall also does not have a cache of the MFT - making it suitable to operate on a changing live filesystem.

Reading compressed files is currently pretty slow since the lznt1 compression algorithm is implemented in pure python. This could be easily accelerated with a C implementation in future.

Rekall Forensics blog

Tuesday, December 23, 2014

Rekall NTFS Support.