Apple Disk Image

Apple Disk Image (commonly referred to by its file extension, .dmg) is the de-facto disk image format on macOS and iOS. The format is comparable to ISO and IMG. They contain a mechanism for loopback mounts. The DMG files are self-contained file-system images, and can be mounted by the hdiutil(1) command, which can also be used to create them (using its -create option). In macOS, a double click or using the command line of open(1) will similarly result in a mount. Contrary to popular misconception, the DMG is not a single file format, and has several sub formats. Some also have a license embedded in them, so as to force macOS to display an acceptance GUI, prior to performing the actual mount.

The handling of the mount operation is performed at the kernel level by the com.apple.driver.DiskImages extension.

iOS is capable of mounting DMG files, and the iPhone SDK indeed contains the "DeveloperDiskImage.dmg", which contains various binaries and libraries for profiling, such as CHUD. Likewise, the recovery, restore, and file system images of iOS (contained in the .ipsw) are .DMG files. In iOS, however, the DMG file format is that of a raw file system, and cannot be readily mounted with hdiutil, unless the -imagekey argument is specified:

root@Ergo (/)# file ~/iOS/5.1.restore.ramdisk.dmg /Users/morpheus/iOS/5.1.restore.ramdisk.dmg: Macintosh HFS Extended version 4 data (mounted) last mounted by: '10.0', created: Wed Feb 15 05:26:23 2012, last modified: Tue Apr 3 11:16:04 2012, last checked: Wed Feb 15 08:26:23 2012, block size: 4096, number of blocks: 4218, free blocks: 0
 * 1) Use file command to show image file type. Mountable DMGs are usually identified as VAX COFF, or bzip2 data
 * 1) Use file command to show image file type. Mountable DMGs are usually identified as VAX COFF, or bzip2 data

root@Ergo (/)# hdiutil attach ~/iOS/5.1.restore.ramdisk.dmg hdiutil: attach failed - not recognized root@Ergo (/)# hdiutil attach ~/iOS/5.1.restore.ramdisk.dmg -imagekey diskimage-class=CRawDiskImage /dev/disk3 /Volumes/ramdisk root@Ergo (/)# hdiutil info image-path image-alias shadow-path icon-path image-path: /Users/morpheus/iOS/5.1.restore.ramdisk.dmg image-alias: /Users/morpheus/iOS/5.1.restore.ramdisk.dmg shadow-path: icon-path: /System/Library/PrivateFrameworks/DiskImages.framework/Resources /CDiskImage.icns ...
 * 1) Normal attachment: No go
 * 1) Normal attachment: No go
 * 1) A little more forceful:
 * 1) A little more forceful: