Apple Disk Image file signature
Magic number at the start of the file · koly (at end of file)
A modern Apple .dmg (UDIF) has no header magic. Its signature is a trailer: the ASCII word koly begins a 512-byte block at the very end of the file, describing the disk image.
What this magic number means
A magic number is a short, fixed run of bytes at a known position that tells a program what a file really is, no matter what the filename says. If a file starts with the bytes 6B 6F 6C 79 (the text koly (at end of file)), it is a Apple Disk Image file.
Because the marker is at the end, not the start, tools must seek to the last 512 bytes to confirm a DMG. Older compressed DMGs may instead begin with an encrypted or zlib header.
End-of-file marker
This format is identified by a trailer, not a header. The bytes 6B 6F 6C 79 appear near the end of the file (koly trailer, 512 bytes from the end).
Formats that use the 6B 6F 6C 79 signature
These file types in our database carry this signature:
How to check a file's signature
You can read the first bytes of any file yourself. The magic bytes are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.dmg
- hexdump -C -n 16 example.dmg
- file example.dmg
Windows (PowerShell)
- Format-Hex -Path example.dmg -Count 16
Python
- open("example.dmg","rb").read(4).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the Apple Disk Image file signature?
Apple Disk Image files start with the hex bytes 6B 6F 6C 79 (koly (at end of file) in ASCII). This magic number identifies the format regardless of the file's name or extension.
How do I check a file's magic number?
Open the file in a hex editor, or run a command such as xxd -l 16 example.dmg on Linux or macOS, or Format-Hex -Path example.dmg -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the Apple Disk Image signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine Apple Disk Image file has these exact bytes; a file with the wrong bytes is not really Apple Disk Image, whatever its name says.