Drop a DMG disk image. Any size.
safe · secure · no server · works offline · fast
…or just start typing at the cursor.
safe · secure · no server · works offline · fast
…or just start typing at the cursor.
OmniViewer opens a .dmg disk image right in your browser. Drop
the file and you get its structure laid bare: every partition
the image carries, each one’s size and compression format,
and the decoded koly trailer. There is no upload and no server
— the image is read directly by your browser, and the heavy work runs
in a background Web Worker so the page never freezes. Crucially, it reads
only the image’s index, so even a multi‑gigabyte installer opens
instantly and privately.
blkx block map), with its sector range, uncompressed size, number of block chunks and a breakdown of how those chunks are compressed. This is what a .dmg actually contains, read without mounting or extracting anything.koly trailer decoded in plain language: the UDIF version, image variant, the total sector count and uncompressed size, the segment UUID, and the data‑fork and master checksums. A disk image’s “header” actually lives at the end of the file; this is it. Read‑only — the trailer is checksummed, so rewriting it would corrupt the image.
Apple has shipped disk images since the late 1980s. Classic Mac OS used
NDIF (the New Disk Image Format) with the .img
extension; Mac OS X replaced it with UDIF, the Universal
Disk Image Format, and the .dmg extension it still uses today.
UDIF is what hdiutil and Disk Utility produce, and it has been
the delivery vehicle for Mac software for two decades. Apple never published
a formal specification — everything known about the format comes from
reverse engineering. Our
deep dive into the koly trailer and block maps
walks the whole structure, with the field tables and real parsing code.
OmniViewer opens every file format; the DMG toolkit sits alongside the other archive tools, powered by the same viewing engine as fastjsonviewer.com and hugecsv.com.
No. OmniViewer is a static page with no server-side processing: your .dmg is read directly by your browser, and every tab — partitions, metadata, stats and hex — runs locally in a Web Worker. The image never leaves your computer.
Yes. OmniViewer reads the UDIF structure — the koly trailer, the property list and the blkx block map — in pure JavaScript, so it works in any modern browser on Windows, Linux, macOS or a phone. No Mac, no hdiutil, no install.
No. It inspects the image’s structure — the partitions, their sizes and their compression — without mounting the filesystem or decompressing the contents. That is deliberate: it lets you see what a .dmg contains without trusting the image enough to mount it, and it is what keeps the whole thing local and fast.
Effectively unlimited. To read an image OmniViewer needs only two small reads — the 512-byte koly trailer at the end of the file, and the property list it points at (kilobytes to a few megabytes). The multi-gigabyte data fork is never read, so a 20 GB installer opens as fast as a tiny one.
A .dmg keeps its index at the end of the file, not the front: the last 512 bytes are a fixed structure whose magic number is the ASCII string "koly". It records the UDIF version, the uncompressed sector count, the checksums, and — most importantly — where the XML property list that maps the partitions begins. OmniViewer decodes it in the METADATA tab.
Each run of sectors is stored with a method named by a four-letter code: UDZO (zlib/DEFLATE, the classic default), UDBZ (bzip2), ULFO (LZFSE, Apple’s modern default) or ULMO (LZMA), plus raw (uncompressed) and zero-fill runs that cost nothing. OmniViewer shows the mix per partition in the PARTITIONS tab, the same codes you pass to hdiutil convert.
UDIF (Universal Disk Image Format) is the modern Apple disk-image format behind the .dmg extension. .smi is a "self-mounting image" variant, .dmgpart files are segments of a split image, and .img is the older NDIF extension — OmniViewer opens all of them in the same toolkit and reads them as UDIF where the koly trailer is present.