Filename extension .dmg, .smi, .img Type of format disk image | Developed by Apple Inc. Website apple.com | |
Internet media type application/x-apple-diskimage Uniform Type Identifier (UTI) com.apple.disk-image,
com.apple.disk-image-smi | ||
An Apple Disk Image is a disk image commonly used by the Mac OS X operating system. When opened, an Apple disk image is "mounted" as a volume within the 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.
Contents
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.
Overview
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.
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. New Disk Image Format (NDIF) was the previous default disk image format in Mac OS 9, 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. 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. 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), HFS Plus, File Allocation Table (FAT), ISO9660 and Universal Disk Format (UDF).
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 using the hdiutil utility.
UDIF data format
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.
UDIF supports ADC (an old proprietary compression format by Apple), zlib, and bzip2 (Mac OS X v10.4 and later only) compression internally.
Trailer
The trailer can be described using the following C structure. All values are big-endian (PowerPC byte ordering)
Here is an explanation:
Support
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 (a spoonerism of FileVault). There are few options available to extract files or mount the proprietary Apple Disk Image format. Some cross-platform conversion utilities are:
In Windows, most dmg images can be opened using several other programs such as 7-Zip, UltraISO, and IsoBuster. MacDrive can also mount simple dmg files as drives under windows, but not sparse disk or encrypted dmgs. A free Apple DMG Disk Image Viewer also exists.
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). darling-dmg is a FUSE module enabling easy DMG file mounting on Linux.
Macintosh
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.
Currently, the only way to open a .dmg disk image in Mac OS 9 is to use either the developer version of Disk Copy (version 6.4), or a beta version of the unreleased 6.5. However, both versions can only open uncompressed images; compressed Disk Images are unusable on Mac OS 9.