Unpack a Mac .dmg installer using guestfish

If you’re not familiar with Mac OS X, it uses a packaging format derived from NextStep where applications are directories and to install an application, you just drag and drop the package directory into the “Applications” folder (or wherever you choose because apps are self-contained and can run from anywhere).

Mac OS X installers use a file format called an “Apple disk image” or *.dmg file, which is a sort of bzip2-compressed filesystem. In the example below, I downloaded the binary installer for the brilliant open source video player VLC, and double-clicked on it, whereupon Mac OS X mounts the packaged filesystem. In the image below, the VLC application looks like a file but in reality is a directory full of application files.

Can we decode and unpack these .dmg files in Linux using libguestfs? Well, not always, but sometimes (because of some limitations in the Linux HFS+ driver described below).

First I should note that this probably doesn’t work with password-protected / encrypted files, but they’re not common for software distribution. DMGExtractor claims it can handle those.

Secondly the image is usually bzip2-compressed, and you have to run bunzip2 on it by hand first:

$ mv Firefox\ 3.5.3.dmg firefox-3.5.3.dmg.bz2
$ bunzip2 firefox-3.5.3.dmg.bz2
bunzip2: firefox-3.5.3.dmg.bz2: trailing garbage after EOF ignored

(Yeah, I don’t know how important that error was either … But it doesn’t seem to affect things.)

Thirdly the file system image is not at the start of the file. You have to find it and strip off some sort of header from the file. Look for the HFS+ superblock in the file:

$ hexdump -C firefox-3.5.3.dmg | grep '  48 2b 00 04'
00008400  48 2b 00 04 00 00 01 00  63 65 72 64 00 00 00 00  |H+......cerd....|

and strip it, less 0×400 bytes:

$ dd if=firefox-3.5.3.dmg of=firefox.img bs=$((0x8400 - 0x400)) skip=1

This should produce an HFS+ filesystem image. Check that:

$ file firefox.img
firefox.img: Macintosh HFS Extended version 4 data last mounted by: 'cerd',
created: Mon Aug 24 10:37:21 2009, last modified: Mon Aug 24 18:37:21 2009,
last backup: Mon Aug 24 18:37:21 2009, last checked: Mon Aug 24 18:37:21 2009,
block size: 2048, number of blocks: 25773, free blocks: 0

Now this can be loaded directly into guestfish:

$ guestfish --ro -a firefox.img -m /dev/sda

Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.

Type: 'help' for help with commands
      'quit' to quit the shell

><fs> ll /
total 180
lrwxrwxr-x  1  502   80     13 Aug 24 13:37   -> /Applications
drwxr-xr-x  1  502   80      7 Aug 24 13:37 .
dr-xr-xr-x 29 root root      0 Oct 25 17:41 ..
-rwxr-xr-x  1  502   80  12292 Aug 24 12:41 .DS_Store
-rw-r--r--  1  502   80 165905 Aug 24 12:41 .VolumeIcon.icns
drwxr-xr-x  1  502   80      3 Aug 24 13:37 .background
drwxr-xr-x  1  502   80      3 Aug 24 13:33 Firefox.app

and in this case, I just wanted to extract the contents to a tarball, so:

><fs> tgz-out / firefox.tgz

$ zcat firefox.tgz | tar tf -

“hfs: failed to load catalog file”

It worked with the Firefox installer, but two other installers I tried (for VLC and Disk Inventory X) didn’t go so well. The HFS+ driver produced this error for those:

hfs: failed to load catalog file

which as far as I can see is either a problem that these .dmg files don’t follow the HFS+ [proprietary] standard by missing out the catalog B-tree, or else the HFS+ driver can’t find that B-tree for unknown reasons.

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