Difference between revisions of "ISO9660 Implementation Notes"

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(Created based on skins_iso9660.txt.)
 
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* File and directory level analysis
 
* File and directory level analysis
  
= ISO9660 OVERVIEW =
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= ISO9660 Overview =
 
This provides a quick introduction to the ISO9660 file system.  The
 
This provides a quick introduction to the ISO9660 file system.  The
 
terms used are different then with other file systems.  For a full
 
terms used are different then with other file systems.  For a full

Revision as of 18:25, 26 October 2008

Note: This was copied from the skins_iso9660.txt file from TSK. It will need some help to integrate it into the Wiki.


Introduction

The ISO9660 file system is used on many platforms and has many variations and extensions. At the most basic level of ISO9660 there are several differences than traditional filesystems due to the type of media available.

This document gives a quick overview of ISO9660 and how it was implemented.

The Sleuth Kit allows one to investigate an ISO9660 image in the same ways as any UNIX image, including:

  • Creation of ASCII timeline of file activity
  • File and directory level analysis

ISO9660 Overview

This provides a quick introduction to the ISO9660 file system. The terms used are different then with other file systems. For a full overview of the file system, refer to the document "Volume and File Structure of CDROM for Information Interchange"

http://www.ecma-international.org/publications/standards/Ecma-119.htm


Volume Descriptors

ISO9660 uses structures called Volume Descriptors to store information about the directory hierarchy of an ISO9660 volume. At 32768 bytes into the image there is a contiguous list of volume descriptors. A primary volume descriptor contains an address of a Path Table which is a list of every directory on the volume. In this path table each directory record has a single run of contiguous bytes known as an Extent. Each directory's single data extent contains a group of contiguous directory descriptors which represent files, directories or other standard file types.

Primary volume descriptors only allow uppercase filenames in the 8.3 format (8 chars dot 3 chars).

Supplementary volume descriptors are very similar to primary volume descriptors. The main difference is that supplementary volume descriptors store filenames as UCS-2 characters and are used in Microsoft Joliet extensions to allow mixed case filenames up to 103 characters.

All volume descriptors are stored at least once, with there being a requirement to have only a single primary volume descriptor for an image to be valid. Supplementary volume descriptors usually contain the same data as primary volume descriptors.

Files

ISO9660 file are stored in an extent whose size is measured in bytes.

A file is considered unique if its extent address is unique.

Directories

Directory names are only stored in the path table of the volume descriptor. As a directory is encountered as a directory descriptor inside another directory's extent, the address of its data extent is examined by the ISO9660 implementation to see if we've seen this directory before and figure out what its name is.

Directories are unusual in the way they are identified as a unique inode. If we examine the root directory using a primary volume descriptor then its extent address is where on the volume the extent containing the list of directory descriptors with 8.3 encoded names exists. If we examine the root directory of that same volume using a supplementary volume descriptor we will find that the extent address is different because these directory descriptors are UCS-2 encoded, even though each directory descriptor will point at the same data extent for each file.

This last paragraph is quite complicated. Lets simplify:

Imagine a CD with 3 files on it: file-1.txt, file-2.txt, file3.txt.

The path table in a primary volume descriptor has one directory in it and its extent contains 3 directory descriptor structures with 8.3 uppercase encoding. The path table in a supplementary volume descriptor describing this same volume has one directory but its extent is different because those 3 directory descriptor structures are different than the previous 3. The files are not considered unique because their extent addresses (where their data lies) is not unique.

Of Note:

Due to many reports of mastering software errata, there are some issues that The Sleuth Kit handles that the specifications for ISO9660 say will never happen. The specs say that there is only one unique primary volume descriptor per volume. The Sleuth Kit handles the possibility of finding more and alerts the user to this.

Inodes don't really exist in ISO9660 so the implementation is improvised based on anything thats extent is unique is a different file. The pseudo inode strucutre is stored in a linked list to make viewing an entire image faster.

ISO9660 stores many fields as both byte order. A 32 bit number will take 8 bytes, the first 4 are little endian, the last 4 are big endian.

Using The Sleuth Kit with ISO9660

The Sleuth Kit allows one to view all aspects of the ISO9660 structure.

All Sleuth Kit commands should work the same as their counterparts.

Note that Autopsy can automate this process for you and allows you to view all attributes.

  http://www.sleuthkit.org/autopsy

What TSK Cannot Currently Do

There are a few things that The Sleuth Kit is not yet able to do with ISO9660:

  • Multisessions CDs are not handled.
  • High Sierra is not handled.
  • Files that are stored with an interleave gap

Original Version By: Wyatt Banks, Crucial Security