UFS1 Superblock

The superblock contains the basic file system category of data in a UFS file system and UFS1 and UFS2 use different data structures. The UFS1 superblock is located in sector 16 and is allocated 2,048 bytes, but most of it is not essential or is zero. I am going to discuss only the essential data, but all the fields are included in the following table, if only to show how many non-essential values there are. The fields in the UFS1 superblock that are used by FreeBSD, NetBSD, and OpenBSD are shown in Table 17.1.

Table 17.1. Data structure for the UFS1 superblock.

Byte Range

Description

Essential

07

Unused

No

811

Offset to backup superblock in cylinder group relative to a "base"

Yes

1215

Offset to group descriptor in cylinder group relative to a "base"

Yes

1619

Offset to inode table in cylinder group relative to a "base"

Yes

2023

Offset to first data block in cylinder group relative to a "base"

No

2427

Delta value for calculating staggering offset in cylinder group

Yes

2831

Mask for calculating staggering offset (cycle value) in cylinder group

Yes

3235

Last written time

No

3639

Number of fragments in file system

Yes

4043

Number of fragments that can store file data

No

4447

Number of cylinder groups in file system

Yes

4851

Size of a block in bytes

Yes

5255

Size of a fragment in bytes

Yes

5659

Size of a block in fragments

No

6063

Minimum percent of free blocks

No

6467

Milliseconds of rotation for next block

No

6871

Disk RPMs

No

7275

Mask used to calculate the address for a block

No

7679

Mask used to calculate the address for a fragment

No

8083

Shift used to calculate the byte address for a block

No

8487

Shift used to calculate the byte address for a fragment

No

8891

Maximum number of contiguous blocks to allocate

No

9295

Maximum number of blocks per cylinder group per file

No

9699

Number of bits to convert between a block address and a fragment address

No

100103

Number of bits to convert between a fragment address and a sector address

No

104107

Size of superblock

No

108111

Offset to cylinder summary area (no longer used)

No

112115

Size of cylinder summary area (no longer used)

No

116119

Number of indirect addresses per fragment

No

120123

Number of inodes per block in inode table

No

124127

Number of sectors per fragment

No

128131

Optimization technique

No

132135

Sectors per track

No

136139

Hard disk sector interleave

No

140143

Hard disk track skew

No

144151

File System ID

No

152155

Fragment address of cylinder group summary area

No

156159

Size of cylinder group summary area in bytes

No

160163

Size of cylinder group descriptor in bytes

No

164167

Hard disk tracks per cylinder

No

168171

Hard disk sectors per track

No

172175

Hard disk sectors per cylinder

No

176179

Cylinders in file system

No

180183

Cylinders per cylinder group

No

184187

Inodes per cylinder group

Yes

188191

Fragments per cylinder group

Yes

192195

Number of directories

No

196199

Number of free blocks

No

200203

Number of free inodes

No

204207

Number of free fragments

No

208208

Super block modified flag

No

209209

FS was clean when it was mounted

No

210210

Mounted read only flag (set to 1 if read only)

No

211211

General flags (see Table 17.2)

No

212723

Last mount point

No

724727

Last cylinder group searched

No

7281115

Unused

No

11161195

Array inode addresses for snap shot inodes

No

11961199

Expected average file size

No

12001203

Expected number of files per directory

No

12041311

Unused

No

13121315

Last time fsck was run

No

1316 1319

Size of cluster summary array in group descriptors

No

13201323

Maximum length of internal symbolic link

Yes

13241327

Format of inodes

Yes

13281335

Maximum file size

No

13361343

Mask used to calculate the offset in a block for an address

No

13441351

Mask used to calculate the offset in a fragment for an address

No

13521355

File system state

No

13561359

Format of positional layout tables

No

13601363

Number of rotational positions

No

13641367

Rotational block list head

No

13681371

Blocks for each rotation

No

13721375

Signature value (0x011954)

Yes

Table 17.2. Flag values for the superblock general flag field.

Flag Value

Description

Essential

0x01

Uncleanset when file system is mounted

No

0x02

Soft dependencies are being used

No

0x04

Needs consistency check next time it is mounted

No

0x08

Directories are indexed using hashtree or B-Tree

No

0x10

Access Control Lists are being used

No

0x20

TrustedBSD Mandatory Access Control multi-labels are being used

No

0x80

Flags have been moved (used by UFS2 in the "old" field)

No

The first fields give the offsets that are used to locate data structures inside each cylinder group. We also can see the delta and cycle values that are used to calculate the base address for each cylinder group in UFS1. The standard total number of blocks, fragments, and cylinder groups is given, sometimes in multiple formats. The number of inodes and fragments per cylinder group is given later in the superblock.

Byte 128 has a field for the block allocation optimization technique. There are currently two values for this field. If set to 0, the OS should try to save time when allocating new blocks, which might cause wasted space and fragmentation as the file system fills up. If set to 1, the OS should try to save space when allocating new blocks and find an ideal-sized location. This might take more time though when creating files. Knowing this might help during file recovery.

There are several flags starting at byte 208. The first flag is set when the superblock has been modified and is cleared when mounted. The flag in byte 209 is set to 0 if the file system was clean when it was mounted. The flag in byte 210 is set to 1 if the file system was mounted read only. Lastly, the flag in byte 211 is a general flag and can contain any of the values given in Table 17.2.

There is also a field at byte 1234 to 1237 that identifies which type of inode is used to store file metadata. If the field is 2, it is the 4.4BSD inode, and if it is 0xffffffff, it is the 4.2BSD inode. The other data in the superblock are non-essential and might or might not contain valid data.

Let's take a look at an OpenBSD system. The superblock is in sector 16 and is allocated four sectors, so we use the following dd command:


# dd if=openbsd1.dd bs=512 skip=16 count=4 | xxd

0000000: 0000 0000 0000 0000 1000 0000 1800 0000 ................

0000016: 2000 0000 1001 0000 2000 0000 f0ff ffff ....... .......

0000032: dc9d 0f41 1027 0000 ff24 0000 0200 0000 ...A.'...$......

0000048: 0020 0000 0004 0000 0800 0000 0500 0000 . ..............

0000064: 0000 0000 3c00 0000 00e0 ffff 00fc ffff ....<...........

0000080: 0d00 0000 0a00 0000 0700 0000 0008 0000 ................

0000096: 0300 0000 0100 0000 0008 0000 00fe ffff ................

0000112: 0900 0000 0008 0000 4000 0000 0200 0000 ........@.......

0000128: 0000 0000 3f00 0000 0100 0000 0000 0000 ....?...........

0000144: 2c9d 0f41 8f5a 19a2 1001 0000 0004 0000 ,..A.Z..........

0000160: 0008 0000 1000 0000 3f00 0000 f003 0000 ........?.......

0000176: 1400 0000 1000 0000 8007 0000 801f 0000 ................

0000192: 0400 0000 fe03 0000 e50e 0000 1000 0000 ................

0000208: 0001 0000 2f6d 6e74 0000 0000 0000 0000 ..../mnt........

0000224: 0000 0000 0000 0000 0000 0000 0000 0000 ................

[REMOVED]

0000832: 0000 0000 0000 0000 0000 0000 00e6 d2d0 ................

0000848: 0038 d3d0 003c d3d0 0100 0000 0000 0000 .8...<..........

[REMOVED]

0001184: 0000 0000 0000 0000 0000 0000 0040 0000 .............@..

0001200: 4000 0000 0000 0000 0000 0000 0000 0000 @...............

[REMOVED]

0001312: 0000 0000 0700 0000 3c00 0000 0200 0000 ........<.......

0001328: ff7f 0101 0840 0000 ff1f 0000 0000 0000 .....@..........

0001344: ff03 0000 0000 0000 0000 0000 0100 0000 ................

0001360: 0100 0000 6005 0000 6205 0000 5419 0100 ....`...b...T...

0001376: 0000 0101 0101 0101 0101 0101 0101 0101 ................

This file system is from an IA32 system, so the bytes are in a little-endian ordering. If the file system were from a big-endian system, such as a Sparc system, the bytes in each numeric field would be reversed.

Bytes 8 to 11 show that the backup superblocks are located at an offset of 16 fragments (0x10) from the base of each cylinder group. Bytes 12 to 15 show that the group descriptor is located at an offset of 24 fragments (0x18) from the base, and bytes 16 to 19 show that the inode table starts 32 fragments (0x20) from the base.

To calculate the base of a UFS1 cylinder group, we start with bytes 24 to 27, which shows the delta value as 32 (0x20). This means that group 0 will have its base at fragment 0, and group 1 will have its base at fragment 32. Bytes 28 to 31 in the superblock show that the cycle mask is 0xfffffff0, which means that we care only about the final four bits of the group number. Therefore, after every 16 groups, we cycle back to the beginning and the base offset returns to fragment 0. In this case, group 15 will have a base offset of fragment 480, and group 16 will have a base offset of fragment 0.

Bytes 32 to 35 is the time that this was last written to, and the format is the number of seconds since January 1, 1970 GMT. Bytes 36 to 39 show that there are 10,000 (0x2710) fragments in the file system, and bytes 44 to 47 show that there are only two cylinder groups in this file system. The size of each block is given in bytes 48 to 51, and we see that it is 8,192 bytes (0x2000). The fragment size is in bytes 52 to 55, and it is 1,024 bytes (0x0400). To ensure you don't have to divide these two numbers, the number of fragments per block is given in bytes 56 to 59, which is 8.

Bytes 104 to 107 show that the superblock is 2,048 bytes (0x0800). Bytes 152 to 155 give the location of the cylinder group summary area, and it is located in fragment 272 of this file system. Its size is given in bytes 156 to 159, and we see that it is 1,024 bytes, or one fragment. Bytes 184 to 187 give the number of inodes per cylinder group, and there are 1,920 (0x0780) in this file system. The number of fragments per cylinder group is in bytes 188 to 191, and there are 8064 (0x1f80).

The flags start at byte 208 and the first byte is 0, which means that the superblock has been written after the last modification. Byte 209 shows that soft dependencies are used, and the flags in 210 and 211 are in their default state. Byte 212 starts the location of the last mount point, and this file system claims to have been last mounted at /mnt/. The remaining fields are left for you to enjoy processing. The fsstat tool output for this image was previously given in the "File System Category" section of Chapter 16.

Part I: Foundations

Digital Investigation Foundations

Computer Foundations

Hard Disk Data Acquisition

Part II: Volume Analysis

Volume Analysis

PC-based Partitions

Server-based Partitions

Multiple Disk Volumes

Part III: File System Analysis

File System Analysis

FAT Concepts and Analysis

FAT Data Structures

NTFS Concepts

NTFS Analysis

NTFS Data Structures

Ext2 and Ext3 Concepts and Analysis

Ext2 and Ext3 Data Structures

UFS1 and UFS2 Concepts and Analysis

UFS1 and UFS2 Data Structures

Summary

Bibliography

Bibliography



File System Forensic Analysis
File System Forensic Analysis
ISBN: 0321268172
EAN: 2147483647
Year: 2006
Pages: 184
Authors: Brian Carrier

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