How an Apple DMG disk image is structured
An Apple Disk Image (DMG) is a container format created by Apple for distributing software and disk snapshots on macOS.
Modern DMG files use the Universal Disk Image Format (UDIF). The core metadata of a UDIF image is stored at the
very end of the physical file in a 512-byte block known as the koly trailer, starting with the 4-byte signature
koly (0x6B6F6C79).
The koly trailer stores relative offsets and byte lengths pointing to an embedded XML Property List (plist). This plist contains the blkx (block chunk table), which defines partition layouts, Apple Partition Maps (APM) or GUID Partition Tables (GPT), and chunk-level compression mappings (such as ADC, zlib/DEFLATE, and bzip2). Inside the primary partition sits an Apple HFS+ (Hierarchical File System Plus) or modern APFS (Apple File System) volume.
How to open and extract DMG files online
- Open the Apple disk image. Drop your macOS .dmg file into the box above or click to browse. The parser seeks to the final 512 bytes of the file to read the Universal Disk Image Format (UDIF) 'koly' trailer.
- Inspect partition maps and app bundles. View the partition tables, embedded Apple HFS+ or APFS file systems, and macOS application packages (.app).
- Audit embedded assets and documentation. Inspect bundle structures, application names, icons, and software license agreements (SLic).
- Export partition images and payloads. Export extracted disk partitions, raw disk images, or file catalogs directly to your device with one click.
What you get out
- Complete partition directory — volume labels, partition types (Apple_HFS, APFS, Driver Descriptor Map), and byte sizes.
- macOS application detection — identify embedded
.appsoftware bundles, frameworks, and installer packages. - Container architecture analysis — verify UDIF version, sector count, and chunk compression codecs.
- Direct partition exports — download raw partition images (.img) for mounting or forensic analysis.
- Exportable partition manifest — copy partition tables to the clipboard or export a tab-separated TSV manifest.
Which Apple DMG files this opens
This extractor opens standard Apple disk images:
- UDIF read-only compressed images (UDZO) — standard software distribution DMGs compressed with zlib.
- UDIF read-only uncompressed images (UDRO) — raw flat disk images.
- Internet-enabled disk images (UDBZ / ULFO) — distribution packages for macOS applications.
- macOS installers and developer SDK packages — software bundles distributed for OS X and macOS.
For optical CD/DVD disc images, use our ISO extractor. For Windows installer packages, use our MSI extractor.
Why zero-upload client-side extraction matters
DMG installers often hold complete proprietary application codebases, configuration files, and software licenses. Sending a 500 MB DMG file across the internet to a third-party conversion website consumes massive bandwidth and hands your software assets to unknown cloud servers.
EasyExtract processes DMG files 100% in your browser's private memory. File chunks are inspected locally using client-side JavaScript. No data is ever transmitted over the network or saved on remote hard drives.
Format limits and boundary edge cases
Three specific boundary cases are handled with clear notifications:
- FileVault encrypted DMG images. Encrypted sparse bundles or DMGs protected by Apple FileVault encryption require password authentication through macOS security frameworks and cannot be opened without native decryption keys.
- APFS proprietary snapshot trees. DMGs formatted with encrypted or multi-container APFS volumes are cataloged at the container level; deep file extraction requires an APFS-compatible file system driver.
- Segmented (.dmgpart) archives. Multi-part split DMGs must be concatenated before reading the trailing koly header block.
When opening DMG files on Windows or Linux helps
- Windows developers inspecting Mac apps — inspect assets, plists, and binaries inside a macOS DMG without owning a Mac or installing commercial utilities like TransMac.
- Linux sysadmins auditing Apple software — verify file contents, hashes, and licenses of software packages before deploying them to Apple fleets.
- Retrieving documentation or icons from a Mac download — extract high-resolution icons (.icns), PDF manuals, or READMEs without booting macOS.
- Safe pre-execution analysis — check what an Apple installer package contains before running it on a production workstation.
DMG images compared with ISO, EXE, and Archive formats
A DMG is a virtual hard drive formatted specifically for Apple ecosystems (HFS+/APFS), whereas an ISO is an optical disc image standard (ISO 9660) compatible across all operating systems. While DMG delivers software on macOS, Windows software is typically packaged in executable installers. To inspect Windows executables without running them, use our EXE file opener to safely extract EXE file contents on any OS.
For standard disk images, use our ISO extractor. For standard file archives, use our ZIP extractor or TAR extractor.
Apple DMG UDIF standards and partition edge cases
Apple Disk Images (.dmg) use the Universal Disk Image Format (UDIF). The primary metadata container
is the 512-byte koly trailer located at the final 512 bytes of the file, containing
big-endian pointers to the embedded XML Property List (plist) and resource forks.
Three extraction edge cases: (a) the XML plist contains base64-encoded mish block
tables describing partition chunk runs (zlib, bzip2, or uncompressed raw sectors); (b) DMGs often enclose
Apple HFS+ or APFS file systems — extracting files requires parsing the HFS+ volume header or exporting the
partition image for secondary inspection; (c) Internet-enabled DMGs (UDZO) often feature embedded software
license agreements (SLic) that prompt users on macOS. Read our technical guide on
how to open DMG files on Windows.
Frequently asked questions
How do I open a DMG file on Windows 10 or 11?
Drag and drop your macOS .dmg file into the dropzone above. EasyExtract parses the Apple UDIF container and extracts partition contents directly in your browser without installing TransMac or 7-Zip.
Is my DMG file uploaded to a remote server?
No. The disk image is read directly from your local disk and processed entirely within your browser's private JavaScript sandbox. Your files and data are never transmitted over the internet.
Can this tool extract .app applications from a DMG?
Yes. EasyExtract detects macOS .app application bundles within the partition structure and allows you to inspect and export the application payload.
Why does macOS use DMG instead of ZIP?
DMG files act as virtual disks that support file system permissions, icon positioning, custom folder background graphics, and software license agreements that must be accepted before mounting.
Can I open encrypted DMG files?
DMG images encrypted with FileVault AES-128 or AES-256 are identified as encrypted containers. Decrypting them requires mounting on macOS with the appropriate administrative passphrase or keychain entry.
Can I open DMG files on Linux or Chromebooks?
Yes. Because EasyExtract runs client-side in the web browser, it opens Apple DMG files seamlessly on Ubuntu, Fedora, Debian, ChromeOS, and Android without needing `hfsplus` or `dmg2img` command-line packages.
Can I export the DMG partition table?
Yes. Click 'Copy partition list' to copy the complete partition layout to your clipboard, or click 'Download partition list' to export a tab-separated (.tsv) report.