Apple Disk Image is a disk image format commonly used by the macOS operating system. When opened, an Apple Disk Image is mounted as a volume within the Macintosh Finder.

An Apple Disk Image can be structured according to one of several proprietary disk image formats, including the Universal Disk Image Format (UDIF) and the New Disk Image Format (NDIF). An Apple disk image file's name usually has '.dmg' as its extension.

DMG files are mounted during the installation of programs on a Mac. Mount a DMG file with help from a web and graphics specialist in this free video clip. Expert: Brian Santucci.

Features[edit]

Apple Disk Image files are published with a MIME type of application/x-apple-diskimage.

Different file systems can be contained inside these disk images, and there is also support for creating hybrid optical media images that contain multiple file systems.^[1] Some of the file systems supported include Hierarchical File System (HFS), HFS Plus, File Allocation Table (FAT), ISO9660 and Universal Disk Format (UDF).^[1][2]

Apple Disk Images can be created using utilities bundled with Mac OS X, specifically Disk Copy in Mac OS X v10.2 and earlier and Disk Utility in Mac OS X v10.3 and later. These utilities can also use Apple disk image files as images for burning CDs and DVDs. Disk image files may also be managed via the command line interface using the hdiutil utility.^[3]

In Mac OS X v10.2.3, Apple introduced Compressed Disk Images^[4] and Internet-Enabled Disk Images for use with the Apple utility Disk Copy, which was later integrated into Disk Utility in 10.3. The Disk Copy application had the ability to display a multi-lingual software license agreement before mounting a disk image. The image will not be mounted unless the user indicates agreement with the license.^[5]

An Apple Disk Image allows secure password protection as well as file compression, and hence serves both security and file distribution functions; such a disk image is most commonly used to distribute software over the Internet.

History[edit]

Apple originally created its disk image formats because the resource fork used by Mac applications could not easily be transferred over mixed networks such as those that make up the Internet. Even as the use of resource forks declined with Mac OS X, disk images remained the standard software distribution format. Disk images allow the distributor to control the Finder's presentation of the window, which is commonly used to instruct the user to copy the application to the correct folder.

A previous version of the format, intended only for floppy disk images, is usually referred to as 'Disk Copy 4.2' format, after the version of the Disk Copy utility that was used to handle these images.^[1] A similar format that supported compression of floppy disk images is called DART.^[1][6]

New Disk Image Format (NDIF) was the previous default disk image format in Mac OS 9,^[1] and disk images with this format generally have a .img (not to be confused with raw .img disk image files) or .smi file extension. Files with the .smi extension are actually applications that mount an embedded disk image, thus a 'Self Mounting Image', and are intended only for Mac OS 9 and earlier.^[7][2]

Universal Disk Image Format (UDIF) is the native disk image format for Mac OS X. Disk images in this format typically have a .dmg extension.^[1]

File format[edit]

Apple has not released any documentation on the format, but attempts to reverse engineer parts of the format have been successful. The encrypted layer was reverse engineered in an implementation called VileFault^[8] (a spoonerism of FileVault).

Apple disk image files are essentially raw disk images (i.e. contain block data) with some added metadata, optionally with one or two layers applied that provide compression and encryption. In hdiutil these layers are called CUDIFEncoding and CEncryptedEncoding.^[1]

UDIF supports ADC (an old proprietary compression format by Apple), zlib, bzip2 (as of Mac OS X v10.4), and LZFSE (as of Mac OS X v10.11)^[9] compression internally.

Trailer[edit]

The trailer can be described using the following C structure.^[10] All values are big-endian (PowerPC byte ordering)

Here is an explanation:

Utilities[edit]

There are few options available to extract files or mount the proprietary Apple Disk Image format. Some cross-platform conversion utilities are:

• dmg2img was originally written in Perl; however, the Perl version is no longer maintained, and the project was rewritten in C. Currently, without additional tools, the resulting images may be mounted only under Mac OS X and under Linux (provided hfsplus support has been enabled). UDIF ADC-compressed images have been supported since version 1.5.^[11]
• DMGEXtractor is written in Java with GUI, and it supports more advanced features of dmg including AES-128 encrypted images but not UDCO images.^[12]
• 7-Zip, including the free cross-platform port of its command-line interface, p7zip.

In Windows, most dmg images can be opened using several other programs such as UltraISO and IsoBuster. MacDrive can also mount simple dmg files as drives under windows, but not sparse disk or encrypted dmgs.^[13] A free Apple DMG Disk Image Viewer also exists.^[14]

In Linux and possibly other Unix flavors, most .dmg files can be burned to CD/DVD using any CD-burner program (using cdrecord directly or a front-end such as K3B or Brasero) or directly mounted to a mountpoint (e.g. mount -o loop,ro -t hfsplus imagefile.dmg /mnt/mountpoint).^[15][16] darling-dmg is a FUSE module enabling easy DMG file mounting on Linux.^[17]

See also[edit]

References[edit]

1. ^ ^abcdefg'hdiutil(1) Mac OS X Manual Page'. Archived from the original on 2016-05-14. Retrieved 2016-05-14.
2. ^ ^ab'Mac OS X: Using Disk Copy disk image files'. Archived from the original on 2013-12-19. Retrieved 2013-04-25.
3. ^hdiutil(1) – Darwin and macOS General Commands Manual
4. ^'Re: Some apps refuse to launch in 10.2.8! (OT, but very important)'. Archived from the original on 2014-01-17.
5. ^'Guides'. Apple. Archived from the original on 2009-03-06. Retrieved 2010-10-27.
6. ^'DART 1.5.3: Version Change History'. Archived from the original on 2013-12-19. Retrieved 2013-04-25.
7. ^'Software Downloads: Formats and Common Error Messages'. Archived from the original on 2010-12-24. Retrieved 2009-05-06.
8. ^'VileFault'. 2006-12-29. Archived from the original on 2007-01-09. Retrieved 2010-10-27.
9. ^Michael Tsai (2015-10-07). 'LZFSE Disk Images in El Capitan'. Archived from the original on 2017-04-09. Retrieved 2017-04-09.
10. ^'Demystifying the DMG File Format'. Archived from the original on 2013-03-17.
11. ^'dmg2img'. Archived from the original on 29 November 2010. Retrieved 2010-10-27.
12. ^'DMGExtractor'. Archived from the original on 2011-01-02. Retrieved 2011-01-03.
13. ^MacDrive Features / Boot Camp / System Requirements /. 'MacDrive Home page'. Mediafour. Archived from the original on 4 October 2010. Retrieved 2010-10-27.
14. ^Olivia Dehaviland (2015-03-03). 'Apple DMG Disk Image Viewer'. DataForensics.org. Archived from the original on 2015-04-02. Retrieved 2015-03-10.
15. ^'How To Convert DMG To ISO in Windows, Linux & Mac'. Archived from the original on 2010-03-07.
16. ^'Convert DMG To ISO using PowerISO'. Archived from the original on 2009-05-02. Retrieved 2009-02-15.
17. ^'darling-dmg'. darling-dmg. Retrieved 29 March 2015.

External links[edit]

• Apple Developer Connection A Quick Look at PackageMaker and Installer
• O'Reilly Mac DevCenter Tip 16-5. Create a Disk Image from a Directory in the Terminal

Overview

The Disk Copy or Disk Utility feature for macOS can be used to create an encrypted disk image. Encryption means it requires a password to open or become available (to 'mount'). An encrypted disk image acts as secure storage space and can be used like any other disk image file. It may be copied to or created on network volumes or removable media, including Zip drives, USB flash media, or FireWire hard drives. Each encrypted disk image is protected by a password. The following document explains how to create, mount, and unmount an encrypted disk image.
Important: If you forget the password to your encrypted disk image, the data on that disk image cannot be recovered.

Getting Started

Files can be moved to or from an encrypted disk image as easily as you can from a non-encrypted disk image. Please be aware of the following when using an encrypted disk image:

• Backup programs will need to back up the entire image if any files within it have changed, since the image appears as a single file.
• There is no way to change the password on an image file.
• An encrypted disk image cannot be used as your home directory.

Complete the following steps to create an encrypted disk image for your version of macOS v. 10.4 or greater.

MacOS

1. Click Go, Applications, Utilities, and then click to open Disk Utility.
   Note: You can also click the New Image button and go to Creating a New Blank Image section.
2. Choose File, then New, then Blank Disk Image.
   
3. A New BlankImage window will display.
   
4. Enter a name in the Save As field. This name is used for the disk image (.dmg) file.
   
5. Enter a name in the Name field. This name displays when the disk image file is opened (mounted).
   
6. Select the Size of the image file from the drop-down menu.
   
7. Choose the Encryption option 128-bit AES to encrypt the image.
   
8. Select the volume Image Format from the drop-down menu. The default “read/write” option is recommended. Click the Create button.
   
9. Enter and verify a secure password when prompted and click OK.
   Note: Ensure the box next to Remember password (add to keychain) is unchecked. This ensures that no one will be able to determine the password for your encrypted drive by checking your keychain.

Mounting and Unmounting Encrypted Disk Images

1. After the encrypted disk image has been created, it will automatically mount for the first time and files can be copied to this location. The named volume disk image will display above the mounted source file.
   
2. When you are finished using the encrypted disk image, you must unmount the image. Drag its icon to the trash or select the eject button next to its icon in any finder window. In the example below, the encrypted disk image is named “personal.'
   
3. To mount the image again, simply double-click on the disk image you created. In the image below, the disk image is named personal.dmg. You will see it at the top of the list.
   
4. Enter your password when prompted. Click OK. Your disk image should be mounted.
   Note: Always remember to unmount your disk image when you are finished with it.