What is DMG?

A DMG file is a mountable disk image created for macOS. When opened, it appears as a mounted volume on the desktop, containing files and folders just like a physical disk. DMG files are commonly used to distribute software and can include custom backgrounds, icons, and license agreements.

DMG files replaced the older IMG format in Mac OS X and support advanced features like compression, encryption (password protection), and checksums for data integrity verification.

Did you know? Every Mac application you download likely comes in a DMG file. Apple distributes macOS installers, Xcode, and system updates using DMG images!

History

The DMG format was introduced with Mac OS X 10.0 in 2001 as part of Apple's transition to a Unix-based operating system. It replaced the older IMG and NDIF formats used in Mac OS 9.

Key Milestones

  • 2001: DMG format introduced with Mac OS X 10.0
  • 2003: Mac OS X 10.3 adds encrypted DMG support
  • 2005: Mac OS X 10.4 introduces sparse disk images
  • 2016: macOS Sierra improves DMG security verification
  • 2018: macOS Mojave requires notarization for DMG files

Key Features

Core Capabilities

  • Virtual Mounting: Appears as a real disk when opened
  • Compression: Multiple compression methods (zlib, bzip2, LZFSE)
  • Encryption: 128-bit and 256-bit AES encryption
  • Checksums: Built-in CRC32 and MD5 verification
  • Custom Layout: Custom backgrounds and window arrangements
  • Code Signing: Apple Developer ID signatures

Common Use Cases

Application Distribution

Standard format for Mac software installers

Encrypted Storage

Password-protected secure file containers

System Backups

Complete disk images for backup and recovery

macOS Installation

Operating system installers and updates

Advantages

  • Native macOS support (double-click to mount)
  • Excellent compression reduces file size
  • Strong encryption for security
  • Supports custom installer UI
  • Code signing for verified distribution
  • Automatic integrity verification

Disadvantages

  • macOS only (limited cross-platform support)
  • Proprietary format
  • Requires third-party tools on Windows/Linux
  • Can be large files despite compression
  • Not suitable for non-Mac users

Technical Information

Format Specifications

Specification Details
File Extension .dmg
MIME Type application/x-apple-diskimage
Developer Apple Inc.
Format Type Disk Image
Compression zlib, bzip2, LZFSE
Encryption 128-bit or 256-bit AES

Common Tools

  • macOS: Disk Utility (built-in)
  • Windows: 7-Zip, DMG Extractor
  • Linux: dmg2img, 7z