.DMG

Not to be confused with .dmg or .smi

MIME type: application/x-apple-diskimage

Is binary? Yes

Is encrypted? Unknown

Can execute dangerous code? No

Related extensions: .dmg .smi .img .iso

Internet Media types: application/x-apple-diskimage

Uniform Type Identifier (UTI) com.apple.disk-image

Type code ?

Magic number Unknown

A .DMG file is an Apple Disk Image file. It is used to install software on MacOS.

The uppercase variant has no diference from the original. They are both the same.

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) from Mac OS X and the New Disk Image Format (NDIF) from Mac OS 9. An Apple disk image file's name usually has ".DMG" as its extension.

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Operating system: Any (MS-DOS, DR-DOS, Windows, Windows 9x, Windows NT, Linux, GNU/Linux, Classic MacOS, MacOS, MacOS X, OS X, iOS, Android, ChromiumOS, ChromeOS, OS/2, Solaris, FreeBSD, NetBSD, OpenBSD, DragonflyBSD, etc.)

Opens with:

Finder (MacOS)

Other? Unknown

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Definition 1: Apple Disk Image file

A .DMG file is an Apple Disk image file. It is used to mount and install software on the MacOS operating system. It was introduced in MacOS 9.

Definition 2: Pseudo-extension (null)

A pseudo-extension is a file format that is applied to a file, despite not having a purpose, other than to change the file name (such as thatsalotta.DMG) however this file extension is rarely used as a pseudo-extension

Alternatives

It is not recommended to alternate from this software on MacOS entirely, as it is one of the system defaults. You can use alternative extensions for diferent purposes. Alternatives to the DMG Disk Image file include:

.iso

.exe

.deb

.rpm

.apk

Recommend one

No other definitions

Recommend one

Limitations and problems

Unknown

Recommend one

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 (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", intended only for Mac OS 9 and earlier.

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.

Version history

There are only ? versions of the DMG Disk image file.

The following table needs to be reformatted.

Data unavailable

Metadata

Apple Disk Image files contain metadata. The following is a piece of the disk image header that has been reverse engineered. It is written in the C programming language.


typedef struct {
  uint8_t  Signature[4];           // magic 'koly'
  uint32_t Version;                // 4 (as of 2013)
  uint32_t HeaderSize;             // sizeof(this) =  512 (as of 2013)
  uint32_t Flags;                 
  uint64_t RunningDataForkOffset;
  uint64_t DataForkOffset;         // usually 0, beginning of file
  uint64_t DataForkLength;
  uint64_t RsrcForkOffset;         // resource fork offset and length
  uint64_t RsrcForkLength;        
  uint32_t SegmentNumber;          // Usually 1, can be 0
  uint32_t SegmentCount;           // Usually 1, can be 0
  uuid_t   SegmentID; 
  uint32_t DataChecksumType;       // Data fork checksum
  uint32_t DataChecksumSize;
  uint32_t DataChecksum[32];
  uint64_t XMLOffset;              // Position of XML property list in file
  uint64_t XMLLength; 
  uint8_t  Reserved1[120];
  uint32_t ChecksumType;           // Master checksum
  uint32_t ChecksumSize;
  uint32_t Checksum[32];
  uint32_t ImageVariant;           // Unknown, commonly 1
  uint64_t SectorCount;
  uint32_t reserved2;
  uint32_t reserved3;
  uint32_t reserved4;
} __attribute__((packed, scalar_storage_order("big-endian"))) UDIFResourceFile;