CHFI dd command Practice Question
An analyst recovers a disk image from a Linux server that used ext4. The image shows a superblock backup at multiple offsets. Which dd command would correctly extract the backup superblock located at offset 32768 bytes?
⚠ Common exam trap
Candidates may incorrectly assume that only one command is correct, or they may miscalculate skip values for different block sizes. Both B and C produce the same result; the key is to correctly compute skip and count to read exactly the superblock size (1024 bytes) at the given offset.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
dd if=image.dd of=superblock.bin bs=512 skip=64 count=2
Both options B and C correctly extract the backup superblock from the disk image. The ext4 superblock is 1024 bytes and starts at offset 32768 bytes. With bs=512, skip=64 gives offset 32768 (64*512=32768), and count=2 reads 1024 bytes (2*512=1024). With bs=1024, skip=32 gives offset 32768 (32*1024=32768), and count=1 reads 1024 bytes. Both commands extract exactly the superblock. Option A uses bs=4096 and count=1, reading 4096 bytes, which is too much and may include adjacent data. Option D uses bs=32768 and count=1, reading 32768 bytes, far exceeding the superblock size.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
dd if=image.dd of=superblock.bin bs=4096 skip=8 count=1
Why it's wrong here
Choosing bs=4096 and skip=8 computes the offset as 8 * 4096 = 32,768 bytes, matching the superblock location, but count=1 makes dd read a full 4,096-byte block. Because the superblock is only 1,024 bytes, the output file would be four times larger than the target, capturing the superblock plus the following 3,072 bytes of group descriptors and other metadata that immediately follow in the block group. This over-read prevents the extracted file from being a clean, size-matched superblock, so it fails forensic requirements even though the offset is correct.
- ✓
dd if=image.dd of=superblock.bin bs=512 skip=64 count=2
Why this is correct
Using bs=512 with skip=64 also lands at 64 * 512 = 32,768 bytes, the same superblock offset, but count=2 reads two 512-byte sectors for a total of 1,024 bytes. This approach is explicitly correct because it compensates for the smaller block size by incrementing the count, thereby extracting exactly the superblock without including adjacent sectors. It demonstrates that dd can achieve the same forensic extraction using any divisor of the superblock size, as long as the offset and byte count equal 32,768 and 1,024 respectively.
- ✓
dd if=image.dd of=superblock.bin bs=1024 skip=32 count=1
Why this is correct
With bs=1024 and skip=32, dd seeks 32 * 1024 = 32,768 bytes into the image, exactly the offset of the primary superblock (block 0 of the filesystem is at byte 1024, but the superblock starts at 32,768 bytes after the partition offset in this scenario). Reading count=1 transfers exactly 1,024 bytes, which matches the canonical 1,024-byte size of an ext2/3/4 superblock, producing a pristine superblock.bin with no extraneous data. This is the standard dd method for isolating a superblock because block size, skip count, and read count align perfectly with the target structure.
- ✗
dd if=image.dd of=superblock.bin bs=32768 skip=1 count=1
Why it's wrong here
This invocation sets the skip offset to 1 * 32768 = 32768 bytes, precisely where the primary superblock begins, but count=1 with bs=32768 causes dd to read a full 32,768-byte block into the output file. Since an ext2/3/4 superblock is exactly 1,024 bytes, the resulting superblock.bin would contain the superblock plus ~31,744 bytes of adjacent filesystem metadata, corrupting the extraction. Moreover, using a block size larger than the superblock is invalid for forensic isolation because the output size must equal the object's true size.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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