Difference between pages "Forensic Live CD issues" and "Virtual Hard Disk (VHD)"

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== The problem ==
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{{expand}}
  
[[Tools#Forensics_Live_CDs | Forensic Linux Live CD distributions]] are widely used during computer forensic investigations. Currently, many vendors of such Live CD distributions spread false claims that their distributions "do not touch anything", "write protect everything" and so on. Community-developed distributions are not exception here, unfortunately. Finally, it turns out that many forensic Linux Live CD distributions are not tested properly and there are no suitable test cases developed.
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== Image types ==
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There are multiple types of Virtual Hard Disk (VHD) images:
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* Fixed-size hard disk image
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* Dynamic-size (or sparse) hard disk image
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* Differencing (or delta) hard disk image
  
== Another side of the problem ==
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== External Links ==
  
Another side of the problem of insufficient testing of forensic Live CD distributions is that many users do not know what happens "under the hood" of such distributions and cannot adequately test them.
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* [http://en.wikipedia.org/wiki/VHD_(file_format) VHD (file format)], by Wikipedia
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* [http://technet.microsoft.com/en-us/library/bb676673.aspx Virtual Hard Disk Image Format Specification], by Microsoft
  
=== Example ===
 
  
For example, [http://forensiccop.blogspot.com/2009/10/forensic-cop-journal-13-2009.html ''Forensic Cop Journal'' (Volume 1(3), Oct 2009)] describes a test case when an Ext3 file system was mounted using "-o ro" mount flag as a way to write protect the data. The article says that all tests were successful (i.e. no data modification was found after unmounting the file system), but it is known that damaged (i.e not properly unmounted) Ext3 file systems cannot be write protected using only "-o ro" mount flags (write access will be enabled during file system recovery).
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[[Category:File Formats]]
 
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And the question is: will many users test damaged Ext3 file system (together with testing the clean one) when validating their favourite forensic Live CD distribution? My answer is "no", because many users are unaware of such traits.
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== Problems ==
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Here is a list of common problems of forensic Linux Live CD distributions that can be used by developers and users for testing purposes. Each problem is followed by an up to date list of distributions affected.
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=== Journaling file systems updates ===
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When mounting (and unmounting) several journaling file systems with only "-o ro" mount flag a different number of data writes may occur. Here is a list of such file systems:
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{| class="wikitable" border="1"
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|-
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File system
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!  When data writes happen
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!  Notes
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|-
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|  Ext3
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|  File system requires journal recovery
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|  To disable recovery: use "noload" flag, or use "ro,loop" flags, or use "ext2" file system type
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|-
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|  Ext4
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|  File system requires journal recovery
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|  To disable recovery: use "noload" flag, or use "ro,loop" flags, or use "ext2" file system type
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|-
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|  ReiserFS
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|  File system has unfinished transactions
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|  "nolog" flag does not work (see ''man mount''). To disable journal updates: use "ro,loop" flags
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|-
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|  XFS
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|  Always
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|  "norecovery" flag does not help. To disable data writes: use "ro,loop" flags. The bug was fixed in recent 2.6 kernels.
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|}
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Incorrect mount flags can be used to mount a file system on evidentiary media during the boot process or during the file system preview process. As described above, this may result in data writes to evidentiary media. For example, several Ubuntu-based forensic Linux Live CD distributions mount Ext3/4 file systems on fixed media (e.g. hard drives) during execution of [http://en.wikipedia.org/wiki/Initrd ''initrd''] scripts (these scripts mount every supported file system type on every supported media type using only "-o ro" flag in order to find a root file system image).
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[[Image:ext3 recovery.png|thumb|right|[[Helix3]]: damaged Ext3 recovery during the boot]]
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List of distributions that recover Ext3 (and sometimes Ext4) file systems during the boot:
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{| class="wikitable" border="1"
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|-
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!  Distribution
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!  Version
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|-
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|  Helix3
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|  2009R1
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|-
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|  SMART Linux (Ubuntu)
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|  2010-01-20
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|-
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|  FCCU GNU/Linux Forensic Boot CD
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|  12.1
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|-
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|  SPADA
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|  4
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|}
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=== Root file system spoofing ===
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Most Ubuntu-based forensic Live CD distributions use Casper (set of scripts used to complete initialization process during early stage of boot). Casper is responsible for searching for a root file system (typically, an image of live environment) on all supported devices (because a bootloader does not pass any information about device used for booting to the kernel), mounting it and executing ''/sbin/init'' program on a mounted root file system that will continue the boot process. Unfortunately, Casper was not designed to meet computer forensics requirements and is responsible for damaged Ext3/4 file systems recovery during the boot (see above) and root file system spoofing.
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[[Image:Grml.png|thumb|right|[[grml]] mounted root file system from the [[hard drive]]]]
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Currently, Casper may select fake root file system image on evidentiary media (e.g. [[HDD]]), because there are no authenticity checks performed (except optional UUID check for a possible live file system), and this fake root file system image may be used to execute malicious code during the boot with root privileges. Knoppix-based forensic Live CD distributions are vulnerable to the same attack.
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List of Ubuntu-based distributions that allow root file system spoofing:
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{| class="wikitable" border="1"
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|-
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!  Distribution
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!  Version
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!  Notes
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|-
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|  Helix3
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|  2009R1
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|
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|-
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|  Helix3 Pro
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|  2009R3
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|
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|-
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|  CAINE
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|  1.5
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|
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|-
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|  DEFT Linux
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|  5
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|
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|-
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|  Raptor
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|  20091026
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|
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|-
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|  grml
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|  2009.10
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|  Actually, [[grml]] uses live-initramfs scripts (Casper fork)
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|-
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|  BackTrack
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|  4
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|
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|-
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|  SMART Linux (Ubuntu)
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|  2010-01-20
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|
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|-
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|  FCCU GNU/Linux Forensic Boot CD
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|  12.1
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|
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|}
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Vulnerable Knoppix-based distributions include: SPADA, LinEn boot CD, BitFlare.
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=== Swap space activation ===
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=== Incorrect automount policy ===
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=== Incorrect write-blocking approach ===
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== See also ==
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* [http://www.computer-forensics-lab.org/pdf/Linux_for_computer_forensic_investigators_2.pdf Linux for computer forensic investigators: problems of booting trusted operating system]
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* [http://www.computer-forensics-lab.org/pdf/Linux_for_computer_forensic_investigators.pdf Linux for computer forensic investigators: «pitfalls» of mounting file systems]
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Revision as of 04:01, 13 September 2012

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Please help to improve this article by expanding it.
Further information might be found on the discussion page.

Image types

There are multiple types of Virtual Hard Disk (VHD) images:

  • Fixed-size hard disk image
  • Dynamic-size (or sparse) hard disk image
  • Differencing (or delta) hard disk image

External Links