#What Is Dmg File Mac OS X Disk#

What Is Dmg File Mac OS X Disk

вЂўA file with the DMG file extension is an Apple Disk Image file, or sometimes called a Mac OS X Disk Image file, which is basically a digital reconstruction of a physical disc. AboutDMG File (What It Is and How to Open One) - Lifewire Discover The Best Images Images. The software search & extract data from DMG files like.DeMystifyinG the DMG file format Demystifying the DMG File Format Jonathan Levin, - 6/12/13 1. When you access a disk image, its icon will appear on the desktop as if you have attached an external data storage device.SysTools DMG File Extractor Software to Unpack, Save & Extract DMG files on Windows 10, 8, 7. DMG files can be opened, created or converted using the Apple Disk Utility located in the Utilities folder. Img file format used in old 'classic' Mac operating systems.

The DMG file format has been painstakingly reverse-engineered by several , and this article/addendum aims to consolidate their hard work into a single document. This article, therefore, is an attempt to rectify that shortcoming. I briefly mention DMGs (pages 589-590), but due to the page constraints of an already large book, I had failed to delve into their format sufficiently. I realized, however, that the DMG file format (being Apple proprietary) was woefully undocumented.

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.The koly block, when present, is formatted according to the following:Uint32_t Version // Current version is 4Uint32_t HeaderSize // sizeof(this), always 512Uint64_t DataForkOffset // Data fork offset (usually 0, beginning of file)Uint64_t DataForkLength // Size of data fork (usually up to the XMLOffset, below)Uint64_t RsrcForkOffset // Resource fork offset, if anyUint64_t RsrcForkLength // Resource fork length, if anyUint32_t SegmentNumber // Usually 1, may be 0Uint32_t SegmentCount // Usually 1, may be 0Uuid_t SegmentID // 128-bit GUID identifier of segment (if SegmentNumber !=0)Uint32_t DataChecksumSize // Checksum InformationUint32_t DataChecksum // Up to 128-bytes (32 x 4) of checksumUint64_t XMLOffset // Offset of property list in DMG, from beginningUint64_t XMLLength // Length of property listUint8_t Reserved1 // 120 reserved bytes - zeroedUint32_t ChecksumSize // Checksum informationUint32_t Checksum // Up to 128-bytes (32 x 4) of checksumUint64_t SectorCount // Size of DMG when expanded, in sectors} _attribute_((_packed_)) UDIFResourceFile Note: All fields in the koly block (and, in fact, elsewhere in the DMG format) are in big endian ordering. Images created with the DiscRecording.Framework contain the koly block.Most dmg files are unencrypted. In those cases, however, there is nothing special or noteworthy about the file, which can be read as any disk would, by its partition table (commonly APM, or GPT). Note, that 'most' is not 'all': images created with hdiutil(1), for example, can simply be raw dd(1)-like images of the disk layout, with no metadata. As other references to this trailer call it the 'koly' block, we can do the same. This trailer is identifiable by a magic 32-bit value, 0x6B6F6C79, which is 'koly' in ASCII.

DMG files compressed with BZlib, for example, start with a BZ2 header. Using HFSleuth in verbose mode on a DMG will dump the KOLY header, as shown in the following output:Data fork: from 0, spanning 200307220 bytesXML plist: from 200307220, spanning 56675 bytes (to 200363895)Segment UUID: 626f726e-7743259b-6086eb93-4b42fb65Output 1: Using HFSleuth on the iTunes 11.0 DMGThis method of creating DMGs also explains why commands such as 'file' have a hard time identifying the DMG file type: In the absence of a fixed header, a DMG can start with any type of data (disk or partition headers), which can be further compressed by myriad means. This is shown in figure 1:The Data Fork: Disk blocks, compressed in various ways. Commonly, the plist is placed in the blocks leading up to the koly block, which fits the simple algorithm to create a DMG: First compress the image blocks, then place the XML plist, and finalize with the koly block. This requires DMG implementations to use macros such as be#_to_cpu (16, 32, and 64).The most important elements in the koly block are the fields pointing to the XML plist: This property list, embedded elsewhere in the DMG, contains the DMG block map table.

What makes the blxx data useful, however, is that it allows an implementation to skip past the partition table data, and isolate the partition of interest directly from the DMG.The 'data' in the blxx header is a structure, which (like its sibling, koly) is also identifiable by a fixed signature - in this case 'mish'. The output of such a property list is shown below:BWlzaAAAAAEAAAAAAAAAAAAAAAAAAAABAAAAAAAAAAAAAAIB/////wAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAIAAAAg6P2c0P9/AAAwGb9f/38AAAAAAAAAAAAAwBi/X/9/AAAoAAAAAAAAAAAAAAAAAAAAAKAFAAEAAAAYAAAAAAAAACAkv1//fwAA8Bi/X/9/AACKWZeI/38AACAZv1//FwAAKAAAAAAAAAAAAAAAAAAAAFhAAgEBAAAAABm/X/9/AAAAAAACgAAABgAAAAAAAAAAAAAAAAAAAAAAAAABAAAAAAAAAAAAAAAAAAAAOP////8AAAAAAAAAAAAAAAEAAAAABWlzaAAAAAEAAAAAAAAAAQAAAAAAAAA/AAAAAAAAAAAAAAIBAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAIAAAAgk1UtLv9/AAAwGb9f/38AAAAAAAAAAAAAwBi/AAAAAAACgAAABgAAAAAAAAAAAAAAAAAAAAAAAAA/AAAAAAAAADgAAAAAAAAAu/////8AAAAAAAAAAAAAAD8AAAAABWlzaAAAAAEAAAAAAAAAQAAAAAAAAIRcAAAAAAAAAAAAAAQEAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAIAAAAgbcaqTv9/AAAwGb9f/38AAAAAAAAAAAAAwBi/Listing 2: Sample XML plist in an APM formatted DMG (Flash installer)A detailed discussion of both APM and GPT can be found in chapter 15 of the book , as well as Apple's notes on APM and GPT. This forces all binary (8-bit) data to be encoded using Base-64 encoding (a wiser choice would have been using CDATA blocks). The property list file contains, at a minimum, a 'blkx' key, though it may contain other key/values, most commonly 'plst', and sometimes a service level agreement (SLA) which will be displayed by the OS (specifically, /System/Library/PrivateFrameworks/DiskImages.framework/Versions/A/Resources/DiskImages UI Agent.app/Contents/MacOS/DiskImages UI Agent) as a pre-requisite to attaching the DMG *.Due to XML parser restrictions, data in the property list is 7-bit.

The mish block is formatted like this:Uint32_t Version // Current version is 1Uint64_t SectorNumber // Starting disk sector in this blkx descriptorUint64_t SectorCount // Number of disk sectors in this blkx descriptorUint32_t BlockDescriptors // Number of descriptors// Where each BLXKRunEntry is defined as follows:Uint32_t EntryType // Compression type used or entry type (see next table)Uint32_t Comment // '+beg' or '+end', if EntryType is comment (0x7FFFFFFE).