File System Analysis Techniques
                     Sleuth Kit Reference Document 
                       http://www.sleuthkit.org

                            Brian Carrier
                        Last Updated: July 2005



INTRODUCTION
=======================================================================
Currently, evidence is most frequently found in the file system.
This is because it is non-volatile and remnants of deleted files
can typically be found.  This file will help one to use the low-level
tools in The Sleuth Kit for a forensic analysis.

This document is organized into small scenarios, which provide
examples of how to use The Sleuth Kit.  Most of these functions
are automated with Autopsy, but they are here for reference and
education.  

    http://www.sleuthkit.org/autopsy

The techniques used here apply to both UNIX and Windows file systems.



TIME LINE
=======================================================================
The steps from the timeline Sleuth Kit Implementation Notes are
followed (using both ils and fls) and you notice some interesting
activity from unallocated inodes, namely MFT Entry 5035 from image
c_drive.dd.  To display the contents of this file, use "icat":

    # icat images/c_drive.dd 5035 | less

NOTE: To prevent your terminal from getting messed up, pipe all
output of "icat" through a pager like "less".


SEARCH
=======================================================================
In this scenario, we will search the unallocated space of the
"wd0e.dd" image for the string "abcdefg".  The first step is to
extract the unallocated disk units using the "blkls" tool (as this
is an FFS image, the addressable units are fragments).

    # blkls images/wd0e.dd > output/wd0e.blkls

Next, use the UNIX strings(1) utility to extract all of the ASCII
strings in the file of unallocated data.  If we are only going to be
searching for one string, we may not need to do this.  If we are going
to be searching for many strings, then this is faster.  Use the '-t d'
flags with "strings" to print the byte offset that the string was found.

    # strings -t d output/wd0e.blkls > output/wd0e.blkls.str

Use the UNIX grep(1) utility to search the strings file.  

    # grep "abcdefg" output/wd0e.blkls.str | less
    10389739: abcdefg

We notice that the string is located at byte 10389739.  Next,
determine what fragment.  To do this, we use the 'fsstat' tool:

    # fsstat openbsd images/wd0e.dd
	<...>
    CONTENT-DATA INFORMATION
    --------------------------------------------
    Fragment Range: 0 - 266079
    Block Size: 8192
    Fragment Size: 1024

This shows us that each fragment is 1024 bytes long.  Using a
calculator, we find that byte 10389739 divided by 1024 is 10146
(and change).  This means that the string "abcdefg" is located in
fragment 10146 of the "blkls" generated file.  This does not really
help us because the blkls image is not a real file system.  To view
the full fragment from the blkls image, we can use dd:

    # dd if=images/wd0e.dd bs=1024 skip=10146 count=1 | less

Next, we will identify where this fragment is in the original image.
The "blkcalc" tool will be used for this.  "blkcalc" will return the
"address" in the original image when given the "address" in the
blkls generated image.  (NOTE, this is currently kind of slow).  The
'-u' flag shows that we are giving it an blkls address.  If the '-d'
flag is given, then we are giving it a dd address and it will
identify the blkls address.

    # blkcalc -u 10146 images/wd0e.dd
    59382

Therefore, the string "abcdefg" is located in fragment 59382.  To view
the contents of this fragment, we can use "blkcat".

    # blkcat images/wd0e.dd 59382 | less

To make more sense of this, let us identify if there is a meta data
structure that still has a pointer to this fragment.  This is achieved
using "ifind".  The '-a' argument means to find all occurrences.

  # ifind -a images/wd0e.dd 59382
  493

Inode 493 has a pointer to fragment 59382.  Let us get more information
about inode 493, using "istat".

    # istat images/wd0e.dd 493
	inode: 493
	Not Allocated
	uid / gid: 1000 / 1000
	mode: rw-------
	size: 92
	num of links: 1
	Modified:       08.10.2001 17:09:49     (GMT+0)
	Accessed:       08.10.2001 17:09:58     (GMT+0)
	Changed:        08.10.2001 17:09:49     (GMT+0)
	Direct Blocks:
	  59382

Next, let us find out if there is a file that is still associated with
this (unallocated) inode.  This is done using "ffind".

    # ffind -a images/wd0e.dd 493
	* /dev/.123456

The leading '*' identifies the file as deleted.  Therefore, at one point,
the file '/dev/.123456' allocated inode 493, which allocated fragment
59382, which contained the string "abcdefg".

If "ffind" returned with more than file that had allocated inode 493,
it means that either both were hard-links to the same file or that one
file (chicken) allocated the inode, it was deleted, a second file (egg)
allocated it, and then it was deleted.  The string belongs to the second
file, but it is difficult to determine which came first.  On the other
hand, if "ffind" returns with two entries where one deleted and one not,
then the string belongs to the non-deleted file.

As previously mentioned, Autopsy will do all of this for you when
you do a keyword search of unallocated space.  



DELETED CONTENT
=======================================================================
To view all of the deleted file names in an image, use the "fls" tool.
For all deleted files, use the '-r' flag for recursive and '-d' flag
for deleted.

    # fls -rd images/hda9.dd | less
    d/d * 232: 	/TEMP-823450
    r/d * 293: 	/TEMP-131100
	
This shows us the full path that the deleted files are located.  On some
systems, such as Windows NTFS, the file content may be recovered
(depending on how much system activity has occurred).  On other
systems, such as Solaris UFS and Linux Ext3, deleted files can not
be easily recovered.   The number at the beginning of the line is
the inode number.  The '*' shows that it is deleted and the 'd' and
'r' show the type (directory and file).  The first letter identifies
the directory entry type value (which does not exist in all file
system types) and the second letter is the type according to the
inode.  In most cases these should be the same, but it may not for
deleted files if the inode has been reallocated to a file of a
different type.  If we do an "istat" on the directory (232) we will
notice that the size is 0.

    # istat images/hda9.dd 232
	inode: 232
	Not Allocated
	uid / gid: 0 / 0
	mode: rwxr-xr-x
	size: 0
	num of links: 0
	Modified:       08.23.2001 21:52:33     (GMT+0)
	Accessed:       08.23.2001 23:05:39     (GMT+0)
	Changed:        08.23.2001 21:52:33     (GMT+0)
	Deleted:        08.23.2001 23:05:39     (GMT+0)
	Direct Blocks:


Linux does this to all of its deleted directories.  It should also
be observed that no block addresses are shown in the "istat" output.
This is because the size is 0 and the program thinks that the address
is bogus.  Using the '-b' option of "istat", we can force it to
output the block address.  With Linux Ext3, the block pointers would
be 0, but Linux Ext2 kept the old addresses.

    # istat -b 2 images/hda9.dd 232
	inode: 232
	Not Allocated
	uid / gid: 0 / 0
	mode: rwxr-xr-x
	size: 0
	num of links: 0
	Modified:       08.23.2001 21:52:33     (GMT+0)
	Accessed:       08.23.2001 23:05:39     (GMT+0)
	Changed:        08.23.2001 21:52:33     (GMT+0)
	Deleted:        08.23.2001 23:05:39     (GMT+0)
	Direct Blocks:
	  388 0

Now we can examine the contents of block 388 and see the file
names that were in that directory:

    # blkcat -h images/hda9.dd 388 | less



MANUAL UNIX FILE RECOVERY
=======================================================================
A UFS/FFS or EXT2FS/EXT3FS file system is organized into groups.
Each group has its own inodes and blocks to store data in.  When
a new file is created, it is given an inode in the same group that
the parent directory inode is in (if there are still inodes
available).  When a new directory is created, it is given an inode
in a new group.  An inode allocates blocks from the same group that
its inode is in.

When recovering a file from one UFS or EXTxFS, the group layout
can be used.  When a deleted file is found with 'fls', notice the
inode of the parent directory:

    # fls -r images/hda1.dd
	d/d 30789:      doc
    + r/r * 0:    doc/.a/ssh.tar
    + r/r 30792:    doc/.a/install
	<...>

We want to recover the 'ssh.tar' file and notice that the parent
directory is 30789 and the deleted file has a cleared inode pointer.
To identify the group that it is in, the 'fsstat' tool is used:

    # fsstat images/hda1.dd
    FILE SYSTEM INFORMATION
    --------------------------------------------
    File System Type: EXT3FS
	<...>

    Group: 0:
      Inode Range: 1 - 15392
      Block Range: 0 - 32767
        Super Block: 0 - 0
        Group Descriptor Table: 1 - 1
        Data bitmap: 2 - 2
        Inode bitmap: 3 - 3
        Inode Table: 4 - 484
        Data Blocks: 485 - 32767

    Group: 1:
      Inode Range: 15393 - 30784
      Block Range: 32768 - 65535
        Super Block: 32768 - 32768
        Group Descriptor Table: 32769 - 32769
        Data bitmap: 32770 - 32770
        Inode bitmap: 32771 - 32771
        Inode Table: 32772 - 33252
        Data Blocks: 33253 - 65535

    Group: 2:
      Inode Range: 30785 - 46176
      Block Range: 65536 - 98303
        Data bitmap: 65536 - 65536
        Inode bitmap: 65537 - 65537
        Inode Table: 65540 - 66020
        Data Blocks: 65538 - 65539, 66021 - 98303

    <...>

The inode is in the range of inode addresses for group 1.  To search
for the deleted file, we extract the unallocated space using 'blkls':

    # blkls images/hda1.dd 32768-65535 > output/hda1-grp1.blkls

If we wanted to extract all of the data for the group, we could
use 'dd':

    # dd if=images/hda1.dd of=output/hda1-grp1.dd bs=4096 skip=32768 \
      count=32767

Where, the fragment size is 4096 (which can also be found in the
'fsstat' output).  Either of these images can then be analyze for
keywords or using other data carving tools such as 'foremost'.
This process allows one to reduce the amount of data that must be
analyzed.

    http://foremost.sourceforge.net

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Copyright (c) 2002-2005 by Brian Carrier.  All Rights Reserved