Mac OS disk image types

Summary

Apple Mac OS supports disk image types used for various purposes by the operating system, e.g. software distribution in the form of .dmg files. This specification is based on available documentation and was enhanced by analyzing test data.

This document is intended as a working document of the data format specification for the libmodi project.

Document information

Author(s):	Joachim Metz <joachim.metz@gmail.com>
Abstract:	This document contains information about the Analysis of Mac OS disk image type formats
Classification:	Public
Keywords:	Mac OS disk image type formats

License

Copyright (C) 2012-2021, Joachim Metz <joachim.metz@gmail.com>.
Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.3 or any later version
published by the Free Software Foundation; with no Invariant Sections, no
Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included
in the section entitled "GNU Free Documentation License".

Revision history

Version	Author	Date	Comments
0.0.1	J.B. Metz	July 2012	Initial version.
0.0.2	J.B. Metz	January 2016	Switched to asccidoc format.
0.0.3	J.B. Metz	October 2016	Textual changes and additional information about formats.
0.0.4	J.B. Metz	November 2016	Textual changes and additional information about formats.
0.0.5	J.B. Metz	January 2017	Additional information about UDIF data format.
0.0.7	J.B. Metz	December 2018	Changes to formatting and addtional information about formats.
0.0.8	J.B. Metz	January 2019	Additional information about formats.
0.0.9	J.B. Metz	December 2019	Additional information about UDIF format.
0.0.10	J.B. Metz	March 2021	Additional information.
0.0.11	J.B. Metz	August 2021	Changes to UDIF formats.

1. Overview

Apple Mac OS supports disk image types used for various purposes by the operating system, for example software distribution in the form of .dmg files.

Currently Mac OS is known to support:

Identifier	Extension	Description
DC42		Unknown (Disk Copy 4.2)
SPARSEBUNDLE UDSB	.sparsebundle	Sparse bundle disk image
SPARSE UDSP	.sparseimage	Sparse disk image
UDBZ	.dmg	bzip2 compressed Universal Disk Image Format (UDIF) image
UDCO	.dmg	Apple Data Compression (ADC) compressed Universal Disk Image Format (UDIF) image
UDIF	.dmg	Read-write uncompressed Universal Disk Image Format (UDIF) image
UDRO	.dmg	Read-only uncompressed Universal Disk Image Format (UDIF) image
UDxx	.dmg	Uncompressed Universal Disk Image Format (UDIF) image
UDZO	.dmg	zlib/DEFLATE compressed Universal Disk Image Format (UDIF) image
ULFO	.dmg	LZFSE compressed Universal Disk Image Format (UDIF) image
ULMO	.dmg	LZMA compressed Universal Disk Image Format (UDIF) image

#   UDRW - read/write
#   UDTO - DVD/CD master
#   UFBI - entire device, uncompressed UDIF
#   IPOD - iPod image, similar to UDZO
#   RdWr - NDIF read/write
#   Rdxx - NDIF read-only
#   ROCo - NDIF compressed
#   Rken - NDIF compressed (KenCode)
#   UNIV - hybrid image (HFS+/ISO/UDF)

New Disk Image Format (NDIF)

1.1. Test versions

The following version of programs were used to test the information within this document:

Mac OS X 10.7 (Lion)
macOS 10.14 (Mojave)
macOS 11.2 (BigSur)

TODO add other Mac OS test versions

2. Universal Disk Image Format (UDIF)

Universal Disk Image Format (UDIF) are either uncompressed or compressed:

2.1. Uncompressed UDIF image format

Uncompressed UDIF images are equivalent to the RAW storage media data (CRawDiskImage).

2.2. Compressed UDIF image format

A compressed UDIF image consist of:

Data fork
Optional resource fork
Optional XML plist
Resource file at the end of the image file

Characteristics	Description
Byte order	big-endian

2.3. UDIF resource file

The UDIF resource file (UDIFResourceFile) is 512 bytes of size and consists of:

Offset	Size	Value	Description
0	4	"koly"	Signature
4	4	4	Format version
8	4	512	Resource file size Contains number of bytes
12	4		UDIF image flags
16	8		Unknown (RunningDataForkOffset)
24	8		Data fork offset The offset is relative from the start of the image file
32	8		Data fork size
40	8		Resource fork offset The offset is relative from the start of the image file
48	8		Resource fork size
56	4		Unknown (SegmentNumber)
60	4		Number of segments Contains 0 if not set
64	16		Segment identifier Contains an UUID
80	4		Type of the data checksum
84	4		Size of the data checksum
88	128		Data checksum
216	8		XML plist offset The offset is relative from the start of the image file
224	8		XML plist size
232	120		Unknown (Reserved1)
352	4		Type of the master checksum
356	4		Size of the master checksum
360	128		Master checksum
488	4		UDIF Image type (or variant)
492	8		Number of sectors
500	4		Unknown (reserved2)
504	4		Unknown (reserved3)
508	4		Unknown (reserved4)

Note	The XML plist size can be 0, such as in an UDIF stub (UDxx) image.

2.4. UDIF image flags

Value	Identifier	Description
0x00000001	kUDIFFlagsFlattened	Unknown (flattened?)

0x00000004	kUDIFFlagsInternetEnabled	Unknown (internet enabled?)

2.5. UDIF image types

Value	Identifier	Description
1	kUDIFDeviceImageType	Device image
2	kUDIFPartitionImageType	Paritition image

2.6. UDIF XML plist

TODO complete section

The UDIF XML plist contains the following key/value pairs:

Identifier	Description
resource-fork	dictionary

The resource-fork dictionary contains the following key/value pairs:

Identifier	Description
blkx	array of dictionaries
plst	array of dictionaries

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>resource-fork</key>
	<dict>
		<key>blkx</key>
		<array>
			<dict>
				<key>Attributes</key>
				<string>0x0050</string>
				<key>CFName</key>
				<string>Protective Master Boot Record (MBR : 0)</string>
				<key>Data</key>
				<data>
				bWlzaAAAAAEAAAAAAAAAAAAAAAAAAAABAAAAAAAAAAAA
				AAgIAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAIAAAAgQfL6MwAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAACgAAABQAAAAMAAAAAAAAAAAAAAAAAAAABAAAA
				AAAAIA0AAAAAAAAAH/////8AAAAAAAAAAAAAAAEAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAA=
				</data>
				<key>ID</key>
				<string>-1</string>
				<key>Name</key>
				<string>Protective Master Boot Record (MBR : 0)</string>
			</dict>
                        ...
		</array>
		<key>plst</key>
		<array>
			<dict>
				<key>Attributes</key>
				<string>0x0050</string>
				<key>Data</key>
				<data>
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEAAQAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
				AAAAAAAAAAAA
				</data>
				<key>ID</key>
				<string>0</string>
				<key>Name</key>
				<string></string>
			</dict>
		</array>
	</dict>
</dict>
</plist>

A block (blkx array entry) the following key/value pairs:

Identifier	Description
Attributes	string that contains a hexadecimal formatted integer value
CFName	string
Data	string that contains base-64 encoded data of the blkx table
ID	string that contains a decimal formatted integer value
Name	string

The block table (BLKXTable) is variable of size and consists of:

Offset	Size	Value	Description
0	4	"mish"	Signature
4	4	1	Format version
8	8		Start sector Contains the sector number relative to the start of the media data
16	8		Number of sectors
24	8		Unknown (DataOffset) Seems to be always 0
32	4		Unknown (BuffersNeeded)
36	4		Unknown (BlockDescriptors) Does this value always correspond to the number of block table entries?
40	4	0	Unknown (reserved1)
44	4	0	Unknown (reserved2)
48	4	0	Unknown (reserved3)
52	4	0	Unknown (reserved4)
56	4	0	Unknown (reserved5)
60	4	0	Unknown (reserved6)
64	136		Checksum Contains an UDIF checksum
104	4		Number of entries
108	… x 40		Array of block table entries

The block table entry (BLKXChunkEntry) is 40 bytes of size and consists of:

Offset	Size	Description
0	4	Entry type
4	4	Unknown (comment)
8	8	Start sector Contains the sector number relative to the start of the start sector of the block table
16	8	Number of sectors
24	8	Data offset Contains the byte offset relative to the start of the UDIF image file
32	8	Data size Contain the number of bytes of data stored, which is 0 for sparse data

Value	Identifier	Description
0x00000000		Unknown (sparse)
0x00000001		Uncompressed (raw) data
0x00000002		Sparse (used for Apple_Free)

0x7ffffffe		Comment

0x80000004		ADC compressed data
0x80000005		zlib compressed data
0x80000006		bzip2 compressed data
0x80000007		LZFSE compressed data
0x80000008		LZMA compressed data

0xffffffff		Block table entries terminator

struct UDIFChecksum {
  uint32_t type;
  uint32_t size;
  uint8_t data[128];
};

Is the maximum compressed chunk size 2048 sectors?

Comment seems to reference compressed data but has no size or number of sectors
value.

2.7. UDIF comment

TODO complete section

00000000 28 73 2a c1 1f f8 d2 11 ba 4b 00 a0 c9 3e c9 3b |(s*......K...>.;|
00000010 c7 09 25 40 27 b1 f6 46 a9 fa 91 35 3b a9 78 c3 |..%@'..F...5;.x.|
00000020 28 00 00 00 00 00 00 00 27 40 06 00 00 00 00 00 |(.......'@......|
00000030 00 00 00 00 00 00 00 00 45 00 46 00 49 00 20 00 |........E.F.I. .|
00000040 53 00 79 00 73 00 74 00 65 00 6d 00 20 00 50 00 |S.y.s.t.e.m. .P.|
00000050 61 00 72 00 74 00 69 00 74 00 69 00 6f 00 6e 00 |a.r.t.i.t.i.o.n.|
00000060 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00000080 00 53 46 48 00 00 aa 11 aa 11 00 30 65 43 ec ac |.SFH.......0eC..|
00000090 98 31 f0 dc 6a e3 8a 4c bf b1 eb 53 fb f1 8c dd |.1..j..L...S....|
000000a0 28 40 06 00 00 00 00 00 d7 ff 7b 02 00 00 00 00 |(@........{.....|
000000b0 00 00 00 00 00 00 00 00 64 00 69 00 73 00 6b 00 |........d.i.s.k.|
000000c0 20 00 69 00 6d 00 61 00 67 00 65 00 00 00 00 00 | .i.m.a.g.e.....|
000000d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00004000

2.8. UDIF data fork

TODO complete section

2.9. UDIF resource fork

TODO complete section

3. Sparse disk image (.sparseimage) format

The sparse disk image consists of:

file header
image data

Characteristics	Description
Byte order	big-endian

3.1. File header

The file header is 4096 bytes of size and consist of:

Offset	Size	Value	Description
0	4	"sprs"	Signature
4	4		Unknown (version?) Seen: 3
8	4		Number of sectors in band The bytes per sector is 512 bytes
12	4		Unknown Seen: 1
16	4		The non-sparse image size in sectors The bytes per sector is 512 bytes
20	12		Unknown (empty values)
32	4		Unknown
36	28		Unknown (empty values)
64	…		Array of band indexes
…	…		Unknown (empty values)

The array of band indexes contains 32-bit values that contain the band index where the band should be located in the non-sparse image. The first band is 1 and 0 indicates an unset value in the array.

Offset	Size	Value	Description
0	4		Band index

E.g. if the first band index is 1, then the sparse image data at offset 0x00001000 maps to offset 0 in the non-sparse image. A band non defined in the sparse image contains 0-bytes in the non-sparse image.

band data offset = 4096 + ( array index x sectors per band x 512 )
original data offset = 4096 + ( ( band index - 1 ) x sectors per band x 512 )

4. Sparse bundle disk image (.sparsebundle) format

The sparse bundle disk image was introduced in Mac OS X v10.5. It consists of a directory (bundle) with the .sparsbundle suffix containing:

sub directory:
- bands
files:
- Info.bckup
- Info.plist
- token

4.1. Info.plist and Info.bckup

The Info.plist and its backup (Info.bckup) is also referred to as "Information Property List". The file is an XML plist file that contains a single dict that contains the following key/value pairs.

Identifier	Description
CFBundleInfoDictionaryVersion	The version of the information property list format. Consists of a string containing a floating point The value should be 6.0 as of Mac OS X 10.0
band-size	The (maximum) size of a band (file) in bytes Consists of a string containing an unsigned integer
bundle-backingstore-version	Unknown Consists of a string containing an unsigned integer The value should be: 1
diskimage-bundle-type	The bundle type Consists of a string The value should be: com.apple.diskimage.sparsebundle
size	The media size in bytes Consists of a string containing an unsigned integer

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>CFBundleInfoDictionaryVersion</key>
        <string>6.0</string>
        <key>band-size</key>
        <integer>8388608</integer>
        <key>bundle-backingstore-version</key>
        <integer>1</integer>
        <key>diskimage-bundle-type</key>
        <string>com.apple.diskimage.sparsebundle</string>
        <key>size</key>
        <integer>4194304</integer>
</dict>
</plist>

4.2. Token

The token file is empty.

4.3. Bands

The bands sub directory contains files containing the actual data of the bands. The files are named using a hexadecimal naming scheme where 0 is the 1st band, a the 10th, f the 15th, 10 the 16th, etc.

5. Notes

https://github.com/planetbeing/libdmg-hfsplus/tree/master/dmg

Appendix A: References

[OSXDEVEL]

Title:	CFBundle Reference(s)
Author(s):	Apple Inc.
URL:	https://developer.apple.com/library/mac/#documentation/CoreFoundation/Reference/CFBundleRef/Reference/reference.html

Title:	CFBundle.h
URL:	http://opensource.apple.com/source/CF/CF-550/CFBundle.h

Title:	SparseBundle.c
URL:	http://www.opensource.apple.com/source/hfs/hfs-191.1/CopyHFSMeta/SparseBundle.c

[WIKIPEDIA]

Title:	Apple Disk Image - UDIF data format
URL:	https://en.wikipedia.org/wiki/Apple_Disk_Image#UDIF_data_format

Appendix B: GNU Free Documentation License

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You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an "aggregate" if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

8. TRANSLATION

Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

If a section in the Document is Entitled "Acknowledgements", "Dedications", or "History", the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.

However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.

10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.

11. RELICENSING

"Massive Multiauthor Collaboration Site" (or "MMC Site") means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A "Massive Multiauthor Collaboration" (or "MMC") contained in the site means any set of copyrightable works thus published on the MMC site.

"CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.

"Incorporate" means to publish or republish a Document, in whole or in part, as part of another Document.

An MMC is "eligible for relicensing" if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

Author(s):	Joachim Metz <[email protected]>
Abstract:	This document contains information about the Analysis of Mac OS disk image type formats
Classification:	Public
Keywords:	Mac OS disk image type formats

Files

Mac OS disk image types.asciidoc

Latest commit

History

Mac OS disk image types.asciidoc

File metadata and controls

Mac OS disk image types

Summary

Document information

License

Revision history

1. Overview

1.1. Test versions

2. Universal Disk Image Format (UDIF)

2.1. Uncompressed UDIF image format

2.2. Compressed UDIF image format

2.3. UDIF resource file

2.4. UDIF image flags

2.5. UDIF image types

2.6. UDIF XML plist

2.7. UDIF comment

2.8. UDIF data fork

2.9. UDIF resource fork

3. Sparse disk image (.sparseimage) format

3.1. File header

4. Sparse bundle disk image (.sparsebundle) format

4.1. Info.plist and Info.bckup

4.2. Token

4.3. Bands

5. Notes

Appendix A: References

Appendix B: GNU Free Documentation License

0. PREAMBLE

1. APPLICABILITY AND DEFINITIONS

2. VERBATIM COPYING

3. COPYING IN QUANTITY

4. MODIFICATIONS

5. COMBINING DOCUMENTS

6. COLLECTIONS OF DOCUMENTS

7. AGGREGATION WITH INDEPENDENT WORKS

8. TRANSLATION

9. TERMINATION

10. FUTURE REVISIONS OF THIS LICENSE

11. RELICENSING