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 from Mac OS X and the New Disk Image Format from Mac OS 9. An Apple disk image file's name usually has ".dmg" as its extension.
Features
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. Some of the file systems supported include Hierarchical File System, HFS Plus, File Allocation Table, ISO9660 and Universal Disk Format. 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. In Mac OS X v10.2.3, Apple introduced Compressed Disk Images 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. 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
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. A similar format that supported compression of floppy disk images is called DART. New Disk Image Format was the previous default disk image format in Mac OS 9, and disk images with this format generally have a .img or .smi file extension. Files with the .smi extension are actually applications that mount an embedded disk image, thus a "Self Mounting Image", intended only for Mac OS 9 and earlier. Universal Disk Image Format is the native disk image format for Mac OS X. Disk images in this format typically have a .dmg extension.
File format
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. Apple disk image files are essentially raw disk images 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. UDIF supports ADC, zlib, bzip2, and LZFSE compression internally.
Metadata
The UDIF metadata is found at the end of the disk image following the data. This trailer can be described using the following C structure. All values are big-endian typedef struct __attribute__ UDIFResourceFile;
The XML plist contains a key, with information about how the preceeding data fork is allocated. The main data is stored in a base64 block, using tables identified by the magic. This structure contains a table about blocks of data and the position and lengths of each "chunk". The data and resource fork information is probably inherited from NDIF.
Encryption
The encryption layer comes in two versions. Version 1 has a trailer at the end of the file, while version 2 puts it at the beginning. Whether the encryption is a layer outside of or inside of the metadata is unclear from reverse engineered documentation, but judging from the demonstration it's probably outside.
Utilities
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. It extracts the raw disk image from a DMG, without handling the file system inside. UDIF ADC-compressed images have been supported since version 1.5.
DMGEXtractor is written in Java with GUI, and it supports more advanced features of dmg including AES-128 encrypted images but not UDCO images.
The Sleuth Kit. Handles the DMG format, HFS+, and APFS.
Most dmg files are unencrypted. Because the dmg metadata is found in the end, a program not understanding dmg files can nevertheless read it as if it was a normal disk image, as long as there is support for the file system inside. Tools with this sort of capacity include:
* Transmac can handle both UDIF dmgs and sparsebundles, as well as HFS/HFS+ and APFS. It is unknown whether it handles encryption. It can be used to create bootable macOS installers under Windows.
* A free Apple DMG Disk Image Viewer also exists, but it is unknown how much what it actually supports..
Unix-like:
* darling-dmg is a FUSE module enabling easy DMG file mounting on Linux. It supports UDIF and HFS/HFS+.
