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The FAT File System

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  1. The FAT File System CSC 414

  2. Objectives • Understand the structure and components of the FAT (12/16/32) File Systems • Understand what happens when a file is created, deleted, moved, etc. • Understand what forensic tools show you about the FAT file system and why.

  3. First Some Basics • Common Integral Data Types • Little Endian vs. Big Endian • ASCII/Hex/Decimal/Binary/Unicode • MS DOS Date/Time Format

  4. Common Integral Data Types Note: different hardware architectures support and use different length integer data types. The above is only intended for the X86 (Intel) platform.

  5. Little Endian vs. Big Endian • Little Endian= the first byte of an integer is the one of lowest value, the last byte the one of highest value. So we read the bytes in the reverse order that we see them in a HEX Editor. • Big Endian= the first byte of an integer is the one of highest value, the last byte the one of the lowest value. So we read the bytes in the order you see them in a HEX Editor. • When looking at “Signed Integers”, any multi-byte HEX value starting with the first HEX digit 8-F is a negative value. Any starting with 0-7 as the first digit is a positive value.

  6. ASCII/Hex/Decimal/Binary/Unicode • Look at .XLS table of ASCII and Extended ASCII characters. • Make it easy on yourself when converting from one to the other…USE A CALCULATOR! • When using the Windows Calculator, leading 0 digits do not show in Hex, Binary or Decimal. • i.e. 8 Decimal = 00001000 Binary • Shows in Calculator as 1000

  7. MS DOS Date/Time Format

  8. FAT (12/16/32) • Boot Record (boot sector plus any additional reserved sectors) • File Allocation Table 1 • File Allocation Table 2 • Root Directory • Data Area (divided into clusters, starting with Cluster #2) • In FAT32, the root directory is part of the data area.

  9. Boot Sector (FAT 12/16)

  10. BIOS Parameter Block (BPB) FAT12/16

  11. Boot Sector (FAT32)

  12. BIOS Parameter Block (BPB) FAT32

  13. FSInfo Sector (FAT32 only) • The FSInfo sector immediately follows the first sector, containing the BIOS Parameter Block, on a FAT32 partition. • The FAT32 boot record is made up of these two sectors and one more reserved sector, though this 3rd sector is not used for anything, that immediately follows the FSInfo sector. • A backup copy of the three sectors that make up the FAT32 boot record exists and is usually located at sector 6 of the partition. • Note: with everything you know about FAT boot sectors at this point you should be able to manually search a disk for lost FAT boot records (i.e. for “MS” (4Dh 53h) at sector offset 03h.

  14. File Allocation Tables (FAT) • 2 (supposedly identical) copies • Copy 1 immediately follows the boot record • Copy 2 immediately follows copy 1 • 1st entry identifies type of disk (i.e. fixed or removable) • 2nd entry identifies whether Chkdsk is set to be run or not. • All other entries either point to the next cluster in a file (NN), identify the end of the file (FF), identify a bad cluster (F7), or identify the cluster as being free (00).

  15. File Allocation Tables (FAT)

  16. File Allocation Table (FAT12) FAT12 – entries are 12 bits long (little endian) More complicated and harder to visually identify than FAT16 or FAT32, but Runtime Software’s Disk Explorer for FAT does a great job of interpretation! FF8 FFF FFF 004 005 006 007 008 009 00A 00B 00C 00D 00E 00F 010 011 012 013 014 015 FFF 017… FFF = eof

  17. File Allocation Table (FAT32) FAT32 – entries are 4 bytes (32 bits) long (little endian) 0F FF FF F8 = fixed FF FF FF FF 0F FF FF FF = eof 00 00 00 00 04 = 4 00 00 00 05 = 5 00 00 00 06 = 6 00 00 00 07 = 7 00 00 00 08 = 8 0F FF FF FF = eof 00 00 00 00 = free 00 00 00 00 0F FF FF 0F = eof Notice the pattern… Very easy to visually identify on disk. F8 = fixed media F0 = removable

  18. FAT Directory Entries (basic) Filename, Ext, Attributes, Uppercase/LFN/lowercase, Create time, fine resolution: 10ms units, values from 0 to 199, Creation Time/Date, Accessed Date, Higher 2 bytes starting Cluster, Modified Time/Date, Lower 2 bytes starting cluster number, File Size • 32 bytes in length • Subdirectories always have “.” and “..” as the first two entries • Note: Knowing the above, you should be able to search a disk for lost directory entries

  19. FAT Directory Entries (basic) 00100000 = 0x20

  20. FAT Directory Entries (VFAT/LFN)

  21. FAT Directory Entries (VFAT/LFN) LFN Sequence # (bit 6 on for last LFN entry in name – makes the 13thentry “4D” instead of “0D” Filename in Unicode Attribute – always “0F” for LFN entry Checksum of DOS filename

  22. Data Area • FAT 12/16 – Files or Sub-Folders start at “Cluster 2”, the first cluster available for data • On a DOS bootable FAT partition, io.sys must be located at Cluster 2 • FAT 32 – The Root Directory starts at Cluster 2. Files/Sub-Folders start after that.

  23. FAT File System Operations - Demo • We will now take a look at a FAT file system and what actually happens on your disk when you perform normal file operations, as seen by WinHex and other forensic tools: • Create a file/folder • Rename a file/folder • Move a file/folder • Delete a file/folder • How file/folder fragmentation occurs and what it looks like

  24. Questions??? • Use the discussion board, as usual…