SKILL.md

Acquiring Disk Image with dd and dcfldd

When to Use

• When you need to create a forensic copy of a suspect drive for investigation
• During incident response when preserving volatile disk evidence before analysis
• When law enforcement or legal proceedings require a verified bit-for-bit copy
• Before performing any destructive analysis on a storage device
• When acquiring images from physical drives, USB devices, or memory cards

Prerequisites

• Linux-based forensic workstation (SIFT, Kali, or any Linux distro)
• dd (pre-installed on all Linux systems) or dcfldd (enhanced forensic version)
• Write-blocker hardware or software write-blocking configured
• Destination drive with sufficient storage (larger than source)
• Root/sudo privileges on the forensic workstation
• SHA-256 or MD5 hashing utilities (sha256sum, md5sum)

Workflow

Step 1: Identify the Target Device and Enable Write Protection

# List all connected block devices to identify the target
lsblk -o NAME,SIZE,TYPE,MOUNTPOINT,MODEL

# Verify the device details
fdisk -l /dev/sdb

# Enable software write-blocking (if no hardware blocker)
blockdev --setro /dev/sdb

# Verify read-only status
blockdev --getro /dev/sdb
# Output: 1 (means read-only is enabled)

# Alternatively, use udev rules for persistent write-blocking
echo 'SUBSYSTEM=="block", ATTRS{serial}=="WD-WCAV5H861234", ATTR{ro}="1"' > /etc/udev/rules.d/99-writeblock.rules
udevadm control --reload-rules

Step 2: Prepare the Destination and Document the Source

# Create case directory structure
mkdir -p /cases/case-2024-001/{images,hashes,logs,notes}

# Document source drive information
hdparm -I /dev/sdb > /cases/case-2024-001/notes/source_drive_info.txt

# Record the serial number and model
smartctl -i /dev/sdb >> /cases/case-2024-001/notes/source_drive_info.txt

# Pre-hash the source device
sha256sum /dev/sdb | tee /cases/case-2024-001/hashes/source_hash_before.txt

Step 3: Acquire the Image Using dd

# Basic dd acquisition with progress and error handling
dd if=/dev/sdb of=/cases/case-2024-001/images/evidence.dd \
   bs=4096 \
   conv=noerror,sync \
   status=progress 2>&1 | tee /cases/case-2024-001/logs/dd_acquisition.log

# For compressed images to save space
dd if=/dev/sdb bs=4096 conv=noerror,sync status=progress | \
   gzip -c > /cases/case-2024-001/images/evidence.dd.gz

# Using dd with a specific count for partial acquisition
dd if=/dev/sdb of=/cases/case-2024-001/images/first_1gb.dd \
   bs=1M count=1024 status=progress

Step 4: Acquire Using dcfldd (Preferred Forensic Method)

# Install dcfldd if not present
apt-get install dcfldd

# Acquire image with built-in hashing and split output
dcfldd if=/dev/sdb \
   of=/cases/case-2024-001/images/evidence.dd \
   hash=sha256,md5 \
   hashwindow=1G \
   hashlog=/cases/case-2024-001/hashes/acquisition_hashes.txt \
   bs=4096 \
   conv=noerror,sync \
   errlog=/cases/case-2024-001/logs/dcfldd_errors.log

# Split large images into manageable segments
dcfldd if=/dev/sdb \
   of=/cases/case-2024-001/images/evidence.dd \
   hash=sha256 \
   hashlog=/cases/case-2024-001/hashes/split_hashes.txt \
   bs=4096 \
   split=2G \
   splitformat=aa

# Acquire with verification pass
dcfldd if=/dev/sdb \
   of=/cases/case-2024-001/images/evidence.dd \
   hash=sha256 \
   hashlog=/cases/case-2024-001/hashes/verification.txt \
   vf=/cases/case-2024-001/images/evidence.dd \
   verifylog=/cases/case-2024-001/logs/verify.log

Step 5: Verify Image Integrity

# Hash the acquired image
sha256sum /cases/case-2024-001/images/evidence.dd | \
   tee /cases/case-2024-001/hashes/image_hash.txt

# Compare source and image hashes
diff <(sha256sum /dev/sdb | awk '{print $1}') \
     <(sha256sum /cases/case-2024-001/images/evidence.dd | awk '{print $1}')

# If using split images, verify each segment
sha256sum /cases/case-2024-001/images/evidence.dd.* | \
   tee /cases/case-2024-001/hashes/split_image_hashes.txt

# Re-hash source to confirm no changes occurred
sha256sum /dev/sdb | tee /cases/case-2024-001/hashes/source_hash_after.txt
diff /cases/case-2024-001/hashes/source_hash_before.txt \
     /cases/case-2024-001/hashes/source_hash_after.txt

Step 6: Document the Acquisition Process

# Generate acquisition report
cat << 'EOF' > /cases/case-2024-001/notes/acquisition_report.txt
DISK IMAGE ACQUISITION REPORT
==============================
Case Number: 2024-001
Date/Time: $(date -u +"%Y-%m-%d %H:%M:%S UTC")
Examiner: [Name]

Source Device: /dev/sdb
Model: [from hdparm output]
Serial: [from hdparm output]
Size: [from fdisk output]

Acquisition Tool: dcfldd v1.9.1
Block Size: 4096
Write Blocker: [Hardware/Software model]

Image File: evidence.dd
Image Hash (SHA-256): [from hash file]
Source Hash (SHA-256): [from hash file]
Hash Match: YES/NO

Errors During Acquisition: [from error log]
EOF

# Compress logs for archival
tar -czf /cases/case-2024-001/acquisition_package.tar.gz \
   /cases/case-2024-001/hashes/ \
   /cases/case-2024-001/logs/ \
   /cases/case-2024-001/notes/

Key Concepts

Tools & Systems

Common Scenarios

Scenario 1: Acquiring a Suspect Laptop Hard Drive
Connect the drive via a Tableau T35u hardware write-blocker, identify as /dev/sdb, use dcfldd with SHA-256 hashing, split into 4GB segments for DVD archival, verify hashes match, document in case notes.

Scenario 2: Imaging a USB Flash Drive from a Compromised Workstation
Use software write-blocking with blockdev --setro, acquire with dcfldd including MD5 and SHA-256 dual hashing, image is small enough for single file, verify and store on encrypted case drive.

Scenario 3: Remote Acquisition Over Network
Use dd piped through netcat or ssh for remote acquisition: ssh root@remote "dd if=/dev/sda bs=4096" | dd of=remote_image.dd bs=4096, hash both ends independently to verify transfer integrity.

Scenario 4: Acquiring from a Failing Drive
Use ddrescue first to recover readable sectors, then use dd with conv=noerror,sync to fill gaps with zeros, document which sectors were unreadable in the error log.

Output Format

Acquisition Summary:
  Source:       /dev/sdb (500GB Western Digital WD5000AAKX)
  Destination:  /cases/case-2024-001/images/evidence.dd
  Tool:         dcfldd 1.9.1
  Block Size:   4096 bytes
  Duration:     2h 15m 32s
  Bytes Copied: 500,107,862,016
  Errors:       0 bad sectors
  Source SHA-256:  a3f2b8c9d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0a1
  Image SHA-256:   a3f2b8c9d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b0c1d2e3f4a5b6c7d8e9f0a1
  Verification:    PASSED - Hashes match

Supporting file: LICENSE

This file is part of the acquiring-disk-image-with-dd-and-dcfldd skill package. Use it when SKILL.md references LICENSE.

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Supporting file: references/api-reference.md

This file is part of the acquiring-disk-image-with-dd-and-dcfldd skill package. Use it when SKILL.md references references/api-reference.md.

API Reference: dd and dcfldd Disk Imaging

dd - Standard Unix Disk Duplication

Basic Syntax

dd if=<source> of=<destination> [options]

Key Options

conv= Values

• noerror - Continue on read errors (do not abort)
• sync - Pad input blocks with zeros on error (preserves offset alignment)
• notrunc - Do not truncate output file

Output Format

500107862016 bytes (500 GB, 466 GiB) copied, 8132.45 s, 61.5 MB/s
976773168+0 records in
976773168+0 records out

dcfldd - DoD Forensic dd

Basic Syntax

dcfldd if=<source> of=<destination> [options]

Extended Options

Output Format

Total (sha256): a3f2b8c9d4e5f6a7b8c9d0e1f2a3b4c5...
1024+0 records in
1024+0 records out

sha256sum - Hash Verification

Syntax

sha256sum <file_or_device>
sha256sum -c <checksum_file>

Output Format

a3f2b8c9d4e5f6...  /dev/sdb
a3f2b8c9d4e5f6...  evidence.dd

blockdev - Write Protection

Syntax

blockdev --setro <device>   # Set read-only
blockdev --setrw <device>   # Set read-write
blockdev --getro <device>   # Check: 1=RO, 0=RW
blockdev --getsize64 <device>  # Size in bytes

lsblk - Block Device Enumeration

Syntax

lsblk -o NAME,SIZE,TYPE,MOUNTPOINT,MODEL,SERIAL,RO
lsblk -J   # JSON output
lsblk -p   # Full device paths

hdparm - Drive Identification

Syntax

hdparm -I <device>   # Detailed drive info
hdparm -i <device>   # Summary identification

Supporting file: scripts/agent.py

This file is part of the acquiring-disk-image-with-dd-and-dcfldd skill package. Use it when SKILL.md references scripts/agent.py.

#!/usr/bin/env python3
"""Forensic disk image acquisition agent using dd and dcfldd with hash verification."""

import shlex
import subprocess
import hashlib
import os
import datetime
import json


def run_cmd(cmd, capture=True):
    """Execute a command and return output."""
    if isinstance(cmd, str):
        cmd = shlex.split(cmd)
    result = subprocess.run(cmd, capture_output=capture, text=True, timeout=120)
    return result.stdout.strip(), result.stderr.strip(), result.returncode


def list_block_devices():
    """Enumerate connected block devices."""
    stdout, _, rc = run_cmd("lsblk -J -o NAME,SIZE,TYPE,MOUNTPOINT,MODEL,SERIAL,RO")
    if rc == 0 and stdout:
        return json.loads(stdout)
    return {"blockdevices": []}


def check_write_protection(device):
    """Verify a device is set to read-only mode."""
    stdout, _, rc = run_cmd(f"blockdev --getro {device}")
    if rc == 0:
        return stdout.strip() == "1"
    return False


def enable_write_protection(device):
    """Enable software write-blocking on the target device."""
    _, _, rc = run_cmd(f"blockdev --setro {device}")
    if rc != 0:
        print(f"[ERROR] Failed to set {device} read-only. Run as root.")
        return False
    if check_write_protection(device):
        print(f"[OK] Write protection enabled on {device}")
        return True
    print(f"[ERROR] Write protection verification failed for {device}")
    return False


def compute_hash(path, algorithm="sha256", block_size=65536):
    """Compute the SHA-256 or MD5 hash of a file or device."""
    h = hashlib.new(algorithm)
    try:
        with open(path, "rb") as f:
            while True:
                block = f.read(block_size)
                if not block:
                    break
                h.update(block)
    except PermissionError:
        print(f"[ERROR] Permission denied reading {path}. Run as root.")
        return None
    except FileNotFoundError:
        print(f"[ERROR] Path not found: {path}")
        return None
    return h.hexdigest()


def acquire_with_dd(source, destination, block_size=4096, log_file=None):
    """Acquire a forensic image using dd with error handling."""
    dd_cmd = [
        "dd", f"if={source}", f"of={destination}",
        f"bs={block_size}", "conv=noerror,sync", "status=progress"
    ]
    print(f"[*] Starting dd acquisition: {source} -> {destination}")
    print(f"[*] Block size: {block_size}")
    start = datetime.datetime.utcnow()
    if log_file:
        dd_proc = subprocess.run(dd_cmd, capture_output=True, text=True, timeout=120)
        combined = (dd_proc.stdout or "") + (dd_proc.stderr or "")
        with open(log_file, "w") as lf:
            lf.write(combined)
        rc = dd_proc.returncode
    else:
        result = subprocess.run(dd_cmd, text=True, timeout=120)
        rc = result.returncode
    elapsed = (datetime.datetime.utcnow() - start).total_seconds()
    print(f"[*] Acquisition completed in {elapsed:.1f} seconds (rc={rc})")
    return rc == 0


def acquire_with_dcfldd(source, destination, hash_alg="sha256", hash_log=None,
                        error_log=None, block_size=4096, split_size=None):
    """Acquire a forensic image using dcfldd with built-in hashing."""
    cmd = [
        "dcfldd", f"if={source}", f"of={destination}",
        f"bs={block_size}", "conv=noerror,sync",
        f"hash={hash_alg}", "hashwindow=1G",
    ]
    if hash_log:
        cmd.append(f"hashlog={hash_log}")
    if error_log:
        cmd.append(f"errlog={error_log}")
    if split_size:
        cmd.extend([f"split={split_size}", "splitformat=aa"])
    print(f"[*] Starting dcfldd acquisition: {source} -> {destination}")
    start = datetime.datetime.utcnow()
    result = subprocess.run(cmd, text=True, timeout=120)
    rc = result.returncode
    elapsed = (datetime.datetime.utcnow() - start).total_seconds()
    print(f"[*] dcfldd completed in {elapsed:.1f} seconds (rc={rc})")
    return rc == 0


def verify_image(source, image_path, algorithm="sha256"):
    """Verify image integrity by comparing hashes of source and acquired image."""
    print(f"[*] Computing {algorithm} hash of source: {source}")
    source_hash = compute_hash(source, algorithm)
    print(f"    Source hash:  {source_hash}")
    print(f"[*] Computing {algorithm} hash of image: {image_path}")
    image_hash = compute_hash(image_path, algorithm)
    print(f"    Image hash:   {image_hash}")
    if source_hash and image_hash:
        match = source_hash == image_hash
        status = "PASSED" if match else "FAILED"
        print(f"[{'OK' if match else 'FAIL'}] Verification: {status}")
        return match, source_hash, image_hash
    return False, source_hash, image_hash


def generate_report(case_dir, source_device, image_path, tool_used,
                    source_hash, image_hash, verified, elapsed_seconds=0):
    """Generate a forensic acquisition report."""
    report = {
        "report_type": "Disk Image Acquisition",
        "timestamp": datetime.datetime.utcnow().isoformat() + "Z",
        "case_directory": case_dir,
        "source_device": source_device,
        "image_file": image_path,
        "acquisition_tool": tool_used,
        "block_size": 4096,
        "source_hash_sha256": source_hash,
        "image_hash_sha256": image_hash,
        "hash_verified": verified,
        "duration_seconds": elapsed_seconds,
    }
    report_path = os.path.join(case_dir, "acquisition_report.json")
    with open(report_path, "w") as f:
        json.dump(report, f, indent=2)
    print(f"[*] Report saved to {report_path}")
    return report


if __name__ == "__main__":
    print("=" * 60)
    print("Forensic Disk Image Acquisition Agent")
    print("Tools: dd / dcfldd with SHA-256 verification")
    print("=" * 60)

    # Demo: list block devices
    print("\n[*] Enumerating block devices...")
    devices = list_block_devices()
    for dev in devices.get("blockdevices", []):
        name = dev.get("name", "?")
        size = dev.get("size", "?")
        dtype = dev.get("type", "?")
        model = dev.get("model", "N/A")
        ro = "RO" if dev.get("ro") else "RW"
        print(f"    /dev/{name}  {size}  {dtype}  {model}  [{ro}]")

    # Demo workflow (dry run)
    demo_source = "/dev/sdb"
    demo_case = "/cases/demo-case/images"
    demo_image = os.path.join(demo_case, "evidence.dd")

    print(f"\n[DEMO] Acquisition workflow for {demo_source}:")
    print(f"  1. Enable write protection: blockdev --setro {demo_source}")
    print(f"  2. Acquire with dcfldd: dcfldd if={demo_source} of={demo_image} "
          f"hash=sha256 hashwindow=1G bs=4096 conv=noerror,sync")
    print(f"  3. Verify: compare SHA-256 of {demo_source} and {demo_image}")
    print(f"  4. Generate acquisition report with chain-of-custody metadata")
    print("\n[*] Agent ready. Provide a source device and case directory to begin.")