Chapter Two File Systems. CIS 4000 Intro. to Forensic Computing David McDonald, Ph.D.

Transcription

1 Chapter Two File Systems CIS 4000 Intro. to Forensic Computing David McDonald, Ph.D. 1

2 Learning Objectives At the end of this section, you will be able to: Explain the purpose and structure of file systems Describe Microsoft file structures Explain the structure of New Technology File System (NTFS) disks List some options for decrypting drives encrypted with whole disk encryption Explain how the Windows Registry works 2

3 File Systems File system or file management systems are systems that the OS uses to organize and locate data stored on a hard disk. Gives OS a road map to data on a disk File system is a set of data types, which is employed for storage, hierarchical categorization, management, navigation, access, and recovering the data File system can use storage devices like hard disks, CD-ROM or floppy disk Command line or graphical user interface can be used to access the files When you need to access a suspect s computer to acquire or inspect data You should be familiar with the computer s platform 3

4 Clusters Preliminary Discussion Exploring Microsoft File Structures Cluster is defined as the smallest amount of space allocated by the operating system to hold a file Cluster is more efficient if size of the cluster is small There is no default size for the cluster The cluster address allocated by the operating system is called logical address The physical addresses are the addresses that exists at firmware or hardware level 4

5 System Partitioning(revisited) A system partition stores files that are used to boot (start) the computer The OS is installed on the system partition A boot partition is a volume of the computer that contains the system files used to start the OS The information regarding the files on the disk, their location, size and other important data is stored in the Master Boot Record (MBR) file Every disk has Master Boot Record that contains the information about partitions on the disk (partition table) 5

6 Partitioning Concerns Partitioning of hard disk drive is done for effective storage management of data Partition is logical part of the disk that holds data. It can be divided into Primary Partition on which an OS can be installed Used when computer starts to load the OS Extended Partition Can be divided into additional logical drives Windows look for primary partition to start the computer. This active partition contains the boot files used to start an operating system Inter-partition gap is unused or void space between the primary and first logical partition (later) 6

7 File System Functions Tracks files on across storage Keeps track of allocated/unallocated sectors Keeps track of bad sectors Stores metadata (e.g. date & time stamp) 7 7

8 File Systems Focus on Microsoft Windows file systems FAT (File Allocation Table) 16 bit file system developed for MS-DOS Used in consumer versions of Microsoft Windows till Windows Me Considered relatively uncomplicated and became popular format for devices like floppy disks, USB devices, Digital cameras, flash disks FAT32 32 bit version of FAT file system with storage capacity up to 2 GB 8

9 File Allocation Table (FAT) FAT was originally developed for floppy disks (written to the outermost track) Filenames, directory names, date and time stamps, starting cluster, attributes (e.g. hidden, read-only, system) 9 Evolution FAT 12: for floppy disks FAT 16: MS-DOS, Windows 95 (early), WinNT 4.0, maximum capacity = 2.02 GB FAT 32: Windows 95 ~ XP, maximum capacity = 2 terabytes 9

10 Examining FAT Disks Sectors and Bytes per Cluster 10 10

11 Boot Sector Boot Sector Boot Sector is the first sector (512 bytes) of a FAT file system 11

12 Examining FAT Disks File slack space 12 12

13 Deleting FAT Files Filename in FAT database starts with HEX E5 FAT chain for that file is set to zero Free (unallocated) disk space is incremented Actual data remains on disk Can be recovered with computer forensics tools 13 13

14 File Systems Focus on Microsoft NTFS (New Technology File System) First introduced with Windows NT Provides improvements over FAT file system Stores more information about a file In NTFS, anything such as file name, creation date, access permissions and even contents is written down as metadata Stored in Master File Table (MFT ) = meta data Reduces slack space 14 NTFS uses Unicode 14

15 NTFS Partition Boot Sector When you format an NTFS volume, the format program allocates the first 16 sectors for the boot sector and the bootstrap code Used to store information: about the file system On the location of the MFT MFT mirror file is stored in the boot sector 15

16 NTFS Master File Table (MFT) Each file on an NTFS volume is represented by a record in a special file called the master file table (MFT). NTFS reserves the first 16 records of the table for special information. The first record of this table describes the master file table itself, followed by a MFT mirror record. If the first MFT record is corrupted, NTFS reads the second record to find the MFT mirror file, whose first record is identical to the first record of the MFT. The locations of the data segments for both the MFT and MFT mirror file are recorded in the boot sector. A duplicate of the boot sector is located at the logical center of the disk. The third record of the MFT is the log file, used for file recovery. The seventeenth and following records of the master file table are for each file and directory 16

17 MFT Structure 17

18 NTFS Attributes Every file has a unique identities like: Name Security information and It can also contain metadata of file system in the file. 18

19 NTFS Attributes 19

20 NTFS Compressed Files Improve data storage File, folders, or an entire volume can be compressed Need to decompress it when analyzing Advanced tools (e.g. EnCase) do it automatically 20 20

21 Deleting NTFS Files On deletion from Windows Explorer the file is moved into the recycle bin If the file is deleted from command prompt then recycle bin is bypassed. It can be recovered only by using the forensic tools When a file is deleted the following tasks are performed by the operating system in the NTFS: The clusters are made available for the new data File attribute of the MFT is marked available The list of links to the cluster locations is deleted 21

22 File Systems Exploring Microsoft File Structures: FAT vs. NTFS File Allocation Table (FAT) New Technology File System (NTFS) A table, which tracks all the system storage changes. A latest file system developed specially for Windows Versions available are FAT12, FAT16, FAT32. Supported in all versions of windows operating system. NTFS is the only version. Supports all the operating systems after windows Doesn t support large file names. Supports large file names. Doesn t support extremely large storage media. Supports extremely large storage media. 22

23 File Systems - Other MacOS X file systems HFS (Hierarchical File System) Developed by Apple Computer to support Mac Operating System Traditionally used by floppy and hard disks but now also used by CD-ROMs UFS (UNIX file system) Derived from the Berkeley Fast File System (FFS) that was originally developed at Bell Laboratories from first version of UNIX FS All BSD UNIX derivatives including FreeBSD, NetBSD, OpenBSD, NeXTStep, and Solaris use a variant of UFS Acts as a substitute for HFS in Mac OS X 23